ECN-RX--04-080
August 2004
SOLID-SORPTION COOLING WITH INTEGRATED
THERMAL STORAGE:
The SWEAT prototype
R. de Boer
W.G. Haije
J.B.J. Veldhuis
S.F. Smeding
Paper presented on the international conference on
Heat Powered Cycles, HPC 2004, Larnaca, Cyprus
SOLID-SORPTION COOLING WITH INTEGRATED THERMAL STORAGE:
The SWEAT prototype
R. de Boer*, W.G Haije, J.B.J. Veldhuis and S.F. Smeding
Energy research Centre of the Netherlands, ECN, PO Box 1, 1755 ZG, Petten,
the Netherlands. E-mail: [email protected]
ABSTRACT
INTRODUCTION
The
SWEAT
(Salt
Water
Energy
Accumulation and Transformation) project is
focused on the development of a prototype of
a modular solid-sorption cooling system for
residential and industrial applications. The
solid-sorption cooling system uses sodium
sulphide-water (Na2S-H2O) as the working
pair. It requires low-grade heat to drive the
cooling process and has intrinsic long-term
storage capacity for heat and cold.
The shell-and-tube design of the SWEAT
module contains sorbent-filled copper-wirefin tubular heat exchangers, a condenser and
an evaporator coil. The design of the modular
system, the corrosion protection and
production technology were developed with
the aim of using standard industrial
manufacturing procedures, in order to obtain a
low-cost system, fit for serial production.
Results of performance measurements and a
comparison with a Simulink® simulation
model are given.
Solid-sorption heat-pump technology is an
alternative to conventional heat-pumping
technology
having
energetic
and
environmental benefits. The SWEAT solidsorption system has specific properties that
make it attractive for use as a cooling system
driven by waste heat. It uses the salt sodium
sulphide (Na2S) as the solid sorbent for water
vapour. Sodium sulphide has a high
absorption capacity and a high heat of
absorption (reaction), making it in principle
possible to achieve a solid-sorption machine
that can combine a high thermal power
density with high thermal energy storage
density, competitive with conventional heatpumping technology.
KEYWORDS: Solid-sorption cooling, Na2SH2O, prototype development, cooling power,
thermal energy storage.
However, sodium sulphide is a very corrosive
salt making it difficult to handle in heatexchange applications. In addition, to operate
this solid-sorption cooling machine, vacuum
conditions are required, and the presence of
even the smallest amounts of noncondensable gasses strongly decreases the
performance of the SWEAT system. The
corrosion reactions however always lead to
the formation of non-condensable gasses, and
should therefore be avoided.
The aim of this work is to demonstrate the
technical feasibility as well as the feasibility
of the production procedures and processes
for the SWEAT modules.
For this purpose the design, manufacture and
testing of a functional prototype of the
SWEAT technology was undertaken. Two
SWEAT modules were produced and put
together in the technical prototype. The
performance of the prototype is measured
This paper describes the material selection,
the design and construction of the SWEAT
prototype, as well as the results of the
measurements performed with it and the
analysis.
THEORY
The thermochemical properties of the Na2SH2O working pair have been determined in
previous studies, de Boer et al. (2003). The
useful equilibrium reactions in this system
are:
Na2S·5H2O(s) ↔ Na2S·2H2O(s)+3H2O(g)
Na2S·2H2O(s) ↔ Na2S·½H2O(s)+1½H2O(g)
The enthalpies of the reactions and the
equilibrium temperature differences are
summarised in Table 1. Based on the heats of
the reactions, the total theoretical heat storage
capacity is 3.84 MJ/kg Na2S (or 1.1 kWh/kg)
and the cold storage is 2.54 MJ/kg Na2S (0.70
kWh/kg).
The
theoretical
maximum
efficiency for cooling, COPcooling (cooling
output/heat input) is 0.66.
The equilibrium temperature difference in
Table 1 is the temperature difference between
pure water and the respective Na2S·xH2O
having equal vapour pressures. This means
that in order to charge the SWEAT system the
salt must have theoretically a temperature of
at least 56 K or 61 K higher than the
condenser temperature.
The absorption-desorption cycle for the
SWEAT system is shown in a temperature
pressure diagram in Figure 1. The lowest
eutectic melting temperature of the salt
hydrate within its operating range is 83°C.
