Spray cooling on micro structured surfaces

Spray cooling experiments have been performed on a smooth heater surface. The working fluid water was atomized through a full cone nozzle into a low pressure spray chamber. Different coolant mass fluxes were studied by varying the distance between nozzle and heater. Observations obtained with an infrared camera indicated that the length of the three phase contact line increased as the heat flux in the experiments was raised. Motivated by this finding, the effect of different micro structured surfaces, which lead to an increase in the three phase contact line, on the spray cooling performance was studied. The micro structures consisted of micro pyramids with different heights. A significant enhancement in the heat transfer performance due to the surface structures could be observed, especially at low coolant fluxes.Additionally, high spatial resolution temperature measurements on a smooth heater using thermochromic liquid crystals were obtained. These measurements indicate high local temperature gradients for a regime where the coolant film on the heater is ruptured.     

Performance of different structured surfaces in nucleate pool boiling

Several structured surfaces have been developed in-house for the augmentation of boiling heat transfer using distilled water as test fluid under atmospheric pressure. These surfaces have either a number of parallel tunnels or orthogonally intersecting tunnels. Effect of design parameters like tunnel inclination and different cavity structure at the tunnel base on the boiling heat transfer has been investigated. Three different structures namely circular groove, rectangular groove and rounded base have been used at the end of the tunnels. Heat flux is varied in the range of 0–250 kW/m². Experimental results showed tunnels inclined at an angle 60° with the horizontal provide better augmentation compared to straight vertical tunnels. Amongst different base geometry the circular pocket produced most conducive condition for the boiling heat transfer. The use of tunnels also increases the degree of augmentation. The highest augmentation was obtained from the surface having intersecting inclined tunnels with a circular base.     

A simple model for PV module reflection losses under field conditions

PV module power ratings are determined at standard test conditions, which require perpendicular incident light. Under field conditions larger incidence angles occur, resulting in higher reflection losses than accounted for in the nominal power rating. In this article we will present a model to take these losses into account, and discuss some results for practical situations. From our model we conclude that the reflection losses relative to STC are determined mainly by the air glass-interface. Limited validation assuming certain spectral losses showed a rough correspondence between calculated reflection losses and experimental values on a yearly averaged basis (1.2% difference between model and experiment). Model calculations show that for modules faced towards the equator, and with a tilt angle equal to the latitude, yearly reflection losses relative to STC are about 3%. For this tilt and orientation, the losses seem to be only slightly dependent on the geographical latitude of the location. Tilt, orientation and seasonal dependence are significant. For vertically mounted PV modules (facades) near the equator the reflection losses can be quite large (up to 8%)     

Enhanced boiling heat transfer simulation from structured surfaces: Semi-analytical model

A semi-analytical model of the bubble dynamics is proposed based on the experimental results reported in the literature on boiling from porous enhanced surfaces. The model considers the ‘flooded mode’ regime of enhancement boiling and is validated for data covering a range of tunnel and pore dimensions. The dynamic model accounts for the temporal evaporation rate variation inside tunnels to arrive at the latent heat flux due to internal evaporation and frequency of bubble formation. The population density is predicted using an empirical formulation, and in turn used to estimate the total heat flux from the porous enhanced surface. The model predicts the heat flux for pool boiling from structured surfaces within ±30% of the experimental data. The model is subsequently used in the prediction of the thermal performance of a novel two-phase heat spreader that employs porous structured surfaces for enhancing boiling heat transfer.     

Reduction of optical losses in colored solar cells with multilayer antireflection coatings

Solar modules are becoming an everyday presence in several countries. So far, the installation of such modules has been performed without esthetic concerns, typical locations being rooftops or solar power plants. Building-integrated photovoltaic (BIPV) systems represent an interesting, alternative approach for increasing the available area for electricity production and potentially for further reducing the cost of solar electricity. In BIPV, the visual impression of a solar module becomes important, including its color. The color of a solar module is determined by the color of the cells in the module, which is given by the antireflection coating (ARC). The ARC is a thin film structure that significantly increases the amount of current produced by and, hence, the efficiency of a solar cell. The deposition of silicon nitride single layer ARCs with a dark blue color is the most common process in the industry today and plasma enhanced chemical vapor deposition (PECVD) is mostly used for this purpose. However, access to efficient, but differently colored solar cells are important for the further development of BIPV. In this paper, the impact of varying the color of an ARC upon the optical characteristics and efficiency of a solar cell is investigated. The overall transmittance and reflectance of a set of differently colored single layer ARCs are compared with multilayered silicon nitride ARCs, all made using PECVD. These are again compared with porous silicon ARCs fabricated using an electrochemical process allowing for the rapid and simple manufacture of ARC structures with many tens of layers. In addition to a comparison of the optical characteristics of such solar cells, the effect of using colored ARCs on solar cell efficiency is quantified using the solar cell modeling tool PC1D. This work shows that the use of multilayer ARC structures can allow solar cells with a range of different colors throughout the visual spectrum to retain very high efficiencies.     

