[set fair (OP)]

Supposing you electolyse a chemical and then recombine what you get to produce electricity. The main energy loss is in the recomnination? What is the % loss in each process?

[Bored chemist]

In principle, zero.
In practice , sometimes quite close to zero and sometimes the loss is so huge that the reaction is effectively irreversible.

[alancalverd]

Electrolytic cells generally get hot, but as BC says, the energy loss is extremely variable. To some extent it depends on closely matching the source and electrolyte impedances.   

[chiralSPO]

Often it depends on how patient you are. If you want to move or exchange lot of energy very quickly, you have to pay for it (energetically). Energy efficiency for splitting water into hydrogen and oxygen can be done with >90% efficiency using precious metal catalysts and low current densities. But once the currents involved are large enough that it is limited by the resistance of the solution, then you can basically view it as a resistor in the cell, producing heat with a power of current times overpotential (applied potential minus the thermodynamic potential). Energy efficiencies of <70% are acceptable if the energy/time is more important than energy/cost.

You didn't specify whether the recombination is electrochemical or not, or how you would like your energy.

If you want heat, I can get you a virtually 100% efficient source as any power you like up to a few thousand degrees °C. (ie a hydrogen oxygen flame, anywhere from a 50 W jewler's torch to a 2,000,000,000 W space shuttle engine).

If you want that energy in the form of DC electricity, you can use an acidic proton exchange membrane-based fuel cell to get 50–60% of your energy back from combining hydrogen and oxygen. (the rest goes to heat)

If you want AC, then either you burn it and drive a turbine, or convert DC to AC and take yet another conversion hit.

If you want light energy...

[Bored chemist]

Electrolytic cells generally get hot, but as BC says, the energy loss is extremely variable. To some extent it depends on closely matching the source and electrolyte impedances.   

In a way, yes.
Because you need to be very careful not to match the impedances.

Alan has remembered what the maximum power theorem says, but forgotten when it is applicable.

If you want a nearly reversible process- i.e. one with the least losses, you use a very high load impedance.

[Bored chemist]

If you want light energy...

...Heat a piece of quicklime with your oxyhydrogen torch.

[alancalverd]

If you want a nearly reversible process- i.e. one with the least losses, you use a very high load impedance.

and wait a very long time....(Faraday's Law)

[evan_au]

recombine what you get to produce electricity

There are a number of important cases where the main product is not electricity.

There are proposals to replace coal with hydrogen in steel manufacture. Heat is needed to melt the iron ore, and oxygen is used to reduce the carbon content to the required level. So you could electrolyse water from solar cells during the day, and use it + wind to power a 24-hour steel plant.

Another case is where the output product may be a chemical, and economical hydrogen itself may be a useful chemical ingredient. But this will require replacing the current production processes.

[alancalverd]

Politicians and environmental campaigners seem to be obsessed with electricity, but it accounts for less than a quarter of energy used in the UK, the majority of which is involved in prime movers (transport and site machinery) or heating - including industrial heat and space heating. Hydrogen is potentially a valuable replacement for fossil fuels in these applications, although it is a lot easier to manage if combined with a bit of carbon!

[Bored chemist]

it is a lot easier to manage if combined with a bit of carbon!

In reality, it turns out we were not managing it in that system, were we? We kept letting go of the carbon.

[Bored chemist]

There are proposals to replace coal with hydrogen in steel manufacture.

Not just a proposal.
https://thedriven.io/2021/10/14/volvo-says-this-is-the-worlds-first-vehicle-made-from-fossil-free-steel/

[alancalverd]

Still an interesting idea. Polestar say that making one of their mid-size electric cars releases 29 tonnes of CO2, about as much as an existing diesel car will emit in 100,000 miles. Around 50% of UK electricity is generated from fossil fuels, and that number will almost certainly have to increase in the short term to meet the demand of electric cars.So the CO2 break-even point between keeping your old oilburner and buying a new "electric" gasburner is about 150,000 miles - about twice the expected life of the battery.

