High Temperature Pump for Solar Thermal and Geothermal Power Generation

Solar thermal and geothermal power plants use high-temperature pumps to circulate heat transfer fluids through collectors, heat exchangers, and power cycles. These pumps operate at 100-600°C with unique challenges including thermal energy storage, variable heat input, and corrosive fluids. This guide covers high-temperature pump selection for solar thermal and geothermal power generation.

Solar Thermal Power Systems

Parabolic Trough Systems

Heat Transfer Fluid

• Synthetic thermal oil (e.g., Therminol VP-1, Dowtherm A): biphenyl/diphenyl oxide
• Temp range: 12-400°C (liquid phase)
• Vapor pressure: high at 400°C (~11 bar) — system must be pressurized
• Flammable, toxic (Dowtherm A) — require zero-leakage pumps
• Freeze point: 12°C (Therminol VP-1) — need freeze protection in cold climates

Pump Selection

• HTF (heat transfer fluid) circulation pump: high-temp chemical pump (OH2, centerline support)
• Flow: 500-5,000 m³/h (large plants), head: 50-150 m
• Material: carbon steel (WCB) or 2.25Cr-1Mo (F22), 316 for corrosive
• Seal: double mechanical (Arrangement 3, Plan 53) or magnetic drive (zero leakage)
• Key: centerline support (thermal growth), low NPSHr (hot oil near vapor pressure), high reliability
• Multiple pumps in parallel (N+1 redundancy)

System Considerations

• Expansion tank: nitrogen-blanketed, accommodates thermal expansion, maintains NPSH
• Minimum flow: protect pump during low solar irradiance (early morning, evening, cloudy)
• Freeze protection: heat tracing, jacketing, or drain system (for cold climates)
• Variable flow: VFD to match solar irradiance (pump speed follows collector output)
• Thermal cycling: daily startup/shutdown — fatigue-resistant design

Solar Power Tower (Molten Salt)

Molten Salt System

• Nitrate salt: 60% NaNO₃ / 40% KNO₃ (Solar Salt), melting point 220°C, max 565°C
• Two-tank storage: cold tank (290°C), hot tank (565°C)
• Salt circulates through receiver (heats from 290→565°C), stored in hot tank
• Hot salt pumped to steam generator (cools to 290°C), returns to cold tank
• Storage allows 24/7 operation (dispatchable power)

Pump Selection

• Hot salt pump: Vertical long-shaft, submerged in hot salt tank (565°C)
  • Flow: 500-3,000 m³/h, head: 50-150 m
  • Material: 316/304 stainless (nitrate salt <565°C)
  • Shaft: long (5-15m), with line bearings (product-lubricated)
  • Motor: above tank, cooled by ambient air
  • Key: maintain salt >220°C (heat tracing on shaft, jacketing)
• Cold salt pump: Vertical long-shaft, submerged in cold salt tank (290°C)
  • Similar design, lower temp (less severe)
• Receiver circulation pump: vertical canned pump or horizontal high-temp
• Key: no elastomers (metal gaskets), freeze protection, preheat before startup

System Considerations

• Freeze protection: all salt piping, pumps, valves must be >220°C (heat tracing, jacketing)
• Preheat: pumps must be preheated to >220°C before introducing salt (prevent freeze)
• Thermal expansion: salt expands ~5% from 290→565°C — overflow tank needed
• Corrosion: nitrate salt is mildly corrosive at >500°C — use 316/304, monitor wall thickness
• Reliability: salt freeze = catastrophic (solidified salt, expensive cleanup) — redundant heat tracing

Geothermal Power Systems

Geothermal Brine Pumps

• Brine: high TDS (total dissolved solids), silica, scaling, corrosive
• Temp: 80-350°C (depending on resource)
• Pressure: high (artesian flow or pumped)
• Pump type: vertical turbine (submersible or lineshaft) for production wells
• Material: duplex stainless, 316, nickel alloy (corrosion/scale resistance)
• Key: handle scaling (silica, calcium carbonate), corrosion, high temp

Injection Pumps

• Inject cooled brine back into reservoir (sustainability, pressure maintenance)
• High pressure: 50-200 bar (overcome reservoir pressure)
• Pump type: horizontal multistage (BB3/BB4/BB5) or vertical multistage
• Material: duplex, 316, nickel alloy (corrosive brine)
• For high-pressure injection: multistage centrifugal (similar to boiler feed)
• Key: handle scaling, high pressure, variable flow (reservoir pressure changes)

ORC Working Fluid Pumps

• Binary cycle: geothermal brine heats organic working fluid (refrigerant, hydrocarbon)
• Working fluid pump: magnetic drive or canned motor (zero leakage)
• High pressure ratio: condenser (low P) → evaporator (high P)
• Flammable/toxic fluid (depending on type) — zero leakage required
• Key: precise flow control, high efficiency, hermetic design

Material Selection

Solar Thermal (Thermal Oil)

• Casing: WCB (carbon steel) up to 350°C, 2.25Cr-1Mo (F22) up to 450°C
• Impeller: 316, CF8M, or WCB
• Shaft: 316, 17-4PH, or 2.25Cr-1Mo
• Seal: double mechanical (Plan 53) or magnetic drive (zero leakage)
• Gaskets: spiral wound (316/graphite), camprofile, RTJ (high pressure)

