Analysis of Natural Ventilation in a Passive House Located in Cold Conditions- a Case Study of Denmark

Transcription

1 Analysis of Natural Ventilation in a Passive House Located in Cold Conditions- a Case Study of Denmark Ivan Oropeza-Perez* 1, Poul Alberg Østergaard #2, Arne Remmen #3 * Department of Civil Engineering, Aalborg University, Sohngårdsholmsvej 57, 9000 Aalborg, Denmark 1 iop@civil.aau.dk # Department of Development and Planning, Aalborg University, Vestre Havnepromenade 9, 9000 Aalborg, Denmark 2 poul@plan.aau.dk 3 ar@plan.aau.dk Abstract This article shows the potential of using natural ventilation as a passive method of cooling buildings that are located in cold climate countries using Denmark as a case study. The energy saving potential of natural ventilation is found by performing thermal simulations of a household located in Vejle during cooling season, in the months of June, July and August. The dwelling belongs to a Danish project of passive houses denominated Komfort Husene, where its occupants claim there is no thermal comfort in summer time. The results show that the use of natural ventilation helps to reduce the demand of mechanical ventilation within the dwelling hence there is a potential of energy saving. Keywords - Natural ventilation, passive house, cold conditions, energy saving potential 1. Introduction As is well known, a reduction of greenhouse gases emission is necessary in order to counter global climate change. The European Commission aims to reduce greenhouse gas emissions by 20 % compared with emission levels of 1990 among the European Union by 2020 [1]. Of the total European CO 2 emissions, buildings contribute to about 35 %, and out of this share, about 77 % corresponds to residential buildings [1]. The residential sector is thus an important target for energy savings, and energy savings are also important elements in many scenarios for sustainable development [2-4]. Passive houses i.e. houses whose objective is to minimize their energy needs keeping a good indoor microclimate - are seen as a good solution for achieving thermal comfort and indoor air quality (IAQ) while maintaining low energy consumptions in the building hence there is low CO 2 emissions [5]. However, according to qualitative and quantitative assessments, during some seasons, especially summer time, the indoor air temperature could rise

2 above the comfort limit [6,7] giving cause to active methods of cooling and thereby an increasing energy consumption. The objective of this paper is to assess the potential of natural ventilation in cold conditions taking a passive house located in Vejle, Denmark as a case study [8]. In this household, both qualitative and quantitative surveys show that there is an uncomfortable period of overheating during the summer time. Therefore, natural ventilation is analyzed as a means to achieve the thermal comfort. The analysis is done by running simulations with the coupled thermalairflow program Energy Plus made by the U. S. Department of Energy [9] and using a model of natural ventilation which takes into account the design of the dwelling, the occupancy and the outdoor conditions of one case study [10]. Simulations calculate the hourly indoor temperatures of the dwelling during summer time using and not using natural airflow as a cooling method. The energy saving potential is gotten when there is a comfortable indoor air temperature enabling the mechanical ventilation systems such as fans to be turned off. The potential energy saving is thus the consumption of power if the equipment would have been turned on. 2. Theoretical Reference The model of natural ventilation used in this article starts with the thermal energy balance on the zone air i.e. within the dwelling - assuming that the indoor temperature is well-mixed [11]: dt c pv E dt (1) Conv E Int E AC E The energy stored in the zone air shown in (1) is equal to the sum of the convective heat transfer from the surfaces E Conv, the internal heat loads E Int, the heat transfer due to mechanical ventilation E AC, and the heat transfer due to natural ventilation E Vent, respectively. The heat transfer by natural ventilation is given by (2) [11]. As (1), the internal temperature is assumed to be uniform: Vent E Vent c q T T ) (2) p f ( Int Out The natural ventilation rate qf is a function of the wind speed and the thermal stack effect, as it is shown in (3). The model formulation used is from the ASHRAE Handbooks of Fundamentals [12,13]. The ventilation rate driven by wind is given by (4)

