When I was younger, I used to believe that science alone could solve any technological challenge – and that whatever solution was technically the best would win out. It was only after spending a few years in industry that I came to realize how economics, market forces and competition play equally important (and sometimes bigger) roles. A technological solution that might seem excellent on paper can, I learned, be dragged down by practical difficulties and poor timing.
For anyone developing new technology, it’s therefore important to set review points on projects, continually assess the market and the competition, and regularly look at the readiness of your technology in a dispassionate and balanced way. I can think of dozens of technologies that didn’t pan out as intended. But here I’m going to explore “solar concentrator” photovoltaics (CPVs) – devices that generate electricity using lenses or curved mirrors to focus sunlight onto tiny solar cells.
I was reminded of this topic whilst writing about the world’s most efficient solar cell, which was developed in 2019 at the US National Renewable Energy Laboratory. It is a CPV device and has a record-breaking efficiency of 47.1% when illuminated by 143 suns. On the face of it, the device sounds amazing given that today’s silicon photovoltaic (PV) flat panels have an efficiency of only 22%.
Why did concentrator photovoltaics, which were once touted as the next big thing, fall by the wayside?
Sure, most CPV installations look impressive and futuristic, but flat-panel silicon PVs have fallen so much in price that attempts to deploy CPVs have almost ground to a halt since 2017 (even if research on them has continued). Is this why CPVs, which were once touted as the next big thing, have fallen by the wayside?
Research into CPVs began in the mid-1970s after the shock of the Middle East oil embargo (Prog. Photovolt. Res. Appl. 8 93). Most work took place at Sandia National Laboratories in New Mexico, with the first system consisting of an acrylic Fresnel lens that focused sunlight onto silicon PV cells. The cells were cooled by water to stop them from warming up and losing efficiency; they also used a tracking system so they always faced the Sun.
Other companies quickly tried their hand, with CPV systems developed by everyone from Motorola and Boeing to GE and RCA. Several successful large-scale demonstration projects emerged from this early work, notably the 350 kW Soleras Project in Saudi Arabia and the 300 kW Entech system in Austin, Texas. The former ran continuously from 1981 for over 15 years and provided valuable insights into the practicalities and operating costs of CPVs.
During the early 1980s, however, work stalled as the urgency of the energy crisis passed. With oil and natural gas proving much more abundant than expected, the cost of these fuels plummeted. So once US federal funds for CPVs became scarce, most of the participants dropped out. Research was scaled back, although a dedicated few continued to pursue the dream.
It was easy to blame the loss of interest in CPVs on low natural gas prices or a lack of political will. But the biggest problem was that CPV systems didn’t sell. Regular, flat silicon solar PV panels, in contrast, have hundreds of applications, ranging from navigation to telecommunications. They are incredibly reliable, lack moving parts, and need very little maintenance.
Solar PVs are particularly useful for people in developing nations, who now use them for lighting, refrigeration and water pumping – especially if in remote areas where other sources of power are not available. Simply put, none of these applications are particularly suitable for CPVs, which were only cost effective for installations larger than 100 kW.
A new dawn?
The market for CPVs did improve following the development in the early 2000s of high-efficiency “tandem” multi-junction CPVs, which combine silicon with a III-V semiconductor such as gallium arsenide. In fact, various multi-megawatt CPV projects have been commissioned around the world since 2010 using such devices. Modern commercial systems have efficiencies of up to 42% and the International Energy Agency thinks this could rise to 50% by the mid-2020s.
And yet even these superior CPVs are not perfect, needing active tracking systems so they always face the Sun as well as special cooling. That’s a lot of added complexity for the extra efficiency. What’s more, the cells don’t work as well in hazy or polluted conditions because the spectrum doesn’t match the spectrally “tuned” cells. Cloudy days are another problem because the sunlight isn’t concentrated enough.
These limitations of multi-junction PV systems reduce their power output and impact the economics with the higher capital costs and maintenance bills. It’s hard to see how they can improve on solar PV panels, the price of which has dropped by 82% between 2010 and 2019, according to the International Renewable Energy Agency.
The fall in cost of flat-panel solar PV has mostly been driven by economies of scale, but the Chinese government has played a role too. By heavily subsidising its solar PV industry, some have argued that China has been able to sell solar panels in the US and Europe for less than it costs to make and ship them. Known as “dumping”, the practice has driven out competitors and allowed Chinese suppliers to corner the market.
With the economics now favouring flat-panel solar PV so strongly, the near-term outlook for the CPV industry has faded. Several of the largest CPV manufacturing facilities have closed operations including those of Suncore, Soitec, Amonix and SolFocus. Flat-panel solar PVs, despite being less efficient, have won the day due to simple economics.
However, all is not lost. Perhaps a second golden age for solar concentrator technology is on the cards using the concentrated energy to warm up liquids, with the stored heat converted to electricity when the Sun isn’t shining. It’s a fascinating possibility that I’ll discuss next month.
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