3 ms·

> The process of filling the empty tanks to 100 psi takes 5-10 pedaling sessions per day for roughly a week. The main thing I wanted to know from this article

> The process of filling the empty tanks to 100 psi takes 5-10 pedaling sessions per day for roughly a week. The main thing I wanted to know from this article is how much peddling it takes to keep the tank filled, and I don't feel like he answered that: how long is a peddling session? If it's 5 minutes, that's about four and a half hours of peddling to fill the tank. If it's 15 minutes, that's about 13 hours. And how often does he empty the tank? How long does it take just to keep it topped up in an average week? Details, man! He must know this if he uses the thing.

brnt 3y ago
I love lowtechmag, but it's almost a rule they are economical with details too, and often to hide a (pretty obvious) compromise that not everyone would be willing to make.
- antisthenes 3y ago
  The extremely obvious compromise is that you need to have a high-PSI air tank on hand as well as a donor bike. The donor bike can probably be gotten for very cheap or free, but the tank... I looked at the prices of 80-gal 200 PSI rated tanks online, and the conclusion is...not great. It's going to run you roughly $1000, for which you can realistically buy a whole set of cordless battery powered tools AND an outlet-powered air compressor on top of that.
  - tspike 3y ago
    The point is that this solution works without access to electricity and is resilient to the loss of industrial supply chains
    - sp332 3y ago
      How are you going to make an air tank that can handle 200 PSI without industrial supply chains?
      - bagels 3y ago
        There are a bunch that are already made. I think this setup is really impractical, but once you have it, you aren't reliant on the supply chain to feed the power plant, but you are reliant on it for finding a source of food to power the human pedaling.
  - littlestymaar 3y ago
    > I looked at the prices of 80-gal 200 PSI rated tanks online, and the conclusion is...not great. Or you can get a bunch of empty propane tanks for a few bucks and call it a day…
  - aziaziazi 3y ago
    For the tank: > There are many used air compressors for sale that have faulty motors or pumps, and are therefore quite inexpensive And the pneumatic parts/equipment: > An additional benefit of using compressed air for energy is the low cost of air tools. The current trend is pushing toward more battery powered equipment, so people are selling off their “outdated” pneumatic tools.
- ars 3y ago
  On top of that humans are very bad at converting fuel into work. "In a rapidly changing world where inexpensive and reliable energy going forward is no longer a given" In such a world a steam engine will give you more useful output for the same input - you could literally burn your food in the steam engine and get more useful work vs eating the food. This kind of thing only makes sense in a world that has not invented steam engines, or at the minimum using an animal that can eat grass that you can't. (And even then burning the grass will give you more work.)
  - slow_typist 3y ago
    According to [1], human ‘fuel’ efficiency on a bike is about 20%. That is way ahead of any simple piston powered steam engine (should be below 10%, citation needed) that you would install in a workshop. [1] https://physics.stackexchange.com/questions/46788/how-efficient-is-the-human-body https://physics.stackexchange.com/questions/46788/how-effici...
    - ars 3y ago
      Steam engines are closer to 40% efficient, not 10%. And they can burn the entire fuel, unlike a person who can only eat the choice parts and must waste large portions. (For example corn - the person can only eat the kernels, the steam engine can productively use the husk and the cob.) Also, your 20% figure is for energy absorbed after digestion (they check via oxygen consumption), but digestion itself is wasteful - lots of usable fuel is not absorbed when someone eats, and is emitted as waste. It's really not even close. It's a problem also for comparing bicycles vs cars - even though they are heavier, cars are surprisingly efficient compared to the entire production process of delivering food to humans (lots of energy is used in the farm). If I remember my figures correctly 2 people in a car is more efficient than 1 omnivore on a bicycle. And a single person in a car is more efficient (less CO2 emitted) than a carnivore on a bicycle. (The car comparison depends a lot on what the person eats.)
      - mrob 3y ago
        Steam turbines can easily reach 40% efficiency. Simple (single expansion) steam piston engines are much less efficient, usually well below 10%.
KennyBlanken 3y ago
The hint is in him having a 7hp (quite powerful) compressor that apparently runs enough to be running up his electric bill. I'm perplexed by the comment about "empty tanks." In these systems the tank is almost never empty because pneumatic tools are pretty useless below a certain pressure; you can't just use them until they're empty. The impact guns I've used became tonka toys around 40 psi or so, if that. The compressor might get shut off for the night, and sure, there are some air leaks - but the next morning when the shop opens, it's not empty. Maybe the reason this dude's compressor is eating him out of house and home is because he's got a bunch of leaks he isn't addressing...
- anon84873628 3y ago
  Huh? Nowhere does the author suggest that the electric powered system was "eating him out of house and home". The motivation here was clearly resiliency and creativity. He also mentions the importance of addressing leaks too. "Empty tanks" is probably just shorthand for "pressure too low to run the tools".
oh_sigh 3y ago
If it takes 7hp(5200w) for 5m, let's say a person can do 100w, so they will take 52x as long, so 260m total. Broken up between 7 sessions a day for a week means each session is only 5 minutes.
juujian 3y ago
Also, I'm curious whether it takes more energy to fill the tanks from 60 to 80 psi vs 80 to 100 psi. Filling the tank until its full is not really a useful metric, would be more interesting to learn how much pedaling continuous operation requires.
- function_seven 3y ago
  I think it’s harder to go 80–100 than it is from 60–80. I’m both cases, you need to stuff the same volume of additional air into the tank, but in the 80–100 case, the resistance to the stuffing is higher. At least I think that’s how it works. Assuming spherical cows and an ideal gas.
  - ars 3y ago
    In theory if the resistance to the stuffing is higher, then it also pushes the air out harder - i.e. it's a wash. Except that it's not, because when you compress air you heat it, and all the heat energy is gone. This is why compressed air is a terrible energy storage method. The hotter you heat the air (higher PSI) the more energy you waste.
    - function_seven 3y ago
      Wouldn’t the air not be pushing against my piston? It’s already in the tank and the piston is being pushed by atmospheric pressure for the next gulp. Good note on the heat. I’ll wrap my whole setup in a blanket to recover that ;)
    - raverbashing 3y ago
      It does heat and you do lose some energy there, but not enough to lose all of the compression applied
  - Someone 3y ago
    It must be harder. If it wasn’t, you’d be able to get to arbitrary pressures with a constant force. So, in the extreme, if you can get the air to 15 psi with your lungs, you’d be able to get the air to 1000 psi with your lungs, too. Another way to see that is to consider two tanks at different pressures that are connected to each other, and ask yourself why air will flow from the higher pressure one to the lower pressure one.
gnopgnip 3y ago
A healthy cyclist might average 200-250w over an hour, or an FTP of 200-250. One horsepower is ~735w, so ~5kw for the 7hp compressor. The cyclist putting in a really high degree of effort needs to pedal for 20x-25x as long as the compressor to do the same amount of work. A 7hp compressor delivers in the range of 1m3/min@6bar or roughly 35cfm@90psi. The output can vary a lot depending on how the compressor is optimized though. Air tool air use varies as well, a sand blaster or paint sprayer will have different needs. But an air tool like a disc sander or die grinder or air hammer or impact wrench will be in the 5cfm range or a little under .2m3/min. So for an hour of tool use the compressor needs to run for 8.5 minutes or 2 hours and 50 minutes of cycling.

