What is a geothermal heat pump? A geothermal heat pump is a way of heating and cooling buildings by exchanging their heat with the ground.
Natural objects obviously gain and lose heat depending on the amount of sunlight, what they are made of, the weather, where they are, and a whole host of other conditions. When humans first evolved, we lived in caves because they kept us out of the wind and away from the elements. Rock is also a good insulator, and it kept a lot of the heat from our fires inside the cave, rather than wicking it to the air outside. It also kept a lot of the heat from the summer sun out. That is why caves tend to be warmer than the surrounding air in the winter and cooler in the summer. The problem with rock caves, though, is that you cannot move them. As we became more mobile, we started to build our houses, and we wanted to make use of the insulating properties of different materials. In a poorly insulated house, a huge amount of heat is lost through the walls, roof, and windows. Good insulation can cut this heat loss by 30–40% compared with an older, poorly insulated house. However, no matter how well insulated your house is, you will never get it to 100%, and you will eventually lose all of the heat you put in, which means that you are paying for fuel to heat or cool the air around your house because this is where the heat is lost to. Your house might be powered by renewable energy, but most likely, the power comes from either natural gas or a coal or oil powered power station. A geothermal heat pump can change all of that.
A geothermal heat pump makes use of the fact that the temperature of the ground below about 10 meters doesn’t vary. This is because of the insulating properties of all of the rock, sand, and clay that make up the top layers of the ground. Sunlight hits the ground, and it imparts its energy, becoming thermal energy. This energy can penetrate many materials, but the thicker and the denser these materials are, the more quickly the thermal energy stops propagating. Depending on the makeup of the ground, the depth to about one meter is directly affected by the sun and warms and cools with the seasons. Below that, down to about 10 meters, the thermal energy takes longer to pass through, and it decreases as you go deeper. Because it takes so long to go in and out, the ground at this depth tends to take many months to warm and cool. That means when the air is cool, the ground is still warm, and when the air has warmed up, the ground is still cool. This happens to the sea throughout the day, and it is why areas near the coast have a milder temperature. Once you go below 10 meters, the temperature of the ground doesn’t change. If you keep going deeper, however, the temperature will start to rise as you get closer to the heat source within the Earth.
A geothermal heat pump was first proposed by Lord Kelvin, who was one of the scientists who helped come up with the first and second laws of thermodynamics. These laws state that you cannot create or destroy energy, only change it, and the entropy of any system will always increase. A geothermal heat pump is an example of the second law of thermodynamics. Thermal energy will always move from an area of high thermal energy to an area of low thermal energy. If you open your oven door after baking a cake, the hot air will leave the oven and the temperature will equalize with that of the room. You will never find the room getting colder because more hot air is moving into the oven. Conversely, if you go away and leave your freezer door open, the cold air will equalize with that of the room, and the food in the freezer will melt. You will never find the room warming up because cold air is moving into the freezer. This is entropy.
A geothermal heating system has two main parts. Outside, there is a buried loop of pipe filled with water mixed with antifreeze. This loop runs horizontally a few metres below the surface, or vertically down tens of metres. Inside the house, there is a heat pump unit, which works a bit like a refrigerator in reverse. In winter, the fluid in the ground loop picks up low-grade heat from the ground, which is warmer than the outside air. The heat pump uses electricity to compress a refrigerant, raising its temperature, and then transfers that heat into the house. In summer, the process can be reversed: the heat pump takes heat out of the house and dumps it into the cooler ground. You still have to supply electrical energy to run the compressor and pumps, but because the system is moving existing heat instead of making new heat, it can deliver three to five times more heat energy than the electrical energy it uses. This way, a house can be kept warm in the winter and cool in the summer and the only power needed is for the pump and compressor. This could be run from a solar-powered generator. Because a geothermal heat pump can move three to five times more heat energy than the electric energy the pump uses, they are considered to be 300% efficient, as compared to modern heating systems, which can never be more than 100% efficient. And this is what I learned today.
Sources
https://www.energy.gov/energysaver/geothermal-heat-pumps
https://en.wikipedia.org/wiki/Ground_source_heat_pump
https://en.wikipedia.org/wiki/Lord_Kelvin
https://en.wikipedia.org/wiki/Laws_of_thermodynamics
https://www.nachi.org/rockwool.htm
Image By Mark Johnson (Marktj at English Wikipedia) – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7926670