Organic Rankine Cycles (ORCs), HFO-1336mzz-Z, power generation, working fluids, refrigerants
Global demand for power continues to grow as populations and living standards continue to increase around the world. One example is mechanical power from mobile internal combustion engines to drive heavy-duty vehicles (e.g Diesel trucks), ships and rail-cars. Another example is electrical power from decentralized power plants co-generating heat and power from a variety of primary energy sources. Increasing energy prices and increasing awareness of the environmental impacts associated with energy use are motivating an exploration of opportunities to generate power through Organic Rankine Cycles (ORCs) from abundantly available and largely underutilized low temperature heat. Heat at temperatures lower than about 250 oC can be extracted from gases exhausted from truck or marine engines, flue gases from boilers or industrial furnaces or incinerators, hot brine from underground reservoirs (often co-produced with petroleum and natural gas) or solar collectors. The attractiveness of an ORC application depends greatly on the availability of a suitable working fluid. Low Global Warming Potential (GWP) has been added to a long list of specifications the working fluid must meet including no ozone depletion potential, high energy efficiency, high volumetric capacity for power generation, low toxicity, low or no flammability, high chemical stability and compatibility with available lubricants and with common materials of equipment construction. Hydro-Fluoro-Olefins (HFOs) have been identified as a new class of compounds that could enable the formulation of working fluids with GWPs substantially lower than incumbent working fluids. The presence of a double bond in HFO molecules enables the rapid decomposition of HFOs in the atmosphere and reduces drastically the GWP of HFOs relative to similar saturated Hydro-Fluoro-Carbons (HFCs). This paper evaluates HFOs as working fluids in Rankine Cycles for the generation of power from low temperature heat. A wide range of thermo-physical properties of the new fluids were determined. The thermodynamic performance of the new fluids in sub-critical and trans-critical Rankine power cycles was evaluated and compared with incumbent working fluids. The safety and health properties of the most promising candidates as well as their thermal and chemical stability and compatibility with lubricants and common materials of equipment construction were also investigated. Fluid candidates have been identified with attractive environmental, safety and performance properties. One candidate, DR-2, offers both very low GWP and non-flammability, thus breaking an early stereotype about fluids based on HFOs. Moreover, DR-2 shows remarkable thermal stability in sealed glass tube testing at least up to 250 oC (the highest temperature tested to date) despite its unsaturated chemical nature. DR-2 is currently under lab and field testing for various targeted applications.