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Performance of a new solar air heater with packed-bed latent storage energy for nocturnal use
Applied Energy 110 (2013) 267–275
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Performance of a new solar air heater with packed-bed latent storage energy for nocturnal use Salwa Bouadila ⇑, Sami Kooli, Mariem Lazaar, Safa Skouri, Abdelhamid Farhat The Research and Technology Center of Energy, The Thermal Processes Laboratory, Hammam Lif, B.P. 95, 2050 Tunis, Tunisia
h i g h l i g h t s A new solar air heater collector using a phase change material. Experimental study of the new solar air heater collector with latent storage. Energy and exergy analysis of the solar heater with latent storage collector. Nocturnal use of solar air heater collector.
a r t i c l e
i n f o
Article history: Received 11 October 2012 Received in revised form 4 March 2013 Accepted 18 April 2013 Available online 15 May 2013 Keywords: Latent heat storage Packed bed solar air heaters Energy efficiency Exergy efficiency
a b s t r a c t An experimental study was conducted to evaluate the thermal performance of a new solar air heater collector using a packed bed of spherical capsules with a latent heat storage system. Using both first and second law of thermodynamics, the energetic and exegetic daily efficiencies were calculated in Closed/ Opened and Opened cycle mode. The solar energy was stored in the packed bed through the diurnal period and extracted at night. The experimentally obtained results are used to analyze the performance of the system, based on temperature distribution in different localization of the collectors. The daily energy efficiency varied between 32% and 45%. While the daily exergy efficiency varied between 13% and 25%. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Successful solar system design is an iterative process involving consideration of many technical, practical, reliability, cost and environmental considerations. The Solar Air Heater (SAH) is less expensive than other solar collectors and it uses less materials [1]. One of the first studies of SAH was conducted by Close [2]. The author included a corrugated absorber surface and used a trapped layer of air between the single glazing surface and the absorber. Gupta and Garg [3] tested the performance of four different SAH using absorber surface as well as wire mesh packing over the absorber. Solar air heating has attracted increasing interest after the oil crisis, many air heaters were realized in these periods including novel designs [4– 6]. Sharma et al. [7] used a wire matrix packing above the absorber plate to enhance heat transfer. Choudhury and Garg [8] obtained a SAH thermal efficiency about 70% by using a packing material placed above the absorber. In the last century, air heater designs have focused mainly on improving the convective heat transfer at
⇑ Corresponding author. Tel.: +216 71 97 772 206; fax: +216 71 430 934. E-mail address: [email protected] (S. Bouadila). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.04.062
the absorber. Mohamad [9] designed a solar air heater, which consists of pre-warming air in its first pass between the upper and the lower glass channel. The main idea of this work is to minimize the heat losses from the collector top cover and to maximize the heat extraction from the absorber. The thermal efficiency of this collector exceeds 75%. Experimental and comparative studies of a selectively coated absorber plate and a black-painted absorber plate have been carried out by Hachemi [10]. Koyuncu [11] compared SAH flat plate designs and glazing configurations. Consequently, the author proved that the best efficiency was obtained using the flat black metal plate with a single polymer glazing. In order to increase the air heater efficiency authors [12,13] realized a doubleglass and a double-pass with a steel packed-bed above the heater absorber plate. Esen et al. [14,15] tested and compared different absorbing obstacles on a flat plate with an experimental study and artificial neural network models. They showed that the heat transfer efficiency is given by breaking up the boundary layer and reducing the dead zones in the collector. The thermal energy can be stored as sensible heat, latent heat, reaction heat or combination of those forms. Many thermal storage energy systems are cited in literature [16–22], with applications in cogeneration, building and solar heating of water or air.
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Nomenclature Ac Ap Cp
N h hc hr hw IT L L1 L2 L3 _ m Nu P Pr _ m R Re Ra 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) exergy rate (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 rate (W) universal gas constant (J/kg °C) 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 db bottom insulation thickness (m) de edge insulation thickness (m) D difference in time e emissivity
The use of Phase Change Material (PCM) storage in a solar air heater is less common, the majority of the studies were numerical. Hence, authors [23–25] used numerical tools to determine the solar air heating systems performance using PCM energy storage, to examine the effect of the latent heat and the PCM melting temperature and to identify the optimum phase change materials physical properties. Ghoneim and Klein [26] evaluated the theoretical performance of solar heating collectors with PCM and sensible heat storage in water and in air. Fath [27] used a 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 [28] performed a parametric study to evaluate effects of various thermal and geometrical parameters and different PCMs on the stored solar heat quantity. Esen et al. [29] analyzed a solar assisted latent heat storage tank by using two different numerical models. Enibe [30] has realized a single-glazed flat plate solar collector integrated with a paraffin wax PCM. Alkilani et al. [31] 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. [32] presented a comparative and experimental study of a typical solar air heating system with and without temporary thermal energy storage material for en-
g g0 k ki
l ls q r s w
energy efficiency (%) optical yield (dimensionless) thermal conductivity (W=m CÞ thermal conductivity of insulation (W=m CÞ PCM dynamic viscosity (N s/m2) PCM dynamic viscosity at T = TPCM (N s/m2) density (kg/m3) Stefan –Boltzmann constant, 5.670 108 W/m2 K4 transmittance exergy efficiency (%)
Subscripts A absorbed a ambient av average b bottom c cold ch charging d destroyed dis discharging e edge fin_ch final of the charging process fin_dis final of the discharging process g glasses h hot in inlet 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
ergy and exergy analysis. Castell et al. [33] presented an experimental study of a PCM tank for cold storage applications. Different configurations and flow rates of the heat transfer fluid were studied. A novel conceptual system called mobilized thermal energy storage system (M-TES) was proposed for distributed heat supply [34]. These systems have been studied for many years, focusing on PCM stability and improving heat transfer [35,36]. 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 [37–39]. In order to provide an amount of storage heat nocturnal use we will present an analytical and experimental investigation 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. Charging and discharging processes, energy and exergy analysis of the collector are carried out. 2. Description of SAHLSC A schematic arrangement of solar air heater with phase change energy storage using spherical capsules is given in Fig. 1 and a photograph of the SAHLSC is shown 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. Capsules have an outer diameter of 0.077 m and are blow molded from a blend of polyolefin with an average thickness of 0.002 m. The
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2
4
3 5 6 7
2
(1) Fan (2) Diverged section (3) Glass cover (4) Air inlet (5) Packed bed (PCM) (6) Collector box (7) Insulating material (8) Fixed steel matrix (9) Air outlet
Global solar radiation (kw/m )
1
8
2007 2008 2009 2010 2011
200
150
100
50
0 Jan
Feb
Mar
Apr
May
9 Fig. 1. Schematic view of SAHLSC.
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month Fig. 3. Monthly variation of the global solar radiation for the 2007–2011 periods in Tunisia.
