In a video call from the US, SolarPACES caught up with Ranga Pitchumani, Editor-in-Chief of Solar Energy, a leading journal in the field.

Dr. Pitchumani was the Chief Scientist at the U.S. Department of Energy (DOE), SunShot Initiative, Founding Director of its Concentrating Solar Power (CSP) program, and Director of the Initiative’s Systems (Grid) Integration program. A former Ex-Co member of the IEA SolarPACES, he has also served as an advisor to several solar energy programs in different countries including the Australian Solar Thermal Research Initiative of ARENA. In his academic role, he is the George R. Goodson Endowed Chair Professor of Mechanical Engineering at Virginia Tech where he directs the Advanced Materials and Technologies Laboratory and has authored numerous scientific papers in the field of solar energy.

SK: How has CSP changed since you led the DOE SunShot program for the Obama administration?

RP: The pursuit of making CSP cost-competitive with grid parity, that began with the SunShot Initiative, has advanced quite well. There has been a systematic exploration of the various options or pathways for Generation 3 CSP namely the liquid pathway (using molten salt as the heat transfer fluid and storage medium), the gas pathway (directly capturing concentrated solar energy using supercritical CO2), and the solid pathway (using particles for direct capture, transport and storage of concentrated solar energy). Of these, the particle pathway has emerged as a viable approach to realizing high-temperature next-generation CSP. Sandia is leading the development of the next-generation particle receiver and CSP system. There’s tremendous momentum in particle-based CSP now.

Apart from the engineering marvel and challenges in this technology, there’s a lot of interesting fundamental physics that we need to understand and incorporate into our traditional analysis. Particles are a new domain for CSP and likewise high-temperature Gen3 CSP brings unique challenges to particle flow, heat transfer, and interactions with confinement surfaces. So, it is an important learning and discovery exercise as we develop the technology driven by the science.

And we are just starting to “scratch the surface” of the problem.

SK: Literally, in some of your work..

RP: I have always argued that CSP is an amalgam of multiple disciplines, including physics, optics, chemistry, chemical engineering, mechanical engineering, materials science – and with particles also high temperature tribology. Now, more than ever, is a fertile opportunity for the broader community to engage in advancing this fantastic technology.

SK: Yes, it really is astonishing to me how many other disciplines are involved in particle CSP. How best to hoist sand up to the top of a tower is not what somebody imagines they’ll figure out when they get into concentrated solar research, right?

RP: Absolutely, absolutely. And to me, that makes it interesting and fun, because it keeps me young, because I am always needing to learn new things.

SK: Do your Virginia Tech students come from many kinds of engineering backgrounds?

RP: In my lab we have quite a mix. I have physicists, chemical engineers, mechanical engineers, electrical engineers, and materials scientists. It’s just getting the brightest minds to work on problems. It’s wonderful. My research group is passionate, whether I’m there or not. I mean, they’re not doing it because their supervisor gave them this problem. They’re doing it because they love it. And so they go above and beyond. And most of the time I’m learning from them.

It’s really a fantastic way to do work, because you excite them, you give them the broad picture, and say, here’s a big problem to solve. And they’re off. They’re not asking me what to do next. They are in the driver’s seat, and my role is to fuel their curiosity by asking the critical questions. It’s an extremely fun exchange. Quite unlike a classroom setting where the professor knows the answer to the problem and is testing to see if the student can arrive at the answer. Research is more of a two-way exchange. Very often, I am the student learning from them and discovering with them. These are the thought leaders, entrepreneurs and change makers of the future. I simply have the privilege of having them in my group now.

SK: That is great. It sounds really gratifying. So last time I covered your work, it was on coatings, if I recall. Has your focus always been on these physical surfaces?

