Journal of Solar Energy Engineering, Transactions of the ASME

Effect of 1,8-diiodooctane on the performance of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solar cells with glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/P3HT: PCBM/Ca/Al structure was studied. The morphology and thickness of the active layer were investigated using atomic force microscopy (AFM). The UV-visible spectroscopy and X-ray diffraction (XRD) analysis were used to study the absorption behavior (of the solutions and coated layers) and crystallinity of the active layer, respectively. The results show that the existence of 1,8-diiodooctane reduced the open circuit voltage from 0.81 to 0.52 V and increased the short circuit current by about three folds; the fill factor (FF) and power conversion efficiency were increased from 36.0 to 54.1% and 0.47% to 1.54%, respectively. These changes can be attributed to the enhanced crystallinity of P3HT or the doping effect of 1,8-diiodooctane on P3HT chains. UV-visible analysis demonstrated that the addition of 1,8-diiodooctane to the solution did not change the absorption onset, whereas in the coated layers, the maximum absorption peak shifted to higher wavelengths. The XRD analyses demonstrated the enhancement of crystallinity of P3HT upon the introduction of 1,8-diiodooctane.

Current technologies of concentrated solar power plants (CSP) are under extensive industrial development but still suffer from lack of adapted thermal energy storage (TES) materials and systems. In the case of extended storage (some hours), thousands of tonnes of materials are concerned leading to high investment cost, high energy and GHG contents and major conflicts of use. In this paper, recycled industrial ceramics made by vitrification of asbestos containing wastes (ACW) are studied as candidates to be used as sensible TES material. The material presents no hazard, no environmental impact, good thermophysical properties (λ= 1.4 W m−1 K−1; Cp = 1025 J kg−1 K−1; ρ= 3100 kg m−3) and at very low investment cost. Thanks to the vitrification process of the wastes, the obtained ceramics is very stable up to 1200 °C and can be directly manufactured with the desired shape. The vitrified ACW can be used as TES material for all kinds of the CSP processes (from medium up to high concentration levels) with properties in the same range than other available materials but with lower cost and without conflict of use. The proposed approach leads also to sustainable TES allowing a pay back of the energy needed for the initial waste treatment. Furthermore, this new use of the matter can enhance the waste treatment industry instead of land fill disposal.

A numerical study was conducted to examine the impact of rotor solidity and blade number on the aerodynamic performance of small wind turbines. Blade element momentum theory and lifting line based wake theory were utilized to parametrically assess the effects of blade number and solidity on rotor performance. Increasing the solidity beyond what is traditionally used for electric generating wind turbines led to increased power coefficients at lower tip speed ratios, with an optimum between 3 and 4. An increase in the blade number at a given solidity also increased the maximum Cp for all cases examined. The possibility of a higher aerodynamic power extraction from solidity or blade number increases could lead to a higher overall system power production. Additional advantages over current 5% to 7% solidity, high speed designs would include lower noise, lower cut-in wind speed, and less blade erosion.

This paper discusses procedures for creating calibrated building energy simulation programs. It begins with reviews of the calibration techniques that have been reported in the previous literature and presents new hourly calibration methods including a temperature bin analysis to improve hourly x−y scatter plots, a 24-hour weather-daytype bin analysis to allow for the evaluation of hourly temperature and schedule dependent comparisons, and a 52-week bin analysis to facilitate the evaluation of long-term trends. In addition, architectural rendering is reviewed as a means of verifying the dimensions of the building envelope and external shading placement as seen by the simulation program. Several statistical methods are also presented that provide goodness-of-fit indicators, including percent difference calculations, mean bias error (MBE), and the coefficient of variation of the root mean squared error (CV(RMSE)). The procedures are applied to a case study building located in Washington, D. C. where nine months of hourly whole-building electricity data and sitespecific weather data were measured and used with the DOE-2.1D building energy simulation program to test the new techniques. Simulations that used the new calibration procedures were able to produce an hourly MBE of –0.7% and a CV(RMSE) of 23.1% which compare favorably with the most accurate hourly neural network models (Kreider and Haberl, 1994a, b).

Steady natural convection heat transfer has been studied in porous wall, solar collector systems. The boundary conditions were: two isothermal walls at different temperatures, two horizontal bounding adiabatic walls, and either uniform or nonuniform heat generating porous layer without vents. The aspect ratio A was from 0.5 to 1.4. The Rayleigh number varied from 103 to 108. The Darcy number was from 10−8 to 10−2 but the detailed studies were carried out only for 10−4, an optimum value. The results are presented in terms of thermal parameters (θ, θmax, Nu) as a function of Ra and other nondimensional parameters (A=H/L, B=1/L, F=d/L, kr). The isotherms and streamlines within the system are also produced. The overall results indicate that geometrical parameters are the most important parameters affecting the system performance.

