Build Your own Passive Solar Water Heater
(Page 2 of 7)
October/November 2007
By David A. Bainbridge
Incoming water enters the breadbox near the bottom of the tank, and sun-warmed water is drawn from near the top and routed to the backup unit in the house. A horizontally oriented breadbox is easier to build and less visible than a vertical one, but tilting the tank increases the stratification of the water and improves the solar angle, resulting in higher temperatures.
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The vertical three-tank batch heater is similar but increases performance with more storage and collector area. For the triple-tanker, three water heater core tanks are enclosed in a large, well-insulated box. Glazing material covers the south-facing side and the top of the box. An insulated lid or roll-down cover could be installed to improve heat retention, but I never found it to be necessary. The three tanks are plumbed in series, with the central (therefore best insulated) tank serving as the final stage before the heated water is discharged.
The greater collection surface area, improved thermal stratification caused by the tilted position and the series hookup provide better heating than the smaller, horizontal, single-tank breadbox. To test the efficiency of the three-tank heater, I installed and monitored one for a full year at my home in north-central California and found that it supplied an impressive 70 percent of my family’s hot water!
A greenhouse batch heater may be the best option for performance and freeze protection in colder climates or during the winter months. Almost any configuration can be used inside your sunspace or greenhouse. Usually, the solar water heater is tucked up near the peak of the greenhouse roof to take advantage of the warmest air and best sun exposure (see gallery illustration). For maximum efficiency and freeze protection in cold climates, an insulated, glazed box may be desirable. Insulate your pipes if the sunspace gets cold at night.
A building-integrated batch heater is built into the attic or a south-facing wall, where the tank is more easily protected against freezing. Pipes can run in heated space. When it’s time to reroof, there is no need to remove and replace the heater. (This is when many solar systems are junked, even if they still work.) An insulated box is created in the attic with a glass skylight. A drain pan may be a good idea to minimize leak risk. Make sure you can reach the heater easily for repairs or replacement if necessary.
How Batch Water Heaters Work
A few basic principles and considerations govern the design, installation, use and maintenance of batch heaters. IPSWHs are called batch heaters because the heart of the system is the “batch” of water stored in the tank(s). The basic batch heater design is based on a tank or a series of large diameter pipes or tanks. The tank is painted flat black or coated with a selective surface that absorbs solar heat easily and transfers it to the water stored inside. To increase heat collection and reduce heat loss, the tank is enclosed in an insulated box covered on the south-facing side or top with a glazing material.
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