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Potential of natural ventilation in temperate countries – A case study of Denmark
Applied Energy 114 (2014) 520–530
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Potential of natural ventilation in temperate countries – A case study of Denmark Ivan Oropeza-Perez a,⇑, Poul Alberg Østergaard b a b
Department of Civil Engineering, Aalborg University, Sohngårdsholmsvej 57, 9000 Aalborg, Denmark Department of Development and Planning, Aalborg University, Vestre Havnepromenade 9, 9000 Aalborg, Denmark
h i g h l i g h t s Thermal simulations were run of a dwelling located in a temperate country. The dwelling is a passive house which has problems of overheating during the summer. Natural ventilation was used as a strategy to cool down the household. A new method to assess the saving potential in temperate conditions countries is applied. A potential of saving in a large-scale scenario is assessed for temperate countries.
a r t i c l e
i n f o
Article history: Received 25 March 2013 Received in revised form 1 October 2013 Accepted 6 October 2013
Keywords: Natural ventilation Passive house Temperate conditions Energy saving potential Constant cooling rate
a b s t r a c t The objective of this article is to investigate the energy performance of natural ventilation as a passive cooling method of buildings within houses located in temperate countries using Denmark as a case study. The method consists in running simulations with a thermal–airflow program of a household located in Vejle, Denmark during the months of June, July and August calculating the indoor air temperatures during this period. The dwelling belongs to a Danish project of passive houses named Komfort Husene where its users claim there are periods of overheating during the summer time. Then, after the simulations are validated with measured data, and by applying a new assessment method presented in this article as the cooling rate due to natural ventilation instead of a constant mechanical ventilation rate in the thermal balance within the dwelling, the energy saving is calculated. Results show that there is a reduction of 90% of hours of a possible use of mechanical ventilation, showing the feasibility to achieve thermal comfort within the house by using passive ventilation. The conclusion is therefore that the results contribute to an assessment of the economic and environmental benefits of using natural ventilation rather mechanical one on large-scale scenarios located in temperate conditions. Finally, as a practical implication example, an assessment for Denmark is carried out. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction A reduction of greenhouse gas emission is necessary in order to counter global climate change [1] and consequently, the European Commission aims to reduce greenhouse gas emissions by 20% compared with emission levels of 1990 among the European Union by 2020 [2]. Greenhouse gasses encompass a group of gasses that all enhance the greenhouse effect, and the main gas in terms of anthropogenic enhanced greenhouse is carbon dioxide (CO2) from energy use and cement production. Of the total European CO2 emissions, energy use from buildings contributes about 35%, and out of this share, about 77% corresponds to residential buildings [2]. The residential sector is thus an important target for ⇑ Corresponding author. Tel.: +45 99407234. E-mail address: [email protected] (I. Oropeza-Perez). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.10.008
CO2 emission reductions through energy savings. The literature addresses residential energy savings and CO2 emission reductions from three main perspectives; through systems analyses with focus on a switch to low carbon fuels [3–6], from a conversion system optimisation perspective [7–16], and finally with a focus on the end-use demand efficiency including building envelopes and user behaviour [17–21]. Passive houses – i.e. houses whose objective is not using or to minimise their energy needs keeping a good indoor microclimate including thermal, lighting and acoustical comfort, among others, by not using artificial heating/cooling/lighting systems through the building design such as thermal mass, orientation, shading and natural ventilation – are seen as a good solution for achieving indoor comfort while having low CO2 emissions related to their energy consumption [22].
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Nomenclature
aeffective effective angle of the wind entrance (°) awind wind direction (°) q density of the air at sea level and 20 °C (1.2 kg/m3) DHNPL Aopening CD Cp cp COP DAC qcp V dT dt EConv EVent EAC EAC-Vent EInt
height from midpoint of lower opening to the Neutral Pressure Level (m) opening area (m2) discharge coefficient for opening (dimensionless) pressure coefficient (dimensionless) specific heat of the air (J/kg K) coefficient of performance of the air conditioning (dimensionless) electrical demand of the mechanical ventilation (W) energy stored in the zone air (J/s) convective heat transfer from the surfaces (J/s) heat transfer due to natural ventilation (J/s) cooling rate due to mechanical ventilation (J/s) cooling rate due to both mechanical and natural ventilation (J/s) internal heat loads (J/s)
However, according to both qualitative and quantitative assessments, during some seasons, especially summer time, the indoor air temperature in some cases could rise above the comfort limit [23,24]. Although these cases are not common, the case that refers this article has this characteristic because even though the dwelling is considered as a passive house, it was mainly focused on heating savings. Hence, problems of overheating are presented during the cooling period. Thereby, for the right environmental conditions, natural ventilation can be one of the ways to obtain thermal comfort while maintaining low energy consumption in the building [25–33]. In this context, natural ventilation and other passive methods of cooling have been analysed with simulations and measurements in households located in both cold and warm conditions [34–41]. Furthermore, the performance of ventilation for improving indoor air quality (IAQ) has been analysed in low-energy houses under temperate and cold conditions [42–44]. However, analyses of the energy saving potential of natural ventilation as a passive cooling method within solar passive houses located in temperate or cold conditions have not been carried out. In addition, for the Danish case, the number of residential buildings is growing every year approximately 0.45% [45,46]. With this growth, the energy consumption of the buildings increases as well. As part of the consumption stem from the use of mechanical ventilation to cool down the building, natural ventilation is proposed and investigated in this article. The objective of this article is to assess the potential of natural ventilation taking the passive house located in Vejle, Denmark as a case study [47]. 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 analysed as a means to achieve thermal comfort. The analysis is done by running airflow–thermal simulations with the program EnergyPlus made by the US Department of Energy [48] 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 [49]. Simulations calculate the hourly indoor temperatures of the dwelling during the 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
ESav EnSav ElCon ElConNV
cooling rate saved by natural ventilation (J/s) energy saving potential due to natural ventilation (W h) energy consumption of mechanical ventilation (W h) energy consumption of mechanical ventilation after using natural ventilation (W h) Fschedule value from a user-defined schedule [0 1] h number of hours NalConNV energy consumption of mechanical ventilation after using natural ventilation in a large-scale scenario (W h) Ow opening effectiveness (dimensionless) qf volume natural ventilation rate (m3/s) swind wind speed (m/s) T indoor air temperature at the time step n (K) TInt indoor air temperature (K) TOut outdoor air temperature(K) volume flow rate due stack effect (m3/s) V_ Stack volume flow rate driven by wind(m3/s) V_ wind V volume of the building (m3)
is thus the consumption of power if the equipment would have been turned on. 2. Characteristics of the Danish building stock As it is proposed in this article, the use of mechanical ventilation increases along with the growth of the building stock – particularly in a future situation with improved building envelopes optimised for the colder seasons. Hence, in order to establish the energy savings potential of the Danish residential sector, it is necessary to analyse its characteristics. 2.1. Number of dwellings in Denmark The number of dwellings in Denmark in 2009 was 2,548,240 [45,46]. Since 2004 this number has grown by almost 70 thousand (see Fig 1), with an approximate average growth of 0.45% per year, and an increased building mass will results in an increased energy consumption in the Danish house sector, although efficiency improvements through renovation as well as replacement of some older dwellings included in the numbers to some extent counter balances this. These data include different kinds of existing dwellings in Denmark – semi-detached, multi-dwelling, student hostels, farm houses, etc., but only include occupied dwellings for each year. 2.2. Built area The distribution of dwellings in Denmark according to their built area can be seen in Fig. 2. It can be seen that 50.5% of the share of the building stock have 100 m2 or more of built area. This implies that either for heating or cooling purposes, the demand of energy could be relatively high due to the relatively large size of the households. 2.3. Number of occupants The distribution of dwellings according to the number of occupants is shown in Fig. 3. The largest percentages are gathered among 1 to 2 occupants per dwelling which together represent 71.7% of the building stock. The cooling and heating consumption
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Fig. 1. Number of dwellings and population in Denmark in the period 2004–2009 [45,46].
Table 1 Average area per person in the dwellings of Denmark.
Fig. 2. Danish dwellings distributed by built area (m2) [46].
Year
m2/person
2004 2005 2006 2007 2008 2009
49.75 49.86 50.21 50.57 50.79 51.05
2009 is found and presented in Table 1. The table was made without exact data for the dwellings either with less area than 50 m2 or with more area than 175 m2. Due to this lack of data, sizes of 30 m2 and 200 m2 were set for finding the average area per person. The increase of built area per person in Denmark shown in Table 1 has an average growth of 0.25 m2 or 0.5% annually. This means that the building stock is growing regardless the development of the size of the population. Moreover, with an increasing population, the more built area will be necessary, and thus the demand of cooling will increase in the next years unless other measures are taken e.g. natural ventilation. 2.4. Materials of construction in traditional Danish dwellings
Fig. 3. Danish dwellings distributed by number of occupants [46].
per person is therefore high considering that the demand is attributed to only one or two persons in the dwelling – conversely, the few occupants per dwelling also result in a higher number of dwellings compared to countries with more occupants per dwelling. Therefore, with Figs. 2 and 3, by taking the numerical average of the built area and the number of occupants, the mean area that uses one person in the Danish residential sector from 2004 to
With natural ventilation, materials of construction are very important because they act as a thermal store which may influence the indoor air temperature [50]. Thus, it is important to know the thermal characteristics of the building materials used in the building stock. In Denmark, the roof is in general a wooden construction either as flat or sloped [51]. The most common cladding is in the first case asphalt or tar-and-gravel, and in the second case tiles or metal. The walls are commonly made of brick [51]. The materials of construction in the floor, roof, walls and windows are shown in Tables 2 and 3. These materials are compared with typical materials from the neighbouring countries Sweden and Germany. Comparing Tables 2 and 3, it can be seen that German, Swedish and Danish dwellings have similar materials of construction. However, of the four elements which influence most in the thermal performance – i.e. windows, walls, floor and roof – Danish dwellings have walls with massive brick masonry and lightweight panels which usually have low U-values [51].
