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Experimental energy and exergy analysis of a flat plate solar air heater with a new design of integrated sensible heat storage
Energy 111 (2016) 609e619
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Experimental energy and exergy analysis of a flat plate solar air heater with a new design of integrated sensible heat storage G. Kalaiarasi a, b, *, R. Velraj a, Muthusamy V. Swami b a b
Institute for Energy Studies, Department of Mechanical Engineering, Anna University, Chennai 600025, India Building Research Division, Florida Solar Energy Center, University of Central Florida, Cocoa, FL 32922, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 January 2016 Received in revised form 4 May 2016 Accepted 27 May 2016
This paper presents an experimental energy and exergy analysis of a novel flat plate solar air heater (SAH). It has a specially designed absorber plate made up of copper strips (copper tubes with extended copper fins on both sides), welded longitudinal to one another. This structure acts as an integrated absorber-cum-storage unit, where a high quality synthetic oil (Therminol-55) is filled within those copper tubes as a sensible heat storage (SHS) medium. To study the impact of this novel design and the sensible heat storage over the performance of the SAH, the results were compared with the output of a conventional SAH of similar dimensions. For the precise comparison of their performances, the experiments were conducted on both the SAHs at same location, simultaneously. It ensures identical testing conditions such as the amount of solar radiation received and surrounding environment of the experimental setup. Exergy analysis is a powerful thermodynamic tool and it helps in computing the actual output of a system, theoretically. It helps the researchers to optimize the system design to compensate the present and also the future needs. Experiments were conducted for two different mass flow rates (0.018 kg/s, and 0.026 kg/s). The results showed that the maximum energy and exergy efficiency obtained was in the range of 49.4e59.2% and 18.25e37.53% respectively, for the SAH with sensible storage at m_ ¼ 0.026 kg/s. Besides, the SAH with sensible heat storage was observed to perform better than the conventional flat plate SAH without storage. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Flat plate solar air heater Integrated sensible heat storage Exergy analysis Energy analysis
1. Introduction In recent years, India has been making several impressive progress in various fields, such as agriculture, transport, communication and industrial sectors, thereby striding towards an economical self-reliance among other developing nations [1]. The advancement in all these fields has lead to an ever growing demand for energy. To succeed in its planned economic growth, India has to derive adequate energy supply from various sources [1,2]. Though India has a sufficient supply of fossil fuels at present, they are continually being depleted. It will not sustain for a long duration [3,4]. Thus, the need for energy alternatives become inevitable. Various researches are in progress to effectively utilize different renewable energy resources for this purpose. Among them, solar energy is considered as a suitable option because of its value added
* Corresponding author. Institute for Energy Studies, Department of Mechanical Engineering, Anna University, Chennai 600025, India. E-mail address: [email protected] (G. Kalaiarasi). http://dx.doi.org/10.1016/j.energy.2016.05.110 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
benefits, such as eco-friendly, economic, easy installation and so on. Though solar energy is available only during day time, it could still be utilized as an effective energy resource, provided with an efficient solar collector design and storage unit [2,5]. The common methods of solar energy harvesting are converting the solar radiation into thermal energy with a help of solar heating system. A typical solar air heating system consists of a collector frame, absorber plate, top glazing, Insulation layer and support structures. Its performance is based on various factors such as collector material, design, surface area, tilt angle, mass flow rate of the heat transfer fluid, environmental conditions and so on [4e6]. Their optimal design is essential to achieve an efficient and economical operation of various applications. One of the precise methods to evaluate the performance of a solar air collector is by conducting the 1st and 2nd law of thermodynamic analysis, viz. energy and exergy analysis. Both the analyses are valuable tools to study the thermal performance of a system. Energy could be represented as the sum of exergy and anergy, where anergy is the amount of energy spent on the system to obtain the useful exergy. Analyzing a system based on exergy
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Nomenclature h S _ Ex P U Q M m_ I_ Q_ c I C cp A T t1,t2 W Z1 ,..,Zn R
enthalpy (kJ kg1) entropy (kJ kg1 K1) exergy rate (kW) fluid pressure (Pa) heat loss coefficient (W m2 C1) incident solar energy (kJ) mass (kg) mass flow rate of air (kg S1) rate of irreversibility (kW) solar energy absorbed by collector surface (kW) solar radiation (W m2) specific heat capacity (kJ kg1 K1) specific heat capacity of air at constant pressure (kJ kg1 K1) surface area (m2) temperature (K) time interval total uncertainty in the measurements (%) uncertainty factors universal gas constant (kJ kg1 K1)
accounts for the irreversibility in a system during the energy conversion process [7,8]. It is a tool for assessing the efficient usage of solar energy, which gives the maximum power that can be extracted from a system [9]. Besides, it helps in quantifying the collection of solar energy and the amount of energy exchanged to the heat transfer fluid [5]. Both energy and exergy efficiencies could be improved by changing the design and operation of SAH, such as the collector surface area, mass flow rate and so on [6,7]. For example, the higher collector surface area could lead to increased exergy efficiency, but it will also increase the capital cost of the solar air heater. Thus, an optimal collector area should be chosen. Similarly, the higher mass flow rate not only improves the exergy efficiency, but it also decreases the outlet temperature. Besides, higher blower power input is needed for higher mass flow rate. So, an optimal mass flow rate should also be chosen for economic and efficient operation [10e15]. Saidur et al. [10] reviewed the energy and exergy analysis of various solar applications and conveyed that the 1st law efficiency is alone not sufficient to select a desired system, but the exergy (2nd law) efficiency is also important. Exergy analysis determines the source and magnitude of irreversibility in detail and plays a vital role in understanding the system performance. The study shows exergy destruction is higher in solar heating and air conditioning systems among various solar applications. Akpinar et al. [11] experimentally investigated the energy and exergy performance of a SAH with and without different types of obstacles (typeI to IV). The study showed that the 1st law efficiency varied between 20% and 82%, whereas, the 2nd law efficiency varied from 8.32% to 44%. Among the case studies, the SAH with leaf shaped obstacles yielded the maximum output compared to the SAH without any obstacle. Besides, the results showed that the thermal efficiency is significantly dependent on solar irradiation, surface geometry, area of the air channel and air mass flow rate. TorresReyes et al. [12,13] presented a paper on optimizing the performance of a flat-plate solar air heater by minimizing the entropy generation. A preliminary design of an efficient solar air heater was made based on the entropy generation number, mass flow number and stagnate air temperature, which are subjected to a
Abbrevaions SAH solar air heater SHS sensible heat storage IST Indian standard time Greek Symbols absorptance of absorber plate transmittance of glass cover inclination angle of solar collector (degree) first law/Energy efficiency second law/Exergy efficiency
a t q h J
Subscripts c collector pl absorber plate avg average con convection e environment out output in input s surface
€ thermodynamic optimization procedure. Oztürk et al. [14] conducted an experimental investigation of the thermal performance of a SAH with packed bed Raschig rings. From the results, the net energy efficiency ranges from 2.05 to 33.78% whereas, the net € exergy efficiency ranges from 0.01 to 2.16%. Moreover, Oztürk et al. [15] experimentally analyzed the energy and exergy efficiencies of SAH with paraffin wax as storage for greenhouse heating. The mean daily rate of thermal exergy transferred and stored in latent heat storage was found to be 111.2 W and 79.9 W, respectively. The average net energy and exergy efficiency were computed as 40.4% and 4.2% respectively. Esen et al. [16] conducted an experimental investigation on the energy and exergy efficiencies of double-flow SAHs with several obstacles and without obstacle. It was observed that obstacles in the air duct of the double-flow collector is an efficient method of adapting air exchange according to the user demands. Alta et al. [17] analyzed the energy and exergy performance of SAHs with and without fins. Among them, the double glass finned type heater found to be operating with higher exergy efficiency compared to other types. Sami et al. [18] presented the microscopic exergy and energy analysis on the dynamic mathematic model of an indirect cabinet solar dryer. The analysis reveals that the indirect solar cabinet has a relatively lower exergy efficiency compared to energy efficiency and it was also found that the exergy losses were maximum during the mid-day. Impacts of various factors like collector surface, its length and mass flow rate over the exergy destruction and efficiency were investigated. Tyagi et al. [19] reviewed the solar heating system with and without thermal storage. To overcome the limitations of solar energy such as its time dependent and intermittent nature, this paper focused on phase changed material (PCM) based thermal energy storage to improve the performance. It was found that the latent heat storage is more efficient compared to the sensible heat storage. Saxena et al. [20] reviewed the various ways to improve the thermal performance of SAHs such as dimensions of the air heater components, different types of obstacles, usage of latent and sensible heat storage material, usage of concentrators to multiply the available insolation and so on. Bahrehmand et al. [21] performed
