Biofuels | Batteries

Hybridisation and back-up

Synergies in hybridisation and energy backup

The integration of biofuels and batteries in hybrid systems represents a promising approach to face the challenges of energy transition and decarbonisation. Biofuels obtained from organic matter (agricultural waste, vegetable oils, among others) have established themselves as a renewable alternative to conventional hydrocarbons due to their ability to leverage existing infrastructure and offer backup solutions when intermittency occurs.

In turn, battery energy storage systems (BESS) based on lithium-ion technology enable instantaneous responses (<10 ms) and rapid deployment, making them central to grid flexibility. These systems can provide services such as frequency control, operational reserve and contingency response.

The hybrid biofuel-battery combination takes advantage of the best of both worlds: batteries respond to instantaneous peaks in demand or compensate for fluctuations, while diesel/biodiesel generators ensure a stable supply during prolonged periods of low renewable generation. Power electronics —bidirectional inverters, multifunctional DC–DC converters— are essential for the efficient management of these resources, ensuring interoperability between the generator, battery bank and electricity grid.

Future opportunities in global decarbonisation

The hybridisation of batteries with renewables and biofuel-based backup components is a strategic pillar in the energy decarbonisation paradigm. In Spain the PNIEC 2030 sets a target of 74% renewables and 6 GW of storage, including 2.5 GW in batteries. In addition, the National Storage Strategy envisages up to 20 GW of capacity by 2030, indicating a strong push towards firm and flexible hybrid systems.

From a technological perspective, the cost of battery storage has fallen significantly (~84% since 2010), and in 2023 the LCOE fell to approximately 117 USD/MWh, with payback periods of around 4.1 years. On the other hand, biofuels provide continuous energy backup and leverage existing infrastructure, avoiding highly costly investments.

The combination of both offers multiple benefits:
– Optimisation of the overall LCOE of the system.
– Energy security against high intermittency.
– Dynamic responsiveness and sustainability.

In the context of Spain’s National Energy and Climate Plan and strategies such as the European PPA (Power Purchase Agreements), this hybridisation is emerging as a competitive tool for efficiently closing the renewable energy loop.

Projects under development and future perspectives

a) A study conducted with HOMER and PVSYST for a telecommunications station in Greece showed that the optimised configuration (9 kWp PV + 20 kW biodiesel + large capacity battery banks) improved supply reliability from 93% to 100%, while avoiding the emission of 27.7 tCO₂ per year.

b) Integration with hydrogen and fuel cells:
The Calistoga Resilience Centre (California) combines lithium-ion batteries with hydrogen fuel cells and a cryogenic tank, creating a microgrid system capable of acting as an autonomous backup (‘black-start’) to supply municipal power in the case of a grid failure.

c) Advanced hybridisation with power electronics:
Recent research promotes integrated multi-port DC–DC converters for hybrid systems (battery+ultracapacitor+fuel cells), enabling bidirectional and modular operation, with efficient power control and component reduction.

d) Optimisation and energy management algorithms (EMS):
Studies show that applying optimisation algorithms (PSO, GA, ACO) improves component sizing and minimises COE, achieving renewable fractions of 97–98% in battery/stack/photovoltaic systems. Real-time management systems based on Mixed Integer Quadratic Programming (MIQP) can control multiple FC+battery stacks, optimising service life and operational efficiency.

Role of power electronics in each process

Power electronics perform critical functions in hybrid biofuel-battery systems:

• Bidirectional DC–DC/DC–AC converters: facilitate conversion between battery, generator and grid, enabling charge, discharge and parallel modes. They are key in integrated multi-source topologies.
• Smart inverters: manage energy flow, ensure voltage/frequency quality, and enable black start or grid support with high reliability. In the case of Calistoga, the synergy between BESS and inverters enables a rapid transition to fuel cells.
• Energy management systems (EMS): algorithms embedded in power electronics dynamically allocate energy resources, optimising the use of batteries, biofuels and even hydrogen, within cost and emissions constraints.
• Hybrid multi-technology storage systems: examples such as VRFB+Li-ion use DC–DC converters to balance power flows and extend the system’s service life. In maritime applications, weight reductions of 30% and cost reductions of 28% have been achieved compared to single-technology systems.