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I. Oropeza-Perez, P.A. Østergaard / Applied Energy 114 (2014) 520–530 Table 2 Main floor and roof materials in three different European countries [51]. Country
Floor structure
Roof structure
Roof covering
Sweden
Reinforced concrete slabs
Denmark
Wooden beams spanning from façade to façade, supported on the spine wall Reinforced concrete slabs Reinforced concrete slabs
Wooden structure Reinforced concrete slabs Wooden structure Reinforced concrete slabs Wooden structure
Sloped: Brick tiles or asbestos cement tiles Flat: Roofing felt Sloped: Panes or tiles or cement Flat: Asphalt felts on board Sloped: Brick, steel or copper, bituminous material steel or cooper
Germany
Reinforced concrete slabs
Table 3 Typical main wall, window frame and fabric materials in three different European countries [51]. Country
Walls
Windows frames
Outside finishing
Sweden
Brickwork Precast concrete panels Massive brick masonry Precast concrete panels, lightweight panels Massive/perforated brick work Concrete panels
Wood
Concrete, plaster, metal sheets
Wood Plastic, Aluminium Wood
Facing bricks Fibre reinforced plaster Brick, stone metal or wood coverings Concrete Plaster, pains
Denmark Germany
The case study which is shown in this article has the same materials of construction existing in traditional Danish dwellings; however, as it is a passive house, these materials have lower U-values than the regular ones. Therefore, with high heat gains – e.g. solar gains and internal heat sources – the household is prone to get more heated during summer time [51] thus its cooling demand potentially tends to increase. 2.5. Electricity consumption in the Danish household sector The final energy consumption in the Danish household sector for 2009 was 192 PJ including electricity [52]. In a break down, the electricity consumption in the same sector and year was 36.42 PJ or 10.11 TW h [53]. The remainder is mainly for heating. Regarding consumption of active methods of cooling in the house sector, dwellings with air-conditioning systems are practically non-existent in Denmark [46,54]. Nevertheless, the expenditure for mechanical ventilation systems such as fans – including e.g. extractor fans and oven fans – has been growing over time [46]. Danish Energy Authority statistics do not include a breakdown of the electricity consumption for ventilation purposes, so the assessment in Fig. 4 is based on an economic assessment from
[46,55]. The electricity consumption per year is thus calculated with the respective annual prices in €/kW h from 2004 to 2009 [55] in Fig. 4. By multiplying the consumption in 2009 in each dwelling – i.e. 111.98 kW h/year – by the number of dwellings at the same year, a consumption of approximately 285 GW h is assessed, which represents 2.8% of the total electricity consumption in the household sector. Furthermore, according to Fig. 4, fans consumption tends to increase with time. Therefore, it is relevant to find a manner to reduce this electricity consumption with methods of passive ventilation. While modest, the share can be expected to increase with the increase of passive houses in Denmark unless the summer overheating is addressed. 2.6. Development of passive houses in Denmark The first certified passive house in Denmark was occupied in 2008 by the architect Olav Langenkamp [56]. This house was built according to the standards of the Passive House Institute in Darmstadt, Germany. Since then, Langenkamp Architects have had around 40 projects of passive houses around Denmark [57].
Fig. 4. Yearly expenditure and electricity consumption of fans per dwelling in Denmark [46,55].
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Other projects of passive houses have been carried out such as the case used in this article, denominated Komfort Husene – The Comfort Houses in English – which has 10 existing dwellings [58]. According to Brunsgraad et al., the development of passive houses in Denmark has a great potential and is increasing over the time [56], but it is important to note that although with passive houses under cold and temperate conditions there would be energy savings for heating during winter time, there would be a potentially more demand for cooling during summer time as the houses were designed for thermal comfort under low outdoor temperatures hence inevitably they will heat up under high outdoor temperatures.
In Eq. (1), the internal temperature is assumed to be uniform, thus the heat transfer by ventilation is given by Eq. (2) [50]:
3. Energy saving potential
V_ wind ¼ Ow Aopening F schedule swind
The potential of the use of natural ventilation when there is thermal discomfort have been carried out in previous works by setting an operative indoor temperature of comfort and calculating the natural airflow necessary to reach this temperature [59–64]. However, these studies have been carried out neither in temperate conditions countries nor with passive houses. Also, the potential of energy saving in a large-scale scenario depends to a great extent on the particular characteristics of each dwelling – i.e. design, occupancy and outdoor conditions – in order to get the most accurate result. For doing that, it is necessary to run several simulations thus the task could be very time-absorbing. Another way to calculate the energy saving is by using the room air balance equations, where, during daytime, the air changes per hour that should be provided into the building to cool it down can be calculated by combining the necessary cooling power and the difference between the outdoor and the indoor temperature. In this way, one can estimate the cooling demand with and without ventilation, respectively [65]. Nonetheless, applying this to a largescale scenario would imply a very complex task due to the large number of combinations between air changes per hour and indoor/outdoor temperature differences that would have to be determined in every case study of the country. Hence, a new method to calculate the energy saving potential of natural ventilation instead of mechanical ventilation is proposed in this article. This method estimates the mechanical ventilation demand and the energy saving for a large-scale scenario by considering the passive house of the case study as an extreme case where the highest indoor temperature could be reached in any house located in Denmark. Therefore, during the summer time, if the use of natural ventilation helps to cool down the dwelling, it is considered that natural ventilation is also sufficient to cool down other households. Also, the analysis of this case study could help to achieve a potential energy demand level in projected dwellings by driving both the building design and the occupancy to a better use of natural airflow for thermal comfort purposes. 3.1. Thermal–airflow balance in EnergyPlus As the method proposed in this article is supported by the coupled thermal–airflow building simulation programme EnergyPlus made by the U.S. Department of Energy [48], the description of the program is presented. The description starts with a thermal energy balance on the zone air – i.e. within the dwelling – assuming that the indoor temperature is well-mixed [50]:
qcp V
dT ¼ EConv þ EInt þ EAC þ EVent dt
ð1Þ
When the heat transfer in each term of Eq. (1) is negative, the heat flux goes from inside to outside the dwelling and the dwelling is thus loosing heat, while when is positive, the dwelling is gaining heat.
EVent ¼ qcp qf ðT Int T Out Þ
ð2Þ
The natural ventilation rate qf is a function of the wind speed and the thermal stack effect, as is shown in Eq. (3). The model formulation used is from the ASHRAE Handbooks of Fundamentals [66,67].
qf ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðV_ stack Þ þ ðV_ wind Þ
ð3Þ
The ventilation rate driven by wind is given by Eq. (4)
ð4Þ
This volumetric rate can be also expressed by the surface pressure distribution. This distribution is a function of the pressure coefficient (Cp) which depends on the wind direction & speed, among other factors [50]. However, in order to achieve an accurate value of Cp, the variant values of wind direction and speed – boundary conditions – must be as closed to reality as possible [50]. If this data is not available, techniques such as wind tunnel experiments or CFD simulations must be carried out [50]. In this article, due to the boundary conditions are given only hourly by the closest meteorological weather station, they have a level of uncertainty if the data is considered to be exactly outside the building. Therefore, the volumetric flow due to wind is calculated with a fixed opening effectiveness which is seasonal averaged [68]. This opening effectiveness is calculated based on the angle between the actual wind direction and the effective angle of the wind entrance using Eq. (5) [68]:
Ow ¼ 0:55 þ
jaeffectiv e awind j 0:25 180
ð5Þ
The difference between the effective angle and the wind direction should be between 0° and 180°. This equation is a linear interpolation using the values recommended by the ASHRAE Handbook of Fundamentals [68] – i.e. 0.5–0.6 for perpendicular winds and 0.25–0.35 for diagonal winds. The ventilation rate due to stack effect is given by Eq. (6) [68]:
V_ stack ¼ C D Aopening F Schedule
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðjT Int T Out jÞ 2g DHNPL T Int
ð6Þ
The discharge coefficient for opening is given by Eq. (7) [68]:
C D ¼ 0:40 þ 0:0025jT Int T Out j
ð7Þ
Thereby, the entire set of Eqs. (1)–(7)is iterated in EnergyPlus within one time-step to determine the indoor temperature setting a given temperature set-point. Then, the subsequent time step is iterated. EnergyPlus runs the simulations depending on whether the output(s) are to be hourly, daily, monthly or yearly. For this article, hourly simulations are done. Furthermore, the inputs and boundary conditions – i.e. initial outdoor and indoor temperatures, wind speed, wind direction, etc. – are taken from the case study. It is important to mention that although there are simulation tools such as COMIS, AirNet and AIOLOS, which deal with the airflow within the building, EnergyPlus was used because its integrated simultaneous solver of both the thermal and airflow models gives in every time-step the cooling rate due to both natural and mechanical ventilation as well as the convective heat transfer from the walls and the energy stored in Eq. (1). These outputs are necessary in order to assess the indoor air temperature of the dwelling by using natural ventilation using the model proposed in this article. The aforementioned programs do not deal with the building thermal model at a coupling manner with the airflow model thus are not in the scope of the study presented here.
