Training-Report-FINAL

– Laboratory Internship –
Stage d’Initiation à la Recherche et Développement (SIRD)
2014 -2015
Trainee: Anas MAJDOULI
Supervisor(s): Assoc. Prof. Runa T. HELLWIG / Dr. Muhammad ARIFEEN WAHED
Tutor: Mr. Maxime ROGER (MCF)
06/04/2015 – 04/09/2015
A simulation-based study on
Domestic Water Heating systems
in Singapore
Département Génie Energétique et Environnement
Solar and Energy Efficient Buildings (SEEB) Cluster - SERIS

2
Acknowledgments
First of all, I would like to express my deep sense of gratitude to Associate Prof Runa
HELLWIG, my supervisor at SERIS, for giving me the opportunity to undertake this laboratory
internship in SERIS, a leading solar energy research institute that is respected all around the
world. She provided me all the necessary assistance and advice to begin my training in the
best conditions. Throughout my training, she has always shown keen interest and
encouragement.
I am very much thankful to Dr. Muhammad ARIFEEN WAHAD, my co-supervisor, for
his valuable guidance at every step of my project. He regularly took care to follow my
progress, always provided help when I asked for, and never stopped to encourage me.
This made me achieve my goals as best as possible.
In particular, I would like to thank Dr. Frédéric LEFEVRE, the head of the department
GEN at INSA de Lyon, for his special devotion to help us take the necessary steps during the
application and recruiting process of this internship.
I would also like to thank Région Rhône-Alpes for providing financial support.
I am very grateful to all my colleagues at SERIS for contributing every day to our
wonderful work atmosphere, for offering help and sharing their experiences. My work would
not have been completed as expected without the assistance of Dr .Lu ZHAO.
Finally, I express deep and sincere gratitude to my family and my friends. Their undying
support and affection is the greatest gift anyone has ever given me.

3
Table of Contents
Acknowledgments...................................................................................................................................2
Table of Contents....................................................................................................................................3
Abstract...................................................................................................................................................5
Résumé ...................................................................................................................................................6
Nomenclature .........................................................................................................................................7
Abbreviations..........................................................................................................................................7
I- Introduction ....................................................................................................................................8
II- Literature review.............................................................................................................................9
1. Singapore’s energy landscape and fuel mix................................................................................9
i. Energy imports........................................................................................................................9
ii. Energy consumption ...............................................................................................................9
2. Common types of water heaters in Singapore’s market ..........................................................11
i. Instantaneous Tankless Water Heaters [Electric, Gas ITWH] ...............................................11
ii. Storage Tank Water Heaters (STWH)....................................................................................12
iii. Standard ratings....................................................................................................................14
iv. Comparison between Tankless and Storage Water Heaters:...............................................15
3. Renewable energies on the rise in Singapore...........................................................................16
i. Solar Photovoltaic energy.....................................................................................................17
ii. Solar Thermal energy............................................................................................................18
4. Singapore’s carbon footprint....................................................................................................22
III- Methodology.............................................................................................................................24
1. Research design ........................................................................................................................24
i. Summary of the project........................................................................................................24
ii. Cases of the study.................................................................................................................24
2. Approach and modeling............................................................................................................25
i. Overall methodology of the study........................................................................................25
ii. Simulation software: TRNsys ................................................................................................27
iii. TRNsys components..............................................................................................................27
iv. TRNsys worksheets of the simulated cases ..........................................................................38
3. Assumptions..............................................................................................................................45
4. Input Data .................................................................................................................................45
i. Weather Data........................................................................................................................45
ii. Domestic Hot Water (DHW) profile generation ...................................................................46

4
IV- Results and Discussion..............................................................................................................52
1. Daily performance of water heaters:........................................................................................52
i. Case 1: Electric Instantaneous Tankless Water Heater (Electric ITWH) ...............................52
ii. Case 2: Gas Instantaneous Tankless Water Heater (Gas ITWH) ...........................................55
iii. Case 3: Electric Storage Tank Water Heater (Electric STWH) ...............................................60
iv. Case 4: Thermal Solar Domestic Water Heating system (Thermal SDWH)...........................63
2. System comparison:..................................................................................................................66
i. Energy savings.......................................................................................................................66
ii. Cost savings...........................................................................................................................67
iii. Environmental impact: carbon footprint..............................................................................69
V- Conclusions ...................................................................................................................................72
VI- Limitations of the study & Future work........................................................................................74
List of Figures ........................................................................................................................................75
List of Tables .........................................................................................................................................76
References ............................................................................................................................................76
Appendixes............................................................................................................................................80
1. TRNsys Documentation.............................................................................................................80
2. Generation, Distribution and Usable temperatures.................................................................92

5
Abstract
In Singapore, water heating accounts for 21 % of the total consumed electricity in a
household, as it is the second biggest energy guzzler after air-conditioning. In order to be able
to shift into more sustainable energy systems and reduce reliance on imports of fossil fuels,
Singapore is investing on different alternative technologies. Solar energy is one of the major
fields of research.
Within this context, this study was conducted to compare the performances of four
different water heating systems, in order to ascertain the financial and environmental impacts
of improvements such as fuel switching and solar thermal retrofit.
Three systems are commonly used water heaters: Electric, Gas Tankless and Electric Storage
water heaters. Another system is a solar thermal water heater that uses electricity as a backup
energy source.
The four systems of interest were modeled and simulated in the TRNsys software for
one year time period, considering the same Domestic Hot Water (DHW) profile.
The latter was elaborated to meet the hot water needs of an average landed property for
more than four persons. Also, reliable weather data over 2014 were collected and sorted from
the weather station of NUS. The DHW profile and the weather data were both implemented
in each model. All of the systems are supposed to deliver hot water at a same constant
temperature.
A previous investigation into domestic water heating in Singapore showed that Electric
tankless and storage water heaters are the most commonly used appliances in Singapore.
Therefore, for this study, these two systems were designated as base cases for tankless and
storage waters heaters respectively. Accordingly, for tankless water heaters, the comparative
study was conducted on a source energy basis to capture the differences in energy, cost and
carbon emissions related to the use of electricity or natural gas. For storage tank water
heaters, by comparing a base Electric STWH to a solar thermal retrofitted one, the interest
was on quantifying the benefits of the solar thermal technology.
The results show that, when aggregated to all landed properties of Singapore,
appreciable cost savings and significant carbon reductions may be achieved when natural gas
replaces electricity in tankless water heaters. Also, regarding the storage water heaters,
potential energy savings can reach 40 % in energy use when comparing the solar thermal
solution to the base Electric Storage water heater, resulting in substantial annual cost savings
and carbon reductions at the household and national levels. The solar thermal solution
(Electric backup) was also compared to tankless water heating systems and proved to be a
superior solution for energy and cost savings, and that it could be improved if natural gas
replaces electricity as source energy for the backup heater (e.g. gas furnace).

6
Résumé
À Singapour, la production d’eau chaude sanitaire (ECS) représente 21 % de la
consommation totale d’électricité dans le milieu résidentiel, et se place deuxième principal
consommateur d’énergie après la climatisation. Afin de faciliter sa transition vers des
systèmes énergétiques plus durables, et par conséquent diminuer sa dépendance vis-à-vis
des importations de combustibles fossiles, la cité-État a fait de la sécurité et de l'efficacité
énergétique les priorités de sa politique en investissant sur de nouvelles technologies.
L’énergie solaire constitue l’un des plus grands pôles de la R&D, étant donné le climat tropical
favorable de Singapour.
Dans ce cadre, cette étude a été réalisée pour examiner et comparer les performances
de quatre systèmes de production d’ECS, dans le but de cerner les impacts financiers et
environnementaux liés au remplacement de l’électricité par du gaz naturel comme source
d’énergie. L’étude permettra également d’évaluer le potentiel d’un couplage à un système
solaire thermique. Les trois premiers systèmes sont très couramment utilisés : chauffe-eau
instantané électrique, ou au gaz ; et un ballon d’eau chaude électrique. Le dernier système
étudié est un chauffe-eau solaire à appoint électrique.
Ces systèmes ont tous été modélisés et simulés sur TRNsys sur une période d’un an,
en considérant le même profile journalier de consommation d’eau chaude domestique,
représentatif d’une propriété privée abritant plus de quatre personnes. De même, des
données météorologiques ont été implémentées dans chaque cas d’étude, après les avoir
recueillies et triées auprès de la station météo de NUS. Tous ces systèmes sont supposés
délivrer de l’eau chaude à la même température de consigne.
Une enquête précédente avait montré que les chauffe-eaux électriques - instantané
et à réservoir de stockage - sont les plus utilisés à Singapour. Par conséquent, ces deux
systèmes ont été pris comme cas de référence afin de mener une étude comparative ayant
pour critères le type de source d’énergie utilisée et l’intégration d’un système solaire
thermique. Le but est de déterminer les économies potentielles d’énergie, les réductions du
coût de fonctionnement et d’émissions de CO2.
Les résultats de cette étude montrent que l’utilisation du gaz naturel comme source
d’énergie dans les chauffe –eaux instantanés permet de réaliser des économies appréciables
et des réductions d’émission de CO2 considérables à l’échelle nationale.
De même, le chauffe-eau solaire à appoint électrique permet d’économiser près de 40 % de
l’énergie consommée par un ballon d’eau chaude électrique de référence. Par conséquent,
cette solution génère des profits importants et des réductions d’émissions CO2 substantielles,
à l’échelle domestique comme à l’échelle nationale. Une amélioration possible consisterait à
utiliser un chauffe-eau solaire avec une chaudière à gaz.

