A comprehensive analysis of why ORC power generation chooses turbine expanders over screw expanders

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The so-called ORC (Organic Rankine Cycle) power generation refers to the process in which an organic working fluid absorbs heat from waste heat streams through a heat exchanger, resulting in the formation of steam at a certain pressure and temperature. This steam then enters an expansion engine where it expands to perform work, thereby driving a generator to produce electricity. The steam discharged from the turbine releases heat to the cooling water in the condenser, condensing into a liquid state, and is then returned to the heat exchanger via a working fluid pump, thus continuing this cycle endlessly. ORC power generation technology was developed earlier in Europe and the United States, and has since reached a certain scale; it represents an important application in the field of low-temperature and low-heat power generation. In developed countries in the West, the effective utilization rate of low-temperature and low-heat technologies can reach 60%, while in China it is currently between 30% and 40%, showing a significant gap. On the application side, although ORC power generation technology was introduced in China at an early stage (ORC technology was first used for low-temperature power generation in Yangbajing, Tibet, in 1977, with a capacity of 1 MW), its development has been slow, and there are only a few manufacturers in the country that possess this technology. The turbine ORC power generation system jointly developed by Shanneng Ecology Group and Yangtze Power Group has been successfully applied in areas such as industrial waste heat utilization and coal mine gas treatment; it is one of the few cases in China where turbine ORC technology is put into practical use. Wuhan Hanshi Environmental Protection utilizes Changdong’s ORC power generation system, which has played a key role in dealing with large amounts of medical waste during this pandemic. ORC power generation technology is primarily used in applications involving low temperatures and low heat levels. Over the past nearly two decades, China’s economic development has progressed at a rapid pace. With the onset of the energy crisis and increasing demands for environmental protection, ORC power generation has begun to receive more attention. ORC power generation technology can be applied in a wide range of scenarios, including industrial waste gases, waste heat, combustion of coal mine gas, geothermal energy, and solar thermal energy. Experts predict that the photothermal sector will be a very significant application area for ORC power generation in the future. However, the \"Several Opinions on Promoting the Healthy Development of Power Generation from Non-hydro Renewable Energy Sources\" issued jointly by three ministries this year stipulate that new solar thermal projects will no longer be covered by central government financial subsidies. The future prospects for concentrated solar power generation are uncertain; therefore, the author believes that industrial waste heat will remain the most important application area for ORC power generation. The term \"industrial waste heat\" covers a very wide range of substances, generally including high-temperature flue gases emitted from various process equipment, cooling water from high-temperature process equipment, as well as certain high-temperature wastewater and waste gases. For a long time in the past, this heat was often discarded outright due to high reuse costs and immature technologies, resulting in significant waste of energy. Later, in large thermal power generation units, high-temperature flue gas was made use of effectively through technologies such as reheat and double reheat, which constitutes an effective method of energy reuse. However, for some energy-intensive and polluting enterprises in industries such as chemicals, steel, and cement, such treatment methods are still insufficient. The emergence of ORC technology has provided a new approach to the utilization of industrial waste heat. Thanks to its high coupling efficiency and high waste heat utilization rate, it has become an option for many companies to reduce energy consumption and lower costs. Experts analyze that once the ORC power generation market is launched, it will have a market scale in the trillions. However, given the limited number of current applications of ORC technology in China, this also deters many companies. By analyzing the current ORC power generation technology in China, this article aims to provide some insights. Given the author’s limited expertise and weak writing skills, any points that are not clear can be discussed together. We conduct a comprehensive analysis based on the ORC power generation principle, turbine selection, working fluid selection, and economic considerations. I. Principle of ORC power generation: This process was briefly introduced earlier; some additional details are provided here. The working fluid used in the steam turbine generator sets that we refer to in daily life is water; its principle can be abbreviated as WRC. The working fluid used in ORCs is an organic fluid, with materials that have a low boiling point, high heat transfer efficiency, and low cost being generally chosen. The organic working fluid transforms into recyclable steam by absorbing wastewater, waste gas, or other heat (which we collectively refer to as waste heat), thereby driving rotating components to perform work and powering a generator to produce electricity. In summary, the required equipment mainly includes ORC generator sets, evaporators, condensers, cooling systems, as well as some pump and valve devices. Since ORC generator sets generally have low power output, the equipment required