Modelling the Performance of Air Source Heat Pump Systems

Department of Mechanical Engineering Modelling the performance of Air Source Heat Pump Systems Author: Michael Eric Baster Supervisor: Professor John Counsell A thesis submitted in partial fulfilment for the requirement of the degree Master of Science Sustainable Energy: Renewable Energy Systems and the Environment 2011 Copyright Declaration This thesis is the result of the author’s original research. It has been composed by the author and has not been previously submitted for examination which has led to the award of a degree. The copyright of this thesis belongs to the author under the terms of the United Kingdom Copyright Acts as qualified by University of Strathclyde Regulation 3.50. Due acknowledgement must always be made of the use of any material contained in, or derived from, this thesis. Signed: M E Baster Date: 09/09/11 . 2 Abstract The objective of this project is the development of an air source heat pump-based heating system model which can be used to assess the impact of different methods of providing supplementary heating on heat pump performance. Generalised heat pump performance is represented by regression models developed from publically-available independent test data of 30 heat pump units. An inverse model of a radiator-based distribution system is used to integrate the heat pump model within the IDEAS methodology (Murphy et al, 2011). IDEAS is a newly-developed dynamic approach for the estimation of building heating energy requirements. It allows perfect control of internal temperature through the use of inverse dynamics, and enables the constraints placed on air source heat pump performance by different supplementary heating configurations to be assessed independently of any particular method of practical control. Two fundamental supplementary heating configurations are investigated: in the first, supplementary heating is delivered separately from the heat pump heating system; in the second, supplementary heating is integrated into the heat pump heating system. It was found that integrating supplementary heating placed constraints on heat pump performance; combined with radiator systems of less than optimal size, it lead to a 20% drop in system efficiency compared to separate supplementary heating. The model is also used to compare the energy costs and CO2 emissions of an air source heat pump system with optimally-sized radiators to those estimated for a gas condensing boiler. Emissions savings of 3 to 13% were found, depending on heating pattern used. Cost savings were dependent on an off-peak electricity tariff being used. 3 Acknowledgements I am grateful to my supervisor Professor Counsell and to Gavin Murphy for the valuable advice they provided over the course of this project. I would also like to thank my family for their support and encouragement. MEB 4 5 Section Table of contents Page 1 Introduction 8 1.1 Project objectives 9 1.2 Project overview 10 1.3 Note on terminology 11 2 Factors affcting air source heat pump performance 12 2.1 Principles of the heat pump cycle 12 2.2 Practical heat pumps 14 2.3 Improvements to the basic practical heat pump cycle 16 2.4 Defrost cycles 17 2.5 Heating systems and seasonal performance factor 19 3 Literature review 24 4 Methodology 28 4.1 IDEAS methodology 28 4.2 ASHP system model - overview 32 4.3 Radiator system model 34 4.4 ASHP model 37 4.5 Climate data and internal gains calculations 40 4.6 Heat pump sizing methods 41 5 Results 44 5.1 Supplementary heating configuration 44 5.2 Intermittent versus continuous heating 47 5.3 Comparison of methods of deriving radiator heat transfer 50 coefficients 6 Conclusions 52 7 Further work 54 Annex A Description of the heat pump model’s implementation in 55 Simulink Annex B Calculation of radiator heat transfer coefficient 72 Annex C Calculation of solar gain 76 References 79 6 List of figures 2.1 Idealised (reverse Carnot) heat pump cycle 2.2 Heat pump systems with different supplementary heating configurations 4.1 IDEAS model response to a step change in set point temperature 4.2 Simplified block diagram of heat pump model integrated into IDEAS 4.3 Manufacturers’ and calculated radiator heat transfer coefficients 4.4 COP regression model 4.5 factor regression model 5.0 Demonstration of ASHP model outputs 5.1 Heating system electricity use – ASHP capacity 6.3kW, continuous heating 5.2 Heating system electrical power consumption and return temperature over the first two weeks of December 5.3 Electricity use under intermittent and continuous heating patterns 5.4 Comparison of the methods of deriving radiator heat transfer coefficient A.1 Heat pump model integrated into IDEAS – Simulink block diagram A.2 Radiator subsystem block diagram – separate supplementary heating A.3 Heat pump system block diagram – separate supplementary heating A.4 Radiator subsystem block diagram – integrated supplementary heating A.5 Heat pump system block diagram – integrated supplementary heating B.1 Calculated radiator