Solar Energy

DLR testing the use of molten salt in a solar power plant in Portugal

Published

3 years ago

October 26, 2021

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DLR testing the use of molten salt in a solar power plant in Portugal

Engineers from the German Aerospace Center
have taken an important step towards using molten salt as a heat transfer medium in parabolic trough solar power plants. Together with the University of Evora and industrial partners, a team from the DLR Institute of Solar Research has for the first time begun operating the solar field of the Evora parabolic trough test plant in Portugal with molten salt.

This innovative technology is helping to further reduce the costs of operating solar thermal power plants. With their integrated storage systems, solar thermal power plants are the only technology able to generate large amounts of power from solar energy around the clock.

Current state-of-the-art commercial parabolic trough power plants use a special thermal oil as the heat transfer medium. The oil absorbs concentrated solar radiation collected using mirrors, converts it into heat and transfers it via pipelines to a heat storage unit or a steam turbine to generate electricity. The heat storage tank, filled with molten salt, can hold the thermal energy at temperatures of up to 560 degrees Celsius for a period of 12 hours and release it again when the demand for electricity increases.

The power plant needs heat exchangers to transfer the heat from the oil to the salt in the storage tank, but some energy is always lost during this transfer before it can later be converted into electricity. The maximum possible operating temperature of the oil used is approximately 400 degrees Celsius, which limits the efficiency of the energy conversion. Researchers and industry are therefore looking for ways to further increase the temperatures in solar power plants in order to lower the costs of electricity generation.

One promising way to raise temperatures in parabolic trough power plants is to use molten salt not only as a heat storage medium, but also as the heat transfer medium in the collector field. Depending on the composition of the molten salt, it can withstand significantly higher temperatures than thermal oil – up to 565 degrees Celsius. Another advantage is that the storage tanks can be filled directly with molten salt from the solar field – eliminating the need for a heat exchanger.

In order to demonstrate this approach, the DLR Institute of Solar Research, together with the University of Evora and companies from Germany and Spain, has been building a solar parabolic trough test facility using molten salt as its heat transfer medium. The work started in 2016 and has taken place as part of the High Performance Solar 2 (HPS2) research project, which is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi). The aim of the project is to demonstrate that parabolic trough power plants can be operated safely and economically with molten.

A technical challenge when using molten salt as a heat transfer fluid is that heating of all the pipelines is necessary. To prevent the molten salt from solidifying as the plant is filled, electrical trace heating must be used to preheat all salt-carrying components.

Successful initial filling and test operation of the system at 300 degrees Celsius

The collector modules of the HelioTrough 2.0 generator from project partner TSK Flagsol, which are now filled with molten salt and connected to each other, provide a total thermal output of up to 3.5 megawatts across a total length of 684 metres.

Currently, the plant operates with a ternary salt mixture from the project partner Yara, which has the advantage of a lower melting temperature compared to a binary salt solar salt mixture and can absorb heat up to a temperature of approximately 500 degrees Celsius. In addition to its use in solar thermal power plants for electricity generation, this salt mixture is also of interest for solar process heat supply systems.

Starting from an initial temperature of 300 degrees Celsius, the engineers want to gradually increase the operating temperature up to 500 degrees Celsius. In the coming weeks, the other components of the salt circuit will be brought into operation in Evora. In addition to the two-tank storage system, this includes the steam generator and the measurement equipment.

“We are very satisfied with the way the first filling went. Our next goals are to gain operating experience, fill all further components with molten salt step by step and test regular operations and also critical operating scenarios,” says Jana Stengler, head of the Fluid Systems Group at the DLR Institute of Solar Research, on the results of the initial testing.

The HPS2 plant is designed to also be operated with solar salt, a mixture of potassium nitrate and sodium nitrate, to achieve even higher temperatures of up to 565 degrees Celsius. Higher temperatures in the solar field allow for higher efficiencies in the conversion of solar energy into heat and heat into electricity, which lowers the cost of generating electricity.

“Power plants using the technology from HPS2 can be built more easily and operate more efficiently. This reduces electricity production costs by up to 10 percent,” says Mark Schmitz from the project partner TSK Flagsol, underlining the importance of the project for future solar thermal power generation. “That is an enormous step for a single technical change. At the same time, it makes longer storage durations of 12 full-load hours and more economically achievable.”

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Solar Energy

3D-printed microstructure forest enhances solar steam desalination

Published

3 days ago

July 24, 2024

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3D-printed microstructure forest enhances solar steam desalination

by Clarence Oxford

Los Angeles CA (SPX) Jul 24, 2024

To address the global freshwater scarcity issue, researchers in Singapore have developed advanced solar steam generators (SSGs) for seawater desalination. This method, powered by renewable energy, mimics the natural water cycle by using solar energy to evaporate and purify water, offering a potentially cost-effective solution compared to traditional, energy-intensive desalination techniques. However, current SSG technologies face limitations due to the complexity of fabricating designs that maximize surface area for optimal water evaporation.

