Compost Waste Heat to Power Organic Rankine Cycle Design and Analysis

This paper presents the research conducted for an industry partner with the goal of developing a waste heat driven, fully renewable and green, power plant. The industry partner was desirous of a compost waste heat driven power plant with the possibility of supplemental solar thermal energy boost, but with the main energy source being compost. The goal is for the plant to operate with a duty of 24/7 on compost waste heat and utilize solar thermal energy to boost power output during the day. This paper discusses the design of a suitable Organic Rankine Cycle (ORC) power plant, the design of a compost driven heat exchanger/boiler, compost pile thermal analysis, Concentrated Solar Power (CSP) hybrid plant analysis, and expected power output analysis for this concept. Furthermore, the selection of isobutane as the baseline refrigerant for this ORC plant will be justified. Analysis was conducted and a feasibility study was carried out in order to determine if the concept is feasible and competitive in the open market. As such, a Levelized Cost of Energy (LCOE) analysis was also performed to ensure that the energy produced at this plant would come at a reasonable, competitive cost. The results shown herein for a hybrid CSP / compost waste to heat isobutane driven ORC powerplant afford an LCOE on the order of 4 ¢/kWh for compost alone and 10.7 ¢/kWh for compost and CSP solar. These are in comparison to the average LCOE for solar stations of approximately 10 ¢/kWh. Paramount to the operation of the compost waste heat to power plant presented herein is the correct design and selection of the heat exchanger which interfaces the compost waste heat stream to the isobutane ORC. The design and analysis of this heat exchanger is given in detail herein.