heat transfer coefficient B.2 Schematic of assumed radiator construction: view from above List of tables 4.1 Dimensions and thermal characteristics of building modelled 4.2 Sample of WPZ test data 5.1 Results of simulations of separate and integrated supplementary heating 5.2 Annual electricity costs under different tariffs and heating patterns – 4m2 radiators 7 1. Introduction Domestic heat pumps make use of the vapour compression cycle to absorb low grade heat from the external air and reject it at higher temperature inside a building. They can provide a highly efficient method of providing space and water heating: it is often stated that they can provide three times as much energy in the form of heat as they require in the form of electricity. They have the potential to reduce the costs, emissions and resource intensity of UK heating. The use of heat pumps for domestic heating is common in several Northern European countries, as well as Japan and North America. A report published by the European Heat Pump Association (2009b) states that nearly 130,000 units were sold in Sweden in 2008 and around 80,000 in both Norway and Germany. The report also found that air source heat pumps (ASHPs) are gaining in popularity relative to their more-established ground source cousins; in 2008, sales of ASHPs in the “space heating only” market segment overtook sales of ground source heat pumps for the first time. Though ground source heat pumps can be more efficient than ASHPs, they are more costly to install and more restricted in their applicability. The UK market for heat pumps, and ASHPs in particular, is relatively under- developed. A Scottish Parliament research report contained an estimate that in 2008 as few as 150 ASHPs were installed in the UK (Scottish Government Social Research, 2009). Government support for ASHPs, in the form of grants towards installation costs, has only been intermittently available since 2007 in the UK (2006 in Scotland). However, against a background of the ambitious emissions reductions targets introduced in the Scottish and UK Climate Change Acts, interest is growing in their potential benefits. In his book Sustainable Energy – Without the Hot Air, David MacKay shows that a heat pump powered by electricity generated at a centralised, highly efficient gas power station represents a significantly smaller gas use than a condensing boiler (Mackay, 2009, p146-154). He argues that all fossil-fuel heating should be replaced with heat pumps, and estimates that doing so in conjunction with building insulation and heating system control improvements would reduce the primary energy used for heating by 75%. From 2012, long-term support for ASHPs may be provided by the Renewable Heat Incentive. 8 MacKay’s arguments are based on heat pumps operating with an efficiency of 300 to 400%. However a recent UK field trial revealed that the efficiencies achieved in real installations varies widely (Energy Saving Trust, 2010). Most of the trial subjects were retrofit installations using radiator systems to distribute heat. A greater understanding of the reasons for this variation is required if the cost and emissions savings ASHPs could potentially provide is to be realised. 1.1 Project objectives The objective of this project is the development of an air source heat pump-based heating system model which can be used to assess the impact of different methods of providing supplementary heating on heat pump performance. A key requirement for the model is that it works within the IDEAS methodology. IDEAS is a newly-developed, simplified dynamic approach to the modelling of building heating requirements, designed to integrate with and augment the UK government’s SAP method for the energy rating of dwellings (Murphy et al, 2011). IDEAS is used for three main reasons: it has been calibrated against SAP, which in turn is based on extensive monitoring work of real houses; it incorporates inverse dynamics-based perfect control, meaning that heat pump performance can be assessed separately from the effect a particular control method; and the version used in this project has been implemented in the Matlab software package, and makes use of the Simulink graphical modelling environment. The flexibility this provides enables different heat pump system configurations to be examined. A second requirement is that the model must take account of the varying constraints placed on heat pump output and efficiency by the dynamic conditions in which it is operating. To determine these conditions, an inverse model of a radiator-based distribution system is developed. 9 1.2 Project overview The work undertaken for this project is broken down into five key stages. These form the main sections of this report. 1: Identification of the fundamental factors upon which heat pump performance depends. This involved gaining an understanding of the principles of the vapour compression cycle and important aspects of practical heat pump systems.

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