Use Solar Energy to Heat Water

Solar Hot Water

This article can be viewed in full in the May 2008, edition 194 of Fine Homebuilding. "Solar Hot Water" was originally published by Scott Gibson.

"A spike in energy prices and short-lived government incentives created a solar hot-water boomlet in the 1970s and 1980s. The interest withered when energy prices dropped and government subsidies dried up, sticking homeowners with systems that didn't always work and couldn't be serviced for lack of qualified technicians. Rising energy prices are once again making solar attractive. But this time around, the industry is offering more-dependable, better-designed hot-water systems the give homeowners in all parts of the country a reliable way to cut energy bills.

"Heating water with the sun can be pretty simple. In the right climate, a 55-gal. drum painted black and perched on the roof provides plenty of hot water. Collectors like that, called batch heaters, are producing hot water all over the world. But technology has a lot more to offer these days, making solar hot water feasible for any region of the country and for just about any application, from swimming pools and hot tubs to domestive hot water and even space heating.

"No matter where you live, a solar system can reduce energy costs and provide a reliable supply of domestice hot water."

Solar Hot Water Panels

There are many ways to heat water, but keeping it hot is another story

"Although solar hot-water systems vary widely in design and complexity, they share some basics. The sun heats water, or another liquid capable of transferring heat, in a collecto Specialized materials called selective coatings are made to absorb available solar radiation. They include black chrome, black nickel, and aluminum oxide combined with nickely or titanium nitride oxide.

"Once water is hot, it's moved either to a storage tank or is piped directly to where it's needed. That much seems timple, but the trick ismaking sure the water doesn't cool down too much or freeze. To cover the wide range of temperatures and solar potential that hot-water systems can encounter, manufacturers offer a variety of equipment and plumbing options.

"In general, systems are either active or passive, meaning they operate with or without electric pumps. They also can be direct or indirect (open loop or closed loop), which means the collectors heat the water that's used in the house, or heata nonfreezing transfer medium that in turn heats potable water in a heat exchanger. In virtually all cases, solar-heated water is routed through a conventionalwater heater, where it gets a temperature boost before being distributed to its point of use."

Servamatic solar hot water heaters

How much hot water do you need?

"Most Americans use about 20 gal. of hot water a day, a standard industry benchmark. Most hot-water tanks are sized for a single day's consumption, so an average family of four, for example, might end up with an 80-gal. tank. Solar hot-water systems should have no trouble delivering that kind of volume, but there aren't any safe generalizations about whether it will be enough to satisfy household demands."

Example number 2: "A 3000 sq.-ft. house occupied by two elderly people will have substantially less demand than the exact same houes next door that has five people, including two teenage girls. Their dend is tenfold what it is in the other house."

"Other variables include when demand for hot water is highest, whether use comes all at once (morning showers, dishes, etc.) or is distributed over the day, how many appliances are in the house and when they are used, and how much solar potential the house has. 'It's different for each day and each house.'

"If there's such a thing as an average, the Arizona Solar Center estimates that a solar hot-water system should be able to deliver 100% of hot water in the summer about about 40% on a year-round basis. Performance varies by region. A household of four people would need 40 sq.-ft. of collectors for an 80-gal. tank in Arizona, 55 sq.-ft. in South Carolina, and 106 sq.-ft. in Vermont.

"How this translates into savings on gas or electric bills is also a wild card. Most solar hot-water systems are used to heat watter before it goes into a conventional water heater, not as an outright replacement for a water heater fuled by gas or electricity. Careful consumers who are flexible about when they use hot water will see more solar benefit than a family that wants a lot of hot water at once. Under the right circumstances, virtually all of a household's hot-water needs can be met by a solar system. But that's no guarantee."

Solar Hot Water Heating System

Composed of a 55-gal. drum, plastic irrigation tubing, and a piece of plywood. It is designed as a "thermosiphon unit" meaning that no pump is required to move the water, natural convection currents should do the trick!
Composed of a 55-gal. drum, plastic irrigation tubing, and a piece of plywood. It is designed as a "thermosiphon unit" meaning that no pump is required to move the water, natural convection currents should do the trick!

Using solar for space-heating

"Solar collectors are commonly used for domestic hot water, but they also can supplement both forced-air and hydronic-heating systems. In Europe, pacakge systems that do both are relatively common. But due to heavy winter-heating loads and reduces solar potential, homeowners in the country shouldn't explect to get much more than one-third of their winter heat from solar soucrces with today's technology.

"The president of Synepex Energy in Cambridge, Mass., says the proportion of winter heat from solar depends on the type of heating system, the amount of insulation installed, and the tightness of the house. A best-case scenario in New England, land of snowy winters and cold, dreary springs, is that a solar system meets 40% to 70% of the heating load. That's a well-sealed house with a radian-floor heating system.

"Radiant-floor heat is especially well suited to solar hot-water systems because it requires lower water temperatures, 120F versus 180F that would be pumped through a typical baseboard hydronic system. Solar hot water also can be used for newer forced-air systems that use a technology called "hydro air." These boilers heat water forced through a heat-transfer coil, where it warms out-going air.

"A big drawback with trying to heat the house with solar hot water is that demand is highest when the heat potential of the system is lowest. On an overcast day in northern New England, the sun is long gone by late afternoon, and the call for heat goes up accordingly. The answer is to store hot water generated during the day in storage tanks so that it can be used for heat when the sun goes down or when the days are cloudy. Tanks can be very large, 2000 gal. or more, although the newer systems can use much-smaller tanks, as little as 200 gal."

Researchers also are looking down the road at promising new possibilitys. For exapmle, one experimental project in Canda includes using solar collectors to heat the ground when solar potential is abundant in summer. "In winter, geothermal heat pumps can be used to extract the stored heat. This seasonal storage of heat is one idea that could make 100% solar heat possible in the furture."

Fine Homebuilding

Fine Homebuilding Magazine

The Fine Homebuilding magazine shows in-depth techniques for various home projects. You'll find hands-on construction tips that are more than fully explained for easy comprehension.

The projects range from building foundations to from carpentry and interior finishing to system installations (whether plumbing, heating, or air).

Each issue contains in-depth reviews about tools, techniques, and materials that you may need for fine homebuilding projects.

You'll find information from various departments covering builder's tips, remodeling ideas, and answers to reader's questions.

This is a magazine that I love to peruse on my days off to get ideas for my next project. I have learned wood staining tips that I used to build my corner desk, and I've found tool reviews that I've used when looking for better tools to effectively build various projects around my home.

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Comments 1 comment

Eileen Hughes profile image

Eileen Hughes 8 years ago from Northam Western Australia

You appear to have really researched this subject. We bought a solar king hot water many years ago. As we were all out early in morning and all home late at night. It was no good. We were forever putting the power booster on.

It may have been a dud. But it turned us off buying one again Instead we went to infinity gas heater. Only heats when needed. no storage. Very good hub

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