Cycle Design

It has taken centuries to improve on the 2-4% efficiencies of the very first steam cycles. Both the nuclear and fossil steam cycles are not very different from the very first designs some 300 years ago. Water is boiled at one part of the cycle (where the fuel is consumed) and condensed in another cluster of hardware (with the optimal efficiencies occurring with the lowest condenser temperatures). Cycle efficiencies are found in the range of 30-40% net efficiency for vapor power cycles (plant auxiliary equipment, including emission controls hardware can quickly degrade the plants output and overall performance). Combine the vapor and air cycles together, and efficiencies reach almost 60%. Pushing cycle performance to higher levels is a serious materials and process design challenge. One thought might be to consider changing the cycle completely, and this approach is central to the idea of Chemical Looping.

Table 10.1 summarizes how these technologies compare with and without the ability to chemically isolate the CO2 component, starting with the most widely used technologies (fossil steam), and ending with consideration of technologies not yet on the market (e.g. Chemical looping and Oxy-Fuel). Chemical looping comprises a series of controlled oxidation-reduction reactions, with pathways that incur lower entropy losses than encountered with the combustion process. If this technically challenging process could be deployed, power systems might reach operating with efficiencies in the range of 50-70%, and without the requirement for specialized carbon separation technologies (or air separation). However, the most advanced processes are only on the drawing boards, or using laboratory scale reactors. Demonstrations using calcium as the working medium are underway at universities on the kilowatt scale [14].

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