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apolloman
2008-Sep-15, 08:29 AM
Morning all.

I read an article on Space.com this morning about an experiment carried out by a Mr. John C. Mankins who demonstrated solar power transmission by transmitting 20 watts of power between two islands.
Despite the fact that less than one one-thousandth of a percent of the power actually arrived, technology like this could have a huge impact if developed to beam energy from orbiting solar panels to power stations on Earth.

Could somebody explain how this works ? how do you beam power back to Earth from space ? By laser ?
Is it a feasible tecnology to invest in or could it become one?

It sounds like the perfect "free" fuel for the future but I'm no expert so wanted some opinions or ideas.

Thanks

Van Rijn
2008-Sep-15, 08:56 AM
Could somebody explain how this works ? how do you beam power back to Earth from space ?


Not much different from the way the sun "beams" power to Earth from space, and receivers are used to convert it to electricity. Though in this case, it might be microwaves, or a tight laser beam. Years ago, Peter Glaser suggested building solar power satellites that would convert sunlight to microwaves, then send the microwaves to large rectifying antennas on Earth. There are some advantages and some disadvantages over just using PV panels on Earth.

Some advantages: There is no weather in space, the solar power satellite can be put in an orbit where it receives constant sunlight, and the microwaves sent to earth don't care that much about weather (there is some loss of energy in heavy weather). So it's a constant power source.

The key disadvantages: Complexity and cost. Right now, cost is far too high to compete with other energy options.



Is it a feasible tecnology to invest in or could it become one?


For the next few decades, for general energy use, I'd say no. Perhaps for specialized purposes, like powering spacecraft or even some aircraft. Eventually, late in the century, maybe, but I think it's likely that other options will always have economic advantages over this.

astromark
2008-Sep-15, 09:00 AM
You ask for opinion and ideas....
So no, I do not see this as a working technology yet. I would not invest in this. The space elevator sounds reasonable compared to this....
As for ideas... Yes a recent article regarding solar powered electrical energy was clean and green and almost free. Cobalt electrodes in water powered from solar panels converting water to hydrogen and oxygen... sounds good for me.

Grashtel
2008-Sep-15, 09:04 AM
The idea for Solar power satellites (http://en.wikipedia.org/wiki/Solar_power_satellite) has been around for quite a while. The most commonly proposed way of getting power from the satellite to the ground is by microwave power transmission (http://en.wikipedia.org/wiki/Microwave_power_transmission) while has a theoretical maximum efficency of 90%, though other methods have been proposed including lasers and running super conducting cable down a space elevator (http://en.wikipedia.org/wiki/Space_elevator).

AndreH
2008-Sep-15, 10:47 AM
Morning all.

I read an article on Space.com this morning about an experiment carried out by a Mr. John C. Mankins who demonstrated solar power transmission by transmitting 20 watts of power between two islands.
Despite the fact that less than one one-thousandth of a percent of the power actually arrived, technology like this could have a huge impact if developed to beam energy from orbiting solar panels to power stations on Earth.

Could somebody explain how this works ? how do you beam power back to Earth from space ? By laser ?
Is it a feasible tecnology to invest in or could it become one?

It sounds like the perfect "free" fuel for the future but I'm no expert so wanted some opinions or ideas.

Thanks

Regarding feasibility: Even if the transmission of the power is solved, PV satellites have a problem that can not be overcome in principle.
You already need a lot of energy to bring everything into orbit.
Obviously Solar cells make only sense if they produce during theri life time more energy as is needed to manufacture the cells (or more correct the whole module including mounting brackets, frames, screws...).
Only relatively shortly these was achieved. Todays modern thin film panels are said to reach the "break even" within 3-8 years.

I feel (without calculating) this will change dramatically if you put them into orbit (geo sync?).

If you are interested and have some patience, I could dig up some numbers for you.

This is one of those solutions which only look elegant at the first glance (like the shuttle;) )

apolloman
2008-Sep-15, 11:33 AM
Thanks for the replies !

AndreH : I have the patience and interest to wait if you're willing to do the numbers bit... that would be really great.
:lol:

JustAFriend
2008-Sep-15, 02:01 PM
I read an article on Space.com this morning about an experiment carried out by a Mr. John C. Mankins who demonstrated solar power transmission by transmitting 20 watts of power between two islands.


You need to watch basic cable.

Discovery Channel just ran a program as part of their "Project Earth" series showing the whole thing last week.
Explained the process and showed the beam and they even flew a helicopter through the beam at various points along the way to make sure the beam was staying focused.
- You may even be able to see the episode online at discoverychannel.com, but I had problems downloading their video player plugin

Here's some more history from Wikipedia:

http://en.wikipedia.org/wiki/Wireless_energy_transfer

Note that they did tests up to tens of kilowatts back in 1975....

marsbug
2008-Sep-15, 02:01 PM
My gut reaction is that there may be some niches it could fill, but for the foreseeable future it is not feasable for large scale enrgy production.

AndreH:

You already need a lot of energy to bring everything into orbit.
Obviously Solar cells make only sense if they produce during theri life time more energy as is needed to manufacture the cells (or more correct the whole module including mounting brackets, frames, screws...).
Only relatively shortly these was achieved. Todays modern thin film panels are said to reach the "break even" within 3-8 years.

Thats a good argument, but the ability to access the transmitted energy from anywhere (assuming a ring of sattelites in geostationary orbit, and that they can focus the energy transmitted with some accuracy when asked to do so) with a line of sight to geostationary orbit might be enough in some cases to make that overlookable. Disaster relief comes to mind.

apolloman
2008-Sep-15, 02:30 PM
Thanks just a friend, no cable available unfortunately. I'll check wikipedia though.

AndreH
2008-Sep-15, 04:54 PM
Thanks for the replies !

AndreH : I have the patience and interest to wait if you're willing to do the numbers bit... that would be really great.
:lol:

Maybe some of the space interested guys can give a figure for the energy to bring 1 kg into geo sync without doing the maths?

mugaliens
2008-Sep-15, 05:35 PM
On the sunniest of days, the Earth's atmosphere attenuates 83% of the Sun's energy. On average, including nights, seasons, and weather, it's closer to 95%. Thus, one would conclude that putting these bad boys in space would mean an increase in efficiency by a factor of 20x, right?

No.

As another already pointed out, one must factor in the cost of manufacturing, assembly, and transport to desired location. Right now the payback period is 3-5 years. When you throw them into orbit, even with the 20-fold increase in power output per cell, you're also looking at an elongation of the payback period from 3-5 years to something beyond 20 years.

Payback period isn't a measure of rate of return on investment. Rather, it's a measure of investment risk. The shorter the payback period, the lower the risk.

