Convection heat and mass transfer along a vertical heated plate with film evaporation in a non-Darcian porous medium

A numerical analysis has been carried about to study the heat and mass transfer of forced convection flow with liquid film evaporation in a saturated non-Darcian porous medium. Parametric analyses were conducted concerning the effects of the porosity ε, inlet liquid Reynolds number Re_l, inlet air Reynolds number Re_a on the heat and mass transfer performance. The results conclude that better heat and mass transfer performances are noticed for the system having a higher Re_a, a lower Re_l, and a higher ε. Re_l plays a more important role on the heat and mass transfer performance than Re_a and ε. For the case of ε = 0.4 and Re_a = 10,000, the increases of Nu and Sh for Re_l = 50 are about by 33.9% and 35.3% relative to the values for Re_l = 250.     

New organic photosensitizers incorporating carbazole and dimethylarylamine moieties for dye-sensitized solar cells

Four organic dyes (XS10–13) employing carbazole unit as electron donor and N,N-dimethylarylamine moieties as electron-donating groups were designed and synthesized for nanocrystalline TiO₂ dye-sensitized solar cells. The electron-donating groups of dimethylarylamine increase the electron density of donor moiety and enhance the molar extinction coefficient of dyes. For a typical device the maximum IPCE value could reach 86%, with a short-circuit photocurrent density (J_sc) of 9.8 mA cm^?2, an open-circuit photovoltage (V_oc) of 642 mV, and fill factor (FF) of 0.63, which corresponds to an overall conversion efficiency (η) of 4.0%. For a comparison, the N719-sensitized TiO₂ solar cell showed an efficiency of 6.4%.     

Effect of ultra-thin polymer membrane electrolytes on dye-sensitized solar cells

In-situ ultra-thin porous poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF–HFP) membranes were prepared by a phase inversion method on TiO₂ electrodes coated with Ru N-719 dye. These membranes were then soaked in the organic liquid electrolyte to form the in-situ ultra-thin porous P(VDF–HFP) membrane electrolytes. Dye-sensitized solar cell (DSC) using the membrane electrolyte exhibited an open-circuit voltage (V_oc) of 0.751 V, a short-circuit current (J_sc) of 16.260 mA cm^?2 and a fill factor (FF) of 0.684 under an incident light intensity of 1000 W m^?2 yielding an energy conversion efficiency (η) of 8.35%. The V_oc, FF and η of the solar cell using the membrane electrolyte increased by about 5.8%, 2.2% and 5.7%, respectively, when compared with the corresponding values of a cell using liquid electrolyte. However, the J_sc decreased by about 2.1%.     

Synthesis of triarylamines with secondary electron-donating groups for dye-sensitized solar cells

Three organic dyes XS24–26 containing N,N-dimethylaniline and butoxybenzene have been designed, synthesized and applied in the dye-sensitized solar cells (DSSCs). The influence of secondary electron-donating groups on the performance of DSSCs is discussed. The dimethylaniline is beneficial to extend absorption spectrum, whereas butoxybenzene is useful to suppress electron recombination. XS26 containing butoxybenzene and thiophene unit gives the highest power efficiency η of 5.67%, with a J_SC of 12.36 mA cm^?2, V_OC of 680 mV, and ff of 0.67.     

A theoretical study on area compensation for non-directly-south-facing solar collectors

