FACULTY OF ENGINEERING AND SUSTAINABLE DEVELOPMENT Department of Building Engineering, Energy Systems and Sustainability Science

Martxel Lizaso

2020

Student thesis, Advanced level (Master degree, one year), 15 HE Energy Systems

Master Programme in Energy Systems

Supervisor: Björn Karlsson Examiner: Mattias Gustafsson

A review of cooking technology around the world and the potential of solar cooking

i

Abstract This report studies the importance of solar cooking when moving towards a

more sustainable and egalitarian future. The problems regarding its

implementation, in fact, technological, social and economic problems, are

presented. A historic overview is offered as well as the theory behind the

technology. After studying the current situation concerning cooking, different

devices have been analysed and considered, from small ones suitable for

households to bigger ones for institutions. Furthermore, several backup

systems are also proposed aiming to obtain an ideal Integrated Solar Cooker

(ISC). Possible hybrid systems have also been evaluated during the work.

Furthermore, state-of-the-art technology in Thermal Energy Storage (TES) is

also commented and taken into account for the most efficient combination of

technologies. Several photovoltaic kitchens are mentioned in this report as

well.

Two main conclusions have been drawn: trying to solely rely on the sun is a

mistake and the ideal and universal ISC does not exist. Other factors besides

the income are determinant when choosing an energy source, therefore, a

thorough investigation in every particular case is completely necessary for a

successful implementation of an ISC.

However, the countless devices available make the adoption of solar

technology possible in every situation, helping to achieve some of the

Sustainable Development Goals.

Keywords: solar cooking, thermal energy storage, Phase Change Materials,

energy consumption cooking, institutional cooking, energy sources cooking.

ii

Preface

First of all, I would like to express gratitude to Stefan Karnebäck, from

Engineers Without Borders Sweden, for introducing me to the amazing world

of solar cooking and for providing me with valuable knowledge and help

throughout the course of this work. He was the one who proposed the subject

of the thesis, and even though in the beginning I was sceptical about solar

cooking, after our first meeting I realised that it is a really powerful

technology capable of changing millions of people’s lives.

I also wish to thank Prof. Björn Karlsson, my supervisor from University of

Gävle.

I would like to express thanks to Tamera Peace Research and Education

Center and to Sunseed Desert Technology for the online meetings in which

they have kindly provided information about their systems as well as answered

all of my questions.

This work would not have been possible without the constant support and

encouragement of my family and friends, who have always been an important

pillar in my life.

Finally, I could not miss the opportunity to express my most sincere gratitude

to Helena Bergman, a really close friend of our family, who has always been

ready to help since I told her I was coming to Sweden. She has solved all the

inconveniences I have faced during my stay in this beautiful country.

iii

Nomenclature

Abbreviation and Acronyms

Letters Description

ASABE American Society of Agricultural and

Biological Engineers

EU European Union

INR Indian rupee

IR Infra red

ISC Integrated solar cooker/cooking

ISSBH Improved small scale box hybrid

LCC Life Cycle Cost

LHTES Latent heat thermal energy storage

LPG Liquefied petroleum gas

MNRE Ministry of New and Renewable Energy

PCM Phase change material

PNG Pressurised natural gas

PSC Parabolic solar cooker

PV Photovoltaic

SBC Solar box cooker

SCI Solar Cookers International

SDG Sustainable development goals

SFSC Small family solar cooker

SHTES Sensible heat thermal energy storage

SSB Small scale box

iv

SSBH Small scale box hybrid

TES Thermal energy storage

UNHCR United Nations High Commissioner for

Refugees

USD United States Dollars

UV Ultra violet

VIS Visible

WHO World Health Organisation

Latin

Symbol Description Unit

A Area m2

c Specific heat capacity kJ kg-1 K-1

F1 First figure of merit K m2 W-1

F2 Second figure of merit -

h Convection coefficient W m-2 K-1

I Solar irradiance W m-2

m Mass Kg

P Power W

r Reflectance -

R Thermal resistance m2 K W-1

T Temperature ºC or K

t Time S

v

Greek

Symbol Description Unit

ɛ Emissivity -

α Solar altitude / Absorbance º / -

Δ Difference -

θz Zenith angle º

σ Stefan Boltzmann’s constant W m2 K4

𝜏 Transmittance -

vi

Table of contents

1 Introduction ................................................................................................ 1

1.1 Background........................................................................................... 1

1.2 Aims and limitations ................................................................................ 3

2 Theory ...................................................................................................... 5

2.1 Solar radiation ....................................................................................... 5

2.2 Properties of materials.............................................................................. 8

2.3 Efficiency of solar cookers ........................................................................ 11

2.4 Thermal Energy Storage (TES) ................................................................... 13

3 Method .................................................................................................... 15

4 Results and discussion ................................................................................... 16

4.1 Current situation ................................................................................... 16

4.2 Household cooking ................................................................................. 20

4.2.1 Solar technology .............................................................................. 20

4.2.2 Backup system ................................................................................ 35

4.3 Institutional cooking ............................................................................... 37

4.3.1 Solar technology .............................................................................. 37

4.3.2 Backup system ................................................................................ 41

5 Conclusions ............................................................................................... 45

5.1 Study results ........................................................................................ 45

5.1.1 Households .................................................................................... 45

5.1.2 Institutions .................................................................................... 47

5.2 Outlook .............................................................................................. 49

5.3 Perspective .......................................................................................... 49

References ....................................................................................................... 51

vii

Figures index

Figure 1. Types of solar cookers: (a) panel cooker; (b) concentrating cooker; (c) box

cooker. Source: ResearchGate............................................................................. 2

Figure 2. The world population in 2017. Billions of people on different income. .............. 4

Figure 3. Solar radiation per wavelength outside the atmosphere. ................................. 5

Figure 4. Schematic illustration of a pyranometer (a) and a pyrheliometer (b).................. 6

Figure 5. Solar altitude and zenith angle. Source: Itacanet.org. .................................... 7

Figure 6. Latitude angles. Source: Quora.com......................................................... 7

Figure 7. Global horizontal irradiation. Source: Global Solar Atlas. .............................. 8

Figure 8. Properties of a selective surface. Source: Energyprofessionalsymposium.com. .... 9

Figure 9. Schematic image of reflectance, transmittance and absorbance. Source:

Biologywiki. ..................................................................................................10

Figure 10. Standardised cooking power against temperature difference (Funk, 2000). ......13

Figure 11. LCC per meal cooked in rural Kitui, Kenya. ............................................17

Figure 12. LCC per meal cooked in rural Moshi, Tanzania. .......................................18

Figure 13. Efficiencies of different energy sources for cooking (Ramanathan and Ganesh,

1994). ..........................................................................................................19

Figure 14. CooKit. Source: SCI. .........................................................................20

Figure 15. Standard cooking power of the CooKit with a Pyrex bowl as transparent cover.

Source: SCI. ..................................................................................................20

Figure 16. Potential designs: (a) polyhedral, (b) semi-cylindrical, (c) bi-rectangular, (d)

parabolic. ......................................................................................................21

Figure 17. Schematic diagram of the solar cooker proposed by El-Sebaii. ......................23

Figure 18. 3D sketch of the solar cooker proposed by Guidara et al.: (a) with outer

reflectors, (b) without outer reflectors. ................................................................24

Figure 19. Prototype designed by Coccia et al.. ......................................................25

Figure 20. Water temperatures of both cookers from sunrise to sunset. ........................26

Figure 21. Temperature of water in the cooker with black paint coated stone pebbles. .....27

Figure 22. Variation of water temperature in cooker 1 and cooker 2. ...........................28

Figure 23. SK-14 cooker along its inventor Dr. Dieter Seifert (second from left). ...........30

Figure 24. Parabolic trough and storage unit. .........................................................31

Figure 25. Test 1: boiling 1 L of water in both cookers. ............................................32

Figure 26. Test 2: frying test in heat storage unit. ...................................................32

Figure 27. Schematic and real image of Edmond’s cooker. .........................................33

Figure 28. Novel indirect solar cooker with indoor PCM thermal storage......................34

Figure 29. Real cooking test. .............................................................................34

Figure 30. Rocket-stoves. Source: homesthetics. ....................................................36

Figure 31. Schematic view of a paraboloid. ............................................................39

Figure 32. Working principle of Scheffler cooker. Source: SCI. ..................................39

Figure 33. Indirect flat plate collector cooker developed by Schwarzer and Vieira da Silva. 41

Figure 34. Details of a biogas digester. Source: Tamera.org. ......................................42

viii

Tables index

Table 1. On-stove time and energy to cook 1kg of beans with a pressure cooker with

insulating box or without it. ..............................................................................16

Table 2. Important parameters of SFSC-1 and SFSC-2. .............................................22

Table 3. F1 and F2 for different cooker configurations..............................................26

Table 4. Commercially available institutional solar cookers in India. ............................38

1

1 Introduction

Humankind is completely dependent on fossil fuels, since more than 80% of the

global energy consumption is provided by these non-renewable sources. Their

volatile price as well as their future scarceness urges the necessity of a global change.

In developing countries their share is even higher, which makes this population even

more vulnerable. Cooking accounts for a considerable part of household’s and

institution’s energy consumption, relying mainly on kerosene, coal, wood and other

polluting sources due to its low cost and availability. However, the massive use of

these sources causes serious health and ecological problems (Toonen, 2009). This is

a more common issue in remote rural areas where the energy required for cooking

is supplied by non-commercial fuels like firewood, agricultural waste, cow dung and

kerosene (Wentzel and Pouris, 2009; Cuce and Cuce, 2013). Besides the global

warming potential and the deforestation caused by these fuels, their users are

exposed to harming fumes. According to the World Health Organization (WHO)

1.6 million deaths are caused by indoor air pollution every year, mostly women and

children (Household Energy and Health WHO Library Cataloguing-in-Publication Data,

2006).

A considerable share of household’s energy consumption in the European Union

(EU) as well as the United States accounts for electricity consumption due to

electrical appliances such as microwaves and ovens for cooking or heating food.

Furthermore, consumers in developed economies tend to replace these appliances

before they fail generating tons of electrical and electronic waste, which leads to the

loss of already scarce resources. In American households cooking processes consume

around 6.9x108 GJ/year. It is estimated that 133 million microwaves in the EU

consume 148 PJ of primary energy, emit 6.9 Mt of CO2 eq. and generate 184,000 t

of waste every year (Hager and Morawicki, 2013; Mendoza et al., 2019). All the

aforementioned demonstrate the improvement potential of solar technology in the

whole world.

