Hybridization: Pairing Technologies for Synergistic Enhancements (Including sCO2 Hybrids)

 

     Hybridizing technologies can offer some practical advantages. One of the most well-known of “hybrids” is hybrid vehicles where an electric motor is paired with an internal combustion engine to improve vehicle mileage, emissions, and fuel costs. The Toyota Prius, introduced in the early 2000’s is still a popular hybrid choice. Now there are many others. Then came plug-in hybrids, pairing a plug-in EV of lesser range with an ICE engine, which enable further improvements in mileage, emissions, and fuel costs while avoiding issues with pure EVs like range limitations and longer charging times.

     In energy there are several other kinds of hybrids. One of the most well-known is the combined-cycle natural gas plant where a combustion turbine is paired with a steam turbine to take advantage of the waste heat from combustion. Another in the pairing of solar energy and energy storage, or solar-plus-storage that takes advantage of excess solar generation during peaks in the sunniest parts of a day when production is more than needed and saves it for later in the day by feeding it to a battery. In a sense a grid-tied rooftop solar system is a kind of hybrid since it provides power for a house or business and any excess during peak production is fed to the power grid.

    

Diesel-Electric Hybrid Drilling Rigs, Fuel Blending, E-Frac, and Other Hybrids in Oil & Gas

 

     One kind of hybridization used in the oil and gas industry and in other applications is dual fuel engines. Some reciprocating engines and turbines can accommodate dual fuels and blends of natural gas, diesel, propane, and hydrogen. Blending hydrogen with natural gas in pipelines, storage fields, and power plant combustion turbines can be considered a hybridization. Diesel-Electric hybrid drilling rigs have been pioneered by Equinor and other operators and rig fleet companies like Maersk active in the North Sea. These hybrids can utilize braking similar to how automobile hybrids do in order to save energy expenditure and lower emissions. Maersk first deployed their retrofitted diesel-electric hybrid rig Intrepid in November 2020. The Intrepid utilizes battery power to address variable power and the high peak loads that rigs encounter. Braking energy is recovered. A digital energy management system optimizes rig energy use. Energy use and emissions are both reduced significantly. In 2021 a 2^nd retrofitted Maersk rig, the Integrator was deployed. The rigs utilize Siemens Blue-Vault lithium-ion battery storage system which is designed for offshore vessels. Siemens thinks that 300 of the world’s 500 ultra-harsh environment jack-up rigs can be outfitted with these hybridized features. They are also exploring powering with shore power.

     Electric fracking (E-frac) and other oilfield electrification applications can be considered to be hybrids. E-fracking often involves field-treated field natural gas powering gas turbines which in turn power efficient and powerful electric motors for pressure pumping. These are very high horsepower electric pumps that are integrated with digital energy management systems. These can be considered to be natural gas and electric hybrids. Energy management systems like those deployed by Equinor and Maersk for their rigs are also deployed in onshore drilling rigs to optimize power use with assistance from strategically deployed battery storage. The batteries charge when loads are low and discharge when loads are high and can eliminate the need for an extra generator. One such management system is made by EcoCell. It can charge when a rig makes a connection while the mud pumps are off and discharges when the mud pumps are running. Efficiency gains resulting in less energy use and less emissions are the result. Electrification is now utilized routinely and integrated with diesel and/or natural gas power in many different parts of drilling and completion of wells.

    

 

Hybridization in Power Generation

 

     As mentioned, a combined-cycle natural gas plant is a hybrid system where a gas turbine is paired with a steam turbine. There are many other possibilities with varying practicalities. One is simply a solar-plus-storage setup that can aid a homeowner in going “offgrid.” Solar-plus-storage is also used at utility-scale and more of these projects are being proposed to the interconnection ques. There are several other examples of hybrid power plants. Some plants that burn gas also retain coal units for use in cold snaps and others can burn fuel oil during cold snaps. Gas turbine plants can be paired with short duration battery assist. Several kinds of microgrids and combined-heat-and-power, or co-generation can be considered to be hybrids. Waste-heat recovery is a common feature. Hybrids pairing natural gas and renewables in microgrids of various sizes have been deployed. Solar, wind, or batteries are employed.

