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High heat losses via exhaust gas and coolant in internal combustion engines (ICE) are the basis for numerous investigations regarding downstream processes for power generation. The most promising concepts are Organic Rankine Cycles (ORC) and Steam Rankine Cycles (SRC). In previous work of the Center of Innovative Energy Systems, Düsseldorf (Germany), the technical and economic feasibility of a low pressure SRC has been investigated and the advantages in comparison to organic Rankine cycles are highlighted. A distinctive feature of the cycle is the use of the ICE coolant heat for evaporation, which limits the cycles maximum steam pressure to values below atmospheric pressure. This work presents first results of a test rig with data reconciliation according to DIN 2048 to validate the simulation results and design calculations. As a basis for the experimental investigations, a gas fired CHP plant was selected. The design of the test rig is optimized for the operation in the laboratory, where the coolant heat is emulated by a tempering device and the exhaust heat of the CHP plant is emulated by a gas burner. With the designed test rig, it is possible to control the volume flow and the temperatures of the coolant and exhaust gas, so that different load conditions of the gas fired CHP plant can be investigated. For initial tests, the turbine in the SRC is replaced by a throttle to achieve the pressure drop of the turbine. The experimental results show, that the exhaust and coolant heat of a 36 kWel CHP plant can be emulated and the performance expectations of the cycle can be met in stable steady-state conditions. Based on the measurement results and the turbine design calculations an electric power output of the cycle of 3.8 kW will be possible, which results in a cycle efficiency of about 6.5 % and an increase of the electrical power output of the CHP plant of about 10 %. The results show that the plant concept is technically feasible and, with further optimization, also represents an alternative to ORC plants in terms of increasing the efficiency of a cogeneration plant.
This paper deals with the design of a micro-scale ORC plant for teaching and research including the development of an automated control concept. The aim is to provide a safe and environmentally acceptable micro-scale heat engine, which can be developed, implemented and used in university labs for the education of students as well as for small research projects. The test rig allows the support of several learning outcomes on a multi-disciplinary level particularly for the implementation and simulation of small power systems. Special attention was given to the organic working fluid characteristics and its selection process. In addition to the above-mentioned performance goals, favourable safety properties and low global warming potential were decisive in the selection of the novel organic fluid called 3M™ Novec™ 649. The performance and the fluid behaviour of Novec 649 in a micro-scale power cycle are of major interest and the research goal for the test rig presented in this paper. Due to the expected power output of 1 kW, a scroll expander was chosen as the generator drive for the micro plant. In order to design the major parts of the ORC, the thermodynamic simulation software EBSILON®Professional was used. The supply temperature was set to 140 °C. As a result of the simulation, feasible expander inlet pressures spread from 5.5 bar to 8.5 bar. This leads to thermal efficiencies of the ORC in the order of 5 %. Adding a recuperator to the cycle system decreases the operating pressure range but in the end, the thermal efficiency can be increased by 1.5 %-points up to 6.2 %. Finally, an automated control concept is introduced, where the pump is controlled via the fill level measurement system.
The organic Rankine cycle (ORC) is an established thermodynamic process that converts waste heat to electric energy. Due to the wide range of organic working fluids available the fluid selection adds an additional degree-of-freedom to the early design phase of an ORC process. Despite thermodynamic aspects such as the temperature level of the heat source, other technical, economic, and safety aspects have to be considered. For the fluid selection process in this paper, 22 criteria were identified in six main categories while distinguishing between elimination (EC) and tolerance criteria (TC). For an ORC design, the suggested method follows a practical engineering approach and can be used as a structured way to limit the number of interesting working fluids before starting a detailed performance analysis of the most promising candidates. For the first time, the selection process is applied to a two-stage reference cycle, which uses the waste heat of a large reciprocating engine for cogeneration power plants. It consists of a high temperature (HT) and a low temperature (LT) cycle in which the condensation heat of the HT cycle provides the heat input of the LT cycle. After the fluid selection process, the detailed thermodynamic cycle design is carried out with a thermodynamic design tool that also includes a database for organic working fluids. The investigated ORC cycle shows a net thermal efficiency of about 17.4% in the HT cycle with toluene as the working fluid and 6.2% in LT cycle with isobutane as the working fluid. The electric efficiency of the cogeneration plant increases from 40.4% to 46.97% with the both stages of the two-stage ORC in operation.
The organic Rankine cycle (ORC) is an established thermodynamic process that converts waste heat to electric energy. Due to the wide range of organic working fluids available the fluid selection adds an additional degree of freedom to the early design phase of an ORC process. Despite thermodynamic aspects such as the temperature level of the heat source, other technical, economic and safety aspects have to be considered. For the fluid selection process in this paper, 22 criteria were identified in six main categories while distinguishing between elimination and tolerance criteria. For an ORC design, the suggested method follows a practical engineering approach and can be used as a structured way to limit the number of interesting working fluids before starting a detailed performance analysis of the most promising candidates. For the first time the selection process is applied to a two-stage reference cycle which uses the waste heat of a large reciprocating engine for cogeneration power plants. It consists of a high temperature and a low temperature cycle in which the condensation heat of the high temperature (HT) cycle provides the heat input of the low temperature (LT) cycle. After the fluid selection process the detailed thermodynamic cycle design is carried out with a thermodynamic design tool that also includes a database for organic working fluids. The investigated ORC cycle shows a net thermal efficiency of about 17,4% in the high temperature cycle with Toluene as the working fluid and 6,2% in low temperature cycle with iso-Butane as the working fluid. The electric efficiency of the cogeneration plant increases from 40,4% to 46,97% with the both stages of the two-stage ORC in operation.
This paper deals with the investigation in the technical and economic feasibility of a low-pressure steam cycle for the waste heat utilization of a combined heat and power (CHP) plant. The proposed steam cycle uses a pressure level below atmospheric pressure which allows to use the heat from the motor’s coolant for evaporation. The cycle is designed to increase the maximum electrical efficiency of a gas fired reciprocating engine and to increase operational flexibility for varying heat demand. Since organic Rankine cycle (ORC) plants are already being successfully used for waste heat recovery, advantages of the low-pressure steam Rankine cycle (SRC) over an ORC are highlighted. Many ORC fluids are toxic or flammable and thus have an increased risk potential compared to water. An additional advantage of water compared to ORC fluids is the possibility of superheating above 500 °C. The technical feasibility of the proposed cycle is studied by the detailed design of the individual system components such as turbine, heat exchangers and auxiliary parts. This includes the detailed thermodynamic design of the full cycle for the calculation of plant efficiencies. As a basis for future experimental investigations, a cogeneration plant with an electrical output of 50 kWel was selected as a heat source for the steam cycle. The design geometries of the turbine and the heat exchanger are presented along with efficiency and cost predictions. A subsonic radial turbine with a rotational speed of 155,000 rpm was selected. The maximum turbine output is about 7.5 kW. The use of a flash evaporator is investigated to reduce the size and the costs for the evaporator. The suggested steam cycle helps to increase the electrical efficiency of the CHP plant by 4.5% points from an original 36.5% to 41.0%. The steam cycle itself has an electrical efficiency of approximately 9%. Additionally, the condensation temperature in this steam cycle is high enough to be extracted as useful heat output to generate domestic water with a temperature of 50°C.