Introduction
A hybrid solar-hydrogen energy system is an excellent alternative for off-grid facilities and potentially one of the central pillars of sustainable industries.
Among the various types of renewable resources, solar energy is particularly well-received due to its availability and cost-free nature. Adding to that is the increased demand for clean electricity and low-carbon fuels in energy-intensive industries, as well as far-flung, off-grid regions, which highlights the value of solar power production technologies. Consequently, the utilisation of solar photovoltaic structures coupled with storage in areas not connected to the grid has increased substantially. Despite its benefits, the high capital requirements and energy reliability remain the key preventative factors in using this technology for off-grid locations. Therefore, optimising the type and size of both the power production and storage structures is essential in providing a sustainable solution to this problem.
Solar-Hydrogen Technology
Solar Water Splitting
Solar water splitting (referred also to as artificial photosynthetic water splitting) can be achieved by electrolysis of water via photovoltaic cells, photocatalysis (PC) and photoelectrocatalysis (PEC) approaches in the presence of catalysts.
Electrolytic water splitting has produced hydrogen commercially since the early 1900s, with about 4% of hydrogen production. Electrolysis of water is an auspicious technique for producing H2 gas as a renewable energy source. This process occurs in an electrolyser and employs electricity to split water into hydrogen and oxygen. An electrolyser unit consists of an anode, a cathode, and an electrolyte. Different types of electrolysers based on the applied electrolytes include alkaline
electrolysers, solid oxide electrolysers, and polymer electrolyte membrane (PEM) electrolysers. In alkaline electrolysers, hydroxide ions (OH) transfer from the cathode to the anode through the electrolyte, generating hydrogen gas on the cathode. Solid ceramic materials are used as electrolytes in solid oxide electrolysers, which conduct O2 ions selectively at high temperatures. In a PEM electrolyser, a specific solid plastic material is used as the electrolyte, across which the protons can be penetrated. A PEM electrolyser attracts considerable attention as it operates at much lower temperatures (70–90°C) than other electrolysers (700–800°C solid oxide electrolysers and 100–150°C alkaline electrolysers).
The kinetics and the efficiency of water electrolysis are improved by using electrocatalysts as anode and cathode materials to catalyse electrolytic water splitting. The most common electrocatalysts for anode reactions (water oxidation reaction) are iridium and ruthenium, as well as their oxides, while platinum is commonly used as a hydrogen evolution catalyst at the cathode (water reduction reaction). However, the effective nonnoble metal electrocatalysts for water splitting are transition metal oxide, sulphides, selenides, phosphides, and nitrides. Moreover, composites of these materials can further improve water electrolysis.
PEC and PC are promising techniques for solar-to-chemical energy conversion. PEC and PC take the privilege among other water-splitting methods due to the following advantages: i) they require no wires or external electronics; ii) they are closed-loop cycles; iii) they utilise natural resources and renewable energy: water and sunlight; and iv) they need only low-cost semiconducting absorbers and involve direct energy storage in chemical bonds. However, it is not easy to find the right materials for efficient PEC and PC processes and to scale. PEC is the electrolysis of water by the direct use of light in the presence of a catalyst; that is to say, the conversion of light into electrical current and then the splitting of H2O into H2 using that current.
Descrition of the Typical S-H System
The concept of off-grid hybrid solar-based energy systems includes the utilisation of hydrogen and battery storage.
A solar-hydrogen (S-H) system usually supplies electric power to a hydrogen generator (electrolyser) by an arrangement of solar panels (photovoltaic (PV) system). This coupling must meet the following conditions:
- It should supply the electrolyser with a minimum voltage for splitting water (hydrogen and oxygen generation), theoretically 1.23 V and experimentally around 1.45–2 V.
- It should contain a minimum of auxiliary systems to increase global efficiency (e.g., minimise power conditioning and other auxiliaries).
- To achieve maximum global efficiency, each system (electrolyser and PV solar panels) should work at its maximum power point (MPP).
Electrolytic processes use electricity to cause chemical reactions. The most common method is the electrolysis of H2O, where water is split into hydrogen and oxygen using electricity. This is a sustainable way of producing hydrogen because there would be no carbon emissions during production when integrated with renewables like wind or solar. Photoelectric chemical techniques use sunlight energy to split water. In a photoelectric chemical cell, the semiconductors are similar to those used in solar PV electricity generation. They are immersed in a water-based electrolyte where sunlight energises the water-splitting process into hydrogen and oxygen, like in the electrolysis of water.
The S-H interconnection system commonly uses batteries and power electronic DC-DC converters (auxiliary systems) to adjust and modulate the voltage supplied to the electrolyser.
