Home Savings from Ground Source Heat Pumps
Alternative Energy Blog reader Bruce Stenswick from Minnesota emailed me about his experience of installing a ground source heat pump. Here's what he says to say:
For the winter of 2003-2004, from early October to early May I used 7620 kwh of electricity. That is equivalent to 26MBtu of natural gas. My heat pump has a coefficient of performance of 3.3, which means I needed 26 x 3.3 = 85 MBtu of heat for the winter. In order to obtain that amount of heat from a 95% efficient natural gas furnace, I would have to purchase 85/.95 = 90 Mbtu of natural gas. 95% efficient is close to the tops in efficiency. Natural gas this last winter varied from $8.60 to $9.90 /MBtu. At $9.50, 90 MBtu is $850. The 7620 kwh of electricity at Xcel Energy's electric space heating rate of $0.054...../kwh would be $413. So I cut my heating bill in half.
From an energy perspective, I assume a 35% efficient power plant and 7% line losses. Doing the calculations on this I come up with 80 MBtu of natural gas needed to produce the 7620 kwh of electricity delivered to my house.
From a CO2 perspective, I calculate I cut my CO2 output about 30%. 11% because of the efficiency gain, and another 20% due the fact that Minnesota gets 20% of it electricity from sources that do not produce CO2, mostly nuclear.
Other pollutants, I have no idea how to calculate.
In reality, I like to claim I put nothing into the air when I heat my house. I buy all of my electricity from Xcel Energy's WindSource program, which means I pay $0.02/kwh extra. Xcel then goes out and buys or builds that much extra wind energy above and beyond what has been mandated by the legislature. Even doing that, I still cut my heating bill 33%.
These numbers might not hold for everyone. Heat pumps do not work well in cold temperatures with setback thermostats. That does not affect me since someone is always home and the thermostat is never below 68 in the winter. People with setback thermostats often have the house at 60 when no one is home.
This is an overlooked area. At the gas rates we had this winter, I cut my heating bill in half because of the low electric rates that utilities give for heating with electricity. My house was a retrofit. Geothermal heat pumps beat the pants off of natural gas if you are building from scratch and can put in radiant floor heat.
(please note I haven't verified his figures)
Grants for Geothermal Ground Source Heat Pumps
Author of The Tipping Point & Blink on Geothermal Energy
Labels: carbon emissions, geothermal, ground source heat pumps, minnesota
posted by James at 5:22 am
26 Comments:
Where does the COP of 3.3 for the heat pump come from? That seems high for the Minnesota winter. Does the ground really retain that much latent heat?
McLeod: The ground is much warmer than winter air, and the use of a ground-source loop also eliminates the power for evaporator fans and overhead for defrost cycles that you have with air-source heat pumps.
That said, a CoP of 3.3 does not do much for the environment. Nuclear plants are fully subscribed and natural gas is getting more and more expensive, so until large parts (10% or more) of the grid's juice comes from sources like wind, the marginal watt comes from coal.
The usual coal-fired steam plant is about 33% efficient, and transmission losses are on the order of 5%. Fed to the above heat pump the overall efficiency of coal to space heat is 1.03; barely greater than the condensing furnace, and far higher in CO2 and other pollutants.
If you burned gas in a cogenerator of 30% electric efficiency and 90% overall efficiency, then used the electricity to run that same heat pump, your overall efficiency of gas to space heat would be 1.59. No, that is not a typo; it would be 159% efficient (0.60 direct heat + 0.30 * 3.3 heat pump output). If the heat pump was not directly coupled to the engine you could run it independently when e.g. wind power was available. This would increase capital costs but the overall fuel consumption would drop steeply.
If the coal plants were converted to IGCC and piped syngas to local users, you could achieve similar efficiency boosts for coal-fired electricity.
Has anyone tried powering a geothermal system with a low-temperature Stirling engine/generator?
I wrote the original text. I live in a Minneapolis suburb. engineer-poet is correct in that a COP of 3.3 does not help the environment alot, but that is a conservative figure. It is often operating better than that. The 3.3 is a rating with entering water temperature of 32. With a closed loop system, it gets down to 32 after running awhile. It starts out at about 45, so when it starts up it has a higher COP since there is more heat in the water/antifreeze. Also, when building a new house and installing radiant floor heat, you can get COP's of 4.7. I realize that much higher efficiencies can be achieved with combined heat and power systems. I am not familiar with them, but I am sure for alot of homeowners doing retrofits, there would be space and noise constraints. My heat pump takes up no more space than a furnace.
(OT: James, you do realize that changing post titles breaks article links, don't you?)
