Façade design for night cooling by natural ventilation in different climate zones

SBE16 Hamburg International Conference on Sustainable Built Environment Strategies – Stakeholders – Success factors 7th - 11th March 2016 Conference Proceedings Organised by )$6 )BGFO$JUZ6OJWFSTJUÊ U )BNCVSH 4 International Conference on Sustainable Built Environment SBE16 Hamburg Imprint Conference organisers In cooperation with ZEBAU – Centre for Energy, Construction, Architecture and the Environment GmbH www.zebau.de )$6 )BGFO$JUZ6OJWFSTJUÊ U )BNCVSH Edited by: ZEBAU – Centre for Energy, Construction, Architecture and the Environment GmbH, Große Elbstraße 146, 22767 Hamburg, Germany This Proceedings are published under the following Creative Commons license: http://creativecommons.org/licenses/by-sa/4.0/ This booN was prepared from the input ¾les supplied by the authors. The publisher is not responsible for the use which might be made of the following information. 2016 Printed on 100% recycled paper. Druckerei in St. Pauli, Große Freiheit 70, 22767 Hamburg, Germany ISBN 978-3-00-052213-0 DOI: 10.5445/IR/1000051699 Supported by 572 International Conference on Sustainable Built Environment SBE16 Hamburg Façade design for night cooling by natural ventilation in different climate zones Matthias Friedrich, M.Sc., HafenCity University, Germany, [email protected] Prof. Lucila Chebel Labaki, State University of Campinas, Brazil, luci[email protected].br Luciana Fernandes, M.Sc., State University of Campinas, Brazil, lu[email protected]nicamp.br Abstract Depending on the climate region, the local comfort standard and the efficiency of existing building services, the share of energy for cooling, heating and artificial lightning of buildings is between 30% and 50% of the overall final energy consumption of the country. The required energy for cooling of buildings can be significantly reduced by night ventilation. In this case cool air is ventilated through the building to discharge heat energy stored in walls, floor slabs, and furniture. The efficiency of this method depends mainly on the air exchange rate between the outdoor environment and the indoor air volume. Driving forces for the air flow through façade openings are the outside air temperature drop during night, wind pressure distributions acting on the façade due to local wind and the availability of cross wind flow through the building. Resistances for the air flow are given by the size and the geometry of facade openings which can be quantified by discharge coefficients. In the facade design phase the relation between window size and effective opening area must be considered. With predicted discharge coefficients the overall energy efficiency and indoor temperature of buildings with natural ventilation can be analyzed in transient multi zone models. Exemplary buildings in Campinas, Brazil and Hamburg, Germany are analyzed concerning their potential of this method to save energy. Keywords: energy efficiency, facade opening, discharge coefficient, night cooling, natural ventilation Wellershoff Frank Univ.-Prof. Dr.-Ing. HafenCity UniversityHamburg Germany [email protected] 573 Strategies – Stakeholders – Success factors 1. Background The city of Campinas is located in the Southeast of Brazil with summer average temperatures exceeding 30°C and high relative humidity levels throughout the year. Protection against overheating is one of the most important aspects to be considered in building physics. The city of Hamburg is located in the North of Germany. The moderate climate zone has strongly distinct seasons with cold winter and hot summer days. A thermal insulated building envelope is as important as an intelligent concept to avoid overheating. It is aimed to use the temperature amplitude between day and night in both countries for night ventilation to reduce the cooling loads. Massive concrete elements are heat storages during the day time. For night cooling the heat convection from inside to outside shall be maximized, but it is limited by the air exchange rate. 2. Methods The energy efficiency and indoor temperatures are analyzed with transient multi zone simulations. The thermo-energetic performance using night ventilation in exemplary office rooms of nonresidential buildings is simulated with EnergyPlus. EnergyPlus is a building energy simulation program to model energy consumption and water use in buildings. It is a free and open-source software funded by the U.S. Department of Energy (DOE) Building Technologies Office (BTO) and managed by the National Renewable Energy Laboratory (NREL) [1]. The studied rooms are located in Brazil (Campinas) and Germany (Hamburg) in university facilities with typical office activity. Fig. 1: Weather profile of Campinas, Brazil (left) and Hamburg, Germany (right) Fi g . 