SiC stands for Silicon Carbide and the recent commercialization of silicon carbide power semiconductors is one of those sneaky, esoteric paradigm shifts that can be disruptive in an industry. With CREE Semiconductor’s recent introduction of SiC Schottky diodes and more recently MOSFETs, a new era in power semiconductors and solar inverters is being ushered in. Inverters are complex electronic devices that are necessary for connecting solar modules to the utility. SiC Semis (ha ha that sounds pretty funny, SiC Semis) anyway, SiC Semis allow for inverters that, in order of importance, last longer, cost less and perform better. To fully appreciate the significance of this a little industry background may be helpful.
Solar photovoltaic (PV) modules typically have 20 year warranty s. Think about that! How many products can you find that are outdoors, exposed to the elements and carry that kind of warranty. And most in the industry think the lifetime will end up more like 30 – 50 years. For many years the PV module was, by far, the most expensive component in a solar array. So their lifetime was pretty much the primary driver in PV array cost analyses. This allowed the industry to keep our dirty little secret, well . . . secret.
The dirty little secret was/is that you should figure on replacing your inverter at least once over the 20 year design lifetime of your solar array. Even when you had an installer that was honest and told you this, 5 years ago module prices were so high that it represented a manageable percentage of the system cost and people did not worry about it much. Now, however, module costs have declined dramatically and this represents a significant cost consideration on a percentage basis. What to do?
Well we are getting better at making inverters and component technology is improving so we are seeing ongoing incremental improvements in inverters. The industry is moving from a 5 year standard warranty to 10 year but the holy grail is to hit that 20 year number. Also, with module costs dropping, people are looking more closely at the inverter cost. They want to buy only one inverter and they want it to cost less. And that is what the industry HAS to achieve in order to hit grid parity (cost competitive electricity from renewables).
As an aside, I used to be an inverter designer. I have led two inverter design efforts. The thing that kills inverters is heat. It is expensive to get rid of heat so the inverter designer works very hard to make inverters efficient. Higher efficiency means there will be less heat to get rid of reducing the inverter cost. Another inverter design consideration is thatyou need to turn the power on and off very rapidly. The rate at which you turn the inverter on and off is called the switching speed. There are components referred to as filters. Filters are costly. The faster you switching speed, the lower the cost of your filters. Unfortunately, faster switching speed means more heat. So it is an optimization problem trading off more expensive thermal management for more expensive filters.
It turns out that there is a device called a Schottky diode that allows you to have your cake and eat it too. The Schottky allows for higher switching speeds without significant increases in losses. This means that you can make more efficient inverters with lower cost filters. That is a good thing. The problem is that silicon Schottky diodes are low voltage devices. And just to show you how there is never a dull moment for engineers, inverter performance also increases if you have higher voltage inputs. Unfortunately, since the Schottky diodes are low voltage devices, you are limited. So if you could have a high voltage schottky diode you would be able to make a high voltage, high frequency, high efficiency inverter and its cost would go down because the high frequency would allow for lower cost filters and the high efficiency would make it run cooler allowing for lower cost thermal management. Finally, since heat kills inverters and your high frequency, high voltage circuit has less heat, you will have longer lived inverters. So a high voltage Schottky diode would be the enabling technology for solar inverters to hit the cost, lifetime, performance numbers needed for PV to solve its remaining technical problem.
Some time ago I was an invited participant at a Sandia National Laboratories inverter designers meeting. It was a who is who of inverter designers. I remember a guy talking about these experimental Schottky Diodes that operated at high voltage. Now things in the laboratory can take a LONG time to hit the marketplace but I made a note that this is something to watch for. If they come to market SiC Schottky diodes will be a game changer. Having left the inverter design business for some time, I was somewhat out of the loop.
Not too long ago I became interested, from an investment standpoint, in another disruptive technology, LED lighting. In particular I researched CREE the widely acknowledged leader in this field. As it turns out they are using SiC to make super bright, super efficient LEDs (light emitting diodes) . As I researched CREE I began to get very excited, they had just begun production of …. You guessed it High voltage Schottky diodes. This is a really Big deal. Not so long ago inverters that were 95% efficient were state of the art. We are now hearing numbers like 98% efficient. Look at that, that increase in efficiency means you better than half the waste heat generated by the inverter. As an inverter designer, I assure you, the implications for lifetime and cost are substantial and it means we are on the verge of inverters with 20 year + lifetimes and for people interested in renewable energy, that is a Big Big deal.
Bottom line – that, combined with the dramatic price reductions of solar module is putting PV within striking range of traditionally generated electricity.
Copyright Clayton Handleman all rights reserved 2011