Friday, December 12, 2014



April 2019

    A viable carbon free energy storage concept, utilizing heat source (hydrogen) and heat sink (liquid air) for transport and electric power generation, is presented. Recovery of recycled heat and let-down pressure during synthetic fuel gasification drives production of hydrogen and liquefaction of other gases, respectively. The "third way" heat source replaces problematic solar and nuclear heat sources for hydrogen synthesis, and liquefied gases provide sub-cooling of solar or wind powered air liquefiers. Energy storage has in general been dominated by electro-chemical batteries, which are hazardous, mostly dead weight, resource-intensive to manufacture and limited by charge capability. 

Features
    Features of the advanced heat source and heat sink storage concepts include:
1. Gasifier with heat recovery to thermo-chemical hydrogen 
    (TCH) reactor and pressure let-down to compress hydrogen and  
    liquefy other gases,
    a. low temperature solar pre-heat of thermo-chemical reactor   
        fluids,
    b. selective synthesis of hydrogen, syngas and methane,
    c. cryo-capture and methanation recycle of residual carbon from  
        gasifier and prime mover,   
    d. sub-cooling of air liquefier by liquefied gases.
2. Stationary and motor vehicle gas turbine (GT) prime mover  
    fired by methane and hydrogen with carbon monoxide, as   
    required,
    a. cryo-compression by liquefied air,
    b. vehicle induced aspiration to power cryo-GT compressor,
    c. high cryo-GT regenerative braking potential,
    d. cryo-capture of carbon dioxide to dry ice,
3. Combined magneto-caloric/vapor-compression liquefier for  
    gasifier fuel product and for prime mover air, 
    a. absorption of sensible heat by expanded air and absorption of 
        latent heat by magneto-caloric,    
    b. auxiliary drive by dry ice cooled photo-voltaic (PV) panels,   
        and by structure aspirated wind turbine-generator,

Advantages of liquid air storage over battery storage include:
* low weight with approximately two times specific capacity,  
   increasing as consumed, 
* low capital cost,
* long service life with no disposal requirement, 
* consistent efficient performance, 
* less hazardous in terms of toxicity and fire safety, and
* global availability of air.

Background                                                                                           

   A "liquid nitrogen economy" was proposed in 1974 [1] and some  engines with pressurized liquid air working fluid were built and tested, including a fired gas turbine [2], a reciprocating air engine [3] and two air turbines [4, 5]. Subsequently, liquid nitrogen storage began gaining acceptance as indicated by an operating 300 kW pilot planting and development of a peaking turbine [6]. The Cryo-GT has a unique vaporizing cryo-compression system [7] designed to economize liquid air consumption. It is well suited for smaller low pressure motor vehicle application and for distributed generation in an integrated gasification combined cycle (IGCC) [8]. Gasifiers and IGCC plants experienced a period of strong growth [9], which was interrupted by decommissioning and change-over to natural gas due to high carbon release. Conversion to TCH offers an opportunity to revive this technology, while availability of decommissioned plants adds a strong economic incentive.

Prime Movers and Infra-structure
   This post describes the stationary and motor vehicle Cryo-GT prime movers and the infra-structure for supplying compressed or liquefied fuel and sink refrigerant. The distributed generation and vehicle fuel is selected from TCH, syngas (hydrogen and carbon monoxide) and synthesized methane, as required. When methane is selected, a recycle process is employed to react captured carbon with TCH back to methane. It is noteworthy that cryo-GT hydrogen consumption and storage is minimal, especially in vehicle application with draft recovery to supplement regenerative braking. The sink refrigerant is nitrogen enriched liquid air, which is readily condensed using power generated by gasifier let-down pressure, as well as wind and solar. 
   Exemplary household electric consumption is 35 kWh/d, requiring 1.6 kg/d (3.5 lb/d) of liquid methane and 13.6 kg/d (30 lb/d) of liquid airExemplary compact vehicle consumption at 80 km/h (50 mph) is 8 kWh/d, requiring 18 kg/d (40 lb/d) of liquid air and 1.4 kg/d (3 lb/d) of liquid methane, equivalent to 80 mpg of gasoline. Cryo-GT performance is exemplified by 70% station and 45% compact vehicle thermal efficiency (exclusive of recovered draft and deceleration). The Cryo-GT ranges in capacity from about 10 kWe for small vehicles to 1 MWe for a distributed electric station with gasifier and fuel and air liquefiers. Gasifier performance, exemplified by 70% fuel conversion efficiency, requires estimated 15 kg/d (32 lb/d) of wood to supply the combined household and two vehicle requirement. 
   The cryo-sink reduces gas turbine fuel consumption by one–half and by two-thirds as compared to a normally aspirated Brayton and Otto Cycle engine, respectively. Small gas turbine application is enabled, while reducing compressor/turbine work and pressure ratio. 

