# Pulsed-Supplied Water Electrolysis via Two-Switch Converter for PV Capacity Firming

## Abstract

**:**

## 1. Introduction

## 2. System Component Models

_{2}gas occupies a volume of 11 m

^{3}. Within the operating range of 60–80 °C occurred at AEC, the produced hydrogen possesses 99% purity and requires 2.2 kWh/kg to be stored as compressed gas in over-ground vessels with volumetric densities of up to 40 kg/m

^{3}. Even with perfect insulation, a daily loss of 3% is considered to determine the releasing hydrogen into the atmosphere [30]. The minimum cell voltage (${E}_{c}^{o}$) required to initialize electrolysis is given via the following equation under constant temperature T and pressure P.

^{o}is the change in Gibbs’ free energy, F is the Faraday’s constant and n the number of transmitted electrons. Hence, the free reaction enthalpy $\Delta {G}^{o}$ can be calculated by taking into account the reaction enthalpy ${\Delta}_{R}H$, the temperature, the reaction entropy ${\Delta}_{R}S$, and the number of electrons exchanged and equals to 2 according to Equation (5), while F equals to 96,485 C/mol.

_{M}is the molar volume of ideal gas. The real volume of hydrogen can be estimated via Equation (8), where V

_{(measured)}is the volume obtained by the displacement of alkaline solution, T

_{s}is the temperature in standard conditions and T

_{o}is the operating temperature [31].

_{C}~60%), the energy provided to the AEC (e

_{c}) during interval t and hydrogen higher heating value (HHV = 141 MJ/kg) [h9]. Equation (12) shows the balance in the storage tank, taking into account the previous state-of-charge (SOC), the tank size (m

_{max}), the depth of discharge (DoD), the hydrogen consumption (m

_{conc}) and dynamic losses (m

_{loss}).

_{FC}), Equation (15) takes place considering the AFC efficiency (η

_{FC}).

_{PV}is calculated by Equation (16). N

_{PV}indicates the number of panels, P

_{STC}the PV panel power under standard test conditions, η

_{PV}the solar-to-electricity conversion efficiency, G

_{A}the global solar irradiation on the PV array, G

_{STC}the solar radiation under standard test conditions (usually 1 kW/m

^{2}), Tc the temperature of the PV cells, T

_{STC}the temperature under standard test conditions (25 °C) and C

_{T}the PV temperature coefficient [15].

_{PV}and current I

_{PV}, the energy from PV generators e

_{PV}during each time interval t can be obtained by making use of single diode solar PV mathematical models expressed by Equations (17)–(19).

_{MPPT}is the maximum power point voltage, I

_{ph}is the photo current, I

_{sc}the saturation current, q the charge of electrons, N

_{c}the number of cells in series and A

_{i}the ideal diode factor. The size of the inverter is chosen according to the maximum load need (P

_{L}). Considering the efficiency of the applied inverter η

_{inv}, its power output P

_{inv}is estimated as [32]:

_{amb}is the ambient temperature.

_{C}includes the efficiency of the proposed two-switch converter η

_{conc}. The conversion efficiency of a DC-to-DC converter is of vital importance for the overall system’s lifetime and energy consumption. Although a two-switch buck boost (TSBB) converter is composed of more components than a single switch buck boost (SSBB), it has seen a widespread use in power applications. In contrast to SSBB, which can operate in inverting mode, the output polarity achieved with TSBB is the same as the input voltage.

_{1}and d

_{2}, respectively. In buck mode, M2 is always off, while M1 is switching to regulate the output voltage resulting in ${V}_{in}\ge {V}_{o}$. During the conducting state of M1, D2 is forward biased allowing the current to flow, charging the capacitor. When it turns off, D2 is reversely biased and the load is satisfied by the charged components through the D1, which is now conducting. In boost mode, M1 is always on whereas the M2 switch is controlled to regulate V

_{o}. When M2 is on, the isolated DC source charges the coil, while the load is supplied only by the already charged capacitor. Therefore, ${V}_{o}\ge {V}_{in}$ is valid for every d

_{2}. M1 is always off at buck-boost mode where M2 explicitly controls the output voltage. The output voltage is less than the input for ${d}_{2}<0.5$, whereas it becomes greater if ${d}_{2}>0.5$ [33]. The buck-boost modes of operation are listed in Table 1.

