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H2 from seawater 7
- Thread starter thorangle
- Start date
Unclear, supercaps are still in their infancy.
I would further argue that transporting anything material or physical is another loser in the overall system efficiency. How is that more efficient than using power lines?
How many trucks does it take to transport "thousands of megawatt-hrs" of energy?
TTFN
I would further argue that transporting anything material or physical is another loser in the overall system efficiency. How is that more efficient than using power lines?
How many trucks does it take to transport "thousands of megawatt-hrs" of energy?
TTFN
- Thread starter
- #22
Power lines are not with out losses. In fact, transporting hydrogen via pipeline is more effcient (less energy lost) than transmitting electricity via transmission lines for distances over approximatly 200 miles. The exact distance escapes me, the point is that over realtivily short distances (several hundred miles) the transportation of hydrogen is economical. With electrical transmission you have transformer losses at the generation stepup transformer, line losses from I^2R, and several stepdown transformer losses. Hydrogen gas transport via pipe line incounters pipeline friction losses and compressor station losses.
Trucking is not required (an option to remote areas), the natural gas pipeline infastructure is already in place. Utilities in Austraila are experimenting with mixing hydrogen and natural gas, and delivering that mixture, via pipelines to their customers, both residential and commercial.
Trucking is not required (an option to remote areas), the natural gas pipeline infastructure is already in place. Utilities in Austraila are experimenting with mixing hydrogen and natural gas, and delivering that mixture, via pipelines to their customers, both residential and commercial.
HydroScope
Mechanical
thorangle,
my words excatly, except 200 miles is not a long distance and power lines would be used to. The problem 9 times out of ten with useing renewable energy like tidal energy etc. is that the renewable energy energy source is no where near a in place grid system thus by the time you install the power lines etc and power systyem that can cope with major fluctuations with the power generated then, you find that most of the energy does not make it to it's useful point of end use. that is where hydrogen comes in it can be piped 1000kms with minimal loss and essential is storing 100'sMWhours as it runs down the pipe
my words excatly, except 200 miles is not a long distance and power lines would be used to. The problem 9 times out of ten with useing renewable energy like tidal energy etc. is that the renewable energy energy source is no where near a in place grid system thus by the time you install the power lines etc and power systyem that can cope with major fluctuations with the power generated then, you find that most of the energy does not make it to it's useful point of end use. that is where hydrogen comes in it can be piped 1000kms with minimal loss and essential is storing 100'sMWhours as it runs down the pipe
moltenmetal
Chemical
You've forgotten about one of the major losses of high voltage transmission lines: radiation of the energy off into space via the generation of radio waves.
Hydrogen transportation in a pipeline isn't the main loss in hydrogen as a storage medium for energy. Rather, the main energy losses are in making the hydrogen, and then in compressing the hydrogen up to a pressure which makes pipeline transportation economical, and then in the fuelcell (provided you can afford it!). If the route is to make electricity from tidal energy, then make hydrogen from it to make it cheaper to transport to destination, I'd say you're off your tree. If the goal is the storage of off-peak generation energy, that's a different story, because there are so few alternatives for energy storage.
I'd wager that the capital cost differences between building an electrical transmission line and a high pressure hydrogen pipeline are not insignificant either, with electricity being the clear winner on that front.
The best use would be to generate some electricity-intensive product nearby- aluminum, say. That would permit you to "transport" the energy without so many losses, while displacing product which is made by more conventional means.
Hydrogen transportation in a pipeline isn't the main loss in hydrogen as a storage medium for energy. Rather, the main energy losses are in making the hydrogen, and then in compressing the hydrogen up to a pressure which makes pipeline transportation economical, and then in the fuelcell (provided you can afford it!). If the route is to make electricity from tidal energy, then make hydrogen from it to make it cheaper to transport to destination, I'd say you're off your tree. If the goal is the storage of off-peak generation energy, that's a different story, because there are so few alternatives for energy storage.
