cryocar.doc

8/11/2019 cryocar.doc

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Cryocar Seminar

ABSTRACT

Cryogens are effective thermal storage media which, when used for

automotive purposes, offer significant advantages over current and proposed

electrochemical battery technologies, both in performance and economy. An

automotive propulsion concept is presented which utilizes liquid nitrogen as the

working fluid for an open Rankine cycle. The principle of operation is like that of

a steam engine, ecept there is no combustion involved. !iquid nitrogen is

pressurized and then vaporized in a heat echanger by the ambient temperature of

the surrounding air. The resulting high " pressure nitrogen gas is fed to the engine

converting pressure into mechanical power. The only ehaust is nitrogen.

The usage of cryogenic fuels has significant advantage over other fuel.

Also, factors such as production and storage of nitrogen and pollutants in the

ehaust give advantage for the cryogenic fuels.

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CONTENTS

Chapters Page No

#.$ %&TR'()CT%'& #

*.$ +%T'R- *

.$!&*$$$/ !%0)%( &%TR'12& 3R'3)!%'& C-C!2 4

.#3ART '5 A !%0)%( &%TR'12& 3R'3)!%'& C-C!2 6

.#.# Cryogen torage 7essel. 6

.#.* 3ump. 8 .#. 2conomizer. 8

.#.4 2pander 2ngine. 8

.#.6 +eat echanger. 9

4.$3':2R C-C!2 #$

6.$ 32R5'R;A&C2 '5 T+2 '32& RA&.$ T2C+&%CA! %)2 #9

>.# Range 2tension and 3ower ?oosting #9

#$.$ C'&C!)%'& #=

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Chapter 1

INTRODUCTION

The importance of cars in the present world is increasing day by day. There

are various factors that influence the choice of the car. These include performance,

fuel, pollution etc. As the prices for fuels are increasing and the availability is

decreasing we have to go for alternative choice.

+ere an automotive propulsion concept is presented which utilizes liquid

nitrogen as the working fluid for an open Rankine cycle. :hen the only heat input

to the engine is supplied by ambient heat echangers, an automobile can readily bepropelled while satisfying stringent tailpipe emission standards. &itrogen

propulsive systems can provide automotive ranges of nearly 4$$ kilometers in the

zero emission mode, with lower operating costs than those of the electric vehicles

currently being considered for mass production. %n geographical regions that allow

ultra low emission vehicles, the range and performance of the liquid nitrogen

automobile can be significantly etended by the addition of a small efficient

burner. ome of the advantages of a transportation infrastructure based on liquid

nitrogen are that recharging the energy storage system only requires minutes and

there are minimal environmental hazards associated with the manufacture and

utilization of the cryogenic @fuel@. The basic idea of nitrogen propulsion system is

to utilize the atmosphere as the heat source. This is in contrast to the typical heat

engine where the atmosphere is used as the heat sink.

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Chapter 2

HISTORY

Researchers at the University of Washington are developing a new zero-

emission automobile propulsion concept that uses liquid nitrogen as the fuel. The

principle of operation is like that of a steam engine, ecept there is no combustion

involved. %nstead, liquid nitrogen at "*$ 5 B"#>8 C is pressurized and then

vaporized in a heat echanger by the ambient temperature of the surrounding air.

This heat echanger is like the radiator of a car but instead of using air to coolwater, it uses air to heat and boil liquid nitrogen. The resulting highDpressure

nitrogen gas is fed to an engine that operates like a reciprocating steam engine,

converting pressure to mechanical power. The only ehaust is nitrogen, which is

the maEor constituent of our atmosphere.

The !&*$$$ is an operating proofDofDconcept test vehicle, a converted #>=4

1rummanD'lson


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with gaseous nitrogen to develop system pressure but a cryogenic liquid pump will

be used for this purpose in the future. A preheater, called an economizer, uses

leftover heat in the engine/s ehaust to preheat the liquid nitrogen before it enters

the heat echanger. The specific energy densities of !&*are 64 and =9 :DhGkgD!&*for the adiabatic and isothermal epansion processes, respectively, and the

corresponding amounts of cryogen to provide a $$ km driving range would be

46$ kg and *=$ kg. ;any details of the application of !&* thermal storage to

ground transportation remain to be investigatedH however, to date no fundamental

technological hurdles have yet been discovered that might stand in the way of

fully realizing the potential offered by this revolutionary propulsion concept.

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Chapter 3

LN2000'S LIUID NITRO!EN PROPULSION CYCLE

"#g$re 1% LN2000 $#( )#troge) prop$&s#o) *+*&e,



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.# 3ART '5 A !%0)%( &%TR'12& 3R'3)!%'& C-C!2

The main parts of a liquid nitrogen propulsion system areI

#. Cryogen torage 7essel.

