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Energy and exergy analysis of a new solar air heater with latent storage energy
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Energy and exergy analysis of a new solar air heater with latent storage energy Salwa Bouadila*, Mariem Lazaar, Safa Skouri, Sami Kooli, Abdelhamid Farhat Research and Technology Center for Energy, Thermal Processes Laboratory, Hammam Lif, B.P. 95, 2050 Tunis, Tunisia
article info
abstract
Article history:
In this paper, we propose a new solar air heater with a packed-bed latent storage energy
Received 6 February 2014
system using PCM spherical capsules. At daytime, the solar heating system stored the
Received in revised form
thermal solar energy as sensible and latent heat, however, at night it restored. Some pa-
31 March 2014
rameters, such as the global solar radiation and the mass flow rate are varied to investigate
Accepted 1 April 2014
their effect on the absorbed, used, and recovered heat from the system. An optimization
Available online xxx
study using the first and second laws of thermodynamics is also carried out to obtain the energy and exergy efficiencies. The experimental study was conducted, designed, and
Keywords:
realized in the Research and Technology Center of Energy (CRTEn) in Tunisia. The exper-
Latent heat storage
imentally obtained results are used to analyze the performance of the system, based on
Packed-bed solar air heaters
temperature distribution in different parts of the collectors, absorbed, instantaneous
Energy efficiency
stored and used thermal energy. The daily energy efficiency varied between 32% and 45%.
Exergy efficiency
While the daily exergy efficiency varied between 13% and 25%. The effect of the mass flow rate of air on the outlet temperature of the solar air heater is examined. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The world energy demand is projected to be doubled by 2050 and will be tripled by the end of the century [1]. The actual classic types of energy will not be capable to supply this demand in a sustainable way. Many countries have been directed to the use of renewable energies resources as a promising alternative for their energy need. Tunisia has an important solar energy potential with an annual average global solar radiation exceeding 2000 kW h/m2/year [2]. This immense resource will be able in the future to provide an important portion of energy needs. The simplest and the most
efficient way of using solar energy is to convert it into thermal energy for heating applications by using solar collectors. The solar air heater is one of the successful solar technologies is less expensive than other solar collectors and it uses less material [3]. However, the intermittent characteristic of the solar radiations leads to the improvement of suitable collection and storage technologies [4]. Thermal energy storage is simply the storage of high or low temperature energy for later use [5]. It can be stored as sensible, latent heat storage, reaction heat, or a combination of these [6e9]. The use of Phase Change Material (PCM) storage in solar air heater is less common; Fath [10] has used a
* Corresponding author. Tel.: þ216 97 772 206; fax: þ216 71 430 934. . E-mail addresses: [email protected], [email protected] (S. Bouadila). http://dx.doi.org/10.1016/j.ijhydene.2014.04.074 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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Nomenclature Ac Ap Cp Exd Exin Exlos Exout Exst h hc hr hw IT L L1 L2 L3 : m Nu P Pr : Q .Q Re a S t T T Tm U V
surface area of the collector, m2 surface area of the absorber packed-bed, m2 specific heat of air at constant pressure, J/kg C destroyed exergy, W inlet exergy, W loss exergy, W outlet exergy, W stored exergy, W heat transfer coefficient, W/m2 C convection heat transfer coefficient, W/m2 C radiation heat transfer coefficient, W/m2 C wind convection coefficient, W/m2 C solar radiation, W/m2 latent heat, J/kg collector length, m collector width, m collector depth, m mass flow rate, kg/s Nusselt number fluid pressure, Pa Prandtl number heat, W Reynolds number Rayleigh number section area of the duct, m2 time, s temperature, C average temperature, C melting temperature, C heat loss coefficient, W/m2 C velocity, m/s
Greek symbols a absorptance bottom insulation thickness, m db edge insulation thickness, m de
thermosyphon solar air heater with a series of packed tubes containing a PCM with different melting temperatures of 61, 51, 43, and 32 C. The tubes were arranged parallel to each other in order to make a flat-plate-style configuration with air flowing over and under them simultaneously. Fath has found that the heaters with melting temperatures of 51 and 43 C have the best performances. Esen and Ayhan [11] performed a parametric study to evaluate effects of various thermal and geometrical parameters and different PCMs on the stored solar heat quantity. Enibe [12] has realized a single-glazed flat plate solar collector integrated with a paraffin wax PCM. Alkilani et al. [13] was presented a theoretical solar air collector model with cylinder container in a cross flow of pumped air, they have achieved a prediction of output air temperature for eight different values of mass flow. Tyagi et al. [14] was presented a comparative and experimental study of a typical solar air heating system with and without temporary thermal energy storage material for energy and exergy analysis. Castell et al. [15] was presented
D ε h h0 l li m ms r s s j
difference in time emissivity energy efficiency (%) optical yield (dimensionless) thermal conductivity, W/m C thermal conductivity of insulation, W/m C PCM dynamic viscosity, Ns/m2 PCM dynamic viscosity at T ¼ TPCM, W/m C density, kg/m3 StefaneBoltzmann constant, 5.670$108W/m2 K4 transmittance exergy efficiency (%)
Subscripts A absorbed a ambient av average b bottom c cold ch charging dis discharging e edge fin_ch final of the charging process fin_dis final of the discharging process g glasses h hot in outlet ini_ch initial of the charging process ini_dis initial of the discharging process l liquid phase los leakage, lost out outlet p absorber packed-bed PCM phase change material s solid phase st storage
an experimental study of a PCM tank for cold storage applications. Different configurations and flow rates of the heat transfer fluid were studied. This work will address packed beds of encapsulated PCM in the spherical capsules system which is one of the most effective and compact latent thermal energy systems [16,17]. The objective of the present study is to investigate experimentally the amount of latent and sensible storage heat for a nocturnal use of a new Solar Air Heater with Latent Storage Collector (SAHLSC) using spherical capsules as a packed-bed absorber. The SAHLSC designed and realized in the Research and Technology Center of Energy (CRTEn) in Tunisia. We will describe in Section Research methodology, the site selection, the experimental set-up, the measurement procedure, the energy and exergy analysis of the solar air heater latent storage collector, and the uncertainty analysis. In Section Results and discussion, we will report the experimental results and discussion. The main remarks of this work will be reported in the conclusion.
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
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Research methodology Site selection The experimental study and measurements were performed in the Research and Technology Center of Energy (CRTEn) in Borj-Cedria. The site is a sunny region situated on the Mediterranean coast of North Africa, near the city of Tunis (Tunisia), with the latitude 36 430 N and the longitude 10 250 E. Tunisia’s climate is temperate in the north, with rainy winters and hot summers. Temperatures in July and August can exceed 40 C. Winter and Spring are mild with temperatures that can be exceeded 20 C. The average monthly ambient temperature on the month of March is about 20 C and the average monthly global solar radiation ranges between 180 and 190 kW/m2; there’s a huge solar potential in Tunisia [18].
Experimental set-up An experimental set-up of the solar air heater latent storage collector was designed and constructed to investigate the charging and discharging processes (Fig. 1). A schematic arrangement of the solar air heater with phase change energy storage using spherical capsules is given in Fig. 2. The experimental apparatus consists of a packed-bed absorber formed of spherical capsules with a black coating and fixed with steel matrix. The PCM is confined inside spherical capsules. The packed-bed absorber is the most important component of the solar air heater collector, which absorbs the sun radiations and stores the solar thermal energy as sensible heat and latent heat. The length, the width, and the total volume of the collector are 2 m, 1 m, and 0.28 m3, respectively. A 0.004 m thick transparent glass cover was placed 0.015 m apart from the absorber. A 0.05 m thick polyurethane insulation, with heat conductivity 0.028 W/m K, is placed in the bottom of the collector to decrease thermal losses through the bottom.
