Improving Load Calculations for Fenestration with Shading Devices

Abstract

Activities and findings arising from ASHRAE Research Project 1311-RP are summarized. This project included three main goals, (a) development of models for pleated drapes, venetian blinds, roller blinds and insect screens, (b) implementation of these models in the ASHRAE Loads Toolkit, and (c) compilation of results suitable for direct application in building cooling load methods such as Radiant Time Series (RTS). The solar and heat transfer interactions present in multilayer systems are complicated and the corresponding models entail significant complexity. This work produced the ASHRAE Window Attachment (ASHWAT) model that uses a simplified approach to the way in which radiation interacts with each glazing or shading layer. Each layer is assigned spatially-averaged “effective” optical properties so that glazing and shading layers can be arranged in any combination. ASHWAT offers wide scope in the design process, the possibility of active control (e.g., slat angle adjustment), fast computation, and facilitates the implementation of additional shading layer types. Very few input data are needed to model any layer. Measurement-based validation was undertaken at both the subcomponent level and at the complete system level with documentation in the technical literature. The ASHWAT model has been added to the ASHRAE Loads Toolkit and coupled to the heat-balance room model, supporting accurate calculation of cooling load impact of fenestration shading. Simplified correlation models were developed to allow shaded fenestration performance estimates via spreadsheet-tractable formulas. The model was also used to generate greatly expanded simplified data for inclusion in Fundamentals and suitable for direct use in widely-used engineering procedures