This article is the 2nd in a series covering my heatpump project. Last time I covered the very basic design and some critical design numbers. This time is a quick explanation of why geothermal heat pumps work and why they can be more cost effective than any other commercial viable solution at present.

The concept of a heat pump is a bit non-intuitive, at least not like other sources of heating or cooling. When heating your house with a heatpump, the device is pumping heat energy from a source and dumping it into your house. When cooling, it sucks the heat out of your house and dumps it to the sink ("source and sink" are the same, the terms change based on the direction of the heat flow). To contrast this quickly, a typical natural gas furnace or burn natural gas and use that heat to warm the air. A heat pump does not use electricity to generate heat, it uses electricity to move heat. In the case of a geothermal installation the heat comes from the Earth.

The Energy Efficiency Rating

So if heat pumps do not use energy to create heat directly, there must be some ratio by which we can measure how much electricity it takes to move a certain amount of heat. In a natural gas furnace the efficiency is a simple rating. when it burns natural gas a portion of that heat ends up in the house and a portion goes up the exhaust, a good furnace dumps about 95% of the heat into the house and the other 5% goes out with the exhaust. Since a heat pump isn't using the input energy to directly heat it isn't limited to that source energy, it can output more heat energy than the electricity needed to run it. This is actually completely normal for heat pumps. This ratio of input electricity to output heat is the Energy Efficiency Ratio, and is typically given in BTUs per Watt.

It's important to keep the costs of energy in mind when considering a geothermal installation. Both electricity and natural gas are sources of energy, but are measured and billed very differently. Fortunately we can calculate equivalence and compare the costs. Below is a simple calculator to help figure out this ratio. My costs are currently $0.791 per CCF of natural gas, and $0.154 per kWh of electricity.

A heat pump that uses electricity must have an EER of about 20, or more, to compete with natural gas, in terms of operational energy costs. The operational costs tend to be the largest factor in the total cost of ownership for a household heating device. For example the furnace in my house cost about $800; and consumes about $540 in gas each year. It's expected to last about 15-20 years, so the total cost of ownership is expected to be about $8,900 to $11,600.

The heat pump I am designing should have an EER between 20 and 25, it's a bit difficult to pin down without building one for testing. I do expect the final product to be at the upper end of that spectrum. The operational costs might comparatively be $308 to $154 (note: the unit I'm building has less capacity than the furnace I'm comparing it to, so this is a slightly unfair). The cost to build the heat pump will be less (and still would be with equal capacity) - however the cost of the ground loops must be considered and will push the initial cost well above that of the furnace. The ground loops will be the subject of forthcoming articles, particularly how I'm going to minimize their cost as ground loops with the capacity to heat a house in a Michigan winter are typically quite expensive.

Coefficient of Performance

The CoP is how many heatpumps are rated. The technical measurement procedure is somewhat complicated that the result is a ratio that more accurately depicts the ratio of input energy to output energy (whereas the EER uses different units in it's "ratio", I consider the COP to be more accurate as it uses no units). The Energy cost Ratio from the calculator above is the number you're targeting. In my situation that Cost Ratio is about 5.8, and the best heat pumps available today get into the mid 5s, but not quite to even. My design should run around 5, which is less efficient than a furnace at heating (assuming costs stay about the same).