Ground-Source Heat Pumps Don’t Save Energy
Ground-source heat pumps are sometimes seen as a magic technology that can provide an endless supply of environmentally friendly energy. There’s only one problem: ground-source heat pumps aren’t necessarily more energy efficient than traditional home heating systems.
Posted on Apr 6 by Henry Gifford
Henry Gifford is an energy consultant in New York City. His website is www.henrygifford.com
To understand how a heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. works, imagine using your refrigerator to keep your kitchen warm all winter. Let’s say you haul a bucket of 55° water from a backyard well and put it in your refrigerator to cool it down to 50°. Because the heat removed from the water escapes from the coils at the back of the refrigerator, you have just delivered heat to your kitchen. If you get tired of replacing the bucket of 50° water with new 55° water, you could
just install a pump to circulate water to and from the pond.
With a large enough pump and a large enough refrigerator, this method could keep your kitchen warm all winter. A ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. works in a similar way, except it has a larger refrigeration capacity and delivers its heat through ducts or radiators.
Do heat pumps save fossil fuel?
The catch is that while the heat in the groundwater is almost infinite, the electricity used to run the pump and the refrigeration unit is not. For a heat pump to actually save fossil fuel, it must deliver more heat to the house than is consumed in the process of making the electricity it uses, including the electricity used to operate the water pump.
Burning fossil fuel in power plants and transporting it through the electricity grid is about 31% efficient. If fossil fuel is burned in your home using a good sealed-combustion boiler or furnace, the annual efficiency might be about 92%—almost three times more efficient.
A heat pump that delivers 3 watts of heat for every watt of electricity it consumes is said to have a coefficient of performance (COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. ) of 3. This is about the level necessary if the heat pump is to actually save fuel compared to a good (not even super efficient) boiler or furnace that burns natural gas or fuel oil.
Don’t forget to factor in the water pump’s energy use
Most ground-source heat pumps are rated by their manufacturers at a heating COP of about 3.5 to 4. There is a catch, though: COP ratings do not include the energy used by the water pumps. Because pump size and electricity use varies significantly from installation to installation, it is impossible to know the COP of an actual system without measuring. But, in general, when the electricity used by the pump is added to the picture, the COP probably drops to 3 or lower, which means that heat pumps actually use more fossil fuel than a furnace or boiler.
Surprisingly, there has apparently never been a U.S. study measuring the COP of randomly selected residential ground-source heap pumps. One study that came close (“Measured Performance of Five Geothermal Systems,” NAHBNational Association of Home Builders, which awards a Model Green Home Certification. Research Center, November, 1999) was prepared for the Ground Source Heat Pump Consortium — surely not a neutral party. The studied heat pumps were not normal installations: the air-handler blowers had very efficient electronically commutated motors (ECM, which in 1999 were rare and are unusual today), and the ducts were PVC pipe cemented together (almost leak-free). Even under these best-case installations, the heat pumps only managed to produce COP values a little over 3.
In other words, the best-case installations the industry could come up with barely saved any energy compared to burning fossil fuel in the building. And those measurements were for new installations; after months of use the groundwater will cool off, lowering the COP still further.
Because ground-source heat pumps probably use more fossil fuel than simply installing a good boiler or furnace, green builders shouldn’t see heat pumps as a renewable energy source. Remember, it’s always better to reduce the amount of energy a building requires than to look for a new source of energy.
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Apr 6, 2009
1:55 PM EDT
1:55 PM EDT
GSHP- costs low now, energy saved soon
by Robert Alf
Henry has excellent points that few think about. This merely points out however that no single option necessarily makes sense without detailed analysis and comparing to other options. I would claim that GHSP make sense for at least some of my clients in MN. The following can be added to Henry's points to give a full picture for at least one of my clients:
* The source fuel for our electricity in MN is NOT 100% fossil fuel so the efficiency rating is likely higher than 31%. We have (sadly in my view) a large % of nuclear in addition to coal so let's say our efficiency is closer to 40%-45%
* Detailed usage analysis for my client shows a $1200-$1500 year savings in energy costs ... yes that is partly due to rate cuts by the electric utility so after Federal Tax Credits, this client has a rough payback of 5-7 years and will never run out of propane fuel again (very frustrating in our COLD climate)
* This client plans to install solar PV to generate much of their electricity within 5 or so years. The GSHP and related Tax Credits will help make this happen sooner
Overall, this means that for some clients, a GSHP paired with PV is a very powerful and sustainable option both from an energy and financial perspective. Since PV has a relatively long payback in our region, this pairing with GSHP helps to jump that hurdle.
