Bio-mimic energy storage system with solar light conversion to hydrogen by combination of photovoltaic devices and electrochemical cells inspired by the antenna-associated photosystem II

ABSTRACT

Global warming caused by anthropogenic activity is one of the serious problems today. In order to suppress the global warming, the shift from fossil fuel–based energy source to the nature-oriented sustainable energy is encouraged. In this concept paper, possible biomimetic engineering approach inspired by the efficient and sustainable natural energy utilization in living plants is demonstrated. The focal features in plants include (1) the light-harvesting and energy condensing apparatus, (2) water splitting O₂ evolving apparatus, (3) storage of energy-related chemicals, and (4) reversal conversion of storage into the “energy in use” by meeting the demands. Demonstration of solar-driven chemical energy conversion was performed using a system consisted of (i) photovoltaic power-generating device, (ii) an electrochemical unit converting electric power into chemical energy, (iii) storage of H₂, and (iv) polymer electrolyte cells converting H₂ back to electricity by meeting the demands on site. The present concept paper presenting a technical perspective based on the plant-inspired knowledge (conceptual similarity between natural photosynthesis and solar-to-H₂ conversion) is a fruit of interdisciplinary collaboration between the team of chemical energy conversion renown for the world highest record of solar-to-hydrogen conversion efficiency (24.4%, as of 2015) and a group of plant biologists.

Introduction

Global warming caused by anthropogenic activity is one of the serious problems today. In order to suppress the global warming, the shift from fossil fuel–based energy source to the nature-oriented sustainable energy (natural energy) is being proposed. However, the natural energy sources, chiefly, solar light and wind power have tendency to be fluctuated and not suitable for user-on-demand applications. As a result, the use of the systems based on the natural energies is still limited as the supplementary to or secondary to the fossil fuels. In order to buffer the fluctuations of the naturally flowing energy, proper strategy for storing energy is required for manifesting the society based on the natural energy, handled as the true and major sources of energy.

In the present study, inspirations from the efficient and sustainable natural energy utilization in living plants (as summarized in Figure 1) were engineered into biomimetic approach for reproducing the plant functions, namely, (1) the light-harvesting and energy condensing apparatus, (2) water splitting O₂ evolving apparatus, (3) storage of energy-related chemicals, and (4) reversal conversion of storage into the “energy in use” by meeting the demands on site, through combination of devices and units, namely, (i) photovoltaic power-generating device,¹ (ii) an electrochemical unit converting electric power into chemical energy, (iii) storage of H₂, and (iv) polymer electrolyte cells converting H₂ back to electricity by meeting the demands on site.

Figure 1.

Energy handling strategies in plants. (a) Components of light reaction known as Z-scheme. (b) Energy managements for meeting the changing demands consisting of (1) light harvesting and energy condensation, (2) oxygenic light-to-chemical energy conversion, (3) storage of chemicals with high calories, and (4) re-conversion of stored compounds into the useful form of energy.

The present concept paper presenting a perspective on the plant-inspired knowledge on the development of novel technology for sustainable and efficient solar energy utilization is a fruit of interdisciplinary collaboration between the team of chemical energy conversion renown for the world highest record of solar-to-hydrogen (StH) conversion efficiency [24.4%, as of 2015; ²] and the group of plant biologists.

Concepts and background

Similarity between solar-to-hydrogen system and photosystem II (PS-II)

The best model for utilization of solar energy could be found in natural photosynthesis performed by living plants. Photosynthesis is the natural system converting the solar energy into chemical energy by transforming inorganic carbon (CO₂) into the energy-rich forms of organic substances, chiefly, carbohydrates as below

$$6C{O}_2+12H_{2}O\to {\left(C{H}_{2}O\right)}_6+6H_{2}O+6O_2,$$ (1)

Photosynthesis is consisted of two distinct processes known as light and dark reactions where photon-driven electron transport and light-independent carbon fixation are performed, respectively. The photosynthetic electron transport known as Z-scheme consisting of the actions of two photosystems, namely, PSI and PS-II takes place on the thylakoid membrane in chloroplasts (Figure 1a).

