Thermoeconomic analysis method for optimization of insulation thickness for the four different climatic regions of Turkey

Thermal insulation is one of the most effective energy-conservation measures in buildings. For this reason, the energy savings can be obtained by using proper thickness of insulation in buildings. In this study, the optimum thickness of insulation considering condensed vapor in external walls are found by using exergoeconomic analysis. The four various cities from four climate zones of Turkey, namely, Antalya, ?stanbul, Elaz?? and Erzurum are selected for the analysis. The optimum insulation thickness for Antalya, ?stanbul, Elaz?? and Erzurum are obtained as 0.038, 0.046, 0.057 and 0.0739 m at indoor temperature of 20 °C, respectively. The results show that the optimum insulation thickness at the indoor temperature of 18 and 22 °C are determined as 0.0663 and 0.0816 m for the city of Erzurum, respectively. The energy saving for the city of Erzurum is found as 77.2% for the indoor temperature of 18 °C, 79.0% for the indoor temperature of 20 °C and 80.6% for the indoor temperature of 22 °C, when the optimum insulation is applied.     

3-D numerical and experimental models for flat and embedded heat pipes applied in high-end VGA card cooling system

The present study utilizes the three-dimension numerical and experimental methods to investigate the optimum thermal performance of a flat heat pipe-thermal module application in high-end VGA card cooling system, and compares that with a traditional copper metal based plate embedded three 6 mm diameter heat pipe-thermal module under three dissimilar inclination angles of 0°, 90° and 180°. The optimization for the thermal modules researches into various fin material, thickness and gap. Results show that the flat heat pipe-thermal module has the best thermal performance at high power GPU of 180 W and inclination angle of 180°. Simulation results show in good agreement with experimental results within 5%. Therefore, the thermal performance of a flat heat pipe-thermal module can be accurately simulated and analyzed by employed the manner introduced in this paper and is able to cope with the higher heat flux GPU over 62.5 W/cm² in the future.     

Effect of decoration film on mold surface temperature during in-mold decoration injection molding process

In-mold decoration (IMD) during injection molding is a relatively new injection molding technique and has been employed for plastic products to improve surface quality and achieving colorful surface design, etc. During IMD processing, the film is preformed as the shape of mold cavity and attached to one side of the mold wall (usually cavity surface), then molten polymer is filled into the cavity. Heat transfer toward the mold cavity side during molding IMD part is significantly retarded because the film is much less thermal conductive than metal mold. To investigate the effect of film on temperature field, polycarbonate (PC) was injection molded under various conditions including coolant temperature, melt temperature, film material and film thickness. Simulations were also conducted to evaluate the melt–film interface temperature and its influence from film initial temperature and film thermal properties. For PC film, it was found that the heat transfer retardation results in the mold temperature drop in cavity surface and the maximum temperature drop as compared to that of conventional injection molding without film may be as high as 17.7 °C. For PET film, this maximum mold temperature drop is about 13 °C. As PC film thickness increases, the retardation-induced mold temperature difference also increases. The initial film temperature (30 °C and 95 °C) may affect the melt–film interface temperature at the contact instant of melt and film by about 12 °C to 17 °C. When thermal conductivity of film increases from 0.1 W/(m–k) to 0.2 W/(m–k), melt–film interface temperature may vary by 22.9 °C. The simulated mold temperature field showed reasonable agreement with experimental results.     

Improving thermal performance of building walls by optimizing insulation layer distribution and thickness for same thermal mass

Dynamic thermal characteristics of insulated building walls with same thermal mass are studied numerically with optimized insulation thickness under steady periodic conditions using the climatic data of Riyadh. Insulation is effected through use of one, two and three layers of insulation, the locations of which are varied in order to achieve the best performance. Insulation layer(s) thicknesses are optimized by minimizing the total cost of insulation and energy consumption using the present worth method. The results show that the optimum thickness of a single insulation layer is independent of its location in the wall; and that, when more than one insulation layer is used, their total optimum thickness is the same as the optimum thickness of a single layer. As a consequence, walls thermal resistances (R-values) are equal under optimum conditions; however, peak load, time lag, and decrement factor are found to be substantially different. The best overall performance is achieved by a wall with three layers of insulation, each 26-mm-thick, placed at inside, middle and outside followed closely by a wall with two insulation layers, each 39-mm-thick, placed at middle and outside. Comparing performance of the best wall with that of a wall with one layer of insulation, 78-mm-thick, placed on the inside, the following improvements are achieved: 100% increase in time lag from 6 h to 12 h; 10-fold decrease in decrement factor; 20% decrease in both peak cooling and heating transmission loads, and 1.6% and 3.2% decrease in yearly cooling and heating transmission loads, respectively. It is emphasized that all walls have the same optimized R-value and same thermal mass and therefore all improvements achieved are solely due to the developed distribution of insulation layers.     

