Paper Conference

Proceedings of BSO Conference 2020: Fifth Conference of IBPSA-England


Comparison of Methods for Modelling Complex External Shading in EnergyPlus and IESVE

Matthew Tee, Thajnu Rashid, Anna Ioannidou-Kati

Abstract: Passive cooling measures, such as shading devices, offer a means of controlling overheating risk while reducing the energy use for cooling and subsequently operational carbon emissions. As such, shading devices are becoming increasingly common features of new building facades in the UK. Such devices may include detailed geometries and a range of materials, adding complexity to the solar performance analysis. Modelling the performance of such devices is well suited to the parametric modelling environment Grasshopper and environmental plug-ins Ladybug and Honeybee. This allows the solar transmission of these devices to be calculated on an hourly basis also considering reflections within the device. However, it is currently unclear how to model the dynamic hourly performance most accurately in whole building energy simulations, which creates uncertainty for facade and building services engineers alike. This may lead to simplified geometries or static performance assumptions being made and errors in the estimated energy loads leading to a performance gap. This study compares the solar performance of an example external shading device calculated using Radiance, with methods available in energy modelling software IESVE and EnergyPlus. A simplified ‘shoe box’ energy model has been used to compare the total solar energy transmission and energy loads calculated with each method. It was found that while EnergyPlus offers more ways to model shading devices than IESVE, ApacheSim in IESVE results in more accurate dynamic performance calculations than the EnergyPlus methods ‘Full Exterior’ and ‘Full Exterior with Reflections’. While EnergyPlus supports the use of BSDF IDF glazing definitions for energy simulations, unlike IESVE, this was found to consistently overestimate solar transmission compared to the Radiance shading coefficient method yet offered the most accurate method overall. Background For heating dominated climates, such as the UK, reduced thermal transmittance (U-value) and air permeability of building fabric typically produces a net reduction in overall Energy Use Intensity (EUI) for heating and cooling as the reduction in heating demand exceeds any consequential increase in cooling demand. Therefore, in order to improve the energy performance of the UK building stock, there has been a gradual reduction of the minimum U-value requirements for external walls through subsequent revisions of the UK Building Regulations. In addition to regulations, there is a growing body of guidance suggesting lower fabric performance targets are voluntarily adopted in order to achieve more ambitious reductions in the operational carbon emissions of the future UK building stock. An example of such guidance is the Climate Emergency Design Guide (LETI, 2019). While improved U-value and air permeability performance tends to reduce heating energy demand in winter months, it may increase the severity of overheating in summer due to reduced dissipation of solar gain transmitted externally through the building envelope. This is particularly valid for South, South West, and West oriented facades. To reduce the risk of overheating and energy demand for cooling, passive cooling measures are becoming increasingly important in building projects in the UK. Examples of passive measures include reduced window to wall ratio (WWR), lower solar transmittance (g-value) requirements for glazing, and the
Pages: 146 - 153