brnt 3y ago

I love lowtechmag, but it's almost a rule they are economical with details too, and often to hide a (pretty obvious) compromise that not everyone would be willing to make.

antisthenes 3y ago
The extremely obvious compromise is that you need to have a high-PSI air tank on hand as well as a donor bike. The donor bike can probably be gotten for very cheap or free, but the tank... I looked at the prices of 80-gal 200 PSI rated tanks online, and the conclusion is...not great. It's going to run you roughly $1000, for which you can realistically buy a whole set of cordless battery powered tools AND an outlet-powered air compressor on top of that.
- tspike 3y ago
  The point is that this solution works without access to electricity and is resilient to the loss of industrial supply chains
  - sp332 3y ago
    How are you going to make an air tank that can handle 200 PSI without industrial supply chains?
    - bagels 3y ago
      There are a bunch that are already made. I think this setup is really impractical, but once you have it, you aren't reliant on the supply chain to feed the power plant, but you are reliant on it for finding a source of food to power the human pedaling.
- littlestymaar 3y ago
  > I looked at the prices of 80-gal 200 PSI rated tanks online, and the conclusion is...not great. Or you can get a bunch of empty propane tanks for a few bucks and call it a day…
- aziaziazi 3y ago
  For the tank: > There are many used air compressors for sale that have faulty motors or pumps, and are therefore quite inexpensive And the pneumatic parts/equipment: > An additional benefit of using compressed air for energy is the low cost of air tools. The current trend is pushing toward more battery powered equipment, so people are selling off their “outdated” pneumatic tools.
ars 3y ago
On top of that humans are very bad at converting fuel into work. "In a rapidly changing world where inexpensive and reliable energy going forward is no longer a given" In such a world a steam engine will give you more useful output for the same input - you could literally burn your food in the steam engine and get more useful work vs eating the food. This kind of thing only makes sense in a world that has not invented steam engines, or at the minimum using an animal that can eat grass that you can't. (And even then burning the grass will give you more work.)
- slow_typist 3y ago
  According to [1], human ‘fuel’ efficiency on a bike is about 20%. That is way ahead of any simple piston powered steam engine (should be below 10%, citation needed) that you would install in a workshop. [1] https://physics.stackexchange.com/questions/46788/how-efficient-is-the-human-body https://physics.stackexchange.com/questions/46788/how-effici...
  - ars 3y ago
    Steam engines are closer to 40% efficient, not 10%. And they can burn the entire fuel, unlike a person who can only eat the choice parts and must waste large portions. (For example corn - the person can only eat the kernels, the steam engine can productively use the husk and the cob.) Also, your 20% figure is for energy absorbed after digestion (they check via oxygen consumption), but digestion itself is wasteful - lots of usable fuel is not absorbed when someone eats, and is emitted as waste. It's really not even close. It's a problem also for comparing bicycles vs cars - even though they are heavier, cars are surprisingly efficient compared to the entire production process of delivering food to humans (lots of energy is used in the farm). If I remember my figures correctly 2 people in a car is more efficient than 1 omnivore on a bicycle. And a single person in a car is more efficient (less CO2 emitted) than a carnivore on a bicycle. (The car comparison depends a lot on what the person eats.)
    - mrob 3y ago
      Steam turbines can easily reach 40% efficiency. Simple (single expansion) steam piston engines are much less efficient, usually well below 10%.

