CO₂ savings of ground source heat pump systems – A regional analysis

Abstract

In the current study the savings of CO₂ emissions due to the use of ground source heat pump (GSHP) systems was investigated in comparison to conventional heating systems. Based on a subsidy program for GSHP systems in southwest Germany, the regional, average, and total CO₂ savings of 1105 installed GSHP systems were determined on a regional scale. The emitted CO₂ per kWh of heating demand for the studied scenario resulted in 149 g CO₂/kWh for GSHP using the German electricity mix and 65 g CO₂/kWh using the regional electricity mix, which results in CO₂ savings of 35% or 72%, respectively. Similar CO₂ avoidances of GSHP systems were found in American and European studies ranging between 15% and 77% strongly depending on the supplied energy for the heat pumps and the efficiency of installation. The resulting CO₂ savings for one installed GSHP unit in the present study therefore range between 1800 and 4000 kg per year. Nevertheless, the minimum average total annual CO₂ savings of all installed GSHP systems due to the subsidy program amounted to 2000 tons per year. The maximum regional avoided additional CO₂ emissions are primarily associated with the affluent suburbs of the most densely populated area in the region. In 2006 the total contribution of CO₂ savings due to GSHP systems in Germany was only about 3.4% of the total renewable energies. However, continuously rising numbers of installed GSHP units and the increasing use of renewable electricity demonstrate that there is a fine opportunity to substantially avoid additional CO₂ emissions associated with the provision of heating (and cooling) of buildings and other facilities.

Introduction

The impact of renewable energies in the global energy market is significantly increasing (e.g.[1]). They are considered to be a very important factor in avoiding of additional CO₂ emissions while helping to enhance energy security and improve sustainability [2]. Geothermal energy is a renewable energy resource that can be used to provide electricity, heating, and cooling of commercial and domestic buildings and other facilities (e.g. [3], [4], [5], [6]). In 2000, for example, the electrical energy generated by geothermal energy was 49.3 billion kWh/year, representing only 0.3% of total global electrical energy [6].

Shallow geothermal ground source heat pumps (GSHP), which are mainly used for heating and cooling of buildings, have had the most significant impact on direct use of geothermal energy. In 2004 a total of around 1.3 million GSHP systems were installed worldwide with a total thermal capacity of around 15.3 GW (Fig. 1). In 2005 GSHP systems had the largest installed capacity, accounting for 54% and 32% of the worldwide geothermal energy capacity and use, respectively [7]. They are very widespread in the United States, Denmark, and Sweden, and their numbers keep increasing throughout Europe [8]. In Germany, for example, from 2005 to 2006 the number of installed GSHP more than doubled from 10,965 to 24,195 units (BWP [9]). A total of around 100,000 GSHP systems are currently installed in Germany. Despite this fact, in 2006 shallow geothermal energy amounted to only 3.8% of the total heat supply from renewable energies in Germany, from a total of 72.7 TWh/year. Hence, knowing that renewable energies only constitute 5.3% of the total heat supply in Germany, the heat generated by GSHP systems is currently insignificant. Nonetheless, increasing numbers of installed GSHP demonstrate clearly the prospective future of this technique.

GSHP systems use most commonly electricity and less frequently gas to operate their heat pumps. The ratio between output heat to supplied energy of GSHP is defined as the Coefficient of Performance (COP). A typical heat pump has COP of around 4 (e.g. [10]) indicating that the heat pump produces four units of heating energy for every unit of electrical energy input [11]. Nevertheless, a COP of a heat pump can even be higher depending mainly on the difference between target and input temperature of the brine from the ground circuit and, therefore, on geological conditions and technical parameters of both the heat pipe and the building [8]. In contrast, an electric heater has only a COP of 1. Hence, due to the generally high COP of a heat pump and the utilisation of solar and geothermal energy stored in the subsurface, GSHP systems are capable to lower additional CO₂ emissions in comparison to other conventional heating methods such as oil-fired heating. Thus, the use of GSHP for heating and cooling of residential and commercial buildings can significantly reduce the emissions of global greenhouse gases such as CO₂ and SO₂. A study by the American EPA [12] could demonstrate that residential fossil fuel heating systems in the USA produced anywhere from 1.2 to 36 times the equivalent CO₂ emissions of GSHP systems. Hence, avoiding additional CO₂ emissions from 15% to 77% could be achieved through the application of GSHP systems [12]. A European study using an average European CO₂ emission for electricity production of a CO₂ equivalent of 550 g CO₂/kWh showed that electrically driven heat pumps avoid additional CO₂ emissions by 45% compared with an oil boiler and 33% compared with a gas fired boiler [13].

The main objective of the current study is to determine the avoidance of additional CO₂ emissions due to the use of GSHP in comparison to conventional heating systems on a regional scale. Furthermore, the study demonstrates how much CO₂ can be saved by the application of ground source heat pumps. A regional analysis based on a subsidy program of GSHP was performed in southwestern Germany using a geographic information system (GIS), showing the regional achieved CO₂ savings. The subsidy program was carried by the EnBW Energie Baden-Württemberg AG (EnBW) starting in June 2005, continuing an initiative from the Ministry of Environment of the state of Baden-Württemberg [14]. The regional environmental impact and in particular the CO₂ savings due to the subsidy program and the installed GSHP systems is discussed for the state of Baden-Württemberg (BW) in the southwestern part of Germany.