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DLR techno-economic valuation of power-to-liquids finds reducing electrolyzer and electricity costs key to cost-competitive liquid hydrocarbons

In 2012, the Helmholtz Association of German Research Centers launched a three-year project on the production of synthetic liquid hydrocarbons from electricity (i.e. Power-to-Liquids, PtL) using a multistage process (SynKWS), in cooperation with the German Aerospace Center (DLR) – Institute of Combustion Technology Stuttgart; the University of Stuttgart IFK; and the University of Bayreuth – Chair of Chemical Engineering.

As part of the SynKWS work, DLR researchers have now published a techno-economic study of a modeled PtL process in the journal Fuel. The multi-stage process uses renewable power to produce hydrogen using a proton exchange membrane (PEM) electrolyzer. The hydrogen from electrolysis and CO2, delivered by a pipeline, are fed to a plant where the gases are converted in a reverse water–gas shift (RWGS) reactor to syngas (H2 and CO). The syngas is then further converted to hydrocarbons using Fischer-Tropsch (FT) synthesis. The hydrocarbon syncrude is upgraded and separated from unreacted feed and gaseous hydrocarbons to make the final product.

In conventional energy systems, power generation follows the energy demand. In contrast, wind and solar power generation follows natural conditions, with hourly, daily, weekly or seasonal fluctuations. Hence, long-term seasonal storage applications with a high capacity, low storage losses, well-established and safe storage tanks and low space requirements are required. Liquid hydrocarbons are considered an option to store renewable energy while decoupling supply and demand. They are characterized by a high energy density, are used in the transportation sector and exhibit little to no loss during long-term storage. Additionally, liquid hydrocarbons have an existing infrastructure, can be easily transported and also be used as transportation fuel or as feedstock for the chemical industry.

The present work investigates the techno-economic effect of an option to couple continuous fuel production with fluctuating energy sources, considering present realistic assumptions and future technological developments.

—König et al.

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Block flow diagram and system boundary of the process concept. König et al.

For the modeled process, they set the capacity of the plant to 1 GWLHV of hydrogen input, using PEM water electrolysis, reverse water–gas shift reaction (RWGS) and Fischer–Tropsch (FT) synthesis. A feed of 30 t/h of H2 generated 56.3 t/h (12,856 bbl/d) of liquid hydrocarbons.

A storage cavern acts as the link between the highly fluctuating source, the electrolyzer unit and the hydrocarbon synthesis. Hydrogen is stored if excess power is available and used when the hydrogen demand exceeds its generation. The liquid product is stored in tanks for later use.

Because the focus of the project is on renewable—and fluctuating—sources of power, the electrolysis unit need to be extremely flexible, the team noted. The PEM electrolyzer can be operated at high current densities and cover a nominal power density range from 10% to 100%.

In their analysis, the team found the cost of the synthetic fuel depends on a number of factors, especially the capital cost of the electrolyzer and the cost of electricity. Basically, a constant power source, low electricity prices and low capital investment lead to low prices for the liquid hydrocarbons.

1-s2.0-S0016236115006651-gr4
 
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Top: (A) Percentage distribution of the total capital investment; (B) percentage distribution of net production cost (NPC).

Bottom: Sensitivity of cost parameters on NPC. König et al. Click to enlarge.

Broadly, the researchers calculated:

  • A PtL efficiency of 44.6%for the base case scenario.

  • Net production cost ranged from $12.41/GGE to $21.35/GGE for a system powered by a wind power plant with a full load fraction of about 47%, depending on the assumed electricity feedstock price and electrolyzer capital cost.

  • For systems with full load fractions between 70% and 90%, the production cost was in the range of $5.48/GGE to $8.03/GGE.

… reducing the costs for electrolyzer systems and electricity feedstock are the key factors for creating an economically viable production scenario of liquid hydrocarbons from renewable power and CO2. Additionally, economic viability is more realistic for systems operating at high FLFs [full load fractions], as they decrease the capital costs for high installed capacities of the electrolyzer and the intermediate storage. Hence, a minimum FLF of 70% is recommended for liquid hydrocarbon processes based on renewable power.

