COMBINED HEAT & POWER


District energy: designing for effective integration


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Low return temperatures and wide delta Ts are the key to making district energy efficient, and are essential to the maximisation of renewable energy use. In this article Neil Parry, global head of district energy, Armstrong Fluid Technology, discusses how multiple energy sources can be integrated and controlled effectively through successful stratification of the thermal store


istrict energy is truly energy source/


technology agnostic,


and multiple renewable and non- renewable energy sources and types can be integrated into the LTHW system. District energy treats waste heat from processes as ‘just’ another energy source that can be utilised on the network. The use of these multiple sources can be phased, based on the grid demand, their availability, and/ or the carbon intensity of each source at any given time. It is also important to remember that heat energy can be utilised by an


absorption heat pump, for example, to create chilled water for a district cooling network. In this way, all that is required is a temperature differential and heating or cooling can be utilised. Should wind energy be abundant, then the electricity can be used to run heat pumps or perhaps electric boilers. For district heating, air source heat pumps may raise the flow temperature to say 55˚C as an example, and then another technology, or different heat pump refrigerant can be used to lift the 55˚C higher if required by the buildings served.


The key to optimising these multiple energy


sources, given their different operational criteria, is effective integration of thermal stores, be that on the network or inside/outside of the plant-room/energy centre. The thermal store should not just be thought of as a simple store of energy, but also as a window on the relationship between energy output or availability and energy demand on the network. Thermal store essentials The thermal store needs to allow energy


sources to ‘run on’ after the demand has dropped, to maximise their use as they refill the thermal store. The thermal store is also crucial in the sizing of the energy sources. In a district


heating network, the peak demand period is extremely short, so it does not make sense to use the peak demand to calculate the energy source size. The energy sources can be sized below peak and the thermal store utilised to bridge the difference. The thermal store should be hydraulically connected in such a way that it can augment the source output thereby increasing total output to meet the peak. Once the peak is over, the thermal store can be replenished by the energy sources during the lower demand times. To do this optimally, the thermal store needs to be connected in a ‘two pipe’ arrangement. 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 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 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. Another reason for maintaining a clear stratification layer within the store is that, once achieved, this layer can be used to control the connected energy sources. The thermal store should be equipped with multiple temperature sensing points down the side of the vessel, five probably being the minimum for a single vessel. The readings from these temperature points can be sent back to the BMS and used to control the energy sources. As an example, if the stratification layer is detected by the temperature points to be in the middle of the vessel and its direction of travel is downward, it can be surmised that the demand in the building is less than the output of the currently operating energy source(s). In other words, the thermal store is ‘filling’ with hot water. At this point, any connected carbon intensive energy sources that are running should be switched off and only renewable, LZC or waste energy source utilised. If the stratification layer continues to travel down the vessel then the waste energy sources should continue to operate until the stratification layer reaches the bottom of the vessel. At this point, all the energy sources should be switched off.


Once the stratification layer starts to rise in the vessel i.e. the demand is now greater than the supply and the hot water in the vessel is being utilised, then the waste energy and possibly the LZC sources should be utilised. The stratification layer continues to be monitored and once/ if it nears the top of the vessel, then the more carbon intensive sources can then be switched on. This ‘control by stratification’ is applicable to any number or variety of energy sources. However, we would recommend a slight change to the philosophy when solely utilising heat pumps. Heat pumps should have their run- time maximised wherever possible. Therefore it is best to have as many heat pumps running for as long as possible, rather than cascading them as would be applicable for other or mixed energy sources. Only if the stratification layer has reached the bottom temperature point would all the heat pumps be switched off and only when the top temperature point is reached, should the heat pumps be switched on. Connecting a thermal store(s) and controlling in this manner, while ensuring that flow never circulates through the store and around the energy sources ensures that the energy sources can be utilised fully and that they are not compromised by a high return temperature. Some sources, such as heat pumps for example, require relatively small operating delta Ts.


18 BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2023 Read the latest at: www.bsee.co.uk


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