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Ventilative Cooling in a School Building: Evaluation of the Measured Performances

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Ventilative Cooling in a School Building: Evaluation of the Measured Performances fluids Article Ventilative Cooling in a School Building: Evaluation of the Measured Performances Hilde Breesch * , Bart Merema and Alexis Versele Department of Civil Engineering, Katholieke Universiteit Leuven, Construction Technology Cluster, Sustainable Building, Technology Campus Ghent, 9000 Ghent, Belgium; [email protected] (B.M.); [email protected] (A.V.) * Correspondence: [email protected]; Tel.: +32-9-331-6592 Received: 26 August 2018; Accepted: 18 September 2018; Published: 23 September 2018 Abstract: The test lecture rooms on Katholieke Universiteit Leuven (KU Leuven) Ghent Technology Campus (Belgium) are a demonstration case of Annex 62: Ventilative Cooling of the International Energy Agency’s Energy in Buildings and Communities programme (IEA EBC). The building is cooled by natural night ventilation and indirect evaporative cooling (IEC). Thermal comfort and the performances of ventilative cooling are evaluated. Long-term measurements of internal temperatures, occupancy, opening of windows and IEC were carried out in the cooling season of 2017. The airflow rates through the windows in cross- and single-sided ventilation mode were measured by both tracer gas concentration decay and air velocity measurements. In addition, the air flow pattern is visualized by measuring air temperatures in the room. The results show that good thermal summer comfort was measured except during heat waves and/or periods with high occupancy. Both nighttime ventilation and IEC operate very well. IEC can lower the supply temperature by day significantly compared to the outdoor temperature. The Air Changes Rates (ACR) of the night ventilation greatly depends on wind direction and velocity. The air temperature profile showed that the air is cooled down in the whole lecture but more in the upper zone. The extensive data monitoring system was important to detect malfunctions and to optimize the whole building performance. Keywords: ventilative cooling; nearly zero-energy buildings (nZEB); school building; measurements; thermal comfort 1. Introduction In the recently approved second recast of the Energy Performance of Buildings Directive (EPBD) [1], the member states of the European Union committed themselves to develop a sustainable, competitive, secure, and decarbonized energy system by 2050. An important aspect to achieve this goal is to find a cost-efficient equilibrium between decarbonizing energy supplies and reducing final energy consumption of the building stock. While the first recast of the EPBD [2] required that from 2020, all new buildings in the European Union have to be nearly zero-energy buildings (nZEB), this second recast focusses on renovation strategies. An average renovation rate of 3% annually towards nZEB, where nearly means cost effective, is needed [1]. One of the major new challenges in these highly insulated and airtight buildings is the increased need for cooling and the risk on overheating not only during summer but all year round. In addition, this cooling demand depends less on the outdoor temperature and more on the internal and solar heat gains [3]. A shift is noticed from reduction in heating to reduction in cooling demand. As a consequence, conceptual and building technical measures as well as energy efficient cooling systems are needed in these nZEB buildings to guarantee good thermal comfort all year round. Ventilative cooling is an example of an energy efficient cooling method that can contribute to achieve the goal Fluids 2018, 3, 68; doi:10.3390/fluids3040068 www.mdpi.com/journal/fluids Fluids 2018, 3, 68 2 of 14 of the 2nd recast of the EPBD and was extensively studied within IEA EBC Annex 62 [4]. Ventilative cooling is defined as “the application of the cooling capacity of the outdoor air flow by ventilation to reduce or even eliminate the cooling loads and/or the energy use by mechanical cooling in buildings, while guaranteeing a comfortable thermal environment” [3]. Ventilative cooling strategies depend on the indoor-outdoor temperature difference. For temperate outdoor conditions, i.e., mean outdoor 2 to 10 ◦C lower than indoor temperature, an increased airflow rate by day and night is recommended. For hot and dry, i.e., indoor–outdoor temperature difference between −2 ◦C and +2 ◦C, only the airflow rate during nighttime, i.e., night ventilation, is maximized. Extra natural cooling, like evaporative cooling and earth-to-air-heat-exchanger, or mechanical cooling can be provided to reduce the air intake temperature during daytime [3]. Santamouris and Kolokotska [5] reviewed the state of the art technologies for passive cooling including ventilative cooling and concluded that “the efficiency of the proposed passive cooling systems is found to be high while their environmental quality is excellent. Expected energy savings may reach 70% compared to a conventional air conditioned building”. Night ventilation was found to be very effective to reduce