 

The following sections are excerpted from my 2022 book: Natural Gas and Decarbonization so may be just slightly outdated.   

 

Hybrid Gas Peakers with Battery Power for Start-up, Ramp-up, Spinning Reserve, and Lower Emissions: GE’s Hybrid Electric Gas Turbines

 

     In 2017 Southern California Edison and General Electric began operations on what GE calls hybrid electric gas turbine (EGT) units. The model is the LM6000 GE Hybrid Electric Gas Turbine (EGT). These were installed as 50 MW gas peakers with 10MW/4MWh of lithium battery power. Peaker plants are usually designated as “energy service’ resources, either ramped up or off but with added battery power they can provide “spinning reserve,” which is required for quick response. The batteries provide an ideal ramping resource for quick start and quick ramp-up. That allows the plant to respond quickly to short time signals in the 5 to 15-minute range. The batteries increase the flexibility of the plant quite a bit. The turbines run less since they are not required for the power consuming start-up and ramp-up processes. Thus, fuel use is reduced as are carbon and NOx emissions. It also makes these plants cost-effective to run as the batteries can be charged with lowest cost curtailed wind and solar generation. The quick start-up and quick ramp-up provided by the batteries also decrease maintenance costs compared to the turbines powering those functions. These plants can potentially make peaker plants more economic investments for utilities and since they also provide significant decarbonization that makes them attractive for company and regional decarbonization goals. The software-based digital controls and associated equipment can reduce both fuel use and water use and provide seamless operation. The SCE and GE project is expected to reduce greenhouse gas emissions and air pollution by 60%. These ‘digitalized’ gas plants show that gas turbines and energy storage can be quite complementary in optimizing plant function and increasing capacity factors, making such plants less likely to become stranded assets.^{[i] [ii]}

     At the end of September 2020, just a month or so after the California rolling blackouts, a similar gas peaker and battery hybrid plant with the same companies involved, the Stanton Reliability Center, began operations in the state. This one involves two 49 MW gas combustion turbines paired with two 10MW lithium batteries. Cost was $150 million with a 20-year resource adequacy contract. The California Energy Commission explained: “Stanton is designed to operate during periods of peak power demand, providing “greenhouse gas-free spinning reserve, high-speed regulation, primary frequency response, and voltage support with the combined response of the gas turbine and the battery storage system.”^[iii]

 

     Pintail Power’s Natural Gas Combined Cycle and Thermal Energy Storage Hybrid Systems

 

     Low utilization rates of thermal power plants, both coal and gas, leads to more emissions per unit of energy produced. Using these plants, most efficient for baseload operation, to follow and firm intermittent generation makes them more emissions intense as they may have to start a couple times in a day and start-up is emissions intense. One solution specifically for peaking plants is utilizing battery power for start-up as described in the previous section. Another is utilizing thermal storage by combining liquid molten salt thermal storage with natural gas combined cycle (LSCC). Peaking plants as simple-cycle plants with combustion turbines but no recovery of waste heat for a steam cycle can be outfitted. Pintail Power’s patented LSCC system utilizes combustion turbine exhaust heat. A big factor in the design is moving the boiler outside the gas stream, resulting in improved output and efficiency: “A key innovation is moving the boiler outside the gas path, reserving exhaust gas for heating feedwater to the boiler and superheating steam from the boiler. This increases the available steam flowrate by 2.5 to 3 times compared to conventional heat recovery steam generation, resulting in substantially higher steam cycle power output.” That is quite a boost for the steam cycle. This happens “due to the synergistic use of stored energy for evaporation and exhaust energy for sensible heating.” The LCSS system can yield about 50% improvements in plant heat rate. Redesigned flow parameters and a non-reheat steam cycle enable fast start-up and system readiness is designed to be enabled by curtailed renewables (which in places like solar-heavy California is most available just hours prior to solar generation drop-off in the typical evening duck curve). They market it as an optimal way to use curtailed renewables, provide resiliency by providing dispatchable generation, having a storage component that can provide islanding and reduced fuel costs and delivery for remote applications, and for reducing emissions considerably. The system can work with any turbine. Molten salt storage has long been used in concentrated solar plants and is considered proven safe and effective. “Rather than using solar salt, which freezes at 460F (238C), LSCC uses a lower freezing point 288F (142C) eutectic salt, such as the HITEC heat transfer and storage medium from Coastal Chemical. This mixture of water-soluble, inorganic salts of potassium nitrate, sodium nitrite, and sodium nitrate is safe, non-flammable, non-explosive, and non-toxic. It provides exceptional heat transfer performance in a low-cost, reliable, and compact system.” The salt is heated with electric heaters and stored in insulated carbon steel hot tanks with thermal losses less than 1 deg C per day. The electric heaters help the system provide demand response and frequency control to the grid, and temperature control for the system. System components like the flexible electric heaters and the molten salt steam generator for discharging are widely available on the market.