3 q f V V 2 stack 2 wind (3) V O A F s (4) wind w opening schedule It is seen the volume flow rate depends on the opening effectiveness, the opening area, a multiplier fraction schedule and the wind speed. The natural ventilation flow rate can be controlled by a multiplier fraction schedule applied to the user-defined opening area and through the specification of minimum, maximum and delta temperatures. The opening effectiveness is calculated based on the angle between the actual wind direction and the effective angle of the wind entrance using (5) [14]: wind O w effective wind (5) The difference between the effective angle and the wind direction should be between 0 and 180 degrees. This equation is a linear interpolation using the values recommended by the ASHRAE Handbook of Fundamentals [14] i.e. 0.5 to 0.6 for perpendicular winds and 0.25 to 0.35 for diagonals winds -. The ventilation rate due to stack effect is given by (6) [14]: Int Out stack CDAopeningFSchedule 2gH (6) NPL TInt V T T The volume flow rate depends on the discharge coefficient for the opening, the opening area, the multiplier fraction schedule, the gravity acceleration, the height from the midpoint of the lower opening to the neutral pressure level; and the indoor and outdoor temperatures. The discharge coefficient for opening is given by (7) [14]: C D T T (7) Int Out Thereby, the entire set of equations (1)-(7) is iterated within one hour to determine the temperature which satisfies all conditions. Then, the subsequent hour is calculated. For this article, hourly calculations are done. Furthermore, the inputs and boundary conditions are taken from the case study.

4 Finally, the energy saving potential is an indoor temperature function because the mechanical ventilation device in this article is considered as a fan with a constant airflow rate i.e. with both constant cooling rate and electrical input -, therefore, the energy saving will be only in the case when the indoor air temperature is below to a maximum comfort temperature thus the fan is turned off. When the indoor temperature is over this comfort limit, it is considered that the fan is turned on. If the mechanical ventilation device were an air-conditioning unit with a variable cooling rate this one should be calculated in the thermal balance therefore the energy saving would be estimated. 3. Case Study The aim of the case study is to assess natural ventilation in a specific dwelling and then to analyze its feasibility on a large-scale scenario i.e. Denmark. Thereby, the case study is a passive house located in Vejle, which is located in the southeast of Jutland, Peninsula of Denmark (see Fig. 1). Fig. 1 Vejle, Denmark (Source: [15]) The dwelling to analyze is a two floors construction (see Fig. 2) with m 2 of floor area. It has a nuclear family as occupants i.e. four persons, two parents and two children -. Its schedule consists in leaving the house around 7:00 am and arriving at 4:00 pm during week days. On weekends, they remain in the house almost all the time unless special occasions [16]. Absence due to holiday has not been factored into the analyses. Regarding the design, the house was conceived to have the most solar gains in the south and east facades. None of the four facades has got some kind of shading device. However, these devices are proposed in the article as placed in both south and east facades in order to analyze their influence on the indoor air temperature and as a means to obtain thermal comfort.

5 Fig. 2 Image of the case study (Source: [17]) and the sketch used for the simulations The size of the windows is considered as large openings [11] thus there are more driving forces due to the wind pressure than due to temperature differences [11]. The wall construction consists of 120 mm lightweight concrete panels and 108 mm girders. The roof is constructed with 450 x 45 mm concrete slabs to unilaterally decrease 7 degrees. In general, its characteristics i.e. layout, number of occupants, size etc. - are similar to the rest Danish building stock. The difference consists in the high solar gains and low thermal conductivity of the materials of construction. According to the occupants it is too hot in the house [16]. Moreover, one inconvenience is that the occupants claim that natural ventilation is not effective due to it is difficult to keep the windows open as a result of the lock mechanism [16]. Furthermore, the physical characteristics of the construction elements are given in Table 1. Table 1. Properties of the construction elements of the case study (Source: [16,18]) Thickness Wall 0.56 m, roof 0.59 m, glazing 0.06 m U-value Wall W/(m 2 K), roof W/(m 2 K) Density Wall 1920 kg/m 3, roof 800 kg/m 3 Specific heat Wall 790 J/(kg K), roof 900 J/(kg K) Conductivity Wall 0.05 W/(m K), roof 0.04 W/(m K) Transmittance of glazing Reflectance of glazing Colour of glazing Transparent Glazing area North 11%, East 38%, South 36%, West 15%