As illustrated, PS-II is the only mechanism in charge of water electrolysis by extracting electrons from water as follows:

$$2H_{2}O\to O_2+4H^{+}+4e^{-},$$ (2)

which is equivalent to hydrogen production by electrochemical reactions.

Biomimetic approaches

Biological systems could be often the good sources of inspiration for energy utilization mechanism and energy managements backed by chemical transport due to similarity between animal models possessing the energetic metabolic properties fueled chiefly by carbohydrates and lipids and centralized cardio-vascular systems supporting and controlling the transport of energy-related chemicals such as glucose, and the artificial energy production models and their energy-distributing systems such as a fossil fuel–driven thermal power station and centralized grid system. On the other hand, apart from animal models, plants seem to employ highly distinct modes and mechanisms of energy production and managements. Plants utilize solar light as the primary and sustainable source of energy and the apparatuses for light energy utilization (chloroplasts) are systemically distributed. Then, energy captured are converted as sugars and locally stored or transported to the sites of consumption by meeting the changing demands. Therefore, the key inspirations from the energy handling strategy in plants are (i) utilization of natural energy, (ii) decentralized modes of production and managements, (iii) chemical form of energy storage, and (iv) on-demand re-conversion of chemical storage (sugars) into useful form of energy (ATP).

There has been a series of attempts for translating the behaviors and functions of living plants into the algorithms mimicking the actions of machines and/or computers such as finite state automata.³ Vice versa, a variety of artificially developed systems could be regarded to be inspired from biological systems including plants, or at least key similarity could be found among the artificial systems and plants.

As one of such plant-inspired fields of engineering, one may name the vast examples related to artificial photosynthesis in which functions and structures of the light capturing pigments and electron-transporting systems are mimicked or partially reproduced.^4,5 Development of titania-based photocatalytic system light-dependently splitting water molecule concomitantly evolving gaseous form of hydrogen and oxygen could be the first example of artificial systems resembling the photosynthesis.^6,7

For designing the engineering approaches to the solar light-to-hydrogen conversion, the sophisticated system in PS-II could be an excellent guide. In this report, the comparison of the PS-II and the photosynthesis-inspired photovoltaic real solar light-to-hydrogen conversion system is discussed. One of the important key factors for engineering the functioning system was found to be the path for energy flow between each device. In addition to the solar light-to-hydrogen conversion, future development of autonomous energy management system smoothly utilizing the natural energy is discussed based on the idea for energy flow optimization.

Designing the minimal system for demonstration

Rechargeable battery is most expected energy storage device since it performs high efficiency for conversion of electricity into the storage (potential) and vice versa. Therefore, the rechargeable battery was chosen as a key component in the plant-inspired energy management system to be demonstrated here. In addition, the fast demand response of the battery is also suitable for the stabilization of natural energy fluctuation. In general, the cost of battery is, however, proportional to the storage amount, thus, the cost becomes high when the storage amount is increasing. In addition, the battery has the problem of natural discharge, therefore, long term storage in battery should be avoided. Considering these problems of cost and long-term storage, we propose here that chemical energy storage is one of the functioning strategies for storing the energy on site. Hydrogen is expected as a suitable medium for the energy storage partly because it is readily produced through the water splitting reaction. However, there are some difficulties to achieve efficient transportation because the energy density of hydrogen per volume is not large enough even for liquid hydrogen stored below 20 K. Thus, the energy storage in the form of hydrogen is obviously suitable for the decentralized on-site system. However, there are only limited number of reports on the development of this type of hydrogen storage to date. One of the big challenges for vitalization of the idea is the selection of the proper methods for converting the solar light into hydrogen, among a number of techniques proposed to date, including the uses of photocatalysts, photoelectrochemical water splitting, and the combination module of photovoltaic cells and electrochemical cells. This is the key concept to be discussed through model demonstrations in the present report.

Novel concepts for PS-II-mimicking system

Systematic interpretation of PS-II in the plants

PS-II is a complex of proteins in which light-dependent reactions of oxygenic photosynthesis occurs in plants (Figure 1, left). The key reaction is robust oxidation of water leading to generation of oxygen (O₂), protons (H⁺), and electrons (e^–). The light energy-assisted overall reaction can be summarized as in Equation [2].

