Zero-Boil-Off Hydrogen Densification System
Hello, my name is Jong Baik.
At the Florida Solar Energy Center we are researching processes for
increasing the density of cryogenic propellants for launch vehicle
applications. This work is supported by NASA Glenn Research Center.
Technologies that provide for the densification,
conditioning, transfer and storage of cryogenic propellants can reduce gross
lift-off weight of a launch vehicle by up to 20% or increase its payload
capacity. By using densified propellants, we can expect reduced external tank volumes,
decreased vapor pressures, and increased enthalpy gain before boil off. NASA Kennedy Space Center has years of
experience handling cryogenic propellants, but all with saturated liquids. This work focuses on using existing
cryogenic technology to densify hydrogen, and developing a test bed where
densified propellant handling techniques can be researched. Florida
Solar Energy Center and NASA Kennedy Space Center researchers designed, and a
contractor fabricated our hydrogen densification system. Following is a short
description of the densification system components.
The hydrogen
densification system consists of cryostat, cryocooler with helium compressor,
vacuum pump system, gas and liquid cryogen supply system and data acquisition
system.
The cryocooler
constitutes the heart of the hydrogen densification system. Cryomech AL-330
single stage Gifford-McMahon cryocooler was selected and installed on the top
of the densified hydrogen storage tank.
It has an expected cooling capacity of 40W at 20K and 25W at 15K. The
cryocooler is integrated into the cryostat Dewar neck.
The water-cooled type
helium compressor uses 7kW of power and provides 300 psi of gas to the
cryocooler.
The double-walled
cryostat has been designed to store 150 liters of densified hydrogen in its the
stainless steel tank, with additional ullage space. It is 20” in diameter and 40” in height, with multi-layer
insulation and high vacuum space between inner tank and outer jacket.
To minimize
convective heat transfer between inner tank and outer jacket, a combination of
mechanical and turbo molecular vacuum pumps generates 10-6 torr of
high vacuum. Total loss including
radiation, conduction through the support structure and instrumentation lines
is less than 8.3W at 15K.
The entire assembly is designed to be easily modified, if needed, with flanged connections on the cryocooler interface and the outer jacket.
Since the cold head
of the cryocooler is not long enough to reach the bottom of the storage tank,
the heat pipe is used.
The heat pipe is
located at the bottom of the cold head and extends the cryocooler cold head to
the bottom of the inner vessel. It is 3” in diameter and 27” in length. This
pipe uses hydrogen gas as working fluid.
Three silicon diode
temperature sensors are installed on the cryocooler cold head, upstream and
downstream of the heat pipe. From our
preliminary performance tests of the heat pipe using nitrogen, the maximum
temperature difference between the cold head and the bottom end of the heat
pipe was less than 1.5K.
A gas supply line is
wrapped around the bottom of the heat pipe so that supplied gas can be
precooled, liquefied and densified at heat pipe temperature.
Also, more than 2000
pieces of thin copper braids are attached to the bottom of the heat pipe to
increase the contact surface area between heat pipe and fluids.
One capacitance-type
liquid level gauge is installed to measure liquid level in the storage tank.
Five calibrated
silicon diode sensors are installed on a 30” long fiber glass tube along the
vertical axis of the storage tank to gauge liquid level in the tank.
A pressure buildup
unit controls internal pressure by evaporation of stored liquid without any
vent loss.
The pressure buildup unit consists of a
cryogenic valve, a pressure regulator and evaporation coils at the bottom of
cryostat.
Opening the pressure
buildup valve allows stored cryogenic liquid to be evaporated in the pressure
buildup coil by exchanging heat with ambient temperature and an increase in
pressure of storage tank without imposing any external pressure.
On the top flange, a
vacuum-jacketed cryogenic liquid transfer line supplies cryogen to the storage
tank or drains liquid out of the storage tank.
Using an additional
gas supply line and a pressure buildup unit, we can easily transfer cryogenic
liquid into or out of the storage tank.
Adjustable relief
valve, rupture disk and manual venting valve are installed in the gas supply
line, pressure build up coil and manual vent for safety.
In this densification
system, various sensors are installed to study the thermophysical behavior of
densified fluid.
Three pressure
transducers measure the inner storage tank, annular vacuum space and heat pipe
pressure.
Low and high vacuum
gauges depict the pressures of the vacuum pump.
Two mass flow meters
are installed at gas supply and vent lines measure the rate of liquefaction and
heat leak.
Eight temperature
sensors measure the liquid level, cryocooler cold head temperature and heat
pipe temperatures.
Signals from these
instruments are sent to National Instrument field point data acquisition
modules.
Labview 7 Real-time
module performs all the data processing, display and storage of the data on a
PC. All data acquisition modules can be monitored and controlled by intranet
and internet from any remote location.
Researchers at the Florida Solar Energy Center and NASA Kennedy Space Center are collaborating on a test bed designed to gain operational experience in handling densified hydrogen. This test bed has the capability to refrigerate, liquefy, densify and store hydrogen to temperatures near 15K. In preliminary densification tests, the G-M cryocooler has liquefied room temperature nitrogen gas to a 65K densified LN2 at a rate of 15 gallons/day. The system has successfully demonstrated continuous zero-boil-off storage at 65K for several weeks, requiring less than 1 hour a day of cryocooler operation. With our continued work in this area, we expect the hydrogen liquefaction and densification rates at 15K to be 1.2 gallons per day without liquid nitrogen pre-cooling, and 4.4 gallons per day with liquid nitrogen pre-cooling from room temperature gas. This excludes the energy release from the ortho-para hydrogen conversion process. Our next step is to use hydrogen and demonstrate its liquefaction, densification and storage technologies.
Thank you for your
attention.