Insolation is the power density of solar energy at ground level. It is measured in Watts per square metre.
In Melbourne, the average insolation over a year is roughly 200 Watts per square metre.
My shed roof is roughly 30 metres square. If I could capture all the incident solar energy on my shed, I would be capturing around 6,000 Watts, or 6 Kilowatts. Thats 6 KJ per second. Over the average hour (this included night-time hours, so peak generation is higher) that’s 21,600 Kilojoules.
How much energy is this? Let’s think of it in terms of the energy my car produces. My V6 family wagon (yeah, I drive a big CC wagon – you try carting 2 dogs and a kid around in a Prius) supplies a maximum of roughly 155 Kilowatts of power to the wheels. So in other words the power supplied by the sun to my shed is about a 25th of the maxmimum power produced by my car.
Of course I haven’t mentioned efficiency. There is no way to capture all of the energy supplied by the sun – photovoltaic panels capture about 10% from memory. And the car loses energy to heat, vibration and noise, so it needs to burn fuel containing more than 155KW to provide this power at the wheels.
But think of it as the highest upper limit. If you were able to capture all that energy, it would be enough to provide sufficient power to work a V6 engine to capacity for an hour or so. That’s not a trivial amount of energy, given that the car of course doesn’t supply/consume at it’s maximum power very much at all. The problem is being able to capture enough of it, sufficiently cheaply to compete with cheap fossil fuels.
Part 2: Second law upper limit. I’ve somehow found the time, and the envelope, for this rough calculation.
Effective temperature of sunlight is 5700K (or so). Temperature of my shed roof is around 300K. The second law efficiency limit (1 – Tc/Th) is 95%. If I fit the latest model Sadi-matic Solar Engine to my shed, with a surface area of 30m^2, at around 38 degrees S, where average insolation is 200 W/m^2, I can generate roughly 6 KW. That’s a not insignificant amount of power.
Barring an error in my working (which is entirely possible) the second law by itself is a pretty insignificant constraint on achieving steady-state solar paradise.
