LATENT HEAT STORAGE WITH MELTING TEMPERATURE 29 °C
SUPPORTING A SOLAR HEATING AND COOLING SYSTEM
S. Hiebler, H. Mehling, M. Helm, C. Schweigler
Bavarian Center for Applied Energy Research (ZAE Bayern e. V.)
Walther-Meißner-Str. 6, 85748 Garching, Germany
Tel: 49-89-329442-35
hiebler@muc.zae-bayern.de
ABSTRACT
Solar cooling systems with absorption chiller based on lithium bromide/water need a cooling
tower to reject the waste heat to the ambient. Typically a wet cooling tower is used but water
consumption, effort of conditioning water, fogging and risk of growth of legionella bacteria
are hindering the market penetration of small solar cooling systems.
To avoid a wet cooling tower and get on with a dry cooling tower, we support them with a
low temperature latent heat storage. At high ambient temperatures the storage supports the
dry cooling tower and stabilizes the temperature in the reject heat loop at moderate
temperatures. This way, part of the heat rejection is shifted to lower temperatures during
nighttime. During winter the storage is used as buffer storage for solar heating.
The system concept and the development of the storage are presented, followed by a report on
the operation of a first pilot installation.
1.
INTRODUCTION
In solar thermal installations, both solar cooling and solar heating can be provided
synergistically, yielding a complete annual utilisation. During the cold season, solar heat
serves for space heating. During the warm season, solar heat can be converted into useful cold
by means of sorption cooling. A favourable situation is given when low temperature heating
and cooling facilities, e.g. floor or wall heating systems or activated ceilings, are applied for
heating and cooling. During heating operation, a low temperature latent heat storage can be
used to balance the heat generation by the solar system and the supply to the heating system.
Thus, a low operating temperature of the solar thermal system is accomplished yielding
efficient operation with optimum solar gain. In the cooling mode, the same storage is used in
combination with a dry cooling tower in order to replace a wet cooling tower. By that means
heat rejection of the chiller is shifted partly to periods with lower ambient temperatures, i.e.
night time, or to off-peak hours.
2.
SYSTEM CONCEPT
In conventional absorption cooling installations, wet cooling towers designed for a coolant
supply/return temperature 27/35 °C are applied. To use a dry air-cooler, cooling water
temperatures have to be increased to 40/45 °C. As a consequence of the increase of the
cooling water temperature, the temperature level of the driving heat supplied to the
regenerator of the absorption chiller has to be increased accordingly.
This principal dependency of the required driving temperatures on the coolant temperatures of
an absorption chiller can easily be estimated from the linear “characteristic-equation” model
(Furukawa. 1983, Hellmann et al. 1998, Storkenmaier et al. 1999). The external heat carrier
parameters for the discussed system configurations – governed by linear correlation according
to the characteristic-equation model – are given in Figure 1. For chilled water 18/15 °C,
starting from a chiller design with 80/75 °C, the required driving hot water temperature rises
with increasing cooling water temperature to 90/85 °C when a dry air cooler together with a
latent heat storage is used and further to 105/100 °C when solely a dry air cooler is applied.
Similar, for chilled water temperature 12/6 °C the required driving hot water temperature
increases from 90/85 °C to 100/95 °C for the system with dry air cooler assisted by a latent
heat storage. For this chilled water layout, i.e. standard temperature 12/6 °C, complete dry aircooling is not feasible.
driving heat
temp.
lled
chi
105/100°C
100/95°C
90/85°C
ll
chi
ed
°C
2/6
r1
e
t
wa
C
15°
18/
r
te
wa
80/75°C
reject heat
temp.
27/35°C
wet cooling
tower
32/40°C
PCM + dry
air-cooler
40/45°C
dry
air-cooler
Figure 1: Influence of chilled water temperature and reject heat temperature on driving heat
temperature.
By integration of a heat storage into the heat rejection system of the absorption chiller, a part
of the reject heat of the chiller can be buffered during the operation of the solar cooling
system, allowing lower coolant temperatures during peak load operation of the chiller. Then
the stored reject heat can be discharged during off-peak operation or at night time when more
favourable ambient conditions, i.e. lower ambient temperatures, are available. As discussed
above, as a consequence of the reduced coolant temperature arising from the integration of the
latent heat storage, lower temperatures of the driving solar heat are feasible to operate the
absorption chiller. Thus a higher solar gain is obtained for a given size of the solar collector
system. A detailed analysis of the overall system design has been presented earlier
(Schweigler et al. 2007).
During heating operation, the latent heat storage balances the heat generation by the solar
system and other heat sources and the heat supply to the heating system. Thus, a low
operating temperature of the solar thermal system is accomplished yielding efficient operation
with optimum solar gain.
