HYBRID HEAT PUMP SYSTEM | OPTIMISING PERFORMANCE Figure 3: Heat injection, return header The control strategy warrants greater discussion than can be covered in this article. Our advice, however, is to ensure a clear controls methodology is aligned during the design stage. Care should also be taken in the selection of the thermal store, to avoid mixing by making use of baffles, sparge pipes, and so on. Injection method This approach uses the gas boiler to boost the flow temperature to set point at times when the ASHP is unable to satisfy the demand differential. There are many permutations of this method, a variant of which is used on the flow header to correct underperforming biomass boiler set-ups. This particular method, however, works on the return header prior to a low loss header (LLH). The thermal store discharge pump controls the charge and discharge cycle of the thermal store. Under low load conditions the store may be charged to system temperatures, to give usable heat without the requirement of the boilers cascading on (albeit for a finite period). During periods of high demand the thermal store injection will pre-heat the return for top up by the boilers. Ensuring proper discharge/charge cycling of the store is critical to the performance of this method. Pre-heat efficiency loss, net efficiency gain. We lose approximately 1-2.5% overall efficiency in a gas boiler when preheating a 30C return by 5-10K, because of slightly reduced condensing efficiency. This modest loss in efficiency is massively countered by the renewable element of the ASHP efficiency. Provided the ASHP is not preheating the boiler return above condensing temperatures, the slight loss is acceptable to achieve a net gain. Avoiding conflict We have often seen primary and secondary heat-generation methods fight against each other, at the cost of system efficiency. So, its crucial that flow and return temperatures, Ts, controls and, ultimately, the detailed hydronic design are taken into careful consideration when blending the technologies. Addressing and avoiding this conflict at the outset will help ensure optimal system performance and outcomes. Along with standalone, purpose-designed ASHP systems, hybrid systems offer the opportunity for important efficiency gains and emission reduction from heating in existing and new commercial premises. In future articles, I will explore DHW generation and where this sits within standalone and hybrid heat pump systems, and take a detailed look at more innovative methods of hybrid heat generation. CJ RYAN KIRKWOOD is the heat pump business development manager at Baxi Heating 46 November 2021 www.cibsejournal.com CIBSE Nov21 pp43-44, 46 Hybrid heat pumps.indd 46 22/10/2021 18:22