AR/IA 241 LN 231 Lecture 6: Passive Cooling

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1 Faculty of Architecture and Planning Thammasat University AR/IA 241 LN 231 Lecture 6: Passive Cooling Author: Asst. Prof. Chalermwat Tantasavasdi 1. Passive Cooling Methods and Application Passive cooling refers to the use of free energy or no energy for the purpose of cooling people in a space (direct cooling) or a component of a building (indirect cooling). Methods generally involve the use of environment to naturally cool people or buildings, e.g. soil, water, wind, sky, and vegetation. In general, passive cooling can be categorized into five types: ventilative cooling, mass-effect cooling and nocturnal ventilation, radiative cooling, evaporative cooling, and dehumidification. Each of the passive cooling methods will be discussed along with its application. Ventilative Cooling Ventilative cooling is the simplest cooling method by opening windows to let the outdoor air into the buildings. Ventilative cooling directly makes people feel cooler by increasing evaporation and convection rates. Its success is generally measured by the indoor air velocities, giving the air change rates (ACH) of between 5 and 500. ACH refers to the volume of air to be exchanged comparing to the volume of the space. Since it directly and naturally comforts people, ventilative cooling is also known as comfort ventilation or natural ventilation. Ventilative cooling has been the most effective passive method for hot-humid climates since ancient times because the temperature and humidity in these regions slightly exceed the thermal comfort zone. In today s conditions where urban construction further raises the environment temperature and blocks the prevailing wind, the method is dramatically limited. Efficient use of natural ventilation in hot-humid climates is now limited to suburban and rural areas where people can still rely on the wind from cooler and sparser environments. In moderate climates, comfort ventilation is a very well-proven passive method to save a large amount of energy in place of conventional air-conditioning systems, especially during transitional seasons (spring and autumn). 1

2 Mass-Effect Cooling + Nocturnal Ventilation Building constructions with high heat capacity, e.g. concrete and masonry brick, delay the heat gain into the interior spaces. The time lag allows the indoor conditions to stay within the comfort level during the hot hours of the day. At night, when the heat finally reaches the interior spaces, cooler outdoor air is introduced to the spaces to cool the constructions, rejuvenating its ability to slow down the heat gain for the next day. Some may include the effect of earth and soil cooling in this method because they use the same principle. The method is successfully used in hot-arid regions because of the large diurnal change. It has also been used in hot-humid regions, especially for buildings that are generally occupied during the day, e.g. religious buildings. For today s conditions, the method could still work for buildings containing mainly daytime functions, e.g. schools and offices. Radiative Cooling Building exterior surfaces that are exposed to the sky, especially the roof, emit long-wave radiation at night, lowering its temperature. The design generally involves protecting the solar radiation gain during the day and taking advantage of cooled roof at night. In most cases, operable roofs and thermal mass are involved in the systems. The effectiveness of radiative cooling mainly relies on the clearness of the sky. In regions where the sky is clear, e.g. hot-arid climates, it is found that the system work very well. In most of the hot-humid climates, the sky is mostly cloudy, thus dropping the effectiveness of this method. Evaporative Cooling Water droplets increase humidity and cool the air along the enthalpy line. They can both directly cool the air, shifting the air towards comfort zone, and indirectly cool the building surfaces, lowering the heat transfer into the buildings. Traditional methods involve creating water elements as landscape environments, e.g. fountains, ponds, and waterfalls. Modern methods display more integration to buildings, e.g. cooling towers, evaporative cooling walls, and roof ponds. 2

3 Evaporative cooling shows a high potential in hot-arid climates despite the scarce water resources. It can reduce ambient air temperature by several degrees because of the low moisture contents in the air. In hot-humid climates, the method has worked quite well along with natural ventilation for vernacular architecture. Warmer modern environments limit the use of direct evaporative cooling. Indirect evaporative cooling, however, becomes more effective, especially for air-conditioned buildings. Water cooled by evaporation, convection and radiation Night Day Dehumidification Moisture can be removed from the air by chemical desiccant or absorptive materials. The process is the inverse of evaporative cooling, also along the enthalpy line but in the opposite direction. During such process, the humidity decreases, while the temperature increases. Materials usually become saturated at a certain time and need moisture-releasing processes in order to renew their absorptive abilities. Although dehumidification does not cool the air or buildings, it is a very important method for hothumid climates because most of the efforts for conventional systems are dedicated to moisture removal. Dehumidification techniques, however, require extra or complicated processes due to its nature of increasing the air temperature. Warm dry air from the method needs some free-cooling energy from other process, e.g. by exchanging the heat with cooler evaporated air without exchanging the moisture contents. The moisture-releasing processes of saturated air also need extra efforts because they generally involve the use of heat. 3

