| Keywords |
water, binary, cycle, utilization, efficiency, superhot rock, SHR, organic Rankine cycle, ORC, optimization, working fluid, heat exchanger, steam, turbine, exergy, stage, blade, LSB |
| Abstract |
A water-based geothermal binary cycle may be defined as a power cycle in which geofluid transfers heat to pure water in a non-mixing heat exchanger, producing clean steam suitable for a turbine-generator. This work shows that these cycles are an attractive alternative to organic Rankine cycles (or ORCs) for geothermal resources producing at around 300-350 °C, especially if geofluid characteristics preclude a dry steam or flash cycle. It is known in both theoretical and practical terms that higher resource temperatures entail improvements in thermal efficiency, utilization (or exergetic efficiency), and power output. At currently accessible temperatures, typically 100-250 °C, the preferred type of binary cycle is an ORC, which uses a liquid hydrocarbon as its working fluid. An example is given of how to define a typical ORC and determine its optimal design parameters, using a gradient ascent algorithm with the criterion of maximizing utilization. Curve matching by pinch point equalization within the heat exchanger is noted as an emergent feature of optimized cycles in temperature-heat transfer (or T-q) coordinates. Special attention is given to the sizing of the turbine, in the sense of its number of stages and last-stage blade length. This methodology is then generalized to multi-stage (i.e. multi-pressure) and superheated cycles, with wider ranges of production temperatures and working fluids. It is shown that water outperforms most ORC hydrocarbons except cyclopentane for production temperatures above about 300 °C, and is preferable in terms of cost, safety, and scalability. The performance of these water-based binary cycles is then compared in greater detail against cyclopentane ORCs for a variety of production scenarios at 350 °C, including saturated liquid, saturated vapor, and superheated vapor. Water was found to confer more flexibility in cycle design because of its higher critical temperature and non-retrograde condensation, yielding superior thermal efficiency and utilization, ranging from about 22-27% and 61-75% respectively. It was also found that although ORCs entail a somewhat more compact turbine, water seems to confer more significant advantages relating to the costs of the wells, heat exchanger, and condenser. |