Micro Gas Turbines

This work describes the research activity conducted by the authors to enhance micro gas turbines performance, focusing on inlet air cooling, bottoming organic Rankine cycles, micro STIG and trigeneration

Bottoming Organic Rankine Cycles for Passenger Cars

Organic Rankine Cycles (ORC) are very efficient and flexible conversion systems with a high degree of maturity. They can be used with different heat sources, mainly using exhaust heat from different processing also with low temperature level sources. They have been mainly considered suitable for stationary power plants. Furthermore, the limitations of layout and size are less stringent if compared with road transportation propulsion systems, in particular when passenger cars are considered. In this paper, the authors numerically investigate an ORC system as a bottoming solution for energy recovery from exhaust gases for internal combustion engine (ICE) passenger car. A passenger car was tested over a Real Driving Emission (RDE) cycle. Exhaust gas mass flow rate and temperature have been sampled allowing calculation of the thermal power available for the ORC plant at realistic driving conditions. The engine operational range was discretized using engine torque and speed values. As a result, a grid of 10 points was set up in the operational plane and the running conditions assigned to the closest discretized point in the grid, each one characterized by a residence time. The ORC recovered power resulted between 0.5 and 2.5 kW, the Rankine cycle efficiency ranged from 11 to 12% while engine efficiency increase varied from 2.5 to 12%. By considering the permanence time in each discretized operating condition the engine efficiency increment resulted slightly higher than 6%

Organic rankine cycle with positive displacement expander and variable working fluid composition

Organic Rankine Cycles are often used in the exploitation of low-temperature heat sources. The relatively small temperature differential available to these projects makes them particularly vulnerable to changing ambient conditions, especially if an air-cooled condenser is used. The authors have recently demonstrated that a dynamic ORC with a variable working fluid composition, tuned to match the condensing temperature with the heat sink, can be used to achieve a considerable increase in year-round power generation under such conditions [1]. However, this assumed the expander was a turbine capable of operating at multiple pressure ratios for large scale applications. This paper will investigate if small scale ORC systems that use positive-displacement expanders with fixed expansion ratios could also benefit from this new concept. In this paper, a numerical model was firstly developed. A comprehensive analysis was then conducted for a case study. The results showed that the dynamic Organic Rankine Cycle concept can be applied to lower-power applications that use that use positive-displacement expanders with fixed expansion ratios and still result in improvements in year-round energy generation

Potential of Organic Rankine Cycles (ORC) for waste heat recovery on an Electric Arc Furnace (EAF)

The organic Rankine cycle (ORC) is a mature technology to convert low temperature waste heat to electricity. While several energy intensive industries could benefit from the integration of an ORC, their adoption rate is rather low. One important reason is that the prospective end-users find it difficult to recognize and realise the possible energy savings. In more recent years, the electric arc furnaces (EAF) are considered as a major candidate for waste heat recovery. Therefore, in this work, the integration of an ORC coupled to a 100 MWe EAF is investigated. The effect of working with averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC (SCORC) can generate an electricity output of 752 kWe with a steam buffer working at 25 bar. However, the use of a steam buffer also impacts the heat transfer to the ORC. A reduction up to 61.5% in net power output is possible due to the additional isothermal plateau of the steam

Linear active disturbance rejection control of waste heat recovery systems with organic Rankine cycles

In this paper, a linear active disturbance rejection controller is proposed for a waste heat recovery system using an organic Rankine cycle process, whose model is obtained by applying the system identification technique. The disturbances imposed on the waste heat recovery system are estimated through an extended linear state observer and then compensated by a linear feedback control strategy. The proposed control strategy is applied to a 100 kW waste heat recovery system to handle the power demand variations of grid and process disturbances. The effectiveness of this controller is verified via a simulation study, and the results demonstrate that the proposed strategy can provide satisfactory tracking performance and disturbance rejection

Effect factors of part-load performance for various Organic Rankine cycles using in engine waste heat recovery

The Organic Rankine Cycle (ORC) is regarded as one of the most promising waste heat recovery technologies for electricity generation engines. Since the engine usually operates under different working conditions, it is important to research the part-load performance of the ORC. In order to reveal the effect factors of part-load performance, four different forms of ORCs are compared in the study with dynamic math models established in SIMULINK. They are the ORC applying low temperature working fluid R245fa with a medium heat transfer cycle, the ORCs with high temperature working fluid toluene heated directly by exhaust condensing at low pressure and high pressure, and the double-stage ORC. It is regarded that the more slowly the system output power decreases, the better part-load performance it has. Based on a comparison among the four systems, the effects of evaporating pressure, condensing condition, working fluid, and system structure on part-load performance are revealed in the work. Further, it is found that the system which best matches with the heat source not only performs well under the design conditions, but also has excellent part-load performance

