Heat management in CSP

Joyce Laird

The PRATT & Whitney Rocketdyne (PWR) name is normally connected to rocket science and high-powered engine technology, but this aerospace leader is now lending its extreme heat management expertise to the renewable energy industry…and the CSP sector in particular. Could this be an indication that the much heralded “demise” of CSP is somewhat premature?

Pratt & Whitney Rocketdyne (PWR) chose to focus on renewable energy when it became apparent that the energy management and materials-design capability that keeps rocket systems operating efficiently in outer space could provide benefits to terrestrial solar systems.

Randy Parsley, program area manager of PWR's Renewable and Alternative Energy division, Canoga Park in California, says that while the company is looking at a wide range of potential solar technologies, currently there is one flagship solar thermal project on its slate: Concentrating Solar Power (CSP), using molten salt thermal storage technology.

Why start with CSP technology?

To truly understand the new focus, says Parsley, one needs to look at PWR's heritage – which includes many years working with the Department of Energy (DOE) on energy projects: “This includes cooling nuclear reactors with molten salt or liquid metal, and of course our years of success with thermal control situations in the space program,” he says. “Moving into the renewable energy industry was a natural step for us. We took our existing capability and configured it into a solar/thermal receiver configuration”.

The first limited CSP tests with molten salt were done on a small scale in the early 1990s. The project, called Solar Two, was a DoE-funded project, and this proved the viability of thermal energy storage with molten salt, which forms the basis of PWR's current commercial activity.

PWR's molten salt technology has been exclusively licensed to SolarReserve, headquartered in Santa Monica CA. This US-based international developer of solar power projects develops commercial scale solar projects using PWR's technology: “Today, the first commercial scale project using the technology is under construction in Nevada. It leverages this experience but is 10 times larger than the original demonstration project,” Parsley says. SolarReserve received a US$737 million U.S. Energy Department loan guarantee to build the project – the 110 MW Crescent Dunes project, near Tonopah, Nevada.

PWR's high temperature molten salt technology

The type of salt used in the PWR CSP concept has a different chemistry, but is not unlike, basic fertiliser, which is solid while at room temperature. However when the salt used in CSP plants is heated to 550 degrees F, it transforms into a liquid that looks and behaves somewhat like water. Further, it remains a liquid at low pressure up to over 1,050 degrees F, at which point it creates high-temperature steam to drive a standard (though more efficient) steam generator:

The way this plant works – the “cold” liquid salt returns from the steam generator to be stored in a large tank at 550 degrees F, and is then pumped up the tower to the top of the receiver.

In the receiver, the sun's concentrated solar power heats the liquid salt to over 1,050 degrees F, and that is “the key”, says Parsley: “That is where PWR has the knowledge and expertise because of former applied aerospace and rocket technology. That is the difficult part of the design.”

The 550-degree salt runs through tubes that have special materials and high-heat transfer rates, which correlate with many of the issues that PWR rocket projects have faced and solved. When heated to 1,050 degrees F, the molten salt comes back down from the top of the receiver to a “hot tank” that, over the course of a day, fills up with this 1,050-degree salt to store thermal energy.

“Now, what you have is a large tank of high-temperature salt, storing the sun's thermal energy, so that it can be used to create steam any time it is needed,” Parsley says. “You just pump some of that hot salt through a heat exchanger to create steam. While you are creating this steam, the salt cools down back to 550 degrees. It is then cycled back into the cold tank to be reheated by the sun's thermal energy the next day.”

What makes this technology unique for CSP is that the salt is a completely closed loop system and requires little or no replenishment. Parsley says that it doesn't ever change in basic composition. “Water turns to steam and back. Salt is different because it does not ever change its phase from liquid to gas. It's pure liquid salt at 550 degrees and when it is heated to 1,050 degrees, it is just the same liquid salt – only hotter. Also, since there is no phase change, there is no significant change in the pressure or volume of the salt. This allows overall system pressures that are much lower. You just use the same salt over and over, every day, 365 days a year.

