Using steam power to generate energy
From a thermodynamic point of view, every steam power process used to generate energy is based on the same cyclic process. Water is evaporated under pressure, re-expanded in a turbine before the steam is condensed again. This cycle is known as Clausius Rankine process. The Organic Rankine Cycle is also based on this functional principle. The main difference is in the used working fluid, which is an organic fluid rather than water. Learn more about the ORC process and the technologies employed by GETEC.
Development of the ORC process
Meanwhile, established ORC system manufacturers have accumulated more than 30 years of experience with the development and production of ORC systems. Consequently, the availability of the systems technology is high and can be used for individual as well as multifaceted applications.
The ORC process was developed to make heat usable for the generation of energy at a relatively low temperature level. A conventional water/steam circuit process requires a relatively high temperature level in order to be able to generate the specific energy required for the evaporation of water. In so doing, the efficiency of the entire process is highly dependent on the parameters pressure and temperature of the water vapour that is expanded in a turbine.
The working medium used in the Organic Rankine Cycle consists of organic components. The evaporation pressure and evaporation temperature vary, depending on the composition of the working medium. However, these two parameters are generally much lower than the evaporation parameters of water. As a result, it is also possible to use heat sources with a low temperature, as a smaller amount of specific energy is required for the evaporation.
Over the course of many years, the ORC technology was adapted to diverse and numerous areas of application, and a broad scope has thus been created for ORC systems. By varying the working media, it is possible to manufacture modules that are adapted specifically to the temperature level of the heat source, thereby achieving the best possible efficiency of the system.
With high-temperature use (temperature level near 300 °C), the efficiency of ORC systems is close to 20%. Efficiencies of 24% may be achieved with special developments.
With low-temperature systems (temperature levels between 90 and 150 °C), the efficiency is normally around 6 to 10%.
The electrical power range of ORC modules currently ranges from small-scale systems with < 100 kWel to large-scale systems with > 10 MW.
Functional principle of the ORC process
The thermodynamic cyclic process in essence resembles the one of the Clausius Rankine Cycle.
The working fluid is heated in the heat exchanger by means of the heat transfer medium (thermal oil or compressed water) of the intermediate cycle and evaporated (1 → 2). The superheated steam is then expanded in the turbine (2 → 3). The heat released in the context of the isobaric cooling of the steam is used to preheat the working fluid in the regenerator (3 → 4). The expanded steam of the working medium is cooled further and condensed in the condenser (4 → 5). The heat of condensation is either conducted away via cooling towers or used further as hot water for heating purposes. A pump re-increases the pressure in the working medium (5 → 6). After the working fluid has been preheated in the regenerator (6 → 1), the working fluid is re-evaporated in the evaporator and superheated. This completes the cyclic process. The difference compared with the Clausius Rankine Cycle is the incorporation of an additional heat exchanger, the regenerator, which is crucial for increasing the efficiency.
Technical characteristics, advantages and disadvantages of ORC technology
As described above, an organic medium is used as working medium, which can be evaporated at very low pressures and temperatures. By using a variety of fluids with correspondingly different characteristics, the process can be adapted optimally to the respective application. This guarantees the greatest possible efficiency of the ORC system at all times.
Another striking feature of ORC modules is their compact design. All components, such as the evaporator, turbine, regenerator and condenser are usually installed tightly in a case (or on a steel frame). Therefore, the thermal energy required for the evaporation of the working medium is normally transported to the ORC module via an intermediate cycle. Thermal oils are usually used as heat transfer medium in this intermediate cycle. Alternatively, compressed water is used to transport heat from the exhaust heat source to the Organic Rankine Cycle.
In regard of energy generation, the ORC competes with the conventional highly-developed technology of steam turbines. However, the Organic Rankine Cycle has several advantages that make ORC technology an interesting proposition for a number of applications.
- Temperature levels of 90°C to 800°C can be utilised for the conversion into electricity.
- As the expansion takes place virtually fluid-free, the turbine is not affected by any signs of erosion and corrosion.
- ORC systems display excellent partial load behaviour. In individual cases, a working status can be achieved with 10% of the rated load.
- The operation of the system is fully automated and unsupervised. The ORC module does not contain any pressure elements according to the Technical Rules for Steam Boilers [Technische Regeln für Dampfkessel, TRD], for which supervision would be necessary.
- In contrast to steam turbines, the start-up and shut-down is relatively simple and user-friendly.
