Measuring solar reflectance—Part I: Defining a metric that accurately predicts solar heat gain

Solar reflectance can vary with the spectral and angular distributions of incident sunlight, which in turn depend on surface orientation, solar position and atmospheric conditions. A widely used solar reflectance metric based on the ASTM Standard E891 beam-normal solar spectral irradiance underestimates the solar heat gain of a spectrally selective “cool colored” surface because this irradiance contains a greater fraction of near-infrared light than typically found in ordinary (unconcentrated) global sunlight. At mainland US latitudes, this metric R_E891BN can underestimate the annual peak solar heat gain of a typical roof or pavement (slope ? 5:12 [23°]) by as much as 89 W m⁻², and underestimate its peak surface temperature by up to 5 K. Using R_E891BN to characterize roofs in a building energy simulation can exaggerate the economic value N of annual cool roof net energy savings by as much as 23%.We define clear sky air mass one global horizontal (“AM1GH”) solar reflectance R_g,0, a simple and easily measured property that more accurately predicts solar heat gain. R_g,0 predicts the annual peak solar heat gain of a roof or pavement to within 2 W m⁻², and overestimates N by no more than 3%. R_g,0 is well suited to rating the solar reflectances of roofs, pavements and walls. We show in Part II that R_g,0 can be easily and accurately measured with a pyranometer, a solar spectrophotometer or version 6 of the Solar Spectrum Reflectometer.     

Measuring solar reflectance—Part II: Review of practical methods

A companion article explored how solar reflectance varies with surface orientation and solar position, and found that clear sky air mass 1 global horizontal (AM1GH) solar reflectance is a preferred quantity for estimating solar heat gain. In this study we show that AM1GH solar reflectance R_g,0 can be accurately measured with a pyranometer, a solar spectrophotometer, or an updated edition of the Solar Spectrum Reflectometer (version 6). Of primary concern are errors that result from variations in the spectral and angular distributions of incident sunlight.Neglecting shadow, background and instrument errors, the conventional pyranometer technique can measure R_g,0 to within 0.01 for surface slopes up to 5:12 [23°], and to within 0.02 for surface slopes up to 12:12 [45°]. An alternative pyranometer method minimizes shadow errors and can be used to measure R_g,0 of a surface as small as 1 m in diameter. The accuracy with which it can measure R_g,0 is otherwise comparable to that of the conventional pyranometer technique.A solar spectrophotometer can be used to determine , a solar reflectance computed by averaging solar spectral reflectance weighted with AM1GH solar spectral irradiance. Neglecting instrument errors, matches R_g,0 to within 0.006. The air mass 1.5 solar reflectance measured with version 5 of the Solar Spectrum Reflectometer can differ from by as much as 0.08, but the AM1GH output of version 6 of this instrument matches to within about 0.01.     

Methodology for estimation of potential for solar water heating in a target area

Proper estimation of potential of any renewable energy technology is essential for planning and promotion of the technology. The methods reported in literature for estimation of potential of solar water heating in a target area are aggregate in nature. A methodology for potential estimation (technical, economic and market potential) of solar water heating in a target area is proposed in this paper. This methodology links the micro-level factors and macro-level market effects affecting the diffusion or adoption of solar water heating systems. Different sectors with end uses of low temperature hot water are considered for potential estimation. Potential is estimated at each end use point by simulation using TRNSYS taking micro-level factors. The methodology is illustrated for a synthetic area in India with an area of 2 sq. km and population of 10,000. The end use sectors considered are residential, hospitals, nursing homes and hotels. The estimated technical potential and market potential are 1700 m² and 350 m² of collector area, respectively. The annual energy savings for the technical potential in the area is estimated as 110 kW h/capita and 0.55 million-kW h/sq. km. area, with an annual average peak saving of 1 MW. The annual savings is 650-kW h per m² of collector area and accounts for approximately 3% of the total electricity consumption of the target area. Some of the salient features of the model are the factors considered for potential estimation; estimation of electrical usage pattern for typical day, amount of electricity savings and savings during the peak load. The framework is general and enables accurate estimation of potential of solar water heating for a city, block. Energy planners and policy makers can use this framework for tracking and promotion of diffusion of solar water heating systems.     