Air Blown Gasifier                                                                  

   Growing deployment of synthetic fuel gasification [10] affords the potential to combine hydrogen production, utilizing gasifier heat, with liquid air storage, utilizing gasifier let-down pressure. The Integral Gasification/Thermo-Chemical (IGTC) electric plant is a "third way" primary heat source to replace problematic solar and nuclear heating for TCH synthesis.                                                           

   Recovered heat in the air blown gasifier powers a thermo-chemical reactor, which may operate on selected cycles in the gasifier temperature range; including Hybrid Sulfur [11] (Westinghouse), Sulfur-Iodine [12] and Manganese Oxide [13]. The exemplary cycle of this post is the Hybrid Sulfur cycle, which is under advanced development by Savannah River National Laboratory and others. Gasifier outlet temperature of ~ 1100 oC (2010 oF) at ~ 60 bar is well above required sulfur decomposition temperature of ~ 850 oC (1560 oF). The thermo-chemical reactor replaces the steam evaporator of a Rankine bottoming cycle, such as used in commercially available Mitsubishi gasifiers to recover heat of recycled char. Recycling the char above slag liquefaction temperature of 1200 oC (2200 oF) delivers syngas in parallel with TCH. There is further potential to deliver additional TCH by recovery of gasifier waste heat of ~ 30%.                                                                                                 

   With wood feedstock, a typical syngas mixture (mol fraction) from the gasifier is 51% N2, 27% CO and 14% H2, which equates to N2/H2 mass ratio = 50 and CO/H2 mass ratio = 27. With bituminous coal feedstock, a typical syngas mixture (mol fraction) from the gasifier is 56% N2, 31% CO and 11% H2, which equates to N2/H2 mass ratio = 67 and CO/H2 mass ratio = 41Thermal yield of the gasifier/thermo-chemical reactor is approximately 80% TCH and 20% syngas. High yield of TCH reduces gasifier carbon discharge by 70 - 80% and is sufficient to methanate 100% of prime mover and residual gasifier carbon discharge. 

   Downstream of the gasifier, water cooled syngas undergoes clean-up before depressurization through an expander-air compressor to power fuel and air liquefiers. In parallel, compressed hydrogen and oxygen are discharged from the thermo-chemical reactor while, sulfur dioxide depressurizes through an expander-air compressor to supplement liquefier power. The liquefied gases fuel both station and motor vehicle Cryo-GT's. 

   In stationary application, liquid syngas from the gasifier vaporizes during heat sink sub-cooling before firing the cryo-compression gas turbine (Cryo-GT) prime mover. In motor vehicle application, liquid air made with let down pressure assist from the gasifier, provides the Cryo-GT sink. The cryo-sink reduces gas turbine fuel consumption by one–half and by two-thirds as compared to a normally aspirated Brayton and Otto Cycle engine, respectively. Small gas turbine application is enabled, while reducing compressor/turbine work and pressure ratio. Liquid air storage previously relied on intermittent off-peak grid, solar and wind to provide liquefied refrigerant. 

   The methanator operates according to the exothermic reaction; CO + 3H2 = CH4 + H2O. Thermo-chemical reactor hydrogen and separated syngas hydrogen combine with reactor and Cryo-GT exhaust carbon dioxide to yield methane for Cryo-GT fuel. The mass ratio of converted carbon dioxide to hydrogen is 5.5, yielding methane with 85% of the hydrogen lower heating value. Residual carbon dioxide is minimal, eliminating the need for water gas shift upstream of the methanator.

