Installation hybrides PV plus Generatrice diesel

1. 20th –EU-PVSEC, Barcelona, Spain, 6-10 Jun 2005
STRAIGHT FORWARD TECHNIQUE FOR SIZING STAND-ALONE PV HYBRID SYSTEMS
Mohamed Ibrahim, Dr.
System Technology and Projects, SMA Technologie AG,
Hannoversche Strasse 1-5, D-34266 Niestetal, Germany
e-mail: mohamed.ibrahim@sma.de
ABSTRACT: The main purpose of this paper is to introduce a simple but effective technique for sizing stand-alone
PV hybrid systems. Most of the available sizing methods and software-tools require lots of data on energy resources,
demand and system technical specifications. Owing to the complexity of this topic, the presented sizing technique
needs only very little information about the application of interest and location. It proceeds in practical and plausible
steps to design the rated powers of the desired components (e.g. the peak power of the PV plant, inverter, battery, bi-
directional battery inverter and diesel generator). An energy balance equation for estimating the required daily PV
energy is developed in this work. This equation integrates several characteristic parameters (e.g. solar fraction, solar-
load-mismatching factor and conversion efficiencies). In this way, it can be adapted to user requirements,
components characteristics and the technical specifications of the system components. Based on a case study, the
necessary knowledge for the implementation of this technique and several helpful sizing curves are introduced.
Keywords: Photovoltaic, Stand-Alone Hybrid Systems, Sizing
1 INTRODUCTION
Stand-alone hybrid systems are often used to supply
remote and rural applications which do not have any
access to public electricity grid, or even to replace a
conventional diesel genset of high running costs. Before
designing the system (i.e. selecting the suitable structure,
system configuration, components sizes) several
preliminary steps are necessary. These design steps start
from estimating locally available energy resources and
the expected load and extend to cover the socioeconomic
development of the region. However, in this context the
main point of interest is the sizing issue, i.e. the optimal
rated powers of the supply components are to be chosen.
Usually, hybrid systems include several types of
converters (conventional and renewable) and battery
storage media. Moreover, in renewable systems the
generated power depends on continuously fluctuating
energy sources which further increases the complexity of
the sizing process. Therefore, the sizing issue is a subject
of challenge to achieve cost-effective stand-alone
systems. Accordingly, several sizing techniques have
been developed. Some of them depend on statistical
methods, others use time-series with optimizations
techniques, refer to studies by [1] and [2]. Not only
mathematical techniques are deduced for system sizing
tasks but a lot of simulation software has been developed
to support the designer during the sizing process.
However, there is a need for a sizing technique that
requires only very little information concerning the
energy resources and load, and this is often the case in
developing and threshold countries where PV stand-alone
systems are to be used. Moreover, this technique should
be characterized by a plausible procedure which allows
direct understanding of its parameterization process,
implementation possibilities and tolerance. In this
context, a practical technique for sizing AC-coupled PV
hybrid systems is introduced which requires information
usually available in components manufacturer data
sheets, radiation atlas and roughly defined user data.
2 FEATURES OF THE SIZING TECHNIQUE
The suggested sizing technique is characterized by
the following features:
• Suitable for sizing PV hybrid systems for remote and
rural locations
• Usable for AC-coupled stand-alone hybrid systems,
refer to Figure 1
• Simple but practical in implementation
• Uses components technical characteristics (e.g.
efficiency, operating boundaries, etc. )
• Considers user requirements and behavior (e.g. solar
fraction, night loads, autonomy factor, etc.)
• Requires little information concerning energy
resources and location
The calculation procedure implements roughly
defined user and location requirements and uses
component technical characteristics available in
manufacturer datasheets. Hence, component sizes can
then be directly calculated. The calculation procedure
can be implemented according to the following steps:
1. estimating the user daily average energy
consumption (kWh) and critical loads (kW),
2. calculating the required PV peak power (kWp) and
the PV inverter nominal power (kW),
3. Calculating the battery capacity (Ah) and the bi-
HG,t
EPV,MPP
EC,Daily
h PV, Inv
h Bat, Inv
h Wh, Bat
PDiesel, rated
P PV-Inv, nom
P PV, Peak
C Bat, nom
P Bat-Inv, nom
Consumer
Loads
h PV
Figure 1: Configuration of the modular AC-coupled
PV hybrid system

2. 20th –EU-PVSEC, Barcelona, Spain, 6-10 Jun 2005
directional battery inverter power (kW) and
4. defining the diesel rated power (kW).
