analysis of pcm storage unit for night coolness storage in summer season
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This article was downloaded by: [New York University]On: 10 October 2014, At: 17:44Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
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Analysis of PCM storage unit for nightcoolness storage in summer seasonS. K. Shukla a & S. K. Singh aa Department of Mechanical Engineering , Institute of Technology,Banaras Hindu University , Varanasi, 221005, IndiaPublished online: 21 Sep 2011.
To cite this article: S. K. Shukla & S. K. Singh (2011) Analysis of PCM storage unit for night coolnessstorage in summer season, International Journal of Sustainable Energy, 30:sup2, S142-S154, DOI:10.1080/14786451.2011.605950
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International Journal of Sustainable EnergyVol. 30, Suppl. No. S2, 2011, S142–154
Analysis of PCM storage unit for night coolness storage insummer season
S.K. Shukla* and S.K. Singh
Department of Mechanical Engineering, Institute of Technology, Banaras Hindu University, Varanasi221005, India
(Received 11 April 2011; final version received 8 July 2011)
This paper presents a theoretical analysis of a thermal storage unit, which consists of parallel rectangularchannels for the air flow each separated by a phase change material (PCM). The purpose of the storageunit is to absorb night coolness and to provide cooled air at comfort temperatures during daytime in thesummer season. A MATLAB simulation tool has been used to compute the air temperature variation withlocation as well as time, charging and discharging times of the storage unit. Effect of PCM mass, air flowrate and different inlet temperatures are considered for daytime and nighttime operations of the storageunit. Maximum daytime and minimum nighttime ambient temperatures for the selected location are 40◦Cand 24◦C, respectively, for the month of June. Results show that air temperature at the exit increases anddischarging time decreases on increasing the air flow rates during the daytime operation.
Keywords: PCM; storage unit; charging time; discharging time; air temperature; comfort temperature;air flow rate; NTU; night coolness
Nomenclature
a air gap (m)Ac cross-sectional area of air channel (m2)As surface area of heat exchanging plate (m2)b width of the storage unit (m)cp,air specific heat of air (kJ/kg K)cp,pcm specific heat of PCM (kJ/kg K)Dh hydraulic diameter (m)f friction factor (–)hair convection coefficient for air (W/m2 K)l length of the heat storage unit (m)L latent heat of PCM (kJ/kg)Vair mass flow rate of air (kg)Mpcm mass of PCM (kg)Nu Nusselt number (–)
*Corresponding author. Email: [email protected]
ISSN 1478-6451 print/ISSN 1478-646X online© 2011 Taylor & Francishttp://dx.doi.org/10.1080/14786451.2011.605950http://www.tandfonline.com
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Re Reynolds number (–)�t time interval (s)T temperature (◦C)Tpcm temperature of PCM (◦C)Tc comfort temperature (◦C)To outdoor mean temperature (◦C)Tm melting point of PCM (◦C)uair velocity of air in the channel (m/s)Vair volume flow rate of air (m3/h)
Superscripts and subscripts
in inlet condition to the storage uniti number of nodes in the x-directionmin minimum value of the variablemax maximum value of the variablen previous time stepn + 1 current time stepout outlet condition from the storage unit
Introduction
Energy storage not only plays an important role in the conservation of energy but also improvesthe performance and reliability of a wide range of energy systems and becomes more importantwhere energy sources are intermittent. Due to the increased environmental concerns and limitednature of fossil fuels, passive ways of space conditionings are getting more attention, such assolar cooling/heating, storage of night coolness, earth-coupled cooling/heating systems, phasechange material (PCM)-based cooling/heating systems, etc. Since many of the resources, onwhich passive techniques depend, are intermittent in nature, storage of these resources alwaysplays a vital role for continuous utilization of these resources. Therefore, an efficient and reliablethermal energy storage (TES) system plays a significant role when considering passive techniques.TES can be achieved in the form of sensible heating of liquid or solid, storage of high pressuresteam, heat of hydration or utilization of heat of fusion or heat of evaporation. Among all above-mentioned storage techniques, latent heat storage energy technique is getting more attraction dueto its very high energy storage densities and smaller temperature difference when storing theenergy and releasing it when compared with sensible storage techniques.
