|
The high temperature fuel cells offer higher electricity generating efficiencies, but their introduction to the commercial market will occur later, and probably at higher cost.
Of special importance in the design of biofuel - fuel cell systems is the selection of the type of fuel cell and reformer (for converting methane to hydrogen) to suit the heat demand. The reforming reaction takes place at high temperature as described elsewhere and recovery of this heat provides a potential source of high grade heat irrespective of the type of fuel cell used. As a result of the lower electrical generating efficiency and the separate reformer heating and cooling requirements, more waste heat is available from the low temperature than the high temperature fuel cell systems.
Overall Thermal efficiency
This is defined as- eff = W/Q
Where W = the net work output
(electrical power)
Q = the net heat input to the reformer and the change in
enthalpies of formation of the products and reactants in the fuel cell.
(See
step 13 of the mathematical model)
Note that this gives a lower efficiency than for
the fuel cell alone, since there is a requirement to put heat into the reformer.
If waste heat from the reformer is also captured then the efficiency will
increase.
Energy Utilisation Factor
The energy utilisation factor, or EUF, is a measure of the extent of productive use of the energy source without considering the heat/power ratio.
EUF= (W + Qu)/Qin
Where W = the net work output (electrical power)
Qu = the useful heat
produced from the fuel cell plus useful surplus heat from the reformer
Qi =
the net heat input to the reformer and the change in enthalpies of formation of
the products and reactants in the fuel cell
It may be possible to achieve EUF figures of over 80% fro fuel cell plant. The best chance of this are in high temperature fuel cells, where the useful heat is of high grade. If this heat is used with a bottoming cycle (eg in a gas turbine) then significant EUF figures above 80% may well be possible.
Heat to Power Ratio
The mathematical models used in the assessment area of this web site, demonstrate the ratio.
Financial assessment methods and an analysis spreadsheet are included in our assessment methods section.
|
|