30
2007 2008 2009 2010 2011
Ambient temperature (ºC)
25
20
15
10
5
0 Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month Fig. 4. Monthly variation of the ambient temperature for the 2007–2011 periods in Tunisia.
Fig. 2. The photograph of experimental SAHLSC.
PCM is confined inside spherical capsules. This geometry is the most method of encapsulation [40]. 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. 3. Description of the experimental set-up and measurement procedure An experimental set-up of the solar air heater latent storage collector was designed and constructed to investigate the charging and discharging processes. The experiments were performed during March 2012 in the Research and Technology Center of Energy (CRTEn) in Borj Cédria. The experiment’s site is a sunny region,
situated on the Mediterranean coast of North Africa, near the city of Tunis in Tunisia, with the following coordinates: Latitude 36°430 N and Longitude 10°250 E. The meteorological station on the site permits the measurement of the global sun flux on a horizontal plan, ambient temperature, wind speed and wind direction. Figs. 3 and 4 give the variation of meteorological data of the average monthly global solar radiation and ambient temperatures in a period of 2007–2011. The sunshine period of Tunisia is a 3500 h/ year and 350 sunny days per year with a daily insolation range from 5 h in December to 13 h in July. It receives an average global solar radiation intensity of 4.3 kW h/m2/day [41]. A schematic view of the experimental setup SAHLSC is shown in Fig. 5. Charging and discharging process are carried out in this experimental study. The charging process starts when the absorber is exposed to the solar radiation. 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 temperature distribution along the absorber packed bed was measured using four K-type thermocouples T2, T12,T18 and T25 situated in the middle of the second bed, bed number 12, bed number 18 and bed number 25, respectively. Fig. 2 shows the localization of thermocouples inserted inside capsules in different beds. Two temperature sensors for absorber surface of one capsule
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2
The radiation absorbed flux of the absorber packed-bed is defined as:
6
Q A ¼ Ap ðasÞIT
3 4
5
(1) SAHLSC (2) Fan (3) Diverged section (4) Pyranometer (5) Ambient temperature (6) Data aquisation system (7) Support
ð4Þ
where (as) is the effective product transmittance–absorptance that is equal with the optical efficiency (g0) [43]. The stored heat flux during the charging and discharging phases is given by:
Q ch ¼ ½mPCM C p;s ðT m T ini ch;PCM Þ þ mPCM LþmPCM C p;l ðT fin ch;PCM T m Þ=Dt ch
ð5Þ 7
Q ads ¼ ½mPCM C p;l ðT m T fin dis;PCM Þ þ mPCM LþmPCM C p;s ðT in dis;PCM T m Þ=Dt dis
ð6Þ
1
Fig. 5. Schematic view of experimental set-up.
(TP) and the glass temperature (Tg) were used. The charging process tests are performed from 9:00 to 16:00 (local time). The thermophysical properties of the capsules and air, which were used, are given in Table 1. 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 m s1. The inlet and outlet air temperatures of the SAHLSC Tin and Tout, 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 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 [42]:
Q los ¼ U los Ac ðT P T a Þ
ð7Þ
U los ¼ U t þ U b þ U e
ð8Þ
The top loss coefficient from the collector to the ambient is:
1 U t ¼ 1=ðhc;Pg þ hr;Pg Þ þ 1=ðhw þ hr;ga Þ
The flow is laminar during the charging phase; and the appropriate correlation is given by Churchill [44]:
Nuch ¼ 2 þ ð0:589Ra1=4 Þ=ð1 þ ðð0:469Þ=PrÞ9=16 Þ4=9
The energy and exergy analysis was carried out to evaluate the system efficiency.
11
Ra 6 10
2 Nudis ¼ 2 þ ð0:4Re
4.1. Energy analysis 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:
ð1Þ
where QA, Qu, Qst, and Qlos are the absorbed, useful, stored and lost energy, respectively.Based on Duffie and Beckman [42], the useful heat gain from the collector is:
ð2Þ
where
_ ¼ qV av S m
Pr P 0:7
Then the flow is turbulent during the discharging phase; the appropriate correlation is given by Whitaker [44]:
1=2
_ p ðT out T in Þ Q u ¼ mC
ð10Þ
4. Energy and exergy analysis of the solar air heater latent storage collector
Q A ¼ Q u þ Q st þ Q los
ð9Þ
ð3Þ
Vav and S are average air velocity and cross sectional area of the duct at the inlet of the solar air heater, respectively.
2=3
þ 0:06Re
3 0:71 6 Pr 6 380 47 4 3:5 6 Re 6 7:6 10 5 1 6 ðl=ls Þ 6 3:2
1=4 6
0:4
ÞPr ðl=ls Þ
ð11Þ
hc;Pg ¼ Nuðk=tÞ
ð12Þ
hr;Pg ¼ ðrðT 2P þ T 2g ÞðT P þ T g ÞÞ=ðð1=eP Þ þ ð1=eg Þ 1Þ
ð13Þ
The heat transfer coefficient due to convection at the top of cover due to wind is given by Hottel and Woertz [42]:
hw ¼ 5:67 þ 3:86V1
ð14Þ
hr;ga ¼ eg rðT 2g þ T 2sky ÞðT g þ T sky Þ
ð15Þ
The appropriate approximation to sky temperature is given by Swinbak [42]:
T sky ¼ 0:0552T 1:5 a
ð16Þ
Table 1 Thermal properties of PCM and air. Material
Melting point (°C)
Heat of fusion (kJ/kg)
Specific heat (kJ/kg°C)
Density (kg/m3)
Thermal conductivity (W/m°C)
Capsule (SN27)
27
192.6
Liquid 2.22
Liquid 1710
Liquid 0.58
Air (at 25 °C)
–
–
1.0048
Solid 1.42
1.137
Solid 1530
2.49 102
Solid 1.05
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In addition the energy loss through the bottom (Ub) and the edges (Ue) equation as follows:
U the heat transfer rate from absorber packed-bed to the agent fluid across finite temperature difference [47,51]:
U b ¼ ki =db
ð17Þ
_ p T a ðlnðT out =T in Þ ððT out T in Þ=T p ÞÞ Nd ðp f Þ ¼ mC
U e ¼ ðL1 þ L2 ÞL3 ki =ðL1 L2 de Þ
ð18Þ
Nst: The stored exergy rate is both sensible and latent change within the PCM, for the charging state, so it becomes in the discharged process [53]:
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:
g ¼ Q u =Ac IT
Z
Q dis
Z
dis
Ac IT
ð20Þ
T a lnðT fin
w¼
X
_ in ¼ m
X
_ out m
ð21Þ
The general exergy balance can be expressed below as the total exergy inputs equal to total energy outputs.
