Page 343 - Handbook of Energy Engineering Calculations

Gainesville, FL; Santa Maria, CA; St. Cloud, MN; Washington;
Albuquerque, NM; Madison, WI; Oak Ridge, TN. The separate curve above
the shaded band in Fig. 14 is the result for Seattle, which is distinctly
different from other parts of the country. Hot-water loads used in the
computer computations range from 50 gal/day (189.2 L/day) to 2000 gal/day

(7500 L/day). The sizing curves in Fig. 14 are approximate and should not be
expected to yield results closer than 10 percent of the actual value.
Remember that the service hot-water load is nearly constant throughout the

year while the solar energy collected varies from season to season. A hot-
water system sized for January, such as that in Fig. 14 with collectors tilted at
the latitude angle, will deliver high-temperature water and may even cause
boiling in the summer. But a system sized to meet the load in July will not
provide all the heat needed in winter. Orientation of the collector can partially

overcome the month-to-month fluctuations in radiation and temperature.
Solar-energy water heaters cost from $300 for a roof-mounted collector to
over $2000 for a collector mounted on a stand adjacent to the house. The
2
latter are nonfreeze-type collectors fitted with a draindown valve, 50 ft (4.7
2
m ) of collector surface area, an 80-gal (302.8-L) water tank, and the needed
pumps and controls. Estimates of the time to recover the investment in such a

system can range from as little as 3 to as long as 8 years, depending on the
cost of the fuel saved. The charts, used in this procedure, were originally
published in DOE/CS-0011, Introduction to Solar Heating and Cooling—
Design and Sizing, available from the National Technical Information

Service, Alexandria, VA, 22312.
DOE/CS-0011 notes that a typical family of four persons requires, in the
United States, about 80 gal (302.8 L) of hot water per day. At a customary
supply temperature of 140°F (60°C), the amount of heat required if the cold

inlet water is at 60°F (15.5°C) is about 50,000 Btu/day (52,750 kJ/day).
Further, there is a wide variation in the solar availability from region to
region and from season to season in a particular location. There are also the
short-term radiation fluctuations owing to cloudiness and the day-night cycle.

Seasonal variations in solar availability result in a 200 to 400 percent
difference in the solar heat supply to a hot-water system. In the winter, for
2
2
example, an average recovery of 40 percent of 1200 Btu/ft (3785 W/m ) of
2
2
solar energy of sloping surface would require approximately 100 ft (9.3 m )