ENERGY CHP & DISTRICT HEATING


Variation of heat emissions factor for various heat sources


CO2 emission per kWh electricity (g/kWh)


Comparative carbon savings achieved by CHP-led district heating (Source: Paul Woods paper to the CIBSE Technical Symposium 2011. Visit http://cibse.org/symposium2011)


It makes sense to use CHP to decarbonise now, rather than taking a wait-and-see approach that fails to reduce emissions at all


as grid electricity supplies decarbonise in the future, the emissions savings achieved by fossil fuel CHP will diminish. However, significant savings are achievable today and will continue to be achievable for far longer than the lifetime of a single CHP unit, based on current forecasts. It makes sense, therefore, to use CHP to decarbonise now, rather than take a wait-and-see approach that completely fails to reduce emissions at all.


Energy sources CHP and district heating are not inextricably linked and gas-fired CHP is not the only viable technology used in a modern district energy system. For example, two of the UK’s largest and most successful district energy schemes, in Sheffield and Nottingham, already supply heat from their energy from waste plants, each delivering thousands of tonnes of emissions savings to their heat consumers. In addition to gas-fired CHP, local waste


DEFINITIONS


Though the terms district energy and CHP (see below) are often confused, it is worth noting for clarity that district energy (both district heating and cooling) refers to the insulated pipe networks that carry thermal energy from a source to consumers, in conjunction with the plant used to produce that thermal energy.


Combined heat and power (CHP) is the simultaneous generation of useful thermal and electrical energy. CHP is frequently used to generate the low carbon heat used in district energy networks, though it is by no means the only technology that is used.


to energy and biomass plant with good- practice emissions control systems can all operate at the centre of a district energy scheme today. Once the original low- carbon energy plant has reached the end of its lifetime, the district energy scheme can utilise new best available technologies to continue providing the lowest carbon heat that is feasible. In this way, every one of an energy scheme’s consumers can be collectively and progressively decarbonised without relying on piecemeal action by individual parties. This future-proofing approach will be central to the success of the district energy


40 CIBSE Journal May 2012


scheme on the Olympic Park site in east London (see the Journal, August 2011, page 16). Developers are already bidding to build on the land that will be freed up after the event, and their developments will be utilising the low carbon heat supplies available from the district energy system. The Kings Yard energy centre has substantial space provision for an increase in energy plant to meet future loads. Low carbon technologies that are technically feasible to use with district energy networks are not limited to the few mentioned in the March Journal article, and we can expect that there may be more developments in the approach to 2050, potentially including financially viable fuel cells or small-scale carbon capture and storage. Additionally, the potential future role of CHP plus heat storage acting as operating reserve for the grid should not be ignored.


Whereas an individual building might


have its own low carbon plant (such as a biomass boiler or heat pump) that has to modulate against its individual heat demand profile, a district energy scheme brings multiple consumers into a diversified load profile, providing a significant baseload against which a larger item of low-carbon plant can operate at peak design output for a far higher proportion of the year. Consequently, efficiency savings


are achieved not only from substantial economies of scale but also from the far more efficient operation of low carbon plant. The low carbon plant in a well- designed district heating scheme can provide the majority of the annual energy needs, with fossil-fuel boilers merely providing top-up heat at times of peak demand (or at heat demands too low for the turndown ratio of the low carbon plant) and also acting as back-up plant to ensure system resilience. Assessment of a district energy scheme’s feasibility is essential on a case-by-case basis. The implications of thermal distribution losses and pumping energy requirements should always be considered as part of the feasibility. It is important to note that installation of a district energy network can be very expensive, and a system’s operational efficiencies will need to be good enough to make the network installation feasible, in both financial and carbon terms. When it comes to heat loss, the 1C loss per km figure mentioned in the August


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CO2 emission per kWh heat (g/kWh)


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