Solar thermal systems have been the water-heating method of choice for homes near the equator for decades — back when many North Americans relied on a woodburning stove and kettle! Spared the threat of freezing, “batch” equatorial systems are simple, cheap, and effective. North Americans also have a long history with solar hot water, but wintertime freezes — found in much of the United States and Canada — forced the development of freeze-ready “flat-plate” collectors, which went mainstream in California and Florida in the 1970s. Today, the innovative solar thermal industry produces highly efficient models capable of heating water as far north as the Arctic Circle! As a solar water heating system installer, I’ve met hundreds of people who’ve turned to solar water heating as a way to save money and to reduce their carbon footprint.
The solar thermal systems that I promote provide a return on investment between 5 and 15 percent, depending on the type of fuel you’re replacing, your location, and which state utility incentives are available. A residential solar water heater will offset greenhouse gas emissions by three-quarters of a ton of carbon dioxide every year — the equivalent of an average driver cutting 1,300 miles from their annual commute. Moreover, solar water heaters qualify for the 30 percent federal tax credit until 2020, so the time to install is now!
If your plumbing and wiring skills are up to snuff — and code — you’ll have no trouble assembling a solar water heater for your home. For everyone else, a commercially available system is the best recourse. This article will present four initial steps to take and the general options to consider when shopping for solar thermal systems.
Your first step toward hot water self-sufficiency is to reduce waste. Address your household’s water-wasting habits. Shut off the hot water when shaving, soaping up, and rinsing the dishes; better yet, embrace the thrill of a cold shower! Insulate as many hot water pipes in your home as possible. Mineral wool and fiberglass insulation are ideal for high-heat applications, such as indoor solar piping, and polyethylene is recommended for temperatures up to 180 degrees Fahrenheit. Finally, choose high-efficiency washing machines and dishwashers, and low-flow showerheads and faucet aerators to reduce demand.
After reducing waste and determining your hot water needs, you’ll need to size the best system for your household and your wallet. Solar water heaters rarely provide 100 percent of household hot-water needs — there are just too many cloudy days during the year, especially at my home in Wisconsin. Instead, solar thermal systems typically provide 50 to 75 percent of your annual load depending on location and season. Check with a local solar specialist about what to expect in your area.
Whichever capacity you choose, you’ll need a separate solar storage tank and backup water heater — the same tank can’t do both jobs efficiently. Collectors are typically mounted on a roof, but you can also mount them on the ground near a dwelling. Wherever you choose to site them, remember that collector boxes are highly vulnerable to wind — make sure they’re securely braced and fastened!
In the Northern Hemisphere, most solar collectors for water heating will tolerate a small amount of shade and should be mounted within 30 degrees of true south. Optimally, mount your collector at an angle above the horizon that equals your latitude — just shy of 45 degrees for me.
In cool climates, I recommend 20 square feet of collector and 20 gallons of storage for each person in the household. For large families, this can be reduced by 10 percent for each person over four. In warm climates, I estimate 15 square feet and 25 gallons of storage for each person in the household, with a 10 percent reduction for each person after the fourth. This sizing method will give you your best return on investment. Smaller systems than this will certainly work well, but your savings will be less. Larger systems will cost more, while supplying an unnecessary excess.
In climates that never experience freezing, Integral Collector Storage (ICS), or “batch,” systems are acceptable and often the cheapest option. However, when water-filled pipes are exposed to freezing conditions, they may burst. For the coldest climates, closed-loop antifreeze systems offer the best protection, while drainback setups work well for most of the United States.
ICS “no freeze” systems. For climates that never experience freezing conditions or for applications that are only for summertime use, such as summer homes and campgrounds, having water in the solar collector at all times isn’t a problem. For these sunny or seasonal applications, an ICS system will be your best option. Batch systems are simply water tanks exposed to the sun and are especially popular near the equator because of the warm weather and the system’s ease of use.
Commercial ICS collectors have a single tank or multiple tanks inside an insulated box with glass on the side facing the sun. ICS tanks are often painted black or covered with a special coating that’s very good at absorbing the sun’s energy. Unlike closed-loop and drainback systems, ICS systems have no pumps or controls and are plumbed directly into your home water system, so they’re the simplest and cheapest option.
Freeze-ready systems. Two kinds of solar water heating systems are appropriate for climates that experience freezing conditions: closed-loop antifreeze systems and drainback systems.
The most popular and versatile type of system installed worldwide is a closed-loop antifreeze system. Closed-loop systems consist of insulated piping filled with nontoxic, water-diluted antifreeze (often called “solar fluid”), a collector array, a circulating pump, an expansion tank, a solar hot water storage tank, a heat exchanger, and a controller, in addition to valves and gauges.
