HEAT PUMPS | SOLAR ARRAYS PV and solar thermal arrays have been proposed as potential supplemental technologies to heat pumps PANEL GAINS Incorporating solar energy arrays in district energy networks powered by heat pumps has the potential to reduce carbon emissions, but by how much and at what cost? FairHeats Mark Reynolds crunches the numbers and considers the options H eat networks will be a key feature of the UKs future energy strategy, with around 20% of heat forecast to be delivered via such networks, many of which are likely to use heat pump technology as the lead heat source. To further reduce the operational carbon emissions of these systems and mitigate the additional burden that the electrification of heat will place on the electrical Grid infrastructure, rooftop photovoltaic (PV) and solar thermal arrays have been proposed as potential supplemental technologies. This article investigates the technical feasibility and economic benefits of replacing or supplementing heat pump load with solar thermal or PV. As this study is based on urban heat networks in London, where roof-space availability is limited, it was unclear which technology would provide the best carbon reductions and if they offer feasible business cases for use on new and existing developments. In London, the capital costs of a communal heating system are heavily influenced by the carbon offsetting cost required by the Greater London Authority (GLA), currently priced at 95 per tonne of CO2 over a 30-year lifetime. Reducing operational carbon emissions can, therefore, reduce capital cost significantly. This research asks three central questions: 1. Is it more beneficial to use available roof space to generate heat directly from a solar In comparison with the solar thermal, the PV offsets annual electrical demand rather than onsite heat generation thermal array or use a PV array to generate electricity and reduce the heat pump Grid electricity consumption? 2. Can greater carbon and cost reductions be achieved by increasing the heat pump size alone? 3. What is the lowest capital expenditure (capex) achievable when considering the GLA carbon offsetting payments? Modelling approach An hourly demand model was created, based on a standard, fourthgeneration, 2-pipe, low temperature heat network that supplies heating and hot water to end users via heat interface units (HIUs). The base case is a heat network with a 55C flow temperature where air source heat pumps (ASHPs), in combination with a thermal store, are sized to contribute 80% of the annual heat demand (annual heat fraction). This is typical for meeting the GLA minimum decarbonisation requirements. The remaining 20% annual heat demand is provided by peaking gas boilers. The load profile used within the hourly load model was based on heat demands for domestic hot water (DHW) and space heating taken from live operational data of 337 occupied dwellings across 2019 and 2020. 40 January 2022 www.cibsejournal.com CIBSE Jan 22 pp40-42 Heat pump solar top up.indd 40 23/12/2021 15:50