Galactitol hexa stearate and galactitol hexa palmitate as novel solid-liquid phase change materials for thermal energy storage

Galactitol has a melting point of 187.41 °C and a fusion enthalpy of 401.76 J g⁻¹. Its melting temperature is not suitable for many thermal energy storage applications although it has good latent heat storage capacity compared to the several traditional phase change materials (PCMs). The galactitol also has high supercooling degree as about 72 °C. These unfavorable properties limit the usage potential of galactitol in thermal energy storage applications. However, the phase change temperature and supercooling degree of galactitol can be reduced to a reasonable value and therefore its feasibility for energy storage systems can be increased. For this aim, in this study, galactitol hexa stearate (GHS) and galactitol hexa palmitate (GHP) were prepared as novel solid-liquid PCM by means of esterification reaction of the galactitol with palmitic acid and stearic acid. The GHP and GHS esters were characterized chemically using FT-IR and ¹H NMR techniques. By using DSC analysis method, the melting temperature and latent heat value of the PCMs were determined as 31.78 °C and 201.66 J g⁻¹ for GHP ester and 47.79 °C and 251.05 J g⁻¹ for GHS ester. Thermal cycling test showed that the prepared PCMs had good thermal reliability after thermal 1000 melting-freezing cycles. Thermogravimetric analysis (TGA) results revealed that the PCMs have good thermal stability over their working temperatures. In addition, thermal conductivity of the prepared PCMs was increased as about 26.3% for GHP and 53.3% for GHS by addition of 5 wt.% expanded graphite. Based on all results it can be concluded that the prepared GHP and GHS esters can be considered as promising solid-liquid PCMs for many energy storage applications such as solar energy storage, indoor temperature controlling in buildings, production of smart textile and insulation clothing due to their good energy storage properties.     

Natural convective characteristics of a heat source module at different angles in the closed and ventilated cabinets

A numerical analysis is performed to study the characteristics of heat transfer from a block heat source module at different angles in two-dimensional cabinets. Great efforts are carried out to conduct the effects of thermal interaction between the air steams inside and outside the cabinet on the conjugate conduction–natural convection phenomena. Moreover, the enhancement of cooling performance of the heat source module through the construction of air vents on cabinet wall is rigorously examined. The computation domain covers the cabinet and the surrounding area, and the temperature and velocity fields of the cabinet and surrounding area are solved simultaneously. Comparing the results for cases with and without the consideration of thermal interaction between the air streams, the difference in hot spot temperature of module can be up to 26% for Pr = 0.7, K_bf = K_wf = 100, 0 ≦ K_pf ≦ 100, 10⁵ ≦ Ra ≦ 10⁷ and φ = 0°, 90°, 270°. The maximum reduction in hot spot temperature is about 41% when two air vents are constructed on the cabinet wall. The variation of module angle results in the maximum difference of the hot spot temperature is 17% for closed cabinet, and 10% for ventilated cabinet. In addition, the hot spot temperatures for cases with K_pf = 10 are about two times of that for K_pf = 100.     

A micro-solid oxide fuel cell system as battery replacement

The concept and the design of a micro-solid oxide fuel cell system is described and discussed. The system in this study is called the ONEBAT system and consists of the fuel cell PEN (positive electrode – electrolyte – negative electrode) element, a gas processing unit, and a thermal system. PEN elements of free-standing multi-layer membranes are fabricated on Foturan^® and on Si substrates using thin film deposition and microfabrication techniques. Open circuit voltages of up to 1.06 V and power of 150 mW cm⁻² are achieved at 550 °C. The membranes are stable up to 600 °C. The gas processing unit allows butane conversion of 95% and hydrogen selectivity of 83% at 550 °C in the reformer and efficient after-burning of hydrogen, carbon monoxide, and lower hydrocarbons in the post-combustor. Thermal system simulations prove that a large thermal gradient of more than 500 °C between the hot module and its exterior are feasible. The correlation between electrical power output – system size and thermal conductivity – heat-transfer coefficient of the thermal insulation material are shown. The system design studies show that the single sub-systems can be integrated into a complete system and that the requirements for portable electronic devices can be achieved with a base unit of 2.5 W and a modular approach.     

