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. 2019 Dec 31;6(1):e03094. doi: 10.1016/j.heliyon.2019.e03094

Dependence of electric power flow on solar radiation power in compact photovoltaic system containing SiC-based inverter with spherical Si solar cells

Yuji Ando a, Takeo Oku a,∗, Masashi Yasuda b, Kazufumi Ushijima c, Hiroshi Matsuo d, Mikio Murozono e
PMCID: PMC6940697  PMID: 31909264

Abstract

A photovoltaic power generation system suitable for mobile applications was developed. A SiC integrated converter with the maximum power point tracking circuit provided the smallest photovoltaic inverter in ~200 W level. The SiC-based inverter exhibited a peak direct current (DC)-alternating current (AC) conversion efficiency higher than that of conventional Si inverters. A Li-ion laminated battery was mounted in the same housing as the inverter. The weight of entire system containing spherical Si solar cell panels was well below 6 kg. Continuous operation measurements of this system were carried out using four solar cell modules connected in parallel under irradiation by natural sunlight. The total inverter efficiencies under realistic operation conditions were slightly decreased compared with the DC-AC converter values because of loss by the maximum power point tracking device. Even under unstable weather conditions, the system provided power stability without ripples. The behaviors of the output powers of the solar cell, storage battery, and inverter modules were analyzed as a function of the solar radiation power density. The substantial efficiencies of the solar cell modules were dependent on the weather conditions and were approximately 10% on cloudy days. The present compact photovoltaic power generation system with SiC device and spherical Si solar cells is viable for sub kW-class inverter.

Keywords: Electrical engineering, Energy, Power converter, Energy storage technology, Renewable energy resources, Solar energy, Photovoltaics, Energy sustainability, Materials physics, Solar cell, Maximum power point tracking charge controller, Lithium-ion battery, Direct current-alternating current converter, Silicon carbide, Spherical silicon, Field effect transistor, Schottky diode, Inverter

Electrical engineering; Energy; Power converter; Energy storage technology; Renewable energy resources; Solar energy; Photovoltaics; Energy sustainability; Materials physics; Solar cell; Maximum power point tracking charge controller; Lithium-ion battery; Direct current-alternating current converter; Silicon carbide; Spherical silicon; Field effect transistor; Schottky diode; Inverter

1. Introduction

1.1. Advantages of wide bandgap semiconductor materials

Current power device should meet the conditions such as a low ON-resistance, high blocking voltage, and high switching frequency, even at high temperatures. Wide gap semiconductors such as SiC and GaN have been examined as the power devices [1, 2, 3], and they have high voltage endurance, and a thin epitaxial layer is enough to intercept the voltage. Therefore, SiC and GaN switching devices provide lower ON-resistances than the Si devices. Efficiencies of inverters can be improved by a low ON-resistance, and the module size can be reduced by the low ON-resistance and the increased power density. The GaN and SiC devices can be operated at high temperatures, which is due to their good thermal stabilities.

1.2. Reports on photovoltaic inverter with SiC device

After the reports on photovoltaic (PV) inverter equipped with SiC diodes [4] and SiC transistors [5], many works have been reported for the SiC power converters [6, 7, 8]. Interleaved direct current (DC)-DC converters built with SiC bipolar junction transistors (BJTs) and Schottky barrier diodes (SBDs) were reported [9, 10]. DC-alternating current (AC) converters using SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) and junction field effect transistors (JFETs) were reported [11, 12]. 3 phase inverters equipped with SiC MOSFETs and JFETs were also developed[13, 14]. Furthermore, inverters using all SiC devices were reported [15, 16]. Some authors have reported comparative studies on inverters with SiC and Si devices [17, 18, 19], and SiC converters with photovoltaic system were also reviewed [20]. These SiC devices have been used as switching devices like MOSFETs, JFETs, and BJTs. The commercially obtainable SiC MOSFET is a normally-off device controlled by voltage, which is easy to use. Although the SiC JFET and SiC BJT are not so advanced devices, these devices are expected to provide a reduced resistivity for switching-on, because they have good carrier transport comparing with SiC MOSFET. Properties of normally-off SiC BJT depends on the current, and need a specific circuit. SiC JFET has a pn junction with a gate voltage of ~3 V corresponding to the bandgap. The JFET needs a high voltage for switching the gate, and needs a specific circuit. Most of reported works on SiC-based inverters are concerned with applications in multi-kW level.

