Energy and Buildings 42 (2010) 1552–1560 Contents lists available at ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild The development of a finned phase change material (PCM) storage system to take advantage of off-peak electricity tariff for improvement in cost of heat pump operation Francis Agyenim ∗ , Neil Hewitt School of The Built Environment, University of Ulster, Newtownabbey, Co. Antrim, BT37 0QB, UK article info Article history: Received 12 February 2010 Received in revised form 2 March 2010 Accepted 13 March 2010 Keywords: RT58 phase change material Heat pump Economy 7 Off-peak electricity tariff abstract An experimental system consisting a longitudinally finned RT58 phase change material (PCM) in a hori- zontal cylinder has been conducted to evaluate the heat transfer characteristics of RT58. The investigation forms part of a wider study to investigate a suitable PCM to take advantage of off-peak electricity tar- iff. The system consisted of a 1.2 m long copper cylinder filled with 93 kg of RT58 with an embedded finned tube at the centre to serve as a heat transfer tube. The experimental data has been reported using hourly temperature profiles, isotherm plots, overall heat transfer coefficients and energy stored. The results show a quadratic relationship between heat transfer coefficient and the inlet HTF temperature within temperature range (62-77 ◦ C) investigated. Increasing charge inlet heat transfer fluid temperature by 21.9% increased heat transfer coefficient by 45.3% during charging and 16.6% during discharge. The potential implication of integrating PCM storage system to an air source heat pump to meet 100% resi- dential heating energy load for common buildings in UK has demonstrated that with an improvement in heat transfer, store size can be reduced by up to 30%. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Variable electricity rates are charged by utility companies to reflect the high cost of providing electricity during the peak periods of the day. For example in Northern Ireland (United Kingdom), the economy 7 system charges 15.89 p/kWh during the peak period (8 am to 1 am) and only 6.39 p/kWh during off-peak period (1 am to 8 am) [1]. Cost of saving can therefore be achieved for both the consumer and the utility company if heat pumps can be operated during the off-peak periods in the night to store heat for use dur- ing the peak periods of the day. Most studies conducted on heat pumps have investigated the potential for efficiency improvement and capacity adjustment through the improvements in compres- sors, evaporators, condensers, expansion valves, heat exchangers and working fluid mixtures [2–22]. Hewitt et al (2008) [7] devel- oped a high temperature air source heat pump and investigated the performance using air temperatures of -12 ◦ C to +15 ◦ C to heat water up to 60 ◦ C. Performance across the temperature range was presented, as well as detailed defrost energy analysis. Results from a field trial were also presented. The authors acknowledged cost, adoption of ground source heat pump to existing buildings (lim- ited to more affluent areas) and the suitability of radiators to heat ∗ Corresponding author. Tel.: +44 2890 368 227; fax: +44 2890 368 239. E-mail address: aboffour@hotmail.com (F. Agyenim). pump operations as the main challenges in heat pump develop- ment. They also concluded that the provision of a buffer tank or energy storage system would allow the use of low cost off-peak electricity tariffs to reduce running costs. Few studies have been conducted to investigate the use of energy storage to improve the performance of heat pumps (heating and cooling) systems. Wang et al (2009) [23] conducted a study of underground thermal stor- age in a solar-ground coupled heat pump system for residential buildings. The authors reported an experimental efficiency of the underground storage based on solar energy collected to be up to 70%. The performance of the store was reported to depend on inten- sity of solar radiation and the optimal ratio between the store (tank) volume and collector area and recommended a ratio of 20 l/m 2 to 40l/m 2 . Okamoto (2006) [24] reported a two year study on the performance of a heat pump system utilizing heat collected from seawater installed in an aquarium to provide simultaneous heat- ing and cooling. Graphical presentations of the results included seawater and air temperatures, daily and hourly loads, electrical consumption, energy output, consumption of buildings in summer and winter and the coefficient of performance (COP) of the sys- tem. Okamoto (2006) [24] reported a COP of 3.4 for cooling and 2.8 for ice formation with a significant fraction of peak electri- cal demand during the day shifted to the off-peak period during the night thereby helping to save cost as a result. Little attention has been devoted to the improvement in cost of operation of heat pumps through the use of phase change energy storage system. 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.03.027