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Thereby, by solving the differential equation of the energy stored in Eq. (1) it is possible for EnergyPlus to calculate the indoor temperature of the dwelling in every time-step of the simulation without and with ventilation, respectively. Furthermore, EnergyPlus uses Eqs. (2)–(7)to make the calculations when natural ventilation is applied. 3.2. Assessment of the energy saving potential As it was mentioned before, the energy saving is calculated with the method proposed and by using EnergyPlus as a tool. In the method, to estimate the energy saving potential, the estimated hourly energy rate savings are given by the difference between the mechanical ventilation demand without natural ventilation and the demand using ventilation.
ESav ¼ EAC EAC-Vent
ð8Þ
As in EnergyPlus the schedule of the openings is set in such a way that, with natural ventilation, in every time-step, it is possible to reach the temperature of comfort, EAC-Vent is considered with a value of zero.
ESav ¼ EAC
ð9Þ
For calculating the electricity demand required to take out the heat, an average simplified coefficient of performance (COP) of the mechanical ventilation system is given [50]
COP ¼
jEAC j jDAC j
ð10Þ
Therefore, as in this article it is considered the use of a fan with a constant EAC, the energy saving could be calculated with a constant COP in the hours when the fan is turned off as
EnSav ¼
n X ESav i¼1
COP
h ¼ DAC
n X h
Materials of construction. Building shape and orientation. Openings sizes, orientations and shapes. Surroundings – Adjacent constructions, trees and heat/cold sources. HVAC systems. – Fans. – Heating systems.
Outdoor conditions Outdoor temperature. Relative humidity. Wind speed and direction. By setting the input parameters of the occupants’ behaviour, building design and outdoor conditions, the model is adapted to the study case shown in this article. Subsequently, simulations are run with this model using the coupled thermal–airflow program EnergyPlus [48]. 5. Description of the case study The case study is a passive house located in Vejle, which is located in the southeast of Jutland, Peninsula of Denmark (see Fig. 5) at 55°430 N 9°320 E. The dwelling to be analysed is a two-floor construction (see Fig. 6) with 112.2 m2 of floor area. It has a nuclear family as occupants – i.e. four persons, two parents and two children. The schedule of the family consists in leaving the house around 7:00 am and returning at 4:00 pm during week days. On weekends, they remain in the house most of the time unless special occasions [70]. 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 any kind of shading device. However, these devices are
ð11Þ
i¼1
Thereby, the energy consumption of mechanical ventilation within a dwelling after using natural ventilation is given by Eq. (12)
ElConNV ¼ ElCon EnSav
ð12Þ
For a large-scale scenario, e.g. in a national level with m number of dwellings, the total consumption is thus given by Eq. (13)
NalConNV ¼
m m X X ElCon EnSav i¼1
ð13Þ
i¼1
4. Model of natural ventilation A model to assess natural ventilation on a given case study is used for this article [49]. This model includes the thermal energy balance given in Eqs. (1)–(7)and uses three sets of inputs relative to behaviour of the occupants, building design and outdoor conditions. These inputs are [49]: Behaviour of the occupants
Number and schedule of occupants. Internal heat sources. Window openings and solar shading operation. Temperature set-point. Building design
525
Fig. 5. Location of Vejle, Denmark [69].
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Fig. 6. Image of the case study [57] and the sketch used for the simulations.
proposed in the article as placed in both south and east facades in order to analyse their influence on the indoor air temperature and as a means to obtain thermal comfort. It is important to say that the size of the windows are considered as large openings [50] thus there are more driving forces – i.e. to propel the wind – due to wind pressure than driving forces due to temperature differences [50]. The wall construction consists of 120 mm lightweight concrete panels and 108 mm girders. The roof is constructed with 450 45 mm concrete slabs to unilaterally decrease 7°. The materials of construction are similar of the Danish building stock (cf. Tables 2 and 3), with the difference that the U-values are even lower (see Table 4). In general, its characteristics – i.e. layout, number of occupants, size, etc. – are similar to the rest of the 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, being the dwelling with the most problems of overheating among the houses of the project Komfort Husene [70]. Moreover, one inconvenience is that the occupants claim that natural ventilation is not effective as it is difficult to keep the windows open as a result of the lock mechanism [70]. The physical characteristics of the construction elements are given in Table 4. Internal heat sources were set at 3 W/m2 for electrical equipment. In this time of the year, summer time, operation of heating systems is not considered. Table 4 Properties of the construction elements of the case study [71]. Thickness U-value Density Specific heat Conductivity Transmittance of glazing Reflectance of glazing Colour of glazing Glazing area
Wall 0.56 m, roof 0.59 m, glazing 0.06 m Wall 0.083 W/(m2 K), roof 0.068 W/(m2 K) Wall 1920 kg/m3, roof 800 kg/m3 Wall 790 J/(kg K), roof 900 J/(kg K) Wall 0.05 W/(m K), roof 0.04 W/(m K) 0.898 0.075 Transparent North 11%, East 38%, South 36%, West 15%
Regarding surroundings, there are no objects – i.e. trees, buildings, etc. – that can make shade to the house or change the wind speed & direction during the year. Weather data – i.e. hourly and daily average outdoor temperature, wind speed, wind direction and sky clearness – was obtained from the historical database of the observing station 06102 located at an elevation of 22 m above sea level in Vejle (55°510 N, 9°470 E) and operated by the Danish Meteorological Institute for the year 2009 [72]. Additional data come from the weather database of EnergyPlus which takes the hourly values of the weather data from its database of Copenhagen, as this one is the weather data point closest to Vejle. Outdoor conditions are shown in Table 5 giving June 11th, July 30th and August 13th as examples. The year 2009 was chosen because there is already measured data of the indoor temperature of the case study for the same year [70,71]. Continuous measurements of indoor CO2 level and temperature were made every 5 min throughout a measurement period of three years. The equipment was placed centrally in the master bedroom, living room, kitchen and bathroom at a height of 0.16 m [70,71]. For this article only the indoor temperature on the living room is taken account of. This is because in this part of the house is where thermal discomfort is more common rather other rooms [70,71] thus where a cooling method is more necessary.
6. Case study simulations Taking the input parameters from the study case description, two kinds of simulations were run: modelling the existing data and with the use of natural ventilation. Therefore, by performing simulations with EnergyPlus and by setting the maximum operative temperature given by the European guideline EN 15251:2007 for residential buildings during the cooling season – i.e. 27 °C – [73], hourly indoor air temperatures are calculated. Results can be seen in Fig. 7. In Fig. 7 it can be seen the comparison through June, July and August between outdoor temperature (green1 line), modelled indoor temperature (dotted red line), and modelled indoor temperature with the use of natural ventilation (blue line). Measured indoor temperature is not shown in the plot as it is available neither numerically nor graphically, only as a figure with the number of hours with and without thermal comfort into the dwelling [70,71]. When indoor temperature is below 27 °C (black line), both temperatures with and without natural ventilation overlap each other. However, in the simulations, when indoor temperature rises above 27 °C, the use of natural ventilation decreases the temperature to be lower than the maximum operative temperature. Nevertheless, sometimes when the outdoor air temperature is above 27 °C, indoor temperature cannot be lower than this threshold. With the temperature set point of 27 °C, there are 33 hours of discomfort in the 2208 hours that represent the three months of the case study: in June 3 hours and in July 30. In August thermal comfort is achieved all hours. Nevertheless, some studies advice that comfort temperatures might vary through the year as people adapt to changes in outside temperatures [74]. As comfort temperatures vary, heating and cooling set-points should be adjusted in harmony to maintain optimum comfort. This is in keeping with most experience of people. Also, standards such as the ASHRAE 55rev.-2003 consider that an 1 For interpretation of colour in Fig. 7, the reader is referred to the web version of this article.
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I. Oropeza-Perez, P.A. Østergaard / Applied Energy 114 (2014) 520–530 Table 5 Outdoor conditions for Vejle, Denmark on days of June, July and August 2009 [72]. mm/dd
Mean temp. (°C)
Max. temp. (°C)
Min. temp. (°C)
Sky clearness
Mean wind speed (km/h)
Mean wind direction (°)
06/11 07/30 08/13
13.3 14.4 17.1
16.0 16.1 22.0
7.0 13.0 13.0
0.21 0.32 0.28
11.5 20.0 8.5
27.8 129.4 74.8
acceptable indoor operative temperature should be between 25 and 20 °C approximately for naturally conditioned spaces located in cold/temperate conditions [75]. Moreover, the occupants of the dwelling indicate that they do not feel comfortable at a temperature of 27 °C [70], suggesting that perhaps the 27 °C threshold is not set with cold or temperate climate dwellers in mind. Therefore, the adaptive comfort temperature is set at 25.5 °C for free running buildings – i.e. without mechanical ventilation – in Denmark for summer time [74]. This corresponds to Category A by the guideline CR 1752 [75]. With this new indoor temperature set-point, the results of the simulation are shown in Fig. 8. In this case the number of hours on thermal discomfort is increased, especially during July with 71 hours. In June there are 12 hours and in August 2 hours. All together there are 85 hours which represent 3.39% of the 2208 hours of these three months. With that, a comparison between the measured number of hours without thermal comfort [70,71] – using the same guideline CR 1752 – and the simulations carried out in this article could be seen in Table 6. 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 and NV simulated). The shading device consists in two overhangs 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 occupants’ behaviour 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 modelled day. Thereby, according to the magnitude of the differences between the measurements [70,71] and the simulations, the model is validated.
