Solar radiation estimation and site analysis

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"Turn your face to the Sun and the shadows fall behind you."
(Maori proverb)

Shadowing decreases yield significantly

Vegetation, snow and surrounding objects are the most common causes of shadows
(photo: Denis Lenardic)

The choice of a proper location is the first and the very essential step in solar system design procedure. Even the most carefully planned solar system doesn’t work satisfactory, if the location wasn’t properly chosen. It is critical that the modules are exposed to sunlight without shadowing at least from 9 am to 3 pm; therefore, the properties and values of solar insolation should be studied. The modules have to be fixed with proper tilt angle allowing the system efficient operation.


Solar radiation potencial estimation
For estimation of electricity generated for predefined location (country and city) a PV Potencial Estimation Utility is available on IES web pages. Calculations for Europe, Africa and Mediteranian possible. The tool offers many different calculation options: PV potencial with given inclination and orientation Calculation of optimal inclination for given orientation Calculation of optimal inclination and orientation
PV-Calculator - With PV-Calculator you may calculate yield of particular PV systems. Calculation is based on precise global radiation data for choosen location. Tool is available in german language only.
Additional information can be found in: A Quasi-Physical Model for Converting Hourly Global Horizontal to Direct Normal Insolation; Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces;

Sun path

Understanding the motion of the Sun is essential for proper solar systems design and the choice of proper location for solar collectors or photovoltaic modules. The solar path is described with the Sun path diagram. The Sun path diagram is a very useful tool in the early photovoltaic system design phase for shadowing determining. The most important geometrical parameters, which describe Earth-Sun relations, include declination /δ/, sun height /α/ and Solar azimuth /φ/. They are defined as follows:

Declination equation

Solar azimuth equation

h = ±15^o· ( number of hours from/to solar noon )

Sun path diagram, created with SunOrb 1.2

Solar azimuth

Sun height

According to the equations above we can estimate the most proper photovoltaic modules tilt employing the following equation:

Module tilt - equation 2

Declination

Module tilt angles

Tilt angle for different latitudes

Declination changes throughout the year, therefore, the optimal tilt angle changes as well. The optimal tilt angle of 46° N latitude can be seen in the picture above.

Tilt and azimuth angle of photovoltaic modules

The proper tilt and azimuth angle choice is by far more important for photovoltaic systems design than solar thermal system design. Manual or automatic tilt angle adjustment can increase the total light-electricity conversion up to 30 % and more in locations with high values of solar radiation. Incidence angle should be as close to 90° as possible. Photovoltaic module tilt angle and location choice in general require more care than solar collectors tilt angle and location choice. Shaded locations, including partially shaded, are not suitable for photovoltaic module fixation. Modules should be south oriented. The following general recommendations should be considered, if you design a photovoltaic system:

Yearly average maximum output power – the photovoltaic modules tilt angle should equal local latitude.

Maximum output power in winter – the photovoltaic modules tilt angle should equal local latitude + 15° (max +20°). Such a tilt angle is a good solution in areas, where the winter load is greater than the summer load. The electricity consumption for lighting is greater during winter than summer.

Manual photovoltaic module tilt angle adjustment – in small systems modules should be fixed in a way, which allows manual adjustment of the module tilt angle. In March the tilt angle should be adjusted to equal latitude, in May the tilt angle equals latitude minus 10 degrees, in September the tilt angle equals latitude and in December the tilt angle equals latitude plus 10 degrees. With such an adjustment the maximal efficiency could be obtained throughout the year.

Accurate and maximum energy output of larger systems should be based on exact calculations, because energy output is influenced by different factors, such as local climatic conditions (solar radiation availability in different seasons, local cloudiness or fogginess in winter, temperature and so on). You will need a long-term solar radiation data for the chosen location.

