Collector time constant
A last aspect of collector testing is the determination of the heat capacity of a collector in terms of a time constant. It is also necessary to determine the time response of the solar collector in order to be able to evaluate the transient behaviour of the collector, and to select the correct time intervals for the quasi-steady state or steady-state efficiency tests. Whenever transient conditions exist, Eqs. (69)-(74) do not govern the thermal performance of the collector since
part of the absorbed solar energy is used for heating up the collector and its components.
The time constant of a collector is the time required for the fluid leaving the collector to reach 63% of its ultimate steady value after a step change in incident radiation. The collector time constant is a measure of the time required for the following relationship to apply :
T — T 1
= - = 0.368 (83)
Toi — Ti e
where Tot is the collector outlet water temperature after time t (°C); Toi is the collector outlet initial water temperature (°C); Ti is the collector inlet water temperature (°C).
The procedure for performing this test is to operate the collector with the fluid inlet temperature maintained at the ambient temperature. The incident solar energy is then abruptly reduced to zero by either shielding a FPC, or defocusing a concentrating one. The temperatures of the transfer fluid are continuously monitored as a function of time until Eq. (83) is satisfied. Results of tests carried out on a PTC constructed by the author are given in Ref. .