Introduction

CO2 capture and transformation to microalgae promise technological sets in installed carbon capture and utilization technologies. Open pond, tubular, flat plate or airlift bioreactors belong among the most frequent laboratory and pilot microalgae cultivation systems. Nowadays, the microalgae biomass is almost produced in open pond systems due to their economic friendliness (Nwoba et al. 2019). Nevertheless, the installation of raceway microalgae cultivation systems is sharply limited by weather conditions and their degree of sterility at a given geospatial location. regarding carbon capture and utilization technologies, the microalgal cultivation technology has to provide a decentralized solution to continuously and annually capture and transform emitted CO2 to microalgae as an intermediate product. This demand leads to decentralized technologies based on closed microalgal cultivation systems placed outdoors or indoors.

Both Vo et al. (2019) and Belohlav et al. (2018) present that flat-panel photobioreactors belong among one of the most effective microalgal cultivation systems due to their high illuminated surface, the highest CO2 capture rate, irradiation of the whole volume, and due to the prevention of batch contamination. Thus, the lighting of the photobioreactor represents one of the critical parameters that affect efficient CO2 transformation to microalgae. Vo et al. (2019) overviewed that photosynthesis rate is maximized corresponding to microalgal biomass yield from 0.2 to 2 g L−1 thanks to microalgae cells' insignificant mutual shading effect. As stated by Fernandez et al. (2018), the light of wavelengths ranging from 400 to 700 nm with irradiances represented by photon flux densities being between 100 and 2000 μE m−2 s−1 is typically applied. Nevertheless, either sunshine or artificial lighting can be used to irradiate photobioreactors. In their indoor studies, Romero-Villegas et al. (2018) reported the maximal productivity of up to 0.4 g L−1 d−1 of an artificial photon flux density of 500 μE m−2 s−1. On the other hand, Chlorella Pyrenoidosa's cell concentration of 0.5 g L−1 and the light intensity of 540 μmol m−2 s−1 were reached by Huang et al. (2015) under intermittent artificial illumination. Based on the overview in Table 1, it is clear that similar microalgal production can be reached either by artificial or by sunshine lighting in flat panels. Nevertheless, significant economic differences can be found between the laboratory and natural environmental conditions, i.e., the usage of natural sunlight or artificial irradiation. Norsker et al. (2011) calculated that microalgal biomass production costs for naturally irradiated cultivation systems, including dewatering, were 4.95 € kg−1 for the open pond and 4.15 € kg−1 for the horizontal pond tubular and 5.96 € kg−1 for flat-panel photobioreactors. Tredici et al. (2016) reported for 1-ha plant in Tuscany equipped with flat-panel photobioreactors irradiated by natural light microalgal biomass productivity of 36 t ha−1 y−1 with microalgal biomass production cost being 12.4 € kg−1. Romero-Villegas et al. (2018) reported that microalgal biomass production cost 5.96 € kg−1 using flat-panel photobioreactors for 100-ha plant size.

Table 1 Microalgae growth in flat-panel photobioreactor systems
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Laboratory and pilot plant studies are working under sunshine or artificial lighting conditions. Nevertheless, it is known that the sunshine duration is sharply limited by location and weather. Based on Fig. 1, there is sunshine around 1200–2,600 h for Middle Europe or around 2,000–3,000 h for Southern European countries. Nevertheless, to convert waste CO2 to microalgae as efficient continuously, sustainable carbon capture and utilization technology, ideally non-stop operating mode through year-round, should be reached. This means that artificial lighting is strongly demanded. Banerjee and Ramaswamy (2019) present a techno-economic study evaluating microalgae production costs for given locations. They found that microalgae production costs depend significantly on the geospatial site and the potential of microalgae productivity. Rezvani et al. (2019), in their general techno-economic study, reported that microalgae productivity, followed by fixed and variable operating costs, are the main factors that affect the economics of the process. The fixed operating cost in the lighting of the photobioreactor represents one of the crucial parameters that can significantly affect the whole economic process. Cost–benefit analysis, provided by Mendella et al. (2020), manifests that the critical economic parameters are microalgae production and the product's price.

