Comparing Low Pressure CO₂–Water Hydro Accumulators and Supercritical CO₂ Turbines for Power Generation

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Comparing Low Pressure CO₂–Water Hydro Accumulators and Supercritical CO₂ Turbines for Power Generation

Introduction

Supercritical CO₂ (sCO₂) turbine systems are widely studied for waste heat recovery and next-generation power cycles due to their very high energy density and efficiency. However, the need for extreme pressures, typically 100 to 200 bar, raises cost and complexity. An alternative approach is to use CO₂ at moderate pressures as a “gas piston” to pressurize water, which then drives a conventional hydro turbine. This article examines how the two approaches differ in design and efficiency, and where each is best suited.

The CO₂–Water Hydro Accumulator Concept

Instead of running CO₂ through a high speed turbine, the gas is stored in pressure vessels separated from water by bladders or pistons. When the CO₂ is compressed or heated, pressure builds and displaces the water side. By opening the water outlet to a hydro turbine, steady pressure and flow are produced without requiring ultrafast turbomachinery.

Key design elements:

Pressure range: typically 3 to 15 bar (low compared to sCO₂).

Water head produced: 30 to 150 meters, depending on charge pressure.

Cycle: two accumulators alternate between charging and discharging for continuous flow.

Advantages: safe pressures, proven hydro turbine technology, easier scaling.

Limitations: modest energy density, large tank volume required for multi-MWh output.

Why CO₂ and Water Must Be Separated

Direct mixing of CO₂ and water dissolves CO₂ into carbonic acid, causing corrosion and loss of pressure. Instead, a physical barrier such as a bladder or piston ensures that gas and liquid do not contact, preserving cycle efficiency and equipment longevity.

Expected Efficiency of the CO₂–Water Hydro Approach

Isothermal expansion losses: efficiency depends on how well the CO₂ side can exchange heat during expansion.

Round-trip electrical efficiency: about 30 to 60 percent when mechanically compressed; higher if waste heat is used for charging.

Scaling constraint: requires large tanks; for example, 10 m³ of CO₂ expanding from 15 bar to 10 bar yields only ~1.7 kWh.

Supercritical CO₂ Turbine Systems

Supercritical CO₂ cycles operate above 73 bar and 31 °C, with common operating pressures of 100 to 200 bar. The fluid’s high density and favorable thermodynamic properties allow compact, high speed turbomachinery with excellent cycle efficiency.

Key characteristics:

High energy density: small volumes deliver significant power.

Efficiency: thermal-to-electric efficiency of 45 to 55 percent for waste heat recovery.

Challenges: turbomachinery must withstand very high speeds and pressures, requiring advanced seals, bearings, and materials.

Applications: concentrated solar, nuclear, and industrial waste heat where compact, high efficiency systems justify higher costs.

Efficiency Comparison

sCO₂ Turbine: 45–55 percent efficiency; compact; very high pressure and mechanical complexity.

CO₂–Water Hydro Accumulator: 30–60 percent electrical round-trip if optimized for isothermal conditions; simpler hydro turbines; much lower pressures; but low energy density and large tank volume required.

Practical Applications

CO₂–Water Hydro:

Suited for sites with abundant low-grade heat and available space.

Works as a hybrid between compressed gas storage and pumped hydro.

Attractive for safe, moderate power where simplicity outweighs energy density.

Supercritical CO₂ Turbines:

Best for high temperature heat recovery in space-constrained facilities.

Delivers maximum efficiency and compact power blocks.

Higher capital costs but competitive at scale.

Conclusion

Using CO₂ to pressurize water for hydro turbines offers a low pressure, mechanically simple alternative to high pressure supercritical CO₂ turbines. While efficiency is lower and energy density is modest, the hydro approach avoids extreme pressures and leverages proven hydraulic turbine technology. For applications where safety, simplicity, and integration with waste heat are priorities, the CO₂–water hydro accumulator is an innovative path. For maximum efficiency in compact systems, supercritical CO₂ turbines remain the benchmark.