Sustainable buildings

New heat pumps addressing carbon emissions and renewable energy integration

It has been well established across central and northern Europe that building space heating is a significant energy user of fossil fuels, typically through gas boilers, oil boilers, solid fuels and electric resistance heating. An opportunity exists to address space heating contributions to the carbon agenda by substituting these with alternative, renewable resources and/or higher efficiency technologies.

Expected savings

The natural gas distribution network is well established across Europe, and gas absorption and gas adsorption heat pumps with efficiencies of up to 140% are possible for space heating systems. There would be a corresponding saving in both gas resources and emissions, although the current technologies would appear to be better suited to commercial applications. However, while it is anticipated that variable time of day gas prices may be a possibility during periods of extreme winter stress on the gas network and supplies, such a variable time of day tariff is more likely with electricity supplies, and therefore the electric heat pump will be considered in more detail.

An increasing renewable energy penetration into the electricity supply network decarbonises the electricity supply network. Thus, an electric heat pump, typically an air source unit for ease of retrofit, appears to be a contender for a near-zero carbon heating system. Initial questions arise around the heat distribution temperature required by the buildings (which is dependent on the quality of the building fabric and the type of heat distribution, e.g. radiators, underfloor etc.) and the coefficient of performance of the heat pump under ambient temperature and heat supply temperature conditions.  Ulster’s research at the Centre for Sustainable Technologies has developed higher temperature heat pumps with the aim of increasing the coefficient of performance of heat pumps operating under low ambient air temperatures and high heat distribution temperatures suitable for radiators, i.e. 60°C+.  It is believed that buildings heated this way will be dominant for the next 50 years, and therefore their heating must be cost effectively addressed.

Testing

A test facility and subsequent field trial utilising an economised vapour injection heat pump initially developed by Emerson saw seasonal coefficients of performance in excess of 3.7, i.e. for every kW of electrical energy, 3.7kW of heat was delivered to the house at temperatures around 60°C. When compared to oil prices at the time of the research, a 20% saving in heating costs was achieved with no discernible loss in thermal comfort within the family home.

Alternative cycles have included the development of a rotary compressor-expansion turbine heat pump cycle with EA Technical Services Ltd. This has advantages in performance over traditional heat pump systems, with coefficients of performance of over 4.0 being achieved in laboratory trials. The next stage of this work will be the evaluation of such systems in a field trial, and Ulster has developed occupied and heavily instrumented test houses on site to facilitate further evaluation.

Energy storage is essential for providing the time dependent element to accommodate variable, non-dispatchable renewable energy. Ulster’s work in the Interreg IVA SPIRE project is developing the relationships between optimised energy storage (e.g. water, phase change materials and/or thermochemical materials) and optimised heat pumps that accommodate the varying needs of buildings. Some of these challenges will include optimised, weather-compensated heat pump control, which normally leads to the heat pump operating at an optimised temperature for the ambient air.  Phase change materials and thermochemical materials operating at a fixed temperature may not avail of lower temperature operation during higher temperature days. Furthermore, the arrangement of heat pump and storage may facilitate smaller-sized heat pumps which operate in tandem rather than independently or in series. Ulster’s research is addressing this integration and sizing issue.

Additional research

Ulster’s research is also addressing new working fluids, including R245fa, and these have been evaluated from a perspective of developing new higher temperature heat pumps, not only for space heating but for industrial process heating applications, as well. A novel use for these fluids has been in the development of a heat pump that is utilised with seasonal thermal energy storage.

Seasonal thermal energy storage is a technology typically associated with solar energy, i.e. solar energy excesses of summer are stored for winter use. Denmark and Germany are successful hosts of such technologies, with working examples in tank or pit forms. Major challenges are space requirements and capital costs, the latter being especially associated with the cost of the tanks and associated insulation, with an estimated figure of 5m³ per square metre of solar collector being a very crude guide.

In the specific case of solar seasonal thermal energy storage, particularly in more extreme latitudes, the solar input is related strongly to summer months. In lower latitudes, there is an expectation that solar energy will be available more often throughout the year, and therefore the ‘seasonal’ thermal store can be charged and discharged more often. This has implications for the size of the store and, potentially, the choice of materials used to perform the storage.

To conclude

It has often been concluded that due to the cost of the materials and the lack of charge/discharge cycling (i.e. use), the application of advanced materials such as phase change materials (PCMs) or thermochemical materials (TCMs) would not be finically successful in seasonal storage applications.  However, the storage is not seasonal.

Under the correct conditions, higher levels of storage can be achieved in summer, and lower levels of storage coupled with discharge in winter is the realistic mode of operation. Discharge elements in summer may be associated with absorption chiller-driven air conditioning (where a smaller level of storage is required for diurnal operation). However in winter, with favourable solar conditions, elements of charge and large portions of discharge are expected. PCMs or TCMs, with their expected characteristics of improved heat storage capacity by up to a factor of ten when compared to water, have the potential to reduce the size of a tank, thus easing the challenges of space/weight, installation costs and heat losses.

The additional challenges are associated with PCM/TCM lifetime, compatibility with materials, the heat exchanger design for cycling ensuring charging and discharging times are as required by the solar supply, and the building’s heating (or cooling) needs. Cost may also be an issue, but reduced volume may compensate in part.

Therefore, seasonal thermal energy storage has possibilities for deployment and potential pathways to be enhanced, especially in areas of more all-year-round dynamic operation. This last point may make such systems applicable to other renewable energy sources, especially where a heat demand from fossil fuels can be displaced, at least in part. The heat pumps developed by Ulster under the FP7 Einstein project will use this heat source when it is insufficient to heat the building and raise it to the correct temperature.

Professor Neil J Hewitt
Director
Centre for Sustainable Technologies
University of Ulster
tel: +44 (0)2890 368566
[email protected]
www.ulster.ac.uk