Friday, December 12, 2014



November 2018

Background

   Since the 1970’s researchers have participated in a global effort to find sustainable energy sources for motor vehicles and electric power generation. This has led to an increased need for clean and reliable energy storage devices, which can store the power generated from clean energy sources, and make it readily available when needed in a wide range of applications. Energy storage has in general been dominated by electro-chemical batteries, which are hazardous, resource-intensive to manufacture, and have limited number of charge cycles.   
   Growth of synthetic fuel gasification [1] now affords the potential to combine hydrogen production, utilizing gasifier heat, with phase change  storage, utilizing gasifier let- down pressure. In stationary application, liquefied syngas vaporizes during heat sink sub-cooling before firing the cryo-compression gas turbine (Cryo-GT) prime mover [2]. In motor vehicle application, liquefied 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. Phase change storage previously relied on intermittent off-peak grid, solar and wind to provide liquefied refrigerant. 
   The integral Gasification/Thermo-Chemical (IGTC) electric plant is a "third way" primary heat source to replace problematic solar and nuclear heating for thermo-chemical hydrogen synthesis. IGTC recovers heat from recycled char in a commercially available gasifier to a thermo-chemical hydrogen cycle, delivering hydrogen and some syngas to fire a station gas turbine-generator while providing liquefied air and liquefied methane for export and vehicle use. Both solar and nuclear primary heat sources are unproven, inefficient and expensive with difficult high temperature design requirements. 
   A "liquid nitrogen economy" was proposed 1974 [3] and some high pressure engines with cryogenic compression were built and tested, including a fired turbine [4] and two fuel-less reciprocating engines[5,6].Subsequently, liquid nitrogen storage began gaining acceptance as indicated by an operating 300 kW pilot planting development of a peaking turbine and a liquid air vehicle engine [7].The Cryo-GT has a unique Cryo-compression system. It is well suited for smaller low pressure motor vehicle application and for distributed generation in an integrated gasification combined cycle (IGCC) [8].
   Advantages of phase change storage over battery storage include:
*long service life with no disposal requirement,
* consistent efficient performance, universal availability,
*low weight and capital cost in a well developed technology,
*less hazardous in terms of toxicity and high temperature fire safety.


   Additional features enhancing fuel and sink refrigerant synthesis include:
* refrigerant pre-cooling of fuel and refrigerant liquefiers,
* motor vehicle induced aspiration to assist Cryo-GT compressor drive,
* refrigerant sub-cooling of photo-voltaic panels [9] to drive liquefiers and auxiliaries,
* structure induced aspiration to assist wind turbine-generator drive, and
*cryo-capture of residual carbon dioxide.

Prime Movers and Infra-structure

   This post describes the stationary and motor vehicle Cryo-GT prime movers and the infra-structure for supplying liquefied fuel and sink refrigerant. The reference vehicle and distributed generation fuels are liquefied methane and syngas, a mixture of carbon monoxide and hydrogen, synthesized from universally available organic materials. The reference refrigerant is liquefied air, which is readily condensed using recovered energy from renewable sources including wind, gasifier pressure and solar. Typical household electric, fuel and liquid air consumption are 35 kWh/d, 1.6 kg/d (3.5 lb/d) liquid methane and 13.6 kg/d (30 lb/d) liquid air, respectively. Cryo-GT performance is exemplified by 45 % compact vehicle (exclusive of recovered draft and deceleration), and 70% station thermal efficiency. The Cryo-GT ranges in capacity from about 10 kWe for small vehicles to 1 MWe for a distributed electric station. Gasifier performance, exemplified by 70 % fuel conversion efficiency, requires estimated 10 kg/d (22 lb/d) of sun dried wood to supply household requirements above. Methanation reaction downstream of the gasifier reduces carbon dioxide emissions from wood or coal combustion to nearly zero.

Stationary Air Liquefier

    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.

