Future Infrastructure: The Environmental Performance of District Open Loop Geothermal Heat Pump Systems
Geothermal heat pumps capitalize on renewable thermal energy exchange in the shallow subsurface to offer well-documented energy savings of 35 40% over conventional heating and cooling equipment. This research was undertaken to evaluate additional savings that may be available by using geothermal heat pumps in a district configuration. A metering system was installed in a district open loop system in Warren, Pennsylvania. Multiple buildings conditioned with distributed heat pump loops are connected to the district and two of these buildings were also metered.
In the initial year of metering, the site energy use intensity (EUI) for those two mixed-use buildings was 38.69 kBTU/ft2 and 32.56 kBTU/ft2 respectively, 36% to 47% savings compared to conventionally-conditioned buildings with similar commercial and residential mix that meet the same energy code. When electricity for the district loop pumps and equipment was added to each building’s energy consumption, energy use increased to 57.81 kBTU/ft2 and 52.14 kBTU/ft2, trimming savings to 3% to 16%. During the time of this study, the district served only about 30% of its capacity and oversized well pumps contributed to energy inefficiency in the central system. If the district were built to capacity and smaller well pumps were installed, the energy savings could be up to 39% compared to conventionally conditioned buildings. Even with these savings, however, the specific district configuration evaluated here does not clearly confer am energy advantage over stand-alone buildings with geothermal heat pump systems. Greater savings may be possible if the district loop allows load balancing among buildings within the district, similar to the load balancing that occurs within a building’s distributed heat pump loop. A possible design is proposed.
Because heat pumps are typically powered by electricity, this research includes an analysis of associated carbon emissions as an important aspect of sustainability. In so doing, it extends existing quantitative approaches to determine the carbon intensity at which all-electric geothermal heat pumps become environmentally preferable to other types of mechanical conditioning in commercial buildings with both heating and cooling. In addition, a site-specific natural gas-to-electricity carbon emissions ratio is presented as a method for rapid assessment of the climate change impact of an all-electric geothermal heat pump system versus a system that uses natural gas for heating. With this method, at 2012 CO2 emissions rates, geothermal heat pumps of average efficiency use less source energy and produce fewer CO2 emissions than natural gas equipment in almost half of US states. By 2030, if states meet US EPA CO2 emissions reduction goals, geothermal heat pumps will be the equipment of choice for lower source energy and lower CO2 emissions in at least 41 states and perhaps as many as 46 states, depending on further improvements in their efficiency. Increased technical education about geothermal heat pumps and investment in ground heat exchanger infrastructure are recommended.
Sustained energy and environmental gains of open-loop geothermal systems can only be achieved with appropriate engineering design and maintenance of the system. Within the first two years of monitoring the Warren system, it became apparent that potential fouling of heat exchangers and pipes is the lynchpin of long-term reliability for these systems. This dissertation links recent developments in microbiology about the role of bacteria and oxygen in fouling with the bacteria and oxygen in open loop geothermal wells. It posits that bio-fouling is almost inevitable regardless of the level of iron in the groundwater and outlines critical associated system design responses. It also includes a new protocol for groundwater analysis and threshold levels of chemical and biological constituents that trigger the need to engage a corrosion specialist during system design. Finally, in collaboration with specialists in the National Ground Water Association (NGWA), the dissertation offers hydrogeologic guidelines for large scale geothermal heat pumps systems since the mechanical engineering literature lacks key fundamental siting considerations for both open loop and closed loop systems. These guidelines support the development of enduring systems that protect owner investments, the nation’s groundwater, and public health.
Overall, low-temperature geothermal heat pump systems offer the potential for a high level of sustainability with the contribution of renewable thermal energy exchange in the shallow subsurface. For that potential to be consistently realized, however, the building sector needs a scientific understanding of how these systems interact with the environment, and engineering guidelines for system design and maintenance to ensure their long term performance. This dissertation is an effort to advance that understanding and systems engineering.
- Doctor of Philosophy (PhD)