RP: The nexus of materials and energy systems has certainly been one of the focus areas of the research in my lab. In our last conversation, we spoke about our work on surface innovations to tame corrosion attack of substrate alloys by molten chloride and carbonate salts at the Gen3 CSP relevant high temperatures. Following up on that, we showed through a rigorous technoeconomic analysis the impressive levelized cost reductions of over 60% compared to Haynes 230, enabled by the corrosion mitigation coatings on low-cost ferrous materials such as stainless steel, while offering corrosion protection on par with or better than the expensive Haynes alloy. So, with these coatings we developed, we can now use common materials like stainless steel, and they can withstand the worst of corrosive elements, such as molten chloride salts, at Gen3 CSP conditions.

We also spoke about our innovation on high temperature solar absorber coatings that feature very high solar absorptance with a low emittance that reduces re-radiation losses from the receiver, overall resulting in an exceptional efficiency for Gen3 systems. We have since subjected the coatings to several rounds of independent testing on sun at Sandia National Laboratories and the National Laboratory of the Rockies (previously, NREL). The coatings continue to amaze us by retaining their excellent optical properties with flying colors—or in spectral parlance, “flying wavelengths”—under on-sun conditions.

Aside from the surface sciences and engineering, the research projects in my laboratory include grid integration of solar energy, solar forecasting under uncertainty, and thermal and thermochemical energy storage.

SK: I’ll add the links to these when I post this. And your papers (below) advance particle-based heat exchangers for CSP. What are the unique challenges in designing heat exchangers for a falling particle system?

RP: One of the things that’s unique about particles, as opposed to the other fluids, is that when they interact with the surfaces on which they move, they introduce a different type of degradation than, say, molten salts and others. And this is really pertaining to the wear on the materials as particles interact with the surfaces. There are two ways that particles interact with a surface. One is when particles impinge on the surface and cause what’s called erosion, or erosive wear. The other is when a bed of particles slide on the surface, and they create what’s called abrasive wear. At high temperatures there’s also oxidation happening on the surfaces at the same time, that gets vigorous as temperature increases. All of these mechanisms occurring simultaneously need to be understood to predict how materials that interact with the particles may degrade or lose mass over time.

SK: Can you adapt abrasion lessons from other industries or are they too different to CSP?

RP: There’s a whole field – tribology – focused on wear mechanisms, but they were focused on particles interactions at low temperatures, room temperature or slightly higher, not at the temperatures that we see and expect to see in next generation CSP. So, there are unique challenges to abrasion in high temperature CSP, and there was a science gap that existed when we started looking at this problem. The question is how the interactions of particles, maybe carbo beads or silica sand, with the various components of the falling particle, CSP system influence material degradation at high temperatures.

A lot of work has been done on the solar receiver component. But there is a whole slew of other components. How the particles interact with the transfer chutes, with the valves and storage bins, and with the surfaces in the heat exchanger? So that’s kind of how we got interested in this problem; finding the wear mechanisms and the wear behavior of the surfaces, as particles are interacting with the various sub-components of the CSP system.

SK: How do you simultaneously maximize heat transfer and minimize abrasion?

RP: We studied the particle s-CO2 shell and tube heat exchanger configuration, and quantified both the heat transfer performance and the surface wear simultaneously. This is the first study of its kind looking at the two aspects together. The study brings forth the tradeoffs: designs and conditions that provide for good heat transfer are not necessarily the ones that are benign to erosion or abrasion wear. For example, you may want close packing of the heat transfer surfaces between which particles flow, as that would be good for heat transfer, but that’s also terrible for abrasion, because the particles are trying to squeeze in through the narrow gaps between surfaces, and rub against the surface and abrade more. The question then is, how do we develop designs that trade off between these physical mechanisms through fundamentally understanding the mechanisms.

To answer the question, we studied the heat transfer and abrasion characteristics quite comprehensively and developed trade-off maps so that designers can select tube layouts that achieve the desired heat transfer while keeping abrasion within whatever their desired limit is. What makes this a first is the coupled thermal and abrasion study for heat exchangers. Heat exchanger analysis is well established and can be found in textbooks, but they are based on fluids, liquids and gases flowing through the passages of the heat exchanger. Abrasion and erosion are at the heart of the field of tribology, but its confluence with fundamental heat transfer analysis, heat exchanger analysis, and in the context of a particle medium, is where the gaps, challenges and opportunities are. Our studies are filling this gap for the engineering community.

