Utilization Of Low-grade Heat Research Articles

A novel cycle engine for low-grade heat utilization: Principle, conceptual design and thermodynamic analysis

Efficient engine technologies to convert low-grade heat to electricity are urgently desired. In this work, a conceptual structure of engine for a novel cycle or one-way oscillating flow cycle (OOFC), which consists of two isochoric and two adiabatic processes, is described for low-grade heat utilization. Characteristics of OOFC allows for the working fluid temperature glide to be matched to the decrease in temperature of low-grade heat. Then, thermodynamic model is developed for evaluating the performance. Theoretical simulation results show that maximum specific output works are in the range of 12.2 kJ kg−1 – 79.7 kJ kg−1. Compared to Stirling cycle system, maximum specific output work in OOFC system could be improved by 16.2 %–24.8 %. Compared to ideal Carnot cycle engine system, maximum specific output works in OOFC system is nearly the same and 1.8 %–2.6 % lower. As Carnot cycle engine is ideal while thermodynamic cycle loss and heat transfer loss in cold heat exchangers are considered in OOFC engine, the ratios of maximum specific output work demonstrate that OOFC system could be very promising for low-grade heat utilization as a result of well-matched temperature profile in hot heat exchanger.

Experimental study on exhaust internal circulation desiccant wheel evaporative cooling chiller

Evaporative cooling chillers reduce the water temperature by absorbing heat during the evaporation of water without the need for compressor assistance or refrigerants. It is an energy-saving and environmentally friendly cooling technology but it needs necessary improvements in the applicable environmental area and refrigeration performance due to ambient air wet-bulb temperature limitations. To obtain chilled water below the dew-point temperature of the ambient air, an exhaust internal circulation desiccant wheel evaporative cooling chiller is proposed and experimentally investigated. A combination of a heat pipe exchanger, a desiccant wheel, an air cooler and an air heater are applied to reduce the wet-bulb temperature of the ambient air before it passes through the direct evaporative cooler. At the same time, the low-temperature and low-humidity exhaust air from the direct evaporative cooler is used to circulate to improve chiller performance. The experimental results show that the average chilled water temperature of the proposed chiller is 10.2 ℃ lower than that of the direct evaporative cooling chiller under five typical environmental conditions. The wet-bulb efficiency of the proposed chiller is the highest at high temperatures and high humidities (36 ℃ and 80% RH), which is 6 times greater than that of direct evaporative cooling chillers. Therefore, the proposed chiller expands the application range of evaporative cooling technology, which is especially suitable for humid areas with low-grade heat utilization such as solar energy.

Thermodynamic analysis of an ammonia-water double absorption-resorption heat transformer system

Absorption heat transformer (AHT) presents an effective solution for enhancing energy efficiency through the utilization of low-grade industrial waste heat. To reduce the high operating pressures of traditional ammonia AHT and elevate waste heat to a higher temperature level, a novel ammonia-water double absorption-resorption heat transformer (DARHT) incorporating a coupled middle absorber-generator is proposed in this paper. A thermodynamic simulation model of the system is established using MATLAB software to analyze the effects of operating pressures and temperatures on the COP, temperature lift, solution circulation ratio and bypass ratio, thereby determining the optimal operating conditions of the system. The simulation results indicate that there exist optimal PH/PL pairs and middle absorber-generator temperatures which lead to the maximum COP of the system. COP decreases with the increase of absorber and resorber temperatures while increases with the increase of generator and desorber temperatures. When PH/PL pair is 1.5/0.36 MPa, the DARHT system can provide a temperature lift exceeding 70 °C, while the working pressures are significantly lower compared to conventional AHT, thereby reducing the requirements of robust components and enhancing the operational stability of the system.

