1 Introduction

Solar cookers are the means to cook food with the help of solar energy. For this purpose, the solar energy can be collected using solar collector and transferred to the cooking vessel. The solar cooking is in practice since seventeenth century with continuous research efforts to improve the performance of cooker. The solar cooking was started with the solar cooker box and has been developed into various forms in due course of time. But, in the present condition the solar cookers are rarely used. Its main reason limits the usefulness of solar cookers in sunshine time only. The cooking at night or in cloudy days is not possible. This has created a necessity for development of solar cookers which can work at night as well as in cloudy days.

As the energy demand is increasing day by day with increasing population and pollution, the need of renewable energy is becoming the very essential in every field. There are various sources for renewal energy which are being widely used now days. Solar energy is the one of a very popular and easily available source of renewable energy. Still its use is only about 4% of total renewable energy used [1]. This solar energy can be used by means of photovoltaic (PV) cell or solar collectors. It has several uses like drying, space heating, cooking, electricity generations etc. [2,3,4,5]. And one of the well-known uses of solar energy is to cook the food [6]. It requires proper mechanism to use this solar energy for solar cooking. And it is done with the help of solar cookers. Till now, lot of solar cookers have been designed and used. It is being developed since seventeenth century [7,8,9]. Still it needs lot of research before selecting any one type for its use at specific region. It depends on geographical area, type of collector & its area, heat requirement, type of food to be cooked; at what time it is to be cooked etc. [10]. Among all those different designs, a simple solar box type cooker is used commonly due to its simplicity. The use of a solar box cooker is limited because cooking of food is difficult due to frequent clouds in the day or unavailability of solar energy in the evening. So cooking at night by this solar box cannot be done. Some have used hybrid energy also to improve the efficiency of cooker [11]. If storage for solar energy can be provided in a box cooker, then there is a possibility of cooking food in the evening and this will increase the effectiveness and reliability of these solar cookers [12,13,14]. This leads to need of indoor cooking system [15, 16]. For a solar cooking system to be accepted and adopted in most of the households, the following characteristics have to be satisfied [17,18,19,20]:

  1. 1.

    Possibility of cooking at any time of day

  2. 2.

    Cooking time must be comparable with conventional cooking

  3. 3.

    Economical aspect

To overcome above cited limitations and gain the desirable characteristics, researchers reported their findings [15, 21]. There are different heat storage systems (Heat Exchangers) available in the market. So, exact selection of most suitable becomes very difficult. Most of the time it become necessary to design and develop customized system which can meet the purpose. The solar collectors are classified as follows [22, 23]:

a. Stationary Collectors

(i) Flat plate collectors

(ii) Evacuated tube solar collectors

(iii) Compound parabolic collectors

b Single axis tracking collectors

(i) Linear Fresnel reflectors

(ii) Parabolic trough collectors

(iii) Cylindrical collectors

c. Two-axes tracking collectors

(i) Parabolic dish reflectors

(ii) Heliostat field collectors

 

According to Soteris A. Kalogirou [23], the temperature up to 400° can be obtained by using parabolic trough collector and even more than that is also possible by using parabolic dish reflector. So, taking this as a reference one can select the suitable solar collector. Hence, in such types of systems, energy storage and transportation is the key work. Solar energy has to be transported to the kitchen by means of a circulating fluid. Therefore, the critical study of heat storage and transfer systems along with their parameters attracts the attention of researchers.

2 Heat storage systems

Heat collected from the sun is used for the solar cooking but if it is directly supplied to cooking then it will be useful in day time/sunshine hours only. To carry the cooking at any time, one needs to store this heat so that it can be retrieved as and when required [24]. To store this heat the storage system is required which will be able to store the heat with minimum losses, so as to store the heat at least in the range of 200–300 °C [25]. Therefore, for the efficient heat storage systems following parameters must be considered:

  1. i.

    Type of solar cooking system (heat exchanger)

  2. ii.

    Heat storage capacity

  3. iii.

    Size and volume of the storage system

  4. iv.

    Heat losses and insulation

  5. v.

    Heat storing Media

  6. vi.

    Temperature range

  7. vii.

    Application (direct use or use for steaming) [26, 27]

2.1 Type of solar cookers with storage

Solar cookers are available in various forms. Its design resembles to the design of heat exchangers which are too much depended on the geometry and its type [28, 29]. Mostly direct box type heat exchanges are used for solar box cooker while shell and tube can be used for the storage systems [30]. Solar Cookers can be classified as below: [8, 31,32,33,34,35] (Fig. 1)

Fig. 1
figure 1

Different Types of Solar Cookers

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i. Solar box cooker

ii. Solar panel cooker

iii. Simple hot box oven

iv. Solar parabolic cooker

v. Fresnel lens Concentrator cooker

vi. Scheffler dish

vii. Solar cooker with PCM

  

There are so many box types of solar cookers available which work in day time only. Its evaluation is done by some researchers [36, 37]. Some other researchers worked on solar cookers with storage systems. And some of them used the conventional box cooker and to which heat storage system is attached. Out of those, double glazed box with PCM was good initiative to cook the food after 5 pm also [38]. But according to the author, it didn’t give the sufficient temperature to cook food only on storage system. General observations made by some researchers are shown in Table 1.

