Simulations using IDA ICE 3.0

4.3.2.1 General description

Before a building thermal simulation tool was chosen, certain perfor-mance criteria were developed. The program was to have the following features:

1. A dynamic building simulation tool 2. User friendly interface

3. Multi-zone capability

4. Simple natural ventilation features

5. Simulation of HVAC systems typical for offi ce buildings 6. Reasonably accurate simulations of different shading devices

7. Possibility of adding new simulation modules developed by the user e.g. a double skin façade module

8. Good support

9. Reasonably well spread among researchers and consultants in Swe-den

10. Known outside Sweden The software candidates were:

• Bsim2000 developed by the Danish Research Institute (SBI)

• IDA ICE 3.0 developed by EQUA (Stockholm, Sweden)

• DEROB LTH developed by the University of Lund

• BV2 available from CIT Management AB (Gothenburg, Sweden) – Bsim2000 has most of the above features (except for 7 and 9)

– IDA ICE has all the above features (was therefore chosen for the simu-lation of the building alternatives)

– DEROB has some of the above features (except for 2, 4, 5, 7 and 9) – BV2 has some of the above features (except at least 3 and 7)

IDA ICE 3.0 is a computational program for indoor climate studies of individual zones within a building, as well as energy use of an entire buil-ding (EQUA, 2002). IDA Indoor Climate and Energy is an extension of the general IDA Simulation Environment. This means that the advanced user can, in principle, simulate any system whatsoever with the aid of the general functionality in the IDA environment.

Validation tests have shown the program to give reasonable results and to be applicable to detailed buildings physics and HVAC simulations (Acherman 2000 and 2003).

4.3.2.2 Description of double façade model

After personal communication with Dr. Bengt Hellström (Division of Energy and Building Design, Lund University) a brief description of the IDA ICE 3.0 double façade model is given below.

The window in IDA ICE 3.0 is divided into frame and glazing. The input data for the frame is the area fraction and the U value. For the glaz-ing, the input parameters are the U value, the solar transmittance (T_sol) and the solar heat gain coeffi cient (SHGC or g) at normal incidence. Also the emittances of the outermost and innermost surfaces of the glazing are given. The solar shadings are specifi ed by coeffi cients, which, when multiplied by U, T and g of the glazing, give the total values of the gla-zing/shading system.

The surface temperatures for the frame and the glazing are calculated from heat balance equations. Absorption of solar irradiation in the gla-zing is assumed to occur only at the innermost pane of the window and the absorbed energy rate is calculated from the difference between the g and the T values of the glazing (with or without solar shading) and the U value.

The double façade is modeled as an external window (with or without a shading device), outside an internal window and a wall. The cavity is assumed to be closed to the outside, except for four openings. One at the top, one at the bottom, whose areas can be chosen; a third opening connects the double façade cavity with the room and, fi nally, it is also possible to have mechanical exhaust ventilation of the cavity and its fl ow rate can be chosen.

The air temperature of the double façade cavity is obtained from an energy balance equation, using convective heat exchange with the surfaces and air exchange with the outside. Temperature stratifi cation is not taken

into account, as the air inside the cavity of the double façade has one temperature node.

The surface convection heat transfer coeffi cients are chosen as the maxi-mum of two values, one calculated from forced convection, depending on the air speed, and one calculated from natural convection, depending on the temperature difference to the surrounding air and the slope of the surface.

The natural convection driven air exchange in the cavity is calculated from the density difference between the air in the cavity and outside air, considering the pressure drops at the inlets and outlets.

4.3.2.3 Validation of IDA ICE 3.0 Double Façade model (IEA SHC Task 34/ECBCS Annex 43)

Concurrently with the “Glazed Offi ce Buildings” project, IEA SHC Task 34/ECBCS Annex 43 (Testing and Validation of Building Energy Simulation Tools) was started. The aim of the Task was to investigate the availability and accuracy of building energy analysis tools and engineering models to evaluate the performance of innovative low-energy buildings.

The scope of the Task was limited to building energy simulation tools, including emerging modular type tools, and to widely used innovative low-energy design concepts. Activities include development of analytical, comparative and empirical methods for evaluating, diagnosing, and cor-recting errors in building energy simulation software.

The objective of Subtask E (Double-Façade Empirical Tests) was to assess the suitability and awareness of building energy analysis tools for predicting heat transfer, ventilation fl ow rates, cavity air and surface tem-peratures, solar protection effect, and interaction with building services systems in buildings with double skin façades.

The validation process was carried out in two steps. First comparative test cases (Kalyanova and Heiselberg, 2005) were simulated and the results were cross compared; then empirical cases (Kalyanova and Heiselberg, 2006) were carried out and the output of the different software were compared with the measurements of the test facility. The empirical tests were led by Aalborg University (AAU), Denmark, using a new facility being constructed at AAU. Detailed description of the test facility can be obtained by (Kalyanova and Heiselberg, 2005).

The double skin façade confi gurations considered for this validation procedure are set out below (Figure 4.8):

• DSF100. All façade openings closed

• DSF200. Openings open to the outside

• DSF300. Openings open to the inside

• DSF400. Bottom opening open to outside; top opening open to in-side

• DSF500. Top opening open to outside; bottom opening open to in-side

Figure 4.8 Double skin façade confi gurations considered for the validation tests.

Within the test cases there are a number of variations to check the infl u-ence of various parameters, including:

• driving force of airfl ow (buoyancy, wind, mechanical fan, combined forces)

• internal (thermal)/External (thermal, solar, wind) boundary condi-tions

• opening area (fully opened, opening area controlled by temperature and/or airfl ow rate)

Some of the output used for the validation of the simulation tools are listed below:

• direct and diffuse solar irradiation on the window surface

• solar radiation transmitted from the outside into the DSF cavity

• solar radiation transmitted from the DSF cavity into the room

• energy used for cooling/heating in the room

• hour averaged surface temperature of external window surface facing outdoors and the DSF cavity

• hour averaged surface temperature of internal window surface facing the room and the DSF cavity

• hour averaged fl oor and ceiling surface temperature and air tem-perature in the room

Simulation results for comparative and empirical tests were obtained by IDA ICE 3.0 (LTH-Lund, Sweden), BSim 2000(Aalborg University,

Den-mark), VA114 (VABI, the Netherlands), TRNSYS-TUD (TUD, Germany) and ESPr (ESRU, UK). Conclusions of the software validation will be araible after the complitation of the IEA Task 34/ECBS Annex 43.

5 Description of the