Saxena et al., 2020 India Real-time outdoor cooking Solar box type Paraffin wax Carbon powder, mixed with paraffin wax and paraffin wax, are used as three different thermal energy
Trang 1Journal of Cleaner Production 324 (2021) 129223
Available online 2 October 2021
0959-6526/© 2021 Elsevier Ltd All rights reserved
Efficient heat batteries for performance boosting in solar thermal
cooking module
aSchool of Renewable Energy and Smart Grid Technology (SGtech), Naresuan University, Phitsanulok, 65000, Thailand
bDepartment of Materials Science & Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong
cInstitute of Research and Applied Technological Science, Dong Nai Technology University, Dong Nai, 76000, Viet Nam
dDepartment of Academic Affairs and Testing, Dong Nai Technology University, Dong Nai, Viet Nam
eFaculty of Environmental Management, Prince of Songkla University, Hat Yai, 90112, Thailand
fEnvironmental Assessment and Technology for Hazardous Waste Management Research Center, Faculty of Environmental Management, Prince of Songkla University,
Hat Yai, 90112, Thailand
gDepartment of Physics, SCSVMV Deemed University, Kanchipuram, 631561, India
hJames Watt School of Engineering, University of Glasgow, Glasgow, G12 8QQ, United Kingdom
A R T I C L E I N F O
Handling editor: Mingzhou Jin
Keywords:
Salt hydrate
Thermal cycling
Corrosion rate
Heat battery
Solar cooking
A B S T R A C T Heat batteries show outstanding charging and discharging thermal energy capability with the latent heat of fusion (Hm) for solar thermal application In this work, novel magnesium nitrate hexahydrate (MNH) based heat batteries are fabricated and tested for 1000 sequential thermal cycles The MNH heat batteries demonstrate a high level of operational stability with the least corrosive rate Real-time performance of the heat batteries was studied by incorporating them in the parabolic solar thermal cooking module The developed MNH heat batteries based solar cooking module illustrates excellent heat retention capacity over 6 h after the sunshine Temperature profiles under no load and full load conditions reveal the moderation and enhancement in the solar thermal cooking module’s operational efficiencies The solar cooking module’s efficiency with the heat batteries reaches
a maximum of 22.8% and 42.5%, under no load and full load conditions, respectively Real-time cooking ca-pacity with different edible materials under both outdoor and indoor environments proves the effective per-formance Further, it is estimated that MNH heat batteries can be in full performance for a minimum of 2 years with maintenance-free and emission-free operations
1 Introduction
With increasing global energy demand, conventional fossil fuel
sources induce global warming by releasing toxic greenhouse gases into
the atmosphere (Sharif et al., 2021) In this aspect, sustainable and
renewable energy sources for day-to-day activities can benefit both
environmentally and economically (Barba et al., 2019; Zayed et al.,
2021) As reported, the process of heating alone consumes about 35% of
the global electricity generated, whereas the total efficiency of any fossil
fuel-based power plant is limited to a maximum of 25% (Ravi Kumar
et al., 2021; Zhou et al., 2020) These factors lead us in search of the surplus source of energy for heating and cooling applications, wide-spread solar insolation in the atmosphere stands as the only potential source of renewable energy (Liu et al., 2020; Ma et al., 2019) Solar insolation in the atmosphere can be harvested into heat or electrical power by solar conversion technologies As most daily processes require
a medium temperature of around 150 ◦C, solar thermal energy conver-sion can fulfil industrial and household applications (Cardenas et al.,
2017; Huang et al., 2019) Solar thermal cooking stands as a powerful application that avoids atmospheric pollution through biomass and
* Corresponding author
** Corresponding author
E-mail addresses: sukruedeen@nu.ac.th (S Sukchai), karthi230407@gmail.com (K Velmurugan)
1 Authors contributed equally
Contents lists available at ScienceDirect Journal of Cleaner Production
journal homepage: www.elsevier.com/locate/jclepro
https://doi.org/10.1016/j.jclepro.2021.129223
Received 3 April 2021; Received in revised form 16 September 2021; Accepted 30 September 2021
Trang 2natural gas combustion (Mawire et al., 2020; Singh, 2021) (
Hosseinza-deh et al., 2021) On the other hand, the solar cooking method maintains
the nutrients level in the cooking, especially as compared to the
con-ventional cooking method, following that taste and aroma are also
maintained at a high degree level (Saxena et al., 2018) Various
tech-nologies in solar thermal energy harvesting for cooking applications
