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A BSRIA Guide www.bsria.co.uk

Heat Pumps

A guidance document for designers by Reginald Brown

BG 7/2009

ACKNOWLEDGEMENTS

This publication has been written by BSRIA’s Reginald Brown with additional information provided by David Bleicher and Mike Smith, and has been designed and produced by Ruth Radburn.

BSRIA would like to thank the following for their help and guidance in reviewing the document:

Heat Pump Association Strategy Committee Steve Pardy, Zisman Bowyer & Partners

This publication has been printed on Nine Lives Silk recycled paper.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means electronic or mechanical including photocopying, recording or otherwise without prior written permission of the publisher.

©BSRIA BG 7/2009 October 2009 ISBN 978 0 86022 686 4 Printed by ImageData Ltd

HEAT PUMPS – A GUIDANCE DOCUMENT

CONTENTS

1 INTRODUCTION 1

2 HEAT PUMP FUNDAMENTALS 2 2.1 Heat pump technology and the economic carbon case 2 2.2 Refrigeration cycles and efficiency 3 2.3 Heat sources and sinks 4 2.4 Combined cycles 6 2.5 Integration of renewable energy 7 3 LEGISLATION 8 3.1 F-Gas Regulations 8 3.2 Handling of refrigerants 8 3.3 Pressure Systems Safety Regulations 9 3.4 Waste Electrical and Electronic Equipment Regulations 9 3.5 Building Regulations 9 3.6 Construction Design and Management Regulations 9 4 OUTLINE DESIGN 11 4.1 Sizing heat pump systems 11 4.2 Selection of heat source and sinks 14 4.3 Estimation of performance 15 5 DETAILED DESIGN OF HEATING AND COOLING SOURCE 16 5.1 Packaged units 16 5.2 Air source 17 5.3 Exhaust air source 20 5.4 Ground source 21 5.5 Ground water 28 5.6 Surface water 30 6 DETAILED DESIGN OF HEATING AND COOLING DISTRIBUTION 31 6.1 Principle 31 6.2 Air-to-air systems 31 6.3 Water (or brine) to air systems 36 6.4 Hydronic systems 36 6.5 Domestic hot water 40 6.6 Integration with conventional heat sources 42 6.7 Integration with renewable energy 44 7 SYSTEM CONTROLS 47

8 INSTALLATION 49 8.1 Delivery and positioning 49 8.2 Electrical installation 49 8.3 Internal pipework installation 50 8.4 Ground loops and boreholes 50

HEAT PUMPS – A GUIDANCE DOCUMENT

CONTENTS

9 COMMISSIONING AND TESTING 54 9.1 Electrical aspects 54 9.2 Refrigeration circuit 54 9.3 Heat source 54 9.4 Heating and cooling distribution 54 9.5 Handover 55 REFERENCES 71

BIBLIOGRAPHY 70

APPENDICES

APPENDIX A: HEAT PUMP THERMODYNAMICS 57

APPENDIX B: COMMISSIONING AND TESTING 64

APPENDIX C: F-GAS REGULATIONS 67

TABLES

Table 1: Heat sources and typical design assumptions 5 Table 2: Heat sinks and typical design assumptions 5 Table 3: Heat pump design factors for water-based heat distribution 11 Table 4: Practical selection issues for heat sources and sinks 14 Table 5: Maximum extraction rates for buried ground coils 21 Table 6: Recommended maximum length for different tube sizes 22 Table 7: Maximum extraction rates for close loop boreholes 23 Table 8: Absorption refrigeration 62 Table 9: Comparison of vapour compression refrigerants 65 Table 10: Absorption refrigerants 66 Table 11: Key dates for implementation of F-Gas requirements 68 Table 12: Common refrigerants subject to the F-Gas Regulations 68 Table 13: Obligations under the F-Gas Regulations 69 Table 14: Frequency of checks on heat pump systems required by F-Gas Regulations 69

HEAT PUMPS – A GUIDANCE DOCUMENT

FIGURES

Figure 1: Heat pump circuit 2 Figure 2: COP versus temperature lift 4 Figure 3: Gas engine heat pump 6 Figure 4: Example annual temperature profile corresponding space heating load 12 Figure 5: Possibilities to reduce noise from free standing external units 19 Figure 6: Buried ground coils 22 Figure 7: Proprietary manifold chamber 23 Figure 8: Preparation of an Energy Piles™ 24 Figure 9: Completed Energy Piles™ 25 Figure 10: Carbon dioxide heat-pipe 26 Figure 11: Schematic summary of air-to-air heat pumps 32 Figure 12: VRF with heat recovery 34 Figure 13: Use of a buffer vessel 38 Figure 14: Passive cooling from a ground loop 39 Figure 15: Tank-in-tank domestic hot water 40 Figure 16: Using a buffer vessel to pre-heat domestic hot water 41 Figure 17: Heat pump with a boiler 42 Figure 18: Solar contribution to ground source 45 Figure 19: Typical solar hot water integration 45 Figure 20: Typical small drilling rig used for heat pump boreholes 52 Figure 21: Components of a typical heat pump circuit 57 Figure 22: Heat pump cycle 58 Figure 23: Process efficiency and the pressure - enthalpy diagram 59 Figure 24: Temperature - entropy diagram 60 Figure 25: Vapour injection cycle 61 Figure 26: Reversible heat pump 62 Figure 27: Schematic of absorption chiller 63 Figure 28: Pressure enthalpy diagram for R134a 64

