34 THE CLIMATE CHALLENGE


DHW STORAGE & TEMPERATURE DIFFERENCES


 storage will, inevitably, reduce the heat network temperature differential. As soon as the store gets even halfway up to temperature, the temperature difference on the primary will be significantly reduced. Data from existing projects show that delta Ts on DHW storage networks typically operate on average around  or less. The smaller the delta T, the more flow rate reuired for a given energy output and therefore the greater energy use. High return temperatures also increase the network losses and can lead to buildings overheating.


There are additional downfalls to DHW stores. One of the most patents of these being that storing the DHW gives an increased likelihood of Legionella growth. Therefore, the temperature of the  store either needs to be ept at, or above,  or regularly cycled to this temperature. This dictates further energy input to achieve , even though the DHW may only be required at 46 -50°C.


f the networ is running at  or , the only way for the store to be lifted to 60°C is to install an additional immersion heater. owever, this immersion heater will need to run far longer than the instantaneous electrical element that is operated by the HIU only when there is a DHW demand.


A single instantaneous HIU will have a higher instantaneous demand compared to a cylinder. owever, the time that the HIU is on that demand will be short. Filling a bath may take around eight minutes, but once filled, there is no demand. f 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.


THE FUTURE


Instantaneous HIUs ensure that the whole system is efficient. owever, 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 networ first. f this is not the focus of an engineer, the networ as a whole and the end users will suffer. Engineers should welcome the ‘new’ heat pump energy source, which will continue to reduce the carbon implication of the electricity that powers them. oving forward, engineers need the networ 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.


Ed Morris is technical manager at Altecnic


THE HEAT INTERFACE UNITS CURRENTLY AVAILABLE TO ENGINEERS ARE EFFICIENT ENOUGH TO BE FITTED ONTO NUMEROUS NETWORKS


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