The principle of geoexchange, or ground source heat exchange,
utilizes the ground or a pond/lake as a source of heat when needed and as a
sink for heat when needed. Geoexchange systems utilize a refrigerant to transfer
heat between ground and building. Heat always moves from an area of high
temperature to one of lower temperature and the greater the temperature
difference the greater the rate of movement. A geothermal heat pump, or ground
source heat pump, moves heat between the ground and the heated area. An air
source heat pump moves heat between the outside air and the heated area. Ground
source heat pumps (GSHPs) are considerably more efficient than air source heat
pumps (ASHPs). ASHPs are, however, cheaper to install than GSHPs and more
efficient than other electric heating systems so are quite applicable for
residential and building heating and cooling as well. GSHPs have the added
advantage that the ground is warm enough to support a secondary function of the
system for water heating, which is done much more efficiently than other
electric water heaters. However, GSHPs have significantly higher upfront costs due
mainly to the added cost of trenching or excavating to lay the underground
loops.
Heat pumps,
both air source and ground source, are expected to be a major focus for home
and building decarbonization going forward. Commercial and residential
buildings make up about 13% of US carbon emissions. Air source heat pumps will
lead the way by far due to lower cost and easier installation. In addition to using
no direct fossil fuels, heat pumps are very efficient, making three to five times
the heat per electricity input. Temperature differences in the heat source
medium, air or ground, are exploited to extract energy that can be said to be
renewable since the temperatures of the mediums remain constant.[ii]
[iii]
The path for
decarbonization of buildings is clearly going to be one of a transition from hydrocarbon
fuels to electrification. Home and building electrification via heat pumps is
economical now, especially in the mid-term as operation and maintenance costs
are lower with electrification and there is some subsidization of the
significant upfront installation costs. It will be more economical for those
home and building owners with oil furnaces to switch to heat pumps as the
payout in comparison would be faster than those switching from natural gas
since the cost for fuel oil is typically much higher than natural gas. That
makes for a good heat pump pitch for those in the US Northeast which is the
region with the most oil furnaces. Upfront costs for installation are cheapest
with fuel oil and natural gas compared to heat pumps with fuel costs making
heat pumps slightly cheaper to operate than natural gas furnaces and far
cheaper to operate than oil furnaces.
The IEA notes
that by 2020, 177 million heat pumps have been deployed globally on a steady
trajectory since 2010. They expect deployment to rise from now to 2025 then
rise more sharply thereafter according to their net-zero pathway. In the U.S.
heat pump sales outpaced gas furnace sales for the first time in 2020 and continued
the trend with 15% market growth in 2021. U.S. tax credits of 20-30% of system
installation cost are one factor for the increase. High natural gas prices in
2022 and beyond are expected to enhance the cost benefits of heat pumps. In
2021 air source heat pump sales were led by China at nearly 12.5 million units,
Japan with about 8 million units, the U.S. with about 4 million units, and the
Eu-27 with about 2 million units.[iv]
While deeper
geothermal heat comes from the planet’s formation and (mostly) radioactive
decay, the heat retained in the soil and upper part of the crust is derived
from solar radiation, conduction, and subsequent soil chemical reactions that
give off heat. The earth acts as a battery, a source and sink of heat. This
stored heat or thermal energy can be tapped. Soil temperature remains constant at
avg. depth of about 20ft, but this varies generally by latitude.
In temperate
environments heating requires more energy than cooling, especially in cooler
regions. Larger buildings have higher heating requirements. The bigger draw of
larger buildings in colder regions means that there is a risk of cooling down
the local subsurface. One remedy for that is to drill shallow vertical wells
for a vertical geoexchange with a closed loop tubing system installed in the
wells. These are typically more than 300 ft so deeper than an average water
well. Size and routing of the piping is important for optimizing loads and
energy use of the heat pump. These deeper vertical ground loops are riskier in
that they require drilling through rock rather than excavating soil. They are
also much more expensive. However, they should be much more efficient which can
offset the costs. Additional requirements can vary quite a bit due to geology
and groundwater and can require steel casing through some or much of the depth.
