This research project demonstrated improved full fills of compressed natural gas (CNG) vehicles using vehicle monitoring and data transmission technology. This technology communicates fuel status to the station, where a smart dispenser and station controller receive data from vehicles to more accurately control CNG fills; a pre-cooling expander then applies the pressure differential between the vehicle and stationary storage to cool the CNG beyond a typical Joule-Thomson cooling. Together, these technologies enabled the station to safely add more fuel to the vehicle before the pressure limits of the CNG tank were reached. By eliminating under-filling, it may be possible to reduce the number of high-pressure storage vessels onboard a CNG vehicle resulting in a reduced capital cost of between 5 percent and 25 percent.
The vehicle monitoring system included a temperature sensor in each storage vessel in addition to the standard pressure sensor already in most vehicles. The data were then monitored and transmitted using a Wi-Fi transmitter. The smart station controller monitored all nearby vehicles with Wi-Fi transmission capability while simultaneously monitoring all station dispensers. While the team was unable to reprogram a commercial dispenser, the project validated the smart controller’s capability to enable more complete fills by providing the dispenser with a new filling target using this real-time transmission of vehicle data.
The pre-cooling piston expander was controlled by linear motors and generators. The motors controlled the speed and stroke of the piston expander to adjust the flow-rate of gas to the vehicle. The expander reached peak efficiencies of 73 percent with hydrogen and about 40 percent with CNG. Furthermore, the project demonstrated that the expander is a net positive energy producer and could lower the energy costs of CNG and hydrogen fueling stations by offsetting some of the power consumed by the compressor.