Bruce, primary reason for not connecting to the grid is because when the grid goes down, so does anything powered by it. The same problem exists for many natural gas and oil furnaces; it's in no way unique to geothermal systems.
I lived in Kingston, Ontario during the '97 ice storm and in Ottawa during last(?) year's summer black-out, so it definitely happens. It is most inconvenient to ones health when it does.
Since a Stirling engine converts a heat differential into mechanical energy directly, what I had in mind was to use it for circulating the fluids directly. That way there's no dependency on anything electrical for the (open-loop control) system to function - K.I.S.S. The electrical subsystem would be purely for closing the control loop, so you can moderate how much or little warmth (or "coolth" in the summer) is being extracted from the ground.
Your grid-intertied system only goes down with the grid if it can't also stand alone; while you are connected, you enjoy advantages such as being able to sell surplus electricity when you need to generate lots of heat, and buy electricity when your heat needs are low and using your own generator would force you to waste fuel and money.
If you're proposing something like the Whispergen, realize that the efficiency I derived from numbers on their website a while back was a mere 11%. That's much better than nothing, but you don't start getting seriously transformative consequences until you hit the neighborhood of 25%.
What about running ground loops for cooling? One day I plan on having a lot of computers in my house and with computers becoming bigger and bigger heat sources it puts more and more strain on the AC unit. I had a fancy of running a few score meters of copper coils six feet under to disippate the heat. Does that make any sense?
If you want to cool your PC's, you might think about getting some water-cooling blocks and doing the heat transfer more directly. Ground loop or groundwater, either would probably do.
In response to stenswick, it is expensive. However, the payback period is less that three years in the energy savings you receive.
In addition, oftentimes the efficiency of the heat pump is not due to the temperature of the climate outdoors as it is the engineer's or installer's calculations. Nine times out of ten the heat pump itself is not to blame.
Please take the time to visit www.igshpa.okstate.edu to speak with an International Ground Source Heat Pump Association accredited installer in your area for further questions on geothermal installations. You may also speak with anyone there in the office about obtaining materials that will help or become an accredited installer or certified geothermal designer yourself.
I also have ground source heat pump. It is rated at having a COP of 3.95.
Mine does not have buried loops, as it's an "open loop" system, as opposed to the closed loop system described earlier.
The source of my winter heat, and summer cooling is well water, which here in SW Oregon, at 100 foot elevation is about 52 degrees year round.
This means that, during the winter, there is no cooling off of the liquid in loops, as in a closed loop system, so it maintains its 3.95 COP all winter.
Even larger savings are possible in the summer, since it has a constant 52 degree source of cooling water.
The one thing I would stress, if you are interested in an open loop, is to make sure to have a return well, into which the 4.5 gpm (in my installation) used by the heat pump is returned to the aquifer. We don't want our aquifers dried up to save electricity!
I'm interested in some further info on the "burning gas in a co-generator". Would this be the power plant, or the homeowner? Pardon my ignorance, but I'm not sure what you're referring to here.
Another bit of confusion, on my part: Poet Engineer stated "If you're proposing something like the Whispergen, realize that the efficiency I derived from numbers on their website a while back was a mere 11%."
Is this 11% figure the efficiency of the wind machine itself? In other words, is it only extracting 11% of the wind energy which passes past its blades? If so, I don't see what possible difference it makes, as it would produce the same power as a 100% efficient wind generator with smaller blades, wouldn't it? Unless there is a size constraint in the location where the wind generator is placed, I don't see what difference the efficiency of the wind turbine makes, other than an inefficient generator would probably cost somewhat more per kwh produced.
Interesting blog; i'm new here, so please forgive me if I'm not as up to speed as some of you folks.
By the way, my electrical power, here in SW Oregon, comes from 80% hydro, I think, plus a year and a half ago I was able to install a 3KW intertie PV system, which has generated over 6000 kwh of electricity so far. I love it, and hope to be able to expand it under the new energy program about to be signed by his highness, GWB. It's a crummy energy program, but i do understand that there will be a two year window of opportunity to get a 30% tax credit for PV from the feds. That, plus perhaps some state tax credit, and it might be a good deal.
I have been trying to get quotes for a 3-ton ground loop system in Connecticut. The local contractor wanted $15k to install the system, including the ground loops (nothing special to deal with).
My complaints to the manufacturer of price gouging brought no alternatives.
In SouthWest Norway I have installed cheap air-to-air heatpump a year ago. In the first year I saved 1500 kWh of hydroelectric electricity as well as 2 cubic metres of firewood. This means the heatpump is now projected to pay for itself in three years.
