Solar Thermal: The Other Way to Use Sunlight
Photovoltaics and solar thermal are often discussed as rivals, and for electricity generation photovoltaics has largely won on cost. But they answer different questions. One produces electrons the instant light falls on it; the other produces heat that can be kept, and about half of final energy demand worldwide is heat in the first place.
Two Technologies Under One Name
The simpler family is the flat-plate or evacuated-tube collector: a dark absorber behind glass, with water or a heat-transfer fluid circulating through it. It reaches 60 to 90 degrees, which is exactly what a household needs for washing and space heating, and it does so at a conversion efficiency of 50 to 70 percent - far higher than any photovoltaic module, because turning light into heat is thermodynamically easy.
Globally this is the larger deployment. Hundreds of gigawatts of thermal capacity sit on rooftops, overwhelmingly in China, with Turkey, Israel, India and Greece following. A solar water heater is often the cheapest carbon reduction available to a household, and it is invisible in electricity statistics because it never produces electricity.
The second family concentrates sunlight to reach temperatures a flat collector cannot. Mirrors focus direct sunlight onto a receiver by a factor of several hundred, producing heat at 400 to 565 degrees. That is hot enough to raise steam at conditions a conventional turbine wants, so the back half of a concentrating solar plant is the same machinery as a coal or nuclear station.
Four configurations dominate. Parabolic troughs focus onto a tube running along the focal line and are the most commercially proven. Solar towers use a field of tracking mirrors aimed at a central receiver, reaching higher temperatures and thus better conversion efficiency. Linear Fresnel systems approximate a trough with flat mirrors at lower cost. Dish systems track the sun in two axes and drive a small engine at the focus.
Why Storing Heat Is the Point
A photovoltaic panel produces electricity, and electricity is difficult and expensive to store. A concentrating plant produces heat, and heat is easy and cheap to store. That single asymmetry is the entire argument for the technology.
The standard method is molten salt, typically a mixture of sodium and potassium nitrate, held in insulated tanks. Hot salt at around 565 degrees can be drawn off to raise steam whenever the operator chooses. Losses are around one percent per day. A plant with a full storage system can generate for 8 to 15 hours after the sun has set, which means it can cover the evening demand peak that photovoltaics misses entirely.
The cost of that storage is remarkably low per unit of energy, because the medium is industrial salt and the container is a steel tank. Compared with the cost of an equivalent battery, thermal storage at this scale is cheaper by a wide margin - though it is worth being precise about what is being compared, since the plant stores heat rather than electricity and can only release it through its own turbine.
The result is a solar plant that is dispatchable. Operators in Spain, Morocco, Chile and the United Arab Emirates run concentrating plants specifically to supply power in the hours after sunset, and several are contracted to deliver around the clock.
Where the Economics Went
For a period around 2010 concentrating solar power and photovoltaics looked like genuine competitors. Then photovoltaic module prices fell by roughly 90 percent in a decade, and the competition ended for straightforward daytime electricity. Concentrating plants are capital-intensive, need large tracts of flat land and a great deal of steel, glass and specialised construction, and they have no equivalent learning curve because each plant is a construction project rather than a manufactured product.
Two constraints shape where they still make sense. The first is direct normal irradiance: mirrors can only focus direct sunlight, so haze, cloud and humidity hurt them far more than they hurt photovoltaics, which still works on diffuse light. The economically viable regions are therefore desert belts - Spain, North Africa, the Middle East, Chile, South Africa, the American Southwest, western China.
The second is that the value lies in the storage rather than the generation. Where a grid needs solar electricity after dark and can pay for it, concentrating plants compete. Where it only needs daytime energy, photovoltaics wins outright.
A growing application avoids the competition altogether: industrial process heat. A great many industrial processes need heat between 100 and 400 degrees, which is above what a heat pump reaches and below what a furnace requires. Concentrating collectors supply that directly, with no conversion to electricity and back, and this is currently the fastest-growing part of the field.
The Complementarity Argument
The useful way to place solar thermal is by what it contributes to a system rather than by its cost per kilowatt-hour in isolation. Photovoltaics delivers cheap energy during daylight. Concentrating solar with storage delivers more expensive energy at a time of the operator's choosing. These are different products, and a grid buys both.
That framing recurs throughout energy systems. Wind and photovoltaics supply energy when the weather allows; hydropower with reservoirs, geothermal and nuclear fission supply it when asked. The economic value of a megawatt-hour depends heavily on when it arrives, and comparisons that ignore timing mislead in both directions.
Low-temperature solar thermal makes a different and simpler point. Roughly half of the world's final energy demand is heat, not electricity, and a large share of that heat is at temperatures a flat collector can reach. Converting sunlight to electricity in order to make hot water involves a conversion the physics does not require.
That principle - use energy in the form the task actually needs - is the same one behind heat pumps, district heating and industrial heat recovery, and it is frequently a larger and cheaper opportunity than generating more electricity.
Frequently asked questions
What is the difference between solar thermal and photovoltaics?
Photovoltaics converts light directly into electricity in a semiconductor. Solar thermal captures sunlight as heat, either at low temperature for hot water and heating, or concentrated to several hundred degrees to raise steam for a turbine. One produces electrons; the other produces heat that can be stored.
Can a solar plant generate at night?
A concentrating solar plant with thermal storage can. Molten salt heated during the day is held in insulated tanks and drawn off to raise steam after sunset, typically giving 8 to 15 hours of generation. Losses are around one percent per day. Photovoltaic plants need separate batteries to do anything comparable.
Why does concentrating solar need clear skies?
Mirrors can only focus direct sunlight. Diffuse light scattered by cloud or haze arrives from all directions and cannot be concentrated onto a receiver. Photovoltaic panels still generate from diffuse light, which is why they work in northern Europe while concentrating plants are restricted to desert regions.
Is solar thermal more efficient than photovoltaics?
For producing heat, yes by a wide margin: a flat collector converts 50 to 70 percent of incident sunlight into usable heat, against 22 to 24 percent for a photovoltaic module producing electricity. But the outputs are not comparable. For producing electricity, a concentrating plant converts roughly 15 to 20 percent overall.
Why did photovoltaics overtake concentrating solar?
Module prices fell about 90 percent during the 2010s because panels are manufactured products with a steep learning curve. A concentrating plant is a construction project, with steel, mirrors, land and site work that do not fall in price the same way. Concentrating solar retains an advantage only where dispatchable output after sunset is what the grid is paying for.