Fig. 2 e Schematic view of SAHLSC.
Measurement procedure The charging process starts when the absorber is exposed to the solar radiation (from 9:00 to 16:00). It was measured by a Kipp and Zonen pyranometer placed facing to the south at the same inclination of the absorber. During this process, air inlet and outlet openings are closed. The discharging process starts from 16:00 to 9:00 (the next day). The inlet and outlet air openings are opened. A fan used to blow the air at a fixed speed equal to 1 ms1. The inlet and outlet air temperatures of the SAHLSC, respectively, were measured with K-type thermocouples. All the measuring instruments were connected to a multi-channel digital Agilent 34970A Data Acquisition. The experimental values are recorded every 5 min. The thermophysical properties of the capsule and air, which were used, are given in Table 1.
Energy analysis of the solar air heater latent storage collector The energy analysis presented in this section is mainly based on the first law of thermodynamics. The theoretical model employed for the study of the SAHLSC consists of using a thermal energy balance during both the charging and discharging phases: QA ¼ Qu þ Qst þ Qlos
(1)
where QA, Qu, Qst, and Qlos are the absorbed, useful, stored, and lost energy, respectively. Based on Duffie and Beckman [19], the useful heat gain from the collector is: :
Qu ¼ mCp ðTout Tin Þ
(2)
where :
m ¼ rVav S Fig. 1 e The photograph of experimental set-up.
(3)
Vav and S are average air velocity and cross sectional area of the duct at the inlet of the solar air heater, respectively.
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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Table 1 e Thermal properties of PCM and air. Melting point ( C)
Heat of fusion (kJ kg1)
Capsule (AC27)
27
192.6
Air (at 25 C)
e
e
Material
Specific heat (kJ/kg C) Liquid 2.22 1.0048
The radiation absorbed flux of the absorber packed-bed is defined as: QA ¼ Ap ðasÞIT
(4)
Density (kg/m3)
Solid 1.42
Liquid 1710 1.137
Thermal conductivity (W/m C) Solid 1530
Liquid 0.58 2.49 102
Solid 1.05
hw ¼ 5:67 þ 3:86 VN
(14)
hr;ga ¼ εg s T2g þ T2sky Tg þ Tsky
(15)
where (as) is the effective product transmittanceeabsorptance that is equal with the optical efficiency (h0) [20]. The stored heat flux during the charging and discharging phases is given by:
The appropriate approximation to sky temperature is given by Swinbak [19]:
Qch ¼ mPCM Cp;s Tm Tini ch;PCM þ mPCM L þ mPCM Cp;l Tfin ch;PCM Tm Dtch
In addition the energy loss through the bottom (Ub) and the edges (Ue) equation as follows:
Qdis ¼ mPCM Cp;l Tm Tfin dis;PCM þ mPCM L þ mPCM Cp;s Tin dis;PCM Tm Dtdis
(5)
(6)
The lost heat flux is given by relation (7), Ulos is the collector overall heat loss coefficient. The thermal energy lost from the collector to the surroundings by conduction, convection, and infrared radiation. Ulos is equal to the sum of energy loss through the top (Ut), bottom (Ub), and edges (Ue) of the collectors given below [19]: Qlos ¼ Ulos Ac TP Ta
(7)
Ulos ¼ Ut þ Ub þ Ue
(8)
Tsky ¼ 0:0552 T1:5 a
(16)
Ub ¼ li =db
(17)
Ue ¼ ðL1 þ L2 ÞL3 li =ðL1 L2 de Þ
(18)
The thermal efficiency based on the first law of thermodynamics is defined as the ratio between the useful energy and the solar radiation incident on the collector: h ¼ Qu =Ac IT
(19)
The daily average thermal efficiency of the SAHLSC is the ratio of the desired energy output during the discharging process to the total energy input during the charging process. Z
Z h¼
Qdis dis
Ac IT
(20)
ch
The top loss coefficient from the collector to the ambient is: 1 Ut ¼ 1 hc;Pg þ hr;Pg þ 1 hw þ hr;ga
(9)
The flow is laminar during the charging phase; and the appropriate correlation is given by Churchill [21]: 4=9
Pr 0:7 1 þ ðð0:469Þ=PrÞ9=16 Nuch ¼ 2 þ 0:589 Ra1=4 11 Ra 10 (10) Then the flow is turbulent during the discharging phase; the appropriate correlation is given by Whitaker [21]: Nudis ¼2 þ 0:4 Re1=2 þ 0:06 Re
2=3
3 0:71 Pr 380 6 47 4 3:5 Re 7:6 10 5 1 ðm=ms Þ 3:2 2
1=4
Pr ðm=ms Þ 0:4
hc;Pg ¼ Nuðl=tÞ hr;Pg
¼ s T2P þ T2g TP þ Tg = ð1=εP Þ þ 1 εg 1
(11)
Exergy analysis of the solar air heater latent storage collector The general exergy balance can be expressed below as the total exergy inputs equal to total exergy outputs. The following assumptions are made for the development of the exergy analysis [22e24]: - steady state, steady flow operation, - negligible potential and kinetic energy effects and no chemical or nuclear reactions, - air is an ideal gas with a constant specific heat, and its humidity content is ignored. The general form of the exergy balance equation is [25,26]:
(12) (13)
The heat transfer coefficient due to convection at the top of cover due to wind is given by Hottel and Woertz [19]:
Exin ¼ Exout þ Exlos þ Exd þ Exst
(21)
where Exin, Exout, Exlos, Exd, and Exst are the inlet, outlet, leakage, destroyed, and stored exergy, respectively. The total inlet exergy, Exin, as includes: - the inlet exergy with air flow:
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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:
:
Exin ðairÞ ¼ mCp ðTin Ta Ta lnðTin =Ta ÞÞ þ mRTa lnðPin =Pa Þ
(22)
- the absorbed solar radiation exergy, assuming the sun as an infinite thermal source [27,28]: Exin ðabsorbedÞ ¼ h0 IT Ac ð1 ðTa =Tsun ÞÞ
(23)
where Tsun ¼ 4077 C is the apparent sun temperature. It represents 75% of black body temperature of the sun [29]. The total inlet exergy is given by the summation of Eqs. (22) and (23) will result in the total inlet exergy of the heater. The outlet exergy, Exout, includes only the outlet air flow [30]: :
:
Exout ðairÞ ¼ mCp ðTout Ta Ta lnðTout =Ta ÞÞ þ mRTa lnðPout =Pa Þ (24) The leakage exergy, Exlos, is caused by the heat leakage from the absorber packed-bed to the environment [31]:
Exlos ¼ Ulos Ac Ta Tp Ta
1 Ta Tp
(25)
The destroyed exergy, Exd, is caused by three terms: - the absorber packed-bed surface and sun temperature difference [29]: Exd ðp sÞ ¼ h0 IT Ac Ta 1 Tp ð1=Ts Þ
(26)
- the duct pressure drop [26,31]: Exd ðDPÞ ¼ ðDPTa =rÞðTa lnððTout =Ta ÞÞ=ðTout Tin ÞÞ
(27)
- the heat transfer from absorber packed-bed to the agent fluid across finite temperature difference [26,31]: : Exd ðp f Þ ¼ mCp Ta lnðTout =Tin Þ ðTout Tin Þ Tp
(28)
The stored exergy, Exst, is both sensible and latent change within the PCM, for the charging and discharging state, so it becomes in the discharged process [32]: Exdis ¼ mPCM Cp;l Tm Tfin
Ta ln Tm Tfin dis;PCM þ mPCM Lð1 ðTa =Tm ÞÞ þ mPCM Cp;s Tini dis;PCM Tm Ta ln Tini dis;PCM Tm Dtdis dis;PCM
(29)
The daily average exergy efficiency, analogous to the energy case, is the ratio of the desired exergy output during the discharging processes by the total required exergy input during the charging processes. ,Z Z j¼
Exin ðabsorbedÞ
Exdis dis
(30)
ch
Uncertainty analysis Uncertainty analysis is needed to prove the accuracy of the experiments. In this study, errors came from the sensitiveness of equipment and measurements uncertainties.