A simplified system structure for cooling (top) and heating operation (bottom) is given in
Figure 2.
COOLING
MODE
CHILLER
Absorptionswä
rmepumpe
bzw. Kä
ltemaschine
15°C
V
18°C
40°C
G
A/C
90°C
85°C
Wä
rmeSpeicher
AUX.
Heizkessel
BOILER
32°C
Luftwä
rme-
40°C
tauscher
32°
C
DRY AIR
COOLER
PCM-
Speicher
36°
C
SolarkollektorSOLAR
anlage
SYSTEM
NT- Kühl-/Heizsystem
HEATING /
COOLING
SYSTEM
HEATING
MODE
LATENT 32°C
HEAT
STORAGE
CHILLER
Absorptionswärmepumpe
bzw. Kä
ltemaschine
WärmeSpeicher
AUX.
Heizkessel
BOILER
Luftwärmetauscher
32°C
PCM-
35°C
Speicher
SolarkollektorSOLAR
anlage
SYSTEM
NT- Kühl-/Heizsystem
HEATING /
COOLING
SYSTEM
LATENT
HEAT
STORAGE
Figure 2: Sketch of a solar heating and cooling system with absorption chiller (evaporator V,
absorber/condenser A/C and generator G) and latent heat storage in cooling mode (top) and heating
mode (bottom).
3.
PILOT INSTALLATION
Within the framework of the German “Solarthermie 2000plus” program, a pilot installation of
the innovative solar heating and cooling system has been built. The installation serves
simultaneously as field test project for a recently developed compact water/LiBr absorption
chiller with 10 kW nominal capacity (Schweigler et al. 2002, Storkenmaier et al. 2003, Kühn
et al. 2005). The chiller has been designed for chilled water supply/return temperature 15/18
°C and allows for utilization of rather moderate driving hot water temperature 85/75 °C when
operated with open wet cooling tower. These conditions have been chosen as nominal data for
the design of the chiller with a nominal chilled water capacity of 10 kW. When operated with
higher driving hot water temperature of maximum 95 °C at the generator inlet the capacity
increases to 160 % of its nominal value. The chiller exhibits a rather efficient part load
behaviour with a COP above 0.7 throughout a wide capacity range.
With respect to the solar cooling capacity of about 10 kW, a waste heat power of about 120
kWh is required in order to cover 50 % of the daily reject heat output of the chiller. The rest
of the reject heat is directly transferred to the ambient by means of a dry air cooler, which
operates with 36/40 °C cooling water temperature under peak load conditions, as given in
Figure 2 (top). Therefore, the design of the latent heat storage will be based on 36/32 °C
cooling water supply/return temperature.
Apart from the solar cooling mode, during the heating season the system will serve for solarassisted heating. Assuming a solar insolation of 500 W/m2 for a duration of 6 hours again a
heat storage capacity of about 120 kWh is required to absorb the solar gain of the 40 m2 solar
collector field. The solar heat has to be stored above the return temperature of the building
floor heating system, which is controlled in dependence of the ambient temperature. Under
moderate winter heating conditions, the heating system return temperature typically ranges
from 22 °C to 26 °C. Of course, phase transition of the PCM has to take place above this
temperature level in order to accomplish heat transfer from the heat storage to the heating
system.
Supply of useful heat and cooling for conditioning of the office building (1000 m2, design
heating demand: 42 W/m2) is provided by activated ceilings which receive chilled water from
the evaporator of the absorption chiller during cooling mode and solar thermal heat during the
solar heating season, respectively.
4.
DEVELOPMENT OF THE LATENT HEAT STORAGE
For the given application in a solar heating and cooling system, heat has to be stored in a very
narrow temperature range in order to fulfil both tasks: support of the heat rejection of the
chiller requires a phase change temperature below 32 °C and contribution to the heating of the
building a phase change temperatures above 26 °C.
Due to this limitation of the available temperature swing, the heat storage has been designed
as a latent heat storage. According to its melting temperature in the range of 27 – 29 °C the
salt hydrate calcium chloride hexahydrate (CaCl2•6H2O) has been chosen as phase change
material (PCM), providing a heat capacity of about 150 J/g or 240 J/L between 26 °C and
32 °C. This value of the specific storage capacity comprises both sensible and latent heat. Of
course the dominating portion is to be attributed to the latent part.
