modelization of a water tank including a pcm module
TRANSCRIPT
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Applied Thermal Engineering 26 (2006) 1328–1333
Modelization of a water tank including a PCM module
Manuel Ibanez a, Luisa F. Cabeza b,*, Cristian Sole b, Joan Roca b, Miquel Nogues b
a Dept. de Medi Ambient i Ciencies del Sol, Universitat de Lleida, Rovira Roure 191, 25198 Lleida, Spainb Dept. d�Informatica i Eng. Industrial, Universitat de Lleida, Jaume II 69, 25001 Lleida, Spain
Received 4 May 2005; accepted 21 October 2005Available online 9 December 2005
Abstract
The reduction of CO2 emissions is a key component for today�s governments. Therefore, implementation of more and more sys-tems with renewable energies is necessary. Solar systems for single family houses or residential buildings need a big water tank thatmany times is not easy to locate. This paper studies the modelization of a new technology where PCM modules are implemented indomestic hot water tanks to reduce their size without reducing the energy stored. A new TRNSYS component, based in the alreadyexisting TYPE 60, was developed, called TYPE 60PCM. After tuning the new component with experimental results, two more expe-riences were developed to validate the simulation of a water tank with two cylindrical PCM modules using type 60PCM, the cool-down and reheating experiments. Concordance between experimental and simulated data was very good. Since the new TRNSYScomponent was developed to simulate full solar systems, comparison of experimental results from a pilot plant solar system withsimulations were performed, and they confirmed that the type 60PCM is a powerful tool to evaluate the performance of PCM mod-ules in water tanks.� 2005 Elsevier Ltd. All rights reserved.
Keywords: Domestic hot water tank (DHW tank); Phase change material (PCM); Thermal energy storage (TES); Modelization
1. Introduction
It seems clear that one of the only ways that our soci-ety can comply with the Kyoto protocol is to promotemore and more the use of renewable energies. But theprice of houses in cities, especially in big cities, does verydifficult the location for domestic hot water (DHW)tanks in buildings. Therefore, any research in the direc-tion of decreasing the size of such tanks is reallyimportant.
Our research group has demonstrated that the use ofphase change materials (PCMs) in DHW tanks is feasi-ble [1,2], but more research is needed to make this tech-nology commercial. The evaluation of the performance
1359-4311/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.applthermaleng.2005.10.022
* Corresponding author. Tel.: +34 973 702742; fax: +34 973 702702.E-mail address: [email protected] (L.F. Cabeza).
of this technology in different systems and locations withdifferent climatology is easier and more reliable whendone with a simulation program.
The aim of this paper is to present the modelizationof a DHW tank with a module of PCM included. Exper-imental work has been detailed in a previous paper [2],but those results are used here to validate the modeldeveloped.
2. TRNSYS store model
Though the effort to simulate the performance ofwater storage tanks with PCM has been previously done[1,3] there are no references studying the modelization ofsuch a tank allowing an easy implementation in a solarinstallation with different elements. The analysis of thereal possibilities of the water tanks including a PCM
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module require the simulation of the operation in differ-ent applications and installations.
Due to its versatility for the complete simulation of abuilding, and to the possibilities of connection of a sim-ulation module with other installations, such as solarand HVAC systems, the program TRNSYS was selectedas a base tool where to develop the new model of water-PCM tanks. The storage tank including PCM moduleswas based in a standard TRNSYS16 component, Type60. This stratified fluid tank assumed that a water-filledsensible energy storage subject to stratification could bemodelled considering that the tank consisted on N fullymixed equal volume segments. The degree of stratifica-tion was determined by the value of N. The stratificationtemperature in the tank was modelled one-dimension-ally. Options of fixed or variable inlets, unequal sizenodes, temperature deadband on heater thermostats,incremental loss coefficients, internal submersed heatexchangers, non-circular tanks, horizontal tanks, andlosses to the flue of a gas auxiliary heater were availablein the standard model. More detailed information isgiven in Klein et al. [4].
