ENERGY CHP & DISTRICT HEATING
In practice, a higher emissions factor
than the grid average is justifi ed, on the basis that the CHP generation is not offsetting the nation’s relatively low-carbon baseload generation (eg nuclear) but is instead affecting the output of the UK’s higher carbon marginal generation, such as from coal-fi red plants. By virtue of the way the grid operates, nuclear or wind energy will not be displaced so the use of an average emissions factor is not as appropriate. This does not mean that coal- fi red generation is used as the sole offset; it simply means that it is justifi ably taken into account. The letter by Paul Woods, in April’s
Journal, clears up the confusion between the actual CO2 savings that CHP can deliver and the relative savings of gas-fi red CHP compared with new gas-fi red power stations. Using realistic performance fi gures, Woods proved in his paper presented to the 2011 CIBSE Technical Symposium that the CO2 content of heat from a gas-fi red CHP district heating system clearly provides a saving when compared to heat from gas-fi red boilers (visit
http://cibse.org/symposium2011). The analysis took account of a range of electricity emission factors and considered typical district heating boiler use (20% of total), heat losses (at 15%) and pumping energy (1% of heat supplied). As Woods reminded us in his April
A combined heat and power installation
is not to argue national fuel strategy on a building-by-building basis but to deliver real projects on the ground that are focused on reducing running costs, achieving a return on investment and striving to go beyond the minimum emissions performance required by Building Regulations.
Emission factors The emissions savings attributed to the cogeneration of heat and power can be a source of confusion for some. The simplest interpretation is to consider that the generated electricity offsets carbon emissions at the grid average factor. Put simply, if the CHP unit were not generating, then the electricity would instead be supplied from the existing electricity grid at the average emission factor.
www.cibsejournal.com
2012 comment, the analysis showed that a signifi cant carbon saving is still achieved when compared with individual gas boilers, even when assuming CHP generation displaces a highly effi cient combined cycle gas power station producing electricity at an emission factor of 400g CO2/kWh. Since research at Imperial College has established that the likely fi gure for displaced electricity (operating marginal) will be around 600g/ kWh in 2016 and 511g/kWh in 2020-20251, it can be seen that appropriate use of CHP will provide signifi cant carbon savings for years to come. Considering the use of primary fuels to
the exclusion of any other analysis (by only allowing comparison between identical fuel types) can result in absurd conclusions when carbon emissions savings are the actual goal. For example, if this approach were correct, then operation of a biomass- fuelled boiler plant could not be responsible for any carbon savings because it could only be compared to the operation of other biomass boilers. The March article was correct to state that
Gas Boilers 26.1%
CHP 49.8%
Biomass 24.1%
Relative contributions of plant to the projected heat loads of the Olympic Park’s legacy developments in the year 2020 (Source: District Heating Concessionaire for the Olympic Park)
WHAT THE GOVERNMENT SAYS…
Heating networks are currently estimated to provide less than 2% of total heat in the UK. The Government believes that heat networks have an increasingly key role to play in the UK energy system and this decade will be crucial in removing the barriers and beginning deployment.’
[Heat networks] can be upgraded over time according to local and national priorities, without impacting on
consumers. For example, it may be economic in the short term to power a network with gas CHP, and to replace this with a lower carbon alternative such as biomass CHP in the medium to long term. In-building heat sources can also be replaced over time, but in many cases it may be easier to replace in-building heat sources once, to switch to district heating, and then replace the central heat source when appropriate, than to frequently replace the in-building heat source.’
Heat networks are best suited to areas with high heat demand density. They can be an excellent choice in urban areas, providing individually controlled and metered heat as reliably as gas boilers. Heat networks can also serve buildings like blocks of fl ats where individual gas boilers may not be an option.’
Source: The future of heating: A strategic framework for low carbon heat in the UK, DECC, March 2012. The quotations can be found respectively on pages 95, 63 and 59
May 2012 CIBSE Journal 39
Phil Jones
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