For the comparison of the storage density of the latent heat storage to a conventional water
heat storage, the specific heat of the liquid, i.e. water, and the temperature swing have to be
taken into account. Given a temperature swing of 6 K only, a conventional water storage
exhibits a storage density of 4.2 kJ/(kg K) x 6 K = 25 kJ/kg. Thus for the given application in
the solar heating and cooling system, the latent heat storage allows for a reduction of the
storage mass by a factor of about 6, requiring only about 10 % of the volume of a
conventional water heat storage.
Figure 3: Capillary tube heat exchanger for activation of the latent heat storage volume (left). 120 kWh
latent heat storage, formed by two 800 L storage tanks (right).
Within the project a commercial capillary tube system commonly used for wall heating
installations has been applied as heat exchanger in the latent heat storage. Because of the high
storage density of the PCM and its low thermal conductivity, a distance between the
capillaries of only a few centimeters had been chosen.
To reach 120 kWh heat storage capacity, two storage modules with a volume of 800 L each
have been designed and constructed. The capillary tube heat exchanger has been properly
configured in order to fully activate the entire storage volume, as shown in Figure 3. The heat
exchanger has finally been immersed in a Polyethylene tank and hermetically sealed in order
to avoid uptake of humidity from the ambient air.
For the sake of economics and in order to achieve a marketable design the heat storage
prototype has been based on standard components. The cost of the PCM is about 0.37
Euro/kg, leading to total cost of the latent heat storage of about 4,000 Euro. Apart from pure
economic aspects, operational issues are of major relevance for the feasibility of small scale
solar heating and cooling installations. Here, the system with dry air cooler and latent heat
storage offers substantial reduction of the maintenance effort and improved performance
during the solar heating season.
40
40
inlet both
36
32
temperature [°C]
temperature [°C]
36
outlet st. 1/2
28
storage temperature 1/2
storage temperature 1/2
28
outlet st. 1/2
24
24
20
00:00
32
03:00
06:00
09:00
20
00:00
12:00
inlet both
03:00
0
10
-2
8
-4
-6
storage 1
storage 2
-8
-10
00:00
03:00
06:00
09:00
12:00
6
4
2
0
00:00
12:00
03:00
06:00
09:00
12:00
time [h:min]
70
0
60
-10
storage 1
storage 2
-20
stored energy [kWh]
stored energy [kWh]
09:00
storage 1
storage 2
Zeit [h:min]
-30
-40
-50
-60
-70
00:00
06:00
time [h:min]
power [kW]
power [kW]
time [h:min]
50
storage 1
storage 2
40
30
20
10
03:00
06:00
09:00
time [h:min]
12:00
0
00:00
03:00
06:00
09:00
12:00
time [h:min]
Figure 4: Operational results of the two 800 L latent heat storage modules. Temperatures (top), power
(center) and stored energy (bottom) during loading (right) and unloading (left) of the latent heat storage.
5.
OPERATIONAL RESULTS
At the beginning, the performance of the storage was determined in standalone tests. A total
of 70 charge and discharge cycles were completed. The evaluation of the recorded data
showed that the storage achieves the design storage capacity of 120 kWh. Figure 4 shows the
temperature-time curves, the power and the stored energy of the two modules during loading
and unloading.
The internal storage temperatures (“storage 1”, “storage 2”) clearly show the effect of the
latent heat in the temperature range of 27 – 29 °C where the PCM melts and solidifies,
respectively. The measured power at charging with a supply temperature of 36 °C was 11 kW,
which is better than the initial thermal design value of 10 kW. At discharge with a supply
temperature of 22 °C the power was about 10 kW. During the 70 cycles, no degradation of the
storage capacity was observed.
After integration of the storage into the new system for solar heating and cooling at the ZAE
Bayern in fall 2007, the performance of the system was monitored during operation. During
the heating period 2007/2008 a significant increase of the solar gain due to the low
temperatures in the system has been observed. Further on it has been proven that this storage
temperature is sufficient for integration into the building heating system, allowing for
substantial contribution to the overall heat supply throughout the heating period. In January
e.g. the solar fraction for heating reached 23 %, rising to 74 % in March, as a consequence of
increased solar gains and lower heating load of the building. In Figure 5 a typical day (10th
Feb. 2008) for heating is shown. During daytime the heat demand is low, so first the solar
collectors can heat the building directly and second the latent heat storage can be loaded.
During nighttime the heat demand is higher and it can be seen, that the stored low temperature
heat can be feed into the heating system.
power [kW]
15
heat [kWh]
unloading
14
loading
unloading
140
13
120
12
11
100
10
9
8
6
80
stored
heat
7
total heat demand
60
5
40
4
3
2
1
PCM storage
output
0
0:00
2:00
4:00
solar heat directly
used for heating
6:00
8:00
10:00
12:00
14:00
16:00
PCM
storage output
20
time [hh:mm]
18:00
20:00
22:00
0
0:00
Figure 5: Typical day for heating (outside temperature: -2 °C to + 5 °C; 10 Feb. 2008). During loading
about 80 kWh are stored.
