High Temperature Heat Pumps with High Energy Efficiency Performance

1
High Temperature Heat Pumps with High Energy
Efficiency Performance
Onofrei R., Stefan D. University Politehnica of
Bucharest, Centre for Technology Transfer in the
Process Industries, 1, Polizu Street, Building A,
Room A056, Sector 1, RO-011061, Bucharest,
Romania, Phone/fax [phone], email
cttip_at_chim.upb.ro
INTRODUCTION To operate at high temperatures, a
heat pump will require a revolutionary approach
if it is to be both efficient and reliable.
Conventional concepts require complex and
critical rotating machines, pumps and turbines,
to compress the working fluid. It is unrealistic
to expect that such machines can be developed and
can operate for a long time at temperatures such
as 1000 K with the reliability needed in an
industrial environment and at the costs demanded
in a competitive economy. Also, the heat pumping
process takes place with relatively small energy
efficiencies when the heat user temperature is
much higher than the surroundings temperature. If
classical mechanical compression heat pumps are
employed, the high difference from the two
temperature levels generally impose, as a
technical solution, the using of a cascade
configuration of such mechanical units, which
greatly increase the complexity of the obtained
system. In literature it is presented a new heat
pumping system that can function at high
temperatures having as working agent
liquid/vapors Na. The functioning of such a
system is quite simple and it relies on the
special property of BASE (Beta-Alumina Solid
Electrolyte) to transport Na ions, making
possible the conversion of electrical energy into
mechanical energy, without the need of any
components in motion. In this paper it is
proposed a system by which such a heat pump is
coupled with a chemical heat pump, with three
chemical reactions, in order to obtain a heating
system with high energy efficiency.
CONSTRUCTIVE AND FUNCTIONAL DESCRIPTION
Na is vaporized in the evaporator. Na atoms are
absorbed on a porous electrode covering the BASE,
where they give up an electron. The Na ions
migrate freely through the polycrystalline
structure of the BASE. After
R reactorsS - separators
Bithermal unit BASE heat pumpTrithermal unit
chemical heat pump
recombination, Na atoms are at a higher
electrochemical potential consistent with a
higher pressure and temperature in the condenser.

Chemical heat pump involved in the couple
The couple configuration
Pseudo-mechanical heat pump with BASE involved in
the couple
MATHEMATICAL MODEL (THERMODYNAMICAL ANALYSIS)
Exergy eficiency of the trithermal unit
Energy eficiency of a hypothetical bithermal heat
pump with a exergy eficiency equal with the one
of the unit in the couple
Other obvious relations
Exergy eficiency of the bithermal unit
Restrictions
Energy balance for the trithermal couple
Energy eficiency of the couple
SIMULATION
Results
Working conditions and thermodynamical features
TS300 K THU400 850 K ?II,b0.7 ?II,t(0.9,
0.8, 0.7)
?II,t 0.9
?II,t 0.8
?II,t 0.7
CONCLUSIONS
The analysis of the simulation results shows that
exergy efficiency of the trithermal unit (?II,t)
affects the applicability of this process as
follows - Leads to the increasing of the
minimum level of temperature of heat user
(THU,min). Under this level the thermodynamical
feasibility of the process is null.- For a given
value for the temperature of heat user (THU,min)
leads to decreasing of the optimum value for
thermal level 2 (T2,O), which generate the
increasing of thermal levels ratio for the
bithermal unit in the couple. - For a given
value for the temperature of heat user (THU,min)
leads to decreasing of the maximum possible value
for the energy efficiency of the couple
(?I,c,max).