Interview by Rikki Stancich

In today’s aggressive solar market, the solar-Brayton combined cycle concept has everything going for it. Of all the CSP technologies, solar-Brayton operates at the highest temperatures and can provide dispatchable power at a remarkable theoretic conversion efficiency of up to 48 %.

Using air as the working fluid, it side-steps CSP’s traditional low-temperature heat transfer fluid handicap; it doesn’t require a single drop of the vast quantity of water for cooling that steam turbine generators need; and is far less restricted by site topography than other CSP technologies, such as parabolic troughs.

But perhaps the most compelling aspect of solar-Brayton technology is its potential for hybridization with biofuels. The Solhyco project, predecessor to the Solugas project currently being demonstrated by Abengoa and DLR near Seville in Spain, did just that. It demonstrated that in certain markets, (e.g. Algeria, California) a combined cycle plant of 21MW operated with a 25% solar share, could produce power at a levelised energy cost of 0.085 €/kWh, using biodiesel as backup for solar. 

CSP Today speaks to Arik Ring, one of the most forward thinking individuals in this space, about the story behind Solhyco and Solugas, and the future potential for solar brayton combined cycle (CC) plants.

CSP Today: What developments led to the Solhyco (hybrid solar Brayton-biodiesel CC) project?

Arik Ring: The idea of developing a solar brayton cycle is not new – it has been around since the 1970-80s.

In the 1990’s some development work was done around the world, in Israel, UK, USA and even Armenia. Toward the end of the 1990s, there was a bilateral programme between Israel and the US to restart the work.

We did a lot of theoretical groundwork, not only in solar Brayton but solar Brayton hybrid that could be backed up by jet fuel.  

But one of the really difficult stones to cross was to switch to Biofuel and get a totally renewable power plant.

The DLR has a group led by Dr. Reiner Buck, which was developing the same idea, but they were developing the receiver side only. Their receiver worked well between 1999-2000 and was demonstrated at Almeria, Spain.

Then the DLR and Ormat (a specialist in geothermal and energy recovery plants) decided to do a new project: the Solgate programme. The main partners were Ormat, DLR, Ciemat, and Abengoa.

I was project manager for the entire project on behalf of Ormat. The project was a success, and it ran until 2004.

By then there was a lot of experience in the system, but we were thinking about where the technology should go. It was then that I first came up with the idea to combine the Solar Brayton Cycle with biofuel technology.

The new project was called Solhyco, it had 2 parts  - we used the Solgate system, but converted it to work with biodiesel. It worked very well, and there were no problems.  

We had actually gained a lot of experience during the testing period. We achieved a high temperature receiver outlet of 1030 degrees C. With Solgate, we only achieved 960 degrees C, so we achieved an even higher temperature, building on the experience of Solgate.

For the second part of Solhyco a microturbine was used, and was tested at Almeria. At that time I was working on a similar microturbine with AORA Solar (Formerly EDIG Solar) in Israel (EDIG was  a partner to the US-ISRAEL bilateral project in the 1990's). Aora had one project in China and gained on that experience developed a modular unit prototype working in a Kibbutz in Southern Israel.

When Solhyco was accepted in 2005 I was then hired by Ormat as a consultant until 2009. I had experience with the Solgate-Solhyco system from 2002, until it was decommissioned in 2009.

Arik Ring working on the Solhyco project.

CSP Today: The project was very successful. How were the project results then used?

Arik Ring: After the success of Solgate and Solhyco, news traveled fast. Several projects had started.  

There is a project running in France –they have 2MW of solar Brayton to be  installed in the Pyrenees. Another program has just started in Brazil. There are two in the US – Wilson SolarPower’s and SouthWest+Brayton Energy’s.

Then there are two  programs in Israel. A new one by Heliofocus, in addition to the ongoing Aora program. Australia’s CSIRO is also going to test the Brayton Cycle in NSW, in cooperation with Mitsubishi and DLR.

Then of course, there is the Solugas programme, which is progressing in Spain. Solugas is the largest from the electrical side – 5MW of electricity. We did a lot of theoretic work on the future programme during Solgate, which was the foundation for the Solugas programme.

CSP Today: In terms of scale, Solugas represents a significant leap from the previous projects. What challenges were presented by scaling up the technology and how is this being addressed in the Solugas project?

Arik Ring:The main challenge was to get to a full-scale gas turbine. 5MW is small for a utility, but it’s a big step up from a 250kw turbine.

Regarding the receiver technology, there are volumetric receivers and tube receivers. In Solgate/Solhyco we used 3 receivers connected in series: one tube and two high temperature volumetric receivers. 

For the new high temperature receiver [at Solhyco], the material was ceramic; it absorbs the radiation in the entire volume of the receiver.

For Solugas, it has to heat a lot of air because of the scale, and they chose a large tube receiver. The problem in the past was that this kind of design produced a higher pressure drop in the flow of air.

High pressure drop is destructive to performance; if you get a pressure drop in the tubes, you lose power. So ultimately you want as much pressure as possible to get to the turbine.

In a gas turbine the path between the compressor and combustor – it is a very short distance with almost no volume. The challenge with the Solar Brayton Cycle is diverting hot air from the compressor to the large solar receiver/s.

In Solgate/Solhyco the original turbine volume without the solar system was roughly 3 litres. But when the solar receiver's volume is 3000 litres, this is a challenge. You have to keep the system large in order to overcome problems of pressure drops  - you have to insulate well so as not to lose heat, but you need to balance between two things, and you are contained by price.

CSP Today: If solar-Brayton CC technology holds so much promise, what is holding it back?

Arik Ring: For the last 20 years, people have predicted that the Brayton Cycle will emerge as the leader technology [for CSP]. But low oil prices and low R&D investment has meant that it has taken a lot of time to get if off the ground.

The Brayton cycle delivers the highest temperature possible and high efficiency (which is dependent on temperature).

In terms of efficiency, it has the potential to be the best solar system. It has a much smaller land footprint. Unlike steam turbines, there is no water requirement (aside from mirror cleaning).

And it is even more efficient if you use it in a combined cycle – it produces more electricity from the same heat source, the gas turbine exhaust heat may also be used to desalinate water, provide cooling, etc.

Once it had been demonstrated in Solgate and Solhyco, it was no longer a question of “if” but “when?”

We know now what needs to be done. The people at Ormat, DLR, Ciemat, AORA, CSIRO all now have a good understanding of the technology as well as practical experience.

The same is true for the receivers, interconnection, and turbines – there is a lot more knowledge today than previously. It is no longer theoretic –we have Solhyco, Solgate, and Solugas in Spain. Aora is about to install a second unit on top of  a working prototype in Israel, so the technology is progressing.

Where we had failure in the 1980s and in the1990s, nowadays we have success stories. The technology has been demonstrated, and we now have solar towers being built in Seville and in California. With mega projects like Brightsource’s Ivanpah, the price of heliostats will go down dramatically.

The technology can only take off. It could be some time before large-scale deployment, but its time will arrive.

To respond to this article, please write to Editor:

Rikki Stancich: rstancich@csptoday.com

Image credit: All images courtesy of Arik Ring