Heat pump systems operating on ground temperature

Transcription

1 Heat pump systems operating on ground temperature Geoheating Many areas, including Klamath Falls as discussed in Extebsion 23.3, Geothermal energy, have used groundwater resources for indoor air conditioning or heating, that is, for temperature equalization. Government and commercial projects are common. Boise first used its hot water resources for indoor heating in In the era of cheap energy, the system was discontinued. In the 1980s in Boise, Idaho, in the wake of the 1970s energy crisis, geothermal systems were installed in the City Hall of Boise, the Boise Capitol Mall Complex, and the Fort Boise Veteran s Hospital. (12) Fig. E A district geothermal climate system. (U.S. Department of Energy, NREL, Ref. 246, p. 6) The Sacramento Housing and Redevelopment Agency installed geothermal climate control systems the 50-year-old River Oaks and Dos Rios complexes (this was a district heating system, Fig. E09.6.1). The Agency was heavily subsidized by the Department of

2 Energy, Ch. 9, extension 6 Heat pump systems operating on ground temperature 2 Housing and Urban Development and rebates from the Sacramento Municipal Utility District. (247) Milgro Nurseries installed a geothermal climate system in its Newcastle, Utah facility. (246) Commercial and government buildings are by no means the only ones to benefit from geothermal energy. Homeowners can benefit as well. Many individual homeowners in Klamath Falls use ground hot water. In Iceland, most homes and businesses are heated by hot water from below the surface. Just as with the large generators, the geothermal systems are cheap to run once they are installed, but very expensive to build. A typical homeowner will have to commit in the neighborhood of $8,000 to $25,000 to construct a system. (247,248) However, if the homeowner plans to air condition the home, an expense of about $4,000 to $5,000 would have been required anyway, and the energy savings and long life of the geothermal system will make the extra investment worthwhile. Is it only where there is hot water lying below the surface that we can use the ground as a resource? The answer is clearly no. Since there is so much thermal mass in the ground (caves maintain just about the same temperature no matter what the time of year, and our ancestors used food cellars that stayed year-round at a cool 13 C (55 F) temperature because they were below the frost line, which is several meters below ground level). We can store thermal energy in the ground in summer to cool a house and use the stored thermal energy in winter to warm the house. More than half a million groundwater heat pumps are in use in the U. S., many in private homes. (246) A manufacturer quotes a price for a geothermal heat pump system of about $2,500 per ton of capacity, and the most typical installation has a 3 ton capacity. (249)

3 Energy, Ch. 9, extension 6 Heat pump systems operating on ground temperature 3 Many Montana residents, for example, were chagrined by high energy prices that followed deregulation, and decided to do something about it. (250) That something is the groundwater heat pump (as the system is called when the loop is open), also sometimes designated as a geoexchange system (when the loop is closed). In the Chapter, we discussed air-based heat pumps and the difficulties sometimes associated with their use. One difficulty is that the efficiency varies because of the outside air temperature (actually, it is due to the difference between inside and outside temperatures), and sometimes the temperature difference is too great for the heat pump to be able to work very well. When the temperature difference between inside and out is more than about 12 to 15, the efficiency drops substantially. A source of supplemental heating may be necessary. That cannot occur with groundwater-based or geoexchange systems because the working temperature is always about at that 13 C (depending on latitude, the ground temperature is somewhere between 7 C and 21 C; this 13 is the ground temperature in my neck of the woods). The groundwater heat pump An electric heat pump that warms the inside of a house by cooling groundwater uses only 45% as much fuel as a furnace that is 75% efficient (not atypical) for the same amount of heating, despite the factor of one-third efficiency loss in translating fuel energy to electrical energy at the generating plant. The groundwater may be in a pool underground from the house (say, in an aquifer), or may be flowing. In either case, water is pumped from the reservoir of groundwater to the house and back. Fig. E shows the concept of a geothermal heat pump system.