On the other hand, the simulations with natural ventilation plus shading have 4 hours less than with only natural ventilation in the three months. According to Brunsgaard et al., during June 10.7% of the time there is no thermal comfort within the building [70]. With natural ventilation these hours would go to 0.5%. In July, 18.8% of no comfort would decrease to 3.2%. For August, the discomfort would decrease from 10.5% of the time to less than 0.1%. In addition, Table 6 shows that with the use of 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. 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 setpoint, wind speed and solar heat gains. Furthermore, according to the model presented here, the total internal heat sources have as the most important factors the electrical equipment and the convention heat flux from the walls, where is found that this flux is higher during night, when the absorbed heat during daytime due to solar gains is released into the building. As both outdoor temperature and wind speed are parameters beyond control, and the temperature set-point depends to a great extent on the preferences of the occupants, it is recommended not using electric equipment with high heat radiation. In addition, the improving of natural ventilation because of the use of some complementary features such as shading devices – presented in this article – or its behaviour during not occupied hours could be addressed in order to achieve a complete solution for the overheating issue within the dwelling. For the first
Fig. 7. Indoor and outdoor temperatures of the case study for June, July and August.
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Fig. 8. Indoor and outdoor temperatures of the case study for June, July and August.
Table 6 Number of hours with thermal discomfort with and without natural ventilation. Month
No. hours (measured) [70,71]
No. hours (simulated)
No. hours (NV simulated)
No. hours (shading and NV simulated)
June July August
237 417 232
243 477 204
12 71 2
11 69 1
Total
886
924
85
81
subject, as it was mentioned, the overhangs proposed in this article achieve only a few more hours of comfort. However, more moveable shading methods such as blinds and curtains would achieve more comfort hours by avoiding the solar heat gains if a proper utilisation by the occupants is carried out. Likewise, the use of natural ventilation during absence is addressed by proposing a window protection which allows the air inlet and avoids breaking into the dwelling. Thereby, the occupants would be able to keep the windows open during the non-occupied periods while natural ventilation could remove the internal heat gains by convention heat transfer. However, these features were not taken into account in the simulations due to they depend to a great extent on the randomness of the occupants´ behaviour which is very difficult to model in this case study. Nonetheless, a proper scheduling of these complementary methods by the users could assure thermal comfort.
7. Energy saving by using natural ventilation With the results it is possible to calculate an energy saving potential for not utilising electricity-based systems. A conventional system of mechanical ventilation would consist in a fan of 50 W which could supply up to 120 m3 of fresh air per hour with a pressure difference of 1 kPa [76] and which could meet the ASHRAE Standard 62.2P for residential buildings – i.e. 120 m3 h1 for a dwelling of 3 bedrooms and 110 m2 floor area [77] as the case study dwelling.
Therefore, by using Table 6, and considering that during hours of thermal discomfort the fan is turned on, the energy saving is calculated with Eq. (11) as is shown in Table 7. Furthermore, for both measured and simulated cases without using natural ventilation, the energy consumption is also estimated by multiplying the number of discomfort hours by its power, i.e. 50 W. In this case only natural ventilation without the shading device was taken account of. The energy saving potential of the dwelling is up to 42 kW h during the cooling season. This means avoiding 839 hours using an electric fan. With an average electricity demand for ventilation in Danish dwellings of 112 kW h in 2009 (cf. Fig. 4), this corresponds to a saving of 37.5% in this year.
7.1. Potential of saving in Denmark As long as the increasing of the building stock continues (cf. Fig. 1); and the ratio of the built area to the number of persons holds its growth rate (cf. Table 1), the cooling demand in the Danish residential sector will be rising within the following years. This is in addition to the fact that the construction of passive houses in Denmark oriented to heating savings is growing and thus may present overheating during the summer time. Therefore, their cooling demand would increase as well. For these reasons, the use of natural ventilation – at the stage of building design and behaviour of the occupants – in the Danish household sector is proposed as a passive method of cooling in
Table 7 Electricity consumption and saving projected with and without the use of natural ventilation. Month
Consumption (kW h) measured
Consumption (kW h) simulated
Consumption NV (kW h) simulated
Energy saving (kW h)
June July August
11.9 20.9 11.6
12.2 23.9 10.2
0.6 3.6 0.1
11.6 20.3 10.1
Total
44.3
46.2
4.2
42.0
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order to save energy. For projected dwellings, the building design is the main feature to take account of, whereas in existing dwellings the occupancy would be the driver to optimise the natural airflow within the building [49]. Thereby, an assessment of the technical feasibility of natural ventilation on a large-scale scenario such as the Danish residential sector could be made based on the analysis of the case study presented in this article. The passive house is used as a reference case where the indoor temperature is usually higher than the temperature within regular dwellings and even so it is decreased with natural ventilation. Thus, its potential could be extrapolated to the remaining dwellings of the sector which share similar characteristics of design and occupancy. Furthermore, outdoor conditions in Denmark do not vary to a great extent over the territory of both Jutland and Zealand (cf. Fig. 5) [72]. Therefore, by calculating the avoided use of mechanical ventilation, an estimated average saving energy can be determined for the whole country by using Eqs. (12) and (13). This saving is calculated in 107 GW h for 2009. With the national consumption of mechanical ventilation for the same year, i.e. 285 GW h (cf. Subsection 2.5) the final consumption after using natural ventilation is set in 178 GW h. Natural ventilation would prevent the excessive use of active methods of cooling such as fans and air-conditioning systems which will increase over time (cf. Fig. 4). Environmental benefits by not emitting greenhouse gases linked to the electricity generation [53] and economic benefits of the users by saving money on their electricity invoices [55] may thus be realised. Hence, with a CO2 emission factor of 0.567 ton CO2eq/MW h for 2009 in Denmark [53], a potential mitigation of 60.7 thousand tons of CO2 equivalent is estimated. Also, as an economic benefit, with a price of 0.1239 €/kW h for Danish households in 2009 [55], a potential saving of 13 million Euros is calculated.
8. Conclusion This article presents an analysis of the feasibility of using natural ventilation as a method of cooling buildings when there are overheating periods within a passive house located in a temperate country. With the correct use of natural ventilation there is a potential reduction in terms of time of 90% of mechanical ventilation use during summer in the presented case of study that represents a saving of 42 kW h for 2009. Although the potential of saving seems low, it is important to say that it would be higher if the possibility of using even more electricity-demanding devices such as air-conditioning systems were more considered by the occupants. Currently in Denmark the use of air-conditioning in the residential sector is practically non-existent; however, its use is gradually growing over the time. On the other hand, for the case study, and 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 of which the internal heat sources are under direct user influence. In addition, the use of large openings in the four facades is recommendable as long as a well-done scheduling of windows openings is carried out as well as the outdoor conditions – outdoor temperature, wind speed and wind direction, mainly – 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
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kind of device. That means that solar gains are not the main factor of the total internal heat gains. In this context, the high heat gains due to electrical equipment that exists within the dwelling along with the convection flux that is presented during night – when the absorbed heat from daytime due to solar gains is released into the building – are considered as the main causes of the high indoor temperature. As the occupants claim that natural ventilation is not effective as 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. For passive houses it might even be considered setting larger requirements for minimum surface area of windows that can open. However, issues such as security must be also taken account of in order to ensure the welfare of the occupants. Perhaps in some periods when the dwelling is not occupied it is safer to keep the windows closed, or to install a protection in such a way that it can allow the air inlet avoiding at the same time objects with a high volume. Thereby, with the energy saving potential of natural ventilation within the passive house – with high solar gains and low thermal conductivity – it is reasonable to suppose that this cooling method would work within regular buildings. In this kind of buildings the use of mechanical ventilation is also present. Therefore, in a largescale scenario, presented here as the Danish residential sector in 2009, an average saving potential of 107 GW h is assessed. Also, a mitigation of 60.7 thousand tons of CO2 equivalent is calculated as environment benefit, whereas a saving of 13 million Euros is estimated as economic benefit. However, these figures are very rough since are based on only the case study presented in this article. Thus, they are just illustrative to show the possible potential of natural ventilation applied onto a large-scale scenario. Thereby, the accurate assessment of the potential on a national level depends on the analysis of several cases where natural ventilation is used accommodating for differences in weather, buildings, occupancy, local topography and other factors. For doing that, all the case studies of the scenario must be analysed which is both challenging in terms of data availability and in terms of modelling time and effort. As last comment, realising the potential saving would increase the possibility of getting both environmental and economic benefits in the Danish large-scale scenario – and furthermore from an implementation perspective increase the likelihood of house buyers to opt for passive houses which may otherwise run the risk of being thwarted if passive houses get a reputation for poor indoor climate. References [1] Pachauri RK, Reisinger A. Contribution of working groups I, II and III to the fourth assessment report of the intergovernmental panel on climate change; 2007. [2] Hamdy M, Hasan A, Siren K. Applying a multi-objective optimization approach for design of low-emission cost-effective dwellings. Build Environ 2011;46(1):109–23. [3] Alberg Østergaard P, Mathiesen BV, Möller B, Lund H. A renewable energy scenario for Aalborg Municipality based on low-temperature geothermal heat, wind power and biomass. Energy 2010;35(12):4892–901. [4] Østergaard PA, Lund H. A renewable energy system in Frederikshavn using low-temperature geothermal energy for district heating. Appl Energy 2011;88(2):479–87. [5] Lund H, Mathiesen BV. Energy system analysis of 100% renewable energy systems—the case of Denmark in years 2030 and 2050. Energy 2009;34(5):524–31. [6] Lund H, Hvelplund F, Østergaard PA, Möller B, Mathiesen BV, et al. System and market integration of wind power in Denmark. Energy Strategy Rev 2013;1(3):143–56. [7] Suárez I, Prieto MM, Fernández FJ. Analysis of potential energy, economic and environmental savings in residential buildings: solar collectors combined with microturbines. Appl Energy 2013;104:128–36.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Potential of natural ventilation in temperate countries – A case study of Denmark Ivan Oropeza-Perez a,⇑, Poul Alberg Østergaard b a b
Department of Civil Engineering, Aalborg University, Sohngårdsholmsvej 57, 9000 Aalborg, Denmark Department of Development and Planning, Aalborg University, Vestre Havnepromenade 9, 9000 Aalborg, Denmark
h i g h l i g h t s Thermal simulations were run of a dwelling located in a temperate country. The dwelling is a passive house which has problems of overheating during the summer. Natural ventilation was used as a strategy to cool down the household. A new method to assess the saving potential in temperate conditions countries is applied. A potential of saving in a large-scale scenario is assessed for temperate countries.