7
Nomenclature
a0 : Intercept efficiency (-)
a1 : 1st
order heat loss W.m-2
.K-1
a2 : 2nd
order heat loss W. m-2
.K-1
Cp : Specific heat of water J.kg-1
.K-1
ΔTH : Upper Dead Band K
ΔTL : Lower Dead Band K
𝑚̇ : Mass flow rate kg.hr-1
m_schedule : Mass flow rate of the domestic hot water provided to the user kg.hr-1
η : Thermal efficiency of the device (-)
Q_Cons : Effective energy consumption of the water heater kWh
Q_Deliv : Net energy delivered to the fluid flowing through the water heater kWh
Q_Loss : Overall thermal losses from the water heater to the environment kWh
Q_Pump : Required energy to run the pump of the solar loop kWh
Q_Solar : Useful energy transferred to fluid with solar collector kWh
Q_NonSolar : Energy consumed by the backup heater in the solar heating systems kWh
Tambient : Ambient temperature °C
t : Simulation time-step h
T_bottom : Temperature at the bottom of the storage tank °C
Tj : Temperature of water at Node j °C
TH : Upper temperature of the differential controller °C
TL : Lower temperature of the differential controller °C
Tin : Temperature of water entering the Electric/Gas ITWH °C
Tout : Temperature of water leaving Electric/Gas ITWH °C
Tsupply : Temperature of water that is supplied to the user °C
Tset : Set point temperature °C
T_top : Temperature at the top of the storage tank °C
UA : Heat transfer value W.K-1
Uside : Average heat loss factor for the edges of the storage W.m-2
.K-1
Ubottom : Average heat loss factor for the bottom of the storage W.m-2
.K-1
Upipe : Average heat loss factor for the piping W.m-2
.K-1
Abbreviations
DB : Dead Band
DHW : Domestic Hot Profile
ECF : Energy Conversion Factor
Electric STWH : Electric Storage Tank Water Heater
ETC : Evacuated Tube Collector
FPC : Flat-Plate Collector
HX : Heat Exchanger
IAM : Incidence Angle Modifiers
ITWH : Instantaneous Tank Water Heater [Electric-; Gas-]
NG : Natural Gas
SDWH : Solar Domestic Water Heating [Thermal-; PV-]
Thermal STWH : Thermal Storage Tank Water Heater
TRNsys : Transient System Simulation program

8
I- Introduction
All around the world, it is now accepted that renewable energies are becoming an
increasingly important issue to ensure a diversified, green and cost-effective energy mix,
facing today’s raising cost of fossil fuels whilst reducing greenhouse gas emissions.
Governments are stepping up efforts to accelerate the transition towards alternative energies
through technological progress and significant cost reductions, combined with novel
applications from R&D outcomes.
For the small city state of Singapore, the lack of significant indigenous energy
resources requires the use of various forms of renewable energy to meet the country’s energy
needs, and consequently reduce reliance on fossil fuels imports; mainly petroleum, natural
gas and coil. It will improve the Singapore’s energy diversity and security.
However, Singapore suffers from some geographic constraints and hence has limited
alternative energy potentials. Indeed, the lack of space, low wind speed and weak marine
currents are among the unfavorable natural conditions, going against Singapore’s energy self-
sufficiency.
Despite this limited resources, and due to its location on a sun belt that receives 50%
more radiation than temperate climate, Singapore has been focusing on solar energy over
this last decade, as a promising renewable energy source. With an average annual solar
irradiance of 1,150 kWh.m-2.year-1 [1], and driven by S$ 1 billion of public money committed
by the government to energy R&D without taking into account private funding,
the deployment of solar energy over the city-state is growing [2]; significantly for solar
photovoltaics and mildly for the solar thermal technology.
Solar PV is already popular in the city-state with a total of 636 PV installations across
Singapore for electricity generation [3]. Researchers are convinced that ‘in the mid to long-
term about 10 to 20 % of the nation’s electricity demand will come from solar PV’ [Dr. Thomas
REINDL, Deputy CEO of Solar Energy Research Institute of Singapore]. The levelized cost of
electricity (LCOE)1 of PV has already fallen below retail tariffs in the city –state and continues
to decrease as the systems costs are falling down.
As for the solar thermal energy, one application in Singapore is water heating for
domestic usage. This technology is slowly gaining popularity, even though the efficiency of
solar thermal collectors is higher than the efficiency of PV modules, for the water heating
applications. By this study, we intend to gain more knowledge on the feasibility of solar
thermal technology for domestic water heating in Singapore.
1
Levelized cost of electricity (LCOE) represents the per-kilowatt-hour cost (in real dollars) of building and
operating a generating plant over an assumed financial life and duty cycle

9
II- Literature review
1. Singapore’s energy landscape and fuel mix
Singapore is a small city state of 710.2 km2. Its population is estimated to have reached
5.4 million by the end of 2014, of which the resident population accounts for approximately
70.7 %, mainly Singapore citizens [4].
i. Energy imports
With limited local energy resources, this country is highly dependent on the import of
oil and natural gas to ensure energy supply security. Indeed, in 2014, Singapore imported
6,781 million GJ of energy products, compared to 3,600 million GJ exported.
Energy imports consist of Crude Oil (CO), Coal and Peat (CP), Petroleum Products (PP),
Natural Gas (NG) in its pipelined (PNG) and liquefied (LNG) forms, and other energy products.
It is important to note the increase of NG imports to 431 million GJ, 4.5% higher than in 2013.
Table 1 below compares, for each source, the amount of energy imported in Singapore over
2013 and 2014. The values are given in million gigajoules (million GJ).
2013 2014
TOTAL 6,678 6,781
Crude Oil (CO) 1,976 1,987
Coal and Peat (CP) 11 17
Petroleum Products (PP) 4,276 4,344
Natural Gas 413 432
- Pipeline NG 370 337
- Liquefied NG 44 95
Other Energy Products 1 2
Table 1- Singapore's energy mix in 2013 and 2014 [3]
ii. Energy consumption
All information contained in this section refers to EMA’s energy statistics of 2015 [3].
Singapore’s total final energy consumption (TFEC) 2 decreased by 6.5 % to 561 million
GJ (i.e. 156 TWh) in 2013, covering mainly petroleum products (PP), electricity and natural
gas demands. PP demand felt of 13 % in favor of an increase experienced by electricity and
natural gas demands, by 1.6 % (161 million GJ, i.e. 45 TWh) and 15 % (54 million GJ i.e. 15
TWh) respectively. Figure 1 shows how TFEC is divided by sector and energy product in 2013.
Industrial-related sector covered around 65 % of TFEC, while households accounted only for
2
Total final energy consumption (TFEC) refers to energy that is supplied to the consumer for all final energy
uses such as heating, cooling and lighting [48]

10
4.9 % (28 million GJ, i.e. 7.7 GWh) mainly in the form of electricity (87 %) and natural gas after
being transformed to town gas (13 %).
By the end of 2014, Singapore’s total electricity consumption reached 46 TWh,
primarily industrial-related (42.6%), commerce and service-related (36.5%),
and for households (14.9 %). For this latter, the electricity consumption rose by 2.5%
to 6.9 TWh, compared to that of 2013. Public housing units account for 60% (4.1 GWh) of
household’s total energy consumption, while landed properties consume the 40% remaining
(2.8 GWh).
Figure 1- Total Final Energy Consumption by Sector & Energy Product, 2013

11
2. Common types of water heaters in Singapore’s market
For Singapore, the weighted energy consumption profile across all housing types
shows that water heating accounts for 20.9% of the total consumed electricity in a household
according to 2012 Household Consumption Survey [5]. It is the second biggest energy guzzler
after air-conditioning.
A domestic hot water system is used to heat a household’s cold mains water to a set
point temperature and deliver it to for domestic usage to appliances such as sinks,
dishwasher, and shower. This study accounts only for shower use as it represents 67% of total
hot water demand [6].
A previous investigation into domestic water heating in Singapore showed that Electric
Instantaneous Tankless Water Heaters (Electric ITWH) are the most commonly used appliance
in HDB apartments, while private housing (including landed properties) are using Electric
Storage Tank Water Heaters (Electric STWH) [7]. Besides these two main types of water
heating systems (Instantaneous-tankless and storage tank water heaters), solar thermal and
solar photovoltaic energy sources are considered as a promising alternatives and gain, at
different speed, popularity in Singapore.
The following sections describe the various water heaters mentioned above.
i. Instantaneous Tankless Water Heaters [Electric, Gas ITWH]
As its name suggests, this water heater produces hot water on-demand and delivers it
at a set point temperature, instead of storing preheated water in a tank.
When the user turns on the hot water tap, the mains water enters the heater and a
sensor detects the water flow. The controller automatically activates the heating element.
The energy source can be either electricity or gas, using either an electric element or a gas
burner respectively. Then, a heat exchanger heats the water to the preset temperature and
delivers it through the pipes for usage. When the tap is tuned off, the unit shuts down.
Figure 2 shows the inside of an electric (2.a) and Gas (2.b) instantaneous water heaters.
Figure 2- Schematic of an Electric (a) and Gas (b) Instantaneous Tankless Water Heaters
[Source: US Department of Energy]

12
In the market, tankless systems exist in two size ranges: the point-of-use and the
whole-house use systems.
The point-of-use installations use small units to provide hot water for only one fixture.
In Singapore, this type of heaters is commonly encountered in hostels to deliver hot water for
each shower block. Indeed, this system operates more efficiently and is less expensive
because of the short distance between hot water production and supply that reduces water
wastage and piping losses to environmental surroundings.
The whole-house-use tankless water heater is designed to deliver the hot water to the
whole dwelling. These units are fairly larger and are installed in a utility room or garage [8].
ii. Storage Tank Water Heaters (STWH)
Conventional storage water heaters consist of a cylindrical vessels or containers that
use electricity, gas or oil to heat a large volume of water from the mains water temperature
to the set point temperature. The hot water is stored inside the tank and can be delivered
continuously when it is needed. As is the case for tankless water heaters, storage heaters use
either gas burner (Figure 3 - Right) or electric resistance element(s) (Figure 3 - left), depending
on the energy source.
To maintain the delivered temperature at the set point temperature, the tank includes
a thermostat and controller to watch the temperature of water at the top of the tank, and
compare it to the set point temperature in order to turn on or off the heating device of the
tank.
When the temperature at the top of the domestic tank (Generation Temperature) is
above the desired usage temperature, a tempering valve is used to mix hot and cold water to
prevent the risk of scalding.
Generally, cold water enters from the bottom of the tank and hot water is supplied
to the various fixtures from the top through insulated pipes. Given that water density varies
with its temperature (hot water is less dense), as it heats, water rises naturally to the top of
the tank. Therefore, the temperature inside the tank is stratified into multiple isothermal
layers, so that as water is drawn from the top of the tank, the user benefits of hot water until
it is depleted.

