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
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-
© DEPARTMENT OF NATuRAl RESOuRCES CANADA, 2009. All RIGHTS RESERVED.
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
Copyright © 2009 by the American Solar Energy Society Inc. All rights reserved.
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
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
solartoday.org SOLAR TODAY May 2009