| HEAT PUMP SIZING To be as flexible as possible, the model allows for easy variation of building size, DHW, space heating loads, expected heat losses, and geographical location. By doing this, the impacts of these factors on heat pump and thermal store sizing can be easily understood and assessed. The final model was then validated against load profiles taken from operational heat networks to provide confidence in the findings of the research. While there are many benefits to oversizing thermal storage, a key constraint will be the spatial requirement within the energy centre. This is especially key when comparing this with traditional CHP thermal storage, which operates at higher temperatures and, therefore, requires a smaller volume. The hourly load model allowed the thermal storage size to be modelled against the heat pump to determine the point at which any increase to thermal storage will have minimal impact on reducing the heat pump size. The study found that, as a rule of thumb, thermal storage should be sized at 50-75LkW-1, as shown in Figure 1. The percentages under the graph represent the annual target heat fraction contributions (for the heat pumps). Because of the impact of the higher electricity tariffs compared with gas, it is critical that heat pump sizing is considered carefully. If the heat pump is undersized and unable to meet the required target heat fraction, end users will see a significant increase in their cost of heat because of the reliance on electric boiler top-up. Equally, oversizing a heat pump presents a number of challenges, particularly when considering the spatial requirements against a gas CHP or gas boiler solution. As such, the hourly load model was used to provide a rule of thumb for a given target heat fraction contribution as a percentage of the site-wide peak load. The findings demonstrate that a target heat fraction that is as high as 99% of the site-wide annual demand can be achieved by installing heat pump capacity of less than 50% of the peak load (see Figure 2). A summary of the rule of thumb findings are presented in Table 1. These findings provide a benchmark to support developers and designers early on in the design stages, to ensure energy 36 August 2023 www.cibsejournal.com Thermal store size vs heat pump size 450 400 Heat pump size (kW) If the heat pump is undersized and unable to meet the required target heat fraction, end users will see a significant increase in costs 350 300 250 200 150 0 20 40 60 80 100 120 140 160 Thermal store size (LkW-1) 80% 90% 95% 97.5% 99% Figure 1: Impact of thermal store size against heat pump size Heat pump size vs target heat fraction 100 Heat pump capacity (kW) as a % of instantaneous peak load demand (sized as per CP1 2020) HYBRID ENERGY CENTRES 90 80 70 60 50 40 30 20 10 0 90% 95% 97.5% 90% 100% Annual target heat fraction based on total annual aggregated consumption 250 units 500 units 1,000 units Units no. of dwellings Figure 2: Heat pump size against target heat fraction centre sizing is fully considered until more detailed design calculations can be carried out at a later stage. In conclusion, the research demonstrates that sizing a hybrid system for a heat network requires careful consideration of various factors, including the capacity of the heat pump and thermal store. Designers must take spatial limitations into account when estimating the required heat pump and thermal store size, to ensure these will not be constrained later in the design development, when the risk of undersizing will be critical. The research provides targeted guidance for sizing heat pumps and thermal storage at concept design, in advance of detailed design, helping to pave the way towards more efficient and sustainable heat networks in the UK. To read the research paper, visit the Research & writing page on FairHeats website: fairheat.com/research-writing CJ ELLIE HISCOCK is a consulting engineer at FairHeat Target heat fraction Heat pump size (% of peak load) 90% 30 10% 95% 35 10% 97.5% 40 10% 99% 45 10% Table 1: Summary of rule of thumb for heat pump sizing