Changes in the temperature coefficients of the characteristics of amorphous silicon solar cells subjected to light degradation and recovery

Measuring changes in temperature coefficients is an effective way to estimate the performance of a solar cell. We investigated changes in the temperature coefficients of the I–V characteristics of amorphous silicon (a-Si) solar cells subjected to light degradation and recovery. There is a good correlation between change in the temperature coefficient (Ψ) and the degradation/recovery state of a cell’s conversion efficiency (η). This relationship can be expressed by Ψ_η=−0.0052Δη−0.45. Therefore, the temperature coefficient corresponding to the degradation/recovery state can be estimated.     

Experimental investigation of silver nano-fluid on heat pipe thermal performance

Nano-fluid is employed as the working medium for a conventional 211 μm wide × 217 μm deep grooved circular heat pipe. The nano-fluid used in this study is an aqueous solution of 35 nm diameter silver nano-particles. The experiment was performed to measure the temperature distribution and to compare the heat pipe thermal resistance using nano-fluid and DI-water. The tested nano-particle concentrations ranged from 1 mg/l to 100 mg/l. The condenser section of the heat pipe was attached to a heat sink that was cooled by water supplied from a constant-temperature bath maintained at 40 °C.At a same charge volume, the measured nano-fluid filled heat pipe temperature distribution demonstrated that the thermal resistance decreased 10–80% compared to DI-water at an input power of 30–60 W. The measured results also show that the thermal resistances of the heat pipe decrease as the silver nano-particle size and concentration increase.     

Electrical and optical properties of CdTe films prepared by vacuum evaporation with close spacing between source and substrate

CdTe films for solar cell application were prepared at various growth temperatures by a vacuum evaporation system having close spacing between the source and substrate. The CdTe thin films had a cubic structure highly oriented with the (1 1 1) direction perpendicular to the substrate surface, regardless of the growth temperature. The crystallites are of random shape and reach up to about 2 μm in size with increasing growth temperature. The higher growth temperature contributed to the reduction of dark resistivity from 6×10⁷ Ω cm at room temperature to 5.4×10⁶ Ω cm at 300°C. The photovoltaic properties of the CdS/CdTe solar cell were considerably improved with the increase in the growth temperature, which was caused by the increase of (1 1 1) texture and grain size in CdTe films.     

Analytical model for the thermal conductance of double-compound honeycomb transparent insulation, with validation

Whereas early thermal models of honeycomb transparent insulation assumed the honeycomb to be bounded by opaque plates on both faces, more recent models have allowed for an air gap between the honeycomb and one of the plates: the “compound honeycomb” configuration. This paper deals with a new configuration, one that is basically the compound honeycomb configuration, but the other bounding plate is diathermous (i.e., partly transparent to long-wave radiation) rather than opaque. This new configuration has arisen in the application of honeycombs in greenhouses. This paper extends the existing compound honeycomb model, by adding a new variable and a new equation. It was found that a 9 × 9 matrix needs to be inverted rather than the 8 × 8 required by the earlier formalism. To test the model, the overall conductance across a set of transparent honeycombs resting on one of two diathermous plastics was measured, using a guarded heater plate apparatus. The honeycombs were fabricated from UV-stabilized polypropylene, and had a cell size of about 10 mm. Although the model tended to slightly over-predict the measurements (by about 10%) it is considered to be accurate enough for design purposes.     

Experimental investigation of performance of a multi-purpose PV-slat window

This paper reports on experimental investigation of performance of a new type of PV-slat window (PV-SW). The main functions of this PV-SW are as follows: to admit sufficient daylight, to act as a shading device for decreasing direct heat gain through window glazing and to ensure indoor air movement, which improves resident's thermal comfort. To assess the performance of this PV-SW, two test rooms of 1×1×1.5 m³ (H:W:L) volume were built using plywood and gypsum boards. At the first, the PV-SW of 0.5×0.6 m² surface area was located at the south-facing wall whereas the other room was equipped with a commercial transparent slat window of the same size.The PV-SW consists of six PV slats. The photovoltaic cells were connected in series giving a maximum electrical power output of 36 W (12 V×3 A). The circuit was connected to a direct current axial fan, located inside the room, that requires a maximum power of 43 W. The analysis of performance of this PV-SW was investigated based on power output, daylight factor and temperature difference between indoor and ambient.The experimental results showed that this multi-purpose PV-SW is extremely interesting as it can produce power up to 15 W, decrease indoor temperature and provide sufficient light for housing. The maximum indoor illumination was about 750 lx with slats angle of 68°. The room temperature was about 2–3^oC lower than that of room equipped with transparent slats.     