a r t i c l e
i n f o
Article history: Received 25 March 2013 Received in revised form 1 October 2013 Accepted 6 October 2013
Keywords: Natural ventilation Passive house Temperate conditions Energy saving potential Constant cooling rate
a b s t r a c t The objective of this article is to investigate the energy performance of natural ventilation as a passive cooling method of buildings within houses located in temperate countries using Denmark as a case study. The method consists in running simulations with a thermal–airflow program of a household located in Vejle, Denmark during the months of June, July and August calculating the indoor air temperatures during this period. The dwelling belongs to a Danish project of passive houses named Komfort Husene where its users claim there are periods of overheating during the summer time. Then, after the simulations are validated with measured data, and by applying a new assessment method presented in this article as the cooling rate due to natural ventilation instead of a constant mechanical ventilation rate in the thermal balance within the dwelling, the energy saving is calculated. Results show that there is a reduction of 90% of hours of a possible use of mechanical ventilation, showing the feasibility to achieve thermal comfort within the house by using passive ventilation. The conclusion is therefore that the results contribute to an assessment of the economic and environmental benefits of using natural ventilation rather mechanical one on large-scale scenarios located in temperate conditions. Finally, as a practical implication example, an assessment for Denmark is carried out. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction A reduction of greenhouse gas emission is necessary in order to counter global climate change [1] and consequently, the European Commission aims to reduce greenhouse gas emissions by 20% compared with emission levels of 1990 among the European Union by 2020 [2]. Greenhouse gasses encompass a group of gasses that all enhance the greenhouse effect, and the main gas in terms of anthropogenic enhanced greenhouse is carbon dioxide (CO2) from energy use and cement production. Of the total European CO2 emissions, energy use from buildings contributes about 35%, and out of this share, about 77% corresponds to residential buildings [2]. The residential sector is thus an important target for ⇑ Corresponding author. Tel.: +45 99407234. E-mail address: [email protected] (I. Oropeza-Perez). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.10.008
CO2 emission reductions through energy savings. The literature addresses residential energy savings and CO2 emission reductions from three main perspectives; through systems analyses with focus on a switch to low carbon fuels [3–6], from a conversion system optimisation perspective [7–16], and finally with a focus on the end-use demand efficiency including building envelopes and user behaviour [17–21]. Passive houses – i.e. houses whose objective is not using or to minimise their energy needs keeping a good indoor microclimate including thermal, lighting and acoustical comfort, among others, by not using artificial heating/cooling/lighting systems through the building design such as thermal mass, orientation, shading and natural ventilation – are seen as a good solution for achieving indoor comfort while having low CO2 emissions related to their energy consumption [22].
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Nomenclature
aeffective effective angle of the wind entrance (°) awind wind direction (°) q density of the air at sea level and 20 °C (1.2 kg/m3) DHNPL Aopening CD Cp cp COP DAC qcp V dT dt EConv EVent EAC EAC-Vent EInt
height from midpoint of lower opening to the Neutral Pressure Level (m) opening area (m2) discharge coefficient for opening (dimensionless) pressure coefficient (dimensionless) specific heat of the air (J/kg K) coefficient of performance of the air conditioning (dimensionless) electrical demand of the mechanical ventilation (W) energy stored in the zone air (J/s) convective heat transfer from the surfaces (J/s) heat transfer due to natural ventilation (J/s) cooling rate due to mechanical ventilation (J/s) cooling rate due to both mechanical and natural ventilation (J/s) internal heat loads (J/s)
However, according to both qualitative and quantitative assessments, during some seasons, especially summer time, the indoor air temperature in some cases could rise above the comfort limit [23,24]. Although these cases are not common, the case that refers this article has this characteristic because even though the dwelling is considered as a passive house, it was mainly focused on heating savings. Hence, problems of overheating are presented during the cooling period. Thereby, for the right environmental conditions, natural ventilation can be one of the ways to obtain thermal comfort while maintaining low energy consumption in the building [25–33]. In this context, natural ventilation and other passive methods of cooling have been analysed with simulations and measurements in households located in both cold and warm conditions [34–41]. Furthermore, the performance of ventilation for improving indoor air quality (IAQ) has been analysed in low-energy houses under temperate and cold conditions [42–44]. However, analyses of the energy saving potential of natural ventilation as a passive cooling method within solar passive houses located in temperate or cold conditions have not been carried out. In addition, for the Danish case, the number of residential buildings is growing every year approximately 0.45% [45,46]. With this growth, the energy consumption of the buildings increases as well. As part of the consumption stem from the use of mechanical ventilation to cool down the building, natural ventilation is proposed and investigated in this article. The objective of this article is to assess the potential of natural ventilation taking the passive house located in Vejle, Denmark as a case study [47]. 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 analysed as a means to achieve thermal comfort. The analysis is done by running airflow–thermal simulations with the program EnergyPlus made by the US Department of Energy [48] 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 [49]. Simulations calculate the hourly indoor temperatures of the dwelling during the 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
ESav EnSav ElCon ElConNV
cooling rate saved by natural ventilation (J/s) energy saving potential due to natural ventilation (W h) energy consumption of mechanical ventilation (W h) energy consumption of mechanical ventilation after using natural ventilation (W h) Fschedule value from a user-defined schedule [0 1] h number of hours NalConNV energy consumption of mechanical ventilation after using natural ventilation in a large-scale scenario (W h) Ow opening effectiveness (dimensionless) qf volume natural ventilation rate (m3/s) swind wind speed (m/s) T indoor air temperature at the time step n (K) TInt indoor air temperature (K) TOut outdoor air temperature(K) volume flow rate due stack effect (m3/s) V_ Stack volume flow rate driven by wind(m3/s) V_ wind V volume of the building (m3)
is thus the consumption of power if the equipment would have been turned on. 2. Characteristics of the Danish building stock As it is proposed in this article, the use of mechanical ventilation increases along with the growth of the building stock – particularly in a future situation with improved building envelopes optimised for the colder seasons. Hence, in order to establish the energy savings potential of the Danish residential sector, it is necessary to analyse its characteristics. 2.1. Number of dwellings in Denmark The number of dwellings in Denmark in 2009 was 2,548,240 [45,46]. Since 2004 this number has grown by almost 70 thousand (see Fig 1), with an approximate average growth of 0.45% per year, and an increased building mass will results in an increased energy consumption in the Danish house sector, although efficiency improvements through renovation as well as replacement of some older dwellings included in the numbers to some extent counter balances this. These data include different kinds of existing dwellings in Denmark – semi-detached, multi-dwelling, student hostels, farm houses, etc., but only include occupied dwellings for each year. 2.2. Built area The distribution of dwellings in Denmark according to their built area can be seen in Fig. 2. It can be seen that 50.5% of the share of the building stock have 100 m2 or more of built area. This implies that either for heating or cooling purposes, the demand of energy could be relatively high due to the relatively large size of the households. 2.3. Number of occupants The distribution of dwellings according to the number of occupants is shown in Fig. 3. The largest percentages are gathered among 1 to 2 occupants per dwelling which together represent 71.7% of the building stock. The cooling and heating consumption
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Fig. 1. Number of dwellings and population in Denmark in the period 2004–2009 [45,46].
Table 1 Average area per person in the dwellings of Denmark.
Fig. 2. Danish dwellings distributed by built area (m2) [46].
Year
m2/person
2004 2005 2006 2007 2008 2009
49.75 49.86 50.21 50.57 50.79 51.05
2009 is found and presented in Table 1. The table was made without exact data for the dwellings either with less area than 50 m2 or with more area than 175 m2. Due to this lack of data, sizes of 30 m2 and 200 m2 were set for finding the average area per person. The increase of built area per person in Denmark shown in Table 1 has an average growth of 0.25 m2 or 0.5% annually. This means that the building stock is growing regardless the development of the size of the population. Moreover, with an increasing population, the more built area will be necessary, and thus the demand of cooling will increase in the next years unless other measures are taken e.g. natural ventilation. 2.4. Materials of construction in traditional Danish dwellings
Fig. 3. Danish dwellings distributed by number of occupants [46].
per person is therefore high considering that the demand is attributed to only one or two persons in the dwelling – conversely, the few occupants per dwelling also result in a higher number of dwellings compared to countries with more occupants per dwelling. Therefore, with Figs. 2 and 3, by taking the numerical average of the built area and the number of occupants, the mean area that uses one person in the Danish residential sector from 2004 to
With natural ventilation, materials of construction are very important because they act as a thermal store which may influence the indoor air temperature [50]. Thus, it is important to know the thermal characteristics of the building materials used in the building stock. In Denmark, the roof is in general a wooden construction either as flat or sloped [51]. The most common cladding is in the first case asphalt or tar-and-gravel, and in the second case tiles or metal. The walls are commonly made of brick [51]. The materials of construction in the floor, roof, walls and windows are shown in Tables 2 and 3. These materials are compared with typical materials from the neighbouring countries Sweden and Germany. Comparing Tables 2 and 3, it can be seen that German, Swedish and Danish dwellings have similar materials of construction. However, of the four elements which influence most in the thermal performance – i.e. windows, walls, floor and roof – Danish dwellings have walls with massive brick masonry and lightweight panels which usually have low U-values [51].
