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How it works: Solar power towers with integrated storage

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The 110MW Crescent Dunes Solar Energy Plant, located near the town of Tonopah in the Nevada desert, will be the largest solar tower plant with integrated energy storage facility built to date.

But what exactly is it? What does storage actually do? And how does it work?

Let’s start with the tower.  There are a bunch of different technologies that come under the umbrella of solar thermal, or concentrated solar power. These include compact linear Fresnel reflectors, and parabolic troughs.

Solar towers use heliostats (or dual-axis sun-tracking mirrors) to reflect the sun’s heat onto a single receiver point.  This technology is  favoured because it can generate more heat than other technologies, has great economies of scale, and can integrate storage.

That heat could be used for industrial processes, such as steam production, as well as generating electricity. Generally, the more heat that is created, the more efficient the plant.

The heliostats track the sun‘s movements through the day. At SolarReserve’s Crescent Dunes facility, the plant will comprise 600 hectares of land, approximately 10,340 heliostats (each one 115 sq metres) with a total of approximately one million square metres of glass.

In the case of Crescent Dunes, the receiver (and the solar tracking algorithms) was derived from rocket engine propulsion technology developed by Rocketdyne, now a subsidiary of Aerojet.

Unlike other solar towers, which heat water directly to create steam and drive a turbine, the Crescent Dunes facility will heat molten salt, which is piped through the receiver located at the top of a tower, which is 180m high.

Two storage tanks are used. A cold tank stores the salt at 280C, pumps it up to the top of the tower where it circulates through the receiver, where the salt’s temperature is taken to 565C and it is then piped back down to the hot storage tank.

There, the energy is stored for use at a later time or released immediately into a heat exchanger that produces steam that powers a standard steam generator.

Here is a schematic of the plant.

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Tom Georgis says that in Nevada, the Crescent Dunes plant will capture and store the sun’s energy throughout the day because the utility does not want the plant to generate electricity in the morning.

This is the key to the technology’s strengths – it can be configured in any way that the customer wants. To underline the point, and the value of dispatchable, emissions free energy, SolarReserve show what output from a solar PV farm looks like, then wind, which varies from day to day, and what a solar tower power plant with storage can deliver.

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At Tonopah, the plant will have 10 hours storage at 110MW capacity, and will deliver under contract between the hours of noon and midnight on average.  An “hour” of storage means that the plant can run for one hour at full output using only stored energy.

It could have installed a bigger turbine with less storage capability, or a smaller turbine with more storage. A 50MW turbine would mean storage for 20-24 hours, and the ability to produce baseload power 24/7. Although, Georgis points out that you don’t need 24 hours storage to run baseload 24 hours, because when the sun is shining, energy can be stored and generated at the same time.

Georgis says that there are three key phases to the plant:

In phase 1, in the morning, it will collect and store the sun’s energy; in phase 2 (in the afternoon) it will start to generate, releasing salt to the heat exchangers and continuing to release cold salt to be recirculated. It is producing electricity, but it still collecting and storing.  In phase 3 (at night), it will not collect any more energy, but will generate electricity from the energy stored in the salts.

This is how SolarReserve explain the technology on the website (click here if you want to see the original with pictures).

SolarReserve’s technology, typically referred to as Concentrated Solar Power (CSP), uses thousands of mirrors to reflect and concentrate sunlight onto a central point to generate heat, which in turn is used to generate electricity.

More than 10 thousand tracking mirrors called heliostats reside in a 1,500 acre field, where they reflect and concentrate sunlight onto a large heat exchanger called a receiver that sits atop a 550-foot tower.

Within the receiver, fluid flows through the piping that forms the external walls; this fluid absorbs the heat from the concentrated sunlight. In SolarReserve’s technology, the fluid utilized is molten salt, which is heated from 500 to over 1,000 degrees Fahrenheit.

Molten salt is an ideal heat capture medium, as it maintains its liquid state even above 1,000 degrees Fahrenheit, allowing the system to operate at low pressure for convenient energy capture and storage. After passing through the receiver, the molten salt then flows down the piping inside the tower and into a thermal storage tank, where the energy is stored as high-temperature molten salt until electricity is needed.

SolarReserve’s technology leverages liquid molten salt as both the energy collection and the storage mechanism, which allows it to separate energy collection from electricity generation. When electricity is required by the utility, day or night, the high-temperature molten salt flows into the steam generator, as water is piped in from the water storage tank, to generate steam.

Once the hot salt is used to create steam, the cooled molten salt is then piped back into the cold salt storage tank where it will then flow back up the receiver to be reheated as the process continues.

After the steam is used to drive the steam turbine, it is condensed back to water and returned to the water holding tank, where it will flow back into the steam generator when needed. After the molten salt passes though the steam generator, it flows back to the cold tank and is re-used throughout the life of the project. The hot molten salt generates high-quality superheated steam to drive a standard steam turbine at maximum efficiency to generate reliable, non-intermittent electricity during peak demand hours.

The steam generation process is identical to the process used in conventional gas, coal or nuclear power plants, except that it is 100 percent renewable with zero harmful emissions or waste. SolarReserve plants provide on-demand, reliable electricity from a renewable source—the sun—even after dark.

  

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