My friend Jack asked me recently about geothermal heat pumps. I have been looking into heat pump technology for a while and wanted to write about it, so I gave him a longer-than-usual reply and figured I’d put some of what I wrote in my blog for anyone else who was curious about ground source heat pumps and net zero homes in general.
Heat pumps are like air conditioners running in reverse. You can use a heat pump to either heat or cool a building by reversing the flow of its refrigerant. Just as air conditioners become less efficient when the outside temperature gets too high, heat pumps get less efficient when the outside temperature gets too low (like around 30F). Of course, this is the worst time for a heating system to start losing its efficiency, that is, as the outdoor temperature gets colder, because it’s precisely when the maximum output from it is required. This is one of the reasons heat pumps have not been as popular in colder climates as they are in mild climates.
The benefit of adding a geothermal source to a heat pump is that the heat exchange loop stays at a consistent temperature. This allows the heat pump to maintain its efficiency because the earth below the frost line at most latitudes in the lower 48 states stays at a consistent 50-60 degrees F year round. This constant source of temperature allows heat pumps to maintain a high ‘coefficient of performance’ (COP). The COP is similar in concept to an air conditioner’s SEER (Seasonal Energy Efficiency Ratio). Basically, the COP is a ratio of watts of electricity input to watts of heat output you get. A typical heat pump has a COP of around 3 if the difference between the indoor and outdoor temperatures is within about 40 degrees F. The less the temperature difference, the higher the COP and conversely, the larger the temperature difference, the lower the COP. A COP of 3 is like getting 300% efficiency compared with simple electric resistive heat which is 100% efficient. A 100% efficient heater has a COP of 1. However, when the outdoor temperature approaches freezing, a heat pump’s COP can drop down below 1, at which time a resistive backup heater takes over. The primary drawback of an air source heat pump is that just when you need heat the most, a heat pump starts to get much more expensive to operate due to the reduction in its COP. For geothermal (i.e., ground source) systems, the COP is closer to 3.5 all the time and so it doesn’t suffer from the problem with air source heat pumps that vary with outdoor temperature.
An air conditioner’s efficiency is measured by its SEER which is basically the COP averaged over a range of typical outdoor temperatures and multiplied by 3.413. Typical values for an air conditioner’s SEER are around 10-15 which corresponds to an COP range of 2.9 – 4.4 . It’s been improving over the past few years, mostly due to government mandates. In Japan, they are now producing heat pumps with COPs as high as 6.
To add a geothermal heat sink to a heat pump system, you need to bury the heat exchanger loop below the frost line. This can be done using a loop in a vertical bore hole or in a horizontal trench. In general, a ground source heat exchange loop for a typical house would be between 1500 to 2500 feet long depending on the size of the system, and buried at least 6′ deep. The costs to install the heat exchange loop are similar to those of drilling a well in the case of a vertical system, or digging a 6′ deep trench several hundred feet long and two feet wide. So the installation of the heat exchange loop can get quite expensive. If the loop is arranged in a coil in a trench, you need about 1 foot of trench length per every 4 feet of loop. As you can imagine, this would not be easy to do unless it’s done during the construction of the house and prior to any landscaping. Also, if anything goes wrong with the loop such as having a leak, it would be very expensive to isolate and fix the problem once it’s buried.
If heat pumps are 300% efficient, why doesn’t everyone use them? After all, a gas furnace is only abut 80% efficient. One reason is that generating electricity from coal, gas, or nuclear power is only about 30-40% efficient. As much as 2/3 of the thermal energy created at an electric power plant becomes wasted heat. So the overall savings due to the multiplicative effect of the heat pump are offset by the losses of converting the fuel to electricity back at the power plant, not to mention the losses of delivering energy over the electrical grid. That’s part of the reason that electricity tends to cost about 3 times as much per unit of energy as buying natural gas and burning it in a furnace to heat your house. The capital and installation costs of a geothermal heat pump system are also significantly more than a gas furnace (about $25K vs. $3K).
I currently spend about $800 per year to heat my home with natural gas and a similar amount on electricity. I figure if I were to use electricity and a geothermal heat pump for heat, it would cost about the same as what I currently pay for natural gas, but I’d have an extra $22K in capital cost for the heat pump over the cost of a gas furnace. Now, if I had a total solar electric home then it would make sense to consider a heat pump, but to do that, I’d need to have about 12 kW of solar panels installed on my roof (at a cost of $86K), based on my annual gas and electric consumption. Even with generous solar rebates (currently $4.50/W by my utility company, up to $45K) and the new solar tax credit just passed by the U.S. Congress (up to 30% of the net solar system cost) that could take my cost of the solar system down to around $29K. But it still would be hard to justify because of the additional capital outlay for the heat pump, bringing the system cost up to $54K.
To get to net zero energy with my existing home using PV solar and a ground source heat pump, it would take about $111K in capital expenditures of which $57K could be foisted off on to my fellow taxpayers and utility customers. But that’s still too expensive to justify it based on its economic return. It would take about $54K in personal expenditures to save $1600/year in utility bills. Ignoring the cost of financing a $54K expenditure, the amortization of the system would be $2700 per year assuming it needed to be replaced in 20 years. But if energy prices doubled, which is certainly possible, it would begin to look much more attractive. They’d need to quadruple for it to be attractive without subsidies.
Energy efficiency initiatives don’t get a lot of attention because most of them have negative economic returns. This is usually due to the low cost of energy in the U.S. which is about half of what Europeans pay due to higher energy taxes. However, if you use energy efficiency as a way to reduce capital costs of a renewable energy system, the picture is quite different, primarily because renewable energy capital expenses are so high. My electricity and heating bills are currently on par with the U.S. average. If I could figure out how to reduce them by 50%, it would allow me to reduce the size of a renewable energy system proportionally to be half the cost. This is usually when efficiency becomes a much more attractive proposition. Getting by with a 6 KW solar system for all our electric and heating needs would cut the previously mentioned $86K pre-rebate cost in half.
If energy prices go up significantly, and there’s good reason to believe they will as oil and gas production peak, you’ll likely see a lot more uptake in the technologies that help create net zero energy homes.