CHAPTER 2 LITERATURE OVERVIEW - Information and...
Transcript of CHAPTER 2 LITERATURE OVERVIEW - Information and...
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CHAPTER 2
LITERATURE OVERVIEW
Apart from power generation, the application of the concentrating
collectors along with a thermal energy storage system in industrial process
heating and institutional cooking, has been gaining momentum recently. In
the present work, a detailed survey has been made on the various aspects of
this field of research, which includes parabolic trough collectors, thermal
energy storage materials, system and applications, solar cookers and also
experimental and computational analyses of various types of cooking units.
2.1 PARABOLIC TROUGH COLLECTOR
Among the solar concentrating collectors, the parabolic trough
collector (PTC) is a well proven technology designed to reach temperatures
above 100 C and up to 450 C. Parabolic trough collectors are employed in a
variety of applications, including industrial steam production for electricity,
for process heat application and hot-water production. PTCs are mostly
preferred for solar steam-generation, because of the ability to obtain high
temperatures without any serious degradation of the collector efficiency. The
main attracting feature of the PTC is its high collector efficiency even at
higher temperatures.
Several developments have been carried out to improve the
performance of the PTC. The design and performance characteristics of a
parabolic trough solar collector system were thoroughly studied by Kalogirou
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et al (1994a). The optimization of the collector aperture and the rim angle as
well as the selection of a receiver diameter was addressed in this study. In a
later study, the authors Kalogirou et al (1994b) described a low cost method
for the mass-production of high accuracy parabolic surfaces with fiberglass.
According to the ASHRAE 93-1986 standard, the performance test of a
parabolic trough collector was conducted by Kalogirou (1996). Almanza et al
(1997) investigated the receiver behavior of PTCs in direct steam generation
under different experimental conditions, and they concluded that by replacing
the steel absorber tube with copper, it is almost possible to eliminate the
thermal stress in the wall of the pipe, due to the smaller circumferential
temperature differences.
Kalogirou (1998) used parabolic trough solar energy collectors for
sea-water desalination, and studied the performance of the PTC desalination
system. Odeh et al (1998) developed the thermal model of a trough collector
to find its thermal loss. Since the model was developed considering the
absorber tube temperature rather than the fluid bulk temperature, the authors
suggested that the thermal model can be used for any working fluid, to predict
the performance of the PTC.
Lokurlu et al (2005) developed a new kind of solar air-conditioning
unit through parabolic trough collectors, combined with a double effect
absorption chiller for the air-conditioning of buildings. The performance and
detection of the optical losses of the PTC are very important issues, in order
to improve the optical efficiency to ensure the desired output energy quality.
Riffelmann et al (2006) developed and established the PARASCAN and the
CAMERA-TARGET methods, to assess the flux distribution in the focal
region of PTCs. The evaluation of the geometric properties of the
concentrating collectors becomes important, when the thermal output is lesser
than expected. The use of the PTC on a small scale model for hot water
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generation was performed by Valan Arasu and Sornakumar (2006).
Brooks et al (2006) conducted the baseline performance study of a parabolic
trough solar collector, using the ASHRAE 93-1986 standard. Lupfert et al
(2007) summarized the various techniques available for analyzing the optical
performance of the PTC.
The thermal analysis of a PTC porous disc receiver was
numerically investigated by Ravi Kumar and Reddy (2009). The investigation
revealed that the use of a porous medium in a tubular solar receiver enhances
the system performance significantly. PTC applications can be divided into
two main groups. The first is applications where the temperature requirement
is in the range of 300-400 C, especially in concentrated solar power plants.
The second group of applications requires temperatures in the range of
100-250 C like various industrial process heat applications. A comprehensive
review of the use of the PTC for various applications of thermal energy up to
400 C was presented by García et al (2010). Tao and He (2010) developed a
unified two-dimensional numerical model for the coupled heat transfer
process in a parabolic solar collector tube, which includes natural convection,
forced convection, heat conduction, and the fluid-solid conjugate problem.
The vacuum solar receiver is the key component of a parabolic trough solar
collector, which plays a prominent role in the system efficiency. Gong et al
(2010) established and optimized a 1-D theoretical model of China’s first high
temperature parabolic trough solar receiver, to compute the receiver’s major
heat loss through a glass envelope, and then they systematically analyzed the
major influential factors of heat loss. Wang et al (2010) introduced an
eccentric type tube receiver, to minimize the thermal stresses of the tube
receiver. The authors suggested that employing an eccentric tube receiver
with optimum eccentricity and oriented angle for the PTC system, can reduce
the thermal stress and enhance the reliability of the tube receiver effectively.
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He et al (2011) established a coupled simulation method, based on
the Monte Carlo Ray Trace (MCRT) and the Finite Volume Method (FVM),
to solve the complex coupled heat transfer problem of radiation, heat
conduction and convection in a parabolic trough solar collector system. The
numerical study of conduction and convection heat losses from a half-
insulated air-filled annulus of the receiver of a parabolic trough collector, was
performed by Ansary and Zeitoun (2011). When a new design for a solar
collector is developed, it is necessary to guarantee that its intercept factor is
good enough to produce the expected thermal jump. García-Cortés et al
(2011) carried out experiments to determine the real shape and the intercept
factor of a new prototype of a parabolic solar trough collector. The
interception coefficient of the collector was calculated by the author and from
the results he concluded that 10% of the incident rays do not reach the
absorber. The author finally suggested that the mounting of some facets
should be revised in order to reduce the energy efficiency degradation. The
heat transfer and optical analysis of the PTC is essential to optimize and
understand its performance under different operating conditions. A detailed
one dimensional numerical heat transfer analysis of a PTC was carried out by
Padilla et al (2011). They validated the numerical results with the
experimental data and concluded that the results showed a better agreement
with the experimental data.