Advantages and disadvantages compared to other technologies

Aspects	Biofuel hybridisation + batteries	Alternatives (batteries only / biofuel only / hydrogen)
Advantages	Instant flexibility + extended backup; high degree of autonomy; reuse of infrastructure.	Batteries only: fast response and lower emissions, but limited storage. Biofuel only: long backup, slow response. Hydrogen: high self-sufficiency, but high cost and low efficiency.
Costs	Moderate capital cost and operational reduction, avoiding large batteries.	Batteries only: increasing cost for extended duration. Hydrogen: high investment in electrolysis and FC. Biofuel only: recurring fuel.
Emisiones	Significant reduction compared to diesel. Depends on the biofuel footprint.	Batteries + renewables: zero direct emissions, but high production footprint. Hydrogen: zero emissions, but limited efficiency.
Grid flexibility	High: fast response + stable support.	Hydrogen has a lag; only batteries are fast but have limited backup time.
Technical complexity	Requires advanced EMS and multifunction power electronics	Batteries: simpler system; hydrogen: requires complex infrastructure.
Technological maturity	High. Biofuels and batteries are already deployed; hybrid projects are in actual operation	Hydrogen: in early stages. Batteries only: extensively tested.

Manufacturing processes and real-life examples

• Production of HVO (Hydrotreated Vegetable Oil) biodiesel
  HVO is obtained through the catalytic hydrogenation of vegetable oils or fatty residues (triglycerides), using hydrogen and catalysts to remove oxygen and produce an alkane similar to diesel fuel. This process used by companies such as Neste (NExBTL/HVO100), complies with European standards EN 15940 and EN 590, and allows up to 100% of fossil diesel to be replaced in Euro 5/6 engines without modifications. Neste has plants in Porvoo (200 kt/a), Singapore and Rotterdam (2×800 kt/a), and HVO100 is available in European countries such as Finland, Sweden and Belgium.

• Second-generation biofuels (2G): pyrolysis and gasification in Europe
  The BioMates project (EU, H2020) applies rapid ablative pyrolysis to lignocellulosic biomass such as straw or Miscanthus at 500°C, generating biopetroleum, biochar and gas for thermal self-consumption. In Spain the University of Seville (Surfcat) has developed processes that convert urban waste, CO₂ and biomass into bio-oil or gases, which are then refined with advanced metal catalysts. A pilot plant is planned in Linares – Jaén.

• European facilities: Cepsa – Bio Oils (Huelva)
  The joint venture is building a 500 kt/a 2G biofuel plant (SAF + HVO) in Palos de la Frontera, with an investment of €1.2 billion, avoiding 3 million tCO₂ per year, zero use of fresh water and IoT/AI digitalisation. The EIB is financing it with €285 million as part of REPowerEU. In Palos Cepsa produced SAF used at airports such as Madrid, Barcelona and Seville, with a significant impact on reducing emissions.

• Examples of HVO use in Spain
  In 2023, Renfe, Maersk, and Cepsa conducted successful tests using 100% HVO in locomotives between Algeciras and Córdoba (130 tonnes, 500 tonnes of CO₂ avoided). ROR and Cepsa are extending the use of HVO to logistics hubs. Nestlé plans to have 42 HVO trucks in 2024 and DHL uses 37 HVO trucks in Formula 1. Bosch–Rhenus are exploring HVO in heavy transport.

The case of ‘methane batteries’

Reformed methane fuel cells (MCFC and SOFC)

Fuel cells that operate with reformed methane are an advanced means of decentralised electricity generation from gases such as biomethane, biogas or even natural gas. These technologies do not use hydrogen directly as an energy vector, but rather reform methane in situ to produce it and then feed it into an electrochemical cell where the redox reaction takes place.

There are mainly two types:

• Solid Oxide Fuel Cells (SOFC): these operate at high temperatures, around 800°C, which allows for high conversion efficiency (over 60% in some cases) and the direct use of biogas. However, their high temperature entails significant costs in terms of materials, maintenance and thermal control, which limits their implementation to medium and large scales, such as industrial facilities or cogeneration projects.