is usually relatively simple and easy to install; the main components are available for production in China. The most important component among these is the expander used in ORC generators, and the investment in ORC generators accounts for over 60% of the total investment in the entire system. There are two main technical issues regarding the entire system: one is the selection of the expander, and the other is the selection of the working fluid. II. Selection of expanders Expanders are mainly divided into two types: one is turbine expanders (which are further divided into radial and axial types), and the other is positive-displacement expanders (with screw expanders being the most commonly used type; no examples exist for other types, so the following discussion will focus on screw expanders). The so-called turbine expander, also known as a turboexpander, features rotating blades within a chamber; the expansion of gas does work to drive these blades to rotate, thereby causing the shafting to turn as well. A turbine expander can, in an adiabatic condition, convert the heat released from gas expansion into kinetic energy of the shaft system to the greatest extent possible, thereby achieving higher thermal efficiency and exergy efficiency. Currently, the main turbine equipment in large thermal power generation units are turbines, so the technology in this area is relatively mature. Screw expanders are currently a popular choice among ORC manufacturers in China. Since there was existing experience with screw expanders when this ORC technology was first introduced to China, it has enabled significant progress in the development of this technology in the country. The screw expander commonly referred to is a twin-screw expander, and the work generation consists of three processes: suction, expansion, and discharge. Due to this twin-screw design, lubrication at the coupling of closed shafting systems becomes a major challenge. Currently, the mainstream ORC expanders in China are mainly turbines and twin-screw machines. The advantages of turbine expanders are their mature technology and the possibility of multi-stage design, which allows them to be used in units with a capacity of over 200 KW. Furthermore, turbine expanders feature high isentropic efficiency, low various types of losses, and a compact structure, along with favorable operation and maintenance cycles. Its drawback is its high rotational speed, which requires a sophisticated control system; it is not suitable for units below 200 KW. Although domestic manufacturers are currently producing screw expanders in bulk, the experience of using them is not good. The main problem is the high lubrication requirements for the screw, as mentioned earlier; in addition, its manufacturing precision and subsequent maintenance costs are higher than those of expanders. More importantly, since screw expanders do not have a multi-stage design like turbine expanders, increasing the active power requires enlarging the volume of the chamber; therefore, they are not suitable for units with high power output. Screw expander machines have also found some application abroad, and the reason for this is the widespread use of ORC power generation technology in those countries. Since residents in many areas abroad live in a scattered manner, the ORC power generation technology can help meet part of their electricity demand. Although screw expanders require high-quality materials for their components, they demand less sophisticated control technology, which is why they have gained some popularity. The application environments at home and abroad are completely different; our ORC technology serves primarily as a supplement to industrial power generation, and its control technology is fully mature. Due to the high power requirements of the application scenario, the turbine expander is the better choice. III. Selection of working fluid: The organic working fluids used in ORC power generation vary, but they fall into a few categories; the most commonly used ones are R245fa and R123. The selection of the working fluid takes into account factors such as heat transfer efficiency, heat loss, and cost-performance ratio. Working fluid Standard boiling point/°C Critical temperature/°C Critical pressure/MPa Latent heat of vaporization/J*g-1 Specific heat capacity/J*g-1*K-1 Molecular weight R245fa: 14.9, 154.1, 3.64, 196.69, 1.318, 134.0 R123: 27.7, 183.8, 3.67, 170.19, 1.022, 152.9 As can be seen from the table above, the basic parameters of these two working fluids are not very different; however, R245fa has a higher latent heat of vaporization and specific heat capacity per unit mass compared to R123. Therefore, it can release more heat, which means that its heat exchange efficiency is higher. Therefore, it is more reasonable to choose R245fa from a technical perspective. Furthermore, R245fa is currently a widely used refrigerant in industry, with more readily available supplies, while R123 is generally used only as an auxiliary product. IV. Cost efficiency: In terms of initial investment, turbine expanders are slightly more expensive, mainly due to a lack of competitiveness in the market previously. As more and more manufacturers begin to focus on turbine expanders, prices have also dropped. Furthermore, turbine expanders have lower operating and maintenance costs, making them an important option for future ORC technologies. In summary, turbine expanders have certain advantages both in terms of technology and economics. However, the ORC market in China is not yet mature, and many companies refer to foreign examples when choosing products. However, there are still slight differences between domestic and international situations, and turbine expanders remain quite competitive in future applications of industrial waste heat.