I'd wager that the capital cost differences between building an electrical transmission line and a high pressure hydrogen pipeline are not insignificant either, with electricity being the clear winner on that front.
The best use would be to generate some electricity-intensive product nearby- aluminum, say. That would permit you to "transport" the energy without so many losses, while displacing product which is made by more conventional means.
Regarding distributing hydrogen by pipeline, if the existing infrastructure for natural gas is to be used, there will be problems with hydrogen embrittlement unless the pipelines are replaced/upgraded. Depends on what those pipelines were made of.
HydroScope
Mechanical
Molten:
When hydrogen produced from electrolysis it is possible to have the hydrogen bubble off at high pressure by placing the electrodes at a water depth, using nat gas pipeline for example, often at 400kpa then this pressure can be acheived in lass than 10M of water. no need for loss of compresson if a long distance is necessary than a larger water depth to begin with can be used. I am unaware of a reduction in electrolysis efficency at depth.
If the hydrogen is to be then used to generate electricity it can be used as a fuel in a conventional Internal combustion engine at high efficency, if the engine's fuel/air mixture is leaned out to equivelence ratio 2.5 engine efficency often is above 40% (ford model U recently realised 38% brake eff). These in turn can turn a generator. At equivelence ratio of 2.5 and above then no NOx is produced, no need for a catalytic convertor
Hydrogen Internal combustion charistics are coming along now. With many benifits of hydrogen addition into even the highly polluting desiel and petrol engines with increases in brake efficency, lower HC , lower CO, lower Co2, lower N0x, thus reduces running cost and good for environment and this set to save the bus companies millions in fuel costs.
Have you heard of the air car? it runs of compressed air. . I meantion it because the compression of hydrogen is a major issue and it shows that the compression energy in hydrogen tank could also be utilised by a correctly designed engine! and hence drive further again.
cost of powerlines coming down because of bad weather hence snow storms or cyclones are a major issue and in countries where these occur regularly most residents don't depend on electricity gas is everywhere because of it realability in such a situation. Hydrogen well ahead there simply because of realiability.
Molten I don't understand this:
The best use would be to generate some electricity-intensive product nearby- aluminum, say. That would permit you to "transport" the energy without so many losses, while displacing product which is made by more conventional means.
Celebur
certainly does depend on the pipelines. pipes in aussie have shown 10% vol is safe while it might be possible to reach 20% by vol no need for upgrade etc.
When hydrogen produced from electrolysis it is possible to have the hydrogen bubble off at high pressure by placing the electrodes at a water depth, using nat gas pipeline for example, often at 400kpa then this pressure can be acheived in lass than 10M of water. no need for loss of compresson if a long distance is necessary than a larger water depth to begin with can be used. I am unaware of a reduction in electrolysis efficency at depth.
If the hydrogen is to be then used to generate electricity it can be used as a fuel in a conventional Internal combustion engine at high efficency, if the engine's fuel/air mixture is leaned out to equivelence ratio 2.5 engine efficency often is above 40% (ford model U recently realised 38% brake eff). These in turn can turn a generator. At equivelence ratio of 2.5 and above then no NOx is produced, no need for a catalytic convertor
Hydrogen Internal combustion charistics are coming along now. With many benifits of hydrogen addition into even the highly polluting desiel and petrol engines with increases in brake efficency, lower HC , lower CO, lower Co2, lower N0x, thus reduces running cost and good for environment and this set to save the bus companies millions in fuel costs.
Have you heard of the air car? it runs of compressed air. . I meantion it because the compression of hydrogen is a major issue and it shows that the compression energy in hydrogen tank could also be utilised by a correctly designed engine! and hence drive further again.
cost of powerlines coming down because of bad weather hence snow storms or cyclones are a major issue and in countries where these occur regularly most residents don't depend on electricity gas is everywhere because of it realability in such a situation. Hydrogen well ahead there simply because of realiability.
