A heat pump uses thermodynamic principles to move heat from one place to another. A geothermal heat pump moves heat from the house to the ground, which is a pretty constant 50°F year-round.
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1. How does a geothermal system cool your house?
A heat pump uses thermodynamic principles to move heat from one place to another. A geothermal heat pump moves heat from the house to the ground, which is a pretty constant 50°F year-round.
2. How is it different from a backyard air conditioner?
A backyard air conditioner is just a heat pump that transfers heat from the house to the outside air. This isn't as efficient, because the outside air is usually hotter than the house air when the AC is running, so the heat pump has to work harder to push the heat outside.
Specifically, the part of a traditional AC system that sits inside the furnace is the same in my geothermal system, but instead of being connected to a compressor in the backyard that has a fan to circulate air outside, my geothermal system is connected to a compressor in my basement with a pump to circulate coolant underground.
3. How does it heat your house?
Essentially, the heat pump's compressor just runs in the other direction, so it pumps heat from the ground into the house. The 50°F ground temperature isn't hot enough to heat our house, but the heat pump uses compression and expansion to boost the temperature.
4. How is it different from a backyard heat pump?
Although they're not useful this far North, many homes down South are heated by a heat pump in the backyard, which transfers heat from the outside air into the house. This isn't as efficient as a geothermal heat pump, because the outside air is usually colder than the ground in the winter (especially in Wisconsin), so the heat pump has to work harder to get enough heat to warm the house.
Specifically, the part of a traditional heat pump that sits inside the furnace is the same in my geothermal system, but instead of being connected to a compressor in the backyard that has a fan to circulate air outside, my geothermal system is connected to a compressor in my basement with a pump to circulate coolant underground.
5. How does compression/expansion change the refrigerant's temperature?
The laws of thermodynamics say you have to add energy if you want to move heat from a colder place to a warmer place. Heat pumps (which are found in air conditioners, refrigerators, dehumidifiers, etc.) use a "vapor-compression refrigeration cycle" to move heat from one place to another, making one place warmer and the other place colder.
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When a gas is compressed (4), it becomes hotter (think of it as concentrating its heat into a smaller space). This hot gas is pushed through a heat exchanger / condensor (1), where it gives up most of its heat (making this place warmer) and cools a little. Then it passes through an expansion valve (2), which cools it more. Gases rapidly become cooler when they expand (feel the hose of a gas grill's propane tank when gas is flowing out of it). This cold (and partially liquified) gas flows through a heat exchanger / evaporator (3) to acquire some heat (making this place colder) and returns to the compressor (4).
6. How much energy does it save?
See the Performance page for detailed numbers. The geothermal system hasn't been installed long enough for me to have enough data to give a good answer yet, but the two utility bills we received in the cooling season of 2010 after the system was installed shows it could use 30% less energy in cooling mode than my previous air conditioner. If my two months of data are typical, this will add up to about 650 kWh saved every summer.
I don't have any data for heating mode yet, but I don't expect it to save as much energy as cooling mode, because Wisconsin winters are very cold, and I'm comparing it to our previous gas furnace, which is a very efficient way to heat a house. I hope to see overall energy savings as high as 10%; however, we will actually use less natural gas but more electricity. To compare natural gas and electricity, we can convert both to units of megajoules (MJ), where 1 kWh = 3.6 MJ and 1 therm = 105.5 MJ. For example, if we use 15% less gas but 400% more electricity than last year's heating season, we would use 365 therms and 570 kWh, which would result in an overall savings of about 5100 MJ, or 10% less than last year.
7. How much CO2 does it save?
According to the most recent EPA eGrid data (compiled in 2006 from data submitted by utilities in 2004), utility companies serving my subregion create 1.556 lb of CO2 for every kWh of electricity generated. If it uses 650 kWh less electricity every summer than my previous air conditioner, this would add up to just over 1,000 lb of CO2 saved every summer.
