标签: solar

Solar-based energy is often associated with electricity-generating solar cells; solar-based heating systems using a working fluid; solid-state thermoelectric-based conversion; or perhaps a solar-concentrator which liquefies some salt-based compound for storage .   Now there's a new entrant in the photon-to-electron race: an optical rectifying antenna, or rectenna. This highly experimental device, discussed here , was developed at the Georgia Institute of Technology (better known as "Georgia Tech"). It captures and rectifies photons to produce electricity directly; they claim this the first time this has been done. You can read a more-detailed technical discussion in this academic paper in Nature , as well. This optical rectenna developed at Georgia Tech uses advanced physics principles and materials, and is claimed to be the first to achieve direct optical-to-RF conversion.   Rectennas are not new: they have been used for forty-plus years in highly specialized situations to direct RF-to-electron conversion (and Google "Nikola Tesla" for even earlier efforts). Another recent story " Rectenna Serves 2.45-GHz Wireless Power Transmission " gives full details on one version which claims 63% conversion efficiency; certainly impressive. Looking at the RF article and all the details, you see that as in so many advanced ideas, the concept is straightforward but the actual implementation is quite complicated with many subtleties. Similarly, reading the Georgia Tech paper in Nature , you get a good sense of the technical challenges of capturing and converting photons. The low efficiency of around 1% is certainly not marketable yet, as conventional solar cells are running in the 10-20% range, but you never know where these things will go, what breakthroughs may occur, or when they may happen (please, let's NOT see a roadmap yet, that's way too premature).   One other point is apparent looking at the optical rectenna: it is made possible due to advances in nanomaterials. I recently attended the annual conference of the Materials Research Society , where 5,000+ attendees discussed, explored, and demonstrated the latest in materials along with their creation, applications, and test at temperatures from near 0 K to thousands of degrees; the test aspect, even at room temperature, is a major challenge for many of these nanomaterials. There's certainly lots of fascinating things happening that may have a significant impact on technology in ways we can't foresee yet. When we casually use packaged devices and products, it’s easy to not realize how much basic materials science are the now-routine enablers, in terms of new materials, ultrapure elemental and created materials, and understanding of their electrical, chemical, physical, optical, and mechanical properties. Even if you see little need to follow developments in material science (we can't do everything ), it's probably a good idea to at least follow developments in optical-related science, engineering, and test. Laser Focus World and Photonics (both available online and in print) are valuable and readable resources with insight, ideas, and even inspiration. Given the major role that optical concepts and components have in so many electronics systems, including LEDs, lasers, photodetectors, optical fiber links, filters, displays, and instrumentation, it's a worthwhile use of precious time.

About once a month, I check my car tyres, since correct pressure is necessary for good car handling, a smooth ride, and good gas mileage. When I checked my front tyres recently with my Topeak digital gauge (image below), one was at the correct pressure (30 psi), but the other was higher, at about 33 psi. What puzzled me was that they had both been at 30 psi the previous time. I know tyres can lose pressure, but I had never heard of a case where the pressure increased on its own.     This inexpensive digital-pressure gauge is a pleasure to use: It reads up to 160 psi/11 bar (useful for bike tyres and suspensions) to three significant figures; can be switched among psi, Bar, or kg/cm² readings; and handles Presta and Schrader valves -- a big improvement over the old "pencil-type" mechanical tyre-pressure gauges.   I gave this some thought and saw only two possibilities at first: Someone was playing a practical joke on me (very unlikely); or my previous reading for just that one tyre alone was in error (also unlikely, as all tyres were measured twice, and at the same time).   Then I looked at the car and saw that the tyre that read high was in full sunlight, while the other was in the shadow, and a black tyre certainly does heat up from solar radiation. Mystery solved -- or maybe not. I pulled out my custom-made "back of the envelope" pad and did a quick calculation using the ideal gas law :   P × V /T = K or P = K × T / V   ...where P is pressure, V is volume, T is the absolute temperature, and K is a constant, which depends on the amount and type of gas (here, the value is irrelevant).     My hand-made "back of the envelope" pad reminds me that doing quick, rough estimates is often a good first step to understanding the parameters of a problem.   Thus if the pressure I measured was about 10% higher than the original, and the volume was constant, then the temperature of the air in the tyre also should have gone up about 10%. The "cold" ambient temperature was about 77°F (25°C) or about 300K (remember, this is a rough estimate we're doing), so the delta rise would be 30K (30°C), or about 55°F.   Then I worried that perhaps a change in the tyre's volume would affect my estimate, but I realized it was a non-issue for two reasons. First, a car tyre is not an easily expandable balloon; it is a rubber enclosure restrained by steel-wire belts. Therefore, its volume stays fairly constant, especially for modest variations around a nominal value (this is a type of assumption we often use in many simplified models).   Second, even if the tyre did expand slightly due to the increase in internal pressure, that would actually cause a decrease in the resultant pressure -- again, the gas law. (I recall seeing a complex differential equation embodying the relationship among a tyre's construction, pressure, and volume, for more advanced modeling.)   Was solar heating the answer to my mystery? I don’t know, as I have no way of measuring the internal air temperature. I suppose I could do some thermal modeling, or even use an application such as COMSOL Multiphysics for a thermal/mechanical simulation, but it's not worth the effort.   So the question of sunlight heating the tyre and raising the pressure remains a slightly open mystery. My "gut" tells me that a 30°C/55°F rise for a black-rubber tyre in full sunlight is possible, but that's where I have to stop.   Do you think it was solar-heating effect? Can you think of any other causes? Have you ever had a similar "simple" measurement mystery, where you are not sure of the actual cause of the observed effect?

