Talk:High-temperature electrolysis

Untitled

Have reworded quite a bit: Removed this bit "which will be used in the future to produce hydrogen for cars through HTE of water." as it's a bit too speculative. Would be nice to get some theoretical hydrogen economy in there if anyone has any ideas on how to word something so far-reaching? Have also removed the 20% conversion efficiency too, as it's really quite variable depending on temperature, process, etc. A nice link to it would help, so long as that first para doesn't get too unwieldy 82.69.54.182 05:01, 18 Jun 2005 (UTC)

The reference to pyrolysis should be removed. just looking at the respective pages for pyrolysis and hydrogen shows that pyrolysis is not used currently to generate hydrogen gas. — Preceding unsigned comment added by 124.168.56.180 (talk) 10:22, 14 July 2015 (UTC)

Conservation of energy

I'm looking at the DOE page referenced from this one, and I'm confused.

If a process requires 225 megajoules (thermal) to produce 145 megajoules (1 kg) of hydrogen, where is the other 80 megajoules? I presume it has to show up as heat at the electrolyzer output, which is at 850 degrees C. The graph on page 2 shows that the process has "thermal energy input", which suggests the reaction is endothermic. So if 850 C requires 40 MJ/kg thermal energy input, does that mean it's actually 120 MJ/(kg output) at the input, at some temperature higher than 850 C, and rejecting 80 MJ/(kg output) at the output at 850 C? If so, how hot is the input?

The DOE page suggests 1 kg of hydrogen produced comes out as 3 kg hydrogen, 8 kg oxygen, and 18 kg steam, and enters as 2 kg hydrogen and 27 kg steam. To carry in 40 MJ of thermal energy, that mix would have to be 690 K hotter, or 1540 C! You'd think the DOE page would mention a scary fact like that.

Iain McClatchie 06:42, 17 October 2005 (UTC)

To produce the electricity using a heat engine heat has to flow from a hot area to a cold area. Because a heat engine is limited in efficiency by the temperature difference according to Carnot's theorem for an idealised heat engine, it can at best achieve an efficiency of: 1-Tc/Th. Because Tc will be the temperature of the heatsink it is in practice set by the temperature of teh environment surrounding your engine, and will normally be about 300 Kelvin. Since this temperature is far too low to be used as input for the high temperature electrolysis ( heat cannot flow from a colder region to a hotter one, and most high temperature electrolysis schemes use about 1000 Kelvin input temperature ) it is just wasted as spill-heat in the coolant. High temperature electrolysis achieves a higher conversion efficiency than normal electrolysis because a high fraction of the energy is supplied as heat, and thus you need less electricity, leading to less heat being wasted in the coolant. Theoretically you could supply all the energy required as heat, but then the temperature would have to exceed 25000 C, which isn't practical. Thus instead one aims to supply a fraction of the energy as heat and a fraction as electricity. In general the conversion efficiency can never achieve more than the Carnot limit of 1 - Tc/Th, but during normal electrolysis the efficiency is generally lower than this since the efficiency of generating the electricity is typically lower than the Carnot limit, and this allows HTE to achieve efficiency gains by using less electricity and supply more heat directly. However, the carnot limit does not apply to energy supplied as electricity, so if the efficiency of the electricity generating heat engine was closer to the Carnot limit, that would offset the benefits of high temperature electrolysis. Thus HTE ( as well as other thermochemical hydrogen production schemes ) is mainly interesting because the turbines we use to create electricity are less than optimal. If highly efficient Stirling engines could be scaled economically the benefit of producing hydrogen thermochemically would be reduced. 85.224.76.122 (talk) 22:51, 8 December 2007 (UTC)

All that aside, as a picky point 141.86/225 = 0.6305 which is not quite 64%. More importantly, the article uses the high heating value (HHV) for hydrogen of 142 kWh/kg when the LHV of 120 kWh/kg may be more applicable, which would make the efficiency only 120/225 = 53%. With 22% efficient solar panels the respective compound efficiencies are .22*.63 = 14% and .22*.53 = 11.7%. The difference matters when compared with the very recently claimed 14% efficiency of direct solar-to-hydrogen production.

But why not go directly from 225 MJ/kg to cost per kg? Isothermally (slowly) compressing 1 kg of H2 to 70 MPa at an efficiency of 50% takes an additional 20 MJ, and cooling is not needed when filling up very slowly. 245 MJ = 68 kWh which at (e.g.) PG&E's off-peak rate of 10 cents/kWh gives a cost of $6.80/kg of 70 MPa H2. This is considerably better than the $16.50 currently charged at 70 MPa hydrogen pumps in California, though not as good as what FCV drivers pay through 2019, namely zero thanks to the auto manufacturers. The capital cost of a 12 kW electrolyzer producing 3.3 g of H2 a minute and a 70 MPa compressor for that flow rate must be added to this. Vaughan Pratt (talk) 00:48, 30 August 2016 (UTC)

The efficiency of solar panels should not even come into this calculation. This is a misuse of a piece of information that is simply an expression of the he area required to generate a given amount of energy under standard conditions. So therefore 12 gy coming from solar panels is 12 kW, and it’s. The transmission loss is also very low to the point of use. Higher efficiency panels would simply require a smaller footprint to do the same thing, which I don’t think is meaningful for this conversation. 65.92.1.136 (talk) 18:23, 19 April 2024 (UTC)

link out of date

the externel link to the DOE is not working— Preceding unsigned comment added by 62.68.29.247 (talk) 13:46, 7 November 2006 (UTC)

Link restored converting from http to https.--Robertiki (talk) 10:52, 2 August 2021 (UTC)

Convention in battery symbol

The battery symbol of the first figure is the reverse of convention where the (+) terminal is shown as the longer line segment. I'm nit picking here but inconsistency can cause confusion in certain contexts {Spyglasses (talk) 02:26, 25 April 2021 (UTC)}

Economic Potential

"At current hydrocarbon prices, HTE can not compete with pyrolysis of hydrocarbons as an economical source of hydrogen, which produces carbon dioxide as a by-product."

Current (written in past) hydrocarbon prices, does not reflect geopolicially inflated natural gas prices that may be current at time of reading. Needs a specific benchmark price. Pyrolysis is not the 20th century economic production method for hydrogen. Steam reforming is. Pyrolysis creates graphite (solid) and hydrogen. No CO2 byproduct. — Preceding unsigned comment added by 2607:FEA8:2260:E00:49E1:4FAC:68F0:6397 (talk) 23:38, 3 December 2022 (UTC)