For example, what would happen if we sank $100 Billion in solar-powered satellite technology, and ten years into the project, the day before the first launch, someone annouced a breakthrough whereby a simple microbe, when watered, would turn an entire desert floor into one giant solar cell in the space of a couple of months (payback period) and at a cost of perhaps $1000 per acre? That would pretty much render the $100 Billion as a sunk cost, and the right decision would be to halt the project and switch gears, as the alternative investment just became a whole lot more attractive than the incredibly expensive approach with the very long payback period.

If we could ever make them light enough, we may well float an entire solar cell farm into the stratosphere, tethered to Earth, but beaming the power back via microwave (cheaper than 50 miles of conducting cable), and the farm would have on-board helium generation to replenish that lost through diffusion. While such an effort would be many times less expensive than putting the cells in orbit, it would still be expensive, and with a longer payback than many current alternatives.

KaiYeves
2008-Sep-16, 01:35 AM
It's a good idea, and eve appeared in Marvel Adventures Iron Man!

AndreH
2008-Sep-17, 12:13 PM
Thanks for the replies !

AndreH : I have the patience and interest to wait if you're willing to do the numbers bit... that would be really great.
:lol:

First step:

I want to calculate the enrgy needed to bring a mass m into an orbit r.

Therefore we need the total energy of this mass on earth surface (lets assume at the equator). And the total energy when it is the orbit.

For the total energy (potential + kinetic) in an orbit with Radius r I have derived Etoto = -1/2 G m M /r

The total Energy on the surface of the Earth at the equator is:

Etots = - G m M / R + 1/2 m vrot^2 where vrot is the rotation speed of the Earthat the equator.

So for the difference between orbit and surface I do get:

Ediff = Etoto - Etots = -1/2 G m M/r + G m M/R - 1/2 m vrot^2 (edited for clarification it must say vrot not v))

= G m M(1/R-1/2r) -1/2 m vrot^2

where G is the gravitational constant, M = Earth's mass, R = radius of the earth and vrot = 42 000 km/24 h which is 486 m/s. r is the radius for the geo sync. orbit.

Before we plug in the numbers, I would kindly ask someone to verify the formula. My math is rusty. My knowledge about formulas also. I did not find a good formula somewhere so I did it all myself.

Thanks for any corrections.

apolloman
2008-Sep-17, 12:20 PM
AndreH : Thanks for the reply. I have no idea if the maths is good, but I appreciate the time you took to do this
I will check it over asap.:lol:

AndreH
2008-Sep-17, 03:01 PM
snip.....where G is the gravitational constant, M = Earth's mass, R = radius of the earth and vrot = 40 000 km/24 h which is 463 m/s. r is the radius for the geo sync. orbit.

.....snip

Thanks for any further corrections.
---

samkent
2008-Sep-17, 03:44 PM
If we assume that the orbit of the collector does not “shade” the Earth from any sun light, here’s another problem.

1 megawatt beamed to Earth would raise the temperature of 822000 gallons of water by 12 degrees in a 24 hour period.

That my friend is global warming!

AndreH
2008-Sep-17, 05:36 PM
If we assume that the orbit of the collector does not “shade” the Earth from any sun light, here’s another problem.

1 megawatt beamed to Earth would raise the temperature of 822000 gallons of water by 12 degrees in a 24 hour period.

That my friend is global warming!

But if we assume that we simply replace all power production on earth, than nothing will change.
All power electrical power we produce currently already ends up as heat.

Meanwhile I dug up a number for power generated by square meter of photovoltaic cells in space.

This link says http://www.zdf.de/ZDFmediathek/content/216230?inPopup=true it are 125.000 kW/4500 m^2. This is rounded up, 30 W/m^2.

So now I need an idea what one square meter of "space type" solarcells does weigh.
Any ideas?

cjl
2008-Sep-18, 12:09 AM
30W/m2 sounds low, as the solar intensity at earth is around 1370W/m2, and solar cells should be more than 2.5% efficient.

Oh, and Mugaliens, where did you get that 80%+ figure for atmospheric attenuation? From what I can find, solar intensity at sea level varies from 1,047W/m2 (ideal case, sun at zenith, low turbidity and water vapor content) to as low as 161W/m2 for high water vapor, high turbidity, and an angle of 80 degrees off of the zenith. For all normal values for a sunny day, 500-800W/m2 seem easily attainable, which is around 40-60% of the 1370w/m2 that is supplied by the sun at this distance, hardly the 80% attenuation figure you quoted.

(source for solar irradiance here (http://www.jgsee.kmutt.ac.th/exell/Solar/Intensity.html#s2))

Van Rijn
2008-Sep-18, 12:24 AM
Regarding feasibility: Even if the transmission of the power is solved, PV satellites have a problem that can not be overcome in principle.
You already need a lot of energy to bring everything into orbit.


Since getting material up into Earth orbit was such a problem, Gerard K. O'Neill argued that most of the material should be brought down to Earth orbit from the moon or asteroids. The energy cost is far less that way, and there are more efficient transport methods (such as using a mass driver instead of a rocket). In practice, of course, building the space infrastructure for all that he proposed would still be very expensive, but if we were to build solar power satellites, I think extraterrestrial material would have to play a part.

Tim Thompson
2008-Sep-18, 12:58 AM
Could somebody explain how this works ? how do you beam power back to Earth from space ? By laser ? Is it a feasible tecnology to invest in or could it become one?
This is an idea that has been around for a long time. Quite regardless of how one gets the power, beaming it back to Earth turns out to be a hurdle too great to cross with existing technology.

My brother worked on this problem 30 years ago as an electrical engineer for a major aerospace company in competition for a contract to do just that, beam power back to earth from orbit, as microwaves (this is the most efficient way to do it). The problem they could not overcome was the need for coherency across the surface of a very large antenna. Destructive interference between different parts of the same antenna kills the power. Since it's microwaves, they have to be spread over a large area to avoid health & safety issues on the ground, and in airplanes that might fly through the beam. So that is another worrisome issue.

Lasers are cutting edge technology for spacecraft communications, but I don't think they will work for this application. A laser requires power to be concentrated over a relatively small area, so they are in effect too powerful. You are basically pointing a weapons grade laser at a receiver that will convert the power from the laser to something useful. What would that be without melting? And what if "oops" points the weapons grade laser somewhere unintended?

They even considered running a cable down from orbit, but aside from impedence issues, there are obvious mechanical problems with this plan.

As far as I know, the technological problems remain sufficiently large that none of the expert companies in the field are working on the problem (at least openly) any more.

samkent
2008-Sep-18, 01:03 AM
In practice, of course, building the space infrastructure for all that he proposed would still be very expensive, but if we were to build solar power satellites, I think extraterrestrial material would have to play a part.

We don't even do that in Antartica. What makes you think we will ever do that in space?

What did the old sailing ships obtain in the new world? Food, water and small wood supplies. How many decades was it before they built entire ships?

Nope we will just haul the solar panels up to orbit. We know how to do that and we know it would be cheaper.