Molten I don't understand this:
The best use would be to generate some electricity-intensive product nearby- aluminum, say. That would permit you to "transport" the energy without so many losses, while displacing product which is made by more conventional means.
Celebur
certainly does depend on the pipelines. pipes in aussie have shown 10% vol is safe while it might be possible to reach 20% by vol no need for upgrade etc.
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1
- #27
I'm going to jump in here - as the manufacture and patent holder of various EC applications - it does not take much to change sea water or any other aqueous solution where sodium chloride is present - into hyperchloric acid - the voltage is small - try like the voltage level of the batteries in your flashlight - so it does not take much soalr produced DC voltage to start the reaction - however, as simply a soure of hydrogen gas - it takes greater voltage to get any economic value of such a treatment - so I think you should look at the process different - such as can the hydrochloric acid be created, then added to another "product" to create an explosion - much like a combustion engine used gasoline - this then would make more sense, especially if the resultant gases/by-product of the explosion is a harmless gas/vapor.
If using an acid - is one better then the other for such a combination - EC can create phosphoric acid, nitric acid, carbolic acid, etc. - so the solution may not be "hydrogen fuel cells" but an acid reaction fuel cell using sea water as the "crude" source.
The other point - sea water is not pure - you have heavy metals, phosphates, carbonates, and other "impurities" that may have to be removed or accounted for in the reaction.
Dave Orlebeke/Aquatic Technologies
If using an acid - is one better then the other for such a combination - EC can create phosphoric acid, nitric acid, carbolic acid, etc. - so the solution may not be "hydrogen fuel cells" but an acid reaction fuel cell using sea water as the "crude" source.
The other point - sea water is not pure - you have heavy metals, phosphates, carbonates, and other "impurities" that may have to be removed or accounted for in the reaction.
Dave Orlebeke/Aquatic Technologies
- Thread starter
- #28
My original question had to do with; what to do with off-peak electrical power produced from a tide stream generator. Given that tidal flows are very predictable (especial comparied to other forms of alternative energy) but frequently will not match up with peak electrical requirements.
Transmission capital costs:
Given that the generators would be located off shore. I have been told by a colleague of mine, that the cost of installing undersea cable or pipe line is in the equipment to do the job, not the actual cable or pipeline. My colleague assures me that for the undersea segment of these "transmission facilities" the cost is approximatly the same. Once on shore, then I believe the electrical transmission line may be cheaper.
Assuming that capital cost are the same to get the energy commodity to shore, what are some thoughts about hydrogen production on shore at one larger facility versus hydrogen production equipment located at each tidal stream generator?
Generator voltages:
A typical AC generator produces electricity in the 13-15 KV range. (FYI, This is typically close to the voltage range selected for ditributing electrical power in suburban areas.) I suppose the generator could be manufactured to produce a voltage idealy suited for hydrogen electrolysis. What is the ideal voltage for hydrogen electrolysis?
Transmission capital costs:
Given that the generators would be located off shore. I have been told by a colleague of mine, that the cost of installing undersea cable or pipe line is in the equipment to do the job, not the actual cable or pipeline. My colleague assures me that for the undersea segment of these "transmission facilities" the cost is approximatly the same. Once on shore, then I believe the electrical transmission line may be cheaper.
Assuming that capital cost are the same to get the energy commodity to shore, what are some thoughts about hydrogen production on shore at one larger facility versus hydrogen production equipment located at each tidal stream generator?
Generator voltages:
A typical AC generator produces electricity in the 13-15 KV range. (FYI, This is typically close to the voltage range selected for ditributing electrical power in suburban areas.) I suppose the generator could be manufactured to produce a voltage idealy suited for hydrogen electrolysis. What is the ideal voltage for hydrogen electrolysis?