X
Nin ¼
X
Nout
ð22Þ
The General form of the exergy balance equation is [46,47]:
Nin ¼ Nout þ Nlos þ Nd þ Nst
ð23Þ
where Nin, Nout, Nlos, Nd and Nst are the inlet, outlet, leakage, destroyed and stored exergy rate, respectively. Nin: The total inlet exergy as includes: U the inlet exergy rate with air flow:
_ p ðT in T a T a lnðT in =T a ÞÞ þ mRT _ a lnðPin =P a Þ Nin ðairÞ ¼ mC
ð24Þ
U the absorbed solar radiation exergy rate, assuming the sun as an infinite thermal source [48–51]:
Nin ðabsorbedÞ ¼ g0 IT Ac ð1 ðT a =T sun ÞÞ
Z dis
Exergy is the amount of maximum work which can be produced by a system or a flow of matter or energy as it comes to equilibrium with a reference environment [45]. The mass balance equation can be expressed in the rate form as:
ð25Þ
where T sun ¼ 4077 C is the apparent sun temperature. It represents 75% of black body temperature of the sun [52]. The total inlet exergy is given by the summation of Eq. (24) and Eq. (25) will result in the total inlet exergy rate of the heater. Nout: The outlet exergy rate includes only the rate of outlet air flow [45]:
_ p ðT out T a T a lnðT out =T a ÞÞ þ mRT _ a lnðPout =P a Þð26Þ Nout ðairÞ ¼ mC
T a lnðT m =T ini
dis;PCM ÞÞ
dis;PCM
Tm
dis;PCM =T m ÞÞDt ch
Ndis
Z
Nin ðabsorbedÞ
the absorber packed–bed to the environment [51]:
Nlos ¼ U los Ac T a ðT p T a Þð1 ðT a =T p ÞÞ
ð27Þ
Nd: The destroyed exergy rate is caused by three terms:
ð32Þ
ch
5. 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. 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 rate and the thermal efficiency are presented here. _ for the mass flow rate Eq. (3) The fractional uncertainty, wm_ =m, is written as [54]:
_ ¼ ½ðwV av =V av Þ2 þ ðwT a =TÞ2 þ ðwPa =Pa Þ2 1=2 wm_ =m
ð33Þ
The density of air q 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 rate, wQ/Q, _ considering Cp as from Eq. (2) is a function of (Tout Tin) and m, constants [54].
_ 2 þ ðwðT out T in Þ =ðT out T in Þ2 1=2 wQ =Q ¼ ½ðwm_ =mÞ
ð34Þ
The fractional uncertainty for efficiency, wg/g, from Eq. (19) is a _ and IT, considering Cp and Ac as constants [54]. function of (Tout Tin), m
_ 2 þ ðwðT out T in Þ =ðT out T in Þ2 þ ðwIT =IT Þ1=2 wg =g ¼ ½ðwm_ mÞ
Nlos: The leakage exergy rate is caused by the heat leakage rate from
ð31Þ
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.
ch
4.2. Exergy analysis
dis;PCM
þmPCM Lð1 ðT a =T m ÞÞ þ mPCM C p;l ðT fin
ð19Þ
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.
g¼
Ndis ¼ ½mPCM C p;s ðT m T ini
ð30Þ
ð35Þ
The total uncertainty in determining flow rate, heat rate and efficiency were estimated by Eqs. (33)–(35), respectively. The total uncertainty calculated of the mass flow rate of air is 0.0039, the heat rate is 0.0087 and efficiency is 0.0096.
U the absorber packed-bed surface and sun temperature difference [52]:
6. Results and discussion
Nd ðp sÞ ¼ g0 IT Ac T a ðð1=T p Þ ð1=T s ÞÞ
ð28Þ
6.1. The SAHLSC performances of the operation mode: Closed/Opened cycles
ð29Þ
Collector performance tests were conducted in the month of March 2012. During these experiments, inlet, outlet and the
U the duct pressure drop [47,51]:
Nd ðDPÞ ¼ ðDPT a =qÞðT a lnððT out =T a ÞÞ=ðT out T in ÞÞ
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packed bed temperatures of SAHLSC, the PCM and the ambient temperatures and global solar radiation were recorded. In closed cycle during the charging periods from 9:00 to 16:00 (local time), both inlets and outlets are closed and the packed bed absorber stored the energy provided from the sun. In the charging cycle, the internal laminar flow collector is due to difference of the air density. The Nusselt number is determined by using the Eq. (10): Nuch = 2.67. The convective heat transfer coefficient hc can be calculated by using the Eq. (12): hc ¼ 3:43½W=m2 C. In opened cycle, the stored energy of the solar air heater is recovered once the cold air passes through the bed. Therefore, the bed is fully discharged when the outlet and inlet air temperature at the collector are similar. In the discharging process, the turbulent flow into the collector is caused by the fan. The value of Nusselt number is determined by using the Eq. (11): Nudis = 7.15. The convective heat transfer coefficient hc ¼ 27:34½W=m2 C is calculated from Eq. (12). The heat convection transfer is more important due to the air flow through capsules. Figs. 6a and 7a show the outlet SAHLSC temperature, wind velocity and ambient temperature as a function of day times. It is seen from Fig. 6a that the temperature into the collector (closed cycle), under clear sky conditions, increases gradually at the beginning of the charging period and remains nearly constant around 35–42 °C, the PCM is completely charged. The 17 March 2012 is characterized by severely solar radiation fluctuation with ambient air temperatures from 16 °C, and wind velocity vary from 1 m s1
Fig. 7a. Ambient temperature, air temperature into SAHLSC and wind velocity as a function of day times (17 March 2012).
