Solar position algorithm for solar radiation applications

There have been many published articles describing solar position algorithms for solar radiation applications. The best uncertainty achieved in most of these articles is greater than ±0.01° in calculating the solar zenith and azimuth angles. For some, the algorithm is valid for a limited number of years varying from 15 years to a hundred years. This report is a step by step procedure for implementing an algorithm to calculate the solar zenith and azimuth angles in the period from the year −2000 to 6000, with uncertainties of ±0.0003°. The algorithm is described in a book written by Jean Meeus in 1998. This report is written in a step by step format to simplify the complicated steps described in the book, with a focus on the sun instead of the planets and stars in general. It also introduces some changes to accommodate for solar radiation applications. The changes include changing the direction of measuring azimuth angles to be measured from north and eastward instead of being measured from south and eastward, and the direction of measuring the observer’s geographical longitude to be measured as positive eastward from Greenwich meridian instead of negative. This report also includes the calculation of incidence angle for a surface that is tilted to any horizontal and vertical angle, as described by Iqbals in 1983.     

Solar radiation transposition models applied to a plane tracking the sun

Many solar radiation transposition models, from a horizontal plane to any fixed inclined surface, have been compared with measurements from different sites. The tests here were carried out for four top-ranked models and with measurements taken on a plane tracking the sun. The best precision obtained in this study is ±7% on the global solar radiation and ±20% on the diffuse solar radiation with a zero bias.     

Five new algorithms for the computation of sun position from 2010 to 2110

Five algorithms for sun position computation, with validity from 2010 to 2110, are proposed and discussed. The algorithms have different accuracy levels, with maximum errors in the solar position spanning from 0.19° to 0.0027°, thus covering a wide range of possible applications. The algorithms are optimized in order to reduce their computational cost as much as possible. The extended time range of validity allows users to employ these algorithms even in long-term projects.     

Calculating the position of the sun

The procedure for calculating the position of the sun by computer is discussed. The equations used to generate the values in The American Ephemeris and Nautical Almanac are presented in a simplified form that allows the position of the sun to be calculated rapidly to an accuracy of 0.01°.     

Design and development of efficient multipurpose domestic solar cookers/dryers

A truncated pyramid-type solar cooker is designed, fabricated and tested. The truncated pyramid geometry concentrates the incident light radiations towards the bottom and the glazing glass surface on the top facilitates the trapping of energy inside the cooker. One of the salient features of the proposed design is to completely eradicate the need for tracking the sun during cooking, as tracking of sun does not yield better performance. During testing, the highest plate stagnation temperature, under no-load condition, approached 140 °C and under full-load condition, water temperature inside the cooker reached 98.6 °C in 70 min. Two figures of merit, F₁ and F₂, were calculated and their values were 0.117° C m²/W and 0.467 °C l, respectively, meeting the standards prescribed by the Bureau of Indian Standards for solar box-type cookers. Minor modifications in design are recommended to achieve higher temperatures and reduce cooking times. The design also allows trays to be retained for use as a household dryer.     

Novel, low cost CaCl2 based desiccants for solar crop drying applications

Drying with solar-heated air is satisfactory so long as the sun is shining. To continue this process through the night-time and periods of cloud cover, it is necessary to either store some of this energy in a thermal mass or incorporate desiccants within the drying system. This paper reports the results from studies undertaken to develop three low cost, solar regenerative clay–CaCl₂ based solid desiccant materials; establish their moisture sorption and regeneration characteristics; assess their performance when compared with commercial desiccants; and integrate these within a low cost solar drying system for small-scale village-based crop drying. The moisture sorption and desorption performance of the desiccants were characterised in a Fison Environmental Cabinet at conditions of 85% (RH) and 25°C for 120 h for moisture sorption and 50°C and 20% (RH) for 8 h for regeneration. These conditions were representative of the environmental conditions monitored in the solar drying system. The bentonite–CaCl₂ (type 1) desiccant gave a maximum moisture sorption of 45% dry weight basis (dwb) while bentonite–CaCl₂ (type 2) and kaolinite–CaCl₂ (type 3) solid desiccants each gave moisture sorption values of 30% (dwb). It was concluded from the moisture sorption and regeneration characteristics that their application in solar crop drying and air dehumidification is highly useful due to their low regeneration temperatures, sub 100°C.     

