Interview by Rikki Stancich

CSP Today: ARC’s Compound Parabolic Concentrating (CPC) units are designed for water and space heating, and cooling. What power generating capacity do these units have?

James Riddell: It depends on design of the generator you use. It is possible to generate power at 90-120°C with a low temperature ORC. Our CPC units are rated to operate up to 250°C, so maintaining 120°C temperatures is well within the collector working range. Our CPCs don’t use copper tubing in the collector tube; they use cupro nickel, so they are not a standard evacuated tube design. ARC designs the collectors, and the components are made in Germany.

CSP Today: What differentiates ARC from other micro CSP solutions?

James Riddell: A combination of things. We offer 3 different kinds of reflectors including compact linear Fresnel and parabolic trough collectors. If power output required is a under 250kWe (true micro CSP) – which is possible to produce at 120°C - you can use the compound parabolic concentrating units. Really, these are an upgraded version of what you would see in the UK producing hot water.

The original work for the evacuated tubes was done at Sydney University in the early 1980’s, and the work on the CPC optics was carried out at the University of Chicago in the late 1970’s, so the technology is not new. It was more a case of adding the two technologies together and picking the right materials.

Our advantage is the end user is not limited to a single technology; instead the solution is designed to around the end users power / hot water / chilled water requirement and land availability.  

More electricity requires more land space and higher operating temperatures. If land is not an issue, then a parabolic trough operating around 250°C is the optimal operating system.  

To limit the land impact, you would opt for the CLFR system, which operates up to 425°C. At that temperature you can generate up to 20MWe using an ORC turbo-generator very efficiently. This is the top end of our organic rankine cycle generation range currently, but not quite hot enough to run a water /steam generation system successfully.

CSP Today: Why did ARC opt against working with steam?

James Riddell: Handling steam has many issues that have plagued the fossil fuel generation industry for years. The problem with water is that if you operate at below 374°C and 22 MPa (under critical point), it is possible to get condensation in the turbine generator, which reduces efficiency considerably.

To be safe, steam systems are usually run supercritical to avoid this problem. As you are working with extremely high pressures and high temperatures, these create health and safety issues. Super critical thermal power stations are not the easiest thing in the world to work with – but no one has come up with a better solution yet.

There are also natural resource issues, with regards to the water requirement. Also, if you don’t work with purified water, there is a high risk of corrosion. Water at these temperatures and pressures is a very aggressive medium to use and causes both corrosion and erosion problems with turbine blades. All of these issues are eliminated in an ORC cycle.

CSP Today: ARC also produces Compact Linear Fresnel Reflector systems using multiple absorbers to track the sun. How does ARC’s solution differ from other CLFR systems and what is its advantage?

James Riddell: The actual operating theory is no different than other designs – most systems are based on the work done by Dr. David Mills [ex- Solar Heat and Power / Ausra / Areva Solar] and Prof. Graham Morrison in the late 1990’s on collector designs that allowed collection on multiple receivers, as an alternative to the CPC design.

We use polymer mirror coating on float glass strips (so it does not need to be low iron glass), coupled to a simple tracking mechanism and a very low wind profile makes the cost per metre remarkably low

We also use a CPC evacuator tube as the receiver, which doesn’t move or rotate. This cuts heat losses at high temperatures -  at 350°C we have a thermal loss of 7- 9%. 

Because it is a compact system, we have a higher density of mirrors on the ground, so the land co-efficient on price becomes smaller.

We are also experimenting with inert gas-based systems, using gas as the working heat transfer fluid. We lose a bit on sensible heat over liquids, but it’s a lot more environmentally friendly in the event of a leak and much easier to handle. 

Also, with inert gas you don’t have to run at a particularly high pressure in the receiver tubing ( ~ 8 bar). There are no stagnation issues, no flammability issues, no chemical interactions and so on.

CSP Today: To what extent is leakage a problem?

James Riddell: In CLFR its not as much of a problem as it is in parabolic trough systems, where you have rotating joints, flexible tubing, pump seals, pump motors. Wear and tear on these parts can lead to leaks.

With a gas transfer fluid, all you need is a small compressor to get it moving but it heats so fast that the parasitic load required to keep the fluid moving is much lower.

CSP Today: What is the optimal operating temperature and output of these systems?

James Riddell: ORC Systems can operate from 90C to 350C. By picking the right collector and coating you can optimise the system for the temperature you want to operate at. It’s a function, in part, of the choice of the cermet coating, whether its stainless steel / aluminium nitride, tungsten / aluminium nitride (which has a very good performance and is a triple layer cermet) or other metal / ceramic combinations. Each one has an optimal temperature working range.

CSP Today: How many /which of ARCs CSP solutions have been deployed to date?

James Riddell: About 30-something around the world. The CLFR systems are very new – we will see the first of these become operational in Australia in the next few months.

CSP Today: Has there been any interest from utilities for larger scale CSP plants?

James Riddell: The interest has been more from regional councils and mining camps in the 1-5MW range. The technology is ideal for remote towns where grid connections are too expensive to run. 

We are usually too small for a utility – below 20MWe we are too small to consider. The solution could be scaled up if we could figure out how to make ORCs more efficient at the higher output ranges.

CSP Today: How robust is the Australian market for micro CSP solutions?

James Riddell: The market is very robust for PV, but not robust at all for micro CSP because of the way Federal and State governments have funded PV, which gets preferential treatment. Currently for PV, four carbon credits are generated for each MW generated, whereas for CSP only one carbon credit is generated per MW of output.

It is all a result of policy decisions that were made 10 years ago. The CSP sector in Australia is very small; we don’t have the same lobbying power as the PV sector.

Things are slowly changing. The Solar Flagships programme is looking at two large scale solar thermal (which will more than likely be steam-based), and two PV installations. There is a good chance we will provide the collectors for one of the solar thermal installations, given that we are the only local manufacturer.

CSP Today: Are any new policies likely to be introduced or revised in order to support the build out of CSP solutions like ARC’s?

James Riddell: Potentially, but it will probably be two years away. Any new policy decisions will be a spin-off from the Solar Flagships programme. We might see greater acceptance of CSP and more balance in how renewable energy credits are allocated.

CSP Today: Why has CSP been slow gain market share in Australia compared to PV?

James Riddell: I think it was simply down to the fact that PV was marketed better in early 2000’s. There were simply more PV installations on rooftops when the policy decisions were made.

CSP Today: To what extent will oil price volatility provide a boost to the renewables sector in Australia?

James Riddell: Despite the fact that Australia is mineral rich, we will get hit quite badly. Currently the cost of renewable energies is twice that of coal, which is heavily subsidized. Currently 87% of Australia’s power is produced by coal, and that is unlikely to change any time soon, due to our extensive coal deposits.

Renewable energies are being pushed forward with Federal and State government renewable energy programmes, which have targets for 2020, but when you consider that the forecasted energy use for 2020 is 210 terawatts consumption - with 30-40 terrawatts from renewable energy sources the picture is not overwhelming.

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

Rikki Stancich: rstancich@csptoday.com