Ex situ measurements of through-plane thermal conductivities in a polymer electrolyte fuel cell

In this paper thermal properties for materials typically used in the proton exchange membrane fuel cell (PEMFC) are reported. Thermal conductivities of Nafion membranes were measured ex situ at 20 °C to be 0.177 ± 0.008 and 0.254 ± 0.016 W K⁻¹ m⁻¹ for dry and maximally wetted membranes respectively. This paper also presents a methodology to determine the thermal conductivity of compressible materials as a function of applied load. This technique was used to measure the thermal conductivity of an uncoated SolviCore porous transport layer (PTL) at various compaction pressures. For the dry PTL at 4.6, 9.3 and 13.9 bar compaction pressures, the thermal conductivity was found to be 0.27, 0.36 and 0.40 W K⁻¹ m⁻¹ respectively and the thermal contact resistivity to the apparatus was determined to be 2.1, 1.8 and 1.1 × 10⁻⁴ m² K W⁻¹, respectively. It was shown that the thermal contact resistance between two PTLs is negligible compared to the apparatus’ thermal contact resistivity. For a humidified PTL, the thermal conductivity increases by up to 70% due to a residual liquid saturation of 25%.     

Thermal modeling of a natural convection greenhouse drying system for jaggery: An experimental validation

The aim of this work is to develop a thermal model so as to predict the jaggery temperature, the greenhouse air temperature and the moisture evaporated (jaggery mass during drying), during the drying of jaggery under natural convection conditions. The experiment was conducted separately for 0.75 kg and 2.0 kg of jaggery pieces having dimensions of 0.03 × 0.03 × 0.01 m³ for complete drying. The jaggery was dried in a roof-type even span greenhouse with floor area of 1.20 × 0.78 m². Experiment was carried out during February 5–8, 2004 at IIT Delhi (28°35′N 72°12′E) from 10 am to 5 pm. A computer program was developed in MATLAB software so as to calculate the jaggery temperature, the greenhouse air temperature and the moisture evaporated and was also used to predict the thermal performance of the greenhouse on the basis of solar intensity and ambient temperature. The software developed was experimentally validated. It was shown that the analytical and experimental results for jaggery drying are in good agreement.     

Free cooling of a building using PCM heat storage integrated into the ventilation system

This article presents a study of the free cooling of a low-energy building using a latent-heat thermal energy storage (LHTES) device integrated into a mechanical ventilation system. The cylindrical LHTES device was filled with spheres of encapsulated RT20 paraffin, a phase-change material (PCM). A numerical model of the LHTES was developed to identify the parameters that have an influence on the LHTES’s thermal response, to determine the optimum phase-change temperature and to form the LHTES’s temperature-response function. The last of these defines the LHTES’s outlet-air temperature for a periodic variation of the inlet ambient-air temperature and the defined operating conditions. The temperature-response function was then integrated into the TRNSYS building thermal response model. Numerical simulations showed that a PCM with a melting temperature between 20 and 22 °C is the most suitable for free cooling in the case of a continental climate. The analyses of the temperatures in a low-energy building showed that free cooling with an LHTES is an effective cooling technique. Suitable thermal comfort conditions in the presented case-study building could be achieved using an LHTES with 6.4 kg of PCM per square metre of floor area.     

The electro-optical properties of amorphous indium tin oxide films prepared at room temperature by pulsed laser deposition

The electrical and optical properties of pulsed laser deposited amorphous indium tin oxide films at room temperature are discussed. The films were grown from indium oxide (In₂O₃) targets of different tin (Sn) doping content (0, 5 and 10 wt%) at different oxygen pressures (P_O2) ranging from 1×10⁻³ to 5×10⁻² Torr. The electrical and optical properties of the films were examined by Hall measurements and optical spectrophotometry. It was found that high conductivity amorphous films could be prepared at room temperature irrespective of the Sn doping content. The properties of these films deposited from 0, 5, 10 wt% Sn-doped In₂O₃ targets show a similar response to changes in P_O2. The maximal conductivity of (4.0, 2.1 and 1.8)×10³ S/cm and optical transmittance (visible) higher than 90% were obtained at P_O2 region of (1–1.5)×10⁻² Torr. An undoped In₂O₃ film produced the highest conductivity of 4×10³ S/cm in these studies.     

