An experimental characterisation of ground and air source heat pump technologies in the Irish maritime climate

Abstract

As energy derived from hydrocarbons now constitutes 87% of worldwide energy consumption there has been mounting concern about the potential negative environmental, economic and sustainability impacts of such over reliance on fossil fuel. Recent national and international policies reflect these concerns by setting targets for renewable energy supply by 2020 and 2050. Ireland has committed to achieve 12% renewable derived thermal energy consumption by 2020 and heat pump technologies are targeted as a key contributor. However, this project responds to the absence of climate specific design knowledge for the Irish Maritime climate and the need for effective source side management. This study addressed this knowledge deficit by developing a unique test facility with the capacity to characterise both ground-source and air-source heat pumps. It consisted of an automated weather station; 15kWth horizontal collector ground source heat pump (GSHP hc); 15kWth vertical collector ground source heat pump (GSHPyc); 8kWth air source heat pump (ASHP), along with 111 sensors and supporting data acquisition system which allowed the characteristics of the climate, collector and heat pumps to be continuously monitored. This facility was operated for 745 test days between 2007 and 2009, during which time 22 tests were conducted and 168,522kWh (606GJ) delivered. The average Seasonal Performance Factor (SPF) for the GSHPhc, GSHPvc and ASHP were 2.90, 2.95 and 3.74 respectively, highlighting the suitability of the Maritime climate for ASHPs. The impact of climate on the ground’s upper layer has been characterised in terms of ground temperature and moisture content and its impact on heat pump Coefficient Of Performance (COP) has been quantified. The influence of heat pump duty and collector design parameters such as soil type and ground cover type has also been assessed. A new parameter known as the Collector Performance Indicator (CPI) has been established to allow horizontal collector performance to be quantified. A numerical model has been developed to assess the performance of a new climate sensitive split level horizontal collector that delivered an 8% higher SPF than the standard collector design. Potential exists to boost performance by a further 7% using more effective source side management techniques. Suggestions were also made to insulate a portion of the vertical collector return pipe; that would boost performance by 5%. The study concludes with a series of recommendations that would further exploit the potential of the test facility and test data and boost the contribution of heat pumps in a sustainable energy fuelled future.