Performance Analysis of a Large-Scale Ground Source Heat Pump System
The UK government’s Carbon Plan-2011 aims for 80% carbon emission reduction by 2050, and the 2009 UK National Renewable Energy Action Plan has set a target of delivering 15% of total energy demand by renewable energy sources by 2020. Ground Source Heat Pump (GSHP) systems can play a critical role in reaching these goals within the building sector. Achieving such benefits relies on proper design, integration, installation, commissioning, and operation of these systems. This work seeks to provide evidence to improve the practices in design, installation and operations of large GSHP systems. This evidence has been based on collection and analysis of data from an operational large-scale GSHP system providing heating and cooling to a university building. The data set is of significance in that it is collected from a large-scale system incorporating fifty-six borehole heat exchangers and four heat pumps. The data has been collected at high frequency since the start of operation and for a period of three years. The borehole heat exchanger data is intended to form a reference data set for use by other workers in model validation studies. The ground thermal properties at the site have been estimated using a novel combination of numerical model and parameter estimation methods. The utility of the reference data set has been demonstrated through application in a validation study of a numerical borehole heat exchanger model. The system heat balances and power consumption data have firstly been analysed to derive a range of performance metrics such as Seasonal Performance Factors. Analysis has been carried out at the system and individual heat pump level. Annual performance has been found satisfactory overall. A series of analyses have been carried out to investigate the roles of circulating pump energy, control system operation and dynamic behaviour. Monitoring data from one of the heat pumps has also been analysed in further detail to make comparisons with manufacturer’s steady-state performance data and with consideration to variations in fluid properties. Some modest degradation from stated performance has been identified. The most significant operational factors accounting for degradation of overall system performance have been excessive pump energy demands and short cycling behaviour. Some faults in operation of the system during the monitoring period have also been identified. A series of recommendations are made as to ways to improve the design and operation of large-scale GSHP systems based on this evidence. These recommendations are chiefly concerned with better design for part-load operation, reduction in pump energy demands and more robust control systems.
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