{"id":3124,"date":"2008-11-06T09:53:09","date_gmt":"2008-11-06T09:53:09","guid":{"rendered":"http:\/\/scienceblogs.com\/principles\/2008\/11\/06\/puzzling-solar-panel-press-rel\/"},"modified":"2008-11-06T09:53:09","modified_gmt":"2008-11-06T09:53:09","slug":"puzzling-solar-panel-press-rel","status":"publish","type":"post","link":"http:\/\/chadorzel.com\/principles\/2008\/11\/06\/puzzling-solar-panel-press-rel\/","title":{"rendered":"Puzzling Solar Panel Press Release"},"content":{"rendered":"

Getting back to science, at least for the moment, I was puzzled by a press release from RPI, with the eye-catching headline Solar power game-changer: ‘Near perfect’ absorption of sunlight, from all angles<\/a>. The article describes work published in Optics Letters<\/cite> (that I haven’t been able to put my hands on yet), developing new anti-reflection coatings to enhance the absorption of light by silicon solar panels:<\/p>\n

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An untreated silicon solar cell only absorbs 67.4 percent of sunlight shone upon it — meaning that nearly one-third of that sunlight is reflected away and thus unharvestable. From an economic and efficiency perspective, this unharvested light is wasted potential and a major barrier hampering the proliferation and widespread adoption of solar power.<\/p>\n

After a silicon surface was treated with Lin’s new nanoengineered reflective coating, however, the material absorbed 96.21 percent of sunlight shone upon it — meaning that only 3.79 percent of the sunlight was reflected and unharvested. This huge gain in absorption was consistent across the entire spectrum of sunlight, from UV to visible light and infrared, and moves solar power a significant step forward toward economic viability.<\/p>\n<\/blockquote>\n

That all makes sense. What puzzles me is the second claim made in the release, which has to do with the problem of angle dependence. Most materials are reflect more light at glancing angles than normal incidence. If you look at a pane of glass straight on, for example, it reflects about 4% of the light striking it, which is why you only see a faint image of your reflection. If you look at a window from the side (put your head next to the frame), on the other hand, the reflection will be much stronger.<\/p>\n

The release notes this, and then goes on to make a very strange claim:<\/p>\n

<\/p>\n

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This same is true of conventional solar panels, which is why some industrial solar arrays are mechanized to slowly move throughout the day so their panels are perfectly aligned with the sun’s position in the sky. Without this automated movement, the panels would not be optimally positioned and would therefore absorb less sunlight. The tradeoff for this increased efficiency, however, is the energy needed to power the automation system, the cost of upkeeping this system, and the possibility of errors or misalignment.<\/p>\n

Lin’s discovery could antiquate these automated solar arrays, as his antireflective coating absorbs sunlight evenly and equally from all angles. This means that a stationary solar panel treated with the coating would absorb 96.21 percent of sunlight no matter the position of the sun in the sky. So along with significantly better absorption of sunlight, Lin’s discovery could also enable a new generation of stationary, more cost-efficient solar arrays.<\/p>\n<\/blockquote>\n

There’s nothing particular wrong with the angle-dependent reflectivity idea, but the claim that this eliminates the need for sun tracking strikes me as really odd. I can believe that increased reflectance at large angles plays a role in this process, but that’s not the only thing going on, and, indeed, it’s not something I’ve heard used as an explanation for the need to have solar panels track the sun.<\/p>\n

The other effect is purely a matter of geometry, and it’s easy to illustrate with a flashlight or laser pointer. If you take a light source, and shine it straight down onto the floor, you’ll see a nice, round spot of light. If you tip the source at an angle, that spot stretches out into an oval shape, getting longer along the direction of the tilt.<\/p>\n

That oval shape covers more area than the original circular spot. Which means that the intensity<\/strong> of the light (the amount of light hitting a given small area in one second) is lower for the light coming in at an angle than for the light coming straight on.<\/p>\n

For solar panels, this means that the intensity of light hitting a horizontal panel changes as the day goes along. It’s highest at noon, and drops off dramatically in the early morning and late afternoons. The amount of electricity that can be produced by a solar cell is directly proportional to the intensity (the electricity produced is some fraction of the energy hitting the panel), so horizontal solar cells generate more electricity in the middle of the day than in the morning or evening.<\/p>\n

This geometric factor is the usual reason given for why you need solar panels to track the sun, and it will still hurt the efficiency of solar power generation even if the reflectance problem is solved by this magic new coating. The reflectance thing can’t be helping the overall efficiency, but it’s not the only reason for tracking systems, and I’m not sure it’s even the main reason.<\/p>\n

It’s conceivable that the increased efficiency gained by lowering the reflectivity of the panels (and eliminating the tracking motors) is enough to offset the losses due to geometric factors. But it’s not immediately obvious to me that that would be the case.<\/p>\n

Which is why, as I say, I’m puzzled by this claim.<\/p>\n","protected":false},"excerpt":{"rendered":"

Getting back to science, at least for the moment, I was puzzled by a press release from RPI, with the eye-catching headline Solar power game-changer: ‘Near perfect’ absorption of sunlight, from all angles. The article describes work published in Optics Letters (that I haven’t been able to put my hands on yet), developing new anti-reflection… Continue reading Puzzling Solar Panel Press Release<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"1","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[48,19,7,11,56],"tags":[],"class_list":["post-3124","post","type-post","status-publish","format-standard","hentry","category-environment","category-experiment","category-physics","category-science","category-technology","entry"],"_links":{"self":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/3124","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/comments?post=3124"}],"version-history":[{"count":0,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/posts\/3124\/revisions"}],"wp:attachment":[{"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/media?parent=3124"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/categories?post=3124"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/chadorzel.com\/principles\/wp-json\/wp\/v2\/tags?post=3124"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}