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I. Oropeza-Perez, P.A. Østergaard / Applied Energy 114 (2014) 520–530 Table 2 Main floor and roof materials in three different European countries [51]. Country
Floor structure
Roof structure
Roof covering
Sweden
Reinforced concrete slabs
Denmark
Wooden beams spanning from façade to façade, supported on the spine wall Reinforced concrete slabs Reinforced concrete slabs
Wooden structure Reinforced concrete slabs Wooden structure Reinforced concrete slabs Wooden structure
Sloped: Brick tiles or asbestos cement tiles Flat: Roofing felt Sloped: Panes or tiles or cement Flat: Asphalt felts on board Sloped: Brick, steel or copper, bituminous material steel or cooper
Germany
Reinforced concrete slabs
Table 3 Typical main wall, window frame and fabric materials in three different European countries [51]. Country
Walls
Windows frames
Outside finishing
Sweden
Brickwork Precast concrete panels Massive brick masonry Precast concrete panels, lightweight panels Massive/perforated brick work Concrete panels
Wood
Concrete, plaster, metal sheets
Wood Plastic, Aluminium Wood
Facing bricks Fibre reinforced plaster Brick, stone metal or wood coverings Concrete Plaster, pains
Denmark Germany
The case study which is shown in this article has the same materials of construction existing in traditional Danish dwellings; however, as it is a passive house, these materials have lower U-values than the regular ones. Therefore, with high heat gains – e.g. solar gains and internal heat sources – the household is prone to get more heated during summer time [51] thus its cooling demand potentially tends to increase. 2.5. Electricity consumption in the Danish household sector The final energy consumption in the Danish household sector for 2009 was 192 PJ including electricity [52]. In a break down, the electricity consumption in the same sector and year was 36.42 PJ or 10.11 TW h [53]. The remainder is mainly for heating. Regarding consumption of active methods of cooling in the house sector, dwellings with air-conditioning systems are practically non-existent in Denmark [46,54]. Nevertheless, the expenditure for mechanical ventilation systems such as fans – including e.g. extractor fans and oven fans – has been growing over time [46]. Danish Energy Authority statistics do not include a breakdown of the electricity consumption for ventilation purposes, so the assessment in Fig. 4 is based on an economic assessment from
[46,55]. The electricity consumption per year is thus calculated with the respective annual prices in €/kW h from 2004 to 2009 [55] in Fig. 4. By multiplying the consumption in 2009 in each dwelling – i.e. 111.98 kW h/year – by the number of dwellings at the same year, a consumption of approximately 285 GW h is assessed, which represents 2.8% of the total electricity consumption in the household sector. Furthermore, according to Fig. 4, fans consumption tends to increase with time. Therefore, it is relevant to find a manner to reduce this electricity consumption with methods of passive ventilation. While modest, the share can be expected to increase with the increase of passive houses in Denmark unless the summer overheating is addressed. 2.6. Development of passive houses in Denmark The first certified passive house in Denmark was occupied in 2008 by the architect Olav Langenkamp [56]. This house was built according to the standards of the Passive House Institute in Darmstadt, Germany. Since then, Langenkamp Architects have had around 40 projects of passive houses around Denmark [57].
Fig. 4. Yearly expenditure and electricity consumption of fans per dwelling in Denmark [46,55].
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Other projects of passive houses have been carried out such as the case used in this article, denominated Komfort Husene – The Comfort Houses in English – which has 10 existing dwellings [58]. According to Brunsgraad et al., the development of passive houses in Denmark has a great potential and is increasing over the time [56], but it is important to note that although with passive houses under cold and temperate conditions there would be energy savings for heating during winter time, there would be a potentially more demand for cooling during summer time as the houses were designed for thermal comfort under low outdoor temperatures hence inevitably they will heat up under high outdoor temperatures.
In Eq. (1), the internal temperature is assumed to be uniform, thus the heat transfer by ventilation is given by Eq. (2) [50]:
3. Energy saving potential
V_ wind ¼ Ow Aopening F schedule swind
The potential of the use of natural ventilation when there is thermal discomfort have been carried out in previous works by setting an operative indoor temperature of comfort and calculating the natural airflow necessary to reach this temperature [59–64]. However, these studies have been carried out neither in temperate conditions countries nor with passive houses. Also, the potential of energy saving in a large-scale scenario depends to a great extent on the particular characteristics of each dwelling – i.e. design, occupancy and outdoor conditions – in order to get the most accurate result. For doing that, it is necessary to run several simulations thus the task could be very time-absorbing. Another way to calculate the energy saving is by using the room air balance equations, where, during daytime, the air changes per hour that should be provided into the building to cool it down can be calculated by combining the necessary cooling power and the difference between the outdoor and the indoor temperature. In this way, one can estimate the cooling demand with and without ventilation, respectively [65]. Nonetheless, applying this to a largescale scenario would imply a very complex task due to the large number of combinations between air changes per hour and indoor/outdoor temperature differences that would have to be determined in every case study of the country. Hence, a new method to calculate the energy saving potential of natural ventilation instead of mechanical ventilation is proposed in this article. This method estimates the mechanical ventilation demand and the energy saving for a large-scale scenario by considering the passive house of the case study as an extreme case where the highest indoor temperature could be reached in any house located in Denmark. Therefore, during the summer time, if the use of natural ventilation helps to cool down the dwelling, it is considered that natural ventilation is also sufficient to cool down other households. Also, the analysis of this case study could help to achieve a potential energy demand level in projected dwellings by driving both the building design and the occupancy to a better use of natural airflow for thermal comfort purposes. 3.1. Thermal–airflow balance in EnergyPlus As the method proposed in this article is supported by the coupled thermal–airflow building simulation programme EnergyPlus made by the U.S. Department of Energy [48], the description of the program is presented. The description starts with a thermal energy balance on the zone air – i.e. within the dwelling – assuming that the indoor temperature is well-mixed [50]:
qcp V
dT ¼ EConv þ EInt þ EAC þ EVent dt
ð1Þ
When the heat transfer in each term of Eq. (1) is negative, the heat flux goes from inside to outside the dwelling and the dwelling is thus loosing heat, while when is positive, the dwelling is gaining heat.
EVent ¼ qcp qf ðT Int T Out Þ
ð2Þ
The natural ventilation rate qf is a function of the wind speed and the thermal stack effect, as is shown in Eq. (3). The model formulation used is from the ASHRAE Handbooks of Fundamentals [66,67].
qf ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ðV_ stack Þ þ ðV_ wind Þ
ð3Þ
The ventilation rate driven by wind is given by Eq. (4)
ð4Þ
This volumetric rate can be also expressed by the surface pressure distribution. This distribution is a function of the pressure coefficient (Cp) which depends on the wind direction & speed, among other factors [50]. However, in order to achieve an accurate value of Cp, the variant values of wind direction and speed – boundary conditions – must be as closed to reality as possible [50]. If this data is not available, techniques such as wind tunnel experiments or CFD simulations must be carried out [50]. In this article, due to the boundary conditions are given only hourly by the closest meteorological weather station, they have a level of uncertainty if the data is considered to be exactly outside the building. Therefore, the volumetric flow due to wind is calculated with a fixed opening effectiveness which is seasonal averaged [68]. This opening effectiveness is calculated based on the angle between the actual wind direction and the effective angle of the wind entrance using Eq. (5) [68]:
Ow ¼ 0:55 þ
jaeffectiv e awind j 0:25 180
ð5Þ
The difference between the effective angle and the wind direction should be between 0° and 180°. This equation is a linear interpolation using the values recommended by the ASHRAE Handbook of Fundamentals [68] – i.e. 0.5–0.6 for perpendicular winds and 0.25–0.35 for diagonal winds. The ventilation rate due to stack effect is given by Eq. (6) [68]:
V_ stack ¼ C D Aopening F Schedule
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðjT Int T Out jÞ 2g DHNPL T Int
ð6Þ
The discharge coefficient for opening is given by Eq. (7) [68]:
C D ¼ 0:40 þ 0:0025jT Int T Out j
ð7Þ
Thereby, the entire set of Eqs. (1)–(7)is iterated in EnergyPlus within one time-step to determine the indoor temperature setting a given temperature set-point. Then, the subsequent time step is iterated. EnergyPlus runs the simulations depending on whether the output(s) are to be hourly, daily, monthly or yearly. For this article, hourly simulations are done. Furthermore, the inputs and boundary conditions – i.e. initial outdoor and indoor temperatures, wind speed, wind direction, etc. – are taken from the case study. It is important to mention that although there are simulation tools such as COMIS, AirNet and AIOLOS, which deal with the airflow within the building, EnergyPlus was used because its integrated simultaneous solver of both the thermal and airflow models gives in every time-step the cooling rate due to both natural and mechanical ventilation as well as the convective heat transfer from the walls and the energy stored in Eq. (1). These outputs are necessary in order to assess the indoor air temperature of the dwelling by using natural ventilation using the model proposed in this article. The aforementioned programs do not deal with the building thermal model at a coupling manner with the airflow model thus are not in the scope of the study presented here.
