Thermal performance of an oscillating heat pipe with Al2O3–water nanofluids

An experimental investigation was performed on the thermal performance of an oscillating heat pipe (OHP) charged with base water and spherical Al₂O₃ particles of 56 nm in diameter. The effects of filling ratios, mass fractions of alumina particles, and power inputs on the total thermal resistance of the OHP were investigated. Experimental results showed that the alumina nanofluids significantly improved the thermal performance of the OHP, with an optimal mass fraction of 0.9 wt.% for maximal heat transfer enhancement. Compared with pure water, the maximal thermal resistance was decreased by 0.14 °C/W (or 32.5%) when the power input was 58.8 W at 70% filling ratio and 0.9% mass fraction. By examining the inner wall samples, it was found that the nanoparticle settlement mainly took place at the evaporator. The change of surface condition at the evaporator due to nanoparticle settlement was found to be the major reason for the enhanced thermal performance of the alumina nanofluid-charged OHP.     

Thermal characteristics of a resurfaced condenser and evaporator closed two-phase thermosyphon

This paper reports a study on the effect of the condenser and evaporator resurfacing on overall performance of a 1 m height closed two-phase thermosyphon. Water was used as working fluid with a fill ratio and operating pressure was 0.75 and 160 mbar, respectively. The thermosyphon performances for plain and modified thermosyphon were studied at 44 power inputs from 43 W to 668 W. The results show that by making the evaporator more hydrophilic and the condenser more hydrophobic, it will be possible to increase the average thermal performance by15.27% and decrease the thermal resistance by 2.35 times compared with the plain one.     

Experimental investigation of a natural zeolite–water adsorption cooling unit

In this study, a thermally driven adsorption cooling unit using natural zeolite–water as the adsorbent–refrigerant pair has been built and its performance investigated experimentally at various evaporator temperatures. The primary components of the cooling unit are a shell and tube adsorbent bed, an evaporator, a condenser, heating and cooling baths, measurement instruments and supplementary system components. The adsorbent bed is considered to enhance the bed’s heat and mass transfer characteristics; the bed consists of an inner vacuum tube filled with zeolite (zeolite tube) inserted into a larger tubular shell. Under the experimental conditions of 45 °C adsorption, 150 °C desorption, 30 °C condenser and 22.5 °C, 15 °C and 10 °C evaporator temperatures, the COP of the adsorption cooling unit is approximately 0.25 and the maximum average volumetric cooling power density (SCP_v) and mass specific cooling power density per kg adsorbent (SCP) of the cooling unit are 5.2 kW/m³ and 7 W/kg, respectively.     

An experimental investigation of a three-dimensional flat-plate oscillating heat pipe with staggered microchannels

Using water or acetone as the working fluid, the thermal performance of a three-dimensional flat-plate oscillating heat pipe (3D FP-OHP) with staggered microchannels was experimentally investigated by varying heating area, cooling temperature and operating orientation. It was found that when the heating area is larger at the same input power, the heat pipe is less orientation-dependent. When the heating area was decreased, to form a localized heating condition and higher heat flux, the thermal resistance and peak-to-peak amplitudes of temperature oscillations in the evaporator increased. The utilization of water as the working fluid generally provided the lowest thermal resistance for all experimental conditions investigated, but – unlike acetone – resulted in more severe temperature fluctuations in the evaporator during localized heating. The 3D FP-OHP, with overall dimensions of 130.18 × 38.10 × 2.86 mm³, demonstrated to efficiently manage heat fluxes as high as approximately 300 W/cm² at a total heat load of 300 W.     

A vapor chamber using extended condenser concept for ultra-high heat flux and large heater area

We proposed an extended vapor chamber (EVC), consisting of an evaporator part and an extended condenser part. A layer of compressed copper foam was sintered on the inner evaporator surface. The extended condenser includes a circular-straight groove network and a fin region. The groove network distributes generated vapor everywhere in the internal volume of EVC. A set of capillary holes are machined within fins. A sliced copper foam bar is inserted in each of capillary hole. The peaks of copper foam bar are tightly contacted with the evaporator copper foam piece. Water is used as the working fluid with a heater area of 0.785 cm². A minimum thermal resistance of 0.03 K/W is reached for the bottom heating. The heat flux is up to 450 W/cm² without reaching dryout. The transition point of thermal resistances versus heat fluxes is significantly delayed with the heat flux exceeding 300 W/cm², beyond which thermal resistances are only slightly increased. EVC not only improves temperature uniformity on the evaporator and fin base surfaces, but also evens the temperature distribution along the fin height direction to increase the fin efficiency. Inclination angles and charge ratios are combined to affect the thermal performance of EVC. An optimal charge ratio of 0.3 was recommended. EVC can be used for ultra-high heat flux and larger heater area conditions.     

