Utilization Of Low-grade Thermal Energy Research Articles

Progress on Material Design and Device Fabrication via Coupling Photothermal Effect with Thermoelectric Effect.

Recovery and utilization of low-grade thermal energy is a topic of universal importance in today's society. Photothermal conversion materials can convert light energy into heat energy, which can now be used in cancer treatment, seawater purification, etc., while thermoelectric materials can convert heat energy into electricity, which can now be used in flexible electronics, localized cooling, and sensors. Photothermoelectrics based on the photothermal effect and the Seebeck effect provide suitable solutions for the development of clean energy and energy harvesting. The aim of this paper is to provide an overview of recent developments in photothermal, thermoelectric, and, most importantly, photothermal-thermoelectric coupling materials. First, the research progress and applications of photothermal and thermoelectric materials are introduced, respectively. After that, the classification of different application areas of materials coupling photothermal effect with thermoelectric effect, such as sensors, thermoelectric batteries, wearable devices, and multi-effect devices, is reviewed. Meanwhile, the potential applications and challenges to be overcome for future development are presented, which are of great reference value in waste heat recovery as well as solar energy resource utilization and are of great significance for the sustainable development of society. Finally, the challenges of photothermoelectric materials as well as their future development are summarized.

Design and Optimization Strategies for Flexible Quasi-Solid-State Thermo-Electrochemical Cells.

Currently, efficient utilization of low-grade thermal energy is a great challenge. Thermoelectricity is an extremely promising method of generating electrical energy from temperature differences. As a green energy conversion technology, thermo-electrochemical cells (TECs) have attracted much attention in recent years for their ability to convert thermal energy directly into electricity with high thermal power. Within TECs, anions and cations gain and lose electrons, respectively, at the electrodes, using the potential difference between the hot and cold terminals of the electrodes by redox couples. Additionally, the anions and cations therein are constantly circulating and mobile via concentration diffusion and thermal diffusion, providing an uninterrupted supply of power to the exterior. This review article focuses mainly on the operation of TECs and recent advances in redox couples, electrolytes, and electrodes. The outlook for optimization strategies regarding TECs is also outlined in this paper.

High-performance thermal management system for high-power LEDs based on double-nozzle spray cooling

Thermal management is one of the main challenges for high-power LEDs. As one of the state-of-the-art thermal management approaches, spray cooling can meet the requirements of fast cooling speed, large heat transfer coefficient, and less energy consumption when it provides thermal management for electronics with high heat flux. In this study, a high-performance spray cooling system for high-power LEDs is designed and established. The influence of nozzle configuration, flow rate, and nozzle-to-surface distance on the heat transfer characteristics are experimentally and numerically investigated. The system shows excellent heat dissipation capacity. Two different nozzle configurations are compared and the results show that the double-nozzle configuration possesses the highest single-phase heat transfer coefficient, reaching up to 20.7 kW/m2·K with the flow rate of 150 mL/min. Interestingly, double-nozzle, which owns better heat transfer performance compared to single-nozzle, shows worse temperature uniformity inside the testing module. Simultaneously, the junction temperature of double-nozzle at the center chip can also be controlled to 85 °C corresponding to a 10% reduction for single-nozzle. The proposed cooling system may enable the further application of spray cooling in electronics like high-power LEDs, potentially reducing the energy consumption in cooling electronics and promoting the utilization of low-grade thermal energy.

Advances in Thermoelectric Composites Consisting of Conductive Polymers and Fillers with Different Architectures.

Stretchable wireless power is in increasingly high demand in fields such as smart devices, flexible robots, and electronic skins. Thermoelectric devices are able to convert heat into electricity due to the Seebeck effect, making them promising candidates for wearable electronics. Therefore, high-performance conductive polymer-based composites are urgently required for flexible wearable thermoelectric devices for the utilization of low-grade thermal energy. In this review, mechanisms and optimization strategies for polymer-based thermoelectric composites containing fillers of different architectures will be introduced, and recent advances in the development of such thermoelectric composites containing 0- to 3-dimensional filler components will be presented and outlooked.

