Introduction

Solar energy is a boon for everyone on the earth. It serves in different ways to fulfill the energy demands of people from different regions in a non-polluting manner. Solar energy technology is commonly used worldwide for heating, cooling, and power generation. Figure 1 shows the world’s capacity for solar thermal and electrical power. Besides this, solar-rich countries use this free energy source to fulfill their heating and cooling demands through solar water heaters, solar cookers, solar air heaters, solar stills, solar dryers, solar ponds, PV panels, and solar thermal power plants.

Fig. 1
figure 1

World status of solar thermal and power generation capacity (Weiss and Spork-Dur 2021)

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Among the above applications, solar cookers are the only source to cook food and to reduce the biomass consumption generally used as a cooking fuel in remote locations or hilly areas (Weiss and Spork-Dur 2021). Through solar cookers, people from remote backgrounds or hilly regions can keep themselves healthy and save a lot of their time which is wasted on collecting biomass for cooking. It is the best application for sustainable development, which operates on clean energy technology. Continued research is going on solar cookers since the 1800s. Different researchers have made numerous efforts worldwide to optimize the design of solar cookers and for more efficient cooking performance under variable ambient conditions to achieve a fast-cooking rate (Saxena et al. 2011).

Literature review

This section focuses on the commonly used thermal heat storage (TES) materials that are used in solar thermal systems to enhance thermal efficiency and to maintain the thermal stability of the system after getting charged (through solar energy) to provide long-term heating and cooling effect. A literature review has been carried out for two major factors, i.e., the definition and classification of thermal heat storage and performance enhancement techniques for solar cookers.

Thermal heat storages

Storage of thermal energy (THS) is an effective technique for end-usage energy demand by energy redeployment. The energy available in heat/cold can be stored for a specific period and recovered from the same place for later usage. It is the standard concept of thermal energy storage, in which the “thermal” denotes either heat or cold (Dincer and Rosen 2011). This technique not only supports a thermal system for efficient operation but also extends the duration of operation due to energy storage. The classification of the THS has been given below, which is based on the state of matter and mode of heat storage in Figs. 2 and 3, respectively.

  1. 1.

    Sensible heat storage (SHS). In sensible energy storage, energy is stored through a temperature change of the storage intermediate such as rocks, bricks, water, and sand. The volume of energy input (Q) to energy storage media from a sensible heat system is just proportionate to the temperature difference (∆t) of the storage’s final and initial state, the mass of the TES material (m), and its heat capacity (Cp). There is no phase change occurred in sensible materials during the temperature variation during the heat storage process (Dincer and Rosen 2011; Alva et al. 2017). The amount of heat energy stored can be obtained as \(Q=m{C}_{p}.\Delta t\).

  2. 2.

    Latent heat storage (LHS). In latent energy storage, energy storage is stored through a phase change of the storage medium, and material is known as PCM. Heat transfer takes place if a matter changes its phase from one to another (which is known as the latent heat of the matter). This heat change is typically much higher than the sensible heat transformation for a specified media (i.e., related to its Cp). Like the SHS materials, the TES capacity (Q) of an LHS is equal to the sum of the latent heat enthalpy (H) at the temperature of phase change of the storage material {\({({C}_{p}.\Delta t)}_{\text{latent}}\)} and the sensible heat existence over the temperature swipe {\({({C}_{p}.\Delta t)}_{\text{sensible}}\)} of the TES process (Kalaiselvam and Parameshwaran 2014). This process can be expressed as\(Q=m\left[{({C}_{p}.\Delta t)}_{\text{sensible}}+H+{({C}_{p}.\Delta t)}_{\text{latent}}\right]\).

  3. 3.

    Thermo-chemical energy storage (TCES). In TCES, the chemical potential of a few matters is utilized on the base of storage and discharging heat energy with minute thermal losses. The revocable chemical interactions occur between the sensitive mechanism of TCES materials and the heat energy recovery in those materials (Bauer et al. 2013).

Fig. 2
figure 2

Thermal heat storage based on state of matter (Weiss and Spork-Dur 2021)

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Fig. 3
figure 3

Thermal heat storage based on mode of heat storage

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Apart from this, it is a fact that solar thermal systems require a large TES capacity to cover at least 1–2 days of heat demand. Such demand is usually fulfilled by SHS in large-size water tanks. However, an alternative is offered by LHS systems, where the heat is stored as latent heat in matters undergoing a phase transformation. The great advantage of LHS is its high heat storage capacity compared to SHS materials; therefore, in the present study, paraffin wax is considered for solar cooking operations.

