CHP EFFICIENCY | THERMAL STORES OPTIMISING THERMAL STORE DESIGN FOR CHP In his second instalment on modern CHP design, Remehas Ryan Kirkwood explores use of thermal stores and how this can maximise CHP efficiency T . A well-stratified, charged tank can have thousands of litres of water at system flow temperature Figure 1: A buffer vessels average temperature will be a few degrees less than the heat source flow temperature because of mixing his part of the guide to combined heat and power (CHP) efficiency looks at thermal stores, the control of heat injection, and the factors affecting sizing. The first article (A condensing guide to CHP efficiency, CIBSE Journal, November 2019) analysed different design configurations of CHP, the detrimental effect on performance of pre-heat designs (CHP in series return with boilers), and the advantages of four-pipe and two-pipe thermal store configurations. To optimise a CHPs thermal and electrical capacity, its essential to understand the buildings profile and base load through dynamic building simulation or hour-by-hour calculation and by looking at how a thermal store or buffer vessel is to be used. Having a properly designed thermal store allows the CHP to provide thermal space for discharged heat and give a volume of heat that can be injected into the system when the CHP is at 100% output, reducing boiler firing time. temperature as the heat source. When heat is required, fluid is removed from the warm layer on the top of the store. Under steady-state conditions where heat in = heat out the thermocline layer is static. A well-stratified, charged tank can have thousands of litres of water at system flow temperature (or greater), which can inject heat into the system when the CHP is at maximum load. This will delay the boilers from firing, allowing peak demands greater than CHP maximum thermal output to be satisfied. A thermal store can also provide heat if the systems output is less than the CHPs minimum downturn. This principle applies if the thermal load is small compared with the electrical requirements. In both cases, some manufacturers use a dry-air cooler to dump heat to atmosphere, a practice that Remeha cannot condone. In both examples, the thermal store capacity and charge status dictate how long the boost and alternate thermal path functions can operate. Thermal store Buffer vessel A thermal store stratifies a thermal layer, offering usable heat at the same flow The average temperature of a buffer vessel will be a few degrees less than the heat source flow temperature due to mixing. The usable energy (or exergy) in the non-stratified buffer vessel will be less than a stratified thermal store although both have the same overall stored energy. Turbulence and interacting flows of different temperature water within the tank must be minimised for the thermocline to form. Entering water velocities of less than 0.15m.s-1 and internal sparge pipes or diffusers to reduce incoming water velocities in the tank will improve the opportunity for a stratification and useful thermal storage. Topping the peaks First, decide whether the CHP should be thermally or electrically led. Many design guides offer advice on how to choose the 42 March 2020 www.cibsejournal.com CIBSE Mar20 pp42-43 Rehema thermal store 2.indd 42 24/02/2020 09:50