.Buildings.Fluid.SolarCollectors.BaseClasses.ASHRAESolarGain

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 ASHRAE93. The solar heat gain is calculated using Equations 555 - 559 in the referenced EnergyPlus documentation.

The solar radiation absorbed by the panel is identified using Eq 559 from the EnergyPlus documentation. It is

Q_Flow[i]=A_c/nSeg (F_R(τα) K_(τα)net (G_Dir (1-shaCoe)+G_Dif,Sky+G_Dif,Gnd))

where Q_Flow[i] is the heat gain in each segment, A _c is the area of the collector, nSeg is the user-specified number of segments in the simulation, F_R(τα) is the maximum collector efficiency, K_(τα)net is the incidence angle modifier, G_Dir is the direct solar radiation, shaCoe is the user-specified shading coefficient, G_Dif,Sky is the diffuse solar radiation from the sky, and G_Dif,Gnd is the diffuse radiation from the ground.

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 (parameter: 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 incidence angle modifier used in the solar radiation equation is found using Eq 556 from the EnergyPlus documentation. It is

K_(τα),net=(G_Beam K_{(τα), Beam}+G_Dif,Sky K_(τα),Sky+G_Dif,Gnd K_(τα),Gnd) / (G_beam+G_Dif,Sky+G _Dif,Gnd)

where K_(τα),net is the net incidence angle modified, G_Beam is the beam radiation, K_(τα),Beam is the incidence angle modifier for beam radiation, G_Dif,Sky is the diffuse radiation from the sky, K_(τα),Sky is the incidence angle modifier for radiation from the sky, G_{Dif, Gnd} is the diffuse radiation from the ground, and K_(τα),Gnd is the incidence angle modifier for diffuse radiation from the ground.

Each incidence angle modifier is calculated using Eq 555 from the EnergyPlus documentation. It is

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

where x can refer to beam, sky or ground. θ is the incidence angle. For beam radiation θ is found via standard geometry. The incidence angle for sky and ground diffuse radiation are found using, respectively, Eq 557 and 558 from the EnergyPlus documentation. They are

θ_sky=59.68-0.1388 til+0.001497 til²
θ_gnd=90.0-0.5788 til+0.002693 til²

where θ_sky is the incidence angle for diffuse radiation from the sky, til is the tilt of the solar thermal collector, and θ_gnd is the incidence angle for diffuse radiation from the ground.

These two equations must be evaluated in degrees. The necessary unit conversions are made internally.

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.
ASHRAE 93-2010 -- Methods of Testing to Determine the Thermal Performance of Solar Collectors (ANSI approved)

Contents

Name	Description
Medium	Medium in the system

Revisions

• 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.
• November 20, 2014, by Michael Wetter:
  Added missing each keyword.
• Jan 15, 2013, by Peter Grant:
  First implementation