I check my car tyres about once a month just like many people, since correct pressure is necessary for good car handling, a smooth ride, and good gas mileage. When I checked my front tyres recently with my Topeak digital gauge (image below), one was at the correct pressure (30 psi), but the other was higher, at about 33 psi. What puzzled me was that they had both been at 30 psi the previous time. I know tyres can lose pressure, but I had never heard of a case where the pressure increased on its own.     This inexpensive digital-pressure gauge is a pleasure to use: It reads up to 160 psi/11 bar (useful for bike tyres and suspensions) to three significant figures; can be switched among psi, Bar, or kg/cm² readings; and handles Presta and Schrader valves -- a big improvement over the old "pencil-type" mechanical tyre-pressure gauges.   I gave this some thought and saw only two possibilities at first: Someone was playing a practical joke on me (very unlikely); or my previous reading for just that one tyre alone was in error (also unlikely, as all tyres were measured twice, and at the same time).   Then I looked at the car and saw that the tyre that read high was in full sunlight, while the other was in the shadow, and a black tyre certainly does heat up from solar radiation. Mystery solved -- or maybe not. I pulled out my custom-made "back of the envelope" pad and did a quick calculation using the ideal gas law :   P × V /T = K or P = K × T / V   ...where P is pressure, V is volume, T is the absolute temperature, and K is a constant, which depends on the amount and type of gas (here, the value is irrelevant).     My hand-made "back of the envelope" pad reminds me that doing quick, rough estimates is often a good first step to understanding the parameters of a problem.   Thus if the pressure I measured was about 10% higher than the original, and the volume was constant, then the temperature of the air in the tyre also should have gone up about 10%. The "cold" ambient temperature was about 77°F (25°C) or about 300K (remember, this is a rough estimate we're doing), so the delta rise would be 30K (30°C), or about 55°F.   Then I worried that perhaps a change in the tyre's volume would affect my estimate, but I realized it was a non-issue for two reasons. First, a car tyre is not an easily expandable balloon; it is a rubber enclosure restrained by steel-wire belts. Therefore, its volume stays fairly constant, especially for modest variations around a nominal value (this is a type of assumption we often use in many simplified models).   Second, even if the tyre did expand slightly due to the increase in internal pressure, that would actually cause a decrease in the resultant pressure -- again, the gas law. (I recall seeing a complex differential equation embodying the relationship among a tyre's construction, pressure, and volume, for more advanced modeling.)   Was solar heating the answer to my mystery? I don’t know, as I have no way of measuring the internal air temperature. I suppose I could do some thermal modeling, or even use an application such as COMSOL Multiphysics for a thermal/mechanical simulation, but it's not worth the effort.   So the question of sunlight heating the tyre and raising the pressure remains a slightly open mystery. My "gut" tells me that a 30°C/55°F rise for a black-rubber tyre in full sunlight is possible, but that's where I have to stop.   Do you think it was solar-heating effect? Can you think of any other causes? Have you ever had a similar "simple" measurement mystery, where you are not sure of the actual cause of the observed effect?

