2000 degrees fahrenheit 2000 degrees fahrenheit
1400-1600 Celcius
Markus Kayser and the Solar sinter
sand heat capacity 0.835 J/gK (Joule per gram Kelvin)
So if one kilo of sand is at 30 degrees centigrade and we want to heat it to 1700 degrees centigrade, we need 1000 x 0.835 x 1670 Joule of energy = 1,395 MJ or Mega Joule..
If the sun has a power of 1000 W/m2, and that light is concentrated on 1 cm x 1 cm with an efficientcy of .80% that 1 cm x 1 cm recieves 800W = 800 Joule per second. It would have to light that spot for 30 minutes to have delivered the 1,396 MJ to melt the 1 kilo of sand (and turn it into white glass).
The actual temperature at the focal point depends on the heat loss possible at that point. If it is a black cavity it will absorb all heat energy and emit very little, then it will heat up a lot. If it is a mirror, it will reflect all heat and heat much less itself.
A pot made with sunlight and sand
The process of emission is captured in the Stefan Boltzmann law
The power loss P is the Area A times the emission J which is 5,67 x 10^8 times the emissivity of sand (epsilon), times the absolute temperature (T)...
"Bare sand occupying above 80% of the surface on the Egyptian side and only about 7% on the Israeli side has an average emissivity between 0.9435 and 0.9543." (bron)
So the sand absorbs sunlight, but then heats up and as a result will lose more energy, until a balance is reached and the amount of energy absorbed is the same as the amount that is emitted. If the Sand can go through a phase change the heat that can be absorbed is much bigger. So the sand must be unable to lose its heat and stay solid because it recieves to much energy, which is where the focussed sunlight comes in.
In an experiment to melt sand one can direct a focused beam of sunlight (1 m2) on a small areay of sand. The energy flux of 800 J/s on 1 cm2 is 800 J/second. If much is reflected (say 95% as seen above) then what is left is 40 J/second, so it could melt 2,8 grams of sand in that 1 second ? This seems to be what we see in the Markus Kayser video..
10 m2 can melt 28 grams of sand per second or 1680 Kg of sand per hour...
From here one can optimize the efficiency and utilility of that melting process..
One could make fiberglass
Or glass bricks
Or a new combination using molten glass as a binding for sand..
One could make walls of glass in situ, maybe hollow or filled with loose sand...
Industry numbers..
"Glass manufacture is a high-temperature energy-intensive process. When using raw materials, glass is manufactured from sand, limestone and soda ash, all of which are abundant natural minerals. However, both limestone and soda ash are carbonates which liberate additional C02 during the melting process. For each tonne of glass produced from virgin raw materials this decomposition produces approximately 185 kg of CO2.
Industrial manufacturers employ large furnaces which operate on a continuous basis and are typically fossil fuel fired. The UK container sector currently operates 32 furnaces each melting an average of 207 tonnes per day of glass and consuming 304 MWh of energy." bron
This means 1,4 MWh per Ton of glass, which comes to 5 MJ per kilo of glass. This is not that efficient one would say..
Melting glass requires less energy than making it..
"So now we have a formed glass, which we can re-melt if we like. In fact, re-melting glass is a key step in making glass since it requires less energy to melt the glass than it did to melt it's original batch. So in commercial production of glass "cullet" (already melted and formed glass) of the same composition is added to the batch to reduce the energy needed to melt the batch. This is why it's always good to recycle!" (bron)
On regenerative heating
"Regenerative furnaces usually have two or more regenerators. A regenerator
consists of a regenerator chamber wherein a checker work of refractory bricks
has been stacked. The bricks form a regular construction with channels for the
flue gases or combustion air. First the flue gases are transported through one
regenerator heating the checker work. After about 20 minutes the checker
work is heated to its optimal temperature. Then the combustion air is led
through this regenerator and the flue gases through the other. The combustion
air is preheated to 1100-1300ºC by the heat of the checker work. Again after
20 minutes the checker work is cooled down too much to heat the combustion
air and the process is reversed.
There are two types of regenerative "(bron)
Recuperative furnaces
Recuperative furnaces are equipped with one or two recuperators. A
recuperator is a heat exchanger, in which heat is transferred directly from the
flue gases to the combustion air in co-current or counter current flow. This
heat exchange is based upon radiation. These exchangers are therefore called
radiation recuperators. The combustion air is preheated to 600-800°C. Higher
temperatures cannot be reached, because the used metallic materials cannot
withstand higher temperature levels.
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