Solar cookers with and without thermal heat storage

This section shows the potential of various techniques through which the overall performance enhancement of solar cookers is possible. Table 1 shows some good designs of box-type solar cookers around the globe with different techniques (such as fins over the solar collector or on the bottom side of vessels, usage of multiple reflectors, dual-energy input, and transparent insulation use) of thermal performance enhancement including the usage of TES materials.

Table 1 Highlights of some recent studies on box cookers
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Table 1 shows that there have been numerous developments made on solar cookers. Still, the contribution of the TES materials (especially PCMs) is highly remarkable in enhancing the overall performance of box cookers. Through a high storage capacity PCM incorporation, the possibility of evening cooking is also possible. But the designs developed for box cookers to date are not in complete favor of consumers. The operation, maintenance, and parts replacement are complex in the above designs. Apart from this, further modifications are not easily possible in such solar cookers, especially those that use PCM beneath the absorber tray. Replacement of the tested PCM after completing its life cycle is not easy. Also, these cookers cannot be further modified or used as conventional cookers (in case of slow cooking requirements) whenever required. Therefore, there is a need to develop a system that acts like a performance booster and can be attached or detached easily per the user’s requirement. Thus, a bed of PCM-filled small containers has been developed (which can be easily attached or detached from the SBC), and RT64 has been considered the potential PCM to operate the solar cooker. The heat transfer was also improved by using a bed of small PCM-filled containers.

Overall, a box cooker has been designed in the present research work to achieve the following objectives,

  1. 1.

    An SBC that has improved cooking efficiency compared with a conventional SBC

  2. 2.

    An SBC that can cook a variety of edibles in reduced time compared with a conventional box-type cooker

  3. 3.

    An SBC that can cook almost edibles without damaging its nutrients, taste, and color

  4. 4.

    An SBC that can further be modified easily (without any change in its standard design)

  5. 5.

    A cost-effective SBC that is easy to adopt for any household, mainly for Indian families.

Upcoming sections focus on achieving the above targets in a simple and informative way.

Materials and methodology

In the present work, a simple conventional design solar hot box cooker (SHBC) was fabricated with locally available materials by considering the standards developed by Mullick et al. (1987), especially for box-type solar cookers. About the specification of the tested SHBC then, it has an aperture area of about 36 cm2. Double glazing (sheet of transparent glass) of the approximate area of about 50 × 48 cm2 was used to trap the solar radiant energy through the dark interior of SHBC. The distance was about 15 cm from the aluminum made blackened cooking plate to the lower glazing sheet. A mirror reflector (51 × 53 cm2) was used for additional thermal gain, which fixed on the lid of the SBC. The specific area of the SBC was about 66 cm2, while the height of the cooker was about 20 cm. Glass wool was used as an insulation material to prevent thermal losses. The cooker’s cooking capacity was about 03–04 kg for different cooking stuff inside the 04 identical cooking vessels (01 L capacity), made of Al and coated dull black.

The main modification to the tested SHBC was encapsulated phase change material (PCM). A bed of PCM-encapsulated small-size copper-made capsules was developed to modify the cooker. All 140 small capsules were placed vertically on a thin blackened sheet (thickness = 0.5 mm) of Al and fixed with the help of metallic adhesive (thermal conductive glue). This bed was also coated dull black and simply placed over the cooking tray of the SHBC, as shown in Fig. 4.

Fig. 4
figure 4

Experimental setup of the present design of box cooker

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About the PCM, then, a commercial-grade paraffin wax RT64 has been considered due to its superior thermo-physical properties. It has a Tmelt of about 64 °C, a thermal storage capacity of about 251 kJ/kg, Cp of about 2.1 kJ/kg.K, a thermal conductivity of about 0.2 W/m.K, and a density range from solid to liquid of about 0.88 kg/L (20 °C) to 0.78 kg/L (88 °C). This PCM was filled approximately 92% in the small capsules due to thermal expansion. A total of about 1.5 kg PCM was used. Noticeably, the specific size of the tested container was about 9.6 mm × 30 mm. The cost of the tested PCM is about $10.2, and the modified SBC costs approximately $48.19 (including PCM).

