Are hydrogen tanks superior to batteries?

I am correct on the round trip efficiency of batteries. I have just double checked it, and suggest you do too. Yes you could have nine powerwalls at £4,500 apiece but I doubt if you'd ever recover that investment and the efficiency would be even worse You'd could also use 7,000 laptop batteries. My point is nobody makes a car charging domestic battery system or a domestic hydrogen filling system yet.

As to plugging into the grid, indeed you could. In fact you'd probably have to! The point of using a home hydrogen system is that you WOULDN'T have to. Transferring the energy into a car as hydrogen is quick and can be done at your convenience, not that of the car. Plugging into the grid is not that easy. You have to find a charging point. It also cost you more.

 

I think Pushmi is confusing what is meant by 'efficiency'. I was talking about round trip efficiency. Yes, you might lose 10% of the energy on charging in the form of heat in the charger and the battery, but you also lose energy in heating in the battery when discharging. So you only get 80% back of what you put in.

The waste heat is due to current through the internal resistance of the cells, and as the heat generated is proportional to the square of the current, fast charging becomes more and more inefficient as you increase the speed. Thus if you charge a battery slowly over - say - ten hours with a heat loss of 100 Watts, you lose a total of 1kWh. If you double the current over five hours, the rate of energy loss will be 400 Watts, meaning that the total loss will be 2kWh. Reducing it to 2.5 hours will give you losses of 4kWh. At 1.25 hours it goes up to 8kW and so on. Fast charging is VERY inefficient.

Batteries used by people like Tesla are designed not for round trip efficiency, but for maximum energy density, for obvious reasons. There ARE lithium batteries that have better round-trip efficiencies - approaching 90% if you charge them carefully - but they hold less energy per kg, so are not so good for cars. Tesla's Powerwall uses his car batteries, partly because the units can be made more compact and saleable, but probably more because Tesla is making car batteries anyway, and the user is unlikely to worry about lower round-trip efficiency.

Well, there is an answer for NeilBlanchard. The cost is ludicrous of course, but I imagine a billionaire wanting power on his remote island paradise might consider it as a possibility. I suspect the key to these things becoming less expensive comes down to getting rid of the compressor. If it can be reduced to an electrolyser and a tank, then the cost can be made far lower perhaps to the level where ordinary mortals can afford it. Honda seems to have a practical demonstrator of this sort of approach.

As to filling your car without the need for pumping it in, I think the key to this would be for the system to have a combination of a bigger tank running at higher pressure. As is well known, for a given mass of gas at constant temperature, the pressure times the volume is a constant. So if the dispenser has a tank the same size as that of the car, containing gas at 140MPa, then the result of connecting it to a car's tank at zero pressure would be both tanks pressurised to 70MPa. If the dispenser's tank were twice the volume of the car's, then you would need it to be pressurised to only 105MPa to fill the car's tank to 70MPa, and so on.

 

I am unconvinced that battery cars are cheaper than hydrogen ones with the same range. I suppose you could take Tesla's most efficient car, stuff it to the gills with yet more flashlight batteries, and achieve similar mileage to the Mirai or the Clarity, but it would cost a lot more.

The killer point, however, is not efficiency but convenience.

Give the consumer a car which gives him the same availability as his ICE car, all the advantages of an electric car, and no need to have to plug it in every time you park, to fiddle about with cables, and worry about how long it takes to charge, or whether the batteries are degrading, or whether he can put the heater on and still get home, and I think you will see which technology prevails.

 

Alas, not so. You need energy to 'warm it up' and that is 100% lost.

It isn't. It is powered by diesel.

 

It doesn't matter where the energy comes from. If you have to supply energy to get the energy out, it is lost energy. It would be pretty hard to have it plugged in as you drive off on a chilly morning anyway. Energy needed to warm it up then would HAVE to come from the battery.

 

Regarding narrowboats, I think a battery powered boat would probably be quite practical. There is no requirement to keep the weight down, indeed the reverse because they have several tons of ballast in them anyway. Lead acid would work fine, and the power needed to keep them gliding along at below the speed limit of 4mph is small.

The fly in the ointment - as usual - is charging them. I suppose the Canal and River Trust could install charging points, but as boaters tend to moor out in the countryside where there is no power available it would be very expensive to do this over well over 2,000 miles of canal. Solar panels are another possibility but it's not possible to collect enough power to run the boat, even on a sunny day. Much of the network is shaded by trees too, which don't help, and anyway, the canals wiggle about so the panels would not be pointing the right way most of the time.

So probably hydrogen would, overall, be less expensive. Unfortunately, there are no facilities for filling up with this along the towpaths either. So, diesel will continue to be used for the foreseeable future. One might speculate about using solar cells and a high differential pressure electrolyser to charge up a tank during periods when it is not in use, but I think that is pretty unlikely to happen for a very long time, if ever.

Interestingly, it would probably be worthwhile recirculating the water emerging from the fuel cell to be used again by the electrolyser were such a system to be devised. Canal water is pretty mucky, and you really need distilled water for electrolysis, so reusing it would make sense. The canal water could be used for condensing the steam that emerges from the fuel cell, so you don't really have a refrigeration problem to worry about.

However, for the time being, diesel is the standard. Mine is a 4 cylinder Vetus (a marinised Mitsubishi!) I use supercapacitors to start it in order to avoid hammering the battery, but vintage 100-year-old single cylinder engines are highly prized by enthusiasts who pay well over £10,000 to acquire one. These things are hand cranked to start, and thump away at about 50 to 100 rpm as they have done since the early part of the 20th century, whilst their owners polish the brasswork, oil the bearings and generally fondle them as they worry about where to get spares from. Not my idea of fun, but they seem to enjoy it.

 

I think fuel cells need to reach an operating temperature which may take some time, measured in a few tens of seconds I imagine. Bit like ICE car, which, by the way, once they warm up are more efficient in cold conditions. The air is denser then and contains more oxygen per unit volume than at warmer temperatures. You may have noticed improved mileage travelling at night due to this, although the difference is pretty small.

If the hydrogen condenses, you are dead and no longer care much! At 70MPa. I think its about -220 degrees centigrade, by which time the air would be liquid too! Most likely starting the car would be the least of your problems!