AndreH
2008-Sep-18, 06:57 AM
30W/m2snip.... sounds low, as the solar intensity at earth is around 1370W/m2, and solar cells should be more than 2.5% efficient.....snip
(source for solar irradiance here (http://www.jgsee.kmutt.ac.th/exell/Solar/Intensity.html#s2))


I agree with you, it seems to be low. But it was the only figure I found so far which states power production AND area. For the ESA ATV I found 3800 Watt from 4 panels. The "span width" is 22 meters, this includes 2 panels plus the ship. The ship itself having a dieameter of around 4+ meter. Looking at the pictures and taking acount of the beams that hold the panels one could. gues one panel has a length of about 8 meters.
But there is no way to tell the width because all pictures I found so have perspective.

Now we can make an estimation. Looking at the picture it is clear that the panels have at least 1 m in width (rather 2, but lets look at the lower limit of the area to get the upper limit of W/m^2).

This yields about 120 W/m^2 which would be4 times the amount of the figures ISS. But as I said this is the upper limit. If the panels are really 2 m wide it would only be.

Eventually they look at average levels because of shading of the Earth or something.
There also might be a problem with the definition of the efficiency.
Todays modern (commercial !) thin film cells have 10 % as an upper limit. Si crytaline cells may come to 15 % (quite high). I AM TLKING ABOUT COMMERCIALY AVAILABLE BIG AREA PANELS, NOT ABOUT LAB THINGS! )Ofcourse there are some special GaAs cells which go up to 30 %. The latter are currently far to expensive and far to energy absorbing in production to be used on earth. In fact these are some times used for space applications. But another source I found while searching the web yesterday said that the ISS has only Si type with around 15%.

Now comes the problem. Solar cells can only use a part of the suns spectrum. So for the commercial cells the efficiency is defined by some standard light conditions on earth. (I do not know which standard conditions exactly). Obviously on earth a good part of the UV spectrum is shielded by the atmosphere.
If the figure you got (1370 W/m^2) includes the complete spectrum is space, the effiency might be quite different.

My lacking knowledge abot the exact definition of the efficiency made me search a source for "practical numbers". Means real power prodcution on real space installations.

I also know figures of earth bound solar panels The power production is defined by by the so called (Wp). This is again power production under certain clearly defined conditions which I do not know exactly how it is defined.

So again I thiught it would be better to find some real numbers instead of digging up definitions.

Anyway, for my further calculations I will stick with 120 Watt/m^2.

If anyone finds better numbers (and is interested in this discussion) I would be happy to use them.

AndreH
2008-Sep-18, 07:07 AM
Since getting material up into Earth orbit was such a problem, Gerard K. O'Neill argued that most of the material should be brought down to Earth orbit from the moon or asteroids. The energy cost is far less that way, and there are more efficient transport methods (such as using a mass driver instead of a rocket). In practice, of course, building the space infrastructure for all that he proposed would still be very expensive, but if we were to build solar power satellites, I think extraterrestrial material would have to play a part.

For my part I would like to stay with the near future. Currently we do not have any space infra structure.
Have a look at the history of the US. From the declaration of independence until there was something as a nation wide rail road net it took approx. 100 years.
I am in big doubt any one on earth would be willing to spent money on this, currently. So for the neaar future (the famous 50 years) I do not see a chance.

AndreH
2008-Sep-18, 07:20 AM
30W/m2 snip....(source for solar irradiance here (http://www.jgsee.kmutt.ac.th/exell/Solar/Intensity.html#s2))

nice link cjl!

AndreH
2008-Sep-18, 10:38 AM
Still was not able to find weight of "space type" solarcells. I have some figures for earth bound ones. They weig about 12 - 15 kg per m^2. Including frames and so on.

To go on and get some idea I will assume that for space applications they are able to make them about 10 times lighter.

So that means I will use 1 kg/m^2 which in turn means 1 kg of solar cells in orbit would produce 120 Watts.

Now we can plug in the numbers and see how long they must stay in orbit to retutn the energy needed to lift them up. No idea what will come out. I'am very curious.

See if I will find the time tonight.

Ronald Brak
2008-Sep-18, 10:48 AM
Ofcourse there are some special GaAs cells which go up to 30 %. The latter are currently far to expensive and far to energy absorbing in production to be used on earth.

Concentrating photoelectric solar systems can make use of high efficiency, high cost multijunction solar cells. They can be economically worth it worth it because sunlight is concentrated by mirrors so they have a large area of low cost mirror and a tiny area of expensive solar cell. Their efficiency can be over 30%. A 154 MW solar power station in Victoria Australia plans to use this technology starting in 2010.


If anyone finds better numbers (and is interested in this discussion) I would be happy to use them.

About a 1,000 watts of sunlight will hit a meter square panel orientated towards the sun on a clear day. You can find out how much will average sunlight energy will hit a fixed square meter by looking at an insolation map. These maps take into account weather conditions. There's one for the United States on the Wikipedia page for insolation:

http://en.wikipedia.org/wiki/Insolation

Looking at the map you can see that in Florida, a square meter angled towards the sun receives about 5 kilowatt-hours a day on average. If that square meter were covered in 10% efficient solar cells it would generate about 0.5 kilowatt-hours a day. Note this map is for January, which is winter in the US.

AndreH
2008-Sep-18, 11:55 AM
Concentrating photoelectric solar systems can make use of high efficiency, high cost multijunction solar cells. They can be economically worth it worth it because sunlight is concentrated by mirrors so they have a large area of low cost mirror and a tiny area of expensive solar cell. Their efficiency can be over 30%. A 154 MW solar power station in Victoria Australia plans to use this technology starting in 2010.

AndreH
I know this type of cells exist. But the concentrator mirrors are far to heavy to be put into space. Thats why I did not mention that. Also I do not want to start nit picking on 2 or 3 percent. The field is hot. So everyday something new can hapenn pushing the efficiency anoter 1 or 2 percent. But it has to make it to mass production. Also very often people give the cell efficiency but not the module efficiency which can be different by several percentages. The module is the whole thing with mounting frames, eventually front electrodes and so on. These reduce the "active" area

About a 1,000 watts of sunlight will hit a meter square panel orientated towards the sun on a clear day. You can find out how much will average sunlight energy will hit a fixed square meter by looking at an insolation map. These maps take into account weather conditions. There's one for the United States on the Wikipedia page for insolation:

http://en.wikipedia.org/wiki/Insolation

Looking at the map you can see that in Florida, a square meter angled towards the sun receives about 5 kilowatt-hours a day on average. If that square meter were covered in 10% efficient solar cells it would generate about 0.5 kilowatt-hours a day. Note this map is for January, which is winter in the US.


AndreH
You miss the point. I was talking about solar cells in orbit. And I wanted to avoid all misunderstanding that can happen by different definitions for efficiency or energy coming from the sun per square meter (what are we talking about? visible only? infrared + UV). I thought it would be faster to get some numbers for real space installations. The numbers for the ATV are from ESA home page. You can feel free to check them.
I want to calculate the "break even" for solar cells in orbit. That menas the time they have to be working until they returned the energy which was needed to put them there.

