Design of a shallow ground geothermal heat pump system for space conditioning of buildings

Abstract

In the EU Directive 2010/31/EU on the Energy Performance of Buildings, the European Union has reinforced that all new buildings shall be nearly zero-energy buildings as of January 2021 [1]. Such a concept is conceived as a solution for the mitigation of CO2 emissions and for the reduction of energy use in the building sector. Given that the majority of new buildings are normally characterised by high rise structures, the roof and façade for harnessing solar energy would still be quite limited to cover the building‘s energy needs to bring it down close to zero-energy. Moreover, given that the major energy consumption in buildings is for space heating and cooling, the use of heat pumps coupled with ground-source heat exchangers can be considered as high efficiency alternative systems that can contribute towards energy reduction as well as produce renewable energy as stipulated in the Renewable Energy Directive 2009/28/EC [2]. The main purpose of this study was to investigate the potential of using shallow ground as a heat exchange medium for space heating and cooling of buildings in Malta. Shallow ground as opposed to deep boreholes offers a particularly important advantage of causing no concern to interference with the water table, which is a particularly sensitive issue in Malta. Moreover, the energy needs to prepare the geothermal field and operate it in terms of pumping power would be lower, thus making the whole setup economically more attractive than deep ground heat exchangers. The study involved the monitoring of the shallow ground temperatures in Marsaxlokk; an energy demand simulation using the software DesignBuilder-EnergyPlus to determine the total heating and cooling demand of an experimental building; a study of the soil thermal conductivity at different moisture content levels by means of hot box experimental tests; and an analysis of the effect of such parameters on ground source heat pump performance. These were followed by a design of an optimal heat pump system coupled to a ground source heat exchanger, whose calculations were based on the standard UNI 11466, to provide space heating and cooling for the experimental room at the Institute for Sustainable Energy in Marsaxlokk. Results showed that the soil thermal conductivity at 10% moisture content is around 1.56 W/mK and that the average soil temperature at 2.3m depth in ground was 15.16°C in February and 22.26°C in August, being the two extreme periods of the year. Findings have also illustrated that buildings in Malta have higher energy loads in summer than in winter. Such loads can be reduced by 45.8% if wall and roof insulation is added to the building and if double glazed apertures with shading are installed. Such reduction in load results in requiring 33.2% less ground heat exchanger to meet the total energy demand. This study concluded that energy efficient buildings can easily opt for this technology, either by installing the ground heat exchanger in a backyard if available, or by utilizing the space area under or around new buildings.