At the heart of any geothermal heating system lies a heat pump. This device can be found in almost any home, yet few people know anything about it. A heat pump is how freezers, air conditioners, and refrigerators work, and it is aptly named, because what a heat pump essentially does is pump already existing heat from on place to another.

In your refrigerator, for example, the heat pump will pump heat from the inside of your fridge to the outside of your fridge, generally dumping the heat at the back of your refrigerator. This is why the back of the fridge is often warm while it is running.

For the mechanically-minded folks, here is a diagram of how a geothermal heat pump works:

In a geothermal system, the heat pump is able to extract heat from the 50 degree well water coming up from underground, and then dump that heat inside your house. This setup is just like a refrigerator running in reverse - the heat pump is cooling the well water, and putting the heat in your home.

The revolutionary thing about heat pumps is that they can pump heat uphill against a heat gradient, or temperature difference. Energy is required to accomplish this, however, and the efficiency of a heat pump becomes a balancing act of making sure that you get more energy out than you put in.

(click graphic for a larger picture)

Listen to the Chewonki Podcast on how heat pumps work. . .

The efficiency of a heat pump is known as the Coefficient of Performance, or COP. A heat pump's COP is often very high, yielding 3, 4, or even 5 units of energy out for every 1 unit of energy in:

Coefficient of Performance (COP) = Energy Out / Energy Put In

 

Looking at Chewonki's Geothermal Data

We have found that the heat pump for Chewonki's geothermal system has a COP of around 4.0 - that is, for every one unit of energy (electricity to run the heat pump) in, we get about three units of energy (heat from the ground) out. The total COP for the entire system - which includes energy inputs for the heat pump, well pump, and associated circulator pumps is 3.0.

If you are interested in seeing what Chewonki's geothermal COP is right now, you can go to our live geothermal data feed. The first four graphs are temperature readings, which are relatively self-explanatory. The next two, labeled Geothermal Output and Heat Pump are the really important ones. Geothermal Output tells us how much energy the heat pump is putting out in BTUs (British Thermal Units). The Heat Pump data stream tells us how much energy we are putting in (electricity in kWh).

To find the COP, we first have to make sure that we use the same units. There are 3412 BTUs in a kWh, so our equation ends up being:

COP = Energy Out / Energy In

COP = Geothermal Output Value / Heat Pump Value * 3412

And that's essentially all there is to it - the other data streams are just for fringe pumps that help move the heat around the building. The well pump is used for the building's water supply as well as the geothermal system.

If you have questions regarding our geothermal system, please feel free to contact us at: pathways@chewonki.org

 

Chewonki's Live Data Feed hosted courtesy of HOBO Dataloggers; geothermal heat pump image courtesy of http://groundsourcegeothermal.blogspot.com