Solar Thermal Information

43
annual basis. The diminishing return on the larger system is
again attributable to lack of load in warmer weather.
The larger house, with its larger load reduces this
excess, but doesn’t eliminate it for the eight-collector
system in either July or August. The annual solar fraction
for the four-collector system on the larger house is now
15.1% versus 26.9% for the eight-collector system.
The most important point to be taken from these
comparisons is that large solar combisystems in typical
residential applications do not offer the same economic
advantages of smaller combisystems. This is due to loads
being far less than the potential amount of heat collection
from a large collector array.
In some installations, the greater summertime energy
delivery of a solar combisystem could be used to heat
to a swimming pool. Heated fluid from the collector array
would be sent through a stainless steel heat exchanger
connected to the pool’s filter system. Assuming this energy
displaced other conventional energy for pool heating, the
economic viability of the system would definitely improve.
Another way to add load to the system in summer is to
dissipate heat into the loop field used for a geothermal
heat pump system. This could allow some quantity of
solar-derived heat to be stored in the earth—heat that
could later be recovered and used during the heating
season. However, the feasibility of this concept precludes
situations where the geothermal loop field is also being
used to dissipate heat from a building cooling system.
It also requires assurance that underground aquifers will
not be carrying away that heat prior to its recovery.
The potential for excess heat production in summer also
favors the use of drainback freeze protection versus
antifreeze in combisystem applications. When no more
energy can be delivered to storage, the drainback system
drains and the collector array “dry stagnates.” In an
antifreeze-based system, a heat-dumping subsystem
must be operational to prevent rapid thermal deterioration
of the antifreeze solution.
COLLECTOR ORIENTATION IN COMBISYSTEMS:
The theoretical optimum collector orientation for a combi-
system is true (polar) south. However, site conditions
that create shading may justify different orientations.
Variations up to 30 degrees East or West of true South
typically result in less than 10% loss of annual solar
energy collection.
An accepted rule of thumb for collector slope in
combisystem applications is latitude plus 15 degrees.
The steeper slope, relative to collectors used solely for
domestic water heating, favors winter sun angles, and
improves performance when heating loads are greatest.
Steeper collector slopes also reduces solar radiation on
collectors during warmer weather when loads are small,
and thus reduce the potential for overheating. Collectors
mounted to a vertical, south-facing, unshaded wall (slope
angle = 90 degrees) yield approximately 20% less energy
gain than collectors mounted at an optimal slope.
figure 6-4