Natural gas has been used as a fuel for piston engine driven vehicles for over fifty years. The desire to improve efficiency and reduce pollution is causing continual change and improvements in the available technology. Some companies are also researching the use of other gaseous fuels, such as hydrogen, as a substitute for liquid fuels.
Some vehicles are designed with fuel systems that store compressed gas in pressure vessels. For example, CNG is commonly stored at ambient temperatures at pressures up to 3600 pounds per square inch (24,925 kPa). CNG can be stored at higher pressures, but this adds to the weight of the storage tanks because they need to be designed and certified for such higher pressures.
Because the energy density of liquefied gas is much greater than that of compressed gas, vehicles designed for longer range sometimes employ fuel systems that store liquefied gas at cryogenic temperatures in special thermally insulated tanks. For example, LNG is normally stored at temperatures of between about −240° F. and −175° F. (about −150° C. and −115° C.), hereinafter generally referred to as “cryogenic temperatures”, and at pressures of between about 15 and 200 psig (204 and 1477 kPa). LNG storage tanks mounted on vehicles can store fuel for several days under common operating conditions. For vehicles in regular use, storing fuel at cryogenic temperatures is not a problem.
Despite the longtime use of natural gas fueled vehicles, these vehicles represent only a small fraction of the total number of vehicles currently in use and compared to the vast number of gasoline and diesel re-fueling stations, there remains a relatively small number of liquefied gas re-fueling stations. Conventional natural gas re-fueling stations are typically designed for supplying only one of LNG or CNG. When a re-fueling station is intended to serve a fleet of vehicles, the fleet can be standardized to use only LNG or only CNG. However, for re-fueling stations that are intended to serve the general public, or a plurality of commercial fleets, there is a need for a re-fueling station that can supply either LNG or CNG.
Since LNG is stored at low pressures compared to CNG, LNG re-fueling stations deliver fuel at relatively low pressures. For cryogenic fluids, centrifugal pumps are suitable for operating within the typical pressure ranges and are capable of operating with high flow rates. Centrifugal pumps designed for cryogenic fluids offer reasonable efficiency in addition to being relatively inexpensive.
Centrifugal pumps typically require fuel to be supplied to the pump suction with a positive value for the net suction head (NSH), which is defined as the difference between the pump inlet pressure and the inlet saturation pressure (expressed in terms of head). NSH is positive as long as the pump inlet pressure is greater than the inlet saturation pressure. Conversely, NSH can be negative when pump inlet pressure is less than the inlet saturation pressure.
Other LNG re-fueling stations use a pressure transfer system where vapor pressure within the LNG storage tank is controlled to provide the means for displacing the LNG from the storage tank. However, such pressure transfer systems result in extra heat being introduced into the storage tank, and may require additional equipment to prevent over-pressurization of the LNG storage tank. For example, some pressure transfer systems further comprise equipment for refrigerating and/or re-condensing vapor, and/or rely on higher quantities of gas being removed through pressure relief systems.
Another disadvantage with a pressure transfer system is that fuel delivery can be delayed since it takes time to build pressure within the storage tank.
On the other hand, CNG re-fueling stations typically employ positive displacement compressors and a cascading CNG storage system for delivering relatively high-pressure gas. Even though conventional CNG compressors operate at relatively high speeds, flow rates are typically relatively low. A cascading CNG storage system is typically used to ensure an adequate supply of high-pressure gas to fill an average-sized vehicle fuel tank in an acceptable amount of time.
The divergent operating conditions between re-fueling stations for LNG (low pressure with high mass flow rate) and CNG (high pressure with low mass flow rate) have presented a challenge for designing a simple re-fueling station capable of delivering both LNG and CNG, especially when it is desirable to have a system with only one fuel pump or compressor for quickly dispensing either LNG or CNG.
U.S. Pat. No. 5,315,831 issued 21 May 1994 (the ‘831 patent) discloses a combined LNG and CNG fueling station. Vapor pressure in the cryogenic tank is employed to deliver LNG to the dispenser and a natural gas fueled internal combustion engine is employed to drive a fuel pump while providing heat to a heat exchanger for producing CNG. In some embodiments, pressure within the cryogenic storage tank is relieved by bleeding gas from the storage tank into the fuel supply system for the internal combustion engine.
Accordingly, the ‘831 patent discloses a pressure transfer system for delivering LNG from the re-fueling station. However, as already noted, there are disadvantages associated with a pressure transfer system, such as more frequent venting from the LNG storage tank when pressure within the storage tank exceeds a predetermined maximum pressure. Venting from the LNG storage tank results in wasted natural gas.
In other arrangements, to avoid frequent venting, refrigeration equipment may be employed for re-condensing the natural gas or at least cooling the gas to collapse some of the pressure within the LNG storage tank. However, such arrangements add to the complexity of the system in addition to increased capital and operational costs.