The Sun Is Bright. Use More Of It.

By Michael P.C. Watts
The sun is bright and warm, there must be a way to use more of it to generate power. Last time the discussion focused on the opportunity for other semiconductor materials, and whether there was any realistic opportunity for new technologies given the state of the industry. This is the third of a series of blogs to try and answer this question. If you hate serials, the complete analysis is available on my web site www.impattern.com.

There are a number approaches to utilize more of the suns radiation: increased absorption, multi-junction cells, photon conversion, heat-absorbing cells, and angular coupling. Increasing absorption has been looked at by an number of groups as a way to make very thin (50-400nm) cells and patterning layers to increase the absorbtion of the film using plasmonic resonance effects. Results published at Photonics West this year suggest a 10% to 20% improvement in efficiency for very thin cells using very little semiconductor. However, in the comparison with polySi cells, these are ways to reduce the amount of semiconductor in the cell, rather than to increase the efficiency of the cell.

The established approach to increasing the utilization of solar radiation is to increase the number of junctions. The multi-junctions cell uses a cascade of progressively lower energy band gaps to collect a wider wavelength range. These are only used in concentrators because the device are so expensive, however there is a performance gain up to 43%, or a gain of 2x relative to polySi cells.

Rather than collect a wider range of photon energies, an alternative is to down convert one higher energy photon to 2 band gap photons, and up-convert 2 low-energy photons to 1 band-gap photon.

Illustration of up and down conversion, from SPIE

Phosphors for down conversion have been developed for LEDs where a blue LED is used to pump a phosphor that emits white light. Analysis suggests another 10% absolute efficiency or 0.3x improvement available for down shift. There is a EU project “NanoPhoSolar” that includes 3G Solar to develop down shift materials. A team from Sydney University has demonstrated efficient visible light upconversion, and has modeled the solar cell application, suggesting a 0.3x gain in a polySi cell is possible. They are working on IR upconverting materials. It looks as if there is a 0.6x opportunity in a cell with both up and down conversion that could result in a 30% cell.

Even a 30% cell would still dissipate the rest of the sun’s energy as heat, so why not utilize the heat directly? It’s less elegant, or high-tech, but can be very effective. SolarWall has developed an integrated PV heat collection system with the heat going to a residential water heater.

The performance of the SolarWall PV/T hybrid technology improves the total solar efficiency to more than 50%, but there needs to be heat exchange modifications to use the heat to warm water. In the greenhouse business this looks like a really interesting alternative.

The fact that the IR is absorbed effectively is simply that a black body will absorb black body radiation very effectively. Using the heat generation directly has been a popular solution for solar power stations. These are large installations where lenses are used to focus energy and generate steam, or drive a Stirling engine. A team at Stanford had proposed an interesting alternative, where a hot surface acts as a thermionic emitter of electrons to directly create electricity, and the residual heat is used to drive a Stirling engine. Melosh from Stanford calculates that “their process can get to 50 percent efficiency or more under solar concentration, but if combined with a thermal conversion cycle, could reach 55 or even 60 percent—almost triple the efficiency of existing systems.”

My take is that the up/down conversion and thermal synergy strategies look very interesting and should be watched with real interest. These will work with any single junction material, so they do not change the competitive balance between polySi and the thin film cells.

About the Author

Mike Watts has been patterning since 1 um was the critical barrier…in other words, for a long time. He describes himself as a tall limey who is failing to develop a Texas accent in Austin. He has a consulting shingle at www.impattern.com.

His blog, “ImPattering,” focuses on the latest developments in the business and technology of patterning. He is particularly interested in trying to identify how the latest commercial applications evolve.