Tools for site analysis
	Sun Position Calculator On Line tool by Volker Quaschning. Calculation of Sun Position, Sunrise and Sunset.
	Panorama master with "Panorama master" you can make exact matched set of photos - base for panorama picture (see also "Horizon" below).
	Horizon software simulates sun path diagrams for arbitrary latitude.
	HORIcatcher HORIcatcher is an easy and fast tool to take outdoor pictures of the horizon. The pictures can be used to determine the solar energy input, sunshine duration and sun exposure reduced by obstacles like trees, houses or mountains. HORIcatcher is supplied with a digital camera.
	Sonnenbahn Indikator Pro is a simple tool for site evaluation.
	SunEye™ The Solmetric SunEye™ (patent pending) is a hand held electronic device that allows users to instantly assess total potential solar energy given the shading of a particular site. Identifying the shading pattern early in the process reduces the expense of system and home design and improves the efficiency of the final system or house.
	Sombrero A PC-tool to calculate shadows on arbitrarily oriented surfaces. For both, active use of solar energy (domestic hot water, photovoitaics) as well as for passive solar architecture, shading or lighting of planes plays an important role SOMBRERO provides quantitative results for the shading of collectors or windows by buildings, trees, overhangs or the horizon.
	Pilkington Sun Angle Calculator This handy tool provides a relatively simple method of determining solar geometry variables for architectural design, such as designing shading devices or locating the position of the sun relative to a particular latitude and time.
	Solar Pathfinder The Solar Pathfinder has been the standard in the solar industry for solar site analysis for decades. Its panoramic reflection of the site instantly provides a full year of accurate solar/shade data, making it the instrument of choice.
	Sun path chart this program creates sun path charts in cartesian or polar coordinates for "typical" dates of each month (i.e.; days receiving about the mean amount of solar radiation for a day in the given month).
Solar radiation and site analysis - related web sites
	think SunSmart - At Think SunSmart, we master the art of solar resource estimation. That's the study of how much energy is provided by the sun under different weather conditions. The use of satellite earth observations has brought a whole new dimension to solar resource assessment. High resolution solar energy data are now available to solar system owners everywhere in the United States offering a wealth of new opportunities. It's now possible to monitor solar systems from space and to spot microclimates on high-resolution maps of solar energy. Languages:
	Renewable resource Data Center - The RReDC provides information on several types of renewable energy resources in the United States, in the form of publications, data, and maps. An extensive dictionary of renewable energy related terms is also provided. Languages: Webmaster's choice - solar radiation terms, glossary, alghoritms, models etc.
	Square One research - Environmental Design Website, compiled by Dr. Andrew Marsh and Caroline Raines of Square One research and the Welsh School of Architecture at Cardiff University. This site provides free information for architects, building designers, students and anyone else interested in energy efficient and sustainable design. Languages: Webmaster's choice - Sun path diagrams explained in detail - very nice web site about renewable energy use and sustainable design.
	Logiciel CalSol Estimation de l'énergie solaire disponible pour une application ´nergétique. Languages:
	GIS Assessment of Solar Energy Resource in Europe - On this site you can find an information on GIS-based inventory of solar energy resource and estimations of the potential photovoltaic (PV) electricity generation in Europe. Languages: Webmaster's choice - PV potencial in Europe - estimations, maps, charts and detailed explanations of calculation methods. Excellent site for profesionals!
	METEONORM - Global Meteorological Database for Solar Energy and Applied Meteorology. METEONORM is a comprehensive climatological database for solar energy applications: a meteorological database containing comprehensive climatological data for solar engineering applications at all points of the globe between the polar circles; a computer program for climatological calculations; a data source for engineering design programs in the passive, active and photovoltaic application of solar energy with comprehensive data interfaces; a standardization tool permitting developers and users of engineering design programs access to a comprehensive, uniform data basis. Languages:
	Volker Quaschning - private web site about renewable energy use and application. Languages: Webmaster's choice - a lot of useful documents available for download. Shadowing analysis and simulation explained in detail.
	Photovoltaics and snow - private PV related web site. Interesting section about snow and photovoltaic modules also included. Available in dutch language only. Languages:

More information and some useful on-line tools

Sun position Calculator.
Meeus, J.: Astronomical Algorithms; Willman-Bell Inc., Richmond, VI, 1999.
Scharmer, K, Greif, J.: The European Solar Radiation Atlas, Vol. 1: Fundamentals and maps.
Scharmer, K, Greif, J.: The European Solar Radiation Atlas, Vol. 2: Database and exploitation software.