“By simply optimising the solar field for more storage, enough energy can be stored to run for 8, 10 or even 24 hours without the sun. It can easily run all the way through the night without the sun. That is our expertise. And that's the technology we exclusively licensed to our customer, SolarReserve,” Parsley adds.
The heat is on

There are of course other CSP plant configurations using molten salt, but they are ‘bolt on' systems, meaning they are added on to the plant. Also, ‘bolt on' salt systems do not achieve the high temperatures that the power tower with integrated storage does. According to Parsley, some trough plants have molten salt bolted onto their main plant but it is much less efficient: “They can't get the temperature much above 700 degrees,” he says, “and there is a big difference between 700 and 1,050 degrees; this is where our experience with rocket engines comes in. We have so much experience in heat transfer under very challenging conditions that we can manage this extreme heat very efficiently.”

The configuration of this type of plant is a large tower (500 to 600 feet high) in the centre of a reflector field. The 100-foot tall PWR receiver sits on top of that tower. This receiver is manufactured to PWR-proprietary specifications, drawing on its space program design expertise.

The receiver consists of 14 panels, each containing 66 tubes. The 14 panels are assembled in a cylindrical formation. Each tube is approximately 80 feet long and made of a proprietary composition with a coating that absorbs the sun. Parsley says the assembled receiver looks like a big black cylindrical radiator:

“When the sun is aimed onto the receiver, the tubes get hotter and hotter. When fully operational capturing the sun's energy during the day, the tubes glow white hot. To ensure tower integrity, PWR has also provided the heat shielding that is placed above and below the panels to keep other parts of the receiver from becoming hot”.

Surrounding the 650-foot tower, with the receiver on top, are thousands and thousands of movable mirrors, called heliostats. Each one of these mirrors moves every 10 seconds to reflect the sun back to the receiver at the top of the tower to heat the salt to 1,050 degrees.

Along with this proprietary receiver and heat-control technology, PWR supplies the control system for all of the mirror assemblies. “Our software tells each mirror where to point every 10 seconds,” Parsley says. “We are responsible for this receiver operation and durability. We also have performance warranties that we provided; we have to be in control of how much energy gets placed upon this receiver. Therefore, we also provide the software for the collector field.”

Along with Crescent Dunes' SolarReserve plant currently under construction in Nevada, U.S., PWR has a separate US$15 million DoE technology contract, specifically focused on reducing the cost of this type of plant in the future.

Crescent Dunes meanwhile, is set to be online and producing power by December 2013.

Currently, this CSP project represents the initial major step for PWR's focus for growing the renewable energy business. However, the company has indicated that there are several things in the pipeline related to other solar and renewable energy avenues.

This is just the beginning for PWR, which is looking beyond CSP into CPV, Waste-to-Energy and some biomass opportunities, but as Parsley says, “these emerging technologies must be competitive in the marketplace without incentives or they are not going to make it.”

It will be interesting to see whether CSP will ever truly fall into this category. One thing is certain: Companies with the expertise of PWR entering the sector can only help strengthen its credentials further.

PWR and the case for CSP

As is well known, CSP projects operate by concentrating the sun's thermal energy to eventually make steam that drives a steam turbine and generator. There are a few different CSP technologies, but the most common are either the trough or power tower systems.

For CSP solar trough systems, the sun is first concentrated to heat oil before the hot oil is used to make steam that in turn, drives a turbine to produce power.

With solar power towers, the sun is reflected to a central receiver that sits atop a single tower in the centre of the mirror field. Some power towers, such as the technology that was developed by PWR, use molten salt as the energy capture fluid, which allows for energy storage, and the ability to generate steam whenever needed, day or night.

Direct-to-steam CSP systems, whether trough or tower, are harder to control, PWR's Parsley says, and it is very difficult and expensive to add even a limited amount of storage, far short of the molten-salt approach.

All CSP has some thermal inertia that improves the ability for grid integration. From this respect, it is better than solar PV and CPV, which exhibit power fluctuations due to clouds passing over the field. But this thermal inertia is really not enough to make CSP technology truly competitive to PV or CPV in a market increasingly driven by cost.

But according to PWR, its molten salt technology brings a large change to the issue of intermittency – with the ability for long-term storage and reliable power generation, making CSP much more competitive in cost, with higher-value power that can be dispatched when needed.

About: Joyce Laird has an extensive background writing about the electronics industry; semiconductor development, R&D, wafer/foundry/IP and device integration into high density circuit designs.


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