Passive cooling

Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or nil energy consumption.[1][2] This approach works either by preventing heat from entering the interior (heat gain prevention) or by removing heat from the building (natural cooling).[3] Natural cooling utilizes on-site energy, available from the natural environment, combined with the architectural design of building components (e.g. building envelope), rather than mechanical systems to dissipate heat.[4] Therefore, natural cooling depends not only on the architectural design of the building but on how the site's natural resources are used as heat sinks (i.e. everything that absorbs or dissipates heat). Examples of on-site heat sinks are the upper atmosphere (night sky), the outdoor air (wind), and the earth/soil.

Overview

Passive cooling covers all natural processes and techniques of heat dissipation and modulation without the use of energy.[1] Some authors consider that minor and simple mechanical systems (e.g. pumps and economizers) can be integrated in passive cooling techniques, as long they are used to enhance the effectiveness of the natural cooling process.[5] Such applications are also called ‘hybrid cooling systems’.[1] The techniques for passive cooling can be grouped in two main categories:

Preventative techniques

Protection from or prevention of heat gains encompasses all the design techniques that minimizes the impact of solar heat gains through the building’s envelope and of internal heat gains that is generated inside the building due occupancy and equipment. It includes the following design techniques:[1]

Modulation and heat dissipation techniques

The modulation and heat dissipation techniques rely on natural heat sinks to store and remove the internal heat gains. Examples of natural sinks are night sky, earth soil, and building mass.[9] Therefore passive cooling techniques that use heat sinks can act to either modulate heat gain with thermal mass or dissipate heat through natural cooling strategies.[1]

Ventilation

Ventilation as a natural cooling strategy uses the physical properties of air to remove heat or provide cooling to occupants. In select cases, ventilation can be used to cool the building structure, which subsequently may serve as a heat sink.

One specific application of natural ventilation is night flushing.

Night Flushing

Night flushing (also known as night ventilation, night cooling, night purging, or nocturnal convective cooling) is a passive or semi-passive cooling strategy that requires increased air movement at night to cool the structural elements of a building.[11][12] Unlike free cooling, which assists in chilling water, night flushing cools down the thermal mass. To execute night flushing, the building envelope typically stays closed during the day, causing excess heat gains to be stored in the building's thermal mass. The building structure acts as a sink through the day and absorbs heat gains from occupants, equipment, solar radiation, and conduction through walls, roofs, and ceilings. At night, when the outside air is cooler and not too humid, the envelope is opened, allowing cooler air to pass through the building so the stored heat can be dissipated by convection.[13] This process reduces the temperature of the indoor air and of the building's thermal mass, allowing convective, conductive, and radiant cooling to take place during the day when the building is occupied.[11] Night flushing is most effective in climates with a large diurnal swing, i.e. a large difference between the daily maximum and minimum outdoor temperature.[14] For optimal performance, the nighttime outdoor air temperature should fall below the daytime comfort zone limits of 22 °C (72 °F) and 60% relative humidity.[15] For the night flushing strategy to be effective at reducing indoor temperature and energy usage, the thermal mass must be sized sufficiently and distributed over a wide enough surface area to absorb the space's daily heat gains. Also, the total air change rate must be high enough to remove the internal heat gains from the space at night.[13][16] There are three ways night flushing can be achieved in a building:

There are numerous benefits to using night flushing as a cooling strategy for buildings, including improved comfort and a shift in peak energy load.[18] Energy is most expensive during the day. By implementing night flushing, the usage of mechanical ventilation is reduced during the day, leading to energy and money savings.

There are also a number of limitations to using night flushing, such as usability, security, reduced indoor air quality, and poor room acoustics. For natural night flushing, the process of manually opening and closing windows every day can be tiresome, especially in the presence of insect screens. This problem can be eased with automated windows or ventilation louvers, such as in the Manitoba Hydro Place. Natural night flushing also requires windows to be open at night when the building is most likely unoccupied, which can raise security issues. If outdoor air is polluted, night flushing can expose occupants to harmful conditions inside the building. In loud city locations, the opening of windows can create poor acoustical conditions inside the building.

Radiative cooling

All objects constantly emit and absorb radiant energy. An object will cool by radiation if the net flow is outward, which is the case during the night. At night, the long-wave radiation from the clear sky is less than the long-wave infrared radiation emitted from a building, thus there is a net flow to the sky. Since the roof provides the greatest surface visible to the night sky, designing the roof to act as a radiator is an effective strategy. There are two types of radiative cooling strategies that utilize the roof surface: direct and indirect:[9]

Evaporative cooling

Main article: Evaporative cooling

This design relies on the evaporative process of water to cool the incoming air while simultaneously increasing the relative humidity. A saturated filter is placed at the supply inlet so the natural process of evaporation can cool the supply air. Apart from the energy to drive the fans, water is the only other resource required to provide conditioning to indoor spaces. The effectiveness of evaporative cooling is largely dependent on the humidity of the outside air; dryer air produces more cooling. A study of field performance results in Kuwait revealed that power requirements for an evaporative cooler are approximately 75% less than the power requirements for a conventional packaged unit air-conditioner.[20] As for interior comfort, a study found that evaporative cooling reduced inside air temperature by 9.6 °C compared to outdoor temperature.[21]

Earth coupling

Earth coupling uses the moderate and consistent temperature of the soil to act as a heat sink to cool a building through conduction. This passive cooling strategy is most effective when earth temperatures are cooler than ambient air temperature, such as in hot climates.

