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PG&E Makes Deal For Solar Power From Space
N!NJA writes "California's biggest energy utility announced a deal Monday to purchase 200 megawatts of electricity from a startup company that plans to beam the power down to Earth from outer space, beginning in 2016. Solaren would generate the power using solar panels in Earth orbit and convert it to radio-frequency transmissions that would be beamed down to a receiving station in Fresno, PG&E said. From there, the energy would be converted into electricity and fed into PG&E's power grid."
This is a horrible idea. What happens when the beamer is hit by a micro meteor nocking out the com and pointing the sat at SF?
They mentioned it in the first Robocop movie.
There is no "I disagree" mod for a reason. Flamebait, Troll, and Overrated are not substitutes.
couldn't this also be used as a weapon?
If you're lucky, you gain a factor of 2-4 in efficiency by going into space, but the costs per photocell are astronomically higher compared to installation in a desert.
That's, of course, assuming you can actually get other nations to agree to let you place a massive power plant and emitter in orbit, something that could easily be weaponized.
As long as you turn off disasters, beamed solar energy is actually a fairly cost effective power solution.
No, these don't work like SimCity. The microwaves are not the frequency used in ovens -- ie that heat up water. Otherwise they wouldn't be much use on a cloudy day.
It's a very positive development. Orbital solar power is the best foothold for the colonisation, industrialisation and settlement of intrasolar space.
Classical Liberalism: All your base are belong to you.
When are we just going to learn to invest (i dunno... like we have in other endevours - think iraq) in our future? Wind farms have been an untapped resource for too long.
Or are they really saying they're going to install roughly 200000 m^2 worth of solar collectors in space? That's a square of roughly 450x450m. And "some startup" is planning a feat like that?
Took us long enough.
"PG&E Makes Deal For Solar Power From Space"
Is there any other kind?
SIG: TAKE OFF EVERY 'CAPTAIN'!!
This is an excellent use for Fresno. I approve.
I saw a documentary on Discovery a few months ago, it was an episode part of "Discovery Project Earth".
I found it extremely fascinating and was wondering if it would just die or if there would be some actual results from the project, seems like we are getting somewhere now!
I remember from the documentary that the biggest problem was the beam being split in two, rather than one focused beam. Hopefully they found a solution to this problem.
Anyways, I strongly suggest watching the documentary if you are interested in this, it really shows how the idea was born and all the small advancements they made which resulted in a successful test.
It's no coincidence that Mr. Freeze was played by the current governor of California...
"Action without philosophy is a lethal weapon; philosophy without action is worthless."
"space satellites could generate power 24 hours a day, unaffected by cloudy weather or Earth's day-night cycle."
That might be true depending on the orbit. If it's in an expensive synchronous orbit it will still be in earth's shadow once a day but I would expect that the beam would have a pretty large diameter at Fresno. If it's in any other orbit Fresno will be in line of sight for only part of the time. So how do they generate and transfer power over 90% of the time?
Nate
I am really a supporter of solar energy - I even have invested some of my money in it - but THIS to me seems like technological masturbation. I do not believe it's cost-effective, and the debris in orbit is only going to increase, so it's a risky investment in any case.
"The agriculture ministry is not in charge of Gundam" - Japanese ministry official.
Maybe a bunch of these could be used to block out the sun, and thus, reverse Global Warming?
Schroedinger's Brexit: The UK is both in and out of the EU at the same time!
One good coronal mass ejection, and these things are toast, I would assume.
As long as you have good fire coverage you should be able to put the fires out fast with little damage.
How is this cheaper or safer than nuclear? Nevermind the costs, would it even produce the same amount of power after transmission? Why is nuclear such a dead end? Before someone asks if I'd like a nuke plant in my backyard, YES, YES I would love it.
Cheap power and a healthy green glow beats go-nowhere plans and whining greenies any day of the week.
--- Do you believe in the day?
I had a lunatic prof in college who advocated this stuff, along with laser powered lightcraft. The technology really works - as he put it, if we can hit an ICBM at Mach 20 with a laser, we can hit a spot on the (relatively) unmoving Earth with a laser.
I want to delete my account but Slashdot doesn't allow it.
These beaming systems have interlocks pointed back from the ground receiver to the satellite. If the two get out of alignment, the satellite immediately loses the ground signal, and immediately stops transmitting.