Solar energy is one of the cleanest energy sources available nowadays. It offers a

wide variety of applications which can be divided in two main groups: electricity

generation and heat production. Among the thermal applications, solar cooking is

one of the simplest, cheapest and most attractive option (Cuce and Cuce, 2013).

1.1 Background

A solar cooker is a device that transfers solar energy to a pot, thus increasing its

temperature until reaching an almost constant value and enabling to cook the food

inside of it. These devices can also be used for other purposes such as pasteurization

and sterilization and water desalination (Cuce and Cuce, 2013).

2

The first solar oven was invented in 1767 by Horace de Saussure, a Swiss physicist.

Since then, it has been widely used all over the world. Based on how thermal energy

is transferred to the pot the systems can be divided into two main categories: direct

and indirect. Alternatively, when considering the system itself, there are three main

categories: panel cookers, box cookers and concentrating cookers (Sedighi and

Zakariapour, 2014; Aramesh et al., 2019). These are shown in Figure 1.

FIGURE 1. TYPES OF SOLAR COOKERS: (A) PANEL COOKER; (B) CONCENTRATI NG COOKER; (C) BOX COOKER.

SOURCE: RESEARCHGATE 1.

However, the continuous research in the field has lead to the development of new

devices such as vacuum tube cookers and conical cookers.

Panel cookers can easily be constructed with low-cost materials. The structure can

be made of cardboard and then covered with aluminium foil to reflect the sun rays

towards the pot, which should be inside a plastic bag or a glass bowl to retain heat

radiation going out. They are easily transportable, their weight does not exceed

0.5 kg and their price is in between 5-15 USD. The negative aspect is that they

hardly reach temperatures higher than 100oC, making the cooking process long.

A box cooker is a box made of an insulating material and with the upper part

covered by a transparent medium. The purpose of this medium is to let sun

radiation in and avoid heat radiation going out. The performance of the device can

be enhanced using booster mirrors to cover a larger area. These cookers are slow to

heat up but perform well even under diffuse radiation and windy conditions.

Cooking temperatures are higher than in panel cookers and some box cookers are

big enough to cook with multiple pots. However, cooking times are still longer

when compared to conventional methods.

1 https://www.researchgate.net/figure/Types-of-solar-cookers-a-solar-panel-cooker-b-solar-parabolic-cooker-and-c-solar_fig1_327228754

3

Concentrating cookers cover a large area and reflect the incoming solar radiation

into a focal point where the pot may be placed, thus, reaching extremely high

temperatures in short times, comparable to conventional cooking methods.

However, this kind of cookers includes higher risks and a more frequent

reorientation. A variation of a concentrating cooker, the Scheffler cooker, allows

inside cooking by means of a double reflection.

A considerable part of the ongoing research is focused on developing more efficient

Thermal Energy Storage (TES) systems, which may be defined as the temporary

storage of thermal energy at low or high temperature. Based on the thermal storage

mode, these can be divided into two main types: Latent Heat Thermal Energy

Storage (LHTES) and Sensible Heat Thermal Energy Storage (SHTES) (Kousksou et

al., 2014; Aramesh et al., 2019). In the former heat is absorbed and then released by

means of a Phase Change Material (PCM) usually changing from solid to liquid when

absorbing heat and the opposite way when releasing it (Mofijur et al., 2019). In the

latter heat absorption and removal occurs by a change in temperature of the storage

material. SHTES is cheaper than LHTES for small volumes but its energy storage

density is lower. However, PCMs have some added disadvantages such as low

thermal conductivity, degradation of the PCM material and higher cost compared to

SHTES (Mawire, Phori and Taole, 2014).

Although solar cooking is technically feasible, there are still several barriers that

need to be overcome. Plenty of projects have faced failure even where sun radiation

is abundant. Otte (2013) divides the variables that influence the adoption of solar

cooking into six categories: economic, social, cultural, environmental, political and

technical.

1.2 Aims and limitations

Trying to rely solely on the sun is a common mistake and leads to stop solar

cooking. It does not always shine, sometimes for days or even weeks. Therefore, it

is completely necessary to include a backup system. Combining a solar cooker with

storage devices and backup systems is the most convenient way of solar cooking and

it is consequently called Integrated Solar Cooking (ISC).

The aim of this study is to analyse the current situation regarding cooking in the

world and evaluate possible combinations of solar cooking devices and TES systems,

always including a backup, to obtain an optimum hybrid system that will be

competitive against traditional cooking technologies as well as environmentally

friendly. The solution must be easy to use, adaptable to different cooking habits and

cultures and affordable for people with low incomes. It should also overcome the

principal hurdle of solar cooking, in fact, being able to cook when the sun is not

shinning.

4

Even though the terms “developing” and “developed” countries are employed during

this report, these are already outdated, the world today is no longer divided in two

categories. Dr Hans Rosling, a late Swedish professor in Global Health, suggests a

simple but more relevant and useful way of dividing up the world into four income

levels as shown in Figure 2. The figure represents the current world population

categorised in four daily income levels (Rosling et al., 2018).

FIGURE 2. THE WORL D POPULATION IN 2017. BILLIONS OF PEOPLE ON DIFFERENT INCOME . SOURCE: GAPMINDER.ORG 2.

Therefore, four different hybrid systems will be suggested according to Dr Hans

Rosling’s classification.

Nevertheless, there are some limitations. This study will be carried out in a

theoretical aspect, based on previous studies and on the experience and

recommendations of users. No experiment will be performed due to the lack of

equipment and testing facility.

2 https://www.gapminder.org/topics/four-income-levels/

5

2 Theory

2.1 Solar radiation

The solar radiation towards the earth’s atmosphere is in average 1367 W/m2, called

solar constant, which varies from a maximum of 1412 W/m2 in the end of

December to a minimum of 1322 W/m2 in the beginning of July. The elliptic orbit

of the earth explains this phenomenon. Figure 3 shows the spectra of the solar

radiation outside the atmosphere.

FIGURE 3. SOLAR RADIATION PER WAVELENGTH OUTSIDE T HE ATMOSP HERE. SOURCE: EUROPEAN SPACE AGENCY (ESA)3.

This radiation can be divided into three parts:

− λ < 0.38 µm : Ultra Violet (UV) radiation

− 0.38 < λ < 0.78 µm : Visible light (VIS)

− 0.78 µm < λ : Infra Red radiation (IR)

A solar cooker at any point of the earth does not receive a power input of

1367 W/m2. The solar flux hitting a horizontal surface of the earth is affected by

two main factors: atmospheric conditions and solar angles.

3 https://www.esa.int/ESA_Multimedia/Images/2017/12/Solar_spectrum

6

Some substances present in the atmosphere such as dust, water, carbon dioxide,

oxygen and ozone absorb part of the solar radiation, reducing the solar flux up to

30%. Thus, the remaining part is composed by two components: direct and diffuse

radiation.

Direct or beam radiation, represents the part of the solar flux that has not been

absorbed or scattered by the atmosphere. It is measured with a pyrheliometer.

Diffuse radiation represents the part that has been absorbed or scattered by the

atmosphere, coming from all angles and corresponding up to 15% of the direct

radiation. The global radiation towards the ground is measured with a pyranometer.

Both measuring devices are shown in Figure 4.

FIGURE 4. SCHEMATIC ILLUSTRATION OF A PYRANOMETER (A) AND A PYRHELIOMETER (B). SOURCE: RESEARCHGATE 4.

In cloudy days most of the radiation is diffuse, which is not reflected by reflective

materials. The majority of solar cookers rely on the radiation received by their

reflective surfaces; therefore, the input power in the cooker is insufficient to cook

under cloudy conditions.

The other important factor affecting the input power in a solar cooker is solar

angles. The relative position of the sun in respect to the earth varies from sunrise

to sunset and this is measured with the solar altitude (α) or the zenith angle (θz).

The former is the angle between a horizontal surface on the earth and a solar ray and

the latter the angle between a vertical line and a solar ray. At solar noon, when the

sun is exactly in the south, the altitude angle reaches its maximum value. When this

value is lower than 90º the solar flux on a horizontal surface is reduced by the sine of

the altitude. Solar altitude and zenith angle are represented in Figure 5.

4 https://www.researchgate.net/figure/A-schematic-illustration-of-a-pyranometer-a-and-a-pyrheliometer-b-based-on-9-and_fig5_332606005

7

FIGURE 5. SOLAR ALTI TUDE AND ZENITH ANGLE. SOURCE: ITACANET.ORG 5.

Latitude angle, shown in Figure 6, measures the north or south position of a specific

site relative to the equator. At any given time two horizontal surfaces at different

latitude have different solar altitude, therefore, different solar flux.

Taking into account that the rotation axis of the earth is tilted 23.5º with respect to

the vertical of its orbit, the angle of solar rays to a horizontal surface changes with

the date of the year. Only the regions in latitudes between -23.5º and 23.5º see the

sun directly overhead (α=90º) in some period of the year, receiving a higher flux

per square meter, as shown in the Figure 6 below.

FIGURE 6. LATITUDE ANGLES. SOURCE: QUORA.COM6.

5 https://www.itacanet.org/the-sun-as-a-source-of-energy/part-1-solar-astronomy/

8

Figure 7 illustrates the global horizontal radiation (kWh/m2) in the world,

demonstrating the potential that solar energy applications have in regions around the

equator.

FIGURE 7. GLOBAL HORIZONTAL IRRADIATION. SOURCE: GLOBAL SOLAR ATLAS7.

2.2 Properties of materials

In the typical operation of a solar cooker, solar radiation hits a reflective surface and

is reflected towards a cookware crossing a transparent cover, which transmits most

of the radiation and absorbs and reflects away a small fraction. The cookware

absorbs the solar energy increasing its temperature and transferring heat to the food.

Simultaneously, heat is lost to the surrounding atmosphere by means of convection

and radiation. The heat loss may be minimised by thermal insulation.

