The technology to extract energy from the earth is as dependable and efficient as it is ecologically conscientious. The earth is a thermal engine. Every day sunlight is absorbed and warms the ground. Every night this thermal energy is radiated into the atmosphere and beyond. Tapping into this cycle can access a virtually unlimited energy source that’s literally as dependable as night and day.
This process is more efficient for a couple of reasons. First, the transfer of heat from the refrigerant-to-water is more efficient than the refrigerant-to-air process. Second, there’s a similar discrepancy between the primary movers, the pump and fan motor, respectively. The fan not only works harder to balance a less efficient heat exchanger but also, since it typically has a larger motor than the equivalent pump for the ground-source system, it uses more electricity. Also, the system holds a much smaller refrigerant charge than a corresponding heat pump, which is ecologically responsible.
Another thermal advantage of the ground-source system is that the earth is usually colder than the air in the summer and warmer in the winter. Consequently, the overall system heat-transfer rate is better than that of conventional heat pumps in both the heating and cooling seasons. One disadvantage of ground-source systems is that they have a higher installation cost than packaged DDX systems because of the well drilling (typically one 300-ft well per ton of cooling) and trenching. Another limitation is that there must be a large land area available for all of the piping and drilling (the wells are spaced at approximate 20-ft intervals). An economical alternative to the ground-source system is to use the same air handlers but to reject heat at a cooling tower and to obtain heat in the winter from a hydronic boiler. The cost of such a system is less than that of a chiller-based system, and its efficiency is equal, if not better. Moreover, a building with both interior and exterior zones on the same main loop will have air handlers simultaneously rejecting heat and extracting heat from the loop, so a significant part of the load is balanced all year long, never requiring any artificial energy in/output from a boiler or cooling tower.
The most common use of ground-source systems is for public and community schools, military installations, and industrial farms. These institutions usually have a large land area available for athletic events and also for parking. (Black tarmac is ideal in climates with a predominant heating load, as it can concentrate heat at the ground loop beneath it.) Schools, in particular, have multiple functions that cause a wide variation in use through the week. In addition to daytime classes when the entire facility is in full use, there are evening sports, community classes, and other public events. For a facility with a central chiller, this type of operating schedule keeps the chiller on-line sixteen or more hours a day, and at inefficient part-loading much of that time.
A typical GSHP retrofit of a conventional HVAC system replaces the chiller with individual WSHP units serving each classroom, office, and other conditioned space. A library, gymnasium, or auditorium may have two or more such units. Multiple-zone air handlers in mechanical rooms can be redesigned so that one vertical unit serves each conditioned zone via the existing ductwork. Console units in each classroom provide local environmental control. Areas such as schools, with a high occupancy rate, usually need 20 percent or more outside air. Where it is impractical to cut outside-air openings to duct air to each individual room or system, the existing air-distribution system can be used to deliver 100 percent tempered outside air to each space, with the balance of the load handled by the individual ground-source heat pumps in each zone.
Controls afford further savings, systemwide. Time clock on/off control of all the units limits operation to the regular workday, with inexpensive mechanical 2-hour override timers for each zone thermostat, allowing after-hours use as required, for meetings or extracurricular functions. Most units can also be provided with a morning warmup cycle when the outside-air damper is closed, until the room reaches setpoint. A random start-stop relay prevents all the units from turning on at once to cause a peak in the electricity demand rate.
Ground-source heat pumps are now marketed by all of the major HVAC manufacturers. They’re dependable, available in a full range of tonnages and configurations, and suitable for virtually any application. A note of caution, for ground-source applications that condition air for large spaces such as cafeterias and gymnasiums. These systems, in which each unit is piped to its own well field, tend to heat up over time (in areas with a predominant cooling load) even when they’re designed by conservative, proven criteria. As a result, several design modifications are recommended: (1) add one more well per loop than with smaller systems; (2) grout the wells from the bottom up with a bentonite slurry (this is recommended for all wells); (3) increase the well depth by 20 percent if possible, and the spacing by at least 25 percent. Finally, where two or more units supply one space, it’s important that the lead unit is rotated among them so that any given unit doesn’t run full-out for extended lengths of time.
As for installations that may already be experiencing such problems— perhaps after an extended dry spell (moist soil improves the heat-transfer process in the ground loop), hot spell, or just a controls malfunction that kept the system operating continuously for a long time— there are some fixes: (1) installing a heat exchanger to extract some heat from the loop, perhaps to preheat domestic water; (2) installing some extra piping, perhaps in the form of horizontal “slinky”-type piping recently on the market; (3) adding more well(s) to the loop.
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Excerpt from Energy Conservation Projects.