A transit is a more precise term and it refers to any event where two objects appear close enough in the sky that their disks overlap.
The term eclipse is reserved for those events where the front object is large enough to significantly cover the back one.
During the transit Venus will only cover about 1/900th of the solar disk and as such this is not usually referred to as an eclipse.
What matters are the apparent sizes of the two bodies not their actual sizes, for example, the Moon is nowhere near half a million miles in diameter but when it transits the face of the Sun the event is called an eclipse. This is because, from the surface of the Earth, the apparent sizes of the Moon and the Sun are very similar and the moon is capable of blocking out a large fraction of the solar disk, sometimes even cover it completely.
Imagine you travelled to Venus during the transit - the disk of the planet would get larger and larger until around 1 million kms (630 thousand miles) from the planet it would be large enough to totally eclipse the Sun.
Ultimately that's right - you can't win against entropy, but in the meantime you can do much better than break even - only the available energy and it's form limits what you can achieve.
If you can collect enough energy to survive it only takes a little more to repair the damage caused by entropic processes.
345 Gallons can only yield around 5 MWh of electrical power.
Yes, you're right. I stuffed up the unit conversion here - my original figure was about 20.667 times too large.
After you allow for that, the corrected end result comes out to 8 * 20.667 = 165 million people, which is in the same order of magnitude as your calculation. The remaining difference is due to two things, I think. The fact that you assumed a conversion from gasoline to electricity, which has about a 40% efficiency, needs to happen, but actually the opposite would need to be done - electricity generated by the PVs would need to be converted to gasoline. The original statement was about how many gas stations would be served by an area the size of NJ. The other difference is that you used the average solar constant instead of a figure specific to the New Jersey area.
Nice to see that someone bothered to check my numbers:)
Average per capita US gasoline consumption per annum (for 1997) - 345 gallons.
Average energy content of that much gasoline - 238 MWh
Area of typical commercially available solar cells needed to generate that much energy: 0.6 acre
(approx. equivalent to a square 50 metres on its side)
Area of New Jersey (according to another post) = 19231 km^2
Therefore, approx. 8 million peoples' needs would be covered by the above area.
The US has about one gas station per 1500 people.
Thus the above area would be able to supply about 5000 gas stations.
Even assuming only 10% efficiency in conversion from electricity to gasoline the guy seems about 1 or 2 orders-of-magnitude out.
Re:The bigger picture -updated version
on
Out of Gas
·
· Score: 1
A typical pv panel today can collect solar energy at the rate of about 100kWh/m^2 per year (at a location like New York, for example, more in a sunnier place). This means you need 170 square metres of panels to collect enough energy in a year to make 2000 pounds of aluminium. That means you get about 11.8 pounds of aluminium to mount each square metre of panels. That sounds like enough to me. Since your panels are likely to last for much longer than a year they will collect far more energy than it took to mount them.
I don't know why someone modded that one up to "Insightful". The point that the article is trying to make is how complex the task was of bringing his Windows machine to a state where he could safely connect it to the internet. Changing the oil in your car is a trivial job by comparison.
Sure some people will still manage to stuff it up but the cost of making a totally foolproof car is one that is a waste of money for most people.
A more accurate analogy would be this:
When you buy a new car from a car dealer you needed to do the following things before it even leaves the showroom:
Cut a new set of keys because the standard ones supplied with it were all alike.
Reprogram the electronic fuel injection computer because it has a bug which can crash it.
Change the tyres because the supplied ones were not all-weather ones.
It would not take 150 million years to get there! Already existing and proven technology would be able to manage a journey to the nearest star in about 10 thousand years. Obviously that's still too slow to bother at the moment but faster methods are at least theoretically possible.