13
In Singapore, and generally in tropical countries, mains water and ambient
temperatures are moderate. Therefore, small capacity of storage is sufficient: the typical size
range of the storage tanks is from 10 to 35 L [9]. However, for solar applications, landed
properties still use the common size range of 75 to 400 L such as in temperate zones (e.g.
United States, Europe)
Less conventional water heating technologies include solar thermal or solar
photovoltaic energies to produce hot water within a storage water tank that is combined with
backup heaters, powered by electricity or fossil fuels.
Indeed, solar thermal collectors can be used to harvest the solar energy.
The latter is then transferred to thermal energy in a fluid circulated loop in order to heat water
through an immersed heat exchanger inside the tank.
Solar Domestic Water Heating (SDWH) is discussed later in this report
[cf. Section II-3].
Figure 3- Electric (left) and Gas (right) Storage Tank Water Heaters (STWH)

14
iii. Standard ratings
The standard ratings of water heaters are based on the Energy Factor (EF) and First
Hour Rating (FHR), in accordance with the 10CFR430 test procedure [10].
Energy Factor (EF):
A water heater’s overall energy efficiency is determined by the Energy Factor (EF),
based on the amount of hot water produced per unit of fuel consumed over a typical day.
It represents the ratio of useful energy output from the water heater to the total energy
consumed over 24 hours under specified test conditions. For the 10CFR430 procedure, the
testing conditions are given in Table 2 below:
Set point temperature 57.2 °C
Ambient temperature 19.7 °C
Ambient Relative Humidity 50 %
Water mains temperature 14.4 °C
Table 2- Standard conditions for 10CFR430 Test of Energy Consumption of Water Heaters [10]
The calculation of the Energy Factor takes into account the following performance
criteria [11]:
Recovery efficiency – the ratio of energy delivered to the water to the energy
consumed by the water heater.
Standby losses (for Storage Tank Water Heaters) – the percentage of hourly
heat loss from the stored water compared to heat content of the water. Standby losses
account for thermal losses to the environment through the top, the bottom and the sides of
the storage tank.
Cycling losses – the thermal losses through inlet and outlet pipes.
Typical Electric Tankless & Storage Water Heaters can reach high efficiencies
(minimum EF ≈ 0.9), while Gas Tankless Water Heaters has commonly a lower Energy Factor
(EF ≈ 0.82) because of heat losses from exhaust gases). [12]
Solar Energy Factor (SEF):
The solar energy factor is defined as the ratio of energy delivered by the system (or
the Useful Energy [QUE] exchanged through the immersed heat exchanger between
circulation fluid of solar loop and the water inside the tank) to the sum of the amount of
energy consumed by the backup heating element (Auxiliary Energy [QAE] and the amount of
energy required to power the pumps, controllers, etc. (Parasitic Energy [QPE]).
𝑆𝐸𝐹 =
𝑄 𝑈𝐸
𝑄 𝐴𝐸 + 𝑄 𝑃𝐸
(1)

15
SEF is generally between 1.0 and 11 and the most common systems have SEF between
2 and 3.
First Hour Rating (FHR)
The First Hour Rating represents the amount of hot water in gallons the heater can
supply per hour, assuming that the tank is full of hot water at the beginning. It depends on
the tank capacity, heating device (burner or electrical resistance) and its size.
For this study, the selected water heaters should satisfy the Energy Star Program
Eligibility Criteria for Residential Water Heaters [13]. It means that each system shall meet all
of the identified criteria. Above are only the main performance criteria. For this purpose, the
parameters of the water heating components used for the simulations are based on an
existing brands, and own the label ENERGY STAR.
iv. Comparison between Tankless and Storage Water Heaters:
Compared to tankless water heater, storage water heaters constantly lose heat to the
environment through the walls of the tank. Therefore, the hot water inside of the tank will
cool down and when water temperature decreases below the set point temperature, the
heating element is turned on by the controller. As a result, more energy is consumed to
maintain the water at the desired temperature [12]. It is then common to use insulation in
order to increase the R-value of the tank and reduce these losses.
One more advantage of tankless water heaters is that they are more durable than
storage water heaters, with more than 20 years lifespan, while a typical storage water tank
last 6-to-12 years [14].
Even with less space and no standby losses, there are still some issues in using tankless
water heaters. One of the major drawbacks is the “cold water sandwich” phenomenon.
It occurs usually between two consecutive draws of hot water.
After the first draw, the tankless water heater is turned off but hot water might still be
available inside the pipes. Therefore, the second draw will start with hot water followed
directly by short cold water flow. When the sensor inside the water heater detects the
required minimum flowrate, the heating device will turn on and then hot water is delivered
again.
Also, tankless water heaters has a higher installation costs. Indeed, recent tankless
systems need electrical outlets for their fan and electronics, upgraded gas pipes, and a new
ventilation system. That can bring average installation costs to US $ 1,200, compared with
US $ 300 for storage-tank models [15]. In addition, tankless water heaters can use only
electricity or fossil fuels as energy sources, while storage tank systems can use renewable
energies such as solar energy to heat directly the water inside the tank (Thermal Solar
Domestic Water Heating – T-SDWH) or to power its electric heating element(s) (PV-SDWH).

16
3. Renewable energies on the rise in Singapore
The total solar energy received at the periphery of the earth’s atmosphere is
approximately 1.74 x 1014 kW (174 PW). Not all of this amount of energy is reaching the
surfaces of the earth. Indeed, around 30 % of it is reflected back into outer space.
In the process of penetration through the atmosphere, the remaining 70 % of solar energy is
either absorbed by the oceans, clouds and lands or dispersed in all directions by the
atmospheric gases and dust particles [16].
As a result, after interaction with the atmosphere (reflection, absorption and
dispersion), the amount of solar energy absorbed by land and oceans is reduced to
approximatively 2.8 x 1021 kJ per year [16].
Figure 4 – Solar radiation breakdown [16]
According to the IEA3, the worldwide energy consumption in 2008 was 5.18 x 1017 kJ
mainly supplied from fossil fuels [17]. Therefore, the solar energy that is absorbed by land
and oceans is more than 5400 times the total energy consumed by the total world energy
consumption in 2008. This shows how promising is the solar energy as a renewable and
environmentally friendly alternative to reduce the reliance on fossil fuel sources.
The main challenge is only about finding a way to convert its useful energy, efficiently and
cost effectively.
Comparing to Europe, Singapore has a very stable climate. All through the year, the
climate in the city state is hot and humid with sunny weather conditions. Indeed, located on
the sun belt near the equator, Singapore receives 50 % more radiation than temperate climate
3
International Energy Agency

17
zones. Previous study of [18] compares monthly solar radiation of Singapore, Abu Dhabi and
Berlin.
Figure 5 shows that Singapore solar irradiance is uniform and that its annual average
solar insolation is 1,634 kWh.m-2. For the city state of 710.2 km2, the amount of solar energy
absorbed on its surface is then estimated to 1,181 PWh [19].
And, according to Singapore Energy Statistics 2015 published by EMA [3], the total energy
consumption in Singapore over 2014 accounted for more than 6.3 PWh, i.e. around 19 times
less than the absorbed solar energy.
Solar energy can be used for both domestic and industrial applications such as heating,
cooling, lighting, electric powering, etc. in order to reduce the dependence on fossil fuel.
It is important to distinguish between solar thermal and photovoltaic technologies. Both are
two different ways to harness the sun’s energy.
i. Solar Photovoltaic energy
Photovoltaic (PV), or PV energy conversion, directly converts sunlight into electricity.
The phenomena taking place in PVs is called the photoelectric effect. It implies that, in certain
materials, the photons of light can be absorbed, causing electrons to release from molecules
within the material. By attaching electrical conductors to the positive and negative side of the
cell, it is possible to capture these free electrons in the form of electric current, so-called
electricity [20].
Photovoltaic technologies are not the area of interest for this work, but it is important to
note that PV systems use directly the solar energy.
Figure 5: Solar irradiance in Singapore, Abu Dhabi and Berlin [18]

18
ii. Solar Thermal energy
Solar thermal energy is another form of harnessing solar energy. It is used for various
thermal applications. This study focuses on water heating for domestic usage (Thermal
SDWH).
The main component of a Thermal SDWH system is the solar thermal collector.
This element absorbs solar radiation and transfers it to a heat transfer fluid (circulating fluid).
In turn, this latter exchanges the heat gained to the mains water. In some configurations,
water may be directly the circulating fluid.
Solar thermal collectors operate even under intermittent cloudy weather conditions.
Indeed, unlike PV cells, they use both direct and diffuse radiations to heat the fluid inside as
it is the surrounding heat that is being absorbed and not the light [21].
Generally, a pump and controller are also required to constitute the solar loop
(active systems), where the heat transfer fluid circulates before entering or after leaving the
collector. The system comprises also a water tank equipped with an immersed heat exchanger
and a backup heater in case solar energy is not enough to reach and/or maintain the desired
temperature of water to be supplied. Figure 9.b shows a schematic of a typical installation of
a solar DHW system using an auxiliary heater and a coiled heat exchanger, immersed inside
the storage tank, to transfer heat from the heat transfer fluid of the solar loop to the mains
water.
When they operate without the use of a pump (passive systems), the circulating fluid
is directly the mains water, and the tank must be placed above the collector, so that cold
water flows down by gravity from the bottom of the tank towards the collector, and hot
water, less dense, rises up naturally to enter the storage tank from the top. Figure 9.a shows
a layout of this configuration.
Among several types of solar thermal collectors, the most common types are Flat-Plate
(FPC) and Evacuated Tube (ETC) collectors:
Flat-Plate Collector (FPC):
A typical FPC consists of a large heat absorbing plate (absorber), usually a large thin
sheet of copper or aluminum for their high thermal conductivity.
This plate is painted black or receives a selective coating, so that it absorbs maximum solar
radiation at the maximum efficiency. Several parallel copper pipes (risers) are arranged across
the absorber, bonded or soldered to maximize the surface of contact.
They contain the heat transfer fluid (working fluid) that is typically water. However, in cold

19
climates, water in the panel is subject to freezing, so the system employs a glycol/water
mixture as working fluid.
When sunlight hits the absorber, the temperature of the surface increases. The heat
is then conducted through the rises to the working fluid. This latter circulates inside the solar
loop from the collector to a heat exchanger (Figure 9.b and 9.c ) in order to transfer heat to
the mains water (Indirect Thermal SDWH), or can be used directly by the household in other
configurations (Figure 9.a).
The risers and absorber plate are enclosed in an insulated metal or wooden box, glazed
on the top to protect the material inside and to create an insulating air space.
The glazing does not absorb the solar heat before it reaches the absorber, but prevent heat
to escape from the absorber to the environment through the air gap.
Also, to reduce this loss of heat, the bottom and sides of the FPC are insulated with high
temperature rigid foam, or aluminum foil insulation [21].
Evacuated Tube collector (ETC):
Evacuated Tube Collectors (ETC) are another type of solar thermal collectors, more
efficient than the Flat Plate Collectors (FPC). Although FPC is cheap and easy to install, its
efficiency is limited by the rectangular shape of the absorber. Indeed, they can operate at
maximum efficiency only when the sun’s rays arrive perpendicularly to the collector’s surface.
Otherwise, the sunlight hits the glazing cover at different angles, and is partially reflected back
to the atmosphere [21].
A typical ETC consists of several glass tubes disposed in parallel and all connected to a
header pipe. The tubes are replacing the dark absorber of the FPC. Each glass tube contains a
Figure 6- Schematic of a typical Flat Plate Collector (FPC)