Bacterial cellulose-based dual chemical reaction coupled hydrogel thermocells for efficient heat harvesting

Reasonable and efficient utilization of low-grade thermal energy in nature is the choice for sustainable energy development. We demonstrate a bacterial cellulose (BC) hydrogel thermocell (TEC) based on BC electrolyte combined with carbon fiber paper and copper composite electrode sheets. The large specific surface area of carbon fiber paper provides a large number of active sites for thermoelectric ions, which drives the redox reaction inside the electrolyte and stimulates the chemical reaction between the electrolyte and the electrode. The combination of the two chemical reactions significantly improves the thermoelectric performance of the thermocell. The thermopower of the BC-TEC reaches 5.9 mV·K−1 at a temperature difference of 50 K. The TEC consisting of 6-units in series produces an open-circuit voltage of about 2 V and a peak power of 535 μW. The TEC shows new potential and prospects in ambient thermoelectric energy conversion by rationally designing the power generation principle.

Shape-stable phase change composites based on carbonized waste pomelo peel for low-grade thermal energy storage

Phase change materials (PCMs), like polyethylene glycol (PEG), have been widely applied to the storage and utilization of low-grade thermal energy, which is a momentous part of energy utilization and conversion. Furthermore, one of the important ways to solve the practical problems of easy leakage and slow speed of heat storage of PCMs is to prepare the phase change composites (PCCs) featured with stable shape, high enthalpy retention and improved thermal conductivity. For this purpose, we focused on the waste bio-based pomelo peel, which was converted into biochar as the supporting material of PEG through a simple carbonization process, then vacuum impregnated with four different molecular weights of PEG to obtain the PCCs characterized by no leakage, high loading and enhanced thermal diffusion ability. The DSC results revealed that the melting enthalpy of several PCCs were about 160 J/g, which were slightly lower than that of pure PEG, but the relative enthalpy efficiency still exceeded 90%. Meanwhile, PCCs had excellent thermal stability and cycling stability, and the mass loss in 200 heat-cooling cycles was only 2.69% at most, which is beneficial to the preservation of enthalpy. The tests of solar-thermal conversion and storage manifested that PCCs could make full use of the solar radiation heat energy, with the efficiency of about 75%∼x223C 80%. With the low-cost and waste-recycling, this work provides a new choice for further efficient and comprehensive utilization of low-grade thermal energy including solar radiation heat energy.

Cascade utilization of low-grade thermal energy by coupled elastocaloric power and cooling cycle

Unlike the conventional elastocaloric cooling system that depends on electricity, the coupled elastocaloric power and cooling cycle proposed in this study exploits low-grade thermal energy in a cascade manner, which opens a new paradigm for the caloric cooling community. Cascade utilization of heat relies on a gradient of transition temperature inside the shape memory alloy actuator that best matches the temperature profile of the material during operation. A transient numerical model is developed to simulate the performance of this new system, and the model is experimentally validated. The transition temperature gradient should exceed 60 K for optimum utilization of input heat. At the same gradient, the distribution of transition temperature can be optimized, where at least 10% improvement over linear profile can be expected in terms of efficiency and cooling power density. Based on the superior performance, this technology could be powered by solar thermal collectors working at 110 °C and could be applied to residential air-conditioners and off-grid refrigerators. The novel system proposed in this study not only complements the existing library of heat-activated cooling technologies, but also provides a new pathway for the expansion of caloric cooling at large.

Comparative Assessment of Various Low-Dissipation Combined Models for Three-Terminal Heat Pump Systems.

Thermally driven heat pump systems play important roles in the utilization of low-grade thermal energy. In order to evaluate and compare the performances of three different constructions of thermally driven heat pump and heat transformer, the low-dissipation assumption has been adopted to establish the irreversible thermodynamic models of them in the present paper. By means of the proposed models, the heating loads, the coefficients of performance (COPs) and the optimal relations between them for various constructions are derived and discussed. The performances of different constructions are numerically assessed. More importantly, according to the results obtained, the upper and lower bounds of the COP at maximum heating load for different constructions are generated and compared by the introduction of a parameter measuring the deviation from the reversible limit of the system. Accordingly, the optimal constructions for the low-dissipation three-terminal heat pump and heat transformer are determined within the frame of low-dissipation assumption, respectively. The optimal constructions in accord with previous research and engineering practices for various three-terminal devices are obtained, which confirms the compatibility between the low-dissipation model and endoreversible model and highlights the validity of the application of low-dissipation model for multi-terminal thermodynamic devices. The proposed models and the significant results obtained enrich the theoretical thermodynamic model of thermally driven heat pump systems and may provide some useful guidelines for the design and operation of realistic thermally driven heat pump systems.