Spray cooling of enhanced surfaces: Impact of structured surface geometry and spray axis inclination

Experiments were conducted to study the effects of enhanced surfaces and spray inclination angle (the angle between the surface normal and the axis of symmetry of the spray) on heat transfer during spray cooling. The surface enhancements consisted of cubic pin fins, pyramids, and straight fins. These structures were machined on the top surface of heated copper blocks with 2.0 cm² cross-sectional areas. Measurements were also obtained on a heated flat surface to provide baseline data. PF-5060 was used as the working fluid. The spray was produced using a 2 × 2 nozzle array under nominally degassed conditions (chamber pressure of 41.4 kPa) with a volume flux of 0.016 m³/m² s and a nozzle height of 17 mm. The spray temperature was 20.5 °C. For the geometries tested, the straight fins had the largest heat flux enhancement relative to the flat surface, followed by the cubic pin fins and the pyramid surface. Each of these surfaces also indicated an increase in evaporation efficiency at CHF compared to the flat surface. Inclination of the spray axis between 0° and 45° relative to the heater surface normal created a noticeable increase in heat flux compared to the normal position (0° case). A maximum heat flux enhancement of 23% was attained for the flat surface. The straight finned surface had a maximum heat flux enhancement of 75% at an inclination angle of 30° relative to the flat surface in the normal position. However, only a marginal increase (11%) was observed in comparison to the straight finned surface in the normal position (0° case).     

Directional characteristics of radiation reflection on rough metal surfaces with description of heat transfer parameters

In this paper we clarify directional characteristics of thermal radiation reflection on rough metal surfaces and establish a technique for determining the parameters for heat transfer computation of radiation energy exchange among surfaces. Directional distribution of bidirectional reflectance ρ of surfaces of root‐mean‐square roughness Σ = 0.1 to 1 µm to the irradiation of a visible laser of wavelength λ = 0.6328 µm and to that of an infrared laser of λ = 3.39 µm is investigated experimentally. The optical roughness (Σ/λ) ranges from 0.028 to 1.27. A measure of the magnitude of specular reflection is presented. A model for describing the ρ distribution is presented, and the experimental results of the ρ distribution are analyzed quantitatively and systematically to determine the values of the specular reflection component R_s and the perfect‐diffuse reflection component R_d of the hemispherical reflectance R_H, which are input parameters for radiation heat transfer computation. © 2002 Scripta Technica, Heat Trans Asian Res, 31(2): 76–88, 2002; DOI 10.1002/htj.10008     

Experimental investigation of flow boiling heat transfer of jet impingement on smooth and micro structured surfaces

Flow boiling heat transfer experiments using R134a were carried out for jet impingement on smooth and enhanced surfaces. The enhanced surfaces were circular micro pin fins, hydrofoil micro pin fins, and square micro pin fins. The effects of saturation pressure, heat flux, Reynolds number, pin fin geometry, pin fin array configuration, and surface aging on flow boiling heat transfer characteristics were investigated. Flow boiling experiments were carried out for two different saturation pressures, 820 kPa and 1090 kPa. Four jet exit velocities ranging from 1.1–4.05 m/s were investigated. Flow boiling jet impingement on smooth surfaces was characterized by large temperature overshoots, exhibiting boiling hysteresis. Flow boiling jet impingement on micro pin fins displayed large heat transfer coefficients. Heat transfer coefficients as high as 150,000 W/m² K were observed at a relatively low velocity of 2.2 m/s with the large (D = 125 μm) circular micro pin fins. Jet velocity, surface aging, and saturation pressure were found to have significant effects on the two-phase heat transfer characteristics. Subcooled nucleate boiling was found to be the dominant heat transfer mechanism.     