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the energy and exergy analysis on the mathematical model of single and two glass cover SAHs. The performance impact due to the suspension of a tin metal sheet in the middle of a airflow channel, longitudinal fins with rectangular or triangular shapes, change in the length and depth of the air channel was investigated. It was found that the double glass SAH is analytically performed better compared to single-glass collector. In addition to that, the SAH with triangular fins are determined to be more energy efficient compared to rectangular fins. Bouadila et al. [22,23] proposed a new SAH with a packed-bed latent heat storage using PCM spherical capsules. The proposed model was experimented at Tunisia climatic conditions and the impact of global irradiation and massflow rate over the absorbed, used and recovered heat from the storage system was investigated. The results showed that the daily energy and exergy efficiency ranges are 32e45% and 13e25% respectively. Bayraka et al. [24], performed exergy and energy analysis on SAH with closed cell aluminium foam as porous material placed sequentially in a staggered manner. Five different types of SAH with various mass-flow rates were analyzed. The result showed that SAH with aluminium foam of 6 mm thickness and air mass flow rate of 0.025 kg/s was yielded higher collector efficiency and temperature output compared to SAH with nonbaffle collectors operated for an air mass flow rate of 0.016 kg/s. Dehghan et al. [25] developed a model of a square and circular solar pond present in Iran. With the solar radiation data collected, the energy and exergy performance of both the ponds at different zones were investigated. From the study, it was demonstrated that the circular pond is efficient compared to the square ponds due to the shading effects. The maximum energy efficiency for nonconvective zone (NCZ) and heat storing zone (HSZ) are 17.25% and 25.8% respectively, whereas 17.39% and 23.65% respectively for the square pond. Similarly, the maximum exergy efficiency for NCZ and HSZ in the circular pond are 0.86% and 2.4%, whereas 0.86% and 2.44% for the square pond. Ranjan et al. [26] reviewed the ongoing research activities in the field of solar distillation system aiming to improve the efficiency using an effective thermodynamic tool, i.e. energy and exergy analysis. Thermodynamic model based on fundamental heat transfer correlations was presented for the simple basin type solar stills to conduct energy and exergy analysis. The energy efficiency of conventional solar still is found in the range of 20e46%. The exergy efficiencies are estimated to be between 19% and 26% for a triple effect system, 17e20% for a double effect and less than 5% for a single effect system. Alta et al. [27] presented a theoretical and experimental analysis of back-pass solar air heaters. It demonstrated the superiority of exergy analysis over energy analysis in the decision of design parameters. The theoretical analysis was performed by running the energy and exergy equations using FORTRAN codes and were compared with the experimental results. Alta et al. [28] studied the energetic efficiency of SAH with aluminium flat plate absorber with different air mass flow rates. experimental results were comparatively analyzed with the results obtained from computational fluid dynamics software and evaluated based on thermographic camera images. It showed that the increasing inlet temperature and lower air flow rates could yield to increase in outlet air temperature. Li et at [29]. investigated the various influencing factors, such as the heat transfer fluid (HTF) mass flow rate and inlet temperature, phase change material (PCM) melting temperature and number, additives for PCMs, storage unit dimension, heat exchanger surface enhancement, and sensible heating and sub-cooling, etc. He presented the exergetic performance assessment of latent heat thermal energy storage systems. Singh et al. [30] presented an exergoeconomic analysis of the double pass packed bed solar air heater theoretically and experimentally. It was observed that the energy efficiency of the heater increases with the increasing mass
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flow rate and recycle ratio, whereas the exergy efficiency increases with an increase in the mass flow rate and the recycle ratio up to a limit and subsequently decreases with further increase in the mass flow rate and the recycle ratio. Hedayatizadeh et al. [31] simulated an exergetic analysis on a double-pass/glazed V-corrugated plate solar air heater. Its maximum exergy efficiency was derived to be 6.27%. Another important conclusion of his study was that the internal exergy loss due to the temperature difference between sun and absorber surface is the most destructive and it is accounted for 63.57% of total exergy loss. Kant et al. [32] reviewed solar drying systems based on thermal energy storage. It summarizes various latent and sensible heat storage materials that can be used for solar drying systems. The thermal storage material reduces the existing load on the gap between energy demand and supply. From the literature survey, it has been found that several researches are focused on thermodynamic analysis on various models of solar air heaters. However, still there is a wide literature gap remained in the field of SAH with integrated storage facility subjected into exergy analysis in different climatic zones. Most of the studies on renewable energy systems are conducted based on energy analysis rather than exergy analysis [33]. So, this study concentrates on detailed exergy analysis of a novel SAH with specially designed absorber unit where synthetic oil is used as temporary heat storage. The results were analyzed for the improvement in exergy efficiency and output air temperature compared to a conventional SAH. Both the experimental setups of novel and conventional SAHs defined in the following section were fabricated and tested. 2. Experimental setup Experiments were conducted on two different types of SAHs, i.e. conventional SAH (type-I) and novel SAH with integrated SHS (type-II). Both the SAHs were constructed and tested at the Anayur locality of Madurai city in India. Its latitude and longitude are 9.9 N and 78.1 E, respectively. The Figs. 1 and 2 show the monthly variations of wind speed and solar radiation in Madurai obtained from RETScreen plus-NASA international climatic database. Fig. 2 shows the annual daily average solar radiation recorded at Madurai as 5.1 kWh/m2/d, with a maximum value of 6.25 kWh/m2/d during the summer month of March and a minimum of 4 kWh/m2/d during the winter month of November [34]. The total incoming solar radiation has been estimated as 1.861 MWh/m2/year. The above mentioned uncertainty parameters, such as solar radiation and wind velocity have direct impact over the thermodynamic performance of a SAH. It is essential to design the SAH in such a way to tolerate their varying ranges to achieve the optimal performance with minimal manufacturing cost. The type-I SAH was designed without any heat storage, whereas the type-II SAH was designed with an absorber and integrated storage unit. The absorber unit of the type-II SAH was composed of a set of copper tubes with extended fins, which were welded longitudinally. The inner and outer diameter of the copper tubes used were 10 mm and 10.8 mm, with a length of 1.8 m. Nine such copper strips were welded to form the absorber unit. A high quality synthetic oil (Therminol-55) of 2.54 L was filled in those copper tubes, which act as a temporary heat storage medium. The cross sectional view of both the SAHs are shown in Figs. 3 and 4. Fig. 5 shows the schematic diagram of type-II SAH with all the measurements. Both the SAHs were made up of 12 mm thick plywood and fabricated for same dimensions, viz. 2 m 1 m 0.180 m. In order to suppress the conductive heat losses at the bottom and lateral sides of the collector, 50 mm thick rock wool layer had been filled. To reduce the convective heat losses and reflections of solar insolation, a 4 mm thick toughened clear glass was fitted over the top of
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Fig. 1. Madurai Climatic data graph: Monthly wind speed variation.
Fig. 2. Madurai Climatic data graph: Monthly solar radiation variation.
Fig. 3. Cross sectional view of type-I SAH.
the absorber plate at a distance of 30 mm. The photographs of the absorber plate showing copper tubes and the entire experimental setup are shown in the Figs. 6e9. To produce the air turbulence in the duct, 0.5 HP blower was used. The setups were mounted properly in order to prevent any shadows or reflected radiations to fall on it. The tests were conducted during the month of May, from 10 AM till evening 6 PM Indian standard time (IST). K-type thermocouples were used to
measure the temperature at various state points, including the inlet, outlet, absorber plate, top glass, bottom sheet and atmospheric temperature. The air velocity was measured using the anemometer, Testo 425. The solar insolation received at the experiment location was measured with a help of SP-110 model apogee pyranometer. For an interval of every 15 min, the outputs from all these sensors were stored in a computer with the help of 34972A Agilent data logger. The readings from the experiments
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Fig. 4. Cross sectional view of type-II SAH.
Fig. 5. Schematic view of the type-II SAH.
Fig. 6. Integrated absorber-cum-SHS of type-II SAH.
were measured and recorded with the help of suitable instruments and their uncertainties are listed in Table 1. Denoting the relative uncertainties in the individual factors as Zn, the total uncertainty could be represented as in Eq. (1).
Fig. 7. Experimental setup of type-I and type-II SAH.
i1 h 2 W ¼ ðZ1 Þ2 þ ðZ2 Þ2 þ ……ðZn Þ2
(1)
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3.1. Energy analysis Energy analysis is a method of thermodynamics to understand the thermal performance based on the 1st law of heat balance equations, which could be expressed as Eq. (2).
Energy absorbed ¼ Energy accumulated þ Energy lost þ Energy output=gain
(2)
The absorbed energy could also be represented as the fraction of solar radiation absorbed at the tilted collector surface [Eq. (3)]. It is mainly impacted by the optical parameters such as absorptance of the plate (a), transmittance of the glass cover (t) and the inclination angle (q) of the collector surface [16].
Energy absorbed; QC ¼ atAC I cosq
Fig. 8. Construction of type-II SAH (Integrated copper tube for heat storage).
(3)
The specific heat capacity of the absorber plate allows it to store a considerable amount of energy, which is represented in Eq. (4)
Energy accumulated ¼ Mpl Cpl
dTpl;avg dt
(4)
The amount of energy lost during the process of SAH while generating the useful thermal output is presented in the Eq. (5). The overall loss coefficient, Uc includes the losses at the top, bottom and lateral sides of the SAH [35,36].
Energy lost ¼ Ucon AC TP;avg Te
(5)
The useful thermal output from the SAH is represented in Eq. (6).
_ p ðTout Tin Þ Energy gain ¼ mc
(6)
The energy efficiency of a SAH is defined as the ratio of the useful thermal gain of the working fluid to the total incident solar insolation on collector area averaged over the same interval of time period which is expressed by the Eq. (7).
Fig. 9. Cross sectional view of type-II SAH.
h¼
Thermodynamic analysis is a powerful tool which helps in understanding the energy influence on the performance of a heating system. It could be carried out widely based on both 1st and 2nd law of thermodynamics, viz. energy and exergy analysis. This paper investigates the performance of type-I and type-II SAHs using both the laws of thermodynamics and compared their results in detail.
