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Abstract The albedo of a roof determines the fraction of incoming sunlight that is reflected, which affects heat transfer into the building and exchange of energy between the built environment and the atmosphere. While the albedo of individual roofs can be easily measured, roof albedo at the city scale is unknown. In this paper we characterize the albedos of roofs in seven cities in California: Los Angeles, Long Beach, Bakersfield, San Francisco, San Jose, Sacramento, and San Diego. The fraction of urban area covered by roofs ranged by city from 10% to 25%. City-wide average roof albedo ranged from 0.17 ± 0.08 to 0.20 ± 0.11 (mean ± standard deviation) for five of the cities; values were higher in Sacramento (0.24 ± 0.11) and San Diego (0.29 ± 0.15). Buildings with small roofs were found to constitute a large fraction of city roof area and to have low mean albedos. This suggests that efforts to increase urban albedo through the use of reflective roofs should include small roofs, which are presumably mostly residential. Roof albedos derived for Bakersfield were used in a regional climate model (Weather Research and Forecasting Model) to estimate temperature changes attainable by converting the current stock of roofs to “cool” high albedo roofs. It was found that seasonal mean afternoon (15:00 LST) temperatures could be reduced by up to 0.2 °C during both the summer and winter. Changes in precipitation were not significant at the 95% confidence level.

Photodegradation of organic polymers by UV radiation exposure, emphasizing polyvinyl chloride photochemistry and radiation stabilization

In this paper, a detailed model for the transient simulation of solar cavity receivers for concentrating solar power plants is presented. The proposed model aims to consider all the major phenomena influencing the performance of a cavity receiver, including radiation, convection and conduction heat transfer mechanisms. For the radiation heat exchange within the cavity, the radiosity method is implemented, where the view factor calculation for all the active and passive surfaces is performed by a ray tracing algorithm programmed in a free software environment for statistical computing, namely R. A one-dimensional modeling approach is used for the tubes constituting the receiver active panels, through which the heat transfer fluid (HTF) is pumped. The governing partial differential equations are solved numerically by applying the finite volume method. Convective heat losses are modeled through different correlations for natural and forced convection heat losses from the specific literature. Once the thermal behavior has been haracterized, the geometry of the model is later fixed to check the consistency of the model and to study its dynamic characteristics. A specific 51.6 MWth, PS10 like receiver is used in this paper, although the implemented model has the flexibility to allow a variable number of panels and geometric configurations. At last, an adaptive neural controller, designed and trained offline, controls the outlet temperature of the molten salts to the desired operating value. Results for transient simulations are shown in the paper, demonstrating the plausibility of the estimations obtained with the developed model. The proposed model has been implemented in the Modelica language and based on the Modelica Standard Library (MSL) modeling approach.

Abstract Solar tower power plants rely on precise calibrations of their heliostats for efficient operation. Open-loop calibration procedures are the most common type due to their cost-effectiveness. Two main approaches to these algorithms exist: geometry-based robotic kinematics and neural network-based models. While the former is reliable and requires little data, it only yields moderate accuracy. The latter, however, promises higher accuracies but is data-hungry and unreliable. In this study, we propose a 2-layer coarse-to-fine hybrid model that combines the strengths of both approaches. Our model uses a rigid-body model for prealignment, then phases in a neural network disturbance model through a regularization sweep. This approach ensures that the prediction accuracy is, in the worst-case, equivalent to that of the rigid-body model. Moreover, it helps to identify deficiencies that may have been overlooked by the physical approach. It especially is capable to compute deviation from the geometry models averaged optimum. For testing, we used real measurement data from daily heliostat calibration at the solar tower in Jülich. We also employed a training/validation data split for evaluation, which allows for a conservative performance assumption over the entire year. Our results demonstrate that the hybrid-model outperforms rigid-body models starting from the first measurement, achieving a top performance below 0.7 milliradians. In conclusion, our proposed hybrid model provides a cost effective in-situ solution for heliostat calibration with highest accuracies on low data in solar tower power plants for all open loop calibration methods.

Abstract Fenestration attachments are anticipated to produce significant reductions in building energy use because they can be deployed quickly at low-cost. New software tools enable users to assess the building energy impacts of optically complex fenestration systems (CFS) such as shades, Venetian blinds, or daylighting systems. However, such tools require users to provide bi-directional scattering distribution function (BSDF) data that describe the solar-optical performance of the CFS. A free, open-source Radiance tool genBSDF enables users to generate BSDF data for arbitrary CFS. Prior to genBSDF, BSDF data for arbitrary fenestration systems could only be produced using either expensive software or with expensive equipment. genBSDF outputs CFS data in the Window 6 XML file format and so can be used with CFS-enabled software tools to model multi-layered window systems composed of glazing and shading layers. We explain the basis and use of the genBSDF tool and validate the tool by comparing results for four different cases to BSDF data produced via alternate methods. This validation demonstrates that BSDFs created with genBSDF are comparable to BSDFs generated analytically using TracePro and by measurement with a scanning goniophotometer. This tool is expected to support accelerated adoption of fenestration attachments and daylighting technologies.

Abstract This paper presents a new index, called the “evaporative capacity”, for rating the performance of the solar air heater in a solar drier consisting of solar air heater and a drying chamber in series. The proposed index complements the widely-used “collector efficiency” as a performance indicator of the solar collector, by taking into account the specific use that is to be made with the heated air. Presented is a detailed method for calculating the evaporative capacity, and a comparison of this new index with the thermal efficiency index, demonstrating its superiority. General charts for a rapid determination of the evaporative capacity are presented, and some possible applications of these charts are described.

Thermochemical cycles are a type of heat engine that utilize high-temperature heat to produce chemical work. Like their mechanical work-producing counterparts, their efficiency depends on operating temperature and on the irreversibilities of their internal processes. With this in mind, we have invented innovative design concepts for two-step solar-driven thermochemical heat engines based on iron oxide and iron oxide mixed with other metal oxides (ferrites). These concepts utilize two sets of moving beds of ferrite reactant material in close proximity and moving in opposite directions to overcome a major impediment to achieving high efficiency – thermal recuperation between solids in efficient counter-current arrangements. They also provide inherent separation of the product hydrogen and oxygen and are an excellent match with high-concentration solar flux. However, they also impose unique requirements on the ferrite reactants and materials of construction as well as an understanding of the chemical and cycle thermodynamics. In this paper, the Counter-Rotating-Ring Receiver/Reactor/Recuperator (CR5) solar thermochemical heat engine concept is introduced and its basic operating principals are described. Preliminary thermal efficiency estimates are presented and discussed. Our results and development approach are also outlined.

This paper presents measurements of the effective specific heat and the extinction coefficient for aqueous nanofluids dispersed with paraffin-filled Multi-Walled Carbon NanoTubes (MWCNTs). The MWCNTs were filled with paraffin wax by capillary action. Centrifugal decanting was used to modify the traditional two-step method so as to produce a nanofluid dispersion that was more stable than that produced by the traditional method. The stability of each suspension was quantitatively evaluated with a laser scattering method over 7 days. A differential scanning calorimetry (DSC) and the three-slap method were used to measure the effective specific heat and the extinction coefficient of the nanofluids, respectively. The measured effective specific heat of the water-based paraffin-filled MWCNTs nanofluid, with a volume fraction of 1%, was up to 5.1% larger than that for the water-based MWCNT nanofluids without paraffin wax. The nanofluid extinction coefficient was shown to increase linearly with the volume fraction for data within the independent scattering regime, which occurred when the nanoparticle-distance/wavelength ratio (c/λ) was less than 2.