Evaluation of single-sided natural ventilation using a simplified and fair calculation method

Transcription

1 Evaluation of single-sided natural ventilation using a simplified and fair calculation method Christoffer Plesner 1*, Tine Steen Larsen, Valérie Leprince 3 1 VELUX A/S, Aadalsvej 99, 970 Hoersholm, Denmark Aalborg University, Sofiendalsvej 9-11, 900 Aalborg SV, Denmark 3 PLEIAQ, 84 C Avenue de la liberation, Meyzieu, France * Corresponding christoffer.plesner@velux.com SUMMARY The overall objective of this paper is to evaluate design expressions for single-sided ventilation and find the most suitable that would in average perform well, while reducing the risk of overestimating air flows in individual cases. The design expression needs to be both simple and fair to fit the scope of standards and regulations in the best way. This has been done by comparing design expressions using parameter variations, comparison to wind-tunnel experiments and full-scale outdoor measurements. A modified De Gids & Phaff method showed to be a simplified and fair calculation method that would improve the support of ventilative cooling in future standards and regulations, by making it simple to predict the air flows in buildings. PRACTICAL IMPLICATIONS Results from this work have been directly used to elaborate the new standard FprEN (revision of EN 154:007) to provide simplified methods to calculate the air flow rate through windows. KEYWORDS Single-sided ventilation, natural ventilation, European standard, calculation method, ventilative cooling 1 INTRODUCTION A great energy saving potential lies within increasing the use of natural and hybrid - ventilation for the cooling of buildings. Overheating in buildings have become a challenge at the design stage and during operation, as buildings have increasingly become more airtight and well-insulated consequently increasing the cooling demands, which are, for some buildings, not only found during summer and midseason periods but also during winter. There are several studies showing overheating issues in both new and existing buildings often due to the lack of natural ventilation, solar shading or proper building design (Kolokotroni & Heiselberg 015; Larsen 011; Brunsgaard et al. 01). This consequently shows there is an increasing need to reduce overheating risks in buildings. Ventilative cooling is an application of natural ventilation, where the outdoor air can be used for cooling also during cold periods and may reduce cooling costs in buildings while achieving a good indoor environment. Ventilative cooling is a low cost solution defined as the use of natural or mechanical ventilation strategies to cool indoor spaces most often utilizing the outside air as a mean of cooling and may even be a critical measure to realize a good indoor environment in renovated or new Near Zero Energy Buildings (Kolokotroni & Heiselberg 015).

2 In this paper only single-sided ventilation has been investigated, where previous works have led to design expressions by (De Gids & Phaff 198), (Warren & Parkins 1985) and (Larsen 006). To help member states meet the requirements of the EPBD recast, the European Commission issued a mandate to European standardisation bodies to revise the first EPBD package standards published in (Bara 010). This includes the revision of calculation standards to assess the energy performance of buildings. The determination of air flow rates in buildings is a very important element to assess the energy performance of a building, and shall include passive cooling techniques. Standard EN 154:007 (CEN/TC ) (which is under revision) includes a simple direct method for single sided ventilation: using the De Gids & Phaff expression (De Gids & Phaff 198). In the revised version of the standard (now called FprEN ) the expression to estimate single-sided ventilation has evolved to be more conservative at low wind speeds, while representing an average of the air flow rate in the building. This new expression, still based on the De Gids & Phaff expression is evaluated in this paper. To our knowledge when windows' opening is taken into account in energy performance regulations and standards it is always through simplified methods (no implicit calculation). For example, French thermal regulation has implemented a modified De Gids & Phaff expression for single sided ventilation (different from the evaluated method from this paper) and another explicit method based on a correlation for cross-ventilation (Anon n.d.). Nevertheless, there are still a lot of countries where natural ventilative cooling is poorly rewarded in energy performance regulations and standards. The implementation of a design method in a standard, which frequently overestimates air flow rates, would jeopardize the implementation of ventilative cooling. Indeed, if the expected air flow rates are frequently not met, this could lead to frequent overheating in buildings, in which ventilative cooling would never meet expectations. This would bring discredit on ventilative cooling techniques. On the opposite, using methods that remain on the safe-side leads to efficient installation and thereby promotion of ventilative cooling. So to ensure a better integration and better understanding of ventilative cooling at the design stage, new simplified direct calculation methods, that correctly can predict air flow rates through windows that generally are on the safe side, are needed. MATERIALS/METHODS In this section existing work and expressions for single-sided ventilation are discussed to set the scene. The aim of this paper has been to evaluate different design expressions for prediction of air flow rates using single-sided ventilation and identify the potentials for improving existing design expressions or make new calculation methods to increase the integration of ventilative cooling in standards and regulation. The methods used in this process have been to first analyse the design expressions using parameter variations of the different constants and variables (not included in this paper). Afterwards the most suited proposed calculation method for singlesided ventilation was compared to wind-tunnel experiments and full-scale outdoor measurements performed by (Larsen 006). The evaluation of the design expressions was based on expressions by De Gids & Phaff and Larsen, in which most of the design expressions are compared with one another. Throughout the process, some of the design expressions are modified to improve the calculation output to fit with the planned use: implementation in a calculation standard where it is considered as more suitable to remain on the safe side.

