Limited Temperature Difference Research Articles

Performance Enhancement of Solar Still Unit Using v-Corrugated Basin, Internal Reflecting Mirror, Flat-Plate Solar Collector and Nanofluids

The conventional solar still (CSS) unit faces challenges such as low productivity (Pd) and thermal efficiency (ηth) due to the limited temperature difference between the hot water and the cold interior glass cover surfaces (ΔTw-gi). This study addresses these issues by introducing enhancements in the CSS unit, incorporating a v-corrugated-type basin, internal reflecting mirror, flat-plate solar collector (FPSC) still, and FPSC nanofluids. A v-corrugated-type basin, internal reflecting mirror, FPSC still, and FPSC nanofluids elicited a significant improvement in the distillate productivity (Pd) up to approximately 22.39%, 41.72%, 70.10%, and 104.13% compared to the CSS unit. This increase in the Pd is attributed mainly to a notable raise in the ΔTw-gi, showing increments of around 34.33%, 52.32%, 77.37%, and 112.87% compared to the CSS unit. Moreover, a v-corrugated basin, internal reflecting mirror, FPSC still, and FPSC nanofluids substantially increased the average daily thermal efficiency (ηth), around 22.01%, 26.71%, 39.57%, and 56.21%, respectively. The results confirmed that integrating the v-corrugated basin, internal reflecting mirror, FPSC still, and FPSC nanofluids within a combined seawater distillation system can significantly enhance the performance of the CSS unit. These different combinations effectively raised the basin water temperature (Tw) and ΔTw-gi, consequently improving the overall performance of the solar still unit.

Thermogalvanic gel patch for self-powered human motion recognition enabled by photo-thermal-electric conversion

Quasi-solid-state electrolytes offer a cheap, leak-free, and scalable way to convert heat directly into electricity for wearable electronics. However, the limited temperature difference with the environment poses a challenge for the extensive application of thermoelectric gels. Currently, photo-thermal-electric (PTE) conversion is an environmentally-friendly technology to exploit the thermogalvanic effect of gels. Here, we report a smart light-driven flexible PTE gel for self-powered human motion monitoring. The whole structure takes advantage of the low thermal conductivity and high output power of poly(vinyl alcohol)/polydimethylsiloxane (PVA-PDMS) gels with Fe2+/3+ as a redox pair, together with the local surface plasmon resonance (LSPR) of Au nanoparticles (AuNPs)@SiO2-PDMS composite film. This enables simultaneous high thermogalvanic and photothermal performance, with an open circuit voltage of up to 12 mV and a short circuit current exceeding 1.25 A m−2 at a temperature difference of up to 9 °C under one sun. The PTE gel is fabricated into a proof-of-concept self-powered patch, which can actively distinguish human motions in real time by recognizing the switching of solar radiation stimulated by arm swing. This work provides thermogalvanic gels with a new opportunity in self-powered wearable sensing that may greatly promote the progress of intelligent medical electronics.

Study on coupling characteristics and parameter optimization of thermoelectric generation via radiative cooling (RC-TE) system

Combining radiative cooling (RC) with Thermoelectric (TE) generators to realize passive power generation has been considered to be one of the potential green energy sources, which can achieve the effect of low temperature power generation without the input of external energy. Although some theoretical and experiments have proved that TE can obtain energy from radiative cooling, the realized power density is very low. That is because the limited temperature difference between the two ends of the TE devices caused by the low energy density of the RC significantly reduces the output performance. In addition, the calculation model in the existing research is often simplified, and the influence of complex structure on the energy absorption of the system cannot be accurately considered. Therefore, this paper presents a detailed three-dimensional study on the effect of the structure and configuration of the RC-TE system, and the influence of environmental factors on the performance of the RC-TE. Here, we built a three-dimensional finite element model of a RC-TE system in COMSOL. The three different heat transfer boundaries model were established on the surface of RC to numerically study the performance of the RC-TE devices. The influence of RC-TE structure and surrounding environmental parameters on the performance of RC-TE is comprehensively explored, including radiative area ratio, wind speed, ambient temperature, and triethylene glycol structure parameters β. The results show that when the radiative area ratio is about 33.18, the maximum voltage of RC-TE can be 234.5 mV under ideal conditions. However, compared with the ideal condition, the output performance under the semi-ideal condition and the convection condition is reduced by 66.62% and 86.63%, respectively. As ambient temperature rises from 0 °C to 30 °C, the maximum voltage is ideal condition about 210.63 mV, while convection condition has greater increase about 49.79 %. As the TE structure β increases, the output performance is significantly weakened and the rate of reduction gets bigger. The purpose of this study is to analyze, verify, simulate and optimize the prototype radiation cooled thermoelectric generator (RC-TE) model to improve efficiency. These findings will have an important positive impact on the optimal design and practical application of RC-TE.

