Validation of a lumped RC model for thermal simulation of a double skin natural and mechanical ventilated test cell

Abstract

Most current building thermal codes impose upper limits to the predicted annual building energy demand for heating, ventilation and air conditioning. In the building design phase these predictions are obtained using thermal simulations with variable complexity. The simplest approach uses a single lumped thermal capacitance to model the high thermal mass building elements, combined with five thermal resistances (known as the 5R1C model proposed in EN ISO 13790 standard). This model is used by many European countries as the reference simplified methodology to assess overheating risk and calculate yearly building energy demand. This paper presents a successful extension of this model that allows for its application to the prediction of the internal air temperature of free-running buildings with double skin façades. The extension consists in an increased number of thermal resistances used to model the double skin façade zone. The extended model is validated using a set of detailed thermal measurements obtained in a free-running double skin test cell. For the case analysed the simplifications used in the RC model do not reduce the overall accuracy: the mean absolute error for room air temperature is approximately 1 °C, the same order of magnitude of more detailed EnergyPlus simulations (1.2 °C).

Introduction

The last two decades have seen an increased public awareness of the environmental and operational costs of building energy consumption. As a result, most current building thermal codes impose upper limits to the predicted annual energy demand for heating, ventilation and air conditioning systems (HVAC). In the building design phase these predictions are obtained using thermal simulation models with variable levels of detail and approximations. The most complex buildings require models with several thermal zones and, in some cases, tri-dimensional computational fluid dynamics simulations (CFD). For small buildings with simple indoor climate control systems, such as single-family homes and apartments, there is an increase in use of single thermal zone simulation models. In response to this increased use, the research community is continuously working on improved models with all levels of complexity.

The main challenges of building thermal simulation are in modelling of room airflow and heat transfer in building elements with high thermal mass (floor, walls, etc.). In the majority of simple thermal models room air is considered perfectly mixed and represented as a single thermal node that is connected to all room internal surfaces via thermal resistances [1]. Heat transfer in building elements with high thermal mass can be modelled using variable levels of detail, ranging from detailed finite difference methods to conduction transfer functions (CTF) [2] or, in the simplest approach thermal resistances connected to a thermal capacitance. The thermal resistance and capacitance used in the models have physical meaning and can be calculated approximately from the thermal properties of the building elements. Fig. 1 shows the combination of a perfectly mixed room air approach with a resistance capacitance model for the high thermal mass elements, this approach is known as an RC model. These models are typically labeled according to the number of resistances and thermal capacities used. EN ISO 13790 standard [3], adopted by many European countries as the reference methodology to assess overheating in buildings during summer and calculate building energy demand, uses a single capacitance that represents all high thermal mass elements and five thermal resistances (making it a 5R1C model).

The widespread use of the 5R1C model makes it a preferred target for further development. This paper presents an extension of this model for application to double skin façade (DSF) buildings. The proposed model is validated using a set of detailed thermal measurements obtained in a free-running double skin test cell [4]. The next section presents a review of existing RC models and model validation studies. The following section presents the model methodology followed by the presentation of free-running test cell measurements. The final section presents the results of the model validation.