Design of a solar-driven methanol steam reforming receiver/reactor with a thermal storage medium and its performance analysis

A novel method of triple line focused on solar-powered receiver/reactor with a thermal storage medium for methanol steam reforming (MSR) hydrogen production is proposed in this paper. The photo-thermal-chemical energy conversion and coupling equations of the receiver/reactor are established, and the dynamic regularity between solar radiation and the hydrogen production characteristics is obtained by numerical simulation. The results show that a high solar radiation intensity helps to stabilize the duration of the reaction. For every 100 W m⁻² increase in the solar radiation intensity, the duration of the reaction maintained at the phase change point temperature of the phase change material (PCM) increases by approximately 11%. The daily hydrogen production performance of the system in Kunming (102°43′E and 25°02′N) during typical solar days is studied. The average annual total hydrogen production per unit of the lighting area is approximately 1300 m³. This research can guide similar issues related to solar thermochemical technology.     

Surface roughness effects on the solar reflectance of cool asphalt shingles

We analyze the solar reflectance of asphalt roofing shingles that are covered with pigmented mineral roofing granules. The reflecting surface is rough, with a total area approximately twice the nominal area. We introduce a simple analytical model that relates the “micro-reflectance” of a small surface region to the “macro-reflectance” of the shingle. This model uses a mean field approximation to account for multiple scattering effects. The model is then used to compute the reflectance of shingles with a mixture of different colored granules, when the reflectances of the corresponding mono-color shingles are known. Simple linear averaging works well, with small corrections to linear averaging derived for highly reflective materials.Reflective base granules and reflective surface coatings aid achievement of high solar reflectance. Other factors that influence the solar reflectance are the size distribution of the granules, coverage of the asphalt substrate, and orientation of the granules as affected by rollers during fabrication.     

Applicability of highly reflective aluminium coil for solar concentrators

Because of their manufacturing flexibility and their low costs, mirrors based on anodized or coated sheet aluminium are a promising alternative as primary or secondary concentrators in a number of solar energy applications. They offer solar weighted reflectances of 88–91%, good mechanical properties and are easy to recycle. However, problems occur due to their limited corrosion resistance. Therefore, prior to application, lifetime tests including outdoor and accelerated ageing tests are necessary to prove their optical durability in terms of achieving a 10-year service lifetime. In this study the optical properties of a number of different aluminized reflector materials after accelerated and outdoor exposure tests have been investigated. Optical testing has been performed by measuring the spectral hemispherical reflectance of exposed samples and calculating the solar weighted value. Additionally, specular reflectance has been measured with a simple mobile reflectometer. Materials involved are standard commercial anodized sheet aluminium with layers of different thicknesses and standard high specular aluminium with a metaloxide layer system plus an anti-oxidation polymer coating. Results show that optical degradation is strongly dependent on climatic conditions. Non-organic coatings involved are primarily attacked by humid climates with higher amounts of atmospheric pollution. Standard anodized materials withstand outdoor and accelerated weathering. However reflectance tends to become less specular, which limits their application in concentrating technologies. Finally, small scale application tests have been performed to demonstrate the applicability concerning handling and mechanical connection with support structures. By measuring power density in the focus of a test collector, minimum specular reflectance requirements for trough systems can be defined.     

Performance of solar still with a concave wick evaporation surface

Surfaces used for evaporation and condensation phenomenon play important roles in the performance of basin type solar still. In the present study, a concave wick surface was used for evaporation, whereas four sides of a pyramid shaped still were used for condensation. Use of jute wick increased the amount of absorbed solar radiation and enhanced the evaporation surface area. A concave shaped wick surface increases the evaporation area due to the capillary effect. Results show that average distillate productivity in day time was 4.1 l/m² and a maximum instantaneous system efficiency of 45% and average daily efficiency of 30% were recorded. The maximum hourly yield was 0.5 l/h. m² after solar noon. An estimated cost of 1 l of distillate was 0.065 $ for the presented solar still.     