Apr 16, 2009
10:20 AM EDT
10:20 AM EDT
not apples to to apples
by Tristan Roberts
Henry, I agree that electricity from the grid is 31% efficient, on average, and that should be factored into evaluations of GSHPs. However, you don't seem to give fuel oil burned on site the same treatment. You assume that sealed-combustion boilers burn that fuel at 92% efficiency, but what about the energy used to deliver the fuel? Can you give us an apples-to-apples comparison?
May 6, 2009
9:17 PM EDT
9:17 PM EDT
Demand Reduction
by Jeff White
Henry has a great point.
"Remember, it’s always better to reduce the amount of energy a building requires than to look for a new source of energy."
I tell my clients demand reduction is the best and number one priority. I often see demand reduction as never making it on their list. We provide super insulated thermal envelopes and can reduce the demand of a given home or building by 50% to 70% as common practice. This gain comes from our process alone. If we pay extra attention to windows and doors we can further reduce the demand. Everyone should be heads down focused on demand reduction.
It literally drives me crazy to see smart people install R13 batts or recycled news print into their walls and ceilings and then spend large sums of money on solar or GSHP systems. We all know traditional insulation systems are not sustainable over time. Even the newer homes we test (5 years old) are generally energy pigs and folks are not comfortable. This is easily solved with super insulated homes. Remember, super insulated homes are more energy efficient and more comfortable than a new source of energy.
May 17, 2009
2:04 PM EDT
2:04 PM EDT
A limited perspective?
by James Morgan
Henry's perspective on ground-source heat pumps makes a certain amount of sense if you a) look only at the heating cycle, b) assume the electricity is fossil-fuel generated and c) compare only to the efficiency of fossil-fueled boilers. Not unreasonable assumptions I suppose for a NYC resident. In other parts of the country though, one or more of those parameters might vary substantially. Here in central North Carolina for example we have a/c comprising as much or more of our residential energy usage as the heating cycle. And while air-source heat pumps are the most common installation here for both heating and cooling, and while these are even less energy-efficient than the ground-source variety, Jeff makes the better point. Load reduction by focussing on better-insulated (and smaller) homes makes much better economic and environmental sense in almost any location than fretting marginal differences in source efficiency.
I'd add that as neither natural gas nor heating oil will be available for ever and, pace bio-fuels enthusiasts, scalable carbon-neutral alternatives for those fuels are not even on the horizon, it's hard to make a case for fossil-fuel point of use heating as a long-term sustainable solution.
Meanwhile renewable non-carbon sources for electricity are gaining ground fast ...
Sep 15, 2009
2:46 AM EDT
2:46 AM EDT
Where is the engineering??
by Brian Haugk, MS, PE
COP is a measure of the number of units of heating obtained for each unit of electricity consumed by the heat pump. We can work out the COP at which it makes economic and environmental sense to use a heat pump (the break even point) in order to reduce carbon emissions and heating costs. At break-even point the CO2 emissions per kWh unit of gas (0.193 kg / kWh) divided by the efficiency of the condensing boiler will be equal to the CO2 emissions per kWh unit of electricity (0.432 kg / kWh) divided by the efficiency (COP) of the heat pump. Therefore, the break-even point for emissions (assuming a 90% efficient condensing boiler) is:
0.90 * (0.432 / 0.193) = 2.24 COP
Therefore, if a ground-source heat pump has a COP greater than 2.24, it will release less carbon emissions than a condensing boiler and have a higher heating output. Enclosed a chart from Department of Ecology understanding that each utility is different based on where the heat source is coming from and has a different kg/kWwh. I agree that pump energy is not included, but this effect is very small compared to the heat pump itself. Most ground source systems are in the 4.0 COP range for heating if designed properly. Cooling as noted above needs to be considered as well as the cooling from a split system used in conjunction with a furnance is normally in the 12 SEER range where a ground source is in the 20+ SEER range.