Fig. 1
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Sunshine duration in hours per year in Europe (Reddit 2020).

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There is a lack of information regarding how sunshine or artificial lighting affects the commercial operation of photobioreactor systems. The paper, therefore, analyzes the dependence of annual operation expenses, revenues and net profit all concerning annual sunshine duration to artificial lighting provided by fossil or renewable energy resources or their mutual combinations.

Materials and methods

The 1-ha microalgae cultivation plant was used as a model technology to evaluate its profitability. Its economic feasibility was determined based on the type of products, i.e., microalgal powder P1.X, microalgal lipids P2.X, or microalgal carotenoids P3.X, and on the form of lighting (see Fig. 2 and Table 2).

Fig. 2
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Technological pathways

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Table 2 Overview of investigated pathways for the economic feasibility study
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It is known that nowadays, operating photobioreactors are typically illuminated only by sunshine or only by solar energy- or electric energy-based artificial lighting. Nevertheless, several lighting pathways are studied to reach an industrial demand of 8,000 working hours.

  • The first configuration of PX.1 respects only sunshine lighting of photobioreactors with annual sunshine hours at a locality.

  • The second configuration of PX.2 combines sunshine hours at a locality plus electric energy-based artificial lighting hours to reach demanded annual working hours.

  • The third configuration of PX.3 supposes that a solar system equips the plant with solar battery storage. As photobioreactors operate under sunlight, batteries are charged. As sunlight is not sufficient, energy to lighten photobioreactors is used instead of electric. If sunlight and solar power are not available, artificial lighting using electrical power is applied to reach demanded annual working hours.

  • The fourth configuration of PX.4 supposes only electric energy-based artificial lighting of photobioreactors with the typical industrial demand of annual working hours.

Mass and energy balances for all the pathways were calculated using the law of mass conservation, respecting no mass losses in the system with solvent recovery—the rule of energy conservation respecting no heat losses in the system, no heat regeneration. Total mass and energy balancing were based on the assumptions as follows.

  • Microalgae cultivation—Flat-panel photobioreactor cultivation systems (Belohlav et al. 2018) were used for the model technology assuming theoretical CO2 fixation rate 1.83 kgCO2 kg−1MICROALGAE volumetric productivity 1 g L−1 d−1. The proposed design of flat-panel photobioreactor was as follows: vertical disposition, the height of panel 2000 mm, the width of unit flat-panel chamber 1000 mm, depth of channel 50 mm, the parallel distance among the row of panels 1000 mm, a light source illuminating the reactor from one side, volumetric batch density per built-up area 990 m3 ha−1, irradiated surface per built-up area 20,000 m2 ha−1 productivity per illuminated area 99 g m−2 d−1, productivity per built-up area 990 g ha−1 d−1. The annual operational cost of sunshine lighted cultivation with harvesting 2.2 Eur kg−1MICROLGAE was used as referenced by Ruiz et al. (2016) for energy balances. The typical artificial light intensity 500 μE m−2 s−1 with corresponded artificial light flux 109 W m−2 were used to model the energy demand of artificial lighting (Vo et al. 2019).

  • Microalgae harvesting—Thickening the microalgal batch from 1 g L−1 to 200 g L−1 was supposed in the design using recommendations given by Dvoretsky et al. (2016) and Tan et al. (2015).

  • Microalgae drying—To evaporate water and produce microalgal powder, there is the energy demand counting 0.611 kWh kg−1 of water based on its heat of evaporation being 2,200 kJ kg−1 of water.

  • Microalgal products extraction—Lipids concentration 30% wt. of dry matter and energy 137 MJ kg−1LIPIDS for lipids extraction, carotenoids concentration 6% wt. of dry matter and energy 65.2 GJ kg−1CAROTENOIDS for carotenoids extraction was used for energy balancing as proposed by Miazek et al. (2017).