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The independent parameters measured in the experiments reported here are: collector inlet and outlet temperature, ambient temperature, PCM temperature, air velocity, and solar radiation. To carry out these experiments, K-type thermocouples with: (i) sensitiveness of data acquisition system, about 0.01 C, (ii) measurement error is 0.03 C, and (iii) sensitiveness of the thermocouple is 0.01 C. The sensitiveness was obtained from a catalog of the instruments. An anemometer with 0.01 ms1 accuracy, and Kipp and Zonen pyranometer with 3% measurements uncertainties were used. The calculated uncertainty of the air mass flow rate, the heat and the thermal efficiency are presented here. : The fractional uncertainty,wm: =m, for the mass flow rate Eq. (3) is written as [33]: i1=2 h : wm: m ¼ ðwVav =Vav Þ2 þ ðwTa =TÞ2 þ ðwPa =Pa Þ2
(31)
The density of air r in the mass flow rate equation Eq. (3) is dependent on ambient pressure Pa and ambient temperature Ta. Similarly, the fractional uncertainty for the heat, wQ/Q, : from Eq. (2) is a function of (Tout Tin) and m, considering Cp as constants [33]. h : 2 2 i1=2 wQ Q ¼ wm: m þ wðTout Tin Þ ðTout Tin Þ
(32)
The fractional uncertainty for efficiency, wh/h, from Eq. (19) : is a function of (Tout Tin), m, and IT, considering Cp and Ac as constants [33]. h : 2 2 i1=2 wh h ¼ wm: m þ wðTout Tin Þ ðTout Tin Þ þ ðwIT IT Þ
(33)
The total uncertainty in determining flow rate, heat and efficiency were estimated by Eqs. (21)e(23), respectively. The total uncertainty calculated of the mass flow rate of air is 0.0039, the heat is 0.0087, and efficiency is 0.0096.
Results and discussion In this study, measurements were performed during the period of March 2012. The external climate conditions for the 4-days period are shown in Figs. 3 and 4. The maximum of the global solar radiation intensity in the horizontal plane attain 1100 Wm2, and outdoor temperature varied from 5 to 22 C (Fig. 3). The wind velocities in this region as a function of days are presented in Fig. 4. The 29th of March, 2012 is a very windy day, where air speed varied between 3 and 8 ms1. The average speed of other days is 2 ms1.
SAHLSC temperature Fig. 5 shows the ambient, the outlet, and the PCM temperature in the 4-days period (26th and 29th March, 2012). Under clear sky conditions and during the charging period, from 9:00 h to 16:00 h (local time), the temperature into the solar collector increases gradually and attained 40 C at noon. During the charging period, the PCM temperature inside the capsule situated in the middle of the collector reach to
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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Fig. 3 e Global solar radiation and ambient temperature as a function of days (26the29th March, 2012).
Fig. 5 e Ambient, outlet, and PCM temperature of the SAHLSC as a function of days (26the29th March, 2012).
27 C, the PCMs starts to melt and after the melting stage (for 2 h) its temperature will reach to 30 C. The PCM temperature attained 45 C (26th March, 2012), the PCMs are completely charged. In fact, at 16:00 h the discharge process starts the PCM temperature starts to decrease. The temperature decreases to 24 C for some minutes, after that it increasing up to 30 C. The solidification stage resist for 6 h (26th March, 2012). The PCM temperature profile illustrates the supercooling behavior as the temperature decreases below the solidification points of the PCM. The supercooled substances are in a metastable phase where the spontaneous formation of the thermodynamically stable solid phase is prevented by a nucleation barrier [34]. The variation between the outlet and the ambient temperatures of the SAHLSC, is about 5e10 C and the PCM temperature is 15e25 C higher than the ambient temperature. The outlet temperature remains approximately constant around 20 C all the night.
The main result of the experimental test is the evaluation of the outlet SAHLSC temperature as a function of the absorber temperature (packed-bed of spherical PCM capsules) (Fig. 6). The results reveal that the outlet temperature of the solar air heater collector varied linearly with the absorber temperature. This curve presents a mathematical correlation which allows deducing, for a fixed air mass flow rate, the outlet temperature as a function of the absorber temperature.
Energy and exergy analysis of the SAHLSC
Fig. 4 e Wind speed as a function of days (26the29th March, 2012).
Fig. 6 e Outlet temperature of the SAHLSC as a function of the absorber temperature.
During collector performance tests the inlet, the outlet, the absorber; the PCM temperatures of the SAHLSC, the ambient temperature, and the global solar radiation were recorded. The energy balance equation Eq. (1) is used for calculating the daily absorbed and recovered heat of the SAHLSC. Fig. 7 presented the daily absorbed heat and the heat recovered from the SAHLSC, during the period from 19th to 30th of
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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Fig. 7 e Absorbed and recovered heat of the SAHLSC as a function of days (19the30th March, 2012).
March, 2012. Under best ambient conditions, on 28th March, 2012, the absorbed and stored energy in the solar air heater was 900 kJ and the recovered useful energy during the discharging period is 550 kJ. On 30th March, 2012 is characterized by severely solar radiation fluctuation with ambient air temperatures from 15 C, and wind velocity vary from 3 ms1 to 8 ms1. We noted that the recovered heat of the SAHLSC was 100 kJ. Variations of absorbed, useful and stored energy are presented in Fig. 8 for different times during 26th March, 2012. During the charging process, the instantaneous stored heat fluctuated at the same time as insolation towards a maximum value of 1 kW at 13:00, we noted that at this time the absorbed heat is 2.2 kW; 45% of the solar energy is stored in the collector. As the discharging process proceeds, the PCM starts solidifying and the used heat is uniform for a longer period. The uniform value of the used heat is about 200 W/m2 during
Fig. 8 e Absorbed, useful, and stored heat rates as a function of day times (26th March, 2012).
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11 h, we can also conclude that the useful heat was not affected by the solar radiations fluctuation during the day. This is the major advantage of a latent storage solar air heater, where a uniform discharging process is possible for a longer period, which will be useful for many applications of heating. The energy and exergy analysis of the SAHLSC were performed with data obtained from the experiments. Fig. 9 shows the values of the daily energy and exergy efficiency as function of days from 19th March 2012 to 30th March 2012 using Eq. (20) and Eq. (30), respectively. The daily average energy efficiency changed between 32% and 45%. The higher daily energy and exergy efficiencies are obtained at 29th March. The daily average value of energy efficiencies is about 40%. The daily average exergy efficiency changed between 13% and 25%. It is observed that the average exergy efficiency is about 22%. This value is enough in terms of exergy efficiency. When values are compared with the daily average energy efficiency, the daily average exergy efficiency of the system was lower than the daily energy efficiency for all days.