Stationary Cryo-GT Prime Mover                                                                 The stationary Cryo-GT operates with phase change storage, in which sub-cooling of working fluid entering the compressor is by liquefied methane and nitrogen. Sub-cooling is via a cryo-heat exchanger. Storage density is approximately 16 times as compared to a Li-ion battery. Source temperature may range from ambient atmosphere in a fuel-less liquid air engine to approximately 890 oC (1630 oF) with sink temperature at -190 oC (-315 oF). The gas turbine is the reference engine for phase change storage because of the simplification afforded by external compression and convertibility of available micro-turbines and turbo-chargers to cryo-compression. The reference design point is based on a 1 MWe distributed generation output. Gas turbine efficiency is 70 % at 25,000 rpm with turbine compression ratio of 4; turbine inlet gas temperature of 894 oC (1640 oF); air compressor inlet temperature of –175 oC (-280 oF) and recuperator effectiveness of 90%. Under these conditions methane consumption is 100 kg/h (220 lb/h). Gas turbine compression work is about one-fourth, as with ambient air intake. Emissions are reduced in proportion to fuel consumption and dry ice may be deposed from engine exhaust for other sink pre-cooling. The stationary air liquefier provides liquid air refrigerant for heat sink cooling of motor vehicle Cryo-GT prime movers. Liquefaction is by a vapor-compression machine, in which a cryo-compressor recirculates air and atmospheric make-up air sub-cooled by the syngas in a cryo-cooler arranged in parallel with the heat sink of the stationary Cryo-GT. The sub-cooled air then discharges through a two phase turbine expander into a liquid air separator, from which a liquid air product portion is drawn-off to an air dewar.

Motor Vehicle Cryo-GT Prime Mover

   The motor vehicle Cryo-GT operates with liquid air storage, in which sub-cooling of working fluid entering the compressor is by injection of liquefied air to the cryo-compressor. Storage density is approximately 16 times as compared to a Li-ion battery. Source temperature may range from ambient atmosphere in a fuel-less liquid air engine to approximately 890 oC (1630 oF) with sink temperature at -190 oC (-315 oF). The gas turbine is the reference engine for liquid air storage because of the simplification afforded by external compression and convertibility of available micro-turbines and turbo-chargers to cryo-compression. 
   Prime mover performance is based on a compact 1600 kg (3500 lb) vehicle with frontal area of 2.3 m2 (25 ft2) and drag coefficient of 0.29. Cruise and top speed design points are calculated for 8 kW output at 80 km/h (50 mph) at 50000 rpm and 51 kW at 160 km/h (100 mph) at 100,000 rpm with no vehicle energy recovery.  Gas turbine efficiency is 50% at 50000 rpm and 60% at 100,000 rpm with turbine compression ratios of 1.4 and 2.0, respectively. Turbine inlet gas temperature is 894 oC (1640 oF), air compressor inlet temperature is –175 oC (-280 oF) and recuperator effectiveness is 90%. Under these conditions methane consumption is 1.4 kg/h (3.0 lb/h) and, 6.1 kg/h (13.5 lb/h) respectively. Gas turbine compression work is about one-fourth, as with ambient air intake. Emissions are reduced in proportion to fuel consumption and dry ice may be deposed from engine exhaust for liquefier pre-cooling.
   With energy recovery, including vehicle induced aspiration [8] and deceleration, pressurized hydrogen fueling may be viable. 

Stationary Air Liquefier

   Phase change using liquid air is unique among storage concepts, providing enhanced prime mover performance by lowering heat sink temperature.    Power to the air liquefier is available from recovered gasifier or external renewable sources and temperature of the liquid air is -193 oC (-315 oF). Required power is about one-third as with a vapor compression machine without sub-cooling and associated Cryo-compression.

Stationary Fuel Liquefier

   Syngas liquefaction from the air blown gasifier is by a proposed two-stage process. The first stage is water cooling of syngas from the gasifier at constant pressure to ambient temperature before sensible cooling by turbine expansion. The second stage is magneto-caloric heat lift of latent heat to a dry ice heat sink at – 80 oC (-110 oF), which requires recirculation of vaporized syngas through the magneto-caloric device. The liquefied product is CO and N2 with dissolved H2. A similar two-stage process may be optionally employed to liquefy the O2by-product of the thermo-chemical reactor, as required. Dry ice for the magneto-caloric sink is available from Cryo-GT exhaust and from external sources. Power to the syngas liquefier is from recovered syngas heat downstream of the gasifier via a steam turbine-generator. The reference design point is based on liquefying the above described syngas mixture from the gasifier of 1% H2, 31% CO, 2%CH4, 8%CO2, and 58% N2 (mass fraction). Estimated temperature of the liquid is -193 oC (-315 oF) and power from the steam turbine-generator to the syngas liquefier is about one-quarter as with a magneto caloric liquefier providing normal heat lift to ambient atmosphere. 

