Design of an Improved Cooking Stove Using High Density Heated Rocks and Heat Retaining Techniques

In sub-Saharan Africa, dependence on wood fuel has caused significant depletion of vegetative resources. Whereas there exist hundreds of improved cooking stoves, many have not reached their maximum potential because their designs are predominantly focused on either fuel efficiency or reduced smoke. This research designed and fabricated an improved charcoal stove using high density rocks and heat retaining techniques. The aim was to retain heat and minimise heat losses in cooking devices with a sole purpose of reducing the amount of fuel used during cooking. The stove design herein incorporates the interaction of physical and thermal properties of granite rocks with heat loss theories to give a thermal efficient unit. The stove was estimated to cost US$ 36 which compared favourably with most of the improved charcoal stoves on international market (US$ 3–50 US$). This study revealed that, by introducing the new stove design and insulation, the granite rocks depicted high thermal storage properties with potential for reducing fuel use by over 78%with reference to the open fire stove.The designed granite rock stove therefore paves way for the use of high density rocks in improved cook stoves to achieve high performance energy efficient systems that can sustainably put to use vegetative resources.


Introduction
Many of the world's population living in developing countries lack access to modern energy services for economic and social development.Besides, its existing energy system is unsustainable [1].A large proportion of the households do not have access to grid electricity.Yet, those relying on electricity for cooking experience intermittent power supply.Although liquid petroleum gas burns quite effectively, it is expensive and not viable for a common man.More so, there has been persistent escalating fossil oil prices and fuel crisis which has drawn attention to the need for producing viable alternatives to kerosene and gas for domestic cooking [2].Solar, another potential alternative source of energy, is noticeably location-specific in terms of utilisation.Its associated problems are linked to energy storage for use during the period of modest or no sunshine, as well as need for technological artifacts, which are currently scarce in the developing countries [3].However, several renewable energy resources including hydroelectricity, solar, and biomass are promoted due to their high availability and responsiveness to the environment.
In Africa, biomass is a traditional and the most reliable fuel source of energy used for cooking by over 69% of the population [4].However, its increased utilisation by inefficient technologies has raised fears over the long-term forest degradation with loss of environmental services (e.g., watershed protection) and biodiversity [5].Biomass is quite difficult to burn completely in the most commonly used traditional household-sized stoves [6].Frequent use of stoves developed by improper combustion designs may result into indoor air pollution, impacting negatively on the health of household members, particularly women and children [1].For instance, Nahar [7] reported that about 2 million annual excess deaths of women and children in developing countries are linked to indoor air pollution, precisely due to exposure to carbon monoxide and the volatiles (benzene and formaldehyde) liberated in the form of smoke [8].Such exposures lead to acute respiratory infections, low birth weights, lung cancer, chronic obstructive lung diseases, and eyes problems [9].Thus, faster technological development is vital in advancing charcoal stove and its environmental performance.
Although charcoal is believed to be an affordable, available, and the most convenient fuel source for households, its use in inefficient stoves would produce significant amounts of indoor air pollution and make it unsustainable.Therefore, continual technology development will suppress charcoal's detriments and enhance its efficient utilisation while reducing significantly environmental impact.
This work therefore aims at developing a more efficient and safe charcoal burning stove that can reduce fuel consumption rates and indoor air pollution.Even though a lot has been done to promote "improved cook stoves" in the developing countries, little has been emphasised on quality combustion during their design and development [10].This is attributed to limited scientific information on design features and materials of stove construction.In sub-Saharan Africa however a number of stove models are designed and fabricated by local artisans using locally available materials such as mud, clay, dry grass, ant-hill soil, and clay bricks.But no attempts have been made on high density rocks such as granite.
This paper deals with the development of a charcoal stove prototype from locally available materials including granite rock, stainless steel, and the glass wool.It describes the design features, thermodynamic performance, and thermophysical properties of the granite rock used in thermal-energy storage (TES) system fabrication.According to previous studies on thermophysical properties of granite rock, a suitable TES system should have high values of thermal conductivity, specific heat capacity, material density, and low values of porosity [11][12][13].High thermal storage efficiencies are as a result of high values of thermal conductivity, specific heat capacity, and density.High density and specific heat capacity values lead to a large volumetric heat capacity hence permitting compact storage in the systems, whereas low values of porosity indicate large bulk density and uniaxial compressive strength [14,15].

