However, there is a challenge that this technology needs to overcome. Due to the limited temperatures that can be achieved (typically maximum 55 degrees, with a decrease of efficiency if higher temperatures were targeted), hot water generation appears to present a significant challenge.
In this blog post one of our engineers has explored a number of different technologies that could overcome this hot water generation challenge:
Imagine a block of flats, one option that could be explored would be distributing the ‘Low temperature hot water’ (LTHW) communal loop at temperatures achievable by the heat pump (with a result of having a decrease in heat losses, given the lower flow and return temperatures) whilst a refrigerant loop within each flat could raise the temperature above 60 degrees, feeding a storage unit that could help facing the peak loads. As the refrigerant charge is low, this would not fall under the refrigerant regulations.
If the air permeability of a building was improved to less than 2m³/m²h@50 Pa, a heat pump could be interlinked to the ventilation system of the dwelling. In fact, a packaged solution could include a Mechanical Ventilation Heat Recovery (MVHR) system, a heat pump and a cylinder. From the extract duct of the MVHR, the heat pump could recover enough energy to cover both the space heating and the hot water generation, which would be supported by a cylinder to cope with the longer periods of time required to generate hot water. The heat pump could also potentially provide cooling. The enhanced air permeability is key in this instance, otherwise the heat losses could be more than the heat that can be provided by the system.
This is a technology already present in the market, suitable in particular for high volumes of hot water generation. The beauty of this system is the use of CO2 as a refrigerant, which implies a GWP (Global Warming Potential) equal to 1, considerably greener than the other refrigerants. Although this technology is capable of achieving temperatures above 60 degrees, this is not suitable for heating applications (especially for radiators), as the return temperature could raise above the trans-critical temperature of the CO2.
Heat pumps require more time to cover the peak load compared to gas fired boilers and their efficiency drops considerably at partial loads. Therefore, a heat storage system could optimise their use. Rather than being based on water, these can use different materials (e.g. sodium acetate, which is basically salt and vinegar) whose phase-change temperature is achievable by heat pumps. These are more compact solutions compared to normal hot water cylinders, with a significantly lower heat loss, meaning that the energy could be stored more effectively for longer periods of time. This technology falls into the instantaneous hot water generation category, as the storage is not based on water.
With all of the technologies discussed above, it is important that designers maintain a life-cycle analysis (LCA) approach to the systems specified, with particular care about the overall carbon footprint of the technologies used. As well known, refrigerants have an impact on NOx and SOx emissions, and the R&D in this sector is rapidly evolving. This means that current refrigerants could be superseded when major maintenance to the system is required, perhaps implying future additional costs if the new technologies may not be compatible.
Last but not least, the climate crisis will also play a role in the design. We need to try to future-proof our systems and make them compatible with the different weather scenarios that UK could experience in a few years’ time.
Rossella, our Mechanical Engineer who wrote this article, will be chairing the ‘Heat Pumps: Specification, Integration and Whole Life Impact’ seminar at this years’ Build 2 Perform event on the 26-27th November at the Olympia, London. Click on the link here for more information and to register to attend.