Experimental investigation of a scroll expander for an organic Rankine cycle

This paper presents experimental investigation of the performance of an organic Rankine cycle (ORC) with scroll expander which utilizes renewable, process and waste heats. An ORC test bench is built with a scroll expander‐generator unit modified from a refrigeration compressor‐electrical drive unit. A detailed experimental investigation within the test bench is performed with the organic working fluid R134a. The results show that scroll expander can effectively be used in low‐power ORC to generate mechanical work or electricity from low‐temperature thermal sources (e.g. 80–200 °C, respectively). The experiments are performed under fixed intake conditions into the expander. The pressure ratio and the load connected to the expander‐generator unit were varied. It is found that an optimum pressure ratio and an optimum angular speed co‐exist. When operating optimally, the expander's isentropic efficiency is the highest. The optimum angular speed is around 171 rad/s which corresponds to a generated voltage of 18.6 V. The optimum pressure ratio is about 4. The isentropic efficiency at optimum operation is found in the range of 0.5 to 0.64, depending on the intake conditions. The volumetric efficiency overpasses 0.9 at optimum operation and degrades significantly if the load is increased over the optimum load. A regenerative ORC equipped with the studied expender‐generator unit that operates under 120 °C heat source and has an air cooled condenser generates 920 W net power with efficiencies of 8.5% energetically and 35% exergetically. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermodynamic analysis of double‐stage biomass fired Organic Rankine Cycle for micro‐cogeneration

A biomass fired double‐stage Organic Rankine Cycle (ORC) for micro‐cogeneration is studied. Focus is laid on optimizing thermal efficiency in summer mode by appropriate working fluid and pressure level selection. Simulation and thermodynamic analysis show that in double‐stage ORC, the working fluid in the low‐temperature circuit (LTC) effects total efficiency more than the working fluid in the high‐temperature circuit (HTC). Within the chosen boundary conditions, isopentane gives best thermal efficiency, whereas R227ea is the least efficient in the LTC. Among the working fluids for the HTC, maximum total efficiency is similar for several working fluids. Simulations demonstrate that a prediction of thermal efficiencies with respect to physico‐chemical characteristics of different working fluids is only feasible within certain chemical classes. In the HTC, low critical temperature, low molar mass, and high critical pressure increase the efficiency, whereas in the LTC, condensation pressure is most crucial for high efficiency. Constructional analysis indicate that in the majority of cases, an increase in thermal efficiency is connected with high‐volume flow rates at the outlet of the turbine, which leads to voluminous expansion units and high investment costs, respectively. Appropriate working fluid combinations within a double‐stage ORC reach total efficiencies of up to 35% at flue gas temperatures from 950 to 150 °C. Copyright © 2012 John Wiley & Sons, Ltd.     

Experimental comparison of R245fa and R245fa/R601a for organic Rankine cycle using scroll expander

An experimental test was conducted to compare R245fa with R245fa/R601a on the organic Rankine cycle performance. The major objective of this paper is to ascertain the highest thermal efficiency and the optimal dimensionless volume ratio using the two working fluids. The experimental system consists of an electrically heated boiler, a vapor generator, a scroll expander, a condenser, a working fluid pump, and so on. For the typical weather conditions of May in Tianjin, the experiment results show that the working fluid charge has an important influence on the organic Rankine cycle performances. The optimal isentropic efficiency of the scroll expander corresponds to the design expansion ratio. Underexpanded and overexpanded processes result in the decline of the isentropic efficiency of the scroll expander, with the former playing a major role. R245fa/R601a improves the heat transfer performance in the vapor generator because of the nonisothermal phase change. The highest thermal efficiency for R245fa and R245fa/R601a is 4.38% and 4.45%, thereby illustrating that R245fa/R601a precedes R245fa. The optimal dimensionless volume ratios for R245fa and R245fa/R601a are 0.38 and 0.41, respectively. The experimental test lays foundation of the 500‐kW geothermal plant for demonstration in the next step. Copyright © 2014 John Wiley & Sons, Ltd.     