Infrared Thermoelectric Nanoantenna with Maximum Output Voltage Using Grounded and Open‐Ended SiO2

AbstractThe potential of a nanoantenna‐coupled thermocouple to convert untapped infrared (IR) energy from low‐temperature heat sources into direct current (DC) energy has recently gained attention. In a typical nanoantenna with a thermocouple, heat spread at the antenna center leads to a limited temperature difference and lowers the open‐circuit voltage (Voc). Therefore, a membrane or air gap under the antenna has recently been used, but when a massive array is constructed for high DC voltage output, stability and fabrication issues can arise. To solve this, this work utilizes the constructively coupled fields at the antenna center in both vertical and horizontal directions via a grounded and open‐ended substrate and maximizes Voc of the thermoelectric nanoantenna. In the antenna structure, a titanium (Ti)‐based bowtie nanoantenna and nickel (Ni)‐Ti bimetal nano‐thermocouple are used for high Seebeck effect. The thermoelectric bowtie nanoantenna is designed and fabricated on a metal‐backed and open‐ended silicon dioxide (SiO2) substrate with optimum dimensions, demonstrating the maximum Voc of 2.03 µV at λ0 = 10.6 µm which is five times the maximum Voc of 0.4 µV from the state‐of‐the‐art substrate‐mounted thermoelectric nanoantennas. It is expected that the fabricated SiO2‐mounted thermoelectric nanoantenna with the maximum Voc can be used for a high DC output IR‐harvesting massive nanoantenna array.

Numerical simulation of heat pipes in different applications

• Background about the main components of HPs is presented. • Fluid flow and thermal performance of HPs are discussed. • Numerical simulations of HPs in various applications are elaborated. • Critical challenges and recommendations are given. Nowadays heat pipes are considered to be popular passive heat transfer technologies due to their high thermal performance. The heat pipe is a superior heat transfer apparatus in which latent heat of vaporization is employed to transfer heat for an extended distance under a limited operating temperature difference. Numerical simulation of heat transfer devices is a principal step before implementing in real-life applications as many parameters can be tested in cost-and time-effective behaviors. The present study provides a review of the numerical simulations of various heat pipes in different applications such as cooling of electronic components, heating, ventilation, and air conditioning (HVAC), nuclear reactors, solar energy systems, electric vehicles, waste heat recovery systems, cryogenic, etc. Firstly, this work introduces a background about the main components of heat pipes such as an evacuated tube, wick, and working fluid. The fluid flow and thermal performance characteristics of heat pips are discussed, considering the optimum parameters. Finally, the critical challenges and recommendations for future work encountering the broad application of heat pipes are thoroughly studied.

Structural design for wearable self-powered thermoelectric modules with efficient temperature difference utilization and high normalized maximum power density

Flexible thermoelectric generators (FTEGs) show great potential as energy harvesters for wearable electronics by directly converting body heat into electricity. However, the limited temperature difference due to the high fill factor and low TE leg aspect ratio, which determines the open-circuit voltage and follow-up thermoelectric output, hinders application of FTEGs in self-powered wearable electronics. Herein, to realize efficient temperature difference utilization and high normalized maximum power density simultaneously, we designed a flexible device with a nonplanar zigzag π-type structure based on rigid Bi2Te3-based alloys with a much lower fill factor and a higher TE leg aspect ratio than those of commercial TE modules, which exhibits efficient body heat collection performance. Meanwhile, passive radiative cooling and wavy radiator fins were applied as a substitute for traditional metal bulky heat sinks, thereby enhancing the heat dissipation. These structural design strategies endow the generator with a thermoelectric voltage of 22.6 mV and a maximum output power of 75.76 μW from human body heat at 23 °C. Besides, with further optimization, the thermoelectric modules can generate an output power density of 12.36 μW/cm2 and a voltage density of 4.04 mV/cm2 at 23 °C, sufficient to drive some microwatt or sub-microwatt wearable electronics. The ultrahigh temperature difference utilization efficiency and the decent normalized power density suggest that these structural design strategies can be used to efficiently harvest energy in contexts in which there are limited temperature differences, such as human body heat.