2.2 LATENT HEAT THERMAL STORAGE
Latent heat thermal storage has been a major topic in research for
the last two decades. In the present section, a review has been carried out on
the storage material, heat transfer, and other studies of the storage system and
its various applications.
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2.2.1 Phase Change Materials
A detailed review of low-temperature phase change materials has
been done by Abhat (1983). A broad review of research in the field of phase
change heat storage, especially on salt hydrates, has been done by Lane
(1983). This book gives a detailed account of the development of phase
change materials, the criteria for their selection and the chemical aspects of
the phase change phenomena. Fouda et al (1984) studied the characteristics of
Glauber’s salt as a heat storage medium in a pilot heat storage system. The
effect of several variables was studied over many complete cycles of the unit,
and the quantitative results were presented in terms of thermal recovery
efficiencies and the volumetric heat transfer co-efficient in the direct contact
storage unit. Vaccarino et al (1985) studied a low–temperature heat storage
system utilizing mixtures of Magnesium salt hydrates and Ammonium nitrate
as the PCM suitable for practical exploitation in connection with commercial
flat plate solar collectors.
Ghoneim et al (1991) studied the behaviour of three phase change
materials including sodium sulphate decahydrate, medicinal paraffin and
P116 wax for the use of thermal storage walls in solar passive systems. The
melting and freezing characteristics of the various organic and inorganic heat
storage materials, classified as paraffin, fatty acids, inorganic salt hydrates
and eutectic compounds were investigated, using the techniques of Thermal
Analysis, and Differential Scanning Calorimetry (DSC). A study was made by
Hoogendoorn and Bart (1992) on organic phase change materials for thermal
storage in solar systems. The latent heat effects of these materials are obtained
from the Differential Thermal Analyser (DTA) measurements. It was
concluded that paraffin based phase change materials are attractive for use in
solar heat storage systems for the temperature range of 25-150oC. Sharma et
al (1999) conducted experiments to study the change in the latent heat of
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fusion, melting temperature and specific heat of commercial grade stearic
acid, acetamide and paraffin wax subjected to repeated melt/ freeze cycles.
The study showed that acetamide and paraffin wax were found to be more
suitable phase change materials.
Dimaano and Watanabe (2001) investigated an LHS system with a
capric and lauric acid mixture. The thermal performance and phase change
stability of stearic acid as a PCM were studied experimentally by Sari and
Kaygusuz (2001), and they compared the heat transfer characteristics of the
stearic acid with other studies given in the literature. Py et al (2001) presented
a new supported PCM made of paraffin impregnated by capillary forces in a
compressed natural graphite matrix. Saito et al (2001) performed an analytical
and experimental investigation on the heat removal process of a thermal
energy storage capsule, using gelled Glauber’s salt. Dincer and Rosen (2002)
and Farid et al (2004) presented a detailed review of thermal energy storage
with phase change materials, heat transfer analysis and applications. Cabeza
et al (2003) studied the suitability and thermal performance of sodium acetate
trihydrate thickened with benotine and starch as phase change energy storage
material. The addition of gellants and thickeners avoided the segregation of
these materials.
A review was carried out by Zalba et al (2003) that focused on the
materials, the heat transfer analysis and applications of PCM based TES
systems. They listed over 150 materials used in research as PCMs, and about
45 commercially available PCMs. Nagano et al (2004) studied the feasibility
of a mixture of magnesium nitrate hexahydrate as a base material, and
magnesium chloride hexahydrate as an additive, to store and utilize urban
waste heat from emerged co-generation systems. He et al (2004) used the
liquid-solid phase diagram of the binary system of tetradecane and
hexadecane to obtain information of the phase transition processes for cool
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storage applications. The analysis of the phase diagram indicates that, except
for the minimum melting point mixture, all mixtures melt and freeze in a
temperature range and not at a constant temperature. Shiina and Inagaki
(2005) studied the enhancement of effective thermal conductivity of phase
change materials by saturating it with porous metals. The authors concluded
that considerable reduction in melting time was obtained, especially for low
conductivity PCMs and for a high heat transfer coefficient. Zukowski (2007)
experimentally studied paraffin wax (RII-56) as a PCM enclosed in a
polyethylene film bag, for a short term thermal energy storage unit. Kenisarin
and Mahkamov (2007) analysed the publications of the last 15 years on the
properties and applications of PCMs, and methods of enhancing the heat and
mass transfer in storage devices.
Every latent heat thermal energy storage system requires a suitable
PCM for use in a particular kind of thermal energy storage application. One of
the important factors to be considered when choosing an appropriate PCM is
the life of the PCM, i.e., its ability to resist change in the melting temperature,
and the latent heat of fusion with time due to thermal cycling. Shukla et al
(2008) performed a thermal cycling test of selected inorganic and organic
PCMs. They concluded that paraffin waxes show reasonably good thermal
reliability, and Erythritol, a sugar alcohol, is a promising PCM for high
temperature thermal energy storage, as its latent heat began to show only
gradual degradation after 500 thermal cycles. According to Kaizawa et al
(2008), Erythritol, Xylitol and D-Mannitol appear to be reliable PCMs for
high temperature applications, due to their large latent heat and good
operational safety. The potential use of D-Mannitol as a phase change
material is supported by its thermal properties. The melting temperature and
highest available temperature of D-Mannitol in the given experimental
conditions were reported by Bruni et al (2009).