This would mean that at condenser
temperatures of 27°C or higher, the
temperature of Na2S must be above its
melting temperature in order to regenerate the
system. This requires the use of a sorbent
support material that will maintain a porous
structure of the salt even when occasionally
part of it becomes melted.
Another aspect of the use of Na2S-H2O in a
solid-sorption cooling system is its corrosive
properties. In contact with metal parts of the
system it will lead to the formation of the
respective metal sulphide and the production
of hydrogen. Hydrogen is a non-condensable
gas that will reduce the performance of the
cooling system by blocking the condenser. It
is thus necessary to avoid any direct contact
of Na2S with metals in the system.
Modular concept
For the SWEAT system a modular concept
has been applied. This concept gives the
benefit of flexibility of system design for a
specific cooling demand. At higher cooling
demands more modules are packed together
in the cooling system. Another benefit of this
modular concept is future low-cost serial
production, enabling competitive prices in
comparison
to
conventional
cooling
machines.
THE SWEAT MODULE
For the design and manufacture of a SWEAT
module the main objective is to achieve a
module that can be built with common
industrial manufacturing procedures and with
standard materials.
Material selection
A SWEAT module basically consists of two
vessels connected through a vapour channel.
Inside the vessels three heat exchangers are
installed: the evaporator, the condenser and
the sorbent heat exchanger. Proper operation
of the solid-sorption cooling process requires
that only water vapour is present inside the
system; all other gases and vapours must be
excluded. A SWEAT module thus has low
pressure inside, ranging from 4 tot 40 mbar.
This internal vacuum gives rise to additional
requirements for the materials to be used in a
SWEAT module.
• Materials must be inert to the chemical
environment of the Na2S–H2O or have a
defect-free corrosion-protection layer.
• Materials must be suitable for maintaining
the vacuum inside the vessels.
•
Materials must have very low out-gassing
rates to maintain the low pressure inside
the module.
Stainless steel was selected as material for
construction of the vessels. This gives
sufficient corrosion protection for those parts
that are not in direct contact with Na2S. The
condenser and evaporator coil are made from
stainless steel tubing as well. The sorbent heat
exchanger is a copper wire-fin heat exchanger
(Spiro-tube, manufactured by the Dutch
company Spirotech BV, Helmond) as shown
in Figure 2. Copper wires are soldered in a
spatial geometry onto a central copper tube
that carries the heat-transfer fluid. The copper
wires transport the heat into the sorbent bed,
which is fixed in the wire structure. The
external surface of this heat exchanger must
have a coating for corrosion protection. The
coating is of crucial importance because this
heat exchanger is in direct contact with the
corrosive Na2S material.
Coating development
The requirements for the coating to be applied
onto the Spiro-tube are:
•
Zero defects
•
Chemically inert to Na2S-H2O
•
100% barrier function
Flat test panels with different types of
coatings were exposed to hot sodium sulphide
and analysed by Electrochemical Impedance
Spectroscopy (EIS). The coatings with the
desired chemical properties were then applied
on Spiro-tubes to find the coating that can be
applied with zero defects. An electrochemical
analysis procedure was used to detect and
locate defects in the coated Spiro tube. Longterm exposure tests under simulated working
conditions were then performed with coated
Spiro-tubes for final selection. The selected
coating is a standard industrial epoxy powder
coating, with additives to enhance its
degassing and flow behaviour during the
curing process at 180°C. The cross-section of
a coated Spiro tube is shown in Figure 3.
Sorbent Support material
Na2S-H2O has a eutectic melting point at
83°C. This imposes the upper limit for
temperature of regeneration of the system. A
support material for Na2S is required, to
achieve a sorbent filling of the space in
between the copper wire structure that has
sufficient open porosity for vapour transport,
mechanical stability to maintain the salt
evenly divided in this space, even with
occasional melting of part of the salt. A
fibrous cellulose material was selected for this
purpose. These fibres are chemically inert and
sufficiently thermally and mechanically stable
to act as support for the salt. Cellulose also
has sufficient absorption capacity to maintain
a molten salt located at its position. The space
in between the copper wires is easily filled
with cellulose-Na2S composite material, when
applied in small grains.
