High Power Density Pyroelectric Energy Conversion in Nanometer-Thick BaTiO3 Films

Solid-state pyroelectric nanomaterials can be used for thermal-to-electrical energy conversion in the presence of temperature fluctuations. This article reports investigation of energy conversion in a 200 nm thick BaTiO₃ film using the pyroelectric Ericsson cycle at cycle frequencies up to 3 kHz. The high cycle frequencies were achieved due to the low thermal mass of the nanometer-scale film, unlike previous studies in which the electrical power output was limited by the rate of heat transfer through the pyroelectric material. A microfabricated platform that allowed precise thermal and electrical cycling enabled us to study the effect of electric field range, temperature oscillation amplitude, and cycle frequency on the electrical power output from pyroelectric Ericsson cycles. We measured a maximum power density of 30 W/cm³ for a temperature range 20–120°C and electric field range 100–125 kV/cm, which represents a significant improvement over past work on pyroelectric cycles. The approach presented in this article could lead to high-power waste heat harvesting in systems with high-frequency temperature oscillations.     

Review of metal hydride hydrogen storage thermal management for use in the fuel cell systems

Thermal management of metal hydride (MH) hydrogen storage systems is critically important to maintain the hydrogen absorption and release rates at desired levels. Implementing thermal management arrangements introduces challenges at system level mostly related to system's overall mass, volume, energy efficiency, complexity and maintenance, long-term durability, and cost. Low effective thermal conductivity (ETC) of the MH bed (~0.1–0.3 W/mK) is a well-known challenge for effective implementation of different thermal management techniques. This paper comprehensively reviews thermal management solutions for the MH hydrogen storage used in fuel cell systems by also focusing on heat transfer enhancement techniques and assessment of heat sources used for this purpose. The literature recommended that the ETC of the MH bed should be greater than 2 W/mK, and heat transfer coefficient with heating/cooling media should be in the range of 1000–1200 W/m²K to achieve desired MH's performance. Furthermore, alternative heat sources such as fuel cell heat recovery or capturing MH heat during charging and releasing it back during discharging have also been thoroughly reviewed here. Finally, this review paper highlights the gaps and suggests directions accordingly for future research on thermal management for MH systems.     

Evaluation of Jet Impingement,Spray and Microchannel Chip Cooling Options for High Heat Flux Removal

Thermal management for high heat flux removal from microelectronic chips is gaining critical importance in many earth-based and space-based systems. Heat fluxes greater than 1 MW/m² (100 W/cm²) have already been realized in high-end server applications, while cooling needs in next generation chips and advanced systems such as high-power electronics and electrical systems, pulsed power weapons systems, solid-state sensors, and phased-array radars are expected to reach 5–10 MW/m² (500–1000 W/cm²). After evaluating the contributions from different thermal resistances in the chip-to-ambient thermal path, this paper presents a critical review and research recommendations for three prominent contending technologies: jet impingement, spray cooling, and microchannel heat sinks.     

Fabrication of n-Cu2O electrodes with higher energy conversion efficiency in a photoelectrochemical cell

We observed an n-type photoresponse in cuprous oxide films, which were prepared by a simple method of immersing copper plates in a HCl solution of 3 pH at 40°C temperature, when they were used in a PEC cell. This photoresponse was much higher than the previously published values for n-Cu₂O electrodes which were prepared by other methods. The photocurrent obtained was in the order of 0.3 mA/cm² when the cell was illuminated with light intensity of 50 mW/cm². (The semiconductor electrode was biased to get a zero dark current.) The power conversion efficiency of the cell was 0.01%. The maximum quantum efficiency obtained was 15%. It is hoped that these values could be improved with a better understanding of the photoelectrochemical properties of the cell.     

