I am doing some experiments on hydrogen production by water electrolysis and trying to calculate the efficiency both in terms of systems efficiency and energy efficiency using the equations included in the pdf file. But the answers I get are really not reasonable.
I would appreciate if someone could shine some lights on this. I want to find out how much energy I use to produce say for example a 45ml of hydrogen gas using electricity and how much energy is available if that amount of hydrogen is used in a fuel cell.
By the way, I am trying to use high heating value of 141.8 MJ/kg of hydrogen that is 141.8KJ / mlilli liter of hydrogen.
The energy efficiency (or thermal efficiency) is just the ratio of output energy divided by the input energy. In the water electrolyzer case, your input energy in the electric power used (kW) consumed over a period in time (t) measured in units of kWh to produce a certain mass of H2. The energy output is the energy content in the H2 mass collected over this same period of time (t).
Output energy = Mass (in kg) of H2 collected during time t times the H2 heating value (MJ/kg).
Accordingly, the energy (i.e., thermal) efficiency = Energy contained in the estimated H2 mass divided by the electric energy (kWh converted to MJ) consumed to produce this mass of H2 collected over a period of time t.
The system efficiency can be calculated when you factor the parasitic power consumed in other equipment associated with the electrolyzer such as gas pumps and other sources of energy losses in the system.
Hope this discussion is helpful to you.
Yehia F. Khalil Thank you for a great suggestion.
Output energy seems to be straight forward, that is to multiply the amount of hydrogen produced by the heating value.
However, the the input energy might be a bit challenging as the current changes both with time and temperature.
I am using a constant 31V power device to electrolyte but the current changes every second for the time of experiment. not really sure if it is enough to use the average current.
Yes, you can take the average current.
Remember that the electric power input = Voltage x Current = V.I
When you multiply the input power my the time (t) to produce (m) mass of H2, you will get the input energy to the electrolyzer.
Power x time = energy
Hope you find this additional details helpful!
I am using HHV for hydrogen but struggle to convert HHV which is 141.7MJ/kg to J/ml.
Any suggestions ?
By the way, energy used is UIt where U is voltage. Voltage in this case is 31 and that doesn't change.
Javad Hikmati
Assuming your calculations are to be done at STP (Standard Temperature and Pressure) conditions, 1 mole of H2 (weighs 2 grams) will occupy 22400ml of volume. So 1 kg will occupy (500*22400)ml.
As per the HHV given 141.7 MJ/kg which means 141.7*10e6 J per (500*22400)ml of Hydrogen quantity, we can find how much will be the Heat value for 1 ml of Hydrogen and that will be
[(141.7*10e6)/(500*22400)]= 12.65 J/ml
Hope this helps
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