Outlet Temperature Of Heat Source Research Articles

"3E" Analysis and Working Fluid Selection for a Cogeneration System for Geothermal Large Temperature Difference Utilization.

Widespread utilization of geothermal energy as a clean energy source can reduce the use of fossil fuels and thereby protect the environment. Previous combined heat and power (CHP) systems using low-temperature geothermal heat as a heat source have limited utilization of geothermal heat. In this study, a cogeneration system based on organic Rankine cycle (ORC) and absorption heat exchanger (AHE) is designed. We analyzed the thermal efficiency, exergy efficiency, and economics of the proposed system utilizing 85 °C low-temperature geothermal heat, and the optimal operating fluid for this system is discussed. The results show that the optimal working fluid for the proposed system is R245fa. Using R245fa as the working fluid, the proposed system can achieve an exergy efficiency of 61.39%, when the heat source outlet temperature can be as low as 23 °C, while the proposed system can achieve the lowest LCOE of 0.082 ($/kW ·h) when using R22 as the working fluid.

Cogeneration system based on large temperature difference heat transfer with stepwise utilization

Organic Rankine cycle (ORC) plays a critical role in systems such as waste heat recovery, cogeneration, renewable energy generation and energy storage. No matter which field ORC is applied in, the evaporator side of ORC has an extremely poor utilization of the thermal energy from heat source, resulting in the outlet temperature of hot side fluid being much higher than the inlet temperature of cold side fluid. In order to fully capitalize on the thermal energy, this study introduces the two-stage absorption heat exchanger (AHE) and integrates it into ORC to obtain an original high-efficiency cogeneration system (AHEORC-CHP), which is parametrically analyzed by constructing and operating thermo-economic model in terms of the expansion pressure ratio and working fluid of ORC, the type and flow rate of heat source, and the inlet temperature of generator in AHE. Due to the existence of pinch point temperature difference, conventional ORC design can only reduce the outlet temperature of the heat source with an inlet temperature of 130 °C to about 70 °C under the condition of ideal heat transfer. The AHEORC-CHP configuration proposed in this work can achieve a significant reduction in the final outlet temperature of waste heat source (as low as 24.69 °C) and realize heat exchange with large temperature difference and efficient utilization of the heat source. Besides, the thermoelectric efficiency and temperature effectiveness can reach 90.9% and 1.2 respectively, while the profit ratio of investment rises to 8.46. Further improving the performance of AHEORC-CHP system requires optimizing the design of the pressure in absorption heat pump (AHP) cycle as well as the concentration and flow rate of LiBr-H2O solution. This study can provide a simulation pathway for the application of two-stage AHE in ORC, and likewise for future experimental investigations.

Performance Analysis of Solar Organic Rankine Cycle with Stable Output Using Cyclopentane/Cyclohexane Mixtures▴

AbstractKeeping output stable is necessary for solar organic rankine cycle (SORC) considering variable solar irradiation conditions. In this study, zeotropic mixtures are introduced into SORC with stable output. Evaporation pressure and turbine inlet temperature are optimized with the overall system efficiency as optimization objective. Comparisons are drawn between the basic SORC and regenerative SORC. The influences of zeotropic mixture composition on the overall system efficiency, ORC efficiency, heat source outlet temperature, mass flow rate of heat source and working fluids and temperature rise of internal heat exchanger (IHE) are analyzed. The results show that zeotropic mixtures can slightly increase the overall system efficiency of the basic SORC system compared with the optimal pure working fluid system, but for the regenerative SORC system, the optimal overall system efficiency is higher when pure cyclohexane is used. When cyclopentane mole fraction is 0.4, the overall system efficiency of the basic SORC system reaches maximal value of 16.08%, which is 0.12% higher than for the optimal pure working fluids system, relatively. Under the optimum conditions, the optimal overall system efficiency of the regenerative SORC system is 52.11% higher than that of the basic SORC system.