SK: So as Gen-3 CSP develops you want to clear any issues in advance?

RP: Right. We are trying to engineer this system without a full understanding of the science behind it. That’s really where our work comes in. We want to put out the science so that anybody can use it, and we make it in a way so that it’s not solving just one problem, but we give a design map that anybody can use for their material, for their system, and so forth.

SK: Which metal is it?

RP: We are studying different alloys, stainless steel, Inconel alloys, Haynes, and ceramics such as silicon carbide, to name a few. And usually the wear rate is a function of material properties such as hardness. So, we try to present results in a general form so that the results can be translated across materials.

SK: Could you make a harder metal?

RP: Yes that’s a whole field in metallurgy, how do you harden surfaces and materials? The other way to harden materials, and make them immune to abrasive or erosive wear, is with coatings. With a carbide coating, for example, you can bump up the surface hardness by a factor of five, ten, or more. Correspondingly, the wear rate can be diminished by about that factor. But then the question is, what is the cost trade-off? Is the cost of coatings worth the reduction they provide, or can we alter the heat exchanger design or its configuration, so that we may not need this coating, but can still reduce the wear? Another factor that comes into play are the oxides that grow on the surface at high temperatures, which may be beneficial in reducing wear, depending on the competing rates of oxidation and abrasion or erosion. Understanding all of these considerations is the crux of cost-effective and viable engineering for particle-based Gen3 CSP.

SK: Might this happy medium negate the potential advantage of using high temperature particles and s-CO2 cycle?

RP: The idea is not to degrade performance. Heat transfer is often the most important part of the thermal system. But within a desired heat transfer range, what is the abrasion or erosion wear for those designs? Then if I tweak the heat exchanger performance a little bit, am I saving a lot on material wear? So you can do the tradeoffs of heat exchanger efficiency – giving up a little bit on the efficiency – if that amounts to considerable increase in component life. Or you may say, material wear is paramount. If you want the material wear to be less than say 10 microns a year, you can determine the upper bound on the heat exchanger performance for this constraint. Either way it’s information for the designer to say, which way should I go? Our experiments and modeling are aimed toward developing happens when the particles interact with the surfaces at high temperatures in a heat exchanger and in other components.

SK: Right. Gen-3 particle CSP has such high temperatures, even up to 1500°C.

RP: As I mentioned before, one of the consequences of the high temperatures is oxidation. The surface grows an oxide layer, and so the particles are now interacting not with the nascent surface, but with the oxide layer on top. Now, there are many fundamental questions. What does the oxide layer do? Does it help to kind of shield the underlying surface from the particles? Does it make it worse? Does it crack and go away, all kinds of things can happen. So we are deliberately uncovering the physics of what happens at high temperatures to the layers. And it is fun part to be working on the problem, both from an experimental side and the computational modeling side. We have very detailed experimental data and very insightful simulations of the oxide layer growth with simultaneous abrasion that explain the data, which we’ll publish soon.

It’s a really fantastic domain of problems. We’re marching along trying to understand as we go and at the end we’ll have really nice portfolio of science that we have discovered, but also solutions enabled by the science. The more we approach the problem with a systematic focus, rather than a hurried “let’s put it together and see” approach, the more we can make advances towards the final goal of achieving cost competitive CSP. CSP is an awesome technology, perhaps less appreciated, whether it generates firm power or heat. And heat – generated cleanly – is very, very important for many applications.

SK: So if you were to predict the future, would you say that particle CSP with the s-CO2 Brayton loop is the future? That tiny little turbine is really a big, major change. I remember seeing the turbine at Crescent Dunes, it was like a 747. How could you make cheap power with something that gigantic?

