The urgent need to decarbonize the global economy while meeting rising energy demands has placed Direct Air Capture (DAC) at the forefront of climate mitigation strategies. However, its high energy intensity, particularly the thermal energy required for sorbent regeneration (100–800 °C), remains a critical cost and sustainability barrier. This study investigates the integration of Concentrated Solar Thermal (CST) technology with low-cost, sand-based Thermal Energy Storage (TES) to power DAC systems. We present a comprehensive techno-economic analysis of both grid-connected and stand-alone Solar-Thermal DAC configurations, evaluating their potential to achieve scalable and cost-effective carbon dioxide removal.
The research employs a systems-level optimization approach to model and evaluate Solar-Thermal DAC.
The core system integrates a solid sorbent DAC unit (requiring ~100 °C regeneration heat) with a parabolic trough CST field. The design prioritizes short-cycle sorbents whose regeneration cycles align with solar availability, maximizing the utilization of diurnal solar energy.
A key innovation is the use of low-cost sand as the TES medium. Sand is heated by the CST system during the day and stored in insulated silos. This stored heat is then dispatched to the DAC unit's regeneration process during nighttime or cloudy periods, enabling near-continuous operation.
A bottom-up cost model was developed, incorporating capital expenditures (CAPEX) for solar field, storage, DAC modules, and balance of plant, alongside operational expenditures (OPEX) including maintenance and parasitic energy loads. The model optimizes system sizing (solar field area, storage capacity) to minimize the Levelized Cost of CO2 Removal (LCOR).
$160 – $200 /ton
Achievable LCOR for optimized systems
> 80%
Enabled by sand TES
< 1 km²
For a modular system
The optimized Solar-Thermal DAC system achieves a Levelized Cost of CO2 Removal (LCOR) between $160 and $200 per ton. This positions it competitively against other leading DAC approaches, such as liquid solvent systems powered by geothermal or green electricity, which often report costs in the $250-$600/ton range (e.g., Carbon Engineering, Climeworks).
The integration of sand TES allows the system to maintain high operational availability, achieving annual capacity factors exceeding 80%. An optimal modular design capturing 6000 tons of CO2 per year requires less than 1 square kilometer of land, making it suitable for deployment in arid, high-solar regions.
While grid-connected systems benefit from backup power, stand-alone configurations—relying solely on solar PV for electricity and CST/TES for heat—prove particularly promising. They eliminate grid dependency and associated Scope 2 emissions, showing minimal performance sensitivity to ambient temperature and humidity variations in suitable climates.
This paper isn't just about another DAC concept; it's a masterclass in pragmatic systems integration. The real breakthrough is the strategic pairing of short-cycle sorbent chemistry with diurnal solar thermal cycles and dirt-cheap sand storage. This triad directly attacks DAC's Achilles' heel: the capital intensity of providing continuous, high-grade heat from intermittent renewables. By accepting the sun's daily rhythm and designing the entire capture cycle around it, they've sidestepped the need for prohibitively expensive week-long storage or massive overbuilding of solar capacity—a common pitfall in renewable-powered industrial design.
The argument is elegantly linear: 1) DAC's cost is dominated by heat. 2) Low-carbon heat sources are geographically constrained (geothermal) or logistically complex (waste heat). 3) Solar is abundant but intermittent. 4) Therefore, the solution is not just solar heat, but solar heat + storage that is specifically cheap enough to make the economics work. The sand TES is the critical enabler here—it's not high-tech, but it brings the storage cost down to a level where the overall LCOR becomes competitive. The paper then rigorously tests this logic through techno-economic modeling of both grid-tied and off-grid scenarios, proving its viability in optimal environments.
Strengths: The focus on a holistic, optimized system rather than a component breakthrough is its greatest strength. The $160-200/ton cost target is credible and disruptive if achieved at scale. The use of sand TES is a brilliantly simple, low-tech solution to a high-tech problem, offering superior cost and scalability compared to molten salt systems common in CSP plants, as noted in NREL's assessments of long-duration storage. The analysis of ambient condition sensitivity is particularly valuable for real-world deployment.
Flaws/Omissions: The paper glosses over potential showstoppers. Sand's thermal conductivity is poor, requiring clever (and potentially costly) heat exchanger design to charge/discharge efficiently—a non-trivial engineering challenge. The analysis seems anchored in ideal, sun-drenched deserts. It doesn't sufficiently address performance degradation across seasonal cycles or during prolonged cloudy periods, nor the water usage for mirror cleaning in arid locales. Furthermore, the comparison to "leading DAC technologies" lacks a detailed, side-by-side breakdown of assumptions, making true apples-to-apples comparison difficult.
For investors and developers: Target sedimentary basins with high DNI (Direct Normal Irradiance). This technology isn't for Germany or the UK; its sweet spot is the MENA region, Chile, Australia, or the US Southwest, especially near potential CO2 storage sites to minimize transport costs. The modular 6k ton/year design suggests a strategy of building multiple smaller units rather than one massive plant, de-risking deployment. The research also implicitly argues for increased R&D into sorbent materials with regeneration cycles under 24 hours—this is a critical co-innovation. Finally, policymakers should note: this approach turns a land-use liability (arid land) into a climate asset, creating a new rationale for investments in transmission infrastructure to these zones.
The techno-economic optimization minimizes the Levelized Cost of CO2 Removal (LCOR), formulated as:
$LCOR = \frac{CAPEX \cdot CRF + OPEX}{M_{CO_2}}$
Where $CAPEX$ is total capital cost, $CRF$ is the Capital Recovery Factor $CRF = \frac{i(1+i)^n}{(1+i)^n - 1}$ (with $i$ as interest rate and $n$ as plant lifetime), $OPEX$ is annual operational cost, and $M_{CO_2}$ is the annual mass of CO2 captured.
The energy balance for the sand TES is crucial. The stored thermal energy $Q_{stored}$ is given by:
$Q_{stored} = m_{sand} \cdot c_{p,sand} \cdot (T_{hot} - T_{cold})$
where $m_{sand}$ is the mass of storage sand, $c_{p,sand}$ is its specific heat capacity (~800 J/kg·K), and $T_{hot}$ and $T_{cold}$ are the high and low storage temperatures, respectively.
The study's key findings are best visualized through several conceptual charts (described here based on the paper's narrative):
Scenario: Evaluating a Site in the Nevada Desert, USA
Objective: Determine the feasibility and optimal configuration of a Solar-Thermal DAC plant.
Framework Steps:
The Solar-Thermal DAC system presents a compelling pathway for large-scale CDR, particularly in the following contexts:
Future Research & Development Directions: