## Friday, July 17, 2015

### A grand challenge for nano: Solar energy harvesting near the thermodynamic limit

As I'd mentioned earlier in the week, the US Office of Science and Technology Policy had issued a call for "Grand Challenges" for nanotechnology for the next decade, with a deadline of July 16, including guidelines about specific points that a response should address.  Here is my shot:

Affordable solar energy collection/conversion that approaches the thermodynamic efficiency limit based on the temperature of the sun (efficiency approaching 85%).

Physics, specifically the second law of thermodynamics, places very strict limits on how much useful energy we can extract from physical systems.  For example, if you have a big rock at temperature $T_{\mathrm{hot}}$, and another otherwise identical big rock at temperature $T_{\mathrm{cold}}$, you could let these two rocks just exchange energy, and they would eventually equilibrate to a temperature $T_{0} = (T_{\mathrm{hot}}+T_{\mathrm{cold}})/2$, but we would not have gotten any useful energy out of the system.  From the standpoint of extracting useful energy, that process (just thermal conduction + equilibration) would have an efficiency of zero.  Instead, you could imagine running a heat engine:  You might warm gas in a cylinder using the hot rock, so that its pressure goes up and pushes a piston to turn a crank that you care about, and then cool the piston back to its initial condition (so that you can run this as a cycle) by letting the gas dump energy to the cold rock.  Carnot showed that the best you can do in terms of efficiency here is $(1 - T_{\mathrm{cold}}/T_{\mathrm{hot}})$.  On a fundamental level, this is what limits the efficiency of car engines, gas turbines in power plants, etc.  If the "cold" side of your system is near room temperature (300 Kelvin), then the maximum efficiency permitted by physics is limited by how hot you can make the "hot" side.

So, what about solar power?  The photosphere of the sun is pretty hot - around 5000 K.  We can get energy from the sun in the form of the photons it radiates.  Using 300 K for $T_{\mathrm{cold}}$, that implies that the theoretical maximum efficiency for solar energy collection is over 90%.  How are we doing?  Rather badly.  The most efficient solar panels you can buy have efficiencies around 35%, and typical ones are more like 18%.  That means we are "throwing away" 60% - 80% of the energy that should be available for use.  Why is that?  This article (here is a non-paywall pdf) by Albert Polman and Harry Atwater has a very good discussion of the issues.   In brief:  There are many processes in conventional photovoltaics where energy is either not captured or is "lost" to heat and entropy generation.  However, manipulating materials down to the nm level offers possible avenues for avoiding these issues - controlling optical properties to enhance absorption; controlling the available paths for the energy (and charge carriers) so that energy is funneled where it can be harnessed.  On the scale of "grand challenges", this has a few virtues:  It's quantitative without being fantastical; there are actually ideas about how to proceed; it's a topical, important social and economic issue; and even intermediate progress would still be of great potential importance.

Anonymous said...

A couple of quibbles about side issues.

1. Carnot only provides limits for closed cycle systems. For instance, it can be used for the cylinder of gas in the two-rock example. But it is not applicable to car engines, which are open-cycle internal combustion engines. I.e., the "working fluid" does not return to the original state at the end of a cycle; instead, the fluid is forced out and fresh fluid is placed in the cylinder. Here is one of the few links that I could find:
http://energy.gov/sites/prod/files/2014/03/f8/deer11_edwards.pdf

2. Gas turbine are similarly open-cycle systems, but most power plants use steam turbines. The steam is heated by burning the fuel, and cooled (usually using water from a river), and these are indeed closed-cycle systems and can be considered imperfect Carnot systems.

Anonymous said...

"...it's a topical, important social and economic issue"

I don't agree. Solar energy's problem is intermittency, not efficiency.

Thus the 'storage problem'...and how to solve it economically at the scale of an utility...which I believe is impossible because it requires directly going from photons-->chemical bonds.

David Strubbe said...

Anon at 2:28, direct conversion of photons to chemical bonds is certainly possible. It is the basis of "solar thermal fuels," which are being developed in my research group using molecules that undergo photochemical reactions to change structure and store energy for later release as heat.

https://www.dropbox.com/s/h0h0cvuq8818fsc/Full_with_apps2.mov

James Brown said...

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T Anil Jain said...

Hm.. Nice Article! Thanks for sharing..

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