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The way that systems are rated can be confusing or misleading. A grid-tied solar-electric (PV) system can be rated in terms of DC watts, STC watts, peak inverter watts, or expected annual output. Heating systems are even more difficult to rate – and to compare. Ground source heat pumps are rated in horsepower and COP (coefficient of performance), solar heating systems in square footage of collector surface area, btu/day, or expected “solar fraction,” natural gas systems in terms of btu-hour input and output, electrical systems in terms of kilo-watt peak output, etc. The thing to remember here is that you need to compare different technologies in terms of a common denominator – a unit of energy, not an efficiency rating (like COP) or a size or a capacity. We were asked once to compare the COP of a ground source heat pump to that of a solar heating system. But once you install the solar heating system, the energy is free, so the COP is something over 100 (for every unit of energy used by the system, you get over 100 units out), and the heat pump is around 3. That’s because it takes 40 watts of pumping energy to get the same amount of heat out of a solar collector array as provided by a heat pump using a multiple horsepower compressor, at about 750 watts per horsepower. Is that a useful comparison? It doesn’t tell us much. It is much more useful to talk about actual energy savings in this case in therms of gas saved by each system per year. 2. Environmental impact. Does a system that uses electricity for heating, even at high efficiency, really provide an environmental benefit? Especially in a part of the country in which our electricity is derived from coal combustion, there is a strong argument for minimizing electrical use above all other objectives. The illustrious Ed Mazria, the Santa Fe architect who has achieved national and international recognition as both a passive solar design expert and for the 2030 Challenge, states that if we do nothing else, cessation of coal combustion will avert attainment of the CO2 threshold considered by much of the scientific community to result in irreversible climate change. Mr. Mazria says that burning all the remaining oil and natural gas will not drive us past this threshold, but burning coal at the rate we are for another few years, will. Ground-source heat pumps can make sense in “hybrid” installations. We’re working on one now in which solar heat, a ground-source heat pump, and natural gas high-efficiency boilers are all used. Solar gets top priority (since it’s “free”) and is used with boiler-assist when available, and the heat pump is used with boiler assist when solar is not available. This minimizes both electrical and gas consumption. |
Remember… The technology is only as good as the company that designs and installs it. To make sure you know what you’re getting, we encourage you to carefully weigh your options and choose a provider, not just a technology, which meets your objectives. We also recommend working with a company that can design, install, and maintain the system. This can save a lot of finger pointing down the road. Can a 4-collector (100 sq. ft.) solar heating system provide 80% of the heating energy for a 3,500 square foot home? It’s possible, but unlikely. To find out, you would want to see how much heating energy the home will require and how much heating energy is annually available, deducting for system efficiency from a solar system of that capacity. When I run the calculations (see our website), I come up with year-round savings of 27.78% for a system of this size installed on a very well insulated home.
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Boaz Soifer is Partner and General Manager of Cedar Mountain Solar, which won the 2007 Santa Fe Small Business of the Year Award, the 2007 Fast-Tracker Award, the 2008 RPA System Showcase Award, and was featured in the 2008 Parade of Homes Grand Hacienda Award and Greenest House award-winning Miller residence. The company installs renewable energy heating and cooling systems. Call: 505-474-5445 or visit: 