Experimental investigation of high temperature congregating energy solar stove with sun light funnel

A solar stove which uses a light funnel to guide light and congregate solar energy has been designed. Its structure and operation principle have been introduced. The performance tests under the real weather have been carried out and the graphic lines of experiment have been given. The experimental result shows that the maximum temperature inside the stove is as high as 250 °C under the condition of 1.5 m² of lighting area, 70% reflectivity of reflecting aluminum foil inside surface of concentrator and no load (without water inside the coil pipe). When reflectivity is 86% the heat collecting efficiency of the device is about 43%. The collecting power that the stove receives can be up to 500 W. It is an ideal medium and high temperature solar energy congregating device suitable for industrial usage or cooking and other domestic usage.     

Swedish solar heated residential area with seasonal storage in rock: Initial evaluation

A partly solar heated building area comprising 50 residential units has been built in Anneberg, Sweden. The system includes low-temperature space heating with seasonal ground storage of solar heat. Heating is supplied by 2400 m² solar collectors and individual electrical heaters for supplementary heating. The ground storage comprises about 60,000 m³ of crystalline rock with 100 boreholes drilled to 65 m depth and fitted with double U-pipes. The collectors will have favourable working conditions but the store is rather small, the estimated heat loss from the heat store is about 40% of stored solar heat and the average solar fraction is estimated to 70% after 3–5 years of operation. An initial evaluation after 2 years of operation shows that, although problems have occurred and several parts seem to work less efficient than expected, the overall system idea works as intended.     

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.     

Transient simulation and comparative assessment of a hydrogen production and storage system with solar and wind energy using TRNSYS

This paper uses the TRNSYS software to investigate the hourly energy generation potential, storage, and consumption via an electrolyzer and a fuel cell in the Canadian city of Saskatoon, which is a region with high solar and wind energy potential. For this purpose, a location with an area of 10,000 m² was considered, in which the use of solar panels and vertical-axis wind turbines (VAWTs) were simulated. In the simulation, the solar panels were placed at specific distances, and the energy generation capacity, amount of produced hydrogen, and the energy available from the fuel cell were examined hourly and compared to the case with wind turbines placed at standard distances. The results indicated energy generation capacities of 1,966,084 kWh and 75,900 kWh for the solar panels and the wind turbines, respectively, showing the high potential of solar panels compared to wind turbines. Moreover, the fuel cells in the solar and wind systems can produce 733,077 kWh and 22,629 kWh of energy per year, respectively, if they store all of the received energy in the form of hydrogen. Finally, the hourly rates of hydrogen production by the solar and wind systems were reported.     

Design of direct solar PV driven air conditioner

Solar air conditioning system directly driven by stand-alone solar PV is studied. The air conditioning system will suffer from loss of power if the solar PV power generation is not high enough. It requires a proper system design to match the power consumption of air conditioning system with a proper PV size. Six solar air conditioners with different sizes of PV panel and air conditioners were built and tested outdoors to experimentally investigate the running probabilities of air conditioning at various solar irradiations. It is shown that the instantaneous operation probability (OPB) and the runtime fraction (R_F) of the air conditioner are mainly affected by the design parameter r_pL (ratio of maximum PV power to load power). The measured OPB is found to be greater than 0.98 at instantaneous solar irradiation I_T > 600 W m⁻² if r_pL > 1.71. R_F approaches 1.0 (the air conditioner is run in 100% with solar power) at daily-total solar radiation higher than 13 MJ m⁻² day⁻¹, if r_pL > 3.     

