Round-trip efficiency

Various simulation tools were also developed based on software including Simulink and Matlab to predict expected production from the RES technologies and to analyse the performance of the components. The efficiency of the electrolyser stack was estimated at 75% but that was reduced to 49% if the BOP and compression stages were taken into consideration. The round-trip efficiency of the hydrogen system was estimated at 16% (RES electricity to hydrogen and then back to electricity) while that of the complete system was 44% (thanks to the direct use of RES electricity by existing demand). 

For users of RFC systems, it is important to know the overall efficiency, or the so-called round-trip efficiency. A measure of this efficiency is the relation between the electrical energy that is applied to the system during the charging period and the electrical energy that can be recovered during the discharging period ... 

This overall or round-trip efficiency is the product of the efficiencies of the system components. These components are the electrolyzer (Ely), the FC, the storage pressure vessels, and the compressors for the compression of hydrogen and oxygen and the electrical converters. If a URFC is used instead of an RFC, there is only one electrochemical device in the system. However, this device operates at different efficiencies in the electrolyzer mode and the FC mode ... 

The mechanisms of oxygen evolution and reduction on Teflon-bonded NiCo204 spinel in alkaline solutions were also examined. The influence on the electrode performance of KOH concentration from 0.1 to 1 M and electrolyte temperature from 25 to 75 °C was also studied [48]. A selection of values measured in real cell operation at an electrolyte temperature of80°Cand 30% KOH is given in Table 8.6. The column round-trip efficiency is the product of the efficiency in electrolyzer (Ely) and PC modes. 

As mentioned in Section 8.4, it is useful to improve the round-trip efficiency by applying the pressure for hydrogen and oxygen storage on to the liquid water. This... 

Alkaline electrolyzers are used in most of the systems described above. Vanhanen et al. [103] described a simple RFC system based on a PEM electrolyzer anda PEMEC (Figure 8.18). The power of the electrolyzer was only 30 W. Furthermore, a metal hydride hydrogen store is used instead of high-pressure tanks. The store can be charged with a pressure of 5-10 bar and discharged at ambient pressure. The round-trip efficiency of this RFC system is 30%. An efficiency of 40% should be possible if the components are further improved. 

Rechargeable batteries are particularly suitable for such large-scale storage of electrical energy because of their high round-trip efficiency and scalability. Suitable t5 es of rechargeable batteries are vanadium-redox, sodium-sulfur, zinc-bromine, zinc-air and lithium-ion batteries. 

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