2: Southwest view of the campus buildin g IFCH Campinas 574 International Conference on Sustainable Built Environment SBE16 Hamburg The analyzed room in the hot Brazil climate demands the use of air conditioning systems during most hours of the day to provide comfort for the users. Natural night ventilation can be used to reduce the cooling loads. The office room in Germany is naturally ventilated and has no mechanical ventilation. The external shading by the balcony construction shown in figure 3 and 4 in the area of the test room is not considered in the simulations for research purposes. To ensure the user comfort, the hours of overheating shall be minimized. According to national German standards [2], the maximum comfort temperature is 26°C. Generally the amount of overheating can be quantified by different methods. Just counting the number of hours above 26°C would be insufficient for the reason that the user comfort decreases with raising temperature. Therefore it is common to consider the delta to the maximum comfort temperature. An appropriate and simple method to use is to determine the energy consumption of an imaginary air conditioning system that cools the room temperature to 26°C if required. Hourly local weather data are used for both entire year simulations. The thermal loads are given by internal gains from occupancy, equipment and lighting systems. Air flow resistances by openings for passive ventilation modes are taken from the literature and adjusted for use in each façade. Details about each room as well as modeling settings are presented in Table 1. Table 1 - Details about the rooms Unicamp - IFCH Campinas/SP, Brazil HafenCity University Hamburg, Germany Use Office Mo - Fr 9 a.m. – 5 p.m. Office Mo - Fr 8 a.m. – 7 p.m. Floor Area 29.4 m² 40.1 m² Internal Gains People number 3 4 activity level [3] 130 W 130 W heat gain 13.3 W/m² 13.0 W/m² fraction radiant [3] 0.58 0.58 sensible heat fraction [3] 0.58 0.58 Lights 13.0 W/m² 7.4 W/m² Equipment 12.1 W/m² 15.0 W/m² Fig. 3: South view of the campus building HafenCity University, Hamburg 575 Strategies – Stakeholders – Success factors Temperature Limits 25°C HVAC (PTAC) COP 2.11W/W during working hours 26°C Natural ventilation only Window Glass Area (see also table 2) 5.14 m² single glazing 15.44 m² triple glazing Openable Area 2.68 m² Horizontally pivoted 2.30 m² Bottom pivoted Discharge Coefficient cd 0.61 0.61 Opening Factor 0.12 0.458 Orientation North-East South Shading Blinds and external fixed side elements Blinds max. 180° close above 300 W/m² Natural Ventilation modes Base “No Natural Ventilation” “No Natural Ventilation” Vent 1 “Static Night Cooling” Night Ventilation from 9 p.m. until 9 a.m. “Static Night Cooling” Night ventilation from 7 p.m. until 8 a.m. Vent 2 “Selective Ventilation” Natural Ventilation when Tin > Tout and Tin > 24°C “Selective Ventilation” Natural ventilation when Tin > Tout and Tin > Tset Tset = 21°C when occupancy, else 18°C 3. Natural Ventilation Simulation in EnergyPlus This research addresses a method to quantify relevant characteristics of façade design relative to its suitability for natural nocturnal ventilation. Hence, two different ventilation strategies are analyzed for both rooms described in Table 1. The static night cooling allows natural ventilation during the night independently of the outdoor air temperature. This method is easy to implement in existing, motorized windows and there is no need for complex façade control software. Additionally, a selective ventilation mode is simulated. This mode allows natural ventilation during the day, especially during early daytime, as well as during the night, if the room air temperature is above the outdoor air temperature and the room temperature is above a temperature setpoint. This setpoint will be chosen by local climate conditions (Table 1). Beside the ventilation strategy, the performance of natural ventilation depends on the design of the airflow path. Changes in geometry and friction among the airflow path are the most significant flow resistances. In energy simulation tools (e.g. EnergyPlus) these resistances are often summarized to discharge coefficients cd which indicate the effectiveness of airflow though openings. For rectangular openings, e.g. 90 degrees open windows, the resistance coefficient ȗ = 1/cd is in a small range between 2.7 and 2.8 [4] and therefore the discharge coefficient is 0.61. For common used window constructions the obstruction of the pane in pivoted cases must also be considered. The tilted pane reduces the effective area Aeff which is available for the airflow. Consequently it is not sufficient to use the discharge coefficient cd = 0.61 to describe all