The truth about CHP efficiency

He must be aware that the CHPQA standard has been revised (in November) to accord with EU Directive 2004/8/EC on Co-Generation, which states CHP comparisons should be made against a “same fuel category”.

This directive, and the CHPQA Standard issued by DEFRA (and acknowledged by BERR and DUKES), gives engineers a clear calculation path that ensures real primary energy savings (PES).

The EU Directive acknowledges that all fossil fuel energy sources are a finite resource, and that it is no longer acceptable to consume any fuel (including biofuel and fuel from waste) inefficiently. If we burn vast quantities of fossil fuel inefficiently we will not be reducing our overall carbon consumption, and we will be shortening the time taken to exhaust the world’s fuel reserves. This is a fact.

The article states that heat from a CHP installation has a very low (or zero) carbon factor; however if we apply the principle of comparing against a same fuel category, we find that CHP heat has a carbon factor of about 0.13kgCO2/kW.h for a typical “good quality” 500 kWe gas-fired CHP installation. This is lower than the carbon content of heat supplied by a SEDBUK A boiler (0.22kgCO2/ kW.h) and is the reason why CHP can still make modest energy savings, provided all available heat is used efficiently. To be clear, we should continue to encourage insulation of existing housing stock to reduce energy consumption.

The sad fact is that CHP can only make primary energy savings when there is a heat load, and as a result, CHP systems should be controlled so all heat is used efficiently. This means CHP systems must be shut down when there is no heat demand (and should certainly not be used for driving inefficient absorption chillers).

Modulating CHP to be heat-led is not a new concept, it is what happens in Denmark’s much-admired systems and should be the template for any district heating system installed in Europe. Review of DUKES 2007 (Table 6D) shows that the average of all UK gas-fired combined cycle CHP installations in 2006 made overall primary energy savings of minus 1%.

Clearly these CHP installations need to be significantly improved to start making real energy savings. This is what engineers should be doing – not arguing that the targets should be easier. It is time for all engineers to accept that we need to improve energy efficiency in all our designs. If engineers cannot agree among themselves, what hope is there for those we seek to advise?

Perhaps we can all agree to comply with the EU Directive before the 10th anniversary of the introduction of “greenwash” into the Oxford English Dictionary. Now, who is going to tell the politicians the truth about CHP efficiency?

James Thonger, associate director, Arup