The purpose of this research project is to address the challenge of providing domestic hot water (DHW) using low carbon heat pump technology given the overwhelming trend away from conventional hot water tanks in homes and the inability of present heat pumps to provide instant hot water.

Academics of the Universities of Warwick, Loughborough and Ulster intend to develop in this project a suite of heat pump / storage / control technologies, using either electricity or gas that function without conventional storage cylinders and can deliver energy efficient affordable hot water at the desired rate to a wide range of dwellings well into the future.

Latest work update


Topic: The key section of the TEC call addressed by this proposal is the ‘delivery of domestic hot water (DHW) solutions in compact form’ but any solutions to this challenge must also of necessity include ‘Miniaturisation of thermal energy conversion technologies for use in the domestic sector’ and ‘Domestic scale energy storage technologies’. It also requires elements of smart user adaptive control.

Context: The importance of this topic is highlighted by the fact that DHW accounts for 14% of heat use in the home. This compares with 63% in space heating but is much more intractable to major reduction than is space heating. Domestic hot water accounts for 80 TWh delivered energy per year.

As we move towards a low-carbon future we can expect a combination of improved building insulation and the application of heat pumps to space heating to drastically reduce carbon emissions. However, present heat pump systems are unable to supply instantaneous hot water; a combination boiler (‘combi’) can deliver 20-30kW or more for this purpose and heat pumps sized to meet typical house space heating demand are rated around the 10kW level. A larger heat pump would be uneconomic and would have an unacceptable start up time to meet normal requirements. Thus the only way that present heat pump systems can provide hot water is to charge a conventional DHW tank. This is not an issue except that new build houses tend to be built without space for tanks and opt for instantaneous hot water via a gas combi boiler. Also there is a strong trend when replacing gas boilers to choose combi boilers and discard the storage tank to provide more space within the home. At present almost two-thirds of the 23 million gas boilers in UK homes are ‘combi’ boilers with no large hot water tank. It should be noted that combi condensing boilers do not deliver DHW with laboratory test efficiency due to start up transients, hot water left in pipe runs etc. The mean efficiency of a trial set of combination boilers tested for the Energy Saving Trust was 82.5%. The dynamics of state-of-the-art combi condensing boilers were noted by Atmac et al who observed for example that hot water production at 55°C from a 10°C inlet was achieved in 100 seconds. Much is made of ‘hybrid’ heat pumps to provide both space and water heating but essentially they are an electric heat pump plus gas boiler with integrated control system. When supplying DHW the load is met by the gas boiler and is no more efficient than a combi. Off-gas grid consumers are disadvantaged through higher fuel costs.

A ‘hidden’ DHW load is the demand by appliances such as dishwashers and washing machines which use 14% of domestic electricity [3]. In recent years the trend has been towards cold-fill appliances with electric heating. The argument in favour of this is that efficiency is higher than if direct heating is used in a DHW cylinder and losses occur from the cylinder and pipework between appliance and cylinder. This should be revisited if DHW is supplied by a high efficiency heat pump and also if future appliances can request water at the required temperature from a smart supply.

More efficient alternatives must be made available to respond to these market drivers.

Assuming that we do not see government regulation to decree storage tanks in all houses, we need technical options that are compact, appealing to consumers, economically viable and deliver DHW with much reduced energy consumption/carbon emissions. DECC and other projections [2,7,8] imply that we will still have to use a significant quantity of gas well into the 2030s and even the 2040s. This is partly due to the intermittent nature of the renewables used to decarbonise electricity and also due to the cost. DECC [9] suggest that in an all-electric future the peak load of 55GWe could rise to 90-120GWe, possibly higher, and that reinforcing the transmission and distributions networks in line could cost £46bn NPV (2012-2050).Whatever the future does hold regarding gas supply, whether conventional, fracked, hydrogen or biogas it is important to have a suite of technologies that will between them constitute a future-proofed and flexible response to the evolving energy infrastructure.

Thus there is a need to develop energy efficient gas and electric heat pump systems that do not need conventional large DHW tanks and can meet consumer needs. Such systems must initially utilise the existing infrastructure but be adaptable to uncertain future infrastructure and change.

Research hypothesis and objectives

As stated above our aim is to develop energy efficient heat pump based systems (both electric and gas) that do not need conventional large DHW tanks but can meet the consumers’ requirements for DHW based on their experiences with combi boilers. There are a number of interlinked areas that must be investigated in order to deliver the required solutions; these are illustrated in Figure 1 below.

Figure 1: Low carbon DHW overview

The requirement is to supply low-carbon DHW via either electricity or gas whilst avoiding the need for a conventional storage tank wherever possible. Some dwellings may use e.g. solar thermal as a low carbon source of DHW but for the majority of dwellings, particularly in the winter months, only an air-source heat pump system is feasible, whether electric or gas powered.