Table 1 Uncertainty in measurements. Measurement parameters
Error limits
Uncertainty in the measurement of temperature Ambient air temperature Absorber surface temperature Collector inlet temperature Collector outlet temperature
±1 ±1 ±1 ±1
Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty
±0.03 psi ±0.03 m/s ±5% ±1 min ±1e2%
the measurement of pressure loss the air velocity measurement the measurement of solar energy time measurement of temperature values reading values of table
Z
t2
ððTout Tin ÞdtÞ Z t2 atAC cosq Idt t1
(7)
t1
3. Thermodynamic analysis
in in in in in
_ p mc
C C C C
3.2. Exergy analysis Exergy Analysis or 2nd law of thermodynamics deals with the irreversibility of energy at each stage of operation in SAH. In other words, exergy losses are irreversible, which cannot be utilized or transferred to the working medium. It can be occurred when the system undergoes a process involving heat exchange, changes in pressure and chemical reaction. Thus, it helps in quantifying the quality of the energy exchanged at each level in the system and provides a more realistic useful output of the system. Therefore, it could help in the design optimization of SAH for operating at better efficiency. Exergy analysis is based on the below assumptions [7,16,35,36]. i. It is a steady state and steady flow process. ii. Kinetic and potential energy are negligible. iii. The directions of heat transfer to the system and the work transfer from the system are positive. iv. Air is considered as an ideal gas, so specific heat is constant and its humidity content is negligible.
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v. Exergy balance of a SAH under a steady state, steady flow operation is given in Eq. (2).
_ _ _ _ I_ ¼ Ex heat Exwork þ Exin Exout
(8)
Where,
_ Ex heat ¼ _ Ex in ¼
X Te Q 1 Ts C
X
_ out ¼ Ex
(9)
m_ in ½ðhin he Þ Te ðSin Se Þ
X
m_ out ½ðhout he Þ Te ðSout Se Þ
(10) (11)
Based on the above assumptions and mass balance we have,
_ Ex work ¼ 0 X
m_ in ¼
(12)
X
m_ out ¼
X
m_ air
Fig. 10. Hourly mean energy and exergy variations of Type-I SAH @ m_ ¼ 0.018 kg/s.
(13)
On substitution of Eqs. (9)e(11) in Eq. (8),
Te _ Q m_ air ½ðhout hin Þ Te ðSout Sin Þ I_ ¼ 1 Ts C
(14)
Where, Q_ c is the total rate of exergy received by the collector absorber area from solar radiation [Eq. (15)].
Q_ c ¼ atAc I cosq
(15)
The change in enthalpy and entropy is given in Eq. (16) & Eq. (17)
hout hin ¼ Cp;out Tout Cp;in Tin Sout Sin ¼ Cp ln
Tout Pout Rln Tin Pin
(16)
(17)
After substitution of Eqs. (15)e(17) in Eq. (14), the final form of irreversible rate of solar collector will be [16,36],
Te _ Tout Q C m_ air Cp;out Tout Cp;in Tin þ m_ air Te Cp ln I_ ¼ 1 Ts Tin Pout m_ a Te Rln Pin
during the sunshine hours, from 10:00 AM to 06:00 PM IST. The SAHs were inclined at an angle of 13 , facing south side according to the geographical location of Madurai. As per the RETScreen plusNASA international climatic database, the monthly and total annual average wind speed in Madurai city is shown in the Fig. 1 [34]. From the graph, its annual average wind speed was observed to be 3.2 m/s. Similarly, Fig. 2 shows the monthly and annual daily average solar radiation received at Madurai, which was recorded as 5.1 kWh/m2/d. Using K-type thermocouples all the temperatures, such as the inlet, outlet air temperature, ambient temperature, top and bottom of collector surface temperatures were measured at different state points. The data were recorded for every 15 min using a data logger and stored in a personal computer. Tests were conducted for two mass-flow rates, 0.018 kg/s and 0.026 kg/s on two consecutive summer days, May-29 & 30, 2015 respectively. The useful heat output and exergy & energy efficiencies of both the SAHs were derived from the data obtained. Figs. 10e13 show the hourly variations in the solar radiation, energy and exergy efficiencies of both the solar air heaters tested for different mass-flow rates, viz. 0.018 kg/s and 0.026 kg/s. The insolation received at the collector surface showed a parabolic characteristic with its peak value attained during the mid-day and latter it decreases as the time increases. From Fig. 10, the irradiation
(18) The idea of increasing the exergy and decreasing the irreversibility is by improving the exergy efficiency (J). It is defined as the ratio of exergy gain of the system to the exergy of absorbed solar radiation [Eq. (19)].
_ Ex
J ¼ _ out ¼ Exin
m_ air ð△h Te △SÞ 1 TTes Qc
(19)
4. Results and discussion A comparative thermodynamic performance analysis of typical SAH with and without sensible heat storage (viz. therminol-55 synthetic oil) has been conducted at Madurai city in India. Both the SAHs were fabricated with similar dimensions and simultaneously tested in the same location. Experiments were carried out
Fig. 11. Hourly mean energy and exergy variations of Type-II SAH @ m_ ¼ 0.018 kg/s.
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Fig. 12. Hourly mean energy and exergy variations of Type-I SAH @ m_ ¼ 0.026 kg/s.
Fig. 13. Hourly mean energy and exergy variations of Type-II SAH @ m_ ¼ 0.026 kg/s.
found to be increasing from morning, reaching its maximum value of 917 W/m2 at 12:30 IST on May-29, whereas the Fig. 12 shows the peak value of 850 W/m2 was reached at 12:00 IST on May-30. It was also noted that the ambient and inlet air temperature are directly dependent on the solar radiation and exhibit almost the same characteristics. On the first day of the experimental study, both the type-I & II SAHs were operated with m_ ¼ 0.018 kg/s and its performance graph is illustrated in Figs. 10 and 11. From the Fig. 10, it could be noted
that the mean energy efficiency of the type-I SAH started from 19.18% at 10:00 h and reached its first peak value of 20% obtained at 14:00 h. Similarly, the computed mean exergy efficiency was 8.59% at 10:00 h and reached its first peak value of 9.64% around 14:00 h. Besides, both the 1st and 2nd law efficiencies were increased during late evening hours as the solar radiation decreases because of the residue heat energy available in absorber plate being transferred to the air medium. There were fluctuations seen in the efficiency characteristics of type-I SAH during late evening hours. It could be justified as there were no thermal storage available in type-I SAH. On analyzing the type-II SAH, the Fig. 11 shows that the calculated mean energy and exergy efficiencies are higher in values compared to type-I SAH. It was due to the higher absorption rate of the absorber plate of type-II SAH and the integrated sensible thermal storage. The energy efficiency of the type-II SAH was calculated as 44.39% at 10:00 h and operated with its first peak value of 54.28% at 14:00 h whereas, the exergy efficiency was found to be 14.87% at 10:00 h and increased to its first peak value of 18.51% at 14:00 h. A sample calculation of energy and exergy efficiency from various parameters has been provided in Table 2. Both the efficiencies increased steeply with the decrease in solar insolation after 16:00 h and also exhibited smoother characteristics with the help of integrated thermal storage. It supplied a part of stored heat energy to the working medium and the atmosphere in order to attain the thermal equilibrium. Thus, type-II SAH had operated with extended hours compared to type-I SAH. To better understand the thermodynamic characteristics of both the SAHs, another set of tests were carried out on May-30 for a mass-flow rate of 0.026 kg/s. The hourly variations in the mean energy and exergy efficiencies both type-I and type-II systems are shown in the Figs. 12 and 13 respectively. The graph [Fig. 12] depicts the energy efficiency of type-I SAH varied from 21.76% at 10:00 h to its first peak value of 32.07% at 14:00 h. Its exergy efficiency varied from 10.3% to its first maximum of 13.96% at 14:00 h. In case of type-II SAH, Fig. 13 shows the energy efficiency varied from 56.80% at 10:00 h to its first maximum value of 59.02% at 14:00 h, whereas the exergy efficiency varied from 23.41% to its first maximum value of 33.93% at 14:30 h. The statistical analysis of exergy and energy performance of both the SAHs are presented in the Table 3. In all the cases [Figs. 10e13], energy efficiency was always remained higher than the exergy efficiency throughout the operation period irrespective of the SAH types. It could be due to exergy destruction within the system and exergy losses dissipated to the surrounding. As exergy is the quality of energy which cannot be conserved, it accounts for the amount of energy which were lost during the heat exchange process. Furthermore, the change in enthalpy (△h) and entropy (△S) with respect to time were calculated for type-II SAH and presented
Table 2 Type-II SAH e sample calculation for various parameters with respect to time on May-29. Time Solar intensity
Ambient temp
Inlet temp
Outlet temp
Enthapy change
Entropy change
Hours I (W/m2)
Ta (K)
Ti (K)
To (K)
△ h (kJ/kg)
10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
312.02 313.51 315.3 315.35 314.1 313.59 312.45 311.41 310.15
315.62 317.11 318.9 318.95 317.7 317.19 316.05 315.01 313.75
344.19 348.11 352.65 354.58 355.58 349.78 340.1 342.1 339.34
29.3026 31.8654 34.7735 36.7292 39.0541 33.5292 24.6078 27.7426 26.1674
763.74 800.63 841.68 917.59 832.54 769.93 572.43 369.69 108.21
Input energy
Input exergy
Output energy
Output exergy
Energy efficiency
Exergy efficiency
△ S (kJ/kg.K) Qc (kJ)
Ec (kJ)
Qo (kJ)
Eo (kJ)
hII (%)
JII (%)
0.08724 0.09392 0.10133 0.10665 0.11345 0.0985 0.07383 0.08303 0.07893
0.28002 0.35305 0.39458 0.43375 0.36938 0.30889 0.20009 0.10519 0.02587
0.586 0.637 0.695 0.735 0.781 0.671 0.492 0.555 0.523
0.0416 0.0484 0.0565 0.0619 0.0683 0.0528 0.0308 0.0377 0.0337
44.39 46.03 47.78 46.34 54.27 50.42 49.73 86.85 279.68
14.87 13.72 14.32 14.27 18.51 17.10 15.39 35.88 130.54
1.32 1.3838 1.4547 1.586 1.439 1.3307 0.9894 0.639 0.187
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Table 3 Statistical analysis of performance of solar air heaters during the experiment time frame e 10:00 AM e 04:00 PM IST. Statistical parameters
Type-I SAH
m_ ¼ 0.018 kg/s m_ ¼ 0.026 kg/s
Type-II SAH
m_ ¼ 0.018 kg/s m_ ¼ 0.026 kg/s
N
hI (%) JI (%) hI (%) JI (%) hII (%) JII (%) hII (%) JII (%)
12 12 12 12 12 12 12 12
Mean
19.779 7.952 28.313 13.637 48.738 15.522 55.209 28.683
Standard deviation
2.213 1.385 6.669 4.002 3.265 1.603 3.283 6.007
Fig. 14. Hourly change in enthalpy (△h) and entropy (△S) in type-II SAH @ m_ ¼ 0.018 kg/s.