The modified SHBC was tested for its cooking performance in Dehradun city of the Uttarakhand state of India. The cooking trials were carried out on on-load and no-load conditions. The results are compared to the other existing models to determine the cost-effectiveness, cooking power, and mainly for the thermal efficiency. Following are the essential performance parameters to assess the thermal response of improved SHBC.

The first (F1) and the second figure of merit (F2) have been calculated by the given standard equations developed by (Mullick et al. 1987) as

$${F}_{1}=\frac{{\eta }_{o}}{{U}_{L}}=\frac{({T}_{p}-{T}_{\text{amb}})}{{I}_{\text{in}}}$$
(1)
$${F}_{2}={F}^{^{\prime}}{\eta }_{o}\cdot {C}_{R}=\frac{{F}_{1}({M}_{w}{C}_{P-w})}{{A}_{\text{sbc}}.t}{\text{In}}\frac{\left[1-\frac{1}{{F}_{1}}\left(\frac{{T}_{w1}-{T}_{\text{amb}}}{I}\right)\right]}{\left[1-\frac{1}{{F}_{1}}\left(\frac{{T}_{w2}-{T}_{\text{amb}}}{I}\right)\right]}$$
(2)

It is notable that Mullick et al. (1987) established the F1 for a no-load condition and F2 for an on-load condition for solar cookers. They recommended the values of these figures to be in the range of 0.12 to 0.16 and 0.4 to 0.6, respectively.

The heat transfer coefficient (h) from the cooking plate of SHBC has been estimated by using the following equation (Hall et al. 1978):

$$h=\frac{{Q}_{U}}{{A}_{\text{sbc}}\left({T}_{p}-{T}_{f}\right)}=\frac{\tau .{I}_{\text{avg}}.{A}_{\text{sbc}}}{{A}_{\text{sbc}}\left({T}_{p}-{T}_{f}\right)}$$
(3)

The coefficient of overall heat loss (UL) has been estimated by summing the bottom and top loss coefficient (Mullick et al. 1997) as

$${U}_{L}=\left[\frac{2.8}{\frac{1}{{\varepsilon }_{P}}\left(\frac{1}{{N}_{c}^{0.025}+{\varepsilon }_{c}}-1\right)}+0.825{\left({x}_{m}\right)}^{0.21}+a{V}_{\text{win}}^{b}-0.5\left({N}_{C}^{0.95}-1\right)\right]{\left({T}_{\text{pm}}-{T}_{\text{amb}}\right)}^{0.2}+\frac{{k}_{i}}{{t}_{i}}$$
(4)

where “a” and “b” are constant.

The cooking power of SHBC has been estimated through the standard relation between mass, specific heat, and the temperature of cooking stuff (Funk and Larson 1998) as

$${P}_{\text{sbc}}=m.{c}_{p}\frac{\left({T}_{wf}-{T}_{iw}\right)}{600}$$
(5)

The thermal efficiency (ηtherm) of the tested SHBC is estimated by considering the following equation (Hall et al. 1978):

$${\eta }_{\text{th}}=\frac{{E}_{\text{out}}}{{E}_{\text{in}}}=\frac{m.{c}_{p-w}.\Delta T}{t.{I}_{\text{avg}}({A}_{\text{sbc}}+{A}_{\text{cap}})}$$
(6)

where \({A}_{\text{cap}}\) is about 5 cm.

Uncertainty analysis

It is essential to analyze the errors from the obtained outcomes of an experimental study. Notably, error and uncertainty occur naturally due to instrument selection, its condition, calibration, testing conditions, physical observations, and measurement of readings. Table 2 shows the instruments used in the present research work for taking readings of cooking trials, along with their accuracy and resolution.

Table 2 Measuring instruments used for experiments
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The possible uncertainty in required thermal performance parameters due to instrumentation error has been estimated through a well-known method followed by Negi and Purohit (2005) and with the help of the following equations.

