Trilateral Flash Cycle System Research Articles

Using a Partially Evaporating Cycle to Improve the Volume Ratio Problem of the Trilateral Flash Cycle for Low-Grade Heat Recovery.

This study examined the trilateral flash cycle characteristics (TFC) and partially evaporating cycle (PEC) using a low-grade heat source at 80 °C. The evaporation temperature and mass flow rate of the working fluids and the expander inlet’s quality were optimized through pinch point observation. This can help advance methods in determining the best design points and their operating conditions. The results indicated the partially evaporating cycle could solve the high-volume ratio problem without sacrificing the net power and thermal efficiency performance. When the system operation’s saturation temperature decreased by 10 °C, the net power, thermal efficiency, and volume ratio of the trilateral flash cycle system decreased by approximately 20%. Conversely, with the same operational conditions, the net power and thermal efficiency of the partially evaporating cycle system decreased by only approximately 3%; however, the volume ratio decreased by more than 50%. When the system operating temperature was under 63 °C, each fluid’s volume ratio could decrease to approximately 5. The problem of high excessive expansion would be solved from the features of the partially evaporating cycle, and it will keep the ideal power generation efficiency and improve expander manufacturing.

Modelling and off-design performance optimisation of a trilateral flash cycle system using two-phase twin-screw expanders with variable built-in volume ratio

This research work presents a numerical chamber model of a two-phase twin-screw expander and its further integration in a one-dimensional model of a Trilateral Flash Cycle (TFC) system for low-grade heat to power conversion applications. The novel feature of the expander is the capability of changing the built-in volume ratio (BIVR) of the machine through a sliding valve in the casing that opens an additional suction port. Lowering the BIVR from 5.06 to 2.63 results in an improvement of the volumetric efficiency from 53% to 77% but also in a reduction of the specific indicated power from 4.77 kJ/kg to 3.56 kJ/kg. Parametric analysis on several degrees of freedom of the full TFC system concluded that expander speed and BIVR are the variables that mostly impact the net power output of the unit. An optimisation study enabled the net power output of the TFC system, at design point, to increase from 81 kW to 103 kW.

Experimental study on the prospect of low-temperature heat to power generation using Trilateral Flash Cycle (TFC)

In this paper, a trilateral flash cycle (TFC) based system has been developed and studied to find out its prospect for utilizing more energy and enhancing the power generation capacity. To hold simplicity and minimize the cost of system construction, an impulse turbine and a converging-diverging (CD) stationary nozzle setup have been used as the expander. The experimental study introduced impulse turbine incorporates with a stationary CD nozzle and organic working fluid, which showed a promising power generation capability from a heat source below 80 °C in spite of the increasing size of the heat exchanger, condenser, and pump. In addition to the use of proper impulse turbines, however, the power generation capacity of such type of system is basically a function of the nozzle isentropic efficiency, which lies on the nozzle geometry and the alignment with respect to the turbine. A case study on the application of the TFC system for commercial power generation in associate with economic analysis has also been included in this study which shows a payback time less than 10 years with typical operational life of 20 years, considering only 40% nozzle isentropic efficiency and by using more efficient nozzle, the capital cost per unit power generation could be minimized.

Comparison of the Trilateral Flash Cycle and Rankine Cycle with Organic Fluid Using the Pinch Point Temperature

Low-temperature heat utilization can be applied to waste heat from industrial processes or renewable energy sources such as geothermal and ocean energy. The most common low-temperature waste-heat recovery technology is the organic Rankine cycle (ORC). However, the phase change of ORC working fluid for the heat extraction process causes a pinch-point problem, and the heat recovery cannot be efficiently used. To improve heat extraction and power generation, this study explored the cycle characteristics of the trilateral flash cycle (TFC) in a low-temperature heat source. A pinch-point-based methodology was developed for studying the optimal design point and operating conditions and for optimizing working fluid evaporation temperature and mass flow rate. According to the simulation results, the TFC system can recover more waste heat than ORC under the same operating conditions. The net power output of the TFC was approximately 30% higher than ORC but at a cost of higher pump power consumption. Additionally, the TFC was superior to ORC with an extremely low-temperature heat source (<80 °C), and the ideal efficiency was approximately 3% at the highest work output condition. The TFC system is economically beneficial for waste-heat recovery for low-temperature heat sources.

