Published on Apr 02, 2024
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 fuel. Also, factors such as production and storage of nitrogen and pollutants in the exhaust give advantage for the cryogenic fuels
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 "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.
The LN2000 is an operating proof-of-concept test vehicle, a converted 1984 Grumman-Olson Kubvan mail delivery van. Applying LN2 as a portable thermal storage medium to propel both commuter and fleet vehicles appears to be an attractive means to meeting the ZEV regulations soon to be implemented. Pressurizing the working fluid while it is at cryogenic temperatures, heating it up with ambient air, and expanding it in reciprocating engines is a straightforward approach for powering pollution free vehicles. Ambient heat exchangers that will not suffer extreme icing will have to be developed to enable wide utility of this propulsion system.
Since the expansion engine operates at sub-ambient temperatures, the potential for attaining quasi-isothermal operation appears promising. The engine, a radial five-cylinder 15-hp air motor, drives the front wheels through a five-speed manual Volkswagen transmission. The liquid nitrogen is stored in a thermos-like stainless steel tank. At present the tank is pressurized 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 exhaust to preheat the liquid nitrogen before it enters the heat exchanger.
The specific energy densities of LN2 are 54 and 87 W-h/kg-LN2 for the adiabatic and isothermal expansion processes, respectively, and the corresponding amounts of cryogen to provide a 300 km driving range would be 450 kg and 280 kg. Many details of the application of LN2 thermal storage to ground transportation remain to be investigated; 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
Figure 1: LN2000 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:
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. Crash-worthy cryogen vessels are being developed for hydrogen-fueled vehicles that will prevent loss of insulating vacuum at closing speeds of over 100 km/h.18 Moderately high vacuum (10-4 torr) with super insulation can provide boil-off rates as low as 1% per day in 200 liter (53 gal) containers. Using appropriate titanium or aluminum alloys for the inner and outer vessels, a structurally reinforced dewar could readily have a seven-day holding period. The cost of a mass produced, 200 liter automotive tank for liquid hydrogen containment has been estimated to be between $200 and $400 (in 1970 dollars). Thus the expense of a 400 liter LN2 tank (or two 200 liter tanks) is expected to be reasonable.
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.
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.
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.
Vehicle power and torque demands would be satisfied by both throttling the mass flow of LN2 and by controlling the cut-off point of N2 injection, which is similar to how classical reciprocating steam engines are regulated. The maximum power output of the propulsion engine is limited by the maximum 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 on-board computer. The transient responses of the LN2 power plant and the corresponding operating procedures are topics to be investigated.
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.
To estimate the mass and volume of the primary heat exchanger, it was modeled as an array of individually fed tube elements that pass the LN2 at its peak flow rate without excessive pressure drop. Each element is a 10 m long section of aluminum tubing having an outside diameter of 10 mm and a wall thickness of 1 mm. They are wrapped back and forth to fit within a packaging volume having 0.5 m x 0.4 m x 0.04 m dimensions and are arrayed in the heat exchanger duct. Incoming air will pass through a debris deflector and particulate filter before 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 exterior 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 N2 pressure drop and variations in its thermodynamic properties in the tube elements. Some of the important phenomena not considered at this stage of analysis were the effects of transient LN2 flow rates, start up, frost accumulation, tube fins, and axial 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 extremely dry air to warm up the coldest parts at the rear of the heat exchanger where the LN2 enters. Surface coatings such as Teflon can be used to inhibit ice build up and active measures for vibrating the tube elements may also be applied. However, these approaches may not be necessary since high LN2 flow rates are only needed during times of peak power demand and the heat exchanger elements are much longer than necessary to elevate the LN2 temperature to near ambient at the lower flow rates required for cruise. Thus, the frosted tube rows may have ample opportunity to de-ice once the vehicle comes up to speed.
Even though inclement weather will certainly degrade the performance of the cryomobile, it will not preclude effective operation. If the propulsion system operating conditions were such that the LN2 could only be heated to 250 K prior to injection, the flow rates of LN2 for the isothermal and adiabatic cycles to generate 30 kW would be 115 gm/sec and 187 gm/sec, respectively. The previously described heat exchanger configuration can theoretically heat the higher LN2 flow rate to 250 K with 25 radiator elements when the vehicle is traveling at 25 km/sec (16 mi/h) and the ambient air temperature is only 0°C. The LN2 viscous pressure drop would be about 0.05 MPa, which is easily compensated for with the cryogen pump.
The electric fan would require approximately 1.5 kW to accelerate the air and overcome the 400 Pa pressure drop through the heat exchanger if the vehicle were standing still. Since each element is 0.76 kg, the total tubing mass would be 19 kg. If the same mass was added by the manifolds and duct then the net mass of the heat exchanger would be less than 40 kg. When operating on a typical California day, it is expected that this over-designed cryogen vaporizer will readily heat the LN2 up to ambient temperature without any appreciable icing.
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