Roadmap to Supercritical CO2 turbines

Here is a presentation on Closed Brayton Cycle (supercritical CO2) Research Progress and Plans at Sandia National Labs

The EU also sees a role for Supercritical Carbon Dioxide Power Cycles in power generation with CCS (carbon capture and storage, both in terms of efficiency increase and costs reduction.

The reasons of growing interest toward this technology are manifold:
* simple cycle efficiency potentially above 50%;
* near zero – emissions cycle;
* footprints one hundredth of traditional turbomachinery for the same power output due to the high density of working fluid;
* extraction of “pipeline ready” CO2 for sequestration or enhanced oil recovery, without both CO2 capture facilities and compression systems;
* integration with concentrating solar power (CSP), waste heat, nuclear and geothermal, with high efficiency in energy conversion;
* applications with severe volume constraints such as ship propulsion

There is a DOE project to a make a 10 MWe supercritical CO2 turbine that should be completed in 2015.

DOE-NE 2020 10 MWe RCBC Vision
* Develop and commercialize an RCBC by 2020
* Address all perceived risks and concerns of investors.
* Promote and operate intermediate projects that lead to the 2020 vision.
* Design and test a nominal 10 -15 MWe RCBC that industry agrees scales to 300 MWe
. Advance TRL to 7
* Take TA to the field to implement a Pilot Facility and advance to a turn -key operation (TRL 8)
* Convert a current steam facility or newly dedicated heat source
* Mostly utility operation
* Technology Transfer

Sandia began studying these turbines more than five years ago as part of the lab’s work on advanced nuclear reactors. They selected supercritical CO2 as the working fluid operating at approximately 73 bar and 33 °C at the compressor inlet. Under those conditions, the CO2 gas has the density
of 0.6-0.7 kg per liter—nearly the density of water. Even at the turbine inlet (the hot side of the loop) the CO2 density is high, about 0.1 kg/liter.

The high density of the fluid makes the power density very high because the turbomachinery is very small. The machine is basically a jet engine running on a hot liquid, though there is no combustion because the heat is added and removed using heat exchangers. A 300 MWe S-CO2 power plant has a turbine diameter of approximately 1 meter and only needs 3 stages of turbomachinery, while a similarly sized steam system has a diameter of around 5 meters and may take 22 to 30 blade rows of turbomachinery.

Supercritical CO2 gas turbine systems promise an increased thermal-to-electric conversion efficiency of 50 percent over conventional gas turbines. The system is also very small and simple, meaning that capital costs should be relatively low. The plant uses standard materials like chrome-based steel alloys, stainless steels, or nickel-based alloys at high temperatures (up to 800 °C). It can also be used with all heat sources, opening up a wide array of previously unavailable markets for power production.

It is quite easy to estimate the physical size of turbomachinery if one uses the similarity principle, which guarantees that the velocity vectors of the fluid at the inlet and outlet of the compressor or turbine are the same as in well-behaved efficient turbomachines.

Using these relationships, one finds that a 20 kWe power engine with a pressure ratio of 3.1, would ideally use a turbine that is 0.25 inch in diameter and spins at 1.5 million rpm! Its power cycle efficiency would be around 49 percent. This would be a wonderful machine indeed. But at such small scales, parasitic losses due to friction, thermal heat flow losses due to the small size, and large by-pass flow passages caused by manufacturing tolerances will dominate the system. Fabrication would have been impossible until the mid-1990s when the use of five-axis computer numerically controlled machine tools became widespread.

The alternative is to pick a turbine and compressor of a size that can be fabricated. A machine with a 6-inch (outside diameter) compressor would have small parasitic losses and use bearings, seals, and other components that are widely available in industry. A supercritical carbon dioxide power system on that scale with a pressure ratio of 3.3 would run at 25,000 rpm and have a turbine that is 11 inches in its outer
diameter. It would, however, produce 10 MW of electricity (enough for 8,000 homes), require about 40 MW of recu perators, a 26 MW CO2 heater, and 15 MW of heat rejection. That’s a rather large power plant for a “proof-of-concept” experiment. The hardware alone is estimated to cost between $20 million and $30 million.

