HEAT NETWORKS | OPTIMAL DESIGN Making the transition to electric heat networks As the electrification of heat networks gathers pace, FairHeats Tom Burton compares the cost of having a 100% heat pump-led network with that of a hybrid system, which uses top-up electric or gas boilers to cover peak loads T he UK faces a major task to decarbonise the heat demand of its buildings to have any hope of meeting its ambitious, but necessary, carbon-reduction targets. This presents a big technical challenge for the building services and energy sectors to solve: how to transition to more sustainable and lower carbon heating solutions quickly? Setting legislation and regulatory targets for carbon emissions is one thing, but designing and installing technologies to meet these targets while delivering reliable and affordable heat to building owners and tenants is another. The regulatory trajectory is clear: heat must be electrified, but to what degree, and how? It is estimated that around 18% of UK heat will have to come from heat networks by 2050 if the UK is to meet its carbon targets cost-effectively1. This requires around 500,000 new customers on heat networks a year. For these heat networks, gas CHP has been the dominant low carbon generating asset for the past decade. Electrification means the introduction of heat pumps to largely replace and displace CHP, and this presents several challenges: 1. Heat pump technology is more expensive per kilowatt than gas boilers and CHP. Most of the time, demands from the heat network occur at levels significantly lower than the peak design load, so sizing heat pumps to cope with these conditions results in high capital expenditure on plant that operates rarely and for short durations. 2. For the residential sector, heat pumps have not been deployed at the scale required by UK policy ambitions, so, in many applications, they should be considered novel. 3. The cost of heat generated for heat pump systems has the potential to be significantly higher than gas because of its reliance on the electricity Grid. There is a certain inevitable cost to decarbonising heat, but there are also design decisions that can impact on how expensive heat becomes, with many decisions becoming a trade-off against other metrics such as carbon intensity. these challenges. More specifically, it looks at how to determine the optimal electrification of heat to deliver, or exceed, the required carbon savings, while mitigating the impact of increased costs. The use of peaking plant, such as gas or electric boilers, to cover the intermittent peak loads on a heat network can offer major reductions to capital and running costs, with marginal impacts to carbon reductions and air quality. Six energy strategies were compared for a typical 500-dwelling residential heat network scheme: air source heat pump (ASHP) and gas boiler top-up; ground source heat pump (GSHP) and gas boiler top-up; ASHP and electric boiler top-up; GSHP and electric boiler top-up; ASHP only; and GSHP only. All hybrid systems were investigated with heat pump heat fractions ranging from 50-95%. An hourly demand model based on operational data from hundreds of existing heat networks was used to optimise heat pump and thermal store sizes, to achieve variable target heat fractions. The hourly load profile has the following inputs: annual domestic hot water (DHW) and space heating loads; DHW hourly profile; space heating hourly profile; and network heat loss. The annual DHW and space heating loads, together with the heat-use profiles, have been used to calculate the required demand for every hour throughout the year (see Figure 1). Many planning authorities, such as the Greater London Authority (GLA), now require carbon-offset payments, which means the cheapest solution is not always the one with the lowest heat pump size. For ASHP-led networks, the increase in capex because of an increase in heat fraction is compensated for by a reduction in carbon-offset payments beyond a 95% heat fraction for a gas hybrid system, and is cost neutral up to this heat fraction for an electric hybrid system. For GSHP-led networks, the increase is compensated for by a reduction in carbon-offset payments up to around 80% heat fraction with gas boilers and around 60% heat fraction with electric boilers. This shows that the current GLA carbon pricing, at 95/ tonne over 30 years, is effective at incentivising onsite The use of peaking plant to cover the intermittent peak loads can offer major reductions to capital and running costs This article investigates how potential heat pumpled energy strategies for heat networks can address www.cibsejournal.com August 2022 29