*. 3ump.

. 2conomizer.

4. 2pander 2ngine.

6. +eat echanger.

The parts and their functions are discussed in detail belowI

3,1,1 Cr+oge) Storage -esse&%

The primary design constraints for automobile cryogen storage vessels areI

resistance to deceleration forces in the horizontal plane in the event of a traffic

accident, low boilDoff rate, minimum size and mass, and reasonable cost. CrashD

worthy cryogen vessels are being developed for hydrogenDfueled vehicles that will

prevent loss of insulating vacuum at closing speeds of over #$$ kmGh.#=

;oderately high vacuum B#$D4 torr with super insulation can provide boilDoff

rates as low as #J per day in *$$ liter B6 gal containers. )sing appropriate

titanium or aluminum alloys for the inner and outer vessels, a structurally

reinforced dewar could readily have a sevenDday holding period. The cost of a

mass produced, *$$ liter automotive tank for liquid hydrogen containment has

been estimated to be between K*$$ and K4$$ Bin #>9$ dollars. Thus the epense

of a 4$$ liter !&*tank Bor two *$$ liter tanks is epected to be reasonable.



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3,1,2 P$.p%

The pump is used to pump the liquid nitrogen into the engine. The pump

which are used for this purpose have an operating pressure ranging between 6$$ "

8$$ 3si. As the pump, pumps liquid instead of gas, it is noticed that the efficiencyis high.

3,1,3 E*o)o.#/er%A preheater, called an economizer, uses leftover heat in the engine/s ehaust

to preheat the liquid nitrogen before it enters the heat echanger. +ence the

economizer acts as a heat echanger between the incoming liquid nitrogen and the

ehaust gas which is left out. This is similar to the preheating process which isdone in compressors. +ence with the use of the economizer, the efficiency can be

improved. The design of this heat echanger is such as to prevent frost formation

on its outer surfaces.

3,1, Epa)(er%

The maimum work output of the !&*engine results from an isothermal

epansion stroke. Achieving isothermal epansion will be a challenge, because the

amount of heat addition required during the epansion process is nearly that

required to superheat the pressurized !&*prior to inEection. Thus, engines having

epansion chambers with high surfaceDtoDvolume ratios are favored for this

application. Rotary epanders such as the :ankel may also be well suited. A

secondary fluid could be circulated through the engine block to help keep the

cylinder walls as warm as possible. ;ultiple epansions and reheats can also be

used although they require more complicated machinery.

7ehicle power and torque demands would be satisfied by both throttling the

mass flow of !&* and by controlling the cutDoff point of &* inEection, which is

similar to how classical reciprocating steam engines are regulated. The maimum

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power output of the propulsion engine is limited by the maimum rate at which

heat can be absorbed from the atmosphere. The required control system to

accommodate the desired vehicle performance can be effectively implemented

with either manual controls or an onDboard computer. The transient responses ofthe !&*power plant and the corresponding operating procedures are topics to be

investigated.

3,1, Heat E*ha)ger%

The primary heat echanger is a critical component of a !&*automobile.

ince ambient vaporizers are widely utilized in the cryogenics and !&1

industries, there eists a substantial technology base. )nfortunately, portable

cryogen vaporizers suitable for this new application are not readily available at

this time. To insure cryomobile operation over a wide range of weather conditions,

the vaporizer should be capable of heating the !&*at its maimum flow rate to

near the ambient temperature on a cold winter day. ince reasonable performance

for personal transportation vehicles can be obtained with a $ k: motor, the heat

echanger will be sized accordingly. 5or an isothermal epansion engine having

an inEection pressure of 4 ;3a, the heat absorbed from the atmosphere can, in

principle, be converted to useful mechanical power with about 4$J efficiency.

Thus the heat echanger system should be prudently designed to absorb at least

96 k: from the atmosphere when its temperature is only $C.

To estimate the mass and volume of the primary heat echanger, it was

modeled as an array of individually fed tube elements that pass the !& *at its peak

flow rate without ecessive pressure drop. 2ach element is a #$ m long section of

aluminum tubing having an outside diameter of #$ mm and a wall thickness of

# mm. They are wrapped back and forth to fit within a packaging volume having

$.6 m $.4 m $.$4 m dimensions and are arrayed in the heat echanger duct.

%ncoming air will pass through a debris deflector and particulate filter before

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encountering the elements. An electric fan will draw the air through the duct when

the automobile is operating at low velocities or when above normal power outputs

are required.