An additional factor that provides opportunities for novel solutions are the differing types of hot water load. They tend to be moderate quantities and flow rates used in washbasins, sinks etc but the extreme load is that for a bath-full (100-120 litres) of hot water. We will look at solutions that target all of these loads at high efficiency but may compromise for the extreme loads at a slightly reduced efficiency. There is no evidence that baths are both numerous and synchronised enough to give rise to peak electric grid load problems, which means that electric heat pumps, if capable of meeting extreme loads, might be used in instantaneous heating mode. This will however be confirmed in our research.

Strategically there are different options (and challenges) for electric and gas heat pumps related to capacity control, integration with advanced storage and peak distribution capacity (for electric heat pumps only). The technologies that must all be researched and then integrated are:

Stores There are three broad types of store that we will investigate for use in our set of solutions for different house sizes and energy vectors. The smallest is a buffer store of water that need only be large enough to overcome the start up time of the heat pump (electric or gas) when it is supplying heat directly. The second is a Phase Change Material (PCM) store with a four-fold volume advantage over conventional water cylinders that could for example be located in an otherwise unusable kitchen corner or similar unutilised space. LU have an existing prototype. The third possibility is unconventionally shaped ‘flat’ stores of hot water with either vacuum insulation (VI) or aerogel insulation which give excellent heat retention with only a very small thickness. Spirax Sarco has VI manufacturing experience and is a partner. Two or more such modular stores could fit (for example) under kitchen units or in similar unused spaces. Measurable objectives for stores alone are 1) PCM store of storage capacity equivalent to 150 litre tank but in 50 litre package 2) VI or aerogel ‘flat’ store modules developed with <150 mm depth (to fit under kitchen units) and indicative heat loss coefficient of less than 0.5 W/m2K.

For electric heat pumps (EHPs) extreme capacity control is a technical challenge; compressors have limited permissible speed variation. A conventional system sized for space heating of 10kW would not be able to deliver even 20kW and would have an unacceptably long start–up time. Possible solutions include new scroll compressor types being developed by partner Emerson that can operate under much higher speed ranges and could be combined with a small buffer store to overcome the start up transient. Emerson will donate design expertise and two newly developed ZHW compressor units to the project for integration into systems in test houses.

Measurable objectives for electric heat pumps alone are 1) Demonstration (in test house) of an advanced variable speed scroll compressor system that can deliver either 25kW DHW at 50°C from 10°C feed or space heating output as low as 5kW, both with an annual average COP (Coefficient of Performance = Heat output/Electricity input) of >4.0 , 2) testing of an air source evaporator capable of operating long enough to provide the extreme instant load (1 bath-full of water, typically 100-120 litres) without excessive frosting/freezing.

Gas heat pumps do not have such capacity control issues since high turn down ratio burners and variable speed liquid pumps are available. However there is an efficiency penalty at higher powers and a need to reduce their physical size. The adsorption heat pump developed at UW has adsorption heat exchangers that are far more compact the those in commercial products or in other research establishments worldwide but our measurable objectives are to employ a radical approach using newly developed composite adsorbents and heat exchanger designs that will result in a fourfold increase in power per unit volume and a time to reach full output of no more than one minute. This will allow operation as an instantaneous water heater with only a small buffer tank.
The other essential technical element of our suite of solutions is intelligent control technology. UU already have experience with low cost wireless sensor systems (through NPP OCTES (Opportunities for Community groups Through Energy Storage and NPP SuLA (Sustainable Living Assistant)). We propose a smart DHW control system that knows which appliance is calling for hot water and adapts to learned consumer behaviour patterns. In the case of systems with larger storage, the system will also perform a demand side management role through energy pricing signals from market operators. Given the importance to the consumer of the extreme load of a bath-full of water and its demand on the system we will also investigate the feasibility of a smart bath tap that can communicate with the supply technology. One-touch options might include set levels and temperatures for children or adults. If extra functionality can be offered together with running cost savings the solutions are more likely to appeal to the consumer.

Integration objectives The major components above will be integrated into complete systems and tested as laid out in WP7 below, with test house demonstration of one or more electric systems and laboratory tests of a gas based system. Firm measurable objectives for systems are more complex because we need to show benefits both with the current energy mix and (ideally) a number of possible future mixes. A simplified target is to say that our gas based system should have a gas to DHW efficiency at least 40% better than a combi system and that that annual average COPs for electric based systems should exceed 4.0.