Using coolness of night to achieve comfort temperatures in any space is one of the passiveways of cooling. If this night coolness is stored and being used during daytime, to achieve thecomfort temperatures, mechanical cooling can be either totally eliminated from daytime or atleast limited to certain periods of daytime. The same can be practised during winters if solarenergy during daytime is stored and used for nighttime heating. A large amount of fossil fuelscan be saved, which in turn helps in the reduction of pollutant gases such as NOx and CO2. Hedand Bellander (2006) presented a method to simulate a PCM air heat exchanger. PCM was usedfor its phase change nature in a given temperature range. The aim was to find a model that willfit into a finite difference-based indoor climate and energy simulation software. In this analysis,a fictive heat transfer coefficient was established which includes aspects of the geometry andthe air flow in the heat exchanger as well as the properties of the PCM. Alkilani et al. (2009)presented a theoretical investigation of a solar air heater integrated with a PCM to obtain outputair temperatures in a discharge process. The PCM unit consists of an inline single row of cylinders
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containing PCM. The PCM comprises paraffin wax with mass fraction 0.5% aluminium powderto enhance the heat transfer. Stritih and Butala (2007) presented an experimental and numericalanalysis of cooling the buildings, using nighttime cold accumulation in PCM with constant inlettemperatures. The comparison of experimental and numerical results shows good agreement.Arkar and Medved (2007) presented a numerical study of the free cooling concept with varyinginlet temperatures using RT20 paraffin as PCM, integrated into the ventilation system of thebuilding. A cylindrical latent heat TES (LHTES) device was filled with spheres of encapsulatedRT20 paraffin. In this research, a parametric study of storage unit was carried out using ambientair as inlet air. The correlation between the climatic conditions and the free cooling potential wasinvestigated by Medved and Arkar (2008) for different cities of Europe. The case of a cylindricalLHTES device with a packed bed of spheres encapsulated with PCM, which was integratedinto a building’s mechanical ventilation system, was considered during investigation. For anexperimental verification of the LHTES’s numerical model, a commercially available PCM (RT20paraffin, Rubitherm GmbH) with a latent heat of 142 kJ/kg was used. This PCM has a relativelylarge phase change temperature range. Morrison and Abdel-Khalik (1978) developed a theoreticalmodel for studying the transient behaviour of a phase change energy storage unit. They studiedthe model performance of solar heating systems using both air and liquid as working fluids,based on the following three assumptions, i.e. axial conduction in the flow mode is negligible,the Biot number is very low and heat loss from the unit can be ignored. Solomon (1981) studiedthe behaviour of an array of PCM cylinders as a thermal storage unit, by looking at a single rowof N cylinders, under some assumptions such as the heat transfer process in every cylinder isradially symmetric. This method can be used to design and simulate the systems to predict the airtemperature and freezing time. Virgone et al. (2011) performed the assessment of PCM wallboardusage for the renovation of a tertiary (i.e. lightweight) building. For this purpose, two identicalrooms of a renovated tertiary building have been tested: one equipped with PCM wallboard andthe other being ‘classically’ renovated. The results show that the PCM wallboards enhance thethermal comfort of occupants due to air temperature and radiative effects of the walls. Susman et al.(2011) constructed PCM modules from a paraffin composite and tested in an occupied Londonoffice in the summer season. Design variations tested the effect of heat transfer on a black paint oraluminium surface, the effect of different phase transition zones and the effect of discharging heatinside or outside. During this test, module temperatures were monitored along with air flow rate,air temperature and globe temperature. It was found that black modules transfer heat and exhaustlatent storage capacity significantly quicker than aluminium modules due to radiant exchange.Tyagi and Buddhi (2007) presented a review on various possible methods for heating and coolingin buildings for various types of systems such as PCM trombe wall, PCM wallboards, PCMshutters, PCM building blocks, air-based heating systems, floor heating and ceiling boards. Arkaret al. (2007) studied free cooling efficiency in heavyweight and lightweight low energy buildingswith two LHTES devices: one for cooling the fresh supply air and the other for cooling therecirculated indoor air. In this study, cylindrical heat storage filled with spheres with encapsulatedPCM was used. Stritih and Butala (2007) presented an experimental and numerical analysis ofcooling buildings using nighttime cold accumulation in PCM. Experiments were conducted usingparaffin with a melting point of 22◦C as the PCM to store cold during the nighttime and to cool hotair during the daytime in summer. Dolado et al. (2011) presented models developed to simulatethe performance of the TES unit in a real scale PCM air heat exchanger. The models are based onone-dimensional conduction analysis, utilizing finite difference method, and implicit formulation,using the thermo-physical data of the PCM measured in the laboratory.