Fig. 7b. Absorbed, useful and stored heat rates as a function of day times (17 March 2012).
Fig. 6a. Ambient temperature, air temperature into SAHLSC and wind velocity as a function of day times (26 March 2012).
Fig. 6b. Absorbed, useful and stored heat rates as a function of day times (26 March 2012).
to 7 m s1. We noted that the temperature into the solar collector is ranging from 25 °C to 35 °C and the PCM is also charged during the closed cycle (Fig. 7a). In fact, during the discharging periods; after 16:00 (local time), the outlet temperature decrease progressively at the beginning of the opened cycle due to the coldness of the liquid PCM (sensible heat). After 2 h, the temperature remains constant; nearly 18 °C, during the solidification process (latent heat) for 2 days. Variations of absorbed, useful and stored energy are presented in Figs. 6b and 7b for different times during 2 days, 26 and 17 March 2012. Therefore, to evaluate the stored energy, the energy balance equation Eq. (1) has been used. A great part of the absorbed solar heat was stored inside the PCM. It is observed that during the initial period of charging, the instantaneous heat stored increases with insolation (Fig. 6b) and 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. During the charging process, Fig. 7b shows the instantaneous stored heat fluctuate at the same time as insolation. 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 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.
S. Bouadila et al. / Applied Energy 110 (2013) 267–275
Fig. 8. Daily energy and exergy efficiency as a function of days (19 March to 30 March 2012).
The energy and exergy analysis of the SAHLSC were performed with data obtained from the experiments. Fig. 8 shows the values of the daily energy and exergy efficiency as function of days from 19 March 2012 to 30 March 2012 using Eq. (20) and Eq. (32), respectively. The daily average energy efficiency changed between 32% and 45%. The higher daily energy and exergy efficiencies are obtained at 29 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. The packed bed latent heat thermal energy storage of a solar air heater system using PCM spherical capsules, presented a high degree of stratification. The variations with time of the PCM temperature inside a spherical capsule in the bed during discharging process are shown in Fig. 9. The discharge period varies with the level of the heater bed. Beds near the entrance are discharging during 1 h or 2 h and the above bed is discharging during long time (8 h). When the above bed is fully discharged bed temperature begins to decrease until becomes equal to ambient temperature. The discharged time process is the interval between discharging process beginning and the instant when the outlet temperature is equal to the ambient temperature. Moreover, we can see that the exchanged capacity between the heat transfer fluid and capsules, decrease when the bed is discharged.
273
Fig. 9. Temperature distribution in a bed while discharging process as a function of day times.
Fig. 10a. Outlet temperature for SALHSC and SAHC respectively and global solar radiation as a function of day times (29 March 2012).
6.2. Comparison investigation between SAHLSC system and a commercial solar air heater The performances of the SAHLSC were compared with a commercial Solar Air Heater (SAHC) available in the local market. The comparative investigation has been conducted for two operating mode: Opened cycle and Closed/Opened cycle. During the opened mode the air inlet and outlet openings are opened throughout the day. The outlet temperature and global solar radiation are analyzed and compared for two systems (Fig. 10a). The energy efficiency of the SAHLSC system and the SAHC operating in opened cycle is shown in Fig. 10b. For a peak global solar radiation (1080 W/m2), the mean value of the temperature of the supplied hot air was about 39 °C and 34 °C for the SAHC and the SAHLSC, respectively. We concluded that when there was a diminution of global solar radiation after 14:30, the SALHSC did not decrease seriously while
Fig. 10b. Thermal efficiency for SALHSC and SAHC respectively and global solar radiation as a function of day times (29 March 2012).
the SAHC system showed a great decrease, about 3–5 °C (Fig. 10a). The collector, with the latent heat storage, could maintain outlet temperature oscillating around melting temperature during 5 h more than a collector without storage. In fact, it allowed us to obtain a hot air with less ambient temperature.
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Acknowledgment We thank the Cristopia Energy Systems society for providing us the PCM capsules. References
Fig. 11. Air temperature into SAHLSC and SAHC and global solar radiation as a function of day times.
On 29 March 2012, the SAHC carried out average thermal efficiency about 45%, nearly to the SAHLSC, 35% (Fig. 10b). The SAHLSC showed very stable values of efficiency around 30%, during the whole experimental period, as a consequence of the thermal inertia of the packed bed absorber. A comparative investigation has been conducted in Closed/ Opened cycle for a representative day of the packed bed solar energy storage system and the SAHC. Variations of temperature into the SAHLSC and the SAHC operating in the charging and discharging modes and the global solar radiation are presented in Fig. 11. During the charging periods from 9:00 to 16:00 (local time), the temperature into the collectors increases gradually at the beginning and attains a maximum value of 49 °C and 42 °C at 13:00 pm for the SAHC and the SAHLSC, respectively. At 16:00 pm, the discharging process starts, the temperature into the commercial solar air heater decrease quickly and remain lower than 10 °C after 4 h. While the temperature into the SAHLSC decrease gradually at the beginning of the discharging process and remains constant during all the night and keeps around 17 °C, due to the stored heat in the PCM during the charging process. The SAHLSC device is useful and has the capability to heating a greenhouse system, for example in winter climates.
