Sizing of the Electrolyzer in Wind Hydrogen System

For a given wind power installed, the sizing of the electrolyzer is not trivial. In the case of “stand-alone” systems, a one-to-one approach is often proposed, with the electrolyzer power input being equal to the nominal power output of the wind turbine. In this way, the electrolyzer should be able to retrieve all the wind power in the absence of load. In gridconnected systems, the same approach leads to the choice of an electrolyzer with a power supply equal to the power output of the wind turbine minus a “base load.”

However, the specific capital cost of the electrolyzer being almost equal to the cost of a wind turbine, it is important to take into account the capacity factor of the electrolyzer that will always be smaller than that of the wind turbine.

Electrolyzers are generally current-controlled, which means that a certain DC is imposed according to the desired hydrogen production. In a wind-hydrogen system, the wind turbine

power available for the operation of the electrolyzer is generally known; therefore, the power input should be transformed to a current input.

The voltage–current relation of an electrolyzer is not very simple because it depends on the temperature, pressure, and other construction characteristics. For a given electrolyzer, it is possible to experimentally establish the I–V curve at different temperatures and pressures, and deduce a temperature-dependent current–power curve.

The sizing of the wind turbine, electrolyzer, and auxiliaries is greatly affected by whether the wind-hydrogen system is grid connected or stand alone. The cells can be made in any size, and any number of cells can be stacked in a series depending on the desired output and design point selected. The design point, namely the current density for the cell, is case sensitive and depends on whether the system is operational cost critical or capital cost critical. The cell voltage and therefore the specific power consumption of the electrolyzer depend on the current density according to a I–V curve.

If the capital cost is more important than the operating cost, which depends on electricity price, the optimum current density should be shifted toward the higher range with a lower efficiency. If the operating cost is more significant than the cost of money, the current density should be shifted toward the lower range, or expensive activated electrodes may be used to increase the efficiency. Module material costs usually represent between 44% and 56% of the total material costs in wind-hydrogen system.

Commercial water electrolyzers cover a wide range of hydrogen production rates from 0.001 to 750 Nm3/h. Small hydrogen generators are intended for laboratory use, where hydrogen is often used as a carrier in analytical instruments, whereas large units are used in different fields of the chemical industry.

The actual capital cost of different electrolyzers operating at pressures between atmospheric pressure and 30 bars is presented in Figure above based on offers from manufacturers. There is a wide variation in the specific capital cost of small-size electrolyzers due to the different technical characteristics and options included. The specific capital cost of very small laboratory electrolyzers may exceed 30,000 €/kW, but has not been shown here for simplicity.

There are no large variations in the specific cost of medium-to-large-size electrolyzers, that is, above 200 kW, mainly because there are very few manufacturers around the world that are actually supplying equipment of such a size. The manufacturing process of electrolyzers in wind-hydrogen system is still intensely manual and the prices shown here refer to individual items without taking into account any volume effect.

The specific capital cost of very small laboratory electrolyzers, producing <>

Generally, in wind-hydrogen system the electrolyzer cost increases with hydrogen purity and delivery pressure. In fact, a purification section for the removal of oxygen and the reduction of humidity to a concentration below 10 ppm may represent 30–40% of the total cost.

Advanced electrolyzers actually supply hydrogen at pressures up to 30 bars and there are prototypes with a delivery pressure of up to 100 bars. They are more expensive than atmospheric electrolyzers, but their use decreases the cost of storage: hydrogen may be directly stored at the delivery pressure, and if a compressor is used for filling high-pressure cylinders, it only needs one stage instead of three or four.

source: (2009) Hydrogen Fuel, Production, Storage & Transportation by Ram B Gupta (ed)