By Rikki Stancich in Paris

In April this year, Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), announced its intention to build a new CSP technology demonstration plant that will generate super-heated compressed air to drive a 200kW Brayton Cycle turbine.

The Brayton Solar System’s zero water requirement promises to resolve a major issue that has dogged the water-dependent CSP sector to date – namely acute water scarcity in high DNI areas.

Like all turbine engines, the Brayton Solar System’s has the core components of a compressor, combustion section and power turbine, which drives the compressor. In this case, around 450 heliostats will direct solar heat onto a 30m-high tower. The concentrated solar heat from the solar field will replace the combustion element needed to drive the turbine.

Meanwhile, its modular design means makes it an attractive, lower risk investment compared to other large-scale CSP technologies.

CSP Today's Rikki Stancich speaks in-depth to James McGregor, energy system manager for the Solar Brayton Cycle demonstration field, about the technology’s advantages and limitations, as well as its staggering market potential.

CSP Today:  Within CSIRO's Brayton Cycle solar tower concept, CSP generates the
heat to expand the compressed air. Is water required during any part
of the process?

James McGregor: No, it has zero water requirement, given that it doesn’t require cooling as do most steam turbines.

CSP Today:  So effectively the only water requirement will be for mirror cleaning?

James McGregor: Not necessarily. As part of our heliostat development programme, we are looking at developing a coating system that does not require washing (which ‘self-washes’ when it rains).

We are looking at the optical degradation over time before the glass needs to be replaced. We are also assessing the optical performance of mirrors that are not washed, and weighing it against the economics of the energy and resource requirement to clean the glass.

CSIRO has already developed glass coating using nanotechnology for building surfaces and we’ll be looking at whether this can be modified for the heliostats.

Our research on this has been running for about a year now, so we have gathered a lot of data, but we haven’t yet reached any conclusions.

CSP Today:  What are the advantages of using compressed air, over direct steam generation or heat transfer fluids?

James McGregor: Most solar thermal plants use a steam-based cycle. Water is a key component, though dry cooling is possible at a cost. If the steam turbine is water-cooled, it would require 2-3 cubic metres of water /MWh.

The problem is that the best solar regions often have the lowest mean annual rainfall. The better the solar resource, the better the economics – but water is the constraint on site selection. In this sense, the Brayton cycle resolves a major issue.

Another advantage is the higher operating temperature. The conventional steam cycle’s efficiency is limited by temperatures of around 540 degrees Celsius.

The Brayton cycle, on the other hand, operates at temperatures of above 900 degrees Celsius, which enables a significant step-up in power block efficiency.

If you consider that a power block operating at 30% conversion efficiency can be increased to 40%, then notionally you would only need to build a solar field two thirds of the size. In this sense, you get a big leverage in capital cost reduction, in materials and in efficiency.

A geographical advantage is that Australia has a strong resource and mining sector. A lot of the operators in this sector have a considerable electrical load – around a 30MW point source load – which is typically powered by diesel generators.

Many mining sites are located in areas with very good DNI, but have a limited water resource. So effectively the Australian mining sector presents a ready-made market for the Brayton cycle.

Another advantage is that solar thermal turbines, like fossil fuel plants, have a spinning mass that generates power onto an electrical system. As such there is enough inertia in the system to allow you to ride out cloud cover interruptions and provide smoother generation compared to, say, PV modules.

Yet another important advantage is that the operating temperatures allow us to take advantage of breakthroughs in the gas-fired turbines.

CSP Today: CSIRO’s prototype Brayton Cycle solar tower will generate 200kW of electricity. How scalable is the technology and what is its optimal size?

James McGregor: With steam-based CSP, the bigger you get, the more efficiently you produce power. With the Brayton cycle, we are looking at a 1-10MW module size. Our next step is to develop a 1MW demonstration plant and after that, something in the 10MW range.

It is a modular solution, so you could group the turbines in accordance with the energy requirement. There is no reason why you couldn’t go into the hundreds of megawatts range.

CSP Today: Can the waste heat from the Brayton Cycle be captured?

James McGregor: Yes, you could use the waste heat for cooling – to generate chilled water for cooling industrial buildings. The attraction of the modular system is that you can scale it down and bring it to large consumption sites – you could build it next door to an industrial park, for example.

CSP Today: Have you identified any constraints at this stage, such as receivers that can handle higher temperatures and pressures?

James McGregor: The primary limit is the receivers. We will use commercially available material to begin with, but will develop more advanced receivers to handle higher temperatures.

CSP Today: The project includes molten salt storage capacity. Were any other types of storage materials considered or is molten salt deemed to be the most efficient and compatible?

James McGregor: Molten salt is the most understood medium, but it presently does not work at the temperatures involved and we are looking at alternative options. We have just started looking at this – we are trying to define the characteristics of the ideal storage medium.

CSP Today:  CSIRO’s Brayton Cycle demonstration uses a solar tower and heliostats. Is it restricted to this technology or could the concept be adapted to other CSP technologies?

James McGregor: It could be adapted to dishes, though the engine would have to operate on a moving axis. But it certainly could be done.

But troughs generate nowhere near the temperature requirement, so that is not an option.

CSP Today: Is CSIRO's Brayton Cycle solar tower designed to be a stand-alone
plant? How easily could it be bolted onto or used to augment an existing coal or gas-fired plant?

James McGregor: The Brayton Cycle can be co-fired with gas. In this sense it is a particularly good transitional technology as we move toward lowering emissions.

Bolting on to existing plants would require modification to the existing turbine, which would be very costly, but straight co-firing a gas plant with the Brayton Cycle offers a significant advantage.

CSP Today:  What are the major cost drivers likely to be and how could
cost efficiencies be achieved?

James McGregor: Cost reductions could be achieved through a number of areas. Firstly, scaling up and achieving economies of scale could bring down costs.

Then there is technology for improving efficiency and lowering the cost of components; heliostats account for around 40% of the project’s total cost, so we are working toward high-precision, low cost heliostats.

We are also looking at improvements in efficiency through higher operating temperatures, which ultimately lowers the levelised cost of electricity. Improving efficiency gives you big leverage in capital cost reduction.

Then there is the power block – we are trying to optimise the overall efficiency by matching the power block with the receiver.

So, we are looking at incremental improvements in those 3 key areas.

To respond to this article, please write to the editor:

Rikki Stancich: rstancich@gmail.com