Drawing inspiration from nature, the team utilized 3D printing to create innovative SSGs. Their findings, published in Applied Physics Reviews, highlight a novel technique for manufacturing efficient SSGs and introduce a groundbreaking method for printing functional nanocomposites using multi-jet fusion (MJF).

“We created SSGs with exceptional photothermal performance and self-cleaning properties,” said Kun Zhou, a professor of mechanical engineering at Nanyang Technological University. “Using a treelike porous structure significantly enhances water evaporation rates and ensures continuous operation by preventing salt accumulation – its performance remains relatively stable even after prolonged testing.”

The technology works by converting light to thermal energy, where SSGs absorb solar energy and convert it to heat to evaporate water. The porous structure of the SSGs aids in self-cleaning by removing accumulated salt, ensuring sustained desalination performance.

“By using an effective photothermal fusing agent, MJF printing technology can rapidly create parts with intricate designs,” Zhou added. “To improve the photothermal conversion efficiency of fusing agents and printed parts, we developed a novel type of fusing agent derived from metal-organic frameworks.”

The SSGs feature miniature tree-shaped microstructures that mimic plant transpiration, forming an efficient, heat-distributing forest.

“Our bioinspired design increases the surface area of the SSG,” Zhou explained. “Using a treelike design increases the surface area of the SSG, which enhances the water transport and boosts evaporation efficiency.”

In both simulated environments and field trials, the SSGs exhibited a high rate of water evaporation. The desalinated water consistently met drinking water standards, even after extended testing.

“This demonstrates the practicality and efficiency of our approach,” Zhou said. “And it can be quickly and easily mass-produced via MJF commercial printers.”

The team’s work shows significant potential for tackling freshwater scarcity.

“Our SSGs can be used in regions with limited access to freshwater to provide a sustainable and efficient desalination solution,” said Zhou. “Beyond desalination, it can be adapted for other applications that require efficient solar energy conversion and water purification.”

Research Report:3D printing of bio-inspired porous polymeric solar steam generators for efficient and sustainable desalination

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Solar Energy

Renewables overproduction turns electricity prices negative

Published

3 days ago

July 24, 2024

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Renewables overproduction turns electricity prices negative

By Nathalie Alonso and Catherine Hours

Paris (AFP) July 24, 2024

With the proliferation of solar panels and wind turbines an unusual phenomenon is becoming more and more frequent: wholesale electricity prices turn negative.

While that may brighten the mood of consumers whose power bills have surged in recent years, it could undermine the further development of renewables, a key element in the fight against global warming.

The increasingly frequent phenomenon is “extremely problematic” for the wind and solar sector, said Mattias Vandenbulcke, strategy director of the renewables industry group France Renouvelables.

“It allows some to have harmful, even dangerous rhetoric which says ‘renewables are useless’,” Vandenbulcke said.

In southern Australia, wholesale electricity prices have been negative some 20 percent of the time since last year, according to the International Energy Agency.

The share of negatively priced hours in southern California was above 20 percent in the first half of the year, more than triple from the same period in 2023, the IEA said.

In the first six months of the year in France, there were negative prices around five percent of the time, beating the record set last year, according to the electricity grid operator RTE.

In Switzerland the price tumbled as far as -400 euros (-$436) per megawatt hour on July 14. The lowest prices are usually recorded around midday during the summer when solar production is at its peak.

– ‘A warning signal’ –

The trend has been accelerating for the past three years as demand in Europe has unexpectedly dropped since the Covid pandemic and the war in Ukraine.

Prices turn negative on the spot wholesale electricity market when production is strong while demand is weak.

Around a fifth of the total is traded on this market, where electricity is bought for the following day.

Negative prices help reduce the bills of consumers, said Rebecca Aron, head of electricity markets at French renewables firm Valorem, but the impact is delayed and difficult to discern among the other factors that send prices higher and lower.

Large, industrial consumers that can shift production to times when prices are negative and buy on wholesale markets can reap the biggest rewards.

Negative prices are “a warning signal that there is way too much production on the electrical grid”, said energy analyst Nicolas Goldberg at Colombus Consulting.

Electricity grids need to be kept constantly in balance. Too much can lead to the electricity to increase in frequency beyond norms for some equipment. Too little can lead to some or all customers losing power.

There are currently few options to stock surplus electricity production so producers have to reduce output.

Many renewable producers stop their output when prices are set to turn negative. It takes one minute to stop output at a solar park, two to three minutes for a wind turbine.

