The energy crisis has emerged as the most significant impediment to the advancement of human civilisation (Ahmed S. F. et al., 2021). Rapidly diminishing fossil fuel reserves have been posing a threat to the world’s energy security (Mahlia et al., 2020). Therefore, attempts have been made to transition away from an overreliance on fossil fuels and utilize renewable energy sources such as solar energy, wind energy, geothermal energy, biogas (Mahmudul et al., 2020), ethanol, and biodiesel (Mofijur et al., 2020). With a production of 55 million tonnes every year, hydrogen is an important eco-friendly industrial feedstock due to its high energy density (Wang and Yin, 2018; Bolatkhan et al., 2019; Gielen, 2019). The hydrogen market is experiencing up to 10% growth year on year and is forecast to reach over $191.80 billion in 2024 (Bolatkhan et al., 2019; Nagarajan et al., 2021). However, the commercialization of hydrogen as a fuel suffers major bottlenecks in terms of the techno-economic and environmental feasibility of conventional methods (Das et al., 2019), the dependence on fossil fuels to drive some processes (Dawood et al., 2020), and challenges related to transportation and storage (Nagarajan et al., 2017; Abdalla et al., 2018). Biological techniques for hydrogen synthesis, such as using organisms like microalgae, can effectively address these limitations. Unlike most biological sources, microalgae have many benefits, such as a high growth rate (Muhammad et al., 2021), low requirements for energy, high-quality land and water; a lower environmental impact (Ahmed SF. et al., 2021); and the absence of a need for costly pretreatment processes due to the lack of lignin (Singh and Das, 2020).

Microalgal biohydrogen production has occupied much interest, especially in recent years after it was revealed that various useful metabolites are also produced as co-products (Siddiki et al., 2020). Nevertheless, difficulties in process engineering, low productivity of microalgae, oxygen sensitivity, and operational costs, and inadequate insights on strain capacity, hinder the commercialization of microalgae-based biohydrogen production. Even though there are several metabolic routes to produce biohydrogen from microalgae with an improved yield, recent reviews (Buitrón et al., 2017; Sharma and Arya, 2017; Shobana et al., 2017; Show et al., 2018; Razu et al., 2019; Show et al., 2019b; El-Dalatony et al., 2020; Jiménez-Llanos et al., 2020; Mona et al., 2020; Salakkam et al., 2021) have not provided the comprehensive economic evaluation of such strategies, which has resulted in a dearth in the understanding of process feasibility. Therefore, in addressing this knowledge gap, this mini-review explicates the metabolic routes that enhance microalgae-based biohydrogen production and provides insights into their economic considerations to guide future work in the scaling-up of these technologies.

Biohydrogen production, as an alternative to many conventional energy sources (Foong et al., 2020), is viewed as a strong prospective contender in the energy economy (Wang and Yin, 2018). Microalgae biomass is frequently utilized to generate various types of bioenergy (Ge et al., 2020), with biohydrogen being one of the most promising categories of bioenergy that can be generated from microalgae. Biohydrogen can be generated from microalgae using two alternative processes: i) direct production of biohydrogen by microalgae, and ii) microalgal biomass utilisation as a fermentation substrate by other microbes. The first process can produce biohydrogen ranging from 0.015 to 1.084 mmol/L.h whereas for the second process this may vary from 0.35 to 10.26 mmol/L.h, depending on the feedstocks, culture conditions, and microorganisms used (Oncel, 2015). However, these two processes can be combined to improve the biohydrogen production rate. The metabolic pathways of biohydrogen production from microalgae are illustrated in Figure 1. These technologies for producing biohydrogen using microalgae are discussed in the following sub-sections.

Even though biohydrogen production has increasingly exhibited promising results at the pilot and laboratory scale, studies focused on optimization of process parameters and the feasibility of scaling up to the industrial level are largely absent (Oncel, 2015). Unlike the chemical synthesis of hydrogen, biohydrogen production has many challenges. Solving the technical and engineering problems may cost approximately $1.42 million (El-Dalatony et al., 2020). Thus, even though 1 kg of hydrogen is equivalent to 3.79 kg gasoline (Show et al., 2018), for hydrogen to be competitive with an energy price of gasoline ($2.50 GJ⁻¹), the commercial goal for hydrogen production cost needs to be $0.30 kg⁻¹ (Goswami et al., 2021).