BTW: You don't have to convince me. I am pro solar. It is feasible on earth, absolutely!!!:)

ETA: THis http://www.solarcontact.de/content/news/detail.php4?id=1492German link from mid of May this year states a new record for concentrator cells (carefull, cells not modules). "230 Watt of Sunlight have been converted into 70 Watt of power at IBM Watson research centre". So this is the lab state of the art, obviously. Just for the records this are ~33 %.

Larry Jacks
2008-Sep-18, 12:35 PM
A satellite in geostationary orbit will be in sunlight over 99% of the time. The only times it is shaded is during eclipse season or during a lunar eclipse. There are two eclipse seasons each year centered on the vernal and autumnal equinox. The maximum duration occurs on the equinox and lasts about 72 minutes. The eclipse season starts a few weeks before the equinox (lasting only a few minutes) and builds in duration as you approach the equinox, then tapers off over the following few weeks. A lunar eclipse isn't as regular but can last longer.

Silicon solar cells probably wouldn't be a good choice. At geosynch, the environment is harsh and silicon cells degrade about 3% per year. GaAs cells are more expensive but they're also more robust and efficient. Combined with concentrator mirrors such as a Freznel lens (this has already been used successfully in space on DS-1 and others), you get more power for a given collector size and mass.

Geosynch has a big advantage over other orbits in that your antenna pointing requirements are very simple. I've heard some people discuss putting the solar cells on the moon but this makes the antenna design much harder.

Preventing destructive signal interference would be a major challenge. You'll also either want the signal to be diffuse enough to not harm aircraft passing through the beam or to set aside prohibited airspace around the antenna farms.

AndreH
2008-Sep-18, 01:09 PM
[QUOE=AndreH;1325457]= G m M(1/R-1/2r) -1/2 m vrot^2

where G is the gravitational constant, M = Earth's mass, R = radius of the earth and vrot = 40 000 km/24 h which is 463 m/s. r is the radius for the geo sync. orbit.

Now lets plug in some numbers:
R = 6 370 000 m
r = 42 157 000 m
M = 5.9376 x 10^24 kg
G = 6.6743 x 10^-11 m^3/(kg s^2)
m = 1 kg

putting the numbers in I get = 57,405,047 Joule to put 1kg into orbit.

If I go with my 120W/kg than I get 134 hours to break even!

This is several magnitudes shorter than I thought (if the math is correct).
Well I must admit I am really surprised. So this is obviously no point.

So than we have to look at the cost to put something there, physics itself seems not to contradict.





[/QUOTE]

AndreH
2008-Sep-18, 01:21 PM
A satellite in geostationary orbit will be in sunlight over 99% of the time. The only times it is shaded is during eclipse season or during a lunar eclipse. There are two eclipse seasons each year centered on the vernal and autumnal equinox. The maximum duration occurs on the equinox and lasts about 72 minutes. The eclipse season starts a few weeks before the equinox (lasting only a few minutes) and builds in duration as you approach the equinox, then tapers off over the following few weeks. A lunar eclipse isn't as regular but can last longer.

AndreH:
That is clear. I did discuss shading because I do not understand really the figures I get from ISS and ATV in terms of W/m^2.

Silicon solar cells probably wouldn't be a good choice. At geosynch, the environment is harsh and silicon cells degrade about 3% per year. GaAs cells are more expensive but they're also more robust and efficient. Combined with concentrator mirrors such as a Freznel lens (this has already been used successfully in space on DS-1 and others), you get more power for a given collector size and mass.

AndreH:
I agree concentrator cells would work in space. My concern was the weight. Now obviously if my calculation is correct, this is not such a big concern.

Geosynch has a big advantage over other orbits in that your antenna pointing requirements are very simple. I've heard some people discuss putting the solar cells on the moon but this makes the antenna design much harder.

Preventing destructive signal interference would be a major challenge. You'll also either want the signal to be diffuse enough to not harm aircraft passing through the beam or to set aside prohibited airspace around the antenna farms.

AndreH:
I am still very sceptic about the feasibality when I look at the cost of going into orbit. Also repair and maintenance

----

AndreH
2008-Sep-18, 03:40 PM
:o:doh::wall: There is a big flaw in my calculation. Basic space technology! I also have to carry the fuel which is needed to bring the 1 kg up.....

My math is for sure to rusty to do calculation for that. I remember we did that once in the first semester.
Maybe someone finds it somewhere in the net or knows it right away

cjameshuff
2008-Sep-18, 04:26 PM
If we could ever make them light enough, we may well float an entire solar cell farm into the stratosphere, tethered to Earth, but beaming the power back via microwave (cheaper than 50 miles of conducting cable), and the farm would have on-board helium generation to replenish that lost through diffusion. While such an effort would be many times less expensive than putting the cells in orbit, it would still be expensive, and with a longer payback than many current alternatives.

How do you propose to generate helium in the upper atmosphere? Not that it'd be that necessary, hydrogen from atmospheric water should do the job fine. You still only get a few hours out of every 24 of peak sunlight, though.

A similar concept I've mulled over is a tethered wind farm in one of the stable and predictable jetstreams. Essentially a really, really big kite with turbines and, for weight and cost reasons, microwave power links to the ground. Possibly with helium balloons that could be deployed temporarily to lift it to altitude or bring it down for service. Should give more power than a planet-bound solar farm, and more consistently.

Note that solar power satellites don't have to be in GEO. Stations in lower orbits would have to switch to ground stations around the globe, but efficiently transferring power to those stations would be easier, servicing the stations would be more practical, and lifting components from Earth would be cheaper. Also, that last cost can drop a great deal if lunar or near-earth asteroid resources are used for the station.

And finally, I am not a fan of photovoltaics, they are unavoidably inefficient, limited in lifespan, and expensive and messy to produce. Solar thermal faces its own problems in space (the lack of a good place to sink large quantities of waste heat into...you need big radiators), but could achieve greater efficiency and longer life at lower initial cost. And the parts are easier to make from materials readily available in space...largely steel, aluminum, iron, and glass, as opposed to precisely doped and layered ultra-pure semiconductor materials.

mugaliens
2008-Sep-18, 07:08 PM
Inadvertant duplicate - please delete.

mugaliens
2008-Sep-18, 07:08 PM
:o:doh::wall: There is a big flaw in my calculation. Basic space technology! I also have to carry the fuel which is needed to bring the 1 kg up.....

My math is for sure to rusty to do calculation for that. I remember we did that once in the first semester.
Maybe someone finds it somewhere in the net or knows it right away

Here - Try this (http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation).

AndreH
2008-Sep-18, 08:45 PM
Here - Try this (http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation).