Materials and Methods
. .Construction Materials . . .Fiber Glass.A section of the stove wall was made from glass wool because this material is a poor conductor of heat due to its low thermal conductivity and thus will prevent heat loss by conduction.Through saving as much heat as possible, the fuel combustion efficiency of the stove is improved; it means saving even much more fuel.In addition, the fiber glass material is locally available.
. . .Stainless Steel.Stainless steel was chosen as a material because of its availability on the local market.Technically, it undergoes allowable deformation and is resistant to corrosion in high temperatures.Steel has a longer service life than most metals and because of this, its design life costs are minimal.
. . .Granite Rocks.Rocks are considered an attractive storage material for thermal-energy storage at high temperatures due to their thermal physical and mechanical properties [11,12].The rock selected was granite because of its high density, good thermal properties, and ready local availability.
. .Designing and Construction of the Stove.In order to come up with the desired stove design and its desired performance, proper reflection into factors such as fuel type, calorific value, air flow rate, insulation, local resources, stove power output, safety consideration, reactor cross-sectional area, diameter, and height are of great importance.In the present study, the size of the combustion chamber and the amount of fuel required to accomplish the cooking task were evaluated following work done by Kiwana [16] and Maxwell [17], as well as reasonable assumptions depicted in Table 1.Fuel (carbonised agrowaste briquettes and charcoal) for experimental design was selected based on its local availability within Kampala district.
The combustion chamber dimensions were based on the Kenyan ceramic jiko stove because of its prominence on the Ugandan local market and availability of dimensional data.The jiko stove has a basal internal diameter of 160mm and fire box depth of 100 mm [17].A combustion chamber with an internal diameter of 120mm and height 100mm was fabricated.Thus, taking the combustion chamber (inner chamber) as the focal point, the outer chamber (concentric to the combustion chamber) was fabricated at an internal diameter of 170 using granite rock, glass wool, and stainless steel at 23mm, 48mm, and 1.0mm thick, respectively.

. . Testing of ermal Properties of Granite Rock and
the Improved Cook Stove . . .Specific Gravity Test of Granite Rock.Granite rocks used were sampled from the East African granite dealers, Kampala district-Uganda.These rocks were obtained by randomly selecting representative samples from the cut debris stock piles.By hand picking, samples were collected at random points from the stock piles of different rock sizes.Specific gravity test was conducted according to AASHTO [18] to determine the density of granite for the different rock masses.
Initially, the density of the hand crashed 14/10mm granite rock sample was calculated using (1), where M1 is mass of gas jar and plate, M2 is mass of gas jar, plate, and aggregates, M3 is mass of gas jar, plate, aggregates, and water, and M4 is mass of gas jar, plate, and water.
Given 1 g/cm 3 as the density of water, this implies that it buoys up anything within it by 1 gram per cubic centimetre of displacement.Because of this, the weight in air minus the weight in water is equal to the volume of the rock sample in cm 3 . . . .Water Absorption Test.In line with AASHTO [18], the water absorption test was conducted to determine the amount of water absorbed under specified conditions.This is mainly due to the effect water usually has on heat transfer in rocks through conduction.The data obtained therefore helped shed light on the performance of the granite rock in humid environments.During the experiment, the granite rocks were hand crushed to 14/10mm aggregates before the test could be carried out.Water absorption was expressed as increase in weight percent based on (2), whereby A is weight of oven dry sample in air and B is weight of saturated surface dry sample in air . . .ermal Test of Granite Rock.After lining the granite rock chamber with stainless steel, charcoal fuel was ignited within the combustion chamber.The maximum temperature reaching the granite rock (423 K) was attained after 41 minutes on average and this was recorded.
Samples of the rock were as well heated between room temperature (297.8K) and 567 K in an oven to determine their ability to withstand heat without disintegrating.The rocks were heated and observations made for change in weight for the granite material under test.This was done at time intervals of 20 minutes for 120 minutes with increasing temperature in the oven.Also the changes in the physical structure of the rock were noted.

. . . Water Boiling Test (WBT).
WBT assessed the overall performance of a cookstove via three phases which consisted of (1) bringing water to a boil from a cold start; (2) bringing water to a boil when the stove is hot; and, (3) maintaining the water at simmering temperatures.In WBT experiment, water was heated to boiling point; the time taken to boil a given quantity of water, specific firewood consumption besides evaluating thermal efficiency at both high and low energy input was done.The test was conducted in accordance with Volunteers in Technical Assistance [19].

