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Depends. If the purpose of this is to take sunlight when it’s abundant and turn it into hydrogen as a storage mechanism, then the efficiency of the exchange may

Depends. If the purpose of this is to take sunlight when it’s abundant and turn it into hydrogen as a storage mechanism, then the efficiency of the exchange may not matter as much.

thereisnospork 5y ago
Efficiency always matters; opportunity costs. If sunlight to hydrogen is your goal a novel solution has to at bare minimum be more efficient than solar PV -> electricity -> electrolyzer[0] to be useful. [0]Known, boring technology with 99.9% of the engineering warts and technical debt already solved.
- mlyle 5y ago
  It has to be more -cost- efficient, not more thermodynamically efficient.
  - thereisnospork 5y ago
    One tends to follow the other. Arguing that, in the judgement of process for the conversion of one form of energy (sunlight) to another (hydrogen), thermodynamic efficiency, or the measure of how well a process converts one form of energy to another isn't relevant is somewhere between obnoxiously pedantic and wishful thinking. Funnily enough I read the article after your comment, and it is even less useful, interesting, than my initial assessment (not even direct solar to H2). It has approximately the novelty and utility of porting DOOM to <insert processor here> and running it in a refrigerator.
    - angus-prune 5y ago
      In the real world energy isn’t always fungible. The location of the energy can be an important factor in something’s overall efficiency. If you need the end product at a particular location, the transport costs can overwhelm any differences in efficiency. The same also applies if you have excess energy in a particular location that you want to use. In either case, the transport costs can make using the less efficient process, the more energy efficient choice overall.
- new_realist 5y ago
  No, it doesn't; it just has to cost less, overall. Efficiency is only a weak proxy for cost.
  - clomond 5y ago
    *unless we are talking about energy systems and therefore VARIABLE costs matter significantly.
  - mrpopo 5y ago
    Not if you want to consider environmental impact. The petrol in the ground is free, sunlight is free, biodiversity is free. The cost is only a human factor. Higher efficiency on the other hand usually means lower resource consumption.
    - johnchristopher 5y ago
      Why is that downvoted ? You might have the cheapest energy source, if it's not efficient enough to meet demand without ruining the environment for humans then it's not good. A weak proxy for cost ? Or a strong one for survival ? A cheaper steel bike is less efficient than an expensive carbon fiber bike when your goal is to get to the mountain top before the flood submerges the valley.
  - wiz21c 5y ago
    And cost is sure not a proxy for efficiency.
AtlasBarfed 5y ago
Hydrogen is in an economic race with batteries in storage. Hydrogen is hard to store in tanks and has a higher thermodynamic loss and the fuel cells are expensive. Batteries are big and expensive. So the efficiency of hydrogen split-store-generate needs to beat the efficiency of store-release of the battery and the costs of the equipment.
- londons_explore 5y ago
  So far, batteries are pretty competitive on the "store energy for 24 hours" time horizon - where you need to power something at night with solar collected in the day, or evening out peaks and troughs in the electricity grid. But it isn't yet competitive with "store energy collected in the summer for use in the winter". For that usecase, far more energy must be stored for far fewer dollars to make it competitive vs just having a big fuel tank and a winter-only diesel generator. Keeping lakes of water for a few months for hydropower are also competitive - the storage capacity of a big lake on a mountain top is immense.
  - AtlasBarfed 5y ago
    If CATL and other LiFePO battery makers get them to 200wh/kg, which they teased in 2020, and they maintain the same cycle endurance, temperature range, safety, and cost advantages, it will be a real game changer. I've heard pumped water is like 90% efficient, but you need a gradient and space. Does wind power decrease in winter? I also remember schemes for making a huge solar farm in the Sahara and a superconducting conduit to Europe, and that was several paradigm shifts in solar cell performance ago (15 years). But almost all large countries seem to have a sufficiently sized desert to do this with (US, Russia, China). If solar gets good enough, and it is still on some prime parts of the economies of scale price drops, you don't care as much about long-distance transmission loss.
- ArkanExplorer 5y ago
  But we're already going to be building those batteries anyway: Electric cars. What we need is a proper international standard for cars to be able to interact with the grid, to be able to charge and discharge based on grid conditions and pricing. Its stupid to have everyone coming home from work at 6pm and plugging in their cars to charge right at the existing afternoon demand peak, when solar is waning. Just getting those cars to delay charging until around midnight (allowing user override) would shift a huge amount of peak demand. Same concept if you don't use your car during the day - it should be able to discharge right into the morning and afternoon peaks, at your control. The user should be credited for this onto their car account. It might even make sense to have dedicated high-voltage circuitry in houses and apartment blocks, to be used for EVs, solar, etc. And we should really figure that out immediately since EV production is ramping up right now. We can't just rely on single automakers (Tesla) to work out these societal problems.

