Economics of Thermal Power Conversion – A Space for My Projects, Thoughts, and More…

In discussions on the economic viability of Fusion as an energy source, the costs of thermal power conversion are often cited as a limiting factor in the achievable cost of electrcicity.

The argument goes that no matter the cost of the fusion reactor, the capital and operating costs of the power conversion (typically a Rankine steam cycle) would still exceed that of renewable sources like wind and solar.

The logic of this perspective is sound, however at the end of the day the reality of its effect comes down to the numbers. What are the true capital and operating costs of a power conversion system? It is this question which I will attempt to explore in this post.

Capital Cost Estimates for Utility Scale Power Plants – EIA 2016

This report was commissioned by an external consultant for the US Energy Information Agency (EIA) in 2016, as a follow on to an earlier 2013 study. The report breaks down capital and operating costs for a number of generating technologies, including both thermal, and alternative sources (wind, solar).

From the data we can draw a number of interesting conclusions:

Capital costs for gas turbine and combined cycle plants are significantly lower than those of pure rankine cycles (coal, steam, biomass), as well as wind and solar.
Of the four natural gas-fuelled plants, the advanced simple cycle configuration (ACT)has the lowest capital cost at only $677/kW, likely due to the single large 237MVA F-Class turbine, compared to the two-turbine 202MVA layout (CT) at $1100/kW. This implies significant cost savings associated with singular larger gas turbines compared to multi-unit plants.
While the ACT configuration had significantly lower fixed O&M cost of only $6.8/kW-year compared to $17.5/kW-year of the CT, at a 90% capacity factor, the higher variable O&M costs of $10.7/MWh vs $3.5/MWh mean that for baseload use the ACT costs more to run, at $11.56/MWh compared to $5.72/MWh. This could reflect increased overhaul costs for the higher temperature turbine, despite the reduced fixed costs due to the simplified plant design.
Capital costs are similar between the larger 702MW conventional combined cycle (NGCC) compared to the 200MW simple cycle (CT), and significantly higher between the advanced configurations (AG-NGCC vs ACT), which have capacities of 429MW and 237MW respectively. This implies that the specific capital costs of the Rankine bottoming cycle are significantly higher than that of gas turbine, in order for the cost to overwhelm the larger plants’ economies of scale.
While the O&M of USC Coal using a pure Rankine cycle are high at $9.94/MWh (baseload 90% capacity factor), for a conversion to gas-fired (CTNG) the total costs drop down to only $4.09/MWh. This reflects an approximate halving of fixed costs from $42.1/kW-year to $22/kW-year, and an almost 75% reduction in variable costs from $4.6/MWh to $1.3/MWh. The implication is that the coal handling and flue gas treatment equipment contributes much of the total operating costs of the coal plant, with the O&M of the USC power conversion and associated facilities costing no more than $5.85/MWh. Considering the costs of the gas firing equipment, and the larger capacity of the coal fired plant (650MW vs 300MW), the true power conversion costs should be even lower still.

Plant Configurations

(1) Ultra-Supercritical (USC) Coal – 650MWe USC Rankine cycle using HP, IP, and LP turbines, with single reheat after the HP turbine. The HP turbine inlet temperature is 600C at a pressure of 3800 PSI. The condenser is cooled using natural draft cooling tower.

Flue gas is treated using ammonia injection and Selective Catalytic Reduction (SCR) to reduce nitrous oxide, before passing through a recouperator, bag house, and finally being treated with limestone for sulfur dioxide and acid emissions.

(2) USC Coal with CCS – This configuration is identical to the standard 650MW plant in (1), but with the addition of an amine scrubbing system to capture 30% of produced CO2. The captured cas is compressed using a steam-driven compresser to a pressure of 2000 PSI, before being routed to the fenceline.

This compressor is assumed to necessitate a 12% increase in boiler capacity to provide the steam required.

(3) Pulverised Coal to Natural Gas Brownfield Conversion (CTNG) – This configuration takes an existing 300MWe subcritical rankine cycle operating at 2600 PSI and 540C, and replaces the coal silos, pulveriser, and associated handling systems with a natural gas burner.

The turbine uses the same setup as the 650MW unit in (1,2), with three turbines and a single reheat, and an evaporatively cooled condensor.

(4) Pulverised Coal Greenfield with 10-15% Biomass (GCBC) – This configuration takes the same initial 300MWe coal fired plant as (3), but with the addition of a biomass silo to suplement the coal supply.

This is for a greenfield plant which includes construction of the entire facility from bare ground.

(5) Pulverised Coal Brownfield Conversion to Coal with 10% Biomass (CTCB) – This configuration is identical to the 300MWe plant in (4), but assumes a brownfield site where only the biomass handling systems need to be added.

(6) Conventional Natural Gas Combined Cycle (NGCC) – Two seperate combustion turbines burning natural gas, each with its own heat recovery steam generator (HRSG) feeding a three stage steam turbine generator with single reheat, which is itself linked to a forced air cooling tower.

All three generators are independant and operate at 18kV 60Hz, with each of the two gas turbine units rated at 215MVA*, and the steam turbine unit at 310MVA.

If we take these capacities at face value, the steam turbine will be contributing 41.9% of the total electricity output.

* Note: MVA denotes ‘apparent power’, and decribes the internal load experienced by the generator, which due to reactive effects may be higher than the ‘real power’ (in MW) transmitted to the grid. If we assume a resistive grid load then MVA = MW, and the units will have real powers of 215MW and 310MW respectively.

(7) Advanced Generation Natural Gas Combined Cycle (AG-NGCC) – AG-NGCC uses the same basic design as (6), but simplifies the plant by using a single large H-Class gas turbine in lieu of the two F-Class turbines, and placing the steam turbine on the same driveshaft.

In this way only a single gas and steam turbine, HRSG, and electric generator are required.

The gas turbine is also of a more advanced design, allowing it to achieve higher operating temperatures, and thus greater power and efficiency.

The net output of this plant is 429MW, compared to the 702MW of the conventional NGCC.

(6) Conventional Combustion Turbine (CT) – Two gas turbine generator units, each rated 60Hz 101MVA at 13.8kV. There is no rankine bottoming cycle.

(7) Advanced Combustion Turbine (ACT) – A Single F-Class gas turbine generator producing 237MW. The general electrical and control systems are the same as the conventional plant, with the exception of a larger 234MVA rated generator and support unit (GSU).

(8) Advanced Nuclear (AN) – The reference plant is a two-unit facility using Westinghouse AP1000 Pressurised Water Reactors (PWR).

The AP1000 is a 1200MWe-rated Generation III design, which uses a seperate steel containment vessel and concrete impact shield, and features a number of passive safety features including a gravity-fed water reservoir situated above the reactor. The reactor itself is 4 meters in diameter by 12m tall, and alonside the primary coolant loop sits inside a 40m diameter by 82m steel pressure containment vessel, which is rated to a pressure of 65PSI.

The primary coolant loop contains pressurised water at 343 degrees Celsius and 2500PSI (17.1MPa). The water flows through the two 640 tonne steam generators, transferring heat to the secondary loop at a pressure of 836PSI (5.76MPa) and 292 degrees Celsius.

This secondary loop supplies the steam turbine, which features a single HP and three LP turbines, with two intermediate reheaters. The generator is cooled by a combination of hydrogen gas and water, and is rated at 1375MVA.