Direct conversion of solar power into electric power employs the process denominated as the PV effect. The most common solar cells are manufactured from silicon thin films of high purity. These cells are classified into monocrystalline and polycrystalline cells; the silicon films are impregnated with special materials (generally phosphor and boron) to give them PV properties. A PV panel is a group of connected cells sealed tightly with glass, plastic or other materials covering it. The PV modules are extremely durable because they do not have mobile parts. Companies that fabricate them guarantee their life for 10 or 20 years; however, it is not rare that they end up lasting more than 30 years. Other significant advantages are the high reliability and minimum maintenance. In the market, there are panels of different classes and sizes, so an arrangement of them would allow the generation of enough electric power to operate an electrolyser.
Currently, most commercial hydrogen generation is done by reforming different hydrocarbons, mainly the less heavy hydrocarbons (from methane to naphthas). Reforming reactions are highly endothermic; therefore, it is necessary to carry out the process through the combustion of gas or oil, but substantial quantities of CO2 are generated from these, and this continues to affect the environment markedly. The production of H2 using renewable sources such as electrolysis and photoelectrolysis reduces the pollutants emitted to the environment, mainly if the electric power is supplied by solar energy.
Currently, two commercial hydrogen generators use the electrolysis process: the alkaline electrolyser and the solid polymer electrolyte electrolyser, which is called SPE. Most electrolysers used commercially are alkaline because the technology is sufficiently developed, and several suppliers exist for this equipment (Stuart, Hydrogen Systems, Norsk Hydro Electrolyzers).
The temperature and pressure operations of both electrolysers are similar. However, the SPE type offers some advantages, like the higher purity of the hydrogen produced (i.e., no need for a cleaning process), the fact that it does not handle corrosive electrolytes, and its energy consumption is lower. For example, it does not need to keep a voltage across its electrodes as in alkaline electrolysers. Also, an SPE electrolyser is compact, and its electrolyte is chemically stable, while the KOH in alkaline electrolysers is susceptible to carbonation.
S-H systems have been proposed by various research centres and clean tech developers for a few years. One of the main objectives is to store intermittent energy from the sun by generating a fuel with the highest energy content by weight unit. This means an energy storage method with reduced energy losses compared to ordinary battery systems.
Hydrogen can be transformed into heating or electric energy either by a combustion engine or a fuel cell. In the latter, electric energy is generated very efficiently and mainly cleanly. The employment of hydrogen in commercial fuel cells reaches efficiencies up to 50%, producing only heat and water as by-products. Also, if heat in the form of water vapour is used in a cogeneration scheme, the global energy efficiency could reach close to 80%.
The lifetime of a PV system is generally 30 years, becoming the industry standard for PV module production guarantees. Inverter replacement is assumed to occur at the halfway point of the PV system’s lifetime. The electrolyser lifetime is also assumed to be 30 years. The electrolyser stack should last up to 90,000 h, which should be adequate for the 30-year expected lifetime of the PV system in most locations.
Implementation Strategies
Because both rich and developing nations lack comprehensive strategic and economic strategies, hydrogen uses are not widely realised. Recognising hydrogen as a clean energy alternative requires substantial planning, construction, operation, and overcoming challenges, particularly depending on the region. Efforts to achieve zero net carbon emissions involve speeding up the development of clean hydrogen, which is expected to gain prominence in the near-to-medium term alongside traditional fuels. This transition is supported by public-private partnerships that facilitate commercialising hydrogen and fuel-cell technologies. Clean hydrogen is poised to be integrated into various sectors in the coming years, complementing existing fuels, although specific applications in different industries remain undefined. The progress of hydrogen development needs to align with global commitments to achieve domestic and international net-zero carbon targets. Nonetheless, hydrogen’s adoption is anticipated to increase gradually in the short-to-medium term, alongside conventional fuels and other decarbonisation technologies.
The cost of PV systems has decreased dramatically over the past years. Market prices of solar modules have declined by about 90% and system prices by nearly 80% during a decade, making solar PV the cheapest form of power generation in many parts of the world. Single-axis tracking PV is turning into a very common in utility-scale systems, increasing the CAPEX by about 7% with an increased annual yield, and further positive energy system impact. Another trend is bifacial modules, which also increase the yield. The trend suggests that single-axis tracking bifacial PV may become the utility-scale standard in the future.