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A detailed review on the use of a latent heat storage material in
solar cookers was addressed by Sharma at al (2009). A wide range of PCMs
with their properties, advantages and limitations was comprehensively
reported by Agyenim et al (2010). The selection of a phase change material
for any latent heat thermal storage system requires good thermophysical
properties. The possibility of D-Mannitol as a phase change material was
examined by Kumaresan et al (2011) through DSC and TG-DTG/DTA
analysis.
2.2.2 Configurations and Heat Transfer Enhancement in Thermal
Storage Systems
Understanding thermal behaviour during phase change in a storage
system is extremely important for the design of an efficient storage system. A
number of researchers studied the heat transfer performance and various
configurations of latent heat TES systems for energy storage, focusing on the
efficiency of the storage system.
Saitoh (1983) reported that a spherical shape gives the best
performance among the various existing LHS units including flat plate,
helical coil, and cylindrical capsule types. Abe et al (1984) developed a direct
contact LHS unit using form-stable HDPE rods as the PCM, and performed a
series of experiments for different flow rates, PCM initial temperatures and
HTF (Ethylene glycol) inlet temperatures to study the charge and discharge
characteristics of the storage unit on a lab scale. Dietz (1984) experimentally
studied the thermal performance of an LHS unit, consisting of vertically
oriented tubes filled with Calcium Chloride Hexahydrate (CaCl2.H2O) as the
PCM. The variation of the charging and discharging rates was measured for
different flow rates and temperatures of the HTF (air). These rates decreased
with time as a result of the decreasing effective heat transfer area and
increasing thermal resistance of the PCM. Kamimoto et al (1986) extended
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the work of Abe et al (1984) and developed an LHS unit of 30 kWh capacity,
using form stable HDPE rods for solar thermal applications, and the recovery
of industrial waste heat around 140-150oC. From the heat transfer
experiments and a thermal insulation test, it was confirmed that this storage
unit shows excellent performance due to good direct contact heat transfer and
a formation of thermocline. Saitoh and Hirose (1986) investigated the
transient thermal characteristics of a phase change thermal energy storage unit
using spherical capsules. The effects of the variation in the capsule diameter,
the flow rate of the heat transfer fluid, the inlet temperature difference, the
capsule material and the PCM, on the thermal performance of the storage unit
were classified in detail via a computer simulation. They found that the LHS
unit using spherical capsules had a high heat storage capacity 2.5 times larger
than the usual SHS unit for equal storage volume, when the temperature
swing was 40 K (PCM: Na2HPO4/12H2O).
Sozen et al (1991) investigated the thermal energy storage
characteristics of an SHS and LHS packed bed, consisting of a horizontal
channel filled with randomly packed particles of PCM encapsulated spherical
capsules. The HTF was refrigerant-12, which was modelled as an ideal gas.
The SHS material used was 1% carbon – steel, and the PCM was myristic
acid. The investigations showed distinctly different energy storage
characteristics for these two kinds of packed beds. The dynamic thermal
behaviour of a latent heat thermal energy storage was presented by Silva and
Pires (2002). The authors concluded that the correlations developed can be
used for the rapid estimation of the charging and discharging times and so can
be useful in the design of latent heat thermal energy storage. The performance
of a compact latent heat phase change material integrated in solar collector
system was investigated by Mettawee and Assassa (2006). The propagation of
melting and freezing front was studied during the charging and discharging
process. The experimental results showed that in the charging process, the
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average heat transfer coefficient increases sharply with increasing the molten
layer thickness, as the natural convection grows strong. In the discharge
process, the useful heat gain was found to increase as the water mass flow rate
increases. The performance of a packed bed latent heat thermal energy storage
unit integrated with solar water heating system was carried out by Nallusamy
et al (2006). The author concluded that the packed bed LHS system reduces
the size of the storage tank appreciably compared to the conventional storage
system and that the LHS system employing batchwise discharging of hot
water from the TES tank is best suited for applications where the requirement
is intermittent.
The improvement in the heat transfer rates in the storage units
employing PCMs with different temperatures were investigated by many
researchers. Farid and Kanzawa (1989) and Farid et al (1990) proposed the
use of PCMs with different melting temperatures in a LHS module with air as
HTF. The PCM was encapsulated in multirows of vertical cylinders. Both
experimental and numerical results showed some improvements in the heat
transfer rates during both heat charge and discharge when three types of
PCMs were used. Watanabe et al (1993) extended the experiments of Farid
et al (1990) by using water as the HTF and proved that there was obvious
enhancement of the charging-discharging rates in the LHS system using three
PCMs. Adebiyi et al (1996) reported that the efficiency of storage system
using five PCM families in a packed bed LHS system exceeded those using
single PCM family by as much as 13-26 percent. Wang et al (2001) studied
the charging process of a cylindrical LHS capsule with stearic acid, sliced
paraffin and lauric acid as PCMs. Experimental results demonstrated that,
compared to the capsule with single PCM, the charging rate of the capsule
employing three PCMs enhanced obviously.