Performance of a concentrated photovoltaic energy system with static linear Fresnel lenses

A new type of greenhouse with linear Fresnel lenses in the cover performing as a concentrated photovoltaic (CPV) system is presented. The CPV system retains all direct solar radiation, while diffuse solar radiation passes through and enters into the greenhouse cultivation system. The removal of all direct radiation will block up to 77% of the solar energy from entering the greenhouse in summer, reducing the required cooling capacity by about a factor 4. This drastically reduce the need for cooling in the summer and reduce the use of screens or lime coating to reflect or block radiation.All of the direct radiation is concentrated by a factor of 25 on a photovoltaic/thermal (PV/T) module and converted to electrical and thermal (hot water) energy. The PV/T module is kept in position by a tracking system based on two electric motors and steel cables. The energy consumption of the tracking system, ca. 0.51 W m⁻², is less than 2% of the generated electric power yield. A peak power of 38 W m⁻² electrical output was measured at 792 W m⁻² incoming radiation and a peak power of 170 W m⁻² thermal output was measured at 630 W m⁻² incoming radiation of. Incoming direct radiation resulted in a thermal yield of 56% and an electric yield of 11%: a combined efficiency of 67%. The annual electrical energy production of the prototype system is estimated to be 29 kW h m⁻² and the thermal yield at 518 MJ m⁻². The collected thermal energy can be stored and used for winter heating. The generated electrical energy can be supplied to the grid, extra cooling with a pad and fan system and/or a desalination system. The obtained results show a promising system for the lighting and temperature control of a greenhouse system and building roofs, providing simultaneous electricity and heat. It is shown that the energy contribution is sufficient for the heating demand of well-isolated greenhouses located in north European countries.     

Exergetic analysis and optimization of a solar-powered reformed methanol fuel cell micro-powerplant

The present study proposes a combination of solar-powered components (two heaters, an evaporator, and a steam reformer) with a proton exchange membrane fuel cell to form a powerplant that converts methanol to electricity. The solar radiation heats up the mass flows of methanol-water mixture and air and sustains the endothermic methanol steam reformer at a sufficient reaction temperature (typically between 220 and 300 °C). In order to compare the different types of energy (thermal, chemical, and electrical), an exergetic analysis is applied to the entire system, considering only the useful part of energy that can be converted to work. The effect of the solar radiation intensity and of different operational and geometrical parameters like the total inlet flow rate of methanol-water mixture, the size of the fuel cell, and the cell voltage on the performance of the entire system is investigated. The total exergetic efficiency comparing the electrical power output with the exergy input in form of chemical and solar exergy reaches values of up to 35%, while the exergetic efficiency only accounting for the conversion of chemical fuel to electricity (and neglecting the ‘cost-free’ solar input) is increased up to 59%. At the same time, an electrical power density per irradiated area of more than 920 W m⁻² is obtained for a solar heat flux of 1000 W m⁻².     

Thermal performance of a commercial alkaline water electrolyzer: Experimental study and mathematical modeling

In this paper a study of the thermal performance of a commercial alkaline water electrolyzer (HySTAT from Hydrogenics) designed for a rated hydrogen production of 1 N m³ H₂/h at an overall power consumption of 4.90 kW h/N m³ H₂ is presented. The thermal behaviour of the electrolyzer has been analyzed under different operating conditions with an IR camera and several thermocouples placed on the external surface of the main electrolyzer components. It has been found that the power dissipated as heat can be reduced by 50–67% replacing the commercial electric power supply unit provided together with the electrolyzer by an electronic converter capable of supplying the electrolyzer with a truly constant DC current. A lumped capacitance method has been adopted to mathematically describe the thermal performance of the electrolyzer, resulting in a thermal capacitance of 174 kJ °C⁻¹. The effect of the AC/DC converter characteristics on the power dissipated as heat has been considered. Heat losses to the ambient were governed by natural convection and have been modeled through an overall heat transfer coefficient that has been found to be 4.3 W m⁻² °C⁻¹. The model has been implemented using ANSYS^® V10.0 software code, reasonably describing the performance of the electrolyzer. A significant portion of the energy dissipated as heat allows the electrolyzer operating at temperatures suitable to reduce the cell overvoltages.     

