One of the important technical features of EnergyPlus is
that the fundamental zone model includes infrared (IR)
radiation exchange among all surfaces within the zone. A zone
can consist of a single room or multiple rooms having similar
thermal characteristics. The other fundamental characteristic
of a zone in EnergyPlus is that the air within the zone is
modeled with the assumption that it is well-stirred, so it is
characterized by a single uniform air temperature. Normally,
this corresponds to the desired physical state of the air
within a zone. Practically, this is accomplished with the
mixing caused by the air distribution system or even with the
addition of ceiling fans. However, there are situations where
the well-stirred model condition does not fit the physical
conditions within a space (e.g., large atria spaces,
displacement ventilation systems, and under floor air
distribution systems). In these cases, the air temperature
varies spatially within the zone, but the surfaces in the
regions that have different air temperatures still exchange IR
radiation with the other surfaces in the zone, even those in
regions having a different air temperature.
Historically, several approaches have been taken to model
such situations. They all involve unrealistically modifying
the convective heat transfer characteristics within a single
zone to produce reasonable total heat transfer for the zone
while maintaining the zone IR radiation in its basic form. The
IRT model takes a different, more fundamental approach. The
space is divided into subzones, each having the basic
well-stirred air model, but surfaces in these subzones are
able to exchange IR radiation with other surfaces throughout
the original space. Any convective air exchange between
subzones is handled using the existing flexible capabilities
within EnergyPlus. In other words, the subzones are standard
EnergyPlus zones but they have been given the capability of
allowing IR radiation to be exchanged with surfaces in
adjacent zones.
The IR radiation exchange is accomplished by introducing
special IR Transparent (IRT) surfaces to link adjacent zones.
These surfaces transmit the IR radiation incident on them to
the surfaces in adjacent zone. The special surfaces have no
thermal resistance, and they also have zero convective heat
transfer coefficients on both sides. Consider a stratified
atrium as an example. The atrium would be divided into stacked
vertical zones, with each zone having the special IRT surface
between it and the next lower zone. All physical surfaces in
the two zones exchange radiation with the IRT surface, but it
does not impede the transfer of radiation from one zone to the
other. On each side it interacts as a black body surface and
any radiation incident on it is absorbed. Since it has no
other heat transfer mechanisms, it comes to equilibrium with
zero net radiation transfer.
Schematic diagrams of the application of the IRT surfaces
are shown in Figure 1 and
Figure 2. The
first figure shows it applied between two stacked zones that
could be part of an atrium. All radiation incident on the IRT
in the lower and upper zone is completely absorbed by the
surface. It is prevented from participating in the zone air
heat balance by fixing the heat transfer coefficients on
either side at zero.
Schematic View of Stacked Zones
[fig:schematic-view-of-stacked-zones]
The same arrangement can be used for simulating a
underfloor air distribution system (UFAD). The two zones
represent the lower occupied (mixed) zone and the upper
stratified zone. Since the upper zone is being modeled as a
mixed EnergyPlus zone, it is not precisely the stratified zone
concept. However, if a user has concern about having the
entire upper part of the space at a single average
temperature, the space could be modeled with two stacked upper
zones. In that case the stratified temperature profile would
be established by the relative size of the mixing from the
convective plumes. The user would have to supply those
estimates from external knowledge of the behavior of UFAD
systems and plumes.
Schematic Model of a UFAD Zone
[fig:schematic-model-of-a-ufad-zone]
The behavior of the IRT surface can be analyzed by
considering three parallel black plates infinite in extent, as
shown in Figure 3. The
outside plates are held at two different temperatures, and the
intermediate plate assumes an equilibrium temperature when the
net heat flux goes to zero. All plates have black body surface
characteristics.
Radiation Exchange Model [fig:radiation-exchange-model]
Equation [eq:RadExchangeBlackBodyInBetween]
shows that the presence of a black body surface between a
source and a sink reduces the heat flux by a factor of
two.
Ideally, this same doubling of radiation would need to
occur when an IRT surface is between two zones in EnergyPlus.
However, due to the complexity of the EnergyPlus heat balances
and the treatment of an IRT as equivalent to any other surface
in EnergyPlus, simply doubling the impact of the IRT on either
zone is not possible. In testing, this resulted in either net
radiation imbalances within the zones or potentially unstable
conditions. So, the radiation is not doubled in EnergyPlus as
in the simple example in the previous subsection. Despite this
difference between the theory of the ideal case and the actual
implementation in EnergyPlus, an IRT surface will still allow
a zone to be separated into two separate zones with two
different air temperature and the adjacent zones behave as
black body cavities at some equivalent temperature.
It should be noted that for IRT surfaces any visible or
solar short wavelength radiation incident on the surface will
be absorbed and included with the long wavelength (IR)
exchange with the adjacent zone. No energy will be lost, but
zones with IRT surfaces should not be used in any lighting
analyses.