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Several other heat transfer enhancement techniques such as using
fins, inserting a metal matrix into the PCM, using PCM dispersed with high
conductivity particles and micro-encapsulation of PCM were studied and
reported by various researchers. Since shell and tube heat exchangers are
commonly used in LHS systems, phase change around tubes with fin
configurations has attracted many researchers. Most of the studies are
concerned with PCM on the shell side and HTF on the tube side. The use of
finned tubes with different configurations has been proposed by various
researchers such as Eftekhar et al (1984), Padmanabhan and Krishna Murthy
(1986), Marcos (1990), Sadusuke and Naokatsu (1991), Lacroix (1993), Costa
et al (1998), Velraj et al (1997, 1999), Ismail et al (2001), Lamberg and Si en
(2003), Stritih (2004) and Castell (2008).
Hoongendoorn and Bart (1992) reported that the low value of the
thermal conductivity of the PCMs could be greatly improved by embedding a
metal matrix structure in them. Chow et al (1996) evaluated two thermal
conductivity enhancement techniques. The first technique focuses on placing
PCM in capsules of various shapes in a liquid metal medium. The second
technique involves a metal/PCM composite. Bugaje (1997) investigated
methods of enhancing the thermal response of paraffin wax heat storage tubes
by incorporation of aluminium matrices. Velraj et al (1999) studied the
influence of rasching rings dispersed in paraffin on the performance of LHS
unit of 50 litre capacity. Fukai et al (2002) used carbon-fiber brushes to
improve the thermal conductivities of PCMs packed around heat transfer
tubes. The transient thermal responses in the brush/n-octadecane composites
were experimentally measured and the effects of volume fraction of fibers and
diameter of brush on the heat transfer rate discussed. Cabeza et al (2002a)
performed an experiment in a small thermal energy storage device to study
heat transfer improvement in PCM with three different heat transfer
enhancement methods. Koizumi (2004) made an attempt to enhance the LHS
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rate of solid PCM in a spherical capsule and found that by inserting the
copper plates into the capsules is an effective technique for enhancing the
LHS rate, especially in large spherical capsules.
Progress in LHS systems mainly depends on heat storage material
investigations and on the development of heat exchangers that assure a high
effective heat transfer rate to allow rapid charging and discharging. Latent
heat TES systems are broadly classified into the capsule-type and shell-and-
tube type, according to the mode of exchanging heat energy within the
container. Advantages and disadvantages of different geometries of PCM
encapsulation with different materials and their compatibility were discussed
by Lane (1986). A series of numerical tests were undertaken to asses the
effects of the shell radius, mass flow rates, and inlet temperature of the HTF.
The transient performance of a double pipe heat exchanger as a thermal
energy storage container was investigated both experimentally and
theoretically by Fath (1991). The results indicated that increasing the HTF
inlet temperature and flow rate as well as the heat exchanger length increases
the heat transfer rate and stored energy.
Ryu et al (1991) studied the heat transfer characteristics of cool-
thermal storage units during the charging period using vertical and horizontal
tube systems. The two systems were compared with respect to heat transfer
rate, coefficient of performance and super cooling of the PCM and it was
found that the vertical tube system exhibits better thermal performance than
the horizontal tube system.
One of the most effective and compact latent heat TES system is a
packed spherical capsule bed with different diameters. An experimental and
numerical study was carried out by Ismail and Henriquez (2002) on LHS
system composed of spherical capsules filled with water as PCM placed
inside a cylindrical tank. The authors studied the effect of spherical capsule
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materials such as copper, PVC and polyethylene and found that the use of
polyethylene or PVC for spherical capsule facilitates the construction of the
storage and reduces its costs. Dincer and Rosen (2002) dealt with the
problems of heat transfer with phase change materials in simple and complex
geometries and around isothermal finned cylinders. The results were
presented and validated with actual and existing data. Barba and Spiga (2003)
analyzed the discharge process of the LHS system, for constant temperature
conditions, in three different geometrical configurations, i.e. PCM
encapsulated in slab, cylindrical or spherical polyethylene containers and
found that the shortest time for complete solidification is matched for small
spherical capsules.
The performance of latent heat thermal storage systems is limited
by the poor thermal conductivity of the PCMs used. Jagadheeswaran and
Pohekar (2009) reviewed the various techniques employed to enhance the
performance of latent heat thermal storage units, which include using
extended surfaces, employing the multiple PCMs method, thermal
conductivity enhancement and micro-encapsulation of the PCM.
2.2.3 Applications of Thermal Energy Storage System
Thermal Energy Storage (TES) is one of the key technologies for
energy conservation, and therefore is of great practical importance. One of its
main advantages is that it is best suited for solar thermal applications. Dincer
(1999) made a detailed study of the evaluation and selection of sensible and
latent heat storage technologies, systems and applications in the field of solar
energy. Another significant advantage of TES is that, although it may have
been designed primarily for the storage of solar energy, it is not restricted to
that. It may be used to store surplus energy from the power plants, usually in
the form of waste water, waste energy from air conditioners, waste energy
from industrial processes, and so on. Zalba et al (2003) presented an excellent
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review on the various applications using PCM based thermal storage systems.
Table 2.1 gives a literature review of various applications of TES systems.
