# Hydrogen Generation from a Small-Scale Solar Photovoltaic Thermal (PV/T) Electrolyzer System: Numerical Model and Experimental Verification

^{1}

^{2}

^{*}

## Abstract

**:**

^{2}A and 30–65 °C, respectively. While the current and the water temperature varied in the ranges of 400–2350 mA/cm

^{2}and 28.1–45.8 °C respectively, energy efficiency and exergy efficiency were in the ranges of 57.85–69.45% and 71.1–79.7%, respectively.

## 1. Introduction

^{3}/h, was produced in 1939. The first proton exchange membrane (PEM) electrolyzer was produced by General Electric in 1966 and it quickly gained interest from scientists due its compact system design and high voltage efficiency at current densities in comparison with traditional alkaline technology [38,39,40,41,42,43,44,45]. Apart from these features, a PEM electrolyzer can operate at up to 300 bar pressure and reach 95.1% efficiency at high temperatures [46].

## 2. Materials and Methods

#### 2.1. System Design

^{3}/day of H

_{2}production at process pressure without a compressor. A step-down DC/DC converter was used to decrease the voltage level of the PV/T to provide the power input of the PEM electrolyzer. The higher voltage output achieved by the PV/T was reduced and applied to the electrolysis cell. The DC/DC converter had high efficiency and its efficiency was regarded as 98% in the numeric model. The simulation of the system was obtained using the Matlab/Simulink environment. The PV/T electrolyzer system consists of the following major components: a PV/T array, a DC/DC step-down converter, and a PEM electrolyzer system. The schematic diagram of the system is given in Figure 1.

^{®}Powertherm) was used in the system and the features of the PV/T are given in the Table 1.

#### 2.2. Mathematical Model of the System

_{th}):

_{pm}is average temperature of the absorber plate. It is also used for the electrical output and efficiency calculation and is given in Equation (8).

_{0}= 0.0045) [58]:

_{s}) is taken as 5778 K and exergy input ($E{x}_{in}$) can be calculated by Equation (13):

#### 2.3. PEM Mathematical Model

_{elc}) can be calculated as follows:

_{r}refers the ideal supplied electrical potential. V

_{act a,c}and V

_{ohm}are the anode–cathode, activation and ohmic losses, respectively.

_{H2}and p

_{O2}show the partial pressures of the hydrogen and oxygen, respectively.

_{a}and i

_{c}are the exchange current density of the anode and cathode, and i is current density.

_{2}O) reacting with the exergy of the products exiting the reaction to the input electrical power of the PEM electrolyzer [63].

^{−1}, water is 2.5 kJ kg

^{−1}and oxygen is 0. The physical exergy calculation is given in Equation (28) [64]:

_{p}is specific heat and k is specific heat rate. The physical exergy is given in Equation (29) [63]:

#### 2.4. Economic Analysis

- The economic evaluation (N) period is 25 years for the PV/T panels, the DC/DC converter, and the storage systems, and is 15 years for the PEM electrolyzer.
- The economic indicators of nominal discount rate and inflation rate were used as 9% and 12%, respectively, for 2017 [66].
- The installation, operation and maintenance are not included in cost calculation.
- The capital costs per watt for the PV, the PV/T, and the PEM electrolyzer were taken as 0.82, 2, and 0.9 USD/W, respectively.

## 3. Results

^{2}and 11.2–37.9 °C, respectively.

^{2}and 2.54–7.8 kW/m

^{2}, respectively.

^{2}solar radiation and 33 °C ambient temperature is presented in Figure 5a. The short circuit current of the array and the open-circuit voltage were measured as 4.96 A and 39.9 V under these conditions, respectively. Hence, the maximum electrical output of the array was calculated as 139 W with 32 V. The characteristic experimental thermal efficiency depending on (T

_{in}− T

_{a})/I

_{T}was calculated on a daily basis and compared with the simulation data under the same climatic conditions (Figure 5b). The optical efficiency of the experimental system was obtained as 45.8%, and it was 47.9% for the simulation system.

^{2}and 12.8–31.4 °C, the PV temperature changed between 26.3 °C and 50 °C, and the produced electrical, thermal and total energy amounts were calculated as varying in the ranges of 24.75–156.4 Wh, 41.5–558.7 Wh, and 66.25–713.6 Wh, respectively. The results showed that the increase in PV cell temperature led to a decrease in both PV efficiency and electrical output of the system.