SWEAT module design
The SWEAT module consists of two vessels
that are connected through a vapour channel.
One vessel is the evaporator/condenser part;
the other vessel is the accumulator. An open
view of the module is shown in Figure 4. The
evaporator/condenser part contains an
evaporator coil located at the bottom of the
vessel and surrounded by a capillary cotton
fabric; the condenser coil is located in the top
section of the vessel.
The accumulator contains six Spiro-tubes in
hexagonal packing and one diffuser tube
located in the top section. This is shown in
Figure 5. The Spiro-tubes are divided in three
U-branches and connected in parallel to the
heat transfer fluid, water. One module
contains about 3 kg Na2S.
All flanges and feed-throughs of the modules
are welded to limit the risk of leakage in the
vacuum system.
THE SWEAT PROTOTYPE
The prototype of the SWEAT system contains
two modules. The system can operate in
continuous mode, charging one module while
simultaneously discharging the other, as well
as in batch mode, where both modules are
charged or discharged simultaneously. A
photograph of the system is shown in Figure
6. Both modules were connected to the three
water circuits: one hot-water circuit for
regeneration of the sorbent, a cooling-water
circuit for cooling the condenser and the
sorbent, and a chilled-water circuit connected
to the evaporator.
All three temperatures can be set to a desired
value to measure performance at various
working conditions. Also the duration of
charging and discharging the modules can be
varied. In continuous operating mode a heat
recovery phase is implemented. The heat from
the accumulator in charging mode is
transferred to the cooled accumulator, until
temperature equilibrium between both
accumulators is reached.
The water flows going in and out of a
SWEAT module as well as the temperature of
the ingoing and outgoing water are measured
to determine the thermal powers of the
modules.
Before starting the actual measurements the
modules were thermally insulated to prevent
significant heat loss to the surroundings.
PROTOTYPE PERFORMANCE
An example of a typical charge-discharge
power profile of a single SWEAT module is
presented in Figure 7, together with results of
model calculations. These calculations are
based on heat-transfer rates within the sorbent
heat exchanger and sorbent bed, and are
implemented in Matlab Simulink®. Mass
transfer is not considered to be the ratedetermining factor in the performance and is
therefore not taken into account in the model.
The model predicts the charging power
profile quite well, but the actual discharging
power is significantly higher than calculated.
The maximum charging power is limited by
the electrical power of the heating system to
1200 W. The cooling power is around 1500
W at its maximum, but this value can only be
reached at evaporator temperatures of 20°C
and higher, which is not very relevant for
practical applications. Evaporation in the
range 10 to 15°C results in cooling powers of
500 to 700W. The charging power gradually
drops as the charging process proceeds. The
discharging (cooling) power remains fairly
constant but drops to zero the moment all the
water is evaporated. Charging and
discharging times do not match perfectly in
the situation shown.
The average charging power of the SWEAT
module at various charging temperatures is
shown in Figure 8. The charging power varies
between 200 and 1000W with the supplied
charging temperatures. Lowering of the
condenser temperature increases the pressure
difference in the module and thus increases
the charging power. Increasing the
accumulator temperature also leads to an
increase in charging power. In general the
charging power of the module increases with
increasing temperature difference between
accumulator and condenser. A minimum
temperature difference of 57 K is necessary to
start charging the module. However, at least
63 K temperature difference is needed to get a
module completely charged.
The cooling power obtained during
discharging of the modules is plotted in
Figure 9. It was determined at various chilled
water supply temperatures to the evaporator.
The cooling power increases with increasing
cold supply temperature. At higher
evaporation temperatures the pressure
difference
between
evaporator
and
accumulator increases, leading to a higher
cooling power. Another aspect is that
evaporation at higher temperatures generally
leads to higher power, due to the increased
vapour density.
The highest value of SCP is 500 W/kg Na2S.
The measured maximum thermal storage
capacities of the SWEAT module are:
Heat storage capacity:
Heat in
13.3 MJ (3.7 kWh)
Heat out
11.5 MJ (3.2 kWh)
Efficiency
0.84
Cold storage capacity: 7.6 MJ (2.1 kWh)
COPcooling
0.57
The value for COPcooling is 85% of the
theoretical maximum value. When heat losses
in the system due to thermal masses of metal
heat exchangers are taken into consideration,
the values for heat and cold storage are in
accordance with the theoretical values.
