Energy efficiency A
round 3% of UK buildings are connected to district energy networks currently, but this is expected to increase rapidly. The
UK government has stated that, by 2050, 20% of buildings need to be connected to district energy networks. A key component is the effective integration of thermal stores. The thermal store is not just a simple store of energy. It also functions as a window on the relationship between energy output or availability and energy demand on the network. The thermal store is a ‘reservoir’ of energy ready for periods of high demand. It allows low carbon sources to carry on running even when demand is low, maximising the utilisation of the low carbon source as they refill the thermal store and create greater flexibility and reliability of the system as a whole. In a district heating network, the peak demand period is short and, for the majority of the time, the load will be less than half of the calculated peak. This can allow, with careful sizing of the thermal store, for the total capacity of the energy sources to be lower than the calculated peak energy demand. The thermal store is utilised to bridge the difference from total output to peak demand. The thermal store is utilised during peak demand times and, once the peak is over, the thermal store can be replenished by the energy sources during times of lower demand.
Integrating CHP
CHP can be an extremely useful contributor in a district system, when it is integrated, controlled, utilised, and maintained correctly. It offers two different energy outputs, providing greater capability for the energy grid to meet peaks in demand, and giving additional security of energy supply at times when inputs from solar or wind, for example, may not be available. Strategically-located large scale CHP, for example, fits well into a ‘smart grid’ particularly in locations where the nature of the existing housing or building stock requires higher temperatures from the district network than would normally be required for newer building or in locations where the electricity availability from wind or solar is low. The electricity produced by the CHP can also be used to power large scale heat pumps or electric boilers to further increase the thermal input into the local buildings (loads permitting). Locating CHPs as close as possible to where they are needed, reduces the losses associated with
“Around 3% of UK buildings are connected to district energy networks currently, but this is expected to increase rapidly. The UK government has stated that, by 2050, 20% of buildings need to be connected to district energy networks.”
12 September 2024
www.heatingandventilating.net
Figure 1: thermal store hydraulic connections (single and double stores)
Integration of multiple sources into district energy network
A key challenge facing our industry is how to integrate renewable sources (such as solar, heat pumps, waste heat from processes, and CHP employing biogas and biofuels) into district energy networks effectively. Andrew Harrop, global director – regional sales enablement centres, Armstrong Fluid Technology, how asks how this can be achieved?
Left: Andrew Harrop, global director – regional sales enablement centres, Armstrong Fluid Technology
electricity transmission and the thermal losses of LTHW transport. On smaller systems where thermal storage
capacity is limited it is important to maximise the energy contained in a given capacity. On this type of application, small modulating CHP units are ideal. The thermal store allows the CHP to operate for a longer period of time, even when the thermal demand has dropped below the minimum output of the CHP. This allows the CHP to continue providing
electricity, either into the building or back to the grid. To maximise the amount of energy within the thermal store it needs to be connected in a ‘two pipe’ arrangement (see Figure 1). One pipe into the top of the store, from the flow and one into the bottom of the store, from the return. The pipe- work connecting the thermal store is therefore bi-directional. The Flow (hot) or the return (cold) can be passed through to store, charging or discharging the vessel. This method creates stratification within the store. Control is such that flow cannot circulate through the store, safeguarding the operating delta T and thereby ensuring that the amount of energy for the given volume is maximised. There will be a small area in the store where mixing will occur – the stratification layer - but the goal is to prohibit mixing and ensure the wide delta T of the network is maintained in the vessel. It is therefore common to see thermal stores with anti-mixing baffles and diffusion connections in the store.
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