Solar-Powered Direct Air Capture: Techno-Economic and Environmental Assessment Notes

Overview of Direct Air Capture (DAC) and Study Objectives

• Direct Air Capture (DAC) Definition: Technologies that separate and concentrate atmospheric carbon dioxide solely through mechanical and chemical processes, excluding biogenic sources. Unlike biomass-based processes, DAC does not present biophysical limitations that threaten crop production or biodiversity when scaled up.

• Goal of DAC: The captured $CO_2$ can be sequestered (Direct Air Carbon Capture and Storage, or DACCS) or used as a feedstock for carbon-neutral chemicals and fuels. These synthetic fuels are critical for decarbonizing hard-to-abate sectors.

• Primary Technologies:

  • Solid Sorbent DAC (S-DAC): Captures $CO_2$ using solid sorbents, regenerated with vacuum and low-temperature heat ($\approx 100\,^\circ\text{C}$). While it can use waste heat, it consumed more energy than L-DAC in some assessments and requires durable, efficient sorbents.

  • Liquid Solvent DAC (L-DAC): Employs a liquid alkali solution to form carbonates, which are calcined at high temperatures ($900\,^\circ\text{C}$) to release pure $CO_2$.

• The Problem with Conventional L-DAC: Advanced concepts use oxyfuel combustion of natural gas for calcination. This creates a stream containing roughly one-third fossil $CO_2$, hindering its use for fully carbon-neutral products.

• Proposed Solution: This study assesses a solar-powered L-DAC approach using Solar Thermal Energy (STE) to provide green high-temperature heat (> 1000\,^\circ\text{C}), particularly in arid regions with high solar resources.

Technical Process of Solar Liquid Direct Air Capture (L-DAC)

• Carbon Engineering Ltd. Baseline: The study modifies the process originally published by Carbon Engineering Ltd., which captures $CO_2$ in two main cycles.

• Cycle 1: Alkali-Carbonate Cycle

  • Air Contactor: Atmospheric air is propelled by fans through packing material moistened by an alkali solution (potassium hydroxide, $KOH$). The $CO_2$ reacts to yield potassium carbonate ($K_2CO_3$).

  • Pellet Reactor: $K_2CO_3$ reacts with calcium hydroxide ($Ca(OH)_2$) to form calcium carbonate ($CaCO_3$) and regenerates the $KOH$, which is recirculated back to the contactor.

• Cycle 2: Calcination-Slaking Cycle

  • Solar Calciner: Substitutes the gas-fired calciner. $CaCO_3$ is calcined using solar thermal energy at $900\,^\circ\text{C}$ to produce pure $CO_2$ and calcium oxide ($CaO$).

  • Steam Slaker: $CaO$ is combined with water in a highly exothermic reaction to produce $Ca(OH)_2$, completing the loop.

• System Modifications for Solar Power:

  • Elimination of the air separation unit, gas turbine, and flue gas $CO_2$ absorber.

  • Division into a Continuous Section (air contactor, slaker, pellet reactor) and an Intermittent Section (solar calcination).

  • Energy Recovery: Two steam turbines extract electricity from waste heat—one continuous (from the slaker) and one intermittent (from the $CO_2$ gas cooler and heat recovery steam generator, HRSG).

  • Buffering and Storage: Inclusion of solid storage units (for $CaO$ and $CaCO_3$), water/$CO_2$ tanks, and an auxiliary Photovoltaic (PV) plant with batteries to ensure autonomous, stable operation and 100% green power matching.

Modeling and Methodology

• Simulation Software: Aspen Plus V12.1 was used for steady-state simulation; DLR’s HFLCAL VH13 was used for solar field layouts.

• Solar Calcination Parameters:

  • Technology chosen: CentRec centrifugal particle receiver (high Technology Readiness Level, TRL).

  • Design heat output: $41.7\,MW_{th}$ per tower.

  • Reactor efficiency: $87.8\%$ at $900\,^\circ\text{C}$.

  • Heat requirement: $1.51\,MWh/t\,CO_2$.

  • Daily start-up cost: $10\%$ of average daily heat collected is discarded for cold start-up.