But not all stop their production.

– Tripling renewables –

“Renewable energy can be controlled, but depending on production contracts, there might not necessarily be an incentive to stop,” said Mathieu Pierzo at French grid operator RTE, which has the responsibility for balancing the electricity load.

Some producers are paid a fixed price under their contract or are compensated by the state if prices fall below a certain level.

Fossil fuel and nuclear power plants can adjust their production to some extent, but halting and restarting output is costly.

In the future, solar and wind will also have to “participate more in balancing the electricity system”, Pierzo said.

Solar and wind production is set to rise further as nations agreed at the COP28 climate conference last year to triple renewable energy capacity by 2030 as part of efforts to limit warming to 1.5 degrees Celsius compared with pre-industrial levels.

“Rising frequency of negative prices sends an urgent signal that greater flexibility of supply and demand is needed,” the Paris-based IEA warned last week.

“The appropriate regulatory frameworks and market designs will be important to allow for an uptake in flexibility solutions such as demand response and storage,” it said.

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Solar Energy

NREL explores long-term strategies for sustainable perovskite solar panels

Published

3 days ago

July 24, 2024

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NREL explores long-term strategies for sustainable perovskite solar panels

by Clarence Oxford

Los Angeles CA (SPX) Jul 24, 2024

Researchers at the National Renewable Energy Laboratory (NREL) are examining the future of perovskite solar panels, focusing on scaling, deploying, and designing panels to be recyclable.

Perovskite solar panels could play a key role in global efforts to reduce greenhouse gas emissions. With the technology still in its developmental stages, researchers are emphasizing the importance of designing these panels to minimize environmental impact.

“When you have a technology in its very early stages, you have the ability to design it better. It’s a cleaner slate,” said Joey Luther, a senior research fellow at the U.S. Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) and coauthor of the newly published article in the journal Nature Materials. “Pushing perovskite PV toward enhanced sustainability makes more sense at this stage. We’re thinking about how we can make sure we have a sustainable product now rather than dealing with sustainability issues toward the end of its practical life.”

The article highlights the PV research community’s influential position to prioritize remanufacturing, recycling, and reliability efforts, aiming to make perovskite PV one of the most sustainable energy sources available.

“Perovskites could unlock the next evolution of high-efficiency PV, and it is our responsibility to assure they are manufactured, used, and recycled sustainably,” said the lead author of the study, Kevin Prince, a former graduate researcher at NREL who now researches perovskites at Helmholtz Zentrum Berlin in Germany.

While silicon solar panels dominate the industry and cadmium telluride (CdTe) panels have established recycling programs, perovskites are at a critical point where sustainability issues can be addressed from the start.

The most effective circular economy begins at the design stage, considering materials sourcing, product lifetime, and end-of-life management. Researchers suggest assessing environmental impacts by looking at carbon emissions during production, embodied energy, sustainable material sourcing, and module circularity.

The journal article identifies critical sustainability concerns for each component of a perovskite solar panel. For instance, lead can be diluted with metals like tin to reduce lead content, though this may affect PV efficiency and durability. Expensive precious metals such as silver and gold could be replaced with cheaper alternatives like aluminum, copper, or nickel. Fluorine-tin oxide is recommended over the scarcer indium-tin oxide for front electrodes.

“We want to have the lowest amount of embodied energy in the fabrication,” Luther said. “We want to have the lowest amount of emissions in the fabrication. At this stage, now is the chance to look at those components. I don’t think we have to change anything. It’s more a matter of what decisions should be made, and these arguments should certainly be discussed.”

The authors discuss various ways to improve the circularity of perovskite panels. Remanufacturing involves reusing parts from old modules to make new ones, while recycling converts waste materials into raw materials for reuse. Attention is needed for the specialized glass used in perovskite modules, which is crucial for structural support and protection while allowing maximum sunlight penetration. Establishing a recycling pathway for this glass will be essential as PV deployment increases.

Silvana Ovaitt, a PV researcher and coauthor of the paper, noted that cleaner electricity grids will lead to cleaner manufacturing processes, further reducing emissions.

“Another concern is the transportation of the final modules and the raw glass because those are the heaviest items,” Ovaitt said. “Local manufacturing will be a great way to reduce those carbon impacts.”

The researchers explain that increasing the durability of PV modules, thereby extending their useful life, is a more effective approach to reducing net energy, energy payback, and carbon emissions than designing for circularity alone. A longer lifespan means panels won’t need to be recycled as often.

“Ultimately, we want to make them as durable as possible,” Luther said. “But we also want to consider the aspects of whenever that time does come. We want to be deliberate about how to take them apart and to reuse the critical components.”

Research Report:Sustainability pathways for perovskite photovoltaics