Air Blown Gasifier

   Recovered heat from the air blown gasifier powers a thermo-chemical reactor, which may operate on selected cycles in the gasifier temperature range; including Hybrid Sulfur [10](Westinghouse), Sulfur-Iodine [11] and Manganese Oxide [12]. The suggested reference cycle 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) and gasifier waste heat of ~ 40% delivers hydrogen heating potential equivalent to gasifier syngas output. 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 with exemplaryH2/CO normal volume ratio of 0.33 (coal) and 0.67 (wood). Downstream of the gasifier, water cooled syngas (H2, CO and N2), expands through a syngas liquefier to provide liquefied gas with dissolved hydrogen. The liquefied gases vaporize during cooling of Cryo-GT and air liquefier heat sinks, followed by separation of hydrogen from syngas, methanation of hydrogen, and fueling of both station and motor vehicle Cryo-GT's. The methanation process uses dry ice deposed from the Cryo-sink and evaporated during pre-cooling of the fuel liquefier.
   Illustrative gasifier performance is based on reduced system capacity to supply 35 kWh household demand from the Cryo-GT generator, requiring 240,000 Btu Cryo-GT fuel combustion at 50%. With wood feedstock, a typical syngas mixture (mol fraction) from the gasifier is 51% N2, 27% CO, 14% H2, 5% CO2, 3% CH4, which equates to 13.6 kg (30 lb) N2, 7.2 kg (16 lb) CO, 0.3 kg (0.6 lb) H2, 1.9 kg (4.2 lb) CO2, 0.4 kg (0.9 lb) CH4. Methanation then yields 10.5 lb CH4 with lower CH4 heating value = 13000 kJ/kg (5600 Btu/lb). After combustion in the Cryo-GT, 26 lb CO2 enter the methanator and 3.6 kg (8 lb) are vented, equivalent to 13% of CO2 as with normal wood burning.
   Other advantages of “third way primary heat source” are:
*support of gasifier combustion by the oxygen by-product of the thermo-chemical reactor,
* additional heat recovery from gasifier slag discharge, and
* expanded application of Integrated Gasification Combined Cycle(IGCC)plants, some of which are idled or converted to natural gas due to high carbon emissions.

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.

Stationary Cryo-GT Prime Mover

   The stationary Cryo-GT operates with “source + sink” storage”, in which sub-cooling of working fluid entering the compressor is by liquefied syngas prior to water gas shift and methanation. Sub-cooling is via a cryo-heat exchanger. Thermodynamic engine efficiency is a function of the temperature difference between source and sink relative to the temperature of either. Heat sink energy is stored in a refrigerant, just as heat source energy is stored in fuel. 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 source plus sink 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 MW 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.

Methanator

   The methanator operates according to the exothermic Sabatier reaction; CO2 + H2 = CH4 + 2H2O at ~300 oC (-570 oF).Thermo-chemical reactor hydrogen and separated syngas hydrogen combine with reactor and Cryo-GT exhaust carbon dioxide to yield methane for Cryo-GT fuel, while discharging water. The mass ratio of 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.

Motor Vehicle Cryo-GT Prime Mover

   The motor vehicle Cryo-GT operates with “source + sink” storage”, in which sub-cooling of working fluid entering the compressor is by injection of liquefied air to the cryo-compressor.  Thermodynamic engine efficiency is a function of the temperature difference between source and sink relative to the temperature of either. Heat sink energy is stored in liquefied air, just as heat source energy is stored in the fuel. 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 source plus sink 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 [2] and deceleration, pressurized hydrogen fueling may be viable. 

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 

Advanced Solar PV Drive

[Later]

Advanced Wind Drive

[Later]

References

1. National Energy Technology Laboratory, “Entrained Flow Gasifiers, MHI”, 2014
2. Kaufman, J.S., “U.S. Patents 7,398,841, 7,854,278”, 2008, 2010
3. Kleppe, J. and Schneider, R., "A Nitrogen Economy", ASEE, 1974
4. Kishimoto, K. et-al, “Development of Generator of Liquid Air Storage Energy System", Mitsubishi Tech. Review Vol. 35-3, 1998
5. Ordonez, C., “Liquid Nitrogen Fueled, Closed Brayton Cycle Cryogenic Heat Engine", Energy Conversion and Management, 2000
6. Knowlen, C. et al, "High Efficiency Energy Conversion Systems for Liquid Nitrogen Automobiles", U. of Washington, SAE981898, 1998
7. Center for Low Carbon Futures, “Liquid Air in Energy and Transport Systems", ISBN: 978-0-957872-2-9, 2013
8. Gabbar, H. et-al, “Conceptual Design and Energy Analysis of IGCC System", Sustainability 2017
9. Liebert, C. et-al, “Solar-Cell Performance at Low Temperatures and Simulated Solar Intensities", NASA 1969
10. Gorensek, M. et-al, “Solar Driven Thermo-Electrochemical Hybrid Sulfur Process for Hydrogen Production", AIMES 2018
11. Perret, R., “Solar Thermochemical Hydrogen Production Research", SAND 2011-3622-2011
12. Rao, C. et-al, “Solar Thermochemical Splitting of Water toGenerate Hydrogen", PNAS 114(51), 2018