Figure 2-26. Control System Strategy

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Design: Solar Heating of Buildings and Domestic Hot Water

MIL-HDBK-1003/13A

Cooling load/COP

IT [eta]collect [eta]delivery

where:

Cooling load = the portion of the total cooling load provided by

solar calculated using ASHRAE techniques or others.

COP =

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).

IT =

average instantaneous solar insolation on collector

surface (i.e., at tilt angle).

[eta]collect = average collector efficiency under design

conditions.

[eta]delivery =

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,

1979).

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.