Screenshot from model interface of the flexible
split condenser cycle.
Visualization of pulsating two-phase flow
immediately downstream of the expansion valve.
The prototype of ionic liquid piston compressor
(the designed pneumatic system) for compression
of hydrogen in hydrogen refueling stations.
Flexible heat pump configuration with split condenser
The main outcomes of the analyses are that the
split condenser heat pump has a considerable
potential for lower investment or higher coefficient
of performance, COP, than a traditional
system because of the possible optimization
of the plate heat exchanger configurations. In
addition, the split condenser configuration has
an important advantage by being able to supply
heat at different temperature levels, and hence
provides significant flexibility in the design.
The project also shows that the split condenser
configuration has the highest potential for
ammonia as refrigerant, which matches well
with conventionally used refrigeration and heat
pump systems in industry. Ammonia is a natural
refrigerant and does not contribute to ozone
depletion or global warming.
Continuous vs. pulsating flow boiling
High heat exchanger performance is crucial to
meet efficiency standards with low cost and
environmental impact in various applications
such as heat pumps, refrigeration, and air conditioning.
The performance of heat exchangers
may be improved by passive and active heat
transfer enhancement techniques applicable to
air, liquid, and phase change heat exchangers.
In this project, an experimental investigation
of flow boiling heat transfer was conducted in
a traditional round-tube evaporator with the
aim of heat transfer enhancement by means of
fluid flow pulsations. The hypothesis was that
the pulsations increase the flow boiling heat
transfer by means of better bulk fluid mixing,
increased wall wetting, and flow-regime destabilization.
The fluid pulsations were introduced
by a flow modulating expansion device and
compared with continuous flow by a steppermotor
expansion valve in terms of time-averaged
heat transfer coefficient. The cycle time
ranged from 1 to 7 s for the pulsations and the
refrigerant was R134a, a well-examined pure
fluid.
The results showed that the pulsations improve
the time-averaged heat transfer coefficient
by 5.6 % on average at low cycle time (1 s),
whereas the pulsations may reduce the timeaveraged
heat transfer coefficient at high cycle
time (7 s) by 1.8 %. The latter reduction was
attributed to a significant dry-out that occured
when the flow modulating expansion valve
was closed. Furthermore, the flow pulsations
were visualized by high-speed camera to help
understanding the time-periodic flow regimes
and the effect they had on the heat transfer
performance.
Novel liquid piston compressor for hydrogen refueling stations
Global warming, CO2 emissions, and fossil fuel
depletion, highlight hydrogen fuel cell vehicles
as a promising option for the future car industry.
However, one of the biggest challenges
for a widespread introduction of hydrogen
vehicles into the market is the requirement for
pressurizing hydrogen over 700 bar. Current
gas compressors are costly, have complicated
design, low energy efficiencies, and short life
spans. In addition, due to the lack of internal
cooling, the hydrogen temperature increases
significantly during the compression. This
requires consumption of considerable amount of
energy for cooling of hydrogen before entering
the car tank. Replacing the solid piston with a
reliable liquid in the conventional reciprocating
compressor, as a common compression technology
for compressing hydrogen, will provide a
flexibility in the design as well as the possibility
of the internal cooling. The project is part
of Hyfill-fast international project and aiming
at design, modelling, and fabrication of a novel
compressor technology that can overcome some
of the limitations of the existing compressors.
This can eventually lead to a cheaper hydrogen
stations, distribution, and consequently may
lead to a faster penetration of hydrogen vehicles
into the market.
For this context, the promising concept of ionic
liquid piston has been investigated and the
most appropriate and reliable liquid has been
selected as replacement for the solid piston in
the conventional reciprocating compressors.
Through a comprehensive review between
wide varieties of the liquids, as replacement for
the solid piston, the choice has been narrowed
down to a specific ionic liquid that can fulfill the
requirements. Elimination of the solid piston
provides the possibility of internal cooling of
the hydrogen during compression. A thermodynamic
model has been developed to analyze the
heat transfer inside the compression chamber
of the proposed compressor and find the
parameters that play key role in the reduction
of hydrogen outlet temperature. Finally, a small
prototype has been designed and built as a
proof of concept.
Contact:
Brian Elmegaard, e-mail: be@mek.dtu.dk
Contact:
Martin Ryhl Kærn, e-mail. pmak@mek.dtu.dk
Contact:
Masoud Rokni, e-mail: mr@mek.dtu.dk
Nasrin Arjomand Kermani, e-mail. nasker@mek.dtu.dk
42 thermal energy