Performance research of a solution cross-type absorption-resorption heat transformer using NH3/H2O work fluid

Absorption-resorption heat transformer (ARHT) can realize the efficient utilization of low-grade heat under lower operating pressure than traditional absorption heat transformer (AHT), but the application is limited by the low coefficient of performance (COP). In this paper, an ammonia-water solution cross-type absorption-resorption heat transformer is proposed to improve the performance. The rectifier can be eliminated because the component can be balanced with the adjustment of solution's mass flow rate between the absorber and resorber. A thermodynamic model is established to investigate the influences of different parameters on the coefficient of performance. The results reveal that: When the high/low pressure pair is 1.5/0.48 MPa and the temperatures of generator, absorber, resorber and desorber are 105 °C, 80 °C, 80 °C and 4 °C, the COP of the proposed system can reach 0.4049, which is 19 % higher than traditional ARHT, resulting in a corresponding gross temperature lift (GTL) is 25 °C. Besides, the proposed system has wider working temperature ranges than the traditional system. The advantages of the proposed system also include the lower optimal low-pressure values. Therefore, the ammonia-water solution cross-type absorption-resorption heat transformer is appropriate for working in larger pressure differences and makes waste heat recovery more economical and efficient.

Thermodynamic, economic analysis and multi-objective optimization of an improved self-condensing transcritical CO2 Rankine cycle with two-stage ejector for low grade heat utilization

An improved self-condensing transcritical CO2 Rankine cycle (TCRC) with two-stage ejector on the basis of the preliminarily designed one with an ejector is proposed in this study, aiming to enable the condensation of CO2 using conventional cooling source and achieve desirable performance. Thermodynamic and economic mathematical models of the improved self-condensing TCRC are developed, and detailed parametric analysis is carried out to investigate the effect of main parameters on both thermodynamic and economic performances of the system. Then the multi-objective optimization is conducted to examine the potential for performance enhancement. Thereafter, a comparative analysis of the performances between the improved self-condensing TCRC and preliminary self-condensing TCRC is performed. Results show that the improved self-condensing TCRC with two-stage ejector outperforms the preliminary one with an ejector in both thermodynamic and economic aspects, achieving a 3.93% increase in maximum exergy efficiency and a 13.84% reduction in minimum unit net power cost through multi-objective optimization. Under optimal conditions, the exergy efficiency for the improved self-condensing TCRC is 45.48% with a unit net power cost of 0.1241$/kWh.

Techno-economic feasibility study of coupling low-temperature evaporation desalination plant with advanced pressurized water reactor

The increasing demand for freshwater necessitates sustainable desalination solutions, and nuclear power plants offer a promising avenue by utilizing their low-grade waste heat. This study assesses a techno-economic feasibility of coupling a 5 MWth low-temperature evaporation plant with a UAE-based Advanced Pressurized Water Reactor (APR1400). The system addresses freshwater demand, aligning with sustainability goals through low-grade heat utilization. The investigation explores three extraction points for low-grade heat steam, with temperatures ranging from 80 °C to 130 °C. Thermodynamic evaluations using DE-TOP illustrate power requirements and losses, considering variations in maximum brine temperature from 50 °C to 65 °C. Economic analysis using DEEP estimates water production costs. Findings reveal negligible variances in power plant parameters and a minimal reduction in overall efficiency (<0.5 %). The power loss ratio increases proportionally (10 % to 18.6 %) with higher-temperature heat extraction, while the total power requirements for the desalination plant rises by around 30 %, with a negligible power output reduction ratios (0.03 % to 0.07 %). A consistent linear correlation between water production rate and maximum brine temperature doubles water production (∼900 to 1800 m3/day). Applying multiple extraction points across low-grade heat sources demonstrates scalability, reaching three times that of single-point extraction, with marginal increases in power requirements and losses, while maintaining the power reduction ratio below 0.15 %. Economic feasibility indicates competitive water production costs, ranging from 1.53 to 0.87 $/m3 for desalination capacities between 900 and 5400 m3/day. This study provides valuable insights into sustainable water production at the nexus of nuclear energy and desalination, with implications for necessary policy intervention.