3 THE SIZING PROCEDURE
3.1 Estimating the load characteristics, EC,Daily
In order to start the sizing process, basic information
on the consumption type, the load characteristics, and the
user behavior is needed. In this context, the system user
is asked to specify most of these requirements. However,
the experience of the system designer plays a crucial role
in order to infer the basic information (e.g. reliable
autonomy factor, solar fraction, energy availability) that
is needed to achieve an efficient system configuration
and suitable component sizes. The main required load
characteristics are:
• application characteristics
• average expected daily consumption (kWh/day)
• a critical or maximum expected daily load (kW)
3.2 Sizing of the PV plant
In this step, the peak power of the PV generator
(PPV,Peak) and the PV-inverter nominal power (PPV-Inv,nom)
are to be defined. In order to define these powers, two
important quantities have to be estimated: the energy to
be generated by the PV and the solar radiation potential
at the location of interest.
3.2.1 Energy to be generated by the PV, EPV,MPP
The PV plant is used to supply the user with a
specific amount of his daily consumed energy EC,Daily.
This amount can be defined by the solar fraction (SF) of
the system. The energy generated by the PV plant
(EPV,MPP) has to be inverted via DC/AC-inverter (ηPV,Inv).
A part of it goes directly to the user, and due to the solar-
load-mismatching (FSLM) the other part of the generated
energy has to be buffered in the storage medium. The
energy storing process goes along with conversion losses
which also have to be considered (ηBat,Inv and ηWh,Bat).
Based on the energy balance principle and the modular
AC-coupled system technology, the following equation
(1) has been developed.


















−
×
×+×= 1
,,
1
,
1
,, 2
BatWhInvBat
F
InvPV
dailyC
ESF
MPPPV
E SLM
ηηη
This equation integrates several practical parameters
(e.g. solar fraction (SF), solar-load-mismatching factor
(FSLM) and conversion efficiencies). Therefore, it can be
adapted to user requirements and the technical
specifications of the system components.
For the sake of simplicity, losses in cables and in the
battery during discharge are neglected. Figure 2 shows
several curves for estimating the required daily energy to
be generated by the PV in order to supply a specific daily
demand for different solar fractions. These curves are
estimated for a solar-load-mismatching factor of 50%,
the so called European efficiency of the PV-inverter (e.g.
Sunny Boy
) of about 94%, a bi-directional battery
inverter (e.g. Sunny Island
) efficiency of 91% and a
battery Wh-efficiency of 90%.
3.2.2 Solar radiation on the PV surface, HG, t
In order to define the required PV peak power we
need to estimate the solar irradiation on the module
surface per day. Often solar radiation data are available
for a horizontal surface and in different forms, e.g. in
monthly total global irradiation (kWh/m2
/month) or
yearly total global irradiation (kWh/m2
/year) [3]. These
values are usually found in SOL
AR ATLAS, the METONORM databank or on various
home pages on the internet [4]. These values can be
found directly for the location of interest or nearby
location can be used. The radiation data need to be
treated first, in order to get the irradiation on the module
surface:
1. global radiation should be in daily values (HG in
kWh/m2
/d). If monthly values are available, monthly
average values can be used in order to calculate the
daily average value. If the monthly average
irradiation is of strong deviation from the worst
month, a diesel genset is recommended to be used in
the system. Otherwise worst month sizing (i.e. using
the month of the lowest amount of irradiation) will
result in an oversized PV generator and large storage
capacity.
2. the estimated daily average radiation in the previous
step is for horizontal surfaces. Often PV modules are
tilted to a specific angle (usually equal to the latitude
or latitude minus 20° for locations of highly diffuse
parts of the radiation) and directed towards the South.
In some applications, tracking systems are used. All
these measures will increase the energy yield by 10%
to 50% compared to a horizontal surface. The
irradiation value is then:
HG, t = (1.1 to 1.5) x HG (2)
3.2.3 Estimating the PV peak power, PPV,Peak
The following mathematical formula is introduced in
order to calculate the PV peak power.
tGrelPV
MPPPVSTC
PeakPV
H
EE
P
,,
,
,
×
×
=
η
(3)
This formula requires the energy to be generated by
PV, the global irradiation on the module surface,
radiation at standard test conditions (ESTC=1 kW/m2
) and
the relative efficiency (ηPV, rel) of the PV module (i.e.
deviation of the operating efficiency of the PV module
from the STC efficiency). The value of ηPV, rel, depends
on both the module type and the application location, and
it is about 0,83 and 0.87 for Si-cells in tropical and
temperate regions respectively [4].
Figure 2: Relation between required daily energy and
PV generated energy for different solar
fractions, solar load mismatching of 50%

3. 20th –EU-PVSEC, Barcelona, Spain, 6-10 Jun 2005
The curves in Figure 3 show the relation between the
energy to be generated by the PV and the PV peak power
for different solar irradiation values, and relative PV
efficiencies are illustrated.