Yes, this may help!! rocket equation! It is amazing how much one forgets in 25 years. It is always annoying to search something if you do not know the exact name.

cjameshuff
2008-Sep-18, 11:08 PM
My brother worked on this problem 30 years ago as an electrical engineer for a major aerospace company in competition for a contract to do just that, beam power back to earth from orbit, as microwaves (this is the most efficient way to do it). The problem they could not overcome was the need for coherency across the surface of a very large antenna. Destructive interference between different parts of the same antenna kills the power. Since it's microwaves, they have to be spread over a large area to avoid health & safety issues on the ground, and in airplanes that might fly through the beam. So that is another worrisome issue.

30 years ago, it was also generally impractical to mass produce chip antennas you could hide under a fingernail, or to design high frequency filters as etched patterns in printed circuit boards, or incorporate spread spectrum radio links in toys and cheap consumer electronics. The processing power and algorithms available for computer aided design and control systems have advanced enormously in the last 30 years. 30 years ago, GPS was also impossible...there were satellite navigation systems, but they were much cruder and less capable.

However, you are right in that a single large emitter, or more likely (for beam shaping/steering) a tightly packed array of emitters is needed. You can combine a large number of small transmitter elements spread out over a wide area to achieve the same beam width of a single large transmitter, but the beam intensity will be no higher...you would be better off packing them close together and dealing with the wider beam by using larger collectors. You get a fatter beam, but the intensity is the same and you waste less energy in side patterns. (This is called the "sparse array curse".)

Van Rijn
2008-Sep-18, 11:15 PM
For my part I would like to stay with the near future. Currently we do not have any space infra structure.


We were discussing building solar power satellites that would, one way or another, require a large space infrastructure. In that case, a lunar mass launcher, or an expedition to a low delta-v asteroid, look pretty good compared to lifting it all from the Earth.



I am in big doubt any one on earth would be willing to spent money on this, currently. So for the neaar future (the famous 50 years) I do not see a chance.

Yes, and I wasn't saying that any of these methods made sense currently. I simply said that the lunar option makes more sense than lifting it all directly off of Earth.

Ronald Brak
2008-Sep-19, 12:05 AM
You miss the point.

Sorry.

Ronald Brak
2008-Sep-19, 12:25 AM
We already have solar power recieving stations built in various places around the world. These are in the form of photovoltaic and solar thermal power stations. To avoid the cost of building ground receivers, space based solar power could send light onto them. There are many problems with this. It is extremely difficult to concentrate sunlight onto a small area such as a solar farm as the sun is not a point source of light, lasers have some problems and using LEDs and mirrors would be difficult and I'm not sure if it would be practical at all. But the advantage is that the light could be sent to where ever it is needed most and the space based power company could aways get paid peak rates. Of couse then they would need the capacity to aim their light quickly and cheaply.

This sort of thing only makes sense if we can build things cheaply in space and are unwilling to use the same ability to make things cheaply on earth. This seems unlikely, but I suppose it is a very remote possibility.

a1call
2008-Sep-19, 12:45 AM
You could put stuff in orbit with light as the fuel:


spacecraft that ride on a laser beam into space, require little or no onboard propellant and create no pollution. Sounds pretty far-fetched, considering we haven't been able to develop anything close to that for conventional ground- or air-travel on Earth. But while it may still be 15 to 30 years away, the principles behind the lightcraft have already been successfully tested several times. A company called Lightcraft Technologies continues to refine the research that began at Rensselaer Polytechnic Institute in Troy, N.Y.

Source (http://science.howstuffworks.com/light-propulsion1.htm)

Video from the above source (http://www.howstuffworks.com/framed.htm?parent=light-propulsion.htm&url=http://www.lightcrafttechnologies.com/gallery_video.html)

mugaliens
2008-Sep-20, 01:22 PM
Yes, this may help!! rocket equation! It is amazing how much one forgets in 25 years. It is always annoying to search something if you do not know the exact name.

:)

I was lucky - it was one of the "See also" links at the bottom of another Wiki page to which I was referring at the time.

Noclevername
2008-Sep-22, 08:05 PM
You could put stuff in orbit with light as the fuel:



Source (http://science.howstuffworks.com/light-propulsion1.htm)

Video from the above source (http://www.howstuffworks.com/framed.htm?parent=light-propulsion.htm&url=http://www.lightcrafttechnologies.com/gallery_video.html)

At the moment, reaching orbit by laser is blue-sky (pun) technology. A lot of problems would have to be overcome to make it remotely feasible, from a sufficiently reliable and powerful laser to put any payload more than a few grams at orbital height, let alone orbital speed, to the focussing technology to compenstate for atmospheric distortion; for any plausible near-term tech, it's just not happening anytime soon. The 30 year estimate is a very optimistic one.

cjameshuff
2008-Sep-22, 08:49 PM
At the moment, reaching orbit by laser is blue-sky (pun) technology. A lot of problems would have to be overcome to make it remotely feasible, from a sufficiently reliable and powerful laser to put any payload more than a few grams at orbital height, let alone orbital speed, to the focussing technology to compenstate for atmospheric distortion; for any plausible near-term tech, it's just not happening anytime soon. The 30 year estimate is a very optimistic one.

Ground-to-orbit via laser propulsion, yeah. However, give the upper stage some photovoltaic arrays and an ion engine, and it might use ground lasers to get it where it needs to be faster and without as much solar panels as would be needed otherwise. Meaning more of the final payload as payload instead of fuel or unneeded solar panel area. And it would be a good way to work the kinks out of laser power and tracking.

Digix
2008-Sep-22, 09:05 PM
Could somebody explain how this works ? how do you beam power back to Earth from space ? By laser ?
Is it a feasible tecnology to invest in or could it become one?

It sounds like the perfect "free" fuel for the future but I'm no expert so wanted some opinions or ideas.

Thanks

that wont work at all, because lots of energy will be lost during transmission

however it have some potential if we use big mirror to redirect sunlight into earth, if the mirror is very big we can even concentrate sunlight into one relatively small spot of around kilometer in diameter where we use solar cells or we can just use that huge mirror for lighting streets at night.

since there is no weight in space all we need is thin foil to make mirror.

using laser or microwawe is theoretically useless, laser beam is not so perfect when we use high power lasers, and efficiency is too low.
only free electron laser is more or less usable but it weights a lot and still a question how to make gigawatt range lasers

microwave beam is infocusable unless you use incredible size antennas you will not get any spot on earth smaller than many km in diameter

mugaliens
2008-Sep-22, 09:45 PM
Ground-to-orbit via laser propulsion, yeah. However, give the upper stage some photovoltaic arrays and an ion engine, and it might use ground lasers to get it where it needs to be faster and without as much solar panels as would be needed otherwise. Meaning more of the final payload as payload instead of fuel or unneeded solar panel area. And it would be a good way to work the kinks out of laser power and tracking.

Why spend all that money lasering it when we have that massive multi-spectral nuclear illumination we call Sol?

cjameshuff
2008-Sep-22, 10:05 PM
Why spend all that money lasering it when we have that massive multi-spectral nuclear illumination we call Sol?

