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High costs have consistently been identified as the major barrier to the future developments of CSP technology. In the face of challenges relating to energy security and environmental concerns, the US Department of Energy, in conjunction with the SunShot initiative, has announced a plan of action that seeks to invest in revolutionary technology to help speed up the process of making CSP cost competitive.

The Department of Energy, as part of the SunShot initiative, announced in June 2012 that it will award $56 million (USD) to 21 companies with the purpose of developing more cost effective CSP technologies. Amongst the recipients of the awards are NREL (National Renewable Energy Laboratories), BrightSource Energy Inc and 3M Company.  NREL has won two of the awards. One for $8 million to develop a power cycle that will use s-CO2 as a working fluid and another valued at $3.8 million for a project to develop a novel receiver that utilises a black body absorptive feature that uses falling particles instead of liquid for the heat transfer fluid.

CSP Today speaks to Craig Turchi, Senior engineer at NREL, about the impact these investments may have on the future of CSP technologies and whether the ambitions of a 75% reduction in the levelized cost of CSP-produced electricity is achievable by 2020.

CSP Today: What is your role at NREL and how did you become involved in the SunShot project?

Craig Turchi: I am a task leader for the CSP program’s system analysis activities at NREL. In 2008 I began to track alternative power cycles for CSP and identified the supercritical CO2 as a power cycle with good potential to make some positive impacts on CSP. Developing any new power cycle is a huge undertaking, so it was especially notable that much of the basic research into the CO2 cycle had been established, some by the very researchers who would become our project partners. When the SunShot initiative was released last year we saw an opportunity to pull together a team to advance one of the key aspects of that technology. In particular this involves the demonstrations of a large CO2 power turbine which is large enough to incorporate a lot of the design elements that are required to to illustrate a highly efficient and commercially viable design.

CSP Today: How did the bidding process work to secure the award from the DoE and what were the main drivers behind the DoE’s decision to award $56 million USD for the purpose of reducing CSP costs?

Craig Turchi: The first step towards the grant came in the form of a workshop hosted by the DoE on 28 September 2011. The purpose of the workshop was to get ideas from researchers and industry leaders on areas of concern and need in the CSP energy sector. Based on this, the DoE announced in November last year that it would be accepting proposals in February 2012.

In essence, the DoE is trying to foster investment in areas that may be higher risk, but will also offer higher return if successful. For this reason, applicants seeking the grant had to enter into a cost sharing agreement with the DoE ranking between 20 to 50% depending on the category. NREL’s team has had to match the DoE’s grant of a 8 million award to the s-CO2 power cycle, meaning that 16 million US dollars have been invested in this project.

CSP Today: How will s-C02 improve the CSP process?

Craig Turchi: Because this is a s-CO2 Brayton cycle, CO2 is the working fluid in a closed loop cycle. The CO2 will be continuously re-used, much like water in a steam cycle. However, because the CO2 is a Brayton cycle it will resemble the turbo machinery you find in a gas turbine. CO2 has a benefit over steam in that it has a higher efficiency level in its thermal to electric conversion. Potentially it could raise existing efficiency rates of 42% to 44 or 45%, depending on the specific operating conditions.   In addition, CO2 allows the plant to run at higher temperatures which could further increase efficiency to about 50%. That’s better than any gas turbine or steam turbine. The best conversions – up to 60% - are obtained in combined-cycle power plants, but of course they utilize two coupled power cycles.

CSP Today: What are the major challenges you are facing in the development of a 10MW C02 Turbine?

Craig Turchi: The biggest challenge for the turbine itself is durable materials to perform at the high temperature and pressure of the system. We selected the 10 MW size to allow us to use conventional bearing and seal technologies, and allow the unit to operate at a manageable rotational speed, but this is a first-of-its-kind design. Fortunately we have experienced turbomachinery partners on the team.

CSP Today: In terms of using falling particles in place of liquid in the heat transfer process, how established is this technology?

Craig Turchi: This is being researched by another research team here at NREL. It is a topic which has been examined previously at Sandia and other institutions on an international level. The design we have proposed under the SunShot initiative uses a closed receiver as opposed to previous designs which basically had an open cavity through which particles were falling through resulting in thermal losses and decreased efficiency. Our proposed design allows for higher operating temperatures and efficiency in thermal storage and in the power cycle. Also, falling particles are substantially cheaper than current nitrate salts.

To fully realize the potential of the particle receiver project requires a high-temp power cycle. The sCO2 power cycle in turn needs a high-temp receiver for maximum benefit. Thus the two projects are complementary

CSP Today: What will happen once the allocated three years have passed? Will the research be made publically available?

Craig Turchi: NREL has 41 months in order to complete the project. One of the conditions is that the research needs to be submitted to peer reviewed journals. However, much of the research will be held as the intellectual property of the partners in the cost-sharing process and will most likely involve patent applications. 

CSP Today: Do you think the DoE’s goal of reducing the costs of solar energy by 75% is achievable by 2020?

Craig Turchi: The projects we are discussing today are just a step toward the DoE 2020 SunShot target. That is, we don’t expect to get there in 3 years. Still, it is an aggressive goal. I believe the DoE has set the goal to motivate the research community and CSP industry to think beyond evolutionary changes and strive for the breakthroughs that will make CSP a major player in our energy supply. The two NREL projects, especially if successfully combined, can help realize that goal. 

To respond to this article email the editor Jennifer Muirhead

The issues of cost reduction and performance optimisation will be further addressed in the upcoming CSP Today reports due for release in August 2012: 

• CSP Parabolic Trough Report: Cost & Performance
• CSP Solar Tower Report: Cost & Performance