All the TPF mission aims to spot is Earth-like planets. Even if your journey took a thousand years odds are that the planet would still be there and still much like you observed it a thousand years earlier, unless, maybe, there happened to be a civilisation on it that TPF didn't spot.:)
Photovoltaic cell efficiency is controlled by their composition not colour. I believe PV cells are very efficient to light of specific colours but since sunlight is composed of light of all the different colours the net result is less than perfect efficiency. You may be confusing PV cells with solar thermal systems. The latter are able to use all frequencies, even infra-red, and are made as dark as possible to maximise absorption to heat water or some other fluid.
I don't know what you are trying to say by "light carries the same amount of energy at all wavelengths". Actually, photons of light carry more energy at higher frequencies. Eg. each photon of ultra-violet light carries more energy than a photon of red light. If you mean the sun emits the same amount of energy at all wavelengths then that's not exactly true either. Sun's radiation peaks in the yellow-green light and drops off towards the red and blue ends of the spectrum.
Your estimates are too rough to draw a reliable conclusion - there are a lot of factors to take into account which affect how viable solar electricity generation is.
To begin with the earth receives 1367 W/m2 at its mean distance from the sun - this varies by about 2% up and down depending on time of year.
Some of that is absorbed by the earth's atmosphere. The exact amount of energy received varies according to location, sun's elevation above the horizon and local conditions. The net result of all these factors is about 1000kWh/m^2 of solar radiation falling on a location such as New York or Chicago in the course of a year.
Now contrast this with a typical household electricity usage of between 3000kWh per year for "a couple without children" household to 6000kWh for a family with two children.
This means that with 10% efficiency you need
between 30 and 60 square metres of panels to meet your needs. Now, a typical house will have between 50 and 200 square metres of floor space. Thus, whether you have enough suitable roof space depends very much on your local conditions (roof geometry, any trees shading your roof etc.)
These numbers hopefully illustrate why efficiency and cost of solar cells are such a big deal and why advancements in these areas when they come up make everyone involved hopeful of a breakthrough.
If you could make 100% efficient cells then any house would only need about 5 to 10 square metres to meet all its needs. On the other hand if the cells were cheap enough but only 10% efficient you could just cover your entire roof with them and meet your needs that way.
10 years to recoup followed by another 10 years of service - that's a 3.5% per annum return which sounds quite reasonable for what is a pretty secure investment.
The DOE has an example on their site
which has a 7 year payback time which makes it a 5% per annum rate of return (over 20 years.)
These calculations are not taking into account
inflation though, but that would only improve the return.
Actually, your figure of 0.1% efficiency makes using plants as an energy source work out as very inefficient. Comparing that to typical solar cell efficiency of about 10% means you will need 100x the land area to get the same amount of energy out of plants. So if like some other poster suggested we'd need 9% of US land area to meet our energy demand with solar cells then we would have no hope of doing so with plants. In addition, plants need water, fertiliser, harvesting, processing and are vulnerable to pests. All things that need additional energy to take care of which reduces the overall efficiency of the process.
That is pretty screwy but that's probably the result of an ad-hoc evolution of the phone system.
I think the australian system is quite tidy.
Every number has a one digit area code plus an 8 digit local number.
If you're calling from the same area code you can omit it.
The area codes are assigned quite simply:
2 - central east (NSW and ACT)
3 - south east (Victoria and Tasmania)
4 - mobile phones australia wide
7 - north east (Queensland)
8 - the rest of australia (SA, WA and Northern Territory)
The call originator always pays except in special cases (eg. reverse charges calls) including to mobiles - which should discourage indiscriminate spamming.
Apparently Hebrew sometimes does this and it might work for it but in English it would introduce unnecessary ambiguity. To take your example of "clr" - is it meant to say clear, cooler or colo(u)r? In hebrew, you are meant to tell from the context, but that may not always help.
About the other idea - don't we already do it? Most common words are short and the most common ones are very short, and you can do your own "real time language compression" by using small words instead of big ones where possible.
I just want to say this is an awesome technology
and I agree with Witehira's comment that this is a very important technology and it could very well change the world.