Because sunlight is only available at a fixed and rather low intensity, requiring large solar arrays, with the associated weight penalties and the difficulties associated with unfolding and aiming them.

It being multispectral does not help. It makes it harder to absorb at high efficiency...a photovoltaic panel matched to the laser illuminating it will convert far more of the light to electricity. However, even a craft with broad spectrum panels would benefit from increased power while maneuvering into its final orbit, being able to run its engine at higher power during that time without requiring solar panel area sufficient to do it with solar power. A ground-powered ion engine craft might even be able to reach orbit from a suborbital trajectory.

mugaliens
2008-Sep-24, 09:32 PM
Because sunlight is only available at a fixed and rather low intensity, requiring large solar arrays, with the associated weight penalties and the difficulties associated with unfolding and aiming them.

I was responding to: "However, give the upper stage some photovoltaic arrays and an ion engine, and it might use ground lasers to get it where it needs to be faster and without as much solar panels as would be needed otherwise."

Again, why use lasers? Sol's radiation above our atmosphere is 6 times stronger than what makes it through to our surface.

Just use Sol.

cjameshuff
2008-Sep-24, 10:20 PM
Again, why use lasers? Sol's radiation above our atmosphere is 6 times stronger than what makes it through to our surface.

The answer's right there in the portion you quoted: "to get it where it needs to be faster and without as much solar panels as would be needed otherwise". Solar intensity in orbit relative to the 24 hour average ground intensity is irrelevant...it's still broad spectrum and low energy density.

Once again:
With a ground-based beam power assist, you can reduce the panel size and cost, reduce the panel mass even more due to the smaller panels not needing as much structural strength, and allocate that mass and space to useful payload instead, or deliver more power to an ion drive, speeding insertion into the proper orbit or even reaching orbit from a suborbital trajectory.

This is the third time I've said this, and I don't know how to make it any clearer. High efficiency solar panels are expensive, and launch mass is expensive. Laser illumination can deliver far more power per unit area, allowing smaller solar panels to be used, reducing equipment and launch cost, or allowing a more powerful ion drive to be used for the same powerplant mass, getting the payload to the destination faster and allowing more of the work to be done by the high-efficiency ion drive, again saving launch mass and cost.

cjl
2008-Sep-25, 03:11 AM
Again, why use lasers? Sol's radiation above our atmosphere is 6 times stronger than what makes it through to our surface.

Where are you getting this 6x figure? I already posted this in another thread: http://www.jgsee.kmutt.ac.th/exell/Solar/Intensity.html

Solar intensity outside the earth's atmosphere = 1.37kw/m^2.

Solar intensity at sea level: 0.161-1.047 kw/m^2 (0.698-1.047 kw/m^2 for the sun at zenith, depending on humidity and turbulence).

These numbers show that with the exception of the cases where the sun is either quite low on the horizon or when there is an extremely high level of turbulence and water vapor content, the solar irradiance at sea level is well over the figure you keep stating (1/6 that outside the atmosphere), and in reality, 50-70% is a closer figure for the amount that makes it through to sea level (and this number only increases at altitude).

cjameshuff
2008-Sep-25, 11:59 AM
These numbers show that with the exception of the cases where the sun is either quite low on the horizon or when there is an extremely high level of turbulence and water vapor content, the solar irradiance at sea level is well over the figure you keep stating (1/6 that outside the atmosphere), and in reality, 50-70% is a closer figure for the amount that makes it through to sea level (and this number only increases at altitude).

That sounds like a 24 hour average. Half the time, it's dark enough ground panels deliver virtually no power, and most of the rest of the time it is much lower than ideal noontime. But the average power available to a panel on the ground is a completely useless number in this case...what's being compared is solar panels in orbit and laser collector panels in orbit.

AndreH
2008-Sep-25, 01:01 PM
We were discussing building solar power satellites that would, one way or another, require a large space infrastructure. In that case, a lunar mass launcher, or an expedition to a low delta-v asteroid, look pretty good compared to lifting it all from the Earth.

Yes, and I wasn't saying that any of these methods made sense currently. I simply said that the lunar option makes more sense than lifting it all directly off of Earth.

Ok, I see your point. I was biased because looking at the OP (as I understood it) I assumed we are talking about near future possibilities and investment in such technologies.
And I agree that if one has decided to go for a large space infra structure, it will make a lot of sense to use something like a moon mass launcher.

AndreH
2008-Sep-25, 01:05 PM
:)

I was lucky - it was one of the "See also" links at the bottom of another Wiki page to which I was referring at the time.

It didn't help so far. I found that I lack to much information to do a meaningful calculation (remember, I wanted to calculate the energy needed to put 1 kg into orbit). And I do not have the time to dig it all up.
:sad:

AndreH
2008-Sep-25, 01:16 PM
The answer's right there in the portion you quoted: "to get it where it needs to be faster and without as much solar panels as would be needed otherwise". Solar intensity in orbit relative to the 24 hour average ground intensity is irrelevant...it's still broad spectrum and low energy density.

Once again:
With a ground-based beam power assist, you can reduce the panel size and cost, reduce the panel mass even more due to the smaller panels not needing as much structural strength, and allocate that mass and space to useful payload instead, or deliver more power to an ion drive, speeding insertion into the proper orbit or even reaching orbit from a suborbital trajectory.

This is the third time I've said this, and I don't know how to make it any clearer. High efficiency solar panels are expensive, and launch mass is expensive. Laser illumination can deliver far more power per unit area, allowing smaller solar panels to be used, reducing equipment and launch cost, or allowing a more powerful ion drive to be used for the same powerplant mass, getting the payload to the destination faster and allowing more of the work to be done by the high-efficiency ion drive, again saving launch mass and cost.


Again we are at apoint where we would need numbers. How much energy will be needed to lift a given payload into orbit. Note that in the case of laser propulsion the laser would have to be powered first. The same problem as with the electric car or the hydrogene powered car.

Without having the basic idea what the "cost" in terms of energy is to get the stuff there all the discussion is just guessing.

AndreH
2008-Sep-25, 01:23 PM
snip...This sort of thing only makes sense if we can build things cheaply in space and are unwilling to use the same ability to make things cheaply on earth. This seems unlikely, but I suppose it is a very remote possibility.

Exactly what I am thinking. Just wanted to back it up with some numbers.

cjameshuff
2008-Sep-25, 03:28 PM
Again we are at apoint where we would need numbers. How much energy will be needed to lift a given payload into orbit. Note that in the case of laser propulsion the laser would have to be powered first. The same problem as with the electric car or the hydrogene powered car.

It's not at all the same problem. Getting material into orbit is expensive...$10000-20000/kg. Ion drives achieve their higher fuel efficiency by using higher exhaust velocities, and you increase exhaust velocity, momentum per unit mass increases in direct proportion, but energy increases with the square, so ion drives need large amounts of power.