Specifying
an Infrared Transparent (IRT) surface[LINK]
The Infrared Transparent (IRT) surface is similar to a
resistance-only surface. The idd object for this type of
surface is shown below. The fields indicate that the surface
will actually participate in the transfer of visible and solar
radiation by doing a wavelength transformation and making all
short wavelength radiation that is incident on the surface
into long wavelength radiation and having it participate in
the long wavelength radiant exchange. The Material:InfraredTransparent
object requires only a name. All other parameters are set
internally.
The Infrared Transparent surface should not participate in
a convective/conductive exchange between the zones it
separates. In order to minimize this effect, the SurfaceProperty:ConvectionCoefficients
object must be used. Outside and Inside values for the
surface’s convection coefficients should be on the order of
.1. Further examples are given in the Input Output Reference
document.
Infrared transparent surfaces may also be specified using
Construction:AirBoundary
with the IRTSurface option for radiant exchange. This
method automatically generates an internal material layer with
IRT properties and sets the inside convection coefficient to
the Minimum Surface Convection Heat Transfer Coefficient Value
specified in the HeatBalanceAlgorithm
object which defaults to 0.1 [W/m2-K].
The behavior of multi zones separated with infrared
transparent surfaces can be checked with a simple comparison.
Begin with a single zone model as shown below. This model has
a south-facing window, and four walls exposed to wind and sun,
and a roof exposed to wind and sun.
Single Zone Model [fig:single-zone-model]
The single zone model will be compared with a stacked three
zone model that has zones separated by interzone infrared
transparent surfaces. This model is shown below.
Stacked Three Zone Model [fig:stacked-three-zone-model]
The two upper zones have south facing windows whose total
area is the same as the area of the single window in the
single zone model. The top and the sides are again exposed to
sun and wind. The separating surfaces are modeled as IRT
surfaces. All zones in both models are controlled at the same
setpoint temperature using purchased air.
The sensible heating results are shown below. The results
show the sum of the sensible cooling load for the three
stacked zones and the single zone. It is clear that the IRT
surfaces are very effective in transmitting infrared radiation
between the zones. Some small differences, such as those
shown, will occur because of the conversion from short
wavelength solar to infrared through the special IRT dividing
surfaces.
Comparison of Single and Three
Stacked Zones [fig:comparison-of-single-and-three-stacked-zones]
Infrared Radiation Transfer Material[LINK]
One of the important technical features of EnergyPlus is that the fundamental zone model includes infrared (IR) radiation exchange among all surfaces within the zone. A zone can consist of a single room or multiple rooms having similar thermal characteristics. The other fundamental characteristic of a zone in EnergyPlus is that the air within the zone is modeled with the assumption that it is well-stirred, so it is characterized by a single uniform air temperature. Normally, this corresponds to the desired physical state of the air within a zone. Practically, this is accomplished with the mixing caused by the air distribution system or even with the addition of ceiling fans. However, there are situations where the well-stirred model condition does not fit the physical conditions within a space (e.g., large atria spaces, displacement ventilation systems, and under floor air distribution systems). In these cases, the air temperature varies spatially within the zone, but the surfaces in the regions that have different air temperatures still exchange IR radiation with the other surfaces in the zone, even those in regions having a different air temperature.
Historically, several approaches have been taken to model such situations. They all involve unrealistically modifying the convective heat transfer characteristics within a single zone to produce reasonable total heat transfer for the zone while maintaining the zone IR radiation in its basic form. The IRT model takes a different, more fundamental approach. The space is divided into subzones, each having the basic well-stirred air model, but surfaces in these subzones are able to exchange IR radiation with other surfaces throughout the original space. Any convective air exchange between subzones is handled using the existing flexible capabilities within EnergyPlus. In other words, the subzones are standard EnergyPlus zones but they have been given the capability of allowing IR radiation to be exchanged with surfaces in adjacent zones.
The IR radiation exchange is accomplished by introducing special IR Transparent (IRT) surfaces to link adjacent zones. These surfaces transmit the IR radiation incident on them to the surfaces in adjacent zone. The special surfaces have no thermal resistance, and they also have zero convective heat transfer coefficients on both sides. Consider a stratified atrium as an example. The atrium would be divided into stacked vertical zones, with each zone having the special IRT surface between it and the next lower zone. All physical surfaces in the two zones exchange radiation with the IRT surface, but it does not impede the transfer of radiation from one zone to the other. On each side it interacts as a black body surface and any radiation incident on it is absorbed. Since it has no other heat transfer mechanisms, it comes to equilibrium with zero net radiation transfer.
Schematic diagrams of the application of the IRT surfaces are shown in Figure 1 and Figure 2. The first figure shows it applied between two stacked zones that could be part of an atrium. All radiation incident on the IRT in the lower and upper zone is completely absorbed by the surface. It is prevented from participating in the zone air heat balance by fixing the heat transfer coefficients on either side at zero.