The evaluation of the economic feasibility of several pathways concerning technological setup, as defined by Table 2, was based on the assessment of annual profit, i.e., the difference between revenues and cash expenses. Typical current taxes 10 Eur kg−1 of microalgal powder, 50 Eur kg−1 of microalgal lipids and 2,750 Eur kg−1 of microalgal carotenoids defined were assumed in calculations according to the proposal of NREL (2017). Electricity price 0.13 Eur kWh−1 was used to evaluate energy cash expenses as the dominant part of the total operational cost (Piemonte et al. 2014) for drying, microalgal lipids and microalgal carotenoids production.

The profitability factor PF (–) was used to assess and compare the economic feasibility of studied pathways. It was defined as follows.

$$PF=\frac{\mathrm{the\,annual\, profit\, of\, analyzed\, trial\, P}1.\mathrm{X }}{\mathrm{annual\, profit\, P}1.1}$$
(2.1)
$$PF=\frac{\mathrm{the\, annual\, profit\, of\, analyzed\, trial\, P}2.\mathrm{X }}{\mathrm{annual\, profit\, P}2.1}$$
(2.2)
$$PF=\frac{\mathrm{the\, annual \,profit\, of\, analyzed\, trial\, P}3.\mathrm{X }}{\mathrm{annual\, profit \,P}3.1}$$
(2.3)

Thus, the higher the difference between taxes and cash expenses, the better the technology's economic feasibility. Capital investment cost to the sunshine or solar/electric energy-based artificial technology is assumed to be the same. Then profitability factor represents a value, how many times can be decreased payback time of technology compared to its only sunshine lighted setup.

Results and discussion

The economic analysis is based on the following assumptions. First, there is built a 1-ha plant for decentralized microalgae cultivation and processing as a part of the CCU biorefinery. The technology is composed of a flat-panel photobioreactor system to cultivate microalgae with sunshine lighting and a simple roof to protect the system against the effect of the environment. Then, technological set for harvesting and separation of microalgae, and its post-processing being drying, or extraction of lipids, or extraction of carotenoids follow. Therefore, the economic study scoped to evaluate whether to install sunshine or solar/electric energy-based lighting affects the economic feasibility of the model technology in terms of operating investments and annual profit. The operating investment cost is generally defined as the sum of direct operating cost (energy, personals, supervision, maintenance, laboratories, reserve), indirect operating cost (insurance, corporate directions) and distributional costs like transportation and storage fees. The analyzed pathway has given fixed technological setup, indirect and distributional prices. They, therefore, dominantly differ only in energy demand in dependence on analyzed lighting pathways. Thus the effect of energy demand for lighting pathways on annual cash expenses, revenues and profits were studied.

Microalgal powder production technology

The results analyzing the impact of sunshine lighting, solar- or artificial energy-based light or their mutual combinations on the economic feasibility to produce microalgal powder are presented in Fig. 3 and Table 3. Data of Table3 show a state at which 1,600 annual sunshine lighting working hours is typical, like for Middle European countries.

Fig. 3
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Profitability factor in dependence on annual sunshine lighting hours and lighting pathway of microalgal powder production technology—pathways P1.X

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Table 3 Impact of sunshine/artificial lighting on the profit of microalgal powder production technology
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Analyzing the presented data in Table 3, it is evident that the annual profit of the technology can be increased by the use of artificial lighting operating under electric and solar energy. The mutual combination of sunshine, solar energy- and electric artificial energy-based light P1.3 provides the highest profitability factor 2.18, followed by shared sun and artificial electric lighting, P1.2 being 1.24. Therefore, sunlight photobioreactors are much more economically feasible than entirely artificially irradiated. Regarding profitability factor 0.29 for full artificial electric lighting P1.4, it is evident that artificial irradiation is not financially possible due to high cash expenses that generate lower revenues in comparison with base sun lighted technology. A risk to be less economic effective for sunshine lighted working photobioreactors can be reached if the mutual combination of artificial electric lighting P1.2 is applied due to lower annual profit caused by electrical energy demand. The validity of this statement is for the localities showing 6,000 annual sunshine hours and more (see Fig. 3). The microalgal powder is a low-cost product. Therefore, it is clear that the installation of renewable energy systems, i.e., mutual combination of sun and artificial lighting P1.2, or sun, solar and artificial lighting P1.3, can ensure an increase in profitability of the microalgal cultivation system.

