Solar Thermal
Solar thermal collectors or solar hot water heaters use the sun to heat either air, water, or a heat-transfer fluid in collectors that then heats a tank of water. Three common types of solar thermal systems are:
- Passive systems, which are also called batch heaters. The incoming city or well water goes directly to a black tank enclosed inside an insulated box covered with glass. This tank is plumbed so the heated water then goes directly to your existing water tank.
- Active systems, a more common type that uses a pump and controls to move the heated water from the collector to solar hot water storage tank
- Air systems, which use air to either heat the house air or water. But in most cases, heating air is the least effective way to use solar energy.
Almost every state, including Iowa, has done research showing Domestic Hot Water (DHW) systems provide the best return of any renewable energy installation. Today, Solar DHW still has the best return on your money of all renewable energy projects done in the US and Iowa. A typical 2-panel system (64 sq. ft.) used to heat an 80-gallon tank can reduce the need for fossil fuel heated domestic hot water by 50 to 60 percent. The plumbing from a solar heated tank connects to a home's existing water heater, which stays inactive as long as the water coming from the solar heated tank is as hot as or hotter than the temperature setting on the fossil fueled tank. When the solar tank falls below this temperature, the home's water heater will make up the difference. For large commercial and industrial facilities, high-temperature solar water heaters can provide domestic hot water and hot water process heat.
The quality of materials that the panel is made of is very important to both the performance as well as the life expectancy of the solar system. The Solar Rating and Certification Corporation (SRCC) has two designations on solar panels (OG100) and the entire system (OG300). This is a place to start, but a quality system should also have proof of longevity. A quality flat plate panel should be made with an extruded aluminum frame (not plastic). The cover (glazing) should be low-iron, tempered glass. Plastic covers will yellow and become brittle over a very short time. The absorber plate (interior) should be made of copper with a selective black coating that will absorb over 92% of the solar energy available. Everyday black paint will boil off and coat the under side of the glass and severely affect the performance. A good flat plate collector solar panel made of these materials should work just fine for 35 years. Another type of collector is the evacuated tube, which uses a vacuum for insulation. Although a good collector, the vacuum and the seals have presented maintenance issues over time.
Accurate sizing for domestic hot water is 1½ to 2 gallons per square foot of solar panel. With this proper sizing in the Midwest, you can get almost unlimited 145-degree water for 4 to 6 months of the year. This sizing of Solar DHW can lower your natural gas (or electricity) usage up to 50 percent of what you had been using.
Storage of solar energy in water has been a proven method over many years. This water can be used as DHW, but also can be used to heat some or all of your house, garage, or workshop. To install enough solar panels and storage to heat an entire older home would be expensive, so most people try to do just some space heat. If your house is modern and very energy efficient, then solar could do a large part of your heating. An easy method is to install "kickspace heaters," which can be as simple as a car radiator and a fan to distribute the heat from the water storage to one or two rooms. A more modern method is radiant floor heating, where hot (or even warm) water from the solar tank is run through PEX tubing installed in the floor. Running solar-heated water to just a radiator or space heater to heat two of the coldest rooms in your house could save 75 percent on your fuel usage. In-floor radiant heating can do even more of the heating.
Storage of the solar energy for DHW can be in special tanks made for solar. These tanks (usually 80 gallons) have a copper coil in the bottom of the tank and can be expensive. You could use a standard 80 gallon electric DHW tank from your local plumbing big box store. This cheaper tank will require an external heat exchanger and a second pump. If you get serious about storage of solar energy for heating purposes, then consider a stainless steel bulk milk tank. A 250- to 340-gallon tank can still be bought for $1 per gallon. Following the rule of 1½ gallons per square foot of panel, then 6 to 8 4'x 8' panels would work OK.