HEAT PUMPS – A GUIDANCE DOCUMENT

INTRODUCTIONINTRODUCTION 1

1 INTRODUCTION Heat pumps are increasingly being considered as an alternative to combustion-based heating plant as a means to reduce operating costs and carbon emissions. In some countries heat pumps have already taken a significant proportion of the market for heating appliances and this is likely to be a long-term trend throughout Europe. However, while it is true that space heating and hot water needs in housing and most other buildings can be fulfilled by heat pumps, it would be wrong to treat heat pumps simply as a drop-in replacement for conventional boilers. Heat pumps can replace boiler plant in existing houses, but this should not be done without a thorough review of the associated heat distribution system and rarely without system changes. Conversely, a hydronic heat distribution system, optimised for a heat pump should also work well with a condensing boiler.

This guide explains the design of heat-pump based heating and cooling systems to maximise the benefits of reduced operating costs and carbon emissions while avoiding excessive capital costs for plant and infrastructure. The guide’s emphasis is on the application of packaged heat pump plant for residential and small commercial buildings. Some guidance is provided for component-based plant that may be used for larger scale applications.

The information contained in this guide is drawn from a variety of sources including:

• Heat Pump Installer Manual. BSRIA • Heating Systems in Buildings – Design of Heat Pump Heating Systems. BS EN 15450:2007 • Ground Source Heat Pumps, BSRIA TN 18/99, 1999 • Guide to Good Practice - Heat Pumps, HVCA TR30 2007

Other published material is identified in the text, and a full list of bibliographies and references can be found on pages 70 – 71.

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2 HEAT HEAT PUMPPUMP FUNDAMENTALS FUNDAMENTALS

2 HEAT PUMP FUNDAMENTALS 2.1 HEAT PUMP A heat pump is a device for transferring heat from a lower temperature TECHNOLOGY heat source to a higher temperature heat sink. This is opposite to the AND THE ECONOMIC natural flow of heat from a hot source to a cold sink, but is made CARBON CASE possible by the application of an external energy source to drive a thermodynamic refrigeration cycle. The important characteristic of a heat pump is that the amount of heat that can be transferred is greater than the energy needed to drive the cycle.

Figure 1: Heat pump circuit.

The ratio between the heat provided to the sink and the energy required is known as the coefficient of performance (COP). Electrically driven heat pumps used for space heating applications in moderate climates usually have a COP of a least 3·5 at design conditions. This means that 3·5 kWh of heat is output for 1 kWh electricity used to drive the process.

The COP is the determinant of whether the heat pump will be more economic to use than an alternative heating appliance and whether the carbon emissions with will less than an alternative heating appliance.

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HEAT PUMP FUNDAMENTALS 2

For example, consider a space heating load of 100 kWh per week that can be served by a typical electric heat pump operating at an average COP of 3·5 or condensing gas boiler operating with a thermal efficiency of 100 percent (net calorific value basis):

Heat pump energy consumption = Load/COP = 100/3·5 = 28·6 kWh Boiler energy consumption = Load/Efficiency = 100/100% = 100 kWh

In simple terms, such a heat pump will be cheaper to operate provided that the electricity price is no more than 3·5 times the price of an alternative fuel. There are other factors that come into a more detailed analysis of the benefits, such as maintenance costs and equipment life, but the fuel price ratio is the key. This is also the main reason why the heat pump markets in some other countries have developed much more quickly than the UK where, historically, electricity has been more than 3·5 times the cost of natural gas. However, the long-term trend is for gas prices to increase faster than electricity prices, while heat pump COP gradually improves to provide a clear operating cost advantage for heat pumps. Heat pumps are already cheaper to operate than oil and LPG, and much cheaper to operate than direct electric heating.

While the operating costs for heat pumps and condensing boilers are rather similar at current fuel prices, the case for heat pumps as a low carbon technology is more conclusive.

In 2009, the current UK generator mix emitted carbon dioxide (CO2) at a

rate of approximately 0·43 kgCO2/kWh of electricity used (DEFRA 2007 data for greenhouse gas emissions reporting ). The corresponding figure

for natural gas is 0·194 CO2/kWh. Using the example given above for an electric heat pump with a COP of 3·5 and a condensing boiler operating with an efficiency of 100 percent (net calorific value basis), the carbon dioxide emissions are:

Heat pump CO2 emissions = 28·6 x 0·43 = 12·3 kg CO2

Boiler CO2 emissions = 100 x 0·194 = 19·4 kg CO2

The heat pump therefore emits 35 percent less CO2 than the condensing boiler.

2.2 REFRIGERATION Although a detailed knowledge of thermodynamics is not required for CYCLES AND the practical application of heat pumps, a basic understanding of the EFFICIENCY factors that influence efficiency is important for all heating systems designers, whether using packaged plant or component-based solutions.

A brief review of the thermodynamic refrigeration cycle is given in Appendix A – Page 57 and the properties of common refrigerants in Appendix B – Page 64.

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