Microalgal lipids production technology

The results studying the effect of sunshine lighting, solar energy- or artificial energy-based light or their mutual combinations on the economic feasibility to produce microalgal lipids are presented in Fig. 4 and Table 4. Calculated data in Table 4 also reveal that the annual profit of the technology can be increased by the use of artificial lighting operating under electric or solar energy. The mutual combination of sunshine, solar energy- and electrical power-based artificial light provides the highest profitability factor, 3.09, followed by shared sun and artificial electric lighting being 2.46, and only by artificial electric lighting, 1.82. However, the data also show that if only artificial electric lighting is applied at the localities with 3,000 annual sunshine hours and higher, there is a risk of being less economically effective than sunshine lighted working photobioreactors due to reported lower annual profit caused by the reported lower annual profit electric energy demand.

Fig. 4
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Profitability factor in dependence on annual sunshine lighting hours and lighting pathway of microalgal lipids production technology—pathways P2.X

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Table 4 Impact of sunshine/artificial lighting on profit microalgal lipids production technology
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Microalgal carotenoids production technology

The effect of sunshine lighting, solar or electrical energy artificial lighting or their mutual combinations on the economic feasibility of producing algal lipids is presented in Fig. 5 and Table 5. The joint combination of sunshine, solar energy- and electric-based artificial lighting provides the highest profitability factor, 4.11, followed by shared sun and artificial electric light being 3.69, and only by artificial electric lighting being 3.36. However, the data also show that if only artificial electric lighting is applied at the localities with 5500 annual sunshine hours and more, there is a risk of being less economically effective than sunshine lighted working photobioreactors due to reported lower annual profit caused by electric energy demand. Nevertheless, it is impossible to reach more than 3000 yearly sunlight working hours in reality. There is, therefore, essential to operating microalgal biorefinery with electric energy-based artificial lighting on achieving profitability of microalgal carotenoids production technology.

Fig. 5
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Profitability factor in dependence on annual sunshine lighting hours and lighting pathway of microalgal carotenoids production technology—pathways P3.X

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Table 5 Impact of sunshine/artificial lighting on a profit of microalgal carotenoids production technology
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Critical discussion of analyzed pathways

The economic feasibility study was based on the assumptions that the technology is composed of flat-panel photobioreactors with sunshine lighting and a simple roof to protect the system against the effect of the environment. Technological set for harvesting and separation of microalgae, and its post-processing being drying, or extraction of lipids, or removal of carotenoids follow. Concerning the results of data mining for operating investment costs, an additional capital investment cost must also be considered. Mass balance determined that the model 1-ha microalgae cultivation plant reaches flowrate of microalgal suspension 41,250 kg h−1 and production of microalgae 990 kg d−1 under process conditions defined above. The model plant can capture and use 916 Nm3 d−1 of CO2 emissions to a demanded product, i.e., 61,093 Nm3 y−1.

The analyzed pathways considered modifications in the lighting system (solar energy lighting, electric energy lighting) that led to changes in the technical setup of analyzed pathways. These additional capital investment costs should be considered to predict payback time, i.e., artificial lighting system, protection of photobioreactors against the effect of the environment, and solar systems (see Table 6).

  • Respecting specific investment cost for flat-panel photobioreactors without artificial lighting, capital investment costs for the sunshine lighted model technology was evaluated to be 800,000 Eur.

  • Estimating specific investment cost for electric energy-based artificial lighting (dimmable color ambient outdoor LED lights) being 200 Eur m−2 of illuminated surface, the capital investment cost of artificial lighting was estimated as 4,000,000 Eur. Therefore, the investment cost of electric energy-based artificial light represents around 72% of the total capital investment of the model technology.