Effect of mass flow rate on outlet temperature of the SAHLSC Under comparative climatic conditions for 5-days, Fig. 10 shows the effect of air speed or mass flow rate on the outlet temperature of the SAHLSC; the used air speeds are: 0 m/s (natural convection), 0.75 m/s, 1 m/s, 1.5 m/s, and 1.75 m/s. The mass flow rate of air significantly influences the amount of the outlet temperature SAHLSC. However, the higher mass flow rate will lead to supply the air temperature close to the ambient conditions. At lower mass flow rate, the contact duration, of air with absorber increases resulting in higher outlet temperature. Therefore, the mass flow rate should be lower for getting higher temperature of grain. Whereas, increasing the air speed beyond the 1 m/s, there is a little drop in gain temperature. During natural convection (the air speed equals to 0 m/s), the available air is an insufficient to remove all the heat stored in the packed-bed absorber.
Fig. 9 e Daily energy and exergy efficiency as a function of days (19the30th March, 2012).
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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[8]
[9]
[10]
[11]
[12]
[13]
Fig. 10 e Effect of air speed on the outlet temperature of the SAHLSC.
Conclusion The performance studies of solar air heater with phase change materials were conducted and the following conclusions were drawn. - The solar air heater remains a uniform useful heat during the discharging process. The value of the heat was 200 W/ m2 during 11 h at night. - The outlet temperature remains approximately constant around 20 C all the night. - The daily average energy efficiency of the SAHLSC is about 40%. - The daily average exergy efficiency of the SAHLSC is about 22%. - The mass flow rate of air influences the amount of the outlet temperature SAHLSC.
references
[14]
[15]
[16]
[17]
[18] [19]
[20] [21]
[22] [23]
[24] [1] Kreith F, Gowami DY. Handbook of energy efficiency and renewable energy. London, New York: Taylor and Francis Group; 2007. [2] ElOuderni AR, Maatallah T, ElAlimi S, Ben Nassrallah S. Experimental assessment of the solar energy potential in the gulf of Tunis, Tunisia. Renew Sustain Energ Rev 2013;20:155e68. [3] Wazed MA, Nukman Y, Islam MT. Design fabrication of a cost effective solar air heater for Bangladesh. Appl Energy 2010;87:3030e6. [4] Lodhi MAK. Collection and storage of solar energy. Int J Hydrogen Energy 1989;14:379e411. [5] Faith HE. Technical assessment of solar thermal energy storage technologies. Renew Energ 1998;14:35e40. [6] Garrier S, Delhomme B, de Rango P, Marty P, Fruchart D, Miraglia S. A new MgH2 tank concept using a phase-change material to store the heat of reaction. Int J Hydrogen Energy 2013;38:9766e71. [7] Wang Z, Naterer GF, Gabriel KS, Secnik E, Gravelsins R, Daggupati V. Thermal design of a solar hydrogen plant with a
[25] [26] [27]
[28]
[29] [30] [31]
copper-chlorine cycle and molten salt energy storage. Int J Hydrogen Energy 2011;36:11258e72. Cabeza LF, Mehling H, Hiebler S, Ziegler F. Heat transfer enhancement in water when used as PCM in thermal energy storage. Appl Therm Eng 2002;22:1141e51. Fan L, Khodadadi JM. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew Sustain Energ Rev 2011;15:24e46. Fath HES. Transient analysis of thermosyphon solar air heater with built-in latent heat thermal energy storage system. Renew Energ 1995;6:119e24. Esen M, Ayhan T. Development of a model compatible with solar assisted cylindrical energy storage tank and variation of stored energy with time for different phase-change materials. Energy Convers Manag 1996;37:1775e85. Enibe S0. Thermal analysis of a natural circulation solar air heater with phase change material energy storage. Renew Energ 2003;28:2269e99. Alkilani MM, Sopian K, Mat S, Alghoul MA. Output air temperature prediction in a solar air heater integrated with phase change material. Eur J Sci Res 2009;27:334e441. Tyagi VV, Pandey AK, Giridhar G, Bandyopadhyay B, Park SR, Tyagi SK. Comparative study based on exergy analysis of solar air heater collector using thermal energy storage. Int J Energ Res 2011;2:1e13. Castell A, Belusko M, Bruno F, Cabeza LF. Maximisation of heat transfer in a coil in tank PCM cold storage system. Appl Energ 2011;88:4120e7. Eames IW, Adref KT. Freezing and melting of water in spherical enclosures of the type used in thermal (ice) storage systems. Appl Therm Eng 2002;22:733e45. Ismail KAR, Henriquez JR, Silva TM. A parametric study on ice formation inside spherical capsule. Int J Therm Sci 2003;42:881e7. Nation institute of metrological of Tunisia. http://www. meteo.tn. Duffie FJA, Beckman WA. Solar engineering of thermal processes. 2nd ed. New York: USA: John Wiley and Sons; 1991. Sukhatme SP. Solar energy. New York: McGraw-Hill; 1993. Incropera FP, DeWitt DP, Bergman TL, Lavine AS. Fundamentals of heat and mass transfer. 6th ed. New York: USA: John Wiley and Sons; 2007. Kurtbas I, Durmus A. Efficiency and exergy analysis of a new solar air heater. Renew Energ 2004;29:1489e501. Esen H. Experimental energy and exergy analysis of a double-flow solar air heater having different obstacles on absorber plates. Build Environ 2008;43:1046e54. Karsli S. Performance analysis of new-design solar air collectors for drying applications. Renew Energ 2007;32:1645e60. Tsatsaronis G. Definitions and nomenclature in exergy analysis and exergoeconomics. Energy 2007;32:249e53. Suzuki A. A fundamental equation for exergy balance on solar collectors. J Sol Energy Eng 1988;110:102e6. ¨ ztu¨rk HH. Experimental determination of energy and O exergy efficiency of the solar parabolic-cooker. Sol Energy 2004;77:67e71. Torres-Reyes E, Cervantes-de Gortari JG, Ibarra-Salazar BA, Picon-Nunez M. A design method of flat-plate solar collectors based on minimum entropy generation. Energy 2001;1:46e52. Dutta Gupta KK, Saha SK. Energy analysis of solar thermal collectors. Renew Energ Environ; 1990:283e7. Kotas TJ. The exergy method of thermal plant analysis. Malabar, FL: Krieger Publ Company; 1995. Altfeld K, Leiner W, Fiebig M. Second law optimization of flat-plate solar air heaters part I: the concept of net exergy
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
flow and the modeling of solar air heaters. Sol Energy 1988;41:127e32. [32] MacPee D, Dincer I. Thermodynamic analysis of freezing and melting processes in a bed of spherical PCM capsules. J Sol Energy Eng 2009;131:031017.
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[33] Holman JP. Experimental methods for engineers. 6th ed. Singapore: McGraw-Hill Book Co.; 1994. [34] Mullin JW. Crystallization. 4th ed. Oxford: ButterworthHeinemann; 2001. pp. 181e214.
