1-IJMPERD-Dr. S.K.Shukla-p

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD ) ISSN 2249-6890 Vol.2, Issue 3 Sep 2012 1-9 Š TJPRC Pvt. Ltd.,

ANALYSIS OF PCM STORAGE UNIT FOR NIGHT HEATING USING HEAT OF THE DAY DURING WINTER SEASON PRACHI RAI & S.K.SHUKLA Mechanical Engg.Deptt.,I.T.,B.H.U., Varnasi-221005, India

ABSTRACT This paper presents the theoretical analysis of a thermal storage unit which uses phase change materials and provides much higher energy storage densities. The heat is stored and released at an almost constant temperature. The unit consists of parallel rectangular channels for the air flow each separated by phase change material. The purpose of the storage unit is to absorb the heat of day to obtain comfortable temperature at night during winters. MATLAB programming has been used to compute the air temperature variation with location, charging times of the storage unit. Results show that air temperature at the exit increases and discharging time decreases on increasing the air flow rates during the daytime operation. For winter system, phenomenon of natural convection over a vertical wall is used for analysis of temperature with varying wall length.

KEYWORDS: PCM; Storage Unit; Night Heating; Charging Time; Discharging Time; Air Temperature; Comfort Temperature; Air Flow Rate; NTU; Night Coolness

NOMENCLATURES a

Air gap for PCM storage unit(m)

b

Width of storage unit (m)

Cp

Specific heat at constant pressure (kJ/kg-K)

Dh

Hydraulic diameter of non-circular duct (m) Average convection heat transfer coefficient (W/m2-K)

k

Thermal conductivity(W/m-K)

L

Latent heat energy of PCM (kJ/kg)

l

length of heat storage unit (m) Mass flow rate (kg/s)

NTU

Number of transfer units.

Nu

Nusselt number

p

pressure (Pa)

Pr

Prandtl Number

Q

Heat received (J)

Re

Reynolds number

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Prachi Rai & S.K.Shukla

R

Universal gas constant (J/kg.K)

T

Temperature (K)

Tic

Indoor comfort temperature (K)

Tmo

Outside mean temperature (K)

u

Velocity(m/s) Volume flow rate (m3/h)

Gr

Grashof number

g

Acceleration due to gravity (m/s2) Volumetric thermal expansion coefficient.(K-1)

TS

Surface temperature (K)

T∞

Ambient temperature (K)

INTRODUCTION Increased environmental concerns leading to global warming is caused by tremendous fossil fuel usage. The limited availability of conventional fuels, passive ways of space conditioning to reduce fossil fuel usage are getting more attention. Such techniques include cooling/heating using solar energy, utilization of cooler night temperatures, earth coupled cooling/heating systems, Phase change material based cooling/heating techniques, etc. Since many of these mentioned resources on which passive techniques depend are intermittent in nature, storage of these resources is needed to ensure continuous operation. So an efficient and reliable thermal energy storage system is important when dealing with passive techniques. Thermal energy storage can be in form of sensible heat of liquid or solid, storage of high pressure steam, heat of hydration or utilization of heat of fusion or heat of evaporation. Among these storage techniques, latent heat storage energy technique is getting more attention due to high energy storage densities and smaller temperature differences as compared to sensible storage techniques.

The advantages of latent heat storage over other storage techniques in passive cooling and heating of buildings have attracted many researchers. Using coolness of night to achieve comfort temperatures in a space is one of the passive ways of cooling. Also in winter season nights are very cold, so the heat of sun from day is stored in PCM and used in night to obtain the comfort temperature. Hed and Bellander (2006) presented a method to simulate a PCM air heat exchanger. PCM used is having phase change in a given temperature range. The aim was to find a model that will fit into a finite difference based indoor climate and energy simulation software. To do that a fictive heat transfer coefficient is established. The fictive heat transfer coefficient includes aspects of the geometry and the airflow in the heat exchanger as well as the material properties of the PCM.. M.Alkilani et al.(2009) presented theoretical investigation of Output air temperatures due to a discharge process in a solar air heater integrated with a phase change material.