 

“A typical LSCC application requires about 12.25 metric tons of salt per MWh of delivered energy at a cost of roughly $2,000 per metric ton, or about $25/kWh, a fraction of the incremental cost of lithium batteries.”

 

“To minimize the cost of storage, three identical tanks are used in a round-robin scheme, alternately 2-hot/1-empty when fully charged and 2-cold/1-empty when fully discharged.”

 

“By combining low-cost electric charging, low-cost bulk storage media, and proven combined cycle equipment, LSCC can meet the $150/kWh target as a new-build system with a 12-hour duration, while adding LSCC to an existing plant can reduce the cost below $100/kWh. Further cost reductions can be achieved with larger-scale plants, two-pressure steam cycles, or by adopting solid thermal storage media that proponents claim would be lower cost than molten salt.”

 

     The key is the synergies provided by the hybrid model – the hybrid synergies. An analogy might be just as hybrid plug-in electric vehicles enable lower fuel use and emissions while solving range anxiety so too does a hybrid power system like Pintail’s LSCC or Liquid Air Combined Cycle LACC (see below) while solving renewables variability anxiety compared to renewables-only charged storage. In terms of levelized cost of storage (LCOS) this system can beat lithium battery costs by quite a lot, by about 50% according to Pintail. The ability to retrofit existing thermal power plants with LCSS is a key cost savings feature. The safety issues with lithium batteries are eliminated. The salt storage media is also expected to have a long lifetime without the degradation that occurs in batteries. There is also more flexibility in charging and discharging than with batteries. These advantages posit this technology as potentially disruptive to utility-scale battery storage at some point in the future.[iv]  To recap, the performance advantages for LCSS include less fuel usage, higher plant utilization rates, daily load following through discharging short-duration storage, emergency and event response capable of longer-duration storage, solve renewables curtailment issues, and reduce overall emissions. The emissions reduction advantage depends on plant utilization rate and how much power comes from curtailed renewables. Pintail just calls it low-carbon power. One might call these plants Combined-Storage-Heat-and-Power. They can be scaled for all sizes of microgrids. In sum “the LSCC integrates electrically- heated thermal energy storage with combustion turbine exhaust heat to boost power output and fuel efficiency, while also using the exhaust heat to boost storage efficiency.”[v]

     The LSCC system is quite applicable to places where there is high solar penetration and an abundance of natural gas peaking plant capacity like California. However, ideally it can be adapted where there is existing combine cycle(s) since retrofitting on an existing steam system would be cheaper than adding a new steam system. In 2021 an LSCC pilot plant is being designed possibly for deployment in North Carolina, another solar-heavy state. The project is a public-private partnership and collaboration between the National Energy technology Lab (NETL), utility Southern Company, Pintail Power, Electric Power Research Institute (EPRI), and Nextant ECA.[vi]  

     Pintail’s patented Liquid Air Combined Cycle system is another similar hybrid power plant with storage provided by air cooled to cryogenic temperatures and stored in above ground tanks. It is known as cryogenic thermal storage, or cold thermal storage. High-capacity energy storage and very long-duration energy storage of days to weeks are possible with liquid air. This would be applicable to address seasonal variability of solar generation especially, in places where it is heavy on the grid like California. The high energy capacity is due to the high energy density of liquid air. Both exhaust heat from turbines and the cooled air are used for energy conversion, which maximizes efficiency. The system utilizes widely available refrigeration components and cryogenic tanks and processes and equipment from the industrial gas and LNG industries. “There are two air streams involved in Liquid Air Combined Cycle: Air for cryogenic storage, and air for regasification. In addition, there is an Organic Rankine Cycle which elegantly bridges the hot and cold air streams by extracting additional energy during discharge.”[vii]