The photochemical processes at PS-II occur as follows. At first, the photons are captured by chlorophylls and other accessory pigments in the light-harvesting antenna complexes associated with PS-II. The excited states generated upon absorption of light are transferred and concentrated to the special chlorophyll a centered in PS-II. The absorbed light frees an electron by leaving a hole, through the process called “photo-induced charge separation” on the special chlorophyll a. The electron is then transferred to plastoquinone via pheophytin. As a result, the positively charged chlorophyll a extracts a single electron from the oxygen evolution center (OEC), which is consisting of CaMn₄O₅ metal cluster and other pigments, in charge of water splitting into O₂ and H⁺.

The insight provided by this process is the enhanced efficiency in utilization of the energy due to light absorption via up to 300-fold of concentration meeting the catalytic requirements. In summary, there are following steps at PS-II, and from these processes, the desired functioning modes of artificial systems could be suggested.

1. Light energy concentration or condensation occurs (ca. 300-fold) by light-harvesting antenna complexes associated with PS-II.

2. The light-to-electron conversion occurs at special chlorophyll a centered on PS-II (chlorophyll loses one electron).

3. In parallel with water oxidation at OEC evolving O₂ and 4H⁺, the water-derived electrons are captured by special chlorophyll a for further relays.

As above, the processes within PS-II can be interpreted relatively simple as shown here, thus, artificial water splitting systems can be designed by mimicking the processes at PS-II. Notably, the reaction in the artificial water splitting module produces oxygen (O₂) and hydrogen (H₂), but not proton (H⁺), since proton cannot be stably handled in the simplified system lacking natural membrane-based micro-compartments unlike living plants as follows;

$$2H_{2}O\to O_2+2H_2,$$ (3)

In fact, the half of electrochemical reaction as in Equation [3] is identical to the reaction in living plants as shown in Equation [2]. The reducing steps in the artificial reaction are, however, not the reduction of CO₂ as performed by the plants, but instead, hydrogen can be formed. This is technically due that the CO₂ reduction is a difficult process to control and reproduce, and moreover selection of direct products and the reaction pathways are still largely limited.

Photocatalysis is the simplest way for enabling the water splitting, by using small particles of semiconductor (often requiring co-catalysts), with which the conduction band bottom edge and the valence band top edge energies enable the water splitting, by simply irradiating the catalyst dipped in water. Photoelectrochemical water splitting is similar to the photocatalyst-based reaction but the semiconductor is used as a bulk photo-irradiated electrode (working electrode), where water oxidation or reduction is performed, which is dipped into an electrolyte and the opposite reaction is performed at the other electrode (counter electrode), which is electrically connected metal or the type of semiconductor opposing to the working electrode. Both of these water-splitting methods have been studied for a long period since the finding of Honda-Fujishima effect due to its simplicity and the possibility of high light-to-hydrogen energy conversion efficiency.⁶ The conversion efficiency is, however, up to 14–16% and the structure of the photo-irradiated working electrode is almost the same as the complicated 3-junction photovoltaic device.^8,9 The details of photocatalytic and photoelectrochemical cells are found in elsewhere.¹⁰

In fact, the above mentioned photocatalytic and photoelectrochemical processes apparently lack the light energy concentrating process found in PS-II in photosynthetic organisms, the process enabling the collection and condensation of the low-density wide-spread solar energy, for the use at right energy density required for reaction. Concentrated light is used in some cases, but heavier semiconductor corrosion often accompanies in these cases. Note that in these models, the processes for light perception and the water splitting reaction are performed at the same position on the device. This could be additional key difference from PS-II, although the photocatalyst and photoelectrochemical water splitting have the possibility for high energy conversion.

The combination of two distinct processes in photovoltaic cell(s) and electrochemical cell(s) (PV + EC) has also long history from 1970s. As far as clarified from the literature survey, the first report on PV + EC co-application was probably in 1977.¹¹ Although the energy efficiency for solar light-to-hydrogen conversion is limited by the solar-to-electricity conversion efficiency of photovoltaic cell, the process is similar to that of PS-II. The energy concentration can be achieved in the system with electrical configuration and the process of light-to-electricity and electricity-to-water splitting are separately performed by different devices in photovoltaic cell and electrochemical cell.