Phase change material unit consist of inline single row of cylinders containing PCM. The PCM consists

Analysis of Pcm Storage Unit for Night Heating Using Heat of the Day during Winter Season

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of paraffin wax with mass fraction 0.5% aluminum powder to enhance the heat transfer. Stritih & Butala (2007) presented the experimental and numerical analysis of cooling buildings using night-time cold accumulation in phase change material (PCM) with constant inlet temperatures. The comparison of experimental and numerical results shows good agreement. Arkar and Medved (2007) have presented a numerical study the free cooling concept with varying inlet temperatures using RT 20 paraffin as phase change material which is integrated into the ventilation system of the building. The cylindrical latent heat thermal energy storage (LHTES) device was filled with spheres of encapsulated RT20 paraffin. In this research a parametric study of storage unit has been carried out and ambient air is being used as inlet air.

The correlation between the climatic conditions and the free cooling potential was investigated by Medved & Arkar (2008) for different cities of Europe and for the case of a cylindrical LHTES with a packed bed of spheres encapsulated with PCM that is integrated into a building’s mechanical ventilation system. For an experimental verification of the LHTES’s numerical model a commercially available PCM (RT20 paraffin, Rubitherm GmbH) with a latent heat of 142kJ/kg was used. This PCM has a relatively large phase change temperature range.Morison and Abdel-Khalik (1978) developed a theoretical model for studying the transient behavior of phase-change energy storage (PCES) unit and studied the performance of solar heating systems using both air and liquid as working fluid, this model based on three assumptions: axial conduction in the flow mode is negligible, Biot number is very low that temperature variations normal to the flow can be neglected, and heat loss from the unit can be ignored. Solomon studied the behaviour of an array of PCM cylinders as a thermal storage unit under some assumptions which can make this study by looking at a single row of N cylinders. The heat transfer process in every cylinder is radially symmetric, and recommended to use this method to design the systems and their simulation, which is used by the authors in this study to predict the air temperature and freezing time. J. Virgone et al. (2011) performed the assessment of PCM wallboard usage for the renovation of a tertiary (i.e. light weight) building. For this purpose, two identical rooms of a renovated tertiary building have been tested, one equipped with PCM wallboard the other being “classically” renovated. The results show that the PCM wallboards enhance the thermal comfort of occupants due to air temperature and radiative effects of the walls. Gideon Susman et al., (2011) constructed PCM modules from a paraffin composite and tested in an occupied London office, in summer season. Design variations tested the effect on heat transfer of a black paint or aluminium surface, the effect of different phase transition zones and the effect of discharging heat inside or outside. The modules temperatures were monitored along with airflow rate, air temperature and globe temperature. Black modules transfer heat and exhaust latent storage capacity significantly quicker than aluminium modules, due to radiant exchange. R. Velraj, A. Pasupathy (2006) gave a general idea of PCM based thermal storage system for building air conditioning. They used salt hydrates for this purpose. In present work, MATLAB simulation tool has been used to compute the air temperature variation with location as well as time, charging and discharging time of storage unit. Effect of PCM mass, air flow rate and different inlet temperatures are considered both for the day time and night time operation of the storage unit. Melting point of Phase change materials used for analysis is selected in such a way that it’s value always lie between comfort temperature and maximum night ambient temperatures.

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MATERIALS AND METHODS Figures 1 and 2 explain the schematic of the proposed setups for night heating using day temperature in winter climatic conditions. A PCM thermal storage unit is used to store heat during the day so that heat can be supplied at night or when there is no sunshine. In comparison to conventional heating systems, it uses less fossil fuel and produces higher ventilation rates. During day time, the sunlight falls on the reflector and glass. The reflected and refracted beams fall on the thermal mass (Phase Change Material). Thus, the thermal mass gets charged and stores heat for the future uses. Also some of the light rays heat up the in the day time also. Insulation is provided near the roof so that no heat can escape.

Figure 1: Schematic of the solar heating system

Figure 2: Schematic of the solar heating system

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Analysis of Pcm Storage Unit for Night Heating Using Heat of the Day during Winter Season

During night time, the air of room is cooler. The thermal mass discharges giving the heat to the air thus making it warmer. The temperature of room rises to a comfortable level. The air circulates in the room continuously discharging the thermal mass and heating up the room.

ANALYIS OF THE SYSTEM Analysis is based on the following assumptions: 1.

Material changes phase at a single constant temperature.

2.

Properties of PCM in both phases are considered to be constant.

3.

Conduction resistance of material used between PCM and heat transfer fluid is negligible.

4.

Sub cooling of PCM is ignored.