Besides, the beamed power density doesn't have to be very high per square meter. If it's just concentrated 5x from its density in space, it's 6.5KW:m^2. At this system's 2MW transmission rate, is only 308m^2, or a square 17.5m on a side. If it's really RF, even if the interlock failsafe failed, the beam wouldn't do much except fry some unshielded electronics in the way until something else shut it down. I'm sure the multiple layers of government regulators will ensure a lot of "deadman switches" to stop the only thing that everyone guesses could go wrong.
--
make install -not war
This reminds me a lot of the Microwave power from Sim City 2000... Anyone remember the disaster that happened with the beam would occasionally miss?
The musings of just another geek and his junk.
Just when you thought there weren't enough tinfoil hats in CA, the gov't is giving everyone justification to wear one!
These guys made this deal so that they can get investors and loans to build the thing. It's no risk to PG&E, and now these guys have to execute.
This is my sig.
...Solaren was purchased by the Mikado Group, whose chairman, Dauragon C. Mikado, says that the satellite plan will bring ultimate power...
listens to the crickets
Yeah...didn't expect many people to get that reference.
Dan Aris
Fun. Free. Online. RPG. BattleMaster.
...otherwise kiss radio astronomy in North America goodbye. Those guys thought they were getting interference from the Iridium constellation? Heh..wait until they get 200MW of broadband RF interference coming down on them from this monstrosity.
Not to mention, this seems to be a complete waste of resources. I'd wager that at least as much land (if not more) will need to be dedicated to the antenna array as a 400MW (put in twice the power to make up for day-only operations) solar concentrator plant if they want any sort of chance of capturing all of the beam for conversion. Add to that the fact that the increased solar incidence in orbit will be conteracted by the losses in RF transmission (engineers were thrilled when they achieved 54% between ground towers recently...). And lets not forget the rather substantial launch costs (likely hundreds of millions of dollars). All in all...this is a concept best suited to the Sims game than real life. I'm all for alternative/renwewable energy...but this is just a waste of time and money. But hey..if some VC's like watching stacks of hundred-dollar-bills burn in the mean time...more power to them. I just hope this idiotic scheme doesn't get any federal funding. Our DOE Secretary is a pretty sharp guy...I'm sure he sees the folly in it as well and hopefully will steer well clear of it. I would think the FCC would have something to say as well..considering the MASSIVE potential for RF interference. Investment tip: I wouldn't be sinking any retirement money into this outfit ;)
If the beam going off-track is a safety concern, it's a pretty safe bet that there will be safety interlocks designed to engage should such an event occur, up to and including a self-destruct device. In the space shuttle carries self-destruct mechanisms on it in case it veers off course into a populated area.
A work that expires before its copyright never enters the public domain and thus enjoys eternal copyright protection.
Novel by Kevin J Anderson and Douglas Beason. Part of the story sort of unrelated to the main plot was a company using a rail launcher in New Mexico to launch a bunch of satellites into orbit. They beamed the power to a grid on the ground in the desert. Power beamed back had a narrow range of reception so it couldn't accidentally fry anyone on the ground.
That said, /. Is getting really bad about people jumping in and shitting on whatever idea is proposed here. What the fuck is up with that? Almost all of the replies so far are "This will never work", like the people responding are scientists and engineers and have thought it all through, right? Where's your fucking white papers, assholes? "The Infeasibility of Space-based Power Generation Systems." Why don't you people eat shit and die.
Everything you know is wrong, Just forget the words and sing along.
Introducing MORE energy into Earth's natural system is worse than releasing all the carbon energy that has been stored.
The planet is basically a closed system, it has come into balance with the available energy it collects from its surface area. Introducing more energy from an orbiting source will have a significant effect.
This kind of power generating project is better left for a moonbase, or planet which does not already have an ecosystem.
Time to re-read Red Mars (1992), Green Mars (1993), Blue Mars (1996) by Kim Stanley Robinson.
Or, to begin closer to the beginning, start with The Fountains of Paradise (1979) by da man, Arthur C. Clarke.
"We reject as false the choice between our safety and our ideals." --The American President (20.1.2009)
Seeing as terrestrial photovoltaics are loike 8x more expensive than other power sources, orbital ones should be literally and figuratively, out of sight.
This is what happens when they try to legislate green laws with total disregard to the laws of economics and thermodnamics.