3.2.4 Sizing the PV inverter, PPV-Inv, nom
The size of the PV inverter is of vital importance for
the energy yield. Its optimization can result in improving
the total energy usability of the PV generator and high
availability is expected. A ratio (ν) relating the inverter
rated power (PPV-Inv, nom) and the PV peak power is used
as a sizing criterion as follows:
PeakPVnomInvPV PP ,, ×=− ν (4)
This ratio is location dependent (i.e. based on the
solar radiation intensity) and varies between 0.75 to 1.2
for locations of low and high radiation values
respectively. For example, in Germany and central
Europe ν is of about 0.9. For further southern regions, the
ratio ν may reach 1.1. Accordingly, the PV inverter rated
power can be selected. The advantage of using the string
AC-coupled inverters is that they make the system easy
to be expanded and further inverters can be easily
integrated into the system [6].
3.3 Sizing the battery storage subsystem
The battery in PV stand-alone applications is used as
a buffer for solar energy. The stationary lead-acid battery
of different technologies (e.g. gel, AGM or Flooded) is
the dominant type in solar applications. However, NiCd
batteries also play an important role especially in
applications of low temperatures. The suggested sizing
process is valid for different types of batteries, provided
their technical parameters (e.g. allowed depth of
discharge and Wh-effciency) are considered.
3.3.1 Estimating the battery capacity, CBat, nom
The required battery capacity is often defined by the
autonomy factor (AF) which indicates the number of
days for which the consumed energy can be supplied by
the battery only. Several factors influence the value of
AF, among them are:
• application type: e.g. remote communication station,
weekend bungalow or seasonally visited lodge, all of
them require a specific level of energy availability
and consequently a specified storage size is needed.
• System configuration: either PV/battery only or
PV/battery with diesel genset. For systems without
back-up diesel genset the battery storage is the only
energy supplier in case of several days of low solar
insolation. Hence, its size should consider such a
possibility.
• Battery type: lead-acid (gel or flooded) or NiCd also
influence the required capacity, since different types
are combined with different electrochemical
characteristics (e.g. cycle life and temperature range).
A simple equation is given below for sizing the
battery capacity as a function of several technical
parameters, such as autonomy factor (AF), permitted
depth of discharge (DOD), the voltage of the battery
cluster (VBat) and bi-directional battery inverter
efficiency ( ηBat, Inv).
BatInvBat
dailyC
nomBat
VDOD
AFE
C
××
×
=
,
,
,
η
(5)
Figure 4 shows curves relating the required battery
capacity in Ah per kWh of the daily energy consumption
(EC,daily) and the maximum allowed DOD for different
days of autonomy. Selecting a specific DOD highly
depends on the battery technology, system configuration
and the battery management strategy. The Pb-batteries
are preferred to be used in the range of 50-70% DOD,
whereas NiCd batteries are more suitable for deeper
discharge conditions and lower temperature. Moreover,
an optimized battery management strategy such as the
one in the Sunny Island
series assures reliable
operation, even if the battery is occasionally operated
under deep discharge conditions.
3.3.2 Sizing the bi-directional inverter, PBat-Invt, nom
In modular AC-coupled hybrid systems the bi-
directional battery inverter plays a key role. It is used to
build the local grid (i.e. guarantee stable voltage and
frequency, e.g. 230V/50Hz), discharge the battery in
order to supply the load with AC power, start the diesel
genset, recharge the battery and optimize the battery
management. The rated AC power output of the bi-
directional inverter can be one of the following values:
1- the maximum expected load,
Figure 3: Relation between required PV peak power
and generated PV energy for different
irradiation and relative PV efficiencies
(SF=100% and FSLM=50%)
Figure 4: Relation between a specific battery capacity
and the allowed DOD for different autonomy
factors (battery bank of 60V, and
Sunny Island
)

4. 20th –EU-PVSEC, Barcelona, Spain, 6-10 Jun 2005
2- a critical or specific user load (e.g. industrial
equipment etc. ) or
3- the daily average load.
The option to be selected depends on the application
requirements. For three-phases applications at least three
Sunny Islands
4500 have to be used.
3.4 Sizing of the diesel genset
The diesel genset is a robust machine of low
investment cost and improves the PV hybrid systems
performance (e.g. high energy availability, optimizes
battery size and operating lifetime and ensures high
power reliability). However, integrating a back-up genset
with the stand-alone PV systems goes along with air
pollution, noise and continuous dependence on fuel
delivery. In some places, the fuel delivery is problematic
and can increase operating costs to very high levels.
