Summary of Tools

National Fenestration Rating Council provides procedures for rating window systems that are often required by state energy codes:
http://www.nfrc.org

See also a useful flowchart describing procedures for site-built or custom projects: 
http://www.nfrc.org/sb_outline.html

Simulation programs available to determine the thermal and daylighting performance indices of conventional window systems include WINDOW5, Optics5, and THERM. 

WINDOW 5.0 is a publicly available computer program for calculating total window thermal performance indices (i.e. U-values, solar heat gain coefficients, shading coefficients, and visible transmittances). WINDOW 5.0 provides a versatile heat transfer analysis method consistent with the updated rating procedure developed by the National Fenestration Rating Council (NFRC) that is consistent with the ISO 15099 standard. The program can be used to design and develop new products, to assist educators in teaching heat transfer through windows, and to help public officials in developing building energy codes. 

THERM is a state-of-the-art, Microsoft Windows™-based computer program developed at Lawrence Berkeley National Laboratory (LBNL) for use by building component manufacturers, engineers, educators, students, architects, and others interested in heat transfer. Using THERM, you can model two-dimensional heat-transfer effects in building components such as windows, walls, foundations, roofs, and doors; appliances; and other products where thermal bridges are of concern. THERM's heat-transfer analysis allows you to evaluate a product's energy efficiency and local temperature patterns, which may relate directly to problems with condensation, moisture damage, and structural integrity. 

Optics5 allows the user to view and modify glazing data in many new and powerful ways. Optical and radiative properties of glazing materials are primary inputs for determination of energy performance in buildings. Properties of composite systems such as flexible films applied to rigid glazing and laminated glazing can be predicted from measurements on isolated components in air or other gas. Properties of a series of structures can be generated from those of a base structure. For example, the measured properties of a coated or uncoated substrate can be extended to a range of available substrate thickness without the need to measure each thickness. Similarly, a coating type could be transferred by calculation to any other substrate.

These tools have been developed by LBNL and are available over the web:
http://eetd.lbl.gov/btp/software.html

The tools are accepted by NFRC for rating window systems. In some cases, these tools can be applied by NFRC-certified simulators, test labs and inspection agencies to determine ratings for non-standard products.

The Windows Information System (WIS) is a uniform, multi-purpose, PC based European software tool to assist in determining the thermal and solar characteristics of window systems (glazing, frames, solar shading devices, etc.) and window components. The tool contains databases with component properties and routines for calculation of the thermal/optical interactions of components in a window. WIS contains features often not found in other software packages including routines to characterize the performance of solar shading devices and ventilation in glazing cavities. Airflow is not simulated using CFD, but the tools has been found to yield acceptable approximations of center-of-glass U-value and SHGC.
http://erg.ucd.ie/wis/html_pages/diss_techwis.html

Daylighting algorithms and tools are described in detail in the International Energy Agency Task 21 Daylight in Buildings publication: “Daylight in Buildings: A Source Book on Daylighting Systems and Components”. See http://eetd.lbl.gov/Bookstore.html under Practical Guides & Tools for Energy Users for “Daylight in Buildings: A Source Book on Daylighting Systems and Components”. For those residing outside of U.S. or Canada, please visit http://www.iea-shc.org.  Download the hyperlinked report 8.9.3 “Daylight Simulation: Methods, Algorithms, and Resources” from the CD-ROM directory.

RADIANCE is a lighting and daylighting visualization tool developed by LBNL and is available over the web:
http://eetd.lbl.gov/btp/software.html

This program can model very sophisticated window systems and complex systems, given BTDF measured data.

Daylighting and Electric Lighting Simulation Engine (DElight) is a simulation engine for daylight and electric lighting system analysis in buildings. The program’s origin was the LBNL SUPERLITE program from the 1980s, but the new version has updated the code and added new capabilities. It accepts a bidirectional transmittance distribution function (BTDF) and calculates daylight factors. The program can analyze complex systems, where the daylighting window aperture is treated as a directional light fixture and coupled to the interior space. An exterior radiance model is being developed that takes into account how exterior obstructions modify the BTDF incoming flux.
http://eande.lbl.gov/Task21/DElightWWW.html

COMIS is an air flow distribution model for multizone structures. It takes wind, stack and HVAC into account and allows for crack flow, flow through large openings, and single-sided ventilation. The structure of COMIS (Conjunction of Multizone Infiltration Specialists) was developed at an LBNL workshop in 1987-88. The program has been validated by the International Energy Agency’s Energy Conservation in Buildings and Community Systems Programme, Annex 23 on Multizone Air Flow Modeling.
http://www.eren.doe.gov/buildings/tools_directory/software/comis.htm

DOE-2 and EnergyPlus

DOE-2 and the newer EnergyPlus are public domain programs developed by LBNL and other team members:
http://eetd.lbl.gov/btp/software.html