The production of hydrogen with alkaline electrolysis cells has been used in industrial applications since 1920. Future volume growth is expected to be very relevant. Here three different scenarios are reported by market studies: by 2050, the installed capacity is expected to be either 1, 5, or 17 TWel. The first two are based on IRENA scenarios, and the latter will be required for the 100% sustainable energy and industry system by 2050. Electrolyzer CAPEX depends heavily on the scale. Currently, a 200 kW electrolyser has 2.3 times the unit cost of a 1 MW electrolyser, and in turn, a 1 MW electrolyser has 2.4 times the unit cost of a 100 MW electrolyser; combined, this makes a factor of 5.5 between 200 kW and 100 MW. This level covers the entire system cost, including the electrolyser stack, balance of plant (BoP), installation, civil works, grid connection, and utilities.
How Are Solar-Hydrogen Systems Being Optimised Around the World?
Engineers have designed an optimal hybrid power generation block, including photovoltaics, wind turbines, hydrogen production and storage systems, batteries, and diesel generators, for Froan Island in Norway. The proposed structure is optimised using the particle swarm optimisation (PSO) method. They concluded that the hydrogen storage unit plays a vital role in enabling the system’s long-term storage capability and reducing its dependence on fossil fuels.
Utilising HOMER software, researchers techno-economically analysed a hybrid wind-PV-battery‑hydrogen power generation/storage block for a hydrogen refuelling station. Hybrid wind-PV‑hydrogen-battery-diesel generator schemes have also been studied to achieve the lowest cost to meet three non-domestic load demands at different locations in Cameroon. The Cuckoo Search Algorithm optimises the proposed structure, and they concluded that in the short term, the battery storage unit will be more cost-effective than the hydrogen storage unit.
Another team techno-economically analysed a hybrid wind-PV‑hydrogen-diesel structure to achieve the lowest cost and meet three realistic load demands in Figuil, Cameroon. Similarly, they used the Cuckoo Search Algorithm to optimise the size and minimise the cost of hybrid PV and wind turbine structures with different energy storage technologies. By utilising four well-known meta-heuristics techniques, this same team optimised the size of a solar-wind-battery‑hydrogen power structure in Kousseri, Cameroun.
A method was found to design an optimal hybrid system including PV, hydrogen, and batteries to enhance storage efficiency in Rome, Italy. In another work, the techno-economic viability of a PV-wind turbine-battery‑hydrogen structure to satisfy the electrical and hydrogen requirements of an off-grid region in West China was investigated. A group of specialists aimed to design a hybrid structure comprised of PV-wind turbine‑hydrogen to achieve the lowest cost for hydrogen production in Inner Mongolia, China.
In San Francisco, the feasibility of an integrated hybrid solar‑hydrogen energy structure to satisfy coastal areas’ electrical and hydrogen requirements has also been investigated. Different hydrogen supply routes for a hydrogen refuelling station based on an off-grid or grid PV-wind system using HOMER Pro software in Shanghai, China, have also been investigated. This software tool optimises the size and minimises the cost of hybrid PV and wind turbine structures based on diesel and hydrogen storage.
To come up with an approach to satisfy the electrical load demand in a remote region of Egypt, a group of engineers optimised the size of the PV-biomass‑hydrogen energy/storage system via the Mayfly optimisation algorithm. Likewise, in Jeju Island, Korea, another group introduced a model to optimally design an innovative renewable energy system to minimise the total annual cost.
In Stromboli Island, Italy, the size of a solar-battery‑hydrogen power structure was optimised by utilising a mixed integer linear programming technique. According to this project’s results, embedding a hydrogen storage tank in the design is mandatory for achieving a self-sufficient energy system.
Conclusions
Solar-hydrogen implementation strategies involve integrating PV and H2 production technologies to create sustainable and efficient green energy solutions. The most promising approach combines solar energy with electrolysis to generate hydrogen, leveraging the potential of renewable energy to reduce GHG emissions.
The photovoltaic-hydrogen (PV/H2) system is currently the most commonly employed technique due to its low cost, superior efficiency, and ease of implementation. Advanced methods include photovoltaic tracking systems and concentrated PV systems, which can further improve energy conversion efficiency, though at a slightly higher cost. Hybrid approaches that combine solar and wind energy sources have shown exceptional promise. These integrated systems can address the intermittency challenges of renewable energy by providing more consistent hydrogen production.
Sophisticated control strategies have been developed to manage the inherent variability of renewable energy sources. These include multi-layered approaches that address energy fluctuations across different timescales. Short-term responses utilise power electronics and battery storage, while medium-term strategies incorporate predictive algorithms and flexible electrolyser operation.
Sources
- IRENA (2024), Green hydrogen strategy: A guide to design, International Renewable Energy Agency, Abu Dhabi
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