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Energy and exergy analysis of a new solar air heater with latent storage energy Salwa Bouadila*, Mariem Lazaar, Safa Skouri, Sami Kooli, Abdelhamid Farhat Research and Technology Center for Energy, Thermal Processes Laboratory, Hammam Lif, B.P. 95, 2050 Tunis, Tunisia
article info
abstract
Article history:
In this paper, we propose a new solar air heater with a packed-bed latent storage energy
Received 6 February 2014
system using PCM spherical capsules. At daytime, the solar heating system stored the
Received in revised form
thermal solar energy as sensible and latent heat, however, at night it restored. Some pa-
31 March 2014
rameters, such as the global solar radiation and the mass flow rate are varied to investigate
Accepted 1 April 2014
their effect on the absorbed, used, and recovered heat from the system. An optimization
Available online xxx
study using the first and second laws of thermodynamics is also carried out to obtain the energy and exergy efficiencies. The experimental study was conducted, designed, and
Keywords:
realized in the Research and Technology Center of Energy (CRTEn) in Tunisia. The exper-
Latent heat storage
imentally obtained results are used to analyze the performance of the system, based on
Packed-bed solar air heaters
temperature distribution in different parts of the collectors, absorbed, instantaneous
Energy efficiency
stored and used thermal energy. The daily energy efficiency varied between 32% and 45%.
Exergy efficiency
While the daily exergy efficiency varied between 13% and 25%. The effect of the mass flow rate of air on the outlet temperature of the solar air heater is examined. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The world energy demand is projected to be doubled by 2050 and will be tripled by the end of the century [1]. The actual classic types of energy will not be capable to supply this demand in a sustainable way. Many countries have been directed to the use of renewable energies resources as a promising alternative for their energy need. Tunisia has an important solar energy potential with an annual average global solar radiation exceeding 2000 kW h/m2/year [2]. This immense resource will be able in the future to provide an important portion of energy needs. The simplest and the most
efficient way of using solar energy is to convert it into thermal energy for heating applications by using solar collectors. The solar air heater is one of the successful solar technologies is less expensive than other solar collectors and it uses less material [3]. However, the intermittent characteristic of the solar radiations leads to the improvement of suitable collection and storage technologies [4]. Thermal energy storage is simply the storage of high or low temperature energy for later use [5]. It can be stored as sensible, latent heat storage, reaction heat, or a combination of these [6e9]. The use of Phase Change Material (PCM) storage in solar air heater is less common; Fath [10] has used a
* Corresponding author. Tel.: þ216 97 772 206; fax: þ216 71 430 934. . E-mail addresses: [email protected], [email protected] (S. Bouadila). http://dx.doi.org/10.1016/j.ijhydene.2014.04.074 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Ac Ap Cp Exd Exin Exlos Exout Exst h hc hr hw IT L L1 L2 L3 : m Nu P Pr : Q .Q Re a S t T T Tm U V
surface area of the collector, m2 surface area of the absorber packed-bed, m2 specific heat of air at constant pressure, J/kg C destroyed exergy, W inlet exergy, W loss exergy, W outlet exergy, W stored exergy, W heat transfer coefficient, W/m2 C convection heat transfer coefficient, W/m2 C radiation heat transfer coefficient, W/m2 C wind convection coefficient, W/m2 C solar radiation, W/m2 latent heat, J/kg collector length, m collector width, m collector depth, m mass flow rate, kg/s Nusselt number fluid pressure, Pa Prandtl number heat, W Reynolds number Rayleigh number section area of the duct, m2 time, s temperature, C average temperature, C melting temperature, C heat loss coefficient, W/m2 C velocity, m/s
Greek symbols a absorptance bottom insulation thickness, m db edge insulation thickness, m de
thermosyphon solar air heater with a series of packed tubes containing a PCM with different melting temperatures of 61, 51, 43, and 32 C. The tubes were arranged parallel to each other in order to make a flat-plate-style configuration with air flowing over and under them simultaneously. Fath has found that the heaters with melting temperatures of 51 and 43 C have the best performances. Esen and Ayhan [11] performed a parametric study to evaluate effects of various thermal and geometrical parameters and different PCMs on the stored solar heat quantity. Enibe [12] has realized a single-glazed flat plate solar collector integrated with a paraffin wax PCM. Alkilani et al. [13] was presented a theoretical solar air collector model with cylinder container in a cross flow of pumped air, they have achieved a prediction of output air temperature for eight different values of mass flow. Tyagi et al. [14] was presented a comparative and experimental study of a typical solar air heating system with and without temporary thermal energy storage material for energy and exergy analysis. Castell et al. [15] was presented
D ε h h0 l li m ms r s s j
difference in time emissivity energy efficiency (%) optical yield (dimensionless) thermal conductivity, W/m C thermal conductivity of insulation, W/m C PCM dynamic viscosity, Ns/m2 PCM dynamic viscosity at T ¼ TPCM, W/m C density, kg/m3 StefaneBoltzmann constant, 5.670$108W/m2 K4 transmittance exergy efficiency (%)
Subscripts A absorbed a ambient av average b bottom c cold ch charging dis discharging e edge fin_ch final of the charging process fin_dis final of the discharging process g glasses h hot in outlet ini_ch initial of the charging process ini_dis initial of the discharging process l liquid phase los leakage, lost out outlet p absorber packed-bed PCM phase change material s solid phase st storage
an experimental study of a PCM tank for cold storage applications. Different configurations and flow rates of the heat transfer fluid were studied. This work will address packed beds of encapsulated PCM in the spherical capsules system which is one of the most effective and compact latent thermal energy systems [16,17]. The objective of the present study is to investigate experimentally the amount of latent and sensible storage heat for a nocturnal use of a new Solar Air Heater with Latent Storage Collector (SAHLSC) using spherical capsules as a packed-bed absorber. The SAHLSC designed and realized in the Research and Technology Center of Energy (CRTEn) in Tunisia. We will describe in Section Research methodology, the site selection, the experimental set-up, the measurement procedure, the energy and exergy analysis of the solar air heater latent storage collector, and the uncertainty analysis. In Section Results and discussion, we will report the experimental results and discussion. The main remarks of this work will be reported in the conclusion.
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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Research methodology Site selection The experimental study and measurements were performed in the Research and Technology Center of Energy (CRTEn) in Borj-Cedria. The site is a sunny region situated on the Mediterranean coast of North Africa, near the city of Tunis (Tunisia), with the latitude 36 430 N and the longitude 10 250 E. Tunisia’s climate is temperate in the north, with rainy winters and hot summers. Temperatures in July and August can exceed 40 C. Winter and Spring are mild with temperatures that can be exceeded 20 C. The average monthly ambient temperature on the month of March is about 20 C and the average monthly global solar radiation ranges between 180 and 190 kW/m2; there’s a huge solar potential in Tunisia [18].
Experimental set-up An experimental set-up of the solar air heater latent storage collector was designed and constructed to investigate the charging and discharging processes (Fig. 1). A schematic arrangement of the solar air heater with phase change energy storage using spherical capsules is given in Fig. 2. The experimental apparatus consists of a packed-bed absorber formed of spherical capsules with a black coating and fixed with steel matrix. The PCM is confined inside spherical capsules. The packed-bed absorber is the most important component of the solar air heater collector, which absorbs the sun radiations and stores the solar thermal energy as sensible heat and latent heat. The length, the width, and the total volume of the collector are 2 m, 1 m, and 0.28 m3, respectively. A 0.004 m thick transparent glass cover was placed 0.015 m apart from the absorber. A 0.05 m thick polyurethane insulation, with heat conductivity 0.028 W/m K, is placed in the bottom of the collector to decrease thermal losses through the bottom.