Numerical investigation of a PCM-based heat sink with internal fins

The present study explores numerically the process of melting of a phase-change material (PCM) in a heat storage unit with internal fins open to air at its top. Heat is transferred to the unit through its horizontal base, to which vertical fins made of aluminum are attached. The phase-change material is stored between the fins. Its properties used in the simulations, including the melting temperature of 23-25 °C, latent and sensible specific heat, thermal conductivity and density in solid and liquid states, are based on a commercially available paraffin wax.A detailed parametric investigation is performed for melting in a relatively small system, 5-10 mm high, where the fin thickness varies from 0.15 mm to 1.2 mm, and the thickness of the PCM layers between the fins varies from 0.5 mm to 4 mm. The ratio of the PCM layer to fin thickness is held constant. The temperature of the base varies from 6 °C to 24 °C above the mean melting temperature of the PCM.Transient three- and two-dimensional simulations are performed using the Fluent 6.0 software, yielding temperature evolution in the fins and the PCM. The computational results show how the transient phase-change process, expressed in terms of the volume melt fraction of the PCM, depends on the thermal and geometrical parameters of the system, which relate to the temperature difference between the base and the mean melting temperature, and to the thickness and height of the fins.In search for generalization, dimensional analysis of the results is performed and presented as the Nusselt numbers and melt fractions vs. the Fourier and Stefan numbers and fin parameters. In some cases, the effect of Rayleigh number is significant and demonstrated.     

Energy efficiency enhancement of natural rubber smoking process by flow improvement using a CFD technique

A non-uniform flow and large temperature variation in a natural rubber smoking-room cause an inefficient use of energy. Flow uniformity and temperature variation can be improved by using a computational fluid dynamics (CFD) simulation. The effects of the size, position and number of gas supply ducts and ventilating lids which were at the inlets and the outlets of the smoking-room were investigated. The optimal rubber smoking-room of size 2.6 m × 6.2 m × 3.6 m contains 154 50 mm-diameter hot gas supply ducts, and four 0.25 × 0.25 m and four 0.25 × 0.20 m ventilating lids. The velocity distribution of this model in the rubber-hanging area was rather uniform. The average monitoring temperature of 54 positions was 62.1 °C. This model could reduce the temperature variation by a factor of three from the original room model, i.e., from 15 to 5.5 °C. In a further study, the heat input of an appropriate room model was finely adjusted to obtain a suitable temperature (60 °C) for the smoking process. It was found that an appropriate heat supply at this temperature is 11 kW. At this rate, the temperature variation is 5.3 °C. This improved model should help the rubber smoking cooperatives to achieve at least a 31.25% saving in energy.     

The preparation and properties of multi-component molten salts

This paper was focused on thermal stability of molten salts and their thermo-physical properties at high temperature. In this experiment, multi-component molten salts composed of potassium nitrate, sodium nitrite and sodium nitrate with 5% additive A of the chlorides were prepared by statical mixing method. The experiments found molten salt with 5% additive A had higher thermal stability and its best operating temperature would be increased to 550 °C from 500 °C when comparing to ternary nitrate salt. Meanwhile, thermal stability and thermal cycling analysis showed molten salts with 5% additive A had lower freezing point and loss of nitrite content and deterioration time of molten salts were reduced at the same time. DSC tests also indicated loss of latent heat in molten salts with 5% additive A was decreased. Besides, thermo-physical properties measured showed molten salt with 5% additive A had a heat capacity of 2.32 kJ/kg °C, lower than 4.19 kJ/kg °C for water between 0 °C and 100 °C and a low viscosity range from 3.0 to 1.4 cp between 150 °C and 500 °C, analogous with 1.8–0.3 cp for water between 0 °C and 100 °C. Other thermo-physical properties, such as thermal conductivity, density and linear thermal expansion, were also determined here.     

Heat dissipation performance of silicon solar cells by direct dielectric liquid immersion under intensified illuminations

A novel cooling method for the solar cells under concentrated solar flux is proposed where the surplus heat is removed from both the front and back surfaces of the module by directly immersing the cells in a dielectric liquid. The direct-contact heat transfer and comparatively larger heat dissipation surface area can achieve a fairly low cell temperature which results in higher sunlight conversion efficiencies. Heat dissipation performance of the modules of both simulation sheets and solar cells were studied under the conditions of an irradiance of 50 and 70 kW/m². In these studies a long-arc xenon lamp was used as the illumination source and dimethyl-silicon oil was used as the dielectric fluid. Experimental results show that in turbulent flow, the temperature distribution of the module along the flow direction is quite uniform, resulting in a rise of about 3 °C. The cell temperature can be cooled to about 30 °C and the corresponding heat transfer coefficient is around 1000 W/(m² °C). The liquid inlet temperature does not significantly change the distribution of the module temperature, but it has a linear relationship with the average module temperature. After liquid immersion, the open-circuit voltages of the modules have small changes but large drops are in the short circuit currents. The electrical performance of the modules immersed in the liquid fits reasonably well with the relationships with the operating temperatures and thermal loads, but clearly with some degradation. The main reason for these is because the usage of common silicon cells under concentrations.     

