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Why are solar panels so inefficient?

Solar cells themselves are 20% efficiency is typical (and what monocrystalline silicon can do). Certain “advanced” III-V and II-VI type semiconductors can achieve 30%. That’s about as good as it gets. The reasons are physics.

The specific analysis that figured out how and why it can’t be better is embodied in the Shockley–Queisser limit which calculates efficiency from “first principles” (low level physics) and basically states that upper bound for Silicon if you could magically control everything, would be 30% at room temperature. This mostly has to do with the fact that once you produce electron-hole pairs with light, they will spontaneously recombine unless you can keep the two particles separated. You can’t perfectly do that because of mobility limits so you can never come close to 100%. Other semiconductors are similarly limited.

So that’s one aspect of efficiency in terms of pure thermodynamics.

There’s another form of net efficiency as defined by the available energy flux from the Sun which is actually shockingly small compared to the energy density and fluxes possible with fossil fuels. So in outer space, the best you can achieve at Earth’s orbit is 4 kW/m^2. Once that light enters the Earths atmosphere, you lose at least 50%-75% due to dot-product sun angles, atmospheric absorption and light wavelength sensitivities of you PV. So you are down to 500W to 1kW per square meter.

On top of that is the 30% Shockley-Queisser limit, so now it’s 150W – 300W. And then the fact that solar insolation is not constant over the diurnal cycle (each day) means you have either 3–4 hours of constant equivalent light or 25W-50W/day net power per square meter.

And finally you can’t generally use that energy when its’ generated so you need to store it and with that come additional losses to overcome the limited insolation/availability of electricity generating light.

Now look at the wattage of normal daily activities ranging from heating a house to driving a car to washing your clothes: 100W-300W to drive a car (that’s the average energy power rating), 1000W-2000W to dry your hair, and 2500W to heat 5 gallons of water within 30 minutes in a hot water heater.

The simple fact is there is “inefficiency” in comparable/substitutable economic terms compared to fossil fuel. Solar is mismatched for many common energy loads that we have come to expect as 21st Century humans.

Of course, there are lots of sources of waste but that hasn’t been a serious focus since the 1970s and the oil embargoes. We could build homes crazy energy efficient but they’d never look like current homes, we’d have to tear down most of the current homes to get the benefits and all this would cost a lot of money.

So to even achieve parity “modern lifestyle and convenience” as powered by energy currently enjoyed via fossil fuel, you need to allocate entire square kilometers of real estate per person to generate enough solar energy collection to sustain a parity energy consumption lifestyle.

Do we currently allocate that much land for housing per person? No. Most people don’t generate enough “economic value” to have that kind of land granted to them as owned either for living space, so it’s unlikely it would be suddenly tolerated for providing for living energy.

This is why I usually rant that Green Energy must inevitably result in a reduction of lifestyle quality UNLESS you eliminate things like cars and embrace mass transit from trains to buses. The numbers simply don’t work otherwise and no amount of wishful thinking or pollyanna techno-utopianism is going to change that.

And because of Peak Oil, it’s not like we have a choice about Green Energy. We have to adopt it. Things like self-driving cars and electric cars are mostly a waste of time, resources and focus that would be better placed on actual and better long-term solutions that matter and can be had cheaply now because they are already mature.

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