Dual-pressure condensation high temperature heat pump system for waste heat recovery: Energetic and exergetic assessment

High temperature heat pump (HTHP) is an energy-saving solution to recover the industrial waste heat. Five configurations of dual-pressure condensation HTHP are proposed and studied to improve thermal matching in the heat sink. The energy and exergy models are established, and the performance of dual-pressure condensation HTHP systems is assessed and compared with that of traditional single-stage, two-stage and cascade HTHP systems in detail. The results show that the dual-pressure condensation HTHP system is superior to traditional HTHP system from the perspective of energy efficiency and exergetic performance. A maximum COP is obtained at the optimum intermediate water temperature. The COP of dual-pressure condensation water-cooled saturated system (DWSAS) is the highest of 4.16, and 3.37–9.34% higher than the traditional HTHPs. R1234ze(Z) shows much higher energy efficiency over the other refrigerants for the applications of HTHP, and the COP is 2.62–4.47% larger than using R600a at the outlet temperature of heat source of 15–35 °C. The exergy destruction is also significantly reduced by using dual-pressure condensation HTHPs due to the improvement of thermal matching in the condensers. DWSAS shows the lowest exergy destruction, which is 4.28–19.35% lower than traditional HTHPs. DWSAS by using R1234ze(Z) is recommended due to its outstanding energetic and exergetic performance.

Effects of critical temperature, critical pressure and dryness of working fluids on the performance of the transcritical organic rankine cycle

Thermodynamic properties of working fluids have significant effects on the system performance of the transcritical organic Rankine cycle, and the optimal working fluid is related to the heat source temperature. In this paper, both separate and combined effects of the critical temperature (Tcri), critical pressure (Pcri) and dryness (ξ) of working fluids on the performance of TORCs were investigated under different heat source temperatures. The trends in variation of maximum net power output (Wmax) based on the increase of the critical temperatures of working fluids were also investigated. The results indicated that a lower Tcri of working fluids would lead to an increase in the expander’s outlet temperature and a worse thermal match in the condenser. However, a higher Tcri would result an increase in the heat source outlet temperature and a worse thermal match in the vapor generator. In addition, a higher Pcri would lead to an uneven temperature curve for the working fluid and a worse thermal match in the vapor generator. A larger ξ would result in high superheat at the condensation pressure and a worse thermal match in the condenser. Working fluids with suitable Tcri, lower Pcri and lower ξ could achieve better net power output performance.

Thermodynamic performance comparison between singlepressure and dual-pressure evaporation organic Rankine cycles for heat sources with outlet temperature limit

Organic Rankine cycle (ORC) is a widely used technology to generate power from renewable energy and waste heat. Dual-pressure evaporation cycle holds immense potential to be used in ORC systems because it can remarkably increase the heat-work conversion efficiency and improve the adaptability of ORC to heat sources with various characteristics. This study compared the thermodynamic performance of single-pressure and dual-pressure evaporation ORCs for heat sources with outlet temperature limit, based on five organic fluids. Effects of the heat source outlet temperature limit on the characteristics of dual-pressure evaporation ORC system were also discussed. Results show that the dual-pressure evaporation cycle can substantially increase the net power output by increasing system efficiency, and is beneficial to reduce the charge volume of organic fluid in the system, compared to the single-pressure evaporation cycle. The increments in net power output of dual-pressure evaporation cycle over the single-pressure evaporation cycle can be 4.9%, 8.3%, 10.8%, 12.8%, and 14.0% at most for R227ea, R1234ze(E), R600a, R245fa, and R601a, respectively. While, compared to heat sources without outlet temperature limit, the increment in net power output of dual-pressure evaporation cycle is remarkably lower for heat sources with outlet temperature limit.

