LIQUID NITROGEN AS A
NON- POLLUTING FUEL
ABSTRACT:
Our paper examines the capability of several energy conversion process to provide sufficient energy in a world where the non-renewable resource are getting depleted. Moreover pollution caused by them is increasing at a rapid rate. One such efficient and non-polluting means of running the vehicles is the use of liquid nitrogen. To use liquid nitrogen as a non-polluting fuel, a multiple reheat open Rankine and a closed Brayton cycle are used.
INTRODUCTION:
In 1997, the University of North Texas (UNT) and University of Washington (UW) independently developed liquid nitrogen powered vehicles in which the propulsion systems in these vehicles are cryogenic heat engines in which a cryogenic substance is used as a heat sink for heat engine.
Cryogenic:
Cryogenic can be defined as the branch of the physics that deals with the production of and study of effects and very low temperature.
Cryogenic Engineering:
It is mainly concerned with temperatures found in range of –150oC to absolute zero (-273.15oC).
Various Cryogenic Fluids:
q Liquid Helium
q Liquid Nitrogen
Cryogenic Heat Engine:
It is a engine which uses very cold substances to produce useful energy. A unique feature of an cryogenic heat engine is that it operates in an environment at the peak temperature of the power cycle, thus, there is always some heat input to the working fluid during the expansion process.
Liquid Nitrogen :
Liquid Nitrogen is the cheapest, widely produced and most common cryogenic liquid. It is mass produced in air liquefaction plants. The liquefaction process is very simple in it normal, atmospheric air is passed through a dust precipitator and pre-cooled using conventional refrigeration techniques. It is then compressed inside large turbo pumps to about 100 atmospheres. Once the air has reached 100 atmospheres and has been cooled to room temperature it is allowed to expand rapidly through a nozzle into an insulted chamber. By running several cycles the temperate of the chamber reaches low enough temperatures the air entering it starts to liquefy. Liquid nitrogen is removed form the chamber by fractional distillation and is stored inside well-insulated Dewar flasks.
How does the Nitrogen Powered car work?
Heat from the atmosphere vaporizes liquid nitrogen under pressure and produces compressed nitrogen gas. This compressed gas runs a pneumatic (compressed gas drive) motor with nitrogen gas as the exhaust.
Main Components of the Engine:
q A pressurized tank to store liquid nitrogen
q A heat exchager that heats (using atmospheric heat) liquid nitrogen to form nitrogen gas, then heats gas under pressure to near atmospheric temperature.
q A pneumatic motor (along with a Volkswagen transmission) that runs the car.
Principle of Operation :
The principle of running the LN2000Car is like that of steam engine, except there is no combustion involved. Instead liquid nitrogen at –320oF (-196oC) is pressurized and then vaporized in a heat exchanger by ambient temperature of the surroundings air. This heat exchanger is like the radiator of a car but instead of using air to cool water, it uses air to heat and boil liquid nitrogen. The resulting high pressure nitrogen gas is fed to an engine that operates like a reciprocating steam engine, converting pressure to mechanical power. The only exhaust is nitrogen, which is major constituent of our atmosphere.
Analysis of CooLN2 Car Performance:
A single-cylinder reciprocating expander that runs on compressed nitrogen gas with the exhaust gas released into the atmosphere was considered. When compressed gas flowed into the expanders cylinder, isobaric work was done on the moving piston by the gas.
The net isobaric expansion work done during a single cycle is gauge pressure of the gas multiplied by the volume of the gas that flows into the cylinder.
The isobaric specific energy is Wi = (Ph-Pi)V= Ph(1-P-1)V
Ph-Pi is the difference in absolute pressure between inlet and exhaust gas.
If Pi is atmospheric pressure, Ph-Pi is the gauge pressure of compressed gas.
V is the volume occupied by the compressed gas per unit mass of gas.
P = Ph / Pi is inlet to exhaust pressure ratio.
The isobaric specific energy is Wi = RTh (1-P-1) /A.
Here Th refers to the temperature of the high pressure inlet gas.
The COOLN2Car which a converted 1973 Volkswagen and runs on liquid nitrogen is an illustrative to the use of isobaric expansion equation.
Open Rankine Cycle Process:
The processes considered are the expansion of nitrogen gas at 300K and 3.3 MPA to near atmospheric pressure. The first process considered is isothermal expansion from 3.3 MPA to 120KPA and the work can be easily computed as
Wisothermal = rT ln (P2/P1)
r = 0.2968 (KJ/KgK) for nitrogen gas and T = 300K.
The result for Nitrogen is 291.59 KJ/Kg. Another limiting process is the simple adiabatic expansion of the gas in which no heat is admitted during. the expansion. The work is calculated as
Wadiabatic = KrT [1-(P2 / P1) K-1/K] (k-1)
Where T = 300K and K = 1.4, the ratio of specific heats for nitrogen.
The resulting Wadiabatic is 180KJ/Kg of Nitrogen exhausted at 150KPA.
Operation of liquid-nitrogen fueled, regenerative, closed Brayton cycle cryogenic heat engine is illustrate. Considering adiabatic expander and compressor, the specific energy provided by the system is given by
W = egm (eewe-wc/ec) ……(1)
Here,
m = AeL/Rt cold(pe-1)] ……(2) is the ratio of the working fluid mass flow rate to the liquid nitrogen vaporization rate.
T cold is the temperature of the heat single.
P is the ratio of the absolute pressures on the high and low pressure sides.
L = liquid nitrogen’s latent heat of vaporization.
R = 8314 J/mol-K universal gas constant
e = 1-1 /r r = working fluid’s ratio of specific heat capacities at constant pressure and constant volume.
The ideal specific energy provided by an adiabatic expander is
We =RThot (1-p-e)/[A.e] ……….(3)
That = temperature of heat source
The ideal work done by an adiabatic compressor per unit mass of gas is
Wc = RTcold (Pe-1)(A.e) ……(4)
By combining equations we get
W = egL [eep-e (Thot / Tcold) - (1/ec)] ……..(5)
The equation (5) considers the energy available from using liquid nitrogen as a heat sink. The cold nitrogen gas that is produced by vaporizing liquid nitrogen can be used a heat sink as well.
Advantages:
q The energy density of liquid nitrogen is relatively low and better than readily available battery systems.
q They have significant performance and environmental advantages over electric vehicles.
q A liquid nitrogen car is much lighter and refilling its tank will only 10-15 minutes.
q The exhaust produced by the car is environmental friendly.
Efficiency:
The cool LN2 car can travel 15 miles on a full (48 gallon) tank of liquid nitrogen going 20 MPH. Its maximum speed is over 35 MPH.
Conclusion :
In a real sense, the more such vehicles are used, the cleaner the air will become if the liquefaction process is driven by non-polluting energy sources. In addition to the environmental impact of these vehicles, refueling using current technology can take only a few minutes, which is very similar to current gas refueling times.
click here to download the paper