Table 2.1 Various applications of TES systems
Sl.No
Type ofapplication References
1 Air heating
Kaygusuz et al (1991), Fath (1995), Mehmet (2000) , Aboul-Enein et al (2000), Enibe (2002), Jain and Jain (2004),Madhlopa and Ngwalo (2007), Mohan raj and Chandrasekar(2008) and Bal et al (2010)
2
Solar waterheating(LHS and SHSsystems)
Bansal and Buddhi (1992), Sodha et al (1997), Reddy et al(1999), Kurklu et al (2002), Mehling et al (2003), Baran andSari (2003) and Lee et al (2006) and Nallusamy et al (2006),(2009)
3 Buildingapplications
Peippo et al (1991), Khalifa et al (1998), Kissock et al (1998),Vakilaltojjar and Saman (2000), Lee et al (2000), Ismail andHenriquez (2001), Mehling et al (2002), Velraj et al (2002),Khudhair and Farid (2004), Lin et al(2005), Pasupathy andVelraj (2006), Tyagi and Buddhi (2007), Arkar and Medved(2007), Shilei et al (2007), Zhang et al (2008) and Zhu et al(2009)
5Cooling(Refrigeration andair conditioning)
Tackett (1989), Ryu et al (1991), Hasnain (1998),Vakilaltojjar and Saman (2001), Cabeza et al (2002b),Cheralathan et al (2006) , Morgan and Krarti (2010), Erekand Dincer (2009), Yau and Lee (2010) and Zhai and Wang(2010)
6Solar powerplants (LHS andSHS systems)
Herrmann and Kearney (2002), Kearney et al (2003) andPacheco et al (2002), Laing et al (2006), Seeniraj andLakshmi Narasimhan (2008) and García et al (2011)
7 Green HouseHeating
Hyun-Kap Song (1997), Kurklu (1998a), Kurklu (1998b), andBascetincelikk et al (1998), Najjar and Hassan (2008), Sethiand Sharma (2008) and Benli and Durmus (2009)
8 Electronic coolingPal and Joshi (1997), Bellettre et al (1997), Laouadi andLacroix (1999), Alawadhi and Amon (2003) , Alawadhi(2005) and Kandasamy et al (2007),(2008)
9 Automobiles
Schatz (1992), Mostafavi and Agnew (1996), Korin et al(1999), Vasiliev et al (2000), Zhang (2000), Desai andBannurm(2001), Talbi et al (2002) Subramanian et al (2004)and Pandiyarajan et al (2011)
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The storage of thermal energy is very important in many
engineering applications, which are mentioned above. Because of the
discrepancy between the energy supply and demand in solar heating
applications, a thermal energy storage device has to be used.
Since the present work is focused on storage based solar cookers,
the applications of thermal energy storage in solar cookers done by various
researchers are listed separately, and given in the following section under the
sub section of solar cookers with storage.
2.3 SOLAR COOKERS
A large number of solar cookers have been developed in many
countries and these cookers are classified based on the type of the collector,
the place of cooking, and the storage medium employed. The detailed
classification of solar cookers is given in section 1.4.3. Though in the present
research an indirect type solar cooker is used, an overview of all the types of
solar cookers studied is reported under two major groups, of without and with
storage.
2.3.1 Solar Cookers without Storage
The very first design of a box type cooker as shown in Figure 1.7a,
was designed by Nicholas-de-Saussure. It was simply an insulated box with
two glass panes forming the cover. This design forms the basis of all the
present designs of box type cookers. Reflectors were added to increase the
efficiency and reliability of the simple box type cooker. Ashok (1998)
summarized the history of the development of the box type cookers. A
performance study of the box-type solar cooker was made by Gaur et al
(1999), Nahar (2001), Amer (2003), Ekechukwu and Ugwuoke (2003),
Narasimha Rao and Subramanyam (2003, 2005) and Sebaii and Ibrahim
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(2005). A comparative thermal performance test of a box type solar cooker
using two different cooking vessels, with fin and without fin, was conducted
by Harmim et al (2008). It was experimentally demonstrated that the cooking
time can be reduced by using a finned cooking vessel. Schwarzer et al (2008)
studied the various types of solar cookers, their basic characteristics and
experimental procedures to test the different types of solar cookers. Mirdha
and Dhariwal (2008) theoretically analyzed the various designs of solar
cookers, with respect to north and south facing booster mirrors.
Concentrating type solar cookers are expected to demonstrate high
performance because of the large collection area employed. However, the net
amount of heat used is still low. Habeebullah et al (1995) developed a
numerical model for concentrating type solar cooker. The author introduced a
new concept of oven receiver to boost the overall cooker efficiency. The
analysis showed that the oven type receiving pot has both a higher fluid
temperature and overall receiver efficiency. The performance evaluation of
spiral type and paraboloidal type point focus solar cooker was conducted and
compared by Taha et al (1988). The experimental results showed that the
utilization efficiency of spiral type solar cooker is higher than paraboloidal
type. Abou-Ziyan (1998) designed, constructed and tested a two different
tracking solar cookers namely paraboloid dish solar cooker (PDSC) and a
booster mirror solar box cooker (BMSBC). The study showed that PDSC had
higher rates of cooking (2 to 6 times) and temperature than the BMSBC
because of the high concentration ratio. The effect of wind speed on
efficiency and temperature of the cooker was also reported in this paper.
A new type of conical solar cooker suitable for cooking different
kinds of meat and legumes was designed and tested by Sharaf (2002). The
author reported that the developed conical solar cooker has the advantages of
low price, ease of manufacture, lightweight, high efficiency, small solar area
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and short maximum time for cooking. Sonune and Philip (2003) developed
and tested a Fresnel type domestic SPRERI concentrating cooker. The
developed cooker has an aperture area of 1.5 m2 and a focal length of 0.75 m
and was found to provide an adequate temperature needed for cooking, frying
and preparation of food stuff for a family of 4 to 5 persons.
The theoretical exergy analysis of a simple solar parabolic cooker
(SPC) and the distribution of the exergy losses in the cooker was first studied
and presented by Petela (2005). The author concluded that the exergy
efficiency of the SPC was found to be relatively very low (~1%), and to be
about 10 times smaller than the respective energy efficiency which is in
agreement with experimental data from the literature.
