Small Smart Sustainable Systems for future Domestic Hot Water (4S-DHW)

Lead Research Organisation: University of Warwick
Department Name: Sch of Engineering


The purpose of the proposed research programme 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.

We intend to develop 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 to a wide range of dwellings well into the future.

Ulster will use a novel compressor being developed by industrial partner Emerson that has an exceptional range of running speeds, enabling the same device to either deliver e.g. 25 kW for instantaneous hot water or 10 kW or less for space heating. This would be used in conjunction with a small buffer store to overcome the delay in start-up before hot water is available.

Present gas fired heat pumps (both commercial and under development at Warwick) are easier to modulate but are physically large if delivering 20 or 30 kW and also have a long start up time (5 minutes). The Warwick goal is to use new composite adsorbent heat exchangers to reduce start up time to one minute, even when meeting a 25 kW load and to reduce key component sizes to achieve a compact system.

Thermal storage is a vital part of DHW provision by heat pumps. A small buffer store may be needed to overcome starting transients, or a large capacity store might be needed to provide a bath-full of water quickly. An intermediate capacity store might work together with a heat pump to meet peak loads. Our research will encompass buffers, compact PCM stores that could be sited in unused spaces such as corners in kitchens and 'flat' stores using vacuum or aerogel insulation that could fit under kitchen cabinets or other available unused spaces.

To bring this all together into a range of integrated systems suited to different housing types etc there needs to be both an understanding of the consumer's needs and preferences plus a smart adaptive control system. In addition to data in the literature we have access to data from detailed monitoring studies previously carried out by Loughborough. Consumer preferences will be investigated by the use of surveys carried out by the User Centred Design Research Group at Loughborough Design School. Ulster will assume overall responsibility for sensor choice, control hardware and software. They will devise a system controller that adapts to and meets consumer needs in an optimal way. In the long term this will be part of a house-wide wirelessly linked system including 'wet' appliances such as dishwashers and washing machines and 'smart taps' that communicate with the DHW system so that it responds optimally to the size and type of load demanded.

Planned Impact

This work addresses the 'Energy Trilema' identified by the Department of Energy and Climate Change, i.e. reducing emissions of greenhouse gases, improving security of supply (by reducing energy inputs) and improving affordability.
Its importance 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. Present heat pump systems, whether gas or electrically driven, cannot supply instantaneous hot water in the same way as a combi boiler. We will develop new heat pump technologies integrated with advanced thermal storage to provide affordable low-carbon DHW without the need of a conventional large DHW storage tank.

Quality of Life
The research will lead to products that will result in reductions in CO2 emissions (with present energy supply infrastructure) from the current 875kg of CO2 per household per year using a gas boiler to between 625kg (electric heat pump) and 540kg per year (gas heat pump), benefitting the environment and addressing policy commitments. In monetary terms, the current annual average hot water cost is £230 and heat pumps could achieve reductions of £30 to £65 per year for electric and gas heat pumps respectively This is in addition to the savings in space heating that these same heat pumps will achieve of between £150 and £300 per annum with potential benefits in alleviating fuel poverty.

Business and Industry
The potential benefits to UK business and industry are highlighted by the TINA report on Heat which estimates the value in meeting emissions targets with heat pump systems at £12bn and in business creation £3bn. For heat pump integrated thermal stores it identifies a further £1.4bn of value in emissions savings and £1.4bn in new business creation. BSRIA's newly released heat pump study shows continued growth in the global heat pump market with the market value reaching $5.4 billion in 2014.

Although it is possible that these technologies could lead to the establishment of new companies, it is more probable that existing companies working in related areas would be in a strong position to exploit. We are working with the following manufacturers who will benefit from inclusion in the research and access to its results:

The gas-fired adsorption heat pump research at Warwick will be facilitated through cooperation with BDR, best known in the UK for the 'Baxi' brand.
The electrically driven heat pump at Ulster utilises novel compressor technology developed by Emerson/Copeland to improve heat pump performance.
Spirax-Sarco is making available its vacuum insulation technology to advance thermal storage options in domestic applications.

Research assistants will benefit from working in large established research groups with excellent facilities and collaborating across different disciplines. Research assistants will be encouraged to write academic journal papers and attend and make presentations at conferences. Papers will be jointly authored as appropriate by different members of the research team across the disciplines providing exposure to different ways of working. The PI and Co-Is will benefit from undertaking and directing stimulating research and the high quality high impact publications that are expected to result.