Solar energy integrated with the building is an important approach for the synchronous development of solar energy and architecture. The energy gain of the solar collector integrated with the pitched roof has been greatly influenced by the roof azimuth and tilted angle. Investment cost of the collectors is mainly decided by the size of the collector area. Accordingly, it is significant for solar building design to economically determinate the area compensation of the solar collector at different azimuth and tilted angles. Take Kunming and Beijing as examples, area compensation for the flat-plate tube-fin solar collector used in southern regions and the evacuated tube collector with cylindrical absorbers used in northern regions in China have been theoretically calculated. The results to some extent show that the daily horizontal solar radiation, ambient temperature, the azimuth and tilted angle of the collector integrated into the roof have an influence on the area compensation. The azimuth angle and tilted angle of the roof are the main factors that influence the A/A₀, which is defined as the collector area ratio of the non-south-facing collectors to the south-facing ones with the optimal tilted angle. Comparative studies found that the range of A/A₀ for the evacuated tube collector used in the northern regions is close to that for the flat-plate tube-fin solar collector used in the southern regions. When the pitched roof tilted angle β ∈ [25°, 45°] and the azimuth angle ∣γ∣ ⩽ 30°, the collectors can intercept a lot of solar radiant-energy. Considering the economic situations of the ordinary consumers in China, the optimal area compensation A/A₀ ⩽ 1.30 is recommended in this paper.     

Novel quasi-solid electrolyte for dye-sensitized solar cells

A novel alkyloxy-imidazole polymer was prepared by in situ co-polymerization of alkyloxy-imidazole and diiodide to develop an ionic polymer gel electrolyte for quasi-solid dye-sensitized solar cells (DSCs). The DSCs with the polymer gel electrolyte of 1-methyl-3-propylimidazolium iodide (MPII) showed good photovoltaic performance including the short-circuit photocurrent density (J_sc) of 3.6 mA cm⁻², the open-circuit voltage (V_oc) of 714.8 mV, the fill factor (FF) of 0.60 and the light-to-electricity conversion efficiency (η) of 1.56% under AM 1.5 (100 mW cm⁻²). As a comparison, the DSCs with the polymer gel electrolyte of 1,2-dimethyl-3-propylimidazolium iodide (DMPII) yielded a light-to-electricity conversion efficiency of 1.33%. The results indicated that the as-prepared polymers were suitable for the solidification of liquid electrolytes in DSCs.     

New approach relevant to total heat transfer coefficient including the effect of radiation and convection at the ceiling in a cooled ceiling room

In this study, radiative and convective heat transfer coefficients at the ceiling are determined for a cooled ceiling room. Firstly, convective heat transfer is simulated numerically neglecting the radiative heat transfer at the surfaces (ε_f = ε_w = ε_c = 0), then, radiative heat transfer is calculated theoretically for different surface emissivities (ε_f = ε_w = ε_c = 0.5, 0.6, 0.7, 0.8 and 0.9) for different room dimensions (3 × 3 × 3, 4 × 3 × 4 and 6 × 3 × 4 m) and thermal conditions (T_f = 25 °C, T_w = 28–36 °C and T_c = 0–25 °C). Numerical data is compared with the results of correlations based on experimental data given in literature. New equations related to convective and total (including the effect of convection and radiation) heat transfer coefficients for ceiling are found in the current study.     

Testing of solar cookers and evaluation of instrumentation error

Solar cooking technologies have large potential in developing countries. Many of the solar cookers (particularly box type and parabolic concentrating type solar cookers) have been commercialized in different parts of the world. An effective quality control is essential for a large-scale dissemination of solar thermal technologies on the products being offered by the industry to the end users. For this, there is a need to establish test procedures and methodologies for developing performance characteristic parameters, which could provide an equitable basis for comparison of performances of the products. A comprehensive review of various test procedures of solar cookers has been undertaken in this study.This study presents results of using various test procedures for characterizing box type and a family size parabolic concentrator solar cooker, based on detailed experimental investigations. The study is supported by a number of experiments carried at the location of New Delhi (latitude = 28.56°N, longitude = 77°E) under various climatic and operating conditions round the year. The overall error associated in the determination of performance parameters due to instrumentation has been estimated by using the root-sum square method. It has been estimated that instrumentation cause 1–5.5 percent error on the thermal performance parameters of solar cookers. The effect of instrumentation error has been evaluated maximum on second figure of merit, F₂, optical efficiency factor, F′η_o, and standardized cooking power P_s.     