20
heat pipe which is covered by a special absorbing material, forming the inner tube. Usually
cylindrical in shape, this latter always receives sunlight perpendicularly, enabling thereby the
collector to operate at good efficiency throughout the day, even when the position of the sun
is low, after sunrise and before sunset. In addition, air is removed between inner and outer
tubes. The vacuum thus created insulates the inner tube and thereby reduces the heat losses
from the absorbing coating.
When sunlight penetrates the glass tube, the heat is absorbed by the inner tube and
is then transferred to the fluid that is contained inside the heat pipe. By convection, the fluid
heats a “hot bulb”, located at the top of each glass tube and connected to the manifold. By
circulating through the header pipe, cold water is heated by these “hot bulbs”.
In another configuration (Figure 8), the liquid contained in the inner tube (heat pipe)
vaporizes due to the insulation of the vacuum, and thereby becomes lighter. Therefore, the
hot vapors rises up to the top of the heat pipe, heating the “hot bulb” at very high
temperature.
By losing energy, the hot vapors condense and flow back down the heat pipe to be
heated again. This type of collectors is called Heat Pipe Evacuated Tube Collector.
Figure 7-Schematic of a typical Evacuated Tube Collector (ETC)

21
Solar thermal collectors can also include a tracking mechanism that detects sun
position, intermittent clouds and day or night conditions, and gives instruction to a DC motor
to adapt the position of the collector so that it follows the sun path during the day, and return
it back to its original position, facing the east, at the end of the day.
Figure 9- Schematic of common configurations of Thermal SDWH systems.
(A) NATURAL CIRCULATION SYSTEM. (B) ONE-TANK FORCED-CIRCULATION SYSTEM. (C) SYSTEM WITH ANTIFREEZE
LOOP AND INTERNAL HEAT EXCHANGER. (D) SYSTEM WITH ANTIFREEZE LOOP AND EXTERNAL HEAT EXCHANGER
SOURCE: [27]
Figure 8- Schematic of a Heat Pipe Evacuated Tube Collector (HP-EVC)

22
4. Singapore’s carbon footprint
One of the objectives of this study is to examine the environmental perspective of
each considered water heating solution and the impacts of switching from a solution to
another. Indeed, when dealing with energy systems, it is important to always consider,
besides energy and cost savings, the carbon footprint as an indicator of environmental
sustainability to which the city state is working towards.
Therefore, it is important to first identify Singapore’s global emissions profile of CO2, and then
after conducting the study, ascertain the carbon footprint related to our specific application
(water heating).
Figure 10 depicts Singapore dioxide emissions over the last two decades.
As it may be seen, depending on whether the indicator examines the micro or macro
perspective, Singapore emissions of CO2 seem to have stabilized, and are even decaying. In
addition, Singapore contributes currently by less than 0.2 % of global emissions. But it is worth
noting that the population of Singapore accounts of only 0.072 % of the world’s population.
Additionally, if the period from 1997 to 2007 is considered, it can be seen that in less than 20
years, Singapore total carbon emissions increased by 83 % to reach 39.9 Mt in 2007, while the
population size grew only by 50.6 % during the same period. Accordingly, the carbon
emissions by capita increased by 21.5 %.
This suggests that an average resident of Singapore emits 2.8 times more Carbon than
an average person in the world, and thereby the city state is ranked 27th out of 137 countries
in terms of emissions per capita based on the latest IEA data [22]. Clearly, some
considerations such as size and density of the population, production economic activity level
and economic growth should be taken into account in measuring and comparing Singapore’s
carbon footprint with the rest of the world.
Figure 10- Singapore's Carbon Dioxide Emissions from 1990 to 2007
SOURCE: [49]

24
III- Methodology
1. Research design
i. Summary of the project
This project is a comparative study of different water heating solutions that can be
suitable for the tropical climate of Singapore. Indeed, the city-state is regarded as a favorable
site for solar installations.
This study focuses on residential water heating to produce hot water for domestic
usage, especially for shower application.
Within this context, four different cases (scenarios) are studied and simulated on the
TRNsys software, a flexible program used to simulate the performance of transient systems.
Three common systems are electric or gas- Instantaneous Tankless Water Heaters (ITWH) and
Electric Storage Tank Water Heater (Electric-STWH). One other system is a Solar Domestic
Water Heating (SDWH) alternative: it uses solar thermal collectors (Thermal-SDWH).
Other systems use PV panels to generate electricity and heat water by an electric
heating device (PV-SDWH). In this study, only Thermal SDWH are considered.
In this study, the four systems of interest are modeled in the TRNSYS software and
simulated for one year time period. The same water draw profile is used in all the cases, and
all of the systems are supposed to deliver hot water at a constant temperature.
The comparison of the water heaters is then conducted from simulation results to determine
the energy and cost savings, as well as carbon reductions, in switching from a water heating
scenario to another. The base cases are Electric ITWH and Electric STWH.
ii. Cases of the study
The simulated cases:
Case 1: Electric Instantaneous Tankless Water Heater (Electric-ITWH)
Case 2: Gas Instantaneous Tankless Water Heater (Gas-ITWH)
Case 3: Electric Storage Tank Water Heater (Electric STWH) using an electrical resistance element.

25
Case 4: A solar thermal collector connected to an Electric STWH. (Thermal SDWH)
Data required
 Reliable weather data of Singapore (Ambient temperature, solar irradiance, etc.)
 Domestic Hot Water (DHW) Profile
2. Approach and modeling
i. Overall methodology of the study
Step 1: Weather Data preparation for simulation
It consists in collecting and sorting of weather data from the Solar Energy System (SES)
group, one of the clusters of the laboratory that provide forecasting for PV electricity
generation. The weather station is implemented at the university (National University of
Singapore), near the Southwest coast of Singapore, approximately nine kilometers from the
city center at height of approx. 90 m above the sea level and therefore receives superb
exposure. The data consists of average ambient temperature and solar global irradiance for
each minute over 2014 in Singapore, and it is used as input in the simulation software
(TRNsys).
The simulation software TRNsys already contains weather data for any place on Earth.
However, this data is not always reliable, especially when it comes to solar applications that
require solar irradiance for the particular weather of Singapore. Indeed, the solar irradiance
is highly fluctuating on a daily scale. Therefore, more precise data is required to expect better
results.
Step 2: Domestic Hot Water Profile (DHW Profile)
This study requires a local DHW schedule to represent the hot water consumption in
Singapore.
First, PUB (Public Utilities Board)4 data on daily water consumption per person is
collected. Then, a previous study which relies on local statistics provides hot water
4
Singapore's national water agency

26
requirement per person, then per dwelling, assuming a landed property of more than 4
persons [4].
Given this daily hot water demand, a literature survey was conducted to compare
various DHW profile generating methods, based on different standards (ISO, SRCC, etc.).
One recent approach is to use a German tool DHWCalc [23] to generate a typical daily DHW
profile on statistical basis. This software can incorporate seasonal variations,
weekend/weekday and holiday periods, etc. However, it requires reliable statistics of
Singapore to ascertain this variability. Therefore, assumptions were made and some default
values were used instead.
Step 3: TRNsys simulation of the selected water heating systems.
From basic worksheet preparations to parameter settings and optimization, this step
requires to consult mainly the software’s documentation and previous studies before turning
the simulations. Also, the library of components is available with a wide variety of “types”
that can be used to model the same system. However, the results might be from slightly to
highly different.
A detailed description of each modeled case is given later in Section III-2.iv.
Step 4: Determination of system performance.
For each scenario, the performance of DHW systems is ascertained by determination
of the overall energy consumption, the useful energy gain and energy savings.
Step 5: Comparison of water heaters and determination of cost savings.
Several comparative studies are conducted to quantify energy savings in switching
from a water heating option to another, or in modifying the most relevant parameters.
Then, from the different tariff charges, operating costs and cost savings are computed.
Step 6: Conclusions and recommendations

27
ii. Simulation software: TRNsys
TRNsys, or Transient System Simulation program is widely used to validate new energy
solutions from simple individual systems to the entire design and simulation of modeled
buildings and their associated equipment [24].
This software is very appropriate for pre studies because of its easy-to-use interface.
It consists in adding existing components, called “types”, from a wild integrated library into a
project, and connect them with other components in order to build the desired system.
The “types” are mathematical models that read user specified or other components
inputs, with preset parameters. All of the mathematical models use their parameters and
inputs and interact with other components to compute the various outputs, that can be
displayed using an external file and/or a plotting component for later analysis [24].
Also, transient simulations require to specify the time step and time period by setting
simulation start and stop times. Simulation time step should be in accordance with input data
time step, in case user-specified data is needed. The time period represents the total duration
of the simulation. Once the outputs over one time step have been calculated, the software
updates the inputs and outputs and the simulation moves to the next time step until it reaches
the stop time.
For this study, the simulation is over one year. Indeed, the weather data collected from
SES cluster of SERIS is given over 2014 for each 6 minutes. Therefore, the simulation time step
is also set to 6 minutes, hence 0.1 hour in TRNsys.
iii. TRNsys components
The four cases of this study were modeled and simulated using TRNsys. Each case
requires creating a separate TRNsys project. In addition, all of the components were selected
from the standard TRNsys or the added TESS libraries. In most of the cases, both provide a
broad variety of models for one same system. However, these “similar” types rarely lead to
the same results and are more or less accurate. Literature survey and some tests were
conducted to choose between the proposed components. Also, TRNsys and TESS
documentations were consulted for this purpose.
In this section, the basic functions in relation with the various parameters, inputs and
outputs are discussed. Further details on mathematical models of each TRNsys component
are available on TRNsys documentation. The documentation of the main components is also
provided in Appendix 1.
Common Types:
Type 15-6
Type 99

