Thermal Solar Goes Where PVs Can’t

Energy storage sparks a concentrating-solar boom

4 min read
Thermal Solar Goes Where PVs Can’t

Global solar energy supplies are growing rapidly, with nearly 10 times as much solar capacity installed today as there was a decade ago. Leading the boom is the photovoltaic (PV) panel, which converts sunlight into electricity using semiconductors. But even as the glossy rectangles become increasingly cheaper and ubiquitous, solar PV alone can't solve the nagging question: What to do when the sun isn't shining? As electric utilities and policymakers seek solutions for storing and dispatching energy on demand, concentrating solar-thermal power (CSP) is once again gaining traction.

Solar-thermal systems use sun-tracking mirrors to reflect sunlight onto a receiver, which contains a high-temperature fluid that stores heat. The heat can drive steam turbines or engines to generate electricity around the clock. Or, solar thermal can directly provide heat for industrial processes to make steel, cement, and chemicals—all energy-intensive sectors that are difficult to clean up. With U.S. states and countries adopting measures to curb greenhouse gas emissions, now "might be the right time for CSP to become more broadly applicable," said Margaret Gordon, the CSP program manager at Sandia National Laboratories in Albuquerque, N.M.

The first utility-scale solar-thermal plants were built in the 1980s. Today, nearly 120 projects operate worldwide, and Spain claims more than a third of total installed capacity. In recent years, however, CSP development stalled amid the rapidly falling cost of solar PV. Massive solar-thermal arrays require steep upfront investment. Unlike solar PV, the mirrors, heat exchangers, and other key components aren't yet "off-the-shelf" ready, which adds time and expense to project development, Gordon said. Even so, solar-thermal developments continue to unfurl across the planet's sunniest expanses, driven by the growing demand for energy storage and cleaner heat sources. IEEE Spectrum looked at four new solar-thermal projects—each representing a different CSP technology—that are curbing emissions by harnessing the sun's heat.

Power Tower

Image of the Cerro Dominador thermal power tower.

Cerro Dominador

Cerro Dominator:100-MW solar-thermal power tower + 100-MW solar PV plant.
Atacama Desert, Chile

The US $1.4 billion project began full operations in June. The 700-hectare complex has 10,600 mirrors (or "heliostats") that direct sunlight to a 252-meter-tall tower. Inside the tower, molten salts are heated to 565 °C and flow down into storage tanks, turning water into steam to drive turbines. The closed-loop system has a record 17.5-hour thermal storage capacity, allowing operators to provide electricity 24/7.

"It's flexible. It's like a big battery of molten salt instead of lithium," said Fernando Gonzalez, CEO of Cerro Dominador.

Bottom line: Among CSP, molten-salt power towers have the greatest cost-reduction potential, thanks to higher operating temperatures and improved efficiencies, according to the International Renewable Energy Agency (IRENA).

Parabolic Dish Concentrator

Image of the Solarflux Focus dish.


Solarflux Focus: 10-kW prototype dish.
Pennsylvania, United States

Solarflux's prototype unit is a simplified, lower-cost version of a decades-old concept, operating at Pennsylvania State University's campus in Berks County. Polished aluminum petals cover a 14-square-meter aperture, beaming sunlight onto a receiver at the focal point. The Focus dish is mounted on a dual-axis tracking system, so it's always facing the sun. The receiver transfers heat to an engine or generator and, depending on the heat transfer fluid, can deliver thermal energy of up to 600 °C, the company claims. A 30-kilowatt, 42-square-meter aperture production unit is in development.

Bottom line: Focus may be best suited for "distributed thermal" applications, with one or dozens of dishes directly providing heat to factories, wastewater treatment plants, or desalination facilities, said Naoise Irwin, CEO of Solarflux.

Linear Fresnel

Image of the Fresnel project in China.

Zhang Xiaoliang/VCG/Getty Images

Lanzhou Dunhuang Dacheng: 50-MW Fresnel project.
Dunhuang, China

Built in an industrial park in Northwest China, this project began commercial operations in June 2020. Flat reflective panels are arranged like the stepped lenses of a lighthouse lamp (which typically also uses a Fresnel lens), concentrating sunlight on a loop of overhead pipes. Panels are oriented north-south to track the sun. In a first for commercial Fresnel projects, this installation uses molten salts—not thermal oil—in the pipes. The fluid heats to above 535 °C and flows to a steam turbine or into two molten-salt storage tanks, with a total thermal storage capacity of 15 hours.

Bottom line: Linear Fresnel hasn't yet scaled to the level of towers or troughs, partly due to lower power-cycle efficiencies and higher electricity production costs, says Ken Armijo, a mechanical engineer at Sandia, which previously operated a molten-salt Fresnel test loop facility in Albuquerque.