6 Internal heat sources were set in 10 W/m 2 for electrical equipment. This figure was set after measuring the actual electrical load of the appliances within the dwelling [18]. Furthermore, in this time of the year, summer time, operation of heating systems is not considered. Regarding surroundings, there are no objects i.e. trees, buildings etc. - that can make shade to the house or change the wind speed and/or direction during the year. Weather data i.e. outdoor temperature, wind speed, wind direction and sky clearness - were taken from the historical database of the observing station located in Vejle which is operated by the Danish Meteorological Institute for the year 2009 [19] and from the weather database of the coupled thermal-airflow building simulation program Energy Plus. The meteorological station has an elevation of 2 m over the sea level. Furthermore, the daily values of the weather data were taken from it. In addition, Energy Plus takes the hourly values of the weather data from its database of Copenhagen, being this one the closest available weather database from Vejle. Thereby, Energy Plus runs the simulations depending on whether the output(s) will be hourly, daily, monthly or yearly. The method used is through simulations with Energy Plus [9] and by using a model of natural ventilation of buildings [10]. This model includes (3) to (7) and uses a deterministic set of input factors comprising outdoor conditions e.g. wind speed building design e.g. openings orientation and occupants behavior e.g. openings schedule [10]. Thus, every energy load in (2) is calculated, with and without ventilation. 4. Discussion Taking the input parameters from the study case description, two kinds of simulations were run: modeling the existing data (without natural ventilation) and with the use of natural ventilation. The maximum operative temperature is given by the ASHRAE 55rev standard, considering that an acceptable indoor operative temperature should be between 25 and 20 C approximately for naturally conditioned spaces located in cold conditions [20]. Hence, the adaptive comfort temperature is set at 25.5 C for free running buildings (without mechanical ventilation) i.e. category A by the guideline CR [20]. With this indoor temperature set-point and by performing simulations with Energy Plus the results of these simulations are shown in Fig. 3. In Fig. 3 it can be seen the comparison through June, July and August between outdoor temperature (green line), indoor temperature modeling the measured data (dotted red line), and indoor temperature with the use of natural ventilation (blue line). Existing data is not shown in the plot due to it

7 does not exist numerically and/or graphically, only as a figure with the number of hours with and without thermal comfort into the dwelling [16,18]. Fig. 3 Indoor and outdoor temperatures of the case study for June, July and August Thereby, the number of hours on thermal discomfort using natural ventilation during July is 71. In June there are 12 hours and in August 2 hours. All together are 85 hours representing 3.39 % of the total 2208 hours of these three months. With that, a comparison between the number of hours without thermal comfort measured [16,18] and the simulations carried out in this document could be seen in Table 2. Also, the comparison is done among the hours with natural ventilation (NV simulated) and a third one which is natural ventilation plus a shading device on the south and east façades (shading & NV simulated). The shading device consists in two eaves of 10.7 m length and 1.5 m width with an angle of 20 from the horizontal line put on the south and east facades, respectively. The difference between measured hours and simulated ones are 2.5 %, 14.3 % and 13.7 % for June, July and August, respectively. These differences are attributed to the occupant s behavior which in the model is considered as a constant parameter but in reality it might change depending on the activities of the occupants. Other factors are the wind speed and direction due to these parameters may also change randomly through the modeled day. According to Brunsgaard et al., during June 10.7 % of the time there is no thermal comfort within the building [16]. With natural ventilation these hours would go to 0.5 %. In July, 18.8 % of no comfort would decrease to