Depends. If the purpose of this is to take sunlight when it’s abundant and turn it into hydrogen as a storage mechanism, then the efficiency of the exchange may not matter as much.

thereisnospork 5y ago

Efficiency always matters; opportunity costs. If sunlight to hydrogen is your goal a novel solution has to at bare minimum be more efficient than solar PV -> electricity -> electrolyzer[0] to be useful. [0]Known, boring technology with 99.9% of the engineering warts and technical debt already solved.

mlyle 5y ago
It has to be more -cost- efficient, not more thermodynamically efficient.
- thereisnospork 5y ago
  One tends to follow the other. Arguing that, in the judgement of process for the conversion of one form of energy (sunlight) to another (hydrogen), thermodynamic efficiency, or the measure of how well a process converts one form of energy to another isn't relevant is somewhere between obnoxiously pedantic and wishful thinking. Funnily enough I read the article after your comment, and it is even less useful, interesting, than my initial assessment (not even direct solar to H2). It has approximately the novelty and utility of porting DOOM to <insert processor here> and running it in a refrigerator.
  - angus-prune 5y ago
    In the real world energy isn’t always fungible. The location of the energy can be an important factor in something’s overall efficiency. If you need the end product at a particular location, the transport costs can overwhelm any differences in efficiency. The same also applies if you have excess energy in a particular location that you want to use. In either case, the transport costs can make using the less efficient process, the more energy efficient choice overall.
new_realist 5y ago
No, it doesn't; it just has to cost less, overall. Efficiency is only a weak proxy for cost.
- clomond 5y ago
  *unless we are talking about energy systems and therefore VARIABLE costs matter significantly.
- mrpopo 5y ago
  Not if you want to consider environmental impact. The petrol in the ground is free, sunlight is free, biodiversity is free. The cost is only a human factor. Higher efficiency on the other hand usually means lower resource consumption.
  - johnchristopher 5y ago
    Why is that downvoted ? You might have the cheapest energy source, if it's not efficient enough to meet demand without ruining the environment for humans then it's not good. A weak proxy for cost ? Or a strong one for survival ? A cheaper steel bike is less efficient than an expensive carbon fiber bike when your goal is to get to the mountain top before the flood submerges the valley.
- wiz21c 5y ago
  And cost is sure not a proxy for efficiency.

AtlasBarfed 5y ago

Hydrogen is in an economic race with batteries in storage. Hydrogen is hard to store in tanks and has a higher thermodynamic loss and the fuel cells are expensive. Batteries are big and expensive. So the efficiency of hydrogen split-store-generate needs to beat the efficiency of store-release of the battery and the costs of the equipment.

londons_explore 5y ago
So far, batteries are pretty competitive on the "store energy for 24 hours" time horizon - where you need to power something at night with solar collected in the day, or evening out peaks and troughs in the electricity grid. But it isn't yet competitive with "store energy collected in the summer for use in the winter". For that usecase, far more energy must be stored for far fewer dollars to make it competitive vs just having a big fuel tank and a winter-only diesel generator. Keeping lakes of water for a few months for hydropower are also competitive - the storage capacity of a big lake on a mountain top is immense.
- AtlasBarfed 5y ago
  If CATL and other LiFePO battery makers get them to 200wh/kg, which they teased in 2020, and they maintain the same cycle endurance, temperature range, safety, and cost advantages, it will be a real game changer. I've heard pumped water is like 90% efficient, but you need a gradient and space. Does wind power decrease in winter? I also remember schemes for making a huge solar farm in the Sahara and a superconducting conduit to Europe, and that was several paradigm shifts in solar cell performance ago (15 years). But almost all large countries seem to have a sufficiently sized desert to do this with (US, Russia, China). If solar gets good enough, and it is still on some prime parts of the economies of scale price drops, you don't care as much about long-distance transmission loss.
ArkanExplorer 5y ago
But we're already going to be building those batteries anyway: Electric cars. What we need is a proper international standard for cars to be able to interact with the grid, to be able to charge and discharge based on grid conditions and pricing. Its stupid to have everyone coming home from work at 6pm and plugging in their cars to charge right at the existing afternoon demand peak, when solar is waning. Just getting those cars to delay charging until around midnight (allowing user override) would shift a huge amount of peak demand. Same concept if you don't use your car during the day - it should be able to discharge right into the morning and afternoon peaks, at your control. The user should be credited for this onto their car account. It might even make sense to have dedicated high-voltage circuitry in houses and apartment blocks, to be used for EVs, solar, etc. And we should really figure that out immediately since EV production is ramping up right now. We can't just rely on single automakers (Tesla) to work out these societal problems.