for two different mass-flow rates in the graphs, Figs. 14 and 15. The study of △h and △S is used to understand the amount of heat energy evolved and the amount of energy lost due to irreversibility at each stage of the operation, respectively. In case of SAH, △h is computed based on specific heat capacity and temperature of the air at inlet and outlet, whereas △S is derived based on the temperature and pressure changes at inlet and outlet of the air duct. The value of △h for type-II SAH is varied from 22.73 to 39.05.48 kJ/ kg for m_ ¼ 0.018 kg/s, and 17.03e29.74 kJ/kg for m_ ¼ 0.026 kg/s,
Fig. 15. Hourly change in enthalpy (△h) and entropy (△S) in type-II SAH @ m_ ¼ 0.026 kg/s.
Standard error
0.639 0.4 1.925 1.155 0.943 0.463 0.948 1.734
95% confidence interval for mean Lower bound
Upper bound
18.501 7.153 24.463 11.326 46.852 14.596 53.314 25.215
21.057 8.752 32.164 15.947 50.623 16.447 57.105 32.152
Min
Max
15.95 5.599 12.58 6.2 44.397 13.338 49.444 22.254
24.689 10.087 37.087 21.791 54.281 18.505 59.863 40.379
during the experimental time interval of 10:00 h to 16:00 h. Similarly, △S of the type-II SAH varied from 0.087 to 0.113 kJ/kg.K for m_ ¼ 0.018 kg/s and 0.067e0.081 kJ/kg.K for m_ ¼ 0.026 kg/s. From the results, △h & △S showed reverse relation with the mass flow rate as the optimal higher m_ decreases the energy loss due to irreversibility. Figs. 16 and 17 depict the exergy efficiency of type-I and type-II SAHs for m_ ¼ 0.018 kg/s & 0.026 kg/s, respectively. For m_ ¼ 0.018 kg/s, the average exergy efficiency of type-I & II SAHs were derived as 8.3% and 15.51%, respectively, taken the time interval of 10:00e16:00 h. On increasing the m_ ¼ 0.026 kg/s, the exergy efficiency of type-I & II SAHs were found as 15.03% and 31.02%, respectively. It was observed that type-II SAH with thermal storage unit showed better exergy efficiency even during solar fluctuations compared to the conventional type-I flat plate SAH. It may be due to the presence of integrated SHS unit, which helped in increasing the thermal capacity of the absorber plate, thereby improved the output of type-II SAH. Moreover, the type-II SAH showed smoother efficiency characteristics compared to type-I. Exergy efficiency and irreversibility of type-I and type-II SAHs for the experimentation duration of 10:00 AM e 04:00 PM IST has been computed for two different mass flow rates and the results are tabulated in Table 4. It can be seen from the table that the exergy efficiency of type-II SAH (JII ¼ 13.34e18.51% @ m_ ¼ 0.018 kg/s & JII ¼ 18.25e37.53% @ m_ ¼ 0.026 kg/s) is higher than type-I (JI ¼ 5.6e12.54% @ m_ ¼ 0.018 kg/s & JI ¼ 6.2e21.79% @ m_ ¼ 0.026 kg/s) for both the mass flow rates. Similarly, the increase in the mass flow rate also increases the exergy efficiency of the corresponding solar air heaters. It could be justified as the higher
Fig. 16. Comparison of exergy efficiency between type-I & type-II system @ m_ ¼ 0.018 kg/s.
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Fig. 17. Comparison of exergy efficiency between type-I & type-II system @ m_ ¼ 0.026 kg/s.
until the thermal equilibrium is reached. It may be due to the fact that there is residual heat available in the integrated absorber-cum-thermal storage. Besides, the integrated thermal storage supports the SAH to operate for extended hours than conventional SAH. ii. For all the cases, exergy efficiency is found to be much lesser compared to energy efficiency irrespective of the types of SAH and mass-flow rate. It is the consequence of the SAH subjected to several thermodynamic processes, which involves in exergy loss and exergy destruction (irreversibility). Exergy efficiency provides more realistic output of a system by accounting the quality of energy at each stage of the process. iii. The exergy characteristics are found to be smoother than energy, which is due to the fact that it is less sensitive to the solar radiation fluctuations. iv. The performance of corresponding SAHs increases with the increase in the mass-flow rate. It could be justified as the higher value of mass-flow rate helps in transferring more
Table 4 Exergy analysis of type-I and type-II SAHs for the experiment time frame e 10:00 AM e 04:00 PM IST. SAH type
Mass flow rate kg/S
Type-I
0.018 0.026 Type-II 0.018 0.026
Exergy input Exergy output
Enthapy change
Entropy change
Irreversibility Dimensionless exergy Improvement loss potential
kJ
kJ
kJ/kg
kJ/kgK
KJ
0.088e0.185 0.054e0.165 0.154e0.434 0.052e0.219
0.009e0.015 10.349e15.482 0.032e0.047 0.078e0.171 0.367e0.771 0.005e0.022 5.809e16.796 0.018e0.051 0.048e0.144 0.11e0.607 0.027e0.068 22.731e39.054 0.069e0.113 0.128e0.372 0.281e0.506 0.026e0.052 17.03e29.742 0.052e0.089 0.026e0.167 0.059e0.217
mass flow rate always aids in the improved exchange and transfer of heat energy from the absorber unit, thereby reduces exergy loss considerably. From the results, the type-II SAH operated with air mass flow rate of 0.026 kg/s was found to be performed better with a maximum exergy efficiency of 37.53%. It is evident that the efficiency of the system is indirectly proportional to the irreversibility and exergy loss. The dimensionless exergy loss of a SAH could be defined as the ratio of the amount of _ exergy destructed (Ex dest ) during the process to the energy gain (Qc) of the respective system [36]. For the type-I SAH, the dimensionless exergy loss is computed as 0.367e0.771% for 0.018 kg/s and 0.11e0.607% for 0.026 kg/s whereas the loss of type-II system has been calculated as 0.281e0.506% for 0.018 kg/s and 0.059e0.217% for 0.026 kg/s. The lowest value of exergy loss (0.059e0.217%) was found for the type-II system operated with a mass flow rate of 0.026 kg/s.
e
Exergetic efficiency (minmax)
KJ
%
0.069e0.16 0.062e0.318 0.106e0.319 0.013e0.127
5.599e10.087 6.2e21.791 13.338e18.505 22.254e40.379
thermal output and thereby reduces thermal loss. Though the higher mass flow rate improves the performance, it is vital to select an optimal value of the flow rate to obtain an useful outlet air temperature. v. For both the mass flow rates, the energy and exergy efficiencies of the type-II SAH were higher compared to the type-I SAH. The maximum energy and exergy efficiencies of the type-I (conventional) SAH observed are 32.07% and 19.79% respectively for m_ ¼ 0.026 kg/s. Similarly, the recorded maximum energy and exergy efficiencies of type-II SAH for the same mass flow rate are 59.02% and 37.53% respectively. Thus, both energy and exergy analysis of the novel SAH revealed that it is performing with higher efficiency than the conventional flat plate SAH. Besides, it will provide useful information for thermal applications using integrated thermal storage in solar air heating system.