7. Conclusion The main findings of the present experimental studies are summarized as follows: – 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. This useful heat was not affected by the severally global solar radiation fluctuation during the charging mode. – In Closed/Opened mode, the net daily energy efficiency of the SAHLSC varied between 32% and 45%, whereas the daily exergy efficiency varied between 13% and 25%. – In Opened cycles, the SAHLSC presents a daily efficiency of 35%. The solar collector with storage energy has a daily efficiency close to those of the commercial collectors (45%). Therefore, further numerical simulation should be carried on a SAHLSC system to evaluate the quantity of PCM needed to be installed inside the latent storage solar heater for heating applications.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Performance of a new solar air heater with packed-bed latent storage energy for nocturnal use Salwa Bouadila ⇑, Sami Kooli, Mariem Lazaar, Safa Skouri, Abdelhamid Farhat The Research and Technology Center of Energy, The Thermal Processes Laboratory, Hammam Lif, B.P. 95, 2050 Tunis, Tunisia
h i g h l i g h t s A new solar air heater collector using a phase change material. Experimental study of the new solar air heater collector with latent storage. Energy and exergy analysis of the solar heater with latent storage collector. Nocturnal use of solar air heater collector.
a r t i c l e
i n f o
Article history: Received 11 October 2012 Received in revised form 4 March 2013 Accepted 18 April 2013 Available online 15 May 2013 Keywords: Latent heat storage Packed bed solar air heaters Energy efficiency Exergy efficiency
a b s t r a c t An experimental study was conducted to evaluate the thermal performance of a new solar air heater collector using a packed bed of spherical capsules with a latent heat storage system. Using both first and second law of thermodynamics, the energetic and exegetic daily efficiencies were calculated in Closed/ Opened and Opened cycle mode. The solar energy was stored in the packed bed through the diurnal period and extracted at night. The experimentally obtained results are used to analyze the performance of the system, based on temperature distribution in different localization of the collectors. The daily energy efficiency varied between 32% and 45%. While the daily exergy efficiency varied between 13% and 25%. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Successful solar system design is an iterative process involving consideration of many technical, practical, reliability, cost and environmental considerations. The Solar Air Heater (SAH) is less expensive than other solar collectors and it uses less materials [1]. One of the first studies of SAH was conducted by Close [2]. The author included a corrugated absorber surface and used a trapped layer of air between the single glazing surface and the absorber. Gupta and Garg [3] tested the performance of four different SAH using absorber surface as well as wire mesh packing over the absorber. Solar air heating has attracted increasing interest after the oil crisis, many air heaters were realized in these periods including novel designs [4– 6]. Sharma et al. [7] used a wire matrix packing above the absorber plate to enhance heat transfer. Choudhury and Garg [8] obtained a SAH thermal efficiency about 70% by using a packing material placed above the absorber. In the last century, air heater designs have focused mainly on improving the convective heat transfer at
⇑ Corresponding author. Tel.: +216 71 97 772 206; fax: +216 71 430 934. E-mail address: [email protected] (S. Bouadila). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.04.062
the absorber. Mohamad [9] designed a solar air heater, which consists of pre-warming air in its first pass between the upper and the lower glass channel. The main idea of this work is to minimize the heat losses from the collector top cover and to maximize the heat extraction from the absorber. The thermal efficiency of this collector exceeds 75%. Experimental and comparative studies of a selectively coated absorber plate and a black-painted absorber plate have been carried out by Hachemi [10]. Koyuncu [11] compared SAH flat plate designs and glazing configurations. Consequently, the author proved that the best efficiency was obtained using the flat black metal plate with a single polymer glazing. In order to increase the air heater efficiency authors [12,13] realized a doubleglass and a double-pass with a steel packed-bed above the heater absorber plate. Esen et al. [14,15] tested and compared different absorbing obstacles on a flat plate with an experimental study and artificial neural network models. They showed that the heat transfer efficiency is given by breaking up the boundary layer and reducing the dead zones in the collector. The thermal energy can be stored as sensible heat, latent heat, reaction heat or combination of those forms. Many thermal storage energy systems are cited in literature [16–22], with applications in cogeneration, building and solar heating of water or air.
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Nomenclature Ac Ap Cp
N h hc hr hw IT L L1 L2 L3 _ m Nu P Pr _ m R Re Ra 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) exergy rate (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 rate (W) universal gas constant (J/kg °C) 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 db bottom insulation thickness (m) de edge insulation thickness (m) D difference in time e emissivity
The use of Phase Change Material (PCM) storage in a solar air heater is less common, the majority of the studies were numerical. Hence, authors [23–25] used numerical tools to determine the solar air heating systems performance using PCM energy storage, to examine the effect of the latent heat and the PCM melting temperature and to identify the optimum phase change materials physical properties. Ghoneim and Klein [26] evaluated the theoretical performance of solar heating collectors with PCM and sensible heat storage in water and in air. Fath [27] used a 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 [28] performed a parametric study to evaluate effects of various thermal and geometrical parameters and different PCMs on the stored solar heat quantity. Esen et al. [29] analyzed a solar assisted latent heat storage tank by using two different numerical models. Enibe [30] has realized a single-glazed flat plate solar collector integrated with a paraffin wax PCM. Alkilani et al. [31] 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. [32] presented a comparative and experimental study of a typical solar air heating system with and without temporary thermal energy storage material for en-
g g0 k ki
l ls q r s w
energy efficiency (%) optical yield (dimensionless) thermal conductivity (W=m CÞ thermal conductivity of insulation (W=m CÞ PCM dynamic viscosity (N s/m2) PCM dynamic viscosity at T = TPCM (N s/m2) density (kg/m3) Stefan –Boltzmann constant, 5.670 108 W/m2 K4 transmittance exergy efficiency (%)
Subscripts A absorbed a ambient av average b bottom c cold ch charging d destroyed dis discharging e edge fin_ch final of the charging process fin_dis final of the discharging process g glasses h hot in inlet 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
ergy and exergy analysis. Castell et al. [33] presented an experimental study of a PCM tank for cold storage applications. Different configurations and flow rates of the heat transfer fluid were studied. A novel conceptual system called mobilized thermal energy storage system (M-TES) was proposed for distributed heat supply [34]. These systems have been studied for many years, focusing on PCM stability and improving heat transfer [35,36]. 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 [37–39]. In order to provide an amount of storage heat nocturnal use we will present an analytical and experimental investigation 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. Charging and discharging processes, energy and exergy analysis of the collector are carried out. 2. Description of SAHLSC A schematic arrangement of solar air heater with phase change energy storage using spherical capsules is given in Fig. 1 and a photograph of the SAHLSC is shown 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. Capsules have an outer diameter of 0.077 m and are blow molded from a blend of polyolefin with an average thickness of 0.002 m. The
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2
4
3 5 6 7
2
(1) Fan (2) Diverged section (3) Glass cover (4) Air inlet (5) Packed bed (PCM) (6) Collector box (7) Insulating material (8) Fixed steel matrix (9) Air outlet
Global solar radiation (kw/m )
1
8
2007 2008 2009 2010 2011
200
150
100
50
0 Jan
Feb
Mar
Apr
May
9 Fig. 1. Schematic view of SAHLSC.
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month Fig. 3. Monthly variation of the global solar radiation for the 2007–2011 periods in Tunisia.
