Parabolic Trough Collector Product

Overview

The parabolic trough is a concentrated solar thermal (CST) technology achieving high operating temperatures (300–400°C) with a single-axis linear concentrator. A parabolic mirror surface focuses direct solar radiation onto a receiver tube running along the focal line, heating working fluid to drive a power cycle (Rankine turbine) or feed industrial thermal processes.

A typical 50 m × 6.6 m field (400–600 m² aperture) concentrates sunlight ~80× and collects 1–2 MW thermal at design irradiance (950 W/m² direct normal irradiance, DNI). Sixty to eighty such fields form a 100 MW solar thermal plant, producing 30–40 MW electricity with 6–10 hour molten salt thermal storage.

Commercial plants are operational in Spain, Morocco, Dubai, and India. The technology is mature, with demonstrated 30+ year service lives. Capital cost is 4–6 USD/W for turnkey CST with storage, competing with coal and natural gas for baseload generation in high-insolation regions.

How it works

Each Mirror Segments assembly is a curved mirror reflecting 94% of incident direct sunlight. The parabolic profile focuses all rays parallel to the optical axis onto the Receiver Tube placed at the focal line.

The receiver is an evacuated borosilicate glass tube (vacuum <0.1 Pa) containing a seamless absorber Inner Tube with black-nickel coating (absorptance >0.95, emittance <0.08). Heat-transfer fluid (synthetic oil or molten salt) flows through the absorber at 1–2 bar overpressure, heated from 240°C inlet to 385°C outlet in a single pass.

The Tracking Drive rotates the entire Torque Tube spine and mirror array to track the sun throughout the day. A Sun Sensor photodiode array detects the position error (e.g., is the shadow of the receiver off-center?), sending feedback to a Drive Control proportional controller. The controller modulates an electric Actuator Motor (5–10 kW) through a Gearbox, maintaining ±0.5° tracking accuracy. Typical drive speed is 0.5–2°/hour (15–60 minutes for a full east-west sweep).

Hot HTF exits via the Outlet Header and flows to a power block where a [[steam turbine]] or [[organic Rankine cycle turbine]] generates electricity. Condensed fluid loops back to the inlet.

The Piping Manifold distributes flow to parallel field rows. Each field can be isolated via Ball Valve manual valves for maintenance. Flow measurement and temperature monitoring feed real-time data to plant SCADA via Thermal Sensor instruments.

The Support Pylons are reinforced concrete and steel foundations anchoring the trough against wind forces. A 1 m diameter mirror at 15 m height experiences cantilever bending moment ~500 ton in 50 m/s wind; diagonal Wind Brace bracing suppresses oscillation and resonance.

Thermal Performance

At solar noon with DNI = 950 W/m²:

• Incident power: 600 m² × 950 W/m² = 570 kW
• Mirror losses (reflectivity 94%): 536 kW available
• Receiver optical losses (transmittance 92%): 493 kW
• Receiver thermal losses (conduction, radiation): ~100 kW (higher at 400°C)
• Useful thermal power: ~390 kW

Early morning and late afternoon (low sun angle) produce proportionally less energy. Daily energy yield in Spain (5 kWh/m²/day DNI) yields 2.5 MWh thermal per 600 m² field.

HTF and Thermal Storage

Synthetic oils (e.g., Therminol VP-1) are stable to 400°C with heat capacity 2.7 kJ/kg·K and viscosity 32 cSt at 300°C. Molten salt (60% NaNO₃ + 40% KNO₃) reaches 565°C at slightly higher cost but lower viscosity and better heat capacity (1.5 kJ/kg·K). Salt's high volumetric heat capacity (3 MJ/m³·K vs. 2 MJ/m³·K for oil) enables compact thermal storage tanks.

A 10 hour molten salt storage tank (2500 m³, 10000 ton salt, ΔT = 200 K) stores ~3 GJ, allowing evening electricity generation after sunset, dramatically improving plant capacity factor from 25–35% (no storage) to 50–65% (10 hour storage).

Degradation and Maintenance

Mirror reflectivity degrades 0.5–1.0% per year due to dust, salt deposition, and surface oxidation. Rainfall removes some dust naturally; in arid climates, quarterly or monthly washing with recycled water improves yield by 3–5% per wash.

Receiver vacuum degrades over time (micro-leaks, out-gassing). A well-maintained tube holds <0.1 Pa for 15–20 years; vacuum failure increases heat loss dramatically and signals tube replacement.

HTF is replaced every 10–12 years; oxidation products ("gums") accumulate and foul heat exchangers.

Economics

• Capital: 4–6 USD/W installed (field + power block + storage)
• Operational: 0.03–0.06 USD/kWh (water, labor, spare parts)
• Lifespan: 30+ years for mirrors/receiver tubes, 20–25 years for HTF circuit

In high-insolation regions (North Africa, Middle East, Southwest US), levelized cost of electricity (LCOE) is 6–10 USD/MWh with 6–10 hour storage, competitive with natural gas peaker plants and significantly lower than diesel.