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Heat Pumps for Heating and Cooling Some basic rules are as follows: • The length of the required piping depends on ground thermal conductivity, ground temperature, and heating and cooling power needed. • A ground loop that is too big (tube diameter and loop length) will heat/cool effectively but will be more expensive on capital costs and require more pumping energy. • A ground loop that is too small can ‘stress’ the energy resource, by trying to extract too much energy from too small an area, thereby reducing efficiency. • A common minimum “design” earth loop temperature is 30oF. Increasing this temperature 3 to 5oF can significantly increase the amount of piping required in the earth loop. Decreasing this temperature will reduce the amount of tubing required, but at the expense of reduced heat pump performance. The designer must than find a balance between the fluid supply temperature and the capital cost of the ground loop. • Most earth loops should be designed to ensure that the flow through them remains turbulent at their minimum fluid temperature. Turbulent flow provides better heat transfer between the tube wall and flow fluid. As a rule of thumb, the flow rate should be at least 2.0 US gpm for 3⁄4” through 11⁄4” pipe, and at least 3 US gpm for 11⁄2” pipe to avoid laminar flow. • As a rule, in cold climates, the earth loop length will usually be set by the heating requirement. Likewise, in warm climates, the earth loop length will typically be determined by the cooling load. 3.10. Earth Loop Working Fluids Because the evaporators in most geothermal heat pumps can operate at refrigerant temperatures well below freezing, it is common to use an antifreeze solution rather than 100% water in earth loops. Several types of antifreeze solutions based on salts, glycols and alcohol additives have been used in geothermal systems. Each of these solutions has strengths and limitations. 3.10.1. Salt based Solutions − Salt-based solutions of calcium chloride and potassium acetate have been used in some earlier generation geothermal heat pump systems. While offering acceptable environmental characteristics, 42PDF Image | Heat Pumps for Heating and Cooling
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