Preparation and thermal properties of zeolite/MgSO4 composite sorption material for heat storage

Thermochemical sorption heat storage based on salt hydrates offers the potential advantages such as high energy density, negligible heat loss, small volume change and flexible working modes. Magnesium sulphate (MgSO4) is a high energy density hydrated salt available for the thermochemical sorption heat storage. In this work, different types of zeolites (including 3A, 5A and 13X zeolites) have been selected as mass-transfer-enhancing and structural stabilizing materials, and a series of zeolite/MgSO4 composites have been fabricated. To investigate the thermal energy storage performance of zeolite/MgSO4 composites, the properties of the composites, such as micro-morphology, pore structure, chemical composition and sorption-desorption behavior, have been characterized and analyzed. Results indicated that the mass transfer performance of the composite materials is significantly better than that of the pure zeolites. As the mass fraction of MgSO4 in the composite sorbents increases, the specific surface area and porosity of the composite sorbent decreases, and the sorption performance has been enhanced. Compared with 3A zeolite-based and 5A zeolite-based composites, 13X zeolite/MgSO4 composite materials show the best heat storage performance. At the condition of 25 °C and 60% RH, the 13XM20 has the highest sorption capacity of 0.21 g/g, which is 24% higher than that of pure zeolite 13X. Accordingly, the maximum heat storage density is 438.4 kJ/kg, which exhibits the superior heat storage performance among of the hydrated salt sorbents. The potential of 13X zeolite/MgSO4 as sorption heat storage material in efficient utilization of solar heat and low-grade industrial waste heat has been revealed.

Multi-objective optimization of a biomass microCHP-ORC system under supercritical conditions

ORC cycle is one of the most efficient technologies for the utilization of low-grade heat. ORC systems cover a wide range of heat sources and power outputs. Apart from increasing the overall efficiency, CHP systems contribute to the decentralization of energy production, the conservation of primary fuel, the reduction of the emission of greenhouse gasses and the re-duction of the cost to the final consumer. This justifies the research activity around CHP-ORC systems. In the present paper, a steady-state thermodynamic model for a 50 kWel biomass microCHP-ORC was developed and four candidate fluids were selected: R124, isobutane, R245fa and isopentane. The multi-objective optimization under supercritical conditions was performed using the genetic algorithm. The thermal efficiency, the exergy efficiency and the total heat exchanger surface were selected as single objectives. The evaporation temperature and pressure and the pinch point temperature differences at the heat exchangers were selected as decision variables. Careful examination of the optimal results revealed a systematic ten-dency for high evaporation temperatures and pressures and low recuperator pinch point tem-perature differences. Recuperation was found beneficial in many aspects, especially at higher evaporation temperatures. Also, the use of cogeneration leads to overall system efficiencies that surpass 90%, while simultaneously saving at least 20% fuel. Lastly, isopentane was found to be the best-performing fluid.

Design and optimization of the novel thermally regenerative electrochemical cycle power device based on machine learning

Utilizing low grade heat and waste heat to generate electricity not only mitigates environmental impacts, meanwhile it enhances energy efficiency and lowers energy expenses. Thermal regenerative electrochemical cycles (TREC) can directly convert low grade heat to electricity, and it has a massive potential for low grade heat utilization. The type and design of a TREC system with practical and continuous power output are of paramount importance, and they can directly influence the performance and efficiency of low-grade heat recuperation systems. In this paper, a novel TREC system of a double layers rotating structure was proposed, and the novel optimal design approach employs CFD and machine learning for this system was presented. The results revealed that the height and number of TREC units had a significant impact on the device's performance. The heat to electricity efficiency of TREC device can be up to 7.34% (equivalent to 48.91% of the Carnot cycle efficiency) when the number of units was 12 and the height of unit was 5 mm. The results indicated that the MLP model was the most suitable, with RMSE values of 0.0077, 0.4415, and 0.5091 for heat to electricity efficiency, heat recuperation efficiency, and heat recuperation, respectively. This work established an effective approach for more rapid and precise estimation of the TREC device's performance metrics. The findings from prediction can serve as a practical guide for the design of TREC devices of various sizes. Furthermore, the method can envision the efficiency before the TREC device is manufactured in case of insufficient numerical calculations, reducing the computational and optimized costs.