DC voltage is the proper type - any voltage ove 2.41vDC will "split" the hydrogen-oxygen bond of water. Howerver, it only takes 1.2vDC to break salt water into hyperchloric acid and chlorate. The "process" becomes more efficient the higher the voltage - and can actually reverse the hyperchloric devlopment by off-gassing of the chloride - into sodium hydroxide and other alkaline componds.
An interesting invention is patent number 3,959,663 - "Tide Powered Electrical Generator". It involves lifting a weight with the incoming tide, thereby storing Potential Energy that can power an electrical generator.
Puget Sound, with its 10 foot or higher tides, could take advantage of this type of device. A float could be installed under any of the many fixed piers in Seattle, Tacoma or Olympia. The rising tide could wind a spring or raise a weight attached to a gear driven generator. The float itself could be of significant enough mass (concrete) allowing it's falling with the outgoing tide to add to the power output. The potential energy could be stored and used to provide electrical power at waterfront properties.
Puget Sound, with its 10 foot or higher tides, could take advantage of this type of device. A float could be installed under any of the many fixed piers in Seattle, Tacoma or Olympia. The rising tide could wind a spring or raise a weight attached to a gear driven generator. The float itself could be of significant enough mass (concrete) allowing it's falling with the outgoing tide to add to the power output. The potential energy could be stored and used to provide electrical power at waterfront properties.
- Thread starter
- #31
Muggle
Is there an upper limit (theoretical and/or practical) of the DC voltage for "splitting" the hydrogen-oxygen bond? What is the upper limit based on?
DubyaDee
Interesting patent, I looked up the patent on the US patent office web site. The pilings on the piers I can look at, appear to be spaced 1 to 2 meters apart in a grid pattern. I wonder if any studies have been done to determine an optimum size of the "float/barge" for the vertical tide range in a given area.
Is there an upper limit (theoretical and/or practical) of the DC voltage for "splitting" the hydrogen-oxygen bond? What is the upper limit based on?
DubyaDee
Interesting patent, I looked up the patent on the US patent office web site. The pilings on the piers I can look at, appear to be spaced 1 to 2 meters apart in a grid pattern. I wonder if any studies have been done to determine an optimum size of the "float/barge" for the vertical tide range in a given area.
There is no known upper limit - mostly one of efficiency - meaning once you reach "x" volts, the process does not get any faster when additional voltage is added - and the other problem is evaporation - too much voltage and the heat builds to the point you get very rapid evaporation.
Some firms uses both - the breakdown of the saltwater to create hydrochloric acid, plus evaporation, evenutally releasing sufficient chloride that sodium hydroxide is created - this i snot a new concept to desalination plants. Figure upper reach is 400vDC.
Some firms uses both - the breakdown of the saltwater to create hydrochloric acid, plus evaporation, evenutally releasing sufficient chloride that sodium hydroxide is created - this i snot a new concept to desalination plants. Figure upper reach is 400vDC.
moltenmetal
Chemical
The "efficiency" of an electrolysis process will depend on the concentration (activity) of hydrogen at the electrode. The activity of hydrogen depends on pressure. Therefore the voltage you require for electrolysis increases with the pressure at which the hydrogen must be evolved. Whether that pressure is a result of depth of electrolyte (i.e. piezometric head) or merely by using a valve to keep the hydrogen constrained within the electrolytic cell makes no difference.
What results is an energy efficiency loss to produce compressed hydrogen. It may be less than the loss required to compress the hydrogen using a multi-stage mechanical compressor, or not- but it's still a loss. The energy you put in to store it in higher density form is gone forever as heat.
