Concentrating solar power offers a built-in energy storage benefit that makes 24/7 solar-powered electricity possible. The downside is that CSP plants are enormous, complex operations, and now an international research team wants to add a solar fuels contraption to the mix, which could be applied to a water-splitting system for producing renewable hydrogen. The question is, why?
Why CSP + Hydrogen?
Part of the reasoning behind the hydrogen angle is that it could cut down on some of the on-site clutter involved in concentrating solar power.
For those of you new to the topic, concentrating solar power plants use specialized mirrors to focus sunlight onto a much narrower field, where it heats a liquid. The liquid (typically, molten salt or a specialty oil) goes to a power station to produce steam for turbines to generate electricity. Since the liquid is hot it can also be stored as thermal energy for use at night, so the power station can operate 24/7.
Without the power generating station, you remove a significant amount of the clutter. Of course part of that would be offset by the hydrogen facility, but the end result, at least potentially, is a smaller footprint for the facility overall, potentially opening up more siting options.
Additional site options could also arise because once you add a hydrogen twist to CSP, you don’t have to locate the plant near an existing high voltage power transmission line, let alone build a new power line.
Hydrogen gas can be transported through pipelines like any other gas, but really all you need is a road, a truck, and transportable storage tanks.
As for end uses, aside from generating zero-emission electricity in fuel cells, hydrogen is commonly used for rocket fuel (thanks, NASA!), numerous industrial processes, medications (hydrogen peroxide!), and fertilizer production. The food industry is also a big fan.
The fuel cell angle is probably the biggest deal in terms of climate management. Aside from the auto industry, stationary fuel cell systems are coming into vogue as an alternative to diesel generators for on-site energy security because they offer energy storage, no emissions, and practically noise-free operation.
Currently most hydrogen is sourced from fossil natural gas, so fuel cells don’t fall into the sustainability slot until a fossil-free substitute emerges. Biogas is one option, but so far the real winner appears to be water-splitting, which can be powered by an electrical current sourced from wind or solar energy.
24/7 Solar Fuel!
Water-splitting is where the new CSP research comes in. There are already reams of R&D on the use of solar energy to produce hydrogen by breaking down water molecules, so the next challenge is to develop a highly efficient, continuous process that can operate by night as well as day.
The research team has been focused on creating a reactor that could hit a temperature between 800 and 900 degrees centigrade, and in the latest development the successfully operated a lab-scale, 5 kilowatt device at 850 degrees.
Okay, so 5 kW is kind of small, but it’s a big step forward for the research team. It provides a platform for figuring out how to control larger operations. The team estimates that the smallest commercially viable operation would weight in at 1-5 megawatts, and they could be scale up to 100 megawatts or more.
For those of you on the go, CONTISOL is described as a “solar reactor that runs on air, able to make any solar fuel like hydrogen and to run day or night.”
The continuous angle is super important for reactor-based hydrogen production. When you have to shut down the operation every night, you waste a whole lot of energy cranking the heat up again every morning.
As for that thing about running on air, that’s where the R&D is for now: a system that uses air as a heat transfer medium to supercharge a chemical reaction at high heat. So far the system has been demonstrated on methane reformation.
Lead author of the study Justin Lapp, currently Assistant Professor of Mechanical Engineering at the University of Maine, explains:
So the main idea of CONTISOL was to build two reactors together. One where sunlight is directly doing chemical processing. The other side for storing energy. In the chemical channels the high temperatures of the material drive the chemical reaction and you get a change from reactants to products within those channels, and in the air channels cooler air goes in the front and hotter air comes out the back.
Water-splitting is among other potential applications. Though that’s a little farther off, Lapp foresees smooth sailing in terms of marrying a water-splitting system to CSP. The reaction would not take place at the CSP tower, but the steam generated by CSP would be useful:
In these high temperature solar reactors, the center spot on the tower where all the mirrors focus is best for high temperature chemistry. We get very high flux at the center for getting to 600 – 800 C. But there’s always a bunch of wasted radiation around the outside; there’s still enough light to heat to 200 – 300 C, not enough for chemistry but plenty to evaporate water to steam.
According to Lapp, the addition of a water-splitting angle would also help resolve some CSP plant design challenges:
Water coming in already as steam makes it a lot easier to design the receiver. You don’t have the problems of steam expansion while its boiling. Its easier to keep it tight for steam than liquid. So to ready water for splitting, it would first be boiled to steam right in the tower.
If this is beginning to sound familiar, the US Department of Energy is all over the concentrating solar power for water-splitting angle.
The cost factor could be a sticky wicket, but the cost of concentrating solar power has been falling in tandem with the general trend for renewable energy, so there’s that.
For the record, the research team is based at the German Aerospace Center (aka DLR) and supported by the Aerosol and Particle Technology Laboratory of CPERI/CERTH in Greece. The work is under the umbrella of SolarPACES, the international Solar Power And Chemical Energy Systems collaboration.
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Photo: “CONTISOL was tested at Cologne, Germany using simulated ‘suns’, rather than an actual solar field, and the storage and heat exchanger was also simulated, because the reactor itself is the innovation being tested” via DLR.
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