Novel marine ejector-compression waste heat-driven refrigeration system: Technical possibilities and environmental advantages

A novel combined ejector-compression refrigeration system for provision rooms of merchant ships was conceptualized and analyzed. The principle possibility of recovery of onboard low-grade heat (85–95 °С) of jacket cooling water was confirmed. Two ejectors of different geometries to ensure the ejector stage operation when the seawater temperature varies in a wide range were designed and proposed to install in parallel. The performance of the combined system was compared to a vapor-compression one and the choice of the rational temperatures in the condenser-evaporator tC/Ev was performed. The combined system COPtotal is considerably higher than the vapor-compression system, and it weakly depends on heat source temperature. At condensing temperature tC = 34 °С the decreasing in tC/Ev from 20/15 to 10/5 °С results in increasing of COPtotal up to 13.5 % at generation temperature tG = 80 °С and up to 15.6 % at tG = 90 °С. Utilizing the combined system for the onboard cold production will contribute to fuel savings of 25 to 33 % per voyage (operation and transport) vs. vapor-compression system. The combined system inherent the lower specific CO2 emission per 1 kW h of cooling energy emrefr than the standard system for all considered operation modes. The value of emrefr for combined system at tC/Ev = 10/5 °C, tG = 80 °C and tC = 36 °C or tC = 42 °C is 23.1% or 18.3%, respectively, less than for the vapor-compression one. The approach to analysis proposed can be used for assessment of the feasibility of utilization of low-grade heat and the retrofit of ship refrigeration systems.

Thermophotonic cells in self-sustaining parallel circuits

Thermophotonic (TPX) cells, which integrate a light-emitting diode (LED) and a photovoltaic (PV) cell, offer potential advantages over thermophotovoltaic (TPV) cells for low-grade waste heat utilization. While TPX cells traditionally operate in an independent circuit, which requires an external battery storage unit, the self-sustaining circuit presents an appealing alternative that eliminates the need for the storage unit. In this study, we conduct a comprehensive analysis of TPX cells operating in the self-sustaining circuit. We provide clear expressions for the electrical currents flowing through the LED and PV cell, considering the different bandgaps of the LED and the PV cell. Our results reveal that the bandgap energy of the LED must exceed that of the PV cell for the TPX cell to function in a self-sustaining parallel circuit. Furthermore, we demonstrate that the performance of the TPX cell in the self-sustaining circuit improves with a narrower bandgap energy for the PV cell and a wider bandgap energy for the LED, regardless of the far-field or near-field configurations. While near-field TPX cells exhibit higher power density, they also experience decreased energy efficiency compared to far-field TPX cells. Additionally, we explore the scenario where the bandgaps of the LED and PV cell are the same, analyzing the performance of a closed-structured self-sustaining parallel circuit with multiple LEDs and PV cells in the near-field configuration. We find that the near-field effect has a marginal impact on TPX cells with identical bandgaps, which means that the near-field TPX cell in the self-sustaining circuit will not be useful in practice. The insights gained from this work serve as valuable design guidelines for TPX cells operating in the self-sustaining parallel circuit, enabling practical operation without the need for an external battery storage unit.

High-Performance Thermoelectric Flexible Ag2Se-Based Films with Wave-Shaped Buckling via a Thermal Diffusion Method.

