IT [eta]collect [eta]delivery
Cooling load = the portion of the total cooling load provided by
solar calculated using ASHRAE techniques or others.
Coefficient of Performance of the cooling unit. COP is
the ratio of heat energy removed to energy supplied from
external sources. Manufacturing data is recommended for
determining COP (3413 Btu = 1 kWh).
average instantaneous solar insolation on collector
surface (i.e., at tilt angle).
[eta]collect = average collector efficiency under design
delivery efficiency which takes into account heat
exchanger efficiency and thermal losses.
In general, the collector area required to provide the majority of the
cooling load is larger than the collector area of typically sized heating
only systems. Collector areas for heat engine systems are larger than the
areas for absorption cooling systems due to the thermal efficiency of the
heat engine, which should be included in the preceding equation (Swindler,
2.8 System controls. System controls are used to turn on a circulating
pump or blower to the collector only when the sun is providing heat.
Differential thermostats are commercially available (typically $50 to $150)
to turn on the collector pump only when the collector plate temperature is a
preset number (usually 20 deg. F) hotter than the storage tank bottom
temperature. A typical control strategy is shown in Figure 2-26 (Rho-Sigma,
undated) and the hookup in Figure 2-27.