The microalgae cultivation system and reactor design primarily determine the production cost of biohydrogen. For instance, when biohydrogen is produced from a biophotolysis system, cultivation in a 140 ha open pond and 14 ha photobioreactor can cost $6 m⁻² and $100m⁻², respectively, with a $43 million capital cost and $10 million annual operational costs, leading to a production cost of $10 GJ⁻¹ (Mona et al., 2020). The capital investment was almost 90% of the total cost with the capital charge calculated at 25% annually. Operating at 90% capacity, the system aims to produce 1200 TJ of biohydrogen per year (Mona et al., 2020). However, the hydrogen handling and storage costs were not factored in the total calculation, even though these are significant cost factors that can amount to about $0.58 million (El-Dalatony et al., 2020).

Following the initial failure of continuous biohydrogen production using S-deprived wild type C. reinhardtii, the National Renewable Energy Laboratory in collaboration with the researchers at UC-Berkeley developed a two-stage continuous phase system that promoted an aerobic growth phase and a subsequent anaerobic hydrogen evolving phase (Nagarajan et al., 2021). Their design reduced the photobioreactor costs to $1 m⁻², determined the selling price of hydrogen at $8.97 kg⁻¹ (or ¢22.80 kWh⁻¹), and further revealed the capital costs of custom-built photobioreactors can be drastically reduced if open ponds are used. Their first design used a photobioreactor costing $10 m⁻² and considered utilization of pressure swing adsorption for hydrogen purification and the compression of biohydrogen to 20 MPa for transporting and storing at a facility with the capacity of 300 kg/day. Changes in the flow in accordance with requirements led to production costs of between $0.57 kg⁻¹ and 13.53 kg⁻¹ which was deemed feasible. However, the production stopped 4 days after sulphur deprivation.

Gholkar et al. (2021) conducted a techno-economic feasibility study of a plant in India that cultivated microalgae in an open pond at the rate of 12,790 kgh⁻¹ and produced 1,239 kg of biohydrogen per hour. The total capital investment was estimated at $144.6 million, 11% of which was used to purchase the gasifier and its accessories. In addition, 76% of the total operating cost, which was $7,692 per year was attributed to the cultivation of microalgae, while the remaining was spent on utilities. Having assumed a market price of $10 kg⁻¹, they concluded that biohydrogen production was economically feasible, with a payback period of 3.78 years and an internal rate of return of 22%. Because profitability was primarily determined by the biohydrogen composition and the cost of microalgae production, the researchers recommended developing and using highly selective catalysts.

James et al. (2009) attempted to estimate the production cost of biohydrogen using oxygen tolerant microalgae and found a cost of $1.38 kg⁻¹, excluding the cost for storage (Jiménez-Llanos et al., 2020). 42% of the cost was derived from photobioreactor operation, 34% from the resources required for the biohydrogen collection, and 13% from recycling and pumping systems. Moreover, 4% of the costs were required for the microalgae feed assembly, with the remaining costs attributable to other control systems and consumables. Storage is an important cost consideration as it serves as a major limitation. To make the production price affordable for direct biophotolysis, it was recommended that the conversion efficiency and biomass production be raised to 13.4 and 9.4%, respectively. Another study (Melis and Happe, 2001), which assumes biohydrogen is produced 100% by photosynthesis, found that with a plant running at half capacity and an estimated production rate of 80 kg biohydrogen acre⁻¹day⁻¹, biohydrogen would cost about $2.80 kg⁻¹ which can be competitive with gasoline (Jiménez-Llanos et al., 2020).

Some bioreactor designs can also significantly raise the production cost, particularly depending on the manufacturing materials (Sathyaprakasan and Kannan, 2015), because the cost of materials and nutrients can make up more than 80% of major expenses (Show et al., 2018). A near-horizontal tubular reactor system built using single-stage algal biophotolysis cost $50 m⁻², assuming a 17% capital charge per year and a 10% efficiency of sunlight conversion. This led to an overall biohydrogen production cost of $15 GJ⁻¹. An assessment of a simplified design of a pilot modular photoreactor for 6 months’ field operation not only provided overall comparative costs of different technology compartments but also established a construction cost of $0.75 m⁻², which was significantly lower than frequently reported costs of $20–$100 m⁻². The study revealed that chemical nutrients and fabrication materials represented 84% of the main expenditure.