Photo-to-thermal conversion and energy storage of lauric acid/expanded graphite composite phase change materials

To make better use of solar energy, lauric acid/expanded graphite (LA/EG) composite phase change materials (PCMs) were synthesized to collect and store solar energy as latent heat thermal energy. The results of thermal characteristics show that when the mass fraction of EG is 5%, 10%, and 15%, the latent heat of LA/EG is 164.5, 156.9, and 148.0 J/g, and the thermal conductivity is 2.73, 7.98, and 10.54 W/(m·K). Leakage test shows that LA/EG PCMs with EG mass fraction of 10% and 15% are form stable after phase change. One thousand thermal cycles prove good thermal reliability of LA/EG. TG analysis indicates LA/EG PCMs have good thermal stability within operating temperature range. The Ultraviolet-visible spectra reveal that the absorbance of LA/EG composite PCMs would increase as the mass fraction of EG increases. Photothermal conversion experiment results indicate that the photothermal conversion efficiency of LA/EG composite PCMs increases as the mass fraction of EG increases, and the efficiency can reach 95% when the mass fraction of EG is 15%. Moreover, it was also found that the process of photothermal conversion can be accelerated with stronger illumination intensity or smaller heat transfer size. All the results show that the prepared LA/EG PCMs can convert solar energy into thermal energy and store it in the form of latent heat at the same time, which indicates it has promising prospect in the application of solar energy conversion and storage.     

Low-Pt-loading acetylene-black cathode for high-efficient dye-sensitized solar cells

We reported on the synthesis, characterization, and photovoltaic/electrochemical properties of Pt/acetylene-black (AB) cathode as well as their application in dye-sensitized solar cells (DSCs). The Pt/AB electrode was prepared through a thermal decomposition of H₂PtCl₆ on the AB substrate. SEM and TEM observations showed that the Pt nanoparticles were homogeneously dispersed on the AB surface. The Pt-loading content in the Pt/AB electrode was only about 2.0 μg cm⁻², which was much lower than 5–10 μg cm⁻² generally used for the Pt electrode in DSCs. Electrochemical measurements displayed a low charge-transfer resistance of 1.48 Ω cm² for the Pt/AB electrode. Furthermore, when this low-Pt-loading electrode was used as the cathode of DSCs, an overall light-to-electricity energy conversion efficiency of 8.6% was achieved, showing commercially realistic energy conversion efficiency in the application of DSCs.     

Interfacial electric properties of beta″-alumina and electrode by AC impedance

In order to enhance cell power density and to study the interfacial electric property between beta″-alumina and an electrode, test cells of Na(l)/beta″-alumina/M, where M=TiN or TiB₂ or Na–Sn or Na–Pb molten alloys as electrode materials, were set up and run within the temperature range of 400°–800°C. The performance of the test cells and the interfacial electric properties were investigated by measuring current–voltage characteristics and AC impedance. The maximum power density of 0.18 W cm⁻² for TiN and 0.24 W cm⁻² for TiB₂ could be achieved with a large electrode-area of 30 cm² at 800°C. A simplified model and equivalent circuit were given, based on the impedance data. The effect of microstructure of the porous electrode and roughness of the beta″-tube on the cell electric performance and impedance has been studied and discussed. The electron-transport through the porous electrode to the interface of the electrode and the beta″-tube surface is the control step for the electrode reaction, Na⁺+e→Na, rather than the mass-transport step, for a cell of Na(l)/beta″-alumina/porous thin film electrode. The AC impedance data demonstrated that wetting of the beta″-alumina electrolyte plays an important roll in reducing the cell resistance for the molten Na–Sn or Na–Pb electrode, and the molten alloy electrodes have a smaller cell-resistance, 0.3–0.35 Ω cm², at 700°C after 10–20 h. The comparison with sputtered thin, porous film electrodes, showed that the microstructure and thickness of electrode, and the interfacial resistance between electrode and the surface of the beta″-alumina is crucial to enhance cell power density.     