When they say they are about "long-term customer value" they mean they are constantly working on locking customers into using their software thus increasing the cost of switching away from it. Thus in the long term the customers become more valueable to them because they are more and more likely to buy their products (because that's the cheapest option)
What do you mean matter transporters aren't real? I own two: a gasoline powered one and one human powered one. The gasoline one can seat 5 humans and carry several hundred pounds of any matter you like:)
Slashdot Mirror
The term eclipse is reserved for those events where the front object is large enough to significantly cover the back one.
During the transit Venus will only cover about 1/900th of the solar disk and as such this is not usually referred to as an eclipse.
What matters are the apparent sizes of the two bodies not their actual sizes, for example, the Moon is nowhere near half a million miles in diameter but when it transits the face of the Sun the event is called an eclipse. This is because, from the surface of the Earth, the apparent sizes of the Moon and the Sun are very similar and the moon is capable of blocking out a large fraction of the solar disk, sometimes even cover it completely.
Imagine you travelled to Venus during the transit - the disk of the planet would get larger and larger until around 1 million kms (630 thousand miles) from the planet it would be large enough to totally eclipse the Sun.
Ultimately that's right - you can't win against entropy, but in the meantime you can do much better than break even - only the available energy and it's form limits what you can achieve. If you can collect enough energy to survive it only takes a little more to repair the damage caused by entropic processes.
Yes, you're right. I stuffed up the unit conversion here - my original figure was about 20.667 times too large. After you allow for that, the corrected end result comes out to 8 * 20.667 = 165 million people, which is in the same order of magnitude as your calculation. The remaining difference is due to two things, I think. The fact that you assumed a conversion from gasoline to electricity, which has about a 40% efficiency, needs to happen, but actually the opposite would need to be done - electricity generated by the PVs would need to be converted to gasoline. The original statement was about how many gas stations would be served by an area the size of NJ. The other difference is that you used the average solar constant instead of a figure specific to the New Jersey area.
Nice to see that someone bothered to check my numbers :)
Average per capita US gasoline consumption per annum (for 1997) - 345 gallons.
Average energy content of that much gasoline - 238 MWh
Area of typical commercially available solar cells needed to generate that much energy: 0.6 acre (approx. equivalent to a square 50 metres on its side)
Area of New Jersey (according to another post) = 19231 km^2
Therefore, approx. 8 million peoples' needs would be covered by the above area.
The US has about one gas station per 1500 people.
Thus the above area would be able to supply about 5000 gas stations.
Even assuming only 10% efficiency in conversion from electricity to gasoline the guy seems about 1 or 2 orders-of-magnitude out.
A typical pv panel today can collect solar energy at the rate of about 100kWh/m^2 per year (at a location like New York, for example, more in a sunnier place). This means you need 170 square metres of panels to collect enough energy in a year to make 2000 pounds of aluminium. That means you get about 11.8 pounds of aluminium to mount each square metre of panels. That sounds like enough to me. Since your panels are likely to last for much longer than a year they will collect far more energy than it took to mount them.
A more accurate analogy would be this: When you buy a new car from a car dealer you needed to do the following things before it even leaves the showroom:
Cut a new set of keys because the standard ones supplied with it were all alike.
Reprogram the electronic fuel injection computer because it has a bug which can crash it.
Change the tyres because the supplied ones were not all-weather ones.
Install seatbelts.
Install indicators and brake lights.
Install a bull-bar.
Install a fire-wall!
It would not take 150 million years to get there! Already existing and proven technology would be able to manage a journey to the nearest star in about 10 thousand years. Obviously that's still too slow to bother at the moment but faster methods are at least theoretically possible. All the TPF mission aims to spot is Earth-like planets. Even if your journey took a thousand years odds are that the planet would still be there and still much like you observed it a thousand years earlier, unless, maybe, there happened to be a civilisation on it that TPF didn't spot. :)
Photovoltaic cell efficiency is controlled by their composition not colour. I believe PV cells are very efficient to light of specific colours but since sunlight is composed of light of all the different colours the net result is less than perfect efficiency. You may be confusing PV cells with solar thermal systems. The latter are able to use all frequencies, even infra-red, and are made as dark as possible to maximise absorption to heat water or some other fluid.
