General construction

Figure 3.9 shows how heat from the sun is collected by the absorber and is carried away by the fluid flowing through the tubes attached to the absorber. Since these collectors are located outside and normally in a cooler environment, the heated absorber can lose heat its surroundings. To reduce this heat loss, a cover is placed over the absorber and the sides and back of the absorber are insulated. The cover must allow solar radiation to penetrate and, since glass is typically used, some of this radiation will be reflected and radiated out to the atmosphere. The following sections will provide a brief overview and details of the solar collector parts.

Absorber

The function of the absorber is to effectively convert solar radiation into heat. The absorber surface is often coated to maximize this energy collection. The absorber coating is thus designed with a high absorption coefficient, α, for the sun’s radiation spectrum (typically α = 0.92 to 0.96). Absorptivity is the fraction of incident sunlight captured (not reflected) by the absorber. The reflectance is the complement of the absorption and is given by: ρ= 1 – α. For best performance, the absorber should have a low emission coefficient ε (typically, ε = 0.05 to 0.1) for infrared radiation to keep the losses from long wave radiation emission low as the collector heats up.

Emissivity is the ratio of radiant heat loss off the absorber relative to that of a perfectly black surface (“blackbody”). Most common materials, such as black paint, have an absorptivity equal to the emissivity, and the second law of thermodynamics requires that all materials have a=e at a given wavelength of incident light. However, special surface treatments (semiconductor coatings, blackened nickel layer) have an absorptivity in the short-wavelength solar spectrum that is much higher than emissivity in the long-wavelength infrared radiant heat loss spectrum. Such surfaces are called “selective surfaces” and improve the performance of solar collectors, especially when operating at elevated temperatures where radiant heat loss is more important.

Absorber coatings that possess high absorptivity and low reflectance are called “selective absorbers.” Figure 3.10 shows this selective effect where the absorption/reflection characteristics of a selective surface are identified at various wavelengths of the solar spectrum. These wavelength values are taken with an atmospheric thickness of 1.5 of the thickness taken directly above (AM = 1.5). This is important since the atmosphere affects the spectral nature (wavelength distribution) of the solar radiation, and properties reported at AM=1 refer to one thickness of atmosphere. If the sun is not directly overhead, the sun’s rays will have to go through more than one thickness of atmosphere. For example, AM = 1.5 corresponds to a zenith angle of around 48.2 degrees, and AM=0 refers to the wavelength outside of the earth’s atmosphere.

For spectral irradiance originating from the sun the solar constant (1.367 W/m2) is defined as AM = 0. AM = 1 is defined as the spectral irradiance on a horizontal plane (zenith angle = 0°). AM = 1.5 is equal to a zenith angle of around 48.2 degrees and the global radiation accounts for 36,700 Btu/hr/sq ft (1000 W/m2). (Tables of these standard spectra are given in ASTM G 173-03. The extraterrestrial spectral irradiance (i.e., that for AM0) is given in ASTM E 490-00a).

Transparent cover

The purpose of the transparent cover is to reduce the convection losses from the absorber, while allowing the maximum amount of radiation to reach the absorber. The cover must also provide the mechanical strength to protect the absorber from the environment.

Special solar glass with low iron content is used. It is occasionally called “water white glass” and its typical transmittance is τ = 0.89 to 0.91 for the wavelength range of the solar radiation. This can be enhanced to τ = 0.94 to 0.96 when anti reflective coatings are applied. This glass should be tempered to reduce breakage by impact.

Housing

The housing of a collector must provide the necessary mechanical strength to protect the absorber and the insulation to minimize heat loss to the environment. It must withstand wind and snow loads that occur in the area where the collector is installed. It also must be tight enough against rain penetration. These features need to be ensured over the entire lifetime of the system (20 to 25 yrs). Housings are typically made from aluminum sheet stock or extruded sections, galvanized and painted steel, molded or extruded plastic parts, or composite wood products.

Insulation

Insulation is added behind the absorber plate and on the sides of the collector to reduce thermal heat losses. The insulation must use a minimum of binders because it is intended for high temperatures (up to about 400 °F [204 °C] for flat plate collector stagnation); otherwise, the binders will outgas and form a film on the underside of the collector glazing blocking solar radiation. Common insulating materials include, for example, mineral fiber, ceramic fiber, glass fiberglass, and plastic foams. Sometimes polyurethane foam is used, though its resistance to temperature and moisture is limited so it should not be allowed to contact the absorber plate inside the collector. The insulation provides low heat conductivity, some mechanical strength, and temperature and fire resistance.