3 .1 Existing design expressions for single-sided natural ventilation Earlier work with single-sided natural ventilation has discussed the dependency of wind velocity, temperature difference and wind direction on the amount of air going through the opening. Some sources only consider a single contribution, some combine the contributions. This section describe some of the existing design expressions found from literature for calculation of air flows in single-sided natural ventilation in cases with air flow driven by thermal buoyancy, wind and both thermal buoyancy and wind..1.1 Air flows driven by wind The calculation of natural ventilation caused by opening a single window has a long history. In (Warren 1977) the results from single-sided ventilation experiments in both full-scale and wind tunnel tests were presented stating that the air flow rate is a combination of turbulent convection caused by the level of turbulence in the wind near the window opening, buoyancy forces and wind speed. In this early work Warren concluded that a difference between the wind and temperature -dominated cases exist. From this it was concluded that the best way to handle the combination of wind and buoyancy was to calculate the effect from each parameter separately and then use the largest of them. In later expressions by Warren and Parkins (Warren & Parkins 1985) did not include the effect of temperature difference but the effect from turbulent convection which is built into the use of local air speed at the surface of the building near the window, U L. Also it was noted that higher rates may be achieved for other combinations of windows, certain wind directions and tall buildings..1. Air flows driven by a combination of thermal buoyancy and wind One of the earliest design expressions for single-sided natural ventilation was developed by W. De Gids and H. Phaff (De Gids & Phaff 198), who split the driving forces up into two parts; the wind and stack effect contribution and added them. This expression is today used in European standard, EN 154:007 where the aim among others is to determine air flows due to windows opening in buildings (CEN/TC ).: U m = C t + C w U ( T T ) 10 + C st H window abs i e (1) Where, Um is the mean air velocity in opening [m/s], Ct is the turbulence constant (0.01), Cw is the dimensionless coefficient depending on the wind effect (0.001), U 10 is the mean wind speed in H=10 m [m/s], C st is the buoyancy constant (0.0035), H window is the height of the opening [m] and (Ti Te) is the temperature difference across the opening [K]. The volume flow rate can be found from (CEN/TC ). Q v = 0,5 3,6 500 Aow Um () where Aow is the window opening area [m ] and Qv is the air flow rate [m 3 /h]. When comparing the original De Gids & Phaff expression with wind-tunnel measurements it has been found that an overestimation of the air flow rates occur at low wind speeds and at no or very low temperature difference. This can be seen from Figure 1 where the blue calculated points, show an overestimation compared to the wind-tunnel measurements (Larsen 006). This issue is tackled in the new calculation method developed for the revised standard, FprEN in which, an improved single-sided ventilation calculation method that can predict air flows through windows, generally on the safe side is needed in standards and regulations.