Herein, an n-type Ag2Se thermoelectric flexible thin film has been fabricated on a polyimide (PI) substrate via a novel thermal diffusion method, and the thermoelectric performance is well-optimized by adjusting the pressure and temperature of thermal diffusion. All of the Ag2Se films are beneficial to grow (013) preferred orientations, which is conducive to performing a high Seebeck coefficient. By increasing the thermal diffusion temperature, the electrical conductivity can be rationally regulated while maintaining the independence of the Seebeck coefficient, which is mainly attributed to the increased electric mobility. As a result, the fabricated Ag2Se thin film achieves a high power factor of 18.25 μW cm-1 K-2 at room temperature and a maximum value of 21.7 μW cm-1 K-2 at 393 K. Additionally, the thermal diffusion method has resulted in a wave-shaped buckling, which is further verified as a promising structure to realize a larger temperature difference by the simulation results of finite element analysis (FEA). Additionally, this unique surface morphology of the Ag2Se thin film also exhibits outstanding mechanical properties, for which the elasticity modulus is only 0.42 GPa. Finally, a flexible round-shaped module assembled with Sb2Te3 has demonstrated an output power of 166 nW at a temperature difference of 50 K. This work not only introduces a new method of preparing Ag2Se thin films but also offers a convincing strategy of optimizing the microstructure to enhance low-grade heat utilization efficiency.

INCREASING THE EFFICIENCY OF HEAT PUMP USING CIRCULATING WATER OF POWER PLANTS

In the article analytical studies of the efficiency of heat pumps in the thermal schemes of thermal and nuclear power plants for centralized heating and hot water supply systems taking into account the climatic conditions of Ukraine have been carried out. This task meets the objectives of energy saving and reduces emissions of harmful substances into the environment. The main focus is on improving the efficiency of the heat pump power supply system, which integrates an alternative energy source. An analysis of the exergy parameters of heat pump cycles has been performed, which have made it possible to establish that the most efficient working fluid of the cycle is freon R152a, at which the highest heat pump conversion coefficient and the lowest compressor power indicator are achieved. Ways and methods leading to reduction to underproduction of the electricity by turbine of of power plant and to increasation of the efficiency of a steam turbine cycle are proposed. It is shown that the efficiency of the system is most affected by losses in the evaporator, so the efficiency of this element of the heat pump should be given the most attention when using the energy of circulating water. An additional aspect of the novelty of the proposed method lies in the possibility of further optimization of the technical and economic parameters of the power system. A methodology based on the use of the exergy method of analysis to substantiate the conditions for the rational use of heat pumps for power plants in the utilization of low-grade heat of circulating water for cooling the turbine condensers is proposed, which can be considered as innovative way. Results open the further prospects for the development of heat pump heat supplyhas been proposed.

Thermodynamic and exergoeconomic analysis of a combined cooling, desalination and power system for low-grade waste heat utilization

As a form of sustainable energy, low-grade waste heat has considerable amount to be recovered, but the energy utilization efficiency is still low. In this paper, a novel multi-generation system is proposed to utilize the low-grade waste heat efficiently and meet the needs for cooling, freshwater and power simultaneously, by integrating vapor compression refrigeration (VCR), humidification and dehumidification (HDH) desalination with transcritical organic Rankine cycle (TORC). Thermodynamic and exergoeconomic performances of the multi-generation system with five working fluids are evaluated. Parametric studies are conducted to identify the key operating parameters. Results show that the highest exergy efficiency ηex and the lowest average unit cost of products (AUCP) can be obtained by using R161, while the highest coefficient of performance (COPtot) belongs to R143a. The proportion of the vapor generator in the total exergy destruction is the largest, followed by the condenser. Higher net power output and freshwater production can be achieved by raising the vapor generation temperature and the ratio of mass flow rates of working fluid, and the corresponding ηex and AUCP are also improved. There is an optimal pressure for the lowest AUCP and the highest net power output and ηex. Performance comparison with systems from the literature is conducted as well.

Numerical simulation of cascaded absorption-two-stage 4-bed adsorption cooling cycles for efficient low-grade heat utilization

