Temperatures fall at night, but in the height of summer in my location
even minimum overnight air temperatures can be too warm to be comfortable.
But it is possible to passively cool things to temperatures substantially
below air temperature.
For example, yesterday I placed a slab of rock 380mm
× 380mm × 100mm thick weighing about
35kg on top of three smaller stones so that there was an air-gap between
the slab and the soil.
This morning, around sunrise, I measured the air temperature as 23°C
and the temperature of the top face of the slab as 15°.
Temperature of what?
The word temperature is often used losely.
Most of the time what we measure is the temperature of our thermometer.
If the thermometer is carefully placed in a well ventilated place out of
direct sunlight and protected from other major sources of thermal radiation
the temperature of the thermometer can be close to the air temperature, which
is what we are aiming to measure when we want to talk about
things like "how hot is it today?".
If we use an infra-red thermometer we can, to some extent, measure the
temperature of whatever we are pointing the thermometer at.
|The slab of rock
|Socrates kindly offered to stand there to give the scale.
How did this happen?
At night, especially when there is a clear sky (as there usually is in the
summer where I live) heat is radiated away into space.
A slab of rock is quite a good radiator, and can lose heat by radiation
faster than the surrounding air can warm it.
(The bottom of the slab, which received radiation from the soil below, was
How can this be used to advantage?
A cellar, or for that matter a room, could be built with a concrete (or
stone) slab roof.
During the day the roof could be covered with insulation to stop it being
warmed by sun light or the warm air; at night the
insulation could be moved away so that the slab could radiate heat away
The experiment with the stone slab suggests to me that it would be possible
to cool the roofing slab to around seven degrees below the minimum
air temperature on a cloudless night.
(The roof slab of
my cellar is 150mm thick, so would be slower to cool than
the 100mm stone slab of the experiment; on the other hand, the air beneath the
roof slab would probably be cooler than the air beneath the stone slab.)
How would you make the insulated covering easily moved?
Perhaps it could be made into a rigid 'slab' which could be rolled on or off
the cellar on rails?
Alternatively the cover could be folded up in the manner of the covers of
a ship's hold.
I don't need this on my cellar; it is sufficiently cool even in the middle
of summer (no more than 23°) without this form of additional cooling, but
in an area of higher temperatures it could be valuable.
If a shallow layer of water was to be ponded on top of the concrete slab this
could increase the cooling effect even more.
The layer of water could be kept topped up using a float valve.
It would be necessary to consider the effect of strong winds on the layer of
water; perhaps a single layer of gravel could be used to stop the water
from being excessively moved about by strong winds.