For every person using air conditioning or air cooling, there are many more who can’t afford either – and have to suffer extreme discomfort when their houses turn into a virtual oven in the summer, resulting in sickness and even death.
This article contains the photos and the results of the experiment on an extremely simple roof cooling system. All it involves is to cover the roof during the day by a low emissivity cover and withdrawing it at night, exposing the roof for cooling by radiation to the sky.
A Building will receive about five kilowatt hours of heat per day for every square metre of its surface exposed to the Sun. The cover blocks over 90% of it.
The experiment was conducted on a 0.5 Sq. Mtr. Slab. The opening and closing were manual. Several variations were tried; the results were recorded using a data logger that gave output in Excel format, which allowed presentation by charts.
The results show that in every case, the slab bottom temperature remained below 34 Deg. C. or lower. Since this is below the human skin temperature, there is no heat transmitted by the roof to the occupants. Cooling the roof also prevents its heat from spreading into the rest of the building. An architect or an engineer can easily transform this flimsy demo into a robust system that covers whole roofs.
Please note that the system does not use any electricity or water. The results hereafter are proof that it actually works. The only energy required is for opening and closing the cover only twice in a day if it is made automatic.
Finally, while this system will provide a huge relief from heat stress to people who can’t afford air conditioners or air coolers, it will also cause a substantial reduction in the energy consumption of air conditioners.
OPERATION
NIGHT TIME
During the night, the cover is retracted and the slab remains open to the sky. This allows radiant heat transfer from the slab to surface that is at 30 Deg C. to the sky that can be minus 40 to minus 60 Deg. C –depending on haze and clouds. The rate of heat loss in a city is about 100 watts per square meter. So, the slab bottom cools down to 27 Deg. C.
COVER OPEN…
DAY TIME
From Sunup to Sundown, this flexible cover, with a very shiny bottom, is drawn over the slab. It shades the slab, while its low emissivity (0.01) due to the shiny surface prevents any radiant heating of the slab. So, even though the slab gains some heat from the ambient air, its bottom still remains below 30 Deg. C. – at least five degrees below the body. So, it feels cool enough to be free of heat stress.
COVER CLOSED…
CONCLUSION
Even though both the setup above and the larger frame below are crude devices for demonstrating a principle, it would be fairly easy to translate it into a practical system.
A fully operational technology is available for blinds and curtains, both manual and automatic. It should not take much R&D or engineering to morph it into a horizontal configuration. If a similar cover is rolled down when a wall is sunlit and pulled up at night, it would then shield the entire house from the sun and keep it very cool. However, the aesthetics must be solved first.
Due to its high penetration potential, this simple idea, based on our traditional wisdom of cooling the structure for thermal comfort, not the air, can save energy, water and the environment to a large extent. It will also save the foreign exchange by reducing fuel and air conditioner imports. It would also greatly reduce CO2 emissions. This would meet the need for thermal comfort. Greed is always insatiable, so air conditioning is here to stay.
CHARTS FROM DATA LOGGER READINGS
EXPERIMENT 1 – COVER ALWAYS OPEN
These readings were taken in early April when the ambient temperatures were low.
The blue line showing the slab bottom is always above 30 Deg. C. The average is very near the body temperature. Therefore, it does not feel cool.
EXPERIMENT 2 – COVER ALWAYS CLOSED
Here the slab bottom is cooler because of shading from the sun.
However, there is no cooling by night sky radiation. So, the temperature is still always above 30 Deg. C
EXPERIMENT 3 – SLAB COVERED BY DAY & OPEN BY NIGHT
In this case, the shade prevents heating during the day and promotes cooling by sky radiation at night.
The combined effect maintains the slab bottom temperature to be always below 30 Deg. C., that is over five degrees below the body, so it is quite comfortable.
A NEW 8FT. X 8 FT. FRAME
Top view…
Cover open at night For sky cooling, this is yet to be tested…
Shiny Bottom Cover closed for shade during day…