• Molten Carbonate Fuel Cells (MCFC): these also rely on high temperatures and, unlike SOFCs, allow the direct use of methane without prior reforming. They are particularly suitable for large-scale industrial applications and continuous operation, thanks to their tolerance to biogas impurities and greater flexibility in fuel quality. However, their size, cost and technical requirements make them less viable in domestic or small-scale environments.

Both technologies offer remarkable potential in applications such as biogas plants, smart buildings with self-consumption, and even the naval sector, where distributed generation and energy efficiency are essential.

Microbial fuel cells (MFCs and MECs)

Microbial fuel cells (MFCs) represent a radically different approach to methane utilisation. These technologies are based on bioreactions driven by microorganisms, which metabolise organic compounds—such as dissolved organic matter or even methane—and generate electricity directly through the release of electrons.

One of the most innovative lines of research is the study of cells that use methanotrophic microorganisms, which are capable of directly oxidising methane. However, their current efficiency is low due to the limited capacity of microorganisms to generate significant flows and the complexity of the biological system and its industrial scaling.

Despite these limitations, MFCs offer an interesting synergy: they combine wastewater or organic waste treatment with energy generation. This dual use makes them ideal candidates for pilot projects in treatment plants or rural contexts where clean, multifunctional solutions are prioritised.

Chemical batteries based on methane reactions

At an even earlier stage, various studies are exploring the use of methane as a reagent in hybrid energy storage systems. In these designs, methane participates in controlled redox reactions that enable batteries to be charged or discharged.

Although initial results suggest a possible use in self-consumption applications—especially in agro-industrial facilities or farms with surplus biogas—these systems present multiple challenges. Among them are the complexity of integrating gas management with the stability and safety of batteries, as well as intermediate energy efficiency losses.

Therefore, this is a promising avenue but still in the laboratory phase, which will require significant advances in materials chemistry, operational safety and life cycle design.

Technical and operational comparison

From an operational and technical point of view, the different technologies have clearly distinct advantages and limitations:

SOFCs offer high efficiency and compatibility with reformed biogas, but their implementation is limited by high operating costs and thermal requirements. They are ideal for industries or buildings with continuous electrical and thermal demand.

MCFCs allow for a higher margin of impurities in the fuel, making them attractive for rural or industrial biogas plants, provided that their scale and maintenance are justified.

Although not yet commercially mature, MFCs point to a clean and sustainable path with parallel benefits in waste treatment. Their low specific power output currently relegates them to the experimental realm or to applications combined with purification.

Conclusion

The hybrid integration of biofuels and batteries, managed by advanced power electronics and optimised EMS, offers a robust, flexible and affordable solution in the context of the energy transition. It facilitates an energy mix that combines immediate response, prolonged backup, reduced emissions, and adaptability to existing infrastructure. Although it presents technical and logistical complexity (multiple energy vector management, need for sophisticated algorithms and electronics), its strategic deployment can accelerate the decarbonisation of isolated sectors or those supporting the grid.
In addition, options such as methane—and especially biomethane—are emerging as a versatile energy resource that can be integrated into both mature technologies and emerging systems, reinforcing the transition towards more sustainable models of decentralised energy generation and storage.

Compared to single-technology configurations, hybridisation is highly competitive in terms of total cost of ownership and reliability, positioning itself as a central element of the energy systems of the future.

References

1. Información sobre HVO100 y NExBTL – Neste, EN‑normas es.wikipedia.org+2es.wikipedia.org+2en.wikipedia.org+2
2. Detalles planta Neste (Porvoo, Singapur, Rotterdam) en.wikipedia.org+1es.wikipedia.org+1
3. Proyecto BioMates (UE H2020, pirólisis lignocelulósica) cordis.europa.eu+1elpais.com+1
4. Surfcat Univ. Sevilla, gasificación/catálisis residuos urbanos – planta en Linares elpais.com
5. Planta Cepsa–Bio‑Oils, Huelva (500 kt/a, 1 200 M €, BEI 285 M €) moeveglobal.com+4esambiental.com+4new.energias-renovables.com+4
6. Usos de HVO en sector ferroviario/logística en España (Renfe, DHL, Nestlé, Bosch) logisticaprofesional.com
7. Impulso UE a baterías e hidrógeno (Fondo Innovación) ec.europa.eu

Photo of limonite, from which iron is extracted, by Björn Wylezich.