5. Conclusion A comparative experimental investigation of SAH with and without a special designed integrated absorber-cum-sensible heat storage unit has been conducted at Madurai city in India. Their performance were analyzed based on both energy and exergy efficiencies. Both the SAHs were tested from 10:00e18:00 h IST for two different mass flow rates, m_ ¼ 0.018 kg/s & 0.026 kg/s. The interesting facts and findings from the comparison study could be summarized as follows. i. Energy and exergy efficiencies of SAH with thermal storage (type-II) are gradually increasing with solar radiation till 14:00 h IST. After 14:00 h IST, the solar radiation starts to drop, but both the efficiencies are still found to be increasing
Acknowledgement The authors wish to thank Ministry of New and Renewable Energy (MNRE), India for their financial support to conduct this research. References [1] TERI. Energy & environment data directory and yearbook (TEDDY) 2013/14. 28th ed. New Delhi: The Energy and Resources Institute; 2014. [2] Ministry of new and renewable energy reports. URL http://mnre.gov.in/ information/reports-3/. [3] Garg P. Energy scenario and vision 2020 in india. J Sustain Energy & Environ 2012;3:7e17. [4] Ghosh D, Shukla P, Garg A, Ramana PV. Renewable energy technologies for the
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Experimental energy and exergy analysis of a flat plate solar air heater with a new design of integrated sensible heat storage G. Kalaiarasi a, b, *, R. Velraj a, Muthusamy V. Swami b a b
Institute for Energy Studies, Department of Mechanical Engineering, Anna University, Chennai 600025, India Building Research Division, Florida Solar Energy Center, University of Central Florida, Cocoa, FL 32922, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 January 2016 Received in revised form 4 May 2016 Accepted 27 May 2016
This paper presents an experimental energy and exergy analysis of a novel flat plate solar air heater (SAH). It has a specially designed absorber plate made up of copper strips (copper tubes with extended copper fins on both sides), welded longitudinal to one another. This structure acts as an integrated absorber-cum-storage unit, where a high quality synthetic oil (Therminol-55) is filled within those copper tubes as a sensible heat storage (SHS) medium. To study the impact of this novel design and the sensible heat storage over the performance of the SAH, the results were compared with the output of a conventional SAH of similar dimensions. For the precise comparison of their performances, the experiments were conducted on both the SAHs at same location, simultaneously. It ensures identical testing conditions such as the amount of solar radiation received and surrounding environment of the experimental setup. Exergy analysis is a powerful thermodynamic tool and it helps in computing the actual output of a system, theoretically. It helps the researchers to optimize the system design to compensate the present and also the future needs. Experiments were conducted for two different mass flow rates (0.018 kg/s, and 0.026 kg/s). The results showed that the maximum energy and exergy efficiency obtained was in the range of 49.4e59.2% and 18.25e37.53% respectively, for the SAH with sensible storage at m_ ¼ 0.026 kg/s. Besides, the SAH with sensible heat storage was observed to perform better than the conventional flat plate SAH without storage. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Flat plate solar air heater Integrated sensible heat storage Exergy analysis Energy analysis
1. Introduction In recent years, India has been making several impressive progress in various fields, such as agriculture, transport, communication and industrial sectors, thereby striding towards an economical self-reliance among other developing nations [1]. The advancement in all these fields has lead to an ever growing demand for energy. To succeed in its planned economic growth, India has to derive adequate energy supply from various sources [1,2]. Though India has a sufficient supply of fossil fuels at present, they are continually being depleted. It will not sustain for a long duration [3,4]. Thus, the need for energy alternatives become inevitable. Various researches are in progress to effectively utilize different renewable energy resources for this purpose. Among them, solar energy is considered as a suitable option because of its value added
* Corresponding author. Institute for Energy Studies, Department of Mechanical Engineering, Anna University, Chennai 600025, India. E-mail address: [email protected] (G. Kalaiarasi). http://dx.doi.org/10.1016/j.energy.2016.05.110 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
benefits, such as eco-friendly, economic, easy installation and so on. Though solar energy is available only during day time, it could still be utilized as an effective energy resource, provided with an efficient solar collector design and storage unit [2,5]. The common methods of solar energy harvesting are converting the solar radiation into thermal energy with a help of solar heating system. A typical solar air heating system consists of a collector frame, absorber plate, top glazing, Insulation layer and support structures. Its performance is based on various factors such as collector material, design, surface area, tilt angle, mass flow rate of the heat transfer fluid, environmental conditions and so on [4e6]. Their optimal design is essential to achieve an efficient and economical operation of various applications. One of the precise methods to evaluate the performance of a solar air collector is by conducting the 1st and 2nd law of thermodynamic analysis, viz. energy and exergy analysis. Both the analyses are valuable tools to study the thermal performance of a system. Energy could be represented as the sum of exergy and anergy, where anergy is the amount of energy spent on the system to obtain the useful exergy. Analyzing a system based on exergy
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Nomenclature h S _ Ex P U Q M m_ I_ Q_ c I C cp A T t1,t2 W Z1 ,..,Zn R
enthalpy (kJ kg1) entropy (kJ kg1 K1) exergy rate (kW) fluid pressure (Pa) heat loss coefficient (W m2 C1) incident solar energy (kJ) mass (kg) mass flow rate of air (kg S1) rate of irreversibility (kW) solar energy absorbed by collector surface (kW) solar radiation (W m2) specific heat capacity (kJ kg1 K1) specific heat capacity of air at constant pressure (kJ kg1 K1) surface area (m2) temperature (K) time interval total uncertainty in the measurements (%) uncertainty factors universal gas constant (kJ kg1 K1)
accounts for the irreversibility in a system during the energy conversion process [7,8]. It is a tool for assessing the efficient usage of solar energy, which gives the maximum power that can be extracted from a system [9]. Besides, it helps in quantifying the collection of solar energy and the amount of energy exchanged to the heat transfer fluid [5]. Both energy and exergy efficiencies could be improved by changing the design and operation of SAH, such as the collector surface area, mass flow rate and so on [6,7]. For example, the higher collector surface area could lead to increased exergy efficiency, but it will also increase the capital cost of the solar air heater. Thus, an optimal collector area should be chosen. Similarly, the higher mass flow rate not only improves the exergy efficiency, but it also decreases the outlet temperature. Besides, higher blower power input is needed for higher mass flow rate. So, an optimal mass flow rate should also be chosen for economic and efficient operation [10e15]. Saidur et al. [10] reviewed the energy and exergy analysis of various solar applications and conveyed that the 1st law efficiency is alone not sufficient to select a desired system, but the exergy (2nd law) efficiency is also important. Exergy analysis determines the source and magnitude of irreversibility in detail and plays a vital role in understanding the system performance. The study shows exergy destruction is higher in solar heating and air conditioning systems among various solar applications. Akpinar et al. [11] experimentally investigated the energy and exergy performance of a SAH with and without different types of obstacles (typeI to IV). The study showed that the 1st law efficiency varied between 20% and 82%, whereas, the 2nd law efficiency varied from 8.32% to 44%. Among the case studies, the SAH with leaf shaped obstacles yielded the maximum output compared to the SAH without any obstacle. Besides, the results showed that the thermal efficiency is significantly dependent on solar irradiation, surface geometry, area of the air channel and air mass flow rate. TorresReyes et al. [12,13] presented a paper on optimizing the performance of a flat-plate solar air heater by minimizing the entropy generation. A preliminary design of an efficient solar air heater was made based on the entropy generation number, mass flow number and stagnate air temperature, which are subjected to a
Abbrevaions SAH solar air heater SHS sensible heat storage IST Indian standard time Greek Symbols absorptance of absorber plate transmittance of glass cover inclination angle of solar collector (degree) first law/Energy efficiency second law/Exergy efficiency
a t q h J
Subscripts c collector pl absorber plate avg average con convection e environment out output in input s surface
€ thermodynamic optimization procedure. Oztürk et al. [14] conducted an experimental investigation of the thermal performance of a SAH with packed bed Raschig rings. From the results, the net energy efficiency ranges from 2.05 to 33.78% whereas, the net € exergy efficiency ranges from 0.01 to 2.16%. Moreover, Oztürk et al. [15] experimentally analyzed the energy and exergy efficiencies of SAH with paraffin wax as storage for greenhouse heating. The mean daily rate of thermal exergy transferred and stored in latent heat storage was found to be 111.2 W and 79.9 W, respectively. The average net energy and exergy efficiency were computed as 40.4% and 4.2% respectively. Esen et al. [16] conducted an experimental investigation on the energy and exergy efficiencies of double-flow SAHs with several obstacles and without obstacle. It was observed that obstacles in the air duct of the double-flow collector is an efficient method of adapting air exchange according to the user demands. Alta et al. [17] analyzed the energy and exergy performance of SAHs with and without fins. Among them, the double glass finned type heater found to be operating with higher exergy efficiency compared to other types. Sami et al. [18] presented the microscopic exergy and energy analysis on the dynamic mathematic model of an indirect cabinet solar dryer. The analysis reveals that the indirect solar cabinet has a relatively lower exergy efficiency compared to energy efficiency and it was also found that the exergy losses were maximum during the mid-day. Impacts of various factors like collector surface, its length and mass flow rate over the exergy destruction and efficiency were investigated. Tyagi et al. [19] reviewed the solar heating system with and without thermal storage. To overcome the limitations of solar energy such as its time dependent and intermittent nature, this paper focused on phase changed material (PCM) based thermal energy storage to improve the performance. It was found that the latent heat storage is more efficient compared to the sensible heat storage. Saxena et al. [20] reviewed the various ways to improve the thermal performance of SAHs such as dimensions of the air heater components, different types of obstacles, usage of latent and sensible heat storage material, usage of concentrators to multiply the available insolation and so on. Bahrehmand et al. [21] performed