The tube eterior heat transfer coefficient is based on that for a cylinder in

cross flow and the internal heat transfer is for fully developed turbulent flow. The

bulk temperature of the air is assumed to decrease across each tube row as

determined from energy conservation and the pressure drop is determined for the

whole tube bank. The heat transfer calculations also account for &*pressure drop

and variations in its thermodynamic properties in the tube elements. ome of the

important phenomena not considered at this stage of analysis were the effects of

transient !&*flow rates, start up, frost accumulation, tube fins, and aial thermal

conduction.

The formation of rime ice is highly probable. The atmospheric moisture

will be removed relatively quickly as the ambient air is chilled over the first few

tube rows, leaving etremely dry air to warm up the coldest parts at the rear of the

heat echanger where the !&*enters. urface coatings such as Teflon can be used

to inhibit ice build up and active measures for vibrating the tube elements may

also be applied. +owever, these approaches may not be necessary since high !& *

flow rates are only needed during times of peak power demand and the heat

echanger elements are much longer than necessary to elevate the !&* temperature

to near ambient at the lower flow rates required for cruise. Thus, the frosted tube

rows may have ample opportunity to deDice once the vehicle comes up to speed.

2ven though inclement weather will certainly degrade the performance of

the cryomobile, it will not preclude effective operation. %f the propulsion system

operating conditions were such that the !&*could only be heated to *6$ < prior to

inEection, the flow rates of !&*for the isothermal and adiabatic cycles to generate

$ k: would be ##6gmGsec and #=9 gmGsec, respectively. The previously

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described heat echanger configuration can theoretically heat the higher !& *flow

rate to *6$ < with *6 radiator elements when the vehicle is traveling at *6 kmGsec

B#8 miGh and the ambient air temperature is only $C. The !&*viscous pressure

drop would be about $.$6 ;3a, which is easily compensated for with the cryogenpump. The electric fan would require approimately #.6 k: to accelerate the air

and overcome the 4$$ 3a pressure drop through the heat echanger if the vehicle

were standing still. ince each element is $.98 kg, the total tubing mass would be

#> kg. %f the same mass was added by the manifolds and duct then the net mass of

the heat echanger would be less than 4$ kg. :hen operating on a typical

California day, it is epected that this overDdesigned cryogen vaporizer will readily

heat the !&*up to ambient temperature without any appreciable icing.

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Chapter

POER CYCLE

There are many thermodynamic cycles available for utilizing the thermalpotential of liquid nitrogen. These range from the ?rayton cycle, to using twoD and

even threeDfluid topping cycles, to employing a hydrocarbonDfueled boiler for

superheating beyond atmospheric temperatures. The easiest to implement,

however, and the one chosen for this study, is shown below. This system uses an

open Rankine cycle. The temperature " entropy diagram for the open rankine cycle

is described below.

"#g$re 2% Te.perat$re 4 e)trop+ (#agra. 5or the ope) Ra)6#)e *+*&e,

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tate # is the cryogenic liquid in storage at $.# ;3a and 99


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Chapter

Per5or.a)*e o5 the ope) Ra)6#)e *+*&e

The thermodynamic and economic performances of the adiabatic and

isothermal modes of the open Rankine cycle are shown in Table #. These figures

are based on the specifications of a modified +onda CRM for which performance

data were available. The cost of *.8N per kgD!&*was derived assuming only the

energy cost of production.

3rocess

Adiabatic %sothermal

3ump :orkI

8 kLGkgD!&* 8 kLGkgD!&*

&et :ork 'utI

#>4 kLGkgD!&* #4 kLGkgD!&*

+eat %nputI

4#> kLGkgD!&* 96$ kLGkgD!&*

2nergy (ensityI

64 :DhGkgD!&* =9 :DhGkgD!&*

!&*5low RateI

#.6 kgGkm $.> kgGkm

'perating CostI

.> Gkm *.4 Gkm

O ?ased on 9.= k: for highway cruise at >9 kmGh.P ?ased on *.8N per kgD!&*production cost.

Ta7&e1% Per5or.a)*e o5 the ope) Ra)6#)e *+*&e,

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Chapter 8

LIUID NITRO!EN 9ANU"ACTURE

The cost of the !&* @fuel@ is epected to be reasonable. The primary

epense for producing !&*is the energy cost for compression of air. Cryogenic

separation of nitrogen from other condensables in air typically requires only a very

small fraction of the total energy, so the ideal work to manufacture !&*from air is

very nearly that for using nitrogen as a feedstock. This work is eactly the

reversible work obtainable from an ideal cryoDengine, 98> kLGkg. The actual work

required in a modern !&*plant is *.$D*.6 times the minimum, or #64$D#>*$ kLGkg.