Table 1 Comparative solar cooker performance of PCM box cooker
Full size table

Prima facie it observes that food can be cooked in large scale with minimum time in outdoor cooker when storage is utilized. This fact helps in developing the new system (rather than box type only) with storage which can be used in the absence of light. For this, design and material selection for storage systems are the key factors.

2.2 Heat storage capacity

While designing the heat storage system, its heat storage capacity is very important parameter to decide. It depends on the application (in this case cooking) [43]. The requirement of heat depends upon the type of food to be cooked and the number of persons for whom food is to be cooked. The designed storage system should be able to meet the requirements of cooking for stated number of persons. For sensible heat storage, the capacity of storage system can be calculated by using following general relationship [44, 45]:

$$\Delta h = m \cdot Cpdt$$
(1)

where m is the mass of sensible heat storing material, Cp specific heat of material, dt is change in its temperature.

Whereas for the latent heat storage system, heat stored by the system can be calculated as [46]:

$$Q = \mathop \int \limits_{T1}^{Tm} m \cdot Cpdt + m \cdot \Delta h_{m} + \mathop \int \limits_{T1}^{Tm} m \cdot Cpdt = m\left[ { \, Cps\left( {Tm - T_{1} } \right) + \Delta h_{m} + Cpl\left( {T_{2} {-}Tm} \right)} \right]$$
(2)

i.e. Q = sensible heat in solid state of PCM + latent heat of fusion + sensible heat of PCM in liquid state.

So using above relations, one can find out the capacity of the system as well as can design the system.

2.3 Size and volume of the storage system

For the solar cooking to be easily adoptive by all the households, it should be easily portable. Hence size and volume of the system should be smaller which is better for short payback periods [47]. For this, the higher density material is preferred. While designing the system, it can be designed according to heat storage requirement or volume of the storage system. The performance of the system can be evaluated on the basis of heat supplied that is cooking power supplied for the number of people [48]. So it becomes difficult to design the system accordingly by calculating the heat required and converting it into volumes. Hence it is preferred to design it volume wise. And then it can be develop for the heat requirement.

2.4 Heat losses and insulation

For solar cooking at any time, the heat storage unit is an essential mean as we want to store the heat so as to use it at required time. Heat storing doesn’t solve the whole purpose as stored heat should not be dissipated. This heat dissipation may occur due to various heat losses [49]. Therefore to prevent these heat losses, a proper insulation is required [8]. Some insulating material like the mineral wool (stone wool), Polystyrene, aerogel, Vacuum super insulation (VSI), vacuum insulation material (VIM), Glass ceramic foam, vacuum insulation panel VIP and Nano insulation material NIM can be used. Properties of the some insulating materials are given in following Table 2 [50,51,52]

Table 2 Insulating materials & its properties
Full size table

Depending on the operating range, size and volume, the insulation material should to be selected.

2.5 Heat storing media

Depending on the media, basically there are two types of heat (thermal) storage systems [53,54,55]

i. Sensible heat storage system and ii. Latent heat storage system

2.5.1 Sensible heat storage

Specific heat capacity of the material is utilized to store the thermal energy in sensible heat storage [56]. An effectiveness of the storage depends on the store material. The material should have following characteristics [57,58,59]:

  1. i.

    long service life, non-corrosive, non-toxic, non-flammable - large heat storage capacity

  2. ii.

    high thermal diffusivity α = k/ρcp in m2/s and heat diffusivity b = k.ρ,cp in kJ/kgK; with k being the thermal conductivity, ρ the density and cp the specific heat capacity

  3. iii.

    capability to withstand charging/discharging cycles without loss in performance, store capacity or change in structure [60]

  4. iv.

    wide availability, simple handling, storage in simple containers

  5. v.

    low cost

2.5.2 Latent Heat storage System

Storing and retrieving the thermal energy is based on the latent heat of fusion of the material. Where storage medium undergoes a phase transformation which can be either solid to liquid or liquid to gas is latent heat storage system. It uses phase change material to store the heat which can retrieve the energy when outside temperature will be less. These phase change material (PCM) possesses some thermal, physical & chemical properties. Like suitable phase-transition temperature, high latent heat of transition, good heat transfer, favorable phase equilibrium, high density, small volume change, low vapor pressure, long-term chemical stability, compatibility with materials of construction, no toxicity, no fire hazard etc. [46, 57]. Latent heat storage has more capacity and ability to retrieve the heat hence it is an area of research since last few years.