It is therefore essential that mining, steelmaking and metalbashing turn to hydrogen power to preserve the credibility of those who advocate electric transport as a means of combatting climate change. Where the hydrogen comes from is awholenother question.

[Bored chemist]

It is therefore essential that mining, steelmaking and metalbashing turn to hydrogen power to preserve the credibility planet

Fixed it for you.

[Bored chemist]

Polestar say that making one of their mid-size electric cars releases 29 tonnes of CO2, about as much as an existing diesel car

Which is why we should make electric ones, rather than diesel.
It's not about the current vehicle stock; we can't change that.
It's about planning for the future.

[alancalverd]

You miss the point. Electric cars demand CO2 generation (a) to make the cars and (b) to run them. Any attempt to phase out diesel by replacing working stock makes things worse, and it is arguable that over 250 - 300,000 miles (no big deal for a modern diesel car) building and running the diesel will actually emit less CO2 than building the electric car, running it, and replacing the battery 3 or 4 times.

It is true that all-electric transport would not emit any CO2 if it were entirely powered by renewables. It is also true that pigs can fly with a sufficiently large engine.

"Entirely powered by renewables" is, frankly, ignoring the weight of the elephant. We have had working electric vehicles since Faraday's time,  but even though the wind has been blowing for ever and the sun shines from time to time, we are nowhere near to meeting likely future energy demands from renewables.

[Bored chemist]

Any attempt to phase out diesel by replacing working stock makes things worse

Which part of

It's not about the current vehicle stock; we can't change that.

did you not understand?

we are nowhere near to meeting likely future energy demands from renewables.

Bollocks.
"As of December 2020, renewable production generated 40.2% of total electricity produced in the UK"
https://en.wikipedia.org/wiki/Renewable_energy_in_the_United_Kingdom
About 15 years ago it was essentially 0%
If we assume a  linear profile- which is simplistic but easy to calculate- we will reach 100% in about another 22 years time.
We are already well on the way to doing what you say is impossible.

[alancalverd]

"As of December 2020, renewable production generated 40.2% of total electricity produced in the UK"

According to Gridwatch, wind peaked at around 10 GW for a few days and averaged about 5 from Dec 2020 to present. The total installed capacity of all renewables is about 50% of average demand, but wind (15GW installed) rarely delivers more than 20% of capacity. Solar (10 GW installed) only works half the time at best, maxed at 3 GW on a couple of lunchtimes this summer, and delivered 0.5GW average over the year. Average demand over the year was 30GW

But what do they know, eh?

[alancalverd]

The problem is government-encouraged aggressive marketing of electric cars as a Good Thing, which has led people to stop using perfectly serviceable internal combustion vehicles because, inter alia, they are cheaper to run whilst the public subsidises them from road fuel tax  at around 70% compared with domestic electricity at 5%. 

[Bored chemist]

"As of December 2020, renewable production generated 40.2% of total electricity produced in the UK"

According to Gridwatch, wind peaked at around 10 GW for a few days and averaged about 5 from Dec 2020 to present. The total installed capacity of all renewables is about 50% of average demand, but wind (15GW installed) rarely delivers more than 20% of capacity. Solar (10 GW installed) only works half the time at best, maxed at 3 GW on a couple of lunchtimes this summer, and delivered 0.5GW average over the year. Average demand over the year was 30GW

But what do they know, eh?

Renewables share of electricity generation was 37.3 per cent in Quarter 2 2021,
...this was largely a result of much less favourable weather conditions for renewable
generation with lower wind speeds and fewer sun hours
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1021952/Energy_Trends_September_2021.pdf

[alancalverd]

Or 27% if you look at the actual figures in the previous paragraph. That's government statistics for you. And how interesting that as the weather got warmer and the days got longer, the contribution of renewables fell from 40.2% in December, assuming you believe that figure.

Problem is that we have no control over wind speed and sun hours. Except, of course, for the assertion that anthropogenic climate change is delivering more of both.

However you look at it, we are still a long way from delivering 75 GW of electricity continuously from renewables