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NEW ABSORPTION CHILLERS FOR CHCP OR SOLAR COOLING
SYSTEM TECHNOLOGY
Stefan Petersen, Alexander Beil, Christian Hennrich, Wolfgang Lanser, Walther Guido Hüls,
Technische Universität Berlin, KT2, Marchstraße 18, D-10587 Berlin, Germany,
Stefan Natzer, Bayerisches Zentrum für angewandte Energieforschung (ZAE Bayern),
Walther-Meissner-Straße 6, D-85748 Garching, Germany, stefan.petersen@tu-berlin.de
Abstract: Sorption cooling technologies are well known as best practice energy efficient
cooling supplying apparatus where heat as driving source is delivered by waste heat,
trigeneration systems, solar thermal plants, etc. Recent European demonstration projects
could not match this prospect due to parasitic electric consumptions.
A research project under participation of science and engineer researchers (TU-Berlin, ZAE
Bayern) and an energy provider (Vattenfall Europe) was set up to develop high efficient
absorption chillers in the range of 50-320 kW. System set ups for the entire cooling
generation including reject heat and hydraulic components and control are within the focus.
While a 160 kW absorption chiller is on the test bench, 50 kW absorption systems are
already running in 2 demonstration plants.
The chiller operates in between 25 and 140% of load at thermal COP´s in the range of 0.80
and can stationary deliver cold down to part load of 5%. While driving heat can be used from
55°C up to 110°C at the inlet (standard operation point is at 90/72°C in/out), reject heat inlet
temperatures up to 45°C are feasible for normal operation mode. Even higher reject heat
temperatures are viable without crystallization, only limited by lower power density. Operation
figures of new 50 and 160 kW absorption chillers as well as energy efficiency ratios of
demonstration plants are presented. System layout, including market available dry reject
heat systems, gains parasitic electric EER values lower than 6% of cooling load. New
volumetric/energetic density benchmarks up to 23 l/kW are proven.
Results of 50 kW absorption system overreached thermodynamical targets while proposing a
lower price than market available. System demonstration and development up to 320 kW
cooling load systems are actually undertaken in Lab and will be lead to real estate
installations in 2013.
Keywords: optimization, efficiency, control, demonstration, tri-generation
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2 ABSORPTION CHILLERS
Main focus of the project as well as in market availability and necessity are single effect
absorption chillers. In case of facility cooling water/lithium bromide is the most used working
pair.
Most of the commercially available water/LiBr absorption cooling machines are working on a
capacity range from 300 kW and up. Here, shell and tube heat exchanger are state of the art.
Copper tubes are rolled into heavy front plates, assuring the inner refrigerant atmosphere to
be vacuum proof. The direction change of the heat transfer medium is done by water boxes
fixed outside onto the front plates. Transferring this technique on smaller capacity ranges
below 200 kW leads to high specific cost, because with smaller tube lengths the fix costs of
the fringe effects dominate (Estiot et al. 2007).
Other approaches, developed for small machines that are available from few manufacturers,
like coiled tube heat exchangers, led to different problems including high size ratio and high
external pressure drops. On the other hand, this pressure drops lead to high parasitic energy
demands, reducing the advantage of heat driven cooling machines (Aprile 2010).
Another point is the part load behavior of common chillers. With fixed volume flows on the
heat transfer medium side and a fix fan speed for the heat rejection unit (both set for nominal
power), the resulting parasitic electric consumptions dominate at part load. Thus, again, the
advantage of low electric energy consumption for sorption systems is reduced, leading to a
low electric energy efficient ratio (electric EER).
Focusing on these aspects, the goals for a new development of an efficient and economic
absorption chiller were set. Among those, a high COP on a wide capacity range was also set
as target for the system. Inter alia, this can be achieved by low heat losses through reduced
thermal bridges. Of course, also the number of working steps for construction (i.e. price), the
compactness and the weight was to be looked at.
As a result, a new and promising idea of U-shaped bended tubes, arranged in a face-to-face
adjustment for absorber and evaporator and cross-shaped parallel for generator and
condenser, with a split condenser right and left of the generator, came to be.
