Molten Salt Storage Tank

Overview

A molten salt thermal storage tank is the energy reservoir of concentrated solar thermal power (CSP) plants, allowing electricity generation after sunset or under cloud cover. The tank maintains a binary pool of solar salt (a eutectic nitrate mixture melting at 225 °C) at two distinct temperatures: hot salt at 565–575 °C from solar collection, and cold salt at 290–310 °C returning from the power cycle. Storage durations of 4–15 hours are economically viable, enabling capacity factors of 40–50% and reducing the marginal cost of solar electricity generation.

The Tank Vessel is a large cylindrical steel container lined with refractory material to isolate the 600 °C salt from the steel, which would creep and lose strength above 500 °C. An array of heaters maintains minimum temperature during stow to prevent salt freezing (crystallization occurs below 225 °C, causing irreversible blockage). The Circulation Pump transports salt at 300–800 m³/h between the tank and the solar field or power cycle heat exchanger. Ancillary systems manage thermal losses, detect faults, and ensure safe operation over 30-year design life.

Tank Vessel

The Tank Vessel consists of a cylindrical carbon-steel Shell Plate, a flat or dished Top Head, and a hemispherical Bottom Head with central drain sump. The Shell Plate is typically 25–40 mm ASTM A516 Grade 70 plate, formed and welded with full-penetration seams post-weld-stress-relieved at 620 °C to restore material ductility after welding heat-affect softening.

The interior surface is lined with a monolithic castable Refractory Lining (alumina-spinel or magnesia-spinel ceramic), 100–150 mm thick. This refractory serves two critical functions: it thermally isolates the salt from the steel shell (reducing metal temperature to ~400 °C even as salt reaches 570 °C), and it withstands thermal shock from hot salt stratification during charging. The Expansion Joint on the top-head nozzle accommodates 100–200 mm of thermal growth as salt heats from ambient to operating temperature, protecting the upper closure from bending stress.

All Welds are radiograph-inspected and stress-relieved per ASME Section VIII Division 1. The design pressure is typically 1.5–2.0 bar gauge, driven by salt static head (tall tanks develop 1 bar per 10 m) and thermal expansion.

Heating System

The Heating System comprises an Immersion Heater (200–400 kW electric resistance element submerged in the salt), a Heater Thermostat that modulates heater power via contactor or SCR to maintain 290–310 °C cold-pool setpoint, and a Heater Bypass Valve relieving overpressure from thermal expansion. Freeze protection is the largest parasitic load: if a 1000 m³ tank cooled to 200 °C (below freeze point) requires ~180 MWh to reheat to 310 °C, the freeze-protection heater must run continuously at low power (20–50 kW) to offset radiative and conductive losses and prevent crystallization during extended cloudiness or nighttime stow.

The Heater Terminal Box houses the contactor, overload relay, and soft-starter, controlling inrush current to prevent voltage dip on the grid connection. Modern designs use variable-frequency drives (VFD) on the heater element pump to modulate flow rate, reducing heating power requirement below full-capacity level.

Circulation Pump

The Circulation Pump is a centrifugal machine rated for continuous 600 °C operation, handling 300–800 m³/h at discharge pressure of 1–3 bar. The Pump Casing and Pump Impeller are cast or forged from carbon steel or 316L stainless steel. A Pump Motor (150–300 kW) spins the impeller via a flexible coupling; the motor is cooled by an external fan or liquid jacket to keep winding temperature below 120 °C despite nearby 600 °C salt.

The critical challenge is the Pump Seal: a double mechanical face seal with an inert gas (argon or nitrogen) barrier separating cool-side oil from hot salt. The cool chamber is maintained at ~100 °C by recirculating oil through a small heat exchanger; the hot chamber sits at the pump inlet, submerged in returning cold salt (~310 °C). Without double seals, salt would carbonize on the seal face, causing fouling and leakage. The Pump Strainer on the suction line removes debris (salt crystals, refractory fines) that could damage the impeller.

Cooling System

The Cooling System is a parasitic cooler rejecting sensible heat from the cold-pool during night stow, when heaters alone cannot offset losses. An air-cooled aluminum Cooler Core (plate-fin radiator, 100–300 kW duty) is blown by an EC Cooler Fan running at variable speed. A Cooler Thermostat modulates fan speed via PWM or 0–10 V output, keeping the cold pool at ~290 °C. The Cooler Bypass routes hot salt around the cooler during day charging and night standby, saving freeze-protection energy.

Instrumentation and Piping

Temperature is monitored at the hot outlet (via Temp Sensor Top, typically a mineral-insulated Type K thermocouple) and cold inlet (via Temp Sensor Bottom), with dual-channel transmitter and alarm relay in the Junction Box. The Level Sensor detects abnormal salt level (overfill or underfill from leaks), and a Pressure Sensor monitors tank absolute pressure. High tank pressure (>2 bar) indicates blockage or pump failure; low pressure suggests a leak or vent blockage.

The Piping Manifold provides:

• Inlet Nozzle: Hot salt entry with integral check valve (prevents backflow at pump shutdown)
• Outlet Nozzle: Cold salt return with double-block-and-bleed isolation (allows maintenance without full drain)
• Pressure Relief: Pilot-operated valve cracking at 1.5–2.0 bar
• Drain Valve: Large-bore ball valve at sump for seasonal drain or emergency decommission

Thermal Insulation and Foundation

The Thermal Insulation wrap of Mineral Wool (ceramic fiber blanket, 50–100 mm) and Cladding (stainless-steel 304 sheet) reduces radiative loss by ~80%, cutting daily loss from ~2% to ~0.8–1.2%. The Tie Rod Insulation clips hold insulation without thermal bridging.

The Foundation is a reinforced-concrete mat, 1.5–2 m thick, supporting static load (a 1000 m³ tank weighs ~1800 tons when filled) and seismic bending. A Drainage Layer underdrain detects seepage; a Spill Berm secondary containment wall captures salt in case of rupture. The Anchor Lugs tie the tank down for wind uplift and earthquake overturning moment.

Operating Practice

CSP plants charge the hot tank during peak insolation (10:00–15:00 local solar time) when the steam generator is running cold. Cool salt from the power cycle (310 °C) returns to the tank continuously. As sun sets, the Circulation Pump gradually reduces flow; the Heating System kicks in to prevent freeze. During the night, the Cooling System runs intermittently to balance parasitic losses against freeze-protection heating. Seasonal drain cycles (every 2–5 years) remove accumulated carbonate and hydroxide salts that reduce thermal conductivity and increase pumping power.

Modern tank control uses SCADA with remote sensors and smart valving, coordinating storage level, solar forecast, and grid demand to optimize discharge timing and maximize revenue from peak-hour electricity pricing.