Ground Source Heat Pumps and Brownfield ReclamationGeoff Grisso
February 14, 2013 — 870 views
This short article is an invitation to the ground source heat pump community to engage with brownfield practitioners involved in remediation of soil or groundwater to enhance the efficiency of both energy exchange and remediation. Brownfield sites have been affected by former uses of the site or surrounding land; are derelict or underused; are mainly in fully or partly developed urban areas; may have real or perceived contamination problems; and require intervention to bring them back to beneficial use.
Far from all brownfields are contaminated. However those that are contaminated and require remediation to ensure they are suitable for their intended use offer opportunities for ground source heat pump that can increase the economic attractiveness of GSHP solutions and contribute significantly to a positive evaluation of the sustainability of the overall reclamation. Process based remediation technologies (Nathanail et al. 2007) can create opportunities for GSHP to re-energise brownfield sites and greatly enhance the cost effectiveness of such solutions but only if remediation and re-energising are considered in a timely and integrated manner.
Groundwater in urban areas is often polluted with volatile organic compounds such as hydrocarbons and chlorinated solvents. The way in which such groundwater is remediated may also allow energy exchange processes to take place. Pump-and-treat involves pumping groundwater out of the ground, treating it and then reinjecting the clean water back into the ground. Since the energy used to pump the groundwater is accounted for against the remediation process, it is ‘free' to any energy exchange process that could exploit the energy storage capacity of the water. On a large scale this being carried out in the Netherlands where water is pumped to protect areas from inundation. On smaller scale industrial sites, pump and treat is seen as an expensive long term – quasi permanent – means of containing groundwater pollution. Existing pump and treat schemes may offer low cost quick win opportunities to demonstrate the benefits of GSHP on industrial sites.
Permeable reactive barriers (PRB) were developed in the 1990s as an alternative to pump and treat. Instead of pumping the groundwater out of the ground, a treatment zone is placed into the ground to intercept natural groundwater flow. Polluted groundwater flows through the treatment zone and emerges clean down gradient of the PRB. The residence time within the PRB is an important design consideration. Heat exchange infra structure can be installed immediately adjacent and downstream of the PRB at little extra cost – the bulk of the permitting and excavation expense is charged to the remediation works.
Contaminated soils are sometimes treated in situ or capped but most often are excavated for treatment or off site disposal. Where excavation is involved, the resulting void is usually backfilled with imported inert material to make up the site levels. However the expensively created void could be seen as an asset. In a GSHP context, the void could allow the installation of shallow GSHP infrastructure with the cost of excavation charged to the remediation.
The above discussion has shown how costs charged against remediation can be piggy-backed by GSHP solutions to enhance the overall sustainability of brownfield reuse and make GSHP more economically viable.