The equivalent low-dissipation combined cycle system and optimal analyses of a class of thermally driven heat pumps

The performance characteristics, operation, and design strategies of a class of thermally driven heat pumps are investigated due to their important roles in the efficient utilization of low-grade thermal energy. In order to establish a more generic thermodynamic model of thermally driven heat pumps mainly including absorption, adsorption, and ejector heat pumps, low-dissipation assumption is adopted. Accordingly, the associated dissipation parameters accounting for the specific information on the irreversibilities in each heat-transfer process are introduced rather than specifying heat-transfer law. Based on the proposed model, the theoretical results of the coefficient of performance and heat load are derived with regard to two key parameters denoting the size ratio of the two involved subsystems and the matching deviation from reversible limit. The performance characteristics and the optimally operating regions of the whole system are determined and the differences between thermally driven heat pump and refrigerator are highlighted. The proposed model and obtained results further develop the low-dissipation model and may provide a useful description for the operation and design of practical thermally driven heat pumps.

A Simulation-Optimisation Method for Targeting the Optimal Placement of Heat Pumps in Heat Exchanger Networks

Heat pumps can enhance the utilization of low-grade thermal energy. Setting heat pumps correctly in a heat exchanger network can reduce the consumption of cold and hot utilities, achieving energy saving and pollution reduction. Therefore, targeting the optimal placement of heat pumps is of great interest in both the academia and industry. Focusing on the vapour compression heat pump, this work proposes a simulation-optimization method to identify the energy-optimal placement of heat pumps in heat exchanger networks. Based on the powerful fluid property database, the vapour compression heat pump system is first modelled in the Aspen HYSYS V10. Next, a mathematical model is built in the Matlab R2018b platform using the Grand Composite Curve of heat exchanger network as constraints for heat pump placement. The two platforms, Aspen HYSYS V10 and Matlab R2018b, are then coupled to realize the data transfer between the simulation and mathematical models. The genetic algorithm is used to target the energy-optimal placement of heat pump (evaporating temperature, condensing temperature, and working fluid flowrate) in the heat exchanger network. A case study is performed to illustrate the applicability of the proposed method. The proposed simulation-optimization method can be extended to other types of heat pumps.

Performance analysis of solar assisted vapour Jet refrigeration system with regenerator (CRMC method)

There are various forms of low-grade thermal energies that could all be useful in powering an ejector refrigeration system. Effective utilization of low-grade thermal energy is feasible from many available sources from industrial areas, solar and the automobile exhausts etc, . The normal Vapour Jet Refrigeration system is compared with the efficient Vapour Jet Refrigeration system fitted with a solar assisted regenerative heat exchanger unit. The ejectoris designed using Constant Rate Momentum change (CRMC) methodology to enhance the Coefficient of Performance by the usage of the refrigerant R134a as the working medium. The ejector cycle has been recognized as the promising technology for utilization of solar energy for cooling. The heat absorbed by the solar flat plate collector is partially used to heat refrigerant in the ejector cycle. Study on the effect of refrigerant temperature due to heat provided by boiler and solar flat plate in the Vapour Jet Refrigeration system on Coefficient of Performance, pumping power, entrainment ratio, diffuser outlet temperature & heat rejected at condenser is carried out. Simulation of ejector designed through Constant Rate Momentum change (CRMC) methodology is done using FLUENT software.

Experimental study on the performance of the internally-heated ultrasonic atomization liquid desiccant regeneration system

Liquid desiccant dehumidification system has been attracting increasing research interests due to its great energy saving potentials. Moreover, the desiccant solution can be regenerated with thermal energy for recycling use. However, due to the limited contact area and significant temperature drops within the regenerator, the regeneration performance has been restrained, and the necessary desiccant temperature tends to be exorbitant. To this end, this work proposed a new internally-heated ultrasonic atomization liquid desiccant regeneration system (IH-UARS) to improve the desiccant regeneration performance. The proposed IH-UARS was thoroughly studied, in comparison with the adiabatic UARS and the conventional internally-heated flat-plate regenerator (IH-FPR), via extensive experiments and a prediction model developed from the conservation laws of mass and energy. It was found that the regeneration performance, together with the thermal efficiency, was substantially improved by 45.2% and 34.6%, respectively, in the UARS after adding internal-heating. Besides, the necessary desiccant regeneration temperature (NRT) was lowered markedly to merely 36.6 °C in the IH-UARS. Further, better regeneration performance was also presented in the IH-UARS than that of the IH-FPR, with the NRT dropped by 5.9 °C. The results may help improve the performance of the liquid desiccant system and facilitate its fuller utilization of low-grade thermal energy.

