Hot Water Output examples for 3 locations

Simulated Performance of Solar Domestic Water Heater Systems

The table below 35 lists the annual solar fraction for various domestic water heating loads in three different U.S. cities (Boston, MA; Milwaukee, WI; and Las Vegas, NV).

In all cases, the following system assumptions were made:

Closed-loop antifreeze system with heat exchanger effectiveness = 0.50

Collectors face directly south

Collector slope = approximately equal to local latitude

Setpoint temperature for domestic hot water = 120 Degrees F

Cold water temperature = typical of location

Annual solar fractions are given for four daily hot water loads (left column) and three solar collector area/storage tank volume scenarios (top row).

As would be expected, the larger collector arrays and correspondingly larger storage tanks yield higher solar heating fractions in all locations. In looking at these results, one might assume the best system is the one supplying the highest percentage of the load. From a pure energy conservation standpoint, this may be true. However, the economic viability of the system is a different matter. Systems that supply very high solar heating fractions may not provide returns on investment as high as systems supplying lesser amounts of the load. The only way to know for sure is through detailed economic analysis of competing system options. Most solar system simulation software can also perform an economic analysis of the system.

Boston, MA. 40 ft2/60 gal. 80 ft2/120 gal. 120 ft2/180 gal.
40 gal/day 72% 91% 96%
60 gal/day 57% 85% 92%
80 gal/day 48% 77% 89%
100 gal/day 41% 69% 84%
 
Milwaukee, WI. 40 ft2/60 gal. 80 ft2/120 gal. 120 ft2/180 gal.
40 gal/day 67% 88% 94%
60 gal/day 54% 83% 90%
80 gal/day 45% 74% 86%
100 gal/day 38% 65% 81%
 
Las Vegas, NV. 40 ft2/60 gal. 80 ft2/120 gal. 120 ft2/180 gal.
40 gal/day 94% 99% 100%
60 gal/day 88% 98% 100%
80 gal/day 77% 76% 99%
100 gal/day 69% 94% 98%