NETL investigated the economic feasibility of developing deep geothermal resources for space heating for two Pittsburgh, Pennsylvania, commercial development sites: one a greenfield application and the second a retrofit of an existing business park. This study follows a previous study conducted by NETL for the West Virginia National Guard, which considered several additional energy utilization and conversion options using geothermal energy as well as on-site natural gas.
NETL researchers are studying deep direct-use (DDU) of geothermal energy to see if DDU could be economically feasible in this region of the country, considering that an adopter could be motivated to pay somewhat higher prices for heat compared to the current lowest-cost option (i.e., natural gas fueled systems). Low-temperature geothermal resources are widely available but have not been used to their full potential. DDU geothermal energy is promising because it can directly be used for space heating by drawing heat from the hot rock and water within deep subsurface formations. Other systems, like the more commonly used shallow systems, only produce usable geothermal energy after increasing the heating-loop’s water temperature through heat pumps.
A benefit to using DDU geothermal energy over shallow systems with heat pumps is that DDU technologies reduce a site’s carbon footprint by reducing the amount of electricity used by the system. DDU technologies will also use fewer wells compared to using shallower well systems with heat pumps.
DDU geothermal energy has greater initial capital costs relative to shallow well GSHP systems since the DDU wells that need to be drilled must be much deeper. The uncertainty that is involved in picking where to drill the wells for DDU geothermal energy is another challenge, since it is difficult to predict with high accuracy the deep subsurface temperatures and, especially, the deep subsurface permeability, which often is dependent on highly variable natural fractures. While creating permeability at depth through hydraulic fracturing is an option, it is both costly and creates high risks of “short circuits” in the flow of water (or other heat mining fluids) between injection wells and production wells.
DDU could be a more competitive option if future analyses find higher geothermal temperatures at shallower depths, lower project costs and risks, or higher costs for competing energy supplies. This research would likely involve:
drilling a test well to measure deep subsurface temperatures and permeabilities (to reduce project risks).
A study was conducted to determine if well size and depth (and hence cost) could be reduced by designing the geothermal system to meet the heat demand most of the time but not all, with natural gas boilers supplementing the heat supply during the coldest time periods. The study found that levelized cost of heat (LCOH) was relatively inelastic to changes in geothermal system size in cases where the natural gas peaking system was used a specified percentage of the time. The findings suggest that for both retrofit and greenfield scenarios that reside within areas of similar subsurface and surface conditions, both will have similar DDU cost comparisons across different percentages of DDU heat use versus fossil energy heat use. Additionally, substantial price increases were seen ($115 and $134 per MMBTU for the greenfield and retrofit cases respectively), which suggests that the additional cost of installing a new peaking system would not be warranted from an economic viewpoint.
Using data from a previous study where an analysis was conducted to determine cost and performance data for installing circa 500 shallow wells for GSHPs, it was found that GSHPs were likely to be only 15% less costly than using a DDU approach. Given that GSHPs are a more mature technology than DDU and given the greater risks associated with the deep subsurface (where less is known), one can expect that once the DDU technology matures and more deep subsurface information becomes available, DDU could become the most cost-effective geothermal solution for district energy in many places.