Buildings.Fluid.SolarCollectors.BaseClasses

Package with base classes for Buildings.Fluid.SolarCollectors

Information

This package contains base classes that are used to construct the models in Buildings.Fluid.SolarCollectors.

Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).

Package Content

Name	Description
ASHRAEHeatLoss	Calculate the heat loss of a solar collector per ASHRAE standard 93
ASHRAESolarGain	Calculate the solar heat gain of a solar collector per ASHRAE Standard 93
EN12975HeatLoss	Calculate the heat loss of a solar collector per EN12975
EN12975SolarGain	Model calculating solar gains per the EN12975 standard
IAM	Function for incident angle modifer
PartialHeatLoss	Partial heat loss model on which ASHRAEHeatLoss and EN12975HeatLoss are based
PartialParameters	Partial model for parameters
PartialSolarCollector	Partial model for solar collectors
Examples	Collection of models that illustrate model use and test models

Buildings.Fluid.SolarCollectors.BaseClasses.ASHRAEHeatLoss

Calculate the heat loss of a solar collector per ASHRAE standard 93

Information

This component computes the heat loss from the solar thermal collector to the environment. It is designed for use with ratings data collected in accordance with ASHRAE Standard 93. A negative heat loss indicates that heat is being lost to the environment.

This model calculates the heat loss to the ambient, for each segment i ∈ {1, ..., n_seg} where n_seg is the number of segments, as

Q_los,i = -slope A_c ⁄ n_seg (T_env-T_flu,i)

where slope < 0 is the slope for the heat loss, as specified in the ratings data, A_c is the collector area, T_env is the environment temperature and T_flu,i is the fluid temperature in segment i ∈ {1, ..., n_seg}.

This model reduces the heat loss rate to 0 when the fluid temperature is within 1 Kelvin of the minimum temperature of the medium model. The calculation is performed using the Buildings.Utilities.Math.Functions.smoothHeaviside function.

Implementation

ASHRAE uses the collector fluid inlet temperature to compute the heat loss (see Duffie and Beckmann, p. 293). However, unless the environment temperature which was present during the collector rating is known, which is not the case, one cannot compute a log mean temperature difference that would improve the UA calculation. Hence, this model is using the fluid temperature of each segment to compute the heat loss to the environment.

References

J.A. Duffie and W.A. Beckman 2006, Solar Engineering of Thermal Processes (3rd Edition), John Wiley & Sons, Inc.
ASHRAE 93-2010 -- Methods of Testing to Determine the Thermal Performance of Solar Collectors (ANSI approved)

Extends from Buildings.Fluid.SolarCollectors.BaseClasses.PartialHeatLoss (Partial heat loss model on which ASHRAEHeatLoss and EN12975HeatLoss are based).

Parameters

Connectors

Modelica definition

Buildings.Fluid.SolarCollectors.BaseClasses.ASHRAESolarGain

Calculate the solar heat gain of a solar collector per ASHRAE Standard 93

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)

Extends from Modelica.Blocks.Icons.Block (Basic graphical layout of input/output block), SolarCollectors.BaseClasses.PartialParameters (Partial model for parameters).

Parameters

Connectors

Modelica definition

Buildings.Fluid.SolarCollectors.BaseClasses.EN12975HeatLoss

Calculate the heat loss of a solar collector per EN12975

Information

This component computes the heat loss from the solar thermal collector to the environment. It is designed anticipating ratings data collected in accordance with EN12975. A negative heat loss indicates that heat is being lost to the environment.

This model calculates the heat loss to the ambient, for each segment i ∈ {1, ..., n_seg} where n_seg is the number of segments, as

Q_los,i = A_c ⁄ n_seg (T_env-T_flu,i) (C₁ - C₂ (T_env-T_flu,i))

where C₁ > 0 is the heat loss coefficient from EN12975 ratings data, C₂ ≥ 0 is the temperature dependence of heat loss from EN12975 ratings data, A_c is the collector area, T_env is the environment temperature and T_flu,i is the fluid temperature in segment i ∈ {1, ..., n_seg}.

This model reduces the heat loss rate to 0 when the fluid temperature is within 1 Kelvin of the minimum temperature of the medium model. The calculation is performed using the Buildings.Utilities.Math.Functions.smoothHeaviside function.

