Heat Pumps

Ground-coupled and groundwater (often called ground-source or geothermal) heat pump systems are being installed in great numbers in the United States, Sweden, Switzerland, and Germany. Ground­water aquifers and soil temperatures in the range of 5-30°C are being used in these systems. Ground – source heat pumps utilize groundwater in wells or by direct ground coupling with vertical heat exchangers (Fig. 16). Nearly every state in the United States, especially in the midwestern and eastern states,

Heat Pumps

FIGURE 16 Typical ground-source heat pump installation.

utilizes these systems, which are in part subsidized by public and private utilities. It is estimated that approximately 70,000 groundwater systems, and more than 210,000 closed-loop vertical and 170,000 horizontal systems, are in use.

Подпись: AПодпись:Подпись: Hot water Warm water Like refrigerators, heat pumps operate on the basic principle that fluid absorbs heat when it evaporates into a gas and likewise gives off heat when it condenses back into a liquid. A geothermal heat pump system can be used for both heating and cooling. Water-to-air and water-to-water heat pumps are adaptable to geothermal energy. Heat pumps with heating capacities ranging from less than 3 kW to more than 1500 kW are available.

Подпись: C Warm airПодпись: Hot waterПодпись:Heat Pumps

2.2 Convectors

Heating of individual rooms and buildings is achieved by passing geothermal water (or a heated secondary fluid) through heat convectors (or emit­ters) located in each room. The method is similar to that used in conventional space heating systems. Three major types of heat convectors are used for space heating: forced air, natural air flow using hot water or finned tube radiators, and radiant panels (Fig. 17). All these can be adapted directly to geothermal energy or be converted by retrofitting existing systems.

2.3 Refrigeration

Cooling can be accomplished from geothermal energy using lithium bromide or ammonia absorption refrigeration systems. The lithium bromide system is the most common because it uses water as the refrigerant. However, it is limited to cooling above the freezing point of water. The major application of lithium bromide units is the supply of chilled water for space and process cooling. They may be either one- or two-stage units. The two-stage units require higher temperatures (approximately 160°C), but they also have high efficiency. The single-stage units can be driven with hot water at temperatures as low as 77°C (such as at Oregon Institute of Technology; Fig. 4). The lower the temperature of the geothermal water, the higher the flow rate required and the lower the efficiency. Generally, a condensing (cooling) tower is required, which will add to the cost and space requirements.

For geothermally driven refrigeration below the freezing point of water, the ammonia absorption system must be considered. However, these systems are normally applied in very large capacities and have seen limited use. For the lower temperature refrigeration, the driving temperature must be at or higher than approximately 120oC for a reasonable performance. Figure 18 illustrates how the geother­mal absorption process works.

B

Heat Pumps

Heat Pumps

FIGURE 17 Convectors: (A) forced air; (B) material convection (finned tube); (C) natural convection (radiator); and (D) floor panel.

Updated: March 14, 2016 — 1:07 pm