Sandia Laboratories, USA, recently published a paper on the heat transfer cost analysis of using sand particles such as bauxite as heat transfer medium to transfer heat at temperatures up to 1500°C in tower solar thermal (CSP) power generation technology.
The subject is one of the first to fully investigate integrated component systems such as jacks and conveyors for transporting sand. The study found that moving a large number of hot particles is feasible, but requires multiple elevators or several conveyor lines, and the levelized cost of electricity for the entire system can be as high as 1 cent/kWh.
Researchers’ design and technoeconomic analysis of high-temperature particle delivery components in 100 MWe solar thermal power projects recommends that system designers use a gravity-based approach wherever possible to meet DOE’s 5 cents/kW by 2030 target LCOE overall goals at the time.
As far as the new generation of tower CSP technology is concerned, it is important to study the sand medium used to collect and transfer solar heat. Since most ceramic particles can withstand temperatures up to 1500°C and beyond, this not only makes electricity production more efficient, but also provides a direct replacement for fossil fuels in industries that are difficult to decarbonize. High temperatures are often required in cement and steel production, making this technology uniquely important for achieving overall CO2 emissions targets.
In this technology, sand particles fall from an open heat sink after being heated by concentrated sunlight reflected by thousands of mirrors in the solar field at the top of the tower. Using gravity, the sand particles fall from the heat sink to the heat storage tank. It falls through the heat exchanger into a cold tank at the bottom and returns to the tower for reheating to restore circulation.
However, carrying so much sand also presented engineering challenges, not only raising the tower 260 meters above the top of the heat sink, but also transporting it horizontally where it could fall. So one of the key considerations is how to move so many grains of sand. If this solar technology using sand as a heat transfer and storage medium were to be used in commercial projects to replace fossil fuel heat on a scale of around 100 – 150 MWe, this would involve more than 50,000 tonnes of continuously circulating sand. And almost 3 tons of sand will free fall for about 5 seconds at any one time, hitting the reactor below at 60 meters per second.
In this new study, Sandia researchers used their experience with mining hoists to identify technology readiness levels, performance constraints, capital and operating costs, and Expected heat loss.
«We looked at horizontal delivery,» noted lead author Jeremy Sment. «How much does it cost to move the sand grains horizontally to the next component? How much does it cost to hold 50 million kilograms of grains at this temperature and lift them to the top of a 250-meter tower. I have worked with crane experts, Working with the conveyor manufacturer and the silo design engineer, we concluded that it was entirely possible to meet SETO’s electricity cost targets.
He added: «We have also investigated tower-integrated systems that move particles by gravity, bypassing the conveying machinery needed to move particles between components. We have developed a tower-integrated storage model that shows the need for How much material to support the particles. A rough estimate would be $9 million to $13 million for additional concrete, additional steel bars, etc. for vertically integrated systems.”
Of course, this design is expected to reduce LCOE. However, solar towers with large amounts of storage space have never been built before, which requires more work to quantify the additional reinforcement to support the full weight of the particles contained in the tower.
While simple chutes intuitively seem to be the cheapest option for unloading sand, they simply rely on gravity to transport the sand. But Sment found that if the extra height of the tower was needed to support the chute from the skip to the heat sink, the cost of building the additional tower was actually not a good option.
The paper says that the heat sink for the pellet system may need to be taller to accommodate the overturning radius of the jump, the discharge box, the sloping piping system of the heat sink, and the extra height of the pellet stack enough to cover the width of the heat sink.
«You have to raise the top of the tower high enough to install a hopper on top of the heat sink that can flow the particles into the heat sink,» Sment said. “It’s expensive steel work. So once these inclinations get so high that the tower walls have to extend, mechanical conveyors start to be a low-cost option for pellet handling.
«The results of the earlier papers were a bit frustrating,» he admits. «We had to recalculate with a few different ideas that didn’t rely on increasing the height of the tower, which I think is more suited to the 6-cent to 5-cent target.
A better approach might be to hop inside the tower and dump its load of pellets directly into a box above the heat sink.
He explained: “If you don’t need horizontal conveying in the tower, because the jumps can be designed to discharge directly into the heat sink. But I don’t know if you want to use chutes or use conveyors. These all require a good understanding of the cost of the heat sink tower. to actually make that decision.”
A thorough investigation of commercialization options indicates that the thermal storage tank must be above the heat exchanger.
«We’ve considered raising the energy storage above the heat exchanger,» Sment said. “Because it’s too hot right now to move particles at 800 to 1000°C. Without expensive metal alloys, anything over 640°C can be prohibitive because we don’t want conveyor cooling systems or open ventilation from Heat is absorbed in the pellets, so they are limited by the softening point of the steel. By raising the storage tank, it can be discharged directly into the heat exchanger, which reduces the heated pellets to about 500°C, where they can be safely moved to the pellet conveyor.
To carry out this horizontal conveyance at the bottom of the tower after the pellets have cooled, the patented Eco Belt technology of Magaldi, an Italian company, will be able to handle pellets at about 640°C with a capacity of up to 1,800 tons per hour.
“We’ll number them all at the same time to get the capacity we need. For about $7 million in equipment, we can use 2 conveyors to move 1,000 kg/sec of pellets from under the heat exchanger to the cold tank.” he pointed out.
Sment stressed that this techno-economic comparison of real-world costs is a work in progress. For example, using two jump hoists to lift sand would exceed DOE cost targets. However, a jump hoist would approach the weight limit of mining industry precedent.
In addition to numbering the delivery system, there are a number of system design factors that affect flow rate; this includes particle type, particle temperature, and thermal energy storage capacity. These changes may affect receiver or heat exchanger selection, or may create engineering rework in related components that may affect optimal performance, requiring more system analysis.
There are many uncertainties that require more research and development, but this gives particle-based CSP developers a sense of the costs and limitations of particle recycling systems.