This analysis presents the effect of limited summer nighttimes and temperature, for the storageof night coolness which can be utilized during daytime, using PCMs, under harsh summer climaticconditions in order to achieve comfort temperatures from the storage outlet. For this purpose, thestorage unit used is composed of multiple rectangular channels for the flow of heat transfer fluid,
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which is air in this case, separated by the phase change storage material. A MATLAB simulationtool has been used to compute the air temperature variation with location as well as time, chargingand discharging times of the storage unit. Effect of PCM mass, air flow rate and different inlettemperatures are considered for both the daytime and the nighttime operations of the storage unit.The melting point of PCMs, used for the analysis, should lie between comfort temperature andminimum night ambient temperatures.
Methodology
For the analysis of storage of night coolness using PCMs, the storage unit configuration is displayedin Figure 1. This system consists of two essential parts: PCM channels and air flowing duct inalternate orders. Numbers of air channels considered are 5. The dimension of the air flowing ductis l × b × a, in which l is the length of the storage unit, b the width of the storage unit which istaken as 0.45 m and a the air gap which is taken as 0.008 m.
Simplicity of this type of storage unit is the main advantage of its selection. Such types ofmodels have been studied by Morrison and Abdel-Khalik (1978) for the TES coupled with thesolar water heating system. This study incorporates a similar system to be studied with nightcoolness storage during daytime utilization for cooling in the summer season. Figure 2 shows theportion of the PCM container and the air gap which is used in the analysis. Half of the thicknessof the plate and half of its air gap, respectively, are considered here, and other portion of the platewith the same thickness and same air gap will behave similarly.
The analysis is based on the following assumptions.
• The material changes phase at a single constant temperature.• Properties of the PCM in both phases are considered to be constant.• Conduction resistance of the material used between PCM and heat transfer fluid is negligible.
Figure 1. TES unit to be analysed for night coolness storage.
Figure 2. Half portion of the PCM and half portion of the air gap used for the analysis.
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• Sub-cooling of the PCM is ignored.• Heat loss from the unit is ignored.• Axial conduction in the flow mode is negligible.• The temperature of the PCM is constant with location but it can vary with time (Alkilani et al.
2009).
The governing equation for air as heat transfer fluid is
maircp,air dTair = hair bdx(Tpcm − Tair), (1)
where
mair = Vair × pair
3600,
in which
Vair = uair × Ac (2)
Equation (1) can be written with finite difference method in the following form:
T ni,air = T n
i−1,air +hair wdx
(T n
pcm − T ni−1,air
)maircp,air
. (3)
The convective heat transfer coefficient is determined by (Incropera and Dewitt 2008)
hair = Nu × kair
Dh, where Dh = 2ab
a + b. (4)
For a fully developed laminar flow, Re < 2300
Nu = 6.0, for
(b
a= 56.25
).
For a fully developed turbulent flow, Re > 2300
Nu = (f /8)(Re − 1000)Pr
1 + 12.7(f /8)1/2(Pr2/3 − 1),
where
f = (0.79 In Re − 1.64)−2 and Re = ρairuairDh
μ.
The heat carried away by air in the time interval of �t at any time is
Q = inair cp,air(Tnout,air − T n
in,air) × �t. (5)
If no phase change, then
Q = Mpcm cp,pcm(T npcm − T n+1
pcm ), (6)
where
T n+1pcm = T n
pcm + Q
Mpcm Cp,pcm. (7)
If phase changes, then
Tpcm = Tm.