  • Sunshine lighted photobioreactors are typically placed outdoor, and simple roofs can equip them to reduce the effects of the environment.

  • If there is a demand to install artificially lightened photobioreactors in a fully enclosed shed, microalgae cultivation's total capital investment cost is sharply increasing. Applying specific investment costs for shedding 75 Eur m−2, including its infrastructure (Greenhouse 2020), the investment cost to build a fully enclosed shed can reach the value of 750,000 Eur only per shed without the technology itself. Therefore, it can significantly affect the whole economic feasibility.

  • If a greenhouse with reduced shading is satisfactory to protect microalgal cultivation technology against the effect of the environment, its capital investment cost is roughly 3,000,000 Eur. This value represents 40% of the investment cost to microalgal cultivation technology. There is, therefore, the potential to reduce investment costs concerning a fully enclosed shed. Nevertheless, local environmental illumination should be taken into account.

  • Solar panels are nowadays a relatively cheap solution to produce green electric energy. Assuming power demand for lighting is 2190 kW, the capital investment cost to the solar panel was evaluated to be 4,376,000 Eur.

  • Solar energy battery storage system represents one of the dominant additional capital investment costs. As discussed above, the investment of 1,400,000,000 Eur is needed to cover the demand of 2190 kW for 1600 h.

Table 6 Estimation of additional capital investment cost to microalgae cultivation technology
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Based on the discussed data about operating and capital investment costs, it was found out that artificial lighting combining solar energy and electricity can improve both annual profit and revenue. Depending on local sunshine hours and individual technical setup of the technology, there is a chance to significantly increase profitability if solar energy- and electric energy-based artificial lighting is used instead of sunshine one. In agreement, Tredici et al. (2016) found the installation of photovoltaic systems and the plant installation to the location with high sunshine hours as crucial prospects of improvement to reach favorable economic feasibility of microalgal biorefinery. Schipper et al. (2021) quantified working days and photosynthetic efficiency as dominant risk factors to improve the process. If no additional investment cost is needed, investment in artificial light excluding the mutual sun, solar energy- and electric energy-based lighting can increase profitability for microalgal powder, microalgal lipids, or microalgal carotenoid production technologies. Such results deal with the final statement that shared sunshine, solar and electric energy can improve economic feasibility only for microalgal powder or microalgal lipids production technologies. Microalgal carotenoid production technologies' profit rises using only electrical power-based lighting or its share with sunlight. Nevertheless, there is still the necessity to consider an increase in capital investment cost that could significantly affect the overall economic feasibility of given technology. The economic feasibility study must be therefore determined for given technologies, given locations, given the weather, given policy, CO2 availability and purity, and algal product demand concerning revenues, as also remarked by Branco-Vieira et al. (2020)

Conclusion

Shared sunshine, solar energy- and electric energy-based lighting of microalgal production technology can improve its economic feasibility for algal powder, lipids or carotenoids production. Depending on local sunshine hours and individual technical setup of the technology, there is a chance to significantly increase profitability if solar energy- and electric energy-based artificial lighting is used instead of sunshine one. Shared sunlight, solar and electrical energy artificial lighting can significantly improve the profit compared to the illumination of photobioreactors only by sunlight. Nevertheless, risk analysis proved that there is still the necessity to consider an increase in additional capital investment cost that could significantly affect the overall economic feasibility of given technology. Therefore, there is a research need to develop artificial lighting system, protection of photobioreactors against the effect of environment, and solar systems with energy storage to be favorable in investment and operating costs and lifetime. Developing cheap and reliable artificial lighting systems, solar energy storage systems, and improving process characteristics of microalgal cultivation (process stability, high concentrated suspension in thin layer photobioreactors avoiding biofilm formation and self-shading) were identified as the essential research needs and challenges to improve the economic feasibility of artificially lighted microalgal biorefinery.