The utility of a parabolic solar cooker on techno-economic, social,
behavioral and common criteria in the present Indian context in comparison
with other contemporary cooking energy devices was studied by Pohekar and
Ramachandran (2006). The study showed that Liquefied Petroleum Gas
(LPG) stove is the most preferred cooking device, followed by microwave
ovens and kerosene stoves. PSC has occupied fifth rank amongst the devices.
In the conventional solar cooker (box type), it is expected that the
food stuff is put once in the box and is taken out at an appropriate time, not
allowing intervention. This is because; whenever the lid, which is also the
energy collecting surface, is opened a large amount of trapped heat goes out.
Though concentrating type cookers deliver high temperature for cooking,
alike in the box type the user to go out in the sun, during its use and for its
hourly tracking. An indirect type solar cooker is highly regarded for indoor
cooking, where cooking is carried out in the shade or inside a building.
Schwarzer and silva (2003) developed a indirect type flat-plate solar cooker
and it is shown in Figure 1.10a. The cookers demonstrated by them can be
incorporated into the construction of kitchen. Peanut or sunflower oil was
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used as heat transfer medium and the cooker was designed with two non-
removing pots. Advantages of this cooker are possibility of fast cooking, large
pot volumes and the possibility of indoor cooking and heat flow control in the
pots. Solar cookers based on conventional flat plate solar collectors suffer
from the drawbacks such as the performance deterioration due to the reversed
cycle during night and cloudy periods of the day and high heat capacity.
Further the disadvantages are non-removable pots, which make cleaning and
dishing food difficult.
Balzar et al (1996) developed a solar cooking system which
consists of a vacuum-tube collector with integrated long heat pipes directly
leading to the oven plate. Solar cookers using vacuum tube collectors have
several advantages. They do not need tracking. They can reach high
temperatures and cooking can take place in the shade or inside a building
because of the spatial separation of collecting part and oven unit. They require
an effective heat transfer system in order to transfer the heat from the
collector to the hot plate without a marked decrease of temperature. Heat
pipes are very appropriate for this purpose. Their thermal conductance is
extremely high and the heat transfer between the evaporator and the
condenser section is nearly isothermal.
Kumar et al (2001) designed the community type solar pressure
cooker based on evacuated tube solar collector (ETSC). It consists of an
evacuated tubular solar collector and a pressure cooker and both units are
coupled together by heat exchanger. The incident solar irradiance falls onto
the collector and heats up the working fluid inside the tubes. The vaporized
fluid rises upwards to the heat exchanger and transfers energy by
condensation to the water flowing in the secondary loop of the heat
exchanger. The condensed fluid return back to the collector tubes and the
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process of heat transfer continues. It was reported that the system based on
ETSC supplied heat at higher temperature (120 C) than normal flat plate
collector.
Oommen and Jayaraman (2001) developed a compound parabolic
concentrator (CPC) based solar cooker. The CPC module was attached with 5
litre capacity of pressure cooker. Cooking experiments were conducted by
using a rice water mixture as the load and water as heat transfer fluid (HTF).
The efficiency of the system was reported for different inlet temperatures of
HTF. The author concluded that the developed steam solar cooker is a
relatively sophisticated device that unites some of the characteristics of
reflector cookers, steam cookers, pressure cookers and heat accumulating
cookers.
2.3.2 Solar Cookers with Storage
The use of solar cookers without storage is limited because these
cookers cannot be used on partially cloudy days and /or in the late evening
hours. The storage option (sensible / latent) in the cooker will increase its
utility and reliability.
Figure 2.1 Sensible heat storage type cookers (a) using engine oil
(b) using sand and (c) using vegetable oil
b ca b c
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Figure 2.1a shows the schematic of a hot box cooker with used
engine oil as a storage material, developed by Nahar (2003). The device
consists of a double walled hot box. The maximum stagnation temperature
attained inside the cooking chambers of the hot box solar cooker with storage
material was the same, as that of the hot box solar cooker without storage
during the daytime, but it was 23 C more in the storage solar cooker from
17.00 to 24.00 hrs.
Figure 2.1b shows the schematic of the flat plate solar cooker,
using sand as the storage medium, developed by Ramadan et al (1998). The
cheapest storage material and the best performance were achieved by making
a jacket of sensible heat storage material such as sand, 0.5 cm thick. Six
hours / day of cooking time was recorded, and approximately 3 hours / day of
indoor cooking was achieved.
Figure 2.1c shows the schematic of the flat plate solar cooker
developed by Schwarzer and Silva (2003), using vegetable oil as the storage
medium. The system consisted of one or more flat plate collectors with a
coated absorber and double glazed covering, cooking pots and a storage tank
to store thermal energy. Vegetable oil was used as the heat transfer fluid. The
oil was heated up in the collectors and circulated by natural flow to the
cooking unit, where it transferred part of its sensible energy to the double
walled cooking pots. The major advantages are the possibility of indoor
cooking, the use of a thermal storage tank to keep the food warm for longer
periods of time or night cooking, and the reach of high temperatures of the
working fluid in a short period of time.
The major limitations of sensible heat storage materials include low
specific heat capacity and a decrease in the effectiveness of cooking, as the
temperature of the storage material decreases during discharging.
