Solar Thermal Information

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system meets 37.4% of the total load. Thus, the smaller
system produced about 63%of the savings associated
with the larger system. Although double in size, the
larger system did not double the energy savings. This is
largely because of the lack of load in warmer weather,
where the higher potential output of the larger system
is of essentially no use.
It follows that the economic return on investment of
the smaller system, per square foot of collector area,
is greater than that of the larger system. The choice
of which system to install should consider this, as well
as the goals of the owners and the available space
required for each system. If the choice is largely driven
by economic considerations, the smaller system is
the better choice. However, if the owner’s desire is to
cover a greater percentage of the load, and both the
space and funds are available, the larger system may
be selected. Ultimately, most combisystem sizes are
selected based on the owner’s weighted preferences
for performance, economic return and philosophical
commitment to renewable energy use.
Figure 6-3 shows the performance prediction for the
larger house in Syracuse, NY, with the same two
combisystems.
Notice that only one bar — that associated with the
larger (8 collector) system — reaches the 100% mark in
July. The larger load — especially that associated with
100 gallons per day of domestic hot water — makes
better use of the collected energy. The eight-collector
system still approximately doubles the monthly fraction
of the four-collector system from November through
March. The annual solar fraction of the smaller system
is 10.1% versus 18% for the larger system. Again,
having twice the collector and storage size does not
double the annual solar yield. However, the difference is
more significant in this application relative to the small-
house application because there is less wasted solar
energy during warmer weather (e.g., the larger domestic
water heating load makes better use of the available
solar energy).
Moving both of these systems to Colorado Springs,
Colorado, increases both the monthly and annual solar
fractions, as shown in figure 6-4.
Given the more favorable solar climate, the system on
the smaller house experiences even more excess energy
in warmer weather, especially if the larger collector array
is used. The annual solar fraction for the four-collector
system is now 34.8% versus 54.6% for the eight-
collector system. Using twice the collector area and
storage yields about 57% more collected energy on an
figure 6-2
figure 6-3