Environmental Engineering Reference
In-Depth Information
Glass reflectance can be reduced either by coating the glass with a thin film with
a low refractive index material, or by etching the surface to create a porous lower
refractive index medium, reducing the reflectance of the glass. For improved insula-
tion of the collector, multiple glasses that trap multiple insulating air layers can be
used, though the optical losses associated with each additional glass sheet reduce the
insolation transmitted.
Many clear plastics, without specific treatments, become yellow and brittle after
long-term exposure to ultra-violet part of the insolation spectrum. Many plastics can
also be damaged by the high temperatures that can be reached in solar energy collectors,
particularly if fluid stagnation occurs in the solar energy collector. Another disadvan-
tage of most plastics is that, compared to glass, transmittance to thermal radiation is
high at longer thermal wavelengths. Altering the optical properties of plastics to reject
heat gain under the influence of heating or an electric field has been studied as a means
of avoiding overheating damage when plastics are used as absorbers in solar energy
collectors (Resch and Wallner, 2008).
Both glass and plastic aperture covers do, despite their different solar optical prop-
erties, will enable absorber heat losses to be ameliorated as they both trap a largely
stagnant insulating air layer between the glazing and absorber (Rommel and Wagner,
1992).
4.1.4.3 Choice of heat transfer fluid
The choice of the heat transfer fluid is determined by maximum operating tempera-
ture, initial and operating costs, toxicity flammability and environmental impact. For
high temperature applications water and aqueous solutions are often inappropriate.
Hydrocarbon and synthetic-based heat transfer oils may be used up to their maxi-
mum temperatures of around 450 C. They are however flammable. The special safety
systems together with the environmental issues in their post-use disposal measure the
operational costs of the use of synthetic heat transfer rate. Steam has been studied for
many central receiver applications with maximum temperature applications 550 C.
Water used to generate steam must be deionized in order to prevent scale buildup on
the inner surfaces of the receiver. Liquid sodium and nitrate salt mixtures can also
be used as both a heat transfer fluid and storage medium, with a maximum operating
temperatures of 600 C and 560 C.
4.1.4.4 Combined photovoltaic - thermal collectors
A photovoltaic panel may convert typically 10%-15% of the incident insolation to
electricity. The rest of the solar energy absorbed heats the panel reducing photovoltaic
electrical energy conversion efficiency. Combined photovoltaic-thermal collectors have
thus been developed with either water (Ji et al., 2006) or air as the heat transfer fluid.
The many such systems that have been proposed all require displacement of both
suitable electrical and heat loads if economic viability is to be achieved. Hot water
produced by such combined systems can be used for space heating or domestic hot
water. When warm air is produced it can be used for pre-heating of ventilation air. The
heat can also be used in absorption chillers to cool a building. A typical PV/T collector
is shown in Figure 4.1.3.
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