In winter, we have to determine the net effect of using less gas but more electricity. Natural gas emits approximately 12 lb of CO2 for every therm burned. (The actual value can vary from 11.6 lb to 13.5 lb, depending on the chemical composition of the natural gas and the method of combustion. DOE recommends 11.6 lb/therm. I round and use 12 lb in my calculations.) If we use 570 kWh more electricity but 365 therms less natural gas every winter, this results in a net savings of about 3500 lb of CO2 every winter.
So, the net CO2 savings in one year could be about 4500 lb.
8. How much money does it save?
In my recent utility bills including all fees and taxes, electricity costs me about $0.16 per kWh and natural gas costs me about $1.40 per therm (although gas costs fluctuate a lot more than electricity costs).
If we use 650 kWh less in the summer, 570 kWh more in the winter, and 365 therms less in the winter, this will result in a net annual savings of about $525.
9. What is the payoff period?
The payoff period is the time at which the money saved by using less energy equals the initial investment of buying and installing the more efficient equipment. Wikipedia says the average payoff period for a geothermal system in the USA is 12 years.
For my calculation, I compare the cost of the geothermal system against the cost of installing a traditional air conditioner and new furnace, because my old air conditioner and furnace needed to be replaced anyway. Since I was going to buy either an air conditioner and furnace or else a geothermal system and furnace anyway, I only need to pay off the difference between the cost of the geothermal system and the cost of a traditional air conditioner-based system.
Conveniently, my installer gave me an estimate for both systems, so I know the difference was about $6000 (after subtracting all the rebates and tax incentives that applied to both systems). Assuming a net annual savings of $525, the payoff period for my system is 11.4 years.
11. Is your house the first geothermal system in Milwaukee County?
No. The first DNR permit for a vertical, closed loop, geothermal well was requested by Steve Weber of Wauwatosa, WI, a few months before we applied for our permit. However, Steve's installer ran into problems during the well drilling phase, so our system was operational before his. However, DNR permits are not required for horizontal, closed loop geothermal systems, so some of those could have been installed long ago. However, they require a lot of space, and most of Milwaukee County is very urban, so few locations have enough space for a horizontal system. Therefore, it's unlikely that there are many of these.
13. How deep are your wells?
There are three wells, each is 180 ft deep, and they are separated from each other and from the house by about 15 ft, so each has enough space to dissipate heat without affecting the others. Since the pipe goes down and back up each well, this totals about 1200 ft of pipe buried under my yard.
14. Was it hard to drill the wells?
This was the most technically challenging portion of the whole installation. My installer used a full-sized professional well-drilling rig, which is about 50 ft long, 9 ft wide, and requires more overhead clearance than the height of my house (i.e. no tree branches or electrical wires in the way). I had to remove a fence so this rig could back up over the curb and sidewalk, between the houses, and into my backyard. (Luckily, neither the curb nor sidewalk was damaged, but I had to re-landscape some serious tire ruts and replace a damaged cement walkway alongside the house.)
The drilling contractor hit bedrock at 60 ft, so two-thirds of each well was drilled through rock instead of dirt. This required them to use a lot of drilling mud, a clay-based slurry that gets pumped down the hole to cool the drill bit, push the rock chips away from the drill bit, and seal the sides of the well against sand or groundwater incursion. It took three days to drill all three wells.
Wells can be drilled to this depth with smaller rigs, which would be easier to get into the backyard, but they lack the power to deal with that much bedrock, so the project would have taken a lot longer and probably cost more in the end.
15. Was the drilling messy?
Very. We were told to expect we would need to clean up our yard afterward by spreading topsoil and planting new grass, but I wasn't prepared for the magnitude of the mess. When the drilling rig left, the entire south half of our backyard was a giant puddle of mud that was six inches deep in some places. It sat that way for a week or two, until it had dried enough to be scraped up with a bobcat and trucked away.
Then we bought a huge load of topsoil, spread it, seeded it, watered it, and waited for the grass to grow. So, it was about two months from the start of the project until our backyard was usable again.
