World pledges to save 'Mother Earth' despite Trump's snub to climate pact

[sirineou]

22 hours ago, ExpatOilWorker said:

 

VERY inefficient way of storing energy. You are basically throwing 50-75% of the energy away. 

yea you are right compressing and decompressing is a good way to heat or cool your house but if the heat exchange Challenger could be worked out.....or re-channeled usefully.

[ExpatOilWorker]

8 minutes ago, sirineou said:

yea you are right compressing and decompressing is a good way to heat or cool your house but if the heat exchange Challenger could be worked out.....or re-channeled usefully.

To look at a definite example, if we take 1 cubic metre of air at 1 atmosphere pressure and 20C and compress it to 10 atmospheres pressure, its temperature will increase very considerably - to 293C.  If we want to store this compressed air at 10 atmospheres pressure and 20C, then more compression will be needed as we cool the gas, or its pressure will drop as its temperature does.  The total work done on the gas, and the total heat lost are both about 91.7 Watt-hours (Wh).  (This assumes that the air is an ideal diatomic gas.)

This gas would now have a lower entropy than the same amount of uncompressed air.   The entropy change is 796 J/K (Joules per degree Kelvin).  Note the units are energy per degree.  This gives a hint of how the entropy change is related to the work that can in principle be extracted from the compressed air.  That work can be calculated by multiplying the entropy change by the temperature of the environment in degrees Kelvin.  20C is 293K, so the amount of work that can in principle be extracted is 233 kJ, or 64.8 Wh.  If we compare this with the work done compressing the gas, we see that the efficiency of the process is about 71%, even if the compressor is perfectly efficient.

Looking at the expansion of the same air back to 1 atmosphere, using a motor to do work in the process, we can work out that the temperature will fall to -121C, and that the work that is done would be 47.5Wh.  The efficiency of ths process is thus 47.5/64.8 = 73%, even with a perfect motor.  The round-trip efficiency for energy storage and use would then be just 52%.  With real compressors and motors it would clearly be considerably worse.  These numbers above are for a compression ratio of 10.  If we instead use a compression ratio of 100, things get worse still, with a round-trip efficiency of 27%.

This actually gives a clue as to how to improve the situation.  The maximum efficiency of the cycle depends on the pressure ratio, and rises to 100% as that ratio approaches 1.  The answer is to use staged compression, with cooling back to ambient temperature between the stages, and staged expansion, with reheat back to ambient temperature between stages.  If we get the 100 times compression by two stages of times 10 each, then half the work goes into the first stage and half into the second, with efficiencies as for 10 times compression - a huge improvement.  If we use four stages (ratio 3.17), then the maximum effficiency would be 72%.  If we take into account that real compressors and engines are not perfect, and neither are coolers and reheaters, we can see that real overall efficiencies achieved are never likely to be very good, even with very complicated equipment

[ExpatOilWorker]

31 minutes ago, sirineou said:

Thank you  Haqlloween

As the passengers of the Hindenburg  quickly found out, Hydrogen is indeed a volatile agent.

But you are correct the crux  of the problem is indeed efficiency.

I said it before

The stone age did not end because they run out of stones. And the same will occur with the petrochemical era.

We are so close, It would be a shame to fail now.

 

 

Ask yourself who make the bulk on money of oil and gas and your dream is suddenly moving a bit further away?

 

The answer is states and governments, through tax, royalties and national oil companies. Poor little ExxonMobil and the other International Operators are just getting a few breadcrumbs left on the floor.