Working fluids for low-temperature organic Rankine cycles

Abstract

A thermodynamic screening of 31 pure component working fluids for organic Rankine cycles (ORC) is given using BACKONE equation of state. The fluids are alkanes, fluorinated alkanes, ethers and fluorinated ethers. The ORC cycles operate between 100 and 30 1C typical for geothermal power plants at pressures mostly limited to 20 bar, but in some cases supercritical pressures are also considered. Thermal efficiencies Z th are presented for cycles of different types. In case of subcritical pressure processes one has to distinguish (1) whether the shape of the saturated vapour line in the T,s-diagram is bell-shaped or overhanging, and (2) whether the vapour entering the turbine is saturated or superheated. Moreover, in case that the vapour leaving the turbine is superheated, an internal heat exchanger (IHE) may be used. The highest Z th -values are obtained for the high boiling substances with overhanging saturated vapour line in subcritical processes with an IHE, e.g., for n-butane Z th ¼ 0.130. On the other hand, a pinch analysis for the heat transfer from the heat carrier with maximum temperature of 120 1C to the working fluid shows that the largest amount of heat can be transferred to a supercritical fluid and the least to a high-boiling subcritical fluid. r

Układy gazowo-parowe z CO2 jako czynnikiem roboczym

“…W tym kontekście można wyróżnić czynniki "mokre" (jak para wodna), czynni-ki "suche" oraz czynniki izentropowe. Wykorzystanie czynników "mokrych" wiąże się najczęściej z zastosowaniem procesu przegrzania pary w celu podwyż-szenia stopnia suchości czynnika po procesie rozprężania [7]. W energetyce oprócz pary wodnej stosuje się także wiele innych czynników roboczych.…”

Thermodynamic comparison of organic Rankine cycles employing liquid-flooded expansion or a solution circuit

“…After the expansion process, the working fluid is separated from the flooding medium (8). The flooded expansion guarantees that the working fluid is still significantly superheated at the expander outlet.…”

“…To this end, there have been numerous studies that have investigated different working fluids for an ORC [4,5,6,7]. It is clear that the choice of working fluid has a substantial impact on cycle efficiency as well as on system and component design [8]. Selection of a working fluid requires the evaluation of tradeoffs involving efficiency, flammability, turbine volumetric flow ratios, vapor pressures in the condenser, toxicity, cost, and more recently, Global Warming Potential (GWP) and Ozone Depletion Potential (ODP).…”

A thermodynamic criterion for selection of working fluid for subcritical and supercritical domestic micro CHP

“…That is unfortunately not in line with International protocols which pushed researchers to investigate new environmentally friendly fluids which could serve as substitutes. Contributing in this direction, performance of carbon dioxide, synthetic and refrigerant mixtures in different types of cycles are being examined by various researchers, [7][8][9][10][11][12]. Especially CO 2 is expected to be a long term substitute and a major fluid in refrigeration and air-conditioning.…”

“…There are examples of papers regarding selection of the fluid for waste heat applications [4,6], solar installations [3,5]. Some selection criteria have been put forward by various authors [7][8][9][10][11], incorporating thermodynamic properties, provided in literature but these do not have a general character. After these papers the expected features of a good fluid are: low specific volumes, high efficiency of thermodynamical cycle, high latent heat, moderate pressures in the heat exchangers, low cost, low toxicity, low ODP and low GWP among others.…”