## 3. Objective Formulation

#### 3.1. Domestic Load Profile

_{D}) for the whole year is depicted in Figure 3.

#### 3.2. PV Generating System

^{2}. In this work, three generating systems composed of polycrystalline solar panels, rated at 3 kW, 5 kW and 7 kW, are assessed based on actual data for the year 2020. To gain a broad overview with respect to the daily contribution of the PV arrays, Figure 5 is utilized to provide the hourly power output (P

_{PV}) of their seasonal production, showing the real generation of the first week of January, April, July and October.

#### 3.3. Power-to-Hydrogen Module

#### 3.4. Fuel Cell System

_{grid}), the activation variable ξ is included, taking the value 1 if AC bus voltage (${V}_{bus}^{AC}$) is less than load voltage (V

_{L}) and 0 otherwise (means that electricity is withdrawn from the grid while ${V}_{L}\ge {V}_{bus}^{AC}$). Therefore, at each time interval the AC balance is achieved via the following formulation:

## 4. Experimental Evaluation

## 5. Conclusions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 6.**Overall power-to-hydrogen efficiency for: (

**a**) T = 15 °C; (

**b**) T = 20 °C; and (

**c**) T = 25 °C.

**Figure 10.**Overall comparisons for different metrics pertaining: (

**a**) 3 kW; (

**b**) 5 kW; and (

**c**) 7 kW PV systems.

Mode | M1 | M2 | D1 | D2 | V_{o} |
---|---|---|---|---|---|

Buck | On | Off | Off | On | ${d}_{1}\xb7{V}_{in}$ |

Off | Off | On | Off | ||

Boost | On | On | Off | Off | $\frac{1}{1-{d}_{2}}\xb7{V}_{in}$ |

On | Off | Off | On | ||

Buck-Boost | Off | On | Off | Off | $\frac{{d}_{2}}{1-{d}_{2}}\xb7{V}_{in}$ |

Off | Off | On | Off |

Appliance | Power (W) | Consumption (kWh) | Energy Category | Operation Time |
---|---|---|---|---|

Washing machine | 1020 | 2.14 | A | 2.1 h |

Dish washer | 1050 | 1.5 | A | 86 min |

Dryer | 4300 | 4.3 | C | 1 h |

Refrigerator | 90 | 1.35 | B | 24 h |

Freezer | 110 | 1.65 | A | 24 h |

Ceramic hob | 2000 | 2 | A | 1 h |

Electric oven | 890 | 0.89 | A | 1 h |

Toaster | 1500 | 0.75 | - | 30 min |

Mixer | 1200 | 0.6 | - | 30 min |

Electric iron | 2400 | 2.4 | - | 1 h |

LCD TV set | 200 | 0.8 | - | 4 h |

AC unit 12,000 BTU | 3519 | 10.557 | A | 3 h |

AC unit 22,000 BTU | 6452 | 12.904 | A | 2 h |

Blow dryer | 2000 | 1 | - | 30 min |

Water heater | 4000 | 4 | - | 1 h |

Vaccum cleaner | 2000 | 2 | - | 1 h |

Computer | 300 | 0.6 | - | 2 h |

Printer | 150 | 0.05 | - | 20 min |

Stereo sytem | 60 | 0.06 | - | 1 h |

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**MDPI and ACS Style**

Nikolaidis, P.
Pulsed-Supplied Water Electrolysis via Two-Switch Converter for PV Capacity Firming. *Electricity* **2022**, *3*, 131-144.
https://doi.org/10.3390/electricity3010008

**AMA Style**

Nikolaidis P.
Pulsed-Supplied Water Electrolysis via Two-Switch Converter for PV Capacity Firming. *Electricity*. 2022; 3(1):131-144.
https://doi.org/10.3390/electricity3010008

**Chicago/Turabian Style**

Nikolaidis, Pavlos.
2022. "Pulsed-Supplied Water Electrolysis via Two-Switch Converter for PV Capacity Firming" *Electricity* 3, no. 1: 131-144.
https://doi.org/10.3390/electricity3010008