Adaptive design of a prototype electricity-producing biomass cooking stove

Abstract   This work is a second iteration of an adaptive design project aimed at developing an appropriate off-grid technology for small-scale electricity generation in rural Malawi, and possibly for other developing countries. Stakeholder and user feedback gathered from the initial technology demonstrator field trial has been used to inform design improvements of a re-engineered technology demonstrator which has subsequently been deployed in a different region of Malawi to assess its viability, robustness and appropriateness. The ultimate aim of the project is to develop a domestic electricity generator that can provide adequate, affordable and reliable electricity for charging low-powered electrical appliances such as mobile phones, LED lanterns and radios. The technology under development is a thermoelectric generator that is powered from the heat produced by biomass-fed cooking stoves. The re-engineered generator utilises a single thermoelectric generator (TEG) to produce up to 4 W of electrical power whilst using significantly less expensive and more robust components than the first demonstrator. Ten generators were fitted to a low cost and locally manufactured clay cooking stoves and then deployed in the predominantly rural Ntcheu district. The TEG-stoves were equipped with sensors and data loggers and remained in the field for up to 6 months. The users were able to charge their mobile phones, LED lanterns and radios from the stove. None of the stoves were used every day, indicating that the users operated other stoves or cooking methods based on preference. The data obtained showed a maximum power consumption of around 4.5 W · h of energy per day, which represents a 50% increase compared to the previous field trial. The user operation of the stove generator and user behaviour has exposed unexpected, yet fixable, issues with the battery discharge protection of the charge control circuit design of the initial technology demonstrator.

section_content: Over 2. 4 billion people worldwide use solid biomass fuels for household cooking and heating in open fires and basic stoves (MacCarty and Bryden, 2015). 

Improved cooking stoves have been identified as an encouraging alternative to traditional open fire cooking methods, and can offer many benefits such as improved fuel efficiency, personal risk reduction, indoor air quality improvements and a range of associated positive health impacts (Ruiz-Mercado et al., 2011). Whilst there are many factors influencing the adoption of any stove design (Pine et al., 2011), the addition of an electrical generator to an improved stove could make it more attractive than the traditional cooking methods whilst simultaneously tackling the energy access problem typically encountered by the very people using these stoves. 

Electricity and other energy access are hugely important factors in establishing economic and social development on both a domestic and industrial scale (Winkler et al., 2011), yet affordable access to electricity remains one of the primary objectives of developing countries. Of the estimated 1. 4 billion people lacking access, over 85% live in developing countries. Africa has the lowest electrification rate in the world (Adkins et al., 2012). 

The energy access problem is particularly problematic in sub-Saharan Africa (SSA), with the population having the least access to electricity compared to emerging countries from other regions (Onyeji et al., (Adkins, Oppelstrup, 2012).
section_name: Introduction

section_content: Thermoelectric generators, or TEGs, are solid state energy devices which convert heat directly into electricity by means of the thermoelectric effect. For succinctness, a detailed explanation of thermoelectricity is not provided here. An excellent overview of thermoelectricity is given by Rowe (Rowe, 1978) and more recently by Hodes (Hodes, 2005). The model adopted in this study is described in detail in (O 'Shaughnessy, Deasy, 2013) and (Kinsella, O'Shaughnessy, 2014) and uses the 'Effective Seebeck Coefficient' method employed by Hsu and Huang (Hsu et al., 2011), which calculates the Seebeck coefficient α under real load conditions. The output electrical power of the generator is dependent on the internal resistance of the TEG, the temperature difference between its hot and cold faces and the resistance of the load, and can be obtained from Equation 1. 

Theoretically, maximum power is obtained when the TEG resistance matches the load resistance; i. e. when RTEG = RL. As an example, the chosen TEG1-12610-5. 1 supplied by Thermal Electronics Corp., Ontario, Canada, can supply up to 8 W at a matched load output voltage of 4. 2 V with hot and cold face temperatures of 275 °C and 30 °C respectively. This TEG also has a graphite layer on both sides to reduce thermal contact resistance and increase heat flow through the module. 