4 2. Natural Ventilation Since natural ventilation represents the simplest and most effective method for hot-humid climates, it will be further elaborated in details. Topics include fundamentals of airflow, types of airflow, airflow equations, outdoor airflow, indoor airflow, airflow design and study tools, and design considerations for natural ventilation. Fundamentals of airflow Airflow principles include pressure, obstruction, momentum, laminar/turbulent flow, Bernoulli effect, Venturi effect, stack effect, and cross ventilation. Pressure Air moves from a high-pressure area to a lower one. Obstruction When an obstruction blocks the air path, it alters the direction of the air. Momentum Air has momentum and tries to continue on its previous direction and speed. Laminar/Turbulent flow Smooth alteration of obstruction is called laminar flow, while abrupt alteration creates eddies and is call turbulent flow. 4

5 Bernoulli effect This is the effect that lifts the airplane wings. When air travels in two different paths, different pressures could be created. The longer path needs more velocity than the shorter one, resulting in lower pressure. Venturi effect When the air passes through a small area, its velocity increases because the large original volume of air has to travel though a smaller volume at an equal time. Stack effect Warmer air naturally rises since it is lighter than cooler air. It can create airflow if two or more openings are located at different heights. 5

6 Cross ventilation Pressure difference from building configuration creates airflow in buildings. Therefore, it is desirable to have two or more openings, preferably located on the most far apart walls. Types of airflow Categorizing by the driving force of air movement, there are two types of airflow wind-driven and buoyancy airflows. In actual situation where physical conditions comply with both of the airflow types, the two types of airflow occur simultaneously. Wind-driven airflow The air is driven by the difference of pressure (ΔP) as a result of building configuration. At the windward side, the pressure increases, or is known as positive pressure. On the opposite side, the leeward side, the pressure decreases, or is known as negative pressure. Strongest airflow in building occurs if openings are placed on these two sides because they create maximum pressure difference. Stack effect/buoyancy airflow The air is driven by the difference of temperature between indoor and outdoor. The heat accumulated in a space generally makes the indoor air a few degrees higher than the outdoor air. The temperature difference (ΔT) creates air density difference (Δρ) and pressure difference (ΔP), thus generating airflow from opening at a lower position to a higher one. 6

7 Combined wind-driven and buoyancy airflow In general, wind-driven airflow dominates the flow because the prevailing wind is normally stronger than the force from the temperature difference created in the interior space. When the wind subsides, however, buoyancy effect dominates the flow. Airflow equations A basic equation to calculate the heat loss due to natural ventilation can be expressed as Q = V cp ΔT ρ (1) where V = volumetric flow rate (m 3 /s) [normally is the product of aperture area (a) and indoor air speed (v)] In cases where velocities are difficult to determine, the flow rate can also be calculated. For buoyancy effect, the equation can be written as V = A g' h * (2) where A* = effective aperture area (m 2 ) g = reduced gravity (m/s 2 ) h = vertical distance between the mid points of apertures (m) A* 2 a c a c = (3) 2 2 ( a1 c1) + ( a2 c2) where a n = aperture area of aperture#n (m 2 ) c n = discharge coefficient of aperture#n ΔT g = g T ' (4) where g = the earth gravity (9.81 m/s 2 ) ΔT = temperature difference between indoor and outdoor air (C) T = air temperature (indoor or outdoor, C) For wind-driven airflow, the equation can be expressed as ΔP ρ V = A* (5) 7

