| Supercritical CO2


dioxide generated by the combustion is taken away from the cycle at the condenser discharge to maintain the cycle mass balance (hence semi- closed). After the condenser with H2


O removal, the CO2


content of the recycled working fluid is 97%(v) CO2


. A simplified


schematic of the semi-closed Allam cycle is shown in Figure 1. Eliminating the ASU and replacing the combustor with a heat exchanger, the cycle configuration is identical to a conventional, simple recuperated (closed) sCO2


Figure 2. Conceptual schematic diagram of a closed cycle gas turbine/heat pump for pumped thermal energy storage


cycle fuel consumption (no heat integration with the power cycle), which comes to about 57 MWe, confirming the trade-off with the HGC.


cycle.


Developers of the technology (NET Power now in partnership with Baker Hughes) have made announcements about potential zero-carbon projects consisting of 300 MWe commercial power plants based on what they now refer to as the Allam– Fetvedt Cycle (AFC). A recently announced project is a plan to develop and build a “utility- scale natural gas-fired power


plant with near-zero atmospheric emissions” at an Occidental hosted site near Odessa, Texas, “expected to be on line in 2026”, with Constellation Energy in the role of plant operator and power off-taker.


The Allam cycle has received significant attention over the past decade. A 50 MWt test facility in La Porte, Texas, was built and operated to demonstrate the oxy-fuel combustor and the turbine. A press release in late 2021 stated that the Allam cycle turbine at La Porte had delivered electricity to the ERCOT grid.


According to a presentation given by NET Power at the International sCO2


Power Cycles


Symposium, 21-24 February 2022, San Antonio, Texas (where the author gave a keynote address), 900 fired hours at 300 bar and about 1000°C turbine inlet temperature was achieved at LaPorte. The unit was synchronised to the ERCOT grid with stable and controlled ramp and shutdown. In addition, the operation of the compact heat exchanger (the recuperator) constructed with a nickel-based alloy was also demonstrated. NET Power also reported that a number of commercial scale projects were under development and a pre-FEED study for a 284 MWe plant had been completed.


The inherent carbon capture capabilities of the oxycombustion sCO2


cycle (ie, without


a complex and costly chemical process plant add-on) with (advertised) 59% net LHV efficiency has created a unique aura around this simple yet powerful zero-carbon power plant concept. Although not mentioned at the International


Power Cycles Symposium, the study report for the Whitetail project in the UK, prepared by 8 Rivers (owner of the AFC technology) and including a study by McDermott, can be found online. The design and configuration of the AFC Whitetail plant does not include a dedicated ASU to generate the oxygen for combustion, which is supplied via a pipeline from another facility. The heat shown as being provided by the ASU in Figure 1 is produced by a dedicated hot gas compressor (HGC), equipped with a 60 MWe electric motor. While the standard AFC cycle is based on 1150°C turbine inlet temperature, this was deemed risky for the first plant. Thus, the report includes a design for a base case at 900°C turbine inlet temperature and an alternative case at 925°C for the UK standard plant (see Table 1). Without heat integration, ASU power consumption is roughly 10% of the


sCO2


Table 1. Allam–Fetvedt cycle UK standard plant performance (LHV = 50.03 MJ/kg) – excludes the ASU but includes a hot gas compressor


Turbine inlet temperature, °C Gross output, MWe Net output, MWe


Aux load, MWe (% of gross) Fuel consumption, MWt Net heat rate, kJ/kWh Net LHV efficiency


Base 900


387.0 279.4


107.6 (27.8%) 549.3 7078


50.9%


Alternate 925


436.4 296.0


140.4 (32.2%) 550.0 6689


53.8%


Ultimate 1200 484.6 324.9


159.8 (33.0%) 550.4 6099


59.0%


Liquid oxygen (LOX) storage can be incorporated into the AFC. In this concept, the ASU is operated during times of low demand (low or negative electricity price) at full load or part load (depending on the price) and the stored LOX is used during periods of high demand and electricity price. Not utilising the ASU during operation with stored LOX can increase the net plant output by 20%. Another energy storage option is combining green hydrogen (via electrolysis) with the AFC by burning the stored H2


Pumped thermal energy storage cycles As reported at the February 2022 International sCO2


Cycles Symposium, sCO2


in the oxy-combustor (with or without LOX storage).


Power power cycles are being


developed for both indirect and direct cycle heat addition (fired) configurations, including those for energy storage. Examples of the latter include the trans-critical CO2


cycle and other pumped


thermal energy storage (PTES) cycles developed by Echogen, GE, and MAN/ABB.


The basic operating principle of PTES cycles is illustrated in Figure 2. For simplicity, only one turbomachine operating on a simple Brayton cycle is depicted in the diagram. During the charge mode, the system runs as a closed cycle heat pump (or refrigerator, depending on your objective, ie, heating or cooling) in a reverse Brayton cycle, receiving heat from a cold reservoir and rejecting it to a hot reservoir. (This is why the technology is referred to as pumped TES.) During the discharge mode, the system runs as a closed cycle gas turbine in a Brayton cycle, receiving heat from a hot reservoir and rejecting it to the cold reservoir.


working fluid goes through both subcritical (ie, the heat pump evaporator) and supercritical states, hence the moniker trans-critical. In the


In practice, there are two turbomachines, a reverse Brayton cycle heat pump driven by an electric motor and a Brayton cycle gas turbine driving an electric generator. For maximum efficiency, both cycles incorporate recuperation, to heat (cool) compressed working fluid in the gas turbine (heat pump). The hot reservoir comprises two storage tanks for the TES medium, eg, molten salt or another suitable heat transfer fluid. Similarly, the cold reservoir also comprises two tanks for storing the cold medium, typically water/ice (sometimes mixed with a suitable liquid, eg, methanol). In the TES heat pump technology developed by Malta, the working fluid of the heat pump and gas turbine cycles is air. In the trans-critical CO2 CO2


cycle, the


www.modernpowersystems.com | January/February 2023 | 11


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