HEAT NETWORKS | THERMAL STORES Typical peak day heat-demand profile 52 November 2023 www.cibsejournal.com Heat network demand (kW) 600 500 400 300 200 100 0 00:00 06:00 12:00 18:00 00:00 Time of day Figure 1: The morning peak heat demand lasts almost two hours, with moderate demand for the rest of the day Impact of further reductions in electric boiler capacity by implementing a boiler-staging strategy 120% Electric boiler capacity as % of CP1 sizing approach (%) heating and hot water demands these networks serve. This data has previously been used to inform and optimise the sizing approach for the heat network distribution pipework and peaking plant, which, historically, was oversized significantly. For example, in Figure 1, the morning peak demand is seen to last almost two hours, and remains with a moderate demand for the rest of the day. Historic designs have sized the thermal storage and allowed for far shorter peak demand durations. Data was acquired from four operational heat networks to use throughout my assessment. In most cases, data from multiple years was available, which could allow us to assess the variation between demand profiles on a colder vs hotter year, and, ultimately, how large our electric boiler would have needed to be. Initially, I analysed how big the boiler would need to be if all the thermal stores were allocated to the heat pump that is, they fully empty before enabling the boilers. In reality, however, a sensible control strategy, allowing the boilers to enable before the thermal store fully discharged, would be applied. So, I used this control strategy as the main backbone of my research. My analysis of operational heat meter data indicates that new residential developments installing a heat network with a hybridgeneration approach of a heat pump and electric boiler may be able to consider small reductions in the electric boiler capacity if an oversized thermal store is used. Figure 2 shows how significant the reductions could have been for the various datasets. However, designers must approach any undersizing of peaking plant with caution. Take Site A, for example (Figure 2). The A1 dataset (2019-20) says we wouldnt have needed a boiler at all during that winter with a 22m3 thermal store. However, the A2 dataset (2021-22) indicates that we would have still needed significant boiler capacity to meet the large demands. The year-onyear and site-on-site variation is highlighting the risks with undersizing electric boilers. If future designs start to allow for oversized thermal stores, that provides us with additional benefits and opportunities. These include maximising generation hours during periods of low energy costs (subject to variable tariffs), or targeting our heat pump run hours when the coefficient of performance can be maximised, which, ultimately, allows for greater flexibility in the way we can operate energy centres. Although the findings show a significant reduction in electric boiler capacity, there is still substantial variation within the data. Further assessments are required on a larger number of heat networks before any definitive A1 2019/20 100% A2 2021/22 B 2021/22 80% C1 2021/22 C2 2020/21 60% D1 2021/22 D2 2020/21 40% Figure 2: Reductions in boiler capacity for various operational datasets in four sites A to D 20% 0% 0 4 8 12 16 20 24 28 32 36 40 44 48 52 Thermal store size (m ) 3 guidance could be provided regarding reducing the boiler size. We may now be thinking about our run costs, and what happens to these if we prematurely enable the boilers, instead of emptying the thermal stores completely. Well, a case study on one of my datasets showed that the premature enabling, or staging, increased the annual energy consumption by around 1.05MWh. This may seem a lot. If we take the electricity price to be 40p/kWh, this equates to about 1.45 per dwelling every year. In my opinion, this seems worth it in the interest of heat-supply security, especially considering the significant capital cost reductions we could achieve in the beginning. The transition to low carbon heat sources is an important step towards achieving the UKs carbon-reduction targets. By oversizing the thermal storage volume and using a hybrid-generation approach of heat pumps and electric boilers, it is possible to reduce the electric boiler capacity and deliver low carbon heat efficiently. Its important to approach any reduction in boiler size with caution, however. I would like to see more collaboration with industry-wide groups using developers and operators data to inform future standards on suitable and optimal sizing, and issue a design guide for the use of data when considering boiler-capacity reductions. CJ JAKE ADAMSON is a graduate engineer at FairHeat The research was selected to feature and be presented at the CIBSE Technical Symposium 2023. Read the full research paper, visit fairheat.com/research-writing