Dr. Michael Naughton is the Chairperson of the Department of Physics at Boston College. He and his colleagues have recently discovered a way to extract energy from “hot” electrons generated by solar cells.
Dr. Naughton is confident that efficiencies as high as 50% could be achieved with solar power. He is currently the cofounder and Chief Technology Officer at Solasta (http://www.solastacorp.com/), which was created to commercialize the hot electron technology.
Question 1: Your research team at Boston College recently made a solar power advance.
Answer: Well, we have discovered a way to make photovoltaic solar cells that can harvest “hot” electrons. It has long been known that “hot electrons” are being continuously created in solar cells by sunlight, but they always “cool down” and lose most of their energy before being harvested. So we have found a way to extract these “hot” electrons before they lose their excess energy by using ultrathin films of silicon, far thinner than is conventionally used.
Question 2: The efficiency of your ultra-thin solar cells is only 3%. To what extent can you increase efficiency?
Answer: Researchers have known for years if not decades that the efficiency of solar cells can be significantly increased by harvesting these “hot” electrons. Some have even claimed that 60% efficient solar cells could be achievable with this method. Efficiencies of 30% are likely achievable, perhaps by combining this ultrathin technology with other technologies.
Question 3: What other technologies are you referring to?
Answer: The challenge with creating efficient solar cells is to make them thin enough to allow the “hot” electrons to get out, but thick enough to collect all the light. That is a difficult challenge, but we have found a way to do that, and we are confident that we can improve efficiencies using our so-called nanocoax technology. Also, if one were to stack solar cell junctions together in a series, 50% efficiencies should eventually be attainable. There are no formidable technical barriers to stacking cells, so the primary issues would pertain to cost-effectiveness.
Question 4: A hot topic in solar development is thermal solar. Is your voltaic technology compatible with solar thermal?
Answer: Yes, it is highly compatible. A large part of solar energy comes from the infrared spectrum, and this is generally outside the realm of photovoltaics. That is, most photovoltaic devices are limited to the visible, or the red-green-blue, photon spectrum. There is a considerable amount of energy in the longer wavelength spectrum, and those longer wavelength photons can be harnessed by thermal solar cells. Although we haven’t done any direct research on thermal solar cells, we are examining the prospect of putting our ultra-thin photovoltaic cells, in our nanocoax configuration, above thermal cells, and letting our nanowire cells absorb the visible wavelength and having the thermal cells absorb the rest.
Question 5: Your department has done work on superconductors. What role could superconductors play in providing low-cost renewable energy?
Answer: Half of the department at Boston College, including myself, work on superconductors. There has been considerable progress during the past two decades on understanding the principals that cause high temperature superconductivity. It’s impossible to predict, but I am somewhat confident that room-temperature superconductors will eventually be found, possibly within the next several decades. High-temperature superconducting could play a vital role in long distance power transmission, allowing a solar farm in the southwest to power major cities that are thousands of miles away.
Question 6: What cost per kilowatt do you anticipate for this technology, once it is developed?
Answer: We have done detailed cost studies, and have found that this system has the potential to be quite cost-effective. Even with 15% efficiency, without taking advantage of “hot” electrons, the cost can be as low as 5 cents per kilowatt hour. That is approximately the same cost as coal, or “grid parity”. So even taking into account the higher fabrication costs associated with these new technologies, the cost per kilowatt hour easily drops below 5 cents.
Question 7: Solar power has grown exponentially during the past decade. Do you foresee exponential growth in the deployment of solar cells continuing for the next decade?
Answer: In spite of the increased interest in solar energy during the past years, the amount of energy created by solar is still relatively tiny. Solar generates considerably less than 1% of the world’s energy. Some are predicting that solar power generation will double every couple of years, similar to Moore’s law, and they are probably correct. The world’s need for energy will continue to , however, particularly in places like Asia.
Question 8: Tell us about your startup company, Solasta.
Answer: Solasta was formed in late 2006 specifically to commercialize the Nanocoax solar cell. Prototype Nanocoax solar cells are now being made at Solasta. I am the CTO and cofounder, along with Kris Kempa and Zhifeng Ren. Drs. Kempa and Ren, like myself, are Professors of Physics at Boston College. Solasta is funded by venture capital, and is just emerging from stealth mode. We are now at the point where our technology is beyond state-of-the-art, so we have decided to publish our findings. We are currently looking for more capital but are not planning on going public in the near future.
Question 9: These nanowires seem quite useful. Could they be used for applications outside of solar power generation?
Answer: We actually refer to these wires as Nanocoax, as they are very similar to the coax cables that, for example, bring cable TV into the home (only 10,000 times thinner). There is a large number of potential uses for these coaxes, such as artificial retinas for retinal diseases, nanoscale microscopy using visible light, and even optical nanolithography.
Question 10: Some have argued that solar could meet all of the world’s energy needs. Is this feasible?
Answer: If we make solar technology a development priority, and commit the necessary funds, then it is certainly a technical feasibility. Moreover, with sufficiently efficient solar, the amount of land needed to meet all of the world’s energy needs would be quite modest. Harnessing something like 1/10,000 of the sun’s energy that hits the earth would be sufficient to meet all of our current energy needs.
Question 11: But won’t this substantially reduce the amount of available arable farmland?
Answer: Not that much. Many of the best spots for solar generation are in deserts, which are poorly suited to agriculture. Moreover, we are researching ways to combine crop growth with solar generation in the same area. In particular, we are examining “hot” electron solar cells that are virtually transparent, and which let much of the sunlight through. So the land area could be used both for electricity generation and food production.
Question 12: What proportion of the world’s energy will be supplied by solar power in 2020?
Answer: With sufficient funding and development, I believe that the world in 2020 could be deriving as much as 10% of all its power from solar. That is an aggressive goal, given that solar currently produces less than 1% of the world’s energy. But if the word’s governments commit to that number, and to the substantial research and development funding such an effort would require, it could be done. Why not?