I wonder what California ratepayers are going to say. They already pay the highest rates in the nation IIRC. This scheme can't come in under 30x the average cost of power. What a clusterfarg.
seen a 'cancer' tag on this article yet?
if it works like simcity, i predict parts of california will experience an occasional uncontrollable deathray from space. ;)
Good people go to bed earlier.
PG&E is not placing any up front capital to these bozos. So all the people who are saying this is a trick to get PG&E to fund fluff, please, RTFA.
Second:
Microwave Power Transmission (MPT) actually being used as a death ray is way too cool to be real. Sadly, this is the case. MPT just doesn't provide enough radiant energy per square centimeter to actually fry someone. In fact it's a little over the amount that leaks out from your microwave oven. Sorry, I would have love the idea of zap, poof, he's dead Jim.
Third:
I don't think these sucker are actually going to get this thing off the ground, seriously. The biggest thing that has kept people from doing this in the 60's was:
As far as I can tell these turkeys have item 1 checked and are praying for the rest. I can only bet they are hoping that number 2 comes from Obama. If they do succeed I'd love to know how they get pass Rectenna size-distance ratio issue.
Cheers!
PG&E Makes Deal For Solar Power From Space
As opposed to making a deal for Solar Power From Subterranea? Or as opposed to making a deal for Solar Power From Outer Space? Hey /. editors/submittors - I think we can all safely assume that Solar Power comes from space....unless we invented a Dyson Sphere?
I do not support "The Man". I also do not support your irrational stupidity
So now we're going to go from incidental anthropogenic climate change to on purpose anthropogenic climate change? The whole problem is that there is more energy coming into earth/atmosphere system than is going out and now we want to take energy, which would have otherwise passed this system by, and deflect it inside? It's nice to know that highschool physics has completely left the building.
I seem to remember discussion here on
http://science.slashdot.org/article.pl?sid=07/07/25/1738239
where the National Security Space Office (NSSO) of the US DoD wanted to open discussion on Space-Based power generation. The NSSO published an interim assessment:
http://www.acq.osd.mil/nsso/solar/solar.htm
One of the points of the assessment is that the DoD is likely to be a big customer for Space-Based power, especially for powering remote facilities.
As an Amateur Radio operator, I have to do an analysis of my transmitting station to insure the field strength in my immediate area and my neighbors areas are in safe levels. As any operator knows, beamed radio frequency energy has a specific width, side lobes, and spill-over. In other words even though 90% of the energy may be hitting its target, a considerable amount will be hitting the surrounding area. The energy doesn't just stay in the desired beam area, especially over that distance. So while I'm concerning myself with documentation of 50W transmissions to insure safe exposure, this power transfer system will likely be exposing many people to much higher levels. I suppose one advantage to residents of the area will be that they no longer need to heat their homes in the Winter. They'll just have that nice warm feeling all year long.
Hi ho silver
Over on The Oil Drum this was hashed out
From Actual "research"
"surface, it has spread out considerably. The energy density is one-sixth that of the noon-day sun."
So we've went from 2 times the 'normal' PV to now 1/6th the energy density of normal sunlight.
Once again the question to ask is - why go through the hassle of putting collectors in space if land based PV collecting regular old sunlight at 6x times the energy density?
Now you wanna play in space? Why not robotic mining/refining and send the results back down the gravity well? All ya need to do is solve AI and some material science issues. *wink*
This really is very safe, and all the technology is known (not at this scale maybe, but known). The only thing that has stopped us from doing it already has been a lack of willpower.
If you are sending microwaves from a smallish antenna (small enough that you can boost it into GEO, for example) all the way back to earth, the receiver needs to be huge, like many acres. Basically you find a good pasture, put posts in the ground every few dozen feet in a grid, run wires and diodes between the poles, and you now have a high efficiency rectenna and the cows grazing underneath won't even notice.
Even if the beam wandered, the power per square meter isn't that high, and to get through the atmosphere with minimal losses, it won't be at a frequency that is easily absorbed by water, which means that it won't be at a frequency that is easily absorbed by you or me.
See that "Preview" button?
First off the energy transmitted from space would see serious loss due to the distance and crap that the RF would have to go through in order hit a ground station. Also beam width of the energy would have to be so precise, because its foot print would create a need for a very very large dish. Novel idea that does not deserve to much credit.
