A model for integrating passive and low energy airflow components into low rise buildings

Abstract

Integrating passive systems such as cross and stack ventilation, a solar chimney, a wind tower or an earth-air tunnel into a building envelope has the potential to significantly reduce the ventilation and air-conditioning demand on buildings. However the application of combination of these features in low rise contemporary buildings is limited. Among the major reasons, is the lack of simple building modelling techniques preventing effective integrated passive systems. Existing coupled multi-zone airflow and thermal modelling software such as COMIS-TRNSYS, CONTAM-TRNSYS or TRNFLOW do not include the passive airflow components which require simultaneous prediction of temperature and airflow rate in the components such as solar chimneys and wind-induced earth-air tunnels. This paper develops an integrated model incorporating these passive airflow components into a coupled multi-zone ventilation and building thermal model. The model is validated against COMIS-TRNSYS software for a lightweight building with large openings. The prediction of each passive airflow component is validated with available published analytical and experimental findings. The solar chimney model is able to predict reverse flow and accurately predicts the air temperature in the chimney. The earth air tunnel (EAT) considers the transient heating up effect of the soil during operation and predicts the outlet air temperature.

Introduction

Buildings contribute over 40% of the total global primary energy use corresponding to 24% of the world CO₂ emissions [1]. Building heating, ventilation and air conditioning (HVAC) systems are responsible for about half of the energy use in buildings [2]. Effective integration of passive features into the building design can significantly minimise the air-conditioning demand in buildings while maintaining thermal comfort [3]. Passive ventilation, heating and cooling systems take advantage of natural heat sinks and sources. Passive features are components which can be integrated as part of the building envelope at the design stage to induce ventilation, heating and cooling without the need for a mechanical system. These elements may include solar chimneys, earth air tunnels (EAT), wind towers, or evaporative cooling towers. Individual passive systems however are inherently variable in performance and their application in contemporary low rise buildings is very limited. Due to variable responses of passive systems, the application of a combination of these features has the potential to significantly increase the ability to meet the cooling energy demanded to achieve thermal comfort in a building [1], [3]. Highly energy efficient fans may also assist the combined passive systems achieving significant improvement in thermal comfort provision. Hence developing an understanding of the integration of passive features will enable effective passive designs to be deployed.

Commercial coupled multi-zone airflow and thermal modelling software such as COMIS-TRNSYS, CONTAM-TRNSYS or TRNFLOW [4], [5], [6] are well known for their capability to assess natural ventilation. Unfortunately passive elements such as a solar chimney, wind driven EAT, and downdraught evaporative cooling cannot be modelled with the available airflow components as the flow through these elements is mainly dependent on wind, buoyancy in each component and building pressure distribution. For example a chimney may be represented as a duct element in both COMIS and CONTAM. However in this assumption the chimney can only act as a stack where as in a solar chimney the main driving force is the buoyancy due to the absorbed solar radiation in the channel. The model hence needs to incorporate a simultaneous prediction of temperature and airflow in the solar chimney as a separate airflow component. Apart from the multi-zone modelling software, most available simplified models of passive systems [7], [8], [9], [10] are specific to a particular building configuration with no consideration to building pressure distribution. Consequently, these models are not applicable when multiple opening configurations, infiltration and hybrid systems require consideration. Using CFD is an option for studying such systems and is commonly used in the design process. However CFD is not a feasible solution to conduct a performance and optimisation study to maximise thermal comfort over a year [11].

The objective of this paper is to develop a mathematical model to incorporate passive features into a multi-zone ventilation model incorporating a simple thermal building model. A separate fluid dynamic and thermal model was developed for individual passive element.