KennyBlanken 3y ago

The hint is in him having a 7hp (quite powerful) compressor that apparently runs enough to be running up his electric bill. I'm perplexed by the comment about "empty tanks." In these systems the tank is almost never empty because pneumatic tools are pretty useless below a certain pressure; you can't just use them until they're empty. The impact guns I've used became tonka toys around 40 psi or so, if that. The compressor might get shut off for the night, and sure, there are some air leaks - but the next morning when the shop opens, it's not empty. Maybe the reason this dude's compressor is eating him out of house and home is because he's got a bunch of leaks he isn't addressing...

anon84873628 3y ago
Huh? Nowhere does the author suggest that the electric powered system was "eating him out of house and home". The motivation here was clearly resiliency and creativity. He also mentions the importance of addressing leaks too. "Empty tanks" is probably just shorthand for "pressure too low to run the tools".

oh_sigh 3y ago

If it takes 7hp(5200w) for 5m, let's say a person can do 100w, so they will take 52x as long, so 260m total. Broken up between 7 sessions a day for a week means each session is only 5 minutes.

juujian 3y ago

Also, I'm curious whether it takes more energy to fill the tanks from 60 to 80 psi vs 80 to 100 psi. Filling the tank until its full is not really a useful metric, would be more interesting to learn how much pedaling continuous operation requires.

function_seven 3y ago
I think it’s harder to go 80–100 than it is from 60–80. I’m both cases, you need to stuff the same volume of additional air into the tank, but in the 80–100 case, the resistance to the stuffing is higher. At least I think that’s how it works. Assuming spherical cows and an ideal gas.
- ars 3y ago
  In theory if the resistance to the stuffing is higher, then it also pushes the air out harder - i.e. it's a wash. Except that it's not, because when you compress air you heat it, and all the heat energy is gone. This is why compressed air is a terrible energy storage method. The hotter you heat the air (higher PSI) the more energy you waste.
  - function_seven 3y ago
    Wouldn’t the air not be pushing against my piston? It’s already in the tank and the piston is being pushed by atmospheric pressure for the next gulp. Good note on the heat. I’ll wrap my whole setup in a blanket to recover that ;)
  - raverbashing 3y ago
    It does heat and you do lose some energy there, but not enough to lose all of the compression applied
- Someone 3y ago
  It must be harder. If it wasn’t, you’d be able to get to arbitrary pressures with a constant force. So, in the extreme, if you can get the air to 15 psi with your lungs, you’d be able to get the air to 1000 psi with your lungs, too. Another way to see that is to consider two tanks at different pressures that are connected to each other, and ask yourself why air will flow from the higher pressure one to the lower pressure one.

gnopgnip 3y ago

A healthy cyclist might average 200-250w over an hour, or an FTP of 200-250. One horsepower is ~735w, so ~5kw for the 7hp compressor. The cyclist putting in a really high degree of effort needs to pedal for 20x-25x as long as the compressor to do the same amount of work. A 7hp compressor delivers in the range of 1m3/min@6bar or roughly 35cfm@90psi. The output can vary a lot depending on how the compressor is optimized though. Air tool air use varies as well, a sand blaster or paint sprayer will have different needs. But an air tool like a disc sander or die grinder or air hammer or impact wrench will be in the 5cfm range or a little under .2m3/min. So for an hour of tool use the compressor needs to run for 8.5 minutes or 2 hours and 50 minutes of cycling.