have been developed in recent years, like parabolic concentrator
mod-ule, box-type modmod-ule, pressurized steam generator module and others
(Naveen et al., 2020; Palanikumar et al., 2019; Vengadesan and Senthil,
2021) However, solar insolation uncertainty around the day reminds
the importance of using a buffered heat storage system for uninterrupted
cooking service ˙In the recent decades, phase change materials based
heat batteries are widely examined for solar cooking module owing to
the high latent heat of fusion with stable charging and discharging
temperature, unlike sensible heat materials (Coccia et al., 2020; Omara
et al., 2020; Saxena et al., 2011)
Initially, selecting PCM types is the key parameter in thermal
application, depending upon the use temperature For medium
tem-perature applications, PCMs like paraffin wax, fatty acids and salt
hy-drates are widely used (Omara et al., 2020) Though paraffin waxes
suffer the limitation in operating temperature, fatty acids are limited to
their low thermal conductivity (KPCM) compared to salt hydrates (
Kar-thikeyan et al., 2020) Secondly, selecting PCM charging or melting
temperature is the necessary task for any thermal application as PCM’s
operation is truly relying on its Tm (Velmurugan et al., 2021) Several
reports prove that salt hydrates based PCMs are widely examined for
medium temperature application owing to higher KPCM and
non-flammable (N Kumar et al., 2019; Yang et al., 2020) Some works
on salt hydrates like calcium chloride hexahydrate were studied for over
1000 heating and cooling thermal cycles to examine the thermal
sta-bility of the PCM for longer period operation, where at every 100th
thermal cycle, PCMs samples were examined for digital scanning
calo-rimeter (DSC) analysis to find the stability of the PCM Tm and Hm Minor
variation in Tm and Hm is noted for the 10th and 100th thermal cycles;
after the 200th cycle, variation becomes negligible in finding PCM
sta-bility Calcium chloride hexahydrate shows stable endothermic and
exothermic peaks at the 1000th thermal cycle, which means this PCM is
highly recommended for storing thermal energy (Tyagi and Buddhi,
2008) El-Sebaii et al examined magnesium chloride hexahydrate for
solar cooking purposes by accelerating continuous 1000 heating and
cooling cycles Noticeable variation in PCM Tm and Hm was observed
over 1000 thermal cycle though it is negotiable in terms of thermal
stability and the supercooling effect occurred up to 300 thermal cycles
within a range of 2 ◦C–3.2 ◦C, beyond 500th thermal cycle supercooling
becomes zero which concludes magnesium chloride hexahydrate is the
best material for thermal energy storage (El-Sebaii et al., 2011)
How-ever, salt hydrate has an unstable property such as super cooling and
phase separation after several hundreds of thermal cycling, which means
some parts of the PCMs will remain solid or liquid (Peng et al., 2019; Tan
et al., 2020) Purohit and Sistla reported incongruence in several
inor-ganic PCMs melting and solidification process for Na2S2O3.5H2O,
Al2(SO4)3.8H2O, Na2SO4.10H2O, Na2HPO4.12H2O, and Na2HPO4.7H2O
(Purohit & Sistla) Evidently, Shukla et al.’s study reveals that sodium
hydroxide and Disodium tetra borate performed the 1st thermal cycle
and for the 2nd thermal cycling, it does not melt even at high
temper-ature Following that, ferric nitrate shows insignificant performance in
cooling cycling, after providing sufficient time, also failed to become
solid and barium hydroxide failed to melt at 1st cycle (Shukla et al.,
2008) To avoid the failure in storing thermal energy, it is mandatory to
examine the stability of novel salt hydrates by accelerating continuous
heating and cooling cycles (Khan et al., 2016; Schmit et al., 2020)
Thirdly, salt hydrates show high corrosive with metal over a longer
period of storage in metal tank (El-Sebaii et al., 2009; Salgado et al.,
2020; Vasu et al., 2017) SP21E inorganic salt shows 7–8 mg/cm2-yr of
corrosion rate (CR) for aluminium (Al) and carbon (C) over the first
week; gradually, carbon CR decreased to less than 2 mg/cm2-yr over
12th week However, aluminum CR increased to 15 mg/cm2-yr Overall, stainless steel (SS) shows a lower CR for SP21E than copper (Cu) and other materials (Ferrer et al., 2015) Supportively, P Moreno et al study also reveals that SS has low CR for MgSO4.7H2O, Zn(NO3)2.4H2O,
Na2S2O3.5H2O and K3PO4.7H2O over 12 weeks immersion of PCMs (Moreno et al., 2014) Salt molecules are initiating the corrosion process with some specific metals and it is necessary to find the suitable metal for the selected inorganic PCM to avoid the corrosion and leakage issue (Calabrese et al., 2019; Dindi et al., 2020; Jaya Krishna and Shinde,