4 Figure 1 De Gids & Phaff expression, measurements vs. calculations A second design expression for single-sided natural ventilation was developed by (Larsen 006), combining thermal buoyancy, wind velocity and also wind direction, which was not included in earlier work. In (Larsen 006) the flow patterns found in the experimental results also show clearly deviation between wind and temperature dominated cases. The situations where these forces are almost similar become more difficult to handle. The design expression developed by Larsen for single-sided ventilation is shown in (3) (Larsen & Heiselberg 008; Larsen 006). Q v C T = A C f ( β ) C U + C T H + C (3) 1 p ref 3 p,opening Uref The wind direction is included in expression (3) by the use of C p, which depends on the wind direction and by f(β), which includes the effect of local velocities near the opening. The fluctuations are defined by the pressure difference across the opening included in Cp,opening. Expression (3) is to be seen as an alternative to a more simplified calculation method, which is the aim of this paper. The new and modified De Gids & Phaff calculation method, which is proposed for FprEN is seen in (4) and is from here referred to as the modified De Gids & Phaff calculation method or MDGM. Q ρ A a;ref w;tot V = 3600 max w ; 10 ρ a;e ( C U C H abs( T T )) 0, 5 st w;st i e (4) Where ρ a;ref and ρ a;e are the reference and external air densities [kg/m 3 ], A w;tot is the total window opening area [m ], H w;st is the useful stack effect height for airing [m].. Wind driven or thermal driven air flows? (Larsen 006) measured the air change rates in a simple full-scale building with a single opening at the Japanese Building Research Institute (BRI), Tsukuba, Japan. These measurements showed a clear difference between cases dominated by wind and temperature difference, but also that the influence of temperature difference decreases and finally disappears with increasing wind speeds and the opposite is found for increasing temperature difference. The results from Larsen thereby clearly show a difference between wind and temperature dominated cases - a difference which was handled by using two different calculation expressions for the two situations by (Warren 1977).

5 3 RESULTS This section contains results from the comparison of different single-sided ventilation expressions by De Gids & Phaff and the MDGM proposed for FprEN with wind-tunnel and full-scale outdoor measurements. 3.1 Modified De Gids & Phaff calculation method One of the methods used in this analysis was a MDGM, because of the found overestimation of air flow rates at low wind speeds and at no or very low temperature difference, which is a problem when a design expression that can predict air flow rates through windows, generally on the safe side is needed to be used in standards and regulations. The overestimation was evaluated and it was found that there always would be positive air flow contribution due to the fixed wind turbulence coefficient, Ct from (4) with a value of 0,01. A modification was to remove the fixed constant, Ct from the De Gids & Phaff expression which makes sense as this is independent of wind or stack, and consequently overestimates the flows at low wind speeds and at no or very low temperature differences. The removal of fixed constant, Ct, is actually already done in the French thermal regulation, RT01 which has implemented a modified De Gids & Phaff expression for single-sided ventilation (Anon n.d.). Another modification of the design expression was to split the De Gids & Phaff expression into two separate parts; the wind and stack contribution and taking the highest air flow value from the two parts to find the design air flow rate. After further analysis, it showed that choosing the design air flow as the highest of stack and wind effect was the most conservative and is in line with the former thoughts of Warren & Parkins where the same idea was initially used. Also (Larsen & Heiselberg 008; Larsen 006) concludes that clearly wind and temperature dominated cases are found from the measurement of flow patterns in the opening based on the balance between wind velocity and temperature difference in each specific case. 3. Measurements from wind-tunnel and full-scale outdoor In this section the original De Gids & Phaff and the MDGM are compared with wind-tunnel and full-scale outdoor measurements made by (Larsen 006). Here it is again important that the air flows would in average perform well, while generally not overestimating which is needed for the use in standards and regulations. Figure shows the wind-tunnel measurements (x-axis) vs. calculated air flows using the MDGM. Different points (measurements, calculations) are shown with a corresponding incidence angle, though not part of the MDGM, but only part of the wind-tunnel measurements. If the points are within the blue hatched area, consequently indicates overestimation of the MDGM compared to the wind-tunnel measurements. There are different horizontal combinations starting from the bottom indicating the measurements/calculations at different temperature and wind conditions. We can furthermore see many points very close to each other, especially in conditions of T=5 C and U=1, 3 and 5 m/s, where the differentiation between wind driven and temperature driven becomes difficult. We can actually see very conservative air flows using the MDGM, for the majority of the time (88%) being below the hatched area indicating underestimation compared to wind-tunnel measurements.

