SOLAR TODAY - May 2009

source heat pump can do even better.

A ground-source, or geothermal, heat pump exchanges heat between the soil and a heating, cooling or water-heating load. This is accomplished using a ground-tempered loop rather than a coil exposed to the ambient air. The even temperature of the earth makes a ground-source heat pump extremely efficient. Studies show that COPs range from 3 to 5 depending on where you are, averaged over the season, and could easily be 6 to 8 if there were a market for this level of efficiency. A ground-source heat pump uses 25 to 50 percent less electricity than conventional HVAC, according to the U.S. Department of Energy.

In climates requiring air conditioning and dehumidification, most homeowners will want

look to Efficient Heat Pumps

a vapor compression machine, and since the

While Kansas City Power and Light’s incremental cost between an AC-only system 1953 electric house was mostly noticed for its and a heat pump is small, it is likely advanta-roundish appliances, automated drapes and the geous to use a heat pump rather than gas heat. TV hidden behind a sliding picture frame, it Although air-source systems have lower initial boasted a humble but crucial invention: the cost than ground-source systems, in ventilated air-source heat pump.

By exploiting the phase-change properties of a refrigerant, a heat pump moves heat in the opposite direction it would tend to move by conduction or convection — from a cold outdoors to a warm indoors. Reverse the flow of the refrigerant, and you have a cooling system as well. This elemental elegance has been complemented in recent years by efficiencies approaching a coefficient of perfor-

mance, or COP, of 2 to 4 depending on where low-load applications there is little need for a you are, averaged over the season. For every heat pump’s compressor to run except when watt of electric energy the heat pump draws, it is very hot or cold. Under these conditions, it transports 2 to 4 watts of heat in the desired ground-source systems have significant capac-direction. Coincidentally, the heat-to-carbon ity, efficiency and comfort advantages. emissions efficiency of grid electricity used for Since water heating accounts for 20 heating is three times lower on average than to 40 percent of a typical home’s energy burning natural gas — so electric air-source requirement, producing it cost effectively is heat pumps with these efficiencies can effec- important. Solar water heating is proven for tively remove the carbon disadvantage of just about any climate, potentially performing electric heat versus natural gas. But a ground well for 20 or 30 years. But backup electric or

neighbors, depending on local electricity and natural gas rates.

Linda Wigington is the special projects director at Affordable Comfort Inc. (afford able comfort.org), which advocates super-insu-lated homes and the systems that heat them. “It’s sort of obvious that electricity gives you all sorts of flexibility,” she says. “I certainly understand the push to get away from fossil fuels. In the meantime, the challenge is the transition strategy.” How do we move forward, at a time when fossil fuel remains cheaper than renewables, without making decades-long commitments to gas-burning appliances we’ll wish we didn’t have when we can get those PV panels on every house?

natural gas water heating is typically required, even with a well-designed system.

Water heating with electric systems alone remains a stumbling block. Electric tankless heaters require 220 volts and generally perform poorly (although their performance improves when paired with solar-heated water). Air-source heat pumps that fit on top of conventional water-heating tanks have COPs of 2 and provide cooling and dehumidification as a byproduct in climates where these are valued. In heating-dominated climates, where these devices effectively raise the house’s heating load, locating the devices in basement buffer zones or where they have access to refrigerator waste heat minimizes this effect. By the end of 2009, heat pump water heaters (tank and heat pump as a single integrated device) will be available nationwide under one of the world’s strongest appliance brands.

Match Strategy to Energy load

As society moves beyond bigger-is-better housing, we recognize that electric systems are more logical the smaller the loads they address. We can reduce energy demand through super-insulation, airtight building envelopes, plug-load conservation, heat-recovery ventilation and solar water heating. Daylighting and passive solar contributions are also beneficial. Load reduction is the prerequisite of any all-electric system, not only because it decreases the size of the PV array required to supply or offset the system, but also because by significantly reducing the load, we can change the type of electric system required. We also may be able to eliminate the need for gas on the site. According to Wigington, “There is an increased awareness that the lower the load, the harder it is to justify a gas line when you consider the cost of venting, the line and the service charge.”

The basic strategies for energy systems at all-electric houses sort by load size.

Small Energy Demand. If the house is compact, superinsulated and tight, electric resistance baseboards may be enough in climates where no air conditioning and dehumidification is required. Electric appliance resistance “losses” become gains and begin to heat the house. The temperature doesn’t drift much during the day, so loads can be shifted to when