Motor Vehicle Cryo-GT Liquefier

   The on-board air liquefier supplies liquid air for cooling the cryo-compressor of the gas turbine prime mover. Supplementary liquid air from an external source may be provided to the cryo-compressor, as required. The liquefier is a magneto-caloric machine, which pumps the latent heat of pre-cooled air to atmospheric temperature. Pre-cooling to -173 oC (-280 oF) is by expansion of air from an isothermal compressor, driven by recovered vehicle energy.

Recovered Station Energy  

   Power to the station air liquefier and to other air liquefiers is provided by recovery of wind and solar energy. Structure induced aspiration enhances wind capture by over 400% as compared to a conventional wind turbine. The air liquefier compressor is coupled to a fan, which is under an estimated 5 differential velocity heads between impact pressure and suction in the wake of leading edges of a building, wall or closed fence 
   Enhanced solar drive of air liquefiers is by sub-ambient cooling of the photo-voltaic panel with dry ice, which is deposed from the gasifier and the gas turbine exhaust. Cooling to - 80 oC (-110 oF) increases panel electric output by approximately 50%.

Recovered Vehicle Energy

   Motor vehicle fuel consumption can be decreased by over 50% with combined recovery including deceleration and vehicle induced aspiration. Deceleration recovery is a well developed technology and operates to drive the on-board vehicle air liquefier. Vehicle induced aspiration operates with the vehicle under power to drive the GT cryo-compressor. The cryo-compressor is coupled to a fan, which is under an estimated 5 differential velocity heads between vehicle impact pressure and suction in the wake of vehicle leading edges. 

References

1. Kleppe, J. and Schneider, R., "A Nitrogen Economy", ASEE, 1974.                                                                                                  2. Kishimoto, K. et-al, “Development of Generator of Liquid Air Storage Energy System", Mitsubishi Tech. Review Vol. 35-3, 1998. 3. Dearman, P., "Liquid Air in Energy and Transport Systems", Centre for Low Carbon Futures, ISBN-978-0-9575872-2-9 UK, 2013.                                                                                                  4. Ordonez, C., “Liquid Nitrogen Fueled, Closed Brayton Cycle Cryogenic Heat Engine", Energy Conversion and Management, 2000.                                                                                                  5. Knowlen, C. et al, "High Efficiency Energy Conversion Systems for Liquid Nitrogen Automobiles", U. of Washington, SAE981898, 1998.                                                                                                  6. Center for Low Carbon Futures, “Liquid Air in Energy and Transport Systems", ISBN: 978-0-957872-2-9, 2013.                     7. Kaufman, J.S. "Motor Vehicle Energy Converter " US Patent 7,854,278 B2, 2010.                                                                           8. Gabbar, H. et-al, “Conceptual Design and Energy Analysis of IGCC System", Sustainability 2017.                                                9. Higman, C., "GSTC Global Syngas Database", Global Syngas Technologies Conf., 2018.                                                               10. National Energy Technology Laboratory, “Entrained Flow Gasifiers, MHI”, 2018.                                                                                                   11. Gorensek, M. et-al, “Solar Driven Thermo-Electrochemical Hybrid Sulfur Process for Hydrogen Production", AIMES 2018.  12. Perret, R., “Solar Thermochemical Hydrogen Production Research", SAND 2011-3622-2011.                                                 13. Rao, C. et-al, “Solar Thermochemical Splitting of Water to Generate Hydrogen", PNAS 114(51), 2018.                                   14. Liebert, C. et-al, “Solar-Cell Performance at Low Temperatures and Simulated Solar Intensities", NASA 1969.                              15. Kaufman, J.S.,"Building with Energy Recovery and Storage Systems", US Patent 9,395,118 B2, 2016.









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