the cooling demand in office buildings and improve thermal summer comfort regardless the climatic conditions [5]. In free floating buildings, amongst others [6–10] monitored that night ventilation can decrease the next day peak internal temperature up to 3 ◦C. In addition, the climatic cooling potential of buildings with night ventilation in Europe was evaluated by Artmann et al. [11]. A very significant climatic cooling potential was found in Northern Europe; a significant in central and Eastern Europe but series of warmer nights can occur. As a consequence, night ventilation may not be sufficient to guarantee good thermal comfort during these periods [11]. Santamouris and Kolokotska [5] also concluded, based on amongst others [12–14], that “indirect evaporative cooling techniques can be low energy solutions for medium and large buildings where passive cooling techniques cannot reach the required comfort conditions”. However, these systems require fan energy, which can be up to 20% less due to lower air velocities [5]. Kolokotroni and Heiselberg [15] give amongst other things, insight in the limitations and barriers to ventilative cooling. The most important are the impact of global warming and heat island effect [6,16,17] noise and pollution [18]. It is expected that the average cooling potential of night ventilation will decrease significantly with regionally varying implications. In Northern Europe, the risk of thermal discomfort in buildings that use exclusively night ventilation is increased while in Central Europe the periods with low night cooling potential are becoming more frequent [19]. Appropriate control is also a requirement to guarantee good thermal comfort. To ensure that the required window opening on a regular basis, the night ventilation strategy should be integrated to the office buildings energy management system [5]. The test lecture rooms of KU Leuven Ghent Technology Campus, which are studied in this paper, were one the demonstration cases of IEA EBC Annex 62: Ventilative Cooling [20]. Natural night ventilation and indirect evaporative cooling techniques are applied. This paper aims to evaluate thermal comfort in this nZEB school building and to discuss the performances of its ventilative cooling systems. First, a description of the building, its systems, and more specifically the ventilative cooling and control strategies are presented. Afterwards, the measurement setup for the evaluation of air flow rates, operation of ventilative cooling and thermal comfort is shown. Section3 presents the results of the measurements and finally the conclusions and lessons learned are presented. 2. Materials and Methods This paragraph describes the building, its systems and the ventilative cooling as well as the measurement setup. Fluids 2018, 3, 68 3 of 14 2.1. Building Description 2.1.1. Building and Use The nZEB school building is realized at the technology campus Ghent of KU Leuven (Belgium) on top of an existing university building. The building contains four zones (see Figure1): two large lecture roomsFluids 2018 (1), 3, andx FOR (2),PEER a REVIEW staircase (3) and a technical room (4). The lecture rooms have a3 floor of 14 area of about 140 m2, a volume of 380 m3. The lecture rooms are designed as identical zones with a different thermal mass.The nZEB The lowerschool building room has is realized a brick at externalthe technology wall camp withus exterior Ghent of insulation KU Leuven while(Belgium) the upper room hason atop lightweight of an existing timber university frame building. external The wall building with contains the same fourU zones-value. (see Both Figure lecture 1): two rooms large have a lecture rooms (1) and (2), a staircase (3) and a technical room (4). The lecture rooms have a floor area concrete slab floor. This results in a light (2nd floor) and a medium (1st floor) thermal mass according of about 140 m2, a volume of 380 m3. The lecture rooms are designed as identical zones with a different to EN ISOthermal 13790 mass. [21 The]. lower room has a brick external wall with exterior insulation while the upper room Tablehas1 a shows lightweight the building timber frame properties. external The wall school with the building same U is-value. designed Both andlecture constructed rooms have according a −1 to the Passiveconcrete slab House floor. standard.This results Thisin a light means (2nd floor) that and the a air medium tightness (1st floor)n50 thermalis lower mass than according 0.6 h and the U-valuesto EN ISO of 13790 the envelope[21]. parts are maximum 0.15 W/m2 K. The windows are constructed Table 1 shows the building properties. The school building is designed and constructed with triple glazing and have a g-value of 0.52. The window-to-wall ratio is 26.5% on both façades. according to the Passive House standard. This means that the air tightness n50 is lower than 0.6 h−1 The window-to-floorand the U-values ratio of the is 13%.envelope The parts windows are maximum are equipped 0.15 W/ withm2 K.internal The windows and externalare constructed solar shading. The externalwith triple solar glazing shading and have are a moveable g-value of 0.52.
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