Highly efficient and durable anode-supported SOFC stack with internal manifold structure

We have developed a solid oxide fuel cell (SOFC) stack with an internal manifold structure. The stack, which is composed of 25 anode-supported 100-mm-diameter SOFCs, provided an electrical conversion efficiency of 56% (based on the lower heating value of methane, which was used as a fuel) and an output of 350 W when the fuel utilization, current density, and operating temperature were 75%, 0.3 A cm⁻², and 1073 K, respectively. The electrical efficiency and the output were maintained for 1100 h. The cell voltage fluctuation was ±2% for 25 cells. The relationship between average cell voltage and current density in the 25-cell stack was as almost the same as that in the 1- and 10-cell stacks, which suggests that our stack provides almost the same cell performance regardless the number of the cells.     

Design for a BlackLight Power multi‐cell thermally coupled reactor based on hydrogen catalyst systems

The design and cost estimates compared with other systems of an energy‐producing reactor system are presented. Heat from hydrino reactions within individual cells provides both the reactor power and the heat for regeneration of the reactants. These processes occur continuously over a plurality of cells in different phases of the processes. The hydrino reactions are maintained and regenerated in a batch mode using thermally coupled multi‐cells arranged in bundles wherein cells in the power‐production phase of the cycle heat cells in the regeneration phase. In this intermittent cell power design, the thermal power is statistically constant as the cell number becomes large, or the cell cycle is controlled to achieve steady power. The conversion of thermal power to electrical power requires the use of a heat engine exploiting a cycle such as a Rankine, Brayton, Stirling, or steam‐engine cycle (Int. J. Energy Res. 1997; 21 :113–127; Int. J. Energy Res. 1998; 22 :237–248; Int. J. Energy Res. 1998; 22 :991–1000; Int. J. Energy Res. 2010; 34 :1071–1087; Int. J. Energy Res. 2009; 33 :1203–1232). Owing to the temperatures, economy goal, and efficiency, the Rankine cycle is the most practical and can produce electricity from a steam source at 30–40% efficiency with a component capital cost of about $300 per kW electric. Conservatively, assuming a conversion efficiency of 25%, the total cost with the addition of the boiler and chemical components is estimated at $1380 per kW electric. The system applications for distributed power (1–10 MW electric) and central generation retrofit and green‐field projects are projected to be very competitive relative to existing power sources and systems. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance evaluation of photovoltaic/thermal–HDH desalination system

The efficiency of photovoltaic (PV) panel drops with increase in cell temperature. The temperature of the PV panel can be controlled with various cooling techniques. In the proposed work the PV panel is cooled by circulating water and the recovered heat energy is used to run a humidification and dehumidification desalination to produce distilled water from sea water (or) brackish water. This work deals with a detailed analysis of performance of combined power and desalination (Photovoltaic/Thermal–Humidification and Dehumidification) system. A mathematical model of PV/thermal–humidification dehumidification plant was developed and simulations were carried out in MATLAB environment. The performance of photovoltaic/ thermal desalination (Photovoltaic/Thermal–Humidification and Dehumidification) system was investigated under various solar radiation levels (800–1000 W/m²). For each solar radiation level the effect of mass flow rate of coolant water (30–110 kg/h) on water outlet temperature, PV efficiency, PVT thermal efficiency, distilled water production, and plant efficiency was studied. Results show that under each solar radiation level increasing coolant flow rate increases efficiency of PV panel and reduces the plant efficiency. The highest PV efficiency (16.598%) was reached under 800 W/m² at mass flow rate of 110 kg/h and the highest plant efficiency (43.15%) was reached under 800 W/m² at a mass flow rate of 30 kg/h. The maximum amount of distilled water production rate (0.82 L/h) was reached under 1000 W/m² at water mass flow rate of 30 kg/h.     