Benefits of the low ON-resistance for the SiC device have generally been considered to be reduced in sub-kW level inverters. Thus, it is necessary to investigate the effectiveness of SiC devices in sub-kW applications. SiC-equipped inverters in sub kW level were recently reported [21, 22, 23, 24, 25], and have been found to be effective and compact inverters [26, 27, 28, 29]. Such small inverters are useful for various compact and transportable devices.

1.3. Aim of this work

The purpose of the present work was to develop a compact, light-weight SiC-based inverter equipped with a maximum power point tracking (MPPT) circuit and a lithium ion laminated battery. A previously reported SiC inverter with a MPPT system [26, 27, 28, 29] was refined in the present study, and a 15-V Li-ion laminated battery was developed. Based on these technologies, a compact photovoltaic PV power system was constructed using spherical Si modules. The spherical Si solar cell panel is flexible and lightweight, and are appropriate for mobile use [30]. This study also aimed to study the influence of the solar radiation power on the electrical power flow of this PV power generation system. For this purpose, constant measurements of the present system were carried out under the natural sunlight irradiation. The behavior of the output powers of the solar cell, storage battery, and inverter modules were analyzed as a function of the solar radiation power density. The substantial efficiencies of the solar cell and inverter modules under realistic operation conditions were estimated.

2. Experimental

Figure 1(a) shows a schematic of the present photovoltaic power generation system. An MPPT charge controller, Li-ion laminated battery, and DC-AC SiC converter were contained in a single battery-converter unit. Figure 1(b) shows a digital photographic image of the developed battery-converter system. This unit consisted of a photovoltaic converter module and a lithium ion laminated battery module. The PV inverter module contained the MPPT circuit and the DC-AC SiC converter circuit. The MPPT charge controller maximized the photovoltaic output power by controlling load of the panels, and controlled the power supply to the DC-AC part by discharging and charging the lithium ion battery [31]. The DC-AC converter was constructed from a front DC-DC stage with a next DC-AC stage. The DC-DC circuit was constructed from 4 SiC Schottky barrier diodes (STPSC10TH13TI, STMicroelectronics, Tokyo, Japan) [32]. A single-phase full-bridge DC-AC converter consisted of 4 SiC metal-oxide-semiconductor field-effect transistors (SCT3060AL, ROHM Semiconductor, Kyoto, Japan) [33]. The switching frequencies of the pulse width modulation were 200 kHz for the DC-DC parts and 100 kHz for the DC-AC parts. These high switching frequencies made it possible to decrease the size of the PV inverter module. The output power of the present inverter was 150 W. Compared with the reported inverter module [26, 27], the volume and weight of the module were reduced more than ~35%. Optimization of the circuit structure and density provided the present, most compact inverter among ~200 level PV inverter systems [34].

Figure 1.

Figure 1

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(a) Schematic of the present photovoltaic inverter system. Digital photographs of the (b) inverter-battery unit, (c) top and (d) bottom of the present system.

Figures 1(c) and 1(d) show digital photographic images of the present inverter system. A battery-converter unit was attached to the bottom of the solar cell module. The maximum power point voltage and current for the spherical Si panels (CVFM-0540T2-WH, Clean Venture 21, Kyoto, Japan) were 16.2 V and 3.34 A, respectively. The rated output power for one solar cell module was 54 W. The photo-generated voltage was ~14 V, and the output power depended on the solar radiation power.

A LiMn2O4-based laminated battery module was also developed. This battery consisted of four 3.8 V cells connected in series. To adjust the specifications of the Li-ion battery to those of the spherical Si solar cell module, the rated voltage of the battery was set to 15.2 V and the current capacity was 12 Ah. The thinness of these cells (<10 mm) realized a thin-type power conversion apparatus in which the PV inverter module and battery module were mounted in the same housing. Sizes and weights of modules and the total system are summarized as Table 1. The weight of the total system with a single PV module was well below 5.7 kg.

Table 1.

Sizes and weights of modules and the total system.