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Thereby, by solving the differential equation of the energy stored in Eq. (1) it is possible for EnergyPlus to calculate the indoor temperature of the dwelling in every time-step of the simulation without and with ventilation, respectively. Furthermore, EnergyPlus uses Eqs. (2)–(7)to make the calculations when natural ventilation is applied. 3.2. Assessment of the energy saving potential As it was mentioned before, the energy saving is calculated with the method proposed and by using EnergyPlus as a tool. In the method, to estimate the energy saving potential, the estimated hourly energy rate savings are given by the difference between the mechanical ventilation demand without natural ventilation and the demand using ventilation.
ESav ¼ EAC EAC-Vent
ð8Þ
As in EnergyPlus the schedule of the openings is set in such a way that, with natural ventilation, in every time-step, it is possible to reach the temperature of comfort, EAC-Vent is considered with a value of zero.
ESav ¼ EAC
ð9Þ
For calculating the electricity demand required to take out the heat, an average simplified coefficient of performance (COP) of the mechanical ventilation system is given [50]
COP ¼
jEAC j jDAC j
ð10Þ
Therefore, as in this article it is considered the use of a fan with a constant EAC, the energy saving could be calculated with a constant COP in the hours when the fan is turned off as
EnSav ¼
n X ESav i¼1
COP
h ¼ DAC
n X h
Materials of construction. Building shape and orientation. Openings sizes, orientations and shapes. Surroundings – Adjacent constructions, trees and heat/cold sources. HVAC systems. – Fans. – Heating systems.
Outdoor conditions Outdoor temperature. Relative humidity. Wind speed and direction. By setting the input parameters of the occupants’ behaviour, building design and outdoor conditions, the model is adapted to the study case shown in this article. Subsequently, simulations are run with this model using the coupled thermal–airflow program EnergyPlus [48]. 5. Description of the case study The case study is a passive house located in Vejle, which is located in the southeast of Jutland, Peninsula of Denmark (see Fig. 5) at 55°430 N 9°320 E. The dwelling to be analysed is a two-floor construction (see Fig. 6) with 112.2 m2 of floor area. It has a nuclear family as occupants – i.e. four persons, two parents and two children. The schedule of the family consists in leaving the house around 7:00 am and returning at 4:00 pm during week days. On weekends, they remain in the house most of the time unless special occasions [70]. 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 any kind of shading device. However, these devices are
ð11Þ
i¼1
Thereby, the energy consumption of mechanical ventilation within a dwelling after using natural ventilation is given by Eq. (12)
ElConNV ¼ ElCon EnSav
ð12Þ
For a large-scale scenario, e.g. in a national level with m number of dwellings, the total consumption is thus given by Eq. (13)
NalConNV ¼
m m X X ElCon EnSav i¼1
ð13Þ
i¼1
4. Model of natural ventilation A model to assess natural ventilation on a given case study is used for this article [49]. This model includes the thermal energy balance given in Eqs. (1)–(7)and uses three sets of inputs relative to behaviour of the occupants, building design and outdoor conditions. These inputs are [49]: Behaviour of the occupants
Number and schedule of occupants. Internal heat sources. Window openings and solar shading operation. Temperature set-point. Building design
525
Fig. 5. Location of Vejle, Denmark [69].
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Fig. 6. Image of the case study [57] and the sketch used for the simulations.
proposed in the article as placed in both south and east facades in order to analyse their influence on the indoor air temperature and as a means to obtain thermal comfort. It is important to say that the size of the windows are considered as large openings [50] thus there are more driving forces – i.e. to propel the wind – due to wind pressure than driving forces due to temperature differences [50]. The wall construction consists of 120 mm lightweight concrete panels and 108 mm girders. The roof is constructed with 450 45 mm concrete slabs to unilaterally decrease 7°. The materials of construction are similar of the Danish building stock (cf. Tables 2 and 3), with the difference that the U-values are even lower (see Table 4). In general, its characteristics – i.e. layout, number of occupants, size, etc. – are similar to the rest of the 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, being the dwelling with the most problems of overheating among the houses of the project Komfort Husene [70]. Moreover, one inconvenience is that the occupants claim that natural ventilation is not effective as it is difficult to keep the windows open as a result of the lock mechanism [70]. The physical characteristics of the construction elements are given in Table 4. Internal heat sources were set at 3 W/m2 for electrical equipment. In this time of the year, summer time, operation of heating systems is not considered. Table 4 Properties of the construction elements of the case study [71]. Thickness U-value Density Specific heat Conductivity Transmittance of glazing Reflectance of glazing Colour of glazing Glazing area
Wall 0.56 m, roof 0.59 m, glazing 0.06 m Wall 0.083 W/(m2 K), roof 0.068 W/(m2 K) Wall 1920 kg/m3, roof 800 kg/m3 Wall 790 J/(kg K), roof 900 J/(kg K) Wall 0.05 W/(m K), roof 0.04 W/(m K) 0.898 0.075 Transparent North 11%, East 38%, South 36%, West 15%
Regarding surroundings, there are no objects – i.e. trees, buildings, etc. – that can make shade to the house or change the wind speed & direction during the year. Weather data – i.e. hourly and daily average outdoor temperature, wind speed, wind direction and sky clearness – was obtained from the historical database of the observing station 06102 located at an elevation of 22 m above sea level in Vejle (55°510 N, 9°470 E) and operated by the Danish Meteorological Institute for the year 2009 [72]. Additional data come from the weather database of EnergyPlus which takes the hourly values of the weather data from its database of Copenhagen, as this one is the weather data point closest to Vejle. Outdoor conditions are shown in Table 5 giving June 11th, July 30th and August 13th as examples. The year 2009 was chosen because there is already measured data of the indoor temperature of the case study for the same year [70,71]. Continuous measurements of indoor CO2 level and temperature were made every 5 min throughout a measurement period of three years. The equipment was placed centrally in the master bedroom, living room, kitchen and bathroom at a height of 0.16 m [70,71]. For this article only the indoor temperature on the living room is taken account of. This is because in this part of the house is where thermal discomfort is more common rather other rooms [70,71] thus where a cooling method is more necessary.
6. Case study simulations Taking the input parameters from the study case description, two kinds of simulations were run: modelling the existing data and with the use of natural ventilation. Therefore, by performing simulations with EnergyPlus and by setting the maximum operative temperature given by the European guideline EN 15251:2007 for residential buildings during the cooling season – i.e. 27 °C – [73], hourly indoor air temperatures are calculated. Results can be seen in Fig. 7. In Fig. 7 it can be seen the comparison through June, July and August between outdoor temperature (green1 line), modelled indoor temperature (dotted red line), and modelled indoor temperature with the use of natural ventilation (blue line). Measured indoor temperature is not shown in the plot as it is available neither numerically nor graphically, only as a figure with the number of hours with and without thermal comfort into the dwelling [70,71]. When indoor temperature is below 27 °C (black line), both temperatures with and without natural ventilation overlap each other. However, in the simulations, when indoor temperature rises above 27 °C, the use of natural ventilation decreases the temperature to be lower than the maximum operative temperature. Nevertheless, sometimes when the outdoor air temperature is above 27 °C, indoor temperature cannot be lower than this threshold. With the temperature set point of 27 °C, there are 33 hours of discomfort in the 2208 hours that represent the three months of the case study: in June 3 hours and in July 30. In August thermal comfort is achieved all hours. Nevertheless, some studies advice that comfort temperatures might vary through the year as people adapt to changes in outside temperatures [74]. As comfort temperatures vary, heating and cooling set-points should be adjusted in harmony to maintain optimum comfort. This is in keeping with most experience of people. Also, standards such as the ASHRAE 55rev.-2003 consider that an 1 For interpretation of colour in Fig. 7, the reader is referred to the web version of this article.
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I. Oropeza-Perez, P.A. Østergaard / Applied Energy 114 (2014) 520–530 Table 5 Outdoor conditions for Vejle, Denmark on days of June, July and August 2009 [72]. mm/dd
Mean temp. (°C)
Max. temp. (°C)
Min. temp. (°C)
Sky clearness
Mean wind speed (km/h)
Mean wind direction (°)
06/11 07/30 08/13
13.3 14.4 17.1
16.0 16.1 22.0
7.0 13.0 13.0
0.21 0.32 0.28
11.5 20.0 8.5
27.8 129.4 74.8
acceptable indoor operative temperature should be between 25 and 20 °C approximately for naturally conditioned spaces located in cold/temperate conditions [75]. Moreover, the occupants of the dwelling indicate that they do not feel comfortable at a temperature of 27 °C [70], suggesting that perhaps the 27 °C threshold is not set with cold or temperate climate dwellers in mind. Therefore, the adaptive comfort temperature is set at 25.5 °C for free running buildings – i.e. without mechanical ventilation – in Denmark for summer time [74]. This corresponds to Category A by the guideline CR 1752 [75]. With this new indoor temperature set-point, the results of the simulation are shown in Fig. 8. In this case the number of hours on thermal discomfort is increased, especially during July with 71 hours. In June there are 12 hours and in August 2 hours. All together there are 85 hours which represent 3.39% of the 2208 hours of these three months. With that, a comparison between the measured number of hours without thermal comfort [70,71] – using the same guideline CR 1752 – and the simulations carried out in this article could be seen in Table 6. 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 and NV simulated). The shading device consists in two overhangs 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 occupants’ behaviour 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 modelled day. Thereby, according to the magnitude of the differences between the measurements [70,71] and the simulations, the model is validated.