Design and performance analysis of a 500‐W heat source for radioisotope thermophotovoltaic converters

A radioisotope thermophotovoltaic (RTPV) system effectively converts the decay heat of radioisotopes into electricity via thermally radiated photons. In this work, a 500‐W thermal heat source unit including ²³⁸PuO₂ radioisotope fuel, shielding material, and selective emitter is designed from the viewpoint of radiation safety, thermal performance, and overall conversion efficiency by considering various shielding materials, fuel configurations, and packing factor (PF), defined as the ratio of fuel region volume to total heat source enclosure volume including fuel cladding and shield. The design study starts with a reference cubic configuration and extends to the more complicated configurations having separate cylindrical fuels. The results of the study showed that the heat source unit design suggested here can reduce the total radiation dose, peak neutron fluence, and maximum temperature using separate cylindrical fuel rods. For example, a design having a separated 3 × 3 cylindrical fuel rod array of 30% PF increases the overall efficiency by ~39% with similar maximum temperature and radiation doses in comparison with the reference heat source unit with a single cubic module and a 10% PF. This demonstrates the importance of the proper design of the RTPV heat source unit.     

Electrochemical conversion of enriched crude glycerol: Effect of operating parameters

The enrichment of crude glycerol (29.8 wt.%) from a biodiesel production plant and its subsequent electrochemical conversion under a galvanostatic mode to added-value compounds was successfully performed at a laboratory scale. The optimal solvent-extraction based enrichment of the crude glycerol, after the acid pre-treatment to remove most free fatty acids and salts, was found using n-propanol:pre-treated crude glycerol at volume ratio of 2, attaining 97.9% glycerol. The effects of the initial glycerol solution pH (1, 7 or 11), type of electrode (platinum (Pt), titanium-coated ruthenium oxide (Ti/RuO₂) or stainless steel (SS)) and applied current density (0.08–0.27 A/cm²) were explored. Using a galvanostatic mode, the enriched crude glycerol could be converted to added-value products, such as ethylene glycol, acetol, glycidol, acrolein, 1,2-propanediol (PD) and 1,3-PD. A Pt electrode, initial glycerol solution pH of 1 and current density of 0.14 A/cm² were found to be optimal giving a complete conversion of 0.3 M glycerol within 14 h with a total product yield of 68.7%. However, each specific product had a different optimal applied current density and electrolysis time. Finally, a simplified diagram showing the possible major reaction pathways of glycerol conversion by this electrochemical conversion over a Pt electrode was presented.     

A bifunctional electrochemical flow cell integrating ammonia production and electricity generation for renewable energy conversion and storage

Renewable energy has rapidly advanced in the global energy system, triggering the visible development of energy storage technologies in recent decades. Among them, the electricity-fuel-electricity approach is an effective way for the storage and utilization of renewable power. In this work, a bifunctional electrochemical flow cell integrating both ammonia production and electricity generation modes is developed for renewable energy conversion and storage. Ammonia, a hydrogen carrier having a high hydrogen content of 17.6 wt %, is relatively easier to convert to liquid phase for large-scale storage. The long-distance ammonia transport can reliably depend on the established infrastructure. In addition, as a carbon-free fuel beneficial for achieving the goal of carbon-neutrality, ammonia is considered as an environmentally benign and cost-effective mediator fuel. This flow cell is able to operate via two modes, i.e., an ammonia-production mode for energy storage in the form of ammonia (via nitrogen reduction reaction) and an electricity-generation mode for energy conversion in the form of electricity (via ammonia oxidation reaction). This flow cell is constituted by a PtAu/C-coated nickel-foam electrode for nitrogen and oxygen reduction reactions, a Pt/C-coated nickel-foam electrode for water and ammonia oxidation reactions, and an alkaline anion exchange membrane for charge-carrier migration. Charging this flow cell with the supply of nitrogen results in a Faradaic efficiency of 2.70% and an ammonia production rate as high as 9.34 × 10^?10 mol s^?1 cm^?2 at 23 °C. Moreover, energizing this flow cell with ammonia results in an open-circuit voltage of 0.59 V and a peak power density of 3.31 mW cm^?2 at 23 °C. A round-trip efficiency of 25.7% is realized with the constant-electrode mode.     