4 Energy, Ch. 9, extension 6 Heat pump systems operating on ground temperature 4 Fig. E Diagram of the operation of a groundwater heat pump using a pond. While a closed system is shown, an open system may be used. (Adapted from Ref. 246, U.S. Department of Energy, NREL) In cooling mode during the summer, the compressor will work as an air conditioner does, and the net result is that inside air will transfer heat to the groundwater, cooling the house while heating the groundwater. Air ducts in the house distribute cool air in the summer, just as a home air conditioner would. If the groundwater pool is not moving or slowly moving water, the surrounding rock will be warmed as the house is cooled. If the water is flowing, the house s excess thermal energy is carried away and distributed a bit at a time to the entire surroundings underground. In heating mode during the winter, the compressor will work as a heater, and overall the water from the ground is cooled and the thermal energy in it plus the work done is transferred to the inside environment. The same air ducts that were used by the cooling system distribute hot air in winter. Flowing water will be cooled slightly, obtaining its equilibrium through transfer from all the surroundings. The still groundwater will be

5 Energy, Ch. 9, extension 6 Heat pump systems operating on ground temperature 5 giving back some of the thermal energy stored in it during the summer as well as cooling the immediate vicinity of the storage volume. The system can be used in closed mode as well as in open mode. In closed mode, a working fluid is circulated inside the piping between the inside of the house and the pool or groundwater source. In the case that a deep well is available, an open system may be run between the well and the house. This is most popular in the northeast. The heat pump, while it is in use, can also be a source of hot water. A heat exchanger called a desuperheater can efficiently provide transfer of heat from the compressor. A desuperheater is an auxiliary heat exchanger that uses superheated gases from the heat pump s compressor to heat water. Of course, in spring and fall, when the system is off, an alternative water heating system must be used. Heat pump manufacturers can and do provide a built-in water heating system that can be used the year around that uses the desuperheater in winter and summer and more conventional water heating when the heat pump is off. Fig. E A groundwater heat pump in use at Georgia Tech showing the details of the commercial-sized system. (U.S. Department of Energy)

6 Energy, Ch. 9, extension 6 Heat pump systems operating on ground temperature 6 Fig. E A geothermal heat pump space heating/cooling and hot water heating technology at the new Printing Center on the Georgia Tech campus. (U.S. Department of Energy) Fig. E shows a commercial sized heat pump system. The scale of such systems is not overwhelmingly large, as may be seen by comparison to the size of the cinder blocks. This heat pump is used in a Georgia Tech building. A geothermal system in its enclosure is shown in Fig. E Again, this is not a great deal larger than a normal heating system. The geoexchange heat pump The principle of the geoexchange heat pump is similar to that of the groundwater heat pump, but it is used in cases when groundwater is inaccessible, brackish, contaminated, or otherwise unavailable. The geoexchange system involves more installation work, and so is inherently more expensive to construct than the groundwater system. The energy costs

7 Energy, Ch. 9, extension 6 Heat pump systems operating on ground temperature 7 while in use are basically identical between the two systems. Fig. E shows the vertical field geoexchange system, while Fig. E shows the horizontal field geoexchange system. Fig. E Diagram of the operation of a vertical field geoexchange heat pump. (U.S. Department of Energy, Ref. 242, p. 5) As shown in Figs. E and E09.6.6, some ground in the neighborhood of the house is excavated and several large loops of high density polyethylene plastic pipes are laid either horizontally or vertically (the vertical orientation is shown in the first picture and the horizontal system in the second). The plastic pipes will carry an antifreeze solution as a working fluid rather than pure water as would be used in the groundwater system. For a residential home, typically four

8 Energy, Ch. 9, extension 6 Heat pump systems operating on ground temperature 8 or five such loops will be put in place. Enough land must be available to spread the loop system. Fig. E Diagram of the operation of a horizontal field geoexchange heat pump. (Adapted from U.S. Department of Energy, NREL, Ref. 246, p. 5) In winter, the liquid in the pipe is warmed to the temperature of the surrounding ground. It is then pumped back to the surface where work is done with a compressor and the work plus the thermal energy is distributed as hot air through the rest of the home through the same sort of ductwork used by an air conditioner or the groundwater heat pump. (246,250) In summer, exactly the reverse process occurs. The system circulates the working fluid so that it collects thermal energy from the home, cooling it in the process, and transfers it back into the ground. Again the cool air is distributed through the ductwork. (246,250)

9 Energy, Ch. 9, extension 6 Heat pump systems operating on ground temperature 9 The heat pump water heater is shown in Fig. E It has features similar to the storage water heater discussed in Extension 9.2, Savings on appliances. Hot water is stored in a tank, not heated on demand, as were some of the water heaters discussed before. These water heaters need to be placed in a normal room; they do not function well in cool spaces, so should be placed in the living area of the home somewhere. Fig. E A heat pump water heater. (U.S. Department of Energy, Ref. 103)