5.

Glass reflects 90% of energy falling on it.

6.

PCM absorbs 70% of energy incident on it.

In this system heat transfer between PCM and air takes place by natural convection. So, for this process

Where

= Nusselt Number Gr = Grashof number = g= Acceleration due to gravity = volumetric thermal expansion coefficient.

TS= Surface temperature T∞ = Ambient temperature L = length of wall = kinematic viscosity is a function of Prandtl number =

Pr is Prandtl number.

In this analysis we have assumed that sunlight initially falls on a glass surface and then strikes the PCM wall.

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Prachi Rai & S.K.Shukla

Energy reflected by glass is Where Gc = incident solar radiation A = area of glass plate NTU (number of transfer units) for the storage unit can be defined as:

Known values Acceleration due to gravity, g = 9.8m/s Conductive heat transfer coefficient of air, ka = 0.024 W/(m-K) Kinematic viscosity,

= 15.89*10-6m2/s

Prandtl number, Pr = 0.7 Volumetric thermal coefficient, β=0.003 K-1 Area of Glass surface, A = 2m2 Mass flow rate of air = 0.0644 kg/s Specific Heat capacity of air, Cp = 1.005 kJ/kg-K PCM used In this case we have used HS 29 as Phase Change Material with following property: Melting point, Tm = 29-32°C Latent heat enthalpy, L = 195kJ/kg Density = 1550 kg/m3 *HS stands for HYDRATED SALT and 29 is its phase change temperature. This PCM is manufactured by a company named PLUSS.

RESULTS AND DISCUSSIONS In this analysis, the different values of convective heat transfer coefficient are considered and graphs of outlet temperature corresponding to length of wall are plotted. It is seen that as the length of wall increases the temperature decreases and comfort range of temperature is obtained. Figure 3 explains this phenomenon in details. Figure 4 shows the time of charging for different mass of PCM. It can be observed that if solar radiation is increased the time of charging is decreased but it always lies in the range of 12-14 hours. In Figure 5, the value of Number of Transfer Units (NTU) corresponding to air temperature is depicted. It is clear from this figure that heat transfer is very less as compared to the case of day cooling with low values of NTU.

Analysis of Pcm Storage Unit for Night Heating Using Heat of the Day during Winter Season

Figure 3: Temperature of room for different wall lengths

Figure 4: Time of charging for various values of solar radiation

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Figure 5: Number of transfer units that will be required

REFERENCES 1.

C. Arkar, S. Medved, (2007): Free cooling of a building using PCM heat storage integrated into the ventilation system, Solar Energy, 81 (9), 1078-1087.

2.

M. M. Alkilani, K. Sopian, M. Sohif , M. A. Alghoul, (2009): Output Air Temperature Prediction in a Solar Air Heater Integrated with Phase Change Material, European Journal of Scientific Research, 27 (3), 334-341.

3.

G. Hed, R. Bellander,(2006): Mathematical modelling of PCM air heat exchanger”, Energy and Buildings, 38, 82-89.

4.

S. Medved, C. Arkar, (2008): Correlation between the local climate and the free-cooling potential of latent heat storage, Energy and Buildings, 40 (4), 429-437.

5.

D. J. Morrison, S. I. Abdel-Khalik, (1978): Effects of phase-change energy storage on the performance of air-based and liquid-based solar heating systems, Solar Energy, 20,57-67.

6.

G. Susman, Z. Dehouche, T. Cheechern, S. Craig, (2011): Tests of prototype PCM sails‟ for office cooling, Applied Thermal Engineering, 31, 717-726.

7.

A. Solomon, (1981): Some of approximations of use in predicting the behavior of a PCM cylinder array”, Letters in Heat and Mass Transfer, 8,237-246.

Analysis of Pcm Storage Unit for Night Heating Using Heat of the Day during Winter Season

8.

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U. Stritih, V. Butala, (2007): Energy saving in building with PCM cold storage International Journal of Energy Research, 31, 1532-1544.

9.

J. Virgone, F. Kuznik, K. Johannes, (2011): In-situ study of thermal comfort enhancement in a renovated building equipped with phase change material wallboard, Renewable Energy, 36, 1458-1462.

10. R. Velraj, A. Pasupathy, (2006): Phase change material based thermal storage for energy conservation in building architecture�, International Energy Journal, 7(2), 123-143.