Topic: Natural Gas CHP Engines
Gas engines emit significantly more NOx than boilers of equivalent thermal output, and as a result this is usually the most significant pollutant when considering the impact of a CHP plant on urban air quality. Again the same advice applies about engaging with the local authority at the earliest possible stage. There are NOx reduction technologies that can be applied to gas engines, but these are generally expensive and require additional space.
Q3. Can absorption chillers be used with CHP to produce cooling as well as heat?
A1. The emissions from a DE/CHP/biomass scheme vary depending on the fuels used.
A2. The Department of Communities and Local Government are currently consulting on the definition of zero carbon. However, the carbon emission factors used for heat from district heating in current building regulations are as follows. Although, all of these heat sources are ‘low carbon’, none of them are zero carbon due to emissions in production, transportation, etc of the fuel.
1: SAP 2005, The Government's Standard Assessment Procedure for Energy Rating of Dwellings, 2005 edition, revision 2. For more information of the definition of zero carbon consultation see: (http://www.communities.gov.uk/)
A3. Combined Cooling Heat and Power (CCHP) is possible with heat generated from the CHP being used by an absorption chiller to provide cooling. Depending on the mix of uses on a district system the absorption chillers can be located in an energy centre and chilled water distributed to buildings in the scheme, or absorption chillers can be located in individual buildings and use heat from the district heating system to provide cooling. In the latter case, the water distributed in the DH system would need to be high temperature as this is required for the absorption chiller to work effectively.
Absorption chillers are however very expensive and require substantial maintenance. Their use generally yields only marginal carbon benefits over highly efficient conventional electric chillers. They are usually only worth considering when there is a true waste heat source where there is no alternative demand for heat. It may be perceived that absorption chillers can be used to make CHP schemes more viable by using heat during the summer but benefits are marginal and usually far outweighed by the substantial capital cost compared to conventional high efficiency chiller systems.