The WSJ's Cassandra Sweet calls Solaren a "stealth startup:"
The company is in talks with two trusts, one in the U.S. and one in Europe, about financing for engineering, design and testing of the system, and launching a pilot system, The company needs funding "in the billions of dollars" [simply for the pilot project] after which it will likely float an initial public offering. UPDATE: PG&E Looks To Outer Space For Solar Power
The faith of the innocent.
Solaren is one guy with a pick-up team of engineers: it has no money and no track record.
Amazing, it is as if all the great feats of space exploration in the 60's and 70's, and the grand plans we had to follow up with space colonies, lunar colonies, space power and asteroid mining are all just strange myths from a distant past.
This was all conceived back in the 70's. http://www.nss.org/settlement/ColoniesInSpace/colonies_chap03.html It was even remotely possible back then. It can certainly be done now.
Would it be safe? Yes. Would it be practical? Possibly, depending on how it is done. Is it less objectionable than other forms of energy? Not to those who understand it, want it, and appreciate the long term benefits... but they are few.
We can't use abundant, cheap coal because of irrational fears of CO2. We can't use nuclear because of only slightly less irrational fears of radioactive waste. We can't use wind because it's big and noisy and no one wants it near them. We won't be able to use Orbital Solar Power because "the microwaves will cook us", "the energy balance of the Earth will be disrupted", "the big satellites will ruin the night sky" and a host of other silly reasons.
Such a pity. Building these first PowerSats could herald the new age of Space Colonization. More likely fear and ignorance will keep us trapped on this muddy little rock for the rest of time.
is somebody better run this by the HAM radio operators.
They say that hell hath no fury like a woman scorned, but that saying came into being before HAMs were on the scene.
Doctors destroy health, lawyers destroy justice, universities destroy knowledge, religion destroys spirituality
I live in Fresno,and to think I was hoping for a local nuclear power plant.
Maybe we can still get one of those too!
Inane Comments are Generously Disregarded
PG&E Makes Deal For Solar Power From Space
So I can't help but ask... is there some other place people have been getting their Solar power from?
Because last time I checked, it ALL comes from space...
Here's a thought.... Rather than converting the light to RF and beaming that down to the surface, why not just reflect the light down to the surface and put your solar panels there? That way, you can take advantage of advances in solar panel efficiency without the cost of launching a new satellite?
why beam from space?
why not beam across the country?
it sure would make maintenance cheaper.
comment directly in my journal
Did someone look at the wrong calendar ?
Sounds exactly like something they tried on Discovery Channel on a show called "Project Earth".
http://www.discoverychannel.ca/Article.aspx?aid=13193
They showed that it works but the power and precision needed to get the RF from orbit to the generating station will be stagering.
It brings up an interesting question... They're talking about converting a whole lot of electrical energy into RF energy, in the vacuum of space. The conversion won't be 100% efficient... How will they dissipate all of the extra heat?
Please keep your rectenna to yourself.
Does anybody want to take a SWAG at the cost per kilowatt hour of this "free energy"?
This idea is quite questionable, adding more energy will only increase the environmental problems...
A scenario somewhat like this occurs in Crawford Killian's Empire of Time (which I enjoyed).
"Not an actor, but he plays one on TV."
"We've talked with United Launch Alliance, and gotten an idea of what's involved and what the cost is," he said.
So they clearly have the entire operation all worked out and ready to go.
First thing I thought of was "Real Genius".
This worked great in Sim City. Just leave disasters turned off.
~Candera
Nuclear? In California? Forget it.
It would take 10 years to build a plant, pretty much anywhere today. It would require years of environmental impact studies and would likely fail to pass in the end.
California will likely be out of electricity long before 2016 anyway, so this is somewhat pointless. Unless they can get the capacity up to the point to equal increasing demand, people are just going to have to learn to do without.
Without 100% reliable electric power - where it gets turned off periodically - all sorts of things change. Kiss your home server goodbye. If they turn it off long enough, plan on buying food differently because refrigerators aren't going to stay cold. Freezers would be OK still, but it will change how people buy food. Think back to air conditioning in the 1950's where you had it in movie theaters and some other places but not at home. This will change all sorts of things in people's lives.