RP: The large power block that you saw is similar to that in fossil powered plants that use steam for power generation. Steam cycles are at their limit of efficiency and pushing the efficiency higher quickly becomes cost prohibitive. s-CO2 based Brayton cycle offers the efficiencies needed for cost-competitive CSP. The genesis of a concerted development of s-CO2 cycle components, the tiny turbines, the compressors, and other subsystems dates back to SunShot. Since then a lot of progress has been made in addressing challenges in terms of materials, being able to handle the high pressures and so forth, and we are advancing closer to viable commercial system. And since a power block is something that is shared with conventional power generation, it really benefits multiple technologies: fossil and nuclear in addition to solar thermal. In reality, the first adopters could be fossil because there’s so many of them.

SK: But why should the fossil industry get to benefit from all the work done by concentrated solar researchers?

RP: That’s one way to see it. I actually see it a different way. If you can reduce the cost of the power block through the larger deployment opportunities in fossil or nuclear, that ultimately benefits CSP. So the more the deployment opportunities in other thermal plants, the more CSP benefits from that lower price. What SunShot brought was a sense of purpose, direction and urgency, in terms of what we need to reach the efficiency target for CSP, and what does the s-CO2 cycle have to look like? Without a purpose, if you just say, oh, this cycle looks like a good idea, let’s just explore, and see what we can develop – then you don’t know where you’re going, or how far you need to go. And all that changed with the DOE Gen-3 solar program that pushed for the s-CO2 Brayton cycle with particles for heat transfer.

Papers:

Analysis and design of a particle heat exchanger for falling particle concentrating solar power

Analysis of abrasion wear in particle storage and valve subsystem for falling particle concentrating solar power

Analysis and mitigation of erosion wear of transfer ducts in a falling particle CSP system

Erosion wear analysis of heat exchange surfaces in a falling particle-based concentrating solar power system

Analysis of erosion of surfaces in falling particle concentrating solar power

Effects of Thermally Grown Oxides on Erosion Wear of Surfaces at High Temperature for Falling Particle Concentrating Solar Power

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A new paper from researchers at PROMES-CNRS in France presents a control strategy to reliably deliver a precise thermal power from concentrated solar to meet industrial heat requirements.

The objective was to design a proof-of-concept algorithm for operating a concentrated solar plant, but one intended primarily to deliver heat, and deliver it from thermal energy storage tanks rather than from each moment’s sunlight, as in a typical CSP power plant.

“There are lots of studies on the control of solar plants, but most of them are on CSP plants, with only power production objectives. So you can find lots of references trying to, for example, use an optimization algorithm to maximize the profit you can make with such a plant. But this is not our objective. Our objective was to satisfy a heat demand from industry.” said lead author Eliott Girard in a call from France.

To do this, the algorithm would control the flow rate of the hot storage liquid to deliver a required thermal power throughout the day.

“We are focusing on operational constraints, such as what happens when you have a larger DNI drop, on a smaller time scale, to determine the best mass flow rate in the plant over a short time horizon,” Girard explained. “This is what’s new about our study.”

The setup would comprise a solar field of parabolic trough collectors, delivering oil heated by focused sunlight to a single thermocline thermal energy storage tank, so that heat would primarily be drawn from the storage tank.

A thermocline storage with particles in oil

In thermocline storage, both the heated and cooled heat-transfer fluid is stored in a single tank. This creates a region of a mixed temperature (a ‘thermocline’) in the middle.

When this thermocline region can be minimized, it reduces costs to store both temperatures in a single tank. The solar-heated oil enters at the top, and once the heat is extracted by a heat exchanger in the middle, the now-cooler liquid sinks to the bottom. This cooler liquid is then routed back to the parabolic trough collector field for reheating by focused sunlight.

The study assumed that the liquid would be the thermal oil, the standard in commercial trough-type CSP plants. Small pebble-like particles inside a basket in the oil would store additional heat.

