There are basically seven different types of water heaters that people may choose to use based on their water usage, location, climate, needs and expenses. These seven are: integral collector/storage, thermo siphon, three-season, drain-back, drain-down, re-circulation, and active closed-loop. Of course some people may make changes or even come up with a new idea or two but these seven are the most generally used types. What exactly is involved with each of these solar hot water heaters?
The integral collector/storage solar water heater is quite possibly the most well known and simplest solar water heating system. Often known as the “bread box” system it was originally produced in the 1970’s but is still in use now. It is simple, efficient and cheap to build. All it takes is a tank, insulation and sun. One name brand of the integral collector/storage solar water heater you may recognize is Servamatic. There are also in-line units, positioned between the well and the shower and this type of heater requires very little maintenance or repair.
Thermosiphon solar water heaters use sunlight to strike tubes and fins within the collector box in which water or glycol is circulating. The collector is parallel with the storage tank causing a loop. The heated fluid moves from the collector to the storage tank and back to the collector through a process called thermo siphon. This is a natural convective action. Those that live in colder climates may find that the collector will freeze and burst. Changes in design can prevent this from happening or another type of solar water heater may be your preference. However if you live in a climate zone without freezing temperatures, an open thermosiphon system could work well.
The three-season system is simply a variation on other solar water heating types. What makes it different is that you use your solar water heater for three out of four seasons, then during the fourth drain it and use an alternate form of water heater.
A drain-back solar water heating system drains water in the panels into a tank when there’s no heat available from the sun. The panels are empty of water, then, and cannot freeze. A non-pressurized tank is used to capture this water, and a pump refills the panels when the sun’s warmth is detected.
The drain-down solar water heating system is a variation on the drain-back system. In the drain down system the water is dumped onto the ground. It uses a Sunspool™ valve to fill the panels for operation. The same valve, when it reaches a lower temperature, opens to dump the water that’s in the panels onto the ground. This is a fairly common design you may have seen in older systems.
In a re-circulation solar water heating system the method used to prevent freezing is to use a pump to circulate a little bit of hot water from the storage tank back into panels when low ambient temperatures are experienced.
The active closed-loop solar water heating system uses any type of fluid in the collector to storage loop that won’t freeze at low temperatures. The heat gathered in the collector is then transferred to the water in the storage tank through a heat exchanger. What are some fluids that don’t freeze that you can use? Glycol, silicon oil, and methanol would work but never automotive anti-freeze. It is poisonous. The most common used heat transfer medium is polypropolene glycol, which is a food-grade dough extender used in the baking industry. It costs about $20 a gallon and is mixed with water. A 10% mixture will protect the collectors down to 20-25°F. Depending on locations a higher amount may be needed.
Each of these types of solar water heaters may be assembled at home or purchased. Did you know that the average family of 4 spends about $551 annually just heating their water? (Based on electricity at 12¢ per kWh). Of course there are other methods of water heating like propane at $1.41 per gallon costs about $26 a month or $307 per year. Natural gas and fuel oil are less, as is electricity in other parts of the country.
So why consider a solar water heater? First look at the savings once a solar water heating system is installed and has returned the investment, the energy from it thereafter is free and it is good for the environment.
Interested in the basics of solar water heating systems? Read The Vocabulary of Solar Water Heating Systems.
US Department of Energy