Optimal angles for harvesting solar electricity in some African cities

In recent years, the quest for renewable and sustainable energy has been extensive while solar energy has been on the vanguard of sustainable alternative and renewable energy sources due to its clean nature and cost effectiveness for most human activities such as water pumping and electric power generation, amongst others. The off beam installation of Photovoltaic (PV) modules has posed a severe challenge to the optimal functioning of these PV cells despite the abundance of solar irradiation receivable in most African cities. This paper presents the Optimal Inclination Angles (OIA) for mounting PV modules in the absences of a mechanized or automated solar tracking device, for optimum yield in solar electricity generation for some selected African cities using the Photovoltaic Geographic Information Systems (PVGIS) dataset. The OIA of the selected African cities has been identified for optimal solar irradiation exploitation and if the modules are mounted on a horizontal plane, it is expected that considerable amount of solar irradiation would not be harnessed as it has been estimated using the difference from the Irradiation on OIA (Hopt) and the Irradiation on horizontal plane (Hh), whose difference shows that the northern African cities, Algiers, Rabat and Tripoli, are seen to have high levels in unutilized solar irradiation of 780 Wh/m², 760 Wh/m² and 680 Wh/m² respectively while Harare, Lusaka, Maiduguri, Khartoum, Maputo and Luanda would have considerably high levels in untapped solar irradiation of 360Wh/m², 330 Wh/m², 180 Wh/m², 260 Wh/m², 570 Wh/m² and 80 Wh/m² respectively if PV modules are mounted on horizontal plane. However cities such as Bangui, Abidjan and Mogadishu have quite low levels in unexploited solar irradiation of 40 Wh/m², 70 Wh/m² and 10 Wh/m² respectively when PV modules are mounted on horizontal plane. These differences show the amount of solar irradiation which if adequately harnessed, adds to the solar energy potentials of the region.     

Estimation of rooftop solar photovoltaic potential using geo-spatial techniques: A perspective from planned neighborhood of Karachi – Pakistan

With an ever increasing population and significant solar resource availability throughout the whole year, Karachi metropolis hold a promising rooftop solar photovoltaic (PV) potential considering its millions of urban households. This research work highlights techniques to combine geographic information systems (GIS) and object-based image recognition approach to identify the available rooftop area for PV deployment in a small scale region of DHA Phase 7 Karachi. A six step methodology has been adopted for the estimation of rooftop PV potential which involves geographic division of high resolution satellite imagery; sampling and rooftop feature extraction using FE tool of ENVI EX software; extrapolation of rooftop areas for the entire ROI using roof area-population relationship; visual inspection to analyze different rooftop factors such as building orientation, shading effect from trees and nearby buildings and other roof uses; a comparison of extracted rooftops to the physically measured sample rooftops, reduction for shading and other uses; and finally conversion to energy and power outputs. A relationship of total roof area and population of 13 m²/capita ±5% has been found. With higher efficiency rooftop PV panels, 12.24 MW of solar power can be generated which is 122.4% of peak power demand of DHA Phase 7.     

Cost analysis of different solar still configurations

The enhancement of the productivity of the solar desalination system, in a certain location, could be attained by a proper modification in the system design. Therefore, different design configurations could be found in literatures. However, the increase in the system productivity with high system cost may increase also the average annual cost of the distillate. Cost analysis of different design configurations of solar desalination units is essential to evaluate the benefit of modification from the economical point of view. The main objective of this work is to estimate the water production cost for different types of solar stills. In this paper 17 design configurations are considered. Systems with higher and lower values of productivity are considered in this investigation. A simplified model for cost analysis is applied in this study. The results show that, the best average and maximum daily productivity are obtained from solar stills of single-slope and pyramid-shaped. The higher average annual productivity for a solar still is about 1533 l/m² using pyramid-shaped while the lower average annual productivity is about of 250 l/m² using modified solar stills with sun tracking. The lowest cost of distilled water obtained from the pyramid-shaped solar still is estimated as 0.0135 $/l while highest cost from the modified solar stills with sun tracking is estimated as 0.23 $/l.     

Optical properties, durability, and system aspects of a new aluminium-polymer-laminated steel reflector for solar concentrators

A newly developed aluminium-polymer-laminated steel reflector for use in solar concentrators was evaluated with respect to its optical properties, durability, and reflector performance in solar thermal and photovoltaic systems. The optical properties of the reflector material were investigated using spectrophotometer and scatterometry. The durability of the reflector was tested in a climatic test chamber as well as outdoors in Älvkarleby (60.5°N, 17.4°E), Sweden. Before ageing, the solar weighted total and specular reflectance values were 82% and 77%, respectively, and the reflector scattered light isotropically. After 1 year's outdoor exposure, the total and specular solar reflectance had decreased by less than 1%. However, after 2000 h in damp heat and 1000 W/m² simulated solar radiation, the optical properties had changed significantly: The light scattering was anisotropic and the total and specular solar reflectance values had decreased to 75% and 42%, respectively. The decrease was found to be due to degradation of the protective polyethylene terephthalate (PET) layer, caused by UV radiation and high temperature. The conclusions are that the degradation is climate dependent and that PET is not suitable as a protective coating under extreme conditions, such as those in the climatic test chamber. However, the results from outdoor testing indicate that the material withstands exposure in a normal Swedish climate.     

