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Steam Generation

4 min read

Turning 20°C water into steam takes about 2.6 MJ per kilogram, 334 kJ to reach 100°C and 2,257 kJ to boil it, so every kilowatt of heat that reaches the water makes about 1.4 kg of steam an hour. A 2 m² trough at 50% efficiency delivers 1 kW and makes about 1.4 kg an hour, so the output of a small trough is a few kilograms of steam an hour at most, and any solar boiler is a pressure vessel that needs a safety valve.

Part of High-Temperature Applications

Solar steam generation focuses sunlight on a pipe or boiler of water to produce steam for an engine, for process heat or for distillation.

Energy Per Kilogram Sets Everything

Inputs (standard values, recalled): specific heat of water 4.18 kJ/kg·K; latent heat of vaporisation at 100°C 2,257 kJ/kg; feed water at 20°C.

  1. Sensible heat = 4.18 × 80 = 334 kJ/kg.
  2. Latent heat = 2,257 kJ/kg.
  3. Total = 2,591 kJ/kg, about 2.6 MJ/kg.
  4. Steam per hour per kilowatt of net heat = 3,600 kJ ÷ 2,591 kJ/kg = 1.39 kg/h.

Check the parent page’s trough. It says a 2 m × 1 m trough on a 5 cm pipe can boil 20-40 L/h.

  • Aperture 2 m² × 1,000 W/m² = 2.0 kW incident. At 50% efficiency, 1.0 kW net, so 1.0 × 1.39 = 1.4 kg/h of steam. At a perfect 100%, 2.0 × 1.39 = 2.8 kg/h.
  • Even if “boil” only meant bringing water to 100°C without vaporising it, 20-40 L/h needs 20-40 × 334 kJ = 6.7-13.4 MJ/h, or 1.9-3.7 kW, which exceeds the 2 kW arriving.
  • Making 20-40 kg/h of steam needs 20-40 × 2,591 kJ/h = 51.8-103.6 MJ/h = 14.4-28.8 kW of net heat, equal to 14-29 m² of aperture at 100% efficiency, or 29-58 m² at 50%.

So the parent page’s figure is high by roughly an order of magnitude or more for the trough dimensions given. The same arithmetic applies to its array of 4-6 troughs of 3 m × 1.5 m (18-27 m² of aperture if the width is 1.5 m, so 18-27 kW incident, 9-13.5 kW net at 50%): about 12.5-19 kg/h of steam (9 × 1.39 = 12.5; 13.5 × 1.39 = 19). As for the engine: 1 kW mechanical from 27 m² of aperture is 1 ÷ 27 = 3.7% of the sunlight arriving, and 5 kW from 18 m² is 5 ÷ 18 = 28%. Small steam engines turn only a few percent of the heat they receive into work (Steam Engine puts a first single-cylinder engine at 0.1-0.3 horsepower), so the upper end is not supported. Budget the lower figures.

Pressure And Temperature Go Together

Saturated steam temperature rises with pressure (steam-table values, recalled): about 152°C at 5 bar absolute, 180°C at 10 bar and 198°C at 15 bar. The parent page’s “200-300°C at 5-15 bar” is therefore steam that has been superheated above saturation, not the saturated steam of a plain boiler. A simple solar boiler gives saturated steam at the pressure you hold it.

What You Need

  • A trough or dish with a tested focus (Parabolic Trough)
  • A receiver pipe of steel, blackened, strong for the pressure
  • A safety valve set below the weakest part (Safety Valve)
  • A pressure gauge (Pressure Gauge) and a water-level indication (Water Level Gauge)
  • A feed pump or gravity head, a steam line, insulation

Method: Run A Small Solar Boiler

Step 1: Build and test the pipe with water at 1.5 times working pressure (Pressure Testing).

Step 2: Fit gauge and safety valve before the first heating, with the valve’s outlet pointing away from people.

Step 3: Fill with water; never aim the focus at an empty or low pipe.

Step 4: Uncover and track; watch the gauge; stop heating by covering the concentrator, not by closing the steam outlet.

Common Mistakes

  • Counting litres of water instead of kilojoules.
  • No water supply while heated. An empty pipe at the focus overheats in seconds.
  • Letting a cloud or tracking error leave the pipe full of cold water and then re-focusing. Heat the pipe gradually.

Safety

Steam pressure, scalding steam and a concentrated focus

A boiler is a pressure vessel: water becomes about 1,700 times its volume as steam, and a pipe that ruptures releases that energy at once (Boiler Construction). Fit a safety valve and gauge, test the pipe with water, and keep the valve outlet pointed away from people. Steam scalds more than boiling water: condensing 1 kg of steam releases 2,257 kJ, 6.8 times the 334 kJ released by 1 kg of boiling water cooling to 20°C. The focus also ignites materials and damages eyes permanently and painlessly.

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