A solar primer

A solar primer

If you are interested in solar energy, the first thing to remember is the number 33. That’s because the North Carolina Solar Center in Raleigh can be reached at 800-33NCSUN.  Established in the 1980s, the center is an invaluable resource that offers information on qualified solar installers, suppliers who provide consultation on solar projects, and solar house design. Their Web site is www.ncsc.ncsu.edu.

There are two types of solar products. Solar heating collectors convert solar energy into usable heat. They usually heat water for domestic hot water and space heating. Solar-electric panels are made of a semiconductor material that converts solar light into electricity. As a rule, solar heating systems will give you the most return for your money.

The most common type of solar heat collector is a flat-plate collector, mounted vertically. These are made with aluminum frames, glass fronts and insulated backs and contain black-coated copper fins. The most common sizes are 4 feet by 8 feet and 4 feet by 10 feet. Another type of collector is the vacuum tube collector, which comes in varied sizes and is usually set at the same pitch as flat plates. Vacuum collector systems usually collect less energy than flat-plate systems in bright sun (due to differences in square footage) but perform better in overcast and very cold conditions.

Concentrating collectors use a reflective surface to aim solar rays onto a solar absorber. In addition, an “invisible solar system” goes behind a metal roof and can also incorporate photovoltaics, or converting the sun’s energy to electricity. Collectors can be mounted on the ground or roof.

Passive solar, a form of solar heating with no moving parts, is a set of design criteria applied to a home. Usually, 70 percent to 80 percent of the glazing in the home is on the south side. The southern glazing is about one-twelfth the square footage of the building’s square footage. The sun through the south windows must land on a solar mass, usually a 4-inch concrete slab, and the home is insulated. A passively heated home will save enough energy in 50 years or less to pay for the entire cost of construction. With a passive-solar home, remember that all “low-e” (low emissivity) windows are not the same. Low-e glazing made for southern climates is designed to keep the sun out, while glazing for northern climates is designed to let the sun through and keep the heat in.

Before you build, it’s a good idea to get a solar survey of your site. Several devices can map the path of the sun at your location, and most solar contractors perform this service. As a rule, solar-heating systems can tolerate some tree shading and can face up to 40 degrees from south with little impairment. Solar-electric systems are more exacting and can’t tolerate any shade.

Solar energy is usually collected when it is least needed, so it must be stored for later use. Generally, you will need a gallon to 1.5 gallons of storage capacity for every square foot of flat-plate collector and even more for vacuum collectors. For example, a system using four 4-by-10 collectors will require a 160- to 240-gallon storage tank.

A 4-by-10 solar collector usually provides enough domestic hot water for two people. Solar hot-water space-heating systems can incorporate up to 10 or more collectors, but payback diminishes if a solar-heating system is sized too large. This is because more collectors lie idle as the weather warms up.

The cooler a collector is, the more energy it can harvest from the sun. Therefore, a system that requires low water temperatures to operate, such as domestic hot-water heating or radiant floor heat, are a perfect match for solar energy. Heating systems that use radiators or hot-water baseboard require high (160 degrees and more) water temperatures and should be avoided. For the best performance, make sure that all solar piping from the collector to the storage tank is insulated with 1-inch wall-thickness insulation.

Solar collectors are designed to withstand the extremes they go through, and the industry has refined and improved the materials used to make them. The average lifespan is 20 to 30 years, and the only maintenance required is periodic cleaning. The other part of the system that is vulnerable is exposed piping above the roof or ground. The piping should be well-insulated and covered with a weatherproof covering such as aluminum flashing.

Solar systems cost about $2,000 for each collector installed. Vacuum tube systems cost a bit more. A square foot of collector will generate about 2 cents of equivalent propane energy in a full sun day. Most solar domestic hot-water systems pay for themselves in four to seven years. Solar-heating systems take longer because the collectors lie idle in the summer.

Solar-electric collectors are called photovoltaic panels, and panels are grouped together in photovoltaic arrays. The panels and arrays come in a variety of sizes that can be mounted on roofs, ground racks or poles.

As with solar-thermal systems, solar-electric systems store power for later use. In stand alone or independent systems, electricity is stored in large batteries. After several days of cloudy weather, the battery may become depleted, and a backup source of energy such as a propane generator is used. These components drive up the cost of a stand-alone system. In a grid-tie system, the utility is used for storage and backup. When the panels make excess energy, it is sold to the utility grid, and when the panels stop making power, the utility returns the power. Grid-tie systems cost about 30 percent less than stand-alone systems, but they usually don’t work during a power outage.

Solar electricity is expensive, and there is no real payback yet. A typical home in the United States uses about 20 kilowatt hours a day. A stand-alone photovoltaic home is designed to consume one-tenth of that amount. A good stand-alone system costs between $20,000 and $24,000. In a photovoltaic home, electricity is treated as a refined energy used for the functions that only electricity can do, including lighting and pumping water. All other chores, including heating and refrigeration, are consigned to a cruder form of energy such as propane, oil or thermal solar. Solar-electric panels are expected to perform well for 20 years. Storage batteries can last from eight to 16 years.

There are tax credits for solar use. The combined state and federal solar-tax credits as of 2006 in North Carolina are 55 percent of the cost of the system. The maximum credit allowed on a residential solar system is $5,500, and if the system augments a heating system, the credit can extend to the heating system as well. There’s also a tax credit for including passive-solar features in a new home. The credit can be rolled over for several years until it is absorbed. The tax credits also apply to solar-electric systems, and they can be expanded on commercial projects.

[Beach Barrett has been in North Carolina since 1979. He started a radiant floor heat and solar-energy business, Thermacraft Energy Services, in 1988.]