Results and Discussion
. .Classification of Granite Rock.As presented in Table 2, the specific gravity of the granite rock was found to be 2.72, implying that the rock was of high density.
Given the fact that there was little (0.42%) water absorbed by the rock (Table 2), negligible amount of heat would be required to drive it off.Therefore, the moisture content of the rock would not have any significant effect on its heat retention properties.This makes it a suitable material for the stove construction.
The effect of temperature on the physical properties of the rock was also evaluated based on the practical experience of constructed stove operations and oven tests.From the oven test experiment, the granite rock recorded an initial temperature of 297.8 K and mass of 401.65g.There was a gradual decrease in mass with increasing temperature over time (Figure 1, see Figure S4 in the Supplementary Materials for comprehensive analysis).
After 120 minutes of oven heating, the granite rock recorded a constant mass of 401.21g at constant temperature of 567 K, indicating maximum moisture loss from the rock.Despite the slight change in colour (dusty pale white) of the rock, there was no visible fracture observed.Similarly, the maximum temperature reaching the granite rock in the constructed stove was recorded as 423 K (after 41 minutes of operation) at minimum moisture content (Figure 1) and no sign of fracture.This implies that the granite rock would be able to perform in the stove without disintegrating after successive cooking times. . .Choice of Fuel for Experimental Design.For the improved stove design, solid fuels were evaluated.The selection to evaluate carbonised agrowaste briquettes and charcoal fuels was taken based on local availability within Kampala district.
Based on (3), we need to transmit 1,538,355J of energy to a pan to boil 4.9-liter of water (≈ 4900g) from room temperature (298 K) to 373 K (at sea level) given its specific heat capacity of 4.186 J/g.K [20,21].
where H is the heat required, m is the mass of substance, and  is the temperature difference.
Assuming a thermal efficiency of 35%, and energy loss of 65%, 2025g of briquette fuel would be required to boil 4.9 liters of water as shown below.70.89 0.35 = 2025 g of briquettes (6) Therefore, the fuel chamber needs to be designed to hold at least 2025g of briquettes.Similarly, 51.6g of charcoal with average calorific value = 29.8MJ/Kg = 29800J/g [16] would generate 1,538,355J of energy required to boil 4.9 liters of water (see (3)) if fuel conversion efficiency was 100%.1538355 29800 = 51.6 g of charcoal (7) For a thermal efficiency of 35%, 147g of charcoal fuel is therefore required, as shown below: 51.6 0.35 = 147 g of charcoal (8) Therefore, the fuel chamber needs to be designed to hold 147g of charcoal.Since 100% efficiency is not practical for improved cook stoves, the decision to use charcoal as a fuel for the stove design was based on a more practical 35% efficiency target.It was observed that more briquettes would be required to achieve the same cooking task as charcoal.Therefore, charcoal was a preferred choice of fuel.
. .Design of the Stove Prototype.The convective and conductive heat transfer through the stove wall were calculated using Fourier's heat relation (see (9)).
where Q is heat flow rate (W/m 2 ), A is total cross-sectional area of conducting surface (m 2 ), X is thickness of specimen (m), and T is temperature (K).According to Baldwin [22], use of Fourier's heat equation for examination of heat transfer across a stove wall generates values that are too large.This is because the heat transferred into and out of an object depends not only on the conductivity to and from the surfaces but also on the conductivity within the object itself, dirt, or oxide layers and air at the surface of the material.Thus, ( 9) is arranged using the thermal resistance concept as shown in  =  (1 − 2) 1/ℎ1 + / + 1/ℎ1 (10) where h is the convective heat transfer obtained from A and b are constants depending on geometry and flow conditions.L denotes length.
This value was used for only stainless steel since granite and fiber glass do not cover the bottom of the stove, whereas, for granite and fiber glass, the surface area was estimated using  Taking theoretical maximum temperature = 2123 K [23] and based on reasonable assumptions, thermal conductivity of granite, Kg = 2.68 W/mK [24]; fiber glass, Kf = 0.04 W/mK [25]; and stainless steel, Ks = 16 W/mK [25], total resistance was estimated as 9.768 K.W −1 (see ( 15 Yet, it has been found that the stove would release a total of 1,538,355J of energy to water at 100% efficiency (see (3)).This implies that more than half (1,538,355 -670,032 = 868,323J) of the energy produced by the charcoal fuel goes to the water.Thus, the losses through the stove wall were workable for the stove design.
. .Features of the Stove.The stove (Figure 3) consists of a combustion chamber (inner retort) constructed using a 2.0mm thick stainless-steel material.
The combustion chamber is designed to receive solid biomass fuel (charcoal) and enhance complete combustion of combustible gas released during charcoal burning process.The combustion chamber is imbedded in a cylinder (outer retort) constructed using granite rock and glass wool (Figure 4).The combustion chamber and outer retort are designed in a way that a 30mm gap is left in between to provide a free movement of air and enhance resistance to heat transfer to the inner wall of the outer retort.This minimises heat loss given the low thermal conductivity (1.6 W.m-1. K-1) of the granite rock and air (0.02 to 0.05 W.m-1. K-1), compared with stainless steel (16 to 64 W.m-1. K-1) used in the construction of the combustion chamber [26].This leads to large amount of heat from the burning charcoal to concentrate at the bottom of the cooking pan, thus increasing thermal efficiency.
. .ermal Performance of the Stove.Indicators including thermal efficiency, water boiling rate, specific fuel consumption, fire power, and fuel use reduction were used to evaluate the performance of the designed stove.Data used in the evaluation (Table 3) were generated by WBT experiment using a recommended protocol [19].
The thermal efficiency, a measure of the fraction of heat produced by the fuel that made it directly to the water in the pot [20,21], was calculated using where Mw is mass of water in sauce pan, Cw is specific heat capacity of water, T is local boiling temperature-initial temperature of water (K), Fcm is fuel consumed (moist) (g), LHV is net calorific value (J/g), Mwv is mass of water vaporised (g), and h is specific enthalpy of vaporisation (J/g).For example, for the 2nd test run, the thermal efficiency was calculated as While thermal efficiency is a well-known measure of stove performance, better indicator may be sought especially during the low power phase of the water boiling test.This is because a stove that is very slow to boil may have a very good looking thermal efficiency since large amount of water was evaporated.However, the fuel used per water remaining may be too high since so much water was evaporated and so much time was taken while bringing the pot to a boil [20,21].With respect to this, indicators such as specific consumption, water boiling rate, fire power, and fuel use reduction were calculated as well.The water boiling rate was obtained the following equation: The water boiling rate for the 2nd test run was calculated as Specific fuel consumption measures the amount of fuel required to boil (or simmer) 1 liter of water.It is calculated (see (22)) by the equivalent dry fuel used minus the energy in the remaining charcoal, divided by the liters of water remaining at the end of the test [20,21].
The specific fuel consumption for the 2nd test was obtained using (24) Table 4 shows a summary of results on stove performances.It was found that, after 4 test runs of the water boiling test, the designed improved stove had a mean thermal efficiency of 28.8%.This means that 28.8% of the total energy produced by the fuel is used to boil water in the pot.Comparisons (Figure 5) were made with locally existing stoves such as the Lorena stove, brick stove, Envirofit  supersaver premium stove, molded 1-pot stove, Kenya ceramic jiko stove, metal stove, trench fire, and the traditional 3-stone stove.Previous studies [27,28] show that the abovementioned stoves had thermal efficiency values of 14%, 17%, 35.7%, 16%, 24.5%, 21%, 13%, and 9%, respectively.With a thermal efficiency of 28.8%, the designed prototype stove achieves tier 2 in the IWA tiers of performance [29].This shows a substantial improvement over the baseline traditional 3-stone stove.
. .Cost of the Granite Rock Cooking Stove versus Conventional Stoves.Every household cooking system incurs different costs and benefits depending on diverse energy technologies employed.These costs are related to the capital cost of a newly developed stove and/or design modifications, cost of fuels required, cost of stove distribution or marketing, money, and time spent for regular stove operation and maintenance [30].In this study, the capital cost of the granite rock stove was estimated at US$36 and compared favourably with conventional charcoal burning stoves (US$3-US$50) in accordance with Jeuland and Pattanayak [30] (Table 5).
Given the fact that the granite rock stove depicts a relatively high thermal efficiency (28.8%) and fuel use reduction (78.8%) (Table 4), there is a higher likelihood for it to operate under reduced cost, making it cheaper than the conventional stoves.

