Popularizing organic Rankine cycle (ORC) power generation technology

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This post was last edited by Su Mu Zhe XCZF on 2020-4-24 09:13. I. Technical principle: The organic Rankine cycle system consists of an evaporator, an expander, a condenser, and a working fluid circulation pump, as shown in the figure below. In the evaporator, the organic working fluid absorbs heat from a low-temperature heat source to produce saturated or superheated vapor. This vapor drives the expansion engine to rotate, which in turn generates electricity. The exhaust gas after performing work in the expansion engine enters the condenser where it is cooled back into a liquid state, after which it is pumped back to the evaporator by a working fluid pump, thus completing one thermal cycle. For low-grade heat, the ORC technology offers many advantages over conventional steam Rankine cycles. The most notable feature is that, compared to water vapor, organic working fluids have lower boiling points; they can be vaporized at relatively low pressures (0.2–1.5 MPa) and temperatures (100°C, or even 40–50°C), thereby enabling the recovery of otherwise wasted low-grade thermal energy, which can then be converted into electrical energy. Organic working fluids also exhibit high efficiency in recovering sensible heat. Since the ratio of sensible heat to latent heat is not equal in the Rankine cycle, whereas the proportion of sensible heat is higher in ORC systems, this technology enables the recovery of more heat compared to the Rankine cycle using steam. The figure below shows the temperature-entropy diagrams of the Rankine cycle for water (a) and organic working fluids (b). II. Working Fluid: Choosing a working fluid that ensures technical feasibility, economic efficiency, and compliance with environmental regulations is one of the key issues in low-temperature waste heat power generation technology. 1. The pressure of the working fluid during the cycle must not be too high nor too low; it must remain within the range permitted by the device’s compressive strength and sealing performance. 2. Try to choose dry working fluids to ensure the safety of turbine operation and also improve the efficiency of the system. 3. Consider factors such as the heat transfer properties, fluidity, stability, flammability, toxicity, and cost of the working fluid to select the most suitable organic working fluid. III. Heat Exchangers: Both evaporators and condensers are essentially heat exchangers, and their heat exchange efficiency is one of the most important factors affecting the overall efficiency of the system. Therefore, optimizing the performance of heat exchangers is an important way to improve the performance of the cycle. The key to improving the efficiency of heat exchangers is to reduce the irreversible losses in the heat transfer process and decrease the temperature difference for heat transfer. From a equipment perspective, it is possible to increase the heat exchange area of the heat exchanger, design its structure appropriately, and use materials with higher heat transfer coefficients ； From the perspective of the working fluid, an organic fluid with good heat transfer properties can be used, and efforts should be made to synchronize the heating process of the fluid with the temperature changes of the heat source, thereby reducing the temperature difference for heat transfer. IV. Selection of the expander: The expander is the most important component in an ORC system; it converts the thermal energy of the organic working fluid into mechanical energy, which in turn drives the engine to generate electricity. Based on their working principles and structures, expanders are divided into velocity expanders and positive-displacement expanders. A velocity-type expander operates on the principle of using nozzles and impellers to convert high-temperature, high-pressure gas into a high-speed fluid, and then converting the kinetic energy of this high-speed fluid into rotational mechanical work. This type of expander is typically suitable for high-flow applications, offering high output power as well as a relatively high speed. Velocity expanders are typically various types of expansion turbines, such as multi-stage axial flow steam turbines, gas turbines, and centrifugal turbines. The ORC power generation system jointly developed by Shanneng Ecology Group and Yangtze Power Group utilizes centrifugal turbines, and it has been put into practical use in areas such as industrial waste heat utilization and coal mine gas treatment. It is one of the few cases in China where ORC turbine technology is applied in actual operations. A positive-displacement expansion engine operates on the principle of converting the thermal energy of steam into changes in the volume of the working fluid, thereby performing work externally. This type of expander is generally suitable for applications with low flow rates and high expansion ratios. Piston expanders, scroll expanders, screw expanders, etc., all belong to the category of positive-displacement expanders. V. Working Fluid Pump: The working fluid pump raises the pressure of the low-pressure liquid working fluid coming from the condenser and sends it to the inlet of the evaporator, where the pressure is relatively higher. Hydraulic diaphragm pumps that allow for continuous measurement are generally chosen. The hydraulic end of the pump forms a working chamber through a diaphragm; a pump that uses the periodic elastic deformation of this diaphragm to replace the reciprocating motion of a piston is called a diaphragm pump. The piston of a diaphragm pump transmits energy to the fluid through a flexible diaphragm; since there is no direct contact between the fluid and the piston, it can also be used to transport liquids that are prone to wear or are corrosive. VI. Conclusion There are a wide variety of low-grade heat sources, including renewable resources such as solar energy, various industrial waste heats, geothermal energy, biomass energy, and ocean thermal gradients. The total amount of these resources is enormous; taking industrial waste heat as an example, 50% of the thermal energy utilized by humans is ultimately released directly in the form of low-grade waste heat. Utilizing and recovering this portion of energy through ORC low-temperature waste heat power generation technology not only helps to address China’s energy challenges but also reduces environmental pollution during the energy production process.