Air Curtain on Whole Building Energy Use

Summary of Energy use in Buildings with Air Curtains 

"The Best Air curtains on Earth Come From Mars"

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This study conducted the whole building energy analysis of the DOE medium office reference building for three different scenarios of building entrance:

1. single door without vestibule or air curtain (hereafter, a single door)
2. single door equipped with air curtain (hereafter, an air curtain door)
3. single door with vestibule (hereafter, a vestibule door)

For the modeled medium office building, the major conclusions were found as follows.

• The whole building annual energy use when the air curtain is installed is less in all the climate zones: it is less than the single door in the climate zone 1-3, and less than the vestibule door in the climate zone 3-8.
• The modeled air curtain door is shown to reduce air infiltration significantly under the same conditions when compared to either the single door or the vestibule door.
• The predicted annual pressure difference across the envelope of the modeled building is mostly within -10 ~ 10 Pa.
• The modeled air curtain door is shown to provide comparable performance as the modeled vestibule door for the climate zone 3 – 8. Compared to the vestibule door, the air curtain door can save 0.3% ~ 2.2% energy for zone 3 ~ 8, corresponding to 1146 kWh ~ 18986 kWh. Better performance will be achieved for colder climate.
• The major saving of the air curtain door comes from the heating saving so, although there is saving, the air curtain total saving in zone 1 – 2 is marginal, which is 0.0% ~ 0.1% (81 kWh ~ 132 kWh) when compared to the single door.
• Building entrance orientation, building pressure, and door usage frequency all affect air infiltration/exfiltration and the resultant energy performance of the air curtain door. Particularly, the effects of building entrance orientation and the balance of the HVAC system cannot be overlooked and they were shown to be as important as door usage frequency.

Investigation of the Impact of Building Entrance Air Curtain on Whole Building Energy Use Executive Summary

BACKGROUND

The U.S. was reported to consume 19% of the global energy in 2011, and the building sector (residential, commercial and government buildings) accounted for about 41% of the primary energy usage. The top four end uses of the building sector are space heating (37%), space cooling (10%), water heating (12%), and lighting (9%), which sums up to about 70% of the buildings site energy consumption. For commercial buildings, air infiltrations can be as high as 18% of the total heat loss. Air infiltrations (or air leakages) are often caused by unintentional or accidental introduction of outside air into a building through cracks in the building envelope and/or entrance doors. Infiltrations through door openings become quite significant when the doors are used frequently such as in restaurants, retail stores, supermarkets, offices and hospitals (DOE 2012).

A common energy code solution to reducing energy loss from air infiltration through open doors has been requiring a vestibule rather than having a single door. Currently based on the American Society of Heating, Refrigerating and Air-Conditioning Engineers Standard 90.1 – Energy Standard for Buildings Except Low-Rise Residential Buildings (ASHRAE 2010), and the International Energy Conservation Code (IECC), in most cases, vestibules are required in climate zones 3 – 8. However, vestibules seem not to cater to building owners’ taste due to the concerns over space and construction cost. A vestibule could cost anywhere from $20,000 to $60,000. In addition, a vestibule becomes ineffective when both entrance doors open simultaneously during heavy traffic periods so as to allow cold outdoor air to penetrate.

Air curtains, which are typically mounted above doorways, separate indoor and outdoor temperatures with a stream of air strategically engineered to strike the floor with a particular velocity and position. The air prevents outdoor air infiltration while also permitting an unobstructed pedestrian entryway. An air curtain for a single six- foot-wide entrance/exit opening is often less than $6,000 plus installation costs. It also helps to block flying insects, dust, wind, cold/warm, and ambient moisture to achieve a better indoor comfort. Furthermore, building entrances equipped with air curtains are believed to be more energy efficient than the entrances with single doors and with vestibules as well. However, an exhaustive literature search reviewed that no previous studies to quantify the impact of building entrance air curtains on whole building energy usage.