I don't know what you are trying to say by "light carries the same amount of energy at all wavelengths". Actually, photons of light carry more energy at higher frequencies. Eg. each photon of ultra-violet light carries more energy than a photon of red light. If you mean the sun emits the same amount of energy at all wavelengths then that's not exactly true either. Sun's radiation peaks in the yellow-green light and drops off towards the red and blue ends of the spectrum.
Your estimates are too rough to draw a reliable conclusion - there are a lot of factors to take into account which affect how viable solar electricity generation is. To begin with the earth receives 1367 W/m2 at its mean distance from the sun - this varies by about 2% up and down depending on time of year. Some of that is absorbed by the earth's atmosphere. The exact amount of energy received varies according to location, sun's elevation above the horizon and local conditions. The net result of all these factors is about 1000kWh/m^2 of solar radiation falling on a location such as New York or Chicago in the course of a year. Now contrast this with a typical household electricity usage of between 3000kWh per year for "a couple without children" household to 6000kWh for a family with two children. This means that with 10% efficiency you need between 30 and 60 square metres of panels to meet your needs. Now, a typical house will have between 50 and 200 square metres of floor space. Thus, whether you have enough suitable roof space depends very much on your local conditions (roof geometry, any trees shading your roof etc.) These numbers hopefully illustrate why efficiency and cost of solar cells are such a big deal and why advancements in these areas when they come up make everyone involved hopeful of a breakthrough. If you could make 100% efficient cells then any house would only need about 5 to 10 square metres to meet all its needs. On the other hand if the cells were cheap enough but only 10% efficient you could just cover your entire roof with them and meet your needs that way.
10 years to recoup followed by another 10 years of service - that's a 3.5% per annum return which sounds quite reasonable for what is a pretty secure investment. The DOE has an example on their site which has a 7 year payback time which makes it a 5% per annum rate of return (over 20 years.) These calculations are not taking into account inflation though, but that would only improve the return.
Actually, your figure of 0.1% efficiency makes using plants as an energy source work out as very inefficient. Comparing that to typical solar cell efficiency of about 10% means you will need 100x the land area to get the same amount of energy out of plants. So if like some other poster suggested we'd need 9% of US land area to meet our energy demand with solar cells then we would have no hope of doing so with plants. In addition, plants need water, fertiliser, harvesting, processing and are vulnerable to pests. All things that need additional energy to take care of which reduces the overall efficiency of the process.
I think the australian system is quite tidy. Every number has a one digit area code plus an 8 digit local number. If you're calling from the same area code you can omit it.
The area codes are assigned quite simply: 2 - central east (NSW and ACT) 3 - south east (Victoria and Tasmania) 4 - mobile phones australia wide 7 - north east (Queensland) 8 - the rest of australia (SA, WA and Northern Territory)
The call originator always pays except in special cases (eg. reverse charges calls) including to mobiles - which should discourage indiscriminate spamming.
The people who cast the votes don't decide an election, the people who count the votes do.
Apparently Hebrew sometimes does this and it might work for it but in English it would introduce unnecessary ambiguity. To take your example of "clr" - is it meant to say clear, cooler or colo(u)r? In hebrew, you are meant to tell from the context, but that may not always help. About the other idea - don't we already do it? Most common words are short and the most common ones are very short, and you can do your own "real time language compression" by using small words instead of big ones where possible.
I just want to say this is an awesome technology and I agree with Witehira's comment that this is a very important technology and it could very well change the world.
When they say they are about "long-term customer value" they mean they are constantly working on locking customers into using their software thus increasing the cost of switching away from it. Thus in the long term the customers become more valueable to them because they are more and more likely to buy their products (because that's the cheapest option)
What do you mean matter transporters aren't real? :)
I own two: a gasoline powered one and one human powered one. The gasoline one can seat 5 humans and carry several hundred pounds of any matter you like