Silica aerogel granulate material for thermal insulation and daylighting

Silica aerogel granulate is a nanostructured material with high solar transmittance and low thermal conductivity. These properties offer exciting applications in building envelopes. One objective of the joint R&D project ISOTEG at ZAE Bayern was to develop and characterize a new glazing element based on granular silica aerogel. Heat transfer coefficients of less than 0.4 W/(m² K) and a total solar energy transmittance of 35% for the whole glazing unit were achieved. The glazing has a thickness of less than 50 mm. Another application for granular silica aerogel is, for example, in solar collectors.The thermal properties of the glazing as well as the optical and thermal properties of the granular aerogels are presented here. The solar transmittance of a 10 mm packed bed of silica aerogel was 53% for semi-translucent spheres and 88% for highly translucent granulate. In our heat transfer experiments the gas pressure, external pressure load, temperature and gas filling were varied. The various thermal conductivity values measured for the glazing and collector applications were compared to the values calculated using two different packed bed models. For the gas-dependent measurements the intergranular voids in the granulate were 1.0 ± 0.1 mm before loading the packed bed, 0.3 ± 0.1 mm at an external load of 3.2 bar (3.2 × 10⁵ Pa) and 0.6 ± 0.1 mm after release.A direct radiative conduction of λ_direct = 4.5 ± 0.5 × 10⁻³ W m⁻¹ K⁻¹ was obtained.     

Effect of yarn properties on thermal comfort of knitted fabrics

In this research, thermal properties of 1×1 rib fabrics knitted by using various yarns of different properties were investigated with all details. The mentioned yarn properties were yarn count, yarn twist and combing process. The thermal resistance, thermal absorptivity, thermal conductivity, water vapour permeability of samples were measured with the aid of Alambeta and Permetest devices respectively. The results of the tests were evaluated statistically and the importance levels of the relationship between the measured parameters were determined. It is observed that yarn properties like yarn count, yarn twist and combing process of cotton have affected different thermal comfort properties of 1×1 rib knitted fabrics. While the yarn twist and yarn count increase thermal resistance values decrease and water vapour permeability values increase. The combing process has the same effect on the thermal properties.     

Effect of spatial variation of incident radiation on spectral response of a large area silicon solar cell and the cell parameters determined from it

Effect of spatial variation of incident monochromatic light on spectral response of an n⁺–p–p⁺ silicon solar cell and determination of diffusion length of minority carriers (L_b) in the base region and the thickness of the apparent dead layer (x_d) in the n⁺ emitter from the spectral response have been investigated. Spectral response of a few 10 cm diameter and 10×10 cm² pseudo-square silicon solar cells was measured with the help of a standard silicon solar cell of 2×2 cm² area in 400–1100 nm wavelength range. Different areas (4, 9, 16, 25 and total area 78.6 or 96 cm²) were exposed. The effect of the radial variation of incident radiation was determined quantitatively by defining a parameter f₁ as the ratio of the average intensity falling on the reference cell to that on the exposed area of the test cell. The value of f₁ varied between 1 and 1.15 (1.25) as the exposed area of the cell varied from 4 cm² to 78.6 (96) cm² indicating that the spatial inhomogeneity of intensity increased with the increase in the exposed cell area. Short-circuit current densities, J_sc, computed from spectral response data for AM1.5 spectrum were less compared to the directly measured values by a factor which was nearly equal to f₁. However, radial variation of intensity does not affect the determination of diffusion length of minority carriers in the base region (by the long wavelength spectral response, LWSR method using the measured spectral response data in 0.85<λ<1.05 μm range) and the thickness of the dead layer (by the method of Singh et al. using the data of 0.45<λ<0.65 μm range) significantly.     

An investigation into the use of a wind shield to reduce the convective heat flux to a nocturnal radiative cooling surface

The effect of a wind shield on the convective heat flux from an ambient air stream blowing over a horizontal surface intended for nocturnal radiative cooling has been studied by computational fluid dynamical calculations and by wind tunnel experiments under conditions appropriate for the climate of Thailand. The test unit was a rectangular plate 312 mm×250 mm, with vertical metal strips for the wind shield having heights up to 100 mm along the edges of the plate. It was found that a wind shield of height 25 mm slightly increased the convective heat transfer due to increased turbulence over the surface, but wind shields of height 50 mm and 100 mm reduced the convection due to a separation of the main airflow from the surface. Radiative cooling was reduced by the wind shields. The net cooling of the surface was best with no wind shield at wind velocities less than about 1 m s^–1, and with the wind shield of height 100 mm at wind velocities greater than about 2 m s^–1.     

