Design Optimization and Global Impact Assessment of Solar-Thermal Direct Air Carbon Capture

TL;DR

This paper tackles the high thermal energy demand of direct air capture (DAC) by proposing a solar-thermal DAC architecture that uses concentrated solar thermal (CST) heating and sand-based thermal energy storage to regen­erate a solid MOF sorbent at ~100 °C. The authors develop a high-resolution thermodynamic model in MATLAB/Simscape, and an optimization framework that operates on a grid-connected basis (5-minute cadence) and a stand-alone, solar PV+battery scenario, achieving capacity factors above $80\%$ and LCCO2 near $160-200/ton-CO2$ in favorable regions. A modular 6000 t-CO2/year design is shown to require <$1\ \mathrm{km^2}$ of land, with global sandy-desert regions potentially delivering up to ~26.9 Gt/year of CO2 abatement for stand-alone deployment; the stand-alone option also reduces exposure to electricity price volatility and interconnection delays. Compared with geothermal heat, solar-DAC can be cost-competitive in sedimentary basins with favorable solar profiles, offering a practical pathway to large-scale, low-emission CO2 removal in desert environments, provided supportive financing and policy frameworks are in place.

Abstract

The dual challenge of decarbonizing the economy and meeting rising global energy demand underscores the need for scalable and cost-effective carbon dioxide removal technologies. Direct air capture (DAC) is among the most promising approaches, but its high energy intensity, particularly the thermal energy required for sorbent regeneration, remains a critical barrier to cost reduction and sustainable deployment. This study explores solar-thermal DAC systems that combine concentrated solar thermal technology with low-cost sand-based thermal energy storage to meet this demand. We analyze the techno-economic performance of such systems in both grid-connected and stand-alone configurations. Results show that solar-thermal DAC can achieve annual capacity factors exceeding 80% and CO2 removal costs as low as 160-200 USD per ton, making it competitive with leading DAC technologies. The proposed system operates most efficiently with short-cycle sorbents that align with solar availability. The stand-alone Solar-DAC systems, which rely solely on solar energy for both electricity and thermal energy, are particularly promising in regions with high solar capacity and sandy terrain, exhibiting minimal ambient sensitivity from temperature and humidity. An optimal 6000 ton/yr modular system design takes <1 km2 land-use requirement and potentially >26 Gt/year DAC capacity is identified for sandy terrain alone globally. In areas with sedimentary basins suitable for CO2 storage, solar-powered DAC offers a lower-cost alternative to geothermal heating, which often faces geological and economic constraints.

Figures (4)

• Figure 1: solar-thermal DAC schematic and sample week thermodynamic simulation. The schematic diagram (upper) shows the single unit solid-sorbent DAC system design. A sample week thermodynamic simulation results using MATLAB Simscape (lower) shows the solar capacity factor input (row-1), the sand energy storage temperature profile (row-2), and resulted DAC sorbent temperature cycles (row-3). The optimization used in this study simplifies the simulation thermodynamic assumption, and the DAC regeneration cycle comparison (row-4) show that operation optimization can achieve $>$99% simulation cycles. This study utilizes a type of MOF azarabadi_sorbent-focused_2019sinha_systems_2017 sorbent enabling a 1-hour cycle time, minimizing solar curtailment by promptly utilizing thermal energy.
• Figure 2: solar-thermal DAC design parameter optimization using TX power market connection as example. (a) optimal CST heating capacity vs sand storage capacity; (b) impact of solar concentration ratio for net-CO2 abatement per unit CAPEX; (c) scaling effect of DAC and CST CAPEX, unit 1 = 6000 ton-CO2/year capacity; (d) optimal sand storage max temperature vs sand storage capacity, higher max temperature meaning higher usable heat for the same storage capacity, but lower solar heating efficiency for CST. Incentive selling price = $200/ton-CO2, which is sufficiently high to support nearly 100% operational capacity factor. Eventual optimal design parameters for 6000 ton-CO2/year modular solar-thermal DAC: CST heating capacity = 3 MWh/5min, sand storage capacity = 70 MWh/100° C operation range, sand storage max temperature = 400 ° C, no additional solar concentration. When capacity factor of solar-thermal DAC is sufficiently high ($>$80%), this optimal design settings are very robust across different power markets and stand-alone solar PV powered systems.
• Figure 3: Stand-alone Solar-DAC System (power+thermal) global deployment cost analysis and potential ambient sensitivity using presentative sorbent technology. (upper) global mapping of solar stand-alone LCCO2. The analysis focuses on regions with solar capacity factor $>$15% with 107713 points globally, some places without calibrated data, such as the Sahara desert, are excluded. (lower) ambient sensitivity to local temperature and relative humidity of a representative sorbent material. The sample sorbent is particularly sensitive to relative humidity, which significantly increases costs in humid tropical regions. In contrast, sandy terrains with lower humidity levels offer more favorable operating conditions for this DAC technology.
• Figure 4: Stand-alone Solar-DAC and Geothermal-DAC system comparison in the continental U.S. region. Comparing the LCCO2 of stand-alone solar-DAC and stand-alone geothermal-DAC by mapping their differences (solar subtracting geothermal). Negative value shown in red color indicating in favor of solar deployment in the West Texas regions.