_{r}, V

_{ohm}, V

_{act}, and V

_{elc}) under the 30 °C operation temperature conditions correlated with the change in the current. The ohmic voltage increased proportionally to the current increase. The anode activation overpotential was calculated as 0.37 V, while the cathode overpotential was 0.25 V for 1600 mA/cm

^{2}. The change in the electrolysis voltage depending on the current and temperature was also investigated by changing the current and temperature in the respective ranges of 200–1600 mA/cm

^{2}and 30–65 °C. The variations in the electrolyzer voltage in relation to different levels of the current and temperature correlated well with the experimental results (Figure 8b). As shown in Figure 8c, the change in electrolyzer voltage was also investigated in relation to the PV/T’s water supply and the current output between 12:00 and 18:00. The decrease in the current and increase in the water temperature caused the electrolyzer voltage to decrease.

_{2}and 1147 Wh/day of electricity were produced with the PV/T-E system (Figure 9).

^{2}and 28.1–45.8 °C, respectively, the energy and exergy efficiency changed between 57.7 and 69.59% and 54.3–60.7%, respectively. The increase in the temperature of the supplied water and decrease in the current caused energy and exergy efficiencies to increase. It is clear that the exergy efficiency of the PEM electrolyzer is higher than the PV/T system’s exergy efficiency. The reason is that the solar radiation has a large capacity for useful work (exergy) due to the high temperature of the Sun. The higher loss of exergy in the PV/T system was caused by irreversibility of the process. In comparison, the PEM electrolyzer exergy losses are around 40–45%. This loss is the difference between the exergy entering the electrolyzer as an electrical energy, and the exergy exiting the electrolyzer in the form of the hydrogen produced in the process.

^{2}of solar irradiation were used for the distribution of hydrogen production. The distribution of the annual hydrogen generation distribution with respect to the global solar radiation was studied and illustrated. It was calculated that the highest hydrogen production occurs in the range of 800–900 W/m

^{2}, which accounts for 11.67% of the total amount of annual hydrogen generation (Figure 13).

## 4. Conclusions

^{2}and 28.1–45.8 °C, the PEM electrolyzer energy and the exergy efficiency were calculated in the ranges of 57.7–69.59% and 54.3–60.7%, respectively.

^{2}, which accounts for 11.67% of the total amount of annual hydrogen generation.

## Author Contributions

## Funding

## Conflicts of Interest

## Nomenclature and Subscripts

Nomenclature | Subscripts | ||

I_{T} | Inclined solar radiation (W/m^{2}) | A | Ambient |

Q_{u} | Useful energy (w) | in | Inlet |

U_{L} | Overall heat loss coefficient (w/m^{2} °C) | o | Outlet |

W | Width of the tube spacing (m) | a | Anode |

D | Diameter of copper tube (m) | c | Cathode |

τ | Transmissivity of glass | rev | Reversible voltage |

α_{g} | Absorptivity of glass | th | Thermal |

C_{p} | Water specific heat, j kg^{−1} k^{−1} | e | Electrical |

k | Specific heat rate | t | Total |

F’ | Collector efficiency factor | m | Membrane |

F_{R} | Collector flow rate factor | H_{2}O | Water |

n | Number of electron | O_{2} | Oxygen |

p | Partial pressure | H_{2} | Hydrogen |

z | Stoichiometric coefficient of electron | ph | Physical |

α | Transfer coefficient | ch | Chemical |

h | Overpotential | elc | Electrolyzer |

δ | Membrane thickness | ohm | Ohmic |

s | Membrane conductivity | act | Activation |

r | Material resistivity | LHV | Lower heat value |

i | Current density | η | Efficiency |

i_{o} | Exchange current density | A | Surface area (m^{2}) |

P | Operating pressure | PEM | Polymer electrolyte membrane |

ΔH | Enthalpy change | PV | Photovoltaic |

ΔS | Entropy change | PV/T | Photovoltaic/Thermal |

R | Universal gas constant | sim | Simulation |

F | Faraday’s constant | Exp | Experimental |

z | Stoichiometric coefficient | TLCC | Total life-cycle cost |

T | Temperature | f’ | Inflation |

λ | Degree of humidification | CoE | Cost of energy |

ṁ | Mass flow rate (kg/s) | N | Lifetime (year) |

Er | Reversible voltage | R’ | Real discount rate |

Ex | Exergy | i’ | Nominal discount rate |

ΔG | Gibbs free energy | CRF | Capital recovery factor |

E_{0} | Reversible potential of water splitting | TPV | Total present value |

δ | Thickness | r | Reactant |

Ė | Corresponding rate of exergy input | p | Product |

HOMER | Hybrid optimization model for electric renewable |

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**Figure 5.**Current-voltage (I-V) and power-voltage (P-V) curve of the PV/T system (