Improvement of polymer/fullerene solar cells by controlling geometry of the ITO substrate surface

We have found that the short-circuit current, J_sc, of polymer/fullerene [RR-P3HT/C₆₀] solar cells has a clear dependence on the surface roughness of the ITO/glass substrate. We prepared an ITO surface with an average roughness, R_a, of 0.7–11 nm by chemical etching. At first J_sc increases with the increase in ITO surface roughness and then gradually decreases. The maximum performance was obtained at R_a≈4 nm. J_sc is also high with a very flat surface of R_a=0.7 nm. This feature can be attributed to the trade-off between the increase in absorption light path length and film-quality deterioration.     

Optimal design of geometric parameters of double-layered microchannel heat sinks

This work uses an optimization procedure consisting of a simplified conjugate-gradient method and a three-dimensional fluid flow and heat transfer model to investigate the optimal geometric parameters of a double-layered microchannel heat sink (DL-MCHS). The overall thermal resistance R_T is the objective function to be minimized, and the number of channels N, channel width ratio β, lower channel aspect ratio α_l, and upper channel aspect ratio α_u are the search variables. For a given bottom area (10 × 10 mm) and heat flux (100 W/cm²), the optimal (minimum) thermal resistance of the double-layered microchannel heat sink is about R_T = 0.12 °C/m²W. The corresponding optimal geometric parameters are N = 73, β = 0.50, α_l = 3.52, and, α_u = 7.21 under a total pumping power of 0.1 W. These parameters reduce the overall thermal resistance by 52.8% compared to that yielded by an initial guess (N = 112, β = 0.37, α_l = 10.32, and α_u = 10.93). Furthermore, the optimal thermal resistance decreases rapidly with the pumping power and then tends to approach an constant value. As the pumping power increases, the optimal values of N, α_l, and α_u increase, whereas the optimal β value decreases. However, increasing the pumping power further is not always cost-effective for practical heat sink designs.     

Light induced degradation in nanocrystalline Si films and related solar cells: Role of crystalline fraction

Silicon thin films with different crystalline volume fractions have been deposited at different power and pressure conditions. Structural properties of the films have been investigated. The effects of crystalline volume fractions and grain sizes on the degradation of photoconductivity have been studied. Single-junction solar cells have been fabricated with protocrystalline and nanocrystalline Si as absorber layer. Protocrystalline silicon solar cells show less than 1% degradation upto 50 h of light soaking. Then the cells degrade upto 500 h and thereafter become steady. Nanocrystalline solar cells show degradation initially and become steady after 10 h of light soaking. Using protocrystalline silicon as absorber layer the solar cell efficiency degrades 9% before stabilization, whereas using nanocrystalline silicon as absorber layer (X_c～65%) the solar cell efficiency degrades 2.9%. Stabilized efficiency of the second type of cell is better than that of the first cell, but initial efficiency is higher for the first cell (η=7.1%).     

0.4% absolute efficiency gain by novel back contact

Replacing the aluminum back contact of screen-printed multicrystalline silicon solar cells by a novel low-temperature layer sequence boosts the absolute power conversion efficiency η by Δη=0.4%. The optimized hydrogenated amorphous silicon (a-Si:H)-based back side junction provides efficient back side passivation and contacting at the same time. The improved passivation quality reduces the effective surface recombination velocity S_eff to S_eff<20 cm s^?1. Due to the optimized back side layer sequence, the open circuit voltage V_OC rises by ΔV_OC=15 mV up to V_OC=622 mV and the short circuit current increases by ΔJ_SC=0.8 mA cm^?2.     

Homogeneous p+ emitter diffused using boron tribromide for record 16.4% screen-printed large area n-type mc-Si solar cell

A record efficiency of 16.4% (156.25 cm²) has been achieved for an n-type wafer-based (hereafter, “n-based”) mc-Si solar cell. A horizontal quartz tube furnace with an industry-compatible scale is employed for forming a p⁺-emitter using boron tribromide (BBr₃) as the boron source, in which system less contamination is confirmed than in other options of boron diffusion. A significantly homogeneous emitter is achieved with the standard deviation of 1.5 Ω/sq. n-Based mc-Si solar cells are fabricated with phosphorus-diffused BSF, SiN deposition, and fire-through screen-printed contacts. The properties of the best cell are; η: 16.4%, V_oc: 607 mV, J_sc: 35.2 mA/cm², and FF: 76.7%.     

