In order to store wind energy in the form of hydrogen, current generated by wind turbines (or solar PV arrays) is run through water (H2O) to create hydrogen (H2) and oxygen (O2) in a process known as electrolysis, using a device called an electrolyser (or electrolyzer).
Ideally, hydrogen is made only when there is no other immediate direct use for the wind generator’s electricity. When that is the case, hydrogen is storing power that would otherwise simply be discarded without any source of payment for the wind generator’s owners. While it is not technically correct to say that this excess power is free when used to make hydrogen, it is accurate to say that the cost is very low.
Technologies differ, but a useful rule of thumb is to say that it takes about 60 kilowatt hours of electricity to make 1 kilogram kg) of hydrogen. Using the kilogram as a unit of H2 measure is helpful because a kilogram of hydrogen contains roughly the same amount of energy as a gallon of gasoline.
Thinking in these terms also focuses our attention on the fact that wind power stored as hydrogen can be used in place of gasoline as a vehicle fuel. A single 6MW offshore wind machine with a 50% utilization factor could make hydrogen equal to 1200 gallons of gasoline every day.
Taking a moment to reflect on electrolysis as a power storage method and hydrogen as a gasoline substitute reveals that the technology of storing wind energy as hydrogen necessarily involves several technologies, once the need to make money, or at least not lose money, is added to our thought process.
The ultimate cost of using electrolysis to store wind energy is affected not just by the design of the electrolyser and size and pressure of the storage tanks, but also by:
a. The technologies used to capture, store and use or sell, the oxygen generated by the electrolysis;
b. The technologies required for the storage, possible transport or possible on-site dispensing of hydrogen for use as a vehicle fuel (see the Transportation section);
c. The technology used to convert the stored hydrogen back to electricity if the facility operator’s goal is to help the grid’s ability to use more renewable power or to create a 24/7 power supply for a use that is directly connected to the wind generator or solar array. Possible choices include:
(i) A typical internal combustion generator running on H2 stored in high-pressure tanks near the wind or PV generator, instead of diesel, gasoline or other fossil fuel
(ii) Using a Fuel Cell to convert the locally tank-stored hydrogen back to electricity. See also this page:
Wind Energy Storage Today's Products
(iii) Using a remote combined cycle natural gas power plant by pumping hydrogen from the electrolyser into a natural gas pipeline that is feeding the power plant. Mixing hydrogen and natural gas in the same pipeline involves well-known technologies that require little if any modification to existing pipelines unless and until the amount of hydrogen in the line exceeds 15% of the total volume.
Similarly, natural gas/hydrogen mixtures at this level can be used with modifying the power plant. The mixture will burn cleaner than natural gas alone and reduce the amount of that fossil fuel that must be burned. This technology is referred to as “Power to Gas” and is discussed in greater detail in the next several paragraphs.
(iv)Converting stored hydrogen back to electricity using an internal combustion engine generator or fuel cell may or may not result in acceptable electricity costs. Inefficient (15% to 30%) internal combustion engines have a far lower purchase price than more efficient fuel cells (40% to 60%).
The critical difference for a particular installation may lie in how well the generator or fuel cell captures and uses excess heat. With total efficiencies, including heat capture, exceeding 90% for many fuel cell installations, the more expensive fuel cell may nevertheless be the better choice.
When trying to picture a wind-to-electricity-to-hydrogen-to-electricity system, several different possibilities arise.
a. An electrolyser that makes the hydrogen, the tanks that store it, and the generators or fuel cells that generate new electricity from the “stored wind power,” are all located in close proximity to the wind generators. Electricity is delivered to end-users through the same wires that the wind machines use for direct delivery of electricity to end users.
b. The installation made up of an electrolyser, storage tanks and a generator or fuel cells receives electricity from wind machines that are many miles away on land or many miles offshore. In this scenario, the eletrolyser/storage/generator or fuel cell facility is just another “end user” when seen from the perspective of the wind machines. The transmission lines from the generator or fuel cells to the consumer-end-user may or may not be the same lines that run between the wind machines and other end users.
c. An electrolyser is located near the wind machines but the hydrogen gas is transported by pipeline, ship, rail or truck to the generator/fuel cell site. If the internal combustion engine generator in this scenario is actually the turbine in a combined cycle natural gas power plant, we have the “Power to Gas” method mentioned in c(iii) above.
Power-to-Gas has several significant aspects:
a. No hydrogen storage tanks are required. The natural gas pipeline system acts as a storage “tank” with a capacity that, for practical purposes in the near and medium term, is unlimited, given the amount of hydrogen that is likely to be produced using this method.
b. There is no need to connect the wind machines to end users using transmission lines. All of the output from the wind machines can go to the electrolysers producing hydrogen that becomes “green fuel” for the natural gas power plant. The efficiency losses from elecrolysis (+/- 50%) are more than offset by saving the cost of new transmission lines – existing lines from existing power plants will continue to carry the combined cycle plant’s output.
c. Although the electroyser must be near a natural gas pipeline, the wind machine can be miles away, even offshore. Recall sub-paragraph (b) in the previous section. A single line from the wind machines to the electrolyser is all that is required.
d. Another attribute of the natural gas system is seasonal storage. In many areas around the world, there is a seasonal pattern to the wind generation profile. Unfortunately, periods of high wind output may come during the shoulder periods of lower electricity demand. Power-to-Gas provides not only TWh of storage, but seasonal storage capability as well. This effectively enables the renewable gas produced from surplus renewable generation as part of the fuel stream of existing gas-fired generators at the most suitable time. It is the best of both worlds.
e. The Achilles Heel of Power to Gas, and utility scale renewable hydrogen systems generally, is right-sizing the electrolyzers. Luckily the technology scales up when smaller units are connected together, making “modularity” the industry’s hoped-for answer to the size question. The largest onshore wind farm (Alta Wind Energy Center in California) has a capacity of 1,000 MW.
Currently, a number of companies are developing megawatt-scale PEM and alkaline electrolysers and the belief is this will facilitate the first steps toward proving electrolyser technology in this relatively new application.
Currently, the world’s largest single stack PEM electrolyser is Hydrogenics’ 1 MW system it recently announced will be installed at an E.ON power-to-gas facility in Hamburg, Germany. Obviously in the future much larger systems will be made, but they are likely to contain multiple stacks, or be connected to form a multi-megawatt system.
This information was posted by Kay Mann for Rick Smith, President of the Hydrogen Energy Center.