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Table 1. Physical properties.
SP 25 A8 (PCM)/Name of property Rubitherm Aluminium Compound
Melting point (Tm) (◦C) 27 – 27Latent heat enthalpy (L) (kJ/kg) 180 – 180Density (ρ) (kg/m3) 1380 2702 1390Thermal conductivity (k) (W/m K) 0.6 237 2.45Specific heat (cp) (kJ/kg K) 2.5 0.896 2.47
The number of transfer units (NTU) for the storage unit can be defined as:
NTU = hair As
mair cp,air. (8)
The following is the relation that has been developed for calculating indoor comfort temperaturesfor any region which is coupled with the mean outdoor temperatures (Nicol and Roaf 1996, Rajaet al. 2001):
Tc = 18.5 + 0.36 × To, (9)
where
To = Tmax + Tmin
2, (10)
in which To is the mean daily temperature and Tc is the indoor comfort temperature. The comforttemperature range may vary about ±2◦C for air conditioned space.
To overcome the low thermal conductivity problem of PCM, we can add a powder of materialwith higher conductivity, such as copper or aluminium powder, with a goal to lower the system cost,and we preferred aluminium powder (1.5%); the physical properties of the stimulated compoundcalculated are as follows (Alkilani et al. 2009):
kc = kpcm vpcm + kAl vAl,
cp, c = cp,pcm mpcm + cp,Al mAl,
ρc = ρpcm vpcm + ρAl vAl,
where vpcm = Vpcm/Vc is the volume fraction of the PCM in the compound; vA1 = VA1/Vc thevolume fraction of the aluminium powder in the compound; mpcm = Mpcm/Mc the mass fraction ofthe PCM in the compound and mA1 = MA1/Mc the mass fraction of aluminium in the compound.The compound formed after mixing is used as the PCM for night coolness storage (Table 1).
Results and discussion
Results, obtained from the MATLAB computer programs, are presented here to show the effect ofdifferent parameters during charging and discharging times of the PCM storage unit. The thermalconductivity of the PCM is increased by mixing the aluminium powder to reduce the Biot numberbelow 0.1 for PCM. The main aim of this work is to obtain comfort temperatures from the storageunit when it is hot in daytime during summer season by utilizing night coolness with air as the heattransfer fluid. Figure 3 shows the maximum, minimum and comfort temperatures for the Varanasilocation on a monthly basis. The comfort temperature as calculated from the above-mentionedrelation and known climatic data for the month of June is 30 ± 2◦C.
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Figure 3. Climatic data for the location Varanasi.
Variation in air flow rates, length of the storage unit and its effect on the air outlet temperatureare shown in Figure 4(a) and (b) for two different inlet air temperatures of 38◦C and 40◦C. Eightair flow rates have been considered varying from 10 to 80 m3/h, with an increment of 10 m3/h.All the flow rates mentioned here are the flow rates with single air channel, i.e. the total flow rateis five times of these values. For the analysis purpose, it is assumed that the PCM is fully chargedat its melting point and no sensible heat is considered for this analysis part only. It is becauseof the interest of our study and dominating action of latent heat over sensible heat as it is only10% of the latent heat, for this type of storage system. The outlet air temperature is exponentiallydecreasing with increasing length of the storage unit. The outlet air temperature should be higherat higher air flow rates because of smaller time availability for heat transfer between PCM and air,although the increase in the convective heat transfer coefficient increases the heat transfer. Thecombined effect decreases the outlet air temperature from the storage unit at higher air flow rates.At lowest flow rate which is 10 m3/h, the length of the storage unit required is less than 1 m forboth inlet air temperatures. The minimum length of the storage unit required for the highest flowrate is 2 m for an inlet temperature of 38◦C and 2.5 m for an inlet temperature of 40◦C.
