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2.An absorption refrigeration system uses a heat source (e.g., geothermal energy, solar energy, and waste heat from steam plants, and even natural gas when it is at a relatively low price.) to provide the energy needed for the cooling process.
3.Quite similar to a vapor compression system.
4.The compressor is replaced by a generator and absorber.
5.Ammonia is used as a refrigerant i.e. R-717 and Water as an absorber.
6.Condensation, expansion and evaporation processes are the same as the VCR system.
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1. eurammon Symposium 2017
DESIGN OF NH3-H2O ABSORPTION CHILLER
FOR LOW GRADE WASTE HEAT RECOVERY
Marco Cefarin
Schaffhausen, 22nd/23rd June, 2017
2. Schaffhausen, 22nd / 23rd June, 2017 Page 2
Background:
• Environmental awareness is increasingly arising and it is pushing for a more sustainable
and less polluted world
• Energy efficiency is one of the principal research topics in the industry, indeed heat
recovery is one of the most investigated subjects in many researches
• An effective solution to increase heat recovery is to convert unused heat in cold. It is
possible to employ heat activated refrigeration systems such as absorption cooling heat
pumps.
• Absorption systems based on ammonia - water (NH3/H2O) mixture can be employed in
below 0°C applications.
• The NH3/H2O couple is composed by natural refrigerants with ODP=0 and GWP=0
1. Introduction
3. Schaffhausen, 22nd / 23rd June, 2017 Page 3
Objectives
The core idea of the research project is to keep the design of the absorption chiller as simple
as possible in order to reduce the required space and thus the costs. To do so it will be used
and tested the following :
• Plate heat exchangers (PHE) have higher transfer rate, more compact design and lower
cost compared to shell and tube.
• A direct expansion evaporator has been preferred to a flooded one.
• Falling film type absorber has been preferred over the bubble type absorber for lower
pressure losses and simpler construction.
• Instead of a membrane pump, a low NPSH water pump has been installed.
• A partial condenser has been selected to replace the distillation column.
1. Introduction
4. Schaffhausen, 22nd / 23rd June, 2017 Page 4
Project goals:
• Develop numerical model to describe the operation of an absorption cooling heat pump that
uses ammonia-water solution
• Identify the principal system parameters values in order to find the optimal working
condition with high global system performance
• Using the obtained numeric values, create sizing charts in order to identify the space of
solutions where an absorption chiller can operate
• Design, build and test an absorption chiller prototype
• Analyse and compare the experimental data with the theoretical values obtained from the
numerical simulations
1. Introduction
5. Schaffhausen, 22nd / 23rd June, 2017 Page 5
2. Absorption cycle
Basic concepts
• Depending on the achieved effect:
• HEATING HEAT PUMP CYCLE = heating by transfer of heat to a
source at a higher temperature (Thermopump).
• REFRIGERATION HEAT PUMP CYCLE = cooling by removing
heat from a source at a lower temperature source (Frigopump)
Coefficient Of Performance (COP)
It can be defined as
The heating and cooling performance are related:
6. Schaffhausen, 22nd / 23rd June, 2017 Page 6
2. Absorption cycle
Representation of vapor compression and absorption
refrigeration cycles.
The absorption cycle can be considered as a compression
cycle where compressor has been substituted by the
generator-absorber assembly.
Vapor compression refrigeration cycle representation
Absorption refrigeration cyce representation
8. Schaffhausen, 22nd / 23rd June, 2017 Page 8
3. State of the art
The main topics in ammonia absorption research from literature:
-A critical component: the Absorber
-Use of microchannels: the application in the absorber and the generator
-Absorption performance enhacement by nano-particles and chemical surfactants
-Absorption cycle in heat recovery applications
-Solar cooling
-Compression-absorption cycles
-Advanced absorption cycles
NH3-H2O GAX cycle NH3-H2O double effect cycle
9. Schaffhausen, 22nd / 23rd June, 2017 Page 9
4. Thermodynamic model
Temperature/mass fraction diagram for NH3-H2O
The most used diagrams for calculations of pure fluid
cycles are T - s, ln(p) - h or h - s.
In absorption processes an additional variable has to
be considered; a mixture has an additional degree of
freedom, if compared to a pure fluid, the mass
fraction. Historically, enthalpy-mass fraction
diagrams (h-x diagrams) were preferred with
temperature and pressure as parameters. The mass
fraction is usually defined as:
Thermodynamical properties can be obtained using
diagrams of mixture properties as shown in the figure
on side.
10. Schaffhausen, 22nd / 23rd June, 2017 Page 10
4. Thermodynamic model
Duhring diagram for NH3-H2O
NH3-H2O cycle representation
Thermodynamical properties are obtained in
EES.
Energy and mass balances are used to
evaluate the properties for each component.
11. Schaffhausen, 22nd / 23rd June, 2017 Page 11
4. Thermodynamic model
SHTR, COP, concentration of
NH3 in poor and rich solution,
solution concentration difference
∆x, recirculation factor f and
pump absorbed power as a
function of Tgen (Tcond = 35°C,
Tevap =-3.5°C)
High gradient area
12. Schaffhausen, 22nd / 23rd June, 2017 Page 12
4. Thermodynamic model
COP as a function of desorber temperature for
different condensation temperatures Tcond (Tevap =-
3.5°C)
COP as a function of concentration difference ∆x for
different condensation temperatures Tcond (Tevap =-
3.5°C).