6 Figure Wind-tunnel measurements (Larsen 006) vs. the MDGM In Figure 3 is shown outdoor full-scale measurements (x-axis) vs. calculated air flows from the MDGM. Different points (measurements, calculations) for all 48 cases made on the nd floor of an office building at Aalborg university, Denmark made by (Larsen 006). This is to properly compare the MDGM with measurements from a real case, such as a full-scale outdoor setup for best fit. Here it was again important that the air flows would in average perform well, while generally being on the safe side. The wind speed in the full-scale outdoor measurements is measured 4 m above the roof, which is then the "wind speed at site at 10 m height" (U 10), and this corresponds to the wind speed used in the De Gids & Phaff expression. We can actually see very conservative values for the MDGM, underestimating for the majority of the time for the outdoor cases (83%) being below the hatched area, not indicating overestimation, like in the original De Gids & Phaff expression. This could be viewed as a strength when having to find a calculation method that in average performs well, while generally being on the safe side, which is needed for the use in standards and regulations. Figure 3 - Full-scale outdoor measurements (Larsen 006) vs. the MDGM 3.3 Findings from the MDGM compared to wind-tunnel measurements Table 1 shows the accuracy of the design expressions calculated as the absolute deviation between the calculated air flow and the wind-tunnel measurements. Input for the calculations are the measured values from the wind-tunnel. The table also shows if over- or underestimations occur. It is shown that the accuracy of the MDGM is 9% but with underestimations in 88% of the wind-tunnel cases and thereby overestimations in only 1% of the time. In comparison, the original De Gids & Phaff expression also has an accuracy of 9%, but overestimates in 50% of

7 the cases, showing a much lower degree of overestimation with the MDGM due to the alternations made. For the MDGM overestimation only occurred for 17% of all outdoor cases, which is very conservative but the overall air flow remains in an acceptable range. Of these overestimating occurrences, the air flow was in average overestimated by 0% for the windtunnel and 14% for the outdoor cases. The MDGM showed good results limiting overestimating air flow rates at especially low or no temperature difference and low wind speeds, while maintaining an acceptable correlation with both wind-tunnel and full scale outdoormeasurements on average. Furthermore, the MDGM was generally on the safe side and the authors consider the calculation method well suited for the use in calculation standards to integrate ventilative cooling effects from single-sided ventilation. For more information on the over- and underestimations of the design expressions see (Larsen et al. 016). Table 1 Accuracy of the design expressions (calculated vs. wind-tunnel measurements) Original De Gids & Phaff 9% (mainly overestimation) Modified De Gids & Phaff (MDGM) 9% (mainly underestimation) Overestimation; % of occurrence of original De Gids/MDGM 50% / 1% Underestimation; % of occurrence of original De Gids/MDGM 50% / 88% 4 DISCUSSION The new standard FprEN proposes four methods to estimate the air flow rate through windows including an iterative method with an implicit equation to determine the internal pressure of the building, the single-sided ventilation method presented here and also a crossventilation explicit method. In theory the iterative method is the most precise one as it relies on well validated physical models and on solving the mass balance equation. Moreover an iterative method allows to take the interaction between windows and other leaks, ventilation system and combustion appliance into account, but the iterative method has its limits and is not suitable for all purposes, such as being a heavier and more time consuming calculation method. First of all, to implement the iterative method adequately, every parameter interacting with ventilation through windows are necessary inputs (combustion appliance, vents, etc.), which are difficult to obtain at the design stage. Furthermore, the iterative method cannot apply to ventilation zones where the only air flows considered are due to single-sided ventilation. Indeed in this case, with a zonal model, the wind pressure results in no air flow rate, but high pressure differences between the inside and outside and then an important instability in the mass balance flow rate. A simplified method allows obtaining a fast overview of results without spending too much time calculating, as in the design phase of a building project. The choice of the design method depends on the objective of the calculation and where in the process you are: For estimation of air flow rates at the design phase it would make sense to use simple direct methods as proposed for standards e.g. FprEN to rather quickly get an output, whereas for projects in a later stage where you need more precise output, more advanced iterative methods may be used. Measurements have shown that the modified De Gids & Phaff method well represents singlesided ventilation phenomena. Nevertheless, it is conservative and remains on the "safe-side", which is considered more suitable for a regulatory calculation method. Therefore, this method is adequate for Energy performance calculations where the iterative method is not suitable. 5 CONCLUSIONS The key issue was to evaluate design expressions for single-sided ventilation to improve the support of ventilative cooling in future standards and regulations in a simplified and fair way. This has been done by parameter variations, comparison to wind-tunnel experiments and fullscale outdoor measurements. Below are the main findings on the performance of the Modified

