Geothermal Basics

Just so everyone is on the same page we will start from the beginning. Our plan is to keep this page updated with answers to your questions. So please feel free to respond to any email we send with your question. If it fits, we will answer it here, so everyone gets the benefit.

**UPDATE** We’ve already added a few reader questions that we thought fit well on this page. Check those out at the bottom.

So what exactly is Geothermal?

To understand geothermal it’s important to break down the actual origin of the word. “Geo” means earth, and “therm” is a measure of heat. This sounds simple but actually describes it well. Geothermal uses the earth as a heat transfer mechanism. Some people see “therm” and automatically think “heat.” The problem with this is that they think geothermal only uses the earth to help with heating.

What makes geothermal so good is that we can use the temperature of earth both ways. We can use it at a heat source (when we need heat added to our space) and a heat sink (when we need to remove heat from our space). We can transfer the heat from the ground several different ways, but in a broad overview, geothermal is just putting to use heat from the earth.

How does it work?

Here’s where we get down to specifics. In order to transfer heat from the ground to our house (or building), something has to travel between the two. Water is typically used as this medium. In many cases, antifreeze or glycol is added as a safety precaution.

The best way to move this water from one place to another is to use plastic piping (we will cover piping specifics in a few days). Piping is buried in the ground and then filled with water. The piping enters the house and connects to a heat pump.

A geothermal heat pump looks like most regular furnace or air conditioning units, if not a little smaller. It will have air ducts connected to it, and a place for an air filter. The piping will connect and have a small water pump on one pipe.

The small pump pushes the water through the pipe, and the actual heat pump transfers heat from the water pipes to the air (or vice versa).

Why is a geothermal heat pump so good?

There are several reasons that make the geothermal heat pump so efficient. The biggest reason is because by the time the water makes it all the way from the buried pipe back to the heat pump it is around 55 degrees F. This means that the heat pump does not have to do much work to provide air conditioning, nor does it have to add much heat in heating mode.

The heat pump itself has a compressor and a small refrigeration circuit inside. The refrigerant pipes intertwine with the water in order to transfer heat back and forth. The refrigerant pipes also have a section that sits inside the air ducts to transfer temperature to the air.

Just to clarify: in cooling mode the 55 degree F water cools the refrigerant, which goes through compression and expansion, making it even cooler. Then the heat pump fans blows air across the cold refrigerant pipe, making the air cold. You feel the nice air conditioning inside your home or office. Meanwhile, after the air blew across the refrigerant pipes, the refrigerant inside got warmer, so it transfers its heat to the water in the geothermal loop pipes. It gets pumped through the buried pipes, which give it time to cool down to the ground temperature, and we start again.

There is really only a subtle difference when we use the heat pump in heating mode. The 55 degree water cools the refrigerant. But this time the refrigerant is going backward in the heat pump, so the compressor makes it hotter, not colder. The fan blows air across the refrigerant pipes (NOTE: refrigerant is just the official name of the fluid, it doesn’t mean it is cold all the time). The air gets warm and is blown through the ductwork, and you feel comfortable as you relax in your living room. While the air is circulating, the heat has left the refrigerant pipes because it was used to heat the air. The refrigerant is now very cold, and it makes the geothermal water cold. The geothermal water is pumped through the buried pipes, which gives it time to warm up to the ground temperature, and we start the process again.

What about using geothermal to generate electricity?

Using geothermal to generate energy is a much more complex story. It’s possible, but extraordinarily costly.

Deep, very deep, miles deep in the earth is very hot water. It’s much hotter than the 55 degrees F that resides between 10 and 500 feet deep within the earth. There are ways to drill down deep enough into the earth’s surface to tap into the water that is several hundred degrees F. This water is under extreme pressure and can be brought to the earth’s surface via open loop piping. The water comes to the surface and is strategically expanded in pressure vessels so that it “flashes” from high pressure, high temperature water into steam. This steam, as it expands and decompresses is pushed through large turbines. The turbines turn, generating electricity. The electricity is supplied to the power grid via the standard electricity system. One of the most famous plants that do this on a continual basis are the “Geysers” in California.

Some people refer to geothermal as actual production of electricity, but this is not cost efficient or safe to attempt on a personal level. It takes hundreds of millions of dollars to develop any electricity generation facility that has any scale, and as of now there is no scaled down version that is applicable to an individual. The equipment is costly, and electricity generation is very highly regulated by all governments.

Other questions

Where does the heat go? Won’t the earth run out?

This is an interesting question. We talk about taking heat from the earth to heat our homes and putting heat into the earth to cool our homes. So the question most frequently asked is something to the extent of “can we continue to do this?” or “will the earth run out of heat?” The short answer is that the earth will not run out of heat for several hundred lifetimes. The center of the earth is extremely hot liquid under huge amounts of pressure, and that heat slowly propagates it’s way to the earths surface. This has been the case since the beginning of time. There are also depleted heavy metals, volcanic material, and other things buried in the earth’s crust that also give off heat. Last but not least, the sun is always heating one side of the earth with conduction and radiative heat. We could dive into a theoretical argument as to how long this will last.

So the more appropriate explanation is there are factors way more important than the (relatively speaking) few Btu’s we trade from the ground. It will take hundreds of generations to make any statistically significant impact on the actual earth’s temperature.

Can I generate electricity at my house?

No. Geothermal can be used to generate electricity, but not on any small scale. You’re much better off to look at do it yourself solar power. It’s not constant, but much more realistic.

I live in Australia, is geothermal a good option?

Yes! I know that climates and average temperatures vary greatly around the world around the calendar. Without getting too technical, the earth’s temperature doesn’t vary that much around the globe. Forget about the fact that the air temperature can be a lot different, because several hundred feet underground it’s not that different. That’s what we are using to heat and cool with.

Of course we are biased, but there isn’t an engineer anywhere that will say using an air cooled device is more efficient than using something at a consistently moderate temperature (like geothermal). It doesn’t exist.

In a perfect world we would air condition with cold outside air and heat with warm outside air, but that’s not how it works. When it’s cold outside we want to be warm inside, so we need heat. Instead of trying to get heat from the cold outside air, we get it from the ground. In the summer it’s the opposite.

How long will geothermal last?

Almost all piping comes with a 50 year warranty. The heat pump inside the house will last typically 20 or 25 years. This is much longer than a conventional system because the compressor sees a more moderate temperature than the drastic high and low temperatures of conventional systems. If your heat pump dies or needs to be replaced, it’s easy to disconnect the water pipes and the air ducts and install a new heat pump in its place.