Why Are Solar Panels So Inefficient?

The 2nd law of thermodynamics prohibits a 100%-efficient solar cellular. More specifically, Carnot’s theorem pertains to photovoltaics and any other solar power system, where the hot part of the “heat engine” may be the temperature of the sun and also the cold side is the background temperature on earth. (This is actually slightly oversimplified. ) In this way, for a system with sunshine concentration (lenses and decorative mirrors and motors to follow sunlight as it moves in the sky), the maximum efficiency is ~85%, and for a system that does not monitor the sun, the maximum efficiency will be ~55%. (For details check in with my calculations here. )

With an overcast day, tracking the sunlight doesn’t work, so ~55% is the theoretical maximum.

Available today, the highest efficiency that money can buy is usually … drumroll … ~35% for unconcentrated photovoltaics (PV) (e. g. Spectrolab), ~35% for concentrated PV (e. g. Amonix), and ~35% for solar thermal (e. gary the gadget guy. Ripasso).

By the way, in unconcentrated PV, there is currently an enormous gap between the highest effectiveness that money can buy (~35% from Spectrolab, for ~$100, 000 for each square meter) and the maximum efficiency that is not insanely costly (~20% silicon modules through SunPower SPWR +4. 35%). I expect that difference to shrink dramatically within the next NXGPY +% 10-20 many years thanks to Alta Devices, that already has a pilot collection creating affordable ~25%-efficient photo voltaic modules, and is moving in the direction of 30% or even beyond. These types of cells will be light and versatile too! This is very exciting. However I’m getting off-topic. The actual question is not primarily regarding what’s affordable, but there is no benefits possible. How to explain the actual gap between ~35% as well as the theoretical maximum?

For unconcentrated PV, the best cells (currently ~35%) have been creeping towards theoretical maximum (~55%) for many years (see chart), and I anticipate they will continue to do so. Really dont mean that they will literally asymptotically approach closer and nearer to 55%; eventually there will be the tradeoff where higher minimal efficiency (under standard examination conditions) comes at the expense associated with lower real-world efficiency (which involves working robustly below a variety of light and temperatures conditions). So there is a roof for unconcentrated PV performance, and it’s somewhere between ~35% and ~55%, but Dont really know where.

For focused PV: In theory, PV tissues should get more and more efficient because light concentration increases. Quite simply, if you double the light strength, it should *more* than dual the electricity generation. That is why the theoretical restrict for concentrated systems (~85%) is higher than unconcentrated (~55%). However , there is a cost in order to concentration too: (1) The contacts / mirrors are not ideal; (2) The solar mobile will get hotter, which reduces its efficiency; (3) You are able to only get power from the light coming directly from sunshine, not the diffuse glowing blue light from the rest of the atmosphere, which accounts for at least 15% of the light, sometimes much more. Thanks to those problems, the best centered PV system that money can buy is definitely more-or-less equally efficient since the best unconcentrated system that you can buy. Will that always be correct? Well, the nominal assumptive limit is ~85%, however the only way to get which high is to concentrate sun light to the maximum possible focus of 50, 000X. At a a lot more realistic concentration like 1000X, the theoretical limit is actually ~75%. Next, we take into account the 15% or more dissipates light, and we’re right down to ~65%. After accounting with regard to imperfect lenses/mirrors and cell phone heating, we are probably to a limit of 55-60%. Therefore I don’t think we should assume a huge divergence between the greatest available concentrated PV compared to unconcentrated PV. The productivity will be basically determined by the particular PV cell, and the concentrator will have only a small impact on the system-level efficiency.

The last main category is solar heating, which uses lenses as well as mirrors and solar-tracking to be able to heat something really very hot, and then use that to operate a heat engine. The particular highest-efficiency solar thermal systems currently available are based on stirling engines and they are ~35% efficient. A Stirling engine can already operate near the Carnot limit, therefore presumably the primary way to improve efficiency of a solar thermal product is to heat the thing to the next temperature. To get that hypothetical ~85% efficiency, you need to focus the sunlight by a factor of fifty, 000, and heat the one thing to 2000C. This heat is insanely high: I believe that no one knows how to create a long-lasting high-efficiency heat motor that can work at such a warm. If you heat to “only” 1000C, the maximum efficiency falls to ~75%; if you temperature to 600C - that is realistic in a solar stirling engine system - then your maximum efficiency is ~65% (or ~55% including the lost 15% diffuse light, since discussed above). That 57% figure is still way over a ~35% that has been achieved up to now, so there seems to be lots of room for improvement when the solar thermal industry continues to grow. However the 85% figure will never occur, and even 70% is extremely not likely.

(For completeness, I should point out that there are solar power systems that will don’t fit in any of the over categories, like thermophotonics and also thermophotovoltaics. These are very early-stage ideas, and I don’t understand enough about them to opinion. )

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