References

  1. 1 2 3 4 5 6 Santamouris, M.; Asimakoupolos, D. (1996). Passive cooling of buildings (1st ed.). 35-37 William Road, London NW1 3ER, UK: James & James (Science Publishers) Ltd. ISBN 1-873936-47-8.
  2. ↑ Leo Samuel, D.G.; Shiva Nagendra, S.M.; Maiya, M.P. (August 2013). "Passive alternatives to mechanical air conditioning of building: A review". Building and Environment. 66: 54–64. doi:10.1016/j.buildenv.2013.04.016.
  3. ↑ Limb M.J., 1998: "Passive Cooling Technologies for office buildings. An Annotated Bibliography". Air Infiltration and Ventilation Centre (AIVC), 1998
  4. ↑ Niles, Philip; Kenneth, Haggard (1980). Passive Solar Handbook. California Energy Resources Conservation. ASIN B001UYRTMM.
  5. 1 2 Givoni, Baruch (1994). Passive and Low Energy Cooling of Buildings (1st ed.). 605 Third Avenue, New York, NY 10158-0012, USA: John Wiley & Sons, Inc. ISBN 0-471-28473-4.
  6. 1 2 Brown, G.Z.; DeKay, Mark (2001). Sun, wind, and light: architectural design strategies (2nd ed.). 605 Third Avenue, New York, NY 10158-0012, USA: John Wiley & Sons, Inc. ISBN 0-471-34877-5.
  7. ↑ Caldas, L. (January 2008). "Generation of energy-efficient architecture solutions applying GENE_ARCH: An evolution-based generative design system". Advanced Engineering Informatics. 22 (1): 54–64. doi:10.1016/j.aei.2007.08.012.
  8. ↑ Caldas, L.; Santos, L. (September 2012). "Generation of energy-efficient patio houses with GENE_ARCH: combining an evolutionary Generative Design System with a Shape Grammar" (PDF). Proceedings of the 30th eCAADe Conference - Digital Physicality. eCAADe. 1: 459–470. Retrieved 26 November 2013.
  9. 1 2 Lechner, Norbert (2009). Heating,Cooling, Lighting: sustainable design methods for architects (3rd ed.). 605 Third Avenue, New York, NY 10158-0012, USA: John Wiley & Sons, Inc. ISBN 978-0-470-04809-2.
  10. ↑ Grondzik, Walter T.; Kwok, Alison G.; Stein, Benjamim; Reynolds, John S. (2010). Mechanical and Electrical Equipment For Building (11th ed.). 111 River Street, Hoboken, NJ 07030, USA: John Wiley & Sons. ISBN 978-0-470-19565-9.
  11. 1 2 Blondeau, Patrice; Sperandio, Maurice; Allard, Francis (1997). "Night ventilation for building cooling in summer". Solar energy. 61: 327–335. doi:10.1016/S0038-092X(97)00076-5. Retrieved 11 November 2015.
  12. 1 2 Artmann, Nikolai; Manz, Heinrich; Heiselberg, Per Kvols (February 2007). "Climatic potential for passive cooling of buildings by night-time ventilation in Europe". Applied Energy. 84: 187–201. doi:10.1016/j.apenergy.2006.05.004. Retrieved 11 November 2015.
  13. 1 2 DeKay, Mark; Brown, Charlie (December 2013). Sun, Wind, and Light: Architectural Design Strategies. John Wiley & Sons. ISBN 978-1-118-33288-7.
  14. ↑ Givoni, Baruch (1991). "Performance and applicability of passive and low-energy cooling systems". Energy and Buildings. 17 (3): 177–199. doi:10.1016/0378-7788(91)90106-D.
  15. ↑ "Night-Purge Ventilation". Autodesk Sustainability Workshop. Autodesk Education Community. Retrieved 11 November 2015.
  16. ↑ Grondzik, Walter; Kwok, Alison; Stein, Benjamin; Reynolds, John (January 2011). Mechanical and Electrical Equipment for Buildings. John Wiley & Sons. ISBN 978-1-118-03940-3.
  17. ↑ Pfafferott, Jens; Herkel, Sebastian; Jaschke, Martina (December 2003). "Design of passive cooling by night ventilation: evaluation of a parametric model and building simulation with measurements". Energy and Buildings. 35: 1129–1143. doi:10.1016/j.enbuild.2003.09.005. Retrieved 11 November 2015.
  18. ↑ Shaviv, Edna; Yezioro, Abraham; Capeluto, Isaac (2001). "Thermal mass and night ventilation as passive cooling design strategy". Renewable Energy. 24: 445–452. doi:10.1016/s0960-1481(01)00027-1. Retrieved 11 November 2015.
  19. ↑ Sharifi, Ayyoob; Yamagata, Yoshiki (December 2015). "Roof ponds as passive heating and cooling systems: A systematic review". Applied Energy. 160: 336–357. doi:10.1016/j.apenergy.2015.09.061.
  20. ↑ Maheshwari, G.P.; Al-Ragom, F.; Suri, R.K. (May 2001). "Energy-saving potential of an indirect evaporative cooler". Applied Energy. 69 (1): 69–76. doi:10.1016/S0306-2619(00)00066-0.
  21. ↑ Amer, E.H. (July 2006). "Passive options for solar cooling of buildings in arid areas". Energy. 31 (8-9): 1332–1344. doi:10.1016/j.energy.2005.06.002.
  22. 1 2 Kwok, Alison G.; Grondzik, Walter T. (2011). The Green Studio Handbook. Environmental strategies for schematic design (2nd ed.). 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA: Architectural Press. ISBN 978-0-08-089052-4.
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