Considering the upper limit of the solar-to-electricity conversion efficiency, application of the photovoltaic cell with high conversion efficiency is the key to improve this limit. From the data of the “best research-cell efficiencies” (NREL, occasionally updated web-site data), it is clarified that the highest energy conversion group is the multi-junction photovoltaic cells and that the solar-to-electricity conversion efficiency is over 40%. Therefore, the usage of the multi-junction solar cell for the StH conversion is expected to achieve high StH conversion efficiency. It has to be noticed that the material cost of this multi-junction photovoltaic cell is expensive compared with that of the conventional photovoltaic cells employing Si as the semiconductor material. In order to fabricate the multi-junction photovoltaic cell with cost effective, the solar light has to be concentrated from ca. 100 times to 1000 times by Fresnel lens. In order to achieve this, the multi-junction photovoltaic cell requires sun tracking because the position of the sun, the concentrating lens, and the cell are required to be properly aligned.

Schematic setups of these photoelectrochemical cell, conventional solar cell convined with electrochemical cell (PV + EC), and concentrated photovoltaic cell (CPV) convined with electrochemical cell are shown in Figure 3. In addition, schematic diagram of the comparisons of the processes of these energy flow are also summarized in Figure 4.

Figure 3.

Schematic setup diagrams of water splitting by (a) photoelectrochemical cell, (b) conventional photovoltaic cell plus electrochemical cell (PV + EC), and (c) concentrated photovoltaic cell plus electrochemical cell (PV + EC).

Figure 4.

Schematic diagram for comparison of light to water splitting processes in photocatalyst and photoelectrochemical cell, conventional photovoltaic cell plus electrochemical cell (PV + EC), concentrated photovoltaic cell plus electrochemical cell (PV + EC), and the natural plant PS-II associated with antenna pigments.

StH conversion efficiency

In the previous section, the similarity between PS-II and PV + EC system was discussed. Since the mechanisms for artificial system is much simpler than that in living plants, the energy flow analysis of the artificial system may be helpful for understanding the plants' system.

The most important parameter of artificial system is the StH energy conversion efficiency. The definition of StH is;

$$S_{t}H=\frac{\mathrm{\Delta }{G}_{f{H}_2}^0{R}_{H_2}}{I_{solar}{A}_{rec}},$$ (4)

where, $\mathrm{\Delta }{G}_{f{H}_2}^0$is standard Gibbs energy of hydrogen formation (237 kJ/mol: 1.23 eV), $R_{H_2}$ is hydrogen generation rate (mol/s), I_solar is solar irradiance (W/m²), and A_rec is light receiving area (m²). This equation can be converted to the device efficiencies;

$$S_{t}H=\left(\frac{P_m}{P_{sun}}\right)\left(\frac{P_{op}}{P_m}\right)\left(\frac{\mathrm{\Delta }{G}_{f{H}_2}^0}{V_{op}}\right)\left(\frac{I_{H_2}}{I_{op}}\right)={\eta }_{PV}{\eta }_{op}{\eta }_{EC}{\eta }_F,$$ (5)

where, P_m is maximum power output of photovoltaic cell, P_sum is the power of the sun, P_op is the operating power of the system, V_op is the operating voltage, I_H2 is the current used for hydrogen formation, I_op is the operating current, η_PV is the energy conversion efficiency of photovoltaic cell, η_op is so-called operation point matching, η_EC is the energy conversion or voltage efficiency of electrochemical cell, and η_F is the current conversion efficiency from input current to hydrogen, that is, the Faradic efficiency.²

The first term of Equation [5] is the maximum power conversion efficiency of the photovoltaic cell. This term indicates that the high conversion efficiency from sun light to electricity largely determines the StH efficiency. Thus, the use of multi-junction photovoltaic cell is in line with the direction for achieving the effective StH conversion as discussed. Since this conversion efficiency is relatively low among other efficiencies in Equation [5] largely due to the properties of solar light spectrum and the structure of photovoltaic cell, improvement in this efficiency generally elevates the whole StH efficiency.

