Performance Analysis of a Solar-Assisted Organic Rankine Cycle

A potential energy crisis prompts the adoption of alternative technologies on our part to address the environmental and economic issues with a sustainable development planning. Renewable energy sources (RES) can be considered as an effective tool towards this direction. Among all RES, solar energy is considered to be the dominant one because of its availability, low cost of use, low CO2 emissions and its multiple ways of being harvested. The objective of this paper is the thermodynamic analysis of a solar powered Organic Rankine Cycle (O.R.C.) and the investigation of potential working fluids in order to select the optimum one. The key thermodynamic properties of a collection of pure organic fluids, which receive great attention in the literature are determined. These properties include the relation of the saturation temperature and pressure, the latent heat and the slope of the curve for the saturated vapor on a temperature-entropy diagram. Next, the elements of a Rankine Cycle are mathematically described and different variations of this cycle are analyzed. By keeping the condensation temperature constant and varying the evaporation and superheating temperatures, important quantities like the cycle thermodynamic efficiency (first law efficiency), the expansion volume ratio and the ratio of the volume flow at the end of the expansion per produced power are calculated. Afterwards, a dynamic model for a solar O.R.C. with a storage tank, which produces electricity is developed. The mathematical model includes all the equations that describe the operation of the solar collectors, the storage tank, the Rankine Cycle and the feedback between them. The model runs for representative days throughout the year, calculating the net produced energy as a function of the selected evaporation temperature for every suitable working fluid. Above that, the temporal variation of the systems’ temperatures, collectors’ efficiency and net produced power, for the optimum organic fluid and evaporation temperature are presented. Primary results show that:  

·        The optimum evaporation temperature at which the maximum work is produced is at least 6οC lower than the critical temperature.

At higher evaporation temperatures, the thermal losses make the system inefficient.

The power production curve follows the trend of the solar irradiance. Thus, the maximum production is observed at the solar noon.

When no Hex is used, the maximum electrical energy production is observed with R245ca being the working fluid (13,15kWhe).

When a Hex is used, the maximum electrical energy production is observed with isopentane being the working fluid (14,84kWhe). R245ca & R245fa are equally effective.