2017) The final step is utilizing the selected PCM for solar thermal cooking application directly as salt hydrate does not require any thermal additives like organic PCMs Several types of solar cooking techniques exist, among which parabolic dish type solar cooking unit gains atten-tion for community cooking As a parabolic dish type cooker is not attached to the cooking unit like a box type, favors handling the cooking container at peak sunshine hours also by non-focusing the container whereas it is difficult for box type cooking (Gonz´alez-Avil´es et al., 2018;
Lecuona et al., 2013) Moreover, once the cooking items are filled in a cooking pot and placed inside the box, it is difficult to open and stir the cooking item which makes parabolic dish type cooker is technical feasible for community cooking (Ahmed et al., 2020; Onokwai et al.,
2019; Senthil, 2021) Other than easy handling, parabolic cooker can track the sun in both single axis and double axis to enhance the thermal efficiency and cooking speed (Al-Soud et al., 2010; Devan et al., 2020) The above literature study and Table 1 reveals that selecting salt hy-drates can simplify the thermal conduction resistance during char-ging/discharging period with the advantage of being non-flammable and low cost ˙Initially few salt hydrates were examined for sequential thermal cycling due to its resistance in supercooling effect and phase separation In this aspect, our study focuses on the use of novel MNH heat batteries for construction of the cooking module Here we have investigated in detail the properties of MNH heat battery’s performance for over 1000 continuous heating and cooling thermal cycles ˙In addi-tion, corrosion study is performed to prioritize the lifetime for solar cooking module without damage or performance degradation We perform the real-time testing for the solar cooking module under load and no-load conditions to emphasis on the advantages of MNH heat batteries We report over 50% increase in the parabolic solar cooking modules performance with the incorporation of heat battery design in the system
2 Material and methods
Thailand lies above the equatorial line with adequate sunshine of
300 days minimum in a year Considering this abundant solar potential
in Thailand, the solar cooking method is widely encouraged to reduce fossil fuel consumption On the other hand, solar cooking should be efficient, maintenance-free and low-budget system As mentioned earlier, several types of solar cooking modules exist in the commercial market Most of them are high cost, fragile, and not convenient for off- sunshine hour cooking due to a lack of thermal energy storage facility This study purposes a parabolic concentrator solar cooking module with
a hassle-free cooking process as the cooking primarily relies on boiling type At the same time, a large area of collector can harvest the heat energy and concentrate it into a smaller receiver/cooking pot without wasting the heat energy
In this study, a standard design of parabolic solar concentrator with a radius of 0.75 m and a depth of 0.27 m are used From this structural
design, the focal length of 0.52 m was calculated using the formula: f =
D2/ 16h, where D and h are the diameter and depth, respectively For
efficient focusing of the solar irradiance, a high-quality solar reflection mirror is attached along the surface area of the parabolic concentrator A parabolic solar concentrator with a focal length of 0.52 m is used in this study with a cooking power of 125 W In this aspect, to develop an efficient and sustainable cooking system for the remote community, we
Trang 3Table 1
Recent literature study of heat batteries associated solar cooker
Saxena et al ( Saxena
et al., 2020 ) India Real-time outdoor cooking Solar box type Paraffin wax Carbon powder, mixed with paraffin wax and paraffin wax, are used as three different thermal energy storage materials filled in
copper tubes and placed in the bottom of the box type cooker for higher heat retention
It is noted that paraffin wax-associated box-type solar cooker yields better performance under real-time conditions
Comparatively, pure carbon powder and paraffin wax attained lower efficiency than paraffin wax-carbon composite
Effective time responses are studied for developing a novel thermal energy storage system for box type cooking of rice, egg and mutton takes 71, 98 and 121 min for complete cooking, respectively
Coccia et al ( Coccia
et al., 2018 ) Italy Real-time outdoor cooking Solar box type Ternary mixture of nitrate and
nitrate salts
This study selected a higher melting temperature PCM (145.14 ◦ C) for faster cooking after sunshine hours It is noted that solar salt maintains the cooking chamber temperature between 170 ◦ C and 130 ◦ C, which is 107.98% higher