• Air Contactor Parameters:

  • Air velocity: $1.4\,m/s$.

  • Pressure drop: $100\,Pa$.

  • Fan efficiency: $70\%$.

  • Atmospheric $CO_2$: $420\,ppm$.

• Geospatial Screening Criteria (282 polygons identified):

  • Coastal limit: Less than $100\,km$ from the ocean (to facilitate desalination).

  • Latitude: Below $45\,^\circ$ in both hemispheres.

  • Slope: Maximum slope less than $2.1\%$.

  • Land Cover: Limited to shrubs, herbaceous, or bare/sparse vegetation; excluded protected areas.

Economic Assessment Framework

• CAPEX Calculation:

  • $CAPEX = \text{Field} + \text{Non-field} + \text{Contingency}$

  • $\text{Field Cost} = 1.127 \times \text{Lang Factor} \times \text{Equipment Cost Correlation}$

  • $\text{Non-field} = 0.233 \times \text{Field}$

  • $\text{Contingency} = 0.2 \times \text{Field}$

  • Lang Factor applied: $4$.

• Levelized Costs:

  • Levelized Cost of Produced CO2 (LCOP): Total annual costs divided by $CO_2$ production.

  • Levelized Cost of Removed CO2 (LCOD): Subtracts indirect greenhouse gas (GHG) emissions and fossil $CO_2$ from the production total. It includes an onshore sequestration cost of $10\,USD_{2022}/t\,CO_2$.

• Natural Gas Scenarios for Comparison:

  • US Scenario: $7.617\,USD_{2022}/GJ_{LHV}$.

  • EU Scenario: $23.590\,USD_{2022}/GJ_{LHV}$.

• WACC (Weighted Average Cost of Capital): Found to be a major cost driver, varying by country (e.g., higher in developing economies).

Environmental Performance and Site Selection Results

• Role of Environmental Conditions:

  • Electricity Consumption: Generally higher on western coasts (North/South America, Middle East) due to dry conditions reducing $CO_2$ removal efficiency, requiring more air to be processed. Low atmospheric pressure at high altitudes also reduces capture per volume of air.

  • Water Losses: High in Red Sea and Persian Gulf regions due to high temperatures and low relative humidity. Conversely, one-third of screened areas show lower water loss (< 4.7\,t\,H_2O/t\,CO_2) than conventional L-DAC estimates.

  • Solar Resource: $60\%$ of analyzed land requires a solar field smaller than $0.3\,km^2$ for a $0.1\,Mt\,CO_2/year$ plant.

• LCOP Global Distribution:

  • With local WACC: Most promising sites are in Southern Europe, Australia, and California (developed economies with low cost of capital).

  • With constant 4.2% WACC: Clusters appear in Peru, Chile, Southern Africa, and the Middle East, reflecting purely technical solar/climatic potential.

• Cost Breakdown: Solarized L-DAC has a higher CAPEX share ($\approx 60-70\%$) than conventional L-DAC. Natural gas can account for over $40\%$ of the LCOP in conventional gas-fired systems.

Impact of Scale and Net Carbon Removal (LCA)

• Economy of Scale: LCOP decreases significantly as capacity increases from $0.05$ to $0.2\,Mt\,CO_2/year$. Solarized plants show a "sawtooth" cost pattern as new towers must be added discretely ($41.7\,MW_{th}$ limits).

• Life Cycle Assessment (LCA) Results:

  • Conventional L-DAC generates significant indirect emissions from natural gas extraction and transportation.

  • Solar L-DAC emissions primarily come from construction materials for solar infrastructure.

  • Net emissions for solar power are more than 1 order of magnitude lower than conventional systems.

• LCOP vs. LCOD Shift:

  • While conventional L-DAC has a lower LCOP (cost per ton produced), solar-powered L-DAC becomes highly competitive or even superior when looking at LCOD (cost per ton effectively removed).

  • Accounting for fossil $CO_2$ generated by natural gas combustion and indirect GHG emissions causes the cost of conventional DAC to escalate dramatically, making solar-powered L-DAC the most cost-effective and environmentally sound alternative for large-scale CDR.