modelLongWaveRadiation
Extends from Modelica.Blocks.Icons.Block (Basic graphical layout of input/output block).
Information
Computes longwave radiation exchange between the sky and a tilted surface using ambient meteorological inputs. The model estimates sky emissivity based on the dew point temperature derived from ambient temperature and relative humidity, following Buck's equation. It then applies the Stefan–Boltzmann law to calculate the net longwave radiation on a tilted surface, accounting for both sky and ground contributions.
Implementation Notes
This model is used in the validation of unglazed photovoltaic–thermal (PVT) collectors ( IDEAS.Fluid.PVTCollectors.Validation.PVT_UI.PVTQuasiDynamicCollectorValidation) where direct longwave irradiance measurements are unreliable or unavailable. It assumes a ground emissivity of 0.95. The clear-sky emissivity is calculated using an empirical correlation based on the dew point temperature. The model is particularly useful in dynamic simulations where longwave radiation must be estimated from standard meteorological data.
References
- Buck, A.L., New equations for computing vapor pressure and enhancement factor, Journal of Applied Meteorology, 1981.
- Meertens, L., Jansen, J., Helsen, L. (2025). Development and Experimental Validation of an Unglazed Photovoltaic‑Thermal Collector Modelica Model that only needs Datasheet Parameters, submitted to the 16th International Modelica & FMI Conference, Lucerne, Switzerland, Sep 8–10, 2025.
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Modelica.Units.SI.Angle | til | Surface tilt (0 for horizontally mounted collector) | |
| Real | pi | Modelica.Constants.pi | Pi constant |
| Modelica.Units.SI.DimensionlessRatio | epsGro | 0.95 | ground emissivity [-] |
| Real | aBuck | 243.5 | Buck constant for dew point [°C] |
| Real | bBuck | 17.67 | Buck constant for dew point [°C] |
Connectors
| Type | Name | Default | Description |
|---|---|---|---|
| Modelica.Blocks.Interfaces.RealInput | Tamb | Ambient temperature [°C] | |
| Modelica.Blocks.Interfaces.RealInput | rH | Relative Humidity [%] | |
| Modelica.Blocks.Interfaces.RealInput | patm | Atmospheric pressure [atm] | |
| Modelica.Blocks.Interfaces.RealInput | Edif_h | Diffuse horizontal irradiation [W/m2] | |
| Modelica.Blocks.Interfaces.RealInput | Eglobh_h | global horizontal irradiation [W/m2] | |
| Modelica.Blocks.Interfaces.RealOutput | lonRad | Longwave radiation [W/m²] |
Components
| Type | Name | Default | Description |
|---|---|---|---|
| Real | Tdew | Dew point temperature [°C] | |
| Real | epsSky | Clear sky emissivity [-] |
Revisions
-
July 7, 2025, by Lone Meertens:
First implementation PVT model. This is for #1436.