(221d) Demonstration of Hydrogen Production By Multi-Typed Solid Oxide Electrolysis Cells System

High-temperature steam electrolysis using solid oxide electrolysis cells (SOEC) can produce hydrogen in low electrolytic voltage as compared with water electrolysis with alkaline and polymer electrolyte. Toshiba aims to establish the hydrogen production technology by renewable energy. We have already developed cell and cell-stack, the key component of a SOEC hydrogen production system, and hydrogen can be more efficiently produced than water electrolysis. On the other hand, for the mass production of hydrogen, it is necessary to connect multi-type cell-stacks and to enlarge electrolysis capacity.
Moreover, it is indispensable to use heat effectively to reduce the consumption energy of the SOEC system since SOEC operation temperature range is from 600 to 800 degrees C.
An objective of this study is to evaluate the following: 1) the effect of scale-up by formation of multi-type cell-stacks, 2) the effect by exhaust heat recovery for consumption energy reduction. 3) the influence of fluctuation of electrolytic power on hydrogen production system in consideration of performing hydrogen production using renewable energy.

The multi-type cell-stacks pilot test plant consists of the following components; an electric furnace (multi-type cell-stacks container), a steam generator, a steam supply line, a produced hydrogen exhaust line, an air supply line for diluting and exhausting by-product oxygen, and oxygen-rich air exhaust line.
Each of the steam supply line and the produced hydrogen exhaust line, the air supply line and the oxygen-rich air exhaust line have the heat exchanger, respectively. And the thermal energy of high temperature fluid exhausted from each multi-type cell-stacks container is given to low-temperature supply gas by a heat exchanger. These heat exchangers were designed so that the energy which heats supply gas might become the smallest. Furthermore, in consideration of the pressure loss of the cell-stacks, orifices are provided in the steam supply line so that gas is uniformly supplied to each cell-stack.
In a multi-type cell-stack container of this pilot test plant, four cell stacks were electrically connected in series, and a total of eight cell stacks were installed. The total electrolysis power is about 10 kW.

The hydrogen production test was carried out under the following conditions: temperature inside multi-type cell-stack container: 700 degrees C., supply water vapor concentration: 95%. Initially, the current-voltage characteristics of the cell stack were measured by applying a current of 0 to 20 A to each cell-stack. Since the eight cell-stacks used for the test showed the same characteristics as the test results with one cell-stack, it was confirmed that steam was evenly supplied to each cell stack. The hydrogen production rate during the test was measured and compared with the theoretical hydrogen production rate calculated from the current applied to the multi-type cell-stacks. The hydrogen production rate in the test was almost the same as the theoretical value. When a current of 20 A (Current value near the electrolysis heat neutral point) was applied to each cell stack, the hydrogen production rate was 3.14 Nm³/h. This is eight times the hydrogen production rate (0.39 Nm³/h) when a current of 20 A is applied to one cell-stack. Thereby, scale-up has been checked.
Moreover, in order to evaluate the effect of exhaust heat recovery on energy consumption reduction, fixed current was applied to each cell-stack to stabilize the temperature and pressure inside the pilot test plant. Since the heating energy of the feed gas was reduced by about 16% compared with the case where the heat exchanger was not installed, it was confirmed that exhaust heat recovery was effective. On the other hand, since heat dissipation from the whole equipment was large, it seems to be necessary to strengthen the heat insulation of the portion into which the large hot gas of heat dissipation flows especially.
Furthermore, two kinds of fluctuation electrolytic currents, A) a combination of lamp and step input, B) simulating photovoltaic power (PV) output waveform were inputted into multi-type cell-stacks and the response of hydrogen generation rate, temperature and pressure was evaluated. Hydrogen production rate was responded well, and the value was almost same as the theoretical hydrogen production rate calculated from an input current. In consideration of the results of the temperature and pressure change in the process, it was confirmed that the followability of hydrogen production to fluctuated input was good.
Since there is no change in the characteristics of each cell-stack by formation of multi-type cell-stacks and there is no loss of manufactured hydrogen, it was proved that scale-up by formation of multi-type cell-stacks is possible. In addition, it is demonstrated that the energy reduction effect by exhaust heat recovery is good, and that hydrogen production is possible using the fluctuated electrolysis current.

A part of this study was carried out in a project commissioned by NEDO.