Global warming’s impact on the performance of GSHP

Abstract

Since heating and cooling systems of buildings consume 30–50% of the global energy consumption, increased efficiency of such systems means a considerable reduction in energy consumption. Ground source heat pumps (GSHP) are likely to play a central role in achieving this goal due to their high energy efficient performance. The efficiency of GSHP depends on the ground temperature, heating and cooling demands, and the distribution of heating and cooling over the year. However, all of these are affected by the ongoing climatic change. Consequently, global warming has direct effects on the GSHP performance. Within the framework of current study, heating and cooling demands of a reference building were calculated for different global warming scenarios in different climates i.e. cold, mild and hot climate. The prime energy required to drive the GSHP system is compared for each scenario and two configurations of ground heat exchangers. Current study shows that the ongoing climatic change has significant impact on GSHP systems.

Introduction

Independent of what causes the global warming (GW), performed analyses of borehole temperature–depth profile evidently confirm that there is continental-scale change in the Surface Air Temperature (SAT) [1], [2]. This is concluded from the general pattern of temperature propagation into the subsurface though there are individual boreholes showing decreases or no significant change in ground temperature profile [1]. Other boreholes reveal that local modification of the surface, e.g. by constructions, results in a significant temperature change [3]. However, in general, the ground temperature change is a direct consequence of a change in SAT [1]. Temperature compilations in Fig. 1 show that SAT has increased 1.4 °C from 1880 to 2008 [4]. In recent decades, studies show that the global temperature increase was greater in high latitudes of the northern hemisphere [5].

By ignoring the hypothesis that current warming is part of a natural cycle [1], [2], two causes of the warming have been suggested: (i) related to the accumulation of greenhouse gases in the Earth’s atmosphere [1], which is generally accepted by a majority of researchers; and (ii) related to heat emissions [6], [7]. Both these explanations imply that current warming is anthropogenic and lead to the conclusion that more efficient use of energy and increased use of renewable energy are the best ways to counteract GW.

Since heating and cooling systems of buildings account for 30–50 % of the global energy consumption, increased efficiency of such systems means a considerable reduction in global energy consumption [8], [9]. Ground source heat pump (GSHP) systems are considerably more energy efficient than conventional heating and cooling systems [10], [11], [12], [13], [14]. The GSHP system is therefore an alternative that is likely to play an important role in slowing down the climatic change. At beginning of 2010 the totally installed capacity in the world was 50,583 MW producing 121,696 GWh/year. Sweden alone, which has the third biggest installed capacity (after USA and China), has about 4460 MW producing 12,585 GWh/year [15]. In such systems, thermal energy is extracted from the ground during the heating season and injected into the ground during the cooling season. Therefore, the thermal performance of GSHP depends on the ground temperature.

The energy demand for heating and cooling largely depends on the ambient temperature [16]. Of the 150 million residential dwellings in the 15 EU Member States approximately 72% were built before 1972. A high proportion was built to standards that required far poorer thermal performance than those imposed under current building regulations [17]. Due to the poor thermal properties of the envelope of these buildings, the heating and cooling demands are very sensitive to any change in ambient temperature. Gaterell and McEvoy [17] show that increasing insulation reduces the impact of climatic change on heating and cooling demand.

As GW affects the ground temperature and energy demand of the buildings, changes in prevailing climate have direct consequences on the GSHP performance. The overall objectives of this study were to study how GW affects the heating and cooling demand of a certain building and thermal performance of a small size GSHP system.

The analysis comprises how GW affects:

• Annual heating and cooling energy demand of a building;

• Monthly share of annual heating and cooling demand;

• Maximum–minimum fluid temperatures extracted from the borehole;

• Efficiency, i.e. COP of existing GSHP systems;

• Driving energy of GSHP system.

Each stage mentioned above was applied for two different scenarios of GW for three different climatic conditions. These climatic conditions were selected based on the fact that they have approximately the same air temperature amplitude over the year (∼10.5 °C), and also represent different heating and cooling demand.