Fig. 2 e Schematic view of SAHLSC.
Measurement procedure The charging process starts when the absorber is exposed to the solar radiation (from 9:00 to 16:00). It was measured by a Kipp and Zonen pyranometer placed facing to the south at the same inclination of the absorber. During this process, air inlet and outlet openings are closed. The discharging process starts from 16:00 to 9:00 (the next day). The inlet and outlet air openings are opened. A fan used to blow the air at a fixed speed equal to 1 ms1. The inlet and outlet air temperatures of the SAHLSC, respectively, were measured with K-type thermocouples. All the measuring instruments were connected to a multi-channel digital Agilent 34970A Data Acquisition. The experimental values are recorded every 5 min. The thermophysical properties of the capsule and air, which were used, are given in Table 1.
Energy analysis of the solar air heater latent storage collector The energy analysis presented in this section is mainly based on the first law of thermodynamics. The theoretical model employed for the study of the SAHLSC consists of using a thermal energy balance during both the charging and discharging phases: QA ¼ Qu þ Qst þ Qlos
(1)
where QA, Qu, Qst, and Qlos are the absorbed, useful, stored, and lost energy, respectively. Based on Duffie and Beckman [19], the useful heat gain from the collector is: :
Qu ¼ mCp ðTout Tin Þ
(2)
where :
m ¼ rVav S Fig. 1 e The photograph of experimental set-up.
(3)
Vav and S are average air velocity and cross sectional area of the duct at the inlet of the solar air heater, respectively.
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Table 1 e Thermal properties of PCM and air. Melting point ( C)
Heat of fusion (kJ kg1)
Capsule (AC27)
27
192.6
Air (at 25 C)
e
e
Material
Specific heat (kJ/kg C) Liquid 2.22 1.0048
The radiation absorbed flux of the absorber packed-bed is defined as: QA ¼ Ap ðasÞIT
(4)
Density (kg/m3)
Solid 1.42
Liquid 1710 1.137
Thermal conductivity (W/m C) Solid 1530
Liquid 0.58 2.49 102
Solid 1.05
hw ¼ 5:67 þ 3:86 VN
(14)
hr;ga ¼ εg s T2g þ T2sky Tg þ Tsky
(15)
where (as) is the effective product transmittanceeabsorptance that is equal with the optical efficiency (h0) [20]. The stored heat flux during the charging and discharging phases is given by:
The appropriate approximation to sky temperature is given by Swinbak [19]:
Qch ¼ mPCM Cp;s Tm Tini ch;PCM þ mPCM L þ mPCM Cp;l Tfin ch;PCM Tm Dtch
In addition the energy loss through the bottom (Ub) and the edges (Ue) equation as follows:
Qdis ¼ mPCM Cp;l Tm Tfin dis;PCM þ mPCM L þ mPCM Cp;s Tin dis;PCM Tm Dtdis
(5)
(6)
The lost heat flux is given by relation (7), Ulos is the collector overall heat loss coefficient. The thermal energy lost from the collector to the surroundings by conduction, convection, and infrared radiation. Ulos is equal to the sum of energy loss through the top (Ut), bottom (Ub), and edges (Ue) of the collectors given below [19]: Qlos ¼ Ulos Ac TP Ta
(7)
Ulos ¼ Ut þ Ub þ Ue
(8)
Tsky ¼ 0:0552 T1:5 a
(16)
Ub ¼ li =db
(17)
Ue ¼ ðL1 þ L2 ÞL3 li =ðL1 L2 de Þ
(18)
The thermal efficiency based on the first law of thermodynamics is defined as the ratio between the useful energy and the solar radiation incident on the collector: h ¼ Qu =Ac IT
(19)
The daily average thermal efficiency of the SAHLSC is the ratio of the desired energy output during the discharging process to the total energy input during the charging process. Z
Z h¼
Qdis dis
Ac IT
(20)
ch
The top loss coefficient from the collector to the ambient is: 1 Ut ¼ 1 hc;Pg þ hr;Pg þ 1 hw þ hr;ga
(9)
The flow is laminar during the charging phase; and the appropriate correlation is given by Churchill [21]: 4=9
Pr 0:7 1 þ ðð0:469Þ=PrÞ9=16 Nuch ¼ 2 þ 0:589 Ra1=4 11 Ra 10 (10) Then the flow is turbulent during the discharging phase; the appropriate correlation is given by Whitaker [21]: Nudis ¼2 þ 0:4 Re1=2 þ 0:06 Re
2=3
3 0:71 Pr 380 6 47 4 3:5 Re 7:6 10 5 1 ðm=ms Þ 3:2 2
1=4
Pr ðm=ms Þ 0:4
hc;Pg ¼ Nuðl=tÞ hr;Pg
¼ s T2P þ T2g TP þ Tg = ð1=εP Þ þ 1 εg 1
(11)
Exergy analysis of the solar air heater latent storage collector The general exergy balance can be expressed below as the total exergy inputs equal to total exergy outputs. The following assumptions are made for the development of the exergy analysis [22e24]: - steady state, steady flow operation, - negligible potential and kinetic energy effects and no chemical or nuclear reactions, - air is an ideal gas with a constant specific heat, and its humidity content is ignored. The general form of the exergy balance equation is [25,26]:
(12) (13)
The heat transfer coefficient due to convection at the top of cover due to wind is given by Hottel and Woertz [19]:
Exin ¼ Exout þ Exlos þ Exd þ Exst
(21)
where Exin, Exout, Exlos, Exd, and Exst are the inlet, outlet, leakage, destroyed, and stored exergy, respectively. The total inlet exergy, Exin, as includes: - the inlet exergy with air flow:
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:
:
Exin ðairÞ ¼ mCp ðTin Ta Ta lnðTin =Ta ÞÞ þ mRTa lnðPin =Pa Þ
(22)
- the absorbed solar radiation exergy, assuming the sun as an infinite thermal source [27,28]: Exin ðabsorbedÞ ¼ h0 IT Ac ð1 ðTa =Tsun ÞÞ
(23)
where Tsun ¼ 4077 C is the apparent sun temperature. It represents 75% of black body temperature of the sun [29]. The total inlet exergy is given by the summation of Eqs. (22) and (23) will result in the total inlet exergy of the heater. The outlet exergy, Exout, includes only the outlet air flow [30]: :
:
Exout ðairÞ ¼ mCp ðTout Ta Ta lnðTout =Ta ÞÞ þ mRTa lnðPout =Pa Þ (24) The leakage exergy, Exlos, is caused by the heat leakage from the absorber packed-bed to the environment [31]:
Exlos ¼ Ulos Ac Ta Tp Ta
1 Ta Tp
(25)
The destroyed exergy, Exd, is caused by three terms: - the absorber packed-bed surface and sun temperature difference [29]: Exd ðp sÞ ¼ h0 IT Ac Ta 1 Tp ð1=Ts Þ
(26)
- the duct pressure drop [26,31]: Exd ðDPÞ ¼ ðDPTa =rÞðTa lnððTout =Ta ÞÞ=ðTout Tin ÞÞ
(27)
- the heat transfer from absorber packed-bed to the agent fluid across finite temperature difference [26,31]: : Exd ðp f Þ ¼ mCp Ta lnðTout =Tin Þ ðTout Tin Þ Tp
(28)
The stored exergy, Exst, is both sensible and latent change within the PCM, for the charging and discharging state, so it becomes in the discharged process [32]: Exdis ¼ mPCM Cp;l Tm Tfin
Ta ln Tm Tfin dis;PCM þ mPCM Lð1 ðTa =Tm ÞÞ þ mPCM Cp;s Tini dis;PCM Tm Ta ln Tini dis;PCM Tm Dtdis dis;PCM
(29)
The daily average exergy efficiency, analogous to the energy case, is the ratio of the desired exergy output during the discharging processes by the total required exergy input during the charging processes. ,Z Z j¼
Exin ðabsorbedÞ
Exdis dis
(30)
ch
Uncertainty analysis Uncertainty analysis is needed to prove the accuracy of the experiments. In this study, errors came from the sensitiveness of equipment and measurements uncertainties.
