Exergetic and Economic Analysis of Energy Recovery from the Exhaust Air of Organic Waste Aerobic Bioconversion by Organic Rankine Cycle

The amount of heat rejected by the exhaust air generated by the aerobic treatment of organic waste (OW) was investigated with the aim of evaluating the amount of electrical energy recoverable by a micro organic Rankine cycle (micro-ORC). Both an energetic and exergetic analysis were performed along with an evaluation of the investment costs. The investigation of the heat content and composition of the exhaust air was experimentally performed on a full scale facility processing 32,000 tonnes/year of OW. Results shows that the average exhaust air rate is of about 4,000 Nm3/h with a temperature of 341 K and a relative humidity of 100%. By cooling thi gaseous stream up to 316 K the net power output of the micro-ORC ranges from about 2 kW to about 20 kW. Contemporary the net electrical efficiency decreases from 5% to about 2% whereas the exergetic efficiency ranges in parallel with the net power output from 11% to 1%. Specific investment ranges from about 2,800 €/kW to about 3,900 €/kW and the cost of the electrical energy results of about 0.1 €/kWh to about 0,13 €/kWh.