ORGANIC RANKINE CYCLE
The Rankine cycle is a thermodynamic cycle used to generate electricity in many power stations and is the practical approach to the Carnot cycle. Superheated steam is generated in a boiler, then expanded in a steam turbine. The turbine drives a generator to convert the work into electricity. The remaining steam is then condensed and recycled as feedwater to the boiler. A disadvantage of using the water-steam mixture is that superheated steam has to be generated; otherwise the moisture content after expansion might be too high, which would erode the turbine blades. Organic substances that can be used below a temperature of 400°C do not have to be overheated. For many organic compounds superheating is not necessary, resulting in a more efficient cycle. In a heat recovery system, it may be shown that if the degree of superheating is reduced, more steam can be generated and hence more energy can be recovered from the heat source as shown in Q8.36. The working fluid superheats as the pressure is reduced, unlike steam, which becomes wet during the expansion process. Organic fluids also have low freezing points and hence even at low temperatures there is no freezing. The ratio of latent heat to sensible heat allows for greater heat recovery than in steam systems.
An Organic Rankine Cycle (ORC) can make use of low temperature waste heat such as geothermal heat to generate electricity. At these low temperatures a steam cycle would be inefficient, because of the enormous volume of low pressure steam, which would require very voluminous and costly piping resulting in inefficient plants. Small-scale ORCs have been used commercially or as pilot plants in the last two decades. Several organic compounds have been used in ORCs (e. g., CFCs, Freon, isopentane, or ammonia) to match the temperature of the available waste heat. Waste heat temperatures can be as low as 70-80°C. The efficiency of an ORC is estimated to be between 10% and 20%, depending on temperature levels. To minimize costs and energy losses it is necessary to locate an ORC near the heat source. It is also necessary to condense the working vapor; therefore, a cooling medium should be available on site. These site characteristics will limit the potential application. Condensing pressure is higher than atmospheric, so there is no need for vacuum equipment. ORC is expensive on the basis of cost per kilowatt-hour compared to other systems, but the main advantage is that it can generate power from low temperature heat sources. ORC plants can also be of large capacity. A 14 MW power plant using Flurinol 85 as the working fluid is in operation in Japan, using the energy recovered from the effluents of a sintering plant. The low boiling point and low latent heat of this fluid compared to steam help recover a significantly greater amount of energy from the hot gases.