The technical assessment revealed a large amount of excess heat, which is assumed to be sold. The approach of the conversion of thermal energy into electricity by a steam cycle is proposed, but this concept reveals only a small overall efficiency increase due to the low efficiency of the steam cycle. In a subsequent study, the direct thermal use of the excess heat and its effect on the process efficiency and economics will be investigated. The technical and economic potential of CO2 sources will be examined to evaluate the limitations of their availability. Additionally, the technical options and the cost of upgrading products from liquid hydrocarbons to jet fuels is of special interest to the authors and will be assessed.

—König et al.

In the paper, the authors note that the use of a solid oxide electrolyzer, which uses excess heat directly to decrease the energy losses due to electrolysis, can increase the overall efficiency of the process.

As a separate example of that, sunfire, Audi’s partner in a PtL e-diesel project that is broadly similar to the modeled DLR process (renewable power for water electrolysis to produce hydrogen, which is then combined with CO2, converted to syngas and used at FT feedstock), claims an overall process efficiency of around 70%, mainly due to its use of its efficient high-temperature solid oxide electrolysis cell (SOEC) technology. (Earlier post.)

In that context, lead author Daniel König separately noted that given the much higher efficiencies of SOEC electrolyzers—since steam is used instead of water as feedstock—PtL process efficiencies of up to 70% may be possible. With optimistic assumptions and assuming a constant power source, a production cost, as claimed by sunfire, of some €1/liter ($4.09/gallon) may also be possible, König observed. He also noted that the DLR assumptions were quite conservative, and checked by the sensitivity analyses reported in the paper.

Resources

  • Daniel H. König, Marcel Freiberg, Ralph-Uwe Dietrich, Antje Wörner, Techno-economic study of the storage of fluctuating renewable energy in liquid hydrocarbons, Fuel, Volume 159, Pages 289-297 doi: 10.1016/j.fuel.2015.06.085

  • Jan Pawel Stempien, Meng Ni, Qiang Sun, Siew Hwa Chan (2015) “Thermodynamic analysis of combined Solid Oxide Electrolyzer and Fischer–Tropsch processes,” Energy, Volume 81, Pages 682-690 doi: 10.1016/j.energy.2015.01.013

Comments

Davemart

So the cost is somewhere between $4gge and $21gge.

Nice to know that is sorted.

HarveyD

With a 70% potential conversion efficiency, many countries with enough solar and wind energy could produce most of the essential (cleaner) liquid fuels required for airplanes and reduced ICEV fleet and clean H2 for FCEVs?

Engineer-Poet

So at 47% FLF, the minimum production cost is roughly twice the current retail price of petroleum motor fuel in Europe.  The FLF will presumably be well under the capacity factor of whatever RE is feeding it (immediate grid demand comes first), so 47% is probably much higher than could be achieved in practice using wind and solar.

As I keep telling people:  if you have to ask what this stuff costs, you can't afford it.

kalendjay

A steam cycle is proposed to recoup additional income, but the steam is too inefficient? That would probably not be the case is a nuclear core were the ultimate power source. The SOEC bears similarity to the familiar MOX nuclear fuel, and might serve as a powerful moderator by way of not contributing much to fission or nuclide byproducts. Steam itself would serve as a moderator, along with CO2, which is the familiar stuff of Gen I gas reactors.

No flammable graphite would be needed a la Chernobyl. The process flows of CO2, steam and free hydrogen by way of intermediating absorbers would keep the reactor under control. The Hyperion Corporation had already patented a reactor concept around the hydrogen management. A compact reactor, yet!

Engineer-Poet
A steam cycle is proposed to recoup additional income, but the steam is too inefficient?

Probably because the heat byproduct comes at too low a temperature.

If you go with a nuclear energy source to bump up the FLF to a level that's marginally economic, you have re-invented Green Freedom.  However, with nuclear power on tap you can charge electric vehicles whenever you want and don't need all that much energy storage.

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