28
Type 24
Type 65a
Type 14 (14b for DHW profile & 14h for pump schedule [Thermal-SDHW] )
Type 31
Type 3d
+ Equation box
Water heaters Types:
Type 6
Type 940
Type 1226 + Type 2
Type 71
Storage tank Type:
Type 534 (Storage No-HX, immersed coiled HX)
a) Common Types
Type 15.6: Weather Data Processor
Type 15 was used to read weather data at regular time intervals from an external
weather data file provided by the software. Many calculations are performed and includes
unit conversion to a desired unit system and interpolations to determine direct and diffuse
radiation at each time step specified by the user. This outputs can be used by other
components such as solar thermal collector, or PV panels. The model also includes calculation
of other useful terms such as mains water temperature that can be used for the purpose of
this study. Weather data is read in a series of standardized formats. The data file used in Type
15-6 is from Meteonorm files (.TM2). This pack provides weather information of Singapore.
However, weather data from Type 15-6 is not regularly updated and thus are not
accurate enough to be implemented for solar applications. If reliable data is available from a
weather station, Type 99 can be used instead.
Type 99: Combined Data Reader and Radiation Processor (User-Defined Data Format)
Type 99 was used to provide accurate weather data for simulation. Also, it uses the
same algorithms as Type 15 to calculate for example solar radiation on titled surfaces. After
collection and sorting, the data is implemented in a text file, following a strict file syntax.
The file is divided into two parts: the header and the data. Both parts are enclosed by
two keywords <userdefined> and <data>. Inside the header, exact geographical information
of the location, data file time interval and time corresponding to first data line must be
specified. Then, weather data variables are indicated by their corresponding keywords. One
line statement is dedicated for each variable. In the same line, information about the position
of the variable in columns, the type of interpolation, addition and multiplication factors are
indicated by additional keywords:

29
<var> NAME <col> VALUE <interp> VALUE <add> VALUE <mult> VALUE <samp> VALUE
Keyword Value Description
<col> Number of the column for variable NAME. If value = 0, the variable
will be skipped and the respective output set to zero
<interp> 0 No interpolation
1 Linear interpolation
2 Five point spline interpolation (Akima)
<add> addition factor ai
<mult> multiplication factor mi
<samp> -1 column value is a mean value related to the time interval Δtd ending
at the time corresponding to actual data line
0 column value is a mean value related to the time interval Δtd/2
before and after the time corresponding to actual data line
1 column value is a mean value related to the time interval Δtd starting
at the time corresponding to actual data line
Table 3- Header settings of TRNsys Weather Type 99
For this study, both Type 15-6 and Type 99 were used. Indeed, Type 99 gives accurate
values for ambient temperature and global radiation from the weather station, while Type
15-6 provides other data such as solar zenith and azimuth angles, angle of incidence for
surface, etc.
Figure 11 shows ambient temperature and global irradiance plots from Meteonorm5
Type 15-6 and from Type 99 (Weather station data of 2014 read on a user-defined file) for the
8th of January 2014 in Singapore.
5
Default global climatological database available on TRNsys.
Figure 11- Comparison between Type 15-6 Meteonorm data and Type 99 Weather Station collected data [TRNsys]

30
Obviously, the plots based on the weather station data better reflects Singapore’s
fluctuating climate than the plots based on Meteonorm data. Indeed, this latter gives results
from interpolated long term monthly values.
Type 24: Quantity Integrator
This component models a quantity integrator that can be used with Type 65-a Online
Plotter and Output File. It integrates a series of quantities over the entire simulation period
using Equation 2, where Yi is the total integrated value of the quantity or rate Xi up to ti.
𝑌𝑖 = ∫ 𝑋𝑖 𝑑𝑡
𝑡 𝑖
0
(2)
By using Type 24 with an output file, the integrated values can be saved in a separate
file for later analysis. For example, for water heating simulations as it is in this project, the
power supplied by an electric heating device can be integrated using this model to determine
the energy quantity consumed over one year simulation. The
output file displays the energy variables integrated to the last time step. Therefore, the last
value of the column represents the total annual energy consumption of the heating device.
Similarly, energy consumptions, heat losses and other energy variables can be displayed for
each system for later comparison.
Integration Period is the only parameter to specify in order to run Type 24. If
integration period equals simulation time, this parameter should be set to “STOP”. The input
and output are respectively the quantity or rate to be integrated and the result of the
integration over the specified period.
Type 65-a: Online graphical plotter with output file
While the simulation is performing, Type 65 allows viewing several selected output
variables from the different components in a separate plot window. It was a very useful tool
since it permits to observe immediately if the simulation is progressing as expected.
When the simulation stops, the user can adjust the window and choose the desired
curves to be displayed. Besides its graphical plotter, Type 65-a is able to save the simulation
results in a text file, and the user can specify the unit on each column under the labels. Each
column is representative of one output variable. All saved values can be extracted from the
text file and used later for analysis.

31
Type 14: Time Dependent Forcing Function (DHW draw & pump schedule)
Type 14 is used to define time-dependent profiles for any period (one day, week, etc.)
and that can be repeated throughout the simulation. The profile consists of a “set of discrete
date points indicating the value of the function at various times over one cycle” [25].
For the purpose of this study, two schedules were necessary to generate: DHW profile
and pump schedule. The same profiles were then used for all four cases.
Type 14-b is an exclusive forcing function for water draw profile generation.
It has two outputs: instantaneous water draw and average water draw. When it is connected
to the tank, instantaneous water draw was used as input for mains water flow rate.
Type 14-h is used for pump control signal in the Thermal SDWH system (Case 4) to
build a daily operating schedule for the pump.
Type 31: Pipe
Type 31 was incorporated in each case to take account of the thermal behavior of hot
water flowing through a pipe, and especially to model heat losses to the environment. The
component models a set of fluid segments that are “pushed” out by the inlet flow entering
the pipe.
By applying conservation of mass, the outlet temperature To is calculated from the
weighted average temperature of the leaving segments, as given in the equation below:
𝑇0 =
1
𝑚̇ ∆𝑇
× (∑ 𝑀𝑗
𝑘−1
𝑗=1
𝑇𝑗 + 𝑎 × 𝑀 𝑘 𝑇𝑘)
(3)
where, 𝑎 represents the fraction of the last segment k that was pushed out at the time step,
and 𝑚̇ ∆𝑇 is the mass of fluid entering the pipe in one time period, creating a new segment.
The total energy losses to the environment are then the summation of individual losses from
each segment, according to Equation 4 below:
𝑄̇ 𝑒𝑛𝑣,𝑗 = (𝑈𝐴) 𝑗 × (𝑇𝑗 − 𝑇𝑎𝑚𝑏𝑖𝑒𝑛𝑡) (4)
where, j refers to a segment of fluid in the pipe; (UA)j, Tj are the associated heat transfer
coefficient and temperature respectively. Tambient is the ambient temperature.
The main parameters of this type are: pipe dimensions (inside diameter and length),
the loss coefficient and fluid properties. As inputs, inlet temperature and flow rate are
connected from another component, and the environment temperature is indicated (or
connected to the weather data Type 99). Among the outputs, Type 31 computes the outlet
temperature and flow rate, and calculates the heat losses to the environment.

32
Type 3d: Pump
Even though the installed pumps have variable speed control, the speed is supposed
fixed for this study. Therefore, Type 3d is used to model a single speed pump that circulates
the heat transfer fluid in the solar loop of the Thermal SDWH system (cf. Figure 13).
The user can specify the input flow rate and the power consumption. Both parameters
are provided by the manufacturer technical specifications.
This component is connected to a time dependent forcing function (Type 14h) that
represents the daily operating schedule for the pump. The input control signal of the pump
comes from the output of Type 14h.
Equation box
The user can add an equation to the simulation. This requires to specify inputs
variables to create intermediates and outputs and build the equation. Then the equation
inputs and outputs are connected to their associated components. This was a very useful tool
for unit conversion from TRNsys outputs units to the International System of Units.
b) Water heaters Types:
Type 6: Auxiliary heater (used as Electric ITWH)
Type 6 models an auxiliary heater that elevates the temperature of a flow stream to a
set point temperature, by setting a maximum heating rate, 𝑄̇ 𝑚𝑎𝑥. This component was
designed to add heat at a rate less than or equal to 𝑄̇ 𝑚𝑎𝑥 , depending on the control function
ɣ.
The efficiency of the device, η, and the overall loss coefficient, (UA), can be set by the
user to take account of the inefficiencies due to the auxiliary components (connections, etc.)
and the heat losses to the environment.
By setting the control function to 1, and a sufficiently large value for 𝑄̇ 𝑚𝑎𝑥, Type 6
operates then like a tankless water heater with internal control to maintain the outlet
temperature at the desired temperature Tset. By specifying a high value for the efficiency of
the device (close to 1), this component can perform like an Electric ITWH, and thereby is used
in Case no.1 in the absence of a specific model for this type of electric water heaters.
Many calculations are performed by this component throughout the simulation period
to compute the outlet temperature, the required heating rate and the heat loss rates at each
time step. The control logic and the associated equations can be found in the TRNsys
document “Mathematical Reference” given in Appendix 1.

33
Type 940: Gas Tankless Water Heater
Type 940 is used in Case no.2 to model a Gas ITWH. According to the description [26],
Type 940 is simply an auxiliary heater with internal controls to modulate the heat input to the
fluid, in order to reach and maintain the set point temperature Tset.
The efficiency of the device in converting the fuel source (natural gas for this study) to
heat is specified.
One of the important features of this component is the minimum flowrate of water
circulating through the device that should be specified as a parameter. Indeed, the burner
will ignite only after the water flow sensor detects water stream at a flowrate above this
threshold. Existing Electric/Gas ITWH operates similarly. Unlike Type 940, Type 6 doesn’t take
account of this parameter, and thereby approaches less the real functioning of tankless water
heaters.
Also, a temperature dead band (DB) is included as a parameter for controlling the
outlet temperature of the device. Indeed, typical tankless water heaters usually provide hot
water at a temperature in the range of Tset ± DB/2.
Both minimum flow rate and temperature DB are important when Type 940 operates.
Indeed, the control logic for this model (Appendix 1) shows that each step of the process is
accomplished only after comparing the flow rate of water to the threshold, and the outlet
temperature to the allowed range of temperature (Tset ± DB/2).
All the equations and steps to model the performance of this component are given in
Appendix 1.
Type 2b + Type 1226: Aquastat (Heating Mode) + Electric Tank Heating Device
An aquastat is a device used for controlling water temperature. In TRNsys, It consists
of a differential controller (Type 2b) that generates an output control function (ɣo) that can
have values of 0 (off) or 1 (on). In heating mode, the value of this control function is set by
comparing, at a time step, the difference between the set point temperature (Upper
temperature TH) and the temperature to be monitored (Lower temperature TL), with two dead
band temperature, ΔTH and ΔTL.
Then, the output control function (ɣo) becomes the input control function (ɣi) for the
next time step and the new value of ɣo is determined depending on the previous state of the
controller as illustrated in Figure 12.

