Solar Thermal CST


Concentrating solar thermal

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210 BC - Ancient Greeks

As far back as the 5th century BC, the Greeks were building their homes in grid patterns to utilize solar power. It was such an accepted technology that the playwright Aeschylus wrote that only pri​mitives “lacked knowledge of houses turned to face the winter sun, dwelling beneath the ground like swarming ants in sunless caves.”

Archimedes was said to have used mirrors to focus sunlight onto invading ships and set fire to them.

How he aimed, and made the mirrors was not specified. Nor their size or design. The one pictured may have delivered 500 W.


This burning mirror was a brainchild of Archimedes between 212-215 BC in Syracuse. With the help of this giant mirror Archimedes incinerated the invading ships in Roman fleets and they were set on fire.

Polybius later accounted for the incident stating that the Romans were set to capture the city if it wasn’t for an old man from Syracuse known as Archimedes. The Roman fleet suffered severe losses and many of their ships were burnt. They stopped their assault on the city but laid a siege of eight months over the city forcing the dwellers to face starvation.

1865 - First solar thermal patent

Auguste Mouchout was the first man to patent a design for an engine running on solar energy.  Worried about his country’s dependence on coal, he received funds from the French government to research solar power. By 1865 he designed a device, which could turn solar energy into mechanical steam power and operate a steam engine.  Source

6 August 1882: The operation of a solar-powered printing press, which produced copies of Le Chaleur Solaire by Augustin Mouchot, a newspaper that he created especially for the event. The press rattled off 500 copies an hour. The experiment was conducted in the Garden of Tuileries, Paris, for the festival of L'Union Francaises de la Jeuenesse

Photograph: /Corbis

Circa 1870

An undated illustration (circa 1870) of inventor John Ericsson's new solar engine, which used concave mirrors to gather sun radiation strong enough to run an engine.      Photograph: /Corbis

1913 - First solar thermal power station

Frank Shuman built the world’s first solar thermal power station in Meadi, Egypt. Shuman’s plant used parabolic troughs to power a 60-70 horsepower engine, which pumped 6,000 gallons of water per minute from the Nile River to adjacent cotton fields.


   Frank Schuman

Parabolic trough solar thermal

An Australian invention is the parabolic trough mirror. A tube of oil or water runs along the trough and the heat is focused on it heating the water for steam production. It seems to be a bit more expensive. The inventor after 10 years finally took his idea to the USA where investors put in $150 million and then sold it to a French nuclear power company. They are now building the first plant in Queensland with Australian Government grant assistance.

White Cliffs power station

Built in 1981 at White Cliffs, an opal mining town in western NSW, may be the world's first commercial power stations. It had 14 dishes 5M in diameter generating steam to run an engine to generate electricity.

It was a particularly efficient reciprocating  steam engine called a Skinner Unaflow Engine. The uniflow type of steam engine uses steam that flows in one direction only in each half of the cylinder. Thermal efficiency is increased in the compound and multiple expansion types of steam engine by separating expansion into steps in separate cylinders; in the uniflow design, thermal efficiency is achieved by having a temperature gradient along the cylinder. Steam always enters at the hot ends of the cylinder and exhausts through ports at the cooler centre. By this means, the relative heating and cooling of the cylinder walls is reduced.

More on Wikipedia

Solar thermal steam engine - Terrajoule

A new company has been formed to use parabolic troughs to produce steam to power a Skinner engine.

The innovation in this venture is that they are using the high pressure steam (594 psig) to power the engine by day, then storing the condensate at 100 psig.  Overnight, this hot water is flashed off to produce low pressure steam for generation.

Terrajoule claims that storing energy as hot water costs less than 20%  of battery storage..

  • Storage has a capital cost of $60 per kilowatt-hour of storage capacity, the typical configuration provides 12 hours of storage.
  • The firm foresees the levelized cost of energy (LCOE) as under 10 cents without government assistance with a five-year payback.
  • CSP troughs have an efficiency of approximately 70 percent and the steam cycle efficiency is above 30 percent for an overall system efficiency of 22 percent. 
  • The steam engine is more efficient than a turbine in the low power range.

How it works

The Andasol power plant in Spain is the first commercial solar thermal power plant to utilize molten salt for heat storage and nighttime generation. It came online March 2009. It uses parabolic trough reflectors.











Saudi Arabia's first solar power plant is expected to begin construction in early 2013, and will take up to 2 years to complete.


1995 First molten salt storage. Solar two

The US Dept of Energy and several utilities built solar towers solar one and solar two. The unique feature of Solar Two was its use of molten salt (Sodium nitrite and nitrate) to capture and store the sun's heat. The very hot salt was stored and used when needed to produce steam to drive a turbine/generator that produces electricity. The system operated smoothly through intermittent clouds and continued generating electricity long into the night.

Solar Two was decommissioned in 1999, (just before Bush left office - probably as a gift to the fossil fuel industry) - demolition video. The area was converted by the University of California, Davis, into an Air Cherenkov Telescope in 2001, measuring gamma rays hitting the atmosphere.

Fortunately the know-how gained here was sold to the Spanish and is in use around the world. 

There are now billions of dollars worth of these towers being built in various countries.