6 https://www.quora.com/If-the-Earth-experienced-aphelion-during-the-Northern-Hemispheres-winter-instead-of-perihelion-as-presently-occurs-how-much-colder-would-winter-in-the-Northern-Hemisphere-be 7 https://globalsolaratlas.info/map?c=14.093957,-21.796875,2

9

The capability of certain material to absorb solar radiation is measured by its

absorbance (α), which is the fraction of the incident radiation that is absorbed by the

material. The rest will be transmitted or reflected away. In order to get heat and

increase food’s temperature, a pot used for solar cooking should have a high

absorbance. Values around 0.9 and above are acceptable while low values would

reflect incident radiation. Taking into account that every surface emits its own

thermal radiation depending on its temperature and emissivity (ɛ), which can be

defined as the fraction of radiation emitted by a blackbody (perfect emitter) at the

same temperature, it is preferable to use a pot that absorbs radiation well and emits

poorly. Hence, the ratio of absorbance to emissivity is an indicator of the capacity of

a surface to get hotter. Surfaces with high values of this ratio are called selective

surfaces, which are characterised by measurements of the reflectance (r). High

reflectance for long wavelengths implies low emissivity and at the same time, low

reflectance for short wavelengths implies high absorbance. Such characteristics are

shown in Figure 8.

FIGURE 8. PROPERTIES OF A SELECTIVE SURFACE . SOURCE: ENERGYPROFESSIONALSYMPOSI UM.COM8.

Several solar cookers use a transparent cover in order to retain heat. Shortwave

radiation coming from the sun passes through the cover and long wavelength

radiation coming from the hot surfaces is reflected back. This is the so called

greenhouse effect. The fraction of the incoming solar flux that gets through the cover is

called transmittance (𝜏). Glass and plastic are usually used for this purpose. An ideal

cover material would have high transmittance for solar radiation and low

transmittance for thermal radiation.

8 http://energyprofessionalsymposium.com/?p=5695

10

FIGURE 9. SCHEMATIC IMAGE OF REFLECTANCE, TRANSMITTANCE AND ABSORBANCE. SOURCE: BIOLOGYWIKI9.

Most solar cookers use reflectors to increase the amount of radiation flux hitting an

absorber surface or food pot. When radiation strikes a reflecting surface, some is

reflected back still being beam radiation, with an angle of reflection equal to the

angle of incidence. A good reflector should have a reflectance over 0.9. The flux

heating the target with reflectors divided by the flux without reflectors is called

concentration ratio.

Heat loss is driven by temperature difference, therefore, the losses increase while

the pot gets hotter. There are three main mechanisms with which heat is lost:

conduction, convection and radiation. The former represents a heat transfer through

a material and is more common in box cookers. The second represents a heat

exchange with a fluid. The latter occurs in absence of a medium between two

surfaces with different temperatures. Convection and radiation are more common in

panel and concentrating cookers.

Thermal resistance (R) may be represented as the temperature difference across a

material subjected to a heat flux. In order to reduce conduction losses materials with

high thermal resistance are used as insulation in the walls of a box cooker. In the

following equation Ac represents the wall area.

𝑄𝑐𝑜𝑛𝑑 = 𝐴𝑐 ∗𝛥𝑇

𝑅 [𝑊]

Convection and radiation losses are calculated with the following expressions:

𝑄𝑐𝑜𝑛𝑣 = ℎ ∗ 𝐴𝑐 ∗ 𝛥𝑇 [𝑊]

𝑄𝑟𝑎𝑑 = ɛ ∗ 𝜎 ∗ 𝐴𝑐 ∗ (𝑇𝑠4 − 𝑇𝑠𝑢𝑟

4 ) [𝑊]

9 http://biologywiki.apps01.yorku.ca/index.php?title=File:Light_outcomes.png

11

Where:

− h: convection coefficient [W/(m2K)]

− σ: Stefan Boltzmann’s constant = 5.67*10-8 [W/(m2K4)]

− Ts: temperature of the surface

− Tsur: temperature of the surroundings

2.3 Efficiency of solar cookers

The performance of a steady system, i.e., a flat plate collector, is easily defined as

the rate of energy absorbed by a fluid flow divided by the power delivered by the

sun. However, solar cookers are unsteady systems where there is not a flow

entering and leaving the system at constant temperatures. An initially cold mass,

comprised by food and pot, receives an energy input increasing its temperature. As

time goes on, the temperature rise slows down until the mass reaches a steady state.

At this point, all the input energy is used to maintain a constant temperature in the

cooker long enough to cook the food, therefore, there is no temperature rise and all

the input heat is lost to the ambient, which would mean a 0% efficiency attending to

the conventional method. However, several authors use the average heating power

of the cooker to obtain a value for the efficiency. Thus, the average power is divided

by the solar irradiance.

𝑄 =𝑚 ∗ 𝑐𝑝 ∗ (𝑇𝑏 − 𝑇𝑎)

𝛥𝑡 [𝑊]

𝜂 =𝑄

𝐼 ∗ 𝐴 [−]

Where, m and cp are mass and specific heat of water, Tb and Ta boiling point and

initial temperature of water and Δt the interval of time. I and A represent solar

irradiance and collector area of the cooker.

It is a hard task to compare solar cooker’s performance due to their high

dependency on uncontrolled variables such as wind speed, ambient temperature,

materials used and available radiation among others. While many technically

successful solar cookers are in operation, there is no broadly accepted basis for

comparing their performance. For this purpose, several attempts to develop

standardised methods to evaluate cookers have been proposed in the last years.

Mullick, Kandpal and Saxena (1987) developed a method consisting of two figures

of merit, F1 and F2. The first figure is obtained performing a stagnation test with no

load.

12

𝐹1 =𝑇𝑝 − 𝑇𝑎

𝐼 [

𝐾 ∗ 𝑚2

𝑊]

Where Tp and Ta are the maximum absorber plate temperature and ambient

temperature respectively and I is the solar radiation on a horizontal surface at the

stagnation time.

The second figure is obtained effectuating a sensible heat test where a mass of water

is heated to certain temperature.

𝐹2 =𝐹1 ∗ 𝑚𝑙𝑜𝑎𝑑 ∗ 𝑐𝑙𝑜𝑎𝑑

𝐴𝑝 ∗ 𝛥𝑡∗ ln [

1 −𝑇𝑙𝑜𝑎𝑑𝑖−𝑇𝑎𝑎𝑣𝑔

𝐹1∗𝐼𝑎𝑣𝑔

1 −𝑇𝑙𝑜𝑎𝑑𝑓−𝑇𝑎𝑎𝑣𝑔

𝐹1∗𝐼𝑎𝑣𝑔

] [−]

Where mload and cload are the mass and the specific heat capacity of water, Ap is the

area of the absorber plate, Δt is the duration of the test and the subscripts i, f and

avg attend to initial, final and average respectively. Thus, the required time to boil

water can be calculated substituting Tload,f by 100ºC and obtaining Δt.

Nevertheless, this standard procedure is more complicated and less universal than

the one proposed by Funk (2000), though the characteristic curve they developed is

a good predictive tool.

The standard test proposed by Funk was presented at the Thirld World Conference

on Solar Cooking (Avinashilingam University, Coimbatore, India, 6-10 January,

1997). The objective of the proposal was to make testing as simple as possible in

order to present cooker’s potential in widely recognised units. Thus, the test

standard committee convened that.

The one figure best representing thermal performance is effective cooking power, which accounts

for both different cooker size and heat gain rates. The unit of power with which most people

are familiar is the Watt. The influence test conditions have on results can be minimized if

uncontrolled variables are held to certain ranges.

They developed the following cooking power expression:

𝑃 =𝑚𝑙𝑜𝑎𝑑 ∗ 𝑐𝑙𝑜𝑎𝑑 ∗ 𝑑𝑇𝑙𝑜𝑎𝑑

𝑑𝑡 [𝑊]

A term called adjusted cooking power was also presented:

𝑃𝑠 =700 ∗ 𝑚𝑙𝑜𝑎𝑑 ∗ 𝑐𝑙𝑜𝑎𝑑 ∗ 𝛥𝑇

600 ∗ 𝐼∗ [𝑊]

13

The measurements must be carried out in 10 min (600 s) intervals and cooking

power for each interval needs to be corrected to a standard radiation of 700 W/m2.

The obtained parameter should be plotted against temperature difference between

ambient and load. The standard cooking power (Ps(50)) is the adjusted cooking power

at a temperature difference of 50ºC. An example of the mentioned plot is shown in

Figure 10. This standard procedure was approved by the American Society of

Agricultural and Biological Engineers (ASABE) in 2003.

FIGURE 10. STANDARDISED COOKING POWER AGAINST TEMPERATURE DIFFERENCE (Funk, 2000).

The results obtained with this procedure appeared to be independent of location and

date as long as the protocol was followed.

2.4 Thermal Energy Storage (TES)

As mentioned in 1.1 Background, the two principal means of storing thermal energy

for solar cooking applications are SHTES and LHTES. The former relies on the

temperature change of the storing material:

𝑄𝑠𝑡𝑜 = 𝑚𝑠𝑡𝑜 ∗ 𝑐𝑠𝑡𝑜 ∗ 𝛥𝑇𝑠𝑡𝑜 [𝐽]

14

Where the terms represent stored energy and mass, specific heat capacity and

temperature change of the storage material from left to right. The performance of

the storage system depends principally on the density and specific heat capacity of

the material, which will define the necessary volume. Two disadvantages of SHTES

systems are the large size usually required and the temperature swing created from

the sensible addition and extraction of energy. Another important parameter is the

rate at which heat can be released (Kousksou et al., 2014).

LHTES is based on the phase change of the storage material:

𝑄𝑠𝑡𝑜 = 𝑚𝑠𝑡𝑜 ∗ (𝑐𝑠𝑠𝑡𝑜∗ 𝛥𝑇1 + ℎ𝑠𝑙 + 𝑐𝑙𝑠𝑡𝑜

∗ 𝛥𝑇2) [𝐽]

Where the terms inside the parenthesis represent the sensible heat stored in the

solid phase of the storage material, the enthalpy of fusion and the sensible heat

stored in the liquid phase respectively. The latent heat, the heat needed during the

phase change, is generally much higher than the sensible heat, so smaller storage

volumes are required. The phase change occurs at nearly constant temperature.

15

3 Method

Taking the limitations expressed in 1.2 Aims and limitations into account, the present

thesis will be a pure literature study, i.e. without any own obtained empirical

material and with no own significant calculation work or theoretical creations.

