Clean Tech 101

Geothermal Heat Pumps

Geothermal heat pumps are otherwise known as water-source, ground-source, earth-coupled or geoexchange heat pumps.

They use earth's constant temperature as the exchange medium instead of outside air temperature. This enables the system to reach high efficiencies of 300% to 600% on cold winter nights, compared to 175% to 250% of air-source heat pumps.

Like any other heat pump, geothermal and water-source heat pumps can also heat, cool and supply hot water for domestic purposes. Some geothermal system models come with variable fans and two-speed compressors.

Types of Geothermal Heat Pumps

The following are the basic types of geothermal heat pumps:

Closed-Loop System

Some geothermal heat pumps employ a closed loop for the circulation of an anti-freeze solution. The closed loop is made of a plastic tubing and is either buried in the ground or submerged in water.

Heat is transferred between the anti-freeze solution in the closed loop and a refrigerant in the heat pump through a heat exchanger. The closed loop can be identified in vertical, horizontal or lake/pond configurations.

However, in direct exchange heat pumps, the refrigerant is pumped through copper tubing buried in the ground in a vertical or horizontal configuration without using the heat exchanger. Direct exchange systems feature a larger compressor, and they can be efficiently operated in moist soils.

  • Horizontal - This configuration is more cost-effective for residential applications. It consists of trenches are 4 ft deep and two pipes, one buried at 6 ft, and the other at 4 f. Two pipes are placed five ft in the ground.
  • Vertical - This type of installation is best-suited for large commercial buildings or schools as the space required for horizontal loops is greater. Vertical loops are used in shallow soil for trenching as they reduce the disturbance to existing landscaping. In a vertical system, holes are drilled 100 to 400 ft deep and about 20 ft apart. Two pipes connected at the bottom with a U-bend are inserted into these holes. The vertical loops are connected with a horizontal pipe in trenches and the heat pump in the building.
  • Pond/Lake - This may be the cheapest option for the site with an adequate water body. An underground supply line pipe is run from the building to the water and coiled into at least 8 ft under the surface for preventing freezing. The coils are then placed in a water source that satisfies criteria of quality, depth and minimum volume.

Open-Loop System

This type of system employs surface body water or well water as the heat exchange fluid circulates via the heat pump system.

Water returns to the ground via the well, surface discharge or recharge well once it has circulated.

Hybrid Systems

Hybrid systems that employ different geothermal resources or a combination of a geothermal resource with outdoor air are another option. This approach is effective when cooling needs are greater than heating needs. This is a variation of an open-loop system where one or more vertical wells are drilled. Water is taken up from the bottom of a standing column and returned back. The system can shed a portion of the return water during peak heating and cooling, resulting to inflow of water to the column. The bleed mechanism cools the column during heat rejection, heats it while extracting heat and reduces the required bore depth.

How do Geothermal Heat Pumps Work?

The following are the basic elements of the geothermal heat pump:

  • Ground loop consisting of fluid-filled plastic pipes buried in the ground near the building. It can be used for capturing and rejecting heat into the ground to warm or cool the building.
  • Eliminating the heat from the fluid in the pipes, concentrating the heat and transferring the heat to the building.
  • Air delivery systems in the form of air ducts/radiant floor systems for distributing hot or cold air throughout the building.

Geothermal heat pumps work more like a refrigerator, with a few extra valves for allowing the heat-exchange fluid to follow two different paths of heating and cooling.

It takes heat from warm regions, exchanges to cool regions, and vice versa. It makes use of electricity for heating and cooling, just like a traditional heat pump. It also uses the Earth's constant temperature as a heat source in winter and repository for heat in the summer.

In the winter, fluid directed through the underground piping loops is heated by the Earth. The heat is then captured and concentrated by the heat pump, and then distributed through the ductwork of the building.

In the summer, the heat pump transfers the heat from the indoor air into the underground loops for relative transmission into the cooler ground.

Also, heat extracted from indoor air can be used for heating swimming pools or water instead of directly passing it to the ground.

Benefits of Geothermal Heat Pumps

Some of the key benefits of geothermal heat pumps include the following:

  • They use nearly 25 to 50% less electricity than conventional cooling or heating systems.
  • Low environmental impact
  • Highly reliable and durable
  • They have greater design flexibility and can be installed under both new and retrofit conditions.
  • Low maintenance and operation costs

Disadvantages of Geothermal Heat Pumps

The limitations of geothermal heat pumps include:

  • The cost of installing the geothermal heat pump is very high even though the extra costs are returned to energy savings within a few years.
  • The area required for setting up pipes can be large and hence they are not suitable for small developments.

References

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