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Latent heat storage makes use of the energy stored when a
substance changes from one phase to another. The use of PCMs for storing
heat in the form of latent heat was recognized as one of the areas to provide a
compact and efficient storage system, due to their high storage density and
constant operating temperature. Few studies have been conducted with latent
heat storage materials in a box type solar cooker to cook food late in the
evening. Domanski et al (1995) studied the use of a phase change material as
a storage medium for a box type solar cooker designed to cook food in the
late evening hours and/or during the non-sunshine hours. They used
magnesium nitrate hexahydrate (Mg(NO3)2.6H2O) as a PCM for the heat
storage. Buddhi and Sahoo (1997) designed and tested a solar cooker with
latent heat storage for cooking food late in the evening. In this design, the
phase change material (PCM) was filled below the absorbing plate.
Commercial grade stearic acid (melting point 55 C, latent heat of fusion
161 kJ / kg ) was used as a latent heat storage material. In such type of design,
the authors observed that the rate of heat transfer from the PCM to the
cooking pot during the discharging mode of the PCM was slow, and more
time was required for cooking food in the evening.
Bushnell (1998) presented a prototype for solar ovens, which
employs pentacrythritol as a solid–solid PCM. The author described the
performance from the efficiency measurement, and determination of the
figure of merit. Sharma et al (2000) developed a PCM based storage unit with
acetamide (melting point 82oC, latent heat of fusion 263 kJ/kg) for a box type
solar cooker to cook the food in the late evening. They recommended that the
melting temperature of a PCM should be between 105 and 110oC for evening
cooking. Buddhi et al (2003) later developed a latent heat storage unit for a
box type solar cooker with three reflectors to store a larger quantity of heat
through a PCM. They used acetanilide (melting point 118oC, latent heat of
fusion 222 kJ / kg) as a PCM for night cooking.
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In a later study, Sharma et al (2005) developed a solar cooker based
on evacuated tube solar collector (ETSC) with PCM storage. The PCM
storage unit used two hollow concentric aluminum cylinders and the space
between the cylinders was filled with 45 kg erythritol (melting point 118oC,
latent heat of fusion 339.8 kJ / kg) used as the PCM. A pump circulates the
heated water (HTF) from the ETSC through the insulated pipes to the PCM
storage unit by using stainless steel tubular heat exchanger that wraps around
the cooking unit by closed loop. During sunshine hours, heated water transfer
its heat to the PCM and stored in the form of latent heat, through a stainless
steel tubular heat exchanger. This stored heat was utilized to cook the food in
the evening time or when sun intensity was not sufficient to cook the food.
They concluded that two times cooking (noon & evening) is possible in a day.
Noon cooking did not affect the cooking in the evening and evening cooking
using PCM storage was found faster than noon cooking. Experiments and
analysis indicated that the prototype solar cooker yielded satisfactory
performance in spite of low heat transfer.
Hussein et al (2008) developed a novel indirect solar cooker with
outdoor elliptical cross section wickless heat pipes, flat-plate solar collector
with an integrated indoor PCM thermal storage and cooking unit. Two plane
reflectors were used to enhance the insolation falling on the collector, while
magnesium nitrate hexa-hydrate (melting temperature 89oC, latent heat of
fusion 134 kJ/kg) was used as the PCM inside the indoor cooking unit of the
cooker. It was found that the average daily enhancement in the solar radiation
incident on the collector surface by the south and north facing reflectors is
about 24%. Different experiments were performed on the solar cooker
without load and with different loads at different loading times to study the
possibility of benefit from the virtues of the elliptical cross section wickless
heat pipes and PCMs in indirect solar cookers to cook food at noon and
52
evening and to keep food warm at night and in early morning. The results
indicated that the present solar cooker can be used successfully for cooking
different kinds of meals at noon, afternoon and evening times, while it can be
used for heating or keeping meals hot at night and early morning. A detailed
review on the research and development of various types of solar cookers was
carried out by Muthusivagami et al (2010).
2.4 INVESTIGATIONS ON THE COOKING UNIT
Owing to the importance of solar cooking, several types of solar
cookers have been developed over the years. One of the major requirements
in using solar energy for cooking applications is the development of a cooking
unit, which should be fast and energy efficient. In the present investigation, a
cooking unit which is an integral part of the storage type indirect solar cooker,
is analysed for its suitability and performance, and hence, a review has been
carried out on the various studies made on the conventional type of cooking
units and its effect on the preparation of food.
A detailed survey on the various designs of cooking equipments,
their behaviour and energy efficiencies were carried out by Probert and
Newborough (1985). Improvements regarding the equipment design, cooking
techniques and consumer education were suggested by the authors from an
energy-thrift perspective. Geller (1982) carried out a detailed study at Ungara
area, in India, to analyse the type of cooking units used there, and the
efficiencies of these cooking units based on the energy balance study. He
found that the cookers have an efficiency of only 6%, and recommended the
use of aluminium pots rather than clay pots for an improvement in efficiency.
All the energy losses incurred during the cooking process are presented in
Figure 2.2.
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Figure 2.2 Energy balance for cooking
Newborough and Probert (1988a) found that, in Britain, in many of
the canteens, cafés, restaurants or hotels, much of the energy expended
during storing, preparing, cooking and serving food was wasted without
achieving any useful purpose. The authors found three interrelated factors
which lead to energy profligacy in catering, analyzed each of these
malpractices from an energy-thrift perspective, and recommended methods to
reduce energy misuse in those areas. In a separate study (Newborough and
Probert, 1988b), the authors developed a test rig to simulate the thermal
behavior of a conventionally-heated electric toaster to measure the steady
state heat transfer. The authors also modified the conventional electric toaster,
and demonstrated a substantial improvement in thermal efficiency.
