Climate Variables That Influence the Thermal Performance of Horizontal Collector Ground Source Heat Pumps

The performance characteristics of new heat pumps are usually evaluated under standard test conditions in certified test laboratories prior to their market release. While this data allows potential customers an opportunity to compare different heat pumps under the same conditions it is difficult to assess how variations in operating conditions, particularly around horizontally oriented ground collectors impact on heat pump Coefficient Of Performance (COP). Indeed, harsh winter conditions of continental climates dictate that horizontal collectors are buried sufficiently deep enough to operate in a thermally stable environment, independent of the weather, but this is not as critical in milder maritime climates and shallower collectors that may be influenced by climate are used. This review paper therefore seeks to identify the key climate variables that have been shown to influence the efficiency of horizontal collector heat pump systems. The literature highlights the significant impact of soil moisture content on COP, but the extended relationship between climate, moisture content and COP has not been established. Historical climate data from both a continental and maritime climate is presented and key aspects of their respective weather patterns are compared to assess their capacity to influence soil condition and COP. A series of empirical models linking changes in soil moisture content to fluctuations in soil thermal conductivity, diffusivity and resistance are also presented so that the impact of climate on soil thermal energy content and heat transfer characteristics might be assessed. However, since no one study has experimentally determined the complex relationship between climate, soil heat transfer characteristics and heat pump performance, this paper concludes with an overview of an experimental test facility that allows this relationship to be established for horizontal collector heat pumps in maritime climates.