WHY IS THE SHALLOW EARTH WARMER than the outside temperature in the winter? Some might argue that it’s because of the hot geothermal deposits deep down. The Earth is actually both a "solar-collector", and a "solar-battery". But I'm getting ahead of myself...
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. This works well because the Earth is warmer than the outdoor temperature in the winter, and the Earth is cooler in the summertime. Because of this, the GHP has high efficiency in both the heating and cooling mode.
Air-source heat pumps (ASHPs) have to work with, or exchange with outdoor air temperatures. This 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. With our efforts to reduce emissions, turning on a combustion furnace just adds to our CO2 footprint, and electric heat strips will cost a fortune to run through a long cold snap. That’s a waste of energy no matter how you look at it. That’s why the Environmental Protection Agency (EPA) and the Department of Energy (DOE) endorse GHP’s. Geothermal is the only economical way to heat without burning fossil fuels for most of the world.
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 most 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 received by Earth (chart to right), you can easily see the overwhelming presence of solar energy. The NASA figure (below) shows that half of Earth’s incoming solar energy is absorbed by the Earth’s surface. When you look a little further, 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, though it doesn’t have a real impact until about 1000’.
It’s Actually Solar
Where does the source of heat for the shallow earth come from? You’ve figured it out; it’s actually solar energy. About 50% 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-energy” 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 summer-time, 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. And since most of the energy that makes our homes “hot” enough that we need air-conditioning is from the sun, we are essentially pumping solar energy into the earth in order to “cool” our homes and businesses. Once again, Earth is a big “solar-thermal-battery”.
Most locations in the world have water underground. It’s useless unless you drill a hole and put in a water-pump that can pump it up where you can use it for drinking, cooking, bathing, etc.
Most locations in the world have solar energy underground. It too is useless unless you drill a hole, and install a heat-pump that can pump the heat up for use in heating and hot water for bathing, 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 answers 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 email@example.com.
*A little math for any doubters: NASA says the earth absorbs 163.3 W/m2, while the steady flow from Earth’s interior (the “hot” core) is 0.09 W/m2. That’s a ratio of 1814 to1 in favor of Solar Energy dominance as a source for geothermal heat pumps. http://science-edu.larc.nasa.gov/energy_budget/