than the conventional solar cooker
Saxena et al (Saxena,
2013) India Real-time outdoor cooking Solar box type Stearic acid Two different configurations of the box-type solar cooker were developed and examined with stearic acid as heat battery for off
sunshine cooking purpose
Maximum stagnation temperature in the cooking layer reached
145 ◦ C
Stearic acid as heat battery retained the heat inside the cooking layer temperature about 64 ◦ C, which can be utilized to cook rice, boiling milk, beans, fishes and other low cooking temperature items during the off-sunshine hours
Saxena and
Karakilcik ( Saxena
and Karakilcik,
2017 )
India Real-time outdoor
cooking Solar box type Sand: Carbon (4:6) The natural and readily available materials like sand and carbon are used as thermal energy storage material in a ratio of 4:6 are
examined as heat batteries for box type cooker
During the peak sunshine hour, cooking layer temperature reached the maximum of 136 ◦ C A sand: carbon as heat battery achieved a cooking power and thermal efficiency of 44.81% and is 37.1%, respectively
Saxena and Agarval
( Saxena and
Agarwal, 2018 )
India Real-time outdoor
condition Solar box type Small hollow copper balls A duct-like solar air heater is introduced to enhance the convection heat mode with the help of DC fans and a 200 W
halogen lamp placed inside the duct to improve the heat convection
A duct-type solar air heat channel fastens the cooking process as compared to the conventional box type cooker
Copper hollow balls are placed over the absorber plate to increase the heat transfer from the absorber plate to the cooking pot as copper thermal conductivity is high
Higher cooking efficiency achieved for sliced potatoes, rice and egg as compared to pulses and boneless mutton
Overall thermal efficiency is attained maximum of 45.11% and cooking power is 60.20 W
Khallaf et al ( Khallaf
et al., 2020 ) Egypt Numerical simulation and real time outdoor
condition
Quonset solar cooker NA Light weight and dome type transparent solar cooker is designed to achieve higher heat absorption Cocking layer is separated into
two for increasing the operational performance of the system
The novel Quonset solar cooker design reduces the radiation heat loss and infrared loss during the sunshine hours
Separation of two cooking compartments increases the incident energy on the absorber plate as it is partitioned by a reflecting mirror, which acts as an additional source to the solar cooker Glycerol is used as cooking fluid to increase the efficiency from 9
to 92% whereas the water lies around 6–35%
Kanyowa et al
( Kanyowa et al.,
2021 )
India Real-time outdoor
condition Scheffler dish type solar
cooker
NA Scheffler dish type solar cooker is examined to find the losses that
occur during the operation as the system which is installed in
2001 at Om Shanti Retreat Center, Haryana, India
Every day 6000 meals are cooked in a day with a maximum of 200 days per year
Due to the age of the system, several losses occur in the system during cooking process
This system performed almost every sunshine hour to cook the meal of 6000 per day
It was noted that thermal efficiency of the system claimed to be 70–80% and overall efficiency of up to 25%
Keith et al ( Keith
et al., 2019 ) Australia Real time outdoor condition Parabolic dish type solar
cooker
Stearic acid Incorporation of stearic acid in the cooking pot enables to
maintain the stable temperature in the active layer, whereas the cooking can be done under the regulated temperature
The amount of PCM filled in the cooking pot is less, favouring the cooked food item hot for later serving purposes
Real-time cooking study reveals that average cooking time for rice
is 71 min which is lower than barely and lentil dishes with an average cooking time of 92 min and 95 min, respectively
It is recommended that the developed parabolic dish type solar cooker payback period is less than 52 weeks for the four membered families
Senthil ( Senthil,
2021 ) India Real time outdoor condition Parabolic dish type solar
cooker
Paraffin wax Internal thermal heat distributions transfer the heat from the
receiving focal point to PCM and the active cooking layer To reach the water temperature at 90
◦ C, with and without PCM takes 120 min and 90 min, respectively
PCM associated cooking pot takes more time to heat the water though it performs stable operation and heat retention for off sunshine cooking purposes
(continued on next page)
Trang 4have fabricated a well-investigated MNH heat batteries-based solar cooking module for uninterrupted thermal energy supply during the day and night Selected novel MNH as heat battery was used for studying the variation in Tmelt and Hm under repetitive thermal cycles Thermal cy-cles were conducted using hot plates and k-type thermocouples for over