Module Size (mm3) Weight (kg)
Inverter 180 × 175 × 25 0.79
Storage battery 492 × 280 × 27 2.77
Solar cell 1020 × 690 × 5 2.13
Total system (1 PV module) --- 5.69
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The output power setting was 150 W, and parallel-connected spherical Si panels were set as the power source. Voltages and currents of the inverter, PV panels, and lithium ion battery were measured simultaneously by power meters (Hioki, PW3336, Nagano, Japan). Interval of the measurement was set as 200 ms, and the data were averaged for every minute. The solar radiation power (Uizin, UIZ-PCM01-LR, Tokyo, Japan), temperature, and humidity (Hioki, LR5001) were measured synchronously. Filament lamps were applied as the load. Conversion efficiencies of individual inverters were estimated by output/input powers.

3. Results and discussion

3.1. Conversion efficiency of the PV inverter

The conversion efficiency of each circuit of the present inverter was characterized independently. The efficiencies of the MPPT circuit (ηMPPT) were measured by supplying alternative power to the cell part with the inactive DC-AC converter. In Figure 2(a), the square symbols show ηMPPT with respect to DC output power in this 15-V battery measurement. When the DC power was above 23 W, the efficiencies surpassed 92%, and the highest efficiency extended to 96%.Efficiencies of the DC-AC circuit (ηDC-AC) was investigated for the inactive spherical Si panels. In Figure 2(b), the square symbols show ηDC-AC regarding the AC output in this measurement. The efficiency surpassed 82% above 75 W, and the highest efficiency extended to 87%. This peak efficiency was more than 3% higher than that of ordinary inverters [26, 28]. In general, high switching frequencies greatly affect the efficiencies of DC-AC circuit because of the increased switching losses. The superior efficiency of this inverter was therefore attributed to decreased switching losses in the SiC device compared to in Si devices.

Figure 2.

Figure 2

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Output power and efficiencies for the (a) MPPT and (b) DC-AC circuits.

To study the influence of the developed 15-V Li-ion battery, similar measurements were carried out using a conventional 12-V LiFePO4-based battery module (O'Cell, IFM12-200E2, Shenzhen, Guangdong, China). In Figures. 2(a) and (b), the circle symbols show the results of the 12-V battery. The difference in efficiencies between the 15-V battery and 12-V battery measurements were below 2% for ηMPPT and below 3% for ηDC-AC. These results indicated that the efficiencies of the DC-AC converter and MPPT circuit were mildly dependent on the battery voltage.

3.2. Power loss analysis

Power loss analysis was carried out for the Si- and SiC-based converters according to a previously reported method [28]. Table 2 summarizes the electrical characteristics (ON-resistance (Rds), turn-on loss (Eon), and turn-off loss (Eoff) of the field effect transistor (FET), and forward voltage (Vf) and recovery time (trr) of the body diode (BD)) assumed for Si and SiC MOSFETs operating at temperature (T) of 25 °C. Reported data for FQPF16N25C (Fairchild semiconductor) and SCT3060AL [31] were used as the parameters. Although the forward current (If) across the BD increased the reverse recovery current (Irr), and the Irr of the SiC MOSFET was reduced by short lifetime of minority carriers compared with that of Si MOSFET. It was assumed that Irr = αIf, where α = 3 and α = 1/3 for Si and SiC, respectively [32].

Table 2.

Properties of Si and SiC MOSFETs of the present and commercially available inverters.

Property Symbol Unit SiC Si Remarks
FET ON-resistance Rds Ω 0.06 0.22 Almost independent of I
FET turn-on loss Eon μJ 70 180 Measured at Vt = 300 V, It = 10 A
FET turn-off loss Eoff μJ 10 120 Measured at Vt = 300 V, It = 10 A
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Comparison between measured and simulated efficiencies for the output AC power. In this calculation, the AC output voltage was set to 100 V and the switching frequencies of the pulse width modulation signal were set to 100 kHz and 20 kHz for the SiC and Si inverters, respectively. Satisfactory agreement was obtained between the measured and simulated efficiencies for both inverters. Open circle symbols in Figure 3 show the DC-AC conversion efficiencies regarding the AC output. The DC-AC efficiency for the SiC device surpassed 80% above 75 W. A Si-based commercial inverter (SXCD-300, Daiji Industry, Osaka, Japan) was similarly investigated for the same system. When the output power was larger than 90 W, the present SiC inverter showed superior efficiency to the Si inverter, and this improvement in efficiency reached 7% at an output power of 150 W. For the lower output power than 90 W, the Si inverter showed superior efficiency to the SiC inverter. Loss analysis was carried out to account for this behavior in the DC-AC conversion efficiency.