This varies much by area but can add additional costs and risks. Rigs that
drill water wells often also drill these vertical geoexchange wells. Two
examples given in a Tradeline article show that larger buildings require quite
a bit of looping space. One, the University of Toronto Scarborough’s new
Environmental Science and Chemistry Building, required for their geoexchange
system 66 holes drilled 600 ft below the building, drilled prior to
construction. Water wells are typically 6-9” in diameter. That is a lot of “hole,”
in this case nearly 40,000 ft or 7.5 miles of hole, for one building, albeit a
large one! And all that hole will only cover about 25% of the building’s
heating and cooling loads! Another project mentioned is the Smithsonian
Environmental Research Center Mathias Laboratory in Edgewater, Maryland. Their
geoexchange field in an adjacent meadow has 250 closed-loop wells that are 435
ft deep. That is a whopping 20.6 miles of hole! I have some idea what a deep
water well costs as I paid to have one drilled once although that was many
years ago. These types of geoexchange systems must be quite costly. These
projects were built between 2015 and 2020.[v]
Universities, hospitals, government facilities, and other buildings often have
a need for non-grid tied reliable heating, water heating, cooling, etc. They
are also trying to meet sustainability goals. The huge upfront investments
require significant subsidization as well. I think that the economics of these
projects should be better studied, compared, and optimized, and compared to
other alternatives and hybrids. I will dig into some economic comparisons later
in the text.
A geothermal heat pump is analogous to a
refrigerator where heat is transferred from one place to another. A
refrigerator is basically comprised of a compressor, a heat exchanger, an
expansion valve, and an evaporator coil. The main advantage of a geothermal
heat pump is that it uses less electricity to transfer a larger amount of heat
energy from the ground. It simply operates more efficiently than an air source
heat pump or another type of electric furnace. The geothermal heat pump system
utilizes three loops. This explanation is for heating mode. First is the ground
loop which moves the heat between the ground and surface. The ground loop is
typically a closed loop, but open loops are also used. Second is the closed
loop containing refrigerant or working fluid. The refrigerant or working fluid is
heated by the ground heat and becomes a gas which is compressed and superheated
then passed through a radiator-like heat exchanger to heating registers. As in
a refrigerator, after the refrigerant transfers heat to the coil it is sent
through a thermostatic expansion valve. The expansion cools the gas to a very low
temperature. It is then sent back into the ground to gather more heat. The
third loop is the loop coming from the heat exchanger through duct work to the
heating registers. As the air cools it goes through the air return and more
warm air supply is sent through the heat pump to the registers. In cooling mode,
a valve is activated that reverses the flow of hot compressed air back to the
ground. A geothermal heat pump in cooling mode is much more efficient than a
typical electric air conditioner.[vi]
[ii]
Rosenow, J., Gibb, D., Nowak, T. et al. Heating up the global heat pump market.
Nat Energy (2022). https://doi.org/10.1038/s41560-022-01104-8
[iii]
Eyre, N. From using heat to using work: reconceptualising the zero carbon
energy transition. Energy Efficiency 14, 77 (2021). https://doi.org/10.1007/s12053-021-09982-9
[iv]
Rosenow, J., Gibb, D., Nowak, T. et al. Heating up the global heat pump
market. Nat Energy (2022). https://doi.org/10.1038/s41560-022-01104-8
[v]
Allen, Jonathon, May 2017. Using Geothermal Exchange Systems to Achieve Zero
Net Energy in Cold Climates. Using
Geothermal Exchange Systems to Achieve Zero Net Energy in Cold Climates |
Tradeline, Inc. (tradelineinc.com)
[vi]
Lloyd, Donal Blaise, 2011. The Smart Guide to Geothermal: How to Harvest
Earth’s Free Energy for Heating and Cooling. PixyJack Press.
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