“We created this algorithm on the assumption that we would have particles in the oil,” Girard explained.

“The advantage of this is that the solid particles increase their storage capacity. And also, solid particles enhance stratification at the thermocline. So you can have a clear thermocline zone, a clear separation between hot and cold. That’s also why this tank has that high capacity, because there are lots of particles which also store some energy.”

A practical solution for many industrial heat uses

The plant would use the control system to ensure the correct amount of heat is delivered by adjusting the flow rate of the heat-transfer fluid through the system. It is possible to increase or slow the flow rate by adjusting the pumps.

The team tested the setup under three scenarios: steady heat needs, on-and-off heat needs, and slowly changing heat needs (similar to what’s needed in paper processing, for example).

The effectiveness of the control algorithm was judged based on two factors:
How much the delivered heat deviated from the target and how much the system overshoots the target when responding to changes.

The team tested three heat-demand scenarios, all simulated under clear-sky, overcast and mixed conditions.

Constant demand: Steady 30 kW thermal power throughout.
Batch demand: Alternates between one hour at 60 kW and one hour with no heat required.
Realistic demand: Gradual changes typical of a paper treatment process during the day.

In all cases, the control system did a good job keeping to the target, with only minor differences from what was required and not much “overshoot” (where the system briefly gives too much heat). The system handled both slow and rapid changes in heat demand and incident solar energy reliably, suggesting it would perform well in other real-world situations.

High ratio of storage to solar field

When concentrated solar power is used primarily to supply a storage system, it should have relatively more storage capacity than the solar field, but determining how much is a work in progress.

Because it would primarily draw on stored solar thermal energy rather than on the sunlight incident on the solar collectors themselves, the prototype plant simulated in the study would have a higher solar ratio than conventional CSP plants. A small 150 kW solar field of parabolic trough collectors would be paired with 1,100 kW of thermal storage, a much higher ratio than in a CSP plant generating power.

“But this might be too extreme a ratio,” Girard considered. “I think the storage tank is actually too oversized in relation to the collectors.”

Next, Girard will be revisiting the effect of the storage tank size relative to the solar field.

The case study is based on a real plant at the PROMES laboratory.
But he emphasized that what is really needed at this point is physical validation:

“There are very few studies concerning the control of CST plants that are based on a physical plant; it’s mostly simulation. Physical validation of the control strategies is lacking. They almost always remain as only validated by simulations, but they need to really implement control strategies,” he said.

Read the Paper; Control of a concentrated solar plant for heat production under various thermal demand and the SolarPACES 2025 Conference Presentation: Model-Based Predictive Control of a Concentrated Solar Plant for Heat Production 

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Solar researchers at Fraunhofer Chile have designed, built, and tested a complete solar collector and crucible system for melting and recycling aluminum, heated by Annular Fresnel solar collectors. In Annular Fresnel, each mirror is flat, like a Fresnel mirror, but arranged in a circle, enabling the very high concentration possible with Dish solar concentrators.

The Annular Fresnel concentrators they had manufactured locally are sliced from silver-coated acrylic sheets. The team built a full system, including a complete crucible furnace and a handling system, which has been operated through multiple aluminum melt/pour cycles under real DNI conditions in Chile for this application‑specific innovation and engineering demonstrator.

They showed that their setup, designed explicitly for aluminum recycling, will operate at the required high temperature at low cost and on a practical scale. As they revealed in a well-received presentation, Fresnel Solar Furnace for Aluminum Melting, and paper at the 2025 SolarPACES Conference in Spain, the team has successfully cast branded ingots during tests.

“We bypassed the standard laboratory phase and moved directly to testing in a relevant environment,” said lead author Pablo Castillo, in a call from Chile.

“It involved significant trial and error—specifically, determining exactly when the aluminum was molten by relying solely on the furnace’s temperature sensors, without visual confirmation.”