Figure 3.

Figure 3

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Measured (circles) and simulated (lines) efficiencies of the developed SiC inverter and commercially available Si inverter.

Contributions of power loss components to the total loss occurring at output powers of 20 W, 75 W, and 150 W are shown in Figures 4(a), 4(b), and 4(c), respectively. In these figures, the total power loss is divided into three components: DC-DC loss (Pdc-dc), DC-AC loss (Pdc-ac), and non-load loss (Pnl) that was independent of the load value. When the power increased, the contributions of the DC-DC and DC-AC losses increased. In contrast, the contribution of the non-load loss became more significant when the output power decreased. Therefore, the decreased DC-DC and DC-AC losses in the SiC inverter were responsible for its superior efficiencies at high power, and the increased non-load loss was responsible for the inferior efficiencies at low power. The non-load loss mainly originated from the filter and transformers equipped in the DC-DC circuit. The decreased DC-DC loss in the SiC inverter was considered to be due to decreased conduction loss in the SiC SBD because of the lower ON-resistance compared with the Si switching diodes. The DC-AC loss was composed of several components.

Figure 4.

Figure 4

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Contributions of DC-DC power loss components to the total loss for (a) 20 W, (b) 75 W, and (c) 150 W.

Contributions of power loss components to the DC-AC loss for 20 W, 75 W, and 150 W are shown in Figures 5(a), 5(b), and 5(c), respectively. The DC-AC power loss is divided into five elements: conduction loss (Pfet), turn-on loss (Pon) and turn-off loss (Poff) of the FET, and conduction loss (Pbd) and reverse recovery loss (Prr) of the body diode. The conduction losses (Pfet and Pbd) were less important since the most of the DC-AC losses were Pon, Poff and Prr. The reduced recovery loss in the SiC MOSFET was mainly responsible for the decreased loss in the SiC DC-AC circuit. To reduce the module size, the switching frequency of pulse width modulation was raised from 20 kHz for the Si inverter to 100 kHz for the SiC inverter, which was expected to significantly increase the switching losses for the SiC device. However, the switching losses were not so much in the SiC inverter, which was due to the smaller turn-on and turn-off delay times of the SiC MOSFET.

Figure 5.

Figure 5

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Contributions of power loss components to the DC-AC loss for (a) 20 W, (b) 75 W, and (c) 150 W.

3.3. Effect of number of solar cell modules

To investigate the substantial output power of the PV modules, one to four PV modules were connected in parallel and the resulting system was operated under irradiation by natural sunlight. The load power was fixed at 74 W. The powers at the solar cell (Ppv), battery (Pbat), and output (Pac) parts of the inverter were measured synchronously. The temperature, humidity, and solar radiation power density (psol) were also monitored during the measurement.

Figure 6(a) shows the measured Ppv as a function of psol in a typical measurement using a single PV module. An ideal curve expected from the rated output power of 54 W is also shown. The measured output power was generally located along this ideal curve. Figure 6(b) shows the results using four PV modules connected in parallel. The measured total power was decreased by 35% compared with that expected from the rated output power of 54 W/module. Figure 6(c) shows the PV module number dependence of the total output power extrapolated to a solar radiation power density of 1 kW m−2. The power density per module decreased with increasing number of PV modules. This decrease in output power density was mainly due to the variation in electrical characteristics of individual PV modules. PV modules were connected in parallel in this measurement setup, and small variations in open circuit voltages would significantly lower the total efficiency.

Figure 6.

Figure 6

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Total output power versus the solar radiation power density for measurements using a (a) single PV module and (b) four PV modules connected in parallel. (c) PV module number dependence of the total output power extrapolated to a solar radiation power density of 1 kW m−2.

3.4. Efficiency changes under the natural sunlight

To study power flow of the photovoltaic power generation system, efficiency changes were measured under the natural sunlight irradiation. First, the measurement was performed under cloudy conditions. Four PV modules were connected in parallel and the load power was set to 20 W. Electric powers Ppv, Pbat, and Pac are shown in Figure 7(a) ηMPPT, ηDC-AC, and the total inverter efficiency (ηTOT) are also shown in Figure 7(b). In the present work, total efficiencies that were originally introduced by Haeberlin et al. [35] was adjusted to contain the battery, which is given by

ηTOT=PacPpv+Pbat (1)

Figure 7.

