70-75% efficient solar powered natural gas to hydrogen production

Solar-to-Chemical Energy Conversion Efficiency is defined as the ratio of the increase in the Higher Heating Value (HHV) in the reacting stream to the direct (non-diffuse) solar energy that is incident upon the parabolic dish concentrator.

(From your link)

I don't know how much energy is used to push the reaction, compared to how much is in the starting reactants.
However,

Molecular Weights
H2 - 2
CH4 - 16
CO2 - 44

Specific Energy
H2 - 141.86 MJ/Kg
CH4 - 53.60 MJ/Kg
Stoichiometry: CH4 + O2 -<> CO2 + 2 H2

So even before we consider the energy input conversion losses,
16 kg of methane (CH4), equal to 16*53.60 MJ,
becomes 4 kg of H2, equal to 4*141.86 MJ

CH4: 16*53.60 = 857.6 MJ
H2: 4*141.86 = 567.44 MJ

 

More calcs:
PV + water + NG -> CO2 + H2
Looks like this:
CH4 + 2 H2O -> CO2 + 4 H2

So 16 Kg of methane results in 8 Kg of H2
The authors say that the reacting stream gains about 30% heat value, and that this amount is 70% of the PV input. That implies that the PV input is 0.3/0.7 = ~ 0.43x of the starting heat value.

If I am getting this right, then in units MJ
16 kg of methane + PV = 16*53.6 + 16*53.6*0.43 MJ results in 8*141.86 MJ of H2

16*53.6+16*53.6*0.43 = 1226.368 MJ
8*141.86 = 1134.88 MJ
Then the conversion efficiency from methane to H2 is 1135/1226 = 92.6%

That sounds quite good but remember that we are starting from PV supplied electricity that does not have a carbon footprint (or at least we ignore it.) However, the conversion loss about equals transmission losses in a grid, and the fuel cell is no where near as efficient as a (battery+motor). So to me the moral is: use the PV as electricity for immediate end use or battery charging if you can, and store it as hydrogen if you cannot.

There is an element of point of view in this analysis by the authors. They view NG as the fuel celebre and want to use it efficiently. I view PV and wind as the fuel celebre, and want to increase its utility to the grid.

 

I don't either, but it does require steam. In typical steam reformation of natural gas, the heat to make the steam comes from burning the NG or H2. This is replacing those fuels with concentrated solar.

Besides the hydrogen production, there was some other applications in the presentation. The methanol from NG would the same type of efficiency boost as hydrogen production. The turbine power production claims to lower carbon emissions; I just don't see how.

Being a wet blanket, the <$2 GGE for hydrogen is sans pressurizing and transport, and it is still just a switch to another fossil fuel.

It isn't PV, but concentrated solar heat. Magnifying glass and ants.

 

Indeed ?
That certainly improves the NG case.

I'll do the arithmetic later.

 

I was thinking more in terms of turning 70% of solar heat into "hydrogen" rather than ~ 40% into electricity.

 

Net Reaction:
CH4 + 2H2O --> CO2 + 4H2

I am having trouble grasping why this is better than conventional steam methane reforming (reaction above) which already makes a whole lot of H2 without too much CO2 yield. But those are huge plants with a lot of heat integration (energy recovery). But maybe this new idea fits in as smaller plant. Lots of times you may have a smaller methane stream (eg; flare gas) but there really is no mini-plant technology capable of making methanol or H2 from that.

 

True, a large plant can employ plenty of tricks to recover heat.
This maybe useful stations with onsite reformation. Though the parabolic dish can make site location problematic.

 

Take #2 ..

To recap from above,
16 Kg of methane + heat results in 8 Kg of H2
The authors say that the reacting stream gains about 30% heat value, and that this amount is 70% of the heat input. That implies that the heat input is 0.3/0.7 = ~ 0.43x of the starting heat value of the methane

If I am getting this right, then in MJ units
16 kg of methane + heat = 16*53.6 + 16*53.6*0.43 MJ results in 8*141.86 MJ of H2

16*53.6+16*53.6*0.43 = 1226.368 MJ
8*141.86 = 1134.88 MJ


Comparison:

• Methane combusted to electricity at 40 - 60% efficiency
• CSP + turbine that makes electricity. 28% conversion efficiency to electricity presumed

Versus

• CSP + Methane to make hydrogen

Arithmetic for use of fuels without hydrogen pathway"

• We start with 16*53.6*0.4, up to 16*53.6*0.6 MJ electricity from methane = 343 – 515 MJ
• 16*53.6*0.43*0.28 electricity from the CSP = 103 MJ

so depending on how efficient the NG power plant is we end up with 446 – 618 MJ of electricity
OR 1134.88 MJ of hydrogen

The break-even point for a hydrogen fuel cell in terms of efficiency (and carbon intensity) then is
446/1138 = 39% if compared to a 40% efficient NG power plant
644/1128 = 55% if compared to a 60% efficient NG power plant

 

^^ This all sounds quite comparable for hydrogen Vs NG combustion if we ignore costs, but I feel compelled to point out that the comparison is somewhat contrived because we are presuming the CSP exists and can be either used for hydrogen or electricity productions. More likely, the heat for the hydrogen conversion is coming from combustion, and a PV field rather than CSP is making electricity.