Geothermal heat pumps are a great way to reduce your energy bill. They use the Earth’s natural temperature gradient to produce hot water or steam for space and water heating applications without using any fossil fuels. In this article we will discuss geothermal heating systems and their benefits, costs, types, and how they work.
What is Geothermal?
In layman terms, geothermal is the harnessing of the earth’s energy through the conversion of heat from deep underground into usable forms such as electricity, hot water, or even hot air. There are two basic methods used in geothermal power generation; closed-loop and open-loop geothermal systems. The main difference between them is whether the process takes place inside or outside of the ground.
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The Closed-Loop System
This type of system is more common because it uses an existing well to generate the heated fluid that can then be piped back underground via a second well. This method has been used successfully on utility scale (over 100 MW) projects around the world including Iceland, New Zealand, USA, Mexico, Indonesia, Australia, and China. However, the closed loop system also has its drawbacks, especially when compared with other geothermal technologies. First of all, the cost of installation is much higher than other geothermal methods. Secondly, since the closed loop system relies on a single well, if the well runs dry, it must be plugged and abandoned. Then the entire project has to be redesigned. Lastly, there is always a risk of having a well blowout during maintenance.
The Open Loop System
Unlike the closed loop system, open loop utilizes an external source of heat to create the heated fluid. One example of an open loop system is a passive solar house where sunlight heats up the ground beneath which is then transferred through the soil to warm the house. Another example would be a greenhouse with a thermal mass underneath the floor. The advantages of this system over the geothermal closed loop system include ease of installation, lower initial costs, and no need for additional wells. On the downside you have a limited amount of usable heat and you cannot use the same well for multiple purposes.
Types of Geothermal Systems
There are different types of geothermal systems depending on the method of generating the heated fluid. Each one has its own pros and cons, so it is important to consider these factors when choosing a geothermal system. We will briefly discuss each type below.
The direct use method consists of pumping hot water through a pipe at the bottom of the borehole and into a storage tank. It requires less equipment and labor than the closed loop system but it does not allow for continuous production.
Using the open cycle method creates the heated fluid by either circulating water through a sealed loop or circulating water through the ground and then transferring the heat to the fluid. The advantage of this method is that it allows for continuous production. The disadvantage is that it requires extensive engineering and construction skills. Also, the initial cost is significantly higher than other geothermal methods due to the large capital investment required.
The constant temperature method generates the heated fluid by circulating liquid through a closed loop or open loop. The main difference between the two methods is the closed loop method transfers the heat directly to the fluid while the open loop method transfers the heat indirectly through the ground. Both systems require extensive engineering and construction skills. The advantage of the constant temperature method is that it is more efficient due to the smaller amount of equipment needed to transfer the heat.
Benefits of Geothermal Systems
There are many benefits associated with geothermal systems including:
Reduced Carbon Footprint
Geothermal systems do not rely on nonrenewable sources like coal and oil. Instead they utilize renewable sources such as the sun and wind. By reducing our reliance on nonrenewable resources, geothermal provides us with a cleaner environment and reduces our carbon footprint.
As mentioned above, geothermal systems do not use carbon based fuel sources. Since geothermal systems use the Earth’s naturally occurring temperature gradients instead of burning fossil fuels, they are considered eco friendly. As a result, geothermal systems reduce pollution which further increases our sustainability efforts.
Because geothermal systems generate energy from renewable sources, they are considered a clean alternative to conventional energy sources. Unlike traditional energy sources, geothermal systems provide us with a steady supply of reliable energy that can be easily purchased and stored.
Because geothermal systems are low cost and easy to install, they help lower the overall household budget. Because homeowners do not need to invest a significant amount of money upfront to get started, homeowners can save thousands of dollars on their initial purchase price.
Homeowner Tax Credit
Some states offer tax credits to homeowners who install geothermal systems. The credits range from $2,000-$4,000 depending on the state.
Costs Associated With Geothermal Systems
Depending on the type of system chosen, the initial cost can vary greatly. For example, installing a closed loop system would cost approximately 4 times more than installing an open loop system. However, the initial cost is offset by lower ongoing expenses. A study conducted by the National Renewable Energy Lab concluded that geothermal systems were cheaper to operate than conventional electric or gas heating systems. Also, geothermal systems require minimal maintenance once installed which helps to lower operating costs.
Types of Geothermal Systems
We now know what geothermal is, and now we can talk about what kind of systems exist. These systems are categorized into three major types of systems:
These systems consist of the following components:
- Heated Water Source – The source of the heated water is usually a well.
- Thermal Mass – The thermal mass is basically a large heat sink that absorbs the heat generated by the water and stores it until it is released.
- Heat Rejection – The heat rejection component is a device that transfers the heat from the thermal mass to the surrounding area.
These systems consist of the following components:
- Pump – The pump circulates the water from the well to the thermal mass and heat rejection areas.
- Heat Exchanger – The heat exchanger converts the heat from the water into usable heat.
- Heat Storage Device – The heat storage device keeps the heat for later use.
- Control Valve – When the control valve detects that the temperature in the thermal mass is too high, the control valve opens and releases some of the heated water.
- Recovery Well – The recovery well collects the excess water created by releasing the heated water.
- Water Tank – The water tank is used to store the heated water.
- Evaporation Well – The evaporation well is used to collect the excess water evaporating from the thermal mass.
- Recycling Wells – The recycling wells are used to recycle the water used to create the heated water.
- Electrical Generator – The electrical generator produces electricity using the heat produced by the heat exchanger.
- Heat Exchanger – The heat exchanger converts the heat from the electrical generator into usable heat.
- Condensate Drain – The condensate drain removes the excess water left in the heat exchanger after the electrical generator is turned off.
- Pressure Relief Valve – The pressure relief valve prevents any excess water from entering the system.
- Safety Valves – The safety valves prevent the system from becoming contaminated with dirt and debris.
- Power Supply – The power supply converts the electricity into usable heat.
- Expansion Vessel – The expansion vessel is used to store the water for later usage.
- Vent Stack – The vent stack is used to release the exhaust gases from the system.
- Expansion Tank – The expansion tank stores the water needed for the system to cycle.
- Heat Pump – The heat pump extracts heat from the ambient air and uses it to cool the water in the expansion tank.
- Air Compressor – The air compressor forces the air through the vents to circulate the air throughout the home.
- Air Filter/Heat Exchanger – The air filter filters out dust and other particles from the indoor air before it enters the heat exchanger.
- Inverter – The inverter converts the power from the electrical generator into alternating current (AC).
- AC Motor – The AC motor converts the alternating current into mechanical movement.
- Belt Drive – The belt drive connects the AC motor to the heat pump unit.
- Heat Pump Unit – The heat pump unit consists of a fan, a compressor, and a condenser coil. The fan blows air across the condenser coil, causing the heat from the condenser coil to be transferred into the incoming air. Once the air is saturated with heat, the fan turns off. Next, the compressor sucks the air through the fan and compresses it. After compression, the air passes through the condenser coil again, this time absorbing the remaining heat. Finally, the air exits the heat pump unit and returns to the room. This entire process is repeated continuously and helps to transfer the heat into the air.