Build Your own Passive Solar Water Heater

Enlarge Image Image Gallery DAVE CHANNON

For the do-it-yourselfer seeking an inexpensive, easy-to-build solar water-heating system, the integral passive solar water heater (IPSWH, pronounced ips-wah) is a dream come true. All you need to build this down-to-earth water warmer is a discarded electric water heater tank, a homemade insulated plywood box to house the tank, a sheet or two of used window glass or clear plastic, a few common plumbing fittings, some pipe and a bit of insulation. Add a few satisfying hours of measuring, thinking, sawing, hammering, painting and wrench-turning, and you’ll have a continuous supply of hot water, provided virtually free of cost by that friendly furnace in the sky (the only safe nuclear reactor, 93 million miles away).

First, let’s review the basics of solar heating for new recruits to the wonderful world of renewable energy. There are two basic types of solar water heaters, active and passive. Active systems depend on external power to run pumps to circulate the heat they gather; passive setups don’t. Passive systems may be less efficient at any given moment, but they are much more dependable and cost less per unit of heat captured. Integral passive solar water heaters, also called batch heaters, are the simplest of the passive systems, and their reliability and independence from external power lead to long-term production at a very low cost. I know of a system in Davis, Calif., that has operated for 30 years at a cost per kilowatt-hour equivalent of about a penny.

Batch heaters have long been known as the best choice in warm climates or for seasonal use in colder areas, and recent work with improved materials and designs suggests they may also be the best choice even in colder areas. For owner-built applications, they outshine their flat-plate and evacuated-tube competition in almost every way, including reliability and ease of installation. They have excellent potential for retrofits and are ideal for a range of farming and commercial applications, providing low-cost hot or warm water for washing or preheating for higher temperature uses.

Five Hot Ones

Now that you know the basics, let’s take a look at five of the many types of batch heaters in use today.

The Solar Shower is really a small-scale batch heater, a classic design using clear and black plastic to make a portable and surprisingly effective solar water heater. Water is placed in the bag and then set out with the clear side facing the sun. Within an hour, the water starts to heat up, providing a delightful shower on even a cool day if the sun is bright. I have used these for years while camping and am always impressed with how well they work.

The single-tank batch heater is often dubbed a “breadbox” heater, because some of them look a lot like an oversized old-fashioned breadbox. Overall, it’s probably the most economical, least complicated of the permanent batch heaters to build, requiring only easily acquired materials and basic construction skills. For the “econo-model” breadbox, a salvaged standard electric water heater tank is housed in an insulated plywood box or an old refrigerator or freezer shell. Glazing material (usually glass, fiberglass or high temperature resistant molded plastic) covers the box.