3 NEW ABSORPTION CHILLER CHARACTERISTICS
Focus of the project has been the development of absorption chillers in small and medium
capacity up to 320 kW. A first 50 kW lab functional model was erected in 2010 being
analyzed for 18 month till late 2011. Based on these results conception studies for the 160
kW model were undertaken, gaining in a new 160 kW lab functional model which is actually
analyzed at the test rig of TU Berlin.
3.1 50 kW lab model
Thermophysical characteristics of the first 50 kW chiller has been well described in 2011
(Petersen et al. 2011). It is stressed that the new chiller design, focusing on heat exchanger
optimization due to pressure losses and surface usage combined with new control strategies
can lead to efficiencies of 0.8 in cooling mode. The new absorption chiller overcomes
limitations of low driving heat temperatures and can therefore start operation at 55°C up to
110°C.
New results do even claim to pull down a second prejudice. Designed for low maintenance
costs there was a need to identify most expensive cost objects. Even in a country like
Germany, where water is not really a limited source, costs for water treatment and bacteria
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Table 2 gives an overview to operation standard conditions of the two models. Currently the
160 kW chiller is examined in the lab for its part load characterization. Both chillers provide
chilled water in between 6°C and 25°C, using hot water from 55°C to 110°C, types for hot
water temperatures up to 135°C will be viable. Hot water temperature spreads at standard
flow rates of 0.014 l/s/kW (standard operation conditions, decentralized CHP) are in the
range of 18 K but also spreads up to 40 K are feasible at low flow system flow rates of 0.006
l/s/kW as used in district heat networks or solar thermal systems.
Table 2: Standard operation conditions
Bee
Bumble Bee
unit
Nominal cooling capacity
50
166
kW
Heat demand
63
208
kW
Reject heat
113
374
kW
COP
0.79
Chilled
water
temperature inlet
21
°C
temperature outlet
16
°C
volume flow
8.5
27.7
m³/h
pressure drop
0.27
0.25
bar
max. pressure
6.0
bar
Hot water
temperature inlet
90
°C
temperature outlet
72
°C
volume flow
3.0
9.7
m³/h
pressure drop
0.12
0.36
bar
max. pressure
16
bar
Reject heat
water cycle
temperature inlet
30
°C
temperature outlet
37
38
°C
volume flow
14.4
39.0
m³/h
pressure drop
0.70
0.57
bar
max. pressure
6.0
bar
4 SUMMARY
The investigative development described in this paper shows up the potential for
modernization of an old fashioned cooling technology. Sorption cooling or heat pump
systems have been a niche-market during electrification period in Europe and northern
American States. Within the actual discussion of energy usage efficiency, solar power usage
and combined heat and power sorption cooling systems offer the possibility to make use of
heat during summer season, diminish additional electric loads caused by compression
cooling systems and with the goal of delivering cold. Within this topic market available
systems suffer by volumetric size, efficiency and prize to be competitive. A new heat
exchanger concept is elaborated.
With respect to research results of the last 15 years specific energy densities are increased
by at least 30% for cooling capacities in the range from 30 kW to 320 kW. Cost reduction is
expected to be in a range of 50% to 70% compared to current cooling costs.
The new generation overcomes conventional operating limits of absorption chillers. Former
limitations regarding minimum hot water temperatures or maximum reject heat temperatures
have been mainly based on construction parameters. Starting operation at 55°C hot water
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inlet there are no limitations to reject heat inlet temperatures up to 50°C. Nevertheless,
system design will be different for high reject heat conditions than for lower ones.
The first 50 kW model is operating in two field tests in Germany, supplying cold for a 44 kW
office cooling driven by district heat and a 35 kW solar cooled datacenter installation. Further
field tests with the final developments in the range of 30 to 200 kW are expected to be set up
in 2013.
5 ACKNOWLEDGEMENT
Authors wish to thank all supporting companies and the scientific community in the field of
absorption technology. Special thanks do belong to our main project partner Vattenfall
Europe and the German Federal Ministry of Economics and Technology represented by
Project Management Jülich (PTJ) under the project number 0327460B.
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