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Optimization of thermal performance of
building integrated solar collector with Phase Change Material
Article · March 2015
DOI: 10.1109/IRSEC.2014.7059759
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Optimization of thermal performance of building integrated solar collector with Phase Change Material
Z. BOUHSSINE*, M. FARAJI, M. NAJAM, M. EL ALAMI Laboratory of Physical Materials, Microelectronics, Automatics and Heat Transfer
Faculty of Science, Hassan II University-Casablanca Casablanca, Morocco
* [email protected] Abstract— In this present paper we study numerically the
optimization of the thermal performance of a building integrated solar collector with Phase Change Materials (PCM). A mathematical model governing the thermal behavior of the solar collector with PCM is developed using the enthalpy method. The model parameters have been defined and the obtained equations are solved iteratively. Several simulations were performed to optimize the conductivity of the PCM for the cold period (January, Casablanca-Morocco).
Keywords- solar collector, optimization, thermal comfort, PCM, concrete.
Nomenclature qu
Tfi
Tfo
Ta
Tc
Tin
Tf
k hin
f Ac
UL
cp
useful flow transferred to the fluid, W/m² fluid inlet temperature, K, °C
fluid outlet temperature, K, °C ambient temperature, K, °C
internal temperature under the slab, K, °C melting temperature, K, °C
building internal temperature, K, °C thermal conductivity, W/mK
internal convective coefficient under the slab, W/m² K liquid fraction
area of solar collector, m² overall conductance, W/m² K heat capacity, J/Kg K Greek letters
ṁ ΔH ρ ϕsa
water flow rate, Kg/s latent heat of melting, KJ/Kg density, Kg/m3
radiation absorbed by the absorbent, W/m² Indices
m b s l
PCM concrete solid liquid
I. INTRODUCTION
Many studies explore how and where the PCM can be used to ensure the thermal comfort in building since before 1980. The passive construction solutions with PCMs provide reduction of energy consumption for heating and cooling, and for
increasing inside thermal comfort due to the reduced inlet temperature fluctuations [1]. Kosny and al. have studied the importance of the PCM and uses thereof to stabilize the temperature inside the building. Thus, the best places for the integration of the PCM is in the construction of surfaces of interior walls, ceilings, or floors [2]. Implementing the PCM in gypsum boards, plaster, concrete or other wall covering materials, thermal energy storage becomes a part of the building structure, useful even for light-weight buildings. Also several researchers have investigated methods for integrated PCM in solar thermal storage system [3,4]. This study explore numerically the optimization of the thermal performance of a building integrated solar collector with PCM. The effect of the PCM conductivity was analyzed for the cold period (January, Casablanca-Morocco).
A. Physical model
Figure 1 shows the proposed model, it is a plan collector, isolated from the sides, carrying forward a glass surface. The interior of the collector comprises an absorbent copper tinged with a dark coating that maximizes the absorption of solar radiation. The energy release is sensed by passing a coolant fluid (water) in contact with the absorber. A PCM layer is connected directly to the collector. The collector is attached to the concrete slab.
Figure 1. The physical model
B. Assumptions
For the mathematical formulation of the problem, the following assumptions were adopted:
- Heat transfer by conduction is one-dimensional and edge effects are neglected;
- The thermal conductivity of concrete is assumed to be constant;
- The PCM is homogeneous and isotropic, the effect of convection is neglected in the PCM (PCM encapsulated);
- The inter-facial resistances between the different layers of the slab are neglected.
C. Equations
The useful flow transferred to the fluid is:
sa L fi a
R
u F U T T
q (1)
The transient energy’s equation in the composite floor (concrete PCM) is written, using the enthalpy method, as follows [5]:
t H f x
k T x t cp T
(2) The second term is a term which takes into account source of the latent heat associated with the phase change when present.
The term liquid fraction f is given by:
f f f
T T
T T
T T f
f f
if 1 0
0 1
(3) The system is subject to the following boundary conditions:
u x
m q
x k T L
x
, (4)
c x
in x
b h T T
x k T
x
0 , (5)
Equality of flows and temperatures at the interfaces:
T T
x k T x k T
x x
, (6)
The thermo-physical properties of the PCM are evaluated as follows:
ms
pm pml
pms lm
m fk f k c f c f c
k . , 1 , , . , 1 , (7) For interfaces, ’i’, between two different materials (‘+’/’-‘), the properties are estimated using the harmonic means [6]:
k k
k
ki k (8)
With δ+ is the distance between the interface and the first node of the material ‘+’, and δ- is the distance between the interface and the first node within the material ‘-‘.
