Electricity storage.

The idea is based on three existing concepts:

A. The high temperature electrolysis requires two times less electricity (see 7.3).

B. The recovering of the heat in high temperature molten salts (see 7.2) : the heat is recovered when operating in generator mode and is used when operating in electrolysis mode.

C. The use of formic acid as a hydrogen tank (see 7.1).

1. Electrolysis.

The electrolysis of H2O, for example in a SOFC electrolyzer to 1000 ° K (727 ° C) requires a electricity amount of 193 KJ / mol (see 7.3)  and a heat input to maintain the desired elevated temperature.


2. Electricity generator.

A SOFC generator can reach an electrical efficiency of 60 to 70%, so:
electrical energy produced by the generator =
245 x 0.7 = 171 KJ / mol (if 70% efficiency)
245 x 0.6 = 147 KJ / mol (if 60% efficiency)


3. Rate recovering of electrical energy.

electric energy generated by the generator / electric energy used for electrolysis =
171/193 = 0.886. (If 70% yield)
147/193 = 0.762. (If 60% yield)

NB: 1) the overall performance would be very high since the bulk of the heat is recovered,
2) by comparison, turbines and pumping stations (STEP) have a rate recovering of about 0.75.


4. Another source of hydrogen.

By steam reforming of methane biogas.

(1) steam reforming of methane: CH4 + H2O -> CO + 3H2 (deltaH = + 206 KJ / m)
        (Conditions: 850 to 950 ° C and 20 to 30 bar)

(2) reaction with CO: CO + H2O -> CO2 + H2 (deltaH = - 41 kJ / m)
              (250 ° C)

Overall reaction: (1) + (2): CH4 + 2H2O -> CO2 + 4H2 (deltaH = 165 KJ / m)

To account for the CO2 content in the biogas, we can write:
CO2 + CH4 + 2H2O -> 2CO2 + 4H2 (deltaH = 165 KJ / m)

For one mole of methane and the supply of heat, it can therefore be 4 moles of formic acid, using 2 moles of CO2 in the CO2 reserves.

5. High-temperature heat (molten salts).

H2O vapor dissociation enthalpy = 245 KJ / mol.

Energy to supply = 245 / 0.9 = 272 kJ / mole.

Heat supply = 272-193 = 79 kJ / mol.


It must be added, considering the heat exchanges:
a) the difference between the heat of vaporization of H2O (41 KJ / mole) and the reaction heat (32 kJ / mol), 9 KJ / mole,
b) the heat to raise the temperature of H2O vapor of 543 ° K (270 ° C t ° saturation of steam at 50 bar) to 1000 ° K, or 20 KJ / mole.

Heat supplied by the molten salt for electrolysis:
79 + 9 + 20 = 108 kJ / mole.

Heat to receive the generator:
0.3 x 245 = 74 kJ / mol (if 70% yield)
0.4 x 245 = 98 kJ / mol (if 60% yield)
But we must deduct 7 kJ / mol, for compensation for the exchange of heat in Z and Y, either:
67 KJ / mol (if 70% yield)
91 KJ / mol (if 60% yield).


For biogas:
heat to be supplied by the reserve of molten salts (for steam reforming of methane): 165 kJ / mol
Heat receive (in generator mode):
4 x 67 = 268 kJ / mol (if 70% yield)
4 x 91 = 364 kJ / mol (if 60% yield)


6. Benefits.

1. These would be used to:
a) regulate the variability of renewable energy (wind, photovoltaic)
b) ensure the extra consumption during peak hours.

2. They become true power stations supplied by:
a) renewable energy in our regions,
b) biogas (converted to formic acid)
c) concentrating solar power plants located in other regions (the CO2 is sent to these areas and, at the same time, the formic acid is received with fewer losses than the transmission of electricity by lines).

3. These mini plants also serve to supply:
a) Vehicles that bring the liquid CO2 and leave with formic acid (such vehicles therefore do not reject CO2)
b) the chemical industry (uptake of CO2 or formic acid).

4. Linking these mini power plants with a line double pipe (formic / CO2 acid) allow total flexibility, by providing sampling points for vehicle filling stations and industry.

7. Notes.

1. Patents have been filed by Gabor Laurenczy and colleagues at the Ecole Polytechnique Fédérale de Lausanne (EPFL).

2. This system is used in concentrating solar power plants in order to provide electricity at night, thanks to the heat accumulated during the day.


3. (Next: ineris.fr STUDY REPORT 10/06/2008)

Changes in the quantity of energy needed for electrolysis of water
function of temperature:

Temperature (K) Energy (kJ / mol)
298                        405
400                        380
1000                      193
1200                      165


4. (Next: AFHYPAC)

"High Temperature Electrolysis: This technology is a direct result of the developments of the SOFC type fuel cell, operating in the range 650 - 1000 ° C.
She reveals interesting if it supplies both electricity and heat to maintain the desired elevated temperature performance can then be greater than 80%.
It is essentially intended to be coupled to a solar system concentration or a high temperature nuclear reactor. "

5. http://www.industrie-techno.com/90-de-rendement-pour-un-systeme-d-electrolyse-du-cea.34969

A system for producing hydrogen by electrolysis of water vapor at high temperature has been validated by CEA Liten. The system has a 90% yield.