Thermal-driven sorption cooling technology was developed to address some pressing challenges including high energy consumption, rising fuel prices, and environmental issues associated with traditional electrical-driven cycles. Nevertheless, this technology needs to be improved in terms of thermal performance and energy efficiency. This study investigates the performance of new cascade single-effect absorption (ABC) and two-stage four-bed adsorption (4ADC) cooling systems powered by low-grade heat source temperatures. Four different configurations are proposed to make proper use of low-temperature heat sources between 45 and 70 °C and improve heat recovery. The use of this temperature range to drive conventional ABC is still considerably lacking in the literature. The ABC does not typically operate below 60 °C. Additionally, the effects of operating conditions on performance are also investigated. The simulation models for both ABC and 4ADC systems are experimentally validated separately and showed excellent agreement. Results showed that the maximum cooling capacity obtained from the proposed system ranges between (5.4–17.1) kW with COP of 0.46–0.47. The COP is found to be relatively insensitive to regeneration temperatures. Increasing the solution pump flow rate increases the system performance by about 25% over the ranges of the regeneration temperature. The proposed configurations provide high energy performance compared with previous related studies. The obtained results proved that the proposed cascade configurations can recover the available low-grade heat source and operate effectively with temperatures between 45 and 70 °C.

Performance analysis of heat and mass transfer, thermodynamic and economic of high gravity direct contact evaporation system

Based on the problem of low heat and mass transfer efficiency and low heat utilization rate in the process of efficient utilization and treatment of low-grade waste heat, the high gravity direct contact evaporation (HG-DCE) system driven by industrial waste heat is proposed. The heat and mass transfer properties in HG-DCE process were studied by simulated with hot water and air at room temperature, and the thermodynamic and economic properties were further analyzed in detail. Results showed that the evaporation intensity of the HG-DCE system could reach 1523 kg/(m3·h), and the volume heat and mass transfer coefficients could reach 270.77 kW/(m3·°C) and 12.23 kg/(m3·s), respectively. According to the established analysis models of entropy generation and exergy loss, the rule of irreversibility of system was obtained, and the highest exergy efficiency was higher than 90.0 % at the gas-liquid mass ratio of 1:1. Simultaneously, the evaporation heat of system accounted for 40.0 % of the supplied heating enthalpy, and the evaporation cost about 0.094 $/kg could be easily achieved. The perfect performance and advantages of HG-DCE system were further confirmed, which provided a new method for using low grade heat for seawater desalination and wastewater treatment.

Negative Pressure in Water for Efficient Heat Utilization and Transfer.

Thermodynamic metastable water in negative pressure provides a possible solution to elevate the upper limit of evaporative heat transfer capacity and the efficiency of low-grade heat utilization, but practical implementations are challenging due to the difficulty in generating and maintaining large negative pressure. Herein, we report a novel structure with a hydrogel film as the evaporation surface and a permeable substrate as the functional layer to suppress cavitation. Based on the structure, we achieve an evaporation-driven flow system with negative pressure as low as -1.67 MPa. Molecular dynamics simulations elucidate the importance of strong water-polymer interactions in negative pressure generation. With the large negative pressure, we demonstrate a streaming potential generator that spontaneously converts environmental energy into electricity and outputs a voltage of 1.06 V. Moreover, we propose a "negative pressure heat pipe", which achieves a high heat transfer density of 9.6 kW cm-2 with a flow length of 1 m.

Performance analysis and optimization of a coupled open and close liquid desiccant sorption system using LiCl solution

The efficient utilization of low-grade heat for air conditioning has been a topic of world concern, especially for low-grade heat below 80℃. In this study, a novel coupled open and close liquid desiccant sorption system driven by 70 ∼ 80℃ low-grade heat is proposed. This system employs a generator to provide strong solution for both the dehumidifier and absorber, achieving the air temperature and humidity independent control to reduce driving temperature and enhance system performance. A mathematical model is developed to simulate the proposed new system, and a comparative analysis is conducted with a traditional combined absorption refrigeration with liquid desiccant dehumidification system using LiCl solution. It is found that the COP of the new system is 0.76, which is higher than 0.65 of the traditional system when the hot water temperature is 75℃, the reason is that the new system avoids the sensible heat consumption of regenerated air which accounts for 17% of the total heat consumption in the traditional system. Furthermore, it is observed that the outlet concentration difference between the absorber and dehumidifier exists in the new system. The cycle is optimized to improve the concentration difference, leading to a 5.5% increase in the COP. After that, economic performance analysis shows a payback period of 2.65 years for a 150 kW cooling capacity system, with a reduction of 86.6 tons of CO2 emissions per year due to the decrease in power consumption.