Simulation and experimental research on energy conversion efficiency of scroll expander for micro‐Compressed Air Energy Storage system

A scroll expander was applied to the Micro‐Compressed Air Energy Storage system, and its energy conversion efficiency was investigated. In order to study the variation mechanism of the volume, mass, pressure and temperature of the air in different chambers, the mathematical model of the expansion process was developed on the base of the geometric model, mass conservation equation, ideal gas equation and energy conservation equation. Then, the mathematical model was implemented in Matlab, and the simulated energy conversion efficiency defined as the ratio between the output shaft power of the scroll expander and the input compressed air power was obtained. Furthermore, a test system was built in order to validate the mathematical model and study the improvement of the energy conversion efficiency. The prototypes of the scroll expander with different cross‐sectional areas of the intake port or the discharge port were fabricated and tested in the experiments. Results show that the simulated torque and energy conversion efficiency agree well with the experimental results. Also, there is a small deviation between the expansion process and the ideal isentropic process due to the gas leakage, intake and discharge loss. In addition, the air supply pressure and the cross‐sectional area ratio of the discharge port to the intake port are two important parameters for the improvement of the energy conversion efficiency. The experiments show that the energy conversion efficiency varies from 23% to 36% at the air supply pressure of 0.35 to 0.65 MPa, indicating that it is proportional to the air supply pressure. It can also be concluded from the experiments that when the air pressure is higher than 0.45 MPa, the ideal ratio range can be determined as 0.6‐0.8. Copyright © 2013 John Wiley & Sons, Ltd.     

Cascaded waste‐heat recovery as a green technology for energy sustainability in power generation

In this work, the Cascaded waste‐heat recovery (WHR) is analyzed from the thermodynamic point of view. Typically, WHR is most effective with small gas turbines and old machines which have a relatively higher design mass flow per kW and higher exhaust temperatures than new designs. The working fluid used in the WHR technology is propane, which vaporizes and condenses at low temperatures. The temperature of the heat source, the outlet pressure of the two expanders, and the mass flow rate of the working fluid are assumed as working variables of the technology. The effect of these variables on the thermal efficiency and power output is evaluated. The obtained results are analyzed and discussed. The results of the calculation are also compared with similar published studies. The overall efficiency considering the gas turbine upstream ranges from about 35% up to 39%. The highest efficiency and power output of the WHR alone at 900 K heat source temperature, 800 kPa condenser pressure, and 100 kg/s mass‐flow rate are 30% and 18 MW, respectively, for two‐expander WHR, and 18% and 9 MW, respectively, for single expander WHR. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance and design optimization of a low-cost solar organic Rankine cycle for remote power generation

Recent interest in small-scale solar thermal combined heat and power (CHP) power systems has coincided with demand growth for distributed electricity supplies in areas poorly served by centralized power stations. One potential technical approach to meeting this demand is the parabolic trough solar thermal collector coupled with an organic Rankine cycle (ORC) heat engine.The paper describes the design of a solar organic Rankine cycle being installed in Lesotho for rural electrification purpose. The system consists of parabolic though collectors, a storages tank, and a small-scale ORC engine using scroll expanders.A model of each component is developed taking into account the main physical and mechanical phenomena occurring in the cycle and based on experimental data for the main key components.The model allows sizing the different components of the cycle and evaluates the performance of the system. Different working fluids are compared, and two different expansion machine configurations are simulated (single and double stage).     

Energy performance and numerical optimization of a screw expander–based solar thermal electricity system in a wide range of fluctuating operating conditions

This paper provides fundamental principles to study the thermodynamic performance of a new screw expander–based solar thermal electricity plant. While steam turbines are generally used in direct steam generation solar systems without admitting fluid in two-phase conditions, steam screw expanders, as volumetric machines, can convert thermal to mechanical energy also by expanding liquid-steam mixtures without a decline in efficiency. In effect, steam turbines are not as competitive as screw expanders when the net power is smaller than 2 MW and for low-grade heat sources. The solar electricity generation system proposed in this paper is based on the steam Rankine cycle: Water is used as both working fluid and storage, parabolic trough collectors are used as a thermal source, and screw expanders are used as power machines. Since screw expanders can operate at off-design working conditions in several situations when installed in direct steam generation solar plants, studying expander performance under fluctuating working situations is a crucial issue. The main aim of the present paper is to establish a thermodynamic model to study the energetic benefits of the proposed power system when off-design operating conditions and variable solar radiation occur. This entails, first and foremost, developing overexpansion and underexpansion numerical models to describe the polytropic expansion phase, which considers all the losses affecting performance of the screw expander under real operating conditions. To assess the best operating conditions and maximum efficiency of the whole power system at part-load working conditions under fluctuating solar radiations, parametric optimization is then improved in a wide range of variable working conditions, assuming condensation pressures of water increasing from 0.1 to 1 bar, under an evaporation temperature rising from 170°C to 300°C.     