Enhancing heat-to-electricity conversion performance of the thermally regenerative electrochemical cycle using carbon-copper composite electrodes

Low-grade heat is ubiquitous but difficult to be recovered. Thermally regenerative electrochemical cycle (TREC) system is expected to be one of the technologies for efficient recovery of low-grade heat. However, due to the limited temperature coefficient and temperature difference, it is difficult to further improve the heat-to-electricity conversion performance of TREC system. Therefore, a TREC system with carbon-copper composite negative electrode is proposed. By studying various factors (acid treatment, current density, and electrolyte concentration) for the deposition of copper metal on carbon cloth, suitable conditions for the preparation of carbon-copper composite electrodes were determined. The proposed method increased the electrode reaction contact area, reduced the electrical energy charged during the TREC system cycle, and increased the heat-to-electricity conversion efficiency of the system to 1.3 times (from 2.97% to 3.86%) compared with the conventional system with copper sheet negative electrode.

Passive Radiative Cooling Enables Improved Performance in Wearable Thermoelectric Generators.

Wearable thermoelectric generators have great potential to be utilized as the power supply for wearable electronics. However, the limited temperature difference across the thermoelectric generators significantly degrades the output performance, which is anticipated to be improved by enhancing the thermal radiation at the cold side without extra energy consumption. In this paper, the impact of thermal radiation on the performance of thermoelectric generators in different environments is simulated and the enhanced performance in a wearable thermoelectric generator combined with a radiative cooling coating is experimentally verified. Compared with the pristine device, the wearable thermoelectric generator with radiative cooling coating can not only achieve an ≈128% improvement of output power in exposed environments, but also exhibit an ≈96% improvement of output power in non-exposed environments. The indoor output performance of the wearable thermoelectric generator with a radiative cooling coating due to its stable voltage output is extensively investigated, which shows an output power density of ≈5.5 μW cm-2 at the indoor temperature of 295 K, doubled that without a radiative cooling coating. This work paves a new way for further enhancing the performance of thermoelectric generators via passive radiative cooling.

Multi-objective optimization on clinker layer thickness of a grate cooler based on entropy generation

The paper proposes and analyzes an entropy generation model of heat transfer and flow in a grate cooler based on convective heat transfer. Entropy generation caused by limited temperature difference and pressure drop are simultaneously regarded as objectives. Employing the genetic algorithm solves the multi-objective optimization model. Cooling air distribution of the grate cooler is fixed and clinker layer thickness on three grate plates are considered as design variables. Optimal thicknesses of the clinker layer are selected from the Pareto front based on minimum energy consumption of cooling fans. Optimal thicknesses of the three plates measure 0.75m, 0.5m, and 0.84m, whereas energy consumption of cooling fans decreases by 19.58% compared with the operating scheme.

P-N conversion in thermogalvanic cells induced by thermo-sensitive nanogels for body heat harvesting

Thermogalvanic cells (TGC) are promising devices for directly converting heat into a stable electric output. The practical applications of TGCs are presently significantly hindered by the low voltage (millivolt level) generated from a limited temperature difference. One general strategy for improving the voltage is to alternately connect n-type and p-type redox units in series. However, the number of the possible redox species is limited, hindering the optimization of the series stacking of devices. In this work, we report a novel concept that enables p-n conversion for the iodide/triiodide (I-/I3-) redox couple induced by poly (N-isopropylacrylamide) (PNIPAM) thermo-sensitive nanogels, with the Seebeck coefficient changing from 0.71 mV K−1 to − 1.91 mV K−1. The results prove that the nanogels enable selective capture of I3- at the hot side followed by the release of I3- at the cold side, yielding a concentration gradient of the free I3-, resulting in the p-n inversion. Furthermore, we designed a wearable device consisting of alternating I-/I3- and I-/I3-/nanogels unites in series that generated the open-circuit voltage of approximately 1 V and output power of approximately 9 μW by utilizing body heat. This work developed a new method for inverting the Seebeck effect of redox couples and is highly important for extending the library of possible redox species in TGCs.

