Good solar cell design

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Light and thermal energy are gained in a solar home or room
through radiation. The transfer of that energy takes the form of
convection. A properly designed space must allow the correct
amount of light in and control the amount of energy dissipated.
Insulation allows for the containment of energy taken in.
Radiation absorption can be increased passively with color. The
albedo of an object refers to how strongly it reflects light. For
example, a white surface will reflect sunlight almost nine times
more than a black surface. This means that your thermal mass,
the area that stores heat energy, should be dark in color. A light-
colored floor or wall may rise in temperature to 80 or 90°F, but a
dark-color thermal mass can rise in temperature to 130 or
140°F.

Types of Solar Gain
In indirect solar gain, heat enters the building through an
aperture and is captured and stored in a thermal mass. The
mass then slowly and indirectly heats the building through
conduction and convection. Isolated solar gain is a separate
space, such as a sunroom, in which the solar energy is captured
and then passively moved as heat through the living space by
natural convection.
A third type of solar gain, concentrated solar power (CSP). While
most applications are large commercial structures, consumers
can use CSP water heaters in their homes. CSP is best in
warm climates where sunlight is a constant and rainfall is low.

Simple Improvements for Solar Gain
Every homeowner and renter can make simple home
improvements to increase the passive solar energy available
and used in the home. Keeping in mind the five elements
required for passive solar, consider the following.
The thermal mass in most homes is already in place and will not
change. This solar storage will be a floor or wall. The larger and
more dense the mass, the more heat storage is available. You
can also change the color to absorb more heat energy. For most
homes, the aperture is in the form of windows or skylights, and
the control will be some type of shade.

Heat and Light Transfer Through Windows
The solar heat gain coefficient (SHGC), refers to the percentage
of solar radiation that passes through a window and is
expressed in a value of 0 or 1. The higher the SHGC, the more
solar gain in the home.

The visible light transmission (VT), refers to the amount of light
transmitted through a window and is also measured by values
between 0 and 1. A window with a VT of 1 will allow no loss of
transmission. The use of a second pane of glass or a coating will
lower the VT value.

Low-E coatings, which stands for low emissivity, refers to the
relative ability of a surface to emit energy by radiation. A Low-E
coating for windows refers to a thin metallic or metallic oxide
layer deposited on one of the glass surfaces. Heat gained or lost
through a window is due to radiation from either outside-in or
inside-out. The coating reflects a portion of the radiation,
reducing heat flow through the window. The coating is normally
invisible to the homeowner. Low-E is also rated on a scale of 0
to 1. A black object would have an emissivity of 1, absorbing all
light, and a perfect reflector would have a value of 0.

A single-pane window is a single sheet of glass with no coatings.
Single-pane windows are energy inefficient, but in the last 30
years, dual-pane and triple-pane windows have become popular
due to their increased efficiency at restricting the transfer of
heat.

The U-value gauges how well a window conducts heat and is
more correctly called the overall heat transfer coefficient. It
measures the rate of heat transfer through a window. A
window’s U-value is rated under standardized conditions. The
usual standard is at a temperature gradient of 75°F, at 50-
percent humidity, with no wind. The lower the U-value, the less
heat transfer occurs, and the more energy-efficient the window
is considered to be.

Here are examples of typical window performance:
• A single-pane window has an average U-value of 1.1 or
greater.
• A dual-pane window has an average U-value of 0.55 or lower.
• A triple-pane window has an average U-value as low as 0.20.

In addition to the glass, the window frame, which is included in
the U-value, must be designed for performance.

A common window used in the 1960s was an aluminum-framed
window with a single pane of clear glass, and this window is
used as our standard for comparison for the following statistics,
which approximate averages:
• A wood or vinyl frame with dual-pane clear glass would be 27
percent more efficient.
• Wood or vinyl frame with dual-pane glass and a Low-E coating
would be 32 percent more efficient.
• An insulated frame window with triple-pane glass and a Low-E
coating would be 41 percent more efficient.

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