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Description (1) Phase-Change Material thermal stores.
Designs for compact phase change material based thermal energy storage systems for the provision of domestic hot water when linked to heat pumps have been developed, simulated and experimentally characterised. The store designs are modular with heat storage capacities and power outputs suitable for most applications excluding at present running a bath realised.

(2) Cost-effective buffer tanks
Work on buffer stores to provide hot water during a heat pump start-up period investigated a range of options. Conventional cylindrical tanks make poor use of the available space. Novel correlations were developed for optimising the volume and minimising heat loss from cuboid tanks The two alternative kinds of store ((i) small modular under-cabinet stores with vacuum insulation, (ii) larger hot water stores which might be located in a loft space or outdoors) were both modelled. Small vacuum-insulated PCM stores were found to be an effective albeit expensive solution to the problem and are optimal where space is limited. Conversely a hot-water store with conventional insulation is more cost-effective in houses with space for a larger tank. Higher capacity stores can make better use of solar thermal collectors.

(3) Improvements in hot water delivery
Large houses tend to suffer from a delay in hot water arrival due to extended pipe runs. Delays of up to 46 seconds were measured in traditional properties with conventional boilers. It is important to avoid such inconveniences with modern heat pump installations as they would mitigate against the uptake of this technology: the user experience should be enhanced to encourage retro-fitting.
The delay could be avoided by using a recirculating system to keep the pipes warm. The required flow rate is very low and could be generated by the thermo-syphon effect i.e. without requiring a pump. A transient modelling method was developed to design such systems and minimise heat losses.
The provision of this double pipe run, combined within an insulating sleeve, would be practical for new-build houses or major refurbishments. The benefits would be apparent wherever the store to tap distance exceeds 3 m. Such a system would enable the hot water store to be located further from the kitchen and bathroom without inconvenience to users.

(4) Improvements in Vapour Compression Heat Pumps
Experimental assessments of a typically sized residential air source heat pump where the variable speed drive has been accessed has revealed a number of possibilities for near-instantaneous domestic hot water. A variable speed air source heat pump (ASHP) system has been studied in the laboratory under a series of ambient conditions at -2, 2, 7 & 15-degree C along with a range of water supplying temperature at 25,30,35,40,45,50,55 deg C. A series of experiments has been conducted in the conditioning chamber at the above mentioned ambient temperature and corresponding humidity as per BS EN 14511 standard to evaluate the performance of the newly developed superheat based controller by Emerson at different heating demands. The operational experiences with the compressor performance, controller response to change in ambient conditions (source and load side), superheat control, loss of control at higher heating demands value has been communicated. The analysis of compressor start-up, operation at six different heating demands of 18kW, 15kW,12kW, 9kW, 6kW, and 3kW, the associated issues with the longer operation at low heat demand of 3kW, conditioning chamber limitations, the effect of varying heating demands on the power consumption and co-efficient of performance (CoP) values has been evaluated. The CoP values for 7 deg C ambient conditions as per standard at 15kW,12kW, 9kW, 6kW, and 3kW are 3.91, 4.21, 4.42, 4.91, 3.85 respectively for 20 deg C water inlet and 30 deg C water supply temperature. Similar trend has been shown for higher water supplying temperature of 40 and 50 deg C but with lower values. The CoP values for heating demands of 15kW,12kW, 9kW, 6kW, and 3kW are in the range of 3.22- 2.95, and 2.41-2.25 respectively. The difference between water inlet and outlet temperature (delta T) is another important parameter for the Heat Pump system design. A comparative testing results of the temperature difference delta T of 5, 10 and 20 deg C at ambient conditions of 15 deg C, the associated effect on the pressure ratio (PR) and the isentropic efficiencies of the compressor have also been analyzed in this study. The CoP values at 9kW heating demand with delta T of 5, 10, and 20 varies in the range of 3.41-5.83, 3.67- 5.87, and 3.58-5.02 for water supplying temperature range of 30-50 deg C. From the testing results, variable speed-based HP system has shown good potential to meet different heating demands by varying compressor speed accordingly.
Exploitation Route Based on results to date prior to in-situ demonstration and performance evaluation under real test conditions additional characterisation for different duty cycles and proof of long term performance and durability is required

The heat-pump integation technology should be demonstrated in a house that includes a modular heat store in the loft, linked to both a heat pump and solar collectors, together with a thermo-syphon recirculating loop to satisfy customer expectations of instant hot water.

The next stage would be for thermal store, pipework and insulation manufacturers to market a range of systems designed to simplify installation.

A technical standard should then be defined to certify the satisfactory operation of such systems and to require new houses to include a thermal store suitable for heat pump and solar collector integration.
Sectors Energy