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.

Modelling, experimental test, and design of an active air permeable wall by utilizing the low-grade exhaust air

The exhaust air insulation (EAI) wall is a novel type of air permeable wall that is characterized by a permeable porous layer. Such a wall design provides a solution to combine the building envelope with exhaust air heat recovery, and allows the exhaust air from a conditioned room to permeate through. The exfiltration process of the exhaust air across the porous layer can partly reduce the inward conductive heat flux through the wall. This makes the temperature at the innermost surface of the EAI wall close to the temperature of the indoor air, and leads to the reduction of cooling and heating loads through the wall. In this study, a comprehensive heat transfer model was developed to analyze the steady state thermal characteristics of the EAI wall. Forty different experiments were performed to verify the proposed model. A quantitative analysis was conducted on the differences when using the unsteady state model and proposed steady state model for the EAI wall with an outdoor thermal disturbance. A sensitivity analysis was carried out to investigate the impact of the design parameters on the thermal characteristics of the EAI wall. The results showed that using a porous material with relatively low thermal conductivity such as phenolic foam is beneficial to improve the heat recovery performance and thermal insulation of the EAI wall. The thermal insulation of the EAI wall is mainly due to the exfiltration process of exhaust air across the porous material. Considering the impact of the design parameters on the thermal characteristic, total thickness and pressure drop, a thickness of 30–50 mm is recommended for the porous material. At an exfiltration velocity of 0.003 m/s, the U-value of the EAI wall was lower than 0.1 W/(m2 K) for a porous material thickness of 30–50 mm. These results demonstrate that the EAI wall is applicable to both energy-efficient retrofitting of existing buildings and new buildings, and can potentially contribute to reduce the cooling and heating loads through the wall by the utilization of low-grade thermal energy in exhaust air.

Performance analysis of a solid-gas thermochemical composite sorption system for thermal energy storage and energy upgrade

Solid-gas thermochemical sorption heat storage has attracted considerable interest in recent years due to high energy density and the ability of long-term storage duration. Operation principle of thermochemical sorption heat storage system is based on reversible solid-gas chemical reaction by storing thermal energy in the form of chemical potential. The comparison of thermochemical adsorption and resorption heat storage cycle has been firstly made. Then, the thermochemical composite sorption heat storage cycle is presented for the effective utilization of the low-grade thermal energy. As a novel thermochemical sorption heat storage cycle, thermochemical composite sorption heat storage has integrated the merits of both thermochemical adsorption and resorption heat storage cycle. Furthermore, the performance of a thermochemical composite sorption heat storage system is analyzed by employing MnCl2-SrCl2-NH3 working pair. Theoretical analysis showed that thermochemical composite sorption heat storage cycle is effective for integrating energy storage and energy upgrade of low-grade thermal energy. It can not only decrease the heat input temperature of external heat source during the heat storage stage, but also upgrade significantly the temperature level of stored thermal energy during the heat release stage. As a result, thermal energy temperature can be upgraded from 81 °C to 170 °C via thermochemical composite sorption heat storage cycle. Therefore, it is very beneficial to enlarge the application temperature range and realize the high-efficient utilization of low-grade thermal energy.

Investigation of a 10 kWh sorption heat storage device for effective utilization of low-grade thermal energy

Heating and domestic hot water for family houses represents a notable share of energy consumption. However, sufficient space for the installation of thermal energy storage (TES) components may not be available in family houses or urban areas, where space may be restricted and expensive. Sorption TES devices seem to be a promising means of replacing conventional TES devices and reducing the occupied space for its high energy density. In this paper, a 10 kWh short-term sorption TES device was developed and investigated. The employed composite sorbent was formed from lithium chloride (LiCl) with the addition of expanded graphite (EG). The principle of sorption TES for the LiCl/water working pair is first illustrated. This prototype was tested under conditions representative of transition or winter seasons. Under the conditions used (charging temperature Tcha at 85 °C, discharging temperature Tdis at 40 °C, condensing temperature Tc at 18 °C, and evaporating temperature Te at 30 °C), the heat storage capacity can reach 10.25 kWh, of which sorption heat accounts for approximately 60%. The heat storage density obtained was 873 Wh per kg of composite sorbent or 65.29 kWh/m3, while the heat storage density of hot water tank was about 33.02 kWh/m3.

