02-11-2011, 05:18 AM
Compressed air, fuel cells, and batteries are all pretty much the same thing. They store energy, they don't make energy. Even the fuel cell only converts stored chemical energy into electrical energy. A fuel cell also requires highly refined fuel (hydrogen) that is usually generated by electrolysis requiring an input of electrical energy.
KonaDave, you are correct that it takes more energy to produce hydrogen than you get back while burning it. This is the way of the universe. There is no free lunch. It also takes more energy to charge a battery than you get back out of it. There is no energy storage method for which this is not true. It would definitely be true for the compressed air car. An interesting angle on the compressed air car is that while the air is expanding in the engine, it will make things cold. Basically the engine block will get cold. You might get air conditioning out of the deal if this could be harnessed.
Dugger, the benefit to what you propose is that you can store energy from pedaling over a long time. Human power is limited. You could get on a bike and pedal to town. That would be the most efficient. A bicycle chain and sprockets drive is the most efficient transmission there is and a person riding a bike is phenomenally efficient, but people but out power at a very low rate so you will travel slowly and carry very little if you are limited to producing the power during the trip. If on the other hand you pedal a pump for an hour a day for 10 days, then assuming 50% efficiency for the storage and engine/transmission, you have 5 hrs worth of energy that you could spend all at once.
I think that you will discover that pedaling is hard work. Once you invest in the system to store the energy whether that is batteries, hydrogen from electrolysis, or compressed air, you are free to take advantage of intermittent energy sources such as solar, wind, and wave power. The advantage of batteries is that they are a comparatively well understood and mature technology, but they are not light nor do they have all that much capacity. The advantage of fuel cells is that they are efficient and "energy dense". That is why they use them on spacecraft, but they are expensive and require highly refined fuel. The advantage of compressed air is that is well understood and light, but does not have a lot of storage capacity. That may be changing now with kevlar and carbon fiber pressure vessels. I deal with compressed gas cylinders at work. We have some aluminum cylinders that weigh 55 lbs empty. I recently had to move some steel cylinders that were slightly larger but that weighed more than 100 lb each. These cylinders hold 2,000 psi and have special caps that protect the valve stem in case the cylinder falls over because if that valve stem broke off the cylinder would take off like a rocket. So pressure vessel technology really controls issues of safety and capacity. A typical economy car that gets 30 mpg might hold about 10 gallons of fuel and have a 300 mile range. Its internal combustion engine is about 25% efficient. Just pulling this out of thin air but I am guessing the efficiency of the air motor is around 50%. Where I am going with this is that if your compressed air car has a range of 300 miles then breaking that air storage cylinder in a crash would be like setting off 5 gallons of gas in a properly mixed fuel/air explosion. Big boom.
KonaDave, you are correct that it takes more energy to produce hydrogen than you get back while burning it. This is the way of the universe. There is no free lunch. It also takes more energy to charge a battery than you get back out of it. There is no energy storage method for which this is not true. It would definitely be true for the compressed air car. An interesting angle on the compressed air car is that while the air is expanding in the engine, it will make things cold. Basically the engine block will get cold. You might get air conditioning out of the deal if this could be harnessed.
Dugger, the benefit to what you propose is that you can store energy from pedaling over a long time. Human power is limited. You could get on a bike and pedal to town. That would be the most efficient. A bicycle chain and sprockets drive is the most efficient transmission there is and a person riding a bike is phenomenally efficient, but people but out power at a very low rate so you will travel slowly and carry very little if you are limited to producing the power during the trip. If on the other hand you pedal a pump for an hour a day for 10 days, then assuming 50% efficiency for the storage and engine/transmission, you have 5 hrs worth of energy that you could spend all at once.
I think that you will discover that pedaling is hard work. Once you invest in the system to store the energy whether that is batteries, hydrogen from electrolysis, or compressed air, you are free to take advantage of intermittent energy sources such as solar, wind, and wave power. The advantage of batteries is that they are a comparatively well understood and mature technology, but they are not light nor do they have all that much capacity. The advantage of fuel cells is that they are efficient and "energy dense". That is why they use them on spacecraft, but they are expensive and require highly refined fuel. The advantage of compressed air is that is well understood and light, but does not have a lot of storage capacity. That may be changing now with kevlar and carbon fiber pressure vessels. I deal with compressed gas cylinders at work. We have some aluminum cylinders that weigh 55 lbs empty. I recently had to move some steel cylinders that were slightly larger but that weighed more than 100 lb each. These cylinders hold 2,000 psi and have special caps that protect the valve stem in case the cylinder falls over because if that valve stem broke off the cylinder would take off like a rocket. So pressure vessel technology really controls issues of safety and capacity. A typical economy car that gets 30 mpg might hold about 10 gallons of fuel and have a 300 mile range. Its internal combustion engine is about 25% efficient. Just pulling this out of thin air but I am guessing the efficiency of the air motor is around 50%. Where I am going with this is that if your compressed air car has a range of 300 miles then breaking that air storage cylinder in a crash would be like setting off 5 gallons of gas in a properly mixed fuel/air explosion. Big boom.