Solar Passive Cooling

Introduction

The term "passive" implies that energy-consuming mechanical components like pumps and fans are not used.

To achieve thermal comfort in the summer in a more sustainable way, one should use the three-tier design approach.

• Heat Avoidance

Historical and Indigenous Use of Passive Cooling

Hot and dry climates typically have buildings with smallwindows, light colors,massive construction. Thera, Santorini, Greece. (From Proceedings of the International Passive and Hybrid Cooling Conference, Miami Beach, FL, Nov. 616, © American Solar Energy Society, 1981.)

The massive materials not only retard and delay the progress of heat through the walls, but also act as a heat sink during the day.

The wind towers in Hyderabad, Pakistan,all face the prevailing wind.

The wind towers in Dubai, United Arab Emirates,are designed to catch the wind from any direction. (Photograph by Mostafa Howeedy.)

In urban settings and other places with little wind, wind scoops are used to maximize ventilation.

Some wind towers inhot and dry areas cool the incoming air byevaporation.

A mashrabiya is a screened bay window popular in the Arabic Middle East.It shades, ventilates, and provides evapo-rative cooling. Cairo, Egypt. (Photograph by Mostafa Howeedy.)

In the evening, the orchestraplatform provided entertain

ment while the water cooledthe air at the Panch Mahal

Palace at Fathepur, India. (Photograph by Lena Choudhary.)

Small domes made of sun-dried mud bricks work wellin very hot and dry climates,such as those found in Egypt.Narrow alleys enable buildin

gs to shade each other. Small courtyards provide outdoor sleeping areas at night.

Dwellings and churches arecarved from the volcanic tuffacones in Cappadocia, Turkey.(Photograph by Tarik Orgen.)

The cliff dwellings at MesaVerde, CO, benefit from theheat-sink capacity of the stonewalls and rock cliff.

Five methods of passive cooling:

I. Cooling with Ventilation

A. Comfort ventilation: Ventilation during the day and night to increase evaporation.

B. Night flush cooling: Ventilation to pre cool the building for the next day.

II. Radiant Cooling

A. Direct radiant cooling: A building's roof structure cools by radiation to the night sky.

B. Indirect radiant cooling: Radiation to the night sky cools a heat-transfer fluid, which then cools the building.

III. Evaporative CoolingA. Direct evaporation: Water is sprayed into the air entering

a building.B. B. Indirect evaporative cooling: Evaporation cools the

incoming air or the building without raising the indoor humidity.

IV. Earth Cooling A. Direct coupling: An earth-sheltered building loses heat

directly to the earth.B. Indirect coupling: Air enters the building by way of earth

tubes.

V. Dehumidification with a Desiccant: Removal of latent heat.

For example, in the South the earth might be too warm for cooling unless its temperature is first lowered by evaporation.

I A. Comfort ventilation

Air Flow through Buildings:

> Site Conditions: Adjacent buildings, walls, and vegetation on the site.

>Window Orientation and Wind Direction

Acceptable wind directions for the orientation that is best for summer shade and winter sun.

Usually indoor ventilation is better from oblique winds than head-on winds because the oblique air stream covers more of the room.

Deflecting walls and vegetation can beused to change air-flow direction so that

the optimum solar orientation can bemaintained.

> Fin Walls

Cross-ventilation between windows on opposite walls is the ideal condition.

Ventilation from adjacent windows can be poor or good, depending on the wind direction.

Some ventilation is possible in the asymmetric placement of windows because the relative pressure is greater at the center

than at the sides of the windward wall.

> Horizontal Overhangs and Air Flow

The greater positive on one side of the window deflects the airstream in the wrong direction. Much of the room remains unventilated.

A fin wall can be used to direct the airstream through the centre of the room.

The solid horizontal overhang causes theair to deflect upward

A louvered overhang or at least a gap in the overhang will permit the airstream to straighten out.

A solid horizontal overhang placed high above the window will also straighten out the airstream.

> Roof Vents

The design of roof ventilator has a great effect on its performance. Percentage show relative effectiveness.

An air speed of 0.5m per second equates to a 3 degreedrop in temperature at relative humidity of 50 per cent.

> Comfort Ventilation

A completed air-flow diagram.

Air flow should also be checked in section.

Comfort ventilation is most appropriate when the indoor

temperature and humidity are above the outdoor level.

The Mayan Indians of the hot and humid Yucat an Peninsula build lightweight,

porous buildings although mud and rocks are available.

I B. Night flush cooling

Since the ventilation removes the heat from the mass of the building at night, this time-tested passive technique is called night-flush cooling. Night-flush cooling works in two stages. At night, natural ventilation or fans bring cool outdoor air in contact with the indoor mass, thereby cooling it. The next morning, the windows are closed to prevent heating the building with outdoor air.

With "night-flush cooling," night ventilation cools the mass of the building.

