Geothermal and Solar: Not so distant cousins

By now, most of us have had enough exposure to geothermal heat pump (GHP) technology to know that by “geothermal,” we’re talking about exchanging heat with planet Earth. We have been educated that the earth remains warmer in the winter and cooler in the summer. Because of this, the GHP has higher efficiency in both the heating and cooling mode.  

With air source heat pumps (ASHPs), exchange with outdoor air temperature can decrease efficiency when it gets too hot outside. When it’s too cold, the heat pump reaches a point at which the back-up furnace or heat strips must come on. That’s a waste of energy no matter how you look at it.

Why is the shallow earth warmer in the winter? Some might argue that it’s because of the hot geothermal deposits deep down. The answer is that it really depends on where you’re looking, but 99 percent of the planet does not have hot geothermal deposits close enough to the surface to really affect the shallow earth temperature. By shallow earth, we’re talking about less than a thousand feet. 

If you look at a comparison of energy inputs on Earth, you see the overwhelming presence of solar energy. The NASA figure shows that half of Earth’s incoming solar energy is absorbed by the Earth’s surface. Then down below the ground, you can see some seasonal variations for the first 50 feet, and then the temperature stabilizes and remains constant until we reach a depth at which the temperature starts increasing incrementally. This is the temperature increase that is the source for “hot-geothermal” resources.

It’s actually solar

Where does the source of heat for the shallow Earth come from? It’s actually solar energy. About 50 percent of the energy that reaches earth is absorbed by the Earth. I like simple, and in its simplest form geothermal heating and cooling technology could be explained like this.

The Earth is both a solar collector and a solar battery — the biggest and best in the world. The size and mass of the Earth make this possible. Pure solar-thermal energy is stored just a few feet below the surface, and is available for extraction by thermal heat pumps, commonly called geothermal heat pumps (GHPs). Every summer, the Earth’s thermal battery is recharged, so we can never run out of this solar energy. 

A benefit in the process is that during the summertime, when we need cooling, we’re helping the solar recharge process with our GHP. The heat pumps are pumping the heat from our homes and businesses back into the ground for use the next winter. Once again, Earth is a big solar-thermal battery.
To many, the concept of the Carnot Cycle (refrigeration cycle) is mystery. The refrigeration cycle (used in most heat pumps) acts in some ways similar to a water-well pump.

Let me explain.

We are on the surface of the Earth, and under our feet, there is (usually) water. The water is useless until the water is “pumped up” to us where it may be put to use for drinking, watering lawns, bathing, etc. 

We are on the surface of the Earth, and under our feet there is “heat”.  It’s useless until the heat is “pumped up” to us where we can use it for heating, hot water, showering, etc.

In the first instance, we need a water-pump to pump water up from the Earth. In the second instance, we need a geothermal heat pump to pump heat up from the Earth.
The Earth holds answ

ers and keys to a lot of things. It stores and keeps our water safe, just as it stores and keeps our solar energy safe. We just need to be smart enough to use these resources wisely.   


Jay Egg is a geothermal consultant, writer, and the owner of EggGeothermal. He has co-authored two textbooks on geothermal HVAC systems published by McGraw-Hill Professional. He can be reached at

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