Model, Simulation and Experiments for a Buoyancy Organic Rankine Cycle

orc2019 Tracking Number 132

Organic Rankine Cycle (ORC) systems are increasingly gaining relevance in the renewable and sustainable energy scenario. Recently our research group published a manuscript identifying a new type of thermodynamic cycle entitled Buoyancy Organic Rankine Cycle (BORC) [1]. Although the cycle itself is not necessarily organic, the publication aimed a renewable energy solution for low temperature systems. In BORC systems, the height of the column-fluid reservoir influences its operation temperature and determines the efficiency of the system. For operation temperatures under the boiling point of water we estimated efficiencies up to 26% for n-pentane (in a 51 m tall water column) and 26.2% for Dichloromethane (in a 71.4 m tall water column). In this work we present two main contributions. First, we propose a refined thermodynamic model for BORC systems accounting for the specific heat of the working fluid. Considering the refined model, the efficiencies for both above mentioned working fluids at temperatures up to 100°C were estimated to be 17.2%. Second, we show a proof of concept BORC system using a 3 m tall, 0.06 m diameter polycarbonate tube as a column-fluid reservoir. We used water as a column fluid. The thermal stability and uniformity throughout the tube has been carefully simulated and verified experimentally. After the thermal parameters of the water column have been fully characterized, we developed a test body to allow an adequate assessment of the BORC-system’s efficiency. According to our BORC model, the working and column fluids are in thermal equilibrium during the expansion. Our experiments focused on test trials tending to the lowest possible temperature of operation, estimated to be 44ºC for a 3 m water column. For the sake of reliability, we reduced the usable height of the column to 2.3 m and we obtained 0.84% efficiency for 46ºC working temperature. This corresponds to 27% of a Carnot cycle working in the same temperature difference. Limitations of the model and the apparatus are put into perspective, pointing directions for further developments.