The length of the storage unit corresponding to an NTU value of 5 in Figure 5 is the minimumlength at which the temperature of air will almost become equal to the material temperature,while only latent heat of the material is being considered (Cengel 2006). Therefore, increasingthe length of the storage unit beyond this value will not affect the outlet temperature of air. TheNTU depends on the heat transfer coefficient and the surface area of the path along which air ismoving. For the surface area, the width of the channel is kept constant, while the length of the airchannel is increased or decreased.
Figures 4 and 5 indicate that the higher air flow rates will require much larger storage lengthto exchange heat with PCM to be in comfort temperature limits at the exit of the storage unit.Comfort temperature limit has been shown in Figure 3. In contrast, limitations of space availabilityand cost are always associated with the length of the storage unit. The best approach will be tofirst choose the minimum length in such a way that it provides comfort temperature at the startof operation at the required flow rate and then use the mass option of the PCM to keep the exittemperature in the comfort range for a desired time period.
Figure 6 shows the comfort temperature timings at the storage unit outlet when plotted againstthe mass of the PCM for different flow rates of air with different surface areas of the heat exchang-ing plate. As the air flow rate increases, the capacity of the storage unit, to provide comforttemperature limits for a given mass and for a given surface area of the storage unit, decreases. Forexample, 12 kg of mass can provide comfort temperatures for about 13, 9, 8 and 6 h, with flow
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Figure 4. Effect of air flow rates and length of the storage unit on air outlet temperatures during daytime operation forinlet air temperatures: (a) Tin = 38◦C and (b) Tin = 40◦C.
Figure 5. Effect of flow rates and length of the storage unit on NTU.
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Figure 6. Effect of flow rates and mass of PCM on comfort temperature timings during daytime operation.
Figure 7. Output air temperature against operating time for different inlet air conditions during daytime operation: (a)Mpcm = 12 kg, l = 2.5 m and V = 40 m3/h and (b) Mpcm = 10 kg, l = 2.5 m and V = 30 m3/h.
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rates of 20, 30, 40 and 80 m3/h at a constant inlet temperature of 38◦C and a constant length of2.5 m of the storage unit for all given cases. At lower air flow rates, comfort temperature versustime plot, for both the length of the storage unit, is approximately the same because both will giveapproximately the same outlet air temperature, and hence the rate of energy carried away by airwill be approximately equal. At higher flow rates, the temperature at the exit will be significantlyhigher for 3.0 m length of the storage unit than that for the 2.5 m length, and hence higher lengthwill give smaller comfort time periods.
Figure 7(a) and (b) shows the effect of mass of PCM and air flow rate on discharge time fordifferent inlet air conditions. It is assumed that initially PCM is in solid form, at its melting pointtemperature, i.e. 27◦C. Since during the melting of PCM, its temperature will remain constant;therefore, we note the constant output air temperature in the comfort temperature range till thetotal mass of PCM is melted. Beyond this point, output air temperature increases rapidly, whichrepresents that melting of PCM will also increase proportionally. It is because sensible heat ofPCM is very less due to very low specific heat when compared with the latent heat of PCM. Foranalysing the variation in sensible heat, the temperature variation of PCM with time is considered,
Figure 8. Charging time against mass of PCM for different air flow rates: (a) l = 2.5 m and Tin = 24◦C and (b) l = 3.0 mand Tin = 24 ◦C.
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but ignored with respect to location (Alkilani et al. 2009). Increased inlet air temperature willalways provide less duration of comfort temperature with the same flow rate when comparedwith decreased inlet air temperature conditions. The combination of air flow rate and mass ofPCM resulted in the same discharge time for different inlet air temperatures. It is noted that thedischarge time is approximately 10 h for different inlet air conditions. Changes in the inlet airtemperature do not have much effect on discharge time of the storage unit. In both the cases, ifthe inlet air temperature reaches beyond 38◦C, the comfort temperature for 8 h cannot be obtainedfor a specified combination of mass of PCM and air flow rate.