First figure of merit (F1):

$${W}_{{F}_{1}}={F}_{1}{\left\{{\left(\frac{{W}_{{T}_{p}}}{{T}_{p}-{T}_{\text{amb}}}\right)}^{2}+{\left(\frac{-{W}_{{T}_{\text{amb}}}}{{T}_{p}-{T}_{\text{amb}}}\right)}^{2}+{\left(\frac{-{W}_{(I)}}{I}\right)}^{2}\right\}}^{0.5}$$
(7)

Second figure of merit (F2):

$${W}_{{F}_{2}}={F}_{2}{\left\{\begin{array}{l}{\left(\frac{\partial {F}_{2}}{\partial {m}_{w}}\frac{{W}_{{M}_{w}}}{{F}_{2}}\right)}^{2}+{\left(\frac{\partial {F}_{2}}{\partial {C}_{w}}\frac{{W}_{{C}_{w}}}{{F}_{2}}\right)}^{2}+{\left(\frac{\partial {F}_{2}}{\partial {F}_{1}}\frac{{W}_{{F}_{1}}}{{F}_{2}}\right)}^{2}\\ {\left(\frac{\partial {F}_{2}}{\partial t}\frac{{W}_{t}}{{F}_{2}}\right)}^{2}+{\left(\frac{\partial {F}_{2}}{\partial {A}_{{\text{sb}}{\text{c}}}}\frac{{W}_{{A}_{\text{sbc}}}}{{F}_{2}}\right)}^{2}+{\left(\frac{\partial {F}_{2}}{\partial {T}_{{w}_{1}}}\frac{{W}_{{T}_{w1}}}{{F}_{2}}\right)}^{2}\\ {\left(\frac{\partial {F}_{2}}{\partial {T}_{{w}_{2}}}\frac{{W}_{{T}_{w2}}}{{F}_{2}}\right)}^{2}+{\left(\frac{\partial {F}_{2}}{\partial \overline{I}}\frac{{W }_{\overline{I}}}{{F }_{2}}\right)}^{2}+{\left(\frac{\partial {F}_{2}}{\partial {\overline{T} }_{\text{amb}}}\frac{{W}_{{\overline{T} }_{\text{amb}}}}{{F}_{2}}\right)}^{2}\end{array}\right\}}^{0.5}$$
(8)

Thermal efficiency (ηtherm):

$${W}_{{\eta }_{\text{th}}}={\eta }_{\text{th}}{\left\{\begin{array}{c}{\left(\frac{{W}_{{M}_{w}}}{{M}_{w}}\right)}^{2}+{\left(\frac{{W}_{{T}_{w2}}}{{T}_{w2}-{T}_{w1}}\right)}^{2}+{\left(\frac{-{W}_{{T}_{i}}}{{T}_{w2}-{T}_{w1}}\right)}^{2}\\ +{\left(\frac{-{W}_{I}}{I}\right)}^{2}+{\left(\frac{-{W}_{{A}_{\text{sbc}}}}{{A}_{\text{sbc}}}\right)}^{2}+{\left(\frac{-{W}_{t}}{t}\right)}^{2}\end{array}\right\}}^{0.5}$$
(9)

Cooking power (Psbc):

$${W}_{{P}_{\text{sbc}}}={P}_{\text{sbc}}{\left\{{\left(\frac{{W}_{{M}_{w}}}{{M}_{w}}\right)}^{2}+{\left(\frac{{W}_{{T}_{w2}}}{{T}_{w2}-{T}_{w1}}\right)}^{2}+{\left(\frac{-{W}_{{T}_{1}}}{{T}_{w2}-{T}_{w1}}\right)}^{2}\right\}}^{0.5}$$
(10)

Heat transfer coefficient (h):

$${W}_{h}=h{\left\{\begin{array}{c}{\left(\frac{{W}_{{T}_{p}}}{{T}_{p}-{T}_{\text{amb}}}\right)}^{2}+{\left(\frac{{W}_{{T}_{w2}}}{{T}_{w2}-{T}_{w1}}\right)}^{2}+{\left(\frac{-{W}_{{A}_{\text{sbc}}}}{{A}_{\text{sbc}}}\right)}^{2}\\ +{\left(\frac{-{W}_{I}}{I}\right)}^{2}+{\left(\frac{-{W}_{{A}_{\text{sbc}}}}{{A}_{\text{sbc}}}\right)}^{2}+{\left(\frac{-{W}_{t}}{t}\right)}^{2}\end{array}\right\}}^{0.5}$$
(11)

Table 3 shows the instruments used for measuring the variations in temperature of various elements of the cooking system, including ambient, insolation, wind speed, and the mass of the cooking substance.

