The Ergosphere
Tuesday, October 09, 2018
 

If power-to-gas (P2G) is the answer, what was the question?

Over at Green Car Congress, "Scottish Scientist" gushes about P2G:
Power to Gas (P2G) is best for (solar, wind etc.) farm-scale energy storage for most farms where there is no possibility of farm-scale pumped hydro.

P2G is excellent for mopping up all the surplus farm power because any energy which P2G can store is an efficiency gain compared to the 100% loss of all curtailed generation.

Grid managers should cease paying curtailment payments and spend the same money more wisely offering incentives to farm operators to install farm-scale energy storage.
In the comment below I had the temerity to ask
Simply not generating surpluses very much or often gets rid of most spilled power too, and also the capital and operating cost of generating it.  What's the goal here?
But mostly I wanted to go into the energetics in greater detail than I did there.

Present-day electrolyzers take around 43 kWh (154.8 MJ) of electricity to produce 1 kg of hydrogen.  This 1 kg of hydrogen has 141.88 MJ higher heating value and 119.96 MJ lower heating value.  Suppose this hydrogen is burned in a non-condensing context, such as a gas-turbine power plant or a kitchen stove.  Almost 1/4 of the input energy is lost between the electrolyzer inefficiencies and the latent heat of the lost water vapor.  Even if burned in a 60% efficient (LHV) CCGT, the net efficiency drops to about 45% before losses in pumping and storage are included.

What IS the goal of this exercise?  Suppose for a moment that it is to displace CO2 emissions.  How effective is P2G for this purpose?  Well, not very.  Replacing 1 kWh generated with OCGTs at 500 gCO2/kWh with 1 kWH generated with best-of-class CCGTs at 320 gCO2/kWh eliminates 180 grams of emissions.  Replacing 1 kWh generated with natural gas with 1 kWH generated by P2G hydrogen eliminates... (working the units)

1 kWh / 43 kWh/kg * 119.96 MJ/kg / 50 MJ/kg(CH4) * 2750 gCO2/kg(CH4) = 153.4 gCO2 eliminated per kWh put into P2G.  This will be roughly the same for any natural gas power plant, as it displaces fuel on a per-MJ LHV basis.

But that's not the end of it.  What's not usually talked about is the effect of "renewables" on the rest of the generating mix.  Due to the high ramp rates of wind and solar, the rest of the generation has to be highly flexible to compensate.  More efficient combined-cycle plants can't ramp quickly due to thermal constraints on the steam side, and they can often only turn their output down by 30% or so.  Given this (absent hydro), less-efficient open-cycle gas turbines are usually the only viable option.  This cuts the maximum thermal efficiency from as high as 62% down to around 40%.

This is a bait-and-switch of enormous size.  To get "renewable energy", you have to increase per-kWh emissions from the NG balancing generators on the order of 55% over what is achievable with CCGTs.  Renewables would require a capacity factor around 35-36% just to break even on emissions; less than that and emissions are WORSE!

America has definitely fallen for the bait-and-switch.  The job now is threefold:
  1. Get to a metric of emissions, period.  Where energy comes from is irrelevant; eliminate all portfolio standards and mandates, FITs, net metering, etc.
  2. Aim at fuel/carbon efficiency rather than RE generation.  RE which forces lower efficiency in the balancing generators can be worse than useless.
  3. Use appropriate market design and system architecture to get efficiency plus resiliency.
There are some options out there which can easily beat 153 gCO2 savings per kWh input.  The problem (if you can call it that) is that they are way, way outside the box of conventional thinking on energy matters.

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Comments:
Putting erratic part-time 'green' power on a grid with full time draw promotes instability when more than a minor percentage of input occurs. The gang at JoNova ( commenters ) have been scathing as to the dysfunction of such an arrangement. South Australia is doing an excellent job of shooting itself in the foot, following the path pioneered in Germany.
 
No argument here, opit.  No argument whatsoever.
 
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