8 3.2 %. For August, the discomfort would decrease from 10.5 % of the time to less than 0.1 %. Furthermore, the simulations with natural ventilation plus shading have 4 hours less than with only natural ventilation in the three months. And although the decrease of discomfort hours seems low, a further analysis regarding shading should be done in order to optimize the avoiding of the solar heat gains, especially during summer. Table 2. Number of hours with thermal discomfort with and without natural ventilation Month No. hours (measured) No. hours (simulated) No. hours (NV simulated) No. hours (shading & NV simulated) June July August Total Furthermore, Table 2 shows that with natural ventilation, thermal comfort is achieved for more hours reducing thus the need to use mechanical ventilation. With the use of natural ventilation there is a reduction of 90.8 % of hours with thermal discomfort compared with the simulated and 90.4 % compared with the measurements. On the other hand, doing a sensitivity analysis, it is found that the input parameters which have the greatest influence in the model are, in this order, internal heat sources, outdoor temperature, temperature set-point, wind speed and solar heat gains. As both outdoor temperature and wind speed are parameters that are not possible to change, and the temperature set-point depends on a great extent on the preferences of the occupants, only for the first parameter it is recommended not using electric equipment with high heat radiation. Also, for the last one the use of proper shading devices is recommended. 5. Energy Saving by Using Natural Ventilation With the results it is possible to calculate an energy saving potential for not utilizing systems that imply the use of electricity. A conventional system of mechanical ventilation would consist in a fan of 50 W which could supply up to 120 m 3 of fresh air per hour with a pressure difference of 1 kpa [21] and which could meet the ASHRAE Standard 62.2P for residential buildings i.e. 120 m 3 h -1 for a dwelling of 3 bedrooms and 110 m 2 floor area [22] as the case study dwelling -. Therefore, using the number of hours in Table 6, and considering that hours with thermal discomfort is when the 50 W fan is turned on, the

9 electricity consumption is calculated as is shown in Table 3. In this case only natural ventilation without the shading device was taken account of. Table 3. Electricity consumption and saving projected with & without using natural ventilation Month Consumption [kwh] Measured Consumption [kwh] Simulated Consumption NV [kwh] Simulated Energy saving [kwh] June July August Total The energy saving potential of the dwelling is up to 42 kwh during cooling season. This means avoiding 839 hours using an electric fan. This is because, according to Fig. 3, outdoor air temperature is almost always below the comfort temperature set-point. Therefore, the use of natural ventilation can achieve at least the same temperature as outdoors and might be able to remove the internal heat gains in an easier manner by using convection heat transfer along the surfaces such as walls and roof. 6. Conclusion With natural ventilation a potential reduction of 90.8 % of mechanical ventilation use can be achieved during summer time in the presented case of study that represents a saving of 42 kwh for In this case, the use of large openings in the four facades is recommendable as long as an adequate schedule of opening of windows is carried out as well as the outdoor conditions are appropriate. Furthermore, the use of a shading device does help to achieve less hours of thermal discomfort within the dwelling. However, thermal comfort does not increase in a great extent by using this kind of device. However, as the occupants claim that natural ventilation is not effective due to it is difficult to keep the windows open as a result of the lock mechanism, in the future it is important to take into account an appropriate design based on the outdoor conditions and the necessities of the occupants. In this case in particular it is recommended to have all windows which can be wide opened with easiness. With the potential of natural ventilation within the passive house with high solar gains and low thermal conductivity in the fabric it is reasonable to say that this mean of buildings cooling would work within regular buildings where in some cases the use of mechanical ventilation is also present. Therefore, a potential for not using mechanical ventilation could be achieved in Denmark.

10 7. Nomenclature Effective angle of the wind entrance [º] effective Wind direction [º] wind Density of the air at sea level and 20 ºC 1.2kg m 3 H NPL Level m opening CD Height from midpoint of lower opening to the Neutral Pressure 2 A Opening area m Discharge coefficient for opening [dimensionless] c Specific heat of the air J kgk p dt c p V = Energy stored in the zone air J s dt EConv Convective heat transfer from the surfaces J s EVent Heat transfer due to natural ventilation J s E AC Cooling rate due to air-conditioning J s EInt Internal heat loads J s Fschedule Value from a user-defined schedule 0..1 Ow Opening effectiveness [dimensionless] q f Volume natural ventilation rate m s 3 swind Wind speed m s T Indoor air temperature at the time step n K TInt Indoor air temperature K T Out Outdoor air temperature K V stack Volume flow rate due stack effect m s 3 V wind Volume flow rate driven by wind m s 3 3 V Volume of the building m 8. References [1] M. Hamdy, A. Hasan, K. Siren. Applying a multi-objective optimization approach for Design of low-emission cost-effective dwellings. Build Environ. 2011, 1, 46 (1) [2] P.A. Østergaard and H. Lund. A renewable energy system in Frederikshavn using lowtemperature geothermal energy for district heating. Appl Energy. 2011, 2, 88 (2)

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