Effect of auxiliary electrical power consumptions on Organic Rankine Cycle system with low-temperature waste heat source
Résumé
The valorization of low-temperature waste heat (< 120°C) into electricity within industrial processes undoubtedly plays a crucial role in improving overall energy efficiency of industrial processes. Organic Rankine Cycle (ORC) using organic fluid in converting low-grade energy is investigated in this study. For wet gas heat sources, i.e. high moisture contents (T water dew point > 50°C), the optimal evaporation temperature at which the overall efficiency is maximal is below the corresponding water dew point temperatures; then a latent heat will be released from the heat source in the boiler leading to high working fluid mass flow rate in the ORC system, involving a high cold water mass flow rate for the condenser. Thus, the overall efficiency of the ORC system will be significantly impacted by the auxiliary electrical power consumptions of the cold water source for the condenser and can be reduced by about a factor 2 when using a cooling tower to cool the water for the condenser. Therefore, the main purpose is to reduce the auxiliary electrical power consumptions by creating a temperature glide in the heat exchangers using an appropriate refrigerant blend as working fluid instead of a pure working fluid. Results show that a refrigerant blend of R-1234yf and R-245fa is the most promising ORC working fluid for the present application. For an ORC using a heat source with an inlet dry temperature of 110°C and water dew point temperature of 60°C for the boiler, and a cooling tower as a cold source for the condenser, an R-1234yf/R-245fa blend at its optimized mole fractions increases the ORC overall efficiency by about 46% compared to R-1234yf (pure working fluid).