Shallow gravel aquifers and the urban 'heat island' effect: a source of low enthalpy geothermal energy

Northern European countries with no high temperature geothermal resources can utilise the urban 'heat island' effect to generate low enthalpy geothermal energy for space heating/cooling systems in buildings, provided a suitable aquifer underlies the urban area. Buried valleys, formed at the height of the Pleistocene glaciation 15,000 years ago, when sea level was 130 m lower than present, and infilled with gravels as sea level rose again at the end of the Pleistocene, underlie many European cities. These high yielding aquifers exist at only a few metres depth, and can provide a supply of groundwater at temperatures elevated 3-4 K above the average rural groundwater temperatures. This can produce a marked improvement both in the output and in the efficiency of a geothermal system making use of this source. When passed through a heat pump operating at a Coefficient of Performance (COP) of 4.5:1, a well yielding 20 1/s of groundwater at 13degreesC can generate 865 kW heat, sufficient to supply space heating for buildings with a footprint in excess of 12,000 m(2) with a peak heating intensity of 70 W/m(2). The economics of this low enthalpy geothermal energy source are outlined. Although development costs are minimal, at current low natural gas fuel prices in Ireland, heating-only applications will be less attractive, and a real cost saving will only accrue if dual heating/cooling functions can be developed. (C) 2003 Published by Elsevier Ltd on behalf of CNR.