modelEN12975SolarGain

Information

This component computes the solar heat gain of the solar thermal collector. It only calculates the solar heat gain without considering the heat loss to the environment. This model uses ratings data according to EN12975. The solar heat gain is calculated using Equations 18.298 and 18.302 in the referenced EnergyPlus documentation. The calculation is modified to use coefficients from EN12975.

The equation used to calculate solar gain is a modified version of Eq 18.302 from the EnergyPlus documentation. It is

Q_flow[i] = A_c/nSeg η₀ (K _(τα),dir G_dir (1-shaCoe)+K_dif G _dif),

where Q_flow[i] is the heat gained in each segment, A_c is the area of the collector, nSeg is the number of segments in the collector, η₀ is the maximum efficiency of the collector, K_(τα),dir is the incidence angle modifier for (direct) beam radiation, G_dir is the current beam radiation on the collector, shaCoe is the shading coefficient, K_dif is the incidence angle modifier for diffuse radiation, and G_dif is the diffuse radiation striking the surface.

The solar radiation equation indicates that the collector is divided into multiple segments. The number of segments used in the simulation is specified by the user via nSeg. The area of an individual segment is identified by dividing the collector area by the total number of segments. The term shaCoe is used to define the percentage of the collector that is shaded.

The main difference between this model and the ASHRAE model is the handling of diffuse radiation. The ASHRAE model contains calculated incidence angle modifiers for both sky and ground diffuse radiation, while this model uses a coefficient from test data for diffuse radiation.

The incidence angle modifier is calculated using Eq 18.298 from the EnergyPlus documentation. It is

K_(τα),x=1+b₀ (1/cos(θ)-1)+b₁ (1/cos(θ)-1)²

where K_(τα),dir is the incidence angle modifier for beam radiation, b₀ is the first incidence angle modifier coefficient, b₁ is the second incidence angle modifier coefficient, and θ is the incidence angle.

This model reduces the heat gain rate to 0 W when the fluid temperature is within 1 degree C of the maximum temperature of the medium model. The calculation is performed using the Buildings.Utilities.Math.Functions.smoothHeaviside function.

References

CEN 2022, European Standard 12975:2022, European Committee for Standardization

EnergyPlus 23.2.0 Engineering Reference

Parameters

Connectors

Contents

Name	Description
Medium	Medium in the system

Revisions

• February 28, 2024, by Jelger Jansen:
  Refactor model.
  This is for Buildings, #3604.
• January 12, 2019, by Michael Wetter:
  Added missing each.
• April 27, 2018, by Michael Wetter:
  Corrected displayUnit.
  This is for Buildings, issue 912.
• May 31, 2017, by Michael Wetter and Filip Jorissen:
  Change limits for incident angle modifier to avoid dip in temperature at shallow incidence angles.
  This is for issue 785.
• September 17, 2016, by Michael Wetter:
  Corrected quantity from Temperature to ThermodynamicTemperature to avoid an error in the pedantic model check in Dymola 2017 FD01 beta2.
  This is for issue 557.
• June 29, 2015, by Michael Wetter:
  Revised implementation of heat loss near Medium.T_max to make it more efficient.
• June 29, 2015, by Filip Jorissen:
  Fixed sign mistake causing model to fail under high solar irradiation because temperature goes above medium temperature bound.
• Jan 15, 2013, by Peter Grant:
  First implementation