- With generated electric powers below 2 MWel, ORC systems often prove to be more economical than conventional steam turbines.
Still, the advantages described above are pitted against several disadvantages.
- When thermal oil is used as heat transfer medium, corrosion may develop on the heat exchanger.
- At high electric powers (> 2 MWel), the system technology is relatively cost-intensive.
Areas of application of ORC technology
Utilisation of industrial waste heat at high- and low-temperature levels
The use of ORC systems for the industrial waste heat utilisation on high- and low-temperature level is one of the main areas of application of ORC technology. Depending on the temperature levels and available exhaust heat quantities, the heat is normally transported to the ORC module via an intermediate cycle. The heat transfer medium releases its heat in the evaporator, thereby evaporating the organic working fluid in the ORC. In most cases, thermal oils or hot water are used as heat transfer medium in the intermediate cycle.
Therefore, fields of application are found in virtually all branches of industry, where a corresponding amount of waste heat is available. Examples include in particular the cement industry, the glass industry and the metal industry.
Depending on the application, amounts of exhaust heat at different temperature levels may be present at the same location. A so-called “split” system was developed for this purpose, which makes it possible to utilise heat sources on two different temperature levels in one ORC module.
Another development on the part of Organic Rankine Cycle module manufacturers is the elimination of the intermediate cycle for the heat transport. The instrument-based time and effort in connection with a conventional exhaust heat utilisation concept that features an intermediate cycle are high and cost-intensive. Therefore, the manufacturers developed a variety of direct evaporators. With them, the medium, for example hot exhaust gas, is guided directly through the evaporator of the ORC module. The instrument-based time and effort are considerably lower. As great diligence must be demonstrated during the extraction of the thermal energy especially when using waste gases derived from combustion processes and manufacturing processes, an intermediate cycle may still be conducive in spite of the higher time and effort involved. With a direct evaporator, there is a risk of corrosion and damage of the evaporator directly at the ORC module in the worst case.
The use in biomass cogeneration power plants is a common field of application of ORC modules. In that application, biomass combustion plants are combined with ORC modules.
The thermal energy generated with the combustion of the biomass is transferred to a thermal oil cycle via the boiler. The thermal oil cycle then supplies heat to the Organic Rankine Cycle.
In addition, district heat becomes available through the cooling water that is required for the ORC. While ORCs are cooled with cooling water that has a temperature of approximately 25°C for the utilisation of waste heat, in order to achieve an electric efficiency that is as high as possible, the ORC in this case is cooled with 70°C-warm water and heated to 90°C. As a result, the removed condensation heat is not released into the environment via cooling towers, but made available for further utilisation instead. This increases the ORC system's overall efficiency. Moreover, the generated electricity may be reimbursed through the Renewable Energy Sources Act (EEG).
Furthermore, the ORC’s outstanding partial load behaviour enables an unproblematic heat-led control of the combined heating and power station.
With geothermal energy, the temperature levels available for utilisation range from 90 to 150°C. The thermal utilisation of geothermal energy by means of a heat pump is a well-established practice in the private sector and in the housing industry. Thanks to ORC technology and the working media that can be used, it is possible to transform heat into electricity, in spite of the low temperature level.
The ORC’s efficiency is around 6 to 10%, which is also due to the low temperatures.
In this application, the exhaust heat of a CHP unit is used in an ORC module for the energy supply, thereby increasing the electrically generated efficiency of the entire system.
Firstly, this combination is used to amplify the generated power, and secondly, the thermal energy of the CHP unit is made available for another use, if the heat was not consumed in its entirety. Under certain conditions, this may be relevant for the fulfilment of the high-efficiency criterion, which used among other things for the decision regarding the allowance of the CHP premium for the energy generated by the CHP unit.
Organic Rankine Cycle technology of GETEC
With the ORC technology, a method has established itself on the energy systems market that is extremely versatile and adaptable to almost any individual application. By using an ORC module, it is possible to consolidate energetic synergies as well as generate economic benefits (CHP compensation, EEG compensation).
The systems technology is considered solid and operationally reliable. As well, ORC systems are characterised by excellent partial load behaviour, thus further expanding their range of applications.
GETEC relies on this technology. For example, several ORC systems are currently being used in biomass cogeneration power plants. However, GETEC also operates reference projects in the field of industrial waste heat utilisation.
In the future, the research and development of ORC systems is expected to advance further and the efficiency of ORC modules will continue to improve.