A techno-economic evaluation of a system producing biohydrogen through supercritical and thermal gasification of microalgae biomass suggested that generating 2,000 dry tonnes of biohydrogen per day required a total capital investment of $215.3 million and $277.8 million for thermal and supercritical gasification, respectively, to yield biohydrogen with a product value of $4.59 and $5.66 kg⁻¹, respectively (Kumar M et al., 2019). The total capital cost comprised individually purchased equipment, installation, and indirect costs, such as construction, engineering, and contingencies. For thermal gasification, the installed capital cost was $131.48 million whereas it was $169.6 million for supercritical gasification. The construction of the thermochemical plant was considered to occupy 20, 35, and 45% of the total capital cost for the first, second, and third years, respectively. Maintenance made up 3% of the total project investment, while labour cost was assumed to be 25% of the total operating cost. Plant overheads comprised half of the total labour and maintenance costs. General and administrative expenses were assumed to be 8% of the operating cost. Overall, the production cost of biohydrogen was reported to be fourfold the cost of hydrogen from natural gas.

The efficient microalgal biohydrogen production is hindered by numerous process parameters that affect the progress of metabolic activities. For instance, light intensity or the accessibility to radiation energy alone can significantly affect biohydrogen synthesis (Oey et al., 2016). In particular, in the artificial environment, the different light intensities reduce the efficiency of the metabolic capacity of microalgae, which is often not an issue in natural habitats where algae can adapt based on needs. Consequently, proper mixing of microalgal culture is crucial for effective light capture. Light can be either an inhibiting factor or a limiting factor depending on the metabolic route utilized for biohydrogen production. Furthermore, pH is also an important physical factor that influences biohydrogen production using microalgae (Razu et al., 2019). Even though the appropriate pH depends on the microalgae species in use, minimal changes in the pH can significantly affect the hydrogen-producing enzyme as well as the growth and metabolic activity of microalgae, affecting their biohydrogen production capacity.

Temperature is another physical parameter that affects biomass productivity and the biohydrogen production capacity within a medium. The optimum temperature for the sustainable proliferation of microalgae and the maximum biohydrogen production has been reported to be 15–85°C in a mixed culture (Razu et al., 2019). The availability of essential micro and macronutrients for enhanced biomass productivity also affects cost. Furthermore, the availability of these nutrients may initiate a stress response within the microalgal cells, which may be exploited to produce different kinds of biofuels. Biohydrogen has been reported to be generated when microalgae are subjected to sulphur (Gonzalez-Ballester et al., 2015), nitrogen (Nagarajan et al., 2017), phosphorous (Batyrova et al., 2015), magnesium (Volgusheva et al., 2015), and potassium deprivation.

The presence of an organic carbon source is also important for cultivation, except for phototrophic strains. The processes involved with microalgal biomass-led biohydrogen production face additional challenges for the smooth and sustainable production of biohydrogen. Oxygen sensitivity is regarded as one of the most challenging obstacles to overcome for many biohydrogen production technologies. The high binding potential of oxygen to the active sites of hydrogenase (El-Dalatony et al., 2020) create irreversible bonds that make it difficult to extract hydrogen and oxygen molecules from the complex compound (Eroglu and Melis, 2016). In addition, issues like altered thylakoid proton gradient, light capture disruption, and low efficiency of photosynthetic activities also contribute to high production costs.

Finally, there are significant differences in the costs of biohydrogen production depending on the routes discussed in the previous section. For instance, utilizing direct biophotolysis for biohydrogen costs an estimated $2.80 kg⁻¹ and generated 20 kg per 1,000 m⁻²day⁻¹ (Show et al., 2018). Resnick (2004), undertook comprehensive calculations of the production costs of each of the metabolic pathways. He estimated that direct biophotolysis, with a capital cost of $1220 GJ⁻¹year⁻¹ calculated at $2.47 m⁻² and an operating cost of over $70,000 million broken down by power ($2.49 million), water ($0.02 million), labour ($50.4 billion), supplies ($7.75 billion), culture production ($5.8 billion), gas separation and handling ($23 million), and a subtotal operating cost (>$63,900 million) with a contingency of 10%, resulted in a biohydrogen production cost of $11,170.33 GJ⁻¹. Biohydrogen produced from indirect biophotolysis with a capital cost of approximately 2.40 GJ⁻¹year⁻¹ and an operating cost of about $102.1 million on an annualized basis had a production cost of $16.26 GJ⁻¹ (Ghosh et al., 2017). The complete breakdown of the operating cost is as follows: power ($2.5 million), water ($0.02 million), labour ($65 million), supplies ($17.3 million), culture production ($8.2 million), gas separation and handling ($0.09 million), and subtotal operating costs (about $93 million) with a contingency of 10% (Resnick, 2004).