An improved modeling for low-grade organic Rankine cycle coupled with optimization design of radial-inflow turbine

Organic Rankine cycle (ORC) has been proven to be an effective and promising technology to convert low-grade heat energy into power, attracting rapidly growing interest in recent years. As the key component of the ORC system, turbine significantly influences the overall cycle performance and its efficiency also varies with different working fluids as well as in different operating conditions. However, turbine efficiency is generally assumed to be constant in the conventional cycle design. Aiming at this issue, this paper couples the ORC system design with the radial-inflow turbine design to investigate the thermodynamic performance of the ORC system and the aerodynamic characteristics of radial-inflow turbine simultaneously. The constrained genetic algorithm (GA) is used to optimize the radial-inflow turbine with attention to six design parameters, including degree of reaction, velocity ratio, loading coefficient, flow coefficient, ratio of wheel diameter, and rotational speed. The influence of heat source outlet temperature on the performance of the radial-inflow turbine and the ORC system with constant mass flow rate of the heat source and constant heat source inlet temperature is investigated for four kinds of working fluids. The net electrical powers achieved are from few tens kWs to one hundred kWs. The results show that the turbine efficiency decreases with increasing heat source outlet temperature and that the decreasing rate of turbine efficiency becomes faster in the high temperature region. The optimized turbine efficiency varies from 88.06% (using pentane at the outlet temperature of 105°C) to 91.01% (using R245fa at the outlet temperature of 80°C), which appears much higher compared to common values reported in the literature. Furthermore, the cycle efficiency increases monotonously with the growth of the heat source outlet temperature, whereas the net power output has the opposite trend. R123 achieves the maximum cycle efficiency of 12.21% at the heat source outlet temperature of 110°C. Based on the optimized results, the recommended ranges of the key design parameters for ORC radial-inflow turbine are presented as well.

Parametric analysis of organic Rankine cycle based on a radial turbine for low-grade waste heat recovery

Organic Rankine cycle (ORC) is one of the promising technologies to convert low-grade waste heat into electricity. This study comprehensively investigates the thermodynamic performance of ORC system and the aerodynamic characteristics of radial turbine. An optimized coupling model for the ORC with a radial turbine is developed, and the particle swarm optimization algorithm is applied to maximize the turbine efficiency dynamically. The model is used to evaluate the effect of heat source temperature on the cycle performance and parameter selection for turbine design optimization. The results show that optimal turbine efficiency can be achieved within the considered heat source outlet temperature range, as high pressure ratio and high mass flow rate can both degrade the turbine efficiency. Among the four studied organic fluids, the ORCs using R123 and R245fa can achieve desirable cycle performances. For the turbine design, lower degree of reaction is beneficial for turbine efficiency gains. The optimized rotational speed and ratio of wheel diameter are dependent on heat source temperature, while which has almost no effects on velocity ratio, loading coefficient, and flow coefficient. The reasonable ranges of partial turbine design parameters are also proposed in this work.

A novel low-grade heat-driven absorption refrigeration system with LiCl–H2O and LiBr–H2O working pairs

A novel low grade heat-driven absorption refrigeration system is proposed, where LiCl–H2O with higher vapor pressure is used in the high-pressure cycle and LiBr–H2O with lower vapor pressure is employed in the low-pressure cycle. Effects of key parameters on the system performance are analyzed, and different heat source utilization modes are considered: parallel modes (PM-1 and PM-2) and serial mode (SM). What's more, comparisons among the PM-1, PM-2, SM and the traditional double-stage LiBr–H2O absorption system (TDS) are made. Results show that the PM-1 has much higher COP than the TDS under small ranges of working conditions, with the maximum COP improvement 26.7%, while the PM-2 shows prominent advantages under wider ranges, with the maximum COP improvement 35%. In addition, The PM-1 shows much higher COP than the SM under higher condensing temperature and evaporation temperature, while the SM has much lower heat source outlet temperature which is at most 12 °C lower than that of the PM-1. The intermediate pressure is important for the system performance and the optimum value is 2.27 kPa in the PM-1 and 2.5 kPa in the SM. For the PM-2, the intermediate pressure should be chosen to achieve low circulation ratio in the low pressure cycle.