Figure 7

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(a) power at the PV (dotted), AC (solid), and battery (broken) parts of the inverter, (b) efficiencies of the MPPT (dotted), DC-AC part (broken), and total system (solid), and (c) solar radiation density. The measurement was carried out using four PV modules at a load power of 20 W. The weather conditions were cloudy and occasionally clear.

Because the weather conditions were cloudy and occasionally clear, considerable variation was observed in psol, as shown in Figure 7(c). Consequently, large instability was seen in Ppv and Pbat, as shown in Figure 7(a). Even under unstable weather conditions, Pac was very stable and no ripples were observed. The total efficiencies were in the range of 50–60%, which was due tothe low power of 20 W. Under these conditions, the efficiencies were predominantly controlled by the transformers and filter circuit in the DC-DC part [34].

A similar measurement was then performed under clear sky conditions. Four PV modules were connected in parallel at 144 W Figure 8(a), (b), and (c) show the changes in electric powers (Ppv, Pbat, and Pac), conversion efficiencies (ηMPPT, ηDC-AC, and ηTOT), and psol, respectively. Because the weather conditions were clear, psol was relatively stable and was around 0.6 kW m−2. Thus, no significant instability was seen for Ppv or Pbat. The total efficiencies were in the range of 86–87%.

Figure 8.

Figure 8

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(a) Power at the PV (dotted), AC (solid), and battery (broken) parts of the inverter, (b) efficiencies of the MPPT (dotted), DC-AC part (broken), and total system (solid), and (c) solar radiation density. The measurement was carried out using four PV modules at a load power of 144 W. The weather conditions were clear.

Total efficiencies were also measured under 30 W, 54 W, 74 W, 90 W, 110 W, and 125 W. The total inverter efficiencies defined by Eq. (1) were calculated as time averages from measurements spanning several hours. Figure 9 shows ηTOT as a function of Pac. A peak ηTOT of 86.1% was obtained at the load power of 144 W. The total efficiencies of the inverter under realistic operation conditions were generally decreased compared with the DC-AC converter values, which is due to the MPPT circuit loss. But, the lowering of efficiencies was only a few percent.

Figure 9.

Figure 9

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Output power and efficiencies for the inverter under realistic operation conditions.

3.5. Output power and solar radiation density

The relation between the solar radiation power density and the output powers was analyzed from the data of the continuous operation measurements. Figure 10(a), (b), and (c) show Ppv, Pbat, and Pac as a function of psol, respectively, which were calculated from the data shown in Figure 4. Because the weather conditions were cloudy and occasionally clear, psol varied widely. As a result, an almost linear correlation was obtained between Ppv and psol. When psol was lower than 0.2 kW m−2, Pbat was positive which indicated discharging of the battery. Under these conditions, the Li-ion battery supplied the power to compensate for the shortage of Ppv. When psol surpassed 0.2 kW m−2, Pbat was negative which indicated charging of the battery. Under these conditions, the electric power supplied by the spherical Si panel was sufficient to supply the system and charge the Li-ion battery. When psol surpassed 0.5 kW m−2, the slope of Ppv tended to decrease. When the battery management circuit sensed an overcharge of the battery, it interrupted the flow of current into the battery, so Ppv was suppressed. The operation of the battery management circuit was therefore responsible for this nonlinearity. The dependence of Pac on psol was very minimal. Figures. 10(a) and (b) also show linear line fit results based on the following equations:

Ppv=ηpvApv(psol−pth) (2)
Pbat=PacηTOT−Ppv (3)

Figure 10.

Figure 10

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(a) Photovoltaic output, (b) battery output, and (c) AC output powers versus solar radiation density.

In these expressions, Apv is the active area of the cell panel. The efficiency of the PV modules (ηpv) and the threshold power density (pth) were used as fitting parameters. Assuming the active area of the solar cell was 2.16 m2, ηpv was estimated to be 9.7%. The threshold power density was approximately 0.04 kW m−2. Similar measurements were performed on different days. A reasonable correlation was obtained between Ppv and psol, and the intersection point was around 0.04 kW m−2. The physical origin of this threshold is currently unclear, and the input voltage regulation scheme used in the MPPT charge controller may have been responsible for this.