The third and fourth efficiencies in Equation [5] are the voltage energy loss in the electrochemical cell and the current–hydrogen conversion efficiency (Faradic efficiency) in the electrochemical cell, which are the key characteristics of electrochemical cell.^12,13 The current versus voltage and hydrogen conversion efficiency from the current characteristics of E103 electrochemical cell are shown in Figure 5a,b as examples. The conversion voltage energy loss inevitably occurs because the reaction is believed to involve some activation energy (turn-on voltage difference from the $G_{f{H}_2}^0$) and current resistance (slope of the current–voltage characteristics). The interesting point on these characteristics in electrochemical cell is that the optimum range exists for the conversion from current to hydrogen. When the current is too low, the existence of parasitic reactions probably pushes down the conversion efficiency. In opposite, when the current density is too high, the presence of the generated bubbles probably interferes with the hydrogen generation reaction. The actual required range of the current may depend on the cell structure, but the optimum range of the current surely exists anyway.

Figure 5.

(a) Current versus bias characteristic and (b) current to hydrogen conversion efficiency versus current for water splitting operation of 4-cm² reaction area E103 electrochemical cell.

The most important operational efficiency is the second term in the Equation [5], which indicates the operation point matching. This is the ratio of the operating power over the maximum power of the photovoltaic cell, which is the ratio of the operating voltage over the voltage of the maximum power conversion point for photovoltaic cell when the connection between the photovoltaic cell(s) and electrochemical cell(s) is direct since the current is the same. The typical examples are shown in Figure 6. The operation point for 18.38-cm² 3-series connection pc-Si photovoltaic cell is largely different from the maximum power point, thus, the operation power loss tends to be large compared with that of 81.12-cm² 4-series connection pc-Si photovoltaic cell. This phenomenon becomes much clearer when the energy conversion efficiencies are compared between solar-to-electricity and StH. The maximum energy conversion efficiencies from the solar simulator light to electricity by 18.38-cm² 3-series connection and 81.12-cm² 4-series connection pc-Si photovoltaic cells were 6.01% and 8.45%, respectively. However, the conversion efficiencies from the solar simulator light to hydrogen remain to be 2.0% and 6.1% for each photovoltaic cell, respectively. As discussed above, the operation point matching between devices are very important for the high energy conversion efficiency when the target performance is defined by system operation like this StH conversion by PV + EC system.

Figure 6.

Current versus bias relationship of 18.38 cm² 3-series cell connection operated with 291 mW/cm² solar simulator and 81.12 cm² 4-series cell connection of p-Si photovoltaic cells operated with 49 mW/cm² solar simulator, and current versus bias relationship of 4-cm² reaction E103 electrochemical cell. The maximum power points of photovoltaic cells (square marked) and the operating points (circle marked) are also shown in the graph. The discrepancy of these two points shows the operation power loss.

This light-to-hydrogen efficiency was improved by ca. 12%, thus, scoring ca. 20% of the maximum efficiency, when a 3-junction CPV connected to a EC of E103 was employed under 10-fold intensive solar light simulator. Similarly, the efficiency was shown to be increased by over 15% when the connection was switched to 2-CPV and 3-EC in series. This result clearly shows that the system optimization could be achieved not solely by optimization of each device.¹²

Comparison between PS-II and artificial StH conversion system

The oxygen evolution process in PS-II system and the artificial StH conversion by PV + EC system share conceptual similarity as discussed above and are summarized in Figure 4. When the PS-II process is compared with the key factors found in the StH efficiency by artificial PV + EC system, we noticed that the energy concentration steps through solar light harvest prior to OEC is thought to be the key factor equivalent in the living plants. Since the OEC consists with metal complex and since the artificial fabrication of this complex may requires a considerbale amont of energies, the process efficiency regarding the OEC should be one of key optimization points. Although the details cannot be discussed as the results available are limited here, this kind of analysis or evaluation for the plant behavior is considered to be one of the approaches to understand the nature of plants.