The energy balance in the water tank is modifiedwhen PCM modules are included. To take into accountthis new element a TRNSYS component was developed,hereafter the proposed type was named Type 60PCM.Fig. 1 shows the different elements considered for theenergy balance in each node of the water tank. The fol-lowing mathematical expression, related to Fig. 1, is adifferential equation which will give the water tempera-ture variation:
midhi
dt¼ _Q
dp
i þ _Qhx
i þ _Qaux
i þ _Qcond
i þ _Qloss
i þ _Qmodules
i ð1Þ
where mi is the mass of node i, dhidt is the enthalpy varia-
tion of node i, _Qdp
i is the energy change of node i due tocharging/discharging via direct inlet/outlet flow includ-ing the flow upward/downward in the tank, _Q
hx
i repre-
Fig. 1. Schematic representation o
sents the energy change due to internal heatexchanger, _Q
aux
i is the energy input due to a built-in elec-trical heater, _Q
cond
i represents the thermal conduction toneighbouring nodes, _Q
loss
i are the losses through the tankenvelop to the ambient, and _Q
modules
i corresponds to theenergy exchange between the storage medium and thePCM modules.
Different modelling simplifications were carefullyevaluated to develop type 60PCM. After a detailedstudy, the following ones were considered acceptablefor PCM modules composed of metallic containers andphase change material mixed with graphite composite:
A. The PCM modules had cylindrical shape and theywere located in vertical position close to the upperbase of the water tank.
B. The thermal conductivity of the metallic contain-ers was high enough to consider their temperaturehomogeneous in normal operating conditions.
C. The heat transfer flux from the phase change mate-rial to the containers was high enough to considerthat heat exchange between PCM modules andwater was only dependent of the free and forcedconvection coefficients in container external sur-face. Internal module resistance to heat transferwere not modelled.
D. Phase change material histeresy was not consid-ered in this simulation work.
Type 60PCM needed additional parameters notrequired in the original type 60. These inputs wererelated to the modules geometry and the phase changematerial thermal properties. The following parameterswere user-defined: the number of cylindrical PCM mod-ules introduced in the tank, the height and diameter ofthe modules and the temperature of the phase changematerial at the initial simulation conditions. The PCMproperties involved in the model were the densities and
f energy flows into a node.
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the specific heat in both material phases. Finally, thephase of change was modelled from the experimentalenthalpy–temperature curve. An experimental data filewas imported and used by type 60PCM.
Fig. 2. Hot water tanks from Lapesa and PCM modules.
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-50
045 50 55 60 65 70 75
Temperature (oC)
En
thal
py
(J/g
)
Fig. 3. Experimental enthalpy–temperature curve for sodium acetatetrihydrate with graphite.
3. Experiments
3.1. Experimental set-up
An experimental solar stand was constructed at theUniversity of Lleida with such a goal [2]. The standhas two thermal solar collectors, two hot water tanksof 146 l and an electrical heater outside the tanks, whichallows electrical heating with a known power whenneeded. The two water tanks are identical, but onewas modified to insert the PCM module and thermocou-ples to measure the temperature of the PCM and thestratification of the water (Fig. 2).
The modules used are commercial aluminium bottlesfilled with almost identical amounts of the PCM–graph-ite composite material. The dimension of the PCM mod-ules were 8.8 cm of diameter and 31.5 cm height, giving1.5 l of capacity. The PCM used was sodium acetatetrihydrate.
The data of the PCM graphite compound was givenby the manufacturers with a density of 1.35–1.4 kg/l, amelting point of 58 �C, a heat capacity of 2.5 kJ/kg K,an enthalpy of 180–200 kJ/kg, and a thermal conductiv-ity of 2–5 W/m K.
The temperature–enthalpy curve was obtained byDSC analysis (Mettler Toledo 822e). To obtain reliabledata, several samples were tested and the obtained curveis presented in Fig. 3.
The total amount of PCM used is about 2.1 kg,meaning two modules of 1.5 l, which result a 2.05% ofthe total volume of the tank.
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27-May 28-May 29-May 30-May 31-May 01-JunTime
Tem
per
atu
re (
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)
PCMeffect
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T-60 T-30T-90
PCM-1
T-110
T-120
PCM-2
PCMeffect
Fig. 4. Cooldown process of the tank with two PCM modules. PCM-1, and PCM-2 are the temperature of the PCM modules. T-0 is the bottomwater temperature, and T120 is the top water temperature.
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)
T-90
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T-60
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PCM-1
T-120
PCM-2
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T-30
T-60
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PCM-1
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PCM-2
Fig. 5. Tank reheating with two PCM modules after its completeemptying. PCM-1, and PCM-2 are the temperature of the PCMmodules. T-0 is the bottom water temperature, and T120 is the topwater temperature.
M. Ibanez et al. / Applied Thermal Engineering 26 (2006) 1328–1333 1331
3.2. Experiences carried out
Several tests were carried out in the experimentalstand. The tests were classified in cooling down processand reheating.