Figure 11(a), (b), and (c) show Ppv, Pbat, and Pac as a function of psol, respectively, which were calculated from the data in Figure 5. Because the weather conditions were clear, a clear correlation was not obtained between Ppv and psol. Pbat was positive throughout the entire measurement. Under these conditions, the Li-ion battery supplied the power to fill in the shortage of Ppv. The dependence of Pac on psol was very minimal. Figures. 11(a) and (b) also show linear line fit results based on Eqs. (2) and (3). The threshold power density was assumed to be 0.04 kW m−2. Assuming the active area of the solar cell was 2.16 m2, ηpv was estimated to be 6.6%. Similar measurements were performed on different days. Measured ηpv values were approximately 6%–7% on clear sky days and approximately 9%–10% on cloudy days. This lower efficiency under clear sky conditions was due to the power loss of the wiring cables and to the heating of the solar cell modules.

Figure 11.

Figure 11

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(a) PV output power, (b) battery output power, and (c) AC output power versus the solar radiation density.

3.6. Estimation of battery duration

Measurements on the battery duration (tbat) were performed while the spherical Si panel was not operated. Figure 12 shows the measured and calculated relationships between tbat and Pac. In the calculation, tbat was estimated by

tbat(Pac,Qbat)=QbatVbatηinv(Pac)Pac (4)

Figure 12.

Figure 12

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Measured and calculated battery durations versus the AC output power for the developed SiC inverter and commercially available Si inverter. The solar cell was inactive in these calculations and measurements.

Vbat is the battery voltage, and Qbat is the battery capacity. ηinv (Pac) is the inverter efficiency as a function of Pac. Qbat, Vbat, and the series resistance were set to 12 Ah, 15 V, and 1 Ω, respectively. The measured values agreed well with the calculated ones. Figure 12 also shows previously reported data for the SiC system reported previously [28]. The developed SiC system exhibited a duration twice as long as that of the reported SiC system.

3.7. Comparison of inverter size

Figure 13 shows the weight of the developed inverter as a function of its volume. Compared with the reported system [26, 27], the volume and weight were reduced by ~40%. Optimization of the circuit design provided the reduced size. Figure 13 also shows the volumes and weights for the ~200 W class inverters, and the developed system was the most compact one [34].

Figure 13.

Figure 13

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Volumes and weights for the ~200 W class inverters.

4. Conclusions

A PV power generation system suitable for mobile applications was developed. The MPPT equipped SiC inverter achieved small volume and light weight among ~200 W level photovoltaicinverters. Compared with the previous system, the volume and weight decreased by ~40%. The conversion efficiencies of the DC-AC part and MPPT circuits increased to 87% and 96%, respectively. The SiC-based inverter exhibited a peak efficiency for the DC-AC circuit that was more than 3% higher than that of the ordinary Si-based inverter. The conversion efficiency of the present SiC inverter was improved by 7% at 150 W. Loss analysis indicated that this improvement was due to decreased conduction loss in the SiC SBD and suppressed the loss by recovery and switchingfor the MOSFET. A Li-ion laminated battery was mounted in the same housing as the inverter. The weight of the total system containing the spherical Si panel was well below 5.7 kg. The measured output powers of four PV modules connected in parallel were decreased by 35% compared to those expected from the rated power of a 54 W/module. Continuous operation measurements of this system were carried out using four PV modules under irradiation by natural sunlight. The total inverter efficiencies under realistic operation conditions were slightly decreased comparing with the DC-AC circuit because of the loss of the MPPT circuit. Even under unstable weather conditions, this system demonstrated a stable output power without ripples. The behaviors of the output powers of the cells, storage battery, and converter modules were analyzed as a function of psol. The substantial efficiencies of the solar cell modules were dependent on the weather conditions and were approximately 9%–10% on cloudy days. The small volume, light weight, and good efficiency and stability showed the usefulness of the SiC devices and spherical Si solar cells for sub kW level compact photovoltaic inverter. Such systems would be useful for various compact and transportable devices.

Declarations

Author contribution statement

Yuji Ando: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Takeo Oku: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.

Masashi Yasuda & Hiroshi Matsuo: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data.

Kazufumi Ushijima: Conceived and designed the experiments; Performed the experiments; Contributed reagents, materials, analysis tools or data.

Mikio Murozono: Contributed reagents, materials, analysis tools or data.

Funding statement

This work was supported by the Super Cluster Program of Japan Science and Technology Agency (JST).

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.

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