Talk:High-temperature electrolysis
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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)
[edit] 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)
[edit] link out of date
the externel link to the DOE is not working
