Reducing the cost of solid sorbent direct air carbon capture by recovering waste heat from water electrolysis under Nordic conditions

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Abstract
Reducing the atmospheric CO2 concentration to a sustainable level requires large-scale carbon dioxide removal (CDR) through direct air capture (DAC). DAC is a heat-intensive process, and waste heat recovery to supply its heat demand has substantial potential to reduce the levelized cost of DAC (LCODAC). This study determines the techno-economic potential of recovering low-temperature waste heat (70◦C) from an off-grid alkaline water electrolyzer (AWE) plant for use in a solid-sorbent-based DAC plant requiring heat at 80◦C–120◦C for desorption. The waste heat recovery system, comprising heat pumps and a heat storage, and the DAC plant are dimensioned to minimize the LCODAC based on optimization at an hourly level, and a comparison with an electric-boiler-based heat supply is performed. It was found that the AWE plant producing 6.3 kt/a of H2 can provide thermal energy for a DAC plant capturing up to 27 kt/a of CO2 under Nordic conditions, where low ambient temperatures are beneficial for the solid DAC process, except for subzero temperatures. The LCODAC can be reduced by 24%–39% when a waste-heat-based heat supply is used instead of an electric boiler. Increasing the desorption temperature from 80◦C to 100◦C or 120◦C significantly reduces the LCODAC, even though the cost of heat supply increases due to the decreasing coefficient of performance of the heat pumps. From an electricity system perspective, the use of waste heat sources to supply thermal energy to DAC plants is highly beneficial, as the primary energy demand and the peak electrical power of the DAC process can be reduced by 60% and 55%, respectively, compared with an electric boiler.

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