The effect of urban heat islands on geothermal potential: examples from Quaternary aquifers in Finland

Hydrogeology Journal (2014) 22: 1953–1967 DOI 10.1007/s10040-014-1174-5 The effect of urban heat islands on geothermal potential: examples from Quaternary aquifers in Finland Teppo Arola & Kirsti Korkka-Niemi Abstract The use of renewable energy can be enhanced by utilising groundwater reservoirs for heating and cooling purposes. The urbanisation effect on the peak heating and peak cooling capacity of groundwater in a cold groundwater region was investigated. Groundwater temperatures were measured and energy potentials calculated from three partly urbanised aquifers situated between the latitudes of 60° 25′N and 60° 59′N in Finland. The average groundwater temperature below the zone of seasonal temperature fluctuations was 3–4 °C higher in the city centres than in the rural areas. The study demonstrated that due to warmer groundwater, approximately 50–60 % more peak heating power could be utilized from populated areas compared with rural areas. In contrast, approximately 40–50 % less peak cooling power could be utilised. Urbanisation significantly increases the possibility of utilising local heat energy from groundwater within a wider region of naturally cold groundwater. Despite the warming in urban areas, groundwater still remains attractive as a source of cooling energy. More research is needed in order to determine the longterm energy capacity of groundwater, i.e. the design power, in urbanised areas of cold regions. Keywords Groundwater management . Urban groundwater . Cold region . Finland Introduction The use of renewable energy systems (RES) is expected to increase globally, as it provides local energy that is not dependent on the international energy transport market Received: 29 January 2014 / Accepted: 21 July 2014 Published online: 17 A ugust 2014 * Springer-Verlag Berlin Heidelberg 2014 T. Arola ()) Department of Geosciences and Geography c/o Golder Associates Oy, University of Helsinki, Apilakatu 13B, 20540, Turku, Finland e-mail: teppo_arola@golder.fi Tel.: + 358 40 7676008 K. Korkka-Niemi Department of Geosciences and Geography, University of Helsinki, Gustaf Hällströmin katu 2a, Helsinki, 00014, Finland and reduces the emission of greenhouse gasses (Andea et al. 2010). The EU is promoting the use of RES in Europe through Directive 2009-28-EN. This directive has set a target of 38 % for the share of RES in the gross overall energy consumption of Finland by 2020. In 2010, RES accounted for 32.2 % of the overall energy consumption of Finland, 3.2 % of which was produced by heat pumps (Statistics Finland 2012). Although heat pumps produced only a minor proportion of the total Finnish RES consumed in 2010, the amount of energy from heat pumps is expected to rise from approximately 4 TWh at present to 8 TWh by 2020 (Ministry of Employment and the Economy 2010). One solution to increase RES is to exploit groundwater energy by means of heat pumps or heat exchangers. One technique used to exploit groundwater energy is called an open loop energy system or open system (Bonte et al. 2011; Haehlein et al. 2010). Open loop heating and cooling systems extract thermal energy from and/or discharge waste heat into bodies of water such as aquifers and lakes. Water is pumped from the body of water through a heat-transfer system and is returned to the environment at a lower temperature for heating applications and a higher temperature for cooling applications. In most cases, groundwater is pumped from an abstraction well and discharged into the subsurface via an injection well (Sanner 2001). If energy is exploited by a heat pump, the term groundwater heat pump (GWHP) system is also used. GWHP systems have been successfully used for energy purposes in North America and Europe since the 1920s (Banks 2012; Ferguson and Woodbury 2006). Large-scale groundwater utilisation experiments were conducted in Finland in the late 1970s and early 1980s with positive results (Iihola et al. 1988). The Finnish geological environment, where high-yielding glaciofluvial sand and gravel aquifers exist at a depth of only a few metres, provides easily exploitable energy reservoirs. Approximately 56,500 ha of aquifers in Finland, comprising 801 groundwater areas (Finnish Environment Institute 2012), are under urban or industrial land use (Finnish Environment Institute 2006). These aquifers are located around the country, and near or under all major cities. However, groundwater is not a widely used energy source in Finland. Yearly and daily groundwater temperature variations are minimal compared with temperature variations in the 1954 air or in lake or river water (Mälkki and Soveri 1986; Silliman and Booth 1993). Groundwater temperatures to depths of approximately 10–25 m are generally equal to the mean air temperature in moderate and warm climates (Banks 2012; Budel 1982; Kasenov 2001). For example, according to the Irish Meteorological Service, mean annual air temperatures in Ireland generally range between 9 and 11 °C. By comparison, the average groundwater temperatures range between 8 and 11 °C (Allen et al. 2003). The mean air temperature in Finland was approximately 2.3 °C during the time period from 1981 to 2010 (Tietäväinen et al. 2010), while the mean groundwater temperature varied from 3.0 to 6.6 °C (Mälkki and Soveri 1986; Oikari 1981). Similar results demonstrating higher groundwater than surface temperatures have been reported from other northern areas such as Russian Siberia (Banks et al. 2004; Parnachev et al. 1999), Canada (Ferguson and Woodbury 2004; Parsons 1970) and Sweden (Rosen et al. 2001). These temperature differences are mainly due to two reasons—firstly, in winter the snow cover functions as an insulator, preventing cold air conduction into the subsurface layers; secondly, frost is formed when topsoil is cooled below the freezing point of water. This change in the state of water releases latent heat into the soil (McKenzie et al. 2007; Soveri 1985; Woo and Marsh 2005). Frost also acts as an insulator, reducing the flow of cold meltwater into deeper soil layers in early spring, when the melting of snow begins (Soveri 1985). Urbanisation increases the air and subsurface temperature in cities (Bornstein 1968; Ferguson and Woodbury 2004; Oke 1973; Preston-Whyte 1970). This phenomenon is called the urban heat island (UHI) effect (Howard 1818 in. Landsberg 1981). The air UHI effect is dependent on numerous factors such as the size of the city and the population (Karl et al. 1988; Oke 1973); even solitary shopping centres have been found to form minor local UHIs (Suomi and Käyhkö 2011). Many studies have suggested that the main reason for the UHI effect is the replacement of natural vegetation by artificial surfaces such as concrete and tarmac (Allen et al. 2003; Cotton and Pielke 1995; Landsberg 1981). According to Ferguson and Woodbury (2004) and Leppäharju (2008), the heat loss from buildings increases the subsurface temperature by several degrees. Since air and subsurface so (...truncated)