Electrolysis. Water electrolysis was industrialized a hundred years ago and remains the most promising method for producing high-purity hydrogen . Water electrolyzers can be classified into three main categories: alkaline electrolyzers, polymer electrolyte membrane (proton exchange membrane; PEM) electrolyzers, and solid oxide electrolyzers (SOE). The alkaline electrolyzer is relatively cheap, stable, and mature . PEM has the advantage of producing high purity hydrogen, and it is also commercialized . The SOE has high energy efficiency and can produce synthesis gas directly; however, this technology has not been commercialized . Each electrolyzer has advantages/disadvantages regarding energy efficiency, stability, and cost. However, in the context of the dynamic electricity markets that we study, one of the most critical features of the electrolyzer is flexibility. Specifically, an electrolyzer must be able to increase/reduce hydrogen production sufficiently fast to respond to electricity prices. Among the three types of electrolyzers, PEM has the flexibility that enables the system to deal fast transients; it takes less than a second to respond and settle to a prescribed set-point change . Given the flexible nature of PEM, we select this technology. After splitting the deionized water into oxygen and hydrogen, the hydrogen is either fed to a Reverse Water Gas Shift (rWGS) unit or stored in a hydrogen tank. Typically, PEM electrolyzers operate at high pressure, with a hydrogen outlet pressure of around 30 bars . This highpressure hydrogen can be directly fed into the rWGS reactor without further compression.