低品位热能驱动有机朗肯循环的变工况特性

构建有机朗肯循环变工况分析模型,研究热源条件对系统变工况性能的影响规律。结果表明:随着热源温度升高,系统的最佳蒸发压力线性增大,而涡旋膨胀机的等熵效率逐渐减小。相比额定工况,热源温度变化-30.0K与30.0K时,净输出功率变化了-32.4%与18.4%,热效率降低了4.0%与11.4%,热回收效率变化幅度分别为-9.8%及8.9%;当热源温度从423增大至483K时,系统不可逆损失的变化率为-37.1%与45.5%,火用效率的变化率为6.7%与-17.5%。相比热源流量,热源温度对系统变工况性能的影响更大。     

有机朗肯循环的研究进展

有机朗肯循环是一种被认为能有效利用低温热能的技术。科研工作者在不同方面（包括工质、膨胀机、换热器的影响、系统的优化）对有机朗肯循环系统效率的影响进行了大量的研究。本文针对不同热源的工质筛选、膨胀机的特点、系统循环优化以及换热器的影响方面进行了讨论和总结,为有机朗肯循环系统的实际应用提供参考。     

A biomass-fired micro-scale CHP system with organic Rankine cycle (ORC) – Thermodynamic modelling studies

This paper presents the results of thermodynamics modelling studies of a 2 kW (e) biomass-fired CHP system with organic Rankine cycle (ORC). Three environmentally friendly refrigerants, namely HFE7000, HFE7100 and n-pentane, have been selected as the ORC fluids. The thermodynamic properties of the selected ORC fluids which have been predicted by commercial software (EES) are used to predict the thermal efficiency of ORC. The results of modelling show that under the simulated conditions (1) the ORC thermal efficiency with any selected ORC fluid is well below (roughly about 60% of) the Carnot cycle efficiency; the ORC efficiency depends on not only the modelling conditions but also the ORC fluid – the highest predicted ORC efficiency is 16.6%; the predicted ORC efficiency follows the following order: n-pentane > HFE7000 > HFE7100 (2) both superheating and sub-cooling are detrimental to the ORC efficiency (3) the electrical efficiency of the CHP system with the selected ORC fluids is predicted to be within the range of 7.5%–13.5%, mainly depending on the hot water temperature of the biomass boiler and the ORC condenser cooling water temperature as well as the ORC fluid, and corresponding to about 1.5 kW and 2.71 kW electricity output (4) the overall CHP efficiency of the CHP system is in the order of 80% for all three ORC fluids although the amount and quality of heating supplied by the CHP system depend on the ORC fluid selected and the modelling conditions.     

Performance of a combined organic Rankine cycle and vapor compression cycle for heat activated cooling

Heat activated cooling has the potential of utilizing thermal sources that currently go unused such as engine exhaust heat or industrial waste heat. Using these heat sources can provide enhanced energy utilization and reduced fuel usage in applications where cooling is needed. The concept developed here uses waste heat from stationary and mobile engine cycles to generate cooling for structures and vehicles. It combines an organic Rankine cycle (ORC) with a conventional vapor compression cycle. A nominal 5 kW cooling capacity prototype system was developed based on this concept and tested under laboratory conditions. In order to maintain high system performance while reducing size and weight for portable applications, microchannel based heat transfer components and scroll based expansion and compression were used. Although the system was tested off of its design point, it performed well achieving 4.4 kW of cooling at a measured heat activated COP of 0.48. Both the conversion and 2nd law efficiencies were close to the model results, proving it to be an attractive technology. The measured isentropic efficiency of the scroll expander reached 84%, when the pressure ratio was close to the scroll intrinsic expansion ratio. The reduced cooling capacity was attributed to off design operation.     

Low‐grade heat‐driven Rankine cycle,a feasibility study

Conversion of low‐grade heat to high‐quality energy such as electricity using the Rankine cycle poses serious challenges. When such conversion is possible, it is invariably expensive or unacceptable due to environmental concerns associated with the working medium. The low‐grade heat can either be from exhaust systems or from solar radiation. Thus, the topic addresses a very useful subject, combining energy efficiency and renewable energy. Although high‐grade heat recovery and energy conversion is a mature technology widely covered by the literature, low‐grade energy conversion, especially using thermodynamic cycles, has not been sufficiently addressed to date. This paper addresses the feasibility of a low‐grade heat‐driven Rankine cycle to produce power using a scroll expander, a low toxicity, low flammability, and ozone‐neutral working fluid. A cost benefit analysis of the recommended system shows that it is a viable option for solar power generation, at about one‐third the cost of a comparable photovoltaic system. Copyright © 2008 John Wiley & Sons, Ltd.     