Apart from the StH conversion scored under artificial light (solar simulator), the PV + EC system employing 3-junction photovoltaic cells achieved 24.4% of StH conversion efficiency under real sunlight.¹ To date, serial connections among a number of photovoltaic cells and electrochemical cells were optimized in order to obtain the high conversion efficiency with the operation point matching. The conversion efficiency recorded with our system was at the world highest level at the reported timing according to the comparison with the other StH efficiencies.¹⁴ In addition, higher StH efficiencies were recently obtained by the PV + EC system with multi-junction photovoltaic cells, especially after our previous report on high StH efficiency.^12,14,15 Current maximum conversion efficiency is 31.7% under artificial sunlight.¹⁶

It is notable that the improved StH efficiency by our multi-junction PV + EC system apparently looks exceeding the energy conversion efficiency during natural photosynthesis by living plants. According to Zhu,¹⁷ the maximum efficiencies for solar-to-biomass conversion ranges from 4.6% (C3 plants) to 4.6% (C4 plants). It likely impresses us that the efficiencies for energy conversion by living plants are relatively low. In fact, it is not fair to compare the StH achieved by the multi-junction PV + EC system with the net photosynthesis in living plants since the living plants consume energy for running the system (as respiration), thus, at the level of gross photosynthesis, the efficiencies can be scored higher (12.6%, C3 plants; 8.5%, C4 plants).

Importantly, biomass (in the form of carbohydrates) is not the primary chemical products from photosynthetic reaction. This could be one of the reasons why plant energy utilization tends to be underestimated. More importantly, in general, 51.3% of energy within the incident solar radiation is outside the photosynthetically active spectrum, 4.9% of energy is lost via reflection by and transmission through the leaves, and 6.6% of energy is lost due to photochemical inefficiency.¹⁷ At this stage, 37.2% of total solar energy could be captured and used for development of chemical potentials involving transport of electrons (and compartmentation of protons); to be followed by consumption of 24.6% (C3 plants) or 28.7% (C4 plants) of total energy in order to provide ATP required for carbohydrate synthesis. This efficiency scored prior to carbohydrate synthesis (37.2%) must be equivalent to and comparable with the maximum StH by the multi-junction PV + EC system (24.4–31.7%).

As discussed above, the effort for understanding the mechanism in plants is highly helpful for newly designing and engineering the artificial systems. We believe that the efficiency in artificial system could be further enhanced by remodeling the system after learning the details in biological systems, as the evolutionarily thrived plants apparently possess more room for optimization via genetical engineering.¹⁸

Energy management system using natural energy sources

Energy management system is required, especially for the electricity supplying, in order to optimize the use the storage energy such as hydrogen derived from nature-based energy sources. In order to evaluate the functioning of the energy management system operation, a small-sized demonstration system was fabricated, as schematic diagram and photographs are shown in Figure 2c,d.

Figure 2.

(a) Schematic diagram of the connection of photovoltaic cell(s) and electrochemical cell(s), (b) hydrogen production modules by connecting CPV and PEEC in Miyazaki, (c) schematic diagram of energy storage system using with light to hydrogen conversion, and (d) energy storage demonstration system with p-Si solar cells (Bottom left shows the front side view of the system).

The key point of this small-sized system is that the system is not targeted high conversion efficiency of StH, but focused on the real use in the future. Thus, the energy input employed as a model was a conventional p-Si photovoltaic cell. In addition, the voltage controls of electrochemical cell input and fuel cell output are required upon the hydrogen generation and output power along with the changes in the voltage as shown in Figure 7a,b. Therefore, the system is equipped with the active voltage control DC/DC converter to tune the operation voltage for EC and the output voltage for FC as shown in Figure 2c.

Figure 7.

(a) The relationship between current input and H₂ generation versus voltage of 2-stack EHC-500 polymer electrolyte electrochemical cell. (b) The relationship between current and power output versus voltage of 32-stack EOS-50 polymer electrolyte fuel cell.