This site:
http://earthobservatory.nasa.gov/Library/EO1/eo1_3.html
quotes 100 W/kg as the hoped-for performance of a new type of solar panel. A 20 kW ion engine would need 200 kg of solar array, costing $2 million at $10000/kg to haul into orbit and taking up almost half the payload of a small launcher like the Falcon 1.

With extremely expensive panels, you can get up to 40% conversion with white light, 20-30% is a more realistic figure, which limits you to around 430 watts per square meter of solar panel. Photovoltaic efficiencies vary by wavelength, however, and can convert light at their most sensitive wavelengths at much higher efficiencies. If the power beam illuminates the panels with just 1370 W/m^2, just equaling the sun, and is converted at 60% efficiency, you could halve the panel mass and increase payload size without increasing time to reach the final orbit, or you can reduce the overall payload and launch directly into higher altitude or higher inclination orbits, reducing time spent moving into orbit. Or you can reduce the size and cost of the second stage, lowering the total price/kg to orbit.

If you quadruple the power per unit area, you can quarter the collector mass, or reduce fuel requirements by more than half: not only do you need less fuel to move the spacecraft, you need less fuel to move that fuel. If you can supply enough thrust early enough, you could significantly reduce the size of the last chemical stage, or maybe even eliminate it entirely without having to deal with the narrow engineering margins of a SSTO launcher.

If you need 200 kg of solar panels for a month while initially moving into orbit, that's equivalent to about $137000/MWH...wholesale energy prices on the ground being more on the order of $50-100/MWH. Even if your laser is 10% efficient and only 10% of the beam actually hits the collectors, costs of running it are dwarfed by the launch costs.

mugaliens
2008-Sep-25, 08:46 PM
The answer's right there in the portion you quoted: "to get it where it needs to be faster and without as much solar panels as would be needed otherwise". Solar intensity in orbit relative to the 24 hour average ground intensity is irrelevant...it's still broad spectrum and low energy density.

Then deploy tiny-mass aluminized mylar panels to amplify the intensity.

That's far more doable and much cheaper than ground-laser illumination.

cjameshuff
2008-Sep-25, 10:29 PM
Then deploy tiny-mass aluminized mylar panels to amplify the intensity.

That's far more doable and much cheaper than ground-laser illumination.

Then you end up with the mass of the extra equipment to unfold and orient the panels properly toward the sun (more precision being necessary because of the concentrator arrangement), and a far more complicated design with a much greater risk of failure. And you still can't gather as much power for a given panel area, because heat will destroy the panels before they reach the power output they could achieve with laser illumination. Yes, you could make the mirrors even bigger and etch diffraction gratings into the aluminization pattern or use a carefully tuned dielectric mirror coating to deliver just the wavelength most efficiently converted to the solar panels, but that solves one problem and makes the others worse.

And this doesn't work at all for early assist into orbit from a suborbital trajectory. You'd need to deploy these big, fragile structures within minutes, you'd be restricted to early morning launch times, and you'd have to get enough delta-v before passing into Earth's shadow to stay out of the atmosphere. A launch using ground power would be able to use much smaller collectors that could be deployed much more quickly...deployment might consist of simply jettisoning the protective covers, perhaps with larger collectors (or, as you said, concentrator mirrors) being deployed as the craft moves into higher orbit.

mugaliens
2008-Sep-27, 02:20 PM
If we assume that the orbit of the collector does not “shade” the Earth from any sun light, here’s another problem.

1 megawatt beamed to Earth would raise the temperature of 822000 gallons of water by 12 degrees in a 24 hour period.

That my friend is global warming!

This is an excellent example of what may prove to be a very strong argument undermining the viability of solar power satellites.

Solar power satellites (http://en.wikipedia.org/wiki/Solar_power_satellite)would reside in geostationary orbit, where they'd be in sunlight 100% of the time except during a few days during the Spring and Fall equinox; even then they would be illuminated for all but 75 minutes out of 1,440 during any 24-hour period, and that, at night, when energy demand is at it's lowest. Furthermore, if they were evenly spaced around the equator, you'd always be receiving power for a minimum of 94.7% of the time during the equinoxes. The rest of the time, they'd be up nearly 100% of the time.

The problem is that that 99% uptime translates into 99% of their energy being sent to Earth, where it will, sooner or later, wind up as heat.

By contrast, ground-based solar takes some of the sunlight which would have wound up as heat, transforms it into energy, where it will eventually wind back up as heat - but not heat in addition to that which would already have struck the Earth.

Now, this isn't quite true, as a solar cell is black, whereas most deserts actually have an albedo (reflectivity) of around 28%. I'm not sure what the albedo of a solar cell is across the EM spectrum - can someone fill that in?

Regardless, even with solar cells, more of the sun's energy is captured by the earth than with desert sand, but it does so without adding additional energy that would otherwise not be captured by the Earth at all.

And therein lies the rub: solar power satellites (SPS) would signficantly contribute to globabl warming, whereas ground-based solar (GBS) would only contribute slightly.

Here's another thing to thing about: regardless of whether we go with SPS or GBS, both would significantly reduce the waste heat that's the natural result of burning fossil fuels! The kicker, though, is that SPS would still likely result in a net increase in heat to the Earth, whereas GBS would likely result in a slight net decrease due to it's lower contribution.

Obviously, for those concerned primarily about global warming, GBS is a preferred route.

Cost-wise, GBS has significant advantages, as well.

Finally, to counter the argument that "we just don't have enough land mass for ground-based solar!" I give you this graphic (http://en.wikipedia.org/wiki/Image:Solar_land_area.png), which depictes various degrees of annual insolation (exposure to solar energy from the Sun), and dots. Each dot represents the area required in it's respective area of insolation to replace, in it's entirely, 100% of the entire world's current energy requirements, which is about 18 TWe. And that's assuming a very low conversion efficiency of just 8%. Imagine how much less would be required if the efficieny jumped to 16%, or 32%!

The colors respresent various amounts of insolation, and it's clear that most population exists near areas of the most insolation.

As for ground-based solar, currently, the most cost-effective, as well as the most efficient in terms of ground use, is the solar trough (http://en.wikipedia.org/wiki/Solar_trough), which uses parabolic troughs permanently aligned E-W, and angled to the sun's mean elevation (seen during the Spring and Fall equinoxes). The troughs focus solar energy on pipes running along the focal point, heating a working fluid, which is in turn used in the production of electrical energy.

Current total efficiency, however, remains about 15%, which is on par with photovoltaics.

cjameshuff
2008-Sep-27, 04:46 PM
And therein lies the rub: solar power satellites (SPS) would signficantly contribute to globabl warming, whereas ground-based solar (GBS) would only contribute slightly.

Are you serious?

