The Thermosiphon: Moving Heat With No Pump

The physics: buoyancy is the pump

A thermosiphon is a gravity engine that runs on a density gradient. Heat the fluid in the lower leg and its density drops; the cooler, denser fluid in the upper/return leg weighs more per unit height. Because the two vertical legs of the loop have different average densities, they exert different hydrostatic pressures at the bottom — and that imbalance is an unopposed pressure difference that pushes fluid around the loop.

The driving head is essentially a hydrostatic difference. For a loop of active height H with a hot leg of average density ρ_hot and a cold leg of average density ρ_cold, the buoyant driving pressure is:

ΔP_drive = (ρ_cold − ρ_hot) · g · H

Using the volumetric thermal-expansion coefficient β (where Δρ ≈ −ρ·β·ΔT), this becomes the form engineers actually design with:

ΔP_drive ≈ ρ · g · β · ΔT · H

Here g = 9.81 m/s², ΔT is the temperature difference between the legs, and β is the fluid property that does the heavy lifting. For water near 60 °C, β ≈ 0.52×10⁻³ K⁻¹ and ρ ≈ 983 kg/m³. In steady state this driving pressure exactly balances the friction losses around the loop, so the flow rate settles where buoyancy in = pipe friction out. Raise the heat input and ΔT rises, buoyancy rises, and the loop speeds up — which is why a thermosiphon is inherently self-regulating.

A worked number: how hard does it actually push?

Take a rooftop solar thermosiphon: water, active loop height H = 1.0 m between the collector and the tank, and a leg-to-leg difference of ΔT = 20 °C.

• ρ ≈ 983 kg/m³, β ≈ 0.52×10⁻³ K⁻¹, g = 9.81 m/s²
• ΔP_drive ≈ 983 × 9.81 × 0.52×10⁻³ × 20 × 1.0 ≈ 100 Pa

That is about 0.01 m of water head — a whisper of pressure, roughly what you feel from a 1 cm water column. A domestic circulator pump, by contrast, produces 20,000–60,000 Pa. So a thermosiphon must be built to move fluid on 1/200th of a pump's push. That single fact dictates every design choice: fat pipes, gentle bends, short runs, and a tank mounted high.

How fast does 100 Pa move water through, say, 6 m of 22 mm copper pipe? Balancing the driving head against laminar/transitional pipe friction lands the loop velocity around 0.05–0.15 m/s and a mass flow of a few tens of kg/h — modest, but for a 1–2 kW solar collector that is plenty to keep the panel from overheating. Two-phase (boiling) thermosiphons cheat this limit entirely: instead of relying on liquid density difference, they exploit the ~1600× density ratio between liquid and vapor, so a small ΔT produces a large buoyant head.

Single-phase vs. two-phase thermosiphons

There are two families, and they behave very differently:

Single-phase (liquid) thermosiphon. The working fluid stays liquid the whole way around — think a solar water heater or an engine's old-style gravity cooling. The driving head is small (β·ΔT is tiny), so these need real vertical height and generous pipe bore. Heat capacity comes from the sensible heat of the flowing liquid.

Two-phase (evaporating) thermosiphon. The fluid boils at the hot end and condenses at the cold end. Vapor rushes up the riser, gives up its latent heat at the condenser, and the condensate drains back by gravity. Because latent heat is enormous (water: ~2260 kJ/kg; many refrigerants 100–350 kJ/kg) and vapor is ~1000× lighter than liquid, a two-phase unit moves far more heat per unit temperature difference. This is the same principle as a heat pipe, but a thermosiphon relies on gravity to return the condensate — so it only works with the evaporator below the condenser, whereas a wicked heat pipe can run in any orientation. A closed two-phase thermosiphon is often called a 'gravity heat pipe' or wickless heat pipe.

Effective axial conductance of a good two-phase thermosiphon can be equivalent to a solid copper bar hundreds of times its cross-section — this is why they show up flattened into laptop and GPU cooling assemblies.

