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Unfinished Business in Geothermal Energy

2008; Wiley; Volume: 47; Issue: 2 Linguagem: Inglês

10.1111/j.1745-6584.2008.00528.x

1745-6584

Grant Ferguson,

Groundwater flow and contamination studies

Geothermal energy has attracted a great deal of attention recently due to concern about greenhouse gas emissions and high energy prices. Low temperature geothermal energy or Earth energy systems have the potential to make a significant contribution to mitigating both of these problems, but there are unresolved issues surrounding our ability to develop these resources from both energy and environmental perspectives. To date, most developments have occurred under relatively pristine subsurface conditions, and there have been few concerns about neighboring systems. There are a few incidents of such problems, but these have received little attention. However, in much the same way that cones of depression from wells commonly extend beyond property boundaries, thermal anomalies associated with subsurface energy developments are also likely to affect adjacent properties. Ground water scientists and engineers are uniquely qualified to address this issue. The analogy between ground water and heat flow allows us to draw on our experiences in ground water resource development and source water protection. In many ways, the problem of distributing subsurface energy rights is similar to water rights. There are probably cases to be made to use first appropriation to assess future developments, but this approach is unlikely to optimize the environmental benefits of ground source heat pumps. A movement toward more intense management of developments to ensure the greatest reduction in greenhouse gases or at least fair access to the subsurface for thermal applications is likely more appropriate. This will require a greater understanding of subsurface heat flow and input from the scientific and technical communities. As with most human activities, there are environmental impacts associated with ground source heat pumps, and these concerns have not been well addressed in all cases. Most guidelines have gone to great length to address concerns about fluids used in heat exchangers and well construction issues, particularly where it is possible to connect otherwise hydraulically separate aquifers. However, a variety of other heat transport–related issues are not typically considered. An ongoing review of the existing National Ground Water Association (NGWA) guidelines for closed-loop heat pump installations (McCray, K., ed. 1997. Guidelines for the construction of vertical boreholes for closed loop heat pump systems. Westerville, Ohio: National Ground Water Association) by an NGWA working group found that guidelines for setback distances to wetlands and surface water bodies are probably too low and most appear to be arbitrary. A broader review of legislation on heat pumps (Hähnlein, S., P. Grathwohl, P. Bayer, and P. Blum. 2008. Cold plumes of ground source heat pumps: Their length and legal situation. Geophysical Research Abstracts 10. EGI2008-A-07946, Vienna: EGU General Assembly) found very few regulations but rather a wide range of guidelines on setback distances and allowable temperature changes, most of which do not appear to be strictly adhered to. The lack of a single guideline reflects the range of heat pump applications and variability in subsurface conditions and indicates that sites should be considered on an individual basis if unacceptable impacts are likely. This has occurred for larger developments in some areas, but criteria for allowing projects to proceed are debatable. A proposal to use heat pumps to heat a significant portion of a new suburb in Winnipeg, Canada, was denied due to potential impacts on aquatic habitats. One only has to examine the number of papers published on ground water–surface water interaction to appreciate the concern that such developments may have a negative impact on ecosystems due to changes in temperature from either closed- or open-loop systems or due to changes in ground water flow patterns from open-loop systems. What is unclear at this point is whether these impacts are acceptable if they result in lower greenhouse gas emissions and if we can develop geothermal energy in a manner that has minimal potential impact on the environment. This is a debate that needs to take place, and ground water scientists and engineers should be active participants in this discussion.