Conclusions
The improved cooking stove was designed and fabricated using locally available materials including granite rocks, stainless steel, and glass wool and was estimated to cost US$ 36.Based on its water boiling test results, 78.8% fuel use reduction was achieved over the baseline open fire stove.This was attributed to the thermal retention storage properties of the granite rock.The granite rock, besides glass wool, significantly contributed to the reduction on estimated heat (about 670kJ) that would be lost through the stove wall.This enhanced its thermal efficiency.Based on comparisons with performance standards and properties of the conventional stoves, the designed granite rock stove is a substantial improvement technology and can thus lessen the pressure put on forestry resources.However, further studies including carbon and particulate matter (PM) emissions are recommended for future design improvements to suit public health standards.Studies on use of other forms of fuel such as briquettes and wood chippings could as well be conducted to establish fuel alternatives to charcoal.this research.The authors also appreciate the personnel at National Forestry Resources Research Institute (NaFORRI), National Agricultural Research Organisation (NARO), for allowing them to use their facilities and equipment to carry out the water boiling test which helped them describe the thermal performance of the designed stove.

Figure 1 :
Figure 1: Relationship between mass of granite rock and temperature with time.

Figure 2 :
Figure 2: Plan and front elevation drawings of the designed stove (all dimensions in cm).

Figure 3 :
Figure 3: The stove made from granite rock.

Figure 4 :
Figure 4: Major components of the stove.

Table 1 :
Thermal design assumptions for the cooking stove.

Table 2 :
Summary of laboratory test results for specific gravity and water absorption tests (see FigureS1-S3 in the Supplementary Materials for comprehensive analysis).

Table 3 :
Data from WBT (see FigureS5in the Supplementary Materials for comprehensive analysis).

Table 4 :
Summary of results on thermal performance of the stove.

Table 5 :
Comparisons between the cost of the granite rock stove and conventional stoves.