Angular distribution of clear sky short wavelength radiance

Over 3000 scans of the clear sky short wavelength (0.3–3.0 μm) radiance were used to produce sky radiance contour maps for solar zenith angles 32° to 80°. The radiance contours are symmetrical about the solar meridian with minimum radiance in the solar meridian at approximately 90° to 110° from the sun. There is no significant change in contour for a change in aerosol optical depth from 0.1 to 0.5. The clear sky radiance is given analytically by N(ψ) = (1.63 + 53.7e−5.49ψ + 2.04 cos² ψ cos θ^*) (1 − e^{−1.90 sec θ}) (1 − e^{−0.53 sec θ}*) where θ^* is the solar zenith angle, θ the zenith angle of the sky radiance direction, and Ψ is the scattering angle between sky and sun directions.     

Enhanced solar energy collection using reflector-solar thermal collector combinations

The amount of direct light gathered by a combination of reflector plus flat-plate collector has been analyzed. The calculations were done allowing variable reflector and collector orientation angles, variables latitude, and arbitrary sun hour angle away from solar noon. The effects of reflection and transmission losses and of polarization of the incident light were included. Correction was also made for the finite size of the reflector. It was found that the optimum orientation has the collector plane almost perpendicular to the plane of the reflector. This optimum orientation is approximately independent of the sun's azimuthal dependence. The optimum reflector angle is found to be between 0° and 10° above the horizon for winter solar conditions. For typical winter operating conditions the enhancement in light gathering power for direct solar radiation is about a factor of 1·4–1·7. This results in an effective increase of 100% in the useful winter heat output from a practical reflector-collector combination with a reflector angle of 0°, over the useful heat output obtained with an optimally oriented simple flat-plate collector. An approximate calculation was also made of the overall enhancement in useful heat output for diffuse solar radiation; an increase by a factor of about 1·5 is predicted. Comparison with the preliminary analysis of the performance of the Coos Bay, Oregon solar house shows substantial agreement with the predictions of the present analysis.     

Site-independent algorithm for obtaining the direct beam insolation from a multipyranometer instrument

The possibility of using the Perez irradiance model for reconstructing the beam and diffuse components of solar radiation from measurements with a fixed multipyranometer system is investigated. It is shown that an iterative algorithm can be derived that enables determination of the direct beam component to about ±50 W/m² without requiring any site-dependent adjustments. The standard error on the derived global irradiance on inclined surfaces of arbitrary orientation is at the ±10 W/m² level. It is concluded that the present degree of precision renders the multipyranometer method an attractive alternative to standard instrumentation for radiation surveys in remote regions. Possibilities for further improvements are discussed.     

塔式太阳能光热电站控制系统研究

  目的  旨在解决塔式太阳能光热电站控制领域的难点问题。  方法  首先基于太阳位置计算天文算法,发明了一种高精度,易于工程实现的定日镜跟踪控制算法;其次对定日镜场控制系统的主要控制技术进行了研究,包括系统架构、硬件与软件设计方案、基于图像处理的反馈测量方案;最后对电站的全厂控制系统进行了整体设计规划并提出了镜场控制系统与DCS主控系统之间的信号接口。  结果  可大幅度降低工程成本,缩短建设周期。  结论  研究成果体系完整、易于工程实现,可用较低的工程造价满足塔式光热电站控制精度的要求。     

Adaptive sun tracking algorithm for incident energy maximization and efficiency improvement of PV panels

In recent times, sun tracking systems are being increasingly employed to enhance the efficiency of photovoltaic panels by constantly tracking the elevation and azimuth angles of the sun. In this paper, a novel adaptive digital signal processing and control algorithm is presented that optimizes the overall PV system output power by adjusting the position angles of the solar panel on both the elevation and azimuth axes. Since the proposed approach is adaptive in nature, the optimal position angles for the solar panel are iteratively computed using the adaptive gradient ascent method, until the incident solar radiation, and hence the output power is maximized. Furthermore, a Taylor’s series approximation is employed for generating a unique optimal position angle increment/decrement at each iteration. Simulation results show that the proposed technique demonstrates fast convergence and excellent tracking accuracy at all times of the day.     