When you let down a compressed gas through a piston engine (i.e. like the "air car"
, you get only the PV energy back. If you want to get a little more than that back, you need to use a Stirling-type engine, or you need a big gas-to-gas exchanger so you can use the atmosphere to heat up your gas after expansion. Hydrogen's a little different because of its negative Joule-Thompson coefficient, but you're still certainly getting nowhere the energy you put in to compress the hydrogen in the first place, back out as work to drive the vehicle. I've run the numbers on the air car and can't understand why anyone would do it, unless
Face it, guys- hydrogen is hydrogen. One of its fundamental properties is that there are only weak Van Der Vaals forces attracting hydrogen molecules to one another- so weak in fact that the normal boiling point is ~ 20 Kelvin. This material would much rather be a gas...To overcome the repulsive forces of molecular collisions resulting from stuffing a lot of molecular hydrogen into a small space, you need to put in some energy- and you only get some of that back, if any. Doesn't matter if you use chemical adsorption by forming hydrides, or if you use physical adsorption using carbon nanotubes, or if you don't bother with adsorption at all and just use massive pressures and store it as a compressed gas or compressed refrigerated liquid. Hydrogen's inherently poor as a high-density energy storage medium.
What results is an energy efficiency loss to produce compressed hydrogen. It may be less than the loss required to compress the hydrogen using a multi-stage mechanical compressor, or not- but it's still a loss. The energy you put in to store it in higher density form is gone forever as heat.
When you let down a compressed gas through a piston engine (i.e. like the "air car"
, you get only the PV energy back. If you want to get a little more than that back, you need to use a Stirling-type engine, or you need a big gas-to-gas exchanger so you can use the atmosphere to heat up your gas after expansion. Hydrogen's a little different because of its negative Joule-Thompson coefficient, but you're still certainly getting nowhere the energy you put in to compress the hydrogen in the first place, back out as work to drive the vehicle. I've run the numbers on the air car and can't understand why anyone would do it, unless Face it, guys- hydrogen is hydrogen. One of its fundamental properties is that there are only weak Van Der Vaals forces attracting hydrogen molecules to one another- so weak in fact that the normal boiling point is ~ 20 Kelvin. This material would much rather be a gas...To overcome the repulsive forces of molecular collisions resulting from stuffing a lot of molecular hydrogen into a small space, you need to put in some energy- and you only get some of that back, if any. Doesn't matter if you use chemical adsorption by forming hydrides, or if you use physical adsorption using carbon nanotubes, or if you don't bother with adsorption at all and just use massive pressures and store it as a compressed gas or compressed refrigerated liquid. Hydrogen's inherently poor as a high-density energy storage medium.
cstallardva
Aerospace
I woulf question the economic transport of hydrogen by pipeline. The energy content of gaseous hydrogen, especially at low pressures that a typical pipeline can withstand is relatively small compared to a petroleum based fuel. To transport a reasonable amount of energy via pipeline hydrogen, the velocity of the hydrogen within thewill become so great that friction losses will become substantial to the point of becoming uneconomic.
- Thread starter
- #35
The Heating Value range for Natural Gas is approximatly 38.1 to 42.5 MJ/kg. The heating value range for hydrogen is approximatly 120.1 to 141.9 MJ/kg. The economics of transporting natural gas via pipeline is well established. Natural gas transmission pipelines are frequently designed and opperated at pressures up to and exceeding 800 pounds per square inch (about 5.5 Mpa).
With a heating value 3 times that of natural gas, one could argue that energy transport via hydrogen would be more economical than natural gas.
With a heating value 3 times that of natural gas, one could argue that energy transport via hydrogen would be more economical than natural gas.
GregLocock
Automotive
Oh. So we'll forget about density then shall we?
Cheers
Greg Locock
Cheers
Greg Locock
- Thread starter
- #37
GregLocock Excellent point!
Hydrogen being about 6 times less dense than Methane. Power consumed in a compressor being directly proportional to density. Everything else being equal (and perhapse a little over simplified). Hydrogen (gas) transport via pipeline should be significantly cheaper!
The issue then become cost of production? Can hydrogen be produced in the range of $4.00 to $5.00 per MMBTU/FT^3?
Hydrogen being about 6 times less dense than Methane. Power consumed in a compressor being directly proportional to density. Everything else being equal (and perhapse a little over simplified). Hydrogen (gas) transport via pipeline should be significantly cheaper!