The effect of annealing temperature on the electrochromic properties of nanostructured NiO films

The nickel oxide (NiO) films have been prepared by sol-gel dip coating route. They were treated at different temperatures ranging from 250 to 450 °C at an optimum number of layers (8 layers) on a conducting substrate (FTO) and glass plate. The X-ray diffraction (XRD) spectrum reveals that the crystallanity increases along the planes (1 1 1), (2 0 0) and (2 2 0) and the crystallite size increases from 7 to 26 nm as the temperature increased from 250 to 450 °C. The FTIR spectrum confirms the formation of Ni-O bond. The SEM images indicate the formation of nanorods in the temperature range 350-450 °C. The electrochromic properties have been studied using cyclic voltammetric (CV) and chronoamperometric (CA) techniques. The Ni(II)/Ni(III) transformation is the possible cause for the reversible colour change from transparent to dark brown. The film prepared at 300 °C with a thickness of 306 nm exhibits maximum anodic/cathodic diffusion coefficient of 11×10⁻¹² cm²/s/6.44×10⁻¹² cm²/s and the same film exhibits the maximum colour change of 68% with a photopic contrast ratio of 5.17. The chronoamperogram reveals that the colouration/decolouration response time decreases with the increase in preparation temperature. The films treated at 300 °C exhibit the optimum electrochromic behaviour.     

Novel heat dissipation design incorporating heat pipes for DC combiner boxes of a PV system

In a Photovoltaic (PV) system, heat is generated by an operating diode. Because DC combiner boxes are waterproof, dustproof, air tight and made of heat-insulating material, thermal energy is easily accumulated, affecting the performance and safety of power cables and other electronic components near the diodes in the DC combiner box. This study utilizes a heat pipe as a channel for heat dissipation to conduct the heat out of a DC combiner box without destroying the air-tightness of the box. An existing DC combiner box was improved upon using this method of heat dissipation. The measured heat flow and temperature demonstrate that the proposed method is feasible. The influence of the condensation section temperature on the maximum heat transfer of the heat pipe was also investigated by experiment. The maximum heat transfer rate of the heat pipe was found to increase with the condensation section temperature of the heat pipe. When the condensation temperature was 20 °C, 30 °C and 40 °C, the maximum heat transfer rate of the heat pipe was 21.6 W, 29.6 W and 39.7 W, respectively.     

Surface-induced thermal decomposition of [Ru(dcbpyH)2-(CN)2] on nanocrystalline TiO2 surfaces: Temperature-dependent infrared spectroscopy and two-dimensional correlation analysis

Thermal degradation mechanism of the self-assembled thin films of [Ru(dcbpyH)₂-(CN)₂] (Ruthenium 505, R505) anchoring on TiO₂ surfaces via its carboxylate group has been examined by temperature-dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The CN stretching bands of R505 at 2000-2100 cm⁻¹ appeared to change drastically at ≈140 °C on TiO₂ surfaces, whereas a major CN peak at ∼2090 cm⁻¹ disappeared at a much higher temperature above ≈250 °C in their solid states. Two-dimensional (2D) correlation analysis was introduced to explain the thermal desorption behaviors of the Ruthenium dye. Multiple peaks of the CN stretching vibrations are more clearly resolved in the 2D correlation analysis. More complicated features in the CN stretching vibrational spectra on TiO₂ than those of the solid states suggest a substantial interaction of the CN groups with the TiO₂ surfaces.     

Procedure for thermal performance evaluation of steam generating point-focus solar concentrators

Solar energy can be used for substitution of the depleting fossil fuels in thermal applications and electricity generation through thermal route. For medium and high temperature applications, solar concentrators are required. Proper sizing and selection of concentrator for any thermal application calls for characterization of the concentrator at the required operating temperature. There are few procedures reported in literature for testing and evaluating solar concentrator performance which are based on sensible heating of the working fluid. One of the limitations of these procedures is requirement of precise operating conditions during testing. A test procedure for characterization of point-focus steam generating solar concentrators based on latent heating at different operating temperatures is proposed. The proposed procedure uses the phase change characteristic of water at constant temperature to measure the thermal performance. This procedure can be used to estimate thermal efficiency of solar concentrator at different operating temperatures above 100 °C. This procedure was used to estimate the efficiency of a point-focus solar concentrator having 25 m² aperture area at 161 °C (equivalent to 5.4 bar (g)). The efficiency was estimated as 47 ± 3.5%. The test procedure can be used for field evaluation of existing systems also with minimum amount of instrumentation and controls.     