5
The independent parameters measured in the experiments reported here are: collector inlet and outlet temperature, ambient temperature, PCM temperature, air velocity, and solar radiation. To carry out these experiments, K-type thermocouples with: (i) sensitiveness of data acquisition system, about 0.01 C, (ii) measurement error is 0.03 C, and (iii) sensitiveness of the thermocouple is 0.01 C. The sensitiveness was obtained from a catalog of the instruments. An anemometer with 0.01 ms1 accuracy, and Kipp and Zonen pyranometer with 3% measurements uncertainties were used. The calculated uncertainty of the air mass flow rate, the heat and the thermal efficiency are presented here. : The fractional uncertainty,wm: =m, for the mass flow rate Eq. (3) is written as [33]: i1=2 h : wm: m ¼ ðwVav =Vav Þ2 þ ðwTa =TÞ2 þ ðwPa =Pa Þ2
(31)
The density of air r in the mass flow rate equation Eq. (3) is dependent on ambient pressure Pa and ambient temperature Ta. Similarly, the fractional uncertainty for the heat, wQ/Q, : from Eq. (2) is a function of (Tout Tin) and m, considering Cp as constants [33]. h : 2 2 i1=2 wQ Q ¼ wm: m þ wðTout Tin Þ ðTout Tin Þ
(32)
The fractional uncertainty for efficiency, wh/h, from Eq. (19) : is a function of (Tout Tin), m, and IT, considering Cp and Ac as constants [33]. h : 2 2 i1=2 wh h ¼ wm: m þ wðTout Tin Þ ðTout Tin Þ þ ðwIT IT Þ
(33)
The total uncertainty in determining flow rate, heat and efficiency were estimated by Eqs. (21)e(23), respectively. The total uncertainty calculated of the mass flow rate of air is 0.0039, the heat is 0.0087, and efficiency is 0.0096.
Results and discussion In this study, measurements were performed during the period of March 2012. The external climate conditions for the 4-days period are shown in Figs. 3 and 4. The maximum of the global solar radiation intensity in the horizontal plane attain 1100 Wm2, and outdoor temperature varied from 5 to 22 C (Fig. 3). The wind velocities in this region as a function of days are presented in Fig. 4. The 29th of March, 2012 is a very windy day, where air speed varied between 3 and 8 ms1. The average speed of other days is 2 ms1.
SAHLSC temperature Fig. 5 shows the ambient, the outlet, and the PCM temperature in the 4-days period (26th and 29th March, 2012). Under clear sky conditions and during the charging period, from 9:00 h to 16:00 h (local time), the temperature into the solar collector increases gradually and attained 40 C at noon. During the charging period, the PCM temperature inside the capsule situated in the middle of the collector reach to
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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Fig. 3 e Global solar radiation and ambient temperature as a function of days (26the29th March, 2012).
Fig. 5 e Ambient, outlet, and PCM temperature of the SAHLSC as a function of days (26the29th March, 2012).
27 C, the PCMs starts to melt and after the melting stage (for 2 h) its temperature will reach to 30 C. The PCM temperature attained 45 C (26th March, 2012), the PCMs are completely charged. In fact, at 16:00 h the discharge process starts the PCM temperature starts to decrease. The temperature decreases to 24 C for some minutes, after that it increasing up to 30 C. The solidification stage resist for 6 h (26th March, 2012). The PCM temperature profile illustrates the supercooling behavior as the temperature decreases below the solidification points of the PCM. The supercooled substances are in a metastable phase where the spontaneous formation of the thermodynamically stable solid phase is prevented by a nucleation barrier [34]. The variation between the outlet and the ambient temperatures of the SAHLSC, is about 5e10 C and the PCM temperature is 15e25 C higher than the ambient temperature. The outlet temperature remains approximately constant around 20 C all the night.
The main result of the experimental test is the evaluation of the outlet SAHLSC temperature as a function of the absorber temperature (packed-bed of spherical PCM capsules) (Fig. 6). The results reveal that the outlet temperature of the solar air heater collector varied linearly with the absorber temperature. This curve presents a mathematical correlation which allows deducing, for a fixed air mass flow rate, the outlet temperature as a function of the absorber temperature.
Energy and exergy analysis of the SAHLSC
Fig. 4 e Wind speed as a function of days (26the29th March, 2012).
Fig. 6 e Outlet temperature of the SAHLSC as a function of the absorber temperature.
During collector performance tests the inlet, the outlet, the absorber; the PCM temperatures of the SAHLSC, the ambient temperature, and the global solar radiation were recorded. The energy balance equation Eq. (1) is used for calculating the daily absorbed and recovered heat of the SAHLSC. Fig. 7 presented the daily absorbed heat and the heat recovered from the SAHLSC, during the period from 19th to 30th of
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Fig. 7 e Absorbed and recovered heat of the SAHLSC as a function of days (19the30th March, 2012).
March, 2012. Under best ambient conditions, on 28th March, 2012, the absorbed and stored energy in the solar air heater was 900 kJ and the recovered useful energy during the discharging period is 550 kJ. On 30th March, 2012 is characterized by severely solar radiation fluctuation with ambient air temperatures from 15 C, and wind velocity vary from 3 ms1 to 8 ms1. We noted that the recovered heat of the SAHLSC was 100 kJ. Variations of absorbed, useful and stored energy are presented in Fig. 8 for different times during 26th March, 2012. During the charging process, the instantaneous stored heat fluctuated at the same time as insolation towards a maximum value of 1 kW at 13:00, we noted that at this time the absorbed heat is 2.2 kW; 45% of the solar energy is stored in the collector. As the discharging process proceeds, the PCM starts solidifying and the used heat is uniform for a longer period. The uniform value of the used heat is about 200 W/m2 during
Fig. 8 e Absorbed, useful, and stored heat rates as a function of day times (26th March, 2012).
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11 h, we can also conclude that the useful heat was not affected by the solar radiations fluctuation during the day. This is the major advantage of a latent storage solar air heater, where a uniform discharging process is possible for a longer period, which will be useful for many applications of heating. The energy and exergy analysis of the SAHLSC were performed with data obtained from the experiments. Fig. 9 shows the values of the daily energy and exergy efficiency as function of days from 19th March 2012 to 30th March 2012 using Eq. (20) and Eq. (30), respectively. The daily average energy efficiency changed between 32% and 45%. The higher daily energy and exergy efficiencies are obtained at 29th March. The daily average value of energy efficiencies is about 40%. The daily average exergy efficiency changed between 13% and 25%. It is observed that the average exergy efficiency is about 22%. This value is enough in terms of exergy efficiency. When values are compared with the daily average energy efficiency, the daily average exergy efficiency of the system was lower than the daily energy efficiency for all days.