Table 3 The uncertainties of required performance parameters
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Uncertainty analysis of all the required performance parameters is carried out with the help of reference Negi and Purohit (2005). Table 3 shows the estimated uncertainties for different performance parameters such as F1 has an uncertainty of about ± 2.27%, F2 has an uncertainty of about ± 4.61%, ηtherm has an uncertainty of about ± 2.48%, Psbc has an uncertainty of about ± 2.41%, and h has an uncertainty of about ± 3.38%. Figure 5 shows the error bar of the necessary TPPs. The significance of errors found within the permissible limit is because of calibrated instruments and correct measurements. The estimated uncertainty values in the required thermal performance parameters lie in an acceptable range and are on par with previous literature (Saxena et al. 2020; Cuce and Cuce 2013).

Fig. 5
figure 5

Error analysis of the necessary performance parameters of the present box cooker

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Regarding the calibration of the instruments, all the instruments are periodically (annually) calibrated through their manufacturer, i.e., Japsin industrial instrumentation, New Delhi. Notably, there is a Solar Radiation Resource Assessment (SRRA) station in Dehradun (installed at UPES, Dehradun). The authors have also measured the values of solar radiation, wind velocity, and ambient temperature from there (during the cooking trials), which were found close to the present experimental readings.

Results and discussion

In the present experimental study, efforts have been made to develop a low-cost and more efficient hot box cooker. A PCM encapsulation technique has been applied impressively to improve its cooking performance. The tested PCM was filled inside about 140 small capsules of aluminum. These capsules were vertically positioned on a flat plate to prepare a bed of the PCM capsules (Fig. 6), which acted like extended fins over the flat plate surface (cooking plate). This bed was placed inside the cooker to attain a high convective heat transfer rate inside the modified SHBC. Notably, a fast-cooking rate is observed through the applied technique for cooking the different edibles through the present design. This design is economical and easy to adopt for the people of remote areas and deep hilly regions.

Fig. 6
figure 6

Different elements of the modified SHBC

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Experiments were conducted on-load (stagnation heating test) and no-load (sensible heat test) conditions to assess the all-important thermal performance parameters for rating the cooking performance of the present SBC. Cooking trials were conducted in the summer season under the climatic conditions of Dehradun city, India, noticeable that the experiments were carried out on consecutive days in April 2022.

Stagnation heating test

In this test, experiments were conducted on no-load conditions, which means there was no cooking stuff inside the modified SBC. The sky was clear, and the day was sunny for cooking trials on 11.05.2022. The cooker was installed on the ground at 10:00 h to achieve the quasi-steady state condition. Around 10:20 h, the readings were taken at an interval of each 20 min during the one-h of standard stagnation heating test. At 10:20 h, the Tamb was observed at about 33.2 °C while the level of solar radiation was noticed to be about 600 W/m2. At this time, the value of Tp was observed about 61 °C. Due to the encapsulated PCM inside the SBC, the Tp was found to be increased with improved ambient conditions. Wind velocity (Wv) was observed at about 1.2 m/s over the horizontal surface near the experimental set-up. The peak value of the plate temperature was noticed at about 146.1 °C (Tamb = 37.2 °C) throughout the day while about 132.3 °C (Tamb = 35.1 °C) during the standard hour testing (recommended by BIS). The first figure of merit (F1) was computed for a range from 0.12 to 0.15, which satisfied the standard. Figure 7 shows the thermal response of tested SHBC under stagnation testing.

Fig. 7
figure 7

Temperature curves of modified SHBC during the stagnation testing

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Sensible heating test

In this test, experiments were conducted on on-load conditions. About 1 kg of water was taken as a cooking substance and filled for an equal amount in the two same-designed cooking vessels inside the tested cooker. The sky was clear, and the day was observed well for cooking trials on 12.05.2022. The cooker was installed on the ground at 10:00 h to maintain a steady state condition. Around 10:20 h, the readings started to be noted with an interval of each 20 min during the 1 h of the standard sensible heating test. Figure 7 shows that at 10:20 h, the Tamb was observed about 33.8 °C while the level of solar radiation was noticed at about 605 W/m2. At this time, the value of Tp was about 67.7 °C, and the Twat was observed to be 51.1 °C. Due to the encapsulated PCM inside the SBC, the Tp was noticed to be improved with improved climatic conditions, which resulted in increased water temperature. Wind velocity (Wv) was observed to be 1.4 m/s over the horizontal surface (over the experimental set-up). The peak value of the plate and water temperature were noticed at about 150.2 °C and 97.7 °C (Tamb = 37.3 °C), respectively. Afterward, during the standard hour testing, these values were observed at about 138.1 °C and 96.88 °C (Tamb = 35.8 °C). The F2 was computed for a range from 0.25 to 0.34.

