Geothermal exchange heat pump

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A geothermal exchange heat pump, also known as a ground source heat pump, is a heat pump that uses the Earth as either a heat source, when operating in heating mode, or a heat sink when operating in cooling mode. All geothermal heat pumps are characerized by an external loop containing water or a water/antifreeze mixture (propylene glycol, denatured alcohol or methanol), and a much smaller internal loop containing a refrigerant. Both loops pass through the heat exchanger.

There are three categories of geothermal heat pumps based on the type of external loop: open loop sytem, closed loop vertical system, and closed loop horizontal system. The open loop system draws water from a well, passes it through a heat exchanger in the building, and then discharges it. The water can be discharged to a stream or lake, or injected into a second well. The pipe in vertical closed loop system uses a single well with the fluid in the pipe constanly recirculated to and from the well. The horizontal closed loop is placed below the frostline (1 to 2 m) underground. Whether a vertical or horizontal closed loop system the pipe is often spiral to increase the contact area per length. A typical traditional 2,000 sq. ft. (185.8 m²) residence will require four tons oil equivalent (14.06 kilowatts) of heating capacity, while a modern residence of equal size should typically consume no more than 1 ton. The amount of vertical or horizontal loop required for each ton of capacity is a function of the soil formation thermal conductivity and deep earth temperature, and also depends on the balance between the amount of heat rejected to, and absorbed from the ground during the course of the year.

Geothermal heat pumps are also used in non-residential buildings, but the variety of loads and load patterns in these applications make it difficult to specify rules of thumb for capacity per unit of building area, or quantity of heat exchanger required per unit of heat pump capacity.

The Earth below the frost line remains at a relatively constant temperature year round, usually between 7-21 degrees Celsius (45-70 degrees Fahrenheit) depending on geographical location. This property allows geothermal heat pumps to perform with far greater efficiency and in a far larger range of extreme temperatures than conventional air conditioners and furnaces.

To understand how a heat pump can heat during the winter and cool during the summer, let us consider each modes:

Contents

1 Heating mode
2 Cooling mode
3 External links
4 See also

Heating mode

In the heating mode, the external fluid is pumped from the well at 8-16 degrees Celsius and passes through the heat exchange unit. Within the heat exchanger the internal fluid is allowed to expand and change state into a gas, which draws heat (heat of vaporization) from the external fluid cooling it. Meanwhile the internal gas is pumped to the compressor where it pressurized causing it to condense into a liquid, which releases the heat and the heat exchanger warms the neigboring air of the house. At the same time, the cooled external fluid is pumped back into the closed loop running into the earth where it is warmed by the soil and recirculated.

Cooling mode

The cooling cycle is very similar except a valve on the internal loop reverses the direction of flow. Now the compressed internal fluid coming from the compressor heats the external fluid, before passing through the evaporator where it vaporizes taking up heat from the air in the house. The heated external fluid is pumped into the ground where it is cooled and recirculated. Alternatively, the heated fluid may pass through a second heat exchanger where water from the house absorbs some of the excess heat. This means that in summer, the house air is cooled and the hot water is heated by the heat pump.

Geothermal exchange heat pump

Heating mode

Cooling mode

External links

See also