Dynamic evaluation of low-temperature metal-supported solid oxide fuel cell oriented to auxiliary power units

A metal-supported solid oxide fuel cell (SOFC) composed of a Ni–Ce_0.8Sm_0.2O_2−δ (Ni–SDC) cermet anode and an SDC electrolyte was fabricated by suspension plasma spraying on a Hastelloy X substrate. The cathode, an Sm_0.5Sr_0.5CoO₃ (SSCo)–SDC composite, was screen-printed and fired in situ. The dynamic behaviour of the cell was measured while subjected to complete fuel shutoff and rapid start-up cycles, as typically encountered in auxiliary power units (APU) applications. A promising performance – with a maximum power density (MPD) of 0.176 W cm⁻² at 600 °C – was achieved using humidified hydrogen as fuel and air as the oxidant. The cell also showed excellent resistance to oxidation at 600 °C during fuel shutoff, with only a slight drop in performance after reintroduction of the fuel. The Cr and Mn species in the Hastelloy X alloy appeared to be preferentially oxidized while the oxidation of nickel in the metallic substrate was temporarily alleviated. In rapid start-up cycles with a heating rate of 60 °C min⁻¹, noticeable performance deterioration took place in the first two thermal cycles, and then continued at a much slower rate in subsequent cycles. A postmortem analysis of the cell suggested that the degradation was mainly due to the mismatch of the thermal expansion coefficient across the cathode/electrolyte interface.     

Performance of a scaled-up Microbial Fuel Cell with iron reduction as the cathode reaction

Scale-up studies of Microbial Fuel Cells are required before practical application comes into sight. We studied an MFC with a surface area of 0.5 m² and a volume of 5 L. Ferric iron (Fe³⁺) was used as the electron acceptor to improve cathode performance. MFC performance increased in time as a combined result of microbial growth at the bio-anode, increase in iron concentration from 1 g L⁻¹ to 6 g L⁻¹, and increased activity of the iron oxidizers to regenerate ferric iron. Finally, a power density of 2.0 W m⁻² (200 W m⁻³) was obtained. Analysis of internal resistances showed that anode resistance decreased from 109 to 7 mΩ m², while cathode resistance decreased from 939 to 85 mΩ m². The cathode was the main limiting factor, contributing to 58% of the total internal resistance. Maximum energy efficiency of the MFC was 41%.     

Surface floating, air cathode, microbial fuel cell with horizontal flow for continuous power production from wastewater

A surface floating, air cathode, microbial fuel cell (MFC) with a horizontal flow is devised and characterized using glucose-based synthetic wastewater. The performance of the MFC is significantly affected by the current-collector of the electrodes. When graphite foil ribbon (150 cm) serves as the current-collector, the respective specific internal resistance and maximum power density are 0.362 Ω m⁻² and 124.0 W m⁻³. The internal resistance can be reduced by increasing the length of the current-collector. For a graphite ribbon current-collector 256 cm long, the specific internal resistance is only 0.187 Ω m⁻² and the maximum power density markedly increases to 253.6 W m⁻³; however, the maximum power density is affected by the current-collector material. When the current-collector is changed to a stainless-steel wire, the maximum power density is reduced to approximately 100 W m⁻³ because of its high liquid|solid interfacial impedance. During three continuous months of operation, issues such as leaking are not observed and as such, the MFC could be easily scaled-up for wastewater treatment by increasing the electrode size and stacking a number of cells without additional ohmic resistance.     

Performance of a gas engine heat pump (GEHP) using R410A for heating and cooling applications

A gas engine heat pump (GEHP) represents one of the most practicable systems which improve the overall energy utilization efficiency and reduce the operating cost for heating and cooling applications. The present work aimed at evaluating the performance of a GEHP for air-conditioning and hot water supply. In order to achieve this objective, a test facility was developed and experiments were performed over a wide range of engine speed (1200 rpm–1750 rpm), ambient air temperature (24.1 °C–34.8 °C), evaporator water flow rate (1.99 m³/h–3.6 m³/h) and evaporator water inlet temperature (12.2 °C–23 °C). Performance characteristics of the GEHP were characterized by water outlet temperatures, cooling capacity, heating capacity and primary energy ratio (PER). The results showed that the effect of evaporator water inlet temperature on the system performance is more significant than the effects of ambient air temperature and evaporator water flow rate. PER of the considered system at evaporator water inlet temperature of 23 °C is higher than that one at evaporator water inlet temperature of 12.2 °C by about 22%. PER of the system decreases by 16% when engine speed changes from 1200 rpm to 1750 rpm.     