Selection and Evaluation of Dry and Isentropic Organic Working Fluids Used in Organic Rankine Cycle Based on the Turning Point on Their Saturated Vapor Curves

The organic Rankine cycle(ORC) is a popular technique used in the utilization of low-grade thermal energy. Among wet, dry, and isentropic organic working fluids, the latter two types are more appropriate for ORC systems. In this paper, the definition of turning point on saturated vapor curve of dry fluid and isentropic fluid was given according to the shape of the saturated curve of working fluids in a T-s diagram. On this basis, the model of near-critical region triangle was established. Using this model, the thermodynamic performance of 57 kinds of dry and isentropic organic working fluids in ORC was evaluated. The performance includes the relation between turning point temperature and cycle thermal efficiency, the relation between near-critical region triangle area and cycle thermal efficiency, the relation between near-critical region triangle area and exergy at turning point temperature, the relation between near-critical region triangle area and reciprocal value of slope of saturated vapor curve. Moreover, working fluid selection was also conducted in terms of heat source type. It was found through theoretical analysis results that the popular R123 is an acceptable choice especially for the utilization of closed type heat source. Considering it will be phased out in near future, then cis-butene, butane, trans-butene, and isobutene are worth studying as its successor. Dodecane is worthy of attention and further research and it can be a good choice for utilization of open type heat source.     

Applicability of scroll expander and compressor to an external power engine: Conceptual design and performance analysis

A scroll expander and scroll compressor for a 10‐kW class Ericsson engine utilizing solar energy as heat source has been suggested. Orbiting scroll members of the expander and compressor were designed to have a base plate with double‐sided wrap structure for compact size and axial gas force cancellation. For axial compliance of the scroll machines, a back pressure chamber was created on each fixed scroll. In order to compensate for the thermal expansion, flexible coupling between the orbiting scroll base plate and the scroll plate holders and sliding keys between the fixed scroll and the supporting frame were designed. Common shafts were shared by the expander and compressor for direct power transmission. For high‐ and low‐side pressures of 6 and 2.5 MPa, respectively, and expander inlet temperature of 700°C, the proposed engine efficiency was estimated to be 7.3%. The engine efficiency strongly depends on the expander and compressor efficiencies as well as on the regenerator efficiency. The shaft output of the designed scroll expander was calculated to be 38.84 kW, while the input power for the scroll compressor was 27.97 kW, yielding 10.87 kW for the engine output. Copyright © 2011 John Wiley & Sons, Ltd.     

太阳能驱动的有机朗肯-喷气增焓（带二次吸气的增效）蒸汽压缩制冷系统性能分析

为有效利用太阳能,以有机朗肯−喷气增焓（带二次吸气的增效）蒸汽压缩式制冷系统为研究对象,建立了系统的热力学模型,分别选取R236fa、R245fa、RC318和R141b作为系统工质,研究了发生温度、凝结温度、冷凝温度、蒸发温度、膨胀机等熵膨胀效率及压缩机等熵压缩效率对系统性能的影响,并以系统性能最佳为目标对工质进行了优选。计算结果表明：对整个系统而言,R141b是最合适的工质,凝结温度和冷凝温度对系统性能有重要影响。以R141b为例,当发生温度在85℃、凝结温度为40℃、冷凝温度为40℃、蒸发温度为 −15℃时,系统COP_s达到0.2528,采用喷气增焓技术对于环境温度很低、太阳能资源丰富的北方地区具有很大的优势。     

Exergy analysis of a 1.2 MW waste heat power generation project

According to systematic features, analysis method based on exergy balance is established. Basic indicators in the system, the subsystem, and facilities are put forward in this paper. By using this method to analyze the generation system of megawatt‐scale in one chemical enterprise, it is found that the objective exergy efficiency of the system is 35.67%, and exergy loss of organic Rankine cycle (ORC) is the highest. The thermal efficiency of the total system is 9.61%. For the condenser, the thermal efficiency is 91.18%, and the exergy efficiency is only 23.44%. The objective exergy efficiency of the evaporator is 74.04%. The influence coefficient of exergy loss of condenser is higher than that of pump and expander, but input exergy of the condenser is lower than that of the expander. It is revealed that ORC subsystem is the part which needs to be focused on, and the condenser is the most important component of ORC subsystem which should be optimized firstly.     