     Pintail also has a patented concentrated solar combined cycle (CCSC). This is similar to the LSCC but instead uses CSP for heat instead of electric heaters to charge the thermal storage. Utilizing a combustion turbine and its waste heat the CSCC is much more efficient than the integrated solar combined cycle (ISCC) used in CSP plants. It also provides better performance. It can be added on to existing CSP plants, increasing their value as grid assets.[viii]   

 

Supercritical CO2 Power Cycles Integrated with Waste Heat Recovery for Gas-Fired Generation and Many Other Thermal Applications

 

     Supercritical CO2 (sCO2) power cycles like those used in Allam Cycle and Brayton Cycle apps can also be used with or without oxy-fuel combustion and carbon capture. The STEP Demo in San Antonio is working with versions of the Brayton Cycle for sCO2. Any fuel or energy source can be utilized for heat. The supercritical CO2 cycle is very efficient. It can be used in industrial waste heat recovery and for shipboard propulsion. The sCO2 is utilized as a closed loop working fluid, above it’s critical point of 1070 psi and 88 deg F, in these applications. The fluid is cooled and recirculated. Compared to water sCO2 is a denser fluid. Since there is high fluid density at relatively low temperature there is less compressor work and more efficient compression. “The thermodynamic properties of sCO2 offer better efficiency than organic Rankine cycles at low temperatures and improved efficiency vs. steam Rankine cycles at turbine inlet temperatures exceeding 1000-1100 °F.” These cycles are in the process of being commercialized and components are being built.^[ix] “It is the unique properties of supercritical CO2 that offer intrinsic benefits over steam as a working fluid to absorb thermal energy, to be compressed, and to impart momentum to a turbine. This higher efficiency results in lower cost and lower emissions for the same amount of power produced.” Another interesting fact about sCO2 cycles is that the components such as heat exchangers and turbomachinery can be considerably downsized, up to 85% in the case of turbomachinery. This is a result of the high sCO2 fluid density. This saves space and reduces cost. The smaller components also help give the cycles a higher ramp rate, an improved response time for adapting to changing power load demands, adding flexibility and reliability. The STEP Demo is currently in an extensive test for demand response. Far smaller components, less fuel use, less water use, and a smaller footprint can reduce capital costs.^[x]

     The STEP Demo is adaptable and is testing different configurations to compare. This should yield some interesting results and new opportunities to both decarbonize and reduce costs. The early testing is of a simple recuperated configuration followed by a higher-temperature recompression cycle configuration. After this “The reconfigurable facility can be adapted to perform validation testing of alternative component designs, cycle layouts or control logic. The system may also be extended to include additional components (such as thermal energy storage, oxycombustion hardware) or to perform validation/qualification testing of full-scale waste heat recovery systems.” Relative to steam cycles, sCO2 cycles can increase power plant efficiency up to 10%. It is thought by some that this tech could revolutionize the power plant industry.^[xi] The key to sCO2 cycles having an effect on overall emissions will be widespread adoption. When scaled-up, costs of an sCO2 cycle are expected to be comparable to a steam cycle. sCO2 with waste-heat recovery is expected to be commercialized soon. The Allam Cycle pilot test facility in LaPorte, Texas, operational since May 2018, proved Allam Cycle viability. Several Allam Cycle projects in different areas have been announced and the STEP project has long been working on supply chain development for sCO2 cycles which should catalyze commercialization.