The TEG is used as a power source to charge a rechargeable lithium-iron-phosphate (LiFePo4) battery, specifically the ANR26650 cylindrical cell manufactured by A123 Systems. LiFePo4 batteries are known for their good safety characteristics and long life cycles. Some battery specifications are provided in Table 1. ). However, if the current design reduces cost, complexity and ease of assembly and integration whilst increasing robustness and still meeting the electricity demand, then the lower power output should be acceptable.
section_name: TEG, battery and stove selection

section_content: To install the generator, a hole is made in the side wall of the stove. Before this, a fire is made in the stove to check for cracks or stress during thermal expansion. The handles of the stove remain intact to ensure portability. 

The generator is assembled and installed as a single unit. 

The thickness of the walls is such that removing a small section does not markedly weaken the stove. In the future, the generator hole will be prefabricated during the manufacture of the stove. 

The TEG is located between two 50x50 mm copper 6. A DC-DC converter boosts the output voltage to a more useful 5 V, and is connected to a male USB port.
section_name: Generator design, assembly & integration

section_content: In total, ten TEG-stoves were manufactured and then were instructed that electricity would be generated as a by-product of normal stove operation, and that there was no need to burn more fuel or for longer periods. If desired, the LED lantern could be recharged during the daily cooking practices.
section_name: Results & discussion

section_content: Since the generators were expected to produce less electricity in Malawi due to the higher ambient temperature, increased sunlight exposure and also due to user behaviour, the TEG-stoves were submitted to a series of trial burns prior to field deployment to verify that all generators operated to a comparable level. Figure 7 shows that all stoves produced a similar power output of approximately 4 W. Although the TEG voltage was not directly measured in the trial the maximum obtainable TEG voltage for this design can be estimated once the apparent temperature difference is known by applying a curve-fit expression obtained from these graphs. Note that TEGs typically display a hysteresis effect, meaning that the heat up and cool down profiles are slightly different. There may be other reasons for this; e. g. lack of food to prepare or a desire to preserve the TEG by only using it at selected times stove. Where possible, later studies will also investigate differences in stove usage based on agriculture or climatic calendars to see changes in usage patterns. 

Field visits to the participant households indicate that many users have more than one cooking stove. This is in accordance with results described by Ruiz-Mercado et al. (Ruiz-Mercado, Masera, 2011) who found that when a new stove is brought into a household, the household members frequently stack stoves and fuels and select a device that best fits the particular cooking practice. Extended periods at higher temperatures result in TEG degradation and eventual failure (Mastbergen, 2008). 

Figure 14 shows that the average daily maximum TEG hot side temperature was kept within the permissible range apart from stoves 1 and 3, which exceeded 300 °C. 

The maximum values for each stove were typically recorded after the generators had failed. 

Following laboratory and pre-deployment testing in Malawi, it was anticipated that cold side temperatures would be maintained in the 80~95 °C range. The higher cold side TEG temperatures observed in Figure 14b indicate that the cooling method was incapable of maintaining the target cold side temperature. Upon revisiting the households it was clear that the plastic fan casings had melted on some TEG-stoves. At the end of the trial all samples were returned to the laboratory for testing where it was determined that the fan and low power DC motors still worked as intended. Six of the ten thermoelectric modules were also still operating correctly. It was concluded that the problem was related to the power delivered to the fan and was not a mechanical issue. 

When the user operates their TEG-stove without connecting a device, the generated power is used primarily to charge the LiFePo4 battery. If the user connects a device to the TEG-stove during cooking, most of the power delivered to the device comes indirectly from the TEG and the energy is not stored in the battery. However, when the user connects a device when the TEG-stove is not in use, the power comes from energy previously stored in the primary battery. As energy is drawn from the battery, its voltage drops. 2015) and the Wonderpot (>$100) (Okamoto, 2013). For future iterations of the generator design in this study, an in-volume price target of $25~$30 is feasible, which would result in a payback period under one year considering how much disposable income is spent on phone and lighting in sub-Saharan Africa (Adkins, Oppelstrup, 2012). 

In collaboration with Concern Universal, the next phase of the research will also involve local manufacturing of some of the heat collection and dissipation components, as well as the development of a business model and engagement of local entrepreneurs regarding the possible marketing, selling and distribution of TEGstoves to the communities. As the number of the TEG stoves available to end users during the pilot phase increases it will be important to track and understand any shifts in social, cultural or power relations between users and their broader community because of the introduction of the TEG.
section_name: Pre-deployment testing in Malawi

section_content: Ten locally-made cooking stoves were retrofitted with a
section_name: Conclusions

section_name: Acknowledgments