Various web databases have been consulted when searching for literature in this

study. University’s (Högskolan i Gävle) Discovery portal has probably been the

most useful source. However, other databases such as Inspec or Google Scholar have

also been great help. The literature mentioned in this report was found entering

some specific keywords, being “Solar cooking”, “Phase Change Materials” and

“Thermal Energy Storage” the ones that gave the best results. Preference has been

given to peer-reviewed articles, making use of the filters available in the databases.

These articles where the solar cookers were evaluated with the procedures

presented in 2.3 Efficiency of solar cookers have also been preferred.

All the references have been handled with Mendeley, software for reference

managing. Specifically, the desktop and the online version as well as the plug-in for

MS Word that it offers have been the most useful.

The first step has been reading as much information as possible, consulting books at

university’s library as well as online sources in order to get acquainted with solar

cooking. Solar Cookers International’s (SCI) wiki has provided really valuable

information in this aspect. After that, available ongoing research has been studied,

thus, recognising the actual problems and thinking about possible solutions.

Eventually, several researchers and people working in the field have been contacted

to ask about more detailed information.

16

4 Results and discussion

This chapter is divided into three subchapters. First of all, the current situation is

described, explaining the actual fuels and technologies in use as well as energy

requirements of cooking processes. The second subchapter deals with household

cooking proposing a wide variety of solar cookers, storage systems and backup

facilities, in fact, Integrated Solar Cooking systems. The devices are ordered from

simplest to more complicated, attaining mainly to cost, ease of use and size.

Eventually, the same procedure followed for households is used for institutions.

4.1 Current situation

De et al. (2013) performed a study in which the amount of energy required for

cooking different meals was obtained. They used a kerosene stove to cook 1 kg of

beans and 1 kg of Irish potatoes separately and measured pot’s internal temperature

every minute until the food was fully cooked. Their energy efficient cooking method

included a kerosene stove, a pressure cooker and a cardboard box whose inner walls

were covered with three layers of highly reflective aluminium foil in order to

minimise radiation losses. Their results, shown in Table 1, demonstrate the

possibility of improving the cooking process regarding time and energy use when

utilising an insulation box for reducing on-stove times and radiation losses.

TABLE 1. ON-STOVE TIME AND ENERGY TO COOK 1KG OF BEANS WITH A PRESSURE COOKER WITH INSULATING

BOX OR WITHOUT IT.

With insulation box Without insulation box

On-stove time (min) 19.04±0.17 49.03±1

Required energy (kJ) 742.0±25 1900±100

However, in most of households in developing countries pressure cookers are not

affordable and ordinary pots are used instead, which require three to four times

more energy and time than pressure cookers. Furthermore, less efficient cooking

stoves or even three stone fireplaces are common. Nevertheless, the results

illustrated above demonstrate the improvement potential of cooking methods when

using a low cost cardboard box with reflector materials, permitting a considerable

reduction in on-stove time and energy. Currently, hay baskets are used with the

same purpose, i.e. to save energy and to maintain food warm until the evening.

17

It is estimated that around 2.5 billion people around the world rely heavily on

biomass to satisfy their energy needs in households and institutions. In sub-Saharan

Africa, wood is the primary energy source for cooking for 85% of the population in

rural areas as well as a considerable part of urban areas. In Kenya biomass provides

68% of the overall energy requirement while in Tanzania charcoal, firewood, dung

and other traditional fuels are the main sources. Their use is growing even more due

to the unstoppable increase in population (Okoko et al., 2008). With the growing

scarcity of firewood, users, mostly women and children, have to walk up to 10 km

and spend 3-4 h for its collection. Furthermore, the efficiencies of the stoves or

means of burning the fuel they use are as low as 12%.

Okoko et al. (2008) carried out a Life Cycle Cost (LCC) analysis of different biomass

fuels focusing on their production and consumption technologies in rural and urban

areas of Kenya and Tanzania. They assumed the required net energy in the pot per

meal to be 5 MJ. Even though the meal itself is not specified, this value corresponds

to the results shown in Table 1 assuming low efficient technology for cooking; the

meal could therefore consist of around 1 kg of beans. Their results included the

energy cost per meal cooked, illustrated in Figure 11 and Figure 12 for rural Kenya

and Tanzania respectively.

FIGURE 11. LCC PER MEAL COOKED IN RURAL KITUI, KENYA.

18

FIGURE 12. LCC PER MEAL COOKED IN RURAL MOSHI, TANZANIA.

Taking a household comprised by around 8 people in rural Moshi as a reference, and

considering that 80% of the rural population rely on firewood their annual cost in

fuel can be calculated. The proposed meal (1 kg of beans) could feed all the

inhabitants and assuming two big meals per day the annual energy cost would be

36.5 USD.

Different factors influence households’ choice of cooking energy source, beyond the

conventional price factor. Other variables such as occupation, household size,

educational attainment, type of house, distance from source and location of the

kitchen influence significantly the decision (Amoah, 2019; Mangula et al., 2019).

The most common ways of cooking in developed countries are surface and oven

cooking mainly driven by electricity. Other energy sources are natural gas and in a

small portion Liquefied Petroleum Gas (LPG), kerosene and wood. Hager and

Morawicki (2013) defined overall efficiency as the product of the production and

transport efficiency of the fuel and the appliance or end-use efficiency, stating that

this is a more comprehensive approach than reporting solely the efficiency of the

device. The efficiencies of different energy sources are shown in Figure 13.

19

FIGURE 13. EFFICIENCIES OF DIFFERENT ENERGY SOURCES FOR COOKI NG (Ramanathan and Ganesh, 1994).

Mendoza et al. (2019) built three home-made solar cookers (panel, box and

parabolic) mainly out of recycled materials and analysed the potential benefits of

using these devices instead of microwaves in Spain. They quantified the

environmental and economic performance of each solar cooker and compared it to

microwaves. Their conclusions included that 42600 t of CO2 eq. could be avoided

and 860 TJ of primary energy could be saved annually at a national level. Moreover,

the electricity consumption and household waste would decrease 67 GWh/yr and

4200 t/yr respectively, saving up to €23.2 million.

In institutional cooking the number of meals can vary from 25 up to a few thousand

meals at a time. Considering the daily energy requirement for cooking in the range

of 1.7–2.7 MJ/(day.person) a considerable amount of energy is needed every time

institutions cook. Institutions in India mostly utilize commercially available fuels

such as Liquefied Petroleum Gas (LPG), Pressurised Natural Gas (PNG) and diesel

due to their ease of operation and faster cooking rate, while in the Kilimanjaro

Region, for example, 57% of institutions rely on firewood (Indora and Kandpal,

2018). In developed countries, however, electricity is the main energy source

followed by natural gas.

20

4.2 Household cooking

A household consists of one or more people who live in the same dwelling and share

meals. The space is usually limited; therefore, the cooker should be of an

appropriate size and capable of cooking enough food for all the members of the

group.

4.2.1 Solar technology

The simplest and cheapest solar cooker available is the panel cooker. The CooKit,

developed by a volunteer group of engineers associated with Solar Cookers

International (SCI), is one of the most spread versions. It is made of cardboard,

aluminum foil and water based glue. Its cost ranges between € 3-7 but SCI published

a construction manual, therefore, it can easily be home made with recycled

materials. A transparent cover surrounds the pot acting like a greenhouse and

retaining heat. The device is shown in Figure 14.

FIGURE 14. COOKIT. SOURCE: SCI.

SCI tested the CooKit with a Granite

Ware cooking vessel in a Pyrex bowl

and in a plastic bag in accordance to

the protocol approved by ASABE. In a

clear day, with an average irradiance

and ambient temperature of around

900 W/m2 and 25ºC respectively and

in a pot surrounded by a Pyrex bowl,

3.1 L of water reached 95ºC in

around 3.5 hours.

FIGURE 15. STANDARD COOKING POWER OF THE COOKIT WITH A PYREX BOWL AS TRA NSPARENT COVER.

SOURCE: SCI.

21

Figure 15Figure 15 shows the standard cooking power of the CooKit with a Pyrex

bowl as transparent cover. The value obtained was 51.2 W. They also tested the

device with a plastic bag as transparent cover resulting in a standard cooking power

of 27.7 W. The experiments demonstrate the importance of the auxiliary devices

used while cooking.

It is convenient that the possible new users receive instructions before acquiring a

CooKit, since even the simplest solar cooker is dependent on configuration

parameters. The device needs to be redirected towards the sun every 30 minutes for

an optimum performance. Pejack studied how the different surfaces of the cooker

reflected solar radiation to hit the pot and demonstrated how important a correct

configuration of the tilt of the front flap is.

Regattieri et al. (2016) developed a device fully integrated to a complete kitchen-set

box enclosed to the humanitarian aids. They combined the problems of providing

effective cooking facilities as well as properly managing the packaging waste. The

main idea was to build a solar cooker by recycling the cardboard box. They studied

the performance of four potential designs shown in Figure 16.

FIGURE 16. POTENTIAL DESIGNS: (A) POLYHEDRAL, (B) SEMI-CYLINDRICAL, (C) BI-RECTANGULAR, (D)

PARABOLIC.

22

Concerning thermal performance and assembly and use complexity, the parabolic

design happened to be the most adequate cooker. 0.5 L of water reached boiling

temperature under 2 hours two times a day, in the morning and in the afternoon,

under Italian meteorological conditions (average solar irradiance of 770 W/m2 and

730 W/m2 respectively). Several tests confirmed an efficiency in the range 14-18%,

comparable to the three stone cooking.

Mealla et al. (2015) evaluated the performance of five panel cookers and a solar box

cooker attaining to international standard procedures. They constructed the cookers

with recyclable materials such as cardboard and aluminium foil. Their results

showed that the Fun Panel reached the highest temperatures (96.65ºC) followed by

the Dual-Setting Panel Cooker (92.7ºC) and the CooKit (89.5ºC).

The Solar Box Cooker (SBC) is the most widespread solar cooker due to its low cost

and ease of operation. It is slow to heat up but work well under intermittent cloud

cover and windy conditions. There is a wide range of variants available from simple

home-made cardboard boxes to more complex devices including outer and inner

reflector and auxiliary heating systems.

Ozturk (2007) evaluated the performance of a SBC consisting of a plastic sheet box.