On the other hand, the simulations with natural ventilation plus shading have 4 hours less than with only natural ventilation in the three months. According to Brunsgaard et al., during June 10.7% of the time there is no thermal comfort within the building [70]. With natural ventilation these hours would go to 0.5%. In July, 18.8% of no comfort would decrease to 3.2%. For August, the discomfort would decrease from 10.5% of the time to less than 0.1%. In addition, Table 6 shows that with the use of 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. 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 setpoint, wind speed and solar heat gains. Furthermore, according to the model presented here, the total internal heat sources have as the most important factors the electrical equipment and the convention heat flux from the walls, where is found that this flux is higher during night, when the absorbed heat during daytime due to solar gains is released into the building. As both outdoor temperature and wind speed are parameters beyond control, and the temperature set-point depends to a great extent on the preferences of the occupants, it is recommended not using electric equipment with high heat radiation. In addition, the improving of natural ventilation because of the use of some complementary features such as shading devices – presented in this article – or its behaviour during not occupied hours could be addressed in order to achieve a complete solution for the overheating issue within the dwelling. For the first
Fig. 7. Indoor and outdoor temperatures of the case study for June, July and August.
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Fig. 8. Indoor and outdoor temperatures of the case study for June, July and August.
Table 6 Number of hours with thermal discomfort with and without natural ventilation. Month
No. hours (measured) [70,71]
No. hours (simulated)
No. hours (NV simulated)
No. hours (shading and NV simulated)
June July August
237 417 232
243 477 204
12 71 2
11 69 1
Total
886
924
85
81
subject, as it was mentioned, the overhangs proposed in this article achieve only a few more hours of comfort. However, more moveable shading methods such as blinds and curtains would achieve more comfort hours by avoiding the solar heat gains if a proper utilisation by the occupants is carried out. Likewise, the use of natural ventilation during absence is addressed by proposing a window protection which allows the air inlet and avoids breaking into the dwelling. Thereby, the occupants would be able to keep the windows open during the non-occupied periods while natural ventilation could remove the internal heat gains by convention heat transfer. However, these features were not taken into account in the simulations due to they depend to a great extent on the randomness of the occupants´ behaviour which is very difficult to model in this case study. Nonetheless, a proper scheduling of these complementary methods by the users could assure thermal comfort.
7. Energy saving by using natural ventilation With the results it is possible to calculate an energy saving potential for not utilising electricity-based systems. A conventional system of mechanical ventilation would consist in a fan of 50 W which could supply up to 120 m3 of fresh air per hour with a pressure difference of 1 kPa [76] and which could meet the ASHRAE Standard 62.2P for residential buildings – i.e. 120 m3 h1 for a dwelling of 3 bedrooms and 110 m2 floor area [77] as the case study dwelling.
Therefore, by using Table 6, and considering that during hours of thermal discomfort the fan is turned on, the energy saving is calculated with Eq. (11) as is shown in Table 7. Furthermore, for both measured and simulated cases without using natural ventilation, the energy consumption is also estimated by multiplying the number of discomfort hours by its power, i.e. 50 W. In this case only natural ventilation without the shading device was taken account of. The energy saving potential of the dwelling is up to 42 kW h during the cooling season. This means avoiding 839 hours using an electric fan. With an average electricity demand for ventilation in Danish dwellings of 112 kW h in 2009 (cf. Fig. 4), this corresponds to a saving of 37.5% in this year.
7.1. Potential of saving in Denmark As long as the increasing of the building stock continues (cf. Fig. 1); and the ratio of the built area to the number of persons holds its growth rate (cf. Table 1), the cooling demand in the Danish residential sector will be rising within the following years. This is in addition to the fact that the construction of passive houses in Denmark oriented to heating savings is growing and thus may present overheating during the summer time. Therefore, their cooling demand would increase as well. For these reasons, the use of natural ventilation – at the stage of building design and behaviour of the occupants – in the Danish household sector is proposed as a passive method of cooling in
Table 7 Electricity consumption and saving projected with and without the use of natural ventilation. Month
Consumption (kW h) measured
Consumption (kW h) simulated
Consumption NV (kW h) simulated
Energy saving (kW h)
June July August
11.9 20.9 11.6
12.2 23.9 10.2
0.6 3.6 0.1
11.6 20.3 10.1
Total
44.3
46.2
4.2
42.0
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order to save energy. For projected dwellings, the building design is the main feature to take account of, whereas in existing dwellings the occupancy would be the driver to optimise the natural airflow within the building [49]. Thereby, an assessment of the technical feasibility of natural ventilation on a large-scale scenario such as the Danish residential sector could be made based on the analysis of the case study presented in this article. The passive house is used as a reference case where the indoor temperature is usually higher than the temperature within regular dwellings and even so it is decreased with natural ventilation. Thus, its potential could be extrapolated to the remaining dwellings of the sector which share similar characteristics of design and occupancy. Furthermore, outdoor conditions in Denmark do not vary to a great extent over the territory of both Jutland and Zealand (cf. Fig. 5) [72]. Therefore, by calculating the avoided use of mechanical ventilation, an estimated average saving energy can be determined for the whole country by using Eqs. (12) and (13). This saving is calculated in 107 GW h for 2009. With the national consumption of mechanical ventilation for the same year, i.e. 285 GW h (cf. Subsection 2.5) the final consumption after using natural ventilation is set in 178 GW h. Natural ventilation would prevent the excessive use of active methods of cooling such as fans and air-conditioning systems which will increase over time (cf. Fig. 4). Environmental benefits by not emitting greenhouse gases linked to the electricity generation [53] and economic benefits of the users by saving money on their electricity invoices [55] may thus be realised. Hence, with a CO2 emission factor of 0.567 ton CO2eq/MW h for 2009 in Denmark [53], a potential mitigation of 60.7 thousand tons of CO2 equivalent is estimated. Also, as an economic benefit, with a price of 0.1239 €/kW h for Danish households in 2009 [55], a potential saving of 13 million Euros is calculated.
8. Conclusion This article presents an analysis of the feasibility of using natural ventilation as a method of cooling buildings when there are overheating periods within a passive house located in a temperate country. With the correct use of natural ventilation there is a potential reduction in terms of time of 90% of mechanical ventilation use during summer in the presented case of study that represents a saving of 42 kW h for 2009. Although the potential of saving seems low, it is important to say that it would be higher if the possibility of using even more electricity-demanding devices such as air-conditioning systems were more considered by the occupants. Currently in Denmark the use of air-conditioning in the residential sector is practically non-existent; however, its use is gradually growing over the time. On the other hand, for the case study, and 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 of which the internal heat sources are under direct user influence. In addition, the use of large openings in the four facades is recommendable as long as a well-done scheduling of windows openings is carried out as well as the outdoor conditions – outdoor temperature, wind speed and wind direction, mainly – 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
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kind of device. That means that solar gains are not the main factor of the total internal heat gains. In this context, the high heat gains due to electrical equipment that exists within the dwelling along with the convection flux that is presented during night – when the absorbed heat from daytime due to solar gains is released into the building – are considered as the main causes of the high indoor temperature. As the occupants claim that natural ventilation is not effective as 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. For passive houses it might even be considered setting larger requirements for minimum surface area of windows that can open. However, issues such as security must be also taken account of in order to ensure the welfare of the occupants. Perhaps in some periods when the dwelling is not occupied it is safer to keep the windows closed, or to install a protection in such a way that it can allow the air inlet avoiding at the same time objects with a high volume. Thereby, with the energy saving potential of natural ventilation within the passive house – with high solar gains and low thermal conductivity – it is reasonable to suppose that this cooling method would work within regular buildings. In this kind of buildings the use of mechanical ventilation is also present. Therefore, in a largescale scenario, presented here as the Danish residential sector in 2009, an average saving potential of 107 GW h is assessed. Also, a mitigation of 60.7 thousand tons of CO2 equivalent is calculated as environment benefit, whereas a saving of 13 million Euros is estimated as economic benefit. However, these figures are very rough since are based on only the case study presented in this article. Thus, they are just illustrative to show the possible potential of natural ventilation applied onto a large-scale scenario. Thereby, the accurate assessment of the potential on a national level depends on the analysis of several cases where natural ventilation is used accommodating for differences in weather, buildings, occupancy, local topography and other factors. For doing that, all the case studies of the scenario must be analysed which is both challenging in terms of data availability and in terms of modelling time and effort. As last comment, realising the potential saving would increase the possibility of getting both environmental and economic benefits in the Danish large-scale scenario – and furthermore from an implementation perspective increase the likelihood of house buyers to opt for passive houses which may otherwise run the risk of being thwarted if passive houses get a reputation for poor indoor climate. References [1] Pachauri RK, Reisinger A. Contribution of working groups I, II and III to the fourth assessment report of the intergovernmental panel on climate change; 2007. [2] Hamdy M, Hasan A, Siren K. Applying a multi-objective optimization approach for design of low-emission cost-effective dwellings. Build Environ 2011;46(1):109–23. [3] Alberg Østergaard P, Mathiesen BV, Möller B, Lund H. A renewable energy scenario for Aalborg Municipality based on low-temperature geothermal heat, wind power and biomass. Energy 2010;35(12):4892–901. [4] Østergaard PA, Lund H. A renewable energy system in Frederikshavn using low-temperature geothermal energy for district heating. Appl Energy 2011;88(2):479–87. [5] Lund H, Mathiesen BV. Energy system analysis of 100% renewable energy systems—the case of Denmark in years 2030 and 2050. Energy 2009;34(5):524–31. [6] Lund H, Hvelplund F, Østergaard PA, Möller B, Mathiesen BV, et al. System and market integration of wind power in Denmark. Energy Strategy Rev 2013;1(3):143–56. [7] Suárez I, Prieto MM, Fernández FJ. Analysis of potential energy, economic and environmental savings in residential buildings: solar collectors combined with microturbines. Appl Energy 2013;104:128–36.
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