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Optimising District Heating

Optimising District Heating

While district heating may have had something of a chequered history in the UK in the past, the last few years have seen a considerable resurgence in the popularity of such systems, often ‘re-styled’ as heat networks.

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This is in line with the move to central energy centres, makes it easier to deploy both traditional and low carbon heat sources together. As a result, district heating is set to play a key role in the UK’s carbon reduction strategy and is also central to the European heating and cooling plan. The UK government is also making funding available to help local authorities explore the feasibility of district heating systems in their areas.

Achieving optimum efficiency requires a whole system approach

Nevertheless, there have been concerns regarding the overall efficiency of district heating. When the situation is examined carefully, however, it is clear that where district heating systems have delivered disappointing performance, this has usually been the result of non-optimal system design.

The key point here is that achieving optimum efficiency requires a whole system approach. Specifying plant that is inherently efficient is certainly one part of the equation but the rest of the system needs to be designed to exploit that inherent efficiency.

In the majority of cases this will also involve integrating different heating technologies and ensuring they work in harmony. Beyond the plant room, as is made clear in the CIBSE/ADE Heat Networks Code of Practice, the management of hot water flow rates and flow and return temperatures is also critical.

District heating in single, multi-occupancy buildings

The majority of district heating systems that are currently being installed are serving single, multi-occupancy buildings or relatively small groups of buildings from a central plant room. As smaller combined heat and power (CHP) plant becomes available, many of these schemes take advantage of the ability to generate both heat and electricity. In addition, the CHP may be backed up by other heat sources that can include biomass boilers, gas or oil fired boilers, heat pumps and solar thermal.

In the case of individual multi-occupancy buildings, space heating and domestic hot water are typically controlled, and possibly metered, by heat interface units (HIUs) within each of the spaces. This is relatively straightforward as the temperatures and pressures will be consistent throughout the building, and well-designed HIUs are able to adjust to variable demand from different spaces.

Where more than one building is involved there can be considerable variation between the temperatures and pressures required by each building. This is especially the case with schemes that incorporate a number of different building types - an approach that is generally desirable because mixed demand patterns spread heat loads more evenly.

In such cases, each building needs to be fitted with a sub-station that is capable of converting the hot water temperatures and pressures provided by the energy centre to the operating conditions required by the building. Each sub-station will typically comprise a plate heat exchanger, pumps and heat meters – ideally in a compact packaged configuration to minimise plant room space requirements.

Central Control

In the energy centre itself it is essential that the various types of heating plant are controlled to take advantage of the performance characteristics of each. For example, the CHP unit could be sized on its thermal output to achieve a constant base load throughout the year, with the power generated being used onsite, exported to the grid or both. Surplus heat in the summer can be used to drive an absorption chiller to generate chilled water for air conditioning.

In addition to this relatively constant load, there will be a higher base load in winter as heating systems are switched on. Here, a biomass boiler will often provide a very effective back-up to the CHP.

In such cases, the control parameters need to recognise that biomass boilers are not designed for rapid on/off firing and allow sufficient tolerances for the biomass boiler to meet the set-point temperature. A buffer vessel should be specified to be fitted between the biomass boiler and the heating system to ensure optimum boiler run times are achieved.

Multiple heat sources

However, it’s also important to make provision for peak loads with heat sources that are highly responsive and efficient at variable heating loads while maintaining constant flow temperatures. An obvious solution is to incorporate fully modulating gas or oil condensing boilers.

If the energy centre is also producing domestic hot water (DHW) then minimum storage high output calorifiers or plate heat exchangers feeding into stainless steel buffer vessels should be considered. This arrangement provides an opportunity to include solar thermal heating or heat pumps in the mix, using them to pre-heat the cold mains water. One of the other heat sources would then be used to bring the water up to the required temperature.

Clearly, with such a diverse mix of heat sources the controls will play a major role in preventing conflicts between the different types of plant. It is therefore important to specify controls that are capable of managing single or multiple conventional and low carbon heat sources, with functionality such as real-time monitoring and visualisation.

The important thing is that with any district heating system served by a multi-source energy centre, it is essential to ensure that all of the systems work efficiently together and that hot water temperatures and pressures are properly controlled at each building. A holistic approach to system design and the use of compatible components within the system is the way to deliver the best solution for the end client.