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High costs and CSP are an unfortunate, but thus far, inseparable pairing. The US DoE and Sunshot Initiative have allocated $56 million to promote cost reductions in CSP Technology.  The latest of these awards has allocated $12.3 million to the Southwest Research Institute (SwRI) for two separate CSP projects (both of which include 20% industry co-funding). The first project worth 8.5 million has seen SwRI partnering with General Electric, Bechtel Marine Propulsion Corporation and Thar Energy LLC to develop a supercritical carbon dioxide (s-CO2) hot gas turbo-expander (the same project CSP Today examined last month in an interview with Craig Turchi from NREL). The second award, for $3.8 million, will see the SwRI along with collaborators Solar Turbines Inc., Oak Ridge National Laboratories and, German Aerospace Center and San Diego State University, developing an open air brayton cycle combustor CSP hybrid gas turbine system that will combine CSP technology and conventional gas turbines to promote an efficient, cost effective, source of energy.

CSP Today: Perhaps we should begin with you giving us a brief overview of the two awards and what this research will entail?

The first project will involve the designing of a high efficiency turbine expander for super critical CO2 cycles. The cycle we are looking at is a double recuperated cycle that will allow us to achieve the above 50% efficiency levels which the DoE is looking for. This is a fairly ambitious task for CSP. To get to this high efficiency rate the turbine needs to be run at a high temperatures and pressures which go beyond the ability of any existing technology. Thus SwRI, together with GE, will design a new turbine expander that runs on s-CO2 that is specifically designed for this high pressure, high temperature CSP Cycle. That is going to be done on a megawatt scale(previous studies have all been limited to kilowatt ranges). Phase one will involve the design of the turbine, phase two will include the fabrication and commissioning of the turbine and phase three the final testing – all of this needs to be done in the next 36 months.

S-CO2 has thermo dynamic advantages that allow you to get to a reasonably high efficiency at cycle temperatures that are lower than those of steam or air cycles. However, s-CO2 cycles are currently underdeveloped: some of the required components don’t exist, including the expander which we are currently developing.  The expander really is leading edge technology as we are combining high pressures and temperatures with an extremely dense gas (S-CO2), so we are moving into technology development that hasn’t been conducted on this scale.

CSP Today: What other challenges are you facing in developing this new technology?

The heat exchanger technology is pretty expensive at the moment so we are working on developing new heat exchange systems which will bring down the costs of the cycle as a whole. There are also dangers of corrosion when using s-CO2. It is important to keep in mind that the awards under the SunShot initiative are not necessarily mutually exclusive. For example, other projects under this initiative are looking at developing receivers. The success of our project depends partially on the success in the development of other projects in the initiative. The entire concept of the SunShot initiative is to identify areas of need in CSP, commission out various projects, and ultimately bring them together at the end to achieve the above 50% efficiency CSP rate desired by the DoE.

A factor which we have identified is the need to promote the communication of knowledge across different areas of research. We are currently organising a workshop along with NREL and NTEL NETL to develop a roadmap for the future of CSP, co-organized by the DoE. For example, research is being conducted into using s-CO2 as a working gas from experts in the nuclear, coal and energy wastage-recovery industries. However, there is a lack of communication between these areas despite the fact that there may be a lot of information that can be shared for the mutual benefit of all involved. The workshop will also provide the opportunity to identify new areas of technology that need to be developed so that the proper funding can be put in place.

CSP Today: How will the open air Brayton Cycle combustor for the CSP contribute to greater CSP efficiency?

It uses a conventional gas turbine. A gas turbine consists of a compressor, combustor and a turbine. You would usually put natural gas into the combustor which then heats and expands the air in the turbine to produce electricity. The concept here is to replace or augment the combustor with a heat exchanger that utilises solar input through a solar receiver.  Then you have your conventional combustor so that you can still run your gas turbine when there is no sunshine. The system will be entirely hybrid meaning that the use of natural gases will be able to supplement the turbine when solar levels are low, allowing the turbine to run at constant optimal efficiency. There is a variant where you could put a Rankine cycle on the back in the form of a combined cycle plant.

For this project we need to create a combustor that can withstand an inlet temperature of up to about 1000°C as this is the temperature which the solar receivers are being designed to support. This means that the turbine will run at about 1400°C. Currently there is no gas turbine combustion technology which can do this.

CSP Today: How will this contribute to achieving the DoE’s goals of bringing down the costs of CSP?

As mentioned, the goal of the SunShot initiative to incorporate a variety of different projects, which when integrated will bring down the overall costs of the CSP Industry and lead to substantial efficiency improvements. The new open air brayton cycle combustor is a stepping stone in the process – a missing link that has been identified as an area of need in CSP. The development of this new combustor, together with the gas-turbo expander, are two steps towards achieving the ultimate goals of CSP cost reduction.

The Supercritical CO2 Technology Road-mapping Workshop is scheduled for February 6 and 7 2013 to be held at Southwest Research Institute in San Antonio, Texas. This event is co-organized by Southwest Research Institute, DOE NETL, and DOE NREL.

To comment on this article contact the editor, Jennifer Muirhead