During the day, the night-flush cooled mass acts as a heat sink. Light colors, insulation,shading, and closed windows keep the heat gain to a minimum. Interior circulating fanscan be used for additional comfort.

II. Radiant Cooling

At night the long-wave infrared radiation from a clear sky is much less than the long-wave infrared radiation emitted from a building, and thus there is a net flow to the sky. On humid nights, the radiant cooling is less efficient but a temperature depression of about 7°F is still possible. Clouds, on the other hand, almost completely block the radiant cooling effect

Direct & indirect radiant cooling

On clear nights with little humidity,there is strong radiant cooling.

Humidity reduces radiant cooling, andclouds practically stop it.

Potentially the most efficient approach to radiant cooling is to make the roof itself the radiator. For example, an exposed-concrete roof will rapidly lose heat by radiating to the night sky. The next day, the cool mass of concrete can effectively cool a building by acting as a heat sink.

At night, the movable insulation is in the “open” position so that the buildings’ heat can be radiated away.

The specialized radiator cools air, which then blown into the building to cool the mass.

During the day, the insulation is in the “closed” position to keep the heat out.

During the day, the radiator is vented outdoors, while the building is sealed

III. Evaporative CoolingWhen water evaporates, it draws a large amount of sensible heat from its surroundings and converts this type of heat into latent heat in the form of water vapor. It works best when relative humidity is lower (70 per cent or less during hottest periods) and the air has a greater capacity to take up water vapor.

A. Direct Evaporative CoolingWhen water evaporates in the indoor air, the temperature drops but the humidity goes up. In hot and dry climates, the increase in humidity actually improves comfort. Misting the air has become quite a popular, direct evaporative-cooling strategy. Water under high pressure is atomized into tiny droplets, which then readily evaporate to cool the air.

Evaporative coolers (swamp coolers)

B. Indirect Evaporative Cooling

This indirect evaporative cooling system uses a roof pond. Note no humidity is added to the indoors.

This indirect evaporation cooling system uses floating insulation instead of a second roof.

Indirect evaporative coolers reduce the indoor air temperature without increasing its humidity.

IV Earth Cooling

Soil temperature varies with time of year and depth below grade.

Before one deciding for earth-cooling techniques one should check the thermal properties of soil must be considered. The temperature of soil near the surface follows the air temperature and may change from region to region.

Cooling the EarthIn dry climates, soil can be cooled significantlybelow its natural temperature by shading it and

by keeping it wet for evaporative cooling.

A. Direct coupling

In earth-sheltered buildings in cold climate, the earth should be insulated from the cold winter air.

In dry climates, soil can be cooled with a gravel bed, which shades the soil while it allows evaporation to occur.

When earth-sheltered buildings have their walls in direct contact with the ground (i.e., there is little or no insulation in the walls), one say that there is direct earth coupling. In regions where the mean annual temperature is below 60°F, direct coupling will be a significant source of cooling. To limit excess heat loss in winters, insulate the earth around the building from the cold winter air but not from the building.

B. Indirect Earth Coupling

Indirect earth cooling is possible by means of tubes buried in the ground. Sloped tubes and a sump are required to catch condensation. An open-loop system is shown, while a closed- loop system would return the air from indoors. Tubes must be absolutely tight to prevent radon gas or water from entering.

V. Dehumidification with a DesiccantIn humid regions, dehumidifying the air in summer is very desirable for thermal comfort and control of mildew. Two fundamental ways to remove moisture from the air exist.

• With the first method, the air is cooled below the dew point temperature. Water will then condense out of the air. For example, in humid climates, water will often condense in earth tubes.

• The second method involves the use of a desiccant (drying agent). A number of chemicals, such as silica gel, natural zeolite, activated alumina, and calcium chloride, will absorb large amounts of water vapor from the air. However, there are two serious difficulties with the use of these materials. First, when water vapor is absorbed and turned into liquid water, heat is given off. The second problem with the use of a desiccant is that the material soon becomes saturated with water and stops dehumidifying. The desiccant must then be regenerated by boiling off the water.

Biblography

• Lechner, Norbert., Heating, Cooling, Lighting : Design Methods for Architects.

• Daniel Halacy, Understanding PassiveCooling Systems.

• AIA Research Corporation, Passive Cooling, Designing natural solutions to summer cooling loads

• Brown, G. Z., and M. DeKay. Sun, Wind, and Light: Architectural Design Strategies.

• N. B. Geetha, R. Velraj / EEST Part A: Energy Science and Research

Topic of research area

The idea is to explore further for temperature reduction techniques by allowing indirect evaporation to take place through building’s outer skin. The phenomenon is similar to that of sweating/ earthen pot, where water will evaporate through a multi layered structure resulting in reduction of temperature due to the loss of latent heat of evaporation. It can be further explored with respect to the factors like porosity of the layer, temperature of water/ air, wind speed, sunlight exposure, rate of water flow, humidity level etc. Some of these can be controlled and for rest, natural range may be taken into consideration.