一种高精度太阳位置算法

在聚光型太阳能热发电系统中,聚光装置需要实时跟踪太阳,以提高发电效率,在开环控制的太阳跟踪系统中,太阳位置的计算精度更为重要。比较了几种常用计算太阳位置的简单方法,并总结出一种算法简单的高精度太阳位置计算方法,此方法的高度角、方位角计算最大误差分别为0.05,°0.13°。     

Spectral reflectance properties of black chrome for use as a solar selective coating

The NASA-Lewis Research Center has determined that a widely available, commercially electroplated decorative finish known as black chrome has desirable solar selective properties. Black chrome electroplated coating has high absorbence in the solar spectrum and low emissivity in the 250°F blackbody thermal spectrum.The discovery of the solar selective properties of black chrome adds another high-efficiency coating to the older, previously known coatings. Additionally, the black chrome is significant as a solar selective coating because the current extensive use of black chrome in the electroplating industry as a durable, decorative finish makes black chrome widely available and potentially lower cost as a solar selective coating.The spectral reflectance properties of a commercially prepared black chrome on steel have been measured. Values are presented for reflectance of the black chrome. These are compared with the reflectance of black paint (Nextel) and with two available samples of black nickel which had been prepared for solar selective properties.The reflectance of black chrome, the two black nickels, and black paint integrated over the solar spectrum for air mass 2 were 0·132, 0·123, 0·133 and 0·033, respectively.The reflectance of the black chrome, two black nickels, and black paint integrated over the blackbody spectrum for 250°F from 3 to 15 μm are 0·912, 0·934, 0·891 and 0·033, respectively. These reflectance measurements indicate absorptivity-to-emissivity ( ) values of 9·8, 13·8, 8·0 and 1·00, respectively.     

A high precision tracking system based on a hybrid strategy designed for concentrated sunlight transmission via fibers

This paper investigates a high precision tracking system that adopts the coordinate calculation algorithm and a photosensitive sensor. This system is designed to satisfy the precision requirement in sun tracking for a concentrated sunlight transmitting system via optical fibers. This system is based on a two-stage tracking process, which consists of a coarse adjustment based on the coordinate calculation algorithm and a fine adjustment using a specially designed photosensitive sensor. The core of the photosensitive sensor is a photodiode matrix that could exactly detect the position of the sunlight focal spot via lens focusing. A predictive control process based on the running trend of sun traces will begin once the fine adjustment is completed. The tracking process is steady and accurate because of the predictability of sun traces based on the coordinate calculation algorithm and the acuteness of the photodiode matrix. The highest tracking precision depends on the compactness of the photodiode matrix and has no limits in the accuracy of the coordinate calculation algorithm. The proposed system can track the sun's focal spot with a position precision of less than 0.3 mm, which is the space between the adjacent photodiodes. The tracking angle precision is determined by the ratio of the position resolution of the photodiode matrix to the focal length, and reaches 0.1°.     

Analysis of wind flow around a parabolic collector (1) fluid flow

Applications of parabolic collectors for solar heating and solar thermal power plant increased in the recent years. Most of the solar power plants installed with parabolic collectors are on flat terrain and they may be subjected to some environmental problems. One of problems for large parabolic collector is their stability to track the sun with respect to time very accurately. Any small off tracking as well as the collector structure stability will be affected by strong wind blowing for the regions where the wind velocity is high.In the present study, a two-dimensional numerical simulation of turbulent flow around a parabolic trough collector of the 250 kW solar power plants in Shiraz, Iran is performed taking into account the effects of variation of collector angle of attack, wind velocity and its distribution with respect to height from the ground.Computation is carried for wind velocity of 2.5, 5, 10, and 15 m/s and collector angles of 90°, 60°, 30°, 0°, −30°, −60°, and −90° with respect to wind directions. Various recirculation regions on the leeward and forward sides of the collector are observed, and both pressure field around the collector and total force on the collector are determined for each condition. The effect of absorber tube on the flow field was found negligible, while the effect of the gap between the two sections of parabola at midsection and the gap between the collector and ground were found considerable on both flow field and pressure distribution around the collector.     

A simple photo-voltaic tracking system

This paper describes a simple photo-voltaic (PV) tracking system which has been designed and manufactured using a pyramidal stand as a base. A rotating unit consisting of two pairs of modules fixed at an angle of 170° between them was installed at the upper edge of the stand. The four modules and a DC motor were connected to a bridge circuit making the system sensitive to solar tracking. The PV tracker has a DC–DC (24/2 V) converter, DC–AC (24/220 V) inverter and a battery. The modules are able to provide a maximum power of 100 W, which feeds the converter, inverter and DC motor. Wind resistance of the tracker is quite low. The total area of the four modules is 1.26 m². The inclination angle between the modules and the horizontal plane is adjustable (34°±11°) and the tracking angle is 120°. The system can track solar motion with an error of ±10°. The designed PV tracking system, with modules fixed at an angle of 170° to feed the load as well as the DC motor, exhibited it to be an efficient energy-conversion system. The fabricated system offers low wind resistance. The cost analysis data revealed that the proposed design was very economical and cost effective.     

