Just putting these here so Michael can't miss them:
michaelsuede said:
If I take a huge parabolic mirror or use a lens to concentrate incident solar radiation 1000 times on to the surface of a pv cell, the incident radiation per meter on the surface of that cell is no longer 1.4kW/m. So why are you arguing as if this is still the case? These cells are built to deal with megawatts of power per meter.
I hope you never attempt to work in a lab in any building that I'm in. Of course if you concentrate the sun's 1.4kW/m
2 with a 1000 sun array, then the peak power density on the silicon will be 1.4MW/m
2, but the area illuminated will be 1000 times less than the total collection area of array, so that the mean power density will still be 1.4kW/m
2. But in the idiotic application, the mean power density will have to be about 230kW/m
2 (depending on its area) which means that the array will be absorbing 165 times more power per unit area than it is exposed to in its normal solar application. Passive cooling will not work. And what's worse the peak power density on the silicon will be 230MW/m
2. Ouch!
Oh - and here's something else. Let's say the graphite dome is 25cm in diameter, so about 0.195m
2 in surface area. Let's say it's heated to 3500K to avoid it sublimating away. The the Stefan-Boltzmann radiation formula gives a total radiated power of 1.7MW, all of which heat will have to be dissipated somehow. Because you're running the dome at a reddish temperature, the radiance will peak in the infrared and only about 14% of the power will be in the visible. The mean total radiance on the PV cells will be 1.7MW/m
2, assuming an array with 1 m
2 area (over 1,000 times the solar case). For a passively cooled system, assuming the cell array is itself radiating as a blackbody, the equilibrium temperature of the array would be about 2350K. That's not going to be good. For a 1000 sun system, you then have a peak power density on the silicon of
1.7GW/m2. Ouch, ouch, ouch!
If the reaction doesn't produce 1.7MW of power then a dome of that size will not get to 3500K.
You can reduce the power output by making the dome smaller, but then you have to engineer all the gubbins inside the dome, molten silver and all into a smaller dome. Or you can run the dome at a lower temperature where the PV cells are even less sensitive and you throw away even more heat.
The steam engine is looking ever so attractive.
