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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.

Abstract Exergy analysis is very helpful for reducing irreversibility and rising the efficiency of solar collectors. The major objective of the present study is to organize a review on exergy analysis of different parabolic solar collectors. The effects of various flows and geometrical parameters of parabolic thermal collectors on the exergy efficiency were presented and discussed. In addition, comparative study was carried out to select the best solar thermal system for maximum exergy efficiency with minimal thermal losses. This study indicated that the hybrid nanofluids enhanced the exergy efficiency significantly as compared to without hybrid nanofluids. Passive techniques comprising twisted tape inserts, fins and insertion of swirl devices in the stream for changing the stream patterns causes to interrupt the thermal boundary layer and accordingly high exergy efficiency. This review would also throw light on the scope for further research and recommendation for improvement in the existing solar thermal collectors. Finally, this work will be beneficial for the scholars working on exergy analyses of solar parabolic collectors.

Abstract In this paper, the experimental performance of a 45 m2 solar field of non-tracking external compound parabolic (XCPC) collectors installed at the University of California, Merced is described. The solar field was operated during July-August 2020 in both clean and dirty conditions and at varying operating temperatures (70, 135, 170 °C) while operating an air heater, thermal evaporator, and double effect absorption chiller. Performance data was used to develop an instantaneous solar field performance model which was then incorporated into an annual performance model using TMY3 data to estimate yearly production from the solar field. The model predicts an annual generation of ∼1100 kWh/m2-year at 80 °C, ∼1000 kWh/m2-year at 100 °C, ∼900 kWh/m2-year at 120 °C, ∼800 kWh/m2-year at 140 °C, and ∼700 kWh/m2-year at 160 °C in California. The XCPC technology is currently expected to have an installed cost of $300/m2 and an annual operations and maintenance cost of $6.5/m2-year. Over a 25 year lifetime it provides a levelized cost of heat at 2–4 cents per kWhth delivered. This is below the cost of commercial natural gas in California and at temperatures ≤ 120 °C below the cost of industrial natural gas, which highlights the potential of the XCPC technology for decarbonizing thermal applications such as water and space heating, drying, sterilization, desalination, evaporation, low pressure steam, double effect absorption chilling, process heating, and more. The lifetime cost of emissions reductions is ∼$169 per metric ton of avoided CO2 when replacing natural gas, ∼$137/MT CO2 when replacing propane, and ∼$83/MT CO2 when replacing electric heating.

Abstract The Gaussian Thermal Flux model is a fast thermal radiation sub-model which operates with minimal input and predicts flux on sloped surfaces for both clear and cloudy days. This paper presents the model's theoretical basis. It is distinguished from empirical models by its detailed two dimensional treatment of clouds, its computation of air layer transmissivity above and below cloud layers and its consideration of cloud height effects. The model improves upon existing flux emissivity and multi-band or line models by being independent of soundings and by its use of gaussian quadrature to speed the numerical integrations. It may be input entirely from standard screen level meteorological measurements.

Abstract This paper describes selectee aspects of the METSTAT (Meteorological/Statistical) solar radiation model. METSTAT was developed specifically to support the production of the National Solar Radiation Data Base for the United States. The model was used to estimate hourly values of direct normal, diffuse horizontal, and global horizontal solar radiation for those times and locations for which measured data were not available. The input parameters for METSTAT include total and opaque cloud cover, aerosol optical depth, precipitable water vapor, ozone, surface albedo, snow depth, days-since-last-snowfall, atmospheric pressure and present weather. The model employs deterministic algorithms to generate accurate monthly means for each element for each hour and statistical algorithms to simulate the statistical and stochastic characteristics of multiyear solar radiation data sets.

Abstract This paper discusses the status of flat-plate photovoltaic (PV) system technology, performance and cost for applications in developing countries. Comparison of key electrical service performance factors is made between PV and conventional generating systems. PV module reliability results from 23 field tests are summarized and the effect of failure on system performance is discussed. Cost algorithms, drawn from historial data, are defined and applied to obtain PV system selling price as a function of worst-month insolation and for several values of loss-of-energy-probability, (LOEP), an index of performance. Comparison is made of levelized energy cost between PV and diesel-generator systems. Projections of PV system selling price, major component costs and system levelized energy cost to the mid-1980s and 1990s are displayed and the implications discussed. It was concluded that PV system technical viability had been confirmed and commercial development is well advanced. PV system electrical service performance is generally superior to service performance provided by the electric grid or diesel in rural and remote areas. At present, for many applications having an annual electrical demand of up to 4 to 10 MWH, the levelized energy cost of PV systems in regions of good insolation is less than for diesel generators. Within 20 yr, if cost projections are borne out, photovoltaics will become the least expensive and most reliable source for most decentralized electric power in the developing world.

Abstract Seasonal solar thermal energy storage (SSTES) has been investigated widely to solve the mismatch between majority solar thermal energy in summer and majority heating demand in winter. To study the feasibility of SSTES in domestic dwellings in the UK, eight representative cities including Edinburgh, Newcastle, Belfast, Manchester, Birmingham, Cardiff, London and Plymouth have been selected in the present paper to study and compare the useful solar heat available on dwelling roofs and the heating demand of the dwellings. The heating demands of space and hot water in domestic dwellings with a range of overall heat loss coefficients (50 W/K, 150 W/K and 250 W/K) in different cities were calculated; then the useful heat obtained by the heat transfer fluid (HTF) flowing through tilted flat-plate solar collectors installed on the dwelling roof was calculated with varied HTF inlet temperature (30 °C, 40 °C and 50 °C). By comparing the available useful heat and heating demands, the critical solar collector area and storage capacity to meet 100% solar fraction have been obtained and discussed; the corresponding critical storage volume sizes using different storage technologies, including sensible heat water storage, latent heat storage and various thermochemical sorption cycles using different storage materials were estimated.

A linear correlation between UV-A and 380 nm was developed by means of the TUV 4.1 radiative transfer model. The prediction error of the correlation was evaluated with data from Buenos Aires, Argentina, 2001, and from 2006, Almeria, Spain. Percent random mean square error (RMSE%) was calculated for intervals of 10° of solar zenith angles, ranging 4.75% at 20° to 37.70% at 90° in clear days and 22.16% at 20° to 26.17% at 90° for cloudy days in Buenos Aires Argentina, and 1.27% at 20° to 11.27% at 90° for clear days in Almeria, Spain. Clouded days were not assessed with the data from Spain. In Argentina, the UV-A radiometer is located in a rural area and the 380 nm radiometer is located in an urban area 6 km away. Hence the real error of the proposed model is closer to that found in Spain were both measurements were performed at the same site. The objective of the work is to achieve a simple and precise method to assess UV-A availability for environmental applications of solar energy, particularly for solar water treatment, at any desired latitude.

Modeling the performance of some concentrating solar systems for thermal power plants may require high temporal resolution irradiance as input, in order to account for the impact of the cloud transient effects. This work proposes a simple method of generating synthetic irradiance of 10-min intervals from the hourly mean values. Boundary conditions are imposed to preserve the expected behavior under clear sky situations. The procedure consists basically on adding a random fluctuation, which characteristic amplitude depends on the sky conditions, to the hourly interpolated values. The assessment of the method with ground data have shown to main aspects to remark: daily and monthly means from the synthetic data are below 5% of root mean squared deviation compared to the original time series; despite the noticeable uncertainty in the 10-min synthetic irradiance values, the dynamic behavior of the fluctuations is comparable to the original data.