Book - School of Science and Technology

Chapter 24Combined heat and power (CHP)The concept of providing heat and electrical power from a single station is by no meansnew. Many private hospitals and other public buildings in the British Isles were equipped,80 or more years ago,with plant able to fulfil the dual function. At a time when interestwas growing in the provision of district heating,immediately after the Second World War,a matter which aroused considerable debate was,if heat were to be provided from anelectrical generating station,which was the primary output and which the recovered wasteproduct?The two earliest editions of this book,of 1936 and 1945,included a chapter entitledCombined Electrical Generating Stations which described plants installed to the designs ofthe original authors prior to 1939.These two editions emphasised electrical output with heat being considered the byproduct.In later editions of the book,this last chapter in the text has been changed bothin emphasis and title,concentrating upon the thermal aspects.At the time of writing the current edition,recent legislation aimed at controllingglobal warming by reducing carbon emissions has generated considerable interestin CHP. In addition to government support for CHP projects,`high quality' CHPinstallations (defined later in this Chapter) will be exempt from the Climate ChangeLevy.Basic considerationsHeat generated by a conventional boiler plant is normally obtained at a reasonably highthermal efficiency whilst delivery of electrical power to the end user is very often at anaverage efficiency,with respect to raw energy,of between 35 and 37%. The intentionalwastage of heat at cooling towers is a feature of most generating stations: some smallattempts have been made to use the cooling water for heating but nothing in the BritishIsles compares with the co-ordinated use of energy resources elsewhere in Europe orScandinavia.It is sufficient to say that the least thermally efficient of power stations could be madethe most efficient if use were to be made of the waste heat. Figure 24.1 illustrates thecomparative efficiency of various forms of electrical,thermal and thermal-electric energyproduction.