Energy System for the Next Millenium: GEOTHERMAL HEATING & COOLING - WHAT IT IS AND HOW IT WORKS

This heat from the sun, stored in the ground, is the energy source used for geothermal heating. Water in a well, of more than 30 feet of depth, will also be at a temperature of 50ˇăF. It is this water, pumped from the well, which is used as the transfer mechanism to move the heat from the ground to the budding. In the case of, what is known as, a "standing column well" system, once the heat has been extracted from the water, it is returned to the well, where it gets warmed up by the ground and again pumped to the building.

To circulate the water through the well, there are 3 main ways to configure the pump and the return pipe in a standing column well, depending on the characteristics of the well:

1. Pump at the bottom of the well and return pipe to the top.

2. Pump at the top of the well and return pipe to the bottom.

3. Pump at the top of the well connected to an induction tube from the bottom and return pipe also to the top.

The pump is connected in the normal way to a pressure tank in the building, from which both domestic water and the water for the geothermal system are drawn. Depending on the different forms of water usage associated with the building, such as irrigation in addition to normal domestic and geothermal use, it may be desirable to use a variable speed well pump to reduce the energy required to pump different amounts of water.

THE HEAT TRANSFER EQUIPMENT:

The main pieces of equipment, used to concentrate the heat from low grade temperatures of around 50ˇăF to more useful building heating temperatures in the region of 120ˇăF, is known as a heat pump. A household refrigerator is a form of heat pump, which pumps heat from the inside of the refrigerator box to the outside radiation coils, either at the back of the unit or underneath the machine. It might therefore be called an air-to-air heat pump.

For the geothermal heat transfer process, it is convenient to think in terms of different loops of fluid flow. For this discussion, we will consider a geothermal heating system in which the heat distribution to the building is by radiant heat with water pipes embedded in the floor. The heat pump used in such an installation is a water-to-water heat pump, because it transfers heat from one heat exchanger with water at a low temperature to another heat exchanger containing water at a high temperature. The 4 separate fluid loops in such a system are:

1. Water loop from the well pump, through a heat exchanger on the well side of the heat pump, where the water gives up its heat, to the refrigerant and then returns to the well to be re-heated by the ground,

2. Refrigerant loop to transfer the heat from the low temperature well side heat exchanger to the high temperature hydronic (radiant heating) side heat exchanger in the heat pump.

3. Water loop from the hydronic side heat exchanger in the heat pump, where the water takes heat from the refrigerant and flows to the hydronic accumulator tank, in which hot water is stored for the radiant heating system.

4. Water loop from the hydronic accumulator tank through the radiant heating pipes in the floor of the building.

The heat pump is the secret to the energy efficiency of the geothermal heating system since it transfers 4 times the heat energy compared with the electrical energy required to run the heat pump.

For cooling and air conditioning, the heat pump flow process is simply reversed. Rather than transferring heat from the ground to the building, the same heat pump, instead, transfers heat from the building to the ground. The exact same heat transfer system is used, except that, for cooling, the water stored in the "hydronic" accumulator tank is cooled down to temperatures around 40ˇăF. This geothermal cooling process has a very high Energy Efficiency Ratio (EER), because of the low constant temperature of around 50ˇăF in the ground, to which the heat pump is depositing the heat from inside the building.