8 De Gids & Phaff method (MDGM), when comparing to the original De Gids & Phaff expression. The MDGM has an accuracy of 9% and underestimates in 88% of the wind-tunnel cases considered which is conservative, being on the safe side applicable to design phase calculations, as e.g. used in standards and regulations, whereas the original De Gids & Phaff also has an accuracy of 9% but mainly overestimates. MDGM overestimates only in 1% of all windtunnel cases, whereas the original De Gids overestimates in 50% of all wind-tunnel cases showing a much lower degree of overestimation with the MDGM. Furthermore, the MDGM overestimates only 17% of all full-scale outdoor cases (of these overestimating occurrences the calculated air flow rate is overestimated by 0% and 14%). The MDGM showed good results limiting overestimating air flow rates at especially very low temperature differences and low wind speeds, which was the case in the original De Gids & Phaff due to the alternations made and is based on good references (De Gids & Phaff (198) and Larsen (006)). The MDGM maintained an acceptable correlation with both wind-tunnel and full scale outdoormeasurements on average. Furthermore, the authors believe the modified method where C t was removed and taking the highest of wind or stack is a better design expression for cases with ventilative cooling, with results being on the safe side, generally underestimating the air flow rate for the majority of the results, which was exactly the aim for the modified design expression, which was developed for the new standard FprEN providing good estimates to air flows through windows in buildings. ACKNOWLEDGEMENT The work by Larsen was supported financially by Villum Kann Rasmussen (VKR) Foundation, Denmark and measurements were carried out at The Japanese Building Research Institute (BRI), Tsukuba, Japan. 6 REFERENCES Anon, Th-BCE (English), Bara, G., 010. Mandate M/480 EN, Brunsgaard, C. et al., 01. Evaluation of the Indoor Environment of Comfort Houses: Qualitative and Quantitative Approaches. Indoor and Built Environment, 1(3) CEN/TC156, 007. EN 154:007 - Ventilation for buildings - Calculation methods for the determination of air flow rates in buildings including infiltration, De Gids, W. & Phaff, H., 198. Ventilation Rates and Energy Consumption Due to Open Windows. Air Infiltration Review, 4(1), pp.4 5. Kolokotroni, M. & Heiselberg, P., 015. Ventilative Cooling (State-of-the-art review), Aalborg, Denmark. Larsen, T.S., 006. Natural Ventilation Driven by Wind and Temperature Difference. Department of Civil Engineering, Aalborg University. Larsen, T.S., 011. Vurdering af indeklimaet i hidtidigt lavenergibyggeri : med henblik på forbedringer i fremtidens lavenergibyggeri, Larsen, T.S. & Heiselberg, P., 008. Single-sided natural ventilation driven by wind pressure and temperature difference. Energy and Buildings, 40(6), pp Larsen, T.S., Plesner, C. & Leprince, V., 016. Calculation methods for single-sided natural ventilation - simplified or detailed? In Proceedings of Clima 016. Aalborg, Denmark Warren, P.R., Ventilation through openings on one wall only. In C. J. Hoogendorn & N. H. Afgar, eds. Energy conservation in heating, cooling and ventilating buildings. Hemisphere, Washington DC, pp Warren, P.R. & Parkins, L.M., Single-sided ventilation through open windows. In Conf. proc. Thermal Performance of the Exterior Envelopes of Buildings, ASHRAE, Florida. ASHRAE, p. 0.