14 June 2002
UL = overall heat loss coefficient. This factor describes the cumulative heat
transfer between the collector and the ambient surroundings.
= transmittance of the glazing.
= absorption coefficient for the absorber plate. Note that this value varies with
wavelength. A selective surface is one that absorbs short wavelength solar
radiation very well while emitting longer wavelength thermal radiation poorly.
(b) Efficiency Parameters. The efficiency of a given solar collector will
vary greatly with ambient temperature, storage tank temperature, and the amount of
solar insolation available. For this reason, each type of collector will perform best under
different select conditions. Two parameters are required to describe the efficiency of a
collector. The first is commonly referred to as FRta. This factor includes the product of
the glazing transmittance and the absorption coefficient and is related to the optical
efficiency of the collector. It takes into account reflection losses both through the cover
glazing and those due to imperfect absorption by the absorber plate coating. For liquid
collectors, the fluid flow rate and collector insulation have very little effect on this factor.
The second factor is related to the thermal losses from the collector to the surrounding
environment. The product of the collector heat removal factor and the overall heat loss
coefficient, FRUL, is used to account for the thermal resistance characteristics of the
collector. Usually, the fluid circulating through the collector is hotter than the ambient
temperature around the collector. This condition means that solar radiation absorbed
by the collector can follow two paths. One path is from the absorber plate to the
circulation fluid. The second path is from the absorber plate to the surrounding
environment. The absorbed solar radiation will be divided according to the temperature
differences of each path and the relative thermal resistances. For a given process,
these temperature differences normally cannot be controlled. Therefore, the thermal
resistances of each path must be considered. The resistance from the absorber plate
to the circulation fluid should be as small as possible (i.e., a good thermal bond should
be made between the fluid circulation tube and the absorber plate). It then follows that
the resistance between the absorber plate and the surrounding environment should be
as large as possible.
(c) Collector Energy Balance. The collector parameters described
above allow an energy balance to be expressed as:
Energy Collected = Solar Energy Absorbed - Thermal Energy Losses to the Environment
The energy balance can be written in a simple equation form using the efficiency
parameters described above:
Energy Collected = (FRta)(I)(Ac) - (FRUL)(Ac)(Ti - Ta)
Equation E-1 shows that heat losses to the environment are subtracted from the net solar
radiation transmitted into, and absorbed by, the collector. Assuming that the efficiency
parameters are fixed for a given collector model, the main factors that affect the amount of
energy collected are I, Ti, and Ta. The geographical location and the season dictate the