A study of organic working fluids on system efficiency of an ORC using low-grade energy sources

Rankine cycles using organic fluids (as categorized into three groups: wet, dry, and isentropic fluids) as working fluids in converting low-grade energy are investigated in this study. The main purpose is to identify suitable working fluids which may yield high system efficiencies in an organic Rankine cycle (ORC) system. Efficiencies of ORC systems are calculated based on an assumption that the inlet condition of the working fluid entering turbine is in saturated vapor phase. Parameters under investigation are turbine inlet temperature, turbine inlet pressure, condenser exit temperature, turbine exit quality, overall irrversibility, and system efficiency. The low-grade energy source can be obtained from a solar pond or/and an ocean thermal energy conversion (OTEC) system. Results indicate that wet fluids with very steep saturated vapor curves in T-s diagram have a better overall performance in energy conversion efficiencies than that of dry fluids. It can also be shown that all the working fluids have a similar behavior of the efficiency-condenser exit temperature relationship. Furthermore, an appropriate combination of solar energy and an ORC system with a higher turbine inlet temperature and a lower condenser temperature (as operated deeply under sea level) would provide an economically feasible and environment-friendly renewable energy conversion system.     

Transient response of waste heat recovery system for hydrogen production and other renewable energy utilization

In this paper, a 1 kW ORC experimental system is built. Using R123 as the working fluid, transient responses of Basic ORC (BORC) and ORC with a regenerator (RORC) are both tested under critical conditions. A total of four experiments are carried out, including: (1) Case 1: the working fluid pump is suddenly shut down; (2) Case 2: the working fluid is overfilled or underfilled; (3) Case 3: the torque of the expander is suddenly loss. (4) Case 4: the cooling water pump is suddenly shut down. All the major quantities such as the output power and torque of the expander, temperatures and pressures at the inlet and outlet of the expander, temperatures at the inlet and outlet of the condenser are measured. The transient responses of the two systems under the controlled critical conditions are tested and compared, some physical explanations are provided. It is found that RORC is more stable than BORC because of the regenerator. Regenerator should act as a “pre-heater” or “pre-cooler” under the critical conditions thus improving the stability of RORC. When the working fluid in the system is underfilled or leaked, the system performance is extremely unstable. Otherwise, when the working fluid is overfilled, the trend of the curves are similar to the optimal working condition but with weaker performances. We also find that if the working fluid pump is shut down when working fluid is overfilled, the rotation speed and shaft power output of the expander will increase significantly, the unique phenomenon can be used to estimate whether the working fluid in the system is overfilled.     

Testing and modeling a scroll expander integrated into an Organic Rankine Cycle

Organic Rankine Cycles (ORC’s) are particularly suitable for recovering energy from low-grade heat sources. This paper first presents the results of an experimental study carried out on a prototype of an open-drive oil-free scroll expander integrated into an ORC working with refrigerant HCFC-123. By exploiting the overall expander performance measurements, the eight parameters of a scroll expander semi-empirical model are then identified. The model is able to compute variables of first importance such as the mass flow rate, the delivered shaft power and the discharge temperature, and secondary variables such as the supply heating-up, the exhaust cooling-down, the ambient losses, the internal leakage and the mechanical losses. The maximum deviation between the predictions by the model and the measurements is 2% for the mass flow rate, 5% for the shaft power and 3 K for the discharge temperature. The validated model of the expander is finally used to quantify the different losses and to indicate how the design of the expander might be altered to achieve better performances. This analysis pointed out that the internal leakages and, to a lesser extent, the supply pressure drop and the mechanical losses are the main losses affecting the performance of the expander.     

Performance analysis of cogeneration systems based on micro gas turbine (MGT), organic Rankine cycle and ejector refrigeration cycle

In this paper, the operation performance of three novel kinds of cogeneration systems under design and off-design condition was investigated. The systems are MGT (micro gas turbine) + ORC (organic Rankine cycle) for electricity demand, MGT+ ERC (ejector refrigeration cycle) for electricity and cooling demand, and MGT+ ORC+ ERC for electricity and cooling demand. The effect of 5 different working fluids on cogeneration systems was studied. The results show that under the design condition, when using R600 in the bottoming cycle, the MGT+ ORC system has the lowest total output of 117.1 kW with a thermal efficiency of 0.334, and the MGT+ ERC system has the largest total output of 142.6 kW with a thermal efficiency of 0.408. For the MGT+ ORC+ ERC system, the total output is between the other two systems, which is 129.3 kW with a thermal efficiency of 0.370. For the effect of different working fluids, R123 is the most suitable working fluid for MGT+ ORC with the maximum electricity output power and R600 is the most suitable working fluid for MGT+ ERC with the maximum cooling capacity, while both R600 and R123 can make MGT+ ORC+ ERC achieve a good comprehensive performance of refrigeration and electricity. The thermal efficiency of three cogeneration systems can be effectively improved under off-design condition because the bottoming cycle can compensate for the power decrease of MGT. The results obtained in this paper can provide a reference for the design and operation of the cogeneration system for distributed energy systems (DES).