OBJECTIVE

The objective of this study is to decide if air curtains can be considered comparable in energy performance to that of buildings with vestibules where they are required by building energy codes and standards in climate zones 3 – 8 by means of whole building annual energy simulations and computational fluid dynamics (CFD) modeling of air curtains. For the climate zones 1 and 2, where vestibules are not required by the codes, this study will also quantify the potential energy savings of air curtains compared to the baseline case of the building entrance without air curtain or vestibule.

RESULTS
The two tasks were divided into the following five sub-tasks.
Task 1 – Air Curtain Infiltration and Exfiltration
The determination of infiltration & exfiltration characteristics of air curtain is the key task. About 350 CFD simulations were conducted to model an air curtain under different settings of outdoor and indoor pressure and temperature differences by using the standard k-ε turbulence model. Both winter and summer operational modes of air curtain were considered. The air curtain jet is supplied with a 20 ̊ angle (outwards as shown in Fig. 1) at 15 m/s and 21 ̊C for the winter mode, and 15 m/s and 24 ̊C for the summer mode. The indoor design temperatures are the same as the air curtain supply temperatures, and the outdoor temperatures varies among -40, -20 and 10 ̊C in the winter mode, and 25, 30, and 40 ̊C in the summer mode. Different door opening angles were also considered including 90 ̊ (fully open), 60 ̊, 30 ̊ and 10 ̊ under the pressure difference (∆P) of -20, -10, -5, 0, 10, 20, 30, and 40 Pa for all the cases and -3.5, -2.5, -1.5, -1, and -0.5 Pa for some cases across the door. Fig.2 shows the three flow scenarios of the air curtain door when the air curtain is in operation compared to the single and vestibule doors during the occupied hours of the building. When the air curtain is not in operation during occupied hours, the air curtain door was modeled as a single door by using CONTAM’s control nodes and schedules. When the building is unoccupied, the building door was assumed to be completely closed and the leakages were zero for the single door, the air curtain door and the vestibule door.

The critical pressure is found to be ∆Plc = -3.3 Pa and ∆Puc = 6.9 Pa. The outflow (negative Q) occurs when ∆P < ∆Puc, and the inflow (positive Q) occurs when ∆P > ∆Puc.
 For the outflow section, there is a sharp increase of flow rate at ∆Plc when the flow switches from “zero infiltration” (Fig. 1a) to “exfiltration” (Fig. 1c) because the indoor air starts to exfiltrate under the air curtain jet as shown in Fig. 1c.

 Compared to both single door and vestibule door, air curtain reduces air infiltration significantly under the same pressure difference across the door, especially for mild ranges of pressure difference.
 Air curtain also causes less outflow than the vestibule door for the negative pressure difference of -7.0 < ∆P < 0 Pa but creates more outflow when ∆P < -7.0 Pa. When the pressure difference ∆P < -15 Pa, air curtain could cause more outflow than the single door.

Task 2 – Building Pressure Difference

Pressure difference across building envelope is one of the major driving forces for infiltration/exfiltration. Nine CONTAM simulations were conducted to calculate the annual pressure differences and infiltration/exfiltration rates through the single door, the vestibule door, and the air curtain door for the medium office building with three scenarios of the HVAC system: a baseline supply and return system (hereafter, 100% supply system), a system with 5% less supply than return (95% supply system) or a system with 10% less supply than return (90% supply system). The 100% supply system is provided by NIST for the medium office building in the study of “Airflow and Indoor Air Quality Models of DOE Reference Commercial Buildings” (NIST Technical Note 1734) (Ng et al. 2012). The last two scenarios were created to consider the impact of the depressurization of the building by the HVAC system, which could cause more infiltrations than the baseline system. The door usage is the baseline case of 100 people/hr. The annual whole building analysis is conducted for 8760 hours including both occupied and unoccupied hours, and considering the on/off schedules of the HVAC system.

 For the 100% supply system,
o the annually average building pressure difference across the entrance door is 0.8 Pa of a range of -9.5 ~

27.4 Pa with a median value of 0.2 Pa. The pressure difference is mostly -10 ~ 10 Pa.
o compared to the single door, the air curtain door reduces 62% of the annual total air in filtration and 3% of

the total exfiltration, and the vestibule door reduces 23% infiltration and 25% exfiltration.