Other required thermal performance parameters were obtained using Eqs. (1) to (6). The thermal efficiency (ηtherm) of modified tested SHBC was found to be a maximum of about 45.7% with an average value of 42.1%, cooking power (Psbc) was estimated at about 54.71 W, the value of HTC (h) between the solar absorber (tray), and the cooking vessel was assessed maximum for a range of about 76.01 to 311 W/m °C, and the value of Uloss was estimated maximum about 5.71 W/m °C. Figure 8 shows the thermal response of tested SBC under the sensible heating test. Notably, the modified cooker took about 105 min to attain the maximum value of the cooking plate and water during the experiments. The time taken “t” can be further reduced by improved ambient conditions during the long sunshine hours, especially in the summer. It has been observed that the Tamb and solar radiation significantly affect the thermal performance of the tested cooker. The Tp of the modified cooker is noted to increase and decrease significantly with a rise and fall in ambient conditions. However, the wind velocity (1.2 m/s) had a marginal effect on the cooking process, as described by Kumar (2004).

Fig. 8
figure 8

Temperature curves of modified SHBC during the sensible heat testing

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The results of the experimental study show that the PCM encapsulation supports the modified SBC to improve its cooking performance. This PCM encapsulation was done by filling the paraffin wax into a small size cylindrical-shaped capsule. With the help of 140 capsules, a bed of PCM-infused capsules was prepared to be placed inside the cooker. These vertically positioned capsules acted like an extended fin over a flat surface, as shown in Fig. 7, and likely geometries over a flat plate enhanced the convective and conductive heat transfer (Das 2011). In other research works, it has been experimentally studied that cylindrical shape fins are easy to design and more appropriate for obtaining a high heat transfer rate (Das 2014) compared with other designs of extended fins (Ranjan et al. 2021). Basically, the fins are the extended surfaces of conductive materials applied to an element to enhance the heat transfer rate to the environment due to increased convection. The amount of RCC of a body determines the approximate volume of the heat it transfers. The higher the temperature gradient between the element and surroundings, the higher the convection HTC. Notably, a large surface area of the element improves the heat transfer. Therefore, adding a fin to a component or system expands the surface area, and it is a cost-effective solution for thermal systems to enhance heat transfer (Nellis and Klein 2009). Figure 9 shows the different fins geometries for thermal systems.

Fig. 9
figure 9

Different types fins commonly used in thermal systems (Nellis and Klein 2009)

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If one can talk about using fins inside the SBC, then these fins enhanced convective heat transfer exclusive in SBC due to the large surface contact area. The hot circulated air inside the cooker supports the incident solar radiant energy. Therefore, cooking time is reduced through the enhanced radiative and convective heat transfer by achieving the boiling temperature in a short span of time.

Apart from this, macro encapsulation of PCM is the best methodology to enhance the thermal performance of solar thermal applications. Therefore, such a technique was applied to a simple design cooker for performance enhancement. RT64HC (Rubitherm 2020) was considered a potential PCM for encapsulation in a small cylindrical capsule due to its large volume capacity among the various extended geometries, as shown in Fig. 6. This encapsulation provides thermal stability to the SBC during interruptions in ambient conditions. The excellent thermo-physical properties of the tested PCM store the solar radiant heat during the solar hours and release the stored at a slow rate to the environment making evening cooking possible through the solar cooker.

Other available works also show that a large quantity of PCM takes much time to charge it completely (Saxena et al. 2020); therefore, a late response occurs in the heating or cooking process. Consequently, to overcome this problem, small-volume capsules has been considered in the present work for PCM encapsulation. Through this, the tested PCM has been charged faster that resulted in fast-rate cooking. The results of this study have been found useful, especially for enhancing the performance of solar thermal systems because of the collective effect of extended geometry and the tested PCM. The heat transfer rate inside the cooker was relatively better than an orthodox box cooker with improved thermal stability. Thus, the results were better, and a fast-cooking rate was observed during the cooking and post-cooking trials.

