Wednesday, February 18, 2015

Passive Solar design consideration for Kathmandu

 By Sabin Chakradhar

Since, we are in the thesis review week; I want to share some of the studies I did for the thesis. I am designing the energy efficient commercial complex in Kathmandu, and my first goal is to use utilize the renewable energy extensively. There are numbers of renewable energy sources but unfortunately not many are available for Kathmandu. Some of the resources that can be used in building are:
Solar Power (Photovoltaics)

 With average 5 sun hours per day, solar Photovoltaics in Kathmandu can produce 5.5 to 6 Kwh/m2/day while in Germany (the world leader in solar panel) it can produce only 2 to 3 Kwh/ m2/day. The latitude of Kathmandu is 27o40’38” N, which gives the highest solar altitude angle of 86o in Summer and lowest solar altitude angle of 40 o in winter. Best thing to do is rotate the panel throughout the year so that the sun rays hit the panel perpendicularly, which is not always possible. So, when fixed in one position, the angle of solar panel is derived by calculating the mean value between winter and summer inclination angle. Since winter require more energy than summer and also the intensity of the sun is less in winter than summer, the panel is tilted by 10o so that it can capture more of the winter sun. Hence, or Kathmandu, the value to the panel inclination is 37o.

Solar Thermal
There are three passive solar techniques that the building can use to cool or heat the living space during summer and winter respectively with the use of the solar heat:
1)     Direct Gain System where sun heats the interior living space.
2)     Indirect Gain System where the heat is transferred to the interior living space by method of conduction.
3)     Isolated Gain System  where the heat is transferred to the interior living space by method of convection.
Solar shading:

The shading device should be of such length that it should block the high sun during the summer and allow the low sun during the winter. The length varies with the height of the window, distance between the window and shading device and the latitude of the site location. The relation is shown in the following figure:

Average wind velocity in Kathmandu is 4.6 kmph which is not enough to drive the big turbines to produce the energy effectively. However there are many types of small wind turbines available in the city and currently used in few small buildings. Also the wind scoops can be utilized in Kathmandu that can catch the wind blowing high above the building and guide them down into the interiors.

Earth Air Tunneling system can be the good choice for the heating or cooling of the building using the geothermal energy. In Pulchowk (site location), the temperature of the earth below 5m is measured to be 19°C and it remains constant throughout the year. 19°C is the temperature close to the average comfortable temperature for human body, so it can be used for cooling during the summer and heating during the winter.

Rain water harvesting system
Rain water harvesting system can be applied to the building, as the city suffers with the water crisis. With the average max rainfall 207.1mm, many water issues can be solved by proper use of the system. Rain water harvesting system needs the capture area to collect the water, conveyance system (pipes and ducts) to transfer the water and storage system to store the water. Depending on the purpose of the use, different types of treatment system can be used to make the water potable. Size of the catchment area is based on the footprint of the roof (in plan). Amount of water collected is given by:

Harvested Water =  Catchment  x  Rainfall depth x 0.623
(Gallon) (Sq. Ft) (Inch) (Conversion factor)


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