February 20, 2018
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Solar hot water heating systems can help save on energy

By Tom Gocze, BDN Staff

Iroducing hot water for home use is a major solar heating topic. We refer to this as solar domestic hot water or SDHW. It is the biggest use of home energy after space heating.

A properly sized solar hot water system can usually save at least 50 percent annually.

There are a lot of different choices when shopping for solar domestic hot water. The major choice is what kind of collector to use.

For years the only choice was the flat plate solar collector. This is a thin box that has glazing on one face, usually glass, insulation on the backside, and an absorber on the inside. The absorber is usually a thin black metal plate that is a conductive metal, such as copper or aluminum, and has tubing attached to it. The tubing carries a coolant, which removes the solar heat from the metal absorber plate and moves it to an insulated tank for later use.

Flat plate collectors can be fairly efficient for producing SDHW and can be manufactured in a variety of forms. They usually cost $15 to $40 per square foot.

A square foot of most solar collectors can generate the equivalent of 1 to 2 gallons of oil energy per year.

The average SDHW system should be about 60 square feet for a family of two to four. This size system should have 1-2 gallons of water storage for each square foot of solar collector.

The other solar collector that is being used for SDHW is the vacuum tube collector. Vacuum tubes are constructed like big glass radio tubes or light bulbs. The metal absorber plate is made into long, thin strips that are inside the tube. They have either a water tube or heat pipe to remove the solar heat.

The beauty of the vacuum tube is the fact that the tube is evacuated of all air. The vacuum creates a high-performance insulated glazing for the collector. The heat loss from a vacuum tube collector can be much lower than a flat plate collector. They are also efficient on cloudy days because of this feature. They cost $30 to $80 per square foot.

When you compare the two collectors objectively, the flat plate actually delivers more solar heat than the vacuum tubes when making SDHW.

There are several things that cause this to happen. First, most of the solar energy we receive occurs on sunny days. This helps favor flat plates. The boost that vacuum tubes have on cloudy days delivers very little energy to a system.

Second, the amount of energy any solar system can deliver as useful energy is a function of the amount of sunlight it can collect. This means we need a decent amount of surface area intercepting the actual rays of the sun. Both collectors do this well, but the actual surface of the absorber plate is what collects sunlight. When you break the plate into little strips to install it in the more efficient vacuum tubes, the net surface of the absorber is a fair bit less, given the space between tubes and the space inside the tube that is not absorber.

This all adds up to 20 percent to 30 percent less net surface for a given spot of roof where the collectors sit. To make up for this, you can add an extra solar collector, but that is expensive.

To compare apples to apples and get a relative feel of the comparable performance of solar collectors for heating hot water that you might be buying, check out www.srcc.org.

Sorry if we got a little too technical this week, but the truth is in the numbers. You need to know what you are buying when someone asks you to lay down $9,000 for a solar hot water system.

Questions for Tom Gocze should be mailed to The Home Page, Bangor Daily News, P.O. Box 1329, Bangor 04402-1329. A library of reference material and a home-project blog are at bangordaily,news.com/thehomepage.

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