The extraction and use of solar energy is one of the most important achievements of man in terms of energy. The main difficulty now lies not even in the collection of solar energy, but in its storage and distribution. If this issue can be resolved, then traditional fossil fuel enterprises can be retired.
SolarReserve is a company that offers the use of molten salt in solar power plants and is working on an alternative solution to storage problems. Instead of using solar energy to generate electricity and then store it in solar panels, SolarReserve proposes to redirect it to thermal storage (towers). The energy tower will receive and store energy. The ability of molten salt to remain in liquid form makes it the perfect thermal storage medium..
The company's goal is to prove that its technology can make solar energy an affordable source of energy that works around the clock (as in any fossil fuel power plant). Concentrated sunlight in the tower heats the salt to 566Β°C, and it is stored in a giant insulated tank until it is used to create steam to run a turbine.
However, first things first.
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SolarReserve's chief technologist, William Gould, has spent more than 20 years developing CSP (concentrated solar power) technology with molten salt. In the 1990s, he was Project Manager for the Solar Two Demonstration Plant, built with support from the US Department of Energy, in the Mojave Desert. A decade earlier, a structure was tested there, which confirmed theoretical calculations about the possibility of commercial energy generation using heliostats. Gould's challenge was to develop a similar project that uses heated salt instead of steam, and to find evidence that energy could be stored.
When choosing a container to store molten salt, Gould vacillated between two options: a boiler manufacturer with experience in traditional fossil-fuel power plants, and Rocketdyne, which made rocket engines for NASA. The choice was made in favor of rocket scientists. Partly because, early in his career, Gould worked as a nuclear engineer for the giant construction company Bechtel, which was working on California's San Onofre reactors. And he believed that he would not find a more reliable technology.
The nozzle of a jet engine, from which hot gases escape, actually consists of two shells (internal and external), in the milled channels of which fuel components are pumped in the liquid phase, cooling the metal and keeping the nozzle from melting. Rocketdyne's experience in developing such devices and working in the field of high-temperature metallurgy was useful in developing a technology for using molten salt in a solar power plant.
The 10 MW Solar Two project operated successfully for several years and was decommissioned in 1999, confirming the viability of the idea. As William Gould himself admits, the project had some problems that needed to be solved. But the core technology used in Solar Two also works in modern stations like Crescent Dunes. The mixture of nitrate salts and operating temperatures are identical, the difference is only in the scale of the station.
Molten salt technology has the advantage of being able to deliver power on demand, not just when the sun is shining. Salt can retain heat for several months, so the occasional overcast day does not affect the availability of electricity. In addition, the emissions from the power plant are minimal and, of course, there are no hazardous wastes created as a by-product of the process.
Work principles
The solar power plant uses 10 mirrors (heliostats) installed on 347 hectares (that's more than 647,5 football fields) to concentrate sunlight on a central tower 900 meters high and filled with salt "filling". This salt is heated up to 195Β°C by the sun's rays and the heat is stored and then used to convert water into steam and to run generators that produce electricity.
The mirrors are called heliostats because each one can tilt and turn to precisely direct its beam of light. Arranged in concentric circles, they focus sunlight onto a "receiver" at the top of the central tower. The tower itself does not glow, the receiver has a matte black color. The glow effect occurs just because of the concentration of sunlight that heats the container. The hot salt flows into a stainless steel tank with a capacity of 16 mΒ³.
heliostat
The salt, which at these temperatures looks and flows almost exactly like water, is passed through a heat exchanger to produce steam to run a standard turbo generator. The tank contains enough molten salt to run the generator for 10 hours. This equates to 1100 megawatt-hours of storage, or nearly 10 times more than the largest lithium ion battery systems that have been installed for renewable energy storage.
Hard way
Despite the promise of the idea, it cannot be said that SolarReserve have been successful. In many ways, the company remained a startup. Although the startup is energetic and bright in every sense. After all, the first thing you see when you look in the direction of the Crescent Dunes power plant is light. So bright that it is impossible to look at it. The light source is a 195-meter tower that towers proudly over the Nevada wilderness about halfway between the small town of Reno and Las Vegas.