The development and testing of small concentrating PV systems

Spreadsheets have been used to compare some 90 possible small PV concentrator designs that might be suitable for use at remote sites. They have apertures of about 2 m², use BP Solar LBG cells, and employ small aperture modules to reduce heat sinking and construction costs. Designs include fixed V-troughs and CPCs, single axis tracked cylindrical lens and mirror systems, and two-axis tracked spherical-symmetry systems. Performance and volume production costs were estimated. Four promising systems were constructed as prototypes:

(A) Point-focus Fresnel lenses, two-axis tracking; Cg=32×; and 69× with secondaries.

(B) Line-focus mirror parabolic troughs, one-axis tracking, Cg=20×.

(C) SMTS (‘single-mirror two-stage’), one-axis tracking, Cg=30×.

(F) Multiple line-focus mirror parabolic troughs, E–W 1/day manual tracking, Cg=6×.

The prototypes were tested at Reading, and three for up to a year’s field trial at ZSW’s test site, Widderstall, in Germany. The best system efficiencies, normalised to 25°C and excluding the end losses of linear systems, were 12.5%, 13.2%, 13.6% and 14.3% for collectors A, B, C, and F, respectively. The collectors were practical and robust, and the performances of collectors B, C and F are only 10% below the estimates in the spreadsheet calculations. The best collectors have estimated production costs between 1.5 and 1.8 US $/Wp, yielding energy costs at a good site (excluding BOS and overheads) of between 5 and 7 cents/kWh (18 and 25 cents/MJ). On the same cost basis a conventional PV array costs 4.3 $/Wp, and 18 cents/kWh (65 cents/MJ).     

Diffusion and influence of Cu on properties of CdTe thin films and CdTe/CdS cells

The effective diffusion coefficients of Cu for thermal and photodiffusion in the CdTe films have been estimated from resistivity versus duration of thermal or photoannealing curves. In the temperature range 60–200°C the effective coefficient of thermal diffusion (D_t) and photodiffusion (D_ph) are described as D_t=7.3×10⁻⁷exp(−0.33/kT) and D_ph=4.7×10⁻⁸exp(−0.20/kT).It is found that the diffusion doping of CdTe thin films by Cu at 400°C results in a sharp decrease of resistivity up to 7 orders of magnitude of p-type material, depending on thickness of Cu film. The comparative study of performance of CdTe(Cu)/CdS and CdTe/CdS cells has been studied. It is shown that the diffusion doping of CdTe film by Cu increases efficiency of CdTe(Cu)/CdS cells from 0.9% to 6.8%. The degradation of photovoltaic parameters of CdTe(Cu)/CdS cell, during testing under forward and reverse bias at room temperature, proceeds at a larger rate than those of CdTe/CdS cell without Cu. The degradation of performance of CdTe(Cu)/CdS cells is tentatively assigned to electrodiffusion of Cu in CdTe, resulting in redistribution of concentration of Cu-related centers in CdTe film and heterojunction region.     

Extending thermal response test assessments with inverse numerical modeling of temperature profiles measured in ground heat exchangers

Thermal response tests conducted to assess the subsurface thermal conductivity for the design of geothermal heat pumps are most commonly limited to a single test per borefield, although the subsurface properties can spatially vary. The test radius of influence is additionally restricted to 1–2 m, even though the thermal conductivity assessment is used to design the complete borefield of a system covering at least tens of squared meters. This work objective was therefore to develop a method to extend the subsurface thermal conductivity assessment obtained from a thermal response test to another ground heat exchanger located on the same site by analyzing temperature profiles in equilibrium with the subsurface. The measured temperature profiles are reproduced with inverse numerical simulations of conductive heat transfer to assess the site basal heat flow, at the location of the thermal response test, and evaluate the subsurface thermal conductivity, beyond the thermal response test. Paleoclimatic temperature changes and topography at surface were considered in the model that was validated by comparing the thermal conductivity estimate obtained from the optimization process to that of a conventional thermal response test.     

