Monday 14 May 2012

More Like a Car Than a Computer - Improvements in Solar PV Panel Efficiencies

Hard work if you can get it
How PV solar efficiency has improved over time

My phone has more computing power than it took to put a man on the moon, doesn't it?

Nowadays we take epic rates of improvement in performance and cost for electronic products in our stride.  It's a commonly held belief that the performance of PV solar panels are following a similar trajectory, but what does the reality look like?

Since 1975, the National Renewable Energy Laboratory in Colorado, USA has been plotting the current world's best efficiency for a range of different types of solar photovoltaic cells. The image above is a simplified version of a more detailed chart. I have shown only the three technologies in most common use and added two recent technical developments of great promise.

The efficiency shown here is not what you would expect to achieve from a commercially available solar pv panel, rather from a small, carefully prepared, no-expenses-spared laboratory sample.

The graph tells a story of hard-won gains over long periods of time more akin to the gradual improvements in fuel efficiency in cars than to improvements in computers, LCD televisions and digital cameras.  In the same time scale that rooms full of mainframe computers have shrunk down to fit into you pocket, the efficiency of solar panels has increased from about 14% to about 24%. 

You see, the bad news is that there's a Law's-of-Physics-type-limit to the efficiency of a solar cell and it's only 34% for the single junction technology that most solar panels use.  As you approach this limit the gains get harder and harder to win. You can see that the world record for silicon solar cell efficiency started to flatten off around 1995, polycrystalline around 1998, and that thin film cells are reaching a similar point right about now. 


Bright New Stars

Two exciting technology developments that have the potential to drive future improvements are organic cells and multi-junction thin film cells.

Organic cells are made from plastic photovoltaic materials, with the benefits of low cost and simple manufacturing processes that such materials bring. As can be seen in the graph, these new materials are just starting out, with low but rapidly rising efficiencies. They are quickly approaching the point where interesting applications will emerge - for example where the larger areas needed for the same power output are not a disadvantage, or the shapes that can be formed are important.

Multi-junction thin film cells sidestep the efficiency limit of conventional solar cells. Layers of photovoltaic material are stacked one on top of the other, each tuned to a different wavelength of light.  As light travels through the stack, each layer slices out the energy associated with the colour of light to which it is tuned.  The theoretical limit to efficiency for such a cell jumps to 64%.  Laboratory samples have already broken through the efficiency limit for a single layer cell.

There's good reason to be optimistic that further progress can be made in PV technology, just remember that today's laboratory record-breaking material can take many years to find its way into proven, real-world products.