.Buildings.Fluid.SolarCollectors.BaseClasses.EN12975SolarGain

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 performs calculations using ratings data from EN12975. The solar heat gain is calculated using Equation 559 in the referenced EnergyPlus documentation. The calculation is modified somewhat to use coefficients from EN12975.

The equation used to calculate solar gain is a modified version of Eq 559 from the EnergyPlus documentation. It is

Q_Flow[i] = A_c/nSeg F_R(τα) (K _(τα),Beam G_Beam (1-shaCoe)+K_Diff G _Diff),

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, F_R(τα) is the maximum efficiency of the collector, K_(τα),Beam is the incidence angle modifier for beam radiation, G_Beam is the current beam radiation on the collector, shaCoe is the shading coefficient, K_Diff is the incidence angle modifier for diffuse radiation and G_Diff 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 using the parameter nSeg. The area of an individual segment is identified by dividing the collector area by the total number of segments. The parameter shaCoe is used to define the percentage of the collector which 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 for beam radiation is calculated using Eq 555 from the EnergyPlus documentation, as

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

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

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

EnergyPlus 7.0.0 Engineering Reference, October 13, 2011.
CEN 2006, European Standard 12975-1:2006, European Committee for Standardization

Contents

Revisions

• 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