Experimental prediction of total thermal resistance of a closed loop EAHE for greenhouse cooling system

The design of an earth to air heat exchanger (EAHE) requires knowledge of its total thermal resistance (R_Tot) for heating and cooling applications. In this research, a 47 m long horizontal, 56 cm nominal diameter U-bend buried galvanized was studied experimental EAHE used for the determination and evaluation of thermal properties of heat exchanger. This system was designed and installed in the Solar Energy Institute, Ege University, Izmir, Turkey. Based on the experimental results, generalized relationships were developed for predicting of thermal resistance of the heat exchanger. Average total heat exchanger thermal resistance was estimated to be 0.021 K-m/W as a constant value under steady state condition.     

Treatment of carbon fiber brush anodes for improving power generation in air-cathode microbial fuel cells

Carbon brush electrodes have been used to provide high surface areas for bacterial growth and high power densities in microbial fuel cells (MFCs). A high-temperature ammonia gas treatment has been used to enhance power generation, but less energy-intensive methods are needed for treating these electrodes in practice. Three different treatment methods are examined here for enhancing power generation of carbon fiber brushes: acid soaking (CF-A), heating (CF-H), and a combination of both processes (CF-AH). The combined heat and acid treatment improve power production to 1370 mW m⁻², which is 34% larger than the untreated control (CF-C, 1020 mW m⁻²). This power density is 25% higher than using only acid treatment (1100 mW m⁻²) and 7% higher than that using only heat treatment (1280 mW m⁻²). XPS analysis of the treated and untreated anode materials indicates that power increases are related to higher N1s/C1s ratios and a lower C-O composition. These findings demonstrate efficient and simple methods for improving power generation using graphite fiber brushes, and provide insight into reasons for improving performance that may help to further increase power through other graphite fiber modifications.     

Preparation and properties of thin Pt-Ir films deposited by dc magnetron co-sputtering

A series of Pt-Ir thin films envisaged for application as fuel cell cathodic catalysts are deposited by dc co-sputtering from pure metal targets. To achieve different metal ratios, the sputtering power applied on the iridium target (P_Ir) is varied in the range 0-100 W at constant power of the Pt target (P_Pt). The influence of the sputtering power on the film composition, morphology, and surface structure is analysed by energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The catalytic activity towards oxygen reduction reaction (ORR) is evaluated in sulphuric acid solutions applying the methods of cyclic voltammetry and potentiodynamic polarization curves. The performed morphological and electrochemical investigations reveal that catalytic efficiency of the co-sputtered Pt-Ir films is superior compared to pure Pt. The ORR is most intensive on the sample deposited at power ratio P_Pt:P_Ir = 100:30 W containing 11 at.% Ir that has also the most developed active surface. The ORR current density for this film achieved at 0.825 V in acid solution (4.1 mA cm⁻²) is about 6 times higher than for pure Pt (0.67 mA cm⁻²). The improved activity of the thin co-sputtered Pt-Ir over Pt allows for essential reduction of the catalyst loading at preserved performance.     

Experimental investigation on performance improvement of electro-osmotic regeneration for solid desiccant

Electro-osmotic regeneration for the solid desiccant has been proved to have many merits such as regeneration without the heat source; energy-saving and simple structure. However; the previous work has revealed that its performance is seriously limited by the severe Joule heating effect and electrode corrosion; which demands further improvement to meet the practical requirement. In this paper; four possible improvement methods are investigated experimentally; including changing the material of anode; changing layout of cathode; applying the interrupted power and optimizing the electrical field strength. Through detailed experiments and analysis; we found that applying the platinum-plated titanium mesh as anode could improve the working lifetime from 6 h to over 120 h and effectively reduce Joule heating effect simultaneously; laying a piece of filter cloth under the cathode could enhance the EO regeneration rate up to 0.0021 g s⁻¹; the application of interrupted power could increase the regeneration rate up to 1.5 times; the optimal on–off-time was found at 30 s:1.3 s with 17 V cm⁻¹ electric field strength and 30 s:0.8 s with 11 V cm⁻¹; and the most suitable value of electric field strength was observed as ranging from 8.5 to 13 V cm⁻¹ in our EO regeneration system.     