Parabolic Trough

Image of the Noor Energy parabolic trough system.

ACWA Power

Noor Energy 1: 600-MW total parabolic trough system + 100-MW power tower + 250-MW solar PV plant.
Dubai, United Arab Emirates

The massive US $4.4 billion complex includes three 200-MW parabolic trough arrays, the first of which will begin commissioning this year. Each unit includes 2,120 mirrored modules, which concentrate the sun's energy onto an absorber tube placed at each module's focal point. A low-viscosity oil in the absorber tubes rises to 393 °C, then flows into the power block. From there, the heat-transfer fluid is used to drive steam turbines, or it's sent to molten-salt thermal energy storage tanks—each with a 12-hour thermal storage capacity. When completed in late 2022, Noor Energy 1 will be the world's largest CSP project.

Bottom line: Parabolic trough systems are considered the most mature and (for now) lowest-cost CSP technology. In 2020, troughs made up two-thirds of the global installed CSP capacity.

The Conversation (3)
Christopher Aoki10 Apr, 2022

On the subject of thermal energy storage, here's an item of potential help

to the renewables industry from an unexpected source:

Article: "Nuclear research leads to breakthrough in grid-scale storage of solar and wind energy"


Seaborg Technology, a Danish molten salt reactor company, developed a method for

controlling corrosion problems associated with sodium hydroxide (a.k.a. "Drano")

as a molten salt. Recognizing that this technology could also be useful for storing

thermal energy in concentrated-solar systems, they spun it off as a separate

subsidiary company ("The name Hyme is a contraction of Hydroxide and to melt.").

Read the press release for more information.

Anjan Saha29 Oct, 2021

Concentrated Solar beam through concave

Reflective mirrors will increase solar insolation on PV panels, thus increasing the output of

SPV module. But thermal heating of SPV panels decreases the efficiency of Solar Cell Modules which need to be cooled by Cooling circuit.

Concentrated Solar Thermal power using proper Heat Exchanger like Acohol in the primary circuit and DM water in the secondary circuit can generate Steam for power plants

or for various processing industries like petrochemicals , food processing , dying also for

Hotel,Restaurant and Residential complexes .

The problem of killing birds by concentrated solar beam can be avoided by

Installation of Decoy Soldiers or fluttering Flags.

Considering the huge benefits of concentrated Solar beam for environmental benefits

and energy conservation, We should go for Solar with

little disadvantage.

Joshua Stern27 Oct, 2021

Tower is dangerous, birds get fried flying through beams they can't see, and it must also radiate a lot of heat, why don't they break it into a dozen local targets with much less intense beams, guide the beams to a common underground target that captures the heat radiated in the big one, or else just duplicate the storage/generation facilities in a dozen/systolic pieces? Similarly the trough idea seems the most scalable, but have to find the right scale factors. I think the radiative aspect is under-managed there, too.

The Inner Beauty of Basic Electronics

Open Circuits showcases the surprising complexity of passive components

5 min read
A photo of a high-stability film resistor with the letters "MIS" in yellow.
All photos by Eric Schlaepfer & Windell H. Oskay

Eric Schlaepfer was trying to fix a broken piece of test equipment when he came across the cause of the problem—a troubled tantalum capacitor. The component had somehow shorted out, and he wanted to know why. So he polished it down for a look inside. He never found the source of the short, but he and his collaborator, Windell H. Oskay, discovered something even better: a breathtaking hidden world inside electronics. What followed were hours and hours of polishing, cleaning, and photography that resulted in Open Circuits: The Inner Beauty of Electronic Components (No Starch Press, 2022), an excerpt of which follows. As the authors write, everything about these components is deliberately designed to meet specific technical needs, but that design leads to “accidental beauty: the emergent aesthetics of things you were never expected to see.”

From a book that spans the wide world of electronics, what we at IEEE Spectrum found surprisingly compelling were the insides of things we don’t spend much time thinking about, passive components. Transistors, LEDs, and other semiconductors may be where the action is, but the simple physics of resistors, capacitors, and inductors have their own sort of splendor.

High-Stability Film Resistor

A photo of a high-stability film resistor with the letters "MIS" in yellow.

All photos by Eric Schlaepfer & Windell H. Oskay

This high-stability film resistor, about 4 millimeters in diameter, is made in much the same way as its inexpensive carbon-film cousin, but with exacting precision. A ceramic rod is coated with a fine layer of resistive film (thin metal, metal oxide, or carbon) and then a perfectly uniform helical groove is machined into the film.

Instead of coating the resistor with an epoxy, it’s hermetically sealed in a lustrous little glass envelope. This makes the resistor more robust, ideal for specialized cases such as precision reference instrumentation, where long-term stability of the resistor is critical. The glass envelope provides better isolation against moisture and other environmental changes than standard coatings like epoxy.

