Heat Pumps Today

CMYK / .ai CMYK / .ai CMYK / .ai www.acrjournal.uk/heat-pumps 23 T R A I N I N G that is operated by the HIU only when there’s a DHW demand. A single instantaneous HIU’s will have a higher instantaneous demand compared to a cylinder. However, the time that the HIU is on that demand will be short. Filling a bath may take around 8 minutes, but once filled, there is no demand. If you fill the same bath from a cylinder, the instantaneous cylinder load will be smaller but the time that the cylinder will be reheating will be around 30 minutes or more. This fact dictates that the diversity used for an instantaneous system is far greater than that used for a cylinder system. The result is a longer peak demand for a cylinder system. The shorter peak demand from an instantaneous system allows for that additional short peak to be supplied via a thermal store. The large delta T of the instantaneous system also allows the size of the store to be reduced, saving on plant-room space. Tackling the second assumption… It has been common for engineers to focus on the plantroom first and decide that the network will be a 70/40 system or some other flow and return values. This is an approach not without its problems. It forces an engineer to select a return value that may or may not be achieved. When the HIU is on hot water demand the return will be lower, when the HIU is on heating, possibly higher. Selecting a return temperature at this point is more of a ‘guestimate’. We need to know what the flow temperature is going to be, and this will be typically dictated by the energy source. For example, 55 degrees from a heat pump. Once an engineer knows this, the HIU can be selected and then the actual return temperature calculated. The fact that most heat pumps typically operate most efficiently with a 10-degree temperature difference should be ignored at this point. The heat network needs to operate on the widest delta T possible, to make it efficient, reduce heat losses and reduce the likelihood of the building overheating. After this is done, an engineer should design in a thermal store, which reduces the peak load on the energy centre and has the capacity ensure that the heat pumps can keep running (even when network demand is low). The thermal store should be connected in a two-pipe arrangement reducing mixing, maximising both stratification and the available energy to the network for that given volume. So, how do we ensure that the heat pumps aren’t subjected to a 20 – 30-degree delta T? The answer is to create a ‘microclimate’ around each heat pump. A mixing valve, preferably electronic, can be configured to mix flow from the heat pump into the return to the heat pump to stabilise the delta T at 10 degrees. Each heat pump then operates in series bringing the flow temperature up to the required 55 degrees, for example. Happy heat pumps and a happy network Each heat pump in the array is working in the same way, at a very good COP and can provide energy to the network and the thermal store. Thus, maximising their run time and restricting short cycling of the heat pump. Parry states, “Designing the network around the heat pump, may give you a happy heat pump, but it will give you a very inefficient network. The losses and inefficiency of the network will far outweigh the gains from the heat pump.” The future Instantaneous HIUs ensure that the whole system is efficient. However, the lower flow temperatures of heat pump networks require careful selection of the HIU to meet the tenants needs. The focus still needs to remain on the design and sizing of the network first. If this is not the focus of an engineer, the network as a whole and the end users will suffer. Engineers should welcome the ‘new’ heat pump energy source, which continue to reduce the carbon implication of the electricity that powers them. Moving forward, engineers need the network to be the most efficient it can be and should continue to reduce the losses by maintaining the widest possible delta T, wherever and whenever they can.

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