Evaluation on energy performance in a low temperature district heating system integrated with organic Rankine cycle

Combined heat and power (CHP) coal-fired plants and heat-only boilers are still working as main heat sources in North China. These provide high temperature water on the primary side of district heating (DH) systems. There can be large temperature differences between the primary side and secondary side deployed in low temperature district heating (LTDH) of buildings. In this paper, a LTDH system, integrated with an organic Rankine cycle (ORC) system, is presented and evaluated on how to utilize the limited temperature difference between the primary and secondary sides in a substation. Two cases are illustrated of the performance of two modes (series or parallel connection) and energy efficiencies of the configurations with or without an ORC system. The results showed that the integrated ORC system could provide sufficient power for the circulating pumps in the DH system. The integration of the ORC leads to only a very slight decrement on the supply water temperature. Generally, the series mode can generate the maximum output power from an integrated ORC system. The parallel mode showed more flexibility on the adjustment of output power from the ORC system, especially where domestic hot water is needed. When the cold tap water was used as a cooling stream in the condenser of an ORC system before preparing to be the domestic hot water, it is very helpful in improving the performance of the ORC and increasing the energy efficiency of the DH system.

Free convection in a parallelogrammic porous cavity filled with a nanofluid using Tiwari and Das' nanofluid model.

The free convection heat transfer of Cu-water nanofluids in a parallelogrammic enclosure filled with porous media is numerically analyzed. The bottom and top of the enclosure are insulated while the sidewalls are subject to limited temperature difference. The Darcy flow and the Tiwari and Das’ nanofluid models are considered. The governing dimensionless partial differential equations are numerically solved using a finite difference code. The results are reported for isotherms and streamlines as well as Nusselt number as a function of the volume fraction of nanoparticles, porosity, types of the porous matrix, inclination angle, aspect ratio and different Rayleigh numbers. It is found that the presence of the nanoparticles inside the enclosure deteriorates the heat transfer rate, which is caused due to the increase of dynamic viscosity by the presence of nanoparticles. Therefore, in applications in which the nanofluids are used for their advantages, such as enhanced dielectric properties or antibacterial properties, more caution for the heat transfer design of the enclosure is necessary.

Experimental study on partially internally cooled dehumidification in liquid desiccant air conditioning system

The dehumidifier is one of the essential components in air-conditioning systems with liquid desiccant. Compared with adiabatic dehumidifiers, internally cooled dehumidifiers can achieve better dehumidification performance. In this study, dehumidification effectiveness and moisture removal rate were adopted to evaluate and compare the performance of these two kinds of dehumidifiers. The results indicated that internally cooled dehumidifiers can significantly improve performance compared with adiabatic dehumidifiers, especially when the solution is at a high temperature and/or low concentration in the downstream section of the liquid desiccant flow. Performance improvement and energy saving can be taken into consideration simultaneously by only supplying cooling water to the latter part of the dehumidifier. Solution temperatures at 20–25°C for adiabatic dehumidifiers are optimal and those at 25–30°C for internal cooled types. Lower temperatures of the cooling water can result in higher values for dehumidification effectiveness and moisture removal rate in an internally cooled dehumidifier; the limited temperature difference between cooling water and solution might seriously weaken the effect of the cooling water. Dimensionless Sherwood number Sh was calculated, and discrepancies between predicted values and experimental data are almost within ±20%.

Study on Operation and Parameters of Urban Sewage Source Heat Pump System with Freezing Latent Heat Collection

Urban sewage is a renewable energy source in the fields of heating and air conditioning engineering, and urban sewage source heat pump system plays an important role on energy-saving and environmental protection. According to the differences of the sewage emission coefficient and heat load of the building, it is not able to satisfy the user heating demand only on the limited temperature difference sensible heat on many situations, especially for the sewage rate of flow in small trunk canals. Therefore, a new heat system named urban sewage source heat pump system with freezing latent heat collection is proposed, in which the freezing latent heat is used as the heating source of the heat pump system at peak heat load in winter as well as the temperature difference sensible heat. According to the status, the system form is brought out and the operation modes of this heat pump system are stated, then the characteristics of the design and operating parameters are discussed.

Thermal behavior of a cryogenic loop heat pipe for space application

This paper discusses a prototype of cryogenic loop heat pipe (CLHP) working around 80K with nitrogen as the coolant, developed at CEA-SBT in collaboration with the CAS/TIPC and tested in laboratory conditions. In addition to the main loop it features a pressure reduction reservoir and a secondary circuit which allow cooling down the loop from the room temperature conditions to the nitrogen liquid temperature and transferring the evaporator heat leaks and radiation heat loads towards the condenser. The general design, the instrumentation and the experimental results of the thermal response of the CLHP are presented, analyzed and discussed both in the transient phase of cooling from room temperature (i) and in stationary conditions (ii). During phase (i), even in a severe radiation environment, the secondary circuit helped to condense the fluid and was very efficient to chill the primary evaporator. During phase (ii), we studied the effects of transferred power, filling pressure and radiation heat load for two basic configurations of cold reservoir of the secondary circuit. A maximum cold power of 19W with a corresponding limited temperature difference of 5K was achieved across a 0.5m distance. We evidenced the importance of the filling pressure to optimize the thermal response. A small heating power (0.1W) applied on the shunted cold reservoir allows to maintain a constant subcooling (1K). The CLHP behaves as a capillary pumped loop (CPL) in such a configuration, with the cold reservoir being the compensation chamber of the thermal link. The radiation heat loads may affect significantly the thermal response of the system due to boiling process of liquid and large mass transfer towards the pressure reduction reservoir.

