Power-to-gas is a way to store extra renewable electricity by turning it into chemical fuel. When wind turbines or solar panels produce more electricity than people need at that moment, the surplus can power machines called electrolyzers. These machines split water into hydrogen and oxygen, making hydrogen gas that can be stored for later use.
This matters because chemical fuels can store energy for days, months, or even seasons.
Key Facts
- Electrolysis overall reaction: 2H2O(l) + electricity -> 2H2(g) + O2(g)
- Power input is P = IV, where P is power, I is current, and V is voltage.
- Electrical energy used is E = Pt, where E is energy, P is power, and t is time.
- Hydrogen can be burned or used in a fuel cell: 2H2 + O2 -> 2H2O + energy.
- Synthetic methane can be made by methanation: CO2 + 4H2 -> CH4 + 2H2O.
- Round-trip efficiency is usually less than 100 percent because heat and conversion losses occur at each step.
Vocabulary
- Power-to-gas
- Power-to-gas is a process that uses electricity to make a gaseous fuel such as hydrogen or synthetic methane.
- Electrolyzer
- An electrolyzer is a machine that uses electric current to split water into hydrogen gas and oxygen gas.
- Hydrogen
- Hydrogen is a light, energy-rich gas that can store energy and later release it in a fuel cell or by combustion.
- Methanation
- Methanation is a chemical reaction that combines hydrogen with carbon dioxide to form methane and water.
- Round-trip efficiency
- Round-trip efficiency is the fraction of stored energy that is recovered as useful energy after conversion, storage, and reconversion.
Common Mistakes to Avoid
- Thinking power-to-gas creates energy from nothing is wrong because it converts electrical energy into chemical energy while losing some energy as heat.
- Forgetting the oxygen product in electrolysis is wrong because water splitting produces both hydrogen and oxygen in a 2 to 1 molecule ratio.
- Assuming hydrogen and methane are the same fuel is wrong because hydrogen is H2 while methane is CH4 and they require different storage and safety systems.
- Ignoring efficiency losses is wrong because electrolysis, compression, methanation, storage, and electricity generation all reduce the final usable energy.
Practice Questions
- 1 An electrolyzer runs at 500 kW for 6 hours. How much electrical energy does it use in kWh?
- 2 If electrolysis produces 20 kg of hydrogen and the process is 70 percent efficient, how much electrical energy was supplied if the hydrogen stores 33 kWh per kg?
- 3 A town has extra solar electricity at noon but needs heat and electricity at night in winter. Explain why power-to-gas could help, and name one disadvantage compared with using a battery.