All outer and inner surfaces, as well as the absorber plate, were painted mat black in

order to absorb solar radiation more effectively. The transparent cover was a 4 mm

thick plastic plate fixed with silicon rubber to make the cooking space airtight. A

black painted commercial aluminium pot was filled with 2.5 L of water and place

into the cooker for the experiments. The tests, based on Funk’s procedure, showed

a maximum temperature of 89.45ºC after 4 hours. The standardised power (Ps(50))

was 26.5 W.

Mahavar et al. (2013) designed two box cookers named Small Family Solar Cookers

(SFSC-1 and SFSC-2). They performed experimental tests in order to study the

thermal performance of the designs and also calculated the costs and saving

potential. Their results include the two figures of merit F1 and F2, the standardised

cooking power Ps(50), energy savings and payback period, the last two depending on

the conventional fuel used. All the important parameters obtained are shown in

Table 2.

TABLE 2. IMPORTANT PARAMETERS OF SFSC-1 AND SFSC-2.

Stag.

Temp.(ºC)

F1

(K.m2/W)

F2 Ps(50)

(W)

Energy saving

(€)

Cost (€) Payback

(yr.)

SFSC-1 144 0.116 0.466 30 12.11-50.76 17.1 0.38-1.8

SFSC-2 144 0.118 0.488 50 12.11-50.76 12.2 0.27-1.25

23

El-Sebaii (1997), Guidara et al. (2017) and Coccia et al. (2017) studied the effect

that inner and outer reflectors have on the performance of the cooker. The former

developed a device with a one-step outer reflector and multi-step inner reflectors

(Figure 17) in order to increase the available power in the pot. His results showed

that the boiling time of 1 kg of water is more than halved (from 168 min to 80 min)

by adding a well configured outer mirror. When comparing the device with a black

box with outer mirror, boiling time is reduced from 130 min to 80 min,

demonstrating the improvement of the inner reflectors.

FIGURE 17. SCHEMATIC DIAGRAM OF THE SOLAR COOKER PROPOSED BY EL-SEBAII.

Guidara et al. developed a device that consisted of two trapezoidal wooden boxes

one inside the other and separated by Rockwool insulation. The inner reflector are

made of polished stainless steel and encased on the inner walls and the absorber

plate is a black painted aluminum plate. The cooker, shown in Figure 18, includes

four removable outer reflectors to enlarge the solar capture area.

24

FIGURE 18. 3D SKETCH OF THE SOLAR COOKER PROPOSED BY GUIDARA ET AL.: (A) WITH OUTER REFLECTORS,

(B) WITHOUT OUTER REFLECTORS.

The absorber plate temperature increases from 81.3ºC in the cooker without

reflectors to 133.6ºC in the cooker with reflectors. The performance of the cooker

was determined using Mullick’s figures of merit. F1 increases from 0.07 K.m2/W,

which is a very low value, to 0.14 K.m2/W, a more acceptable value. Loading the

improved system with 1 kg of water, boiling temperature was achieved after

1 h 8 min under an average solar radiation and an ambient temperature of

830 W/m2 and 33ºC respectively. F2 ranged from 0.34 to 0.39. They also

effectuated real cooking tests, cooking a four people meal consisting of rice in 72

minutes and a meal consisting of beans in 107 minutes.

Coccia et al. designed a high concentration ratio box cooker with a double row of

reflectors in the top shown in Figure 19. The high concentration ratio achieved

allows to classify the prototype between box and concentrating cooker. The system

allows an azimuthal and a zenithal orientation. In a test without load the absorber

plate temperature achieved was 300ºC and the first figure of merit F1

0.39 K.m2/W. When loading the cooker with 4 kg of water in a black vessel,

boiling temperature was achieved after 42 minutes. The best performance of the

device showed a second figure of merit F2 of 0.33. However, it did not reach a

steady state because the input power was still higher than the losses, therefore, they

tested it again with peanut oil. A black vessel with 3 kg of oil was placed in the

cooker and they had had to stop the test when the fluid reached 220ºC. When

loading the cooker with two vessels the F2 obtained was 0.36 and the standardised

cooking power Ps(50) 237.4 W.

25

FIGURE 19. PROTOTYPE DESIGNED BY COCCI A ET AL..

Weldu et al. (2019) evaluated the performance of a box cooker tracking the

reflector angle optimally. Including the tracking system the plate temperatures

increased from 128.2ºC to 148.7ºC and the first figure F1 from 0.127 to

0.154 K.m2/W. 2 kg of water reached boiling temperature 30 minutes faster in the

improved system and the obtained standardised cooking power was 45.25 W.

Misra and Aseri (2012) experimentally investigated the performance of conventional

solar box cookers under two different conditions inside the box: one with natural

convection and the other with forced convection created with a fan powered by a

small photovoltaic panel. All parameters resulted to improve. F1 increased from

0.1204 to 0.1424 K.m2/W, F2 from 0.3124 to 0.4077 and the boiling time of 1 kg

of water decreased from 75 min to 52 min.

Other authors have experimented with finned absorber plates and vessels. The

stagnation temperature and the time required to heat water up to boiling

temperature are 7% higher and 12% lower respectively in a solar box cooker with a

finned absorber plate when compared to the same cooker with a flat absorber plate

(Harmim et al., 2010). Using a finned vessel instead of a non finned one

considerably reduces cooking times as Geddam, Dinesh and Sivasankar (2015)

proved. The second figure of merit F2 obtained was as high as 0.64 in the best case.

They also experimented with Phase Change Materials (PCM) as energy storage,

obtaining promising results.

Solar cookers including Thermal Energy Storage (TES) are already under

development. Both Sensible Heat Thermal Energy Storage (SHTES) and Latent Heat

Thermal Energy Storage (LHTES) are being studied in a wide variety of devices.

26

Cuce (2018) and Nath Shrestha and Ram Byanjankar (2007) experimented with

endemic rocks as SHTES medium. The former used Bayburt stone, a beige tuff

quarried in Bayburt, Turkey. He compared two identical cookers, one filled with

stones and the other without them. The absorber plate temperature in the non

charged cooker showed a sharp rise reaching 130.32ºC in 2 hours while in the

charged cooker it was slow but steady until it reached 124.57ºC after 5 hours. At

this point, the plate temperature of the non charged cooker had decreased to

110.71ºC. As shown in Figure 20, the same tendency can be noticed when loading

the cookers with water, demonstrating the possibility of late evening cooking just by

adding stones as heat storage medium.

FIGURE 20. WATER TEMPERATURES OF BOTH COOKERS FROM SUNRISE TO SUNSET.

Nath Shrestha and Ram Byanjankar collected 7.5 kg of stone pebbles from a local

river. They performed no-load and load tests to a cooker without pebbles, with

pebbles and with black paint coated pebbles to calculate figures F1 and F2

respectively. The results are shown in Table 3.

TABLE 3. F1 AND F2 FOR DIFFERENT COOKER CONFIGURATIONS.

Cooker configuration F1 [K.m2/W] F2 [-]

Without pebbles 0.19 0.55

With pebbles 0.21 0.23

With black paint coated pebbles 0.23 0.27

The first figure F1 is higher in the cookers with pebbles while the second figure F2 is

lower due to the thermal inertia of the storage medium. When loading the cooker

with 1.2 kg of water, the configuration with black paint coated stone pebbles proved

to have the best heat retention capacity. Figure 21 demonstrates the capability of the

cooker to cook two meals per day and maintain food warm for a late evening meal.

27

FIGURE 21. TEMPERATURE OF WATER IN THE COOKER WITH BLACK PAINT COATED STONE PEBBLES.

Several authors have also studied the possibility of adding Phase Change Materials

(PCM) as LHTES. Buddhi and Sahoo (1997) developed a box cooker with a cavity

between the absorber plate and the container for a PCM in which they filled 3.5 kg

of commercial grade stearic acid (melting temperature of 55.1ºC and latent heat of

fusion 160 kJ/kg). They tested the device in real cooking conditions loading it with

rice and water at 10.30, which fully cooked at 13.30, and adding a new load at

16.00, which was fully cooked at 20.00 and with a plate temperature of 65ºC. Solar

radiation ranged from 796 to 924 W/m2.

Coccia et al. (2018) tested their high concentration ratio box cooker, already

mentioned above, provided with a thermal storage unit. The PCM consisted of a

4 kg mixture comprising KNO3, NaNO2 and NaNO3 (melting temperature of

145ºC and latent heat of fusion 101.5 kJ/kg) and was included in a double walled

pot. The heating time (until the load reaches a temperature of 170ºC) increased

when adding the PCM, however, so did the cooling time (time needed for the load

temperature to decrease to 130ºC), increasing from 1.66 h to 3.13 h.

The benefits of adding TES units are obvious; they add value to the cooker

permitting late evening cooking and longer retention of heat. The struggle could be

obtaining the storage materials. In the presented studies, the SHTES materials are

easily available endemic stones while the LHTES materials may be harder to find and

implement. Nevertheless, PCMs perform better than stones.

28

In order to overcome the dependence on meteorological conditions, it is evident

that the future cooker may be a hybrid system. However, this is not a new

approach. Hussain, Das and Huda (1997) modified a solar box cooker supplying it

with an auxiliary heating system composed by built-in electrical heating elements

under the absorber plate and converting light energy into heat by means of a 100 W

electric bulb. They demonstrated the possibility of cooking under very poor

radiation conditions with their new model. Water temperature in cooker 1 and

cooker 2, with auxiliary heating and without it respectively, is shown in Figure 22

along with ambient temperature and global radiation.

FIGURE 22. VARIATION OF WATER TE MPERATURE IN COOKER 1 AND COOKER 2.

Nandwani (2007) designed a hybrid multipurpose device capable of cooking,

pasteurising and drying. It has a black electric absorber plate including a resistance as

well as a thermostat to regulate the plate temperature. On sunny days and without

load, the plate reaches temperatures around 130-150ºC relying only on the sun. On

partially cloudy days the plate is provided with electrical energy and fixed at about

90-120ºC. Therefore, cooking can be done only with solar energy or combining sun

and electricity. The author stated that the device can cook two meals per day for 3-4

people in 3-5 h, allowing the saving of 1160 kWh of electricity, 650 kg of firewood

or 2051 kg of kerosene per year. The hybrid cooker costs around $100-125.