One-Dimensional Modelling of a Trilateral Flash Cycle System with Two-Phase Twin-Screw Expanders for Industrial Low-Grade Heat to Power Conversion

This paper provides an overview of a one-dimensional modelling methodology for equipment and systems for heat to power conversion based on a staggered grid space discretization and implemented in the commercial software GT-SUITE®. Particular attention is given to a newly developed modelling procedure for twin-screw machines that is based on a chamber modelling approach and considers leakage paths between cells and with the casing. This methodology is then applied to a low-grade heat to power conversion system based on a Trilateral Flash Cycle (TFC) equipped with two parallel two-phase twin-screw expanders and a control valve upstream of the machines to adapt the fluid quality for an optimal expander operation. The standalone expander model is used to generate performance maps of the machine, which serve as inputs for the TFC system model. Parametric analyses are eventually carried out to assess the impact of several operating parameters of the TFC unit on the recovered power and cycle thermal efficiency. The study shows that the most influencing factors on the TFC system’s performance are the inlet temperature of the heat source and the expander speed. While the first depends on the topping industrial process, the expander speed can be used to optimize and control the TFC system operation also in transient or off-design operating conditions.

A parametric analysis to evaluate the performance metrics of power generation system involving Trilateral Flash Cycle using three different working fluids for low grade waste heat

With the accelerated growth in population and technology progress in day-to-day life, the traditional fuels demands have increased significantly. Despite the execution of many renewable sources (solar, wind, geothermal, biomass), the modern issues such as high investment and power costs have slowed down their development. In this context, improvisation of the existing power generation ways is required by evolving a highly effective thermal system to extract renewable energy available for low grade waste heat. At present, many of the applicable practices are engrossed on high temperature heat extraction rather than low-grade heat in spite of being expensive. In this work, innovative technology has been recommended by amalgamation of the Trilateral Flash Cycle (TFC) with an expander (reaction turbine) in a binary system to offer improved operation, economical and broader employment of the existing resources. TFC can extract heat more efficiently from hydrothermal means to improve power generation directly and decrease the emissions of greenhouse gas. A theoretical analysis using a computer based model for TFC with simple reaction turbine for three proposed diameters at various rotational speeds and operating fluids is performed. Results of output powers and turbine efficiencies of the recommended system are compared. Also, this design concludes the optimum design factors for the turbine under explicit operational settings and the factors affecting the efficiency and nozzle flow area are discussed for the TFC system.

Development and analysis of a packaged Trilateral Flash Cycle system for low grade heat to power conversion applications

The current research tackles the energy trilemma of emissions reduction, security of supply and cost savings in industrial environments by presenting the development of a packaged, plug & play power unit for low-grade waste heat recovery applications. The heat to power conversion system is based on the Trilateral Flash Cycle (TFC), a bottoming thermodynamic cycle particularly suitable for waste heat sources at temperatures below 100 °C which, on a European scale, account for 469 TWh in industry and are particularly concentrated in the chemical and petrochemical sectors. The industrial test case refers to a UK tire manufacturing company in which a 2 MW water stream at 85 °C involved in the rubber curing process was chosen as hot source of the TFC system while a pond was considered the heat sink. The design of the industrial scale power unit, which is presented at end of the manuscript, was carried out based on the outcomes of a theoretical modelling platform that allowed to investigate and optimize multiple design parameters using energy and exergy analyses. In particular, the model exploitation identified R1233zd(E) and R245fa as the most suitable pure working fluids for the current application, given the higher net power output and the lower ratio between pumping and expander powers. At nominal operating conditions, the designed TFC system is expected to recover 120 kWe and have an overall efficiency of 6%.

Low grade thermal recovery based on trilateral flash cycles using recent pure fluids and mixtures

The current work presents a thermodynamic analysis of a Trilateral Flash Cycle (TFC) system for low grade heat to power conversion applications. Novel aspects of the research are the usage of rotary positive displacement expanders as prime movers of the TFC system as well as the reference to working fluids and their mixtures at the state of the art. In particular, the role of a correct built-in volume ratio of the expander with respect to the pressure ratio of the thermodynamic cycle is emphasized. In fact, a mismatching of these two quantities would lead to an isochoric expansion process which, in turn, might negatively affect the overall power recovery. With reference to a transcritical CO2 stream at 100°C as heat source for the TFC system, parametric and screening studies were carried out using different expander built-in volume ratios and working fluids respectively. Among the fluids analyzed, results showed that pure substances such as R1234ze(E) and propane would provide a greater specific work but, on the other hand, would require built-in volume ratios (8 and 14) that are beyond the capabilities of rotary positive displacement expanders (5). The addition of CO2 to the afore mentioned working fluids would ease the mismatching issue but, at the same time, would reduce the specific power output. Regarding the built-in volume ratio analysis, it was found that optimal values change in accordance to the working fluid and refer to an expansion process with a slight isochoric phase.