30
2007 2008 2009 2010 2011
Ambient temperature (ºC)
25
20
15
10
5
0 Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month Fig. 4. Monthly variation of the ambient temperature for the 2007–2011 periods in Tunisia.
Fig. 2. The photograph of experimental SAHLSC.
PCM is confined inside spherical capsules. This geometry is the most method of encapsulation [40]. 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. 3. Description of the experimental set-up and measurement procedure An experimental set-up of the solar air heater latent storage collector was designed and constructed to investigate the charging and discharging processes. The experiments were performed during March 2012 in the Research and Technology Center of Energy (CRTEn) in Borj Cédria. The experiment’s site is a sunny region,
situated on the Mediterranean coast of North Africa, near the city of Tunis in Tunisia, with the following coordinates: Latitude 36°430 N and Longitude 10°250 E. The meteorological station on the site permits the measurement of the global sun flux on a horizontal plan, ambient temperature, wind speed and wind direction. Figs. 3 and 4 give the variation of meteorological data of the average monthly global solar radiation and ambient temperatures in a period of 2007–2011. The sunshine period of Tunisia is a 3500 h/ year and 350 sunny days per year with a daily insolation range from 5 h in December to 13 h in July. It receives an average global solar radiation intensity of 4.3 kW h/m2/day [41]. A schematic view of the experimental setup SAHLSC is shown in Fig. 5. Charging and discharging process are carried out in this experimental study. The charging process starts when the absorber is exposed to the solar radiation. 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 temperature distribution along the absorber packed bed was measured using four K-type thermocouples T2, T12,T18 and T25 situated in the middle of the second bed, bed number 12, bed number 18 and bed number 25, respectively. Fig. 2 shows the localization of thermocouples inserted inside capsules in different beds. Two temperature sensors for absorber surface of one capsule
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2
The radiation absorbed flux of the absorber packed-bed is defined as:
6
Q A ¼ Ap ðasÞIT
3 4
5
(1) SAHLSC (2) Fan (3) Diverged section (4) Pyranometer (5) Ambient temperature (6) Data aquisation system (7) Support
ð4Þ
where (as) is the effective product transmittance–absorptance that is equal with the optical efficiency (g0) [43]. The stored heat flux during the charging and discharging phases is given by:
Q ch ¼ ½mPCM C p;s ðT m T ini ch;PCM Þ þ mPCM LþmPCM C p;l ðT fin ch;PCM T m Þ=Dt ch
ð5Þ 7
Q ads ¼ ½mPCM C p;l ðT m T fin dis;PCM Þ þ mPCM LþmPCM C p;s ðT in dis;PCM T m Þ=Dt dis
ð6Þ
1
Fig. 5. Schematic view of experimental set-up.
(TP) and the glass temperature (Tg) were used. The charging process tests are performed from 9:00 to 16:00 (local time). The thermophysical properties of the capsules and air, which were used, are given in Table 1. 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 m s1. The inlet and outlet air temperatures of the SAHLSC Tin and Tout, 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 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 [42]:
Q los ¼ U los Ac ðT P T a Þ
ð7Þ
U los ¼ U t þ U b þ U e
ð8Þ
The top loss coefficient from the collector to the ambient is:
1 U t ¼ 1=ðhc;Pg þ hr;Pg Þ þ 1=ðhw þ hr;ga Þ
The flow is laminar during the charging phase; and the appropriate correlation is given by Churchill [44]:
Nuch ¼ 2 þ ð0:589Ra1=4 Þ=ð1 þ ðð0:469Þ=PrÞ9=16 Þ4=9
The energy and exergy analysis was carried out to evaluate the system efficiency.
11
Ra 6 10
2 Nudis ¼ 2 þ ð0:4Re
4.1. Energy analysis 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:
ð1Þ
where QA, Qu, Qst, and Qlos are the absorbed, useful, stored and lost energy, respectively.Based on Duffie and Beckman [42], the useful heat gain from the collector is:
ð2Þ
where
_ ¼ qV av S m
Pr P 0:7
Then the flow is turbulent during the discharging phase; the appropriate correlation is given by Whitaker [44]:
1=2
_ p ðT out T in Þ Q u ¼ mC
ð10Þ
4. Energy and exergy analysis of the solar air heater latent storage collector
Q A ¼ Q u þ Q st þ Q los
ð9Þ
ð3Þ
Vav and S are average air velocity and cross sectional area of the duct at the inlet of the solar air heater, respectively.
2=3
þ 0:06Re
3 0:71 6 Pr 6 380 47 4 3:5 6 Re 6 7:6 10 5 1 6 ðl=ls Þ 6 3:2
1=4 6
0:4
ÞPr ðl=ls Þ
ð11Þ
hc;Pg ¼ Nuðk=tÞ
ð12Þ
hr;Pg ¼ ðrðT 2P þ T 2g ÞðT P þ T g ÞÞ=ðð1=eP Þ þ ð1=eg Þ 1Þ
ð13Þ
The heat transfer coefficient due to convection at the top of cover due to wind is given by Hottel and Woertz [42]:
hw ¼ 5:67 þ 3:86V1
ð14Þ
hr;ga ¼ eg rðT 2g þ T 2sky ÞðT g þ T sky Þ
ð15Þ
The appropriate approximation to sky temperature is given by Swinbak [42]:
T sky ¼ 0:0552T 1:5 a
ð16Þ
Table 1 Thermal properties of PCM and air. Material
Melting point (°C)
Heat of fusion (kJ/kg)
Specific heat (kJ/kg°C)
Density (kg/m3)
Thermal conductivity (W/m°C)
Capsule (SN27)
27
192.6
Liquid 2.22
Liquid 1710
Liquid 0.58
Air (at 25 °C)
–
–
1.0048
Solid 1.42
1.137
Solid 1530
2.49 102
Solid 1.05
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In addition the energy loss through the bottom (Ub) and the edges (Ue) equation as follows:
U the heat transfer rate from absorber packed-bed to the agent fluid across finite temperature difference [47,51]:
U b ¼ ki =db
ð17Þ
_ p T a ðlnðT out =T in Þ ððT out T in Þ=T p ÞÞ Nd ðp f Þ ¼ mC
U e ¼ ðL1 þ L2 ÞL3 ki =ðL1 L2 de Þ
ð18Þ
Nst: The stored exergy rate is both sensible and latent change within the PCM, for the charging state, so it becomes in the discharged process [53]:
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:
g ¼ Q u =Ac IT
Z
Q dis
Z
dis
Ac IT
ð20Þ
T a lnðT fin
w¼
X
_ in ¼ m
X
_ out m
ð21Þ
The general exergy balance can be expressed below as the total exergy inputs equal to total energy outputs.