 For the 95% supply system,
o the annually average building pressure difference across the entrance door is 1.3 Pa of a range of -7.9 ~

29.6 Pa with a median value of 0.5 Pa.
o compared to the single door, the air curtain door reduces 65% of the annual total air infiltration and

increases 3% of the total exfiltration, and the vestibule door reduces 23% infiltration and 25% exfiltration;  For the 90% supply system,

o the annually average building pressure difference across the entrance door is 1.8 Pa of a range of -6.5 ~ 31.5 Pa with a median value of 0.9 Pa.

o compared to the single door, the air curtain door reduces 67% of the annual total air infiltration and increases 11% of the total exfiltration, and the vestibule door reduces 24% infiltration and 24% exfiltration.

Task 3 – Whole Building Energy Simulation

The energy performance of air curtain is a combination effect of infiltration and exfiltration, which can be evaluated by whole building energy analysis. Nine whole building energy analyses were performed by the coupled TRNSYS and CONTAM model for the single door, the vestibule door, and the air curtain door for both summer and winter modes of Chicago, IL. The parameters in consideration include the baseline door usage frequency of 100 people/hr, the 100% supply, 95% supply and 90% supply systems. The air curtain fan power is 1.05 kW. The air curtain is equipped with temperature control and expected to operate only during the occupied hours, when the door is opened and at the same time the ambient air temperature drops below 10 °C for the winter mode, or increases above 30 °C for the summer mode. The major conclusions are as follows.

           Annual energy saving mostly comes from heating saving.

           Small penalty may occur for the vestibule, since during shoulder seasons the vestibule may block the free cooling from the ambient.

           Air curtain leads to better energy performance than the single door and the vestibule door.

           The annual total heating saving of the air curtain varies 7031 kWh (2.8%) ~ 11406 kWh (4.2%) compared to the single door, and 4383 kWh (1.7%) ~ 7359 kWh (2.8%) compared to the vestibule door.

 The reduction of peak heating load by the air curtain varies 28 kW (5.6%) ~ 32 kW (6.2%) compared to the single door, and 18 kW (3.7%) ~ 19 kW (3.8%) compared to the vestibule.
 The reduction of peak cooling load by the air curtain varies 1 kW (0.3%) ~ 2.3 kW (0.8%) compared to the single door, and 0.3 kW (0.1%) ~ 1.3 kW (0.4%) compared to the vestibule.

           The annual air curtain fan energy is 371 kWh.

           Compared to the single door, the annual total energy saving of using air curtain varies 6660 kWh (1.4%) ~ 11085 kWh (2.3%).

 Compared to the vestibule door, the annual total energy saving of using air curtain varies 4169 kWh (0.9%) ~ 7205 kWh (1.5%).
 The variation of the savings depends on how well the HVAC system is balanced. When the HVAC system is less balanced thus tending to cause more infiltration, the energy saving of the air curtain becomes better.

Task 4 – Sensitivity analysis

A sensitivity analysis of 72 whole building energy simulations was performed for the single door and the air curtain door for different climates: climate zone 2 (for cooling dominant climate, e.g. Austin), zone 4 (for heating & cooling climate, e.g. Baltimore), and zone 6 (for heating dominant climate, e.g. Minneapolis). The following key parameters are considered: building entrance orientation, building pressures affected by the 100% supply/95% supply/90% supply HVAC systems, door usage frequencies.

 For climate zones 2,4 and 6, most of the saving comes from heating, and colder climate enjoys more saving from air curtain.
 All the parameters in the sensitivity analysis are important, the variation of which may cause at least 30% difference in terms of annual heating demand saving compared to the according sensitivity test components.

a. Entrance orientation

Building entrance orientation was evaluated for the north, south, east, and west directions. The door usage frequency is set to be 100 people/hr, and the building HVAC is the 100% supply system. The annual heating saving of the air curtain depends on the dominant wind direction for different cities.

