A lot of the future work on R&D will have to be done by manufacturers making the components for a world market and the heat pump manufacturers using these components, supported by scientists and experts from institutes and manufacturers.
At the 13th IEA Heat Pump Conference 31 papers were presented by experts on the topic of HPWH and topics closely related to this. The main topics were refrigerants, PCM’s and Heat Exchangers, with attention to frost formation in cold climates. Together with the findings from the Annex, a further survey of publications and interviews and contacts with involved experts the main outlines of future work can be given.
Refrigerants
‘The energy issue is now at the top of the agenda, moving to a low- GWP refrigerant, without a better energy efficiency, a new design … is not enough’.
Alternative refrigerants impose safety concerns, where these also can be an effective drop-in-replacement for conventional fluids (R134a). Refrigerants with no risks, like CO2 based systems can certainly compete well with conventional fluids. A focus for the R&D will be on appropriate system modifications with heat exchangers in relation to the hot water storage, which can provide further improvement in the system efficiency, with the energy issue on top of the agenda.
Although manufacturers are active and well aware of the need to develop new technologies with alternatives, further international collaboration on alternative refrigerants for HPWH’s is of importance. R&D on refrigerants is done all over the world, however not so much for HPWH’s. The challenge is the transition towards natural working fluids for which the opportunities and challenges are given under Refrigerants.
Modelling the technology
‘Although Heat Pump Water Heaters have reached an important level of maturity, there is still room for improvement of the technology‘.
A large part of the work is done and will have to be done on components by Original Equipment Manufacturers supported by Universities and Institutes.
Stratification in Hot Water Storage tank
- Obtain the maximum yield when drawing hot water. It is of importance to optimize the inflow of cold water in order not to disturb the temperature layers in the tank.
- The flow through and the configuration of the condenser/gas-cooler dependent of the type of refrigerant has effects on the build-up of thermocline.
- Increase the thermocline and separate the storage tank into two layers (upper hot and bottom cold), to decrease the daily heat losses
- Control of re-heating interval of the hot water by increasing the hysteresis of the thermostat or by re-heating once a day
Heat Exchangers
- Heat pump manufacturers gradually adopt microchannel heat exchangers (MCHX) as the outdoor coils due to the compactness and lower cost. The relatively small hydraulic diameter of MCHX also enhances the heat transfer compared to traditional tube and fin coils. However, such benefits may turn to a challenge when heat pumps operate under frost conditions.
- Increasing the heat transfer for condenser plate heat exchangers in order to reduce the refrigerant charge. Successful development have been reported upon by Andersson et al (KTH-Stockholm) reducing the charge ca 100 g of propane for a capacity of 2 – 10 kW.
- Phase change material (PCM) coupled heat exchangers are attracting more attention recently since PCM can absorb or release heat within a narrow temperature range. At the 13th IEA Heat Pump Conference a paper by Tongji University was presented on an successful experimental set up with HPWH’s.
- Cold Climate applications are a challenge for air source HPWH, with a special focus on the evaporators. A number of studies have been done under HPT-Annex 41. Interesting are Case Studies in Alaska.
System Technology
Installer and building focused developments as well as combination with other technologies for integrating the HPWH technology in the building process for individual as well as collective systems. Collaboration with experts from other areas of expertise is important to ensure proper application of the technology.
- Plug-n-play concepts and speeding up the installation process: think of heat pumps, but also shower modules (or sub-components) with heat or hot water recovery.
- Integration of heat pump systems in buildings: such as smaller sub-components with facades or by making systems more compact. In addition, the aesthetics of the system can also be taken into account.
- Modelling the combination with other energy renewable technologies, especially Solar thermal and Solar PVT, as discussed at the Workshop at the 12th IEA Heat Pump Conference. Although already applied by some suppliers some interesting ideas were presented by Stephen Harrison from Queens University.
- Modelling of collective systems in Multi Family Buildings, with new distribution piping arrangements.
- Fresh Water Systems, combining with other heat generators.
Smart Technologies
Future systems need a greater adaption to the grid infrastructure in a development towards electrification. The storage capacity of HPWH’s can give a greater flexibility in the control mechanisms. A number of pilot projects are already installed.
Some topics:
- Modelling of the control mechanisms and smart control where the air source is used at the highest possible outside temperature (mid-day?)
- Reduce peak demand of systems to prevent grid reinforcement or spreading of electricity connections.
- Making systems smarter to provide more flexibility to the electricity grid.
- Optimize the storage size, to make thermal storage possible in relation to locally generated solar electricity.
Boundary conditions
- Legionella, how to deal with it in collective or individual systems
- Seek for harmonisation of the legislation
- Further develop the experience with individual systems and the Volume Limitation Concept for collective systems in Multi Family Buildings.
- Calculation models do hardly take into account innovative technologies and thus are not taken into account in the legislative procedures for calculating energy performance factors are rating the energy use of the building. This is especially the case for Multi Family Buildings.
- Test Procedures, focus on:
- Harmonisation of procedures;
- Develop procedures for different climatic conditions;
- Develop procedures for innovative technologies.
Example projects
Research is great, but unless someone actually buys and uses the product, it does not save any energy (Tony Bouza, U.S. Dept of Energy). An important focus for future work will therefore be on deployment and developing markets.
Monitored example projects will convince end-users, corporations, local policy makers that heat pumping technologies are the ‘best choice’ to generate hot water.
Although HPWH’s have been sold worldwide for over 40 years but still have a very small market, leaving the industry with relatively few case studies, best practices, and expert practitioners to learn from. During the work under the Annex 46 it was noticed that more information is available on Multi Family Buildings but rarely on single family buildings. Next to data on performance the economic data are of importance to convince end users, i.e. installed costs, maintenance costs and energy use in both single-family and multifamily homes for new buildings and retrofit. The available data from monitoring is scarce for individual buildings.
The cost-benefit of these systems will only improve over time as design and installation best practices further reduce installed costs, technological progress and new grid services improve cost savings, and the grid gets cleaner.