Site irradiance levels and the relative sun-collector position are two key parameters for the solar plant designer. Once a system is designed (sized), its heating ability at different times of day, on different days of the year is heavily dependent on the site's solar characteristics.
THERMOFLEX provides four ways to input irradiance and relative sun-collector positioning. Each method is designed to make it easy to use assumptions, actual measurements, or data from statistical analysis, such as TMY3 data.
1. Estimated from Site Data
This method is most useful for up-front scoping studies where the plant designer has limited access to detailed site-specific irradiance data, yet still wants to compute a solar model. It’s a great way to “get started” or to compare relative site performance.
THERMOFLEX estimates the DNI and relative sun-collector angles using a model of relative sun-earth positioning as a function of time of day and day of year. Ground-based irradiance is computed using an estimate of atmospheric transmissivity. This atmospheric representation is most applicable for sites with a large number of sunny days per year, those typically most desirable for solar thermal plant siting.
This method makes it easy to pick a time of day, and a day of year for a specified site, and rely on the program to compute irradiance and solar angles.
The input menu for this method is shown below. The Estimated Irradiance panel along the top includes the solar-specific inputs needed to estimate irradiance. Site altitude is set elsewhere. The daily variation in DNI and ANI (Aperture Normal Irradiance) are shown as a function of solar time as the green and blue lines, respectively. The graph title shows a summary of the conditions used to estimate the irradiance together with the length of the solar day. Daily peak ANI and daily average ANI values are shown to the right.
2. User-defined DNI & Local Time
More detailed plant design and simulation often uses irradiance data measured from ground or satellite. This data is available from a number of sources. In the US, data for hundreds of sites is available in TMY3 datasets available from National Renewable Energy Laboratory (NREL). TMY3 data statistically represent conditions at a specific site by analyzing measurements made over decades. Data sources are available for other locations worldwide, some for free and others on a commercial basis. Regardless of the source, the solar data is characterized by site longitude, latitude, altitude, local time , day of year, and irradiance (DNI, diffuse, total).
THERMOFLEX includes this method of solar data input to facilitate use of TMY3 (and similar) data sets. The Site location and current time panel shown on the input menu below lists the input parameters needed. THERMOFLEX computes the solar time from this data using an equation of time. The relative sun-collector positioning is used to compute azimuth and zenith angles associated with the specified day and time.
3. User-defined DNI & Solar Angles
This method is used to directly specify DNI and location of the sun in the sky. THERMOFLEX computes Aperture Normal Irradiance (ANI) from this data using inputs for collector orientation on the earth (N-S, E-W, or other) and tilt from horizontal.
This method is used to specify “typical” irradiance condition for collector design, or when scanning through a range of conditions for off-design simulation.
4. User-defined ANI
This method is used to directly specify Aperture Normal Irradiance (ANI), that is how much beam irradiance falls normal to the collector aperture. As such, it has a single input value. In this case, THERMOFLEX simply applies this value and ignores collector orientation, solar angles, and other inputs that would be used to ultimately compute this quantity.
This approach requires the least amount of input to THERMOFLEX but usually requires the largest amount of independent calculation outside THERMOFLEX to determine this input value.
The diagram below shows the definition of solar angles relative to collector midpoint. The collector is not shown, but may be located with primary axis along N-S, E-W, or anywhere in between. Large collectors are typically installed with zero tilt, but the model allows specification of tilt away from the horizontal if needed.