Another day, another Gizmodo piece on renewable energy gadgets to debunk. Last week ReWire took a critical look at the gadgetry site's strange lauding of a car-top wind turbine that couldn't possibly work. This week, it's solar orbs: a form of concentrating solar involving glass ball lenses.
In a piece published Monday, Gizmodo writer Kelsey Campbell-Dollaghan waxed lyrical over Rawlemon, a startup currently crowdfunding its concentrating photovoltaic design. Rawlemon uses a large ball lens to focus light on photovoltaic cells. Rawlemon's designer claims the lens can concentrate available light by up to 10,000 times, boosting the output of the small solar cells by a considerable amount.
Campbell-Dollaghan's take on the potential product is somewhat breathless. "Rawlemon," she writes, "by sheer force of numbers, has the power to outperform traditional solar panels by many thousands of times," even offering the possibility of generating power from moonlight. Sadly, a bit of simple math deflates her claim. While Rawlemon's technology may well offer some nifty advantages for specialized uses, the system is unlikely to be competitive with plain old solar panels anytime soon.
In December, Cleantechnica's Jake Richardson offered a somewhat more sober look at Rawlemon's actual promise. Unlike the car-top wind turbine, there's nothing in the laws of physics to keep Rawlemon from working more or less as advertised: a large glass sphere filled with water acts as a lens to concentrate available light on a small array of solar cells, and a microtracker keeps those cells in the focal point of the lens as the sun moves through the sky.
Richardson suggested that Rawlemon's design could boost the outlet of the PV cells by 35 percent. That's a remarkable gain in efficiency if true, though it's half the improvement in efficiency touted by Campbell-Dollaghan in Gizmodo. Richardson also pointed out that Rawlemon has been looking at using arrays of smaller ball lenses to concentrate energy on solar cells.
Using lenses to concentrate sunlight on PV cells is nothing new, though the ball lens is a nifty design twist. It may be that a large ball lens' ability to concentrate ambient light by a large factor will allow some interesting edge-case applications.
But the hype over Rawlemon's technology representing a significant advantage over conventional PV panels for everyday use ignores a bit of basic science.
That basic science: a lens can only concentrate the light that falls on it. Remove the lens, and the same amount of sunlight falls on whatever surface was behind the lens.
Rawlemon's large ball lenses look to be about a meter in diameter, so let's use that guesstimate to make the math easy. Place a meter-wide sphere in the sun: the "footprint" of that sphere that's exposed to direct sunlight is a circle with a diameter of slightly less than a meter.
In perfect conditions, about 1,000 watts of sunlight hits each square meter of the rooftop on which you'd mount your solar panels. The amount of sunlight directly striking your meter-wide ball lens would be about 785 watts, because the surface area getting that direct sunlight would be the square of the sphere's radius (one half meter) times π (pi, or 3.141592etc), or about .785 square meters. Times 1,000 per square meter, that's 785 watts of sunlight directly hitting the ball lens.
That sunlight gets concentrated on a few very small solar cells. And yes, those very small solar cells put out a lot more energy than they would if the lens wasn't there.
Or you could just put out several square meters of flat solar panels without the lens, which gives you more photovoltaic surface at what ReWire expects will be significantly less cost, not to mention avoiding a way to support a meter-wide sphere of glass and water on your roof weighing more than 1,000 pounds.
That's not to say Rawlemon's technology doesn't offer some promise. Ball lenses can collect and focus ambient light that flat-plate solar panels don't use quite as efficiently -- though even there, it takes quite an overcast day for traditional PV panels output to zero-out altogether. Moving a tiny PV array to track the sun will likely take less energy than tilting a large panel. Rawlemon's tech is intriguing at first glance, and may well offer solutions to problems we don't yet know we have.
But it's unlikely, with the ever-accelerating decline in standard PV prices, that Rawlemon will offer gains in efficiency for standard urban generation that would be cheaper than just buying a few more square meters of plain-vanilla PV.
As for Rawlemon's implied promise of lunar power: in ideal conditions the light of the full moon is about one millionth as powerful as sunlight, delivering a milliwatt per square meter of illuminated surface. Even if we charitably assume that ambient and reflected moonlight delivers that intensity of power to the entire surface of the meter-wide ball lens, which could not conceivably happen, that's 3 milliwatts of power the lens could concentrate on its PV cells.
To charge your smartphone by the full moon, in other words, you'd need between 700 and 2,000 Rawlemons all hooked into your charger, depending on which model of phone you own. At which point it's really best to wait until morning and charge the phone with the sun and a couple square feet of conventional solar panel. Oh well. It was nice in theory.