Acoustic field characteristics and performance analysis of a looped travelling-wave thermoacoustic refrigerator

This paper focuses on a looped travelling-wave thermoacoustic refrigerator powered by thermal energy. Based on a simplified model for the regenerator, key issues for a highly efficient thermoacoustic conversion, including both thermal-to-acoustic and heat-pumping processes, are summarized. A looped travelling-wave thermoacoustic refrigerator with one engine stage and one refrigerator stage is proposed, with emphasis on high normalized acoustic impedance, sufficient volumetric velocity and appropriate phase relation close to travelling wave in the regenerators of both engine and refrigerator. Simulation results indicate that for the ambient temperature of 30°C, the looped travelling-wave thermoacoustic refrigerator can be powered by the heat at 210–250°C to achieve the refrigeration at −3°C with the overall coefficient of performance above 0.4 and the relative Carnot coefficient of performance over 13%. The characteristics of the acoustic field inside the loop configuration are analyzed in detail to reveal the operation mechanism of the looped travelling-wave thermoacoustic refrigerator. Additional analyses are conducted on the impact of the cooling and the heating temperatures, which are of great concern to the refrigeration applications and the utilization of low-grade thermal energy.

Thermodynamic characteristics during the onset and damping processes in a looped thermoacoustic prime mover

The onset and damping processes of self-excited working-fluid oscillation in thermoacoustic prime movers, related to the quality of the utilizable thermal energy, have attracted everlasting research interest and effort. This work is intended to observe and analyze the temperature and pressure characteristics of the onset and damping processes in a recently developed looped travelling-wave thermoacoustic prime mover with enlarged thermoacoustic cores. A hysteretic loop, caused by the discrepancy between the onset temperature difference and the damping temperature difference, can be found in the looped structure, which indicates the potential in decreasing the onset temperature difference. Additionally, in order to optimize the acoustic field in the thermoacoustic prime mover, a compliance unit was inserted into the loop. The experimental results show that the onset temperature difference drops by up to 223 K with the appropriately located compliance unit, which is advantageous to the utilization of low-grade thermal energy.

A thermo-activated wall for load reduction and supplementary cooling with free to low-cost thermal water

A building envelope serves as a thermal barrier and plays an important role in determining the amount of energy used to achieve a comfortable indoor environment. Conventionally, it is constructed and treated as a passive component in a building thermal energy system. In this article, a novel, mini-tube capillary-network embedded and thermal-water activated building envelope is proposed to turn the passive component into active, therefore broaden the direct utilization of low-grade thermal energy in buildings. With this proposed approach, low-grade thermal water at a medium temperature close to the ambient environment can be potentially utilized to either counterbalance the thermal load or indirectly heat and cool the space. With the revealing of the idea, effects of water temperature and flow rate on the envelope's thermal performance are investigated using a transient model. The results indicate that the thermo-activated wall can be effective in stabilizing the internal surface temperature, offsetting the heat gain, and supplying cooling energy to the space in summer. Utilization of the innovation should take the cost of total energy, energy benefit and efficiency into consideration. This article illustrates how low-grade energy can be actively used as a means for achieving net-zero energy buildings.

Integrated utilization of low-grade thermal energy in hot coal mine

This paper explores the integrated utilization of low-grade thermal energy in hot coal mines, based on analysis of original heating, refrigerating, mine draining, bath draining and air exhaust systems, and in combination with the actual conditions of Tangkou Coal Mine in Shandong Province. It presents a set of comprehensive and integrated utilization schemes for the various different kinds of low quality heat energy. With heat pumps, the recycling of the low quality heat energy from the drainage, bathing water and the exhaust air can occur in winter, and in summer, there exists condensed heat of the refrigerating system. When in conjunction with solar collectors, the thermal utilization of solar power can be realized for the whole year. The system achieves mine drainage and bathing water purification and recycling, as well as purifying exhaust air by water spraying. It also satisfies the demands of a whole year’s bathing heat for the coal mine, with refrigeration in summer, and heating for the ground house and shaft house in winter. It is able to integrate different kinds of low quality heat energy and low emission drainage and dust, and can replace the traditional boiler heating system. Finally, the system reduces conventional energy consumption and the amount of mine water drainage.