Software Tools

SunThink! is a computer application SunWork built as a resource for designing solar systems. It gives an accurate and all-embracing picture of the solar resource available for a particular project site, and it aids in determining the system size needed to meet specific solar system design goals.

SunThink! – What It Does

SunThink! is an open-source program that assists in the good design of solar electricity systems. SunThink! runs within Scilab, an open-source platform for numerical computation. SunThink! includes modules that enable calculation of key information for solar electricity systems:

• comprehensive orientation factors (comparing available solar irradiation for a plane of a given orientation to the irradiation available to a plane at a south-facing azimuth and tilt equal to the latitude), sensitive to atmospheric thickness and local cloud-coverage history if entered;
• shading profile factors (quantifying the effect of shading from described sources on the amount of irradiation available at one or more points);
• designed system attributes such as wiring losses; and
• aspects of system generation outlook including peak, annual, monthly, and time-of-use weighted generation.

SunThink’s primary purpose is to facilitate the accurate, appropriate sizing of these systems to fit specific design goals.

The computations performed by SunThink! are based on sums of evaluated integrals of direct and diffuse irradiance, involving mathematically-expressed sun position, plane orientation, and global and plane sunrise and sunset, and using a fourth-order cosine-polynomial fit of the Young (1992) interpolative formula for airmass with the Meinel experimentally determined expression for intensity to compensate for atmospheric extinction. This is modified for cloud coverage using a second-order fit of specific local historical cloud-coverage behavior with an expression tuned according to available historical NREL insolation data for sites at different geographic and climatic locations.

SunThink! provides several key aspects of functionality that are not present in PVWATTS, the National Renewable Energy Laboratory performance calculator for grid-connected photovoltaic systems. These include the tailoring of data to the cloud-coverage patterns of a region, the ability to forecast production losses from shading based on sets of angular shade-source data, and the ability to calculate weighted production numbers according to the relative weighting of electricity price in time-of-use utility rate schedules.

SunThink! – How To Use It

To run SunThink!, you must first have Scilab installed on your computer. Scilab is a free, open- source platform for numerical computation. It is available for download for Windows and other operating systems at www.scilab.org. The current version of SunThink was designed with the 5.2.2 release of Scilab, and it has only been tested with this release. Download Scilab 5.2.2 here (you will need to scroll down the page to select the operating system that you need.)

Download SunThink! 1.2 and unzip the compressed files. Next, right click on the SunThink.sce file located inside the downloaded SunThink folder, and select the ‘Run with scilab’ option to load SunThink!