Where thermosiphons actually earn their keep

• Solar water heating. The classic thermosiphon: flat-plate collector on the roof, insulated tank mounted just above it. Millions installed worldwide (huge fleets in Greece, Israel, Australia, China). Zero pump, zero controller — hot water for the price of plumbing and sunlight.
• Trans-Alaska Pipeline thermosiphons. About 124,000 ammonia-charged vertical two-phase thermosiphons (the 'heat-pipe' support piles) pull heat out of the ground in winter to keep the permafrost frozen so the warm oil pipeline doesn't thaw its own footings and sink. A textbook passive-cooling save at continent scale, installed from the mid-1970s.
• Nuclear reactor decay-heat removal. Passive natural-circulation loops carry decay heat from the core to a heat sink with no electrical power — a core safety strategy in modern designs (e.g., passive containment cooling and natural-circulation SMRs). Post-Fukushima, passive circulation became a headline safety feature precisely because it survives a total station blackout.
• Electronics and CPU/GPU cooling, transformer cooling (ONAN — oil natural, air natural), and internal-combustion engine 'thermosyphon' cooling on early cars (Ford Model T).

Design trade-offs, limits, and failure modes

The thermosiphon's superpower — no pump — is also its cage. Because the driving head is only tens to a couple thousand pascals, the loop is exquisitely sensitive to hydraulic resistance and to geometry:

• Height is mandatory. ΔP scales directly with H. In a solar thermosiphon the tank bottom must sit above the collector top (commonly by ≥ 0.3 m) or the loop stalls — or worse, reverses and drains heat back out at night (reverse thermosiphoning / night-time cooling loss).
• Low flow, low pressure only. You cannot force a thermosiphon uphill against a tall building's plumbing or through a fine-fin cold plate; the head simply isn't there. High heat flux plus high resistance stalls the loop.
• Non-condensable gases (air, or hydrogen from corrosion) are the classic killer of two-phase units: they collect in the condenser, blanket the surface, and choke heat transfer. Charging and sealing are done under vacuum for exactly this reason.
• Flow instability. Two-phase loops can oscillate (density-wave and geysering instabilities), and single-phase loops can suffer flow reversal or bistable stalling — an active area of reactor-safety analysis.
• Dryout / burnout. Overdrive a two-phase evaporator and the liquid film breaks down; the wall temperature spikes as the heat-transfer coefficient collapses.

The engineering art is minimizing loop friction: oversize the bore, use sweeping bends instead of elbows, keep runs short, and maximize the vertical separation between source and sink.

A common misconception (and a subtle pitfall)

Misconception: 'It moves heat for free / breaks even on energy.' A thermosiphon has no parasitic electrical power, but it is not free lunch. The circulation is driven by degrading a temperature difference — the same ΔT that could have done thermodynamic work. And crucially, a thermosiphon only moves heat 'downhill' in temperature, from hot source to cooler sink. It cannot pump heat from cold to hot; that requires a heat pump or refrigeration cycle with real work input. A thermosiphon is a transport device, not a heat pump.

Subtle pitfall: orientation and reverse flow. A two-phase thermosiphon absolutely will not return condensate if you invert it — evaporator must be below condenser. And even a correctly built single-phase solar loop will happily run backwards after sunset, dumping tank heat to the cold night sky, unless you add a one-way check valve or a heat-trap loop. Designers also forget that β is temperature-dependent (water's β climbs sharply toward boiling), so a loop that stalls when cold can come alive as it heats — nonlinear startup behavior that trips up naïve steady-state sizing.

Thermosiphon (natural circulation) vs. pumped forced-convection loop

Attribute	Thermosiphon	Pumped loop
Driving force	Buoyancy: ρ·g·β·ΔT·H	Mechanical pump head (kPa–MPa)
Moving parts / power	None, 0 W parasitic	Pump + motor, 20–500 W typical
Flow rate control	Self-regulating with heat load	Set by pump speed / valves
Achievable ΔP	~0.1–2 kPa	10 kPa – several MPa
Failure mode	Flow reversal / stall, non-condensables	Pump/bearing/seal failure, power loss