The contribution of the thermal radiation of the inner surfaces of a room on its thermal losses

The temperature difference between the surfaces of the internal and the external walls of a room causes a heat flux from the internal to the external walls through radiation and finally to the environment through conduction. This thermal flux, which is different to the thermal losses due to the conductance of the external walls, is a function of the thermal and the structural characteristics of the walls. In the present work a simple analytical expression is given, which relates the thermal losses due to radiation and the total thermal losses due to radiation and convection directly to the structural characteristics of the room, that is the reflectivity of the internal surfaces, the conductance of the external walls, the ratio of the external to the internal wall surfaces and the temperatures of the internal space of the room and the environment. This relationship has been verified by experiments carried out on an experimental arrangement which simulates this phenomenon.     

Pressure signal reflection at free liquid surfaces and other non-standard boundaries in liquid reservoir testing

The presence of a free liquid surface in a liquid dominated reservoir generates a special type of boundary condition that is of the convolution type. The mathematical aspects of the pressure signal reflection by this kind of boundary condition is discussed in some detail. We conclude that with regard to short-term effects the free surface boundary can be approximated by a rigid boundary.     

Ultrashort-pulse laser structured titanium surfaces with sputter-coated platinum catalyst as hydrogen evolution electrodes for alkaline water electrolysis

For the first time, we report on micro- and nanostructured Ti surfaces produced by ultrashort-pulse laser processing followed by sputter deposition of Pt aiming at efficient cathode electrodes for alkaline water electrolysis. We studied the laser processing-induced surface morphology, the elemental composition of the surface, the specific surface increase, the wetting behavior as well as the activity of the hydrogen evolution reaction. It is demonstrated that ultrashort-pulse laser structuring in combination with thin layer catalyst deposition can dramatically boost the performance of cathodes for the hydrogen evolution reaction due to the enormous increase in specific surface in combination with superhydrophilic and superwetting properties leading to a rapid gas bubble detachment.     

Lifetime analysis of silicon solar cells by microwave reflection

Microwave photoconductive decay (μPCD) has become a standard technique for measuring the carrier lifetime of silicon used in solar cells. Here, we have used μPCD to examine the carrier lifetimes at common doping levels used in the base region of silicon photovoltaic devices. For the conductivity range used in the p-type base of n⁺–p structures, the microwave penetration depth is less than the wafer thickness. In this case, the reflectance–conductivity relationship is very nonlinear. We will show that quasi-steady-state photoconductivity (QSSPC) and resonance-coupled photoconductive decay (RCPCD) lifetime measurements track over a wide range of injection level, and generally agree at higher injection levels. Our μPCD data will be compared with the transient RCPCD data over the same range. The data from the latter agree at low-injection levels, but show serious disagreement at higher injection levels. The conclusion is that μPCD must be limited to low-injection levels in the doping range used for solar cells.     

Methods to decrease losses of energy generated by solar electrical modules

It is shown that the energy loss of blocking diodes decreases if some part of a solar module surface is shadowed. A powerful blocking diode for solar modules based on the silicon n ⁺ n-pp ⁺ structure is suggested.     

Estimation of steady state ground losses from earth coupled structures by simulation

A thermal simulation method for estimating the ground losses from earth coupled structures has been described. The accuracy of the method has been evaluated by comparing the experimental results obtained for a sphere with the corresponding analytical results. The method has been used to determine the shape factor, which can be used to evaluate the heat loss to the ground for (i) slab-on-grade buildings, (ii) a sunken cylindrical structure, and (iii) sunken and buried parallelepiped structures for different ratios of their dimensions.     