**a**) comparison of experimental and simulation thermal efficiencies as a function of the ratio (T

_{in}-T

_{a})/I

_{T}(

**b**).

**Figure 6.**The hourly variation in the energy–exergy efficiency of the PV/T, energy–exergy input and ambient temperature on a daily basis.

**Figure 8.**The variation of the PEM overpotentials (

**a**) comparison of experimental and simulation system electrolyzer voltages with different water temperatures depending on the electrolyzer current (

**b**) the variation in hourly electrolyzer voltage depending on the current and water temperature (

**c**).

**Figure 10.**The hourly variation of energy–exergy efficiency of the PEM electrolyzer depending on the water temperature and current on 2nd September.

**Figure 11.**The annual hydrogen production amount calculated for 65 °C and actual PV/T water temperature.

Dimensions | 1640 × 870 × 105 mm |
---|---|

Maximum current (A) | 5.43 |

Short circuit current (A) | 5.67 |

Maximum voltage (V) | 36.8 |

Open circuit voltage (V) | 45.43 |

Glass | Extra Solar Glass |

Cell number | 72 |

Type | Monocrystal |

Thermal power (W) | 680 |

Active cell area (m^{2}) | 1.222 |

Material | Feature | Measured Variable | Accuracy |
---|---|---|---|

Pyranometer | Kipp and Zonen CMP11 | Global irradiance | ±2% |

Anemometer | NRG 40 | Wind speed | ±1% |

Temperature sensor | PT100, J type | Ambient & collector | ±0.8% |

Current Sensor | ASC712 (0–75 A) | Current | ±0.15% |

Flowmeter | LZB-15 SL (0–250 L/h) | Mass flow rate | ±1.5% |

Symbols—Parameters | Values |
---|---|

Diameter of the copper tube (D) | 0.01 m |

Width of the tube spacing (W) | 0.092 m |

PV thermal conductivity (k_{pv}) | 149 W/m K |

PV laminate thickness (l_{pv}) | 0.0002 m |

Absorber thermal conductivity (k_{abs}) | 385 W/m K |

Absorber laminate thickness (l_{abs}) | 0.00012 m |

Insulation material thermal conductivity (k_{i}) | 0.04 W/m K |

Insulation material thickness (l_{i}) | 0.05 m |

Transmission–absorption coefficient (τα_{g}) | 0.8 W/m K |

Symbols—Parameters | Values |
---|---|

Transfer coefficient anode–cathode (αa, αc) | 0.5 |

Anode–cathode electrode thickness (δa, δc) | 5 × 10^{−6} m |

Anode exchange current densities (i_{0},_{a}) | 0.0042 A/cm^{2} |

Cathode exchange current density (i_{0},_{c}) | 0.001 A/cm^{2} |

Current density (i) | 1.6 A/cm^{2} |

Operating pressure (P) | 1 bar |

Faraday number (F) | 96,485.3 A/mol |

Operating temperature (T) | 303 K |

Degree of humidification (λ) | 24 |

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**MDPI and ACS Style**

Gül, M.; Akyüz, E. Hydrogen Generation from a Small-Scale Solar Photovoltaic Thermal (PV/T) Electrolyzer System: Numerical Model and Experimental Verification. *Energies* **2020**, *13*, 2997.
https://doi.org/10.3390/en13112997

**AMA Style**

Gül M, Akyüz E. Hydrogen Generation from a Small-Scale Solar Photovoltaic Thermal (PV/T) Electrolyzer System: Numerical Model and Experimental Verification. *Energies*. 2020; 13(11):2997.
https://doi.org/10.3390/en13112997

**Chicago/Turabian Style**

Gül, Metin, and Ersin Akyüz. 2020. "Hydrogen Generation from a Small-Scale Solar Photovoltaic Thermal (PV/T) Electrolyzer System: Numerical Model and Experimental Verification" *Energies* 13, no. 11: 2997.
https://doi.org/10.3390/en13112997