Maximum-power-point tracking control of solar heating system

The present study developed a maximum-power point tracking control (MPPT) technology for solar heating system to minimize the pumping power consumption at an optimal heat collection. The net solar energy gain Q_net (=Q_s ? W_p/η_e) was experimentally found to be the cost function for MPPT with maximum point. The feedback tracking control system was developed to track the optimal Q_net (denoted Q_max). A tracking filter which was derived from the thermal analytical model of the solar heating system was used to determine the instantaneous tracking target Q_max(t). The system transfer-function model of solar heating system was also derived experimentally using a step response test and used in the design of tracking feedback control system. The PI controller was designed for a tracking target Q_max(t) with a quadratic time function. The MPPT control system was implemented using a microprocessor-based controller and the test results show good tracking performance with small tracking errors. It is seen that the average mass flow rate for the specific test periods in five different days is between 18.1 and 22.9 kg/min with average pumping power between 77 and 140 W, which is greatly reduced as compared to the standard flow rate at 31 kg/min and pumping power 450 W which is based on the flow rate 0.02 kg/s m² defined in the ANSI/ASHRAE 93-1986 Standard and the total collector area 25.9 m². The average net solar heat collected Q_net is between 8.62 and 14.1 kW depending on weather condition. The MPPT control of solar heating system has been verified to be able to minimize the pumping energy consumption with optimal solar heat collection.     

Heat transfer on a power law stretching sheet subjected to free stream pressure gradient

The present work deals with the numerical study of temperature distribution in the laminar boundary layer driven by the stretching boundary surface subjected to pressure gradient. The similarity transformation obeying the same power law based on composite reference velocity (union of velocities of the stretching boundary and free stream) has been employed that leads to a single set of equations, irrespective of the condition whether U_w > U_∞ or U_w < U_∞, containing three parameters: β measuring the stretch rate of the moving boundary, ε is the ratio of free stream velocity to composite reference velocity and Pr is the Prandtl number of the ambient fluid. The numerical solutions of the thermal boundary layer equations are obtained for three Prandtl numbers 0.7, 1.0 and 10 for 0 ? ε ? 1 and for 0 ? β ? 2. The heat transfer coefficient show appreciable dependence on the ratio of free stream velocity to union of velocities of the stretching surface boundary and free stream.     

Experimental investigations on heat transfer and frictional characteristics of a turbulator roughened solar air heater duct

An experimental investigation has been carried out to study the heat transfer coefficient and friction factor by using artificial roughness in the form of specially prepared inverted U-shaped turbulators on the absorber surface of an air heater duct. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters.The experiments encompassed the Reynolds number range from 3800 to 18000; ratio of turbulator height to duct hydraulic mean diameter is varied from, e/D_h = 0.0186 to 0.03986 (D_h = 37.63 mm and e = 0.7 to 1.5 mm) and turbulator pitch to height ratio is varied from, p/e = 6.67 to 57.14 (p = 10 to 40 mm). The angle of attack of flow on turbulators, α = 90° kept constant during the whole experimentation. The heat transfer and friction factor data obtained is compared with the data obtained from smooth duct under similar geometrical and flow conditions. As compared to the smooth duct, the turbulator roughened duct enhances the heat transfer and friction factor by 2.82 and 3.72 times, respectively. The correlations have been developed for area averaged Nusselt number and friction factor for turbulator roughened duct.     

