Geothermal Heat Pump System - Advantages and Disadvantages

There is more than one way to install a geothermal heat pump system. Each has its own advantages and disadvantages. After reviewing this page, it should be quite clear which direction best fits your needs.

The three most common methods of using geothermal energy for heating and cooling is:   

Ground Coupled - Horizontal

Ground Coupled - Vertical

Groundwater - Production/Injection

Groundwater - Pump and Dump

Surface water


Ground Coupled - Horizontal

This geothermal heat pump system utilizes a buried network of piping at least 4 ft under the surface of the ground.

A fluid (usually antifreeze, water, or refrigerant) is circulated through this piping network. As it flows through this underground piping, it absorbs or rejects heat.

How so?

Let's take an example:

In the summer the fluid flowing through the buried piping network is typically 90-120 F. Since the temperature of the soil, clay, sand, etc. averages approximately 55 F (give or take 10-20 F in most areas), heat tranfers from the fluid, through the pipe, and into the ground.

As the name suggests, the piping is arranged in a horizontal pattern, as shown below.

geothermal horizontal ground loop

The above arrangement shows each branch loop arranged in a first in, last out piping configuration. It is also known in the pumping world as reverse return. A reverse return piping arrangement allows for equal pressure drop through each branch.

What is the purpose of equal pressure drop?

It ensures that each branch has equal flow, optimizing the heat transfer into the soil.

Advantages

1. It is the least expensive method between the two ground coupled systems (horizontal vs vertical bore).

2. Vertical boring requires special equipment that is typically not commonly owned by a contractor. Horizontal boring can be completed by means a common backhoe.

Disadvantages

1. The ground temperature is not as constant as it is in vertical systems. The horizontal piping is usually 4-6 ft below the surface. During periods of extreme weather, this could negatively impact the heat pump's ability to heat or cool. This usually can be taken into consideration by over-designing the length of horizontal pipe.

2. Horizontal systems are typically less efficient than vertical systems.

3. A typical home needs no less than 1/2 acre for sufficient heat exchange. Larger system require even property.

4. Soil moisture content must be accounted for in the design of this geothermal heat pump system.


Ground Coupled - Vertical

The vertical system is very similar to the design concept of the horizontal system.

However, as its name suggest, the buried piping network is arranged in a vertical formation as shown below.

geothermal vertical ground loop

Advantages

1. The method requires very little property. Even if there is such little space that one piping loop interferes with another, a deeper bore can be dug to overcome this problem.

2. This geothermal heat pump system is more efficient that a horizontal system, as the ground temperature remains quite constant 60 - 600 ft into the earth's crust.

3. Vertical systems need less piping than horizontal.

Disadvantages

1. Very high first cost due to the expensive equipment required for vertical bores.

2. Not widely done, therefore the number of contractors available to perform this kind of work is quite limited.


Groundwater Heat Pump System

Groundwater heat pump systems use the groundwater available in aquifers below the surface of the ground as the medium for heat exchange.

Instead of simply burying pipe in the ground, one or two wells are dug and the well water (at ground temperature) is pumped through a heat exchanger that absorbs or rejects the heat coming off of the refrigerant in the heat pump.

To illustrate, while the heat pump is in cooling mode, heat is absorbed from the home by means of the heat pump. Well water is then pumped from the ground (at approximately 55 F) and into a heat exchanger that puts the well water into contact with 100 - 170 F refrigerant. Naturally, heat from the refrigerant is rejected into the well water. This well water (in production/injection well systems, as described below) is then purged into an aquifer located some distance away from the well water supply point.

There are two types of ground water heat pump systems:

1) Production/injection well systems
2) Pump and dump systems.

Production/Injection Well System

The production/injection system pump water from the production well.

After absorbing or rejecting heat from(to) the heat pump.....

The well water is dumped into the injection well. Since all of the groundwater removed from the production well is pumped back into the injection well, there is a resulting net zero of water loss.

See the picture below for a representation of the above described geothermal heat pump system.

geothermal production injection

Advantages

1. Typically is less costly than ground coupled systems. Of course, if the aquifers are quite deep, this will have a significant bearing on the total cost.

2. Very compact (small amount of property required). If the two wells tap the same aquifer, it is best if they are kept a significant distance away from each other so that the injection well temperature does not impact the temperature of the supply water from the production well.

3. There are many contractors that dig wells. Therefore, this is usually a very simple geothermal heat pump system.

Disadvantages

1. Environmental regulations may prevent the use of an injection well. Depending on local code, special equipment may be required to ensure that contaminated water is not injected back into the groundwater. Of course, people contaminate the ground water everyday by using chemicals to treat their lawns. However, harsher guidelines typically surround the use of injection wells.

2. At times the aquifer available may not be large enough to handle the heat rejection/absorption load required during extreme weather conditions. It is usually advisable to hire a professional to determine if the size of your aquifer is sufficient.

3. If the aquifer is too deep, it may drastically reduce the efficiency of the your heat pump. A deep well requires high pumping head and therefore, more energy. An analysis by a professional engineer can determine the efficiency of this design approach.

4. The minerals and contaminants in the well water requires more maintenance on the heat exchanger when compared to closed loop systems.

Pump and Dump

I've never heard the technical term for this type of geothermal heat pump system. Everybody calls it "pump and dump".

And its just like it sounds.

It is exactly like the production/injection system, but without the injection well.

A well supplies the ground water to a heat exchanger associated with the heat pump. Then the water is dumped into a catch drain.

Advantages

1. This method is the least costly of all the previously proposed systems.

Disadvantages

1. The pump and dump system usually only applies toward residential or light commercial. Otherwise, the large quantity of rejection water may be too much to handle. A hybrid of this type of geothermal heat pump system and the surface water system (discussed next) may be applied toward larger commercial or industrial systems if the lake, pond, or river is large enough.

2. Local drain commission regulations may prevent this type of system. Check with your drain commissioner for more details.


Surface Water

In this type of geothermal heat pump system, a piping loop (usually coiled) is submerged at the bottom of a lake or pond.

Open loop surface water systems pump water from the lake and dispose it back in. Usually the disposal point is as far away as possible from the supply point.

Below is a representation of a closed loop, coiled surface water system.

geothermal surface water

Advantages

1. Usually this is the least costly geothermal method if the the lake, pond, or river currently exists.

2. Low pumping head results in a more efficient system than ground coupled and ground water systems. The open loop surface water system requires a lower pumping energy than the closed loop style.

3. In most climates, a closed loop surface water system coupled with auxiliary heating (in colder climates) is a very attractive and efficient means of heating and cooling your home or building.

Disadvantages

1. The wide range of surface water temperatures can render the heat pump near useless in very cold weather. Usually open loop systems are not practical for regions that experience harsh, below freezing temperatures.

2. Closed loop surface water systems require the pond to be deep enough to keep the water within a mild temperature range.


Well that's it for my review of various systems used in geothermal. If you would like to continue reading about it, the Geo-Heat Center at the Oregon Institute of Technology offers some great case studies showcasing different aspects of a geothermal heat pump system. This really is a can't miss!

But don't forget to come back here!

Return to Geothermal Heat Pump