15-Turn Trimmer Potentiometer

A photo of a blue chip
A photo of a blue chip on a circuit board.

It takes 15 rotations of an adjustment screw to move a 15-turn trimmer potentiometer from one end of its resistive range to the other. Circuits that need to be adjusted with fine resolution control use this type of trimmer pot instead of the single-turn variety.

The resistive element in this trimmer is a strip of cermet—a composite of ceramic and metal—silk-screened on a white ceramic substrate. Screen-printed metal links each end of the strip to the connecting wires. It’s a flattened, linear version of the horseshoe-shaped resistive element in single-turn trimmers.

Turning the adjustment screw moves a plastic slider along a track. The wiper is a spring finger, a spring-loaded metal contact, attached to the slider. It makes contact between a metal strip and the selected point on the strip of resistive film.

Ceramic Disc Capacitor

A cutaway of a Ceramic Disc Capacitor
A photo of a Ceramic Disc Capacitor

Capacitors are fundamental electronic components that store energy in the form of static electricity. They’re used in countless ways, including for bulk energy storage, to smooth out electronic signals, and as computer memory cells. The simplest capacitor consists of two parallel metal plates with a gap between them, but capacitors can take many forms so long as there are two conductive surfaces, called electrodes, separated by an insulator.

A ceramic disc capacitor is a low-cost capacitor that is frequently found in appliances and toys. Its insulator is a ceramic disc, and its two parallel plates are extremely thin metal coatings that are evaporated or sputtered onto the disc’s outer surfaces. Connecting wires are attached using solder, and the whole assembly is dipped into a porous coating material that dries hard and protects the capacitor from damage.

Film Capacitor

An image of a cut away of a capacitor
A photo of a green capacitor.

Film capacitors are frequently found in high-quality audio equipment, such as headphone amplifiers, record players, graphic equalizers, and radio tuners. Their key feature is that the dielectric material is a plastic film, such as polyester or polypropylene.

The metal electrodes of this film capacitor are vacuum-deposited on the surfaces of long strips of plastic film. After the leads are attached, the films are rolled up and dipped into an epoxy that binds the assembly together. Then the completed assembly is dipped in a tough outer coating and marked with its value.

Other types of film capacitors are made by stacking flat layers of metallized plastic film, rather than rolling up layers of film.

Dipped Tantalum Capacitor

A photo of a cutaway of a Dipped Tantalum Capacitor

At the core of this capacitor is a porous pellet of tantalum metal. The pellet is made from tantalum powder and sintered, or compressed at a high temperature, into a dense, spongelike solid.

Just like a kitchen sponge, the resulting pellet has a high surface area per unit volume. The pellet is then anodized, creating an insulating oxide layer with an equally high surface area. This process packs a lot of capacitance into a compact device, using spongelike geometry rather than the stacked or rolled layers that most other capacitors use.

The device’s positive terminal, or anode, is connected directly to the tantalum metal. The negative terminal, or cathode, is formed by a thin layer of conductive manganese dioxide coating the pellet.

Axial Inductor

An image of a cutaway of a Axial Inductor
A photo of a collection of cut wires

Inductors are fundamental electronic components that store energy in the form of a magnetic field. They’re used, for example, in some types of power supplies to convert between voltages by alternately storing and releasing energy. This energy-efficient design helps maximize the battery life of cellphones and other portable electronics.

Inductors typically consist of a coil of insulated wire wrapped around a core of magnetic material like iron or ferrite, a ceramic filled with iron oxide. Current flowing around the core produces a magnetic field that acts as a sort of flywheel for current, smoothing out changes in the current as it flows through the inductor.

This axial inductor has a number of turns of varnished copper wire wrapped around a ferrite form and soldered to copper leads on its two ends. It has several layers of protection: a clear varnish over the windings, a light-green coating around the solder joints, and a striking green outer coating to protect the whole component and provide a surface for the colorful stripes that indicate its inductance value.

Power Supply Transformer

A photo of a collection of cut wires
A photo of a yellow element on a circuit board.

This transformer has multiple sets of windings and is used in a power supply to create multiple output AC voltages from a single AC input such as a wall outlet.

The small wires nearer the center are “high impedance” turns of magnet wire. These windings carry a higher voltage but a lower current. They’re protected by several layers of tape, a copper-foil electrostatic shield, and more tape.

The outer “low impedance” windings are made with thicker insulated wire and fewer turns. They handle a lower voltage but a higher current.

All of the windings are wrapped around a black plastic bobbin. Two pieces of ferrite ceramic are bonded together to form the magnetic core at the heart of the transformer.

This article appears in the February 2023 print issue.