Every so often I get a call from a drilling or mechanical contractor who has been asked to put together a budget price for a "so-called geothermal project". I say "so-called" because what they are looking at is not really a geothermal project but a conventional HVAC system with a token heat pump and GHX. I reviewed a project recently that...
was designed as a conventional system with conventional gas boilers and chillers. Based on information from the mechanical system designer, the peak cooling load was 2,200 kW (7,506 kBtu/hr or 625 tons). Two conventional chillers with cooling towers were specified with a total cooling capacity of 2,400 kW (8,189 kBtu/hr, or 682 tons). One "water cooled heat pump" with 500 kW (1,706 kBtu/hr, or 142 tons) cooling capacity was specified for connection to a vertical GHX. The heat pump was isolated from both the GHX and the building chilled water system with water to water heat exchangers to allow them to be completely isolated from the system. The large heat exchangers required 2 additional pumps and reduced the heating and cooling efficiency of the heat pumps, added approximately $100,000 to the cost of the system and required additional mechanical room space.
The boilers have more than enough capacity to meet the peak heating loads with or without the heat pump operating. The distribution system had been designed for air handlers and fan coils that required 55°C (130°F) to deliver the heating energy needed. Dropping the distribution temperature by selecting different air coils in the air handlers to deliver the energy needed at 43°C (110°F) would have improved the efficiency of the heat pumps by about 20%.
In other words, the system was designed around the parameters suitable for a conventional boiler/chiller system...not the characteristics of a geothermal system.
A more cost-effective approach might have considered using 3 or 4 chillers / heat pumps as an integral part of the system rather than a single heat pump / chiller that is totally isolated from the system, and to use the boiler to inject heat into the building system when the heat pump can't maintain the water temperature needed. The system was designed for a project that required large domestic hot water loads, with a large laundry, kitchens and showers. Taking advantage of the heat pumps to provide simultaneous heating and cooling would improve the system efficiency.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.
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