The cooling down process (Fig. 4) consisted in heatingelectrically the water tank until 80 �C and then cooled itdown naturally with natural convection through the walltank.
In the reheating test (Fig. 5) the water tank washeated until 80 �C to ensure the PCM was melted, andthen the tank was emptied and refilled with cold waterfrom the net. The reheating of the top water by thePCM module could be measured.
4. Comparison between simulated and measured data.
Type validation
For simulation study TRNSYS was applied. Mea-sured initial temperatures and flow rates were used asinput values for the simulation. A simulation time stepDtsim = (1/1200) h was chosen. For simulation purposeswater-PCM was divided in 13 and 39 nodes. Compari-son between both alternatives was done for cooldownand reheating experiences. It was concluded that thereis not significant difference between both simulationsin such cases.
Table 1RMSE comparing measured and modelled water temperatures
RMSE (�C) T110 T90 T30
Cooldown 0.35 0.25 0.17Reheating 0.59 0.92 0.61
4.1. Tuning of type 60PCM parameters
Some parameters used in the original type 60, and byextension in type 60PCM, to simulate the tank can beevaluated theoretically but they are better adjustedempirically. The destratification conductivity and theheat losses to the ambient are significant examples.The first step for validation of type 60PCM was to testthis new type in original condition and to tune the above
mentioned parameters running the simulation with nophase change substance included. Two laboratory andsimulation experiences were compared. The first onewas the cooling down of the water in the tank fromhomogeneous and high initial temperature conditions.The second one consisted in forcing a remarkable strat-ification in the water tank and to study how naturaldestratification develops. The water temperature profilemeasurements and simulation allowed to evaluate theheat losses to the ambient in 11.2 W/m2 K and destrati-fication conductivity in 34.6 W/m K.
4.2. Testing of type 60PCM
Two more experiences were developed to validate thesimulation of a water tank with two cylindrical PCMmodules using type 60PCM, the cooldown and reheat-ing experiments previously described in this paper.
To evaluate the performance of the model it wasdecided to compare water measured and simulated tem-peratures computing the root mean square errors(RMSE). Because the cooldown process is a very slowphenomena comparison between data was done every5 min while in the reheating experience the data werecompare every 10 s. Results shown in Table 1 are theRMSE for some layers previously selected: T110 isrelated with data evaluated at 110 cm from the bottomof the tank and in the middle layer which contains thePCM modules; T90 considers data 20 cm below T110in a layer below the PCM modules; finally the resultsin layer T30 are related to data evaluated 30 cm abovethe bottom of the tank. These RMSE have been com-puted for a time interval of 3 days in the cooldown expe-rience and for a time interval shorter than 1 h in thereheating experiment.
Fig. 6 is plotted to show an example of the perfor-mance of type 60PCM. In this figure upper layer tankmeasured and simulated temperatures are compared.Results are really good tough during the phase changeprocess the two maximum corresponding to each PCMmodule are not drawn. To model the subcooling phe-nomena was not considered in this work.
Fig. 7 shows the temperatures in two different levels.It is observed that the reheating process is properlymodelled while there were found real difficulties to sim-ulate the very fast cooldown in the water tank when heatwater is replaced by the cool one. The flow rates usedwere extremely high to be sure that most of the energycontained in the PCM modules was still stored there
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(oC
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Fig. 6. Cooldown experimental and simulated water temperatures atlevel 110. T-Sim is the simulation temperatures, and T-Exp is theexperimental temperatures.
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Time (hh:mm)
T110 - ExpT110 -SimT90 - ExpT90 -Sim
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emp
erat
ure
(oC
)
Fig. 7. Reheating experimental and simulated water temperatures atlevels 90 and 110. T-0 is the bottom water temperature, and T120 isthe top water temperature. T-Sim is the simulation temperatures, andT-Exp is the experimental temperatures.
0.6y Ef. (55oC)
1332 M. Ibanez et al. / Applied Thermal Engineering 26 (2006) 1328–1333
when the reheating process started. It was latelyobserved that this flow rate were several times higherthat the standard values used in solar systems. There-fore, results shown in Table 1 are good enough to con-firm that type 60PCM can be applied to evaluate thiskind of tank in solar systems.