X
Nin ¼
X
Nout
ð22Þ
The General form of the exergy balance equation is [46,47]:
Nin ¼ Nout þ Nlos þ Nd þ Nst
ð23Þ
where Nin, Nout, Nlos, Nd and Nst are the inlet, outlet, leakage, destroyed and stored exergy rate, respectively. Nin: The total inlet exergy as includes: U the inlet exergy rate with air flow:
_ p ðT in T a T a lnðT in =T a ÞÞ þ mRT _ a lnðPin =P a Þ Nin ðairÞ ¼ mC
ð24Þ
U the absorbed solar radiation exergy rate, assuming the sun as an infinite thermal source [48–51]:
Nin ðabsorbedÞ ¼ g0 IT Ac ð1 ðT a =T sun ÞÞ
Z dis
Exergy is the amount of maximum work which can be produced by a system or a flow of matter or energy as it comes to equilibrium with a reference environment [45]. The mass balance equation can be expressed in the rate form as:
ð25Þ
where T sun ¼ 4077 C is the apparent sun temperature. It represents 75% of black body temperature of the sun [52]. The total inlet exergy is given by the summation of Eq. (24) and Eq. (25) will result in the total inlet exergy rate of the heater. Nout: The outlet exergy rate includes only the rate of outlet air flow [45]:
_ p ðT out T a T a lnðT out =T a ÞÞ þ mRT _ a lnðPout =P a Þð26Þ Nout ðairÞ ¼ mC
T a lnðT m =T ini
dis;PCM ÞÞ
dis;PCM
Tm
dis;PCM =T m ÞÞDt ch
Ndis
Z
Nin ðabsorbedÞ
the absorber packed–bed to the environment [51]:
Nlos ¼ U los Ac T a ðT p T a Þð1 ðT a =T p ÞÞ
ð27Þ
Nd: The destroyed exergy rate is caused by three terms:
ð32Þ
ch
5. 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. 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 rate and the thermal efficiency are presented here. _ for the mass flow rate Eq. (3) The fractional uncertainty, wm_ =m, is written as [54]:
_ ¼ ½ðwV av =V av Þ2 þ ðwT a =TÞ2 þ ðwPa =Pa Þ2 1=2 wm_ =m
ð33Þ
The density of air q 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 rate, wQ/Q, _ considering Cp as from Eq. (2) is a function of (Tout Tin) and m, constants [54].
_ 2 þ ðwðT out T in Þ =ðT out T in Þ2 1=2 wQ =Q ¼ ½ðwm_ =mÞ
ð34Þ
The fractional uncertainty for efficiency, wg/g, from Eq. (19) is a _ and IT, considering Cp and Ac as constants [54]. function of (Tout Tin), m
_ 2 þ ðwðT out T in Þ =ðT out T in Þ2 þ ðwIT =IT Þ1=2 wg =g ¼ ½ðwm_ mÞ
Nlos: The leakage exergy rate is caused by the heat leakage rate from
ð31Þ
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.
ch
4.2. Exergy analysis
dis;PCM
þmPCM Lð1 ðT a =T m ÞÞ þ mPCM C p;l ðT fin
ð19Þ
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.
g¼
Ndis ¼ ½mPCM C p;s ðT m T ini
ð30Þ
ð35Þ
The total uncertainty in determining flow rate, heat rate and efficiency were estimated by Eqs. (33)–(35), respectively. The total uncertainty calculated of the mass flow rate of air is 0.0039, the heat rate is 0.0087 and efficiency is 0.0096.
U the absorber packed-bed surface and sun temperature difference [52]:
6. Results and discussion
Nd ðp sÞ ¼ g0 IT Ac T a ðð1=T p Þ ð1=T s ÞÞ
ð28Þ
6.1. The SAHLSC performances of the operation mode: Closed/Opened cycles
ð29Þ
Collector performance tests were conducted in the month of March 2012. During these experiments, inlet, outlet and the
U the duct pressure drop [47,51]:
Nd ðDPÞ ¼ ðDPT a =qÞðT a lnððT out =T a ÞÞ=ðT out T in ÞÞ
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packed bed temperatures of SAHLSC, the PCM and the ambient temperatures and global solar radiation were recorded. In closed cycle during the charging periods from 9:00 to 16:00 (local time), both inlets and outlets are closed and the packed bed absorber stored the energy provided from the sun. In the charging cycle, the internal laminar flow collector is due to difference of the air density. The Nusselt number is determined by using the Eq. (10): Nuch = 2.67. The convective heat transfer coefficient hc can be calculated by using the Eq. (12): hc ¼ 3:43½W=m2 C. In opened cycle, the stored energy of the solar air heater is recovered once the cold air passes through the bed. Therefore, the bed is fully discharged when the outlet and inlet air temperature at the collector are similar. In the discharging process, the turbulent flow into the collector is caused by the fan. The value of Nusselt number is determined by using the Eq. (11): Nudis = 7.15. The convective heat transfer coefficient hc ¼ 27:34½W=m2 C is calculated from Eq. (12). The heat convection transfer is more important due to the air flow through capsules. Figs. 6a and 7a show the outlet SAHLSC temperature, wind velocity and ambient temperature as a function of day times. It is seen from Fig. 6a that the temperature into the collector (closed cycle), under clear sky conditions, increases gradually at the beginning of the charging period and remains nearly constant around 35–42 °C, the PCM is completely charged. The 17 March 2012 is characterized by severely solar radiation fluctuation with ambient air temperatures from 16 °C, and wind velocity vary from 1 m s1
Fig. 7a. Ambient temperature, air temperature into SAHLSC and wind velocity as a function of day times (17 March 2012).