Development of high-efficiency a-Si solar cell submodule with a size of 30 cm × 40 cm

We have achieved a stabilized conversion efficiency of 8.9% in a single-junction a-Si solar cell and 10.6% in a double-junction a-Si/a-SiGe solar cell for a size of 1 cm², which are the world's highest values achieved so far for this size and structure. We have been investigating the improvement of stability in a-SiGe film with regard to the bottom cell i-layer, and the control of E_opt in a-SiGe film in order to confirm the tandem cell design. On the other hand, uniformity of ± 1% has been obtained in conversion efficiency for many small cells fabricated in a size of 30 cm × 40 cm, evaluated by using a-Si single-junction structure. As a result, we have achieved the stabilized high-effective area conversion efficiency of 8.64% in a 30 cm × 40 cm a-Si/a-Si tandem submodule. The combination of the above techniques and further optimization can be expected to achieve a stabilized conversion efficiency of more than 10% for a 30 cm × 40 cm double-junction a-Si/a-SiGe submodule.     

A thermoelectric generator and water-cooling assisted high conversion efficiency polycrystalline silicon photovoltaic system

Solar energy has been increasing its share in the global energy structure. However, the thermal radiation brought by sunlight will attenuate the efficiency of solar cells. To reduce the temperature of the photovoltaic (PV) cell and improve the utilization efficiency of solar energy, a hybrid system composed of the PV cell, a thermoelectric generator (TEG), and a water-cooled plate (WCP) was manufactured. The WCP cannot only cool the PV cell, but also effectively generate additional electric energy with the TEG using the waste heat of the PV cell. The changes in the efficiency and power density of the hybrid system were obtained by real time monitoring. The thermal and electrical tests were performed at different irradiations and the same experiment temperature of 22°C. At a light intensity of 1000 W/m², the steady-state temperature of the PV cell decreases from 86.8°C to 54.1°C, and the overall efficiency increases from 15.6% to 21.1%. At a light intensity of 800 W/m², the steady-state temperature of the PV cell decreases from 70°C to 45.8°C, and the overall efficiency increases from 9.28% to 12.59%. At a light intensity of 400 W/m², the steady-state temperature of the PV cell decreases from 38.5°C to 31.5°C, and the overall efficiency is approximately 3.8%, basically remain unchanged.     

Miniature loop heat pipes for electronics cooling

The paper is devoted to the development of miniature loop heat pipes (mLHPs) with a nominal capacity of 25–30 W and a heat-transfer distance up to 250 mm intended for cooling electronics components and CPU of mobile PC. It gives the results of investigating several prototypes of mLHPs incorporated into remote heat exchanger (RHE) systems in different conditions. It has been established that in the nominal range of heat loads orientation does not practically affect the mLHPs operating characteristics. Under air cooling the total thermal resistance of such a system is 1.7–4.0 °C/W and depends strongly on the cooling conditions and the radiator efficiency. In this case the mLHP’s own thermal resistance is in the limits from 0.3 to 1.2 °C/W, and the maximum capacity reaches 80–120 Bt. The obtained results make it possible to regard mLHPs as quite promising devices for RHE systems providing thermal regimes for electronics components and personal computers.     

Improvement of proton exchange membrane fuel cell overall efficiency by integrating heat-to-electricity conversion

Proton exchange membrane fuel cells (PEMFCs) have shown to be well suited for distributed power generation due to their excellent performance. However, a PEMFC produces a considerable amount of heat in the process of electrochemical reaction. It is desirable to use thermal energy for electricity generation in addition to heating applications. Based on the operating characteristics of a PEMFC, an advanced thermal energy conversion system using “ocean thermal energy conversion” (OTEC) technology is applied to exploit the thermal energy of the PEMFC for electricity generation. Through this combination of technology, this unique PEMFC power plant not only achieves the combined heat and power efficiency, but also adequately utilizes heat to generate more valuable electricity. Exergy analysis illustrates the improvement of overall efficiency and energy flow distribution in the power plant. Analytical results show that the overall efficiency of the PEMFC is increased by 0.4-2.3% due to the thermal energy conversion (TEC) system. It is also evident that the PEMFC should operate within the optimal load range by balancing the design parameters of the PEMFC and of the TEC system.     