Even if they build up the capacity again a few years later the changes will have taken place. People will never trust electric power again to just be there. This is how it is in all over the planet today, just not in the US and Western Europe. I suspect California will be first to go down this road, but it will be nationwide - we haven't kept up with electric demand and it is starting to show.
Let's put aside the question of whether or not this is actually safe, and discuss how efficient this form of power transmission is? Anyone? Bueller?
Are YOU using the TOOL, or is the TOOL using YOU? Think about it!
By 2016 is kind of optimistic there... As soon as 2016 maybe. That is unless they're waaayy ahead of where they claim to be.
Again, no. The microwaves don't interact with organic matter, they pass through. You're not getting cancer from TV broadcasts or mobile phone towers either.
So my popcorn in my microwave is inorganic?
I've read about this idea years ago.
The space based solar panels basically 'step down' sunlight to radio waves; these can be collected on the ground at a much higher efficiency than the original sun light. Also, they can be concentrated to a strength that is more efficient for collection, but not strong enough to damage anything.
In the book I had read, the author proposed using a wire collector grid with mesh large enough to pass the bulk of the sunlight. The grid could be sited over pasture and it would interfere with neither grass nor livestock.
Because I'm definitely going to believe a random web page, with no citations whatsoever, when they tell me that I can consume as much plutonium as some people do caffeine.
Wikipedia does indicate that the chemical toxicity of plutonium is about the same as caffeine, but that it's toxicity is primarily due to its radioactivity. If in the process of eating the plutonium, you happened to inhale a microscopic particle of it... game over. You'd have lung cancer in short order.
Geez, nowhere in the article does it say that California mandated that electric utilities get their power from orbiting solar collectors. They mandated a certain percentage of renewable power, but the source of that power is up to the utility. If PG&E wants to bet on this pie-in-the-sky technology, that's a bad business decision on their part, not a problem with "legislating green laws with total disregard of economics and thermodynamics".
As to the realism of this particular project - if anyone thinks this contract is going to actually perform... I've got some collateralized debt obligations to sell you.
No. Actually, what's kept us from doing it already is that it's not cost-effective. Lifting machinery into space is very, very expensive. And while space-based collection is more efficient, you give back some of those gains in transmitting the power back down to the surface, and give back some more when you convert the microwaves back to AC power.
"Willpower" has nothing to do with it. Cold, hard, cash does.
did anyone else suddenly start hearing sim city 2000 playing in their head?
Karma is for whores
What is the chances of me creating a receiver to convert this and collecting stay energy as the beam spreads out though the atmosphere the way sunlight does?
Somehow I doubt the beam would be so focused that I or anyone else couldn't harvest some of it would it? Or is it going to be one of those things where it would cost me more to do then I would ever recover from doing it?
The debris in space is not automatically going to increase. It's automatically going to decrease because gravity is working on it. I have also read where there is work underway to build a satellite that uses water jets (or other means) to 'nudge' larger bits into re-entry. Now that scientists are aware of the problem, they are undoubtedly going to much greater lengths not to increase the danger, and given enough time the older stuff will clear. Enjoy.
This ain't no upwardly mobile freeway This is the road to hell
While I personally think space-based solar power is quite cool, unfortunately I'm not so sure the numbers quite work out for any time in the near future. My suspicion is that this announcement is primarily for PR reasons, and PG&E has no plans of actually following through. Some analysis from aerospace engineer (and space advocate) Rand Simberg:
http://www.transterrestrial.com/?p=18069
I just canâ(TM)t see how. Unless there are going to be many satellites, the system has to be in GEO to provide baseload power to any given region on earth. They talk about putting up a 200 MW system with âoefour or fiveâ âoeheavy liftâ launches (where this is apparently defined as 25 tons).
Suppose the conversion efficiency of the cells is a generous 30%, the DC-MW conversion is 90%, the transmission efficiency is 90% and the MW-AC conversion efficiency is 90% (generous numbers all, I think). That gives an overall efficiency of 22% from sunlight to the grid. The solar constant in space is 1.4kW/m2, so that means you need 650,000 square meters of panels to deliver 200 MW to the grid. Suppose you can build the cells (including necessary structure to maintain stiffness) for half a kilo per square meter. That means that just for the solar panels alone, you have a payload of 325 metric tons. Generously assuming that their payload of 25 tons is to GEO (if itâ(TM)s to LEO, itâ(TM)s probably less than ten tons in GEO), that would require over a dozen launches for the solar panels alone.