Assuming a 10-fold increase to 180 TW (microwave), the contribution of SPS would amount to a 0.1% increase over what Earth receives from the sun. At 8% conversion and transmission efficiency, that would require solar collector area of 1.25% the area of Earth's silhouette. 1.6 million square km of solar collector...it's not a near term concern. By the time we can build and maintain an orbital array that large, we can move many of the more energy intensive industrial processes into orbit as well...we will almost certainly have already done so, just to make that construction practical.

And of course, waste heat is not evenly spread over the area of the surface, it instead tends to be concentrated in individual hot spots...an aluminum refinery, for example. Meaning that the heat will be radiated into space more quickly than from sun-warmed ground and atmosphere.

So, SPSs would not be a significant contributor to global warming. On the other hand, a 10-fold increase in global power usage using ground-based solar power would require more than tripling the radius of those dots. Aside from the effect on desert ecologies (deserts are not empty, worthless areas), consider the environmental impact of the industry, transportation, etc that goes into just maintaining those solar farms.

mugaliens
2008-Sep-28, 03:02 AM
Are you serious?

No, as I often spend an hour and a half thoroughly buffing up my research and notes just so that I can read someone brilliantly counter my highly detailed and well-constructed posts with comments like, "are you serious?"

Wow, man - hey! I sure called the shots wrong on that one. Got to hand it to you, as you've totally convinced me. "are you serious?" - Genius, man! Sheer genius.

:wall:

Er... Yes, I was serious.

On the one hand you say it's utterly inconsequential, to which I would respond, "then no one should be woried about global warming at all." On the other hand, there are very sure signs that things are heating up. Personally, I seriously doubt we have anywhere near the component effect of other causes, but why exacerbate the problem when a perfectly workable, low-tech, and entirely doable solution is already being implemented throughout many areas of the world?

I contend (seriously, too), that solar power satellites are the pie in the sky approach for several reasons:

1. Risk. There is a lot more to financial calculations than IRR, NPV, or payback period. Individuals, corporations, and governments which forget this go bankrupt all the time. When faced with a choice between two investments, both with payback periods of 3 years, but one of which offers 5% IRR with a negligable risk and another that offers 100% IRR with a 20% chance of going bankrupt, the 5% investment will be selected most of the time (except by those companies which keep forgetting about risk).

Solar power satellits may very well offer financial rewards greater than ground-based power systems. However, they can a substantially greater amount of risk, in part because they're simply too vulnerable.

2. Most of the calculations on solar powered satellites were based on the designed transport capability of the shuttle, which wasn't just slightly higher than what we've realized in actual practice. It was several hundred percent higher than what we've seen in actual practice.

Why? Was it because we had a bunch of knuckle-heads over at NASA who couldn't do cost analysis worth beans? Not at all. It's because space transport remains full of unknowns.

We call that risk.

For SPSes, it means that their current costs are almost invariably calculated both those who're most exited about the prospect - the very people who're least objective when it comes to figuring up all those little ancilliary expenses which, when taken together, can easily equal the root costs of the endeavor.

3. We already have a viable solution! My father tried to get me to buy a Prius instead of my truck. But with my annual milage, the Prius would have saved me approximately $1,000 a year, whereas the cost of my truck was $15,000 less than the Prius, it was roomier inside (though same pax), can haul half a cord of firewood, can tow a sizeable trailer, and can even go off-road.

Even if they were economically equivalent, my truck just has a lot more utility than does the Prius (trust me - I've driven them both, and the Prius for about 600 miles in all kinds of conditions and traffic).

I'll keep my truck! It's simple, straightfoward, well-designed and built, easy to work on... The risk of owning it is low. There are no hidden expenses, no unknowns, such as the invention of a competing technology which would cause the value of a Prius to drop like a rock.

Well, that's enough econ 101 for today...

cjameshuff
2008-Sep-28, 04:03 AM
No, as I often spend an hour and a half thoroughly buffing up my research and notes just so that I can read someone brilliantly counter my highly detailed and well-constructed posts with comments like, "are you serious?"

It was an honest question. It would not be the first time I've seen someone claim to worry about SPSs contributing to global warming as a joke. If we ever get to the point that we're beaming enough energy to the planet that waste heat is a problem, we can just beam a little more and air-condition the planet.



On the one hand you say it's utterly inconsequential, to which I would respond, "then no one should be woried about global warming at all."

Waste heat *is* utterly inconsequential, on a global scale. What does this have to do with whether one should worry about global warming?



On the other hand, there are very sure signs that things are heating up. Personally, I seriously doubt we have anywhere near the component effect of other causes, but why exacerbate the problem when a perfectly workable, low-tech, and entirely doable solution is already being implemented throughout many areas of the world?

Because we don't. There's just not enough energy in sunlight for ground based solar power to supply civilization's needs without an unacceptable environmental impact. I don't deny that it'd be useful for certain areas, but it's far from a complete solution.



I contend (seriously, too), that solar power satellites are the pie in the sky approach for several reasons:

I would not claim they are something we could do any time soon. Eventually, given much more orbital infrastructure, they might become practical, but I don't think they are now. However, it did not take very long for commercial satellites to go from impossible, to possible but impractically risky and expensive, to commonplace, so I'm a bit leery of making any kind of prediction as to *when* they'll become practical.

Ronald Brak
2008-Sep-28, 05:13 AM
Because we don't. There's just not enough energy in sunlight for ground based solar power to supply civilization's needs without an unacceptable environmental impact. I don't deny that it'd be useful for certain areas, but it's far from a complete solution.

Average roof space per Australian -about 30 square meters. Australian electricity consumption, about 1,150 Kilowatt-hours per person per year. Area needed to generate this from 10% efficient roof mounted PV about five and a half square meters. Less than 20% of the average roofspace per Australian. This should result in minimal enviromental impact. Of course this isn't terribly practical, what with the sun not shining at night and the difficulty of laying a power cable across the pacific for load leveling.

But 25% efficient load following solar thermal would only take about 45 square kilometers to power the entire Australian continent. With 5 million square kilometers of desert and semi-desert here, it won't be difficult to spare that much land and load following solar thermal can provide energy 24 hours a day.

If one wanted to minimize the amount of new land required, it would be possible to meet most electricty use through roof mounted solar PV and use solar thermal to meet night time demand.

Of course, powering civilization entirely off solar is not a serious suggestion, it runs into problems in places like Ireland and Moscow, and there are other practical sources of low emission power such as wind and geothermal that can be used, but there is no particular reason why we couldn't run civilization off solar energy with minimal environmental impact, or at least minimal environmental impact compared to what we currently do to get energy.

Van Rijn
2008-Sep-28, 08:30 AM
On the one hand you say it's utterly inconsequential, to which I would respond, "then no one should be woried about global warming at all."


Why? The global warming concern is not based on the minor amount of energy humans add to the environment, but due to "greenhouse gases" produced by fossil fuels. SPS ground collectors wouldn't do any more to add energy to the environment than producing energy by fossil or nuclear plants, or even the albedo effects of dark ground based solar collectors.