What is Earth Energy?

Terms and Abbreviations

COP – coefficient of performance
EER – energy efficiency rating
EES – Earth Energy System
GSHP – ground-source heat pump
Ton = 3.5 kW of energy

Earth energy (often generalized as geothermal energy) is thermal energy, either heat or cold depending on what is desired, derived from the earth (geo). The thermal energy is contained in the sub-surface materials (rock/sand/gravel/soil), groundwater aquifers, and stable surface water bodies, all of which are referred to as “ground” in the term, “ground-source heat pump”.

In some areas the ground is much hotter than average. This is most apparent where geysers are present. The heat energy can be used for direct heating or, if it is hot enough, even electricity production. This form of energy is also called geothermal energy, but for clarification purposes, it will be called high-temperature earth energy.

However, in most areas, the earth temperature is in the 5 to 15°C range. There is still a way to harness this low-temperature earth energy. This is possible because the temperature of the earth is warmer in the winter and cooler in the summer than the outside air. The resource can be tapped into between the temperature range of 4°C to 38°C using a ground-source heat pump.

What is a Ground-Source Heat Pump?

A ground-source heat pump can be used to provide space heating/cooling for homes and offices. In heating mode, it transfers the thermal energy stored in the ground (subsurface, groundwater, or surface water) into a building using the same principle as a refrigerator that extracts heat from food and rejects that heat into the kitchen. A ground-source heat pump takes heat from the ground at a low temperature, passes it into a liquid that vapourizes at a low temperature, raises that temperature by compressing the vapour, passes that higher heat to water, and distributes it throughout the building. The vapour condenses as it loses its heat and it re-circulated to the heat pump. Typically, the energy supplied to the pump (in electricity) is returned in thermal energy at a rate of 1: 3.

In cooling mode the heat pump works in reverse. The relatively cool temperatures from the ground help condense the vapour, which absorbs heat, thus removing it from a building. In many cases, however, the cooling can happen without the need for the heat pump because the temperature of the ground is cool enough to use directly to cool the building.

Heat pumps are measured in “tons,” whereby one ton is equal to 3.5 kW (12000 Btu/h) of energy. It is important to know the heat demand of a building to properly size the heat pump; therefore, an accurate heat-loss assessment should be completed by a certified professional prior to any installations.

There are a few basic methods for tapping into the earth energy.

1) Open loop, aquifer

Groundwater supplies the thermal energy. A supply well is drilled into an aquifer, which may be the same well supplies the domestic water. Approximately 10 - 12 gal/min are passed through the heat pump and then reinserted into the aquifer, either through a return well or a leaching field (see diagrams below). One concern with the leaching field design is ensuring full replacement of the groundwater to the aquifer. A full assessment by a certified professional is required prior to any installation as the leaching field technique is not possible in many situations.

(Electrical Certifications, 2007)

(Electrical Certifications, 2007)

2) Closed loop, vertical

The thermal energy is supplied by the subsurface of the ground (i.e. bedrock, sand, gravel, soil, etc). Tubes are inserted into boreholes through which a solution, typically antifreeze, is circulated. The boreholes are approximately 46 m/ton (150 ft/ton) of heat pump capacity.

(NRCan, 2002)

3) Closed loop, horizontal

This design is similar to above with the exception that the tubes are laid in the ground horizontally, typically at a depth of 1.2 – 2 meters (4 - 6 ft). This design is simpler to install but takes up more ground area and can be less efficient as the ground closer to the surface is more susceptible to temperature fluctuations from the exterior climate (i.e. ground freeze, rain percolating through, etc).

(NRCan, 2002)

4) Closed loop, ocean

This design passes tubing through the ocean near to shore to gather thermal energy. Again, the ocean temperature is affected by the exterior climate so the system can be less efficient than an aquifer or vertical borehole system.

(NRCan, 2002)

The efficiency of the technology is measured by different indicators in cooling mode versus heating mode. In cooling mode the performance indicator is called the Energy Efficiency Ratio (EER). This is the cooling effect produced by the unit (in Btu/hr) divided by the electrical input (in watts) resulting in units of Btu/watt*hr.

Heating performance is measured by the indicator, Coefficient of Performance (COP). This is the heating effect produced by the unit (in Btu/hr) divided by the energy equivalent of the electrical input (in Btu/hr) resulting in a value without units. For both COP and EER, the larger the numerical value, the less electricity required to operate it.

Why a Ground-Source vs. an Air-Source Heat Pump?

Many people are familiar with air-to-air heat pumps that use outdoor air as the source of heat. These units are well suited for moderate climates but decrease in efficiency as the exterior temperature decreases. A ground-source heat pump is more constant in that it is based on the ground temperature which generally retains a constant temperature of approximately 5 to 8°C throughout the year. In general, the COP of a ground-source heat pump is higher than that of an air-source.

Benefits

• high-efficiency earth energy systems are on average 48 % more efficient than the most efficient gas furnaces and more than 75 % more efficient than oil furnaces
• earth energy systems provide more flexibility in a building design because there are no chillers, air-handlers, or other equipment required on the roof or lawn. Additionally, the equipment takes up less space, thus the size of mechanical rooms can be reduced, and boiler rooms can be eliminated.
• earth energy systems protect against heating fuel price fluctuations
• if renewable sources provide the electricity, earth energy systems can provide an emissions-free heating system (CGC, 2007).

Experiences

• In 2005, more than 650,000 geothermal heat pump units were installed in the U.S., resulting in annual savings of 5.2 billion kWh of fossil fuels, reduced electricity demand by 1.7 million kW and the elimination of nearly 4 million tonnes of CO2
• In the US, installations of earth energy systems has increased in frequency by over 30% in the last two years
• Heat pumps are most attractive for buildings requiring both heating and cooling functions, such as larger commercial or apartment buildings, and specialized buildings with swimming pools or hockey arenas because they can accomplish both heating and cooling with one system (CGC, 2007)

References

Canadian GeoExchange Coalition. (2007). Canadian GeoExchange Coalition. Retrieved on Sept 28, 2007.

NRCan. (2002). Residential Earth Energy Systems, A Buyer’s Guide. Retrieved on Sept 27, 2007.