Performance of working-fluid mixtures in ORC-CHP systems for different heat-demand segments and heat-recovery temperature levels

Abstract

In this paper, we investigate the adoption of working-fluid mixtures in ORC systems operating in combined heat and power (CHP) mode, with a power output provided by the expanding working fluid in the ORC turbine and a thermal energy output provided by the cooling water exiting (as a hot-water supply) the ORC condenser. We present a methodology for selecting optimal working-fluids in ORC systems with optimal CHP heat-to-electricity ratio and heat-supply temperature settings to match the seasonal variation in heat demand (temperature and intermittency of the load) of different end-users. A number of representative industrial waste-heat sources are considered by varying the ORC heat-source temperature over the range 150–330 °C. It is found that, a higher hot-water outlet temperature increases the exergy of the heat-sink stream but decreases the power output of the expander. Conversely, a low outlet temperature (~30 °C) allows for a high power-output, but a low cooling-stream exergy and hence a low potential to heat buildings or to cover other industrial thermal-energy demands. The results demonstrate that the optimal ORC shaft-power outputs vary considerably, from 9 MW up to 26 MW, while up to 10 MW of heating exergy is provided, with fuel savings in excess of 10%. It also emerges that single-component working fluids such as n-pentane appear to be optimal for fulfilling low-temperature heat demands, while working-fluid mixtures become optimal at higher heat-demand temperatures. In particular, the working-fluid mixture of 70% n-octane + 30% n-pentane results in an ORC-CHP system with the highest ORC exergy efficiency of 63% when utilizing 330 °C waste heat and delivering 90 °C hot water. The results of this research indicate that, when optimizing the global performance of ORC-CHP systems fed by industrial waste-heat sources, the temperature and load pattern of the cogenerated heat demand are crucial factors affecting the selection of the working fluid

Multi-Objective Thermo-Economic Optimization of a Combined Organic Rankine Cycle (ORC) System Based on Waste Heat of Dual Fuel Marine Engine and LNG Cold Energy Recovery

“…organic Rankine cycle (ORC) system, uniquely suited for WHR and power generation from low and high temperature heat sources, is regarded as one of the most promising technologies [12,13]. With respect to ORC system researches, efforts are mainly focused on three topics: working fluid (WF) selection [14,15], heat and cold source (e.g., solar, industrial waste heat, geothermal, seawater and LNG cold) [16,17] and system configuration [18,19].…”

Optimum composition ratios of multicomponent mixtures of organic Rankine cycle for engine waste heat recovery

“…Oyewunmi et al 38 proposed a methodology in choosing working fluids in a system with optimum ratio of heat-to-electricity and temperature of heat-source to meet the seasonal changes for different end-users. Oyewunmi et al 38 proposed a methodology in choosing working fluids in a system with optimum ratio of heat-to-electricity and temperature of heat-source to meet the seasonal changes for different end-users.…”

“…For statistically advanced fluid theory, there are many works on the selection of the optimal working fluid for ORC systems. Oyewunmi et al 38 proposed a methodology in choosing working fluids in a system with optimum ratio of heat-to-electricity and temperature of heat-source to meet the seasonal changes for different end-users. White et al 39 presented a novel and effective approach in which the ORC system and working fluid could be thermodynamically optimized in a step, extending to the inclusion of cost consideration.…”

Water Mixtures as Working Fluids in Organic Rankine Cycles

“…In addition to pure fluids as working fluids, many authors also considered the possibility to recourse to mixtures of many different fluids, mainly zeotropic mixtures, and several studies discuss performances of mixture of hydrocarbon, refrigerants, siloxanes and other common ORC working fluids [14][15][16][17][18][19][20][21][22][23]. The main advantage of using mixtures is their temperature glide in evaporation.…”