modelLongWaveRadiation
Extends from Modelica.Blocks.Icons.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. (2026). Development and Experimental Validation of an Open-Source Photovoltaic‑Thermal Collector Modelica Model that Only Needs Datasheet Parameters. Submitted to Mathematical and Computer Modelling of Dynamical Systems, Special Issue on Modelica, FMI, and Open Standards.
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Modelica.Units.SI.Angle | til | Surface tilt (0 for horizontally mounted collector) | |
| 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] |
Components
| Type | Name | Default | Description |
|---|---|---|---|
| Real | Tdew | Dew point temperature [°C] | |
| Real | epsSky | Clear sky emissivity [-] | |
| 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] |
Revisions
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July 7, 2025, by Lone Meertens:
First implementation PVT model. This is for #1436.