Using Solar Water Heaters as a Primary or Auxialliary Source of Hot Water
Since the 1980's, solar water heaters haven't enjoyed as many incentives and rebates as solar electricity through photovoltaics. This is a technology that is decades old, is progressing rapidly, has become reliable and coming down in price.
Solar hot water is not only considered a renewable technology but also often incorporated in energy efficiency upgrades and retrofits, especially for pool heating. Still, based on where you live, what direction your roof faces, and how many people in your house, there are several things to consider with solar water heaters.
How They Work
The typical solar hot water (SHW) system is made of a series of flat-plate collectors which absorb the sun’s radiant energy and transfer it to a working fluid. A circulation system using pipes, pumps and controls transports the working fluid to a heat exchanger which then transfers the captured heat to water in a storage tank.
Types of Solar Water Systems
There are two generic types of solar hot water systems. Open loop, or direct systems circulate water directly through the collectors for heating. Closed loop, or indirect systems pass a working fluid through the collectors to a heat exchanger where heat is transferred to the potable water supply. The circulation of fluid to and from the collectors is done either actively with pumps, or passively through the natural buoyancy difference of the solar-heated fluid. Passive systems generally are not recommended for use in freezing climates.
In northern climates the most common type of SHW system is the closed looped pumped system which uses an antifreeze heat transfer fluid such as water/ethylene glycol or water/propylene glycol. This fluid is non-potable and toxic, and normally requires a double-walled heat exchanger to insure fluid separation.
An alternative to using antifreeze is a direct, all-water drain-back system. The water drains by gravity out of the collectors whenever the circulation pump is turned off. This is a simpler way to prevent freezing, but it also leaves no room for mechanical errors during freezing weather.
Evacuated Tube Collector Technology
The SHW system can be configured to preheat water to a building’s existing water heater. Or, a solar tank with an auxiliary heater can replace the existing heater. SHW systems can heat a fluid within 120-150 degrees F. Again, remember that in colder climates such as the Northeast, SHW output varies from day to day and seasonally, necessitating storage and an auxiliary energy supply.
The optimum size and performance of a solar water heater depends on four factors: 1) how much hot water you use per day 2) the amount of sunlight available, 3) the amount of available roof or ground space for the collectors and 4) the operating characteristics of the solar system. For most purposes, a SHW system is designed to provide anywhere from 30 - 70 percent of the annual water heating load.
Solar Water Heater Costs and Savings
When all is said and done, you can save between 50 - 75 percent of your water heating energy in the long term. But, unless you build the collectors yourself (a great do-it-yourself project), the cost of an installed system still ranges in the $5,000 mark. The average lifespan of a solar water heater is about 15 - 20 years. Areas with a lot of sun (several hours for three seasons) can save even more - and homeowners enjoy a higher variety of products.
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