Energy and Buildings 43 (2011) 2705–2711 Contents lists available at ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild Free-running temperature and potential for free cooling by ventilation: A case study Christian Inard a,∗ , Jens Pfafferott b , Christian Ghiaus c a LEPTIAB, University of La Rochelle, Av. Michel Crépeau, 17042 La Rochelle, France b Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany c INSA-Lyon, CETHIL CNRS, UMR5008, Bât. S. Carnot, 9 rue de la Physique, 69621 Villeurbanne, France article info Article history: Received 17 April 2011 Received in revised form 5 June 2011 Accepted 19 June 2011 Keywords: Office buildings Passive-cooling Thermal comfort Free-running temperature Ventilation abstract Free-cooling by ventilation is one of the most energy efficient techniques for cooling. When ventilation is used for cooling, variable airflow rates should to be used in order to achieve comfortable room tem- peratures and to minimize the energy demand for mechanical ventilation. Thus, free-cooling, requires, obviously, the existence of a potential for cooling and needs control mechanisms for the airflow. In this study, the free cooling potential by ventilation for office buildings is evaluated by the free-running temperature. The free-running temperature approach is based on the energy balance of heat gains and losses. It is adapted to evaluate the potential for free cooling by ventilation of office buildings for which the gains through the walls are negligible as compared to the internal and solar gains. The free-running temperature of each office room considers solar and internal heat gains, outdoor temperature, indoor temperatures and ventilation air flow rates. The approach is applied to 14 office rooms in a passively cooled office building in Germany and is used to estimate the potential and to evaluate the total energy saving by free cooling by ventilation. The good fit between monitoring data and calculation procedure proves that the free cooling potential can be accurately estimated by using the difference between the comfort limits, i.e. the target value of the indoor temperature, and the free-running temperature. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Free cooling by ventilation is one of the most effective tech- niques for cooling since the outdoor air is directly used as an ambient heat sink [1]. The potential of free-cooling concepts should be evaluated rather by dynamic than steady-state methods since the indoor temperature variation should be considered appropri- ately. Nevertheless, steady-state methods based on temperature [2,3] or on heating/cooling curve [4] can be adapted to characterize the dynamic behaviour if the frequency or probability distributions is considered [5]. The building free-running temperature character- izes the thermal building behaviour and the frequency distribution of the outdoor temperature to describe the climate [6]. The free- running temperature method is similar to the bin method [7] and it can be demonstrated that the degree-hours as used in the bin method can be expressed as a function of the free-running temper- ature [5]. ∗ Corresponding author. Tel.: +33 5 46 45 72 46; fax: +33 46 45 82 41. E-mail address: christian.inard@univ-lr.fr (C. Inard). This paper presents some results from experimental investiga- tions performed in a passively cooled low-energy office building located at Freiburg (Germany). Based on these monitoring data, the free cooling potential by ventilation is assessed using the free-running temperature approach for each office room. First, the calculation procedure is validated with the monitoring data. Secondly, the energy saving potential for an optimised free-cooling concept by ventilation is derived from the monitored (real operation) and the computed data (ideal operation). This validation and optimisation project proves the applicability of the free-running temperature approach for the optimisation of passive cooling concepts. 2. Monitoring and data analysis In 2002 and 2003, several experiments on passive cooling were carried out in 14 rooms of the Fraunhofer ISE building located at Freiburg, Germany, Figs. 1 and 2. All office rooms are located in the same south-orientated part of the building. The passive cool- ing concept fosters sun protection, high thermal storage capacity 0378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2011.06.017