1000 cycles For every 200 cycles, the weight change, Hm, melting temperature were recorded Corresponding studies for analyzing the chemical bonding dissociation was performed using PerkinElmer FT-IR spectrometer The UV–Vis absorbance spectrum used to observe water molecules’ dissociation was performed using PerkinElmer UV–Vis spectrometer Differential Scanning Calorimetry was performed using Mettler Toledo DSC1 for every 200th thermal cycle Corrosion analysis was performed by calculating the surface roughness profile analyzed from Olympus BH2 optical microscopy images Performance of MNH based solar cooking module experimented at School of Renewable En-ergy and Smart Grid Technology (SGtech) in Naresuan University, Thailand Step 1: MNH filled in all 28 PCM tubes (heat battery), heat transfer fluid is filled next to the PCM layer and the active layer is empty (without load) and step 2: MNH filled in all 28 PCM tubes, heat transfer fluid is filled next to the PCM layer and active layer is filled with water (with load) K-type thermocouples are used to measure the system’s temperatures, 8 thermocouples in PCM tubes with equal intervals of 4 PCM tubes, one thermocouple inside the heat transfer fluid layer, cooking layer and focal receiver point of the bottom (cooking pot) All temperatures are recorded in 5 min’ intervals using Graphtec Datalogger and solar radiation is collected from PV research unit, School of Renewable Energy and Smart Grid Technology (SGtech) in Naresuan University Table 2 shows the instruments operating range used in this study
3 Result and discussion
Magnesium Nitrate is a naturally occurring inorganic compound with utmost attraction towards water molecules, forming a stable Magnesium Nitrate hexahydrate (MNH) MNH has a monoclinic crystal structure in the space group of P21/c with water molecules attached to the Mg ions, as illustrated in Fig 1 (a) MNH naturally possesses the property to store thermal energy through phase transformation in the material and release the stored energy in latent heat and sensible heat This property makes MNH a low-cost, reliable, and earth-abundant Phase Change Material (PCM) in the salt hydrate group Fig 1 shows the in-depth analysis of the change in material properties at the interval
of 200 cycles up to 1000 thermal cycles One thermal cycle consists of a material solid-liquid melting phase and a liquid to solidification phase This thermal cycle study explains the material’s stability over an oper-ational lifetime, corrosive nature, and dissociation chemistry, which are mandatory before employing the material for real-time applications An excellent Phase Change Material stands without any change in latent heat, weight loss and melting time over 1000 thermal cycles (A Sharma and Shukla, 2015; R K Sharma et al., 2016) Generally, the thermal dissociation in metal salt hydrate based PCM involves the release of weakly bonded water molecules followed by nitrogen, carbon as CO2
and NO2 based on their chemical composition In the case of MNH, the pristine material exhibits phase transformation at around 90 ◦C as observed from the DSC curve plot in Fig 1 (c), the dissociation starts to
Outcomes Overall
parabolic trough collector
◦ heat
Table 2
Experimental instrument range
Sensitive balance (METTLER TOLEDO) 0–310 g Graphtec datalogger 20 channel, − 100 ◦ C–1370 ◦ C
Trang 5Journal of Cleaner Production 324 (2021) 129223
5
appear after the 600th cycle where the Magnesium nitrate hexahydrate
starts to lose the water molecules slowly forming magnesium nitrate
monohydrate structure then after the 800th thermal cycle they start to
form the Magnesium oxide phase as they release NO2 gases from Mg
(NO3)2 composition This thermally induced chemical dissociation is
evidently observed from the XRD pattern recorded at each thermal cycle
stage Fourier Transform Infrared spectroscopy reveals this mechanism
with the disappearance of peaks around 800 cm− 1, 1300 cm− 1 and 1700
cm− 1, which corresponds to the hydrogen and nitrogen bonding in water
molecules and Mg(NO3)2 structure Differential Scanning Calorimetry analysis reveals the linearly decreasing melting temperature of the PCM from around 90 ◦C–80 ◦C The melting temperature broadens clearly, signifying the change in the chemical composition of the material induced by thermal stress The change in the melting temperature directly affects the material’s thermal energy storage capacity, which means the reduction in heat capacity and the inability to steady release
of heat when the PCM discharges However, the chemical stability of MNH up to 800th thermal cycle proves its efficiency as a low-cost