34
Let’s consider that the controller was ON (i.e. ɣi =1). The lower deadband temperature
ΔTL is then compared to the difference between the upper and lower temperature (TH -TL),
and the controller will remain ON until the temperature difference falls below the lower
deadband.
Similarly, if the controller was previously OFF (i.e. ɣi =0), the upper deadband
temperature ΔTL is now compared to (TH -TL). Therefore, the output control function (ɣo) will
remain equal to 0 until the temperature difference becomes larger than the upper deadband.
The model includes also a high limit cut-out temperature to be specified by the user
for safety reasons. The controller will set the output control function to 0, regardless of
whatever else is happening, if the temperature being monitored exceeds this safety limit
temperature.
Generally, in water heating applications, an aquastat is used to control a heating
element. Type 2b “watches” the outlet temperature of the tank (Tout) and compares it with
the set point temperature (Tset), the safety limit temperature TMAX,the turn OFF (ΔTL) and the
turn ON (ΔTH) temperature differences. Afterwards, it will generate an output control
function ɣo which is the input control signal of the heating element.
For this study, Type 1226 is the component used to model the heating element.
It is a tank electric heating device that was used either to provide the total energy required
to heat the mains water to Tset [Case 3], or only to add an amount of energy as a backup heater
to maintain the outlet temperature to Tset [Case 4 & 5].
Type 1226 computes two values as outputs: the power supplied to end use 𝑄̇ 𝑓𝑙𝑢𝑖𝑑 and
the power consumed 𝑄̇ 𝑖𝑛𝑝𝑢𝑡 by the heater (of which only 𝑄̇ 𝑓𝑙𝑢𝑖𝑑 is useful), represented by
Equation 5 and 6 resp. The relation between 𝑄̇ 𝑓𝑙𝑢𝑖𝑑 and 𝑄̇ 𝑖𝑛𝑝𝑢𝑡 is given by Equation 7:
𝑄̇ 𝑓𝑙𝑢𝑖𝑑 = 𝜂 × ɣ 𝑜 × 𝑃̇ 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 (5)
𝑄̇ 𝑖𝑛𝑝𝑢𝑡 = ɣ 𝑜 × 𝑃̇ 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 (6)
Figure 12-TRNsys Aquastat (Type 2b) Controller
Function SOURCE : [25]

35
𝑄̇ 𝑖𝑛𝑝𝑢𝑡 =
𝑄̇ 𝑓𝑙𝑢𝑖𝑑
𝜂
(7)
Type 71: Evacuated Tube Solar Collector (ETC)
Used in Case no. 4, Type 71 models the thermal performance of evacuated tube
collectors for solar thermal water heating (Thermal SDWH). This component uses a quadratic
efficiency curve and a biaxial Incidence Angle Modifiers (IAM) to model the collector
performance.
The quadratic efficiency curve, given in Equation 8, is an extension of a linear collector
model that was obtained from the Hottel-Whilier equation [27], where it is assumed that the
efficiency of the collector is a quadratic function of the difference between inlet and ambient
temperatures, and the solar irradiance.
𝜂 = 𝐹𝑅(𝜏𝛼) 𝑛 − 𝐹𝑅 𝑈𝐿
(𝑇𝑖 − 𝑇𝑎)
𝐼 𝑇
− 𝐹𝑅 𝑈 𝐿
𝑇
(𝑇𝑖 − 𝑇𝑎)²
𝐼 𝑇
(8)
where :
FR : Overall collector heat removal efficiency factor [-]
(τα) : Product of the cover transmittance and the absorber absorptance [-]
(τα)n : (τα) at normal incidence [-]
Ti : Inlet temperature of fluid to collector [°C]
Ta : Ambient (air) temperature [°C]
IT : Global radiation incident on the solar collector (Tilted surface) [kJ/h-m²]
UL : Overall thermal loss coefficient of the collector per unit area [kJ/h-m²-K]
UL/T : Thermal loss coefficient dependency on T [kJ/h-m²-K²]
Equation 8 can also be rewritten as follows:
𝜂 = 𝑎0 − 𝑎1
(𝑇𝑖 − 𝑇𝑎)
𝐼 𝑇
− 𝑎2
(𝑇𝑖 − 𝑇𝑎)²
𝐼 𝑇
(9)
where a0 (intercept efficiency), a1 and a2 are the parameters provided by the manufacturer of
the collectors that were tested according to ASHRAE standards and rated by the Solar Rating
and Certification Company (SRCC), as well as for collectors tested under European Standards
on Solar Collectors (CEN).
The main parameters of this component are the collector’s area, orientation, tilt, and
the properties of the heat transfer fluid (working fluid) that circulates in the solar loop and
hence through the collector.
Also, Type 71 requires 10 inputs:
The inlet temperature and flow rate are the respective outlet temperature and flowrate of
the pipe that connects the pump to the collector in the solar loop (Figure 13).

36
From Type 99 (Weather Station Data, the ambient temperature and the incident radiation are
provided.
By connecting Type 15-6 (TRNsys Meteonorm (default) Weather Data), the incident diffuse
radiation, the solar incident, zenith and azimuth angles, the collector slope and azimuth are
implemented.
The outputs are the outlet temperature, the outlet flow rate and the useful energy gain.
Type 71 is also designed to correct the efficiency curve parameters for flow rate and
angle of incident solar irradiance.
By comparing the operational flow rate to the flow rate at test conditions, analytical
corrections are applied to adjust the value of 𝐹𝑅(𝜏𝛼) 𝑛 and 𝐹𝑅 𝑈𝐿.
In addition, the angle of incidence is corrected using a biaxial incidence angle modifiers (IAM).
These unit-less multipliers are used to represent the angle dependence of the optical
efficiency of a solar collector. Generally, collector tests are conducted on clear days and at
normal incidence so that the product Transmittance x Absorptance (τα) is nearly at the normal
incidence value for beam radiation (τα)n. For non-normal solar incidence, the intercept
efficiency 𝑎0 = 𝐹𝑅(𝜏𝛼) 𝑛 is corrected by the factor (τα)e/(τα)n, that represents the ratio
between the effective absorbed and the incident absorbed radiations (Equation 10).
𝐼𝐴𝑀 ≡
(𝜏𝛼) 𝑒
(𝜏𝛼) 𝑛
= 𝐼𝐴𝑀(𝜃𝑖, 0) × 𝐼𝐴𝑀(0, 𝜃𝑡)
(10)
where 𝜃𝑖 and 𝜃𝑡 are respectively the longitudinal and transverse angles of incidence.
Figure 14: IAM correction for ETC - Transversal and longitudinal directions
SOURCE: [25]
Figure 13- layout of the solar loop of Thermal SDWH system [TRNsys]

37
In TRNsys, the number of longitudinal and transverse angles are specified, as
parameters of Type 71. Also, in a separate text file, the user lists the IAM data as function of
this angles. This data is generally provided by the manufacturer specifications for one (50°) or
different angles. Then, the component can read the file and interpolate the specified data.
c) Type 534: Vertical Cylindrical Storage Tank
All the cases that use water tanks includes Type 534 to model a stratified storage water
tank in a vertical configuration. The tank is divided into isothermal temperature nodes to
model temperature stratification between these layers. Each node has a constant volume and
interacts thermally with the nodes above and below by conduction, convection (forced
convection from inlet flow streams and natural convection by destratification mixing due to
a thermal inversion within the tank).
Depending on the application, the user has the ability to specify if the tank includes an
immersed heat exchanger (HX) or not. Then, he can choose between three different HXs:
horizontal tube bank, vertical tube bank, serpentine tube or coiled tube.
For Thermal SDWH system (Case 4), the tank includes an immersed coiled heat exchanger to
transfer the heat from the working fluid to the mains water.
In addition, an auxiliary heater can be provided to each isothermal layer individually,
by connecting a tank heating element (e.g. Type 1226) to the concerned node.
For Electric STWH (Case 3), Type 534 is used without internal HX to model a simple small
storage water tank heater, using an internal electric heating element.
Type 534 has many characteristics that should be set by the user. These include: tank
volume and height, number of nodes, entry and exit location, fluid properties for the storage,
heat loss coefficient for each node, number and location of the heating element(s) and the
location of the aquastat(s) (temperature of the node to be monitored).
If the component includes an immersed heat exchanger, additional parameters should
be specified: number of HX nodes, fluid properties, dimensions (depending on the type of HX)
and the fraction of the HX length that will be assigned to each HX node.
The inputs of the model are: Inlet temperature and flow rate for the port (for the HX
and the port, if the model includes a HX), the edge loss temperature (ambient temperature)
and the control signal for auxiliary heater(s). The outputs includes: Outlet temperature and
flow rate for port (for the HX and the port, if the model includes a HX), temperature of each
node, thermal losses, energy delivered to the flow, auxiliary heating rate, HX heat transfer
rate, etc.

38
For more details, the mathematical description of Type 534 is given in Appendix 1.
It contains all the equations and energy balances required to compute the performance of a
vertical storage tank water heater.
iv. TRNsys worksheets of the simulated cases
Case 1: Electric Instantaneous Tankless Water Heater (Electric ITWH)
In TRNsys, the worksheet of this base water heating system (Figure 15) was created
with some components listed above in Section II-2.iii. An Electric ITWH (Type 6) receives mains
water following a Water Draw Profile (Type 14b) and heat it to the set point temperature of
45 °C. Then the hot water circulates inside a pipe (Type 31) of 1 m length and U-value of 10.7
kJ.hr-1.m-2.K-1 (Armaflex insulation for 25 mm pipe based on inner surface area of the pipe)
before reaching the shower tap.
The base Electric ITWH has a commonly maximum heating rate of 4.5 kW [7] and an
overall heat loss coefficient (UA) of 8.40 BTU.hr-1.F-1 (i.e. 2.51 W.k-1) according to an American
study on residential water heaters [28] that provides model parameters, based on common
existing brands. The efficiency of the device is set to 0.904 according to Energy Star [29].
The temperature of the surroundings is provided by the Weather Station data (Type 99), and
the mains temperature by TRNsys Meteonorm Weather data (Type 16-5) for Singapore. The
initial temperature is set to 29 °C, as representative of the average ambient temperature in
Singapore. The Fluid specific heat of water equals 4.19 kJ.kg-1.K-1 (TRNsys default value).
A first online graphical plotter (Type 65a) shows the inlet and outlet temperatures of
the heater (Tin; Tout), the outlet temperature of the pipe (Tsupply) and the water draw
profile. Also, after unit conversion to IS unit system, the required heating rate (Q_Cons_kW),
the rate of energy delivered to the fluid (Q_Deliv_kW) and the heat losses rate (Q_Losses_kW)
are displayed throughout the time period of 365 days (i.e. 8760 hours). The values are saved
in a separate text file.
In a second online graphical plotter, after integrating power rates over simulation time
period and converting units to kWh, the energy consumed, delivered and lost to the
environment (Q_Cons; Q_Deliv; Q_Loss) are depicted and saved in a separate text file as well.