3 Heat transfer system

After the heat is gained from the sun by using solar collector, it can be stored in heat storage system. This heat transportation in and out of the system is very important [61]. But here the main difficulty can be observed in transferring heat from solar collector to heat storage and heat storage to actual cooking area. Actual heat transfer is taken as shown in below Fig. 2:

Fig. 2
figure 2

Generalized Heat transfer System for Solar cooking with PCM

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Heat from solar collector can be transferred to heat storage system with gravity effect. But to circulate the fluid from storage tank to cooker and cooker to tank again or to collector it requires external force that is pump. Dependending upon the circuilating fluid pump may be used [62]. Also selection of pump depends on the flow rate of fluid. Generally gear pump is used for the circulation of fluid as shown in Fig. 3. This pump may be DC pump or AC pump depends upon the drive given. Also Bourdon Pressure guage is used to monitor the pressure in the system as shown in Fig. 3. The another important factor is heat loses occurred during heat transfer. This can be minimised by using proper insulation on the pipes [63]. The insulation which can be used is presented in Table 2.

Fig. 3
figure 3

a Ciculating Pump, b bourdon tube pressure gauge

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4 Material selections

Material selection is also one of the main aspect in the development of Solar cooking system.

Material selection should be done for the following component of the system:

  1. i.

    Material for shell & Tubes (Heat Exchangers)

  2. ii.

    Heat transfer fluid (HTF)

  3. iii.

    Material for Storage (PCM)

  4. iv.

    Material for Heat transfer system

4.1 Material for Heat Exchanger

Heat storage & cooking systems are obviously the heat exchangers. Hence its main function is to transfer the heat from one media to another. So it is always preferable to use material having higher thermal conductivity. Some materials along with their properties are given in Table 3 [64]. Generally tubes may be made up of copper and shell from the still sheets. [65]

Table 3 Material with their thermal conductivity
Full size table

4.2 Heat transfer fluid

Heat has to be transferred from solar collector to heat storage system to cookers and then back to collector. This fluid should have the [27, 66,67,68,69,70] following properties:

i. Good stability,

ii. Low vapor pressure,

 

iii. Low freeze point,

iv. Low viscosity

v. Higher mass flow rate

Corrosiveness is also one of the important factors to be considered while selecting the fluid.

4.3 Material for Storage (PCM)

For latent heat storage system, selection of phase change material is very crucial.

Phase change material can be classified as below:-

  1. i.

    Organic- Paraffin & Non Paraffin (fatty acids)

  2. ii.

    Non organic- Salt hydrates and metallic

  3. iii.

    Eutectics—it’s a mixture of two or more component [71]

There are lots of PCM available now days but its selection is very crucial thing [72]. It depends on the availability and applicability of the PCM of the selected purpose. Some of the promising PCM are mentioned in the below Table 4.

Table 4 Some Suitable PCM with melting temp & Latent Heat values
Full size table

The paraffins are safe, reliable, predictable, less expensive and non-corrosive. They are chemically inert and stable below 500 °C, show little volume changes on melting and have low vapor pressure in the melt form. Salt hydrates are the most important group of PCMs, which have been extensively studied for their use in latent heat thermal energy storage systems. The most attractive properties of salt hydrates are: (i) high latent heat of fusion per unit volume, (ii) relatively high thermal conductivity and (iii) small volume changes on melting. They are not very corrosive, compatible with plastics. This makes NaOH-H2O and NaNO3 promising PCM for the required range of melting temperature. Sugar alcohols (D- Manitol, Myo-inositol, Dulcitol, Erythritol and Sorbitol) are suitable PCM for medium temperature applications. The bases of the selection criterion are mainly on two points: (1) high phase change enthalpy and (2) melting temperatures between 150 and 250 °C. Also, HDPE is non-hazardous and economical, hence it can be used as a PCM.

5 Research limitations & implications

This paper is mainly focused on the application of solar energy for solar cooking. The solar collector is very important and can change the total cost evaluation and may affect complete economical aspect. Hence, the study and evaluation of solar collector can be carried out separately. From the literature survey and study of the various aspects, a methodology has been proposed which may be useful for designing and developing new system possibly suitable for all time. This proposed system is a latent heat storage system for solar cooking which seems to be more efficient and economical. The proposed methodology is as shown below:

figure a

Hence, the system which can easily store the heat and transport it to cooking vessel as per requirement has to be developed by considering all the factors discussed till now.

6 Conclusion

For the development of efficient solar cooker, heat storage is very essential and in turn the heat transfer system also becomes necessary. With more reliability, economy and efficiency, latent heat storage system can be the most suitable system for solar cooking. Depending on the suitability and applicability one can select the better method as discussed in this paper. The proposed latent heat storage system has been presented. The material selection and designing of the system are identified as the prominent factors for development of heat storage and transportation system. In this system, PCM to be used for which, Paraffin Wax, Myo-inositol, HDPE, NaOH-H2O and KNO3-NaNO2-NaNO2 materials looks more promising. But its easily availability in market is one of the concern. If this goes on larger scale then these materials can be of better use and can have better systems. Development of highly efficient and economical system can take place the occupancy in every kitchen which can save lot of conventional fuels which in turn will save the lot of economy of country. Further development and experimentation is needed to prove these concepts.