Assuming an industrial electric rate for interruptible power of 6NGk:Dh, the energy

cost would amount to *.8NGkgD!&*, in accord with delivery prices of !&*in large

quantities. ;arketing the other commercially important components of air will

help offset the !&* production costs. ince the equipment needed for air

liquefaction can be powered solely by electricity, it is conceivable to decentralize

the @fuel@ manufacturing process and to place small scale production facilities at

the !&* dispensing sites. A costDbenefit analysis is needed to determine the

smallest air liquefaction machinery that can be used to produce !&* in an

economical manner.



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Chapter :

THER9AL STORA!E E""ECTI-ENESS%

?ecause the design and epense of the cryomobile are driven primarily by

the cryogen heat sink, it is useful to characterize performance by a @sink

efficiency@, Q W/Ql, i.e. the inverse of the coefficient of performance for a

refrigerator. 5or a reversible cycle with fied sink and source temperatures the

ideal sink efficiency is = Th/Tl 1, or 2.9 for Tl= 77 K and Th= 300 K.%f only

the enthalpy of evaporation of !&* BhfgQ #>> kLGkg is used to sink such an

engine, the ideal work output is 698 kLGkg of !&*. This specific energy is

significantly higher than the #=$D$$ kLGkg achieved with leadDacid batteries.

Additional potential remains for heat sinking by the cold &*vapor, and the

ideal work recoverable from an epansion engine as !& *is evaporated at 99


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Chapter ;

AD-ANTA!ES

tudies indicate that liquid nitrogen automobiles will have significant

performance and environmental advantages over electric vehicles. A liquid

nitrogen car with a 8$Dgallon tank will have a potential range of up to *$$ miles,

or more than twice that of a typical electric car. 5urthermore, a liquid nitrogen car

will be much lighter and refilling its tank will take only #$D#6 minutes, rather than

the several hours required by most electric car concepts. ;otorists will fuel up at

filling stations very similar to today/s gasoline stations. :hen liquid nitrogen is

manufactured in large quantities, the operating cost per mile of a liquid nitrogen

car will not only be less than that of an electric car but will actually be competitive

with that of a gasoline car.

;,1, As *o.pare( to 5oss#& 5$e&s%

The process to manufacture liquid nitrogen in large quantities can be

environmentally very friendly, even if fossil fuels are used to generate the electric

power required. The ehaust gases produced by burning fossil fuels in a power

plant contain not only carbon dioide and gaseous pollutants, but also all the

nitrogen from the air used in the combustion. ?y feeding these ehaust gases to

the nitrogen liquefaction plant, the carbon dioide and other undesirable products

of combustion can be condensed and separated in the process of chilling the

nitrogen, and thus no pollutants need be released to the atmosphere by the power

plant. The sequestered carbon dioide and pollutants could be inEected into

depleted gas and oil wells, deep mine shafts, deep ocean subduction zones, and

other repositories from which they will not diffuse back into the atmosphere, or

they could be chemically processed into useful or inert substances. Consequently,



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the implementation of a large fleet of liquid nitrogen vehicles could have much

greater environmental benefits than Eust reducing urban air pollution as desired by

current zeroDemission vehicle mandates.

;,2, Co.par#)g E)erg+ De)s#t#es%

"#g$re 3% Spe*#5#* e)erg+ 5or


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Chapter =

TECHNICAL ISSUES

=,1, Ra)ge Ete)s#o) a)( Po>er Boost#)g%

Range etension and performance enhancement can be realized by heating

the !&*to above ambient temperatures with the combustion of a relatively low

pollution fuel such as ethanol or natural gas. ?y increasing the gaseous &*

temperature to 6$$


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Chapter 10

CONCLUSION

The potential for utilizing the available energy of liquid nitrogen for

automotive propulsion looks very promising. Time to recharge Brefuel,

infrastructure investment, and environmental impact are among the issues to

consider, in addition to range and performance, when comparing the relative

merits of different F27 technologies. The convenience of pumping a fluid into the

storage tank is very attractive when compared with the typical recharge times

associated with leadDacid batteries. ;anufacturing !&* from ambient air inherently

removes small quantities of atmospheric pollutants and the installation of largeD

scale liquefaction equipment at eisting fossilDfuel power stations could make flue

gas condensation processes economical and even eliminate the emissions of C'*.

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RE"ERENCE

:::. aa. washington. edu

+eat transfer, L.3. +olman

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