The issue then become cost of production? Can hydrogen be produced in the range of $4.00 to $5.00 per MMBTU/FT^3?
a) compressor power will increase due to low density.
b) adding H2 to a natural gas pipeline has a lot disadvantages, including changing the combustion stability of any equipment that fires the gas ( Wobbe induex, flashback, etc)- one can kiss goodbye any warranties on gas turbines or large boilers that fire the mixed gas. This is a big issue ( in the opposite direction) now being considered in the USA by pipeline companies and users due to the expected change in natural gas composition associated with increased use of LNG and also reduced recovery of heavies from gas fields
c) H2 will leak through seals in valves and other joints much faster than methane, and may be incompatible with some metals and some sealants
d) H2 can be economically produced using bit coal or Pet coke gasification, but the reliability of individual components is an issue. Each gasification vessel may have a max continuous runnig life of about 6 months ( due to corrosion) prior to major maintenance, so spare vessels are needed .
b) adding H2 to a natural gas pipeline has a lot disadvantages, including changing the combustion stability of any equipment that fires the gas ( Wobbe induex, flashback, etc)- one can kiss goodbye any warranties on gas turbines or large boilers that fire the mixed gas. This is a big issue ( in the opposite direction) now being considered in the USA by pipeline companies and users due to the expected change in natural gas composition associated with increased use of LNG and also reduced recovery of heavies from gas fields
c) H2 will leak through seals in valves and other joints much faster than methane, and may be incompatible with some metals and some sealants
d) H2 can be economically produced using bit coal or Pet coke gasification, but the reliability of individual components is an issue. Each gasification vessel may have a max continuous runnig life of about 6 months ( due to corrosion) prior to major maintenance, so spare vessels are needed .
moltenmetal
Chemical
davefitz: you can make H2 economically from coal ONLY if you discard the product CO2 to the atmosphere. The cost of permanent non-biological sequestration of that CO2 will dwarf the cost of replacement gassification vessels. The CO2 emissions, plus the chained inefficiencies of the hydrogen generation, compression/storage/delivery/storage and fuelcell train, eliminate any net environmental benefit of using hydrogen in the first place. Doubt it's at all better than just burning the coal directly in a cogen plant and doing a good job of off-gas clean-up.
Fuelcells are a great part of a renewable energy infrastructure, but you need the renewable energy sources to replace existing fossil-fuel derived electrical generation capacity FIRST. Fossil-fuel derived hydrogen as a stop-gap measure on the way there is not sensible from an engineering perspective- everything I've seen tells me that the net environmental benefit is marginal at best, and indeed may not exist at all once you take everything into account. Fuelcell technology right now is not driven by hydrogen- it's driven by media hype, generated by people who profit from this technology and kept aloft by politicians looking for the magic "technological fix" to our energy woes.
Fuelcells are a great part of a renewable energy infrastructure, but you need the renewable energy sources to replace existing fossil-fuel derived electrical generation capacity FIRST. Fossil-fuel derived hydrogen as a stop-gap measure on the way there is not sensible from an engineering perspective- everything I've seen tells me that the net environmental benefit is marginal at best, and indeed may not exist at all once you take everything into account. Fuelcell technology right now is not driven by hydrogen- it's driven by media hype, generated by people who profit from this technology and kept aloft by politicians looking for the magic "technological fix" to our energy woes.
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1
- #40
macroscopic
Mechanical
Hi there.
I just wanted to jump in and mention that even though I am very excited about energy storage systems, a tidal energy generator doesn't neccesarily need to store the energy at off peak times does it? And if it does, the storage location doesn't have to be the same as where the generation occurs. Tidal energy is very predictable and can therefore be sold on the electric power spot market pretty easily. The energy can be used to offset other fossil fuel plants.
This is a problem with wind and solar energy, because it is not predictable, and since the energy needs to be sold in advance in most deregulated spot markets, it does not work well within the current energy trading system. Storing the energy makes it much easier to integrate into the grid.