That doesnâ(TM)t include the mass of the conversion electronics, basic satellite housekeeping systems (attitude control, etc.) and the transmitting antenna, which has to be huge to get that much power that distance at a safe power density.
So even ignoring the other issues (e.g. regulatory, safety studies, etc.) that Clark mentions, I think this is completely bogus until I see their numbers. And probably even then.
The cost of the power produced from a space-based generating station could NEVER compete with ground based power sources. Oil is far cheaper than solar. You get about 40% increase in power production by being in orbit. Oil will still be cheaper. What does it cost to put you there? What does it cost to operate the satellite(s)? How long does the system last? This is one of the stupidest ideas (economically speaking) that I've ever seen. From a geek point of view, it's a really cool idea though.
What happens in 20 years time when we have hundreds of these all beeming down more energy from space. This will be importing a great deal more heat into the atmosphere hence adding to the greenhouse effect. In a world where our ice caps are melting at a tremendous rate, doesn't this seem a bad idea. We obviously already have enough energy on the planet to convert to electricity without importing more.
Let me ask a simple question...
If we're so good at doing wireless power, wireless RF power, why isn't my laptop powered through a wireless connection? My MP3 player? My cell phone?! (Tiny power, antenna already attached)
They aren't because this is bullshit.
This stinks of a take the money and run scam. Energy is cheap, even with gas at $4 a gallon energy is too cheap for this. Whoever is stupid enough to invest in this company doesn't deserve pity when they find out its all a scam. The engineering isn't there. The logic isn't there. The ROI isn't there. This is just dumb.
d
all language nazi's will burne in heil!
Hey Didn't Tesla have this idea a long time ago? Is this how long it takes product-to0market in the energy sector? Gee, look at anaerobic digestion as a source of cheap power from methane, the Chinese have been doing it for quite some time. We could be doing it here in America and reducing organic wastes to nearly nothing!
What about the birds, people. Think of your feathery friends roasted as they fly through the beam. The horror, the horror.
Instead of reflecting light it is absorbing light, so as a sail this is about half as efficient as a fully reflective solar sail, but nonetheless I was wondering whether this contraption would not be "blown" out of orbit by all that light it is capturing? I have no time to read the full wikipedia article about solar sails but some sections mention that a solar sail can even travel towards the Sun by aiming its "thrust" against its orbital direction. This trick and similar tricks make the sail capable of manoeuving. I wonder if this capability remains if the surface is 99% absorbing instead of 99% reflective.
I have a better idea, let's get rid of our atmosphere and turn the earth into a flat sheet of solar panels.
Oh, no, i forgot, the self-replicating robots will do that for us in a couple of years.
When I heard of this idea some time ago I wondered: Right now we have a "Global Warming" problem -- X amount of incident energy from the sun being absorbed and Y being radiated from the planet. Due to the "greenhouse gasses" the earth is getting warmer and we see it as a problem
I know! Let's attack the problem by increasing X! Why settle for only the energy that comes directly to Earth from the Sun? Why don't we add some mirrors in space, and beam some more down?
How does that compute?
joe
Earth receives some 174*10^15 Watts in solar irradiation. Unless the additional input is a significant fraction of that (200 MW aren't. Heck, even 200 GW wouldn't be), it's not going to make a significant contribution to global warming.
Sounds reasonable -- but can't the same argument be made for the amount of CO2 added by the next coal fired power plant compared to the total CO2 in the atmosphere? In fact, isn't that the basic argument made by those who insist it's all due to weathering?
No, the argument can't be made for CO2, for several reasons. One of them being the half-life of CO2 in the atmosphere being much longer than that of heat. Another being that one needs to take the total CO2 input to the atmosphere from burning fossil fuels into account, not just "the next coal-fired power plant" (200 GW in orbital solar power would be huge already, and still only be about one millionth of the total solar power input to Earth). Yet another one being that Earth temperature only rises with fourth root of energy input.
A 200MW windfarm cost $2-300 Million, just a wild guess but I think the insurance bill alone for "4-5 heavy lifts" would be comprable. Their SBP project is nothing more than a gee-wizz hook to get brand recognition.
And did you exchange a walk on part in the war for a lead role in a cage? - Pink Floyd.