Opportunities for an oversized heat network
By Patrick Elwell, Sustainability Analyst at CBx
A network can be classed as oversized when the installed heat capacity is greater than the heat demand. This can happen for a number of reasons such as poor network design to changes in building fabric and occupancy. Having an oversized system can have negative consequences. Heating systems typically reach their highest efficiencies when operating at full output rating and for an extended period of time. Depending on how the equipment is setup, an oversized system will either, a) operate at full output for short periods, or b) operate at a low output but for longer periods. Neither options are particularly ideal.
Operating at a full output will mean the system can follow the load and call for heat when it is needed. However, because the capacity is oversized, the peak load is achieved at much faster rate after which the system will shut down. This process can repeat itself many times an hour which can diminish the life of the equipment and result in higher maintenance costs. Operating at a low output is not much better. Whilst this setup will mitigate repeated system cycling, the overall plant efficiency is reduced. This results in higher fuel costs, longer payback periods and fewer carbon savings.
There are options available which can help resolve this issue. Thermal storage can allow heat networks to run at their highest output and for longer periods. Storage acts as a buffer between network and customer, thereby smoothing the load profile. A thermal store can also be advantageous if a CHP is the lead source of heat generation. Operators can charge the store during the day and sell the electricity generated back to the grid, taking advantage of day time rates. As heat demand then increases during the evening, the heat demand can be met by the store. Typical costs for thermal storage is £962 per m3.
Another option is to look for potential network expansion. Some networks are deliberately oversized to make sure they are future proof and can connect to other customers later down the line. The Heat Networks Delivery Unit (HNDU) has made £320m of funding available for heat network development in the UK. This is a Government initiative to lower the UK’s carbon emissions and make heat more available in fuel poor areas. So far, over 120 local authorities (LA) have been awarded finding for exploring heat network opportunities. As a result, there is a lot of potential networks coming on line in the not so distant future. Some of this new heat load can be met by tapping into oversized systems already installed. This may result in some areas that were previously considered uneconomical to have a network finding themselves with a fresh opportunity. However, encouraging the ‘sharing’ of networks is a challenge. There are many barriers that must be overcome if this is to become more widespread, such as risk inheritance from legacy systems, maintenance responsibility, etc.
Heat networks have the potential to supply low carbon heat to fuel poor customers. However, as heat networks are becoming more widespread, customer scrutiny is starting to grow. There are questions being raised as to whether heat networks are indeed low carbon and cost effective. As a result, the UK Government is under pressure to show that heat networks provide good value and contribute to lowering carbon emissions. The industry itself has come a long way by creating best practice guides to demonstrate how networks should be. However, little has been done to optimise the existing networks which is what public opinion is based on. Government should also be looking at how existing oversized/under-performing networks can be tapped into as a resource. By doing so, public faith in heat networks can be restored and the UK can move forward in meeting its low carbon heat targets.