Mathematical model establishment and optimum design of a novel cesium thermionic–thermoelectric hybrid generator

Static power sources, such as thermionic energy converters (TEC), thermoelectric generators (TEG), thermophotovoltaic converters (TPV), and alkali metal thermal to electric converters (AMTEC), have enormous potential application in aerospace, military, electronic devices, and low-grade waste heat recovery and utilization. Aiming to further improve the power output and conversion efficiency, any two of them can be cascaded according to the difference in operating temperature. In this study, a novel mathematical model of the static hybrid power generator composed of a cesium thermionic energy converter (CTEC) and a TEG was proposed. The model considered the irreversible heat loss and the additional energy barrier of the CTEC module, and the Thomson effect of the TEG module, synchronously. To obtain the temperature distribution of CTEC and TEG modules, energy balance equations were determined by nonequilibrium thermodynamic analysis. Then, the effects of the output voltage and anode work function of the CTEC, the heat source temperature, and the electric current of the TEG on the output performance were investigated systematically by means of MATLAB. The results indicate that the maximum efficiency of the hybrid generator reaches 26.82% at heat source and heat sink temperatures of 1800 K and 300 K, respectively, which is significantly higher than that of the individual CTEC module (21.28%). This reveals that the waste heat released by the CTEC is utilized effectively in the TEG module, and the increased heat source temperature will further improve the hybrid generator’s power output and efficiency. The output power and efficiency are obviously improved by reducing the anode work function. When ϕC = 1.0 eV, i = 0.005, the power reaches a maximum of 21.61 W at 0.37 V, and the efficiency reaches a maximum of 25.90% at 0.55 V. Moreover, the curve of the output power as a function of conversion efficiency could be used to determine the optimal operating region of the hybrid generator, i,e., Pη < P < Pmax, ηP < η < ηmax, thereby obtaining the thresholds of the hybrid system parameters, i.e., VP < V < Vη, T1,P < T1 < T1,η, T2,P > T2 > T2,η, T3,P > T3 > T3,η and TCs,P > TCs > TCs,η. This study provides a reference for the optimal design and application of a CTEC–TEG hybrid generator in the future.

Synergistic improvement of Seebeck coefficient and power density of an aqueous thermocell using natural convection for low-grade heat utilization

As a cutting-edge heat-to-electricity technology, thermocells possess the advantages of large Seebeck coefficient, low cost and high scalability, indicating a great prospect in low-grade heat recovery. Previous studies about thermocells focus on the electrode materials and electrolytes, but the internal heat and mass transfer mechanisms are less considered, especially the effects of natural convection on the performance of aqueous thermocells. The purpose of this study is to explore the influence of natural convection on the performance of aqueous thermocells. Firstly, a fundamental test bench for aqueous ferricyanide/ferrocyanide ([Fe(CN)6]3−/4-) thermocells using Pt as electrodes is designed and established. Based on the self-developed test bench, then the performance of thermocells related to electrode orientation θ are experimentally investigated under typical thermal boundary conditions, including the constant electrode temperature, and constant heating and cooling temperature. The experimental results under constant electrode temperature show that natural convection can significantly reduce the internal resistance of thermocells. The thermocell obtains the largest |Se| of 1.75 mV/K at θ = 90°, which is 19.9% higher than that of θ = 0°. Meanwhile, the thermocell achieves the maximum power density of 0.211 W/m2 at θ = 135°, which is 744% higher than that of θ = 0°. In addition, by combining the experimental results under constant heating and cooling temperature, it can be observed that natural convection can also sharply reduce the thermal resistance and then lead to a smaller temperature difference of two electrodes. Overall, natural convection has trade-off effects on internal resistance and thermal resistance, and the proper use of natural convection can succeed in prominent improvement of Seebeck coefficient and power density for thermocells.