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TM69 Dynamic thermal modelling of basic blinds (2022)
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TM69 Dynamic thermal modelling of basic blinds (2022)

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Recently, the UK has experienced air temperatures in excess of 40 °C for the first time, smashing previous records. We initially adapt to excessive heat by moving into the shade. Air temperatures are always read in the shade: the sun's radiation does not directly affect the

temperature reading. The primary adaptation response is therefore to reduce our direct component of solar radiation, moving behind a tree or building, but this has little impact on the diffuse component of solar radiation coming from the clouds and sky (the direct solar beam

scattered by molecules or particulates in the atmosphere) or the air temperature.

This guide highlights some of the issues associated with the discrepancies between blind performance in a model and in a real-life application. Future software applications will need to align with the relevant standards.

This guidance also recognises that shading systems, such as internal blinds, simultaneously affect internal daylight and acoustic conditions as well as the thermal and solar performance of a building.

This publication will examine some of the limitations that are present when including moveable blinds in a dynamic thermal model. The focus is on representing a more straightforward approach, in thermal analysis terms, (or ‘basic’) glass–blind systems in some typical software packages. It also suggests ‘workarounds’ for better representations. In this study, a basic blind is defined as any type of blind, but of only one layer (e.g. a flat fabric) and limited to moving only up or down, such as a roller or closed venetian blind. This publication does not examine some of the more complicated glass–blind systems such as external blinds, open interstitial blind (i.e. between glazing layers), honeycomb blinds, or an internal venetian blind in an extended but open position (e.g. with slats positioned at a 45-degree angle).

Although Approved Document O: Overheating (HM Government, 2021) for newly developed residential buildings currently only accepts external shading as a measure to mitigate solar gains, the content of this guide will be helpful to provide a background on the combined physics of glass–blind systems, recognizing the differences between external and internal blind elements.

1 Context  

2 Heat transfer through glass–blind systems 

2.1 Heat transfer mechanisms and additional considerations 

2.2 Heat gain through a blind 

2.3 Heat loss through a blind 

3 Modelling inputs 

3.1 Solar properties of glass–blind systems 

3.2 Thermal performance properties 

3.3 Sources of input data 

4 Representing a blind in a dynamic thermal model 

4.1 Software limitations 

4.2 Workflow and important parameters 

4.3 Representing real operation and installation of blinds  

4.4 How to choose the most appropriate tool for the study 

4.5 When to use default values from the material libraries 

4.6 How to represent the glass–blind system U-value

4.7 How to represent an air gap 

4.8 How to account for additional thermal resistance 

4.9 How to account for natural ventilation flow rate when opening windows with blinds 

4.10 How to set up blind controls 

4. 11 How to interface the DTM application with a CFD model 

5 Best practice usage with other applications 

6 Emerging themes 

References and bibliography 

Appendix A: Material thicknesses 

Appendix B: Perimeter gaps 

Appendix C: Combined glazing and blind U-values and g-values

Appendix D: Examples of additional thermal resistance of blinds 

Appendix E: Examples of shading coefficient (SC) and short wave radiation fraction (SWRF)  

for blinds 

Appendix F: Software application sheet 

Authors: Darren Woolf (Wirth Research and Loughborough University), Maria Gabriela da Silva Costa (Useful Simple Trust), Bahareh Salehi (Mott Macdonald), Elpida Vangeloglou (Etheras Ltd. and London South Bank University)

Peer reviewers: Amber Banbury (Etude), Claire Das Bhaumik (Inkling LLP), Darren Coppins (Built Physics Limited), Esfand Burman (University College London)

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