Amplified radiative cooling via optimised combinations of aperture geometry and spectral emittance profiles of surfaces and the atmosphere

Net thermal radiation cooling, from surfaces at sub-ambient temperatures, to the night sky is amplified if the aperture to the sky is partially blocked with heat mirrors. The temperature at which radiation loss stagnates (the effective sky temperature) falls continuously as the aperture closes and is derived in terms of the aperture size and the spectral properties and temperatures of the atmosphere and of the emitting surface. Cooling surfaces must have high absorptance between 7.9 μm and 13 μm where the atmosphere is most transparent. The best response for the remainder of the Planck radiation spectrum surprisingly switches between two spectral extremes at a temperature which falls as the aperture gets smaller. A perfect absorber is best above this switch, while surfaces which reflect all of this radiation are best below it. A simple formula is presented for the cross-over temperature as a function of aperture size. With known material properties plus representative non-radiative heat gains a high emittance surface is generally better except when heat mirrors are not used. A known high emittance roof paint at 10° C below ambient, under a 45° aperture lined with shiny aluminium, can achieve a net output power near 135 W m⁻² under a clear sky.     

Study on start-up characteristics of loop heat pipe under low-power

A series of experiments based on a flat loop heat pipe under low-power have been conducted in this work. It is found that the LHP can start up unfailingly under low heat power (Q = 6 W) and its thermal resistance is rather high in some cases of low-power. With the thickness of the sintered capillary interlayer increasing, the performance of start-up in the LHP becomes better because heat leak to compensation chamber is reduced and the temperature difference between compensation chamber and the evaporation room is increased. When compensation chamber in the LHP has a heat sink installed, the temperature difference and pressure difference between compensation chamber and evaporation room are augmented. As a result, it is beneficial to the improvement of start-up characteristics.     

A new version of a solar water heating system coupled with a solar water pump

This research target was to improve the thermal efficiency of a solar water heating system (SWHS) coupled with a built-in solar water pump. The designed system consists of 1.58-m² flat plate solar collectors, an overhead tank placed at the top level, the larger water storage tank without a heat exchanger at the lower level, and a one-way valve for water circulation control. The discharge heads of 1 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 10–18 kPa. It was found that water circulation within the SWHS ranged between 15 and 65 l/d depending upon solar intensity and discharge head. Moreover, the max water temperature in the storage tank is around 59 °C. The max daily pump efficiency is about 0.0017%. The SWHS could have max daily thermal efficiency of about 21%. It is concluded that the thermal efficiency was successfully improved, except for the pump one. The new SWHS with 1 m discharge head or lower is suitable for residential use. It adds less weight to a building roof and saves electrical energy for a circulation pump. It has lower cost compared to a domestic SWHS.     

Water immersion cooling of PV cells in a high concentration system

Temperature control of solar cells at high concentrations is a key issue. Short-term efficiency drop and long-term degradation should be avoided by effective cooling methods. Liquid immersion cooling eliminates the contact thermal resistance of back cooling and should improve cell performance. A 250X dish concentrator with two-axis tracking was utilized to evaluate a new CPV system using de-ionized water for immersion cooling. Time-dependent temperature distributions of the PV module of high power back point-contact cells were measured, as well as the I-V curves. The cooling capacities of the liquid immersion approach are very favorable. The module temperature can be cooled to 45 °C at a 940 W/m² direct normal irradiance, 17 °C ambient temperature and 30 °C water inlet temperature. The temperature distribution of the module is quite uniform, but the electrical performance of the cell module degrades after a fairly long time immersion in the de-ionized water.     

Anode-supported micro tubular SOFCs for advanced ceramic reactor system

In this study, micro tubular SOFCs under 1 mm diameter have been fabricated and investigated at 450–550 °C operating temperature with H₂ fuel. The performance of the 0.8 mm diameter tubular SOFC was 110–350 mW cm⁻² at 450–550 °C operating temperatures. To maximize the performance of the cell as well as to optimize the geometry of tubular cells, a current collecting method used in the experiment was examined. A model was proposed to estimate the loss of performance for single cell due to the current collecting method as functions of anode tube length and thickness. The results showed that the losses of performance were calculated to be 0.8, 2.0, and 4.6% at 450, 500, and 550 °C operating temperatures, respectively, for the 0.8 mm diameter tubular SOFC with the length of 1.2 cm.