ENERGETIC STUDY OF A SOLAR ASSISTED GROUND SOURCE HEAT PUMP SYSTEM FOR DOMESTIC HEATING WITH PARAMETRIC ANALYSES VIA SIMULATION

In this thesis, a solar assisted ground source heat pump system is energetically evaluated for the UK Midlands climate conditions. The conducted research is supported by an experiment made by De Montfort University. This consisted of seven photovoltaic-thermal collectors and a novel very shallow geothermal heat exchanger, supplying a heat pump used to heat a small house. The detail mathematical model of the system was formulated, and parametric analyses were conducted via simulation. Parametric analyses were carried out by assuming two dwelling types, one new and one refurbished, and by varying the number of the solar collectors and the size of the novel very shallow geothermal heat exchanger (1.5 m deep). Additionally, two potential system configurations and two locations of geothermal heat exchanger had been assumed (the one to be exposed and the other to be beneath the dwelling).

The important aspects of the research were: a) to determine the parameter which influence the systems energy performance and to define the energetically better system configuration, b) to identify the heat and electricity independence of the system from conventional energy sources, c) to examine the metrics in which the energy evaluation of the systems can be based on and d) the potential reduction of greenhouse gases emissions against a natural gas boiler-based system and a ground source heat pump system. Within the current work, no economic evaluation of the system has been conducted.

The results analysis has shown that the higher ratio of generated to consumed electricity is the index which illustrates the system with the best energy performance. This ratio is a valid approach only if the auxiliary heat required by the systems is added to the consumed electricity, by being assumed to be offered via electricity. By contrast, the seasonal performance factor, which is used widely, was diagnosed as an unappropriated metric to describe the energy performance of the systems. Additionally, the concept of the specific productivity was used to identify improvements on the performance of the systems caused by parameters variation.

Based on evaluation made via the studied metrics, the topology with direct use of solar heat was found to get lower efficiency than this without direct use. As regards the heat autonomy of the systems, this was found to be up to 83% for the new dwelling and 73% for the refurbished one. Similarly, the electricity offered by the photovoltaic-thermal collectors achieved a self-sufficient stage for most schemes paired with the new dwelling. However, the system paired with the renovated building did not manage to get more than the 90% of its electricity to be offered by the collectors. A potential solution for greater power coverage from the solar energy may be a more efficient photovoltaic-thermal technology. Though, the key role of the system heat coverage was found to be the size of the geothermal heat exchangers and the area of the solar collectors. Hence, for a totally geothermal system paired with the new dwelling, the fractional heat coverage was estimated to be between 0.33 and 0.69 for the smallest geothermal heat exchanger of 16 and the largest one with 40 borehole heat exchangers (1.5 m deep), respectively. By adding 20 PVTs on the larger geothermal heat exchanger, the factional heat coverage was increased to 0.83 and to 0.70 for the smallest number of borehole heat exchangers. Likewise, the refurbished dwelling with only 16 borehole heat exchangers was found with 0.31 heat coverage and with 0.55 for the largest geothermal heat exchanger. By adding 20 PVT collectors on both borefields the heat coverage was increased to 0.65 and to 0.74 for the smaller and the larger geothermal heat exchanger, respectively.

It was found that higher energy performance can be obtained when the geothermal heat exchanger is exposed, instead of being placed beneath the new dwelling. The superiority of the geothermal heat exchanger to be installed at an uncover space instead of being placed beneath the dwelling applies to all the investigated configurations and sizes of the system. Thus, the largest system with 20 PVTs was estimated with ratio between the generated to consumed electricity of 1.9 for the exposed location, against the ratio of 1.5 which was found for the exchanger to be covered by the dwelling. Similarly, for the smallest array of 4 PVTs, the ratio was estimated to be 0.26 and 0.22 for the exposed and the covered by the dwelling choice accordingly.

Lastly, the investigated system was found with lower carbon emissions than the natural gas boilers system at all the investigated configurations and for both dwelling types. The gas boiler system was estimated to release 1509.7 kg CO2e year-1 and 3050.9 kg CO2e year-1, when it is installed in the new and the refurbished dwelling accordingly. With the new dwelling, all configurations with more than 12 PVTs were found capable to decarbonize in total the emissions from the natural gas boiler system, while for the energy renovated dwelling the fraction emission savings were estimated up to 0.8. Also, the proposed system was estimated less emissive than the conventional ground source heat pump system for PVT arrays with more than 4 collectors and with more than 8 collectors for the new and the refurbished dwelling, respectively.