Fig 1 (a) Crystal structure and bonding representation of the Mg(NO3)2.6H2O Thermal cycle dependent changes in the Phase Change Materials properties up to
1000 cycles (b) UV–Visible absorbance spectrum (c) DSC curves showing the melting point of the material (d) XRD pattern of the material explaining the disso-ciations (e) FT-IR spectrum showing the bonding transformations (f) Mass of the material (g) Time taken for melting and solidification of the material
S.M.S Rekha et al
Trang 6high-performance PCM in the salt hydrates group Fig 1 (f) and 1 (g)
show the material mass loss over the thermal cycle as it loses the water
molecules and hardens to form MgO crystals Another advantage over
MNH use is the slower and stable temperature heat discharging
prop-erty, as seen in Fig 1 (g), demonstrating slower solidification over the
melting time Additionally, the UV–Vis absorbance spectra support our
claim for the stable and slower chemical dissociation in MNH over the
long 1000 thermal cycles The absorbance increases with the increase in
the number of thermal cycles because the pristine MNH are transparent
to visible light with more water molecules They start to lose the water
molecules the capacity to absorb light for the wide bandgap MgO
crys-tals Thus, the detailed material property analysis proves that the MNH
are highly efficient over 800 operating cycles which is more than 2 years
of time, making the system economically beneficial
Low and medium temperature phase change materials are mostly
paraffin waxes and metallic salt hydrates in high thermal conductive
aluminum or stainless-steel enclosures Hence their nature to withstand
the entire system lifetime inside the container without corroding or
damaging is mandatory (Ferrer et al., 2015; Vasu et al., 2017) We
performed the corrosion study for the MNH using the stainless-steel
container, in which for every 200 thermal cycles the stainless-steel
container was imaged using optical microscope and scanning electron
microscope as shown in Fig 2 The captured surface images reveal the
impact of MNH etching throughout thermal cycles The optical
micro-scope images captured analyzed the ImageJ software to extract the
surface profile and calculate the roughness parameter Ra The surface
plot clearly demonstrates the increase in the container surface’s
roughness profile over the thermal cycles as the value of Ra varies
be-tween 80 and 220 with an average value of 120 The analysis shows that
no obvious intensified corrosion behaviors are observed between
particular cycles For the thermal cycles after 600, the roughness varies
widely as the MNH becomes more concentrated oxide Since magnesium
nitrate monohydrate formation, the corrosion nature seems to increase
as the MgO concentration increases In the case of 800th and 1000th
thermal cycles the surfaces are observed to be etched deeply, signifying
a larger roughness profile The SEM image shows the in-depth
investi-gation on the etching profile of MNH, which starts after the 600th cycle
Moreover, no aggressive corrosion behaviour was observed at charging
cycle, the heat storage capacity of the PCM is not affected by the
corrosion in the container surface Supporting the material property
shown in Fig 1, the corrosion studies also reveal that the MNH performs
as a good PCM with least corrosive nature for a maximum of up to 800
thermal cycles, which benefits any system economically (Ferrer et al.,
2015)
From the above-detailed analysis of the phase change stability and
operational lifetime, we implement the real-time demonstration of the
MNH-PCM based parabolic solar concentrator cooking module We
intend to enhance the well-established solar cooking module’s efficiency
with incorporating MNH-PCM for stable heat flow around the focal
point, thereby providing an effective low-loss cooking process Heat
Transfer Networks explains the direction of the heat flow and the
po-tential resistance involved in it Here we investigate the difference and
the advantage of using MNH-PCMs in the solar concentrator cooking
module Fig 3 (a) shows the heat transfer mechanism involved in the
parabolic concentrator cooker design with and without MNH-PCM
incorporation In this system, cooking process involves three modes of
heat transfer path: (1) aperture focal at direct cooking pot; (2) aperture
focal at heat transfer fluid layer and (3) aperture focal at PCM tubes
Heat transfer mode 1 mechanism involves direct transfer of heat energy
from the solar energy concentration aperture region at the receiver’s
focal point to the active layer by conduction Depending on the type,
heat energy inside the active layer is transferred via conduction or
convection to the load In heat transfer mode 2, an intermediate buffer