Techno-economic survey and design of a pilot test rig for a trilateral flash cycle system in a steel production plant

In recent years the interest in recovering rejected low-grade heat within industry has intensified. Around 30% of global primary energy consumption is attributed to the industrial sector and a significant portion of this is rejected as heat. The majority of this wasted energy is available at temperatures below 100°C and as such conventional waste heat to power conversion systems cannot economically recover the energy, producing simple pay backs that are unacceptable to industry. The Trilateral Flash Cycle (TFC) is however a promising technology with the ability to harness the rejected heat found in these low grade waste streams. The current research work presents a techno-economic assessment of the installation potential for a low grade heat to power conversion system using a TFC system. In particular, thermodynamic modelling is utilised to estimate the expected energy recovery and, in turn, the potential savings achievable through the TFC solution. The survey investigated three diverse and challenging heat sources at steel production plants. Annual energy recovery from the chosen heat source is expected to be 782 MWh. Prior to the upscaling of the system to the 2MW waste thermal power, a pilot test rig was designed and built. Preliminary tests showed a net electrical power output up to 6.2 kW and an overall efficiency of 4.3%.

Development of the Trilateral Flash Cycle System: Part 3: The Design of High-Efficiency Two-Phase Screw Expanders

An extensive research and development programme carried out at City University, London, has led to an improved level of understanding of how Lysholm twin screw machines may be used to recover power from two-phase flash expansion processes. The mode of operation of such machines is described together with the various types of rotor shapes used. Details are given of a computer simulation of the expansion process which was used to analyse 636 test results. These were obtained from earlier investigations as well as those of the authors and include three different working fluids, varying rotor profiles and sizes and power outputs of 5–850 kW. Good agreement was obtained between predicted and measured performance parameters and statistical analyses of the results indicate that this is unlikely to be improved without the development of more refined methods of two-phase flow analysis than are currently in use. Included in the tests are a set of measurements of pressure-volume changes within the expander carried out by the authors which confirmed a hitherto unappreciated feature of the expansion process. This is the relatively large pressure drop associated with the initial filling of the volume trapped between the rotors and the casing. The analytical technique thus developed was used both to explain the poor results of earlier studies with water expanders and to estimate optimum design performance. It is shown that, when expanding wet organic fluids, adiabatic efficiencies of over 70 per cent may be obtained at outputs of only 25 kW while multi-megawatt outputs are possible from machines no bigger than large compressors with efficiencies of more than 80 per cent. Two-phase screw expanders may be used not only for large-scale power generation in trilateral flash cycle (TFC) systems, but also in place of throttle valves in vapour compression systems to drive screw compressors in sealed ‘expressor’ units. The coefficient of performance of large refrigeration, air conditioning and heat pump systems may thereby be raised by up to approximately 8 per cent.

Development of the Trilateral Flash Cycle System Part 2: Increasing Power Output with Working Fluid Mixtures

The trilateral flash cycle system is a proposed means of power recovery from single-phase low-grade heat sources. Its feasibility depends on the efficient adiabatic expansion of light hydrocarbons from the saturated liquid phase into the two-phase region. Such a process is performed most effectively with a Lysholm twin-screw expander when the exhausted vapour is wet. At higher temperatures, when multi-stage expansion is required, working fluids may be found which complete the process as dry saturated vapour. It is shown that at condensing temperatures of 0–50°C, this is possible with a mixture of n-pentane and 2,2–dimethylpropane (neopentane) for fluid inlet temperatures in the 150–180 °C range. A radial inflow turbine may then be used in place of a screw for the last stage. With such an arrangement, expander adiabatic efficiencies of up to 85 per cent have been predicted for power outputs in excess of 5 MW. The method of fluid property estimation is described and its accuracy confirmed by experiment.

Development of the Trilateral Flash Cycle System: Part 1: Fundamental Considerations

The world market for systems for power recovery from low-grade heat sources is of the order of £1 billion per annum. Many of these sources are hot liquids or gases from which conventional power systems convert less than 2.5 per cent of the available heat into useful power when the fluid is initially at a temperature of 100° C rising to 8–9 per cent at an initial temperature of 200°C. Consideration of the maximum work recoverable from such single-phase heat sources leads to the concept of an ideal trilateral cycle as the optimum means of power recovery. The trilateral flash cycle (TFC) system is one means of approaching this ideal which involves liquid heating only and two-phase expansion of vapour. Previous work related to this is reviewed and details of analytical studies are given which compare such a system with various types of simple Rankine cycle. It is shown that provided two-phase expanders can be made to attain adiabatic efficiencies of more than 75 per cent, the TFC system can produce outputs of up to 80 per cent more than simple Rankine cycle systems in the recovery of power from hot liquid streams in the 100–200°C temperature range. The estimated cost per unit net output is approximately equal to that of Rankine cycle systems. The preferred working fluids for TFC power plants are light hydrocarbons.