I'm currently working on a project looking at adding energy storage systems into large offshore wind farms. I was looking for info about ocean water electrolysis and found this forum. I am excited to join the community.
I also wanted to note that even though there are inefficiencies in storing energy, due to the variations in the market value of energy from peak to off peak hours, it can still make financial sense. The differences can be upwards of 500% in some markets. So even a 50 % loss can still make money. I even see the potential in the future for local (customer side) energy storage systems that would allow people to buy energy from the grid at night (when it's cheap) and sell it during peak periods..
I also wanted to throw in some of the storage systems that are out there that haven't been discussed:
CAES: compressed air energy storage. This is usually done in large underground caverns but can be done in vessels. As mentioned in this thread, there are some vehicles that use compressed air. This is heavily being looked at for wind power, an interesting variation is to couple the aerodynamic rotor directly to the compressor, skipping the mechanical->electrical->mechanical energy conversion process.
Reversible Fuel Cells. (still in infancy)
Flywheels
Pumped Water. (this is really the only utility scale energy storage system in use today, except for a couple of compressed air systems.) Pumped hydro storage could be used with a tidal energy generator and managed by the utility.
For my location, I will be looking at steam reforming of methane that happens to exist under our location. This will generate H2 that will be burned in a gas turbine or fuel cell. The heat from burning the H2 will drive the steam reformer...and... hey.. perhaps it will distill the seawater before it goes into the electrolyser.
(I just thought that.. awesome! )
I still haven't found solid info about salt water electrolysis. Does it really need to be purified? I am looking at a system in the Gigawatt range...I mean big big big.
(i hope i'm not to verbose...)
john
I just wanted to jump in and mention that even though I am very excited about energy storage systems, a tidal energy generator doesn't neccesarily need to store the energy at off peak times does it? And if it does, the storage location doesn't have to be the same as where the generation occurs. Tidal energy is very predictable and can therefore be sold on the electric power spot market pretty easily. The energy can be used to offset other fossil fuel plants.
This is a problem with wind and solar energy, because it is not predictable, and since the energy needs to be sold in advance in most deregulated spot markets, it does not work well within the current energy trading system. Storing the energy makes it much easier to integrate into the grid.
I'm currently working on a project looking at adding energy storage systems into large offshore wind farms. I was looking for info about ocean water electrolysis and found this forum. I am excited to join the community.
I also wanted to note that even though there are inefficiencies in storing energy, due to the variations in the market value of energy from peak to off peak hours, it can still make financial sense. The differences can be upwards of 500% in some markets. So even a 50 % loss can still make money. I even see the potential in the future for local (customer side) energy storage systems that would allow people to buy energy from the grid at night (when it's cheap) and sell it during peak periods..
I also wanted to throw in some of the storage systems that are out there that haven't been discussed:
CAES: compressed air energy storage. This is usually done in large underground caverns but can be done in vessels. As mentioned in this thread, there are some vehicles that use compressed air. This is heavily being looked at for wind power, an interesting variation is to couple the aerodynamic rotor directly to the compressor, skipping the mechanical->electrical->mechanical energy conversion process.
Reversible Fuel Cells. (still in infancy)
Flywheels
Pumped Water. (this is really the only utility scale energy storage system in use today, except for a couple of compressed air systems.) Pumped hydro storage could be used with a tidal energy generator and managed by the utility.
For my location, I will be looking at steam reforming of methane that happens to exist under our location. This will generate H2 that will be burned in a gas turbine or fuel cell. The heat from burning the H2 will drive the steam reformer...and... hey.. perhaps it will distill the seawater before it goes into the electrolyser.
(I just thought that.. awesome! )
I still haven't found solid info about salt water electrolysis. Does it really need to be purified? I am looking at a system in the Gigawatt range...I mean big big big.
(i hope i'm not to verbose...)
john
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