29

The negative aspect of the two hybrid cookers mentioned is that an electricity input

is needed. This requirement cannot be fulfilled in many rural areas living out of the

grid. For this purpose, Joshi and Jani (2015) converted a Small Scale Box (SSB)

cooker into a SSB Hybrid (SSBH) cooker adding five foldable 15 W solar panels, a

45 Ah battery and a dish type DC heater, thus combining thermal and photovoltaic

effects. They also developed an Improved SSBH (ISSBH) cooker replacing the dish

heater by three rod type heaters. When the battery was fully loaded, indoor cooking

proved to be possible with 3 h autonomy. Furthermore, their novel prototypes

include the possibility of lighting at night. The approximate prices of the SSBH and

ISSBH cookers are $125 and $120 respectively.

Other authors have developed cooker prototypes only powered by photovoltaic

energy. Even though these cookers are less efficient than thermal cookers, their

main advantages are the possibility of storing energy in batteries for late evening

cooking or cooking on cloudy days and the option of using the stored energy for

other purposes such as lighting or cooling. However, the components needed are

harder to obtain in many countries and still more in rural areas. Nevertheless, this

might change in the near future due to the fast propagation of renewable energy

across the world.

Watkins, O’Day and Arriaga (2016) implemented a 120 W photovoltaic solar

cooker in a rural of Uganda. Their system consisted of an outer cylinder made of a

reed mat used to keep rice hulls as insulators and an inner cooking chamber built out

of an aluminium cooking pot. The home-made heating element was a 26-gauge

Nickel-Chromium wire immersed in a concrete tile. They performed several tests;

however, the testing conditions are not specified. A report of one of the periodic

check-ins is shown stating the following:

“Hi all, Here's your subject with your cooker. We visited her yesterday and she tells us they've

used the cooker every day. They cooked beef in about 3.5 hours and they've cooked rice, okra

and eggplant. Then, there's was enough daylight left to heat water for bathing. Don't

underestimate the significance of that. They would never waste precious firewood on an

extravagance like bathing water. We all like our hot baths; they do, to o. One note: the water

was too hot and they had to dilute it. You've made a significant improvement in their lives.

Staff will return periodically to check on the cookers. No one was at the other homestead but

the father came by to unlock the hut and tell us that whenever they are home, they use the

cooker. We saw that the panel was there and was kept clean. I asked them to think about how

the cooker could be made better and this family had a ready answer. They want a battery

system to charge cell phones and to use a light. Again, on behalf of these families whose lives

you have impacted, thank you!”

Their system without a battery had a cost of $125.

30

Colom (2018) experimentally tested a bigger system composed of a 300 W

photovoltaic panel, two gel batteries of 12 V and 100 Ah each, a 20 A regulator and

two 2.5 ohm resistances. The fulfilled their purpose, i.e. provide energy for cooking

and at the same time store the excess electricity in the batteries for late cooking or

other uses. The total cost happened to be around 740€. After studying the Nigerian

electricity market, the calculated payback was about 2 years.

Even though all the cookers presented above, i.e. panel, box and photovoltaic

cookers, have their own advantages, only the high concentration ratio box cooker

developed by Coccia et al. is capable of reaching high enough temperatures for

frying. However, this is a complex cooker and not easily constructible. When

talking about high temperatures, concentrating cookers are the best option. Their

negative aspects are that these cookers are usually bulkier and new users need a

deeper training before starting to use them.

The SK-14, shown in Figure 23, is one

of the most widespread parabolic

solar cookers in the world. It can

cook food for up to 15 people per day

with 2-3 hours of sunlight. SCI

evaluated its performance according

to the ASABE S580.1 protocol

presented in 2.3 Efficiency of solar

cookers. 10.8 L of water reached

boiling temperature in less than 2

hours and a standard cooking power

of 347 W was calculated for the

device. It costs around 320€.

FIGURE 23. SK-14 COOKER ALONG ITS INVENTOR

DR. DIETER SEIFERT (SECOND FROM LEFT).

31

Mussard, Gueno and Nydal (2013) experimentally compared the SK-14 cooker to a

prototype of a parabolic trough using a storage unit Figure 24. In the SK-14, the

cooking vessel is directly placed on the focal point of the parabola, without any

previous preparation. The prototype, however, needs several hours of sun (around

5h) to fully charge its heat storage based on the latent heat of nitrate salt (melting

temperature of 220ºC). Once charged, the vessel is put on top of the storage unit.

They performed two tests: comparison of time required to boil 1 L of water in both

systems and a frying test in the heat storage system.

FIGURE 24. PARABOLIC TROUGH AND STORA GE UNIT.

The results of the boiling test are shown in Figure 25. Water reached boiling

temperature in 27 min in the SK-14 (850 W/m2 average radiation and 23ºC

ambient temperature) and in 38 minutes in the parabolic trough. They stated that

the surface contact in the heat storage unit was not the optimum and demonstrated

that improving it could decrease the boiling time by 55%. It can be seen in Figure 26

that the prototype is also capable of frying. Even though, the SK-14 cooker may be

faster, the new system allows indoor cooking as well as late evening cooking,

overcoming some of the problems present in solar cooking.

32

FIGURE 25. TEST 1: BOILING 1 L OF WATER IN BOTH COOKERS.

FIGURE 26. TEST 2: FRYING TEST IN HEAT STORAGE UNIT.

Edmonds (2018) constructed a low cost high temperature solar cooker. Sunlight is

collected by a rectangular aperture and eight planar reflectors provide a

concentration ratio of 12. The cooker, shown in Figure 27, can be manually tracked

in azimuth and altitude. It can be used for water sterilisation, low temperature

cooking and high temperature cooking. On clear days the device can sterilise up to

16 L of water, boil 2 L of water in less than 1 h (905 W/m2 average solar radiation)

or bake, fry or grill at temperatures above 200ºC.

33

FIGURE 27. SCHEMATIC AND REAL IMAGE OF EDMOND’S COOKER.

However, the negative aspects of this cooker are obvious. Its large size and its

complex shape do not make it accessible for everyone. Not every individual has

enough space for such a big device or the materials or tools to build it. Nevertheless,

a communitarian use could be interesting, where several families share it and

arrange a schedule to use it.

Pohekar and Ramachandran (2004) made a multi-criteria evaluation of cooking

technology alternatives in order to promote parabolic solar cookers (PSC) in India.

They designed questionnaires for the evaluation of several criteria such as fuel

consumption, cooking time, size and weight, initial cost and overall safety among

others. 30 experts evaluated nine cooking alternatives: chula (Indian mud stove),

improved chula, kerosene stove, biogas stove, electric oven, micro-wave oven, LPG

stove, solar box cooker (SBC) and PSC. The LPG stove had the highest rank while

the electric oven had the lowest. The SBC occupied the third place followed by the

PSC. The main areas of improvement in PSCs appeared to be size, weight and space

requirements, quality and reliability, initial cost, subsidies, aesthetics and ease of

operation. This kind of studies shed some light on in which aspects future

researchers should focus.

34

Hussein, El-Ghetany and Nada (2008) developed an indirect solar cooker with

indoor PCM thermal storage shown in Figure 28. The collector was of flat plate type

and the PCM magnesium nitrate hexahydrate (melting temperature of 89ºC). Two

cooking pots of 3 and 4 L capacity were built in the inner cooking unit. Their

experiments proved the ability of the cooker to cook in the evening and to retain

heat till the next morning. In a real cooking test (Figure 29) a first meal loaded al

12 pm and a second meal for dinner and breakfast for the next day at 4 pm were

successfully cooked for 8 to 10 people. The novel approach overcomes some of the

main problems of solar cooking, i.e. late evening cooking, retaining heat until the

next morning and indoor cooking.

FIGURE 28. NOVEL INDIRECT SOL AR COOKER WITH INDOOR PCM THERMAL STORAGE.

FIGURE 29. REAL COOKING TEST.

35

This system presents the same negative aspects of the one constructed by Edmonds

(2018), in fact, it is complex and not easy to build. However, its capability to cook

big meals could make it appropriate for big families or small communities.

There are also commercially available devices sold by companies like GoSun10 and

Sun Buckets11. The first one sells vacuum tube solar cookers ranging from 120€

cookers for 1 person to 415€ cookers for 4-6 people including the possibility to

cook with electricity from a 12 V source. The second one sells TES devices based on

PCMs. It has to be heated in a parabolic cooker for about 1-2 hours and it can be

used later in the day or even the next morning. They have reported initial

temperatures up to 400ºC.

4.2.2 Backup system

As previously mentioned, it is not the best approach to rely solely on solar

technology due to the intermittent availability of the resource. A backup system is

completely necessary to cook on cloudy days, in periods when the sun is not shining

or at night.

3 billion people rely on solid fuels (wood, charcoal, coal, animal dung, crop waste)

as primary cooking and heating energy source, particularly in rural households. Less

than one-third of these have access to improved or clean cook stoves, which can

significantly prevent much of the harm created by conventional stoves (THE STATE

OF THE GLOBAL CLEAN AND IMPROVED COOKING SECTOR, 2015).

The term “conventional cooking technology” does not represent the same in all

income levels (classification proposed by Hans Rosling). People in Level 1 (L1)

mainly use three stone inefficient cookers burning firewood or any other kind of

solid fuel. In L2 a simple gas burner may be used, since electricity is still unstable. In

L3 meals are cooked on simple stoves driven by gas or electricity. Eventually, in L4

better stoves are used mainly driven by electricity. The proposed backup system

may use the same kind of fuel, since it usually is the only available one, but in a

more efficient, healthy and sustainable way.

10 https://gosun.co/ 11 https://www.sunbuckets.com/

36

In L1, improving the efficiency of the burning method would significantly enhance

people’s life quality. When reducing fuel consumption, the periodicity of the long

walks to collect fuel as well as the amount of harmful smoke would also decrease.

Basic efficient charcoal or wood stoves show theoretical fuel saving potential in the

range of 25-35%. Intermediate stoves such as built-in or semi-fixed chimney rocket

stoves can bring fuel savings up to 65%. Furthermore, such stoves can significantly

reduce indoor emissions. However, these can be expensive and require a home

improvement project. Portable rocket stoves present fewer benefits than chimney

rocket stoves, but some artisanal and semi-industrial versions can cost around $10-

20. UNHCR (United Nations High Commissioner for Refugees) wrote a handbook

in which practical ideas for household and institution cooking in refugee camps are

provided. Several ideas for improved stoves and energy saving practices are

presented, also applicable to households in L1 (UNHCR, 2002).

FIGURE 30. ROCKET-STOVES. SOURCE: HOMESTHETICS 12.