Brayton-cycle turbines using supercritical carbon dioxide would make a great replacement for steam-driven Rankine-cycle turbines currently deployed. Rankine-cycle turbines generally have lower efficiency, are more corrosive at high temperature, and occupy 30 times as much turbomachinery volume because of the need for very large turbines and condensers to handle the low-density, low-pressure steam. An S-CO2 Brayton-cycle turbine could yield 10 megawatts of electricity from a package with a volume as small as four to six cubic meters.

Four situations where such turbines could have advantages are in solar thermal plants, the bottoming cycle on a gas turbine, fossil fuel thermal plants with carbon capture, and nuclear power plants

For solar applications, an S-CO2 Brayton-cycle turbine is small enough that it is being considered for use on the top of small concentrated solar power towers in the 1-10 MWe class range. Unlike photovoltaics, solar power towers use heat engines such as air gas turbines or steam turbines to make electricity. Because heat engines are used, the power conversion efficiencies are two to three times better than for photovoltaic arrays.

Placing the power conversion system at the top of the power tower greatly simplifies the solar power plant in part because there is no need to transport hot fluids to a central power station.

Supercritical carbon dioxide Brayton-cycle turbines would be natural components of next generation nuclear power plants using liquid metal, molten salt, or high temperature gas as the coolant. In such reactors, plant efficiencies as high as 55 percent could be achieved. Recently Sandia has explored the applicability of using S-CO2 power systems with today’s fleet of light water reactors.

Replacement of the steam generators with three stages of S-CO2 inter-heaters and use of inter-cooling in the S-CO2 power system would allow a light water reactor to operate at over 30 percent efficiency with dry cooling with a compressor inlet temperature of 47 °C.

* Sandia National Laboratories and Lawrence Berkeley National Laboratory are involved with Toshiba, Echogen, Dresser Rand, GE, Barber-Nichols in S-CO2 cycles.

* Toshiba, The Shaw Group and Exelon Corporation are engaged in a consortium agreement to develop Net Power’s gas -fired generation technology with zero emissions target. This approach uses an oxy-combustion, high pressure, S- CO2 cycle, named Allam Cycle. Toshiba will design, test and manufacture a combustor and turbine for a 25MW natural gas-fired plant. A 250MW full-scale plant is expected by 2017.

* Echogen Power Systems has been developing a power generation cycle for waste heat recovery, CHP, geothermal and hybrid as alternative to the internal combustion engine.

* Pratt and Whitney Rocketdyne is engaged with Argonne National Laboratories in a project with aim to integrate a 1000 MW nuclear plant with a S-CO2 cycle

A great match for the Integrated Molten Salt Nuclear Reactor (IMSR) being developed by Terrestrial energy

The 60 MW thermal IMSR would be the size of a fairly deep hottub. The Supercritical CO2 turbine would be about 8-10 cubic meters. The Supercritical CO2 could boost the electrical power to 33 MWe.

IMSR design
* No fuel fabrication cost or salt processing = extremely low fuel costs
* Under 0.1 cents/kwh
* Right size reactors, right pressure steam

Later units that include electricity generation can still send steam for cogeneration (use steam for desalination or the oilsand production. This provides another revenue stream for the IMSR nuclear plants.

Looking at the cost components of current nuclaer reactors

                                 Old Nuclear    Coal  New LWR est  IMSR first IMSR later
1   Fuel                                         5.0          11.0    5.0   0.1  0.1
2   Operating, Maintenance - Labor and Materials 6.0           5.0    8.0   1.0  0.2
3   Pensions, Insurance, Taxes                   1.0           1.0    1.0   1.0  0.2
4   Regulatory Fees                              1.0           0.1    1.0   1.0  1.0
5   Property Taxes                               2.0           2.0    2.0   2.0  1.0
6   Capital                                      9.0           9.0   39.0  20.0  5.0
7   Decommissioning and DOE waste costs          5.0           0.0    5.0   0.5  0.1
8   Administrative / overheads                   1.0           1.0    1.0   1.0  1.0
Total                                           30.0          29.1   60.0  27.6  8.6   

I think the IMSR can get down to 0.86 cents per Kwh.

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