     The 10MWe STEP demonstration plant, a public-private partnership with current federal funding from DOE’s NETL of $115 million and $41 million in private funding, is expected to be up and running in 2022. Other partners in the project include Gas Technology Institute (GTI), Southwest Research Institute, and General Electric Global Research. It is an indirect sCO2 recompression closed Brayton cycle. One goal of the project is to “verify the performance of first-of-a-kind components—including its turbomachinery, recuperators, compressors, and seals—and demonstrate that they can operate at a turbine inlet temperature of at least 700C.” Potential future apps include concentrated solar, nuclear, waste-heat recovery, fossil energy, biomass, long-duration energy storage, closed-loop geothermal energy, and shipboard propulsion. The turbines can be less than one tenth of the size of equivalent output gas turbines which is advantageous in several ways. This is due to the much higher density of CO2 in a supercritical state compared to steam. GTI Senior Program Director John Marion gave an update of the project in October 2021 in an interview with Power Magazine’s Sonal Patel who has been following and writing about sCO2 cycles for a few years now: “Mechanical completion is expected in the spring of 2022. Commissioning and testing in a simple recuperated cycle system configuration is scheduled through 2022. The STEP demo system will then be modified to add additional heat recuperation and operate in an RCBC (Recompression Brayton Cycle) configuration to demonstrate the highest efficiency potential of the technology through 2023. This pilot is a fully operational electric generating power plant and testing is planned that will put power generated on a local grid. Extensive testing is planned to fully explore the operating envelope and confirm performance and control strategies.”^[xii]

     sCO2 power cycles are not new but have been explored for decades. CO2 has clear advantages as a working fluid over steam that result in better efficiency. Its density is nearly twice that of steam giving it a higher volumetric heat capacity which means turbomachinery and components can be less than one tenth in size of steam components. The energy requirements to increase temperature and pressure to get to a supercritical state, with properties of both gases and liquids, are relatively low. Power Magazine’s Sonal Patel, with help from Qian Zhu, an engineer and specialist on clean coal technologies at the IEA Clean Coal Centre, wrote an informative primer on sCO2 power cycles in April of 2019. Zhu noted that sCO2 is “considered an ideal working fluid because it is non-explosive, non-flammable, non-toxic, and relatively cheap.” The sCO2 power cycles are considered to be Brayton Cycles. This includes the special configuration that is the Allam-Fetvedt Cycle. The two types of Brayton cycles are indirectly fired closed-loop sCO2 Brayton Cycles and directly fired cycles. The Allam-Fetvedt Cycle is a direct fired cycle utilizing oxyfuel combustion where the exhaust heat is recycled to be used to re-heat the CO2 recycling system. One might call it a directly fired oxyfuel Brayton cycle with waste heat recovery to re-heat the CO2 recycling system. Among the indirectly fired types there is a simple closed-loop Brayton cycle, a recuperated closed-loop Brayton cycle, and a recuperated recompression closed-loop Brayton cycle. Ohio-based Echogen Power Systems developed a multi-stage recuperated closed-loop Brayton cycle that recovers heat from an industrial plant’s exhaust stream through an sCO2 heat exchanger. sCO2 power cycles lead to more efficient waste heat recovery and are applicable to many thermal energy projects. Directly fired cycles include semi-closed direct oxyfuel Brayton cycle and the Allam-Fetvedt cycle.^[xiii]

     Siemens Energy and Canada’s TC Energy are working on the first commercial deployment of an sCO2 power cycle at a pipeline compressor station in Alberta as mentioned. It is expected to be operational by 2022. They will utilize Echogen’s sCO2 waste heat recovery system which is essentially a heat engine – “the 7.5-MW EPS100—that uses a multi-stage recuperated closed-loop cycle, where heat from an industrial plant’s or gas turbine’s exhaust stream is recovered though an sCO2 heat exchanger. “The turbomachinery pumps the liquid CO2 to high pressure and passes through a combination of recuperators and waste heat exchangers (without using a secondary oil loop) before entering the turbo-expander, which drives the shaft that in turn drives a generator,” the company explained. “Effluent CO2 exits the turbine, and passes through a series of recuperators to exchange more heat, and finally enters the condenser where it is converted back to liquid CO2.” Siemens noted that this type of sCO2 power cycle is quite applicable to many oil and gas operations, including remote ones: “Benefits include a 25% to 40% smaller footprint than steam-based systems, a 10% increase in compressor station efficiency, and the capability to produce clean, emissions-free electricity, Siemens said. “Moreover, because the working fluid is contained within a closed-loop system, no boiler operator is required, making the system suitable for remote operation.” Thus, sCO2 power cycles have much potential to add value at reasonable cost while increasing efficiency and reducing carbon emissions and pollution. Widespread adoption can help decarbonize the oil and gas, power generation, industrial, and transport sectors. The increase in efficiency, especially as costs to deploy the tech come down is expected to help bottom lines too with the potential to make sCO2 waste heat recovery projects more economic than steam so that they will get built and be deployed. There are many candidates where waste heat is simply lost that could be recovered. Larger projects with more cost and emissions benefits are likely in the future.^[xiv]  