Cost of solar thermal plants

First commercial solar thermal with molten salt storage

This was the first concentrating solar power tower to use molten salt to store heat for overnight energy generation. It is located near Seville in Spain.One of the problems is that the power stations will be out in the hot dry areas away from water. So it will be difficult to condense the steam. If air cooling is used the heat exchanges must be much larger than for water and this will add 10-20% to the power cost.

Improved molten salt

Sodium calcium  potassium calcium nitrate bring down the melting point of the molten salt mix.  The current temperature range is 290ºC to 390ºC. Thermal oil’s maximum temperature is 400ºC. 

The new ternary salts using potassium calcium nitrate have a range of 131-560ºC.  The wider temperature range means less salt and storage space are needed to store the heat.

The lower melting point means less blockage caused by molten salt solidification, which can be very costly in terms of plant down-time and repairs.





Gemasolar is the first commercial-scale plant in the world to apply central tower receiver and molten salt heat storage technology.

CST - Concentrating Solar Thermal

There are several versions of solar thermal, this is the power tower. A field of mirrors with each mirror tracking the sun and focusing it’s reflected light onto a central tower. In this tower are pipes full of molten salt (NaNO3 + KNO3) which is then heated to 550 - 650 degrees C. then it heats and boils water to produce steam and generate electricity in a standard steam turbine power plant. For overnight or a week of cloudy days the molten salt can be stored in an insulated tank with a loss of 1% per day.

The molten salt must be kept above 290oC to prevent it solidifying in the pipes.

A new development is to add a third salt to lower the melting point  from 220°C to 131°C. The new salt is potassium calcium nitrate. This give a couple of advantages. It expands the molten salt's effective temperature range meaning less salt is needed and it is les likely solid salt will block pipes. Source.

For long spells of cloudy weather pelletised wheat straw can be burned to heat the molten salt.

CSP uses less land per MW than any other form of renewable energy, producing 1.5 to 3 times more power per hectare/acre than solar PV, and the same amount of land as fossil-fuel generation when mining and production requirements enter the equation.

Beyond Zero Emissions have calculated that Australia could produce 60% of it’s energy this way. More on this website










Graphite heat storage

A proposed power plant in Cloncurry Australia will store heat in purified graphite. The plant has a power tower design. The graphite is located on top of the tower. Heat from the heliostats goes directly to the storage. Heat for energy production is drawn from the graphite. This simplifies the design.

The grapite can be heated up to 2,500 deg C, the heat capacity increasing as the temperture rises.

Lloyd Energy Systems CEO Steve Hollis says the system is about 35 to 40 per cent efficient in recovering thermal energy to produce electricity.

Graphite Energy has been running a 3 MW power plant at Lake Cargellico since 2011. (See right). 

More details (interview for Beyond Zero Emissions)

Graphite Energy 3 MW plant at Lake Cargelligo, NSW.

G1-SSR is a solar superheater and storage receiver

Development of graphite heat storage

Normally a CST will produce electricity at 25c/kWh. By superheating existing low temperature steam, this drops to 8c/kWh.

Steam at 100oC can be converted to electricity at a theoretical maximum 20% efficiency. If this is heated to 650oC, then the maximum theoretical efficiency is 68%. In practice these efficiencies should be halved to 10 and 35%. So over 3 times the energy can be extracted from the low temperature steam if it is superheated.

In steam at 650 oC, two thirds of the energy is in heating and evaporating the water. Only one third is in the heating of the steam to 650.  The cost of the power generated is one third of a solar thermal plant starting with cold water. 8 c/kWh instead of 25c/Kwh.

For an isolated mine using diesel generators, the cost of electricity is 30-40 c/kWh, so the savings can be significant.

By the Carnot theorem, the Max Efficency of a heat source and the cold sink = 1-(Tcold/Thot)  Temperature is in Kelvin (or Rankine for the Fahrenheit scale)

T cold oC T hot  oC Max Carnot efficiency
25 100 0.20
25 150 0.30
25 200 0.37
25 250 0.43
25 300 0.48
25 350 0.52
25 400 0.56
25 450 0.59
25 500 0.61
25 550 0.64
25 600 0.66
25 650 0.68
25 700 0.6

In reality, the efficiencies are normally about half the Carnot maximum.

Compact Linear Fresnel Reflector CLFR

Although the federal government has set aside billions of dollars for clean energy, very little has been spent. One could be excused for thinking the coal industry has them in their pockets.

NSW, Qld, and Victoria are all trying to increase the use of coal and gas.

Solar updraught tower - Nevada 0.2 GW Enviromission

Solar Dawn

Chinchilla SW Qld - 0.25 GW solar thermal 

AREVA Solar’s Australian-pioneered Compact Linear Fresnel Reflector (CLFR) technology.  It combines zero-emission solar generation with a low-emission gas boiler back-up system to efficiently produce clean, reliable power to meet growing energy demand. More


Solar receiver houses steam pipes to collect the concentrated solar energy










Solar receiver houses steam pipes to collect the concentrated solar energy

NASA plan to produce power for Earthlings

Maybe not quite ready to roll. NASA is looking into ways of generating power in orbit. One of the problems is how to send the power safely back to Earth.