Site Power-to-heat Ratio

The most appropriate cogeneration system for a site depends to a large extent on the site power-to-heat ratio, defined as15,16  

WGEN = site power generation (not necessarily equal to the site power demand) 

However, a more useful measure of the utility system performance is the cogeneration efficiency. Of the fuel fired in the utility system, some of this energy produces power, some provides useful process heat and some is lost. The cogeneration efficiency recognizes the amount of fuel consumed to produce both power and useful process heat, and can be defined as1516  

Qsupply = E Qfuel + E Qfuel fired heaters boilers 

Hsteam, Hbfw = specific enthalpies of the steam generated in the utility steam boiler and boiler feedwater respectively 

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Figure 23.44 The definition of cogeneration efficiency and site power-to-heat ratio. (From Varbanov P, Perry S, Makwana Y, Zhu XX and Smith, 2004, Trans IChemE, 82A 784, reproduced by permission of the Institution of Chemical Engineers.)...

Introduction of power generation efficiency and heat rate for both power import and on-site power generation makes us ready to answer the fundamental question raised previously from Figures 19.4 and 19.5 Under what price ratio of fuel to power is on-site power generation more economical than power import ... 

In 1956 Paul Flory contributed a pair of papers based on statistical thermodynamics, which proved to have enormous predictive power (21,22). The model was a rod of x isodiametric segments, each of a size that occupied one lattice cell. The quantity x also constitutes the axial ratio of the molecule. The molecules were assumed to he at some arbitrary angle from the preferred axis. The positioning probabilities were formulated as the product of the number of lattice sites available to the initial segment of the chain times the probabilities that the sites required for each successive segment of the chain would be unoccupied, and hence accessible (23). This was entirely an entropic calculation heats of mixing were ignored. Most important, a transition from complete disorder to partial order was predicted to occur abruptly and discontinuously beyond a critical volume concentration, v of polymer. 

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