The efficiency of CHP-based district energy schemes can reduce energy consumption and carbon emissions. Simon Woodward explains
Combined head and power (CHP) is a technology that has been around for many years, but it is currently undergoing a significant increase in popularity as building operators seek to reduce their energy consumption and carbon emissions.

Clearly, one of the main drivers is the increased efficiency that can be achieved by using CHP in an energy centre serving a district energy network connected to a number of buildings. Indeed, there have been several city-based schemes in the UK for many years - notable examples being Whitehall, Birmingham, Southampton and Leicester - all of which continue to grow. The Olympic Park and associated regeneration of Stratford, London, as well as MediaCity in Salford, have also opted for central energy centres linked to district energy networks.

A range of applications

Now, though, as the technologies have developed, the same principles are increasingly being applied to smaller projects. As a result, CHP and district energy are proving ideal for applications ranging from town centres to housing developments and industrial complexes.

The efficiency of such schemes results from the economies of using large-scale plant and the ability to combine different types of plant. District energy schemes served by interconnected energy centres are also more resilient than stand-alone plant in individual buildings.

Some of these schemes deliver just heating and electrical power, while others also use surplus heat in the summer to drive absorption chillers and generate chilled water (tri-generation). At MediaCity, for example, chilled water from absorption chillers combined with free cooling using water from the Manchester Ship Canal is expected to meet 20 per cent of the cooling load of buildings on the network. This will be supplemented by chillers in the individual buildings.

Nevertheless, the principal demand is generally for heat and, typically, CHP will be one of a range of heat sources in an energy centre. The key to optimising efficiency, therefore, is to operate the system in a way that maximises the efficiency of each heat source.

One common design approach is to size the CHP on its thermal output to meet a constant base load through the year. The power generated by the CHP can be used locally or exported to the grid and, as noted above, surplus heat in summer can be used to drive an absorption chiller.

When CHP is used to meet the base load through the year, additional heating is required to meet the increased base load in colder weather, as well as peak demand at certain times of day. For the winter base load it makes sense to use a low carbon heat source that is efficient at constant loads, and biomass heating is often the preferred choice. Of course, the CHP engine may also be biomass-fuelled and any use of biomass will enable the scheme's owners to take advantage of the Renewable Heat Incentive (RHI). The RHI has the potential to make a significant difference to the financial viability and return on investment of many district energy schemes.

For peak loads a heat source that is able to respond quickly and efficiently to changes in demand is required - such as gas-fired condensing boilers.

Clearly, this is just one possible scenario and each project will vary in terms of its precise requirements and optimum solution. For example, a CHP scheme serving an industrial complex is likely to experience higher heating loads during the day - whereas peak demand in a housing development will be in the evenings and at weekends. A mixed development combining commercial and residential properties offers a more constant base load through the day. So all of these issues need to be considered when selecting the best mix of heating plant.

Another major advantage of CHP-based district energy schemes is that they are very flexible, in terms of both heat sources and scale. For example, a project that starts its life with a combination of CHP and boilers may have a solar thermal system or a ground source heat pump introduced at a later stage. In such cases, careful design to ensure the various heat sources are integrated to optimum effect is essential.

District schemes are scalable

District energy schemes are also scalable so that they can be extended to bring in more properties and many of the schemes that have been mentioned continue to expand. The fact that such schemes use the same temperatures and pressures as conventional heating and cooling systems makes it very simple to add existing properties and will result in 5 to 10 per cent energy savings for most consumers.

For all of these reasons, we can expect to see continued growth of CHP-based district energy schemes and building services engineers can expect to see more of their clients enquiring about CHP. So it pays to be up to speed with the latest developments and, very often, partner with specialists that have the experience to deliver the best solutions.

• Simon Woodward is chief executive office of Cofely District Energy and chairman of the UK District Energy Association