Optimization of cell to module conversion loss by reducing the resistive losses

Conventional silicon based photovoltaic industries, which convert silicon cells to modules attracted the attention of the researchers due to the cell to module conversion power losses. The conversion power losses are due to the various parameters such as shadow effect, inherent properties of the solar cells, properties of the materials used for fabricating the module etc. Among them, the inherent properties of the solar cells play a major role in module power. The electrical properties of the solar cell such as series resistance and fill factor drive the conversion power losses in the module. By increasing the number of contact points, the losses will be reduced. In this work it is found that by introducing an extra bus bar in metallization pattern leads to a great reduction in conversion losses. The fill factor gain is observed in three bus bar based modules compared to two bus bar based modules because of the contact points per cell increase which lead to low resistance losses. It is obvious that the power gain in three bus bar modules dominates shadowing loss due to an additional bus bar. A systematic approach on minimising the cell to module conversion loss by optimizing the front contact bus bar width has been studied. It is important that beyond the optimum value of bus bar numbers and its width the shadowing loss will dominate over the gain.     

Enhancement of ground radiation in greenhouses by reflection of direct sunlight

Substantial gains in greenhouse ground illumination/irradiance were obtained with a reflecting wall that was positioned so as to reflect the incoming sunlight from the northern face of the greenhouse to its ground. The effect was particularly significant in winter when the sun was low throughout the day and most of the radiation passed through the greenhouse so that its ground radiant power was reduced.Four types of greenhouses with reflecting walls exhibiting increasing efficiencies of insolation were developed. A theory describing a several-fold enhancement of ground irradiance and radiant energy was developed. Effects of sky (diffuse) radiation on clear and cloudy days were also considered.An experimental model, as well as a full-scale greenhouse with a reflecting wall with provision for measuring ground illumination was constructed and exposed to the sun: Actual measurements were conducted over several days, supporting the theoretical calculations.     

The ignition of gases by rapidly heated surfaces

A surface heated by a frictional impact was simulated by the discharge of a bank of capacitors through a strip of tungsten foil. The resultant rise and fall of temperature were investigated by means of a two-color pyrometer. The heated foil was exposed to various concentrations of flammable gases, and the probability of ignition was investigated as a function of several variables, including gas concentration, temperature profile, and foil size. It was found that the peak temperature necessary for ignition decreased as the width of the foil was increased, and that 7% was the most easily ignitable concentration of methane in air. The size of the heated surface was of the order of 200 mm² and this necessitated a working range of peak temperatures of approximately 1500°C to 2200°C in the study of the ignition of methane-air. Some preliminary experiments indicated that propane-air could be ignited at much lower temperatures.     

Experimental investigations of frost release by hydrophilic surfaces

Frost formation occurs when water vapor in the surrounding air comes into contact with cold surfaces through heat and mass transfer. It is usually an undesirable phenomenon in most refrigeration and cryogenic systems. A few studies have shown that changing the surface energy, such as increasing the surface hydrophilicity or hydrophobicity, has significant effects on frost growth. In this paper, a kind of hydrophilic polymer paint is formulated to counteract frost deposition on cold surfaces. The coated surface can retard frost formation up to three hours under low plate temperatures (− 15.3°C) and high air humidity (72%). To test the antifrosting performance of the hydrophilic paint under more practical conditions, it is applied to a fin-and-tube heat exchanger and a domestic refrigerator at a coating thickness of 30 μm. Comparisons of frost deposition, pressure drops, and outlet temperatures are made between uncoated and coated heat exchangers. Under conditions of high air temperature (2.2°C) and relative high air humidity (90%), the paint prolongs the defrosting interval from 80 to 137 min. Experimental observations also show that the coated hydrophilic fins are free of frost deposition during the entire course of the test and that the coating has no significant additional thermal resistance.     

Improving thermal efficiency by reducing cooling losses in hydrogen combustion engines

Hydrogen can be readily used in spark ignition engines as a clean alternative to fossil fuels. However, the higher burning velocity and shorter quenching distance of hydrogen compared with hydrocarbons cause a larger heat transfer from the burning gas to the combustion chamber walls. Because of this cooling loss, the thermal efficiency of hydrogen-fueled engines is sometimes lower than that of conventionally fueled engines. Therefore, reducing the cooling loss is a crucial element in improving the thermal efficiency of hydrogen combustion engines. Previous research by the author and others has proposed the direct injection stratified charge as a technique for reducing the cooling loss in hydrogen combustion and shown its effect in reducing cooling loss through experiments in a constant volume combustion vessel. However, it is known that a reduction in cooling loss does not always improve thermal efficiency due to a simultaneous increase in the exhaust heat loss. This paper explains the relation between cooling loss reduction and thermal efficiency improvements by the direct injection stratified charge in hydrogen combustion engines.