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The heart of the control system is a differential thermostat, which measures the difference in temperature between the collectors and storage unit. When the collectors are 10-to-20 ° F (5.6-to-11 C°) warmer than the storage unit, the thermostat turns on a pump or fan to circulate water or air through the collector to heat the storage medium or the house.
The operation, performance, and cost of these controls vary. Some control systems monitor the temperature in different parts of the system to help determine how it is operating. The most sophisticated systems use microprocessors to control and optimize heat transfer and delivery to storage and zones of the house.
It is possible to use a solar photovoltaic (PV) panel to power low voltage, direct current (DC) blowers (for air collectors) or pumps (for liquid collectors). The output of the PV panels matches available solar heat gain to the solar collector. With careful sizing, the blower or pump speed is optimized for efficient solar gain to the working fluid. During low sun conditions the blower or pump speed is slow, and during high solar gain, they run faster. When used with a room air collector, separate controls may not be necessary. This also ensures that the system will operate in the event of utility power outage. A PV system with battery storage can also provide power to operate a central heating system with a storage component, though this is expensive for large systems.
Performance and Maintenance
How well an active solar energy system performs depends on effective siting, system design, and installation, and the quality and durability of the components. The collectors and controls now manufactured are of high quality. The biggest factor now is finding an experienced contractor who can properly design and install the system.
Once a system is in place, it has to be properly maintained to optimize its performance and avoid breakdowns. Different systems require different types of maintenance, but you should figure on 8 to 16 hours of maintenance annually. You should set up a calendar with a list of maintenance tasks that the component manufacturers and installer recommends.
Costs and Benefits
The cost of an active solar heating system will vary. A simple window air heater collector can be made for a few hundred dollars. Commercial systems range from $30 and $80 per square foot of collector area, installed. Usually, the larger the system, the less it costs per unit of collector area. Commercially available collectors come with warranties of 10 years or more, and should easily last decades longer. Most systems offer the greatest return on investment in areas with high heating fuel and electricity costs. The economics of an active space heating system improve if it also heats domestic water, because an otherwise idle collector can heat water in the summer.
Some states offer sales tax exemptions, income tax credits or deductions, and property tax exemptions or deductions for solar energy systems. If you are not sure what benefits your state offers, contact your state energy office. (Check your local phone directory for the address and phone number of your state energy office.)
Heating your home with an active solar energy system can significantly reduce your fuel bills in the winter. A solar heating system will also reduce the amount of air pollution and greenhouse gases that result from your use of fossil fuels such as oil, propane, and natural gas for heating or that may be used to generate the electricity that you use.
If you are planning to purchase an active system you should learn as much as you can about the technology. In this way, you will know how to size and install your system properly, make a wise purchase, and maintain the components, and learn how to avoid problems or pitfalls that can occur.
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© 2004 U.S. Deptartment of Energy
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