FROM THE AIR TO DEEP UNDERGROUND
Direct Air Carbon Capture, (Utilisation) and Storage underground (DACCS) or in products (DACCUS)
The diagram presents a DACCS system in a landscape powered by geothermal energy. Step 1: Fans pull ambient air through modules; operation uses renewable, low‑carbon power (e.g., geothermal). Step 2: CO₂ adheres to a filter. When saturated, the filter is heated with geothermal heat and releases pure CO₂. Step 3: The CO₂ is mixed with water, forming carbonic acid/bicarbonate. Step 4: The carbonated water is injected underground through wells and reacts with calcium/magnesium in basalt. Step 5: Stable carbonate minerals (e.g., calcite) form, locking carbon permanently. Step 6: Monitoring sensors track injection, water quality, and seismic activity. An inset shows the chemical pathway from CO₂ + H₂O to carbonate.
Fans in the DACCS plant in Iceland draw in ambient air to start the process of removing CO₂ from the atmosphere. Photo: Climeworks
The CO₂ is filtered directly from the air and separated through chemical processes. It is then liquefied or mixed with water and pumped underground. There, it (more or less quickly) forms a solid mineral and is thus permanently stored in the rock (DACCS). Alternatively, the CO₂ filtered from the air can also be stored in products such as concrete (DACCUS).
The first DACCS plants in Iceland and the USA have been removing CO₂ from the atmosphere for several years. Further plants are in the planning stage, including in the USA, Kenya and Oman.
DACCS plants can be built almost anywhere where there is a suitable subsurface for storage. This can be basalt rock, as in Iceland, or a former oil and natural gas reservoir. More often, however, it is a deep-lying layer of rock containing salt water.
One challenge is the high energy requirement: since CO₂ only makes up approx. 0.04% of the air, the filtering process is very inefficient and therefore energy-intensive and expensive. For the technology to contribute to combating climate change, the energy used must also come from renewable sources and any CO₂ produced must be offset (CO₂ neutrality).