Sheridan and Shilton (1999) evaluated the efficacy of cooking
hamburger patties by an infra-red source at two different wavelengths. The
authors reported that the gas consumption when using the longer wavelength
infra-red source, was reduced by 55% over that for the shorter wavelength, a
higher energy source. Bizzo et al (2004), discussed several aspects of cooking
54
fuel safety, considering traditional clean fuels such as the LPG, natural gas
and kerosene, and non-traditional and/or renewable fuels such as the DME,
producer gases, biosyngas, ethanol and ethanol-gel. In addition to the safety
aspects, the authors also suggested the type of equipment used for the LPG
and kerosene mode of cooking. A realistic, integrated and comprehensive
software was developed by Halder et al (2011), that can simulate a food
process and its safety by combining a fundamental, physics-based model of
the process, with the kinetics of the microbiological and chemical changes in
the food during processing, to provide the needed information at any time and
at any location. The heat utilisation efficiency of all commonly used
cookstoves in Bangladesh was thoroughly discussed by Lucky and Hossain
(2001), with the emphasis placed on natural gas cookstoves. The authors also
presented the cost of cooking for different fuel–cookstove combinations.
A mathematical framework to model the heat transfer efficiency of
cooking pots was proposed by Hannani et al (2006). The model consists of
combining the experimental results and the statistical data of the Residential
Energy Consumption Survey (RECS) of Iran, with a soft-computing concept
such as the neural network. The results showed that the efficiency increases
with an increasing diameter-to-flame ratio, bottom wall curvature, pot wall
slope, and overall conductivity. With an increasing edge radius and pot
height-to-pot diameter ratio, the efficiency decreases. An attempt to use waste
vegetable oil as a fuel for a cooking stove was made by Natarajan et al (2008).
The authors reported that the efficiency of the stove using vegetable oil as
fuel was 48.9% as compared to 34.9% with that of a conventional stove. The
Water Boiling Test (WBT), the Controlled Cooking Test (CCT) and the
Kitchen Performance Test (KPT) are the different protocols available to
measure and compare the performance and pollutant emissions of various
55
cooking stove. Jetter and Kariher (2009) studied the performance and
pollutant emissions of 14 solid-fuel household cook stove by using WBT
protocol.
Improved cooking stove projects in the developing world have the
potential to reduce deforestation, improve health, and slow down climate
change. To meet these requirements, stoves must be carefully designed
through testing and verification of performance. To compare the fuel use,
carbon monoxide (CO) and particulate matter emissions produced during
cooking, the performance of 50 different stove designs was investigated by
MacCarty et al (2010), following the 2003 University of California-Berkeley
(UCB) revised Water Boiling Test procedure. The role of donor organizations
in promoting energy efficient cook stoves was presented by Kees and
Feldmann (2011). In this paper, some basic facts on cooking energy, clean
and efficient technologies for cooking, role of public sector in dissemination
of clean cooking energy technology and the experience of GTZ’s in this
regard stoves were addressed by the author.
Bottani and Volpi (2009) developed a mathematical model for the
prediction of the cooking time of meat products in industrial steam ovens. The
analytical model was validated by comparing the predicted cooking time with
experimental cooking data related to the time–temperature curves of seven
meat samples and the model had a maximum deviation of 4.6%. The authors
suggested this model for direct implementation as a tool to monitor and
automate the industrial meat cooking treatments by means of computer
control. Wählby et al (2000), carried out an experimental study, to find out if
and how the food quality changes, when the airflow is changed from
traditional hot air (forced convection) to impingement and concluded that the
effect is the biggest at the beginning of the cooling process. Alvis et al (2009)
presented a paper that has been devoted to explore the main effects of
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deep-fat frying on the fried material, and to review the most important
methods used to measure the convective heat transfer coefficient. They
suggested that the coefficient value defines to a great extent the equipment
size, and also establishes the contacts between phases and processing time.
Farinu and Baik (2008) carried out a numerical simulation to determine the
convective mass transfer coefficient (hm) using the finite element method
during the frying of sweetpotato. They established a correlation between the
maximum heat transfer coefficient (h) and the maximum mass transfer
coefficient, and also between the maximum h and the corresponding hm at that
time.
2.5 SPECIFIC OBJECTIVES OF THE PRESENT WORK
In the present research, a detailed survey has been made on solar
collectors, storage systems, solar cookers and cooking units. It is observed
from the literature survey that the parabolic dish type collectors have been
successfully implemented in large scale community level cooking
requirements. In these systems solar energy is directly used for cooking
without storage, as the cooking is required only in the afternoon hours.
However, the success of household applications is possible, only if the
cooking is made available even during the non sunshine hours. Hence, it
requires better storage systems. Moreover a review of the literature infers that
considerable efforts have been invested in research and development of direct
type solar cookers with and without storage. However, limited research works
were reported on indirect types, particularly, parabolic trough type solar
cookers with a storage system.
In order to utilize the solar energy by using a parabolic trough
collector for residential cooking application, it was proposed to redesign the
conventional type of cooking unit, so as to circulate the heat transfer fluid
through the cooking unit for efficient heat exchange. Further, from the
57
literature, it is found that there is no theoretically established procedure for
the design of such cooking units.
Considering the above, the specific objectives of the present
research work are formulated as below.
i) to develop a solar system integrated with a cooking unit
through a PCM based thermal storage tank, and to analyze the
charging / discharging performance of the system.
ii) to analyze the performance and optimize certain parameters
for the newly developed cooking units using the CFD
simulation.