Output of the Evaporation Engine (Sloping Canopy)
Abstract
Thermal technologies for solar power generation include both one-axis concepts (troughs, Compact Linear Fresnel Reflectors) and two-axis concepts (towers, dishes). In general, one-axis concepts collect heat energy at a lower temperature than two-axis concepts. The challenge is to produce electricity as cheaply and reliably as possible, and it is not yet clear which collection temperature or technology will yield the economic optimum, namely the lowest Levelised Electricity Cost (LEC). An alternative third concept is investigated here - power generation from passive solar thermal heat collection. Since the collection temperature will probably be in the range 100-150°C for such a concept, the thermodynamic efficiency will be low, and it will be necessary that both the heat collection process and the heat engine be cheap and simple. The paper describes new simulations for a heat engine powered by passive solar heat collection under a transparent insulated canopy. Two possibilities for the required 'low L1T' heat engine would be the Organic Rankine-Cycle and the Stirling Cycle. In addition to their core mechanical components, these engines need heat exchangers to access the sun's energy and condensers or other heat exchangers to dispose of waste heat, and so their specific capital cost ($/kW) is high. Here, an engine with a thermodynamic cycle based on evaporative cooling of hot air at reduced pressure is adopted (Barton, 2008a). This cycle is summarised in Section 2 and can be achieved in a two-stroke piston-in-cylinder engine that is large, multi-cylinder, slow- revving and lightly stressed, without need for heat exchangers or condensers. An experimental version of this engine has been successfully tested (Barton, 2008b) and many other aspects have been studied, including limitations to the engine speed as a result of incomplete evaporation during re-compression.