A well-designed heat network scheme has the potential to deliver on efficiency and provide accurate metering for end users but designing these systems correctly and ensuring they deliver real savings is crucial. Pete Mills, Commercial Technical Operations Manager at Bosch Commercial and Industrial outlines some of the potential pitfalls that need to be avoided when it comes to sizing a heat network.
The main draw of a heat network is its promise of heat provision at a lower cost and in an efficient way. However, the industry, from designers, contractors and operators cannot afford to be complacent, as a small number of failing schemes have demonstrated.
Many district heating designers oversize heat network schemes and the plant heating source as part of a conservative approach; sometimes overcompensating to allow for future expansion and the connection of further networks. There is a lack of experience and guidance on design best practice for heat networks in the UK, and as a result, designers tend to over-specify to allow extra capacity for unknown elements.
This may seem like a minor error at first, only affecting capital expenditure, but in reality, oversizing a network from the outset will have huge ramifications for the long-term efficiency, overall performance, and return on investment of a system.
An iterative approach is therefore necessary, refining elements of the design to ensure that realistic heating and DHW loads are used, which can mean spending more time modelling loads and accurately assessing heat losses.
Oversized and inappropriate boilers
A problem with the design of many smaller heat networks is a lack of renewable or low carbon energy heat sources within proposed schemes, as it is the efficiency gains from these sources that deliver reduced energy costs. Peak load boilers should only operate when the load cannot be met by the renewable or low carbon heat sources and should be selected based on good turndown ratios.
It is important to note that these heating systems spend a lot of their life operating at between 10% and 25% of their peak due to 24 hour operation with periods of low demand, so pumps and boilers need to be sized correctly in order to perform efficiently at these operating points. Opting for boilers with a large turndown ratio is a smart move as the wider the operational range of the boiler plant, the better it can respond to meet fluctuating requirements in the network.
In this scenario, cascades of smaller boilers can be particularly beneficial as the turndown ratio is based across the entire cascade rather than a single boiler. What’s more, selecting multiple boilers in a cascade arrangement ensures even load matching as each boiler will only operate when required. Good turndown, along with good control, help ensure peak load boilers do not flood a system with thermal energy that could have otherwise been picked up by a renewable or low carbon heat source. Wear and tear is effectively shared between the boilers, enhancing efficiency, extending the operational life of the system, and making the chance of a complete breakdown far less likely.
Rather than designing extra capacity into the system, the CIBSE CP1 Code of Practice suggests that top-up standby boilers can be used for additional capacity when needed rather than continuously operating a boiler which is too large for the network.
Common issues affecting the sizing of both the network pipework and heat producing plant are inappropriate diversity factors for DHW demand, and over conservative delivery temperatures. Whereas in the past, the use of inappropriate standards has led to significant oversizing, CP1 has cleared up any doubt about diversity standards with guidance to use the Danish DS439. For delivery temperatures, the guidance contained within HSE ACOP L8 for instantaneously heated hot water, should allow designers to work with delivery temperatures as low as 50°C. These two factors have a significant impact on the overall design which would otherwise distort the network size, efficiency, and cost.
With the goal of reducing network heat loss, pipe work runs need to be carefully considered. It is normal to optimise the pipe runs to reduce cost, but for a heat network, even greater attention is required.
Experience has shown that common issues with poor installation and insulation occur on the lateral runs, leading to some of the most significant heat loss. CP1 gives guidance based on this to consider breaking up the network into more vertical runs, which tend to be better insulated and installed. Where appropriate, smaller pipe sizes, with their reduced surface area, will give lower heat losses; and whilst there is a knock on effect to pump energy use, it can still work out to give the best efficiency overall. The target should be to reduce network heat losses to below 15% with 10% being best practice.
Learn, don’t copy
It’s clear that European and Scandinavian countries lead the way when it comes to well-performing, efficient district heating schemes, with the likes of Denmark using this method to heat the majority of its residential housing. However, it is important that when it comes to designing heat networks in the UK, we consider typical UK operating conditions rather than directly replicating larger projects in other countries.
There are many good UK examples of how well designed and operated schemes have significantly reduced users’ bills and the industry must learn from these by using them as a template to inform future designs. Heat networks should offer the potential for energy cost reductions over many years, which means a long term view is necessary. This way of thinking should permeate the design and operation strategy adopted. It is vital that the end user is properly considered from the outset, how they will be metered, billed and pay must be considered early on. The Heat Trust scheme has done terrific work to establish itself quickly as a voluntary consumer protection scheme, giving end users the confidence they need.
Ultimately, the efficiency of a heat network is dependent upon correct design procedures from the outset. The UK has the potential to benefit greatly from district heating schemes, particularly local authorities and housing associations which are often faced with tight budgets and high efficiency targets. However, appropriate design is crucial in realising these benefits. Correctly sizing a plant and network according to heat demand and using smaller pipes and less pipework to keep heat loss in a network to a minimum are two key factors to consider.
For more information on Bosch Commercial and Industrial and its district heating solutions, visit www.bosch-industrial.co.uk or call 0330 123 3004.