39
Case 2: Gas Instantaneous Tankless Water Heater (Gas ITWH)
The worksheet of this case (Figure 16) is based on the layout created for the previous
Electric ITWH (Case 1). Only Type 940 replaces Type 6 to model a Gas ITWH. The other
components remain unchanged.
Type 940 models a typical 4.5 kW Gas Tankless Water Heater of 0.217 m² (0.609 cm x
0.356 cm). The efficiency of the device is set to 0.82 and the heat loss coefficient U-value
to 11.6 W.m-2.K-1 based on the common model described in [28]. The outlet temperature is
comprised inside a range which is defined by a temperature deadband of 4 Kelvins, according
to [30]. Auxiliary energy power of the controllers and sensors is set to 0.01 kW (Standby and
Heating Modes), as default value provided by TRNsys.
Similarly to Case 1, the same outputs are plotted and saved in separate files after unit
conversion.
Figure 15- TRNsys: Worksheet of Electric ITWH (Case 1)

40
Case 3: Electric Storage Tank Water Heater (Electric STWH)
For this case (Figure 17), Type 534 –vertical storage water tank is used in combination
with an electric tank heating element (Type 1226-Elec). This latter is controlled by an aquastat
(Type 2).
Type 534-NoHX models a small storage stratified tank of 57 liters capacity and 0.510
meter height, without an immersed heat exchanger. It is based on the existing model
65SVP15S of the manufacturer Rheem [31]. The tank is divided into 10 isothermal nodes (N)
to better illustrate the thermal stratification. The heat loss coefficient, U-value, is set
to 2.88 kJ.hr-1.m-2.K-1 for the sides, based on the parameters given by [32].
However, experimental results with existing brands were conducted by [33] and show that U-
value is assumed to be higher for the top and the bottom of the tank. Utop and U bottom are set
to 1 and 2.5 W.m-2.K-1 respectively.
The fluid enters the tank from the last node (Nentry=10) and leaves from the top
(Nexit=1). Its fluid properties are set to the default values provided by TRNsys to represent
mains water. Type 99 and Type 15-6 provide the ambient temperature and mains water
Figure 16-TRNsys: Worksheet of Gas ITWH (Case 2)

41
temperature respectively. The ambient temperature is used by the model to compute the
heat losses to the environment.
In addition, the tank contains one internal heating element placed at N=7 and modeled
by an electric auxiliary heater (Type 1226-Elec). This latter has a capacity of 3.3 kW [31], and
a thermal efficiency of 1. It is controlled by an aquastat (Type 2b) that “watches” the
temperature at the outlet of the tank (N=1), compares it to the set point temperature
(Tset = 45 °C) within a deadband of 4 Kelvins, and generates an output control function for the
auxiliary heater.
After being heated, the outlet flow stream of the tank circulates through a pipe
(Type 31) to reach the shower tap. The length of the pipe was set to 5 meters, in accordance
with an EPA (Environmental Protection Agency) plumbing layout for hot water delivery
system representative of a landed property given in [34]. The other parameters of Type 31
are the same as in the previous cases.
Temperature profiles at each node (Ti, 1<i<10), the outlet temperature of the pipe and
power rates (Auxiliary, Losses) are the main plotted outputs. The values of each variable are
saved in text files after unit conversion. Both energy consumed (Q_Cons) and lost to the
environment (Q_Loss) are obtained after integration, similarly to the previous cases.
Figure 17- TRNsys: Worksheet of Electric STWH (Case 3)

42
Case 4: Thermal Solar Domestic Water Heating system (Thermal SDWH)
Among various possible configurations for solar water heating (Figure 9), the
traditional closed loop (Indirect) solar water system (Figure 9.b) was created in TRNsys on the
basis of the worksheet of Case 3 (Figure 17) by adding a solar loop to the electric Storage Tank
Water Heater (Electric STWH). The worksheet of the simulation is shown in Figure 19.
The solar loop includes an Evacuated Tube Collector (Type 71), a single speed
circulating pump (Type 3d), a heat exchanger immersed inside the water tank (Type 534 –
Immersed Coiled HX) and a differential controller (Type 2b). Recall that water is used as
working fluid inside the solar loop.
Unlike the Electric SDWH (Case 3), this case uses a hot water tank of higher capacity.
The model is based on the German brand Viessman’s Vitocell 100 VH-NCVA (300 L, 1.7 m).
Indeed, this ensures that during daylight, the system takes advantage of the available solar
energy and stores it in the domestic water tank. This thermal storage would be useful to meet
higher load demands, or to reduce the usage of the electric backup heater on the less sunny
days. The tank is divided into 20 nodes to better illustrate the thermal stratification.
The immersed heat exchanger consists of a coil of 11.3 m length, placed between Node no. 8
and Node no. 15. Figure 18 shows a schematic of this model. The technical specifications are
given in [35].
Figure 18- Schematic of Vitocell 100-VH's hot water tank
SOURCE: [35]
SOURCE :

43
The evacuated tube collector (Type 71) is based on Viessmann’s Vitosol 200-T SPE.
It has a gross area of 2.66 m², an intercept efficiency of a0=0.73, and heat loss coefficients
a1 = 1.21 W.m-2.K-1 and a2 = 0.0075 W.m-2.K-2. More technical specifications are given in [36].
Although the circulating pumps have generally variable speed control, a single speed
pump of 60 kJ/hr power is considered for this case for simplification and is modeled by Type
3d. This latter is connected to a system control to ensure that the solar loop is running only
when enough solar energy can be collected. This system control is modeled by Type 2b
Differential Controller that monitors the temperature of water leaving the tank to the
collector in the solar loop (TL), and the outlet temperature of the collector (TH) to determine
if there is enough energy to harvest. Two dead band are set by the user: Upper DB (set to 10
Kelvins) and Lower DB (set to 2 Kelvins). This means that if the temperature of the working
fluid at the outlet of the collector is 10 °C (Upper DB) or more above its temperature at the
bottom of the tank, the pump is then turned on and the working fluid circulates through the
pipes of the solar loop to transfer energy to mains water. The system will keep operating until
this temperature difference fell below 2 °C (Lower DB). To ensure that the hot water does not
boil in the tank, the controller also monitors the temperature at the top of the tank.
By setting the high temperature cut-off to 100 °C, the pump is supposed to stop when this
temperature is reached.
An electric backup heater (Type 1266) of 3 kW heating capacity is placed at
Node no. 6, and is controlled by a Type 1502 Aquastat (Similar to Type 2-Aquastat). This latter
“watches” the temperature at Node no.1 (outlet of the tank) and compares it with the set
point temperature that is set to 45 °C, with a dead band of 2 Kelvins.
Similarly to the previous cases, temperature profiles, DHW flow rates, backup power
and heat losses are plotted and saved by Type 65a. Also, the solar irradiance and the useful
solar gain are displayed. After integration, different energy variables are plotted and saved in
another Type 65a (energy exchanged in the coiled HX, energy of the backup, heat losses, etc.).
NOTE: Type 14h forcing function was initially implemented to make the circulating pump of
the solar loop follow an operating schedule (from 8 a.m. to 6 p.m.). For safety reasons (boiling
inside solar loop), and to reduce the energy consumption of the pump, the differential
controller Type 2b was used instead, as describes above.

44
Figure 19- TRNsys: Worksheet of Thermal SDWH (Case 4)

45
3. Assumptions
The study is conducted for a landed property in Singapore for the year 2014. According
to the Department of Statistics in Singapore [4], average household size is 4.3 persons within
landed properties in 2014. Therefore, the daily DHW demand is calculated for this
configuration.
Based on water consumption statistics published by Public Utilities Board (PUB) in [6],
the daily average consumption of water is 160 liters per person per day, out of which 29% is
used for shower (i.e. 46.4 liters). Hot water demand for shower is estimated to 67% of this
latter, resulting in a consumption of approx. 31 liters per person and day, hence 134 liters per
household per day.
In Singapore, it is assumed that DHW usage is mainly limited to bath/shower uses.
Given that the tap water temperature is around 29°C, the use of hot water at the basins and
sinks is negligible.
It is important to distinguish between weekday and weekend day demand of hot
water, since the latter is generally higher. Indeed, according to ASHRAE 1995 [7], the average
weekend daily DHW consumption is 7.5 % greater than the average weekday daily
consumption. It is assumed that, during weekends, most individuals are expected to spend
more time at home and/or practice sports and other physical activities. This should increase
DHW demand for shower.
For this study, the set point temperature (temperature leaving DHW system) is fixed
to 45°C (Appendix 2). The dead band range depends on each case. Since the parameters of
DHW systems set on the simulation software are from existing brands available in the market,
the DB range is given by the technical manual of each device. Other important parameters
are also provided: storage capacity, draw-off flow rate, standby losses, etc.
4. Input Data
i. Weather Data
In TRNsys, reading the data collected from the weather station requires creating a text
file. This file follows a specific syntax.
First, exact geographical information of weather station are specified by the user. Also,
the data time interval is set to 0.1 hours (6 minutes) in accordance with the time step at which
the collected data are provided. The first data line is also set to the first time step, hence to
0.1 hours.
Then, keywords from Section III-2.iii (Type 99) are used to write the line statements
for the ambient temperature and the global irradiance.