The newly developed cooker is also tested to cook different edibles after conducting standard cooking trials to obtain the required TPPs. In these cooking trials, commonly cooked dishes by the people of Uttarakhand were cooked on additional days after experimental cooking trials. Table 4 shows the cooking results of some other dishes that offer a fair possibility of adopting the tested cooker.

Table 4 Post experiments cooking results of the modified cooker
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The current design of the modified cooker is also practically useful. One can easily use it as a conventional cooker whenever required. Besides this, this cooker can be further modified accordingly by simply detaching the PCM bed. This model also has the flexibility to perform around the year. A simple electrical coil of about 40 W can raise the PCM bed temperature to 125 °C; thus, cooking is possible in any climate at any geographical location. People can cook food by using the present cooker in a hybrid mode instead of using a microwave oven of 1 kW. Through this, a lot of electrical energy can be saved.

Comparative study

The present design of the cooker is quite economical and efficient for cooking food under low environmental conditions. All the cooking stuff available in local or nearby regions of Uttarakhand can cook inside the modified cooker under low or high ambient conditions. Apart from this, people from the hilly areas waste their time to collect biobased fuels, especially wood pellets and making dung cakes. Noticeably, the combustion of these low conversion efficiency fuels releases a bundle of pollutants that are too harmful to the consumer or nearby people. This cooker is feasible to reduce carbon emissions throughout the year with its potential use. Table 5 shows a comparative study of the current experimental study with some other cookers.

Table 5 Different designs of solar cookers and their results
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From Table 5, it can be seen that there are a lot of modifications made to the solar cooker in which very few cookers are considerable practically for solar cooking. Other solar cookers are complicated in design and occupy a large space, increasing the cost of cooking. Maintenance of complex designs is also too expensive. The cookers that function on PCM (in bulk) are not entirely safe from leakage problems, and a large volume of PCM increases the cost of the cooking system (too) and reduces the reliability. Thus, it is essential to estimate the optimum volume of the tested PCMs to be used in different solar energy applications.

The effect of a high heat storage material can also be noticed in other applications, such as solar stills (Dincer and Rosen 2011), photovoltaic systems (Wen et al. 2023), and solar air heating systems (Verma and Das 2022). All these PCM-integrated solar energy systems perform better than a conventional design of the same for a long duration. Likely, heat storage materials and techniques provide high thermal stability to the thermal systems, which resulted in improved efficiency with extended hours of performance of the tested systems.

The present system is low in cost and efficient in performing in low ambient conditions. Notably, the current cooker not only cooks almost edibles, but this cooker also can be used for dehydration of foods, milk pasteurization, grain sterilization, wax melting, and sterilization of medical apparatus. These types of solar cookers can be installed in hostels or hospitals for cooking and sterilization processes on a large scale. Likely, cooking systems are required for the remote areas and hilly regions of India so that people can use them for their cooking demands, reduce their biobased fuel consumption, and keep themselves healthy.

Overall, it is a good experimental testing research work that serves as a road map to the study and thermal performance assessment methods, helping the audience to better understand how the data were obtained and the performance assessment, therefore, supporting them in appropriately analyzing its outcomes and focusing towards further scope.

Payback period

The present developed SBC is quite economical for adoption for people of different households due to its low cost and payback period. Table 6 shows the prices of other elements of the present SBC.

Table 6 The cost of different elements of the tested SBC
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After the estimation of the capital cost (C) of the present cooker, its payback period has been estimated by the following equation (Saxena et al. 2020).

$${\text{PBP}}=\frac{\mathrm{log}\left(\frac{(E-M)}{(a-b)}\right)-\mathrm{log}\left(\frac{(E-M)}{(a-b)}-C\right)}{\mathrm{log}\left(\frac{(1+a)}{(1+b)}\right)}$$
(12)

where C = ₹ 4,021, annual interest rate (a) = 6%, maintenance charges/anum (M) = 5% of C, inflation rate (b) = 5.5%, and number of the years (N) = 05 years.

In comparison with commonly used cooking fuel, i.e., LPG (which has a cooking efficiency of about 61%, a calorific value of about 45 MJ/kg, and a cost of about ₹61/kg), the PBP for tested SBC is estimated about 3 years and 11 months.

Advantages and limitations

The present model of PCM-based solar cooker has the following advantages.

  1. 1.

