Science1 publisher2 min readPublished
Water sales carry the hydrogen economics in a 200 MW tri-generation reactor design
A modelling study in Progress in Energy takes electricity, hydrogen and desalinated water off one 200 MW-thermal module, and its hydrogen cost drops about 17 per cent once the freshwater is sold externally. Nothing has been built.
The Scientist · Science desk
What happened
- Part of the reactor's steam is diverted into high-temperature steam electrolysis, splitting water into hydrogen and oxygen with less electricity than a conventional electrolyser because the feed is already hot.
- Steam leaving the electrolyser is cooler but still warm enough to run a desalination plant that turns seawater into freshwater.
- The modelled plant produces about 90 MW of electricity, 0.6 kg of hydrogen a second (roughly 52 tonnes a day) and 612 cubic metres of freshwater a day, all at the same time, from one module.
- Hydrogen costs $2.93 to $3.19 a kilogram in the mode where the freshwater is sold externally, and $3.49 to $3.88 in the mode where it is kept to feed the electrolyser.
- The system is a proposal evaluated in a model, published by Fateme Dehghani and colleagues in Progress in Energy.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- cost Co-product revenue, not reactor performance, decides whether this design's hydrogen clears $3 a kilogram, so the water and heat offtake has to be contracted before the hydrogen price means anything.
- constraint Self-sufficiency and water sales draw on the same output stream, so a developer picks which one the plant serves at design stage and lives with the hydrogen cost that follows.
- capability The five-point utilisation gain only exists where a neighbour takes the low-grade heat and the water, which ties this design's economics to coastal industrial sites.
Ninety megawatts of electricity out of 200 MW of heat [1] is 45 per cent [1], below the 48 per cent the paper gives for electricity generation alone [5]. The gap is what diverting steam to the electrolyser costs [2]. High-temperature steam electrolysis was chosen because the steam arrives hot and the cell then needs less electricity than a conventional one [2]. The 53 per cent for the full system counts hydrogen and freshwater as useful output, so the five percentage points [2] come from heat still left in the steam after the electrolyser [3].
That 53 per cent is a design-point figure, with all three products running at once [4]. An electrolyser down for maintenance, or a module following grid demand, moves it, and one steady-state number cannot show by how much.
Splitting water needs at least 8.9 kg of water per kg of hydrogen, which follows from the molar masses [3]. At 0.6 kg/s the plant makes 51,840 kg of hydrogen a day [5], so the electrolyser's own stoichiometric feed is about 462 cubic metres a day [4]. The desalination unit delivers 612 [4]. The 8.9 is a floor: it assumes every molecule fed in is split, and the paper itself notes that the water-splitting route needs large supplies of water [10].
Taking midpoints, hydrogen costs $3.69/kg in the self-sufficient mode and $3.06/kg when the water is sold, a difference of 63 cents, or 17 per cent [6]. Over 51,840 kg a day that is about $32,400 [7]. Divided by 612 cubic metres, the water would have to fetch roughly $53 a cubic metre [8]. The summary credits the difference to revenue from water sales [7] without giving the price assumed for the water.
The cost comparison in the paper is internal, between the two operating modes, not against the steam methane reforming that supplies most hydrogen today and emits significant amounts of carbon dioxide [9]. The authors offer the design to water-scarce regions that want low-carbon energy and industrial hydrogen supplies [11].
What to watch
- Whether the full Progress in Energy paper states an assumed water sale price, and whether it sits near desalinated water market rates or far above them.
- Whether the market-led scenario's freshwater output differs from the 612 cubic metres a day quoted for the integrated system.
- Any SMR licensing submission that pairs high-temperature steam electrolysis with a desalination plant on the same module.