For solar cooling the system is designed to provide chilled water at 15/18 °C operated with
moderate driving temperatures and dry air cooling even at high ambient temperatures. Below
26.3 °C ambient temperature the capacity of the dry air cooler is sufficient to completely cool
the chiller. At increasing ambient temperatures more and more reject heat has to be stored in
the latent heat storage. As shown in Figure 6 for the cooling season 2008 lasting from April to
September, the latent heat storage had to support the dry air cooler only on 43 days. With
minimum night time temperatures permanently below 20 °C, no limitations for the unloading
of the latent heat storage have been experienced.
ambient temperature / °C
35,0
2008
PCM storage
supports dry
air cooler
30,0
daytime
26,3
25,0
15,0
10,0
5,0
unloading PCM
storage possible
20,0
nighttime minimum
date / dd.mm.
09.09.
02.09.
26.08.
19.08.
12.08.
05.08.
29.07.
22.07.
15.07.
08.07.
01.07.
24.06.
17.06.
10.06.
03.06.
27.05.
20.05.
13.05.
06.05.
29.04.
22.04.
15.04.
0,0
Figure 6: Daytime maximum and nighttime minimum temperature during the cooling season 2008.
Figure 7 shows typical temperature and capacity charts of the reject heat loop of the chiller
during a hot day with 30 °C ambient air temperature. After a start-up phase the absorption
chiller continuously provides at least 10 kW chilled water capacity which results in about 24
kW reject heat. At moderate ambient temperatures the dominating part of the heat rejection is
accomplished via the dry air cooler, but during hot ambient conditions like here up to 50 % of
the daily reject heat load is covered by the latent heat storage. During the nighttime the lower
ambient temperatures suffice to regenerate the latent heat storage. Again the dry air cooler
serves for the heat transfer to the ambient.
34
32
30
28
ambient
temperature
26
24
unloading PCM storage
Thunderstorm
temperature / °C
dry air cooler outlet
36
PCM storage
outlet
22
20
2008 Aug 07/08
18
efficient unloading
possible
16
chiller in operation
14
9:00 11:00 13:00 15:00 17:00 19:00 21:00 23:00 1:00
time / hh:mm
3:00
5:00
7:00
9:00
unloading PCM storage
power / kW
24
20
reject heat chiller
16
12
heat dry air cooler
8
4
heat PCM storage
dry air cooler
0
-4
2008 Aug 07/08
PCM storage
-8
chiller in operation
-12
9:00 11:00 13:00 15:00 17:00 19:00 21:00 23:00 1:00
time / hh:mm
3:00
5:00
7:00
9:00
Figure 7: Temperature and power performance of the reject heat loop during a hot day (max. 30 °C).
This day about 34 % of the waste heat was stored.
The storage has been for about 750 days in operation and has passed through about 400
loading-/unloading cycles without any problems up to now.
6.
CONCLUSIONS
By applying a low temperature latent heat storage together with a dry air cooler a substantial
improvement of the design of a solar-driven absorption cooling system is accomplished: the
reject heat of the sorption chiller is buffered by the heat storage and transferred to the ambient
during periods of low ambient temperatures, e.g. night time or off-peak situations. Especially
in low capacity applications, the use of a dry instead of a wet cooling tower substantially
facilitates the introduction of absorption technology.
During the heating season, the PCM storage facilitates low operating temperature of the solar
collectors with a positive effect on the solar gain.
For the latent heat storage the phase change material calcium chloride hexahydrate
(CaCl2x6H2O) with phase transition, i.e. melting and solidification, in the temperature range
of 27 to 29 °C is applied. Due to the limited temperature swing available for the given
application, the latent heat storage provides a 10 times higher volumetric storage density in
comparison to a conventional water heat storage. The latent heat storage provide a capacity of
10 kW and 120 kWh thermal storage content are in good agreement with the thermal design
and thus prove the feasibility of the technical concept. It is now operating for about 750 days
and 400 loading/unloading cycles without any problems.
The latent heat storage supporting the heat rejection of the absorption chiller has been
successfully tested. Operational results gained since fall 2007 confirm the general feasibility
and the thermal design of the latent heat storage and the overall system. Analysis of system
operation and performance will continue until end of 2009.
ACKNOWLEDGMENTS
The project is supported by funds of the German Federal Ministry of Environment (BMU)
under contract number 0329605D.
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