Fig. 7b. Absorbed, useful and stored heat rates as a function of day times (17 March 2012).
Fig. 6a. Ambient temperature, air temperature into SAHLSC and wind velocity as a function of day times (26 March 2012).
Fig. 6b. Absorbed, useful and stored heat rates as a function of day times (26 March 2012).
to 7 m s1. We noted that the temperature into the solar collector is ranging from 25 °C to 35 °C and the PCM is also charged during the closed cycle (Fig. 7a). In fact, during the discharging periods; after 16:00 (local time), the outlet temperature decrease progressively at the beginning of the opened cycle due to the coldness of the liquid PCM (sensible heat). After 2 h, the temperature remains constant; nearly 18 °C, during the solidification process (latent heat) for 2 days. Variations of absorbed, useful and stored energy are presented in Figs. 6b and 7b for different times during 2 days, 26 and 17 March 2012. Therefore, to evaluate the stored energy, the energy balance equation Eq. (1) has been used. A great part of the absorbed solar heat was stored inside the PCM. It is observed that during the initial period of charging, the instantaneous heat stored increases with insolation (Fig. 6b) and 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. During the charging process, Fig. 7b shows the instantaneous stored heat fluctuate at the same time as insolation. 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 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.
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Fig. 8. Daily energy and exergy efficiency as a function of days (19 March to 30 March 2012).
The energy and exergy analysis of the SAHLSC were performed with data obtained from the experiments. Fig. 8 shows the values of the daily energy and exergy efficiency as function of days from 19 March 2012 to 30 March 2012 using Eq. (20) and Eq. (32), respectively. The daily average energy efficiency changed between 32% and 45%. The higher daily energy and exergy efficiencies are obtained at 29 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. The packed bed latent heat thermal energy storage of a solar air heater system using PCM spherical capsules, presented a high degree of stratification. The variations with time of the PCM temperature inside a spherical capsule in the bed during discharging process are shown in Fig. 9. The discharge period varies with the level of the heater bed. Beds near the entrance are discharging during 1 h or 2 h and the above bed is discharging during long time (8 h). When the above bed is fully discharged bed temperature begins to decrease until becomes equal to ambient temperature. The discharged time process is the interval between discharging process beginning and the instant when the outlet temperature is equal to the ambient temperature. Moreover, we can see that the exchanged capacity between the heat transfer fluid and capsules, decrease when the bed is discharged.
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Fig. 9. Temperature distribution in a bed while discharging process as a function of day times.
Fig. 10a. Outlet temperature for SALHSC and SAHC respectively and global solar radiation as a function of day times (29 March 2012).
6.2. Comparison investigation between SAHLSC system and a commercial solar air heater The performances of the SAHLSC were compared with a commercial Solar Air Heater (SAHC) available in the local market. The comparative investigation has been conducted for two operating mode: Opened cycle and Closed/Opened cycle. During the opened mode the air inlet and outlet openings are opened throughout the day. The outlet temperature and global solar radiation are analyzed and compared for two systems (Fig. 10a). The energy efficiency of the SAHLSC system and the SAHC operating in opened cycle is shown in Fig. 10b. For a peak global solar radiation (1080 W/m2), the mean value of the temperature of the supplied hot air was about 39 °C and 34 °C for the SAHC and the SAHLSC, respectively. We concluded that when there was a diminution of global solar radiation after 14:30, the SALHSC did not decrease seriously while
Fig. 10b. Thermal efficiency for SALHSC and SAHC respectively and global solar radiation as a function of day times (29 March 2012).
the SAHC system showed a great decrease, about 3–5 °C (Fig. 10a). The collector, with the latent heat storage, could maintain outlet temperature oscillating around melting temperature during 5 h more than a collector without storage. In fact, it allowed us to obtain a hot air with less ambient temperature.
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Acknowledgment We thank the Cristopia Energy Systems society for providing us the PCM capsules. References
Fig. 11. Air temperature into SAHLSC and SAHC and global solar radiation as a function of day times.
On 29 March 2012, the SAHC carried out average thermal efficiency about 45%, nearly to the SAHLSC, 35% (Fig. 10b). The SAHLSC showed very stable values of efficiency around 30%, during the whole experimental period, as a consequence of the thermal inertia of the packed bed absorber. A comparative investigation has been conducted in Closed/ Opened cycle for a representative day of the packed bed solar energy storage system and the SAHC. Variations of temperature into the SAHLSC and the SAHC operating in the charging and discharging modes and the global solar radiation are presented in Fig. 11. During the charging periods from 9:00 to 16:00 (local time), the temperature into the collectors increases gradually at the beginning and attains a maximum value of 49 °C and 42 °C at 13:00 pm for the SAHC and the SAHLSC, respectively. At 16:00 pm, the discharging process starts, the temperature into the commercial solar air heater decrease quickly and remain lower than 10 °C after 4 h. While the temperature into the SAHLSC decrease gradually at the beginning of the discharging process and remains constant during all the night and keeps around 17 °C, due to the stored heat in the PCM during the charging process. The SAHLSC device is useful and has the capability to heating a greenhouse system, for example in winter climates.
7. Conclusion The main findings of the present experimental studies are summarized as follows: – 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. This useful heat was not affected by the severally global solar radiation fluctuation during the charging mode. – In Closed/Opened mode, the net daily energy efficiency of the SAHLSC varied between 32% and 45%, whereas the daily exergy efficiency varied between 13% and 25%. – In Opened cycles, the SAHLSC presents a daily efficiency of 35%. The solar collector with storage energy has a daily efficiency close to those of the commercial collectors (45%). Therefore, further numerical simulation should be carried on a SAHLSC system to evaluate the quantity of PCM needed to be installed inside the latent storage solar heater for heating applications.
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