Page 345 - Handbook of Energy Engineering Calculations

water in the collector and associated pipe freezing during unexpectedly cold
weather. Since they were introduced on a wide scale, thousands of solar
DHW systems have suffered freeze damage, even in relatively warm areas of
the world. Such damage is both costly and wasteful of energy.
Three ways are used to prevent freeze damage in solar DHW systems:

1. Pump circulation of warm water through the collector and piping during
the night hours reduces the savings produced by the solar DHW heating
system because the energy required to run the pump must be deducted
from the fuel savings resulting from use of the solar panels.

2. Use an automatic draindown valve or mechanism to empty the system of
water during freezing weather. Since the onset of a freeze can be sudden,
such systems must be automatic if they are to protect the collector and
piping while the occupants of the building are away. Unfortunately, there is
no 100 percent reliable draindown valve or mechanism. A number of “fail-

safe” systems have frozen during unusually sharp or sudden cold spells.
Research is still being conducted to find the completely reliable draindown
device.

3. Indirect solar DHW systems use a nonfreeze fluid in the collector and

piping to prevent freeze damage. The nonfreeze fluid passes through a heat
exchanger wherein it gives up most of its heat to the potable water for the
DHW system. To date, the indirect system gives the greatest protection
against freezing. Although there is a higher initial cost for an indirect

system, the positive freeze protection is felt to justify this additional
investment.
There are various sizing rules for solar DHW heating systems.
Summarized below are those given by Kreider and Kreith—Solar Heating

and Cooling, Hemisphere and McGraw-Hill:
2
2
Collector area: 1 ft /(gal · day) [0.025 m /(L · day)]; DHW storage tank
2
2
capacity: 1.5 to 2 gal/ft (61.1 to 81.5 L/m ) of collector area; collector water
2
3
2
flow rate: 0.025 gal/(min · ft ) [0.000017 m /(s · m )]; indirect system
2
2
3
storage flow rate: 0.03 to 0.04 gal/(min · ft ) [0.0002 to 0.00027 m /(s · m )]
of collector area; indirect system heat-exchanger area of 0.05 to 0.1
2
2
2
2
ft /collector ft (0.005 to 0.009 m /collector m ); collector tilt: latitude ±5°;