14 June 2002
SYSTEM SELECTION, PLANNING, AND COORDINATION
INTRODUCTION. This chapter provides criteria for selection of a specific type
and configuration of solar energy system, and discusses special issues that must be
considered. Once the system type is selected, coordination with the architect and
structural engineer is critical for determining estimates of roof area, roof and collector
support, and equipment space requirements. It should be noted that this manual
applies to the design of systems for the northern hemisphere. Appropriate corrections
should be made for the design of these systems in the southern hemisphere.
STANDARD SYSTEM TYPES. To meet the Services' goal of standardizing solar
energy installations, the following system types have been selected for use on all active
Closed-Loop System. The closed-loop solar energy system has proven to
be very reliable when designed and maintained properly, largely due to its ability to
successfully withstand freezing temperatures. Freeze protection is provided by
circulating a solution of propylene glycol and water through a closed collector loop.
Figure 3-1 is a schematic of the closed-loop system.
Solar Loop. The differential temperature controller activates the solar
loop pump in the collector loop when the temperature difference between the collector
and storage is large enough for energy to be collected. The propylene glycol solution
circulates in a pressurized closed-loop through the solar collector to an external heat
exchanger. An expansion tank is provided to account for thermal expansion of the fluid
in the collector loop, stagnation, and over-pressure protection. Refer to APPENDIX F
for a discussion of stagnation conditions in solar systems.
22.214.171.124.2 Storage Loop. The control system activates the storage loop pump
simultaneously with the collector loop pump. Water in the storage loop is heated by the
solution in the heat exchanger and passed to the solar storage tank. When there is a
hot water demand, cold water is drawn into the solar storage (preheat) tank and solar
heated water is sent to an auxiliary water heater where it is heated further (if necessary)
and sent to the load.
Design Precautions. While the closed-loop solar energy system can
provide reliable service in any climate, certain design precautions must be taken.