Another study (Hallenbeck and Benemann, 2002) reported that biohydrogen using direct biophotolysis assuming an optimistic capital cost of $50 m⁻² maintained a production cost of $2.13 kg⁻¹ (Nikolaidis and Poullikkas, 2017; Acar and Dincer, 2018a). This implies that a 20% increase in capital cost would raise the production cost by 33%. However, the calculations considered minimal operating costs, at an overall solar conversion efficiency of 10%, and did not account for many significant cost components such as engineering, gas separation and handling. On the other hand, the capital cost using indirect biophotolysis was $135 m⁻² and the production cost was $1.42 kg⁻¹ (Nikolaidis and Poullikkas, 2017). Dincer and Acar (2015) illustrated that direct biophotolysis yielded biohydrogen at a cost of $7.27 kg⁻¹, whereas photofermentation and dark fermentation cost $7.61 kg⁻¹ and $7.52 kg⁻¹, respectively.

Resnick (2004) calculated a production cost of $30.7 GJ⁻¹ using photofermentation. An operating cost of over $193 million was assumed, incorporating the costs for power ($2.5 million), water ($0.01 million), labour ($23 million), supplies ($3.5 million), culture production ($2.7 million), gas separation and handling ($0.13 million), and a subtotal operating cost (>$175 million) with a contingency of 10%. The Tredeci-style reactor serves as a significant cost-driver, resulting in a capital cost of $1.41 GJ⁻¹year⁻¹. Dark fermentation, on the other hand, was found to produce biohydrogen at a cost of $155.59 GJ⁻¹. Dark fermentation has potentially the lowest capital cost of about $0.64 GJ⁻¹year⁻¹ (Resnick, 2004), attributed to the small footprint of the bioreactor. The operating cost of over $980 million can be broken down into power ($2.5 million), water ($0.02 million), labour ($17.8 million), supplies ($2.7 million), culture production ($2 million), gas separation and handling ($0.6 million), and a subtotal operating cost (about $891 million) with a contingency of 10%.

Overall, photofermentation exhibits a higher yield but is relatively expensive (Bolatkhan et al., 2019). Others have demonstrated that photovoltaic processes are more expensive than direct biophotolysis (Goswami et al., 2021), costing up to $170 m⁻³ whereas the latter resulted in a cost of $25 m⁻³. Many of recent studies (Eroglu and Melis, 2016; Khetkorn et al., 2017; Anwar et al., 2019; El-Dalatony et al., 2020; Peng et al., 2020; Singh and Das, 2020; Goswami et al., 2021; Limongi et al., 2021; Wang et al., 2021) primarily extol the advantages of synthetic biology, as advancements in molecular and genetic engineering have allowed these options to be explored for improving the strains of algae to optimize biohydrogen yields. This is no surprise, as synthetic biological methods have a higher yield and output compared to the metabolic routes discussed (Goswami et al., 2021). In general, however, biohydrogen production tends to be more expensive than other fuel alternatives (Show et al., 2018). Nevertheless, the economic analyses of biohydrogen production have been scanty, especially for electrochemical processes.

Most of the economic studies were conducted between 10 and 20 years ago, making economic feasibility analysis of biohydrogen production using state-of-art technology ambiguous. Moreover, many of the existing studies on the economics of biohydrogen are substantially optimistic. It has been calculated that biohydrogen can be produced at a cost between $10–20 GJ⁻¹ (Show et al., 2019b). Regardless, such production costs are too high for biohydrogen to serve as a competitive alternative to gasoline, which costs $0.33 GJ⁻¹. Future work needs to be directed towards comprehensive evaluations with regards to facility feasibility and operation, material and operation costs, light, weather, and land requirements, the mixing of biomass, and stability. Economic feasibility can be improved by recycling residual metabolites and biomass.

Different techniques for biohydrogen production offer diverse benefits, even though none are currently feasible for large-scale implementation. The fermentation processes are more eco-friendly, however they have low efficiency in sunlight conversion and biohydrogen yield. Biophotolysis methods also have a low biohydrogen yield (<15%) and face several challenges in terms of oxygen sensitivity and the availability of sunlight. Nevertheless, direct biophotolysis offers an efficiency of more than 80%. The electrochemical processes are found to be significantly energy-intensive, with high requirements of pressure, temperature, power, and sophisticated equipment, but yield 90% biohydrogen. The current production cost of biohydrogen is quite high, ranging between $10–$20 GJ⁻¹, depending on the microalgae cultivation system, reactor design, and the metabolic route used. In order to compete with gasoline, this cost must be less than $0.33 GJ⁻¹. On the other hand, most economic assessments are fairly optimistic but fail to account for important cost factors such as storage, handling, and transportation. There is also a gap in current economic evaluations as most studies were conducted decades ago. Thus, future research should be directed towards properly assessing the economic feasibility of biohydrogen production processes in order to scale up the technology.