Solar Power Tower (Molten Salt)

• Casing/impeller/shaft: 316/304 stainless (nitrate <565°C)
• For chloride salt (>500°C): Inconel 625, Hastelloy C
• No elastomers — metal gaskets only
• Bearing: product-lubricated (salt) or external grease (above salt)
• Shaft: 316, with line bearings (carbon graphite or 316)

Geothermal (Brine)

• Production pump: duplex stainless (2205, 2507), 316, nickel alloy (C-276)
• Injection pump: duplex, 316, nickel alloy
• Key: resist chloride stress corrosion cracking (CSCC), scaling, erosion
• Coatings: epoxy, ceramic (for scaling resistance)
• For high-silica brine: use scaling-resistant materials, chemical treatment

Unique Challenges

Variable Heat Input (Solar)

• Solar irradiance varies: daily cycle, clouds, seasons
• Pumps must handle variable flow and temperature
• VFD: match pump speed to collector output (energy savings)
• Thermal storage (molten salt): allows constant pump operation (dispatchable)
• Thermal cycling: daily startup/shutdown — fatigue, seal wear, alignment
• For parabolic trough: HTF temp varies 250-400°C (morning vs noon)

Freeze Protection (Solar)

• Thermal oil (VP-1): freeze at 12°C — heat tracing in cold climates
• Molten salt: freeze at 220°C — critical, redundant heat tracing required
• Salt freeze in pump = catastrophic (solidified salt, shaft locked, expensive)
• Methods: electric heat tracing, steam tracing, jacketing, drain system
• Preheat pump before startup (verify temp >freeze point + 20°C)
• For molten salt: maintain temp >250°C at all times (even shutdown)

Corrosion and Scaling (Geothermal)

• Brine: high TDS, chloride, silica, H₂S, CO₂ — corrosive and scaling
• Chloride stress corrosion cracking (CSCC): use duplex or nickel alloy (not 304/316 at high temp)
• Silica scaling: deposits on impeller, volute, piping — reduces efficiency, causes vibration
• Mitigation: chemical treatment (acid, scale inhibitors), coatings, periodic cleaning
• Material selection is critical — consult geothermal chemistry expert

High Reliability Requirement

• Solar/geothermal plants: 20-30 year life, remote locations, high availability (95%+)
• Pump failure = lost revenue (no power generation)
• N+1 redundancy: one standby pump for each critical service
• Condition monitoring: vibration, temperature, pressure (predictive maintenance)
• Quick-change design: cartridge seal, back pull-out, vertical pump pull-out
• Spare parts: critical spares on-site, rotating spare for major pumps

Common Issues

• Molten salt freeze: Heat tracing failure, cold startup. Maintain temp >250°C, redundant tracing, preheat.
• Thermal oil degradation: Overheating, oxidation. Nitrogen blanket, maintain temp
• Cavitation (HTF):
Hot oil near vapor pressure, variable flow. Expansion tank, low-NPSH pump, min flow.
• Geothermal scaling:
Silica/calcium deposits. Chemical treatment, coatings, periodic cleaning.
• Corrosion (brine):
Chloride CSCC, H₂S. Use duplex/nickel alloy, monitor wall thickness.
• Thermal cycling fatigue:
Daily startup/shutdown. Fatigue-resistant materials, gradual warm-up.
• Bearing overheating:
High temp conduction, long shaft. External cooling, product-lubricated bearings.
• Seal leakage (HTF):
Flammable/toxic fluid. Double seal or magnetic drive, leak detection.

Procurement Recommendations

1. Provide: plant type (trough/tower/geothermal), fluid, temp range, variability, storage
2. For parabolic trough: specify high-temp chemical pump, centerline support, double seal or magnetic drive
3. For molten salt: specify vertical long-shaft pump, 316/304, heat tracing, no elastomers
4. For geothermal production: specify vertical turbine pump, duplex/nickel alloy
5. For geothermal injection: specify multistage centrifugal, duplex/nickel alloy, high pressure
6. For ORC: specify magnetic drive pump (zero leakage), high-pressure rating
7. Specify VFD for variable solar/geothermal output
8. Include N+1 redundancy for critical pumps
9. For molten salt: include heat tracing/jacketing, preheat system, freeze protection
10. For thermal oil: include expansion tank, nitrogen blanket, seal cooling
11. Specify material per fluid chemistry (verify compatibility, corrosion testing)
12. Include condition monitoring (vibration, temperature, pressure)
13. Require performance test per API 610 / ASME PTC 8.2
14. Include spare parts: seal, bearings, wear rings, impeller, shaft
15. Verify supplier has solar/geothermal references

Related Industries

Solar thermal and geothermal pump principles apply to power generation (fossil, nuclear, biomass), oil and gas (upstream, thermal EOR), chemical processing (high-temp, corrosive), metals processing (furnace, rolling), and district heating (geothermal direct use). The core principle — reliable circulation of high-temperature heat transfer fluid under variable and corrosive conditions — is consistent across renewable power applications.