layer is added, which holds a heat conduction fluid that moderates the
heat flow between the solar energy concentrator and the active layer
The heat transfer layer directs the heat flow to the active layer from the
focal point via conduction and convection, enabling a stable tempera-ture supply to the load via the active layer without any heat loss In heat transfer mode 3, we involve the function of MNH-PCM and the heat transfer fluid layers between the incident solar concentrator focal aperture and the active layer for enhancing the efficiency of the para-bolic solar energy concentrated cooking module Here the PCM stored in the stainless-steel tube receives the thermal energy from the concen-trator and stores it in specific heat capacity through phase change, transfers the surplus uniformly to the heat transfer fluid The heat transfer fluid spreads the thermal energy uniformly along the surface of the active layer, thereby increasing the system’s efficiency both under load and without load conditions It should be noted that during real- time performance, the active layer will receive thermal energy from the above mentioned three modes of heat flow mechanisms The active layer of the module transfers thermal energy directly to the load, which
is the cooking materials
Further, the parabolic solar concentrator-based cooking module with the design as mentioned in Fig 3 is performed The increase in the module’s efficiency with the use of MNH-PCM (heat battery) is studied through the temperature profile analysis under a good solar isolation day under no load and full load conditions, as shown in Fig 4 Here the full load conditions are performed with the active layer (i.e., cooking area) fully filled with water and for no-load condition the active layer was left empty MNH-PCM are filled in the PCM tubes as shown in Fig 3 (b) and 3 (c), heat transfer fluid is filled and perfectly sealed to prevent leaks We understand that for the intermediate temperature (120–240 ◦C) solar cooking module, the use of water as load test fluid will not help in determining the total system performance Hence the use
of test load fluid with higher boiling point must be used to determine the system performance as mentioned by Sagade et al.(A A Sagade et al.,
2018) However, our system operating temperature is demonstrated to a maximum of 120 ◦C only And in order to demonstrate the real-time cooking capability of our system, we have used water as load test fluid and cooking medium in our experiment
Under no load condition for the recorded solar irradiance of the day, the active layer temperature (TCOOK) rises to a maximum of 77 ◦C at the peak sunshine hours Whereas the parabolic solar concentrator focal receiver point (TFOCAL) reaches to a maximum of 84 ◦C which helps in charging the MNH heat battery by changing to liquid phase and from there the thermal energy (TPCM) released by the heat batteries are almost steady throughout the day as observed in Fig 4 (d)
On the contrary, it is observed that the temperature of heat transfer fluid shoots to 85 ◦C, equivalent to the TFOCAL The reason behind this massive temperature difference is that the MNH has high energy density
of 121.10 J/g and the low specific heat capacity of air in the active layer, this factor along with the continuous heating source from solar concentrator increases the heat transfer fluid temperature (TFLUID) significantly As the active layer is encapsulated to prevent heat loss it attains vacuum as there is no load, more TFOCAL in creates higher TCOOK,
which is transferred to the heat transfer fluid as the active layer is in a vacuum and it starts to increase the TFLUID abruptly On the other hand, the active layer receives the heat through many scenarios, as shown in
Fig 3 (a) If the heat is transferred by single channel which is through PCM, heat transfer fluid and active layer, there will be a minor initiation
in creating a vacuum under no load test condition But the active layer receives a minor amount of heat directly from the parabolic dish how-ever, this is under control when the active layer is filled with cooking item and it never over-shoot the temperature Unfortunately, leaving the active layer empty with a closed environment behaves opposite to the full load condition Generally, no load test is performed to find the operational mechanism and consistency of the developed system, this study reveals that active layer must be filled with cooking stuff unless the vacuum inside the active layer could increase the heat transfer fluid temperature as well as PCM temperature which could degrade the thermophysical property of both materials Unlike, vacuum inside the active layer could harm the total system and those who are operating the
Trang 7Journal of Cleaner Production 324 (2021) 129223