12 https://homesthetics.net/rocket-stove-plans/

37

Advanced biomass stoves may benefit people in L1 and L2. Biomass gasifiers have

reported fuel savings higher than 40% while significantly reducing emissions. Most

natural draft stoves produce bio-char as a by-product, which is charcoal used as a soil

fertiliser or as cooking fuel (UNHCR, 2002; Najib and Islam, 2018; Rakereng,

Muzenda and Gorimbo, 2019). The prices of industrial natural draft stoves vary

from $25 to $50, while artisanal and semi-industrial stoves are available at $15-20.

Make It Green Solutions AB13, a Swedish company, has developed two stoves based

on this technology. Their cookers utilise wood-based fuel to cook and produce bio-

char as a by-product, thus, reducing costs and even generating an income to users

selling bio-char.

LPG emits little particulate matter and burns efficiently. LPG stoves can reduce

household air pollution up to 90% compared to three stone cooking or simple

stoves, however, few impact evaluations have focused on the end users of these.

Devices with fuel efficiencies of about 55% cost around 30-100 US$.

Electric and induction stoves are limited to areas that have access to electricity.

However, this situation will likely change in the near future, when electricity will

have spread to more remote areas. Induction stoves can reach fuel efficiencies of up

to 90%, however, their high price makes them affordable only to people in L3 or

L4. Electric hot plates are less expensive and may be bought by people in L3 or

maybe even L2, in case of having access to electricity. One of the main advantages is

that they are smokeless at the point of use. Their impact on environment depends

on how the electricity has been produced. Cooking with an efficient induction stove

driven by electricity produced from renewable sources would be the ideal situation.

4.3 Institutional cooking

Institutions are identified with a social purpose, cooking in this context, where a

service is offered to individuals. The group of individuals could range from 25

people up to several thousand. Institutions are expected to have better affordability

than households and less constraints regarding availability of space.

4.3.1 Solar technology

Several attempts have been made to implement larger sized box cookers in

institutional or community kitchens; however, they are preferred in a smaller scale.

Throughout time, parabolic concentrator based designs have been more successful.

Two common designs are parabolic dish and Scheffler dish cookers. In the former,

the pot is located in the focal point of the reflector, while in the latter, sun rays are

reflected towards a secondary reflector and the pot is located in its focal point,

permitting indoor cooking.

13 https://makeitgreen.net/

38

Currently, in Indian institutions, three types of concentrator cookers are being

promoted by the Ministry of New and Renewable Energy (MNRE): manually

tracked parabolic dish cookers (SK, PRINCE), fixed focus automatically tracked

(East-West) Scheffler dishes and fully tracked Fresnel dishes. A comparison of the

mentioned cookers is shown in Table 4 (Indora and Kandpal, 2018).

TABLE 4. COMMERCIALLY AVAILABLE INSTITUTIONAL SOLAR COOKERS IN INDIA.

For institutional cooking, parabolic dishes with large aperture areas ranging from 1

to 10 m2 are required. Commercially available designs could be: SK-20, SK-23, SK-

30 and PRINCE-40. The last two devices, for example, are suitable to cook meals

for about 40-50 people. A total of 360 SK-23 cookers, having a total aperture area

of 1440 m2, were installed in the Indian state of Maharashtra. These are used to

cook meals for more than 25,000 students per day and they have reported saving

54 t of LPG per year.

Scheffler collectors track the movement of the sun concentrating sunlight on a fixed

focal point. The dish is made of a lateral section of a paraboloid (Figure 31), thus, the

focal point is out of the dish (Kumar, Prakash and Kaviti, 2017). This characteristic

permits placing the cooking pot indoors avoiding heat losses present in parabolic

cookers (Figure 32). Thus, due to the high temperature reached, any type of cooking

is possible. A dish with an aperture area of 16 m2 can provide approximately 5.5 kW

of thermal energy in a clear day.

39

FIGURE 31. SCHEMATIC VIEW OF A PARABOLOID.

FIGURE 32. WORKING PRINCIPLE OF SCHEFFLER COOKER. SOURCE: SCI.

40

The main components of a Scheffler dish based direct solar cooker are: the primary

reflector (Scheffler dish), a secondary reflector, a cooking pot and the tracking

system. Sometimes, a solid iron block is used at the focal point to better distribute

heat and store heat energy for late evening cooking. Such a system, with an aperture

area of 7-16 m2, could cook food for 50-100 people.

One of the biggest institutional solar cookers in the world consists of 84 Scheffler

dishes of 9.5 m2 aperture area. It is an indirect solar cooking system, which means

that steam is generated in the receiver (3500 kg per day) and transported through

pipes to a nearby kitchen. 30,000 meals consisting of 600 kg of rice, 250 kg of

pulses and 500 kg of vegetables are prepared every day. The total cost of the system

was INR 7 million (€85000 approx.) with a payback period of 5 years. They have

reported saving 500 L of diesel per day.

The ARUN dish is an indirect solar cooker based on a Fresnel paraboloid

concentrator developed by Clique Solar in India. The main components are the

collector, the receiver and the tracking system. ARUN dishes manufacturers have

reported annual efficiencies in the range of 50-60% and a lifespan of more than 25

years. There are currently three designs available: ARUN30 (30 m2), ARUN100

(100 m2) and ARUN160 (160 m2). A system composed by installed in India

generates 60 kg of steam per hour and is used to cook 3500 meals. 12.5 t of oil are

saved per year.

Schwarzer and Vieira da Silva (2003) developed an indirect flat plate collector

cooker (Figure 33) with the possibility of adding a heat storage system. Its

modularity allows manufacturing small units for households as well as large units for

institutions. About 250 systems have been installed in different countries. The flat

collectors are composed by a coated absorber plate and double glazed covering. Oil,

which moves by natural flow, is used as the heat transfer fluid. The heating power

can reach up to 500 W per square meter of collector area. With 2 m2 collector area

and under a solar flux of 1000 W/m2, the system reached a temperature of 235ºC.

The boiling time for 5 L of water is 10-12 min.

41

FIGURE 33. INDIRECT FLAT PLATE COLLECTOR COOKER DEVELOPED BY SCHWARZER AND V IEIRA DA SILVA.

4.3.2 Backup system

In institution, as well as in households, the meaning of “conventional cooking

technology” differs among different income levels. Institutions in L1 rely mainly on

biomass burnt in inefficient three stone cookers. In L2 improved biomass stoves or

gas burners may be used, while in L3 gas stoves or simple electric stoves are

employed. Eventually, in L4 efficient gas or electric stoves are used.

Adkins et al. (2010) compared the performance of institutional three stone cookers

and rocket stove in rural Kenyan schools in terms of firewood used and cooking

times. Both cookers were tested under the same conditions for several days and the

data collected showed that, on average, rocket stoves consumed 33% less wood than

the three stone cookers. However, the amount saved was not enough to amortize

the high investment cost of the improved stove. Therefore, it is completely

necessary to promote policies to help the implementation of more efficient devices.

An interesting option, affordable for institutions in L2 as well as beneficial for

institutions in L4, would be installing a biogas digester. Beyond energy savings, this

would also reduce waste production, since food and crop waste would feed the

digester. Biogas is a flammable gas produced after the anaerobic digestion of organic

matter. It is a mixture of methane (60-70%), CO2 and smaller proportions of other

gases. Therefore, it can be used as a substitute for natural gas employing the same

appliances. In the production process a high quality liquid fertilizer is also generated.

42

Biogas digesters can be built on any scale, from households to large institutions. A

basic system consists of a tank where organic matter is digested and a system to

collect and store the biogas. The digester needs to be fed with biomass and warm

water. The maximum load of organic matter is around 25 L per 1000 L of capacity

(Igbum, Eloka-Eboka and Adoga, 2019).

In Tamera, a solar village in Portugal, they use solar technology for cooking. The

main cooker is based on a 10 m2 Scheffler mirror, which feeds around 50 people,

and they also use parabolic dishes, parabolic trough cookers and box and panel

cookers. Their principal backup consists of a 3000 L biogas digester designed by

T. H. Culhane, which is typically fed with 40-60 L of biomass daily. Thus, around

90 L of biogas are produced daily, providing 10-12 h of fire on a low flame or 5-6 h

on a high flame. Furthermore, they have a manual on their webpage in which the

digestion process and the construction technique are explained. One of the images

contained in the manual is shown in Figure 34.

FIGURE 34. DETAILS OF A BIOGAS DIGESTER. SOURCE: TAMERA.ORG 14.

The negative aspect of a biogas digester would be its water consumption, making its

adoption difficult in water scarcity areas. The need to maintain a warm temperature

in the inside could also be seen as a barrier. However, most of the countries in L1

and L2 have warm climates all the year round and countries in L3 and L4 have easier

access to warm water.

14 https://www.tamera.org/

43

Institutions in L4 areas where they use efficient electric stoves such as induction

stoves, adopting biogas would suppose a high investment cost since they would also

need new appliances. Instead of that, getting renewable and autonomous electricity

could be more beneficial in these cases. A photovoltaic installation would bring

“free” electricity that they could also use for other purposes. The installation of

batteries or the profitability of selling excess electricity should be studied in every

case, since these factors are highly dependent on the policies of every country or

area. $0.1 per installed watt could be used as a reference for the investment cost.

4.4 Discussion

This report has started introducing the main problems that humankind needs to

overcome, i.e. the high proportion of the energy mix covered by fossil fuels that

strongly contribute to global warming. For this purpose, solar cooking is a powerful

and promising tool driven by a clean and free energy source: the sun.

After presenting the aim of this study, to obtain an Integrated Solar Cooking system

including a solar cooker, a TES system and a backup facility, the current situation

has been revised. A wide variety of solar cookers and their characteristics and

performance details have been presented as well as possible backup systems, all of

them divided in two categories: households and institutions.

After reading past and ongoing research in addition to reports of actual case studies,

two main conclusions have been drawn: trying to rely solely on the sun is a mistake

that leads to stop solar cooking and there is no perfect ISC for every situation, every

case needs to be individually studied.

The conventional approach of only focusing on the income as the major determinant

for choice of energy source is already outdated. Other factors such as kitchen

location, distance to energy source and effect of fuel among others considerably

influence the decision. A thorough investigation in every particular case would aid in

the implementation of new policies and campaigns of government and

nongovernmental organisations (Amoah, 2019).