An integrated optimization for organic Rankine cycle based on entransy theory and thermodynamics

The organic Rankine cycle has been one of the essential heat-work conversion technologies nowadays. Lots of effectual optimization methods are focused on the promotion of the system efficiency, which are mainly relied on engineering experience and numerical simulations rather than theoretical analysis. A theoretical integrated optimization method was established based on the entransy theory and thermodynamics, with the ratio of the net power output to the ratio of the total thermal conductance to the thermal conductance in the condenser as the objective function. The system parameters besides the optimal pinch point temperature difference were obtained. The results show that the mass flow rate of the working fluid is inversely proportional to the evaporating temperature. An optimal evaporating temperature maximizes the net power output, and the maximal net power output corresponds to the maximal entransy loss and the change points of the heat source outlet temperature and the change rates for the entropy generation and the entransy dissipation. Moreover, the net power output and the total thermal conductance are inversely proportional to the pinch point temperature difference, contradicting with each other. Under the specified condition, the optimal operating parameters are ascertained, with the optimal pinch point temperature difference of 5 K.

The Effect of Thermal Diffusivity on the System Efficiency of a DOTEC Cycle

In this study, the effect of deep ocean condenser inlet temperature (T DOI ), condenser inlet pressure (P cond,in ), and thermal diffusivity on system efficiency of some selected refrigerants was analyzed using HYSYS. The proposed DOTEC cycle is similar to the reheat Rankine cycle but eliminates irreversibilities by bleeding a fraction of the steam between certain stages of the turbine. The evaporator inlet mass flow rate, inlet temperature of turbine 1, turbine efficiency and inlet and outlet temperature of heat source were imposed. The working fluids considered are sorted in ascending order of their molecular weights as R717, R600a and R152a. Results indicated that a fluid with a lower boiling point temperature like R717 needs a corresponding high heat source and/or evaporator inlet pressure. Also, the response of thermal diffusivity closely follows the change in TDOI as an increase in T DOI increases P cond,in which reduces thermal diffusivity and system efficiency. Furthermore, the fluid with the nominal boiling point temperature has the highest efficiency with efficiency decreasing with an increase in T DOI .

Performance analysis of organic Rankine cycle based on location of heat transfer pinch point in evaporator

The location of heat transfer pinch point in evaporator is the base of determining operating parameters of organic Rankine cycle (ORC). The physical mathematical model seeking the location of pinch point is established, by which, the temperature variations both of heat source and working fluid with UA can be obtained. Taking heat source with inlet temperature of 160 °C as example, the matching potentials between heat source and working fluid are revealed for subcritical and supercritical cycles with the determined temperature difference of pinch point. Thermal efficiency, exergy efficiency, work output per unit area and maximum work outputs are compared and analyzed based on the locations of heat transfer pinch point either. The results indicate that supercritical ORC has a better performance in thermal efficiency, exergy efficiency and work output while outlet temperature of heat source is low. Otherwise, subcritical performs better. Small heat transfer coefficient results in low value of work output per unit area for supercritical ORC. Introduction of IHX may reduce the optimal evaporating pressure, which has a great influence on heat source outlet temperature and superheat degree. The analysis may benefit the selection of operating parameters and control strategy of ORC.

Performance analysis of double organic Rankine cycle for discontinuous low temperature waste heat recovery

This research proposes a double organic Rankine cycle for discontinuous waste heat recovery. The optimal operation conditions of several working fluids have been calculated by a procedure employing MATLAB and REFPROP. The influence of outlet temperature of heat source on the net power output, thermal efficiency, power consumption, mass flow rate, expander outlet temperature, cycle irreversibility and exergy efficiency at a given pinch point temperature difference (PPTD) has been analyzed. Pinch point analysis has also been employed to obtain a thermodynamic understanding of the ORC performance. Of all the working fluids investigated, some performances between each working fluid are rather similar. For a fixed low temperature heat source, the optimal operation condition should be mainly determined by the heat carrier of the heat source, and working fluids have limited influence. Lower outlet temperature of heat source does not always mean more efficient energy use. Acetone exhibits the least exergy destruction, while R245fa possesses the maximal exergy efficiency at a fixed PPTD. Wet fluids exhibit lower thermal efficiency than the others with the increasing of PPTD at a fixed outlet temperature of heat source. Dry and isentropic fluids offer attractive performance.