I have lived in a home with geothermal heating and cooling for 13 years in Eastern South Dakota. My 4-Ton system utilizes four horizontal slinkies buried about 6 feet deep. The system heated and cooled my home for about $45 per month average, at just under 0.06 $/kWhr. Two years ago I replaced my original heat pump with a two-capacity unit (bypass scroll compressor) and my bills have dropped to about $35 per month average. The efficiency is very high on these two-capacity units, and I would recommend it highly.
Geothermal heat pumps are becoming more popular, but our biggest hurdle is the lack of installer that fully understand proper design and installation. I know because I am a certified IGSHPA trainer that has been working with contractors for over 10 years. It is very frustrating to see such an outstanding heating and cooling technology not be supported by installation infrastructure.
I continue to support the geothermal heat pump industry through my position as professor of mechanical engineering at a major university in SD.
A few comments. First, your carbon dioxide emission reduction calculation is way off the mark. In Wisconsin where I live and Minnesota where you live about 70% of our electricity comes from coal, not natural gas. And coal has a higher carbon emission rate.
Regardless I am fascinated by ground source heat pumps and solar electricity systems at grid connected sites. There is a 1320 ft2 home near Madision Wi with a ground source heat pump that uses 2510 kWhs of electric per year. That could be provided by a 2 KW fix mounted unshaded PV system (installed cost about $15,000 - after incentives about $9,000). And no carbon dioxide emissions.
nielsrw@yahoo.com
Jumpoff joe understandably is confused by the name of the whispergen. It is NOT a wind turbine. It is a gas boiler where the heat is transmitted from the gas flame to the water through a stirling engine. The piston oscillates at 50 Hz and generates 50 hz alternating current. Its a combined heat and pour operation where what it produces is heat with electricity as a by product. Have a look at http://www.whispergen.com/
Hi Mr. Stenswick!! Your faithful high school students just logging on to say: Good morning, starshine! The earth says: Hello! Good luck with your heat pump or whatever it is...
I'm quite new to this and perhaps I'm missing a great deal, but wouldn't the efficiency of the geothermal unit be greatly increased if the closed loop were submerged in a constant 90 degree temperature water medium such as might be maintained in a cistern? Also wouldn't the size of the slinky loop required be greatly reduced?
I like the idea of a wispergen powering a heat pump. why not power a heat pump directly from a windmill. as in the turbine is mechanicly geared to the compreser. a storage tank would help regulate the wind avalable to the heat needed by the house. the whole thing would be suplimental to a reliable heat source.
AP mentioned that a Stirling cycle motor could be used to circulate the fluids, to make the system more independent. I think a much bigger advantage is the gain in efficiency. If the heat pump runs off elecricity, then that electricity was probably produced in a power plant by burning fuel. That conversion is always inefficient, with ~65% of the heat going to warm up air or water near the power plant. It makes much more sense to run your own motor and use that "waste" heat yourself.
It would not have to be a Stirling cycle motor, of course. And it could run off natural gas, propane, oil, or whatever. I would use a water-cooled motor, and heat part of the home's air by blowing it through the motor's radiator. (An air-cooled motor would be even simpler, except you would have to be very careful about leaking exhaust manifold gaskets. I know - I used to drive a Corvair.)
I think we hear only about electric heat pumps because the heat pump industry got started in the south, where they need mostly cooling and only a little heating. Of course, when you are using a heat pump for cooling, then the last thing you want is another local source of heat.
- Jim Van Zandt
hey is this the Bruce Stenswick that taught a group of 7th graders? In particular Gary, Harley, David, and Whippy the necklace.
Can you explain the detriment to a set-back thermostat? I have a ground source heat pump as my heating system here in upstate NY. No one is home during the day so I turn the thermostat down to 55-60 degrees (from 70) manually during the daytime. I am on a day/night meter, so turn the thermostat up at night (contrary to most folks) and count on my insulation to conserve some of the heat during the day when the thermostat is down. This works for me (my total utility bill averages $90/mo), but am I losing efficiency? Thanks for your explanation.
Could I get some feedback from GSHP users who used the slinky system? I live in New England and am curious as to how much pe pipe was used for your particular ground conditions per ton.
I considered this but turns out that in areas where the soil is dry, ground loops are not so effective. Moist soil is preferred.
Cold in NY:
We have beenn looking to do Geo in a house we are renovating, the prices we have been quoted are outrageous any where form 35000 - 42000 for a closed loop system they are telling us we would need a 5 ton machine. Has anyone installed a system themselves,if so how difficult is it.
Cold in NY,
Have you considered using an air source heat pump? Installation will be considerably cheaper and quicker.
James
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