 For Austin (climate zone 2), the annual heating saving varies from 328 kWh (0.7%) for the north to 2514 kWh (4.9%) for the south; the annual cooling saving varies from 192 kWh (0%) for the south to 253 kWh (0.1%) for the north, when compared to the single door.
 For Baltimore (climate zone 4), the annual heating saving varies from 497 kWh (0.3%) for the north to 4933 kWh (3.1%) for the west; the annual cooling saving varies from -25 kWh (-0%) for the south to 217 kWh (0.1%) for the north compared to the vestibule.

 For Minneapolis (climate zone 6), the annual heating saving varies from 5788 kWh (1.6%) for the north to 9023 kWh (2.5%) for the west; the annual cooling saving varies from 134 kWh (0.1%) for the south to 292 kWh (0.2%) for the north compared to the vestibule.
 The annual air curtain fan energy is 256 kWh for Austin, 331 kWh for Baltimore and 385 kWh for Minneapolis.

 The annual total saving of using air curtain varies from 330 kWh to 2449 kWh when compared to the single door in Austin; from 383 kWh to 4709 kWh for Baltimore, and from 5695 kWh to 8822 kWh for Minneapolis when compared to the vestibule door.
 The annual total percentage saving of using air curtain varies from 0 to 0.4% when compared to the single door in Austin, from 0% to 1.1% for Baltimore when compared to the vestibule, and from 1% to 1.5% for Minneapolis when compared to the vestibule.

CONCLUSIONS

In this study, CFD simulations of air infiltration/exfiltration through the air curtain were conducted to obtain the corresponding correlations to be used in the whole building energy analysis of the medium office reference building by the coupled simulation of TRNSYS and CONTAM.

For the modeled medium office building with the 100% supply HVAC system and the door usage frequency of 100 people/hr, the whole building annual energy use when the air curtain is installed is less in all the climate zones modeled: it is less than the single door in the climate zone 1-3, and less than the vestibule door in the climate zone 3-8.

Specifically, the following key conclusions are found.
 The airflow rate through the air curtain can be characterized as the function of pressure difference across the door in three sections: zero infiltration, infiltration and exfiltration. Following Yuill’s method for the single door and the vestibule, the air curtain correlations can be obtained as the function of door usage frequency in people per hour in terms of the discharge coefficients and discharge modifiers, or the flow coefficients and flow modifiers.
 Based on the obtained air curtain correlations, the air curtain is shown to reduce air infiltration significantly under the same conditions when compared to either the single door or the vestibule, whereas it may cause higher exfiltration when the indoor pressure is higher enough than the outdoor pressure (e.g. ∆P < -15 Pa) when compared to the single door.
 The predicted annual pressure difference across the envelope of the modeled building is mostly within -10 ~ 10 Pa with the maximum of about 30 Pa and the minimum of -10 Pa for the case of Chicago, IL.
 The modeled air curtain is shown to provide comparable performance as the modeled vestibule for the climate zone 3 – 8. Compared to the vestibule, the air curtain can save 0.3% ~ 2.2% energy for zone 3 – 8, which corresponds to 1146 kWh ~ 18986 kWh. Better performance will be achieved for colder climate.
 The major saving of the air curtain comes from the heating saving so, although there is saving, the air curtain total saving in zone 1 – 2 is marginal, which is 0% (81 kWh ~ 132 kWh) when compared to the single door.

 Building entrance orientation, building pressure, and door usage frequency all affect air infiltration/exfiltration and the resultant energy performance of the air curtain when compared to the single door and the vestibule. Particularly, the effects of building entrance orientation and the balance of the HVAC system cannot be overlooked and they were shown to be as important as door usage frequency.

Based on the results of the modeled building and the air curtain in this study, considering its lower initial cost and space saving benefit, air curtain should be a good alternative to the vestibule for the climate zones of 3 – 8. Note that the results from this study are based on the specific building, the specific parameters and the modeling method of air curtains.

Anderson-Bolds is the Mars Air Representative in Ohio and Kentucky.

216-360-9800

mars@anderson-bolds.com