7
Fig 2 Corrosion analysis of MNH in Stainless steel container under different thermal cycles using the representation of Optical Microscope images (column 1),
Roughness plot Ra (column 2), Scanning electron microscopy images (column 3)
S.M.S Rekha et al
Trang 8system
A decrease in solar irradiance directly diminishes the TFOCAL though
TCOOK temperature is maintained stably with the steady supply from the
MNH heat batteries As mentioned earlier, total 28 MNH heat batteries
are attached to the thermal receiver module for uninterrupted thermal
supply to the active layer and load material Fig 3 (f) demonstrates the
uniform temperature profile of MNH heat batteries at different places
proves the MNH attached thermal receiver’s effectiveness There is no
major fluctuation in PCM tubes until 14:00 which means the receiver’s
focal point is high and entire cooking pot receives the stable heat source
from the parabolic concentrator Necessity of MNH heat batteries in this study is clearly noticed in Fig 3 (h), MNH assisted solar cooker effi-ciency reached a maximum of 24% and without MNH heat batteries is 8.3% only This no-load study proves that the parabolic solar concentrator-based cooking module without MNH heat batteries fails to utilize the entire received thermal energy with no stabilization in ther-mal receiver temperature, which lowers the system’s efficiency Under full load condition, water is filled in the thermal receiver’s active layer, resulting in huge thermal variation observed under direct sun exposure as shown in Fig 4 (e) As the full load testing was also
Fig 3 (a) Represents the Heat Transfer Network model in the system (b) and (c) shows the cooking container, the PCM container tubes (d) solar concentrator dish
construction model
Trang 9Journal of Cleaner Production 324 (2021) 129223
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Fig 4 (a)MNH-PCM based Parabolic Solar concentrator cooking system (b) and (c) Infrared images of parabolic solar concentrator and thermal energy receiver at
focal point Solar irradiance dependent temperature profile of different solar cooking module layers (d) without load and (e) with load (f) and (g) demonstrate the uniform temperature profile in the PCM storage containers under no load and load, respectively (h) and (i) demonstrates the efficiency of the system under no load and load conditions, respectively
S.M.S Rekha et al
Trang 10performed in a day filled with good solar irradiance, the results show a
maximum TFOCAL of up to 120 ◦C The heat batteries are charged from the
TFOCAL, which reaches a peak temperature of 107 ◦C and TFLUID showing
102 ◦C with the active layer temperature reaching 100 ◦C as the load
keeps drawing the heat constantly The heat batteries provide constant
heat energy for the cooking module even as the solar irradiance starts to
fall at the end of the day Fig 4 (g) demonstrates a uniform temperature
profile for the heat batteries over the day The use of heat batteries in the
solar cooking module hugely benefits by regulating the temperature
supply to the active and load layers, thereby protecting the cooking
materials Under any abnormal fluctuations in the solar irradiance, the
heat batteries help moderate the active layer temperature to maintain
the system’s efficiency constant and be suitable for the cooking
condi-tions throughout the day
The efficiency of the parabolic solar cooking module associated with
MNH heat battery is the ratio of thermal energy in the cooking receiver
pot (Qoutput) (including the thermal energy presents in the PCM (QPCM),
heat transfer fluid (QFLUID) and cooking layer (QCOOK)) to the incident
energy of parabolic dish/cooking pot (Qinput) as expressed in Eq (1)
η with MNH=Q output
Q input
=Q PCM+Q FLUID+Q COOK
Iη o(A p+A c
)
Δt
=mL + mc p− FLUID ΔT FLUID+ mc p− COOK ΔT COOK
Iη o
(
A p+A c
)
For parabolic solar cooking module without the MNH heat battery, the efficiency is calculated using Eq (2) where the cooking or receiving part of the parabolic dish is operated with only heat transfer fluid layer and cooking layer, in this concept, the system encounters sudden ther-mal fluctuations with respect to solar radiation:
η without MNH=Q output
Q input
=mc p− FLUID ΔT FLUID+mc p− COOK ΔT COOK
Iη o(A p+A C
)
Fig 4 (i) demonstrates about 50% rise in the cooking system effi-ciency with load compared to no load condition The investigation shows that the use of MNH heat batteries significantly helps to grow the system efficiency during the day irrespective of the solar irradiance fluctuations
The operational performance of MNH heat batteries and heat
Fig 5 Infrared thermal images of the parabolic solar concentrator and thermal receiver under real-time operating conditions