Beyond technical aspects, other variables influence whether the implementation of

solar cooking will be successful or not. Economic, social, cultural, environmental

and political aspects play a key role in the acceptance of the technology. The

importance of these variables can only be understood through a needs assessment of

the target group.

44

People with low incomes cannot afford to invest their money in solar cookers; it is

too risky or impossible for them. It is therefore necessary that policy makers

implement financing schemes to promote their use. Tax reductions, subsidies or

micro-credits would enhance and permit the adoption of solar cookers (Otte,

2013).

Thus, with a successful implementation of solar cooking, people in L1 will be able

to start saving money and start thinking in a longer term, slowly moving towards

L2. The same applies to people in L2. Eventually, the whole of humanity will live

under acceptable living standards.

Consequently, instead of proposing a unique ISC, four different systems will be

suggested for households as well as for institutions, one per income level proposed

by Hans Rosling. However, saying that a system will cover the needs of all the

population in a level is still generalising too much. A family in L1 obtaining free

firewood 10 minutes away from their household will not have the same motivations

as another family that needs to walk for hours every day and pay for their fuel.

Therefore, the motivation and needs of every family or institution must be taken

into account.

45

5 Conclusions

5.1 Study results

5.1.1 Households

5.1.1.1 Level 1

People in L1 are characterised for living with less than $2 per day, therefore, their

investment capacity is quite low. They currently rely on firewood, cow dung,

charcoal and other non-commercial fuels burnt in inefficient stoves, often three

stone cookers.

Taking into account that their main motivation would be economical, the most

viable option would be constructing a simple but effective solar cooker with locally

available or recycled materials, thus obtaining a low cost device. Mendoza et al.

(2019) constructed a panel, a box and a parabolic cooker mostly with recycled

materials with a cost ranging from €1.3 to €2.6. The cooker integrated to a kitchen-

set box enclosed to humanitarian aids developed by Regattieri et al. (2016) could also

be a possible option. In these cases where there are subsidies a complete CooKit or

Fun-Panel including a pot and two plastic bags costing in the range of € 3-7 might be

affordable. However, the use of plastic bags is being banned in some countries,

which would preclude the introduction of panel cookers unless other cover options

are available.

Locally produced heat retention devices such as cardboard boxes with the inner

walls covered in a reflective material or hay baskets would considerably improve the

experience. Stones, sand or any other endemic material could be used as thermal

energy storage. These options would permit having a warm meal in the late evening

or maybe even the next morning.

Regarding the backup system, home-made or locally produced simple stoves would

perform better than three stone cookers, which along energy saving practices such

as firewood preparation, fire management, diet and food preparation and cooking

management, might save up to 65% of fuel (UNHCR, 2002; Barbieri, Riva and

Colombo, 2017). Once again, in case of having subsidies, simple rocket stoves

costing $10-20 could be afforded.

5.1.1.2 Level 2

People in L2 have a daily income ranging between $2-8. Their cooking devices are

usually improved biomass stoves or simple gas burners.

46

Even if still low, their investment capacity is bigger than people’s in L1. They might

afford prototypes like the Small Family Solar Cookers (SFSC) developed by Mahavar

et al. (2013) that cost around €12-17. Depending on the availability of materials

more reflectors could also be added to improve cookers’ performance.

Heat retention devices as well as endemic materials for energy storage may also

significantly enhance the results (Nath Shrestha and Ram Byanjankar, 2007; Cuce,

2018). PCMs could also be available and/or affordable in these cases. Commercially

available stearic acid, for example, would definitely permit late evening cooking and

retaining heat for next morning’s breakfast (Buddhi and Sahoo, 1997; Coccia et al.,

2018).

Advanced biomass stoves have reported fuel savings around 40%. Furthermore,

taking into account that most natural draft stoves produce bio-char as a by-product,

an extra income could be added to the users in case of selling it. If it is not sold it

can be used as a ground fertiliser or water filter (Najib and Islam, 2018; Rakereng,

Muzenda and Gorimbo, 2019).

5.1.1.3 Level 3

With a daily income between $8-32, the investment capacity of people in L3

considerably increases. Although unstable, electricity is often available in these

areas. Hence, besides economical motivations, a more reliable electricity source

may be an incentive.

Several hybrid systems have been presented in this report. The multipurpose device

designed by Nandwani (2007) offers the possibility of cooking solely with the sun or

using its electric absorber plate on cloudy or rainy days. The Small Scale Box Hybrid

cooker developed by Joshi and Jani (2015) includes five foldable solar panels and a

battery, which permits combining thermal and photovoltaic effects for faster

cooking as well as charging electric devices with stored energy. Another option

could be relying only on photovoltaic energy, like the 120 W system implemented

by Watkins, O’Day and Arriaga (2016). All cookers cost around $100-125.

Even though being more expensive, these systems would increase the flexibility and

usability of solar cooking, thus, reducing the need of a backup. The old cooker could

still be used in long non sunny periods. Depending on the economical level of the

family, the old system could also be substituted with a more efficient stove

depending on what fuel they use.

47

5.1.1.4 Level 4

The motivations of people in L4, with a daily income higher than $32, are no longer

mainly economical but more environmental. Gas or electric stoves are the main

cooking technology. Users do not want to leave the comfort levels reached and solar

cooking can be seen as a hobby.

Commercially available cookers such as the parabolic SK cooker or GoSun’s vacuum

tube cookers could best feed users’ needs. The parabolic cooker would require a

bigger available area not usually found in cities where most people in L4 live, but in

case of having that space, it could cook for a whole family. GoSun offers smaller and

more portable devices usable in reduced areas such as balconies or maybe even

windows, therefore making them usable in almost every situation. High quality

home-made devices could also be interesting since using self-made devices is always

comforting and strengthens the adoption of changes.

The three key aspects of energy efficiency are: employed technology, energy

carriers and users’ behavior. Switching from electrical to solar devices implies

improving the first two aspects and could imply a 40% reduction in life cycle costs

and up to 65% reduction in environmental impact, including greenhouse gas

emissions (Mendoza et al., 2019). Consumer behavior modification is the most

promising energy conservation tactic. Simmering rather than boiling, using a pot lid

or cooking larger quantities could reduce energy consumption by as much as 95%

(Hager and Morawicki, 2013).

5.1.2 Institutions

5.1.2.1 Level 1

Investment costs for institutional solar cookers are still too high, therefore, it would

be necessary to have favorable policies supporting this technology and offering

subsidies for its adoption. Manually tracked parabolic dishes such as the SK or

PRINCE are the less expensive and easiest to use models (Indora and Kandpal,

2018).

Improving the current stoves would also be beneficial saving around 33% of fuel

using rocket stoves instead of three stone cookers as stated by Adkins et al., (2010).

However, the amount saved is not enough to cover the extra cost of the investment.

Thus, it is strictly necessary to promote more efficient cooking stoves by means of

financing schemes.

48

5.1.2.2 Level 2

Institutions in L2 could maybe afford the aforementioned parabolic dishes. With an

adequate promotion, maybe even Scheffler mirrors could be installed. In India,

where most of people live in L2, thanks to promotion programs of the MNRE,

Scheffler mirrors are being widely spread all over the country. In small institutions

direct Scheffler cookers are preferred while in bigger ones indirect cookers that boil

steam to use it as heat carrier are more popular (Otte, 2014; Indora and Kandpal,

2018).

Depending on the investment capacity and available materials, co-digestion of

unavoidable food waste is a viable option to produce renewable and free biogas for

rural or urban institutions (Igbum, Eloka-Eboka and Adoga, 2019). In cases when

biogas production is not enough, commercially available gas could be used.

5.1.2.3 Level 3

The investment capacity of institutions in L3 could be high enough to afford a

Scheffler mirror, which is one of the most efficient and widespread institutional

solar cookers worldwide. Its cost and amount of people fed per square meter are

125-200€ and around 23 people respectively. Iron blocks could also be added as

heat storage for late evening cooking or to make the cooking process more flexible

to short cloudy periods. Case studies in India cook three meals every day for 100

people and have reported saving 3 kg of LPG and 25 kg of wood per day. The

amount of money saved could be invested in other areas such as education or health

(Indora and Kandpal, 2018).

A biogas digester would definitely be affordable and beyond providing free energy it

would also reduce waste production, which in some cases increases costs. The

implementation of efficient gas stove might also be considered.

5.1.2.4 Level 4

Institutions in L4 could afford any of the mentioned devices. Highly efficient

Scheffler or ARUN cookers with automatic tracking systems would be effective as

well as educational. The installation of solar cookers would not only bring energetic

and monetary savings but they would also be an example of sustainability and an

inspiration for future generations and other institutions.

Besides thermal cookers, the installation of photovoltaic panels could also be

considered. In institutions where electric stoves are used, generating their own

electricity would maybe be more reliable and in case of adding batteries to the

system, the excess production could be used for other purposes.

49

Promotion programs offering financing schemes would be a great incentive for

institutions to adopt solar energy and thus reduce their carbon footprint. India is the

perfect example where the MNRE promoted solar cooking and now is widely

spread all over the country.

5.2 Outlook

While working on this thesis and searching for scientific articles, these focusing on

technical aspects of solar cooking have been abundant. However, as previously

stated, technical feasibility does not ensure success. Articles studying

implementation problems or supervising actual projects have been scarcer.

Furthermore, the main cause of failure in the majority of unsuccessful projects has

been only considering technical factors.

Thus, a more thorough work on implementation processes as well as training

programmes and needs’ assessments is needed for the success of future solar

kitchens.

5.3 Perspective

To sum up, solar cooking would be beneficial in all income levels. It would permit

saving money and increasing investment capacity for people in the first levels while

at the same time the surrounding environment is less exploited and harmful

emissions are avoided. Thus, an ascent through levels would be possible.

In higher levels, beyond economical and environmental benefits, it would proof that

a more sustainable way of cooking is possible, a process that is totally driven by a

renewable energy source and does not emit greenhouse gases to the atmosphere,

neither directly nor indirectly.

In 2015, world leaders developed 17 Sustainable Development Goals in order to

create a better world. It is believed that solar cooking meets at least 9 of them:

− No poverty

− Zero hunger

− Good health and well-being

− Gender equality

− Clean water and sanitation

− Affordable and clean energy

− Reduced inequalities

50

− Sustainable cities and communities

− Climate action

51

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