Experimental characterization of single screw expander performance under different testing conditions and working fluids

During the last years, one of the most popular ways to recover low-grade waste heat is the organic Rankine cycle (ORC). This technology is widely studied and continuously optimized and, as a result, there are many efficient installations available on the market utilizing heat with stable parameters such as from geothermal sources or from the biomass combustion process. However, if a variable hot source in terms of either temperature or flow rate is introduced, the expansion devices have to work at non-optimal conditions, which decrease the global efficiency of ORC installations, e.g. in the case of waste heat recovery. In order to characterize the performance of a positive displacement expander close enough to the optimum, the influence of pressure ratios, filling factor, and working fluid properties on power output is studied. In this paper, experimental results obtained on a small-scale ORC test setup based on an 11 kWe single-screw expander are presented. Two working fluids are used during the tests, i.e. R245fa and SES36 (Solkatherm). These working fluids are common for ORC installations exploiting low-temperature waste heat. The waste heat source is simulated by an electrically heated thermal oil loop with adjustable temperature and flow rate. Various waste heat inlet flow rates are considered in order to find an optimal evaporation pressure and to maximize the power output with different heat source profiles. Based on the experimental data, the expander model is developed. For each working fluid, optimal working conditions are determined. In most cases, there is under-expansion due to a relatively small built-in volume ratio, causing certain losses. By means of the model, the ideal expansion process is simulated and compared with the real one obtained experimentally to quantify these losses and conclusions can be drawn whether significant benefits can be offered by using an optimized expander instead of an ”off-the-shelf” reversed compressor

A dynamic organic Rankine cycle using a zeotropic mixture as the working fluid with composition tuning to match changing ambient conditions

Air-cooled condensers are widely used for Organic Rankine Cycle (ORC) power plants where cooling water is unavailable or too costly, but they are then vulnerable to changing ambient air temperatures especially in continental climates, where the air temperature difference between winter and summer can be over 40 °C. A conventional ORC system using a single component working fluid has to be designed according to the maximum air temperature in summer and thus operates far from optimal design conditions for most of the year, leading to low annual average efficiencies. This research proposes a novel dynamic ORC that uses a binary zeotropic mixture as the working fluid, with mechanisms in place to adjust the mixture composition dynamically during operation in response to changing heat sink conditions, significantly improving the overall efficiency of the plant. The working principle of the dynamic ORC concept is analysed. The case study results show that the annual average thermal efficiency can be improved by up to 23% over a conventional ORC when the heat source is 100 °C, while the evaluated increase of the capital cost is less than 7%. The dynamic ORC power plants are particularly attractive for low temperature applications, delivering shorter payback periods compared to conventional ORC systems

Low-Concentration Solar-Power Systems Based on Organic Rankine Cycles for Distributed-Scale Applications: Overview and Further Developments

This paper is concerned with the emergence and development of low-to-medium-grade thermal-energy-conversion systems for distributed power generation based on thermo- dynamic vapor-phase heat-engine cycles undergone by organic working uids, namely organic Rankine cycles (ORCs). ORC power systems are, to some extent, a relatively established and mature technology that is well-suited to converting low/medium-grade heat (at temperatures up to ~300–400°C) to useful work, at an output power scale from a few kilowatts to 10s of megawatts. Thermal ef ciencies in excess of 25% are achievable at higher temperatures and larger scales, and efforts are currently in progress to improve the overall economic viability and thus uptake of ORC power systems, by focusing on advanced architectures, working- uid selection, heat exchangers and expansion machines. Solar-power systems based on ORC technology have a signi cant potential to be used for distributed power generation, by converting thermal energy from simple and low-cost non-concentrated or low-concentration collectors to mechanical, hydrau- lic, or electrical energy. Current elds of use include mainly geothermal and biomass/ biogas, as well as the recovery and conversion of waste heat, leading to improved energy ef ciency, primary energy (i.e., fuel) use and emission minimization, yet the technology is highly transferable to solar-power generation as an affordable alternative to small-to- medium-scale photovoltaic systems. Solar-ORC systems offer naturally the advantages of providing a simultaneous thermal-energy output for hot water provision and/or space heating, and the particularly interesting possibility of relatively straightforward onsite (thermal) energy storage. Key performance characteristics are presented, and important heat transfer effects that act to limit performance are identi ed as noteworthy directions of future research for the further development of this technology