Experimental study on a cryogenic loop heat pipe with high heat capacity

Cryogenic loop heat pipes (CLHPs) are efficient heat transfer devices based on two-phase flow. Loop heat pipes for room temperature applications have achieved satisfactory thermal control functions with the benefits of no mechanical moving part, vibration isolation, thermal insulation, long heat transport distance and so on. While there exist many problems for low temperature applications of loop heat pipes, such as limited heat transport capacity, which could not meet the increasing requirement of instrument heat dissipation. This paper presents an advanced CLHP operating at liquid-nitrogen temperature range. An improved condenser structure is introduced to the CLHP, which greatly reduces the flow resistance and increases the cooling capability of the condenser. Many experiments have been carried out on the CLHP prototype for performance test, and one set of the experimental results with a 3.2 MPa fill pressure at room temperature is presented in this paper. It is shown that the advanced CLHP prototype can be operated reliably with a high heat transfer capacity up to 41 W and a limited temperature difference of 6 K across a 0.48 m transport distance.

Preface—Understanding the coevolution of life and environment in great turning periods of Earth history

Wherever the first life originated, the Earth has grown organisms. The Earth provides life with all the basic elements required for the composition and growth of life. And of more importance is that the Earth has the unique environment for the survival, development and reproduction of life. In the vast universe, the Earth is the only planet so far known for the existence of life. This is because of its exclusive position in the solar system, which makes it the best environmental conditions for life, such as the appropriate planetary gravita-tion, moderate sunshine, limited temperature difference between day and night, and so on. While the earth environment brings up life, biotic activities in turn have been buffering the earth environments and facilitating the environmental evolution. The rise of life triggered the emergence of the biosphere in the earth surface system. The biosphere is the most active sphere in the Earth and it facilitates the earth surface system to circulate much faster. The newly emerging geobiology focusing on the biosphere and its interactions with other spheres through Earth history is essential for the earth system science.

Conductive cooling of triangular shaped electronics using constructal theory

Conductive cooling of electronics falls in the category of the more general “area to point” flow problem. Heat generated in a fixed area is to be discharged to a heat sink located on the border of the heat generating area through relatively high conductive link(s). This will maintain a limited temperature difference between the hot spot inside the heat generating area and the heat sink. The solution procedure starts with heat transfer analysis and geometric optimization of the smallest heat generating area. Assembly of optimized smallest areas in a fixed but larger heat generating area by introducing a new high conductive link and geometric optimization of the new area leads to achieving the goal of conductive cooling of the larger area. The sequence of assembly of optimized areas in a relatively larger area and geometric optimization of this area is continued until the required area size to be cooled is obtained. The process of assembly and optimization steps leads to formation of a tree network of high conductive links inside the heat generating area. Along with geometric optimization of the heat generating area in each step, the tree network of high conductive links is optimized with respect to high conductive material allocation in the heat generating area as well.

Exact solution for cooling of electronics using constructal theory

This article deals with the constructal-theory based solution for conductive cooling of electronics. The problem falls in the category of a more general “area to point” flow problem. Heat generated in a fixed area is to be discharged to a heat sink located on the border of the heat generating area through relatively high conductive link(s). This will maintain a limited temperature difference between the hot spot inside the heat generating area and the heat sink. Constructal-theory based solution in this article is supported by and matched with an analytical and exact heat transfer analysis of the physical problem. The solution procedure starts with heat transfer analysis and geometric optimization of the smallest heat generating area. The assembly of optimized smallest areas in a fixed but larger heat generating area by introducing a high conductive link and geometric optimization of the area leads to achieve the goal of conductive cooling of a larger area. Sequence of assembly of optimized areas in a relatively larger area and geometric optimization of this area is continued until the required area size to be cooled is obtained. The process of assembly and optimization steps leads to the formation of a tree-network of high conductive links inside the heat generating area. Along with geometric optimization of a heat generating area in each step, the tree-network of high conductive links is optimized with respect to high conductive material allocation in the heat generating area as well.