CONCLUSIONS
Based on the results obtained in designing,
manufacturing and testing the SWEAT
modules the following conclusions can be
drawn:
• The selected materials for construction of
a SWEAT module fulfil the technical
requirements.
• Applying a zero-defect epoxy powder
coating on the metal surfaces effectively
prevents corrosion of Spiro-tube heat
exchangers and other parts of a SWEAT
module.
• A SWEAT module can be constructed
using assembly procedures that are
common in industrial serial production.
• The performance of the SWEAT module
is as expected based on previous separate
small-scale experiments and model
calculations.
• The temperature of the condenser should
be kept as low as possible to obtain
sufficient charging power. Outside
(condenser) temperatures of ca. 25°C and
higher prevent the SWEAT system
becoming charged. Night-time charging is
thus favourable because of the lower
condenser temperatures.
• SWEAT application is most feasible in
situations where a cheap heat source is
available to charge the system during
night-time and a cooling demand is
present during daytime.
Future work
For further development and demonstration of
SWEAT technology the following aspects
should be considered:
• The charging power profile shows a
decrease in time. This should be flattened
by an improved control strategy over
•
•
•
temperature and flow through the
accumulator and condenser.
A mechanism to control the cooling
power output is required, in order to
match the output with the cooling
demand.
It is recommended to pay attention to
minimising the primary energy use for all
the auxiliary equipment like fans, pumps,
valves and controls.
It is recommended to develop at least two
systems for field-testing. These 1:1 scale
systems should preferably be tested in two
specific
applications,
one
as
a
discontinuous working system using the
advantage of thermal energy storage in the
system, and one as a continuous working
system, where high specific power output
should be accomplished.
NOMENCLATURE
COP
Coefficient of performance
SCP
Specific Cooling Power
ACKNOWLEDGEMENTS
The Dutch Senter-EDI program financially
supports the SWEAT research project.
REFERENCES
Boer, R. de, Haije, W.G., Veldhuis, J.B.J.,
Determination of structural, thermodynamic
and phase properties in the Na2S-H2O system
for application in a chemical heat pump,
Thermochimica Acta, 395, 3-19, 2003.
Table 1 Sorption reactions in the Na2S–H2O system
Reaction
∆T equilibrium
Heat of reaction
[K]
[kJ/mol]
Na2S·5H2O ↔ Na2S·2H2O + 3H2O (g)
56
189
Na2S·2H2O ↔ Na2S·½H2O + 1½H2O (g)
61
111
100
H2O
p H2O
mbar
desorption
condensation
10
evaporation
absorption
1
Na2S·5H2O
Na2S·2H2O
0.1
0
5
10 15 20 25 30 35 40 45
50 55 60 65 70 75 80 85 90
t / °C
Figure 1 Temperature-pressure diagram for the SWEAT system. The absorption-desorption cycle
for the Na2S-H2O system is indicated with the dashed area, as well as the condensation
and evaporation conditions.
Figure 2 Photograph of a Spiro- tube heat exchanger. The diameter of the central tube is 15 mm,
the outside diameter of the wire structure is 6 cm
Coating
Copper wires
Solder
Copper central
tube
Figure 3 Microscope photograph of a cross-section of a coated Spiro-tube
Figure 4 Open view of a SWEAT module
Figure 5 Detail of the front end of the accumulator
Figure 6 Photograph of the SWEAT prototype system
Power [W]
2000
P acc (sim)
P acc (exp)
P cond/evap (sim)
P cond/evap (exp)
1500
1000
500
0
-500
-1000
-1500
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
0:00
-2000
Time [h:m]
Figure 7 Measured thermal power profile of the SWEAT module during a charge-discharge
cycle and comparison to model predictions.
charging powerr /Watt
1000
800
86°C
600
400
77°C
200
82°C
0
10
15
20
25
30
condenser temperature °C
Figure 8 Charging power of a SWEAT module as a function of the condenser temperature at
different heat-source temperatures
Cooling power / Watt
1200
1000
800
600
400
200
0
0
5
10
15
20
25
30
T cold °C
Figure 9 Cooling power of a SWEAT module at different chilled-water temperatures