Last month CleanTechnica took a look at new high-temperature concentrating solar power research from Sandia National Laboratories, and we promised to follow up with a deeper dive into its “falling particle” approach. Well, here it is. Clifford Ho, a Sandia research engineer whose formal title is Distinguished Member of the Technological Staff, graciously lent us some time over the phone to provide his insights into the project.
We also digressed into a brief discussion of the feasibility of building a falling particle, high temperature concentrating solar power system on Mars. I know, right?! Just a digression! But, an interesting one. We’ll get to that in a bit, but first here’s the interview (edited for clarity and flow).
Why High Temperature Concentrating Solar Power?
Concentrating solar power had its share of skeptics until recent years, when the technology has really taken off. Once perceived as overly complicated and expensive, CSP systems have proven to operate efficiently even in less than optimal conditions.
Here in the US, the Energy Department made a point of showcasing its support for CSP during the Obama Administration. Things have quieted down a bit under the Trump Administration but funding continues for CSP research. The latest round of funding sent Sandia and other research institutions on a years-long effort to ramp the technology up into new levels of efficiency with high-temperature technology.
Considering that conventional CSP has already broken into the commercial market, one obvious question is why the Energy Department is interested in pursuing the technology farther. The answer is quite simple: to leverage more renewable energy into the grid. Here’s Mr. Ho:
The real value of concentrating solar power is energy storage, and we’re trying to address that gap in renewables.
Concentrating solar power systems generate heat first and foremost. That is the important value proposition.
With higher efficiency you can reduce the levelized cost of CSP. Higher temperatures will do that because they enable us to increase the efficiency of the power cycle.
Currently, CSP uses steam Rankine cycle. We’re looking at new power cycles that can be more efficient, but they need over 700 degrees C at the turbine inlet.
The working fluid of choice for CSP today is molten salt, so then the question is why not just configure your system to raise the temperature of the salt?
Molten salt decompose at 600 degrees. So now there is a large push, a lot of research into the next level, which is closed loop supercritical [aka liquid] CO2 Brayton cycle.
To get to that higher temperature, we’re looking at different media. There are three pathways. One is higher-temperature salts, but they are corrosive. Another is gas, but the challenge is storage.
Sandia is investigating the third pathway, which is solid particles. They have a large temperature range, they don’t freeze, and they can go to over 1,000 degrees C.
The general idea behind Sandia’s “falling particle” system is quite literal: Particles are heated as they fall through a beam of concentrated light. That requires some tweaks to conventional CSP systems:
The receiver would be different, and so would the storage tanks and other infrastructure. One main difference is that you would need a lift or some type of elevator.
One of the challenges that we’re facing is to increase the efficiency of the receiver, so we are looking at ways to control the fall and stability of the particles in the receiver.
Other than that, the basic process is similar: heating, storing, using, and then recycling the heat transfer medium.
As for balance of system costs, Ho sees some advantages:
The materials and the media are cheap, and this technology takes advantage of gravity. There is no pumping from the receiver, to storage, to the heat exchanger. It just uses a lift at the end.
Screw-type elevators are one option but there is a lot of frictional loss, so we will probably use a bucket elevator or a skip hoist. The challenge is how to lift high-temperature products.
Leveraging Fossil Fuels And Nukes For CSP
Speaking of skip hoists, one thing that came up in our conversation was technology sharing between renewable, fossil, and nuclear technology.
The flow of technology is bidirectional, so for example a falling particle receiver could be used in a system with fossil or nuclear energy as well as concentrating solar power.
On the CSP side, the benefit is that Sandia engineers can apply proven technology from other fields. In addition to the research knowledge base, Sandia can tap into a pool of industry partners.
“We’re not starting from square one,” Ho concluded.
The next step is a two-year design phase. For this phase the Energy Department selected three teams including the one spearheaded by Sandia. The winning team will get additional funding to build a test facility, so this is going to take a while.
What About Mars?
So, Mars. This came up when Ho mentioned that high temperature CSP is a hot topic in Saudi Arabia, which has lent King Saud University and the Saudi Electricity Company to the Sandia effort.
That brought up the use of sand, and that brings to mind NASA’s idea for 3-D printing habitats on Mars. The idea would be to transport printers and a binding agent to Mars and use local material — sand or dirt — for the main ingredient.
Now that we’re on Mars, the gravity situation could help resolve one of the main challenges for falling particle technology. On Earth, the particles fall too quickly for optimal results, so the lower gravity on Mars would be an easy tweak (Sandia is looking at Earthbound solutions for now).
Lower gravity would also help save energy on the lift part of the system.
On the other hand, solar irradiance is lower on Mars, and then there’s that thing about dust storms.
Our friends over at the First Seed Foundation suggest that CSP + storage could work, based on solar irradiance in parts of the Earth that are similar to Mars.
What do you think? Drop your thinks in the comment thread if you have another angle on that.
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Image via Sandia National Laboratories: In the falling-particle receiver, sand-like particles fall from a bucket-elevator hopper, at the top of the receiver tower, past the focused solar energy from the heliostat array. The hot particles are kept in the top tank and released into the middle one as energy is required for power generation. In the middle tank, thermal energy is extracted for the power-generation cycle (not shown). The now cooler thermal-storage particles are released from the bottom of the middle tank into the lower tank where the bucket elevator scoops them out to return them to the top of the receiver tower. The bucket elevator’s speed and hopper size are optimized to deliver a particle density to the central receiver focal point that can capture the maximum available concentrated solar energy.
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