Improving power density and efficiency of miniature radioisotopic thermoelectric generators

We have built and tested a prototype miniaturized thermoelectric power source that generates 450 μW of electrical power in a system volume of 4.3 cm³. The measured power density of 104 μW cm⁻³ exceeds that of any previously reported thermoelectric power system of equivalent size. This improvement was achieved by implementing a novel thermopile design in which wagon wheel-shaped thermoelectric elements contact the entire circumference of the heat source whereas traditional approaches utilize only one heat source surface. The thermopile consists of 22 wagon wheel-shaped elements (11 P–N thermocouples) fabricated from 215-μm thick bismuth–telluride wafers having ZT = 0.97 at 30 °C. The power source operates on a 150 mW thermal input provided by an electrical resistance heater that simulates a capsule containing 0.4 g of ²³⁸PuO₂ located at the center of the device. Our primary research objective was to develop and demonstrate a prototype thermopile and radioisotopic thermoelectric generator (RTG) architecture with improved power density at small scales. Output power from this device, while optimized for efficiency, was not optimized for output voltage, and the maximum power was delivered at 41 mV. We also discuss modifications to our prototype design that result in significantly improved voltage and power. Numerical predictions show that a power output of 1.4 mW, power density of 329 μW cm⁻³, and voltage of 362 mV, is possible in the same package size.     

Photovoltaic-thermal collectors for night radiative cooling of buildings

A new photovoltaic-thermal (PVT) system has been developed to produce electricity and cooling energy. Experimental studies of uncovered PVT collectors were carried out in Stuttgart to validate a simulation model, which calculates the night radiative heat exchange with the sky. Larger PVT frameless modules with 2.8 m² surface area were then implemented in a residential zero energy building and tested under climatic conditions of Madrid. Measured cooling power levels were between 60 and 65 W m⁻², when the PVT collector was used to cool a warm storage tank and 40-45 W m⁻², when the energy was directly used to cool a ceiling. The ratio of cooling energy to electrical energy required for pumping water through the PVT collector at night was excellent with values between 17 and 30. The simulated summer cooling energy production per square meter of PVT collector in the Madrid/Spain climatic conditions is 51 kWh m⁻² a⁻¹. In addition to the thermal cooling gain, 205 kWh m⁻² a⁻¹ of AC electricity is produced under Spanish conditions. A comparative analysis for the hot humid climate of Shanghai gave comparable results with 55 kWh m⁻² a⁻¹ total cooling energy production, mainly usable for heat rejection of a compression chiller and a lower electricity production of 142 kWh m⁻² a⁻¹.     

Assessment of PrBaCo2O5+δ + Sm0.2Ce0.8O1.9 composites prepared by physical mixing as electrodes of solid oxide fuel cells

The performance of PrBaCo₂O_5+δ + Sm_0.2Ce_0.8O_1.9 (PrBC + SDC) composites as electrodes of intermediate-temperature solid oxide fuel cells is investigated. The effects of SDC content on the performance and properties of the electrodes, including thermal expansion, DC conductivity, oxygen desorption, area specific resistance (ASR) and cathodic overpotential are evaluated. The thermal expansion coefficient and electrical conductivity of the electrode decreases with an increase in SDC content. However, the electrical conductivity of a composite electrode containing 50 wt% SDC reaches 150 S cm⁻¹ at 600 °C. Among the various electrodes under investigation, an electrode containing 30 wt% SDC exhibits superior electrochemical performance. A peak power density of approximately 1150 and 573 mW cm⁻² is reached at 650 and 550 °C, respectively, for an anode-supported thin-film SDC electrolyte cell with the optimal composite electrode. The improved performance of a composite electrode containing 70 wt% PrBC and 30 wt% SDC is attributed to a reduction in the diffusion path of oxygen-ions within the electrode, which is a result of a three-dimensional oxygen-ion diffusion path in SDC and a one-dimensional diffusion path in PrBC.     

Heat transfer characteristics of a two-phase closed thermosyphon to the fill charge ratio

In this study, the heat transfer characteristics of a two-phase closed thermosyphon were investigated. For the test, a two-phase closed thermosyphon (copper container, FC-72 (C₆F₁₄) working fluid) was fabricated with a reservoir which could change the fill charge ratio. The experiments were performed in the range of 50-600 W heat flow rate and 10-70% fill charge ratio. Some findings are as follows.The heat transfer coefficients of the evaporator to the fill charge ratio were nearly negligible. These presented about 1-5 kW/m² K with the increase of heat flux and compared with those of smooth surface, showed some enhancement by the grooved surface. However at the condenser, the heat transfer coefficients showed some enhancement with the increase of fill charge ratio by the expanded working fluid pool. And the heat transport limitations appeared in different ways to the fill charge ratio. For the relatively small fill charge ratio (Ψ<20%), it presented about 100 W (Ψ: 10%) by the dry-out limitation.For the large fill charge ratio, it occurred by the flooding limitation and the maximum heat flow rate was about 500-550 W (Bo: 26-28), 230 W (Bo: 18.3) respectively and the Kutateladze number was about 1.9-2.1.