The system of equations thus obtained is integrated numerically on a mesh by means of Patankar’s control volume method [6].
TABLE 1. THE PCM THERMOPHYSICAL PROPERTIES km
(W/mK)
cp,m (J/Kg K)
ρm
(Kg/m3) ΔH (KJ/Kg)
Tf
(°C)
1.9 1000 1800 200 22
II. RESULTS AND DISCUSSION
Figure 2. shows the time wise variation of the outlet temperature of the fluid, Tout, during typical days representing the cold period of the year (January in Casablanca, Morocco).
The outlet temperature rises due to the sunrise and falls during the night. The increase and decrease of the outlet temperature is due to the alternating day/night every 24 hours. On average, temperature reaches a minimum and maximum range between 4°C and 72°C, respectively.
Figure 3. shows the evolution of the useful flow of the collector, Qu, for typical days during January. The flow increases which causes the outlet temperature increase. The useful flow reaches a maximum value (900 W/m²) between solar noon and 3:00 pm and vanishes at 6:00 pm which present the sunset. Casablanca is characterized by severe temperature fluctuations during winter and lower values of temperature are reached.
Figure 2. The outlet temperature of the fluid
Figure 3. The useful flow of the collector
The time step used is 1s. The temperature inside the building rises from 15 °C to 40 °C while the outlet temperature of the collector increases by 4 °C to 70 °C (Figure 2.). The layer of PCM receives a useful flow Qu from the solar collector (Figure 3.) which increases its temperature to the melting temperature 22 °C and a first layer of the liquid phase appears.
The variation of the thermal conductivity, k, has a significant effect on the thermal behavior of the system. Increasing k from 0.7 W/mK to 1.9 W/mK, the internal temperature Tin
reaches a maximum 40 °C at 4 pm and this is due to the thermal inertia of PCM. During the night, the internal temperature Tin decreases to 16 °C for k=1.9 W/mK, less than the melting temperature (Figure 4.). The increase of thermal conductivity k accelerates the solidification rate of the PCM, and leads to the decrease of the liquid fraction f of from 0.42 (k=0.7 W/mK) to a minimum value 0 (k=1.9 W/mK) overnight (Figure 4.). For good thermal insulation to keep the temperature inside the building next to the comfort conditions, we choose k=0.7 W/mK. The presence of the layer of PCM inside the solar collector increases the heat storage in the material, and therefore, an increase in the liquid fraction.
Figure 4. The internal temperature
Figure 5. The liquid fraction
TABLE 2. VARIATION OF THE INTERNAL TEMPERATURE IN TERMS OF k
k (W/mK) Tin,max (°C) Tin,min (°C)
0.7 30.27 22
1 32.91 22
1.5 36.49 19.84
1.9 40.82 15.15
III. CONCLUSION
The optimization of the PCM thermal conductivity, k, reduces the internal temperature from 40 °C to 30 °C, while the outlet temperature of the collector is 70 °C and the ambient temperature is 15 °C in January in Casablanca. The building integrated solar thermal system with PCM provides thermal comfort conditions for the building.
ACKNOWLEDGMENT
The present work was accomplished according to the InnoTherm II program supported by IRESEN- Morocco (Institut de Recherche en Enérgie Solaire et Enérgie Nouvelle).
REFERENCES
[1] N. Soares, J.J. Costa, A.R. Gaspar, P. Santos, “Review of passive PCM latent heat thermal energy storage systems towards buildings energy efficiency,” Energy and Buildings 59, 82–103, (2013) .
[2] Jan Kosny, David Yarbrough, William Miller, Thomas Petrie, Phillip Childs, Azam Mohiuddin Syed, Douglas Leuthold, “Thermal Performance of PCM-Enhanced Building Envelope Systems”, ASHRAE, 2007.
[3] Alkilani Mahmud, Sopian K., Alghoul M. A. and Mat Sohif, “Using a paraffin wax-aluminum compound as a thermal storage material in a solar air heater”, ARPN Journal of Engineering and Applied Sciences, Vol. 4, No. 10, December 2009.
[4] Shuangmao Wu, Guiyin Fang, “Dynamic performances of solar heat storage system with packed bed using myristic acid as phase change material”, Energy and Buildings, 43, p. 1091–1096, (2011).
[5] V.R. Voller, M. Cross and N.C. Markatos, ‘An Enthalpy Method for Convection/Diffusion Phase Change’, International Journal of Numerical Methods in Engineering, Vol. 24, N°1, pp. 271 - 284, (1987).
[6] S. V. Patankar, “Numerical Heat Transfer and Fluid Flow”, Washington D.C. Hemisphere, (1980).
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