Purely thermal heat pumps can be devised with adsorbate/solid adsorbent pairs, for example for refrigeration purposes. As each cycle consists of two periods, i.e., heating/desorption/condensation and cooling/adsorption/evaporation, this mode of operation is well-suited to solar energy. After experiments with the Zeolite/Water pair, a solar-powered ice maker was designed with the Activated Carbon/Methanol pair, and a prototype was built in Orsay. The solar collectors (6 m2) contain, on the whole, 130 kg of A.C., the condensers are air-cooled, and the evaporator has a net production of 30–35 kg of ice per sunny day. The ice is easily removed, and in principle the machine could be automatically operated. The net solar C.O.P. is 0.12, which makes this machine one of the most efficient solar ice makers.

Mean and peak wind loads on flat rectangular or circular heliostats were measured on models in a boundary layer wind tunnel which included an atmospheric surface layer simulation. Horizontal and vertical forces, moments about horizontal axes at the ground level and at the centerline of the heliostat, and the moment about the vertical axis through the heliostat center were measured. Results showed that loads are higher than predicted from results obtained in a uniform, low-turbulence flow due to the presence of turbulence. Reduced wind loads were demonstrated for heliostats within a field of heliostats and upper bound curves were developed to provide preliminary design coefficients.

Theoretical analysis of a solar desalination system utilizing an innovative new concept, which uses low-grade solar heat, is presented. The system utilizes natural means of gravity and atmospheric pressure to create a vacuum, under which liquid can be evaporated at much lower temperatures and with less energy than conventional techniques. The uniqueness of the system is in the way natural forces are used to create vacuum conditions and its incorporation in a single system design where evaporation and condensation take place at appropriate locations without any energy input other than low grade heat. The system consists of solar heating system, an evaporator, a condenser, and injection, withdrawal, and discharge pipes. The effect of various operating conditions, namely, withdrawal rate, depth of water body, temperature of the heat source, and condenser temperature were studied. Numerical simulations show that the proposed system may have distillation efficiencies as high as 90% or more. Vacuum equivalent to 3.7 kPa (abs) or less can be created depending on the ambient temperature at which condensation will take place.

The present research is directed toward demonstrating the feasibility of producing high-efficiency GaAs solar cells with high power-to-weight ratio by organo-metallic chemical vapor deposition (OM-CVD) growth of thin epi-layers on suitable substrates. Antireflection-coated, metal-oxide-semiconductor (AMOS), GaAs solar cells grown on bulk polycrystalline Ge substrates were initially studied, with the best efficiency achieved being about 9 percent AM1 (7 percent AM0). Subsequently, a new direct deposition method for fabricating ultra-thin top layer, epitaxial n+ /p shallow homojunction solar cells on (100) GaAs substrates (without anodic thinning) was developed by means of which large area (1 cm2) cells were produced with about 19 percent AM1 (15 percent AM0) conversion efficiency. An AM1 conversion efficiency of about 18 percent (14 percent AM0), or about 17 percent (13 percent AM0) with 5 percent grid coverage, was achieved for a single-crystal, GaAs, n+ /p cell grown by OM-CVD on a Ge wafer. These achievements led to the fabrication, for the first time, of thin GaAs epi-layers OM-CVD grown with good crystallographic quality, using a (100) Si-substrate on which a thin Ge epi-interlayer was first deposited by CVD from GeH4 and processed for improved surface morphology.

Solar hot water and space heating systems constructed of commodity polymers have the potential to reduce the initial cost of solar thermal systems. However, a polymer absorber must be prevented from exceeding its maximum service temperature during stagnation. Here, the addition of a thermotropic material to the surface of the absorber is considered. The thermotropic layer provides passive overheat protection by switching from high transmittance during normal operation to high reflectance if the temperature of the absorber becomes too high. A one dimensional model of a glazed, flat-plate collector with a polymer absorber and thermotropic material is used to determine the effects of the optical properties of the thermotropic material on the optical efficiency and the stagnation temperature of a collector. A key result is identification of the reflectance in the translucent state required to provide overheat protection for potential polymer absorber materials. For example, a thermotropic material in its translucent state should have a solar-weighted reflectance greater than or equal to 52% to protect a polypropylene absorber which has a maximum service temperature of 115 °C.