Considering from the point of view on the flow of the electric power, the conversion efficiency in this kind of system should be defined as follows. That is;

$${\eta }_{SYS}=\left({\sum }_{i=1}^n{\alpha }_i{\eta }_{PCi}\right)\left(\lambda {\sum }_{j=1}^m{\beta }_j{\eta }_{TRSj}{\eta }_{PSj}{\eta }_{PGj}+\left(1-\lambda \right)\sum _{k=1}^l{\gamma }_k{\eta }_{TRUk}\right),$$ (6)

where,

$${\sum }_{i=1}^n{\alpha }_i=1,{\sum }_{j=1}^m{\beta }_j=1,\sum _{k=1}^l{\gamma }_k=1,$$ (7)

where as α_i (1 ≤ i ≤ n) is the ratio of the power input for each power generation device, β_j (1 ≤ j ≤ m) is the ratio of the power storage for each energy storage device/system, and γ_k (1 ≤ k ≤ l) is the ratio of the power output for each power divided line to system.

${\eta }_{SYS}$ is overall system efficiency, ${\eta }_{PCi}$ is the nature source to electricity conversion efficiency of the i-^th generation device, λ is the power output ratio for storage (that is, 1– λ the power output for direct use), ${\eta }_{TRSj}$ is the power transfer efficiency (like DC/DC converter) for j-^th storage device/system, ${\eta }_{PSj}$ is the electric power to storage conversion efficiency for j^−th device/system, ${\eta }_{PGj}$ is the storage to electric power conversion efficiency for j^−th device/system, and${\eta }_{TRUk}$ is the power transfer efficiency (like DC/DC converter) for k^−th line for output device/system. The meaning of this Equation (6) is simple, that is;

1. the nature source to electricity conversion (${\eta }_{PCi}$) should be high,

2. the smaller storage ratio (λ) is better because the conversion from electricity to storage (${\eta }_{PSj}$) and storage to electricity (${\eta }_{PGj}$) are less than unity, and

3. the power transfer efficiency like DC/DC converter (${\eta }_{TRSj}$, ${\eta }_{TRUk}$)should be high.

The mode of operation of the real functioning system is complicated even for the artificial one because the system accompanies the fluctuations of the input(s) and user-on-demand output(s) to be coped with, and therefore, the operation has to be satisfied the input and output requirements by utilizing the in-between storage device(s) and/or system(s). However, the technical requirements for the system is not too difficult, thus, these requirements are probably the basis or the guidelines for the system design.

The energy efficiency of the system shown in Figure 2d was evaluated with the efficiency measurement of each device. The efficiency was dependent on the operating power; thus, typical efficiency was selected for the system evaluation. The efficiencies of DC/DC converter, electrochemical cell, and fuel cell were about 0.80, 0.50, and 0.30, respectively. As results, the power efficiency of direct power use from the photovoltaic cell output (except for the photovoltaic cell efficiency) was 0.80; however, the efficiency via H₂ storage was 0.10. The model energy management system used here was just connected to commercial devices and not optimized, thus, this result is realistic and acceptable. The power efficiency for the H₂ storage in this model system was, however, not high enough for the real use as indicated from this result, suggesting that the further improvement of the system design is required.

The relatively large-sized experiments employing similar H₂ storage were reported, showing that the efficiency of the H₂ storage could be achieved up to 0.40.^19,20 The devices selected in the reported experiments had high efficiencies, thus, optimizing the performance. It is conclusive that this efficiency is one of the targets to be achieved even for the small system used here. The plants' energy transfer would be also  another line of guidance considering the analogy between the PS-II system and artificial water splitting system.

Information on the experimental bases

Electrochemical cells with photovoltaics under solar simulators

The system of polymer electrolyte electrochemical cell (PEEC) with conventional polycrystalline Si (pc-Si) solar cell was compared with a combination of PEEC with CPV. The multiple-series electrical junction of PEEC and CPV was also used. The CPV used was a GaInP/InGaAs/Ge 3-junction cell with a surface area of 1.0 × 1.0 cm² square (Spectrolab Inc., C3MJ5 PP-06491-CCC CDO-100”; Typical cell efficiency of 39.2% at 500 times light concentration).^21-23 The PEEC was a platinum-loaded carbon paper electrode with a proton exchange polymer membrane as an electrolyte with 4.0-cm² electrode area (h-tec Educations GmbH, Electrolyzer E103). Pure water was used as a splitting source. The schematic system design is shown in Figure 2a. The CPV was illuminated by concentrated light of AM1.5G solar simulators instead of the concentrated sunlight (Ushio Corp., SXUID502XQ). No cooling equipment was used for the CPV, so the cell temperature rose with increasing light intensity and operation time.