8 where ΔP = pressure difference between windward and leeward sides Combining equation (2) and (5) results in the volumetric flow for buoyancy and wind-driven effects as following. ΔP g' h + ρ V = A* (6) Outdoor airflow Factors influencing airflow around buildings include wind profile and neighborhood configuration. Wind profile City configuration and density affect the prevailing wind speed due to their different terrain roughness. For example, wind speed in dense urban area is generally lower than that in open areas such as airport. The following equation explains the wind speed as a result of terrain roughness. H = U ref H H ref U (7) α where U H = wind speed at height H (m/s) U ref = reference wind speed at height H ref (m/s) α = constant for a terrain type (0.15 for open terrains, 0.28 for suburbs, and 0.40 for urban areas) Neighborhood configuration Nearby buildings and vegetation deflect the prevailing wind and may change its velocity and direction. Tall buildings create wind shadow for a distance of six times of their heights. These are important facts that have to be considered, especially during the site planning process. Indoor airflow Factors influencing airflow in buildings include geometry and orientation, location of openings, window type and size, interior obstruction, and insect screens. Geometry and orientation Elongate shapes have better chance to capture the prevailing wind than compact shapes. Direction of the prevailing wind is also important. For best result, however, the buildings do not have to be located perpendicular to the wind. Studies show that angles between 30 to 120 degrees of building facades and the wind allow maximum ventilation. 8

9 Location of openings It is significant to locate inlets on building facades that face the prevailing wind because of their maximum pressure. The outlets should be on the opposite sides or on the maximum negative pressure areas. Window type and size Effectiveness of openings is measured from the opening areas. Therefore, sliding windows are the worst for airflow because less than half of the areas are able to open. In general, large windows allow more airflow and therefore are more effective than smaller ones. Interior obstruction Interior partitions and furniture alter the behavior of airflow. Ideally, they should not be placed in the positions to block the incoming wind. Insect screens Resistance from insect screens reduces the air velocity up to 30%. Dirty screens further block the airflow. Airflow design and study tools There are several ways to study airflow in and around buildings. Tools include smart arrows, fluid mapping table, wind tunnel, water tank, and computational fluid dynamics (CFD). Smart arrows Architects can make a simple and quick sketch to help the understanding of airflow. This method can roughly explain the concept but cannot explain complicated cases. Fluid mapping table A two-dimensional studying tool that creates color streams of liquid in a water basin, imitating the airflow through a section or a plan of a building is called a fluid mapping table. It gives visualized idea of how the air would move in and out of a building but cannot quantitatively measure any parameters. Airflow study due to buoyancy effect is not possible by this tool. Wind tunnel A wind tunnel is a complicated tool that accurately simulates the airflow around and in scaled buildings. Measurements are possible but the method is expensive and time-consuming. Water tank A water tank represents the three-dimensional version of the fluid mapping table. It allows accurate results and covers buoyancy effect cases. However, measurements are possible only for temperature and perhaps pressure. 9

10 Computational fluid dynamics CFD is a computer program that accurately calculates the airflow according to fluid mechanic equations. It shows all the parameters necessary to know but the program requires advanced knowledge of users. Design considerations for natural ventilation Considerations for natural ventilation design include time of usage (day/night/both) and type of cooling (natural ventilation/air-conditioning/both). They can be presented according to each of the building considerations as followings. Building Characteristic Building Consideration N/V A/C Site Planning Loose Tight Building Form Elongated Compact Maximum surface area Minimum surface area Building Mass Day Heavy Night Light Light Building Insulation Light Day Heavy Night Light Building Opening Maximum Minimum Bibliography American Society of Heating, Refrigerating and Air-conditioning Engineers. (1997) ASHRAE handbook fundamentals (I-P ed.). Atlanta, GA: ASHRAE. Areemit, N., & Sakamoto, Y. (2007). Numerical and experimental analysis of a passive roomdehumidifying system using the sorption property of a wooden attic space. Energy and Buildings, 39, CHAM. (2008). PHOENICS London: CHAM. Chenvidyakarn, T. (2007). Review article: Passive design for thermal comfort in hot humid climates. Journal of Architectural/Planning Research and Studies, 5(1), Ghiaus, C., & Allard, F. (Eds.). (2005). Natural ventilation in the urban environment: Assessment and design. London: Earthscan. Givoni, B. (1994). Passive and low energy cooling of buildings. New York: John Wiley & Sons. Givoni, B. (1998). Climate considerations in building and urban design. New York: Van Nostrand Reinhold. Moore, F. (1993). Environmental control systems: Heating cooling lighting. Singapore: McGraw- Hill. Stein, B. & Reynolds, J. S. (2000). Mechanical and electrical equipment for building (9 th ed.). New York: John Wiley & Sons. 10