27 Geothermal District Systems Ali Irani, Graham Turk Technology Overview District Energy Systems District energy systems are thermal networks used to heat and cool buildings in close geographic proximity. A district energy system has three major components: 1) a thermal energy generating plant, 2) distribution piping (to transport the heat carrier), and 3) building equipment interconnections (to draw heat from or reject heat to the carrier) (US Department of Energy, 2020). By sharing infrastructure among many buildings, district energy systems take advantage of economies of scale to deliver cost and efficiency improvements compared to individually conditioning each building. The technology has undergone several evolutions, related to the thermal energy source, heat carrier and level of efficiency, which are outlined in Table 3. Table 3: Generations of district energy systems Generation Heat Source Heat Carrier Other Characteristics 1 Coal Steam 2 Coal, Oil Steam, Pressurized hot water Combined heat and power 3 Natural Gas, Biomass, Waste Pressured hot water Prefabricated pipes 4 Geoexchange, Centralized Heat Pumps Medium temperature water Seasonal heat storage 5 Geoexchange, Distributed Heat Pumps Ambient temperature water Simultaneous heating and cooling, inclined drilling for borefields MIT’s campus is currently a 2nd/3rd generation district energy system, with electricity, steam, and chilled water generated by the Central Utility Plant (CUP) and distributed to campus buildings for their heating, cooling, water, and electricity needs. This technology pathway considers transitioning to a 4th or 5th generation district energy system by converting to a water-based distribution system, phasing out natural gas as the primary heat source, and leveraging geoexchange technology (described below). Geoexchange Technology Geoexchange systems use the earth as a thermal battery to provide heating and cooling as part of district energy systems. The earth is used as a heat source in winter and heat sink in summer, balanced on an annual basis to maintain system efficiency. A network of closed-loop pipes transports the heat carrier through deep underground “bores” and to buildings connected to the system. Geoexchange is commercially available today; system costs vary widely depending on the bore depth and number of buildings connected to the system. While vertical borefields are the most common configuration, the Framingham pilot project (discussed below) uses “inclined drilling” to reduce surface disruption. A geoexchange-based district energy system has the potential to meet nearly 100% of MIT’s heating and cooling needs with zero direct greenhouse gas emissions.
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