The DOE-2 program for building energy use analysis provides the building construction and research communities with an up-to-date, unbiased, well-documented public-domain computer program for building energy analysis. DOE-2 is a portable FORTRAN program that can be used on a large variety of computers, including PC’s. Using DOE-2, designers can quickly determine the choice of building parameters which improve energy efficiency while maintaining thermal comfort. A user can provide a simple or increasingly detailed description of a building design or alternative design options and obtain an accurate estimate of the proposed building’s energy consumption, interior environmental conditions and energy operation cost. DOE-2 has been used by national labs, universities, and industry for hundreds of studies of products and strategies for energy efficiency and electric demand limiting. Examples include advanced insulating materials, evaporative cooling, low-E windows, switchable glazing, daylighting, desiccant cooling, cogeneration, gas-engine-driven cooling, cool storage, effect of increased ventilation, sizing of thermal energy storage systems, gas heat pumps, thermal bridges, thermal mass, variable exterior solar and IR absorptance, and window performance labeling.

EnergyPlus is a new-generation building energy simulation program based on DOE-2 and BLAST, with numerous added capabilities. The initial version of the program, EnergyPlus 1.0, was released in April 2001. EnergyPlus includes a number of innovative simulation features – such as sub-hour time steps, built-in template and external modular systems that are integrated with a heat balance-based zone simulation – and input and output data structures tailored to facilitate third party module and interface development. Other capabilities include multi-zone airflow, moisture adsorption/desorption in building materials, radiant heating and cooling, and photovoltaic simulation.

With respect to the façade, EnergyPlus has the following capabilities. WINDOW5 algorithms are embedded in EnergyPlus so that for each time step WINDOW5 calculations are done within EnergyPlus. The frame, divider, and sash heat transfer calculations treats these elements essentially as U-values, where solar absorbed on these elements is transferred according to U-value.

For window shading devices, EnergyPlus models an interior or exterior shade as a pull down diffuser or a venetian blind. The venetian blind model uses a radiosity calculation similar to that in ISO 15099 to determine visible transmission and solar transmission and absorption as a function of angle of incidence. Interreflections between glass layers and between glazing and shading device are calculated. A heat balance calculation is performed on the glass and shading device layers. Long wave radiation exchange occurs between the inside glass and the room and between the interior shading device, if present, and the room. The temperature of each glazing layer is computed. Air film coefficients are assumed and these parameters can significantly impact how the shading device layer impacts the radiative and convective split. Shading devices more complex than blinds cannot be modeled.

EnergyPlus currently has some limitations. The window’s air film coefficient must be validated (planned). One can use CFD calculations to determine air film coefficients for applications like ISO 15099 above, but such calculations take significant computation time (several hours) and turbulence models cannot be accommodated. For internal room reflections the daylighting calculation relies on a crude split-flux method and must be improved. Algorithms from DElight, which uses a radiosity-based method for internal reflec-tions and a bidirectional transmittance function approach for transmission through complex fenestration, will be incorporated in 2002. The exterior radiance distribution from the ground and adjacent buildings is not well modeled. For example, if a glass building opposes the modeled building, there can be significant reflected heat gains from this opposing building. Semi-transparent photovoltaics cannot be modeled but normal photovoltaics can be.

Double-façades can be modeled in EnergyPlus, but with limitations. One treats the interstitial space as a thermal zone. EnergyPlus can model the radiation that is transmitted between the exterior glass and interior glass façades. The interstitial space can be treated as a plenum. The difficulty is in predicting how much heat gets picked up as it moves through this plenum. If venetian blinds are placed in this interstitial cavity, EnergyPlus assumes that the blinds are associated with the exterior window. Air movement with venetian blinds needs to be validated. To determine thermal loads on the interior space, the COMIS multizone air flow calculation, which is completely integrated in EnergyPlus, can calculate the coupling that occurs between this façade and the interior zone. EnergyPlus can model wind-driven, buoyancy, and mechanically vented air flow through the plenum, although such calculations have not yet been attempted. If the plenum has openings to the interior space, EnergyPlus can model this condition.

A general list of tools offered by the U.S. Department of Energy are available over the web at:
http://www.eren.doe.gov/buildings/tools_directory/

References

ASHRAE. 2001. ASHRAE Fundamentals Handbook. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, N.E., Atlanta, GA 30329.

Andersen, M., L. Michel, C. Roecker, J.L. Scartezzini. 2001. Experimental assessment of bi-directional transmission distribution functions using digital imaging techniques. Energy and Buildings 33 (2001) 417-431.

Winkelmann, F.C. 2001. Modeling windows in EnergyPlus. Proc. IBPSA, Building Simulation 2001, Rio de Janeiro, September 2001. LBNL-47972, Lawrence Berkeley National Laboratory, Berkeley, CA.

Question/Information: eslee@lbl.gov