# It more efficient to generate heat burning electrolyzed hydrogen, or through an electric resistance?

It's convenient and simple to use electricity to generate heat, doubtlessly, say in en electric kettle or boiler or heater, but is it more efficient to generate heat burning electrolyzed hydrogen, or through the resistance (kettle / boiler / heater)? Assume the same amount of electricity was used.

• Is the hydrogen to be burned in the same "space" where it was generated? or is the hydrogen to be generated in one place, and then stored (compressed? chemically?) and transported to be burned in some other place at a later time? Aug 22, 2021 at 15:56
• Also, you said, "generate heat," but did you have any particular temperature in mind? (i.e., do you want to heat your bedroom in winter time? or do you want to fuse silica? A low-pressure hydrogen/oxygen flame is hot enough to fuse silica, but I'm not so sure that an electric furnace can do that trick.) Aug 22, 2021 at 15:59
• There is a debate in the renewables community about the desirability of green hydrogen (electrolyzed using wind or solar generated electricity) for home heating vs using the electricity directly for heating. i.e if hydrogen has a role to play in home heating, or only in long range shipping and aircraft. (I added this clarification to the question) Aug 23, 2021 at 16:10
• If you're comparing the cost of distributing electric power over a wide area vs. distributing hydrogen over the same, then that's a complicated question. A lot of electric power is lost in transmission. Electric transmission lines have resistive losses, capacitive losses, leakage, and "transformer inefficiencies." With hydrogen, you'd have to account for energy lost in compressor stations, cost of trucking it (if applicable), leaks from the plumbing, in addition to the efficiency with which it ultimately is used by the consumer. Aug 23, 2021 at 16:41
• To maximize efficiency, you need to minimize the number of energy conversion steps. Also note - the "renewables community" needs to learn some science and thermodynamics. Developing good debate skills without a sound scientific foundation for those debates will lead to a lot of errors, wasted time, wasted energy, and wasted effort. Aug 23, 2021 at 18:31

According to this wikipedia article, electrolysis has an efficiency between 70-80%, so not all of the electric energy used for electrolysis would go into dissociating water molecules, but some would be lost as heat.

Therefore, the energy that can be obtained from burning the hydrogen obtained from electrolysis would be less than the electric energy spent for electrolysis.

Sorry for my poor english. My native language is french.

I think there is something missing in your question. The current in the resistance is not enough to find the energy. You also need voltage. 1 A at 1 V is different from 1 A at 1000 V. So I assume the same voltage : the minimum voltage to produce a certain amount of hydrogen and oxygen. It suppose that we produce the hydrogen and oxygen reversibly (very slowly, at the minimum necessary voltage). In this case, the minimum electrical work is equal to the variation of Gibbs free energy : $$W=∆G$$

Since I assume the same voltage and the same current, this energy would be that transformed into heat if it was supplied it to a resistance.

Now, the question is whether the combustion of hydrogen produces more or less heat than the energy required to produce it. The thermal energy released is this time the variation of enthalpy $$W'=∆H$$.

The relation between the two is $$∆G=∆H-T∆S$$ with evidently $$∆S>0$$ for liquid water electrolysis. So we find $$W : If I am not mistaken, you gain more heat when you produce hydrogen and then burn it.

The difference does not seem negligible since we have a Gibbs standard free energy of 237 KJ / mol and a standard enthalpy of 285 KJ / mol.

Obviously all this is theoretical and does not take into account the efficiency lower than 1.

• Merci beaucoup! I added an explanation (" there is an ongoing debate in the renewables community about the desirability of green hydrogen (i.e. electrolyzed using wind or solar generated electricity) for home heating vs using the electricity directly for heating - whether hydrogen has a role to play in home heating, or only in long-range shipping and aircraft.") Aug 23, 2021 at 16:21