13. Schaffhausen, 22nd / 23rd June, 2017 Page 13
4. Thermodynamic model
COP and Specific heat transfer rate at Absorber iso-lines and iso-∆x as
function of evaporation and generation temperatures (Tcond = 35°C).
14. Schaffhausen, 22nd / 23rd June, 2017 Page 14
4. Thermodynamic model
Specific heat transfer rates at SHX and evaporator iso-lines and iso-∆x as
function of evaporation and generation temperatures (Tcond = 35°C).
15. Schaffhausen, 22nd / 23rd June, 2017 Page 15
5. Prototype
Main features:
• Heat plate exchangers
• Falling film type absorber
• No rectifying columns – precooling condenser
• Centrifugal water pump
• DX evaporator
24. Schaffhausen, 22nd / 23rd June, 2017 Page 24
6. Experimental test
Comparison with commercial absorption chillers:
• The design process can be considered satisfactory. In fact it has lead to a functioning prototype that has
confirmed all the design assumptions. There are many aspects that need further investigation and
improvements, but the it has showed a good overall performance as first generation prototype.
• In the figure below, the performance values of a commercial ammonia-water absorption chiller are
reported for comparison. It can be noticed that the COP values for same working range conditions are
comparable to the COP values obtained during experimental activity in this research.
• It is clear that further investigation on inconsistencies between numerical model and experimental values
are strongly recommended.
25. Schaffhausen, 22nd / 23rd June, 2017 Page 25
Project results – 1
• This research project confirmed the validity of the selected design process. An ammonia-water
absorption chiller has been designed, built and tested. The machine is now installed and working in
an industrial facility.
• A numerical model has been developed. The obtained results have been used to, firstly, design the
thermodynamic cycle of the prototype and, secondly, to select the components used to manufacture
the prototype.
The design philosophy is to keep it as simple as possible and the following technical solutions
have been tested:
• shell and tube heat exchangers have been replaced by plate heat exchangers. PHE have higher
transfer rate, a more compact design and a reduced cost.
• A direct expansion evaporator has been preferred to a flooded one
7. Conclusions
26. Schaffhausen, 22nd / 23rd June, 2017 Page 26
Project results – 2
• a falling film type absorber has been preferred over the bubble type absorber for lower pressure losses
and simpler construction.
• Instead of a membrane pump, a low NPSH water pump has been installed.
• A partial condenser has been selected to replace the distillation column.
• The prototype has been installed and tested in an industrial facility. The flue gases, result of the
toasting process, is recovered to produce a stream of hot water used to power the absorption chiller.
• The values of COP obtained during the tests are lower than the values obtained with numerical model.
Overall performances were close to experimental values and differ of almost 20%.
7. Conclusions
27. Schaffhausen, 22nd / 23rd June, 2017 Page 27
Economical evaluation – 1
The figure of €kW is a typical parameter employed in economical estimation regarding the installation of a
new chiller for industrial refrigeration application.
Typical values obtained from market:
- Usually vapor compression chillers are adopted as basic solution for industrial refrigeration. The figure
for compression chiller is usually around 200-220 €kW.
- Absorption chillers, single effect LiBr type is the basic solution and present a value similar to standard
chillers and it is equal to 260-280 €kW.
- The ammonia absorption chillers, due to higher construction requirements, have a typical range of
1200-1500 €kW.
7. Conclusions
28. Schaffhausen, 22nd / 23rd June, 2017 Page 28
Economical evaluation – 2
• The prototype as built, without considering the development costs and including the standard mark up,
has a figure of 850 €kW.
• It has to be considered that a further development can easily reduce the cost of 20%, making its final
price even more competitive, especially if compared with ammonia absorption chillers for below 0°C
applications.
7. Conclusions
29. Schaffhausen, 22nd / 23rd June, 2017 Page 29
Improvements
Errors:
- Capacity control on hot fluid circuit is required to run the prototype completely in automatic mode.
- Temperature gauges missing: on condensed liquid receiver and on expanded liquid pipe injecting in
the evaporator.
- No measures of mixture concentration. The mixtures were supposed in saturated condition.
- To simulate off-design conditions it is necessary to introduce a heat exchanger model.
Suggested improvements:
- Bubble type absorber: not tested
- Rectifier cooling: cooled by rich solution stream from SHX
- Subcooling heat exchanger: it can be installed to increase performance
7. Conclusions
30. Schaffhausen, 22nd / 23rd June, 2017 Page 30
Further investigations and research proposals:
- Surfactants addition to evaluate the increase of performances.
- Microchannel heat exchangers and air cooled absorber configuration.
- Increase of detail of the numerical model adding a heat exchanger model.
- Development of heat exchanger sizing model.
- More complex cycles implementation to increase overall performance of the chiller, i.e. GAX cycle.
- Investigation of an absorption chiller working in heat pump mode.
7. Conclusions
31. Schaffhausen, 22nd / 23rd June, 2017 Page 31
This research project has been supported and sponsored by Zudek srl, Muggia (TS).
Zudek srl is an industrial refrigeration company based in Trieste, Italy. It designs, builds and installs ammonia
chillers and plants for industrial and commercial utilities.
The objective of this research project was to acquire the know-how related to design and construction of
ammonia-water absorption chillers.
Acknowledgment