Novelty – We do not believe that any other group is tackling this problem in such a comprehensive fashion, covering the range of technologies, control systems and human factors needed to meet the DHW challenge in the UK and other countries where conventional storage is not feasible. It should be noted that i-STUTE, of which the investigators are members, is not looking at DHW but only at space heating technologies. It does not have the resource for both and is concentrating on the larger space heating applications at present. It will not branch into DHW technologies during its remaining 2½ year life. This project will however benefit from links with the ‘Working with EUED Centres’ project HOTHOUSE within WP1 below, using results of real patterns of hot water usage.
Timeliness BSRIA’s newly released heat pump study [10] shows continued growth in the global heat pump market with the market value reaching 5.4 billion US dollars in 2014, up 1.7% from 2013. In volume terms, the global sales of hydronic heat pumps rose 16% in 2014 to almost 2.5 million units. Given the time needed to progress from proof of concept and first demonstrations through to commercial products and given also the concerns over future peak loads on the electricity distribution network, and the dominance of the combi boiler in supplying DHW without conventional storage we contend that this research is most timely if future DHW systems are to be truly low carbon and sustainable. It is also true that this research could not have been carried out only a few years ago – R&D into high turndown compressors, super-compact sorption generators and novel storage is only now reaching the point where it can be used for this work.

National importance

The Energy Savings Trust noted pertinent facts to support this proposal, namely that hot water use contributes £228 to the average annual combined energy bill and emits 875kg of CO2 per household per year, showers are the biggest water user in the house (25%) and also that on average and each individual takes 4.4 showers and 1.3 baths each week. This work addresses the ‘Energy Trilema’ identified by DECC: Reducing emissions of greenhouse gases, improving security of supply (by reducing energy inputs) and improving affordability.
The TINA report on heat estimates the value in meeting emissions targets with heat pump system components/processes as £12bn and in business creation £3bn. The R&D areas identified that we will address include: Novel heat pump technology, improvements to existing technologies, control philosophy and monitoring methods, and grid integration. Also identified are ‘small scale demonstration of integrated systems (potentially using advanced heat stores such as phase-change stores) to optimise performance of thermal storage and heat pumps’ and ‘enabling benefits for deployment of heat pumps’ with emissions and business creation values of £1.4bn each.

Academic impact

There is a large cohort of researchers, nationally and internationally who work in electric or gas fired heat pumps, heat storage and their integration to meet domestic demand. Nationally we are engaged with dissemination networks such as SIRACH (Sustainable Innovation in Refrigeration Air Conditioning and Heating) with many SME members, with SusTem (Sustainable Thermal Energy Management Network) and will of course contribute as much as possible with the new network to be set up under the Thermal Energy Challenge call. Internationally, Critoph represents the UK for DECC in IEA Heat Pump Programme Annex 43 (Thermally Driven Heat Pumps), and similarly Hewitt is the UK member International Institute of Refrigeration B2. This gives a unique opportunity to interact with researchers outside the UK. We will present results of our work at conferences such as Heat Powered Cycles (UK) and Int Sorption Heat Pump Conference (Japan) in 2017 for sorption developments, 12th IIR-Gustav Lorentzen Conference on Natural Refrigerants - GL2016 (UK) for electric heat pumps, the 11th Phase-change Materials & Slurries for Refrigeration & Air Con (2016) for storage and Behave; the European behaviour and energy efficiency conference. Journal publication plans are given in ‘WP8 Dissemination’ below.
Good collaboration between the researchers at different universities within this project is assured since they already partner in a number of other projects. Their contributions are explained below.

Programme and methodology

The work programme is structured to achieve the goal of providing a suite of low-carbon DHW generation / storage / control packages that can be used in a wide range of dwellings well into the future. The work packages described below and in the Work Plan first gather the necessary data (‘hard’ and ‘soft’) needed to inform specification and design, then build and test the novel system components in isolation and finally prove the concepts in full system tests.

In detail:

WP1 gathers all the data required to specify requirements and determine constraints

WP2 (UU) Electric Heat Pumps for DHW

WP3 (UW) High power, high temperature, super compact Gas Fired Heat Pump

WP4 (LU) Compact Thermal Store

The Phase Change Material (PCM) store is mainly (but not exclusively) intended for use with the EHP and its shape and total volume will enable it to be fitted into otherwise unused spaces within the consumer’s dwelling

WP5 (LU,UW) Vacuum/aerogel insulated ‘Flat’ DHW store

Work on unconventionally shaped stores using sensible or latent heat that could fit into awkward but available spaces (under kitchen units, etc) has two components; the insulation system itself and the control strategy that works best with perhaps up to four small well mixed but highly insulated stores. LU and Spirax Sarco will deal with the insulation and UW with the energy management and control.

WP6 (ALL) Develop sensors, algorithms, controls for all heat pump/store viable options

UU will provide low cost data acquisition systems (e.g. wireless rumble sensors on pipes) and control hardware for use in the Ulster test houses and liaise with partners to develop the control software for the systems to be evaluated in WP7.1. UW will develop control systems for testing under WP7.2. All will work to ensure that the results from the heat pump and storage systems tested in WPs 2-5 are properly integrated with the consumer needs identified in WP1 to develop control system suited to the demand reduction needs of the project and to potential end-users.

WP7 (ALL) System tests using simulated draw off patterns (extreme, real and standard)

WP8 (ALL) Management and Dissemination


Industrial partners:

Latest progress

Click here to view the latest update of the work carried out by our research team presented in our last Advisory Board in May 2018 at the University of Warwick.

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