What the power plant looked like at different stages of construction
2012, start of construction
2014, the project is close to completion
December 2014, Crescent Dunes almost ready for service
Ready station
About an hour away is the famous Area 51, a secret military facility that the entire Internet threatened to storm this summer in order to "rescue" aliens from the hands of the American government. This proximity leads to the fact that travelers who see an unusually bright glow sometimes ask local residents if they have witnessed something unusual or even alien. And then they are sincerely upset when they learn that this is just a solar power plant, surrounded by a field of mirrors almost 3 km wide.
Construction of Crescent Dunes began in 2011 thanks to loans from the government and investment from NV Energy, Nevada's main utility company. And the power plant was built in 2015, about two years later than planned. But even after the construction, not everything went smoothly. For example, in the first two years, pumps and transformers for heliostats, which were not powerful enough, often broke down and did not work properly. Therefore, power output at Crescent Dunes was lower than planned in the early years of operation.
There was another difficulty - with the birds. Falling under the "sight" of concentrated sunlight, the unfortunate bird . According to representatives of SolarReserve, their power plant managed to avoid the regular and massive "cremation" of birds. Together with several national organizations, a special plan was developed to mitigate any potential threats to the power plant. This program was approved in 2011 and is designed to reduce the potential risk to birds and bats.
But the biggest problem for Crescent Dunes was a hot salt storage tank leak discovered in late 2016. According to the technology, a giant ring resting on pylons at the bottom of the tank distributes the molten salt as it comes from the receiver. The pylons themselves had to be welded to the floor, and the ring needed to be able to move, as temperature changes cause the materials to expand/contract. Instead, due to a mistake by the engineers, the whole thing was welded tightly together. As a result, with temperature changes, the bottom of the tank sagged and leaked.
By itself, the leakage of molten salt is not particularly dangerous. When it hit the gravel layer under the reservoir, the melt immediately cooled down, turning into salt. However, the shutdown of the power plant dragged on for eight months. The causes of the leak, those responsible for the incident, the consequences of the emergency and other issues were studied.
SolarReserve's troubles didn't end there. The power plant performance was lower than planned in 2018, with an average power factor of 20,3% compared to the planned power factor of 51,9%, C. As a result, the US National Renewable Energy Laboratory (NREL) began a 12-month cost study of the project CSP by focusing on performance issues and unexpected costs. As a result, at first the company was sued and forced to change management, and in 2019 they were completely forced to admit their .
It's not over yet
But even this did not put an end to the development of technology. After all, there are similar projects in other countries. For example, similar technologies are used in the solar park named after Mohammed bin Rashid Al Maktoum, the world's largest network of solar power plants, united in a single space in Dubai. Or let's say Morocco. There are even more sunny days than in the US, and therefore the efficiency of the power plant should be higher. And the first results show that this is indeed the case.
The 150 MW CSP Noor III tower in Morocco exceeded production and storage capacity targets in the first few months of operation. And the cost of financing tower energy storage projects is in line with expectations, says Xavier Lara, senior consultant at CSP Engineering Group Empresarios Agrupados (EA).
Power plant Noor III
Launched in December last year, the Noor III power plant is showing remarkable performance. Noor III, installed by Spain's SENER and China's energy construction corporation SEPCO, is the world's largest operational tower plant and second in integrating molten salt storage technology.
Experts believe Noor III's robust early performance data on performance, generation flexibility, and storage integration should reduce CSP tower and storage reliability issues and reduce the cost of capital for future projects. In China, the government has already announced a program to build a 6000 MW CSP with storage. SolarReserve is partnering with Shenhua Group, a state-owned coal-fired power plant builder, to develop 1000MW of CSP molten salt generation. But will such storage towers be built further? Question.
However, just the other day, Heliogen, owned by Bill Gates, announced its breakthrough in the use of concentrated solar energy. Heliogen was able to raise the temperature from 565Β°C to 1000Β°C. Thus, opening the possibility of using solar energy in the production of cement, steel, petrochemical products.
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Source: habr.com