(342n) Exploiting Wind/Sun Energy Surplus for Production of Green-Hydrogen: Combining PEM and AE Electrolyzers for Cost Efficiency

Hydrogen has caught renewed interest from the research community as a fuel suitable for decarbonization of the transportation sector. In particular, applications for mid-size mid-distance transportation vehicles (e.g. trucks, long distance passenger buses) are pursued [1] as (currently) large battery sizes and weights prevent their use in them. In addition, hydrogen can be readily produced by wind/sun-powered water electrolyzers to make 100% carbon-free fuels.

In our previous work [2-3], we discussed that as wind/sun are not available when needed, large capacities have been installed to satisfy the demand of energy, causing surpluses when production and demand do not match. For these instances, electric companies are studying several energy storage alternatives; the use of these surpluses to turn on the electrolyzers is one of them. In [3] we analyzed this approach and presented an optimization framework for establishing the optimal electrolyzer capacity to be installed assuming PEM electrolyzers. PEM electrolysis is a suitable for coupling with renewable generation sources, as they can be easily turned on and off, have fast dynamics, and work at almost any partial load. On the other hand, PEM current prices are high and thus also are the costs of hydrogen produced with this technology. An alternative, more mature technology is alkaline electrolysis (AE). AE is cheaper, but due to safety issues [4] cannot run at low current densities thus is not suitable in all the spectrum of variable loads.

In this work, we propose a scenario in which AE and PEM electrolyzers are coupled in order to both lower hydrogen production costs and take advantage of as much energy surplus as possible. For this, we introduce an MIP optimization framework which takes as an input the availability of energy surplus to establish the capacity that should be installed for each technology, considering safe shutdowns of the AE operation. The objective is to achieve a target H2 production at minimum cost. The framework was implemented in GAMS, and solved with CPLEX.

Uruguay, a country that already has a 97% share of renewable energy in the electric matrix generation, is used as a case study. We assume the production of a mean 900kg/d H2 facility, a capacity that coincides with the hydrogen pilot project in place in the country [5]. We studied different scenarios, including coupling with current wind energy surpluses, coupling with other (i.e. non-surplus) combinations of renewable wind/solar energy, and also allowing the electrolyzer to take energy from the grid under a large-consumer Time-of-Use (TOU) tariff. Employing wind and solar generation factors in Uruguay obtained from actual installed plants [6], and literature costs for the electrolyzers [7] we found that (i) even at large shut-down safety values for AE (40% of rated capacity), capital investment makes them preferable to PEM, (ii) in the taking from the grid scenario hydrogen production targets could be met with slightly lower capacities, enabling overall costs reduction.

References

1. Hydrogen Council, How hydrogen empowers the energy transition, January 2017.
2. M. Corengia, A.I. Torres, Processes 2018, 6(10), 204; https://doi.org/10.3390/pr6100204
3. M. Corengia, N. Estefan, A. Torres, Analyzing Hydrogen Production Capacities to Seize Renewable Energy Surplus, 30th European Symposium on Computer Aided Process Engineering (ESCAPE30), 2020, Milano, Italy.
4. M. Carmo, D. Stolten, 2019. Energy Storage Using Hydrogen Produced From Excess Renewable Electricity: Power to Hydrogen, in Science and Engineering of Hydrogen-Based Energy Technologies.
5. VERNE Project, V congreso de energías renovables-LATAM Renovables, July 2019, Montevideo, URUGUAY.
6. Administración del Mercado Eléctrico del Uruguay. https://adme.com.uy/
7. J. Proost, 2019. International Journal of Hydrogen Energy 44 (9), 4406–4413.