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Canot 1Tc h (4) A. Suitable working fluid A working fluid is an important component of the ORC. It determines the efficiency and the economics of the system. The challenge relies in the commercial availability and choice/selection of the fluid. Most of fluids today are phased out by the international protocols and new ones with environmentally friendly properties should be sought. Few criteria for suitable fluids are [1, 3, 8]: appropriate critical parameters, positive or large slope, high thermal stability and compatibility with materials, low environmental impact, high safety level, good availability and low cost, high performance (power/energy-exergy efficiency), low levelized electricity cost, plant low specific installation cost, etc. Combining these criteria to find a suitable fluid in a set is always a difficult task despite several methods available in the literature. For more efficient plant operation some researchers proposed combining fluids (mixtures) but no conclusion has been drawn from different attempts. B. Low power output expanders Since the 1970s ORC turbomachines have been developed and radial and axial turbines are now commercially available for sizes above 400 kW. However for small capacities, expansion machines are still under development. In recent years some machines have entered the market: 165 kW from tri-o-gen, 125 kW from Calnetix (GE), 280 kW from UTC Power, and 50 kW from Electratherm. Microexpanders are difficult to design and the trend is to convert compressors into expanders by reversing their operating mode [8]. C. Dynamic behavior of an ORC machine Heat is not supplied to the power block at a steady rate. Transient phenomena are very frequent for solar and waste heat recovery applications and appropriate control systems should be designed. D. Heat exchangers integration In industries waste heat is usually found in the form of exhaust gas/liquid. Where the temperature allows, the heat wasted can be recovered using different kinds of devices. Heat exchangers seem to be the most adapted to transfer heat to ORCs but the optimal design of the heat exchanger network is required and no guideline exists and each project requires a specific design. The maximum of heat recovered is measured in terms of the exit temperature that should be as low as possible. E. Project and maintenance guidelines Although ORCs have been implemented since the 1970s, no guidelines exist for project and maintenance. Mature applications like geothermal and biomass ORCs still have no guidelines. This may be explained by the fact that industries involved do not release information. F. Performance mapping and choice of an ORC machine Factors that affect the choice of an ORC machine are the level of the heat source, the mass flow of the heat source and these added of the conditions of the cooling system determine The cycle thermal efficiency will hardly exceed 20%, assuming a second-law efficiency of 50% and heat source temperature below 300 °C – see Fig. 4. III. ORC APPLICATIONS Various heat sources can drive an organic Rankine cycle machine: biomass, solar energy, geothermal energy, ocean thermal gradient, waste heat, etc. Biomass can be found in various forms: wood, sawdust, residues, etc. A biomass boiler would transfer chemical energy in the wood into heat. The heat will be recovered by a thermal oil cycle which will further transfer it to the evaporator of the Rankine cycle. A typical biomass CHP (combined heat and power) system has 18% and overall efficiency close to 90%. Solar ORCs use parabolic trough collectors, storage system, thermal oil, and ORC module. A well known system is the 1 MW Saguaro plant in USA [2]. Smaller systems are being investigated for cogeneration in buildings. Waste heat recovery ORC is a very promising application especially in energy intensive industries such as cement and is becoming popular for biogas plants. Geothermal CHP is the most mature application and continues to grow in popularity. OTEC (ocean thermal energy conversion) knows a new interest as solution for marine resource exploitation and potential solution for energy provision in isolated islands located in the tropical regions. A number of projects have been announced at various places: Reunion Island, Hawaii, China, etc. In organic Rankine cycles, thermal energy is usually supplied in the form of sensible heat: hot liquid/gas. Achieving highest possible efficiency would mean extracting minimum amount of heat i.e. limiting the outlet temperature. Then two cases appear [7]: systems which require high thermal efficiencies and those which require maximum power output. A biomass CHP utilizes solid biomass and minimizing the quantity of fuel is important as using less minimum solar collector field in a solar ORC. Maximum power is desired for waste heat recovery ORCs as for OTEC plants. As for geothermal plants an optimal point should be found between cycle efficiency and minimum temperature of the injected fluid. IV. OPPORTUNITIES AND CHALLENGES In regard to heat resource availability, organic Rankine cycles can be part of rural electrification solutions. However, any place with low-enthalpy resources can benefit from this technology. This technology opens new markets for industries (mechanical and electrical equipments, chemical substances) and offers another alternative to increase the share of the renewable energies in the energy mix. The number of manufacturers/installers/suppliers is growing and more jobs are being created. Still some challenges need to be addressed if ORC technology is to become a common technology: 298PDF Image | Organic Rankine Cycle Systems
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