Owing to an increased shortage of energy resources, much attention has been focused on an improved utilization of the energy input in energy conversion-process. Thus, it is important to formulate the appropriate criteria for thermal efficiency in planning and implementing methods for energy conservation.
This paper is concerned with a low grade vapor power cycle which is suitable for energy sources of low temperature such as a solar-thermal, a geothermal or an ocean thermal energy resources. Using the finite-time optimization method, an analytical formulae are developed for estimating the upper limit of heat engine performance, which can be extracted from the given heat-capacity rates of heating and cooling fluids. Optimization implies proper choice of operating temperatures of the working fluid and the optimal allocation fraction of the heat conductance rates between the heating and cooling fluids to obtain maximum power output. The efficiency at maximum power formulated in this paper is compared with observed efficiencies of actual power plants and also numerical results which are calculated with detailed thermodynamic properties of organic working fluids.
In order to improve the energy-conversion performance of a classical Rankine heat engine, non-azeotropic multi-component working fluid cycle (Lorentz cycle) and multi-staged vapor power cycle are suggested. It is found that the maximum power of the Lorentz cycle is twice as larger as that of the Carnot cycle for a given pinch-temperature difference. This result illustrates the advantages in using the multicomponent working fluid cycle over the Rankine cycle. It is also seen that the available work from a heat source with finite heat capacity rates in increased with the number of heat engines, which represents the importance of recovery or bottoming process in the optimization of work production systems.
Using the heat engine efficiency at maximum power, the overall efficiency of practical solar thermal power plants is investigated for estimating the upper limit of their performances.
The formulae derived in this paper provide a useful guide to the achievable performance for real power-conversion systems.