    The modified cooker can cook almost edibles.

  2. 2.

    The cooked food is healthy and nutritious.

  3. 3.

    Operation and maintenance are easy.

  4. 4.

    Cooking is fast than a conventional cooker or an SHS-based solar box cooker.

  5. 5.

    Food can be kept warm for around 40 °C inside the cooker for up to 200 min after complete charging of the tested PCM.

  6. 6.

    This cooker can simultaneously cook 02–03 dishes.

  7. 7.

    Continual attention is not required.

  8. 8.

    User-friendly and non-polluting.

  9. 9.

    It can also be used as a conventionally designed cooker by detaching the PCM bed.

It has certain limitations also, as follows.

  1. 1.

    Adequate sunny environment is required for cooking and complete charging of the tested PCM.

  2. 2.

    Cooking is not possible under extremely cold conditions, during rain or any entire night without auxiliary support.

  3. 3.

    Taking a longer time to cook compared with other cooking fuels, such as LPG and electricity.

  4. 4.

    Baking, frying, or deep frying is not possible.

  5. 5.

    Requires a direction change to track the sun about 02–03 about times.

  6. 6.

    Cleaning of glazing, cooking chamber, cooking vessels, and mirror booster is essential before starting cooking every time. Dirty surfaces of these elements can reduce efficiency and extend the cooking time.

No doubt that these limitations have some impacts on the conclusion of the present research work, but the main one is its adoption by people around the world. People would be less interested due to its dependency on climatic conditions. Slow cooking process comparatively to LPG or electricity and cooking load capacity is subjected up to 04 kg only at a single day. Further research is essential to overcome these problems.

Conclusion

In the present work, a simple solar cooker has been designed and fabricated to obtain a better performance enhancement over a conventional designed SBC. For this, a bed of encapsulated PCM mass has been designed and developed to place it over the cooking tray of the present model. Small PCM-filled capsules not only improved the thermal performance but also cut off the cooking times due to enhanced heat transfer inside the cooking chamber. The present cooker has been tested as per standard test conditions especially developed for solar cookers (for stagnation and sensible heating test), and the outcomes have been found for a great range of satisfaction. This PCM bed can be easily attached or detached with the solar cooker, which makes the system user-friendly and easy to operate and maintain.

The results of the present experimental study showed that F1 and F2 satisfied the range of recommended values as per the cooking standard. The maximum value of the Tp has been observed about 150 °C, the ηtherm (instantaneous) has been observed at about 45.7%, the Psbc has been computed at about 54.71 W, the maximum value of the HTC has been estimated at about 311 W/m °C, and the maximum value of Uloss has been estimated about 5.71 W/m °C. The total fabricated cost of the cooker is estimated at just approximately $55, which shows a fair possibility of its adoption.

This research work also provides a direction for additional work on the discussed technique. A large-size solar cooker with a cooking capacity of around 12–15 people is under the design process. This cooker will follow the same designed PCM bed but will carry composite PCMs through which the Tboil can be achieved in less time, and it will provide an extended thermal storage capacity for evening/night cooking. The cooker can easily track the path of the sun manually due to the roller coasters. Thus, efficiency will be improved more than the present design. Apart from this, however, the present design is sufficient for a small family of 04–05 members to fulfill their cooking needs (who belong to a full sunshine region), but there are some research gaps, such as.

  1. 1.

    A detailed DSC analysis can be done to select a more appropriate PCM for solar cooking.

  2. 2.

    A year-round study is essential to check the appropriateness of the tested heat storage materials.

  3. 3.

    Cost optimization and design optimization are equally important for a wide adoption of the solar cooker for sustainable development.

  4. 4.

    Efficient box cookers are still required for a large-scale cooking.

Besides this, a lower volume of PCM reduces the cooker’s performance, while the bulk volume raises the thermal loads, which results in overheating. Therefore, PCM’s thermo-physical properties and their complete charging and the discharging process should be well examined. The poor material characteristics of metals significantly affect the performance of box cookers. Lower specific heat, thermal conductivity, and absorptivity reduce the inside heat transfer of a box cooker resulting in a slow cooking process. Thus, a good material selection is necessary for the optimal storage of PCM. Lastly, the cooker should be in the approach of every household, and thus, cost optimization is essential. A good design of cooker with a higher cost is not so useful. So, these are some limitations that should be considered before designing a box cooker.