The same arrangement can be used for simulating a underfloor air distribution system (UFAD). The two zones represent the lower occupied (mixed) zone and the upper stratified zone. Since the upper zone is being modeled as a mixed EnergyPlus zone, it is not precisely the stratified zone concept. However, if a user has concern about having the entire upper part of the space at a single average temperature, the space could be modeled with two stacked upper zones. In that case the stratified temperature profile would be established by the relative size of the mixing from the convective plumes. The user would have to supply those estimates from external knowledge of the behavior of UFAD systems and plumes.
Radiation Exchange Basics[LINK]
The behavior of the IRT surface can be analyzed by considering three parallel black plates infinite in extent, as shown in Figure 3. The outside plates are held at two different temperatures, and the intermediate plate assumes an equilibrium temperature when the net heat flux goes to zero. All plates have black body surface characteristics.
The radiant exchange between surface 1 and 2 is:
\[{q_{1 - 2}} = \sigma A\left( {T_1^4 - T_2^4} \right)\]
The exchange between surfaces 2 and 3 is:
\[{q_{2 - 3}} = \sigma A\left( {T_2^4 - T_3^4} \right)\]
Equating heat fluxes and eliminating T\(_{2}\) gives:
\[{q_{1 - 3}} = \frac{\sigma }{2}A\left( {T_1^4 - T_3^4} \right) \label{eq:RadExchangeBlackBodyInBetween}\]
where:
q is the heat flux in W/m\(^{2}\)
\(\sigma\) is the Stephan Boltzman constant
A is the plate area in m\(^{2}\), and
T is the temperature in K.
Equation [eq:RadExchangeBlackBodyInBetween] shows that the presence of a black body surface between a source and a sink reduces the heat flux by a factor of two.
IRT Radiation Exchange in EnergyPlus[LINK]
Ideally, this same doubling of radiation would need to occur when an IRT surface is between two zones in EnergyPlus. However, due to the complexity of the EnergyPlus heat balances and the treatment of an IRT as equivalent to any other surface in EnergyPlus, simply doubling the impact of the IRT on either zone is not possible. In testing, this resulted in either net radiation imbalances within the zones or potentially unstable conditions. So, the radiation is not doubled in EnergyPlus as in the simple example in the previous subsection. Despite this difference between the theory of the ideal case and the actual implementation in EnergyPlus, an IRT surface will still allow a zone to be separated into two separate zones with two different air temperature and the adjacent zones behave as black body cavities at some equivalent temperature.
It should be noted that for IRT surfaces any visible or solar short wavelength radiation incident on the surface will be absorbed and included with the long wavelength (IR) exchange with the adjacent zone. No energy will be lost, but zones with IRT surfaces should not be used in any lighting analyses.
Radiation Transfer Surface Details[LINK]
Specifying an Infrared Transparent (IRT) surface[LINK]
The Infrared Transparent (IRT) surface is similar to a resistance-only surface. The idd object for this type of surface is shown below. The fields indicate that the surface will actually participate in the transfer of visible and solar radiation by doing a wavelength transformation and making all short wavelength radiation that is incident on the surface into long wavelength radiation and having it participate in the long wavelength radiant exchange. The Material:InfraredTransparent object requires only a name. All other parameters are set internally.
The Infrared Transparent surface should not participate in a convective/conductive exchange between the zones it separates. In order to minimize this effect, the SurfaceProperty:ConvectionCoefficients object must be used. Outside and Inside values for the surface’s convection coefficients should be on the order of .1. Further examples are given in the Input Output Reference document.
Infrared transparent surfaces may also be specified using Construction:AirBoundary with the IRTSurface option for radiant exchange. This method automatically generates an internal material layer with IRT properties and sets the inside convection coefficient to the Minimum Surface Convection Heat Transfer Coefficient Value specified in the HeatBalanceAlgorithm object which defaults to 0.1 [W/m2-K].
Behavior Checks[LINK]
The behavior of multi zones separated with infrared transparent surfaces can be checked with a simple comparison. Begin with a single zone model as shown below. This model has a south-facing window, and four walls exposed to wind and sun, and a roof exposed to wind and sun.
The single zone model will be compared with a stacked three zone model that has zones separated by interzone infrared transparent surfaces. This model is shown below.
The two upper zones have south facing windows whose total area is the same as the area of the single window in the single zone model. The top and the sides are again exposed to sun and wind. The separating surfaces are modeled as IRT surfaces. All zones in both models are controlled at the same setpoint temperature using purchased air.
The sensible heating results are shown below. The results show the sum of the sensible cooling load for the three stacked zones and the single zone. It is clear that the IRT surfaces are very effective in transmitting infrared radiation between the zones. Some small differences, such as those shown, will occur because of the conversion from short wavelength solar to infrared through the special IRT dividing surfaces.
Documentation content copyright © 1996-2026 The Board of Trustees of the University of Illinois and the Regents of the University of California through the Ernest Orlando Lawrence Berkeley National Laboratory. All rights reserved. EnergyPlus is a trademark of the US Department of Energy.
This documentation is made available under the EnergyPlus Open Source License v1.0.