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the energy and exergy analysis on the mathematical model of single and two glass cover SAHs. The performance impact due to the suspension of a tin metal sheet in the middle of a airflow channel, longitudinal fins with rectangular or triangular shapes, change in the length and depth of the air channel was investigated. It was found that the double glass SAH is analytically performed better compared to single-glass collector. In addition to that, the SAH with triangular fins are determined to be more energy efficient compared to rectangular fins. Bouadila et al. [22,23] proposed a new SAH with a packed-bed latent heat storage using PCM spherical capsules. The proposed model was experimented at Tunisia climatic conditions and the impact of global irradiation and massflow rate over the absorbed, used and recovered heat from the storage system was investigated. The results showed that the daily energy and exergy efficiency ranges are 32e45% and 13e25% respectively. Bayraka et al. [24], performed exergy and energy analysis on SAH with closed cell aluminium foam as porous material placed sequentially in a staggered manner. Five different types of SAH with various mass-flow rates were analyzed. The result showed that SAH with aluminium foam of 6 mm thickness and air mass flow rate of 0.025 kg/s was yielded higher collector efficiency and temperature output compared to SAH with nonbaffle collectors operated for an air mass flow rate of 0.016 kg/s. Dehghan et al. [25] developed a model of a square and circular solar pond present in Iran. With the solar radiation data collected, the energy and exergy performance of both the ponds at different zones were investigated. From the study, it was demonstrated that the circular pond is efficient compared to the square ponds due to the shading effects. The maximum energy efficiency for nonconvective zone (NCZ) and heat storing zone (HSZ) are 17.25% and 25.8% respectively, whereas 17.39% and 23.65% respectively for the square pond. Similarly, the maximum exergy efficiency for NCZ and HSZ in the circular pond are 0.86% and 2.4%, whereas 0.86% and 2.44% for the square pond. Ranjan et al. [26] reviewed the ongoing research activities in the field of solar distillation system aiming to improve the efficiency using an effective thermodynamic tool, i.e. energy and exergy analysis. Thermodynamic model based on fundamental heat transfer correlations was presented for the simple basin type solar stills to conduct energy and exergy analysis. The energy efficiency of conventional solar still is found in the range of 20e46%. The exergy efficiencies are estimated to be between 19% and 26% for a triple effect system, 17e20% for a double effect and less than 5% for a single effect system. Alta et al. [27] presented a theoretical and experimental analysis of back-pass solar air heaters. It demonstrated the superiority of exergy analysis over energy analysis in the decision of design parameters. The theoretical analysis was performed by running the energy and exergy equations using FORTRAN codes and were compared with the experimental results. Alta et al. [28] studied the energetic efficiency of SAH with aluminium flat plate absorber with different air mass flow rates. experimental results were comparatively analyzed with the results obtained from computational fluid dynamics software and evaluated based on thermographic camera images. It showed that the increasing inlet temperature and lower air flow rates could yield to increase in outlet air temperature. Li et at [29]. investigated the various influencing factors, such as the heat transfer fluid (HTF) mass flow rate and inlet temperature, phase change material (PCM) melting temperature and number, additives for PCMs, storage unit dimension, heat exchanger surface enhancement, and sensible heating and sub-cooling, etc. He presented the exergetic performance assessment of latent heat thermal energy storage systems. Singh et al. [30] presented an exergoeconomic analysis of the double pass packed bed solar air heater theoretically and experimentally. It was observed that the energy efficiency of the heater increases with the increasing mass
611
flow rate and recycle ratio, whereas the exergy efficiency increases with an increase in the mass flow rate and the recycle ratio up to a limit and subsequently decreases with further increase in the mass flow rate and the recycle ratio. Hedayatizadeh et al. [31] simulated an exergetic analysis on a double-pass/glazed V-corrugated plate solar air heater. Its maximum exergy efficiency was derived to be 6.27%. Another important conclusion of his study was that the internal exergy loss due to the temperature difference between sun and absorber surface is the most destructive and it is accounted for 63.57% of total exergy loss. Kant et al. [32] reviewed solar drying systems based on thermal energy storage. It summarizes various latent and sensible heat storage materials that can be used for solar drying systems. The thermal storage material reduces the existing load on the gap between energy demand and supply. From the literature survey, it has been found that several researches are focused on thermodynamic analysis on various models of solar air heaters. However, still there is a wide literature gap remained in the field of SAH with integrated storage facility subjected into exergy analysis in different climatic zones. Most of the studies on renewable energy systems are conducted based on energy analysis rather than exergy analysis [33]. So, this study concentrates on detailed exergy analysis of a novel SAH with specially designed absorber unit where synthetic oil is used as temporary heat storage. The results were analyzed for the improvement in exergy efficiency and output air temperature compared to a conventional SAH. Both the experimental setups of novel and conventional SAHs defined in the following section were fabricated and tested. 2. Experimental setup Experiments were conducted on two different types of SAHs, i.e. conventional SAH (type-I) and novel SAH with integrated SHS (type-II). Both the SAHs were constructed and tested at the Anayur locality of Madurai city in India. Its latitude and longitude are 9.9 N and 78.1 E, respectively. The Figs. 1 and 2 show the monthly variations of wind speed and solar radiation in Madurai obtained from RETScreen plus-NASA international climatic database. Fig. 2 shows the annual daily average solar radiation recorded at Madurai as 5.1 kWh/m2/d, with a maximum value of 6.25 kWh/m2/d during the summer month of March and a minimum of 4 kWh/m2/d during the winter month of November [34]. The total incoming solar radiation has been estimated as 1.861 MWh/m2/year. The above mentioned uncertainty parameters, such as solar radiation and wind velocity have direct impact over the thermodynamic performance of a SAH. It is essential to design the SAH in such a way to tolerate their varying ranges to achieve the optimal performance with minimal manufacturing cost. The type-I SAH was designed without any heat storage, whereas the type-II SAH was designed with an absorber and integrated storage unit. The absorber unit of the type-II SAH was composed of a set of copper tubes with extended fins, which were welded longitudinally. The inner and outer diameter of the copper tubes used were 10 mm and 10.8 mm, with a length of 1.8 m. Nine such copper strips were welded to form the absorber unit. A high quality synthetic oil (Therminol-55) of 2.54 L was filled in those copper tubes, which act as a temporary heat storage medium. The cross sectional view of both the SAHs are shown in Figs. 3 and 4. Fig. 5 shows the schematic diagram of type-II SAH with all the measurements. Both the SAHs were made up of 12 mm thick plywood and fabricated for same dimensions, viz. 2 m 1 m 0.180 m. In order to suppress the conductive heat losses at the bottom and lateral sides of the collector, 50 mm thick rock wool layer had been filled. To reduce the convective heat losses and reflections of solar insolation, a 4 mm thick toughened clear glass was fitted over the top of
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Fig. 1. Madurai Climatic data graph: Monthly wind speed variation.
Fig. 2. Madurai Climatic data graph: Monthly solar radiation variation.
Fig. 3. Cross sectional view of type-I SAH.
the absorber plate at a distance of 30 mm. The photographs of the absorber plate showing copper tubes and the entire experimental setup are shown in the Figs. 6e9. To produce the air turbulence in the duct, 0.5 HP blower was used. The setups were mounted properly in order to prevent any shadows or reflected radiations to fall on it. The tests were conducted during the month of May, from 10 AM till evening 6 PM Indian standard time (IST). K-type thermocouples were used to
measure the temperature at various state points, including the inlet, outlet, absorber plate, top glass, bottom sheet and atmospheric temperature. The air velocity was measured using the anemometer, Testo 425. The solar insolation received at the experiment location was measured with a help of SP-110 model apogee pyranometer. For an interval of every 15 min, the outputs from all these sensors were stored in a computer with the help of 34972A Agilent data logger. The readings from the experiments
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Fig. 4. Cross sectional view of type-II SAH.
Fig. 5. Schematic view of the type-II SAH.
Fig. 6. Integrated absorber-cum-SHS of type-II SAH.
were measured and recorded with the help of suitable instruments and their uncertainties are listed in Table 1. Denoting the relative uncertainties in the individual factors as Zn, the total uncertainty could be represented as in Eq. (1).
Fig. 7. Experimental setup of type-I and type-II SAH.
i1 h 2 W ¼ ðZ1 Þ2 þ ðZ2 Þ2 þ ……ðZn Þ2
(1)
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3.1. Energy analysis Energy analysis is a method of thermodynamics to understand the thermal performance based on the 1st law of heat balance equations, which could be expressed as Eq. (2).
Energy absorbed ¼ Energy accumulated þ Energy lost þ Energy output=gain
(2)
The absorbed energy could also be represented as the fraction of solar radiation absorbed at the tilted collector surface [Eq. (3)]. It is mainly impacted by the optical parameters such as absorptance of the plate (a), transmittance of the glass cover (t) and the inclination angle (q) of the collector surface [16].
Energy absorbed; QC ¼ atAC I cosq
Fig. 8. Construction of type-II SAH (Integrated copper tube for heat storage).
(3)
The specific heat capacity of the absorber plate allows it to store a considerable amount of energy, which is represented in Eq. (4)
Energy accumulated ¼ Mpl Cpl
dTpl;avg dt
(4)
The amount of energy lost during the process of SAH while generating the useful thermal output is presented in the Eq. (5). The overall loss coefficient, Uc includes the losses at the top, bottom and lateral sides of the SAH [35,36].
Energy lost ¼ Ucon AC TP;avg Te
(5)
The useful thermal output from the SAH is represented in Eq. (6).
_ p ðTout Tin Þ Energy gain ¼ mc
(6)
The energy efficiency of a SAH is defined as the ratio of the useful thermal gain of the working fluid to the total incident solar insolation on collector area averaged over the same interval of time period which is expressed by the Eq. (7).
Fig. 9. Cross sectional view of type-II SAH.
h¼
Thermodynamic analysis is a powerful tool which helps in understanding the energy influence on the performance of a heating system. It could be carried out widely based on both 1st and 2nd law of thermodynamics, viz. energy and exergy analysis. This paper investigates the performance of type-I and type-II SAHs using both the laws of thermodynamics and compared their results in detail.
