COMBINED HEAT & POWER
CHP and sector coupling: practical approaches for district energy networks
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CHP can be a major contributor to district energy networks, providing localised electricity generation and utilisation of its heat by-product, into the district energy network. Neil Parry, global head of district energy, Armstrong Fluid Technology explains to practice of sector coupling and its potential to be used in the UK
he electricity it produces can be utilised within the local buildings or the excess sent into the electricity grid. Utilising this electricity in the grid adds CHP to the overall sector coupling approach where the
electricity produced can be used, stored or converted into a different energy type and thereby better utilised in a smart city energy strategy. This approach – sector coupling – is already well-understood and is starting to be established in some Scandinavian and European countries. It also has immense potential here in the UK. Heat generated by the CHP plant can be combined, on a network-wide basis, with waste heat created by industrial processes, as well as at sites such as hospitals, data centres and wastewater treatment plants. The heat is captured, potentially stored, and then utilised within the district energy network to provide heating, domestic hot water and possibly cooling for the connected buildings, such as housing, apartment blocks, offices and commercial buildings. From both economic and environmental perspectives, sector coupling can make a major contribution, easing fuel poverty, ensuring greater renewable energy utilisation, and reducing our carbon impact, whilst enabling local authorities and national government departments to regain control in a volatile energy market.
So how can CHP be effectively integrated into district energy networks using the sector coupling approach? And what technical and practical factors do you need to consider?
CHP’s role in sector coupling
Sector coupling links energy streams and allows energy to be stored or converted in various forms to match changing demands as well as changing outputs during a typical 24 hour period. District Energy is truly energy source agnostic, and many different types of energy can be harnessed and integrated into the district systems. Integrated, maintained, controlled, and utilised correctly, CHP is an extremely useful tool in a district system, offering two different energy outputs and therefore adding greater flexibility to the energy grid to meet peaks in demand, or providing additional security of energy supply at times when wind or solar, for example, may not be available. Large scale CHP, for example, located strategically, 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 electricity transmission and the thermal losses of LTHW transport.
16 BUILDING SERVICES & ENVIRONMENTAL ENGINEER OCTOBER 2023 Thermal store essentials
One vital component in the successful and efficient optimisation of multiple energy sources 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.
Smaller heat networks and communal systems
On smaller systems where thermal storage capacity is limited, maybe in communal networks, for example, 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 can ‘fill’ the store (hot), or the return (cool) can ‘fill’ the store. 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
Figure 1: Thermal store hydraulic connections (single and double stores) Read the latest at:
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