Electrochemical cells with photovoltaic cell under real sunlight

Hydrogen production by connecting CPV modules and PEEC was attempted in the outdoor test field at Miyazaki University, Japan, as shown in Figure 2b. A CPV monomodule (Sumitomo Electric Industries]) had a 57mm² light-receiving area, and a 2.5mm² GaInP/GaAs/Ge 3-junction cell was placed at the sunlight focus. Two types of polymer-electrolyte EC cells, E103 and E106 (h-tec Educations GmbH) were connected to the CPV modules by copper wiring. The EC cells contained platinum-loaded carbon paper electrodes with a proton exchange polymer membrane, and pure water was fed into the cells. A package of E106 contains two EC cells in series, each with the membrane area of 16 cm², while a package of E103 contains one EC cell with the area of 4 cm²; therefore, a set of four E103s in parallel was treated as being equivalent to one EC cell of E106.

Energy management system with c-Si photovoltaic cell and hydrogen storage

Schematic diagram of energy management system with single crystalline Si (c-Si) photovoltaic cell and hydrogen storage is shown in Figure 2c and of the real system photographs are shown in Figure 2d. The c-Si solar cells were 60 W output ATMA60SG {open circuit voltage (V_oc): 20.8 V, short circuit current (I_sc): 3.68 A, maximum power voltage and current (V_max and I_max): 17.6 V and 3.41 A} (Autumn Technology). The solar cells were connected to 1215BN MPPT (maximum power point tracking) controller (Beijing EP Solar Technology Co., Ltd.) and divided into two-lines. One of the lines was connected to electric loads and the other was connected to a PEEC (2-stack, EHC-500, Enoah Inc.) and converted to hydrogen. The hydrogen was converted back to electricity by a polymer electrolyte fuel cell (PEFC) (32-stack, EOS-50, Pearl Hydrogen Technology Co. Ltd.) and connected to the electric loads, with which connection from solar cells and/or PEEC was controlled by the consuming amount of the load. The connection between each device was a VE-200 series DC/DC converter (Vicor corp.) in order to match the voltages.

Conclusion

The solar-to-chemical energy conversion process between the PS-II in the plants and artificial systems were compared. The energy flow found in the combined PV + EC in artificial system was very close to that in PS-II. Energy conversion analysis revelaed that the energy conversion efficiency by PV + EC system equipped with the energy concentration unit by combining the Fresnel lens and multi-junction photovoltaic cell was largely improved. We view that the process for energy concentration and optimization in the antenna-associated PS-II shares the conceptual similarity with our artificial StH system. The discussion in this article suggests that the analysis of the natural processes in living plants is useful for designing the artificial systems and vice versa, the analysis of the energy flow in the artificial systems may assist the analysis of the natural processes in plants. The energy management system with the use of H₂ storage was also discussed. We showed that not only the performance of each device but also the mode or combinations of devices to be connected must be considered for improving the system. The discussion on the similarity between the natural process in plants and the artificial energy management system has not been completed, but the comparison would be helpful for the system design and engineering.

Acknowledgments

The authors thank all the members of Global Solar plus Initiative and Nakano-Sugiyama-Tanemura Laboratory, the University of Tokyo (especially, Prof. Masakazu Sugiyama), all the members of Institute of Environmental Science and Technology, The University of Kitakyushu, all the members of Advanced Photonics Technology Development Group, RIKEN Center for Advanced Photonics, and all the members of Nishioka Laboratory, University of Miyazaki (especially, Prof. Kensuke Nishioka) for their continuous support. The authors thank Takashi Iwasaki, Rui Mikami, and Makoto Inagaki of Sumitomo Electric Industries, Ltd. for supplying the CPV modules. Dr. Shinichiro Nakamura is acknowledged for fruitful discussion.