Experimental study for natural ventilation on a solar chimney

Thermal performance of a solar chimney for natural ventilation was experimentally investigated. The experimental model was implemented on full scale and real meteorological conditions, so that experimental results will be compared with the simulation results. The results show that for a maximum irradiance of 604 W/m², occurring around 13:00 h on September 15th, 2007, a maximum air temperature increment of 7 °C was obtained through the solar chimney. Also, a volumetric air flow rate ranging from 50 to 374 m³/h was measured on that day. Thus, an average air flow rate of 177 m³/h was achieved from 0:00 h to 24:00 h. The experimental solar chimney discharge coefficient, C_d, was 0.52. This coefficient is useful to determine the mass flow rate in the solar chimney design. It was observed that the air flow rate through the solar chimney is influenced by a pressure difference between input and output, caused by thermal gradients and wind velocity, mainly.     

Induced flow for ventilation and cooling by a solar chimney

An experimental and numerical model of a solar chimney was proposed in order to predict its performance under varying geometrical features in Iraqi environmental conditions. Steady, two dimensional, turbulent flow was developed by natural convection inside an inclined solar chimney. This flow was investigated numerically at inclination angles 15° to 60°, solar heat flux 150–750 W/m² and chimney thickness (50, 100 and 150) mm. The experimental study was conducted using a single solar chimney installed on the roof of a single room with a volume of 12 m³. The chimney was 2 m long; 2 m wide has three gap thicknesses namely: 50, 100 and 150 mm. The performance of the solar chimney was evaluated by measuring the temperature of its glass cover, the absorbing wall and the temperature and velocity of induced air. The results of numerical model showed that; the optimum chimney inclination angle was 60° to obtain the maximum rate of ventilation. At this inclination angle, the rate of ventilation was about 20% higher than 45°. Highest rate of ventilation induced with the help of solar energy was found to be 30 air changes per hour in a room of 12 m³ volumes, at a solar radiation of 750 W/m², inclined surface angle of 60°, aspect ratio of 13.3 and chimney length of 2 m. The maximum air velocity was 0.8 m/s for a radiation intensity of 750 W/m² at an air gap of 50 mm thickness. No reverse air flow circulation was observed even at the largest gap of 150 mm. The induced air stream by solar chimney can be used for ventilation and cooling in a natural way (passive), without any mechanical assistance.     

Land-use requirements and the per-capita solar footprint for photovoltaic generation in the United States

In this report, we estimate the state-by-state per-capita “solar electric footprint” for the United States, defined as the land area required to supply all end-use electricity from solar photovoltaics (PV). We find that the overall average solar electric footprint is about 181 m² per person in a base case scenario, with a state- and scenario-dependant range from about 50 to over 450 m² per person. Two key factors that influence the magnitude of the state-level solar electric footprint include how industrial energy is allocated (based on location of use vs. where goods are consumed) and the assumed distribution of PV configurations (flat rooftop vs. fixed tilt vs. tracking). We also compare the solar electric footprint to a number of other land uses. For example, we find that the base case solar electric footprint is equal to less than 2% of the land dedicated to cropland and grazing in the United States, and less than the current amount of land used for corn ethanol production.     

Performance assessment study of photo-electro-chemical water-splitting reactor designs for hydrogen production

Solar water splitting is considered a greatly promising technique for producing clean hydrogen fuel. However, limited studies have paid attention to the designs of photo-electrochemical (PEC) reactors. In this regard, two different designs of PEC reactor are proposed and studied numerically in the present paper. The effects of important design parameters on the system performance are also investigated. The PEC governing equations of transport phenomena related to water splitting reactor are developed and numerically solved. According to the current results, the rate of the hydrogen volume production and the solar - to - hydrogen conversion efficiency increase as an applied solar incident flux increases for both proposed designs. The solar - to - hydrogen conversion efficiencies are calculated to be 12.65% for design 1 and 12.48% for design 2. The hydrogen volume production rate is performed to achieve 78.3 L/m² h by design 1, and 74.8 L/m² h by design 2.