Local manufacturing limitations led to their choice of Annular Fresnel concentrators

Annular Fresnel concentrators are a cross between linear Fresnel reflectors – long lines of flat mirrors – and the highly concentrating solar Dish, in which each little mirror is curved. Annular Fresnel concentrators have the high-concentration advantage of Dish collectors but are simpler to manufacture because they consist of flat mirrors arranged in concentric reflecting rings. In this case, the mirror surface was achieved with silver-plated acrylic plastic.

“Our challenge was achieving this level of concentration using Chile’s local manufacturing capabilities,” Castillo explained.

“We hit a wall realizing we couldn’t manufacture a three-meter parabolic dish locally. Instead, we adopted a simpler design based on the linear Fresnel concept: 13 concentric rings on a single plane with varying slopes. We were pleasantly surprised to find that, even with local limitations, we could successfully achieve these complex geometries.”

Because Fresnel systems are typically linear, they need only single-axis tracking. However, by combining the low-cost Fresnel flat mirrors into a circular Dish-like configuration, the tracking system requirement also changes. The team innovated a slewing drive system for dual-axis tracking for their concentrator to accommodate the change.

Why Chilean aluminum recycling

With its world-leading DNI, Chile has vast regions where this simple solar technology for aluminum melting can recycle aluminum and other metals with similar melting points. And increasingly, Chilean policy requires metal recycling.

Currently, aluminum is exported to Brazil for recycling. The Fraunhofer team believes that, by building a deployable system for large cities in Northern Chile, where there is abundant solar resource, they can recycle aluminum using solar energy domestically and then sell it to companies looking for aluminum with a zero-carbon footprint.

“We have seen significant interest from major multinationals in the food and beverage sector who are pursuing circular economy goals for their packaging,” said Castillo.

“Our industrial partner, who currently exports scrap aluminum, noted a shifting international demand toward ‘green aluminum’ produced with near-zero carbon emissions. We are working together to develop this technology for coastal cities with major ports, as well as more populated areas in the Central Valley of Chile.”

What the experiment proved

The team proved that this simple-to-manufacture annular mirror geometry, combined with their practical, field‑tested aluminum recycling furnace and control system, is a feasible, low-cost route to industrial aluminum recycling.

Designed to maintain the crucible strictly within the optimal processing range of 520–640°C for repeated aluminum pours, their system utilizes a unique ring‑mirror field on a compact slewing drive. With full two‑axis tracking and custom controls, the setup successfully managed the melt-pour cycles using relatively modest, locally manufactured equipment compared to the giant solar furnaces typically used for research, such as those at Odeillo.

In 21 outdoor trials conducted between November 2024 and May 2025, the team established a reliable industrial workflow. On a representative high-performance day (April 8, 2025), the system achieved a maximum daily melt of 1.71 kg under a median DNI of 902 W/m², successfully executing multiple pour-and-recharge cycles. The results have been validated via a six-node thermodynamic model, confirming the system’s potential for scaling.

The system integrates custom electronics, software, and a sun-tracking mechanism for azimuth and elevation, installed at Parque Caren in Santiago, Chile (33.4351°S, 70.8457°W). Iterative prototyping optimized optical and thermodynamic performance, with the furnace heating a crucible to melt aluminum and pour it into ingot molds.

Next steps

To initially test the concept, it was most straightforward to position the furnace at a focal point above the dish. But in production, for practical reasons, the melting pour would need to be on the ground.

“Manipulating a furnace at 800°C and pouring molten liquid three meters above the ground poses significant safety risks,” he said.

To avoid this high-temperature operation at the natural sky-high focal point, they would add a secondary beam-down reflector, so that the furnace and the molten pour can be stable at ground level.

“While a beam-down reflector might incur slight optical energy loss, we gain stability and reduce exposure to wind and dust. So we believe we can achieve a higher output,” Castillo pointed out.

“We have already developed a ‘2.0’ design where the furnace remains stationary at ground level. Having proven the technology is feasible, our next steps are to refine the design, lower costs, and increase output.”

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