Table 1 Uncertainty in measurements. Measurement parameters
Error limits
Uncertainty in the measurement of temperature Ambient air temperature Absorber surface temperature Collector inlet temperature Collector outlet temperature
±1 ±1 ±1 ±1
Uncertainty Uncertainty Uncertainty Uncertainty Uncertainty
±0.03 psi ±0.03 m/s ±5% ±1 min ±1e2%
the measurement of pressure loss the air velocity measurement the measurement of solar energy time measurement of temperature values reading values of table
Z
t2
ððTout Tin ÞdtÞ Z t2 atAC cosq Idt t1
(7)
t1
3. Thermodynamic analysis
in in in in in
_ p mc
C C C C
3.2. Exergy analysis Exergy Analysis or 2nd law of thermodynamics deals with the irreversibility of energy at each stage of operation in SAH. In other words, exergy losses are irreversible, which cannot be utilized or transferred to the working medium. It can be occurred when the system undergoes a process involving heat exchange, changes in pressure and chemical reaction. Thus, it helps in quantifying the quality of the energy exchanged at each level in the system and provides a more realistic useful output of the system. Therefore, it could help in the design optimization of SAH for operating at better efficiency. Exergy analysis is based on the below assumptions [7,16,35,36]. i. It is a steady state and steady flow process. ii. Kinetic and potential energy are negligible. iii. The directions of heat transfer to the system and the work transfer from the system are positive. iv. Air is considered as an ideal gas, so specific heat is constant and its humidity content is negligible.
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v. Exergy balance of a SAH under a steady state, steady flow operation is given in Eq. (2).
_ _ _ _ I_ ¼ Ex heat Exwork þ Exin Exout
(8)
Where,
_ Ex heat ¼ _ Ex in ¼
X Te Q 1 Ts C
X
_ out ¼ Ex
(9)
m_ in ½ðhin he Þ Te ðSin Se Þ
X
m_ out ½ðhout he Þ Te ðSout Se Þ
(10) (11)
Based on the above assumptions and mass balance we have,
_ Ex work ¼ 0 X
m_ in ¼
(12)
X
m_ out ¼
X
m_ air
Fig. 10. Hourly mean energy and exergy variations of Type-I SAH @ m_ ¼ 0.018 kg/s.
(13)
On substitution of Eqs. (9)e(11) in Eq. (8),
Te _ Q m_ air ½ðhout hin Þ Te ðSout Sin Þ I_ ¼ 1 Ts C
(14)
Where, Q_ c is the total rate of exergy received by the collector absorber area from solar radiation [Eq. (15)].
Q_ c ¼ atAc I cosq
(15)
The change in enthalpy and entropy is given in Eq. (16) & Eq. (17)
hout hin ¼ Cp;out Tout Cp;in Tin Sout Sin ¼ Cp ln
Tout Pout Rln Tin Pin
(16)
(17)
After substitution of Eqs. (15)e(17) in Eq. (14), the final form of irreversible rate of solar collector will be [16,36],
Te _ Tout Q C m_ air Cp;out Tout Cp;in Tin þ m_ air Te Cp ln I_ ¼ 1 Ts Tin Pout m_ a Te Rln Pin
during the sunshine hours, from 10:00 AM to 06:00 PM IST. The SAHs were inclined at an angle of 13 , facing south side according to the geographical location of Madurai. As per the RETScreen plusNASA international climatic database, the monthly and total annual average wind speed in Madurai city is shown in the Fig. 1 [34]. From the graph, its annual average wind speed was observed to be 3.2 m/s. Similarly, Fig. 2 shows the monthly and annual daily average solar radiation received at Madurai, which was recorded as 5.1 kWh/m2/d. Using K-type thermocouples all the temperatures, such as the inlet, outlet air temperature, ambient temperature, top and bottom of collector surface temperatures were measured at different state points. The data were recorded for every 15 min using a data logger and stored in a personal computer. Tests were conducted for two mass-flow rates, 0.018 kg/s and 0.026 kg/s on two consecutive summer days, May-29 & 30, 2015 respectively. The useful heat output and exergy & energy efficiencies of both the SAHs were derived from the data obtained. Figs. 10e13 show the hourly variations in the solar radiation, energy and exergy efficiencies of both the solar air heaters tested for different mass-flow rates, viz. 0.018 kg/s and 0.026 kg/s. The insolation received at the collector surface showed a parabolic characteristic with its peak value attained during the mid-day and latter it decreases as the time increases. From Fig. 10, the irradiation
(18) The idea of increasing the exergy and decreasing the irreversibility is by improving the exergy efficiency (J). It is defined as the ratio of exergy gain of the system to the exergy of absorbed solar radiation [Eq. (19)].
_ Ex
J ¼ _ out ¼ Exin
m_ air ð△h Te △SÞ 1 TTes Qc
(19)
4. Results and discussion A comparative thermodynamic performance analysis of typical SAH with and without sensible heat storage (viz. therminol-55 synthetic oil) has been conducted at Madurai city in India. Both the SAHs were fabricated with similar dimensions and simultaneously tested in the same location. Experiments were carried out
Fig. 11. Hourly mean energy and exergy variations of Type-II SAH @ m_ ¼ 0.018 kg/s.
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Fig. 12. Hourly mean energy and exergy variations of Type-I SAH @ m_ ¼ 0.026 kg/s.
Fig. 13. Hourly mean energy and exergy variations of Type-II SAH @ m_ ¼ 0.026 kg/s.
found to be increasing from morning, reaching its maximum value of 917 W/m2 at 12:30 IST on May-29, whereas the Fig. 12 shows the peak value of 850 W/m2 was reached at 12:00 IST on May-30. It was also noted that the ambient and inlet air temperature are directly dependent on the solar radiation and exhibit almost the same characteristics. On the first day of the experimental study, both the type-I & II SAHs were operated with m_ ¼ 0.018 kg/s and its performance graph is illustrated in Figs. 10 and 11. From the Fig. 10, it could be noted
that the mean energy efficiency of the type-I SAH started from 19.18% at 10:00 h and reached its first peak value of 20% obtained at 14:00 h. Similarly, the computed mean exergy efficiency was 8.59% at 10:00 h and reached its first peak value of 9.64% around 14:00 h. Besides, both the 1st and 2nd law efficiencies were increased during late evening hours as the solar radiation decreases because of the residue heat energy available in absorber plate being transferred to the air medium. There were fluctuations seen in the efficiency characteristics of type-I SAH during late evening hours. It could be justified as there were no thermal storage available in type-I SAH. On analyzing the type-II SAH, the Fig. 11 shows that the calculated mean energy and exergy efficiencies are higher in values compared to type-I SAH. It was due to the higher absorption rate of the absorber plate of type-II SAH and the integrated sensible thermal storage. The energy efficiency of the type-II SAH was calculated as 44.39% at 10:00 h and operated with its first peak value of 54.28% at 14:00 h whereas, the exergy efficiency was found to be 14.87% at 10:00 h and increased to its first peak value of 18.51% at 14:00 h. A sample calculation of energy and exergy efficiency from various parameters has been provided in Table 2. Both the efficiencies increased steeply with the decrease in solar insolation after 16:00 h and also exhibited smoother characteristics with the help of integrated thermal storage. It supplied a part of stored heat energy to the working medium and the atmosphere in order to attain the thermal equilibrium. Thus, type-II SAH had operated with extended hours compared to type-I SAH. To better understand the thermodynamic characteristics of both the SAHs, another set of tests were carried out on May-30 for a mass-flow rate of 0.026 kg/s. The hourly variations in the mean energy and exergy efficiencies both type-I and type-II systems are shown in the Figs. 12 and 13 respectively. The graph [Fig. 12] depicts the energy efficiency of type-I SAH varied from 21.76% at 10:00 h to its first peak value of 32.07% at 14:00 h. Its exergy efficiency varied from 10.3% to its first maximum of 13.96% at 14:00 h. In case of type-II SAH, Fig. 13 shows the energy efficiency varied from 56.80% at 10:00 h to its first maximum value of 59.02% at 14:00 h, whereas the exergy efficiency varied from 23.41% to its first maximum value of 33.93% at 14:30 h. The statistical analysis of exergy and energy performance of both the SAHs are presented in the Table 3. In all the cases [Figs. 10e13], energy efficiency was always remained higher than the exergy efficiency throughout the operation period irrespective of the SAH types. It could be due to exergy destruction within the system and exergy losses dissipated to the surrounding. As exergy is the quality of energy which cannot be conserved, it accounts for the amount of energy which were lost during the heat exchange process. Furthermore, the change in enthalpy (△h) and entropy (△S) with respect to time were calculated for type-II SAH and presented
Table 2 Type-II SAH e sample calculation for various parameters with respect to time on May-29. Time Solar intensity
Ambient temp
Inlet temp
Outlet temp
Enthapy change
Entropy change
Hours I (W/m2)
Ta (K)
Ti (K)
To (K)
△ h (kJ/kg)
10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
312.02 313.51 315.3 315.35 314.1 313.59 312.45 311.41 310.15
315.62 317.11 318.9 318.95 317.7 317.19 316.05 315.01 313.75
344.19 348.11 352.65 354.58 355.58 349.78 340.1 342.1 339.34
29.3026 31.8654 34.7735 36.7292 39.0541 33.5292 24.6078 27.7426 26.1674
763.74 800.63 841.68 917.59 832.54 769.93 572.43 369.69 108.21
Input energy
Input exergy
Output energy
Output exergy
Energy efficiency
Exergy efficiency
△ S (kJ/kg.K) Qc (kJ)
Ec (kJ)
Qo (kJ)
Eo (kJ)
hII (%)
JII (%)
0.08724 0.09392 0.10133 0.10665 0.11345 0.0985 0.07383 0.08303 0.07893
0.28002 0.35305 0.39458 0.43375 0.36938 0.30889 0.20009 0.10519 0.02587
0.586 0.637 0.695 0.735 0.781 0.671 0.492 0.555 0.523
0.0416 0.0484 0.0565 0.0619 0.0683 0.0528 0.0308 0.0377 0.0337
44.39 46.03 47.78 46.34 54.27 50.42 49.73 86.85 279.68
14.87 13.72 14.32 14.27 18.51 17.10 15.39 35.88 130.54
1.32 1.3838 1.4547 1.586 1.439 1.3307 0.9894 0.639 0.187
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Table 3 Statistical analysis of performance of solar air heaters during the experiment time frame e 10:00 AM e 04:00 PM IST. Statistical parameters
Type-I SAH
m_ ¼ 0.018 kg/s m_ ¼ 0.026 kg/s
Type-II SAH
m_ ¼ 0.018 kg/s m_ ¼ 0.026 kg/s
N
hI (%) JI (%) hI (%) JI (%) hII (%) JII (%) hII (%) JII (%)
12 12 12 12 12 12 12 12
Mean
19.779 7.952 28.313 13.637 48.738 15.522 55.209 28.683
Standard deviation
2.213 1.385 6.669 4.002 3.265 1.603 3.283 6.007
Fig. 14. Hourly change in enthalpy (△h) and entropy (△S) in type-II SAH @ m_ ¼ 0.018 kg/s.
