Cryogens Essay

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, except there is no combustion involved. Liquid nitrogen is pressurized and then vaporized in a heat exchanger 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 exhaust is nitrogen. The usage of cryogenic fuels has significant advantage over other fuels. Also, factors such as production and storage of nitrogen and pollutants in the exhaust give advantage for the cryogenic fuels. 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. Here an automotive propulsion concept is presented which utilizes liquid nitrogen as the working fluid for an open Rankine cycle. When the only heat input to the engine is supplied by ambient heat exchangers, an automobile can readily be propelled while satisfying stringent tailpipe emission standards. Nitrogen propulsive systems can provide automotive ranges of nearly 400 kilometers in the zero emission mode, with lower operating costs than those of the electric vehicles currently being considered for mass production. In geographical regions that allow ultra low emission vehicles, the range and performance of the liquid nitrogen automobile can be significantly extended by the addition of a small efficient burner. Some 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 â€Å"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. PARTS OF A LIQUID NITROGEN PROPULSION CYCLE The main parts of a liquid nitrogen propulsion system are: 1.Cryogen Storage Vessel. 2.Pump. 3.Economizer. 4.Expander Engine. 5.Heat exchanger. The parts and their functions are discussed in detail below: Cryogen Storage Vessel: The primary design constraints for automobile cryogen storage vessels are: resistance to deceleration forces in the horizontal plane in the event of a traffic accident, low boil-off rate, minimum size and mass, and reasonable cost. Pump: 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 500 – 600 Psi. As the pump, pumps liquid instead of gas, it is noticed that the efficiency is high. Economizer: A preheater, called an economizer, uses leftover heat in the engine’s exhaust to preheat the liquid nitrogen before it enters the heat exchanger. Hence the economizer acts as a heat exchanger between the incoming liquid nitrogen and the exhaust gas which is left out. This is similar to the preheating process which is done in compressors. Hence with the use of the economizer, the efficiency can be improved. The design of this heat exchanger is such as to prevent frost formation on its outer surfaces. Expander: The maximum work output of the LN2 engine results from an isothermal expansion stroke. Achieving isothermal expansion will be a challenge, because the amount of heat addition required during the expansion process is nearly that required to superheat the pressurized LN2 prior to injection. Thus, engines having expansion chambers with high surface-to-volume ratios are favored for this application. Rotary expanders such as the Wankel 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. Multiple expansions and reheats can also be used although they require more complicated machinery. Heat Exchanger: The primary heat exchanger is a critical component of a LN2 automobile. Since ambient vaporizers are widely utilized in the cryogenics and LNG industries, there exists a substantial technology base. Unfortunately, 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 LN2 at its maximum flow rate to near the ambient temperature on a cold winter day. Since reasonable performance for personal transportation vehicles can be obtained with a 30 kW motor, the heat exchanger will be sized accordingly. For an isothermal expansion engine having an injection pressure of 4 MPa, the heat absorbed from the atmosphere can, in principle, be converted to useful mechanical power with about 40% efficiency. Thus the heat exchanger system should be prudently designed to absorb at least 75 kW from the atmosphere when its temperature is only 0 °C. POWER CYCLE There are many thermodynamic cycles available for utilizing the thermal potential of liquid nitrogen. These range from the Brayton cycle, to using two- and even three-fluid topping cycles, to employing a hydrocarbon-fueled 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 states involved in the temperature – entropy diagram for the open rankine cycle is described below. State 1 is the cryogenic liquid in storage at 0.1 MPa and 77 K. The liquid is pumped up to system pressure of 4 MPa (supercritical) at state 2 and then enters the economizer. State 3 indicates N2 properties after it is being preheated by the exhaust gas. Further heat exchange with ambient air brings the N2 to 300 K at state 4, ready for expansion. Isothermal expansion to 0.11 MPa at state 5 would result in the N2 exhaust having enough enthalpy to heat the LN2 to above its critical temperature in the economizer, whereas adiabatic expansion to state 6 would not leave sufficient enthalpy to justify its use. The specific work output would be 320 and 200 kJ/kg-LN2 for these isothermal and adiabatic cycles, respectively, without considering pump work. While these power cycles do not make best use of the thermodynamic potential of the LN2, they do provide specific energies competitive with those of lead-acid batteries. ADVANTAGES Liquid nitrogen automobiles will have significant performance and environmental advantages over electric vehicles. A liquid nitrogen car with a 60-gallon tank will have a potential range of up to 200 miles, or more than twice that of a typical electric car. Furthermore, a liquid nitrogen car will be much lighter and refilling its tank will take only 10-15 minutes, rather than the several hours required by most electric car concepts. Motorists will fuel up at filling stations very similar to today’s gasoline stations. When 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. Compared to fossil fuels: 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 exhaust gases produced by burning fossil fuels in a power plant contain not only carbon dioxide and gaseous pollutants, but also all the nitrogen from the air used in the combustion. By feeding these exhaust gases to the nitrogen liquefaction plant, the carbon dioxide 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 dioxide and pollutants could be injected 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, the implementation of a large fleet of liquid nitrogen vehicles could have much greater environmental benefits than just reducing urban air pollution as desired by current zero-emission vehicle mandates.