The effect of change in the length of the PCM plate (i.e. change in the surface area) and effectof different flow rates have been considered for the nighttime analysis (Figure 8(a) and (b)). It isclear from the figure that if the same mass is being used in both cases, the mass which is in thelarger surface area can be charged more faster when compared with the mass in the lesser surfacearea at all given flow rates. It is also inferred from Figure 6 that the effect of surface area of theheat exchanger is not much with respect to comfort temperature durations, except at higher flow
Figure 9. Charging time against mass of PCM for different inlet air temperatures: (a) l = 2.5 m and V = 160 m3/h and(b) l = 3.0 m and V = 120 m3/h.
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rates. If 10 kg of mass is considered for solidification during nighttime, it can be totally solidifiedin 7 h for a plate length of 3.0 m, whereas a plate length of 2.5 m will take 8 h with a flow rate of160 m3/h, which is four times of 40 m3/h. It is also clear from Figure 6 that the same mass canprovide comfort temperature for about 6 h duration with a 3.0 m plate length and 6.7 h for a 2 mplate length. Therefore, the larger surface area will be more advantageous for nighttime operationof the storage unit, whereas it has very less effect during daytime operation with the same massof PCM but again cost and space parameters will put some restrictions over it.
Charging time required for PCM is the most important parameter to be analysed in this studydue to limited time and temperature availability during night in the summer season. In Figure 9(a),three different possible ambient temperatures during summer nights are considered, which are23◦C, 24◦C and 25◦C and the flow rate is four times when compared with the daytime flow rateof 40 m3/h. Figure 9(b) is plotted for same inlet temperature variations, considering the flow ratethree times of the daytime flow rate, i.e. 40 m3/h, by keeping the length of the storage unit as3.0 m instead of 2.5 m. As the ambient temperature available during the night increases, the timerequired for charging also increases. According to Figure 9(a) and (b), 12 kg mass of PCM perchannel can be solidified in 8 h at two different flow rates of 160 and 120 m3/h for the sameinlet air temperature of 24◦C, but in the latter case, the length of the storage unit needed is 3.0 minstead of 2.5 m. It is observed that increasing the flow rate will be cheaper than increasing thelength because the latter one results in more pressure drop (more electricity will be consumed)with more base material required, which in turn increases the cost. If the ambient temperature isconsidered to be at 25◦C, then the PCM cannot be charged fully considering the mass of PCM tobe greater than 8 kg per channel. Higher flow rates and larger length of the storage unit will beneeded for 25◦C ambient condition.
Conclusions
Storage of night coolness, for obtaining comfort temperatures of air during daytime under climaticconditions, where summer is very harsh, is a challenging work, but is possible by the use of PCMs.The length of the storage unit should be kept in such a way that at required air flow rate, outlettemperature from the storage unit should be within the defined comfort limits. It is inferred that allflow rates between 10 and 80 m3/h will give the air outlet temperature within comfort limits forthe storage unit with 2.5 m of length. Results show that higher air flow rates will provide higherexit air temperatures during the daytime due to smaller time duration available for heat transferat higher flow rates.
Mass of the PCM and length of the storage unit play a vital role in obtaining the comforttemperatures at certain periods of time during the day operation, but during nighttime operation,it is the air flow rate and night ambient temperature which decides that how much mass of PCMcan be solidified in limited summer time of 8 h. It has also been found that four times of air flowrate will be required when compared with the air flow rate during the daytime so that it can becharged in limited night duration of 8 h and provides comfort temperature for about 8 h duringdaytime. More mass will require higher flow rates, which in turn certainly affects the electricalpower consumption.
Increase in the NTU value beyond 5 has no significance because it will not affect the exit airtemperature for latent heat utilization. At higher inlet temperatures of air during daytime, thissystem will provide comfort temperatures for lesser time periods, and at higher inlet temperature(25◦C) during nighttime, the PCM cannot solidify totally in the limited summer nighttime.
It is seen that when the temperature difference between nighttime ambient temperature andmelting point of the PCM is higher than 3◦C, it requires lower flow rates for complete solidificationof PCM within available nighttime hours. But in case if it is lower than 3◦C, then either it will
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not solidify completely or it will require very higher flow rates, even beyond four times of thedaytime flow rate, to store night coolness, which may not be suitable from electricity consumptionpoint of view.
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