Effect of mass flow rate on outlet temperature of the SAHLSC Under comparative climatic conditions for 5-days, Fig. 10 shows the effect of air speed or mass flow rate on the outlet temperature of the SAHLSC; the used air speeds are: 0 m/s (natural convection), 0.75 m/s, 1 m/s, 1.5 m/s, and 1.75 m/s. The mass flow rate of air significantly influences the amount of the outlet temperature SAHLSC. However, the higher mass flow rate will lead to supply the air temperature close to the ambient conditions. At lower mass flow rate, the contact duration, of air with absorber increases resulting in higher outlet temperature. Therefore, the mass flow rate should be lower for getting higher temperature of grain. Whereas, increasing the air speed beyond the 1 m/s, there is a little drop in gain temperature. During natural convection (the air speed equals to 0 m/s), the available air is an insufficient to remove all the heat stored in the packed-bed absorber.
Fig. 9 e Daily energy and exergy efficiency as a function of days (19the30th March, 2012).
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
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Fig. 10 e Effect of air speed on the outlet temperature of the SAHLSC.
Conclusion The performance studies of solar air heater with phase change materials were conducted and the following conclusions were drawn. - The solar air heater remains a uniform useful heat during the discharging process. The value of the heat was 200 W/ m2 during 11 h at night. - The outlet temperature remains approximately constant around 20 C all the night. - The daily average energy efficiency of the SAHLSC is about 40%. - The daily average exergy efficiency of the SAHLSC is about 22%. - The mass flow rate of air influences the amount of the outlet temperature SAHLSC.
references
[14]
[15]
[16]
[17]
[18] [19]
[20] [21]
[22] [23]
[24] [1] Kreith F, Gowami DY. Handbook of energy efficiency and renewable energy. London, New York: Taylor and Francis Group; 2007. [2] ElOuderni AR, Maatallah T, ElAlimi S, Ben Nassrallah S. Experimental assessment of the solar energy potential in the gulf of Tunis, Tunisia. Renew Sustain Energ Rev 2013;20:155e68. [3] Wazed MA, Nukman Y, Islam MT. Design fabrication of a cost effective solar air heater for Bangladesh. Appl Energy 2010;87:3030e6. [4] Lodhi MAK. Collection and storage of solar energy. Int J Hydrogen Energy 1989;14:379e411. [5] Faith HE. Technical assessment of solar thermal energy storage technologies. Renew Energ 1998;14:35e40. [6] Garrier S, Delhomme B, de Rango P, Marty P, Fruchart D, Miraglia S. A new MgH2 tank concept using a phase-change material to store the heat of reaction. Int J Hydrogen Energy 2013;38:9766e71. [7] Wang Z, Naterer GF, Gabriel KS, Secnik E, Gravelsins R, Daggupati V. Thermal design of a solar hydrogen plant with a
[25] [26] [27]
[28]
[29] [30] [31]
copper-chlorine cycle and molten salt energy storage. Int J Hydrogen Energy 2011;36:11258e72. Cabeza LF, Mehling H, Hiebler S, Ziegler F. Heat transfer enhancement in water when used as PCM in thermal energy storage. Appl Therm Eng 2002;22:1141e51. Fan L, Khodadadi JM. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew Sustain Energ Rev 2011;15:24e46. Fath HES. Transient analysis of thermosyphon solar air heater with built-in latent heat thermal energy storage system. Renew Energ 1995;6:119e24. Esen M, Ayhan T. Development of a model compatible with solar assisted cylindrical energy storage tank and variation of stored energy with time for different phase-change materials. Energy Convers Manag 1996;37:1775e85. Enibe S0. Thermal analysis of a natural circulation solar air heater with phase change material energy storage. Renew Energ 2003;28:2269e99. Alkilani MM, Sopian K, Mat S, Alghoul MA. Output air temperature prediction in a solar air heater integrated with phase change material. Eur J Sci Res 2009;27:334e441. Tyagi VV, Pandey AK, Giridhar G, Bandyopadhyay B, Park SR, Tyagi SK. Comparative study based on exergy analysis of solar air heater collector using thermal energy storage. Int J Energ Res 2011;2:1e13. Castell A, Belusko M, Bruno F, Cabeza LF. Maximisation of heat transfer in a coil in tank PCM cold storage system. Appl Energ 2011;88:4120e7. Eames IW, Adref KT. Freezing and melting of water in spherical enclosures of the type used in thermal (ice) storage systems. Appl Therm Eng 2002;22:733e45. Ismail KAR, Henriquez JR, Silva TM. A parametric study on ice formation inside spherical capsule. Int J Therm Sci 2003;42:881e7. Nation institute of metrological of Tunisia. http://www. meteo.tn. Duffie FJA, Beckman WA. Solar engineering of thermal processes. 2nd ed. New York: USA: John Wiley and Sons; 1991. Sukhatme SP. Solar energy. New York: McGraw-Hill; 1993. Incropera FP, DeWitt DP, Bergman TL, Lavine AS. Fundamentals of heat and mass transfer. 6th ed. New York: USA: John Wiley and Sons; 2007. Kurtbas I, Durmus A. Efficiency and exergy analysis of a new solar air heater. Renew Energ 2004;29:1489e501. Esen H. Experimental energy and exergy analysis of a double-flow solar air heater having different obstacles on absorber plates. Build Environ 2008;43:1046e54. Karsli S. Performance analysis of new-design solar air collectors for drying applications. Renew Energ 2007;32:1645e60. Tsatsaronis G. Definitions and nomenclature in exergy analysis and exergoeconomics. Energy 2007;32:249e53. Suzuki A. A fundamental equation for exergy balance on solar collectors. J Sol Energy Eng 1988;110:102e6. ¨ ztu¨rk HH. Experimental determination of energy and O exergy efficiency of the solar parabolic-cooker. Sol Energy 2004;77:67e71. Torres-Reyes E, Cervantes-de Gortari JG, Ibarra-Salazar BA, Picon-Nunez M. A design method of flat-plate solar collectors based on minimum entropy generation. Energy 2001;1:46e52. Dutta Gupta KK, Saha SK. Energy analysis of solar thermal collectors. Renew Energ Environ; 1990:283e7. Kotas TJ. The exergy method of thermal plant analysis. Malabar, FL: Krieger Publ Company; 1995. Altfeld K, Leiner W, Fiebig M. Second law optimization of flat-plate solar air heaters part I: the concept of net exergy
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
flow and the modeling of solar air heaters. Sol Energy 1988;41:127e32. [32] MacPee D, Dincer I. Thermodynamic analysis of freezing and melting processes in a bed of spherical PCM capsules. J Sol Energy Eng 2009;131:031017.
9
[33] Holman JP. Experimental methods for engineers. 6th ed. Singapore: McGraw-Hill Book Co.; 1994. [34] Mullin JW. Crystallization. 4th ed. Oxford: ButterworthHeinemann; 2001. pp. 181e214.
Please cite this article in press as: Bouadila S, et al., Energy and exergy analysis of a new solar air heater with latent storage energy, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.074