Innovative organic rankine arrangements for water savings in waste heat recovery applications

Abstract

Water is a natural resource of vital importance in the Singaporean context because of its high cost and scarcity. Indeed, 430mil gallons a day are consumed by the Singapore end-users, 45% domestic and 55% industry, with the latter expected to increase in the years to come by more than 60%. A significant amount of water is used for cooling towers which, in a hot and humid environment like that of the South-East Asian countries, have to operate extremely hard to reject heat from waste heat sources. While economics often limits the feasibility of low temperature Waste Heat Recovery (WHR) (40°C – 200°C), the large amount of waste heat available represents a huge opportunity for energy savings; indeed, the gaps remain such as maximise energy output, cooling loop in the condenser, smaller and compact size waste heat recovery systems, stabilising waste heat source and quality of waste heat. One valid alternative to reduce the cooling load in cooling towers (i.e. less water consumption) is represented by Organic Rankine Cycles (ORCs), which are mainly used to maximize the electricity output from a waste heat source. The operating principle of ORCs is the same as that of steam power plants and the main advantages of ORCs are simple and compact system structure and widely used heat sources and availability in recovering low temperature heat energy. In small scale, ORCs have not been commercialised due to techno-economic factors (i.e. low efficiency and payback period) and therefore there is a significant drive to find suitable solutions for electric power output between 10kWe and 100kWe. The paper deals with the development of key enabling components for small ORCs systems aimed to minimize water consumption from cooling towers by means of innovative ORC plant arrangements based on Turbo-Expander (TE) pumping system and internal regeneration processes. By means of this novel arrangement, improvement in global ORC performance are observed, reducing at the same time the cooling load at the condenser. The plant arrangement is described into details and results for different low temperatures WHR are presented, highlighting the benefit in terms of m3 of saved water when Toluene and R245FA Working Fluid (WF) are adopted.