A simple comparison in WA state where energy costs are still quite low, but the comparison between gas and electric is similar:
Unit MMBtu/Unit Lb CO2/MMBtu Cost/Unit
Electricity kWh 0.003413 322 $0.075 (0.063)
Natural Gas Therm 0.1 117.08 $1.100 (0.87)
Below is a comparison of a condensing boiler, ground source heat pump, and (a third option) an air cooled heat pump producing 1,000,000 Btu of heating. All three are assumed to be producing low temperature heating water (so, for a radiant system).
Heating Technology (COP) lbs CO2 per MMBtu Heating Oprtng $ per MMBtu kWh per MMBtu Hting
Condensing Boiler 0.95 123.24 $11.58 308.42
Ground Source HP 3.2 100.63 $6.87 91.56
Air-Cooled HP 2.3 140.00 $9.55 127.39
The seaonal efficiency of the Ground Source Heat Pump accounts for ground loop pumping but is probably low at 3.2.
Conclusions:
1. Ground source heat pump is better from the standpoint of equivalent carbon production.
2. Ground source heat pump is 40% lower in terms of operating costs.
3. Energy used by the ground source heat pump requires less than 1/3 comparitively.
Oct 9, 2009
10:25 AM EDT
10:25 AM EDT
COP is not the whole story,
by John Howland
Henry,
Great Post,
I attended your session at the ACI Conference on Tall stuff, Tall Matters; Elevators, Pumps, Ventilation, and More. It was by far the best session that I attend that week.
12 years ago our Electric COOP said they would give me a reduce rate on my electric to change out our 80% gas forced air furnace and put in a air to air heat pump. So they set a separate meter and hook up the heat pump compressor, as the installer started to leave and I said how about the air handler blower fan motor that will run a lot longer now? He said "no that is on your regular metered rate", not such a great savings after all was said and done. Many verbal at play and even more profit driven motives. I am not saying that Heat pump are a bad Idea, just get all of the facts before you take a vacation with your project savings.
Some of the best ways to save energy don't have a box or sales person.
John
Nov 6, 2009
7:45 AM EST
7:45 AM EST
Moo!
by Steve
Jeez Henry, you'll soon be able to upholster your entire living room set in leather with all the sacred cows you're slaughtering.
Seriously tho' thank you for the sober and well reasoned look at ground-source heat pumps. I've noticed you are getting a decent amount of criticism from the other comments, but for me I'm just happy to see lively and active discussion. It really grinds my gears when an idea or a technology is presented as being above reproach and therefore discussion, or even scholarly debate. I'm really glad you brought both this and the questionable use of underfloor heating in new construction to my attention.
Feb 25, 2010
10:39 AM EST
10:39 AM EST
Re. Moo
by Anonymous
A big thank you to Brian Haugk for actually countering a not-totally-accurate article with an indepth review. It's nice to see actual numbers and evidence being brought together, and we're the better off for it.
The author makes a number of leaps of logic and faith, about the electricity source and efficiencies. He also disregards the full picture for alternatives. All that while making some good points, that we should compare systems based on primary energy usage per square foot, and implying that a lifecycle look is the proper look. The author did omit to mention that the 20+ year life of the average heat pump is two 10 or so year lifetimes for a gas furnace. But why get in to that....
May 10, 2010
7:43 PM EDT
7:43 PM EDT
Pumps, shmumps!
by Leon Kowalski
That 1999 data is gettin' kinda long in tooth, dontcha think?
The current crop of geo heat pumps typically have ECM blowers and
(ISO/ARI 13256-1) COP ratings in the 4.7 - 5.1 range for low speed
operation, and in the low 4's for full speed operation -- with an entry
water temperature of 32F (as required by ISO/ARI).
RE: pumps: The typical closed loop geo circ pump is responsible
for an 8% to 12% increase in power consumption -- and the same
reduction in COP. ...8% to 12%. Pumps, shmumps!
...BTW, how's that natural gas air conditioner workin' out for ya?
LK
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