فن آوری ها و اثرات زیست محیطی مبدل های حرارتی زمین در فنلاند Technologies and environmental impacts of ground heat exchangers in Finland

1. Introduction Together with Scandinavian countries and Canada, Finland belongs to the northernmost countries utilizing ground source heat pump (GSHP) technology on a large scale (Nowak & Murphy, 2012: 73, 101, 118, 132, 140). According to the Finnish Heat Pump Association nearly 8500 GSHP units were sold in Finland in 2016 (SULPU, 2017). GSHPs are installed in new buildings, and retrofitted in place of oil burners, electrical heating, wood furnaces and district heating. Since 2013 more than half of new detached houses in Finland have had a GSHP installed (Motiva, 2016: 11). A typical GSHP system in Finland consists of a borehole heat exchanger (BHE) and a vapor compression heat pump with either an inbuilt or a separate domestic hot water tank. Single U-pipes are most commonly used in BHEs. The GSHP system is connected to hydronic heat distribution, which is usually underfloor heating in new buildings and newer retrofit sites, or wall mounted water radiators in older retrofit sites. Horizontal ground heat exchangers (GHEs), in which a single, linear pipe is installed in series, are also used, while slinky or trench collectors are not used (cf. Florides and Kalogirou, 2007; Omer, 2008). Surface water heat exchangers are installed to a lesser extent, mainly in lakes and coastal areas of the Baltic Sea. Open loop heat exchangers are very rare in Finland. Ethanol is the most commonly used antifreeze in the GHEs whereas glycols are rarely used. Boundary conditions for the sizing and design of GHEs in Finland are set by the northern climate and distinctive geological conditions. The annual average ambient temperature in Finland varies from over 5 °C on the south coast to below −2 °C in parts of northern Finland, where the temperature may drop below −40 °C in wintertime (FMI, 2016a,b). Correspondingly, the annual average temperature of the ground surface varies from 8 °C on the south coast to 2 °C in the far north of Finland (GTK, 2017). The thermal conductivity of Finnish rocks is typically over 3 W/(m*K), and the geothermal gradient is 8–15 K/km (Kukkonen and Peltoniemi, 1998; Kukkonen, 2000). The bedrock in Finland generally consists of hard crystalline rocks, and sedimentary rocks are rare. Practically all of Finland is located on the Fennoscandian Shield, which is relatively unbroken and tectonically stable (Korsman and Koistinen, 1998; Plant et al., 2005). Due to the hard rocks in Finland, down-the-hole (DTH) drilling method is economically superior, and more efficient than any other method (cf. Rebouças, 2004). In practice, it is the only method applied to drill boreholes for BHEs in Finland. The rotating DTH hammer’s percussion is powered by compressed air (typical working pressure 35 bar), and the exhaust air is used to flush the drill cuttings out of the borehole (Jouni Lehtonen, personal communication 12 Nov 2016; Jimmy Kronberg, personal communication 24 May 2017). Another consequence of the hard rocks is that boreholes are mostly left ungrouted and usually remain open. The need for grouting is also decreased by the fact that groundwater table is in most cases within ten meters from the ground surface (Karro and Lahermo, 1999). A completely dry borehole indicates that the rock is solid enough to prevent groundwater movement, in which case the borehole is filled with water. Environmental and functional issues related to GSHP construction and use have been studied since the 1970s.