Site Analysis

INTRODUCTION

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.

SUN PATH

Basic terms

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). Declination (Cooper, 1969) [1] and solar height are defined as follows:

N - day of the year

δ - declination, L - latitude, ω - hour angle [2]

Solar azimuth angle (Φ) is the angle of the sun ray's measured in the horizontal plane due south, westward positive (valid for nothern hemisphere). For southern hemisphere it is measured from due north, eastward positive. In architecture solar azimuth angle is measured from due north, clockwise. For detailed description and additional equations please see literature (Sayigh, 1977).

α - Sun height, L - latitude, ω - hour angle

Notes

Sun height and azimuth

Declination, calculated by Cooper's equation, for details see literature below

Sun path chart

Sun-path chart or Sun-path diagram is simple graphical method to present Sun's apparent movement. Depending on projection used following Sun path charts different sun-path charts are used:

Sun-path chart, equidistant projection, generated by Sun-path Chart Software
(courtesy: University of Oregon, SRML)

TILT AND AZIMUTH OF SOLAR 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. According to the equations above we can estimate the most proper photovoltaic modules tilt (β) employing the following equation (valid only at noon):

δ - declination, L - latitude

Declination changes throughout the year, therefore, the optimal tilt angle changes as well. The optimal tilt angle - at noon - for some latitudes is presented in the picture below. If amount of diffuse radiation is very high then the above calculated tilt angle is not necessary the optimal one. In any case detailed yield evaluation is preferred before construction of PV system(s). For tilt angles at noon for different latitudes please reffer to the image below.

Tilt angle for latitudes 30°, 40°, 50° and 60°

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 AND SOFTWARE FOR SITE ANALYSIS

Tools for site analysis

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.

Solar Pathfinder a powerful non-electronic instrument that includes:
- Printed Manual (online version also available)
- Sunpath Diagrams (latitude & application specific)
- Angle estimator (for determining altitude and azimuth)
- White marking pen with extra leads
- Tool box specifically designed for the Pathfinder™

Solar Pathfinder™ gives an entire year’s solar potential for a given site in just seconds. This non-electronic instrument is ready-to-use upon receipt of shipment. Solar Pathfinder Assistant is companion software that generates monthly sunpaths for each specific site latitude instead of relying on the stock latitude band diagrams. It includes weather data for North America, Central America, Europe, and Southwest Pacific. Click here to learn more.

SunEye™ - The Solmetric SunEye™ 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.

Solmetric iPV - is an iPhone® based site evaluation tool, providing full solar site analysis in an affordable hand held package.

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.

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.

Software for site analysis

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).

Autodesk ECOTECT Analysis - is an industry leading building analysis program that allows designers to work easily in 3D and apply all the tools neccesary for an energy efficient and sustainable future.

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.

Shadow Analyser - Shadow Analyzer is an advanced parametric CAD tool for professionals in the area of Solar Energy Engineering and Architecture.

Shadows - Shadows is a program used to design sundials and astrolabes and it is very useful also in solar energy engineering. Supports plane sundials with polar style of any orientation and reclination, analemmatic sundials, cylindrical and bifilar sundials. Simulates, displays and animates the shadow of the style and offers complete ephemeris of the Sun and draws the Solar Diagram.

Amethyst ShadowFX - Amethyst ShadowFX is a sun and shadow modeling program for architects and town planners. Amethyst ShadowFX enables you to easily generate shadow profiles cast by buildings and other objects for any latitude, longitude and time of year.

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.

Panorama master - with "Panorama master" you can make exact matched set of photos - base for panorama picture.

Horizon - software simulates sun path diagrams for arbitrary latitude.

GOSOL - Mit dem städtebaulichen Simulationsprogramm GOSOL können städtebauliche Planungen solar+energetisch schnell und zielsicher verglichen, bewertet und optimiert werden.
Software available in:

ADDITIONAL INFORMATION

Websites

PhysicalGeography.net - PhysicalGeography.net is an educational web portal that focuses on a specific area knowledge known as Physical Geography.

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.

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.

Logiciel CalSol - Estimation de l'énergie solaire disponible pour une application Energétique.
Website availble in:

Introduction to solar motion geometry - Introduction to solar motion geometry on the basis of a simple model; web site by Vyacheslav Khavrus and Ihor Shelevytsky.

PV and snow - Peter Polder's private web site. Interesting section about snow and photovoltaic modules also included. Available in dutch language only.
Website available in:

SOURCES AND ADDITIONAL INFORMATION

Books

	Sayigh, A.A.M., Editor: Solar Energy Engineering; Academic Press, 1977, ISBN 0-12-620850-6.
	Meeus, J.: Astronomical Algorithms; Willman-Bell Inc., Richmond, VI, 1999., ISBN 0-943396-61-1.
	Muneer, T.: Solar Radiation and Daylight Models; Elsevier, Butterworth-Heinemann, Oxford, 2004, ISBN 0-7506-5974-2.
	Scharmer, K, Greif, J.: The European Solar Radiation Atlas, Vol. 1: Fundamentals and maps, Presses des Mines, 2000, ISBN 2-911762-21-5.
	Scharmer, K, Greif, J.: The European Solar Radiation Atlas, Vol. 2: Database and exploitation software, Presses des Mines, 2000, ISBN 2-911762-22-3.

Papers

	Reda, I., Andreas, A.: Solar position algorithm for solar radiation applications; Solar Energy Vol. 76, 2004, p.577-589.
	Reda, I., Andreas, A.: Corrigendum to "Solar position algorithm for solar radiation applications"; Solar Energy Vol. 81, 2007.
	Reda, I., Andreas, A.: Solar Position Algorithm for Solar Radiation Applications; NREL, NREL/TP-560-34302, Jnuary 2008.
	Blanco-Muriel et al.: Computing the solar vector; Solar Energy Vol. 70, 2001, pp. 431-441.
	Cooper, P.,I.: The absorption of radiation in solar stills; Solar Energy Vol. 12, 1969, p.333-345.
	Grena, R.: An algorithm for the computation of the solar position; Solar Energy Volume 82, Issue 5, May 2008, 462-470.
	Walraven, R: Calculation the position of the sun. Solar Energy Vol.20, 1978, pp. 393-397.
	Walraven, R: Erratum. Solar Energy Vol.22, 1979, p.195.
	Wilkinson, B.J.: The effect of atmospheric refraction on the solar azimuth. Solar Energy Vol.30, 1983, p.295.
	Archer, C.B.: Comments on "Calculating the position of the sun". Solar Energy Vol.25, 1980, .91.
	Kambezidis, H.D.; Papanikolaou, N.S.: Solar position and atmospheric refraction. Solar Energy ol. 44, 1990, pp.143-144.
	Muir, L.R.: Comments on "The effect of atmospheric refraction in the solar azimuth". Solar Energy Vol. 30, 1983, p.295.

Other

Calculation of Sun Position, Sunrise and Sunset: Sun Position Calculator.