TEL: 1-608-238-6001 Email: greg@salgenx.com
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Solar-PV-Driven sCO₂ + Concentrated Solar Thermal: Can It Beat Plain PV Concept Recap (Hybrid PV-sCO₂-CSP)• Use solar PV electricity to drive the compressor in a closed-loop supercritical CO₂ (sCO₂) Brayton cycle.• Use concentrated solar thermal (CSP) to heat the recuperated, compressed sCO₂ to ≥ 500 C before expansion through a turbine-generator.• Aim: offset the high compressor work with PV electricity and exploit the high cycle efficiency of recuperated sCO₂ at moderate-to-high turbine inlet temperatures.Thermodynamic Plausibility• sCO₂ Brayton cycles are well-matched to 500–700 C heat sources and shine with recuperation and tight temperature glide near the CO₂ critical point.• At ~500–550 C TIT with good recuperation and reasonable pressure ratios, net thermal-to-electric efficiency is typically 35–45 percent; pushing closer to ~50 percent needs hotter receivers (~600–700 C), excellent recuperators, and optimized turbomachinery.• Compressor work in sCO₂ is non-trivial (often 30–45 percent of turbine work), but because it occurs near the critical region the specific work is low; supplying that from PV can reduce the “parasitic” internal load on the cycle’s net output.Energy Flow Comparison (High-Level)• Standalone PV (assume 22 percent module, ~20 percent DC-to-AC net):• Every 1,000 W/m² of insolation yields ~200 W/m² AC at noon conditions; land-average depends on capacity factor and tracking.• Hybrid PV + CSP sCO₂ at 500 C:• Thermal path: DNI → receiver → sCO₂ turbine → ~35–45 percent conversion to AC.• Electrical assist path: PV → compressor; this does not multiply energy, it reduces internal cycle load and can slightly raise net electrical output per unit thermal input.• With adequate storage (thermal and possibly electrical buffering), the hybrid can shift generation and smooth output, something PV alone cannot do without batteries.Where the Efficiency Advantage Can Come From• Receiver and cycle integration:• Good recuperation effectiveness (≥ 90 percent) and low pressure drops are vital to reach the upper 30s to low 40s percent net thermal efficiency at ~500–550 C.• PV-to-compressor coupling:• If PV covers most compressor work, gross turbine output minus a smaller compressor draw yields a higher net for the same thermal input. The effective solar-to-wire efficiency (considering both optical-thermal and PV inputs) depends on how you account for the PV energy.• Thermal storage:• CSP with hot-tank storage (molten salt or advanced media) lets you run the turbine at near-optimal load longer, raising capacity factor and grid value compared to PV-only.Practical Efficiency Ranges (Realistic, Not Marketing Best-Case)• Optical & thermal losses (heliostats/collector + receiver): typically 55–70 percent from DNI to receiver outlet heat at operating temperature.• Cycle (receiver heat to AC): 35–45 percent net at ~500–550 C; 40–50 percent net becomes more realistic above ~600 C with excellent recuperation and components.• Effective solar-to-wire for the thermal branch: multiply the two:• Example: 0.63 (optical/thermal) × 0.40 (cycle) ≈ 25 percent from DNI to AC for the thermal branch under solid but not exotic assumptions.• Add PV branch for compressor: the PV electricity is additional solar input; it improves net cycle output but doesn’t change first-law totals. When you combine both solar inputs (DNI + PV), the aggregate solar-to-wire efficiency typically ends up similar to or modestly better than the thermal branch alone, while delivering dispatchability that PV lacks without batteries.Economics (Order-of-Magnitude 2025 Landscape)• PV alone:• Lowest capex per kW and $ per kWh when the grid can accept intermittency.• LCOE widely reported in the $25–40/MWh range for utility-scale in strong-sun regions with tracking (site-dependent).• Firming/storage (batteries) adds substantial capex and OPEX.• CSP + sCO₂ (hybrid or pure CSP):• Higher capex (collectors/heliostats, receiver, thermal storage, turbomachinery, recuperators, tower or field, BOP).• LCOE historically higher than PV but much better with storage value (evening peak, capacity payments, firm power contracts). Roughly $80–120+/MWh is a realistic band absent large policy supports; hybrids vary with design and scale.• Value stacking: thermal storage provides long-duration shifting (4–12+ hours) at lower $/kWh-storage than lithium batteries in many cases; the grid value per MWh at peak can exceed PV’s off-peak value.When the Hybrid Wins vs. PV-Only• Grid needs firm, dispatchable, evening power: CSP-sCO₂ with storage delivers high-value MWh; PV without storage cannot.• High DNI sites (≥ 2,000 kWh/m²-yr) with land availability and receiver temperatures ≥ 550–600 C: cycle efficiency and storage economics improve.• Curtailment risk for PV: using otherwise-curtailed PV energy for the compressor or auxiliaries improves overall plant utilization.• Thermal co-products (process heat, desalination): CSP thermal backbone can co-deliver valuable heat that PV cannot.When PV-Only Still Wins• Lowest-cost daytime kWh where intermittency is acceptable or storage is cheap/available.• Lower DNI or diffuse-sun climates where CSP optics underperform.• Small to medium projects where the fixed cost of towers/fields/recuperators is prohibitive.Back-of-Envelope Scenario (Illustrative, not a design)• Assume receiver/field efficiency 60 percent, cycle efficiency 40 percent at ~520–550 C → 24 percent thermal solar-to-AC.• Add PV for compressor sized so that ~80–100 percent of compressor work is met by PV when the sun is up; net cycle output rises a few percentage points for the same DNI input, and capacity factor increases with thermal storage.• The aggregate plant efficiency (counting both solar inputs) can land in the mid-20s percent to low-30s percent range under good conditions, which is comparable to 22 percent PV modules after BOS and conversion, but with dispatchability.Key Risks and Engineering Must-Haves• High-effectiveness recuperators with low pressure drop (costly but essential).• Receiver durability at 500–600 C+ and flux management.• Tight turbomachinery tolerances for sCO₂ and robust seals/bearings.• Thermal storage integration (e.g., molten salt or next-gen media) with minimal exergy loss.• Controls to orchestrate PV-to-compressor matching, thermal charging, and turbine dispatch.Bottom Line• Pure PV at 22 percent remains the cheapest daytime electricity in most markets.• A PV-assisted sCO₂-CSP plant running at ≥ 500 C TIT is thermodynamically sound and can deliver mid-20s to low-30s percent effective solar-to-wire efficiency while adding firm, dispatchable output via thermal storage.• Economically, PV-only typically wins on LCOE, but the hybrid can win on value where the grid pays for capacity, evening energy, and reliability—especially at high-DNI sites with ≥ 550–600 C receivers and strong recuperation.• If your objective is lowest cost kWh, choose PV. If you need firm power and peak coverage without batteries, the hybrid PV-sCO₂-CSP pathway is competitive and strategically attractive. |
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