NASA World Wind (NASA WW) provides open source client and server technologies, based on open standards, to access models of planet Earth, as well as of other planets. NASA WW is developed using Java programming language and provides a software development kit (SDK) for desktop operating systems and for Android. It implements a virtual globe, which is based on the nominal radius of the Earth and augmented with elevation data to implement a digital terrain elevation model (DTM) from available sources [
1,
2]. To this date, the main elevation data source is the shuttle radar topography mission (SRTM) which delivers elevation in 90 m cells averaged from the original 30 m data [
3];
ad hoc Java classes allow the implementation of specific DTMs in order to provide a more accurate model of earth ground elevation data. The scope of NASA WW is not only to provide means for virtual representation of reality through the web and the media, but also to integrate re-usable functions and modules for scientific modelling and analysis. These can be used without any restrictions by researchers to implement dedicated tools to study the numerous inter-linked dynamics that concur to shape our planet’s ecology. The importance of visual representation of modelled dynamics is just as important as analyzing them, as discussed by [
4,
5].
Solar energy is intrinsically of primary importance for life, but also is important in terms of its capability as a source of renewable energy in our highly (and increasingly) energy-demanding societies. Technology has advanced dramatically, improving the efficiency of solar cells capturing incoming energy from light [
6], and having accurate information on the amount of usable solar energy, is of primary importance. For this reason it is valuable to accurately evaluate the potential amount of average energy reaching the spots where such panels are, or will be, installed. A careful simulation in this sense would allow better planning and optimization and would also reduce expenses and probability of errors. As a matter of fact a significant part of the error budget is due to erroneous panel positioning and orientation, or entirely failing to take into account sources of obstruction, near (e.g., neighboring edifices, trees) or far (e.g., morphology of terrain); repositioning of panels, when possible, is quite expensive.
Of the many aspects that make up our environment, the incoming energy (E) from the Sun is without doubt of primary importance. A broad spectrum of wavelengths reaches the atmosphere with a certain energy content. Recent estimation of the mean value of solar radiation reaching the top of atmosphere (ToA) was carried out by the WRC (World Radiation Center) in Davos–Switzerland, with a value of 1367 W/m
2. The atmosphere acts as a filter with different degrees of abatement of the energy (
i.e., transmissivity of the atmosphere) at different wavelengths (λ) depending on the percentage of distribution of gases, temperature and pressure; see [
7] for more details on physical laws regarding this aspect. This energy (which can be expressed as a function of the wavelength,
i.e., E = f(λ)) reaches the Earth’s surface and is available for primary processes,
i.e., photosynthesis, which is responsible for the primary production in the food-chain and thus is responsible for life itself. Carbon fixing is one very important result of this process and has been a critical topic of investigation in the past years. There are specific features of GIS software to calculate spatial energy intensity, and identify potential sites or land constraints [
8], as well as numerous tools for 2-D solar-radiation calculation and many are available online. SOLARFLUX is developed using ARC/INFO version 6.1 (vector-based) and GRID GIS software (raster-based); those modules define the surface topography as a raster-based array (grid) of elevation data [
9,
10]. Solar Analyst is an extension for ArcGIS Desktop (ESRI
©), in its recent versions to this date (9). It generates an upward-looking hemispherical viewshed,
i.e., the angular distribution of sky obstruction calculated for each cell of the input DEM. It then proceeds with overlaying raster information related to Sun and sky conditions at the specified time and day of the year, with allowance for partially obstructed sky sectors [
11]. Both modules require spatial analyst extension to ArcGIS. The geographic resources analysis support system (GRASS) GIS open source environment [
12] provides the
r.sun module implemented using the C language; it calculates the raster map of solar irradiance (for instant time, W/m
2) and irradiation (as a daily sum, Wh/m
2), considering terrain attributes,
i.e., slope, aspect, latitude, and applying a factor parameterizing the attenuation of cloud cover [
13,
14]. GRASS’s
r.sun module is used in the photovoltaic geographical information system (PVGIS) to develop a solar radiation database from climatologic data standardized for Europe and to produce raster maps with a cell resolution of 1 km
2 [
15]. SolarGIS is an online service based on a high resolution climate database, systematically build from satellite and meteorological sources. Derived solar parameters are calculated at a spatial resolution up 80 m using a novel terrain disaggregation method and a DTM derived from SRTM-3 data [
16,
17]. The Solar Energy Planning system (SEP) is a tool developed to support planning and installation of solar water heating panels, photovoltaic panels, passive solar gain; it is implemented in GIS environment (SEPsis) [
18,
19]. Specific software developed for estimating the yield of photovoltaic systems including effects from near and far shadowing are PVsyst [
20] that uses a meteorological database and isotropic sky model, PV*SOL [
21] that offers 2-D and 3-D analysis module.
Several methods have been proposed in the literature that are related to directly or indirectly to the estimation of solar energy incoming towards a surface, but they have not been developed and wrapped in a final user-friendly tool. In [
22] a 1 m cell size is used to calculate shadow casted at each pixel/cell in the digital surface model (DSM). The shadow is considered to be a 3-D straight line starting at the pixel, sharing its height and having a constant slope equal to the Sun altitude. The shadow line is interrupted whenever, along the line, a DSM cell presents a height value Z that is higher than the shadow line at that position. The analysis for estimating radiation is an empirical relation between air mass and atmospheric transmittance. SORAM [
23] uses anisotropic sky condition in order to calculate direct and diffuse light scattering, integrating values from sunrise to sunset. Values are corrected to account for an inclined plane as well as for shadows projected by surrounding objects. 3D-SOLARIA also uses anisotropic sky with a disadvantage of its approach being its current capability only to work with orthogonal surfaces [
24]. An adequate vector-based 3-D GIS for modeling is described in [
25]; the calculation procedure is based on the combined vector-voxel (volumetric pixel) approach, applying a shadowing algorithm that accounts for neighboring values and conditions. Speed and performance depend on voxel resolution: at common scales of resolution, buildings can be seen as blocks without roof structures, whereas a very high level of detail is needed to resolve roofs.
In this project, we have started to create a framework in NASA WW for estimating solar irradiance availability at specific moments in time and locations, with areas sampled with grids with specific spatial resolutions, and considering sources of obstructions such as terrain morphology and nearby buildings. A simple and intuitive panel allows users to add simple building models that can be positioned by the user; some parameters can be customized as well, i.e., building basic size, and roof type. The goal of this research is to deliver maps of solar radiation, providing estimates of the energy that will be available over an area.