一种高精度的太阳跟踪方法

为提高太阳能利用率,提出了同时使用视日跟踪和光电跟踪的太阳跟踪新方法。为提高视日跟踪精度,对常见的太阳位置算法仿真比较,得到实用的太阳位置高精度混合算法;同时,设计了特制的四象限硅光电池,对视日跟踪进行误差修正,从而实现了太阳精确跟踪。理论分析和设计表明,该方法能够实现高精度、全天候地自动跟踪太阳,易于推广应用。     

Multi-pressure absorption cycles in solar refrigeration:: A technical and economical study

A technical and economical study of regenerative absorption chillers with multi-pressure cycle has been undertaken as solar operated refrigeration systems. Referred to as advanced absorption chillers they represent one of the new technology options that are under development. Advanced absorption cooling technology offers the possibility of chillers with thermal COPs of 1.5 or greater at driving temperatures of 140°C, which reduces the collector area and the heat rejection requirements compared to current absorption cooling technology. Two different absorption systems have been considered. The first is an advanced, double-effect regenerative absorption cooling system, driven at 140°C, whose efficiency is about 55% of the Carnot efficiency. The second is an ideal, single-effect regenerative absorption system that achieves 70% of the Carnot efficiency driven at 140°C or 200°C. To evaluate the solar performance of a thermally driven chiller requires a separate analysis of the solar availability for a given location compared to the required monthly average solar input. In this analysis different systems, including the vapour compression chillers, have been compared in terms of the thermal and electrical energy input. An effective electrical COP may be computed assuming that the ratio of electrical energy cost to thermal energy cost is four, which is typical of today’s fossil fuel costs. The effective electrical COPs of different technical options can then be compared. Those systems with higher electrical COPs will have lower energy costs. If solar is to be competitive, then the cost of delivered solar thermal energy should be less than the cost of delivered fossil thermal energy.     

Design considerations for solar collectors with cylindrical glass honeycombs

A properly designed cell structure placed between the solar absorber and outer cover glass can substantially reduce natural convection and infrared reradiation heat losses. Glass has merit for such a cellular structure or honeycomb because it is an inexpensive, abundant and stable material with low thermal conductivity and outstanding optical characteristics. To optimize the design of a honeycomb structure, i.e. to minimize the cost of the solar energy collected Z, requires the determination of the honeycomb solar transmission as a function of incidence angles of the sun, infrared effective emittance, cell Nusselt number, and cell wall conductance as well as an estimate of appropriate costs. For an array of circular tubes, the design parameters are wall thickness b, cell diameter d_i, and cell length L. It is difficult to make b less than about 0.2 mm. Typically, d_i must be no larger than 1.6 cm. Increasing decreases reradiation and conduction losses, but also decreases solar energy transmission. For d_i = 9.53 mm and b = 0.198 mm, optimum values ranged from 3 to 12 for collector temperatures (above ambient air temperature) between 22°C and 100°C. Since the Z vs curves have fairly broad minimums, values less than 9 can be used with less than a 3 per cent penalty in cost at the higher temperatures.A comparison of collector effiency characteristics indicated that cylindrical glass honeycomb collectors with nonselective-black absorbers were markedly superior to single-glazed selective-black and double-glazed nonselective-black collectors, especially at higher collector temperatures. Cost effectiveness studies also indicated honeycomb collector superiority at temperature differences between the working fluid and ambient air greater than about 35°C.     

LiMoNAED: a limited motion, non-shading, asymmetric, ecliptic-tracking dish

The conventional design of a parabolic dish for a small solar conversion system places the receiver along the line between the center of the dish and the sun. This forces the receiver to move in a large arc during tracking, and produces some shading of the dish. In some applications, such large movement of the receiver is not acceptable. A new concentrator design is proposed for small systems with a constraint of limited mobility of the receiver. This is accomplished by using a first polar axis and a second axis that is aligned with the normal to the ecliptic plane. The new design features limited motion of the receiver, with inclination changing only within ±23.45°; off-axis reflector to eliminate shading; constant rotation speed in both axes; and constant flux distribution on the receiver.