impacts. Bottom pivoted windows are used at the HafenCity University in Germany (Figure 4, right). Hult et al. [5] studied the algorithm used by EnergyPlus for pivoted windows and modified the expression in order to achieve better results for top pivoted windows. For this study the expression is transformed for the calculation of Aeff for bottom pivoted windows to take the different position of the rotating axis into account. W/H is the window width/height, Į is the opening angle and z is the vertical coordinate 576 International Conference on Sustainable Built Environment SBE16 Hamburg starting from the pivoting axis. The effective window area for the window system in Hamburg is calculated according to the equations (1) and (2): ܣ௘௙௙ ൌනܹ ௣௜௩௢௧ሺݖሻ݀ݖ ுୡ୭ୱሺఈሻ ଴൅ න ܹ݀ݖ ு ுୡ୭ୱሺఈሻ  ;ϭͿ ܹ௣௜௩௢௧ ൌቆͳ ܹ;൅ͳ ሺʹݖሺߙሻ൅ሺߙሻܹሻ;ቇି଴Ǥହ  ;ϮͿ The IFCH building in Brazil has horizontally pivoted windows where the rotation axis is eccentrically placed (Figure 4, left). There is no reliable reference available for this window construction. Geometric analysis provides opening factors equivalent for the ratio of effective and geometric opening area. The effective area for ventilation is the rectangular area which remains open when the pane is tilted (Figure 5). The lateral areas for ventilation of the windows are not considered as they are obstructed by shading elements. The ratio between the effective area and the opening area resulted in an opening factor of 0.12 (Table 1). The thermal properties of the envelope are also important for the effectiveness of the natural ventilation, as an additional thermal load is transmitted through the envelope and needs to be removed by natural or artificial ways. Table 2 shows the thermal properties of the envelope for both buildings. Fig. 4: Illustration of the openings for ventilation of IFCH Campinas (left) and HafenCity University, Hamburg (right) Fig. 5: Effective area for ventilation that is considered in IFCH building 577 Strategies – Stakeholders – Success factors Table 2 - Thermal properties of the components of the model Component U-factor (W/(m².K)) SHGC [-] IFCH Campinas External wall 1 2.65 External wall 2 3.09 Slab 2.04 Roof 1.74 Single glazing 3.84 0.818 HafenCity University Triple glazing 0.60 0.450 Internal walls and slabs adiabatic 4. Results and Discussion 4.1 Energy consumption An overview of the monthly energy consumption for cooling of both rooms is presented and discussed for further analysis. From figure 6 it is possible to note that night ventilation for the Brazilian building (IFCH) is not that efficient from energy aspects considering the whole year. Evaluating the peak of cooling in October, natural ventilation allows reductions of the cooling load up to 14% (Table 3). Furthermore in winter, the strategy was more efficient reducing up to 29% of the cooling energy in July. The consumption during this season is not expressive; therefore it gives no big contribution to the annual energy demand (reductions around 6%). On the opposite, significant reductions are possible for the room in Hamburg (HCU), mainly with selective ventilation. The potential reduction reaches up to 87% percent in July through selective ventilation and 23% in times with peak cooling loads. Yearly it is possible to reduce 76% of the cooling demand with night ventilation and 93% with selective ventilation (Figure 7). In general it is found that natural ventilation is an effective method to reduce the energy consumption for cooling as well as the peak loads. In moderate climate with cooler nighttime air temperatures the efficiency is also driven by the ventilation strategy. An adaptive opening control allows the usage of temperature difference between indoor and outdoor for passive cooling but it can also prevent the room against overheating, while the static ventilation does not consider the outdoor temperature. In hot climates the positive effect of natural ventilation is less noticeable. For the case of static night cooling an energy conservation of 6% is calculated. For the flexible ventilation strategy based on the temperature difference the same energy conservation percentage of 6 % is calculated. A higher benefit for a flexible ventilation strategy cannot be determined because the windows are open less frequently. 578 International Conference on Sustainable Built Environment SBE16 Hamburg Table 3 - Peak cooling sensible heat gain. HVAC input sensible air cooling (kW) and date + time of the peak load Room Base-case Night ventilation Selective ventilation HCU -1.87 08/03 15:00 -1.60 08/03 15:10 -1.45 08/07 15:10 IFCH -4.87 10/13 09:03 -4.20 10/12 09:03 -4.21 10/12 09:03 Fig. 6: Monthly air conditioning use for the room in Campinas Fig. 7: Monthly air conditioning use for the room in Hamburg