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0.5
0 2 4 6 8 10 12
Modules volume (%)
So
lar
frac
tio
n/C
olle
cto
r ef
fici
enc Fs (55oC)
Ef. (45oC)
Fs (45oC)
Fig. 8. Annual solar fraction (Fs) and collector efficiency (Ef) fordifferent phase change temperatures (45 �C and 55 �C) and volumefraction (%).
4.3. Application to SDHW systems
The new TRNSYS component was developed tosimulate full solar systems. One of the goals ofthese TRNSYS simulations was the optimization ofthe water-PCM storage tank characteristics. To improvewater-PCM tank designs optimization should be donefor different applications and ambient conditions.
The application considered here was a solar domestichot water installation (SDHW) for a family house inLleida (NE Spain). The parameter studied for optimiza-tion was the annual solar fraction achieved by the solarinstallation. Economical considerations will be taken
into account in a near future. The variables under studyin the tank were the percentage of PCM and the phasechange temperature of the material in the modules.
The SDHW simulated was a standard one where thewater-PCM tank validated in the previous section wasincluded. The solar collector area was 5 m2 and the loadwas 200 l/day of 45 �C hot water. The domestic hot-water load profile used was proposed by Jordan andVajen [5]. The cold water inlet temperature was 15 �Cand constant all over the year.
Fig. 8 shows the mean yearly solar fraction and col-lectors� efficiency depending on the volume fractionoccupied by the two modules respect to the total tankvolume and the two phase change material tempera-tures, 45 �C and 55 �C. Solar fraction is understood asthe solar energy contribution to the total load in termsof the fractional reduction in the amount of energy thatmust be purchased. Collectors� efficiency is the ratio ofthe useful gain over some specified time period to theincident solar energy over the same time period.
Results derived from this study depended on loadprofile and climate so they were not asserted as universalvalues. However, they confirmed that the type 60PCMis a powerful tool to evaluate the performance ofPCM modules in water tanks. The solar fraction achievein a water tank without PCM was around 44.5%while introducing PCM modules it increased from 4%to 8% depending on phase change temperature. FromFig. 8 three observations were derived: (1) The variablesof the PCM-water tank studied did not influence inthe mean yearly collector efficiency; (2) the increasein PCM amount has to be carefully evaluated becauseit was observed a saturation phenomena, large amountsof PCM did not improve the solar fraction substan-tially; and (3) the phase change temperature of thematerial was a critical parameter because a similaramount of material with the same phase changeenthalpy gave important differences. Discussions aboutavailability, price or other thermo-physical properties
M. Ibanez et al. / Applied Thermal Engineering 26 (2006) 1328–1333 1333
of PCM in this temperature ranges were not addressedin this paper.
5. Conclusions
The analysis of the real possibilities of the watertanks including a PCM module require the simulationof the operation in different applications and installa-tions. The storage tank including PCM modules wasbased in a standard TRNSYS16 component, Type 60,and was transformed into Type 60PCM.
After tuning the new component with experimentalresults, two more experiences were developed to validatethe simulation of a water tank with two cylindrical PCMmodules using type 60PCM, the cooldown and reheatingexperiments. Concordance between experimental andsimulated data was very good.
Since the new TRNSYS component was developed tosimulate full solar systems, comparison of experimentalresults from a pilot plant solar system with simulationswere performed, and they confirmed that the type60PCM is a powerful tool to evaluate the performanceof PCM modules in water tanks.
Acknowledgements
The authors would like to acknowledge the compa-nies Lapesa S.A., Takama and SGL Technologies fortheir collaboration in this research. The work was par-tially funded with the project CICYT DPI2002-04082-C02-02.
References
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[2] L.F. Cabeza, M. Ibanez, C. Sole, J. Roca, M. Nogues, Exper-imentation with a water tank including a PCM module, SolarEnergy Materials and Solar Cells, in press.
[3] B.J. Newton, Modeling of solar storage tanks, M.S. Solar EnergyLaboratory, University of Wisconsin, USA, 1995.
[4] S.A. Klein, W.A. Beckman, J.W. Mitchell, J.A. Duffie, T.L.Freeman, J.C. Mitchell, J.E. Braun, B.L. Evans, J.P. Kummer,R.E. Urban, A. Fiksel, J.W. Thornton, N.J. Blair, TRNSYS 15Reference Manual, Solar Energy Laboratory, University of Wis-consin, Madison, USA, 2000.
[5] U. Jordan, K. Vajen, Realistic domestic hot-water profiles indifferent time scales, IEA SHC. Task 26: Solar combisystems,2001.