Heat exchangers, typically a coiled tube or flat plate, transmit heat from your solar fluid to your hot water. When choosing a heat exchanger for your system, make sure it has a large enough surface area to move all the solar energy captured by your collector.
The piping in a closed-loop system makes a loop from the collectors to the heat exchanger and back to the collectors. This closed loop is filled with solar fluid, which stays inside the collectors and piping at all times. This fluid needs to be replaced about every 25 years because the antifreeze solution will degrade, especially when the system is idle for long periods or exposed to hot temperatures.
Whenever the sun shines on the collectors in my closed-loop systems, it also activates a small photovoltaic (PV) panel that’s directly wired to a 12-volt, direct-current (DC) pump. That pump will circulate the solar fluid throughout the closed loop until the sun stops shining on the solar collector and PV panel. In part because of their simplicity, DC-powered systems are quickly becoming a popular closed-loop option on the market. As the solar fluid gets hot inside the collectors, it travels through the piping to the heat exchanger. The heat exchanger transfers the heat from the solar fluid to the water inside the solar storage tank, which stores the solar-heated water for your use at a later time. When the sun isn’t shining, the circulating pump simply turns off and the solar fluid stops circulating.
Popular in moderate and hot climates, drainback systems are very similar to closed-loop systems. The big difference is that these systems have a drainback tank instead of an expansion tank. The solar fluid in these systems can be distilled water or a diluted water and nontoxic antifreeze solution. When the sun isn’t shining on drainback systems, the solar fluid is stored in the drainback tank, leaving the pipes and collectors empty. Like a closed-loop system, the pump comes on when the sun warms the collectors and circulates solar fluid throughout the system. A heat exchanger transfers the heat from the solar fluid into the solar storage tank in the same way as in the antifreeze system. Drainback systems are also attractive because they eliminate the expansion tank and therefore have fewer valves and gauges than closed-loop antifreeze systems.
While drainback systems’ direct plumbing is attractive, many installers bemoan their lack of flexibility. Collectors must be mounted above the drainback tank, limiting placement options, and piping installation must maintain at least a 10-degree slope from collector to tank for drainage. Finally, drainback systems always use a 120-volt AC controller and a high-pressure pump. This “high-head” pump differs from the closed-loop system’s circulation pump, because a drainback pump’s higher pressure allows it to completely fill the piping and collectors every time the system turns on. Then, when conditions dip below freezing, the solar fluid drains from the collector again.
Solar collectors absorb the sun’s rays and convert them to heat energy. In an ICS system, a collector can be as simple as a water tank. However, high-efficiency and freeze-proof units incorporate two popular collector options: flat-plate and evacuated-tube collectors. Each technology has its pros and cons, including price differences, and there’s plenty of information and strong opinions on the subject. Once again, before making your final decision, get your local solar installation technicians’ advice for your roof, your expected usage, and your region’s weather patterns.
Flat-plate collectors use an absorber plate — usually copper painted black — to heat a series of pipes, which contain the solar-transfer fluid. The pipes and plate are contained within an insulated frame that’s mounted with a sheet of tempered glass that faces the sun. Flat-plate collectors are by far the most popular kind of collector because they work well in all climates. Furthermore, they’ve been around longest, are very efficient, and are the cheapest option. Because they lose some of their heat to the surrounding environment, well-designed and well-maintained flat-plate collectors rarely overheat, and during winter months they’ll melt any snow or ice buildup.
Evacuated-tube technology appeared in the late 1970s and, despite some early missteps, has continued to develop. Like flat-plate collectors, evacuated-tube collectors place a series of tubes on an absorber plate. However, evacuated-tubes are airless, which allows for a more efficient transfer of heat from plate to solar fluid. This added efficiency means that evacuated-tube collectors must be designed to handle higher internal temperatures than flat-plate collectors. Take particular care to never oversize the collectors or undersize the storage tank. Finally, while prices continue to fall, evacuated-tube collectors tend to cost more than flat-plate collectors for an equivalent heating capacity.
Solar water heaters can last a lifetime when skilled technicians use high-quality materials to install the best system for a given climate, roof site, and particular demand. If you find a system that’s substantially cheaper than the rest, there’s probably a reason, such as lower-quality components. There’s a lot of information out there on solar thermal, and, frankly, the sheer volume of variables can dissuade people from making a significant financial and environmental choice. Before making any decisions, remember this advice: Reduce consumption; identify the appropriate size and site for your system; plan for freeze protection; choose your collector; and, above all, choose quality parts and labor.
Bob Ramlow has more than 40 years of experience with solar energy systems and is a co-founder and lifetime member of the Midwest Renewable Energy Association. This article was adapted from his book Solar Water Heating.
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