The present invention relates in general to compressed natural gas ("CNG") storage and dispensing systems, and more particularly concerns novel apparatus for dispensing CNG at substantially constant and easily measured mass flow rates.
In light of expanding concerns about emissions and foreign energy dependence, commercial and passenger vehicles are increasingly being designed to operate on CNG. There is a growing need, therefore, for CNG fueling stations able to resupply these vehicles. In order for CNG-fueled vehicles to have commercial appeal, not only must these stations be conveniently located, as gasoline filling stations are today, but in addition they must be able to refuel a constant stream of vehicles, each in a relatively brief period of time. Further, they must be reasonably inexpensive to install and operate. Given these considerations and the high pressures at which CNG is typically stored within a vehicle, these CNG fueling stations therefore must be specially designed.
A paramount concern of the CNG station operator is the station's "storage effectiveness," defined as the fraction of stored CNG that can be transferred, at a particular pressure, to CNG vehicle tanks. Storage effectiveness, together with compressor flow rate characteristics, determine the number of vehicles that a station can fill in a given time period. Today, storage effectiveness can be improved by increasing the CNG storage pressure, the number of "cascade" levels, or both.
In a typical cascade CNG station, empty vehicle tanks are filled first from one of a series of three conventional CNG storage tanks. If the pressures in the vehicle and first storage tanks equalize at a pressure below the maximum desired vehicle tank pressure, a sequential valve then connects the vehicle tank to a second storage tank, which contains CNG at higher pressure. If necessary, this process then repeats using a third tank. A dome valve ensures that the vehicle tank pressure does not exceed the maximum desired pressure. During filling, a priority valve determines, based on the pressures in each, which storage tank should be refilled first by the compressor. Throughout the filling operation, a mass flow sensor monitors the total amount of CNG transferred to the vehicle.
Although they greatly increase both the cost and the complexity of the station, these various valves and the gas flow rate sensor are essential to the operation of the cascade system. Even with this expense, however, the storage effectiveness still remains well below unity. Storage effectiveness can be incrementally improved in this type of system only by increasing CNG storage pressure. However, not only does this require a higher pressure compressor, but any energy used to increase storage pressure above the maximum vehicle storage pressure is necessarily lost during the filling operation. This increases the total operating cost of filling each vehicle.
Vehicle owners and station operators would also prefer to reduce the temperature rise in the vehicle tank during the filling operation. The lower the final temperature of the tank, the greater the mass of CNG that can be stored there. When filled by a cascade system, however, vehicle tanks experience considerable "compression heating."