Twenty-two percent efficiency HIT solar cell

We have achieved the world's highest solar cell conversion efficiency of 22.3% (V_oc: 0.725 V, I_sc: 3.909 A, FF: 0.791, total area: 100.5 cm², confirmed by AIST) by using a heterojunction with intrinsic thin layer (HIT) structure. This is the world's first practical-size (>100 cm²) silicon solar cell that exceeds a conversion efficiency of 22% as a confirmed value. This high efficiency has been achieved mainly due to improvements in a-Si:H/c-Si hetero-interface properties and optical confinement.The excellent a-Si:H/c-Si hetero-interface of the HIT structure enables a high V_oc of over 0.720 V and results in better temperature properties. In order to reduce the power-generating cost, we are now investigating numerous technologies to further improve the conversion efficiency, especially the V_oc, of HIT solar cells, with the aim of achieving 23% efficiency in the laboratory by 2010.     

Thermosyphon heat-pipe evacuated tube solar water heaters for northern maritime climates

Under transient climatic conditions previous research has reported that evacuated tube solar water heaters (ETSWHs) with heat-pipe absorbers are the most effective solution for collection of solar energy. The cost of such systems is greater than the mass produced “water in glass” evacuated tube solar water heater mainly manufactured in China. Previous studies have reported that the costs of solar water heating can be reduced through the adoption of thermosyphon fluid circulation. Well designed thermosyphon systems are as effective as pumped systems but with lower capital and running costs. To investigate if costs could be reduced and performance levels maintained, outdoor testing of three thermosyphon heat-pipe ETSWHs primarily designed for pumped fluid circulation was carried out under a northern maritime climate. Experimental data from a year’s side by side monitoring of two thermosyphon ETSWHs (both with the same area of 2 m²) was collected and used to validate a correlation based on a modified version of the f-chart design tool between the observed and expected performance for both systems. The R² value between measured and predicted monthly solar fractions was greater than 0.99 for both systems. The R² value between measured and predicted diurnal solar fractions was calculated as greater than 0.95 for both systems. The only difference between the two was that one utilised internal heat-pipe condensers whilst the other used external ones. The system with internal condensers was found to be 17% more efficient. A simplifying assumption of a constant temperature rise across the collectors reduced the calculations required to predict the performance of thermosyphon heat-pipe ETSWHs and was also statistically significant. To determine if the assumption was valid for other thermosyphon heat-pipe ETSWHs with different collector parameters a third system with internal condensers an area of 3 m², a heat removal factor (F_R) of 0.816 based on the absorber area and a collector loss coefficient (F_RU_L) of 2.25 W m^?2 K^?1 was assembled and its performance monitored, when the same assumption was applied the R² value between the measured and predicted daily solar fractions was calculated as 0.96 experimentally demonstrating that this relationship was still statistically significant for another heat-pipe thermosyphon ETSWH with different collector parameters.     

Research on the shell-side thermal performances of heat exchangers with helical tube coils

This paper presents the results of experimental research on shell-side heat transfer coefficient concerning 3 heat exchangers with helical coils. Measurements were carried in laboratory and the following correlation was found to be adequate Nu = 0.50 ? Re^0.55 ? Pr ^1/3 ? (η/η_w)^0.14 where Re and Nu are based on shell-side hydraulic diameter.     

Anthocyanins and betalains as light-harvesting pigments for dye-sensitized solar cells

We present the photoelectrochemical properties of dye-sensitized solar cells using natural pigments containing betalains and anthocyanins as sensitizers. The dyes extracted from grape, mulberry, blackberry, red Sicilian orange, Sicilian prickly pear, eggplant and radicchio have shown a monochromatic incident photon to current efficiency (IPCE) ranging from 40% to 69%. Short circuit photocurrent densities (J_sc) up to 8.8 mA/cm², and open circuit voltage (V_oc) ranging from 316 to 419 mV, were obtained from these natural dyes under 100 mW/cm² (AM 1.5) simulated sunlight. The best solar conversion efficiency of 2.06% was achieved with Sicilian prickly pear fruits extract. The influence of pH and co-absorbers on natural sensitizers, were investigated and discussed.