In the provided climate files, irradiance on the horizontal plane is presented in watts per square meter of reference surface. The program converts this value to the tilted surface during simulation and multiplies it by the total reference surface.
The radiation needs to be divided into diffuse and direct components. This division follows Reindl’s radiation model with reduced correlation, which is dependent on the Clearness Index and solar elevation angle. [Reindl, D.T.; Beckmann, W. A.; Duffie, J.A. : Diffuse fraction correlations; Solar Energy; Vol. 45; No. 1, S.1.7; Pergamon Press; 1990]
Subsequently, these components are converted to irradiance on the tilted surface using the anisotropic sky model by Hay and Davis. [Duffie, J.A.; Beckmann, W.A.: Solar engineering of thermal process; John Wiley & Sons, USA; second edition; 1991]
This model considers the anisotropy factor for circumsolar radiation and the ground reflection factor (= 0.2).
The irradiance on the collector area (reference surface) is calculated from the irradiance intensity (W/m²) on the horizontal plane:
The solar height and azimuth are determined based on the date, time, and latitude.
The position of the sun relative to the collector surface is calculated based on the solar height, solar azimuth angle, collector tilt angle, and collector azimuth angle. This allows the conversion of the direct share of solar radiation on the horizontal plane into the direct share of solar radiation on the collector, considering the reference surface. The position of the sun relative to the collector surface is also needed for calculating the reflected irradiance (see angle correction factor in the collector equation).
Variant menu: System Definitions > Flat-Plate / Tube Collector > Losses / Thermal Losses The energy absorbed by the collector and output to the collector loop less heating losses is calculated as follows:
| with | Gdir | Part of solar irradiation striking a tilted surface |
| Gdiff | Diffuse solar irradiation striking a tilted surface | |
| TKm | Average temperature in the collector | |
| TL | Air temperature | |
| fIAM | Incident angle modifier |
After deduction of optical losses (conversion factor and incident angle modifiers), a part of the absorbed radiation is lost through heat transfer and radiation to the environment. These losses are described by the heat transfer coefficient.
The heat transfer coefficient k states how much heat the collector releases into the environment per square meter of active solar surface and temperature difference between the average collector temperature and the environment in degrees Kelvin.
It is split into 2 parts, the simple and the quadratic part. The simple part ko (in W/m²/K) is multiplied by the simple temperature difference, the quadratic part kq (in W/m²/K²) by its square.
The specific heat capacity states the amount of heat per square meter of active solar surface that the collector, including its heat transfer medium content, can store at a temperature increase of 1 Kelvin. It is stated in Ws/m²K. This decides how quickly the collector reacts to the irradiation. This influence of this value is only significant for relative small pipeline networks, as the capacity of the pipeline network otherwise takes priority.