Deposition of transparent and conductive Al-doped ZnO thin films for photovoltaic solar cells

The effect of the substrate temperature on the optoelectronic properties of ZnO-based thin films prepared by rf magnetron sputtering has been studied. Three different targets (Zn/Al 98/2 at%, ZnO:Al 98/2 at% and ZnO:Al₂O₃ 98/2 wt%) have been investigated in order to compare resulting samples and try to reduce the substrate temperature down to room temperature. From the ZnO:Al₂O₃ target, transparent conductive zinc oxide has been obtained at 25°C with the average optical transmission in the 400–800 nm wavelength range, T = 80–90% and resistivity, = 3−5 × 10⁻³ Ωcm. In Al:Zn0 layers, the spatial distribution of the electrical properties across the substrate placed parallel to the target has been improved by depositing at high substrate temperatures, above 200°C. Besides, owing to diffusion processes of CuInSe₂ and CdS take place at 200°C, an AI:ZnO/CdS/CuInSe₂ polycrystalline solar cell made with the Al:ZnO deposited at 25°C as the transparent conductive oxide, has shown a more efficient photovoltaic response, η = 6.8%, than the one measured when the aluminium-doped zinc oxide has been prepared at 200°C, η = 1.8%.     

Low-temperature deposition of polycrystalline silicon thin films by hot-wire CVD

Polycrystalline silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430°C. The material properties have been optimized for photovoltaic applications by varying the hydrogen dilution of the silane feedstock gas, the gas pressure and the wire temperature. The optimized material has 95% crystalline volume fraction and an average grain size of 70 nm. The grains have a preferential orientation along the (2 2 0) direction. The optical band gap calculated from optical absorption by photothermal deflection spectroscopy (PDS) showed a value of 1.1 eV, equal to crystalline silicon. An activation energy of 0.54 eV for the electrical transport confirmed the intrinsic nature of the films. The material has a low dangling bond-defect density of 10¹⁷ cm³. A photo conductivity of 1.9 × 10⁻⁵ Ω⁻¹cm⁻¹ and a photoresponse (σ_ph/σ_d) of 1.4 × 10² were achieved. A high minority-carrier diffusion length of 334 nm as measured by the steady-state photocarrier grating technique (SSPG) and a large majority-carrier mobility-lifetime (μτ) product of 7.1 × 10⁻⁷cm²V⁻¹ from steady-state photoconductivity measurement ensure that the poly-Si : H films possess device quality. A single junction n---i---p cell made in the configuration n⁺-c-Si/i-poly-Si: H/p-μc-Si : H/ITO yielded 3.15% efficiency under 100 mW/cm² AM 1.5 illumination.     

New developments in Cu(In,Ga)(S, Se)2 thin film modules formed by rapid thermal processing of stacked elemental layers

A second-generation process for high-efficiency large-area Cu(In,Ga)(Se,S)₂ thin film (CIGSSe) solar modules has been developed applying controlled sodium doping and rapid thermal processing for absorber formation. The pilot line delivers aperture area efficiencies of 12.2%±0.5% (average) for 30×30 cm² modules and a certified champion efficiency of 13.1% for an unencapsulated 60×90 cm² demonstrator module. The stability of the frameless pilot line modules with a low-cost package against humidity is confirmed externally by passing the damp heat test sequence according to IEC 61646. Substitution of CBD-CdS by CBD-Zn(S, OH) buffer layers yields efficiencies up to 12% on 30×30 cm² circuits. First CIGSSe-cells on flexible substrates were also developed applying 30 μm thin titanium foils, 8×8 cm² in size. Average cell efficiencies on a substrate up to 12.4% were achieved.     

Performance of a solar dryer using hot air from roof-integrated solar collectors for drying herbs and spices

A solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors was developed. The dryer is a bin type with a rectangular perforated floor. The bin has a dimension of 1.0 m×2.0 m×0.7 m. Hot air is supplied to the dryer from fiberglass-covered solar collectors, which also function as the roof of a farmhouse. The total area of the solar collectors is 72 m². To investigate its performance, the dryer was used to dry four batches of rosella flowers and three batches of lemon-grasses during the year 2002–2003. The dryer can be used to dry 200 kg of rosella flowers and lemon-grasses within 4 and 3 days, respectively. The products being dried in the dryer were completely protected from rains and insects and the dried products are of high quality. The solar air heater has an average daily efficiency of 35% and it performs well both as a solar collector and a roof of a farmhouse.