Therefore, a careful study of the application and location
conditions is required before specifying the size of the
genset. The rated power of the diesel genset can be one
of the following possibilities:
1. equal to the daily maximum expected peak load,
2. greater than the daily average load or
3. matches a specific critical load.
The final decision for the back-up genset size is
always case specific, and many technical, geographical
and financial conditions play a role.
4 CASE STUDY
A village power supply consumes about 10 kWh/d,
and 60% of its consumption is at night with a maximum
expected power of 8 kW. The village is to be supplied by
a PV/battery/diesel stand-alone system with a solar
fraction of 95%. The yearly solar irradiation on a
horizontal surface is about 1800 kWh/m2
/a.
1- Based on (eq. 1), the energy to be generated by the
PV plant is:














−
×
×+×= 1
90.091.0
1
6.0
94.0
1
1095.0
, 2MPPPV
E
)/(10.12
,
daykWh
MPPPV
E =
2- Using (eq.2), the irradiation on the module surface is:
HG, t = 1.2 x (1800/365) = 5.9 (kWh/day)
3- Based on (eq. 3), the PV peak power is:
)(5.2
684.0
10.121
, kWpP PeakPV =
×
×
=
4- Based on (eq. 4): the PV inverter rated power is:
)(0.35.22.1, kWP nomInvPV =×=−
accordingly, Sunny Boy 3000
can be selected
5- Using (eq. 5): the battery capacity is
)(523
)(6070.091.0
)(2)/(000,10
, Ah
V
daydayWh
C nomBat =
××
×
=
6- The bi-directional battery inverter is selected to cover
the maximum expected load of 8 kW. Therefore, two
Sunny Island
4500 are to be used.
7- In order to have a redundant power supply for the
maximum load the diesel genset is also selected to
have a rated power of 8 kW.
5 CONCLUSION
In this paper, a practical technique for sizing PV
stand-alone hybrid systems which uses the modular AC-
coupled concept is introduced. Main attention during the
development of this concept has been paid to the
simplicity and easy handling in implementation. The
achieved results represent a basis for effective
components sizes and system configuration. The
implemented concept includes technical parameters that
can be simply defined, just from rough information on
the user, location characteristics, components’
manufacturer datasheets and application requirements.
Also, these parameters are integrated in the calculation
process in a manner which represents technical
necessities by the designer and user. That makes the
sizing technique of wide acceptance as a sizing tool.
Finally, a case study including a calculation example has
been introduced.
6 REFERENCES
[1] M. Ibrahim: “Sizing of Stand-Alone Hybrid Systems”,
final report, ISET, Kassel, Germany (1994).
[2] G. Seeling-Hochmuth: Optimisation of Hybrid
Energy Systems Sizing and Operation Control. PhD.
Thesis, Kassel University, Kassel, Germany (1998).
[3] J. Duffie et al.: “Solar Engineering of Therman
Process”, John Wiley & Sons Inc. New York (1991).
[4] V. Quaschning; http://www.volker-quaschning.de
[5] Andreas Wagner: “Photovoltaik Engineering”
Springer-Verlag Berlin Heidelberg, (1999).
[6] G. Cramer, et al. ”String Technology, a Successful
Standard of the PV System Technology for 10 Years
Now“Proceedings 20th
European Photovoltaic Solar
Energy Conference, Vol. III (2005).
NOMENCLATURE:
AF: Autonomy factor (number of days of
autonomy)
CBat, Nom: Estimated nominal battery capacity (Ah)
DOD: Minimum allowed depth of discharge
EC, daily: Daily average consumed energy in (kWh)
EPV, MPP: Daily energy to be generated by the PV (kWh)
ESTC: Solar radiation at STC =1 kW/ m2
FSLM: Solar-load-mismatching factor
HG: Daily average global irradiation received by the
PV array surface (kWh/m2)
PPV,peak, PPV-Inv,nom, PBat-Inv, nom and PDiesel,rated are the peak
power of the PV generator, PV-inverter, bi-
directional battery inverter and diesel genset
respectively.
SF: Solar fraction
VBat: Nominal voltage of the battery bank, it depends
on the inverter type (V).
ηPV, Inv, and ηBat, Inv are the European efficiencies of the
PV inverter (for Sunny Boy
between 92 to
95.5%) and of the bi-directional battery
inverter (for Sunny Island 4500
is about
91%) respectivly
ηWh, Bat: Battery Watt-hour efficiency (for lead-acid
batteries between 85% to 93%)
ηPV, rel, av: Relative average efficiency of the PV array