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| Fig. 1: The Hybridkraftwerk Prenzlau, a hybrid powerplant in Germany which uses hydrogen electrolysis to store excess energy produced by wind turbines. (Source: Wikimedia Commons) |
One of the many challenges facing renewable energies is the problem of energy storage. Since sources of renewable energy tend to fluctuate greatly, they cannot easily adjust to changes in energy supply or energy demand without energy storage. [1] Power-to-gas addresses this problem by using electrical energy to generate synthetic natural gas, which can be more easily stored and transported for use at a different time or location. Usually, the gas produced is hydrogen gas, which does not produce carbon when combusted, making it a carbon neutral energy source when created using renewable energy.
Several methods for hydrogen gas production exist. Currently, the vast majority of hydrogen produced is known as "grey" hydrogen, which does nothing for carbon emissions as it uses fossil fuels as its feedstock, negating the benefits of hydrogen's cleaner combustion. [2,3] However, since we wish to evaluate the feasibility of power-to-gas as renewable energy storage, we will focus on "green" hydrogen, which is produced by electrolysis.
To extract the energy stored in the hydrogen once it is produced, a few methods are used. Hydrogen fuel cells are the most efficient method. Fuel cells essentially run electrolysis in reverse, producing a small current by combining hydrogen with oxygen into water. Unfortunately, like electrolysis, hydrogen fuel cells are much more expensive than their alternatives. [4] Combined-cycle gas turbines, which use the heat produced by combusting hydrogen, are much cheaper and are by far the preferred method today. This comes at the cost of lower energy efficiency, which we will discuss shortly.
The first metric to consider is the roundtrip efficiency, which is the ratio between the amount of electrical energy used to produce the hydrogen and the amount of electrical energy that can then be reextracted. For the preferred method of combined cycle gas turbines, we can calculate this efficiency by multiplying the electrolysis efficiency of 70% by the turbine efficiency of 58% which gives a maximum round-trip efficiency of around 40%. [4]
In comparison, hydrogen fuel cells perform slightly better, peaking at around 50% round trip efficiency. [2] In comparison, pumped hydro storage, which stores energy in the gravitational potential energy of water reservoirs, has around 80% round trip efficiency. [1] Similarly, another developed field of energy storage, battery storage, has efficiency comparable to that of pumped hydro storage at around 80%, and can reach as high as 95%. [1,4]
As such, power-to-gas is a much more wasteful form of energy storage than its alternatives when only considering energy efficiency.
The second challenge faced by power-to-gas is an issue of costs. Whereas hydrogen as a fuel source costs $8.27 × 10-9 J-1, natural gas costs less than half as much at $3.18 × 10-9 J-1. Given that natural gas exists as a cheaper alternative energy source, there is a heavy disincentive to avoid the additional trouble of converting surplus energy to hydrogen gas. Instead, the money could be spent on purchasing the far cheaper fossil fuels to make up for the energy deficits, instead of relying on energy storage.
Moreover, different sources of hydrogen have different prices, which leads to opportunities for arbitrage. As pointed out above, the most common method of hydrogen production actually emits greenhouse gases since it relies on fossil fuels as feedstock. Although it is less environmentally friendly, this method of hydrogen production is preferred because it is far cheaper than electrolysis. In the US, the cost of this "grey" hydrogen is only $1 kg-1, whereas "green" hydrogen costs between $3-4 kg-1. [2] Thus, even if hydrogen gas were to become a common fuel source, as long as electrolysis and "green" hydrogen remains more expensive than other forms of hydrogen production, there are huge incentives to continue using natural gas indirectly in the process of producing that hydrogen.
Despite this economic barrier to entry, hybrid powerplants such as the Utsira windpower and hydrogen plant or the Hybridkraftwerk Prenzlau (Fig. 1), where hydrogen fuel cells were used as energy storage, do exist. In the case of Utsira, which was the first wind and hydrogen plant of its kind, they successfully supplied power to 10 households on a remote island for several years. Due to its remote location and the naturally strong winds, the island where the experiment was held was well suited for such a powerplant. The results of the experiment demonstrated that wind-hydrogen hybrid power could "be competitive with conventional remote-site power supplies diesel or combined wind and diesel generators" within 5-10 years. [3]
Taking power-to-gas one step further, it is also possible to produce liquid fuels from electric powers via the Fischer-Tropsch process, which converts a mixture of hydrogen gas and a source of carbon like carbon monoxide into alkanes. The hydrogen gas can be obtained from "green" hydrogen, and the carbon monoxide can be obtained in the short term from the flue gases of existing powerplants. [5] These liquid fuels have much higher volume density which make them ideal for vehicles like cargo ships and airplanes. This same process can also be used to produce other chemical feedstocks for plastics, wax, or detergents. [5-7]
However, the costs of producing liquid fuels is much higher than extracting fossil fuels. In one study, researchers found that the limiting factor in producing liquid fuels from hydrogen gas was the price of the hydrogen gas itself. In order for the synthetic fuels to be competitive with existing petroleum sources, hydrogen would need to sell for $0.8/kg, even less than the estimated price of "grey" hydrogen sources from above. [5] As such, if clean hydrogen remains economically impractical, producing clean liquid fuels via the Fischer-Tropsch process is doubly so.
In its current form, power-to-gas faces significant economic challenges towards its adoption as a form of energy storage. It makes far more economic sense to augment energy supply with dirty hydrogen and petroleum sources than it does to go through the expensive hydrolysis/fuelcell processes and Fischer-Tropsch process. However, there are still some potential use cases, such as power storage in remote areas, as demonstrated by the Utsira experiment. With continued research in the field of electrolysis and fuel cells, "green" hydrogen and its derivative synthetic fuels may become economically viable and usher in the hydrogen economy.
© David Wang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] O. Krishan and S. Suhag, "An Updated Review of Energy Storage Systems: Classification and Applications in Distributed Generation Power Systems Incorporating Renewable Energy Resources," Int. J. Energy Res.43, 6171 (2019).
[2] R. B. Laughlin and S. W. Freund, "Economics of Hydrogen Fuel," in Machinery and Energy Systems for the Hydrogen Economy, ed. by K. Brun and T. Allison (Elsevier, 2022).
[3] M. Ji and J. Wang, "Review and Comparison of Various Hydrogen Production Methods Based on Costs and Life Cycle Impact Assessment Indicators," Int. J. Hydrog. Energy 46, 38612 (2021).
[4] V. Jülch, "Comparison of Electricity Storage Options Using Levelized Cost of Storage (LCOS) Method", Appl. Energy 183, 1594 (2016).
[5] "IPHE Renewable Hydrogen Report," International Partnership for Hydrogen and Fuel Cells in the Economy, March 2011.
[6] G. Zang et al., "Performance and Cost Analysis of Liquid Fuel Production From H2 and CO2 Based on the Fischer-Tropsch Process," J. CO2 Util. 46, 101459 (2021).
[7] V. Dieterich et al, "Power-to-Liquid via Synthesis of Methanol, DME or Fischer-Tropsch-Fuels: A Review", Energy Environ. Sci. 13, 3207 (2020).