for two different mass-flow rates in the graphs, Figs. 14 and 15. The study of △h and △S is used to understand the amount of heat energy evolved and the amount of energy lost due to irreversibility at each stage of the operation, respectively. In case of SAH, △h is computed based on specific heat capacity and temperature of the air at inlet and outlet, whereas △S is derived based on the temperature and pressure changes at inlet and outlet of the air duct. The value of △h for type-II SAH is varied from 22.73 to 39.05.48 kJ/ kg for m_ ¼ 0.018 kg/s, and 17.03e29.74 kJ/kg for m_ ¼ 0.026 kg/s,
Fig. 15. Hourly change in enthalpy (△h) and entropy (△S) in type-II SAH @ m_ ¼ 0.026 kg/s.
Standard error
0.639 0.4 1.925 1.155 0.943 0.463 0.948 1.734
95% confidence interval for mean Lower bound
Upper bound
18.501 7.153 24.463 11.326 46.852 14.596 53.314 25.215
21.057 8.752 32.164 15.947 50.623 16.447 57.105 32.152
Min
Max
15.95 5.599 12.58 6.2 44.397 13.338 49.444 22.254
24.689 10.087 37.087 21.791 54.281 18.505 59.863 40.379
during the experimental time interval of 10:00 h to 16:00 h. Similarly, △S of the type-II SAH varied from 0.087 to 0.113 kJ/kg.K for m_ ¼ 0.018 kg/s and 0.067e0.081 kJ/kg.K for m_ ¼ 0.026 kg/s. From the results, △h & △S showed reverse relation with the mass flow rate as the optimal higher m_ decreases the energy loss due to irreversibility. Figs. 16 and 17 depict the exergy efficiency of type-I and type-II SAHs for m_ ¼ 0.018 kg/s & 0.026 kg/s, respectively. For m_ ¼ 0.018 kg/s, the average exergy efficiency of type-I & II SAHs were derived as 8.3% and 15.51%, respectively, taken the time interval of 10:00e16:00 h. On increasing the m_ ¼ 0.026 kg/s, the exergy efficiency of type-I & II SAHs were found as 15.03% and 31.02%, respectively. It was observed that type-II SAH with thermal storage unit showed better exergy efficiency even during solar fluctuations compared to the conventional type-I flat plate SAH. It may be due to the presence of integrated SHS unit, which helped in increasing the thermal capacity of the absorber plate, thereby improved the output of type-II SAH. Moreover, the type-II SAH showed smoother efficiency characteristics compared to type-I. Exergy efficiency and irreversibility of type-I and type-II SAHs for the experimentation duration of 10:00 AM e 04:00 PM IST has been computed for two different mass flow rates and the results are tabulated in Table 4. It can be seen from the table that the exergy efficiency of type-II SAH (JII ¼ 13.34e18.51% @ m_ ¼ 0.018 kg/s & JII ¼ 18.25e37.53% @ m_ ¼ 0.026 kg/s) is higher than type-I (JI ¼ 5.6e12.54% @ m_ ¼ 0.018 kg/s & JI ¼ 6.2e21.79% @ m_ ¼ 0.026 kg/s) for both the mass flow rates. Similarly, the increase in the mass flow rate also increases the exergy efficiency of the corresponding solar air heaters. It could be justified as the higher
Fig. 16. Comparison of exergy efficiency between type-I & type-II system @ m_ ¼ 0.018 kg/s.
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Fig. 17. Comparison of exergy efficiency between type-I & type-II system @ m_ ¼ 0.026 kg/s.
until the thermal equilibrium is reached. It may be due to the fact that there is residual heat available in the integrated absorber-cum-thermal storage. Besides, the integrated thermal storage supports the SAH to operate for extended hours than conventional SAH. ii. For all the cases, exergy efficiency is found to be much lesser compared to energy efficiency irrespective of the types of SAH and mass-flow rate. It is the consequence of the SAH subjected to several thermodynamic processes, which involves in exergy loss and exergy destruction (irreversibility). Exergy efficiency provides more realistic output of a system by accounting the quality of energy at each stage of the process. iii. The exergy characteristics are found to be smoother than energy, which is due to the fact that it is less sensitive to the solar radiation fluctuations. iv. The performance of corresponding SAHs increases with the increase in the mass-flow rate. It could be justified as the higher value of mass-flow rate helps in transferring more
Table 4 Exergy analysis of type-I and type-II SAHs for the experiment time frame e 10:00 AM e 04:00 PM IST. SAH type
Mass flow rate kg/S
Type-I
0.018 0.026 Type-II 0.018 0.026
Exergy input Exergy output
Enthapy change
Entropy change
Irreversibility Dimensionless exergy Improvement loss potential
kJ
kJ
kJ/kg
kJ/kgK
KJ
0.088e0.185 0.054e0.165 0.154e0.434 0.052e0.219
0.009e0.015 10.349e15.482 0.032e0.047 0.078e0.171 0.367e0.771 0.005e0.022 5.809e16.796 0.018e0.051 0.048e0.144 0.11e0.607 0.027e0.068 22.731e39.054 0.069e0.113 0.128e0.372 0.281e0.506 0.026e0.052 17.03e29.742 0.052e0.089 0.026e0.167 0.059e0.217
mass flow rate always aids in the improved exchange and transfer of heat energy from the absorber unit, thereby reduces exergy loss considerably. From the results, the type-II SAH operated with air mass flow rate of 0.026 kg/s was found to be performed better with a maximum exergy efficiency of 37.53%. It is evident that the efficiency of the system is indirectly proportional to the irreversibility and exergy loss. The dimensionless exergy loss of a SAH could be defined as the ratio of the amount of _ exergy destructed (Ex dest ) during the process to the energy gain (Qc) of the respective system [36]. For the type-I SAH, the dimensionless exergy loss is computed as 0.367e0.771% for 0.018 kg/s and 0.11e0.607% for 0.026 kg/s whereas the loss of type-II system has been calculated as 0.281e0.506% for 0.018 kg/s and 0.059e0.217% for 0.026 kg/s. The lowest value of exergy loss (0.059e0.217%) was found for the type-II system operated with a mass flow rate of 0.026 kg/s.
e
Exergetic efficiency (minmax)
KJ
%
0.069e0.16 0.062e0.318 0.106e0.319 0.013e0.127
5.599e10.087 6.2e21.791 13.338e18.505 22.254e40.379
thermal output and thereby reduces thermal loss. Though the higher mass flow rate improves the performance, it is vital to select an optimal value of the flow rate to obtain an useful outlet air temperature. v. For both the mass flow rates, the energy and exergy efficiencies of the type-II SAH were higher compared to the type-I SAH. The maximum energy and exergy efficiencies of the type-I (conventional) SAH observed are 32.07% and 19.79% respectively for m_ ¼ 0.026 kg/s. Similarly, the recorded maximum energy and exergy efficiencies of type-II SAH for the same mass flow rate are 59.02% and 37.53% respectively. Thus, both energy and exergy analysis of the novel SAH revealed that it is performing with higher efficiency than the conventional flat plate SAH. Besides, it will provide useful information for thermal applications using integrated thermal storage in solar air heating system.
5. Conclusion A comparative experimental investigation of SAH with and without a special designed integrated absorber-cum-sensible heat storage unit has been conducted at Madurai city in India. Their performance were analyzed based on both energy and exergy efficiencies. Both the SAHs were tested from 10:00e18:00 h IST for two different mass flow rates, m_ ¼ 0.018 kg/s & 0.026 kg/s. The interesting facts and findings from the comparison study could be summarized as follows. i. Energy and exergy efficiencies of SAH with thermal storage (type-II) are gradually increasing with solar radiation till 14:00 h IST. After 14:00 h IST, the solar radiation starts to drop, but both the efficiencies are still found to be increasing
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