46
Once the header prepared, the values of each variable are listed in the associated
column. For one year simulation, at a time step of 6 minutes, 87 600 values are provided for
each variable.
Figure 20 shows the text document building for the location of the weather station in
Singapore.
ii. Domestic Hot Water (DHW) profile generation
To predict domestic hot water (DHW) energy consumption when using a simulation
tool, a hot water draw profile must be determined, generally based on local statistics.
In some countries, the lack of reliable data makes DHW use ascertainment very
challenging, given that various end use applications and users’ behaviors need to be taken
into account for this purpose. This also implies that varying inlet and set point temperatures,
volumes, and flow rates shall - amongst other things - be considered before running the
simulation.
Figure 20- Type 99: Weather Station text file building for TRNsys

47
a) Type of DHW profiles
Depending on the country and/or standard organization, DHW profiles could be given
in volume units (liters, gallons…), in mass units (kilograms) or in fractions.
In this latter case, daily consumption of hot water must be known.
Moreover, the profile could be hourly or at set intervals of use. For example, in the
U.S., DHW draw profile consists of 6 draws of 11-gallons (approx. 42 liters), one hour apart,
with a flow rate of 3 gpm (approx. 11 L.min-1) [37].
For solar hot water systems rating, the Solar Rating and Certification Corporation
(SRCC) use a simple 24-hour schedule, assumed to be the same pattern every day. Only one
draw is considered for each hour [38]. Therefore, this profile is averaging over hourly data,
which makes it a ‘smooth’ approach to realistic draws which are more precise with shorter
water draw duration. Another example is the ISO daily load pattern [39] for the Solar
Domestic Water Heating (SDWH) systems. It consists of hourly factorized DHW schedule.
The sum of the factors over 24 hours equals 1. The hot water load for each hour is the daily
load volume multiplied by the factor for that hour.
Figure 21- Flow rate (left axis) and hourly volume (right axis) of the current SRCC profile. The width of the flow
rate curve is the duration of that hour’s draw, and is proportional to volume.
SOURCE : [38]
However, several previous DHW studies and water heating standards neglect
variability and employ simple hot water draw profiles for use in energy simulation or testing.
Indeed, weekend-day/weekday probability of use, seasonal variations, holiday event
frequency, fixture use and variable flow rates… are different parameters which may
considerably affect the simulation results, e.g. the energy savings of water heaters to be
compared. Even though those parameters are taken into account,
defining a DHW profile still remains a simplistic approach, because each individual behaves
differently towards hot water use, in addition to day-to-day variability.

48
Therefore, only fairly realistic DHW profiles can be established, and on condition that some
reliable assumptions are predetermined.
One approach consists of using a free German tool, DHWCalc, developed in University
of Kassel, to generate domestic hot water profiles on a statistical basis[23]. The generated
profiles are text-files, containing a list of flow rate values for each time step. They are used
primarily for annual system simulations, but are also suitable to be used for test procedures
of laboratory system or component tests. The program distributes DHW draw-offs throughout
the year with statistical means, according to a probability function. Reference conditions for
the draw-offs (flow rates, draw-off periods, etc.) and reference conditions for the probability
function (daily probabilities for draw-offs etc.), can be set by the user, as well as general
profile parameters like time step period and mean daily draw-off volume [23].
In light of the foregoing, this tool was useful to generate a 24 hours DHW profile, taking
account of one year variability. This daily cycle was then repeated throughout the entire
simulation period. The generation of the profile is detailed below (Section III-4.ii.b)
b) ‘DHWcalc’, a tool for DHW profile generation based on statistical basis
The program DHWcalc was used to generate a DHW profile that was implemented in
each simulated case. Different settings are incorporated in the tool to take account of realistic
event variability that can affect the annual consumption of hot water. This requires reliable
local statistics to make the DHW profile fully representative of hot water consumption in
Singapore. As this data is not available, the variability can only be estimated.
Therefore, for some parameters, default values given by the software are sometimes used.
Below, the relevant parameters can be defined:
Main page parameters:
The user specifies between a Single or Multi Family House (number of households).
The only distinction concerns holiday-periods that vary from one household to another.
Then, the Time Step Duration, Start Day and Total Duration of the profile are set.
 The Time Step Duration means the minimum time duration of a draw-off. It is set to
one hour (60 minutes) as SRCC simplified 24-hour profile with an hourly set flow rate
is taken as a reference for this study.
 If set to 1, the Start Day parameter considers Monday as the first weekday of the
profile. The 1st of January is regarded as the first day of the year.
 The Total Duration of the profile is set to 365 days, as this study is conducted for the
whole year of 2014.
Also, the total mean daily draw-off volume is defined. It represents the daily hot water
consumption. According to the assumptions (Section III-3), the daily average hot water

49
consumption was estimated to 134 liters per day for the considered landed property in
Singapore.
Probability distribution parameters:
To spread the flow rates throughout the time period of the profile, the cumulated frequency
method is applied. This method integrates the following probability function:
𝑝(𝑡) = 𝑝 𝑑𝑎𝑦( 𝑡) × 𝑝 𝑤𝑒𝑒𝑘𝑑𝑎𝑦(𝑡) × 𝑝𝑠𝑒𝑎𝑠𝑜𝑛(𝑡) × 𝑝ℎ𝑜𝑙𝑖𝑑𝑎𝑦(𝑡) (11)
This probability function of draw-offs is described by the product of probability
functions for seasonal, daily, and week-daily variations of DHW consumption.
First, the user specifies the Probability during the day, 𝑝 𝑑𝑎𝑦( 𝑡) . Among different
functions, a step function defined for each day of the week with a step size of 1 hour is chosen.
It means that a mean water draw-off volume (in liters) can be set for each hour of the week.
Since this function applies only for special consumption patterns (e.g. sport halls, hospitals),
default button is pressed.
Then, the weekend-day/weekday probability, 𝑝 𝑤𝑒𝑒𝑘𝑑𝑎𝑦(𝑡), is set by specifying the
ratio between the average weekend daily DHW consumption and the average weekday daily
consumption. According to the assumptions (Section III-3), this ratio was assumed to be 107.5
%, meaning that weekend day consumption is 7.5 % higher compared to weekdays.
In addition, seasonal variations are described by a sinusoidal function, representing
weather variability throughout the year. Therefore, 𝑝𝑠𝑒𝑎𝑠𝑜𝑛(𝑡) is set by specifying the
amplitude of this sinusoidal function and the day of year at which this maximum is reached.
After plotting the average ambient temperature in Singapore over 2014 (Figure 22), the
results show that seasonal variations are not impacting. Indeed, the ambient temperature
was slightly oscillating around an average temperature of 28 °C. The maximum deviation to
this average is of 1.19 °C. Therefore, seasonal variations were not taken into account to
generate the DHW profile.

50
Figure 22- Seasonal Variations: Average Ambient Temperature over 2014 in Singapore
Also, the holiday period probability, 𝑝ℎ𝑜𝑙𝑖𝑑𝑎𝑦(𝑡), is set by specifying up to three
periods. During these periods, the DHW consumption is seen to be reduced to zero, meaning
that the household is not occupied. This parameter was disabled for this study, as no local
statistics on the population trends in term of holidays are available.
Flow Rate parameters:
In this windows, the mean daily DHW consumption is reminded, and the mean flow
rate per draw-off is specified, along with the minimum and maximum flow rates. For
simplification, default values were used.
After setting the parameters above and running the program, the DHW profile is
created in a text file. The values obtained are plotted below in Figure 23.
Figure 23- Daily Domestic Hot Water (DHW) profile for TRNsys
28.0 °C
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12
Temperature
[°C]
Month
Tamb
Average Tamb
0
5
10
15
20
25
30
35
-1 4 9 14 19 24
Flow rate
[kg/hr]
Time [Hours]

52
IV- Results and Discussion
1. Daily performance of water heaters:
All four water heating systems were simulated on TRNsys for 365 days (representative
of the year 2014) and the selected outputs of each studied case were saved in separate text
files.
In this section, the performance of each case is studied. For this purpose, temperature,
power and energy profiles were plotted for one typical day, corresponding to the 1st June of
2014. In TRNsys, this represents the simulation period between 3624 and 3648 hours.
i. Case 1: Electric Instantaneous Tankless Water Heater (Electric ITWH)
The main component of this case is Type 6, it serves to model Electric ITWH. From its
associated box, inlet and outlet temperatures of water, power consumption, delivered power
and heat losses rates were plotted directly. These outlets are depicted in Figures 24 and 25.
Figure 24- System temperature and DHW profiles of Electric ITWH over 24 hours
0
10
20
30
40
50
60
70
0
5
10
15
20
25
30
35
40
45
50
3624 3629 3634 3639 3644
Flowrate[kg.hr-1]
Temperature[°C]
Time [hours]
Tin
Tout
Tsupply
m_schedule

53
Figure 25- Energy use and heat losses profiles of Electric ITWH over 24 hours
From figure 24, the following comments can be made:
 Inlet Temperature (Tin) curve indicates that mains water temperature is almost
constant throughout the simulation period. This profile comes from using TRNsys
weather Type 15-6 that generates automatically mains water temperature from other
weather parameters. Only ambient temperature and solar irradiance data have been
collected, which was not enough to generate a precise mains water temperature
profile. Therefore, Type 15-6’s data (Meteonorm) was used and suggests that an
approximately 30°C mains water is entering the water heater.
 In accordance with the generated DHW profile (m_schedule), from simulation time
3624 hours (12:00 AM) to 3627 hours (03:00 AM), the water heater is turned off.
Equipped with flow sensors, the heating element inside the unit is turned when the
unit detects a water draw. This occurs starting from 03:00 AM to 10:00 PM.
The temperature of water exiting the water heater (Tout) is kept to the set point
temperature (45°C) along with DHW profile.
 The curve representing Tout seems constant throughout the water draw profile.
Additionally, the set point temperature is reached after 6 minutes (0.1 hour) and falls
down almost instantly to the mains temperature, after 5.88 minutes.
This is due to using Type 6 as a simplified model for Electric ITWH. Indeed, Type 6 is
modeled to elevate the temperature of a flow stream by setting a maximum heating
rate (enough to maintain water at the set point temperature) instead of specifying the
real heating capacity of the heating resistance. Therefore, this component will
perform to prioritize maintaining water at Tset, meaning that the outlet temperature
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
3624 3629 3634 3639 3644
Power[kW]
Time [hours]
Q_cons_kW
Q_deliv_kW
Q_losses_kW

Training-Report-FINAL

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (7)

Similaire à Training-Report-FINAL

Similaire à Training-Report-FINAL (20)

Training-Report-FINAL