I was lying in bed last night thinking about my recent blog, Why lunchtime is an illusion . In particular, I was thinking of the dynamic, ever-changing displays on the Poodwaddle.com website showing births and deaths and the total population of the planet (7,222,691,563 when I checked just a moment ago, but let's round it down to 7 billion for the purposes of these discussions). I know these counts are just a best-guess approximation based on what we know (and what we think we know), but I believe them to be close enough to the truth to make no difference. The "deaths" display is one that really sticks in my mind. As I get older, I become increasingly aware that one day I will be an element in that count (sad face). Actually, while we're talking about this, Paul Clayton pointed me at the Frequency page on xkcd.com. This really is awesome. I don't know who is in charge of xkcd, but he/she/it/they (see also my Gender-Neutral Prose blog) is/are absolutely amazing. This particular Frequency image provides a real-time graphical display reflecting the frequency with which a variety of disparate things are occurring as we speak. These include such off-the-wall items as a typical heartbeat, a fire department putting out a fire in the USA, a member of the UK parliament flushing a toilet, and a Sagittarius named Amelia drinking a soda (I couldn't make this stuff up if I tried). But that's not what I wanted to talk to you about... As I noted above, there are 7 billion-plus people on the planet Earth. I can't even wrap my brain around this. It scares me. Suppose we all had the same food to eat each day. Imagine 7 billion bananas being consumed for breakfast; 7 billion cups of coffee being quaffed during a mid-morning break; 7 billion cheese sandwiches being consumed at lunchtime; 7 billion chocolate cookies being nibbled in the afternoon; and 7 billion salmon fillets (along with, say, potatoes and peas) being scarfed for supper. Of course, the above would be an idyllic situation—the vast majority of people in the world should be so lucky—I daren't even think about the number of children with empty stomachs, not knowing where their next meal is coming from. At the other end of the spectrum we have people who consume much more than their fair share. I'm embarrassed to tell you how many cups of coffee I drink a day, for example. My mind is bouncing around from topic to topic like a Ping-Pong ball. Take a look at the following image, which is an artist's impression of what our galaxy, the Milky Way, looks like based on data gathered from a number of sources, including NASA's Spitzer infrared space telescope.   The Milky Way is about 100,000 light years in diameter. Our solar system is located in the Orion Spur approximately three fifths of the way from the galactic centre. I don't know about you, but this sort of thing makes me feel really insignificant (in a magnificent sort of way, of course). The thing is that by one path or another (the human brain is a strange and wonderful thing—it's certainly one of my three favourite organs), all of this this reminded me of Alone in the Universe by John Gribbin (click here to see my review ). Gribbin makes a very compelling case for the fact that we may well be alone (as an intelligent, technological race) in the universe. If we truly are the only intelligent, technological race in the universe, it would behoove us to take good care of each other and of the planet we call home. All I'm saying is that munching our way through 7 billion metaphorical bananas (they're the tastiest ones) a day is quite possibly not the best way to do this. What do you think?  

I strongly support solar power—when it makes sense. For energy harvesting in data loggers, site-based power, spacecraft, and large fixed installations such as houses, for example, it often works out fairly well, or is the only viable solution. At the same time, I get annoyed when the promise of free solar power is used as an attention-getting gimmick, regardless of how impractical or nearly useless that promise is. The latest example of this is a piece I just saw in The Wall Street Journal , " Ford to Show Solar-Powered Hybrid at CES ." (Sorry, it may be behind the paywall.) In brief, Ford is equipping some of their hybrid C-Max concept-car models (based on a commercially available vehicle) with solar panels on the roof, along with some sort of sun-tracking system. It's not clear. It looks like it may have steerable Fresnel lenses in order to provide both basic and booster free power. What I like to do when confronted with this sort of technology claim is perform a quick back-of-the-envelope, order-of-magnitude sanity check. I have a notepad I made just for this function, to use at concept meetings. It's made entirely of envelopes. (See photo .) My analysis: I don't care how good that solar power system is, it won't collect enough power to make it even remotely worthwhile. A few rough numbers tell the solar tale. On the source side, the maximum solar radiation reaching Earth's upper atmosphere is about 1,000 W/m 2 , according to various reputable sources. That's the maximum. Just factor in atmospheric losses, clouds, the angle of the sun at different latitudes, and seasonal effects, and that number drops down rapidly. On the conversion side, you have the efficiency of the solar cells reaching say, 15% at best, minus losses in the dc/dc power system of around 50%. (Remember, this is all an estimate. We are just trying to see where things stand.) A car roof is perhaps four square meters—let's say five square meters, to be generous. If the car roof captures full-intensity solar radiation, you've received 5,000 watts per surface, less the capture/conversion losses. Work those losses in and you may garner 375 W for the car battery to store, assuming you got the full 1,000 W/m 2 at the cell surface, which won't happen. To make a long story short, you'll be lucky to collect a few hundred watts. Now look at the load: One horsepower is about 750 W, and you've collected less than half that amount. How far can you go, then? Although it depends on how long you've been able to collect that energy, the real answer is "not far at all." Well, perhaps you can use it to supplement the car's internal power source? Again, not much help. A car today can use it up easily for all the infotainment sub-systems. The bottom line is you are putting a lot of cost and stuff into the car for very little useful return. The power-harvesting reality is simple, determined by the laws of physics. You collect energy over time, when it is available, but you spend it quickly, as power, to meet the much higher needs of the load. Just because you are collecting it at a slow rate doesn't mean you can get away with spending it at that same rate, since most real-world loads have specific minimum-power requirements to make them functional. (See Confusions on energy and power if you need a refresher.) Ford's announcement worked. It got them attention. I'll give them credit for that. But the solar-powered car story is an oldie but goodie. It always amazes me that it still works. (Check Popular Science from the 1950s and you'll see the same story!) Have you seen any free-power-harvesting schemes that made little or no sense? Have you ever tried to point this out to proponents?