 

 

 

 

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[i] St. John, Jeff, April 18, 2017. Inside GE and SoCalEdison’s First-of-a-Kind Hybrid Peaker Plant with Batteries and Gas Turbines. GreenTech Media. Inside GE and SoCal Edison's First-of-a-Kind Hybrid Peaker Plant With Batteries and Gas Turbines | Greentech Media

 

[ii] Stewart, Kent, August 6, 2017. Digitalized Gas Plants and Battery Storage on the Grid: Integration, Collaboration, Competition, and Implications for Cost-Saving, Dealing with Demand Spikes, and Optimizing Gas Peakers. Blue Dragon Energy Blog. Blue Dragon Energy Blog: Digitalized Gas Plants and Battery Storage on the Grid: Integration, Collaboration, Competition, and Implications for Cost-Saving, Dealing with Demand Spikes, and Optimizing Gas Peakers

 

[iii] Hering, Garrett, September 22, 2021. Gas-battery hybrid peaker nears completion in capacity-hungry California. S&P Global Market Intelligence. Gas-battery hybrid peaker nears completion in capacity-hungry California | S&P Global Market Intelligence (spglobal.com)

 

[iv] Conlon, Bill, December 2, 2019. Decarbonizing with Energy Storage Combined Cycles. Power Magazine. Decarbonizing with Energy Storage Combined Cycles (powermag.com)

 

[v] Liquid Salt Combined Cycle. Pintail Power (website). Liquid Salt Combined Cycle – Pintail Power

 

[vi] Hume, Scott (EPRI), April 6, 2021. Liquid Salt CombinedCycle Pilot Plant Design. National Energy Technology Lab. EPRI Title Slide (doe.gov)

 

[vii] Liquid Air Combined Cycle. Pintail Power (website). Liquid Air Combined Cycle – Pintail Power

 

[viii] Concentrated Solar Combined Cycle. Pintail Power (website). Concentrated Solar Combined Cycle – Pintail Power

 

[ix] Allison, Timothy. STEP Advances Supercritical CO2 Power Cycles for Gas-Fired Generation. Pipeline & Gas Journal. July 2021, Vol 246, No. 7. STEP Advances Supercritical CO2 Power Cycles for Gas-Fired Generation | Pipeline and Gas Journal (pgjonline.com)

 

[x] Benefits of STEP Demo. Gas Technology Institute. Improve Efficiency, Lower Emissions in Commercial Energy Applications • GTI

 

[xi] A Step Toward Transformational Energy: Advanced Supercritical CO2 Power Cycles to Improve Efficiencies, Lower Emissions. Southwest Research Institute. Technology Today, Fall 2020. A STEP Toward Transformational Energy | Southwest Research Institute (swri.org)

 

[xii] Patel, Sonal, October 27, 2021. The POWER Interview: Pioneering STEP Supercritical Carbon Dioxide Demonstration Readying for 2022 Commissioning. Power Magazine. The POWER Interview: Pioneering STEP Supercritical Carbon Dioxide Demonstration Readying for 2022 Commissioning (powermag.com)

 

[xiii] Patel, Sonal, April 1, 2019. What Are Supercritical CO2 Power Cycles? Power Magazine. What Are Supercritical CO2 Power Cycles? (powermag.com)

 

[xiv] Patel, Sonal, April 1, 2021. First Commercial Deployment of Supercritical CO2 Power Cycle Taking Shape in Alberta. Power Magazine. First Commercial Deployment of Supercritical CO2 Power Cycle Taking Shape in Alberta (powermag.com)