Methodological reviews and analyses on the emerging research trends and progresses of thermoelectric generators

Thermoelectric generator is among the earliest initiated electricity‐harvesting methods. It is a very potential power harvester that can convert wasteful thermal energy into electricity. However, it often suffers from low energy conversion rate due to its inconsistent heat source, inefficient thermoelectric material (or thermoelement) performance, and incompetent structural issues. Progressively for the first time, detailed methodological surveys and analyses are made for bulk, thick, and thin films in this review. This is in order to accommodate better insights and comprehensions on the emerging trends and progresses of thermoelectric generators from 1989 to 2017. The research interests in thermoelectric generators have started back in 1989, and have continuously experienced emerging progresses in the number of studies over the last years. The methodological reviews and analyses of thermoelectric generator showed that almost 46.6% of bulk and 46.1% of thick and thin film research works, respectively, are actively progressed in 2014 to 2017. Nearly 86.2% of bulk and 44.1% of thick and thin film thermoelectric generators are realizing in between 0.001 and 4 μW cm^?2 K^?2, while 43.1% of thick and thin films are earning among 10^?6 to 0.001 μW cm^?2 K^?2. The highest achievement made until now is 2.5 W cm^?2 at a temperature difference of 140 K and thermoelectric efficiency factor of 127.55 μW cm^?2 K^?2. This achievement remarked positive elevation for the field and interest in thermoelectric power generation. Consecutively, the research trends of fundamental devices' structure, thermoelement, fabrication, substrate, and heat source characteristics are analyzed too, along with the desired improvement highlights for the applications of thermoelectric generators.     

Allotrope carbon materials in thermal interface materials and fuel cell applications: A review

The performance of allotrope carbon materials has been explored because of their superior properties in energy system applications. This review provides an understanding of the current work focusing on the applications of selected carbon materials in important energy systems, focus on thermal interface materials (TIMs), and fuel cell applications. This article begins with the introduction of TIMs and fuel cell in general working principle and presents details on carbon materials. The discussion focuses on updates from the latest research work and addresses current challenges and opportunities for research toward TIMs and fuel cell applications. The optimum performance of TIMs was seen when thermal conductivity achieved at a maximum of 3000 W (m K)⁻¹ by using vertically aligned carbon nanotubes (CNTs) and a minimum internal thermal resistance of 0.3 mm² K W⁻¹. Meanwhile for fuel cell, the platinum/CNTs catalyst applied proton exchange membrane fuel cell achieved high power density of 661 mW cm⁻² in the presence of Nafion electrolyte membrane. This review provides insights for scientists about the use of carbon materials, especially in energy system applications.     

Effect of phosphoric acid-doped polybenzimidazole membranes on the performance of H+-ion concentration cell

H⁺-ion concentration cell, which can harvest thermal energy to generate electricity by hydrogen concentration difference principle with a fuel cell structure, is an innovative thermoelectric conversion device. In this system, phosphoric acid-doped polybenzimidazole (PA-doped PBI) membrane is a key component influencing the power generation performance of the cell. Herein, 30, 45, 60, 75, and 90 μm thick PBI membranes are successfully synthesized and doped with phosphoric acid. To achieve a good compromise between the proton conductivity and durability, the properties of PA-doped PBI membranes are experimentally evaluated to clarify the effect of the acid doping time and membrane thickness on cell performance. The results indicate that the higher the acid doping level, the worse the dimensional stability of the membrane. Also the thinner the PBI membrane, the smaller the membrane resistance to ions motion, while the poorer the stability. Upon reaction at 170 °C, this cell can boast a power density from 3.0 to 8.0 W m^?2, which results in a thermoelectric conversion efficiency of 5.97–14.32%. This study potentially boosts the practical application of thermal-to-electrical conversion technology.