Implementation

EN 12975 uses the arithmetic average temperature of the collector fluid inlet and outlet temperature to compute the heat loss (see Duffie and Beckmann, p. 293). However, unless the environment temperature that was present during the collector rating is known, which is not the case, one cannot compute a log mean temperature difference that would improve the UA calculation. Hence, this model is using the fluid temperature of each segment to compute the heat loss to the environment. If the arithmetic average temperature were used, then segments at the collector outlet could be cooled below the ambient temperature, which violates the 2nd law of Thermodynamics.

References

CEN 2006, European Standard 12975-1:2006, European Committee for Standardization

Extends from Buildings.Fluid.SolarCollectors.BaseClasses.PartialHeatLoss (Partial heat loss model on which ASHRAEHeatLoss and EN12975HeatLoss are based).

Parameters

Connectors

Modelica definition

Buildings.Fluid.SolarCollectors.BaseClasses.EN12975SolarGain

Model calculating solar gains per the EN12975 standard

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

Extends from Modelica.Blocks.Icons.Block (Basic graphical layout of input/output block), SolarCollectors.BaseClasses.PartialParameters (Partial model for parameters).

Parameters

Connectors

Modelica definition

Buildings.Fluid.SolarCollectors.BaseClasses.IAM

Function for incident angle modifer

Information

Overview

This function computes the incidence angle modifier for solar insolation striking the surface of the solar thermal collector. It is calculated using Eq 555 in the EnergyPlus 7.0.0 Engineering Reference.

Notice

As stated in EnergyPlus7.0.0 the incidence angle equation performs poorly at angles greater than 60 degrees. This model outputs 0 whenever the incidence angle is greater than 60 degrees.

References

EnergyPlus 7.0.0 Engineering Reference, October 13, 2011.

Inputs

Outputs

Modelica definition

Buildings.Fluid.SolarCollectors.BaseClasses.PartialHeatLoss

Partial heat loss model on which ASHRAEHeatLoss and EN12975HeatLoss are based

Information

This component is a partial model used as the base for Buildings.Fluid.SolarCollectors.BaseClasses.ASHRAEHeatLoss and Buildings.Fluid.SolarCollectors.BaseClasses.EN12975HeatLoss. It contains the input, output and parameter declarations which are common to both models. More detailed information is available in the documentation for the extending classes.

Extends from Modelica.Blocks.Icons.Block (Basic graphical layout of input/output block), SolarCollectors.BaseClasses.PartialParameters (Partial model for parameters).

Parameters

Connectors

Modelica definition

Buildings.Fluid.SolarCollectors.BaseClasses.PartialParameters

Partial model for parameters

Information

Partial parameters used in all solar collector models

Parameters

Modelica definition

Buildings.Fluid.SolarCollectors.BaseClasses.PartialSolarCollector

Partial model for solar collectors

Information

This component is a partial model of a solar thermal collector. It can be expanded to create solar collector models based on either ASHRAE93 or EN12975 ratings data.

Notice

As mentioned in the reference, the SRCC incident angle modifier equation coefficients are only valid for incident angles of 60 degrees or less. Because these curves behave poorly for angles greater than 60 degrees the model does not calculatue either direct or diffuse solar radiation gains when the incidence angle is greater than 60 degrees.

The heat capacity of the collector without fluid is estimated based on the dry mass and the specific heat capacity of copper. This heat capacity is then added to the model by increasing the size of the fluid volume. Note that in earlier implementations, there was a separate model to take into account this heat capacity. However, this led to a translation error if glycol was used as the medium, because during the translation, the function T_ph for Modelica.Media.Incompressible.Examples.Glycol47 had to be differentiated, but this function is not differentiable.

References

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

Extends from Buildings.Fluid.Interfaces.LumpedVolumeDeclarations (Declarations for lumped volumes), Buildings.Fluid.Interfaces.TwoPortFlowResistanceParameters (Parameters for flow resistance for models with two ports), Buildings.Fluid.Interfaces.PartialTwoPortInterface (Partial model with two ports and declaration of quantities that are used by many models).

Parameters

Connectors

Modelica definition