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System Integration of Thermoelectric Power from Multiple Biomass Stoves: Feasibility study and Prototype design with preliminary results R. Wong, M. Ahmed, J.S. Wu, M.J. Siew, Z.F. Guo, C.M. Zhao, J. Li Department of Electrical & Electronic Engineering, Imperial College, London

HIGHLIGHTS Improved stoves to reduce hazardous pollution while generating electricity Designed system to combine multiple stoves Looking to merge with existing start-ups and deploy the system in rural Kenya Explored the potential of this system in first-world applications


Table of Contents Introduction ....................................................................................................................................1 Technical Considerations .............................................................................................................1 Motive/Aim ..............................................................................................................................1 Technical Analysis .........................................................................................................................2 Thermoelectric Generation(TEG) Technology ............................................................................2 Maximum Power Point Tracking .............................................................................................3 Product Efficiency ...............................................................................................................4 Product Design ...............................................................................................................................4 Material Properties........................................................................................................................5 Battery......................................................................................................................................5 Heat Sink ..............................................................................................................................6 TEG ..................................................................................................................................6 Tests and Experimentation ...........................................................................................................6 Business Aspects .............................................................................................................................7 Market Prospectus ........................................................................................................................7 Contribution to Sustainable Development ..............................................................................7 Future Market Potential (Limitations) ................................................................................8 Conclusion ......................................................................................................................................8 References .......................................................................................................................................9


1. Abstract Two million people die annually from illness because of indoor air pollution from biomass stove use. These stoves are primitive and inefficient. Innovations such as adding a fan to maximise combustion and reduce pollution required electricity, so improved Biomass Stoves were designed and are now capable of generating electricity. The electricity would be used to charge up other small electrical devices such as mobile phones, but we believe there are much more potential to this idea. Our project aims to maximise the power generated in each stove and through multiple stoves, to power bigger appliances within the grid-less, poverty stricken rural parts of Kenya. Thermoelectric generation involves the conversion of heat to electricity via the Seebeck Effect. We explore various ways to maximise efficiency, reduce costs, and also to combine the generated power so as to charge larger devices.

2. Introduction There have been several initiatives regarding the safer and cleaner use of biofuels in the third world. Recently, a creative start-up developed the BioLite Homestove, reportedly capable of generating 2 Watts of electricity at 5 Volts while reducing hazardous pollution by 90%. This has enabled families to charge mobile phones off the grid at no extra cost, providing electricity in small amounts. Our project aims to create a system that connects these stoves efficiently. This network would then be able to charge a larger battery system, potentially even provide power for a small building, such as the village school. The perks of such an initiative are its relative low costs and maintenance compared to solar power. It would also help introduce technology to an otherwise unsupplied village. People would then be able to donate old electrical equipment to village schools while incapable of using them before. The idea of combining tiny generators in a grid is not new. Many countries have systems where domestic generators such as solar panels and biomass generators can connect to the power grid, “selling” electricity back to the market. The challenge is that our small-scale implementation of this will have to be very efficient to be of any use, as the Thermo-Electric Generators (TEGs) have very low power outputs.

2.1 Technical Considerations There are a number of technical challenges and compromises associated with such an undertaking. Two main approaches were considered. In the first, wires connect each stove to a central station where the big batteries will be charged, much like a transmission network. The second involves charging a small highcapacity battery (Lithium-Polymer) at each station, and physically bringing them together in a convenient interface to power bigger devices. There are pros and cons to each approach. The transmission network will be inefficient, as DC transmission over long wires incurs relatively high losses. Furthermore, power generated by TEG is small, and such a loss will become significant. However it has the advantage of not having to create mobile battery plants. On the other hand, a battery system will cause inconvenience as users are required to physically transport batteries around, although the absence of long transmission wires could minimize losses and thus we generate power more efficiently.

2.2 Motive/Aim Looking at the bigger picture, one can see that the first approach is better suited to developed countries with systems already in place for providing electricity back to the market. Systems for Solar and Wind power are commonplace. Examples include Ecotricity, Good Energy, EDF Energy’s “Green Power, and more. Solutions for domestic waste heat are less common because of the relative inefficiency of TEG. However, it is not terribly hard to envision “Add-On” modules to domestic cook-stoves and water boilers easily generating energy from waste heat back to the grid. 1


Our experiments will be focused on the developing world’s perspective as it is more important for them to start harnessing energy than for the developed world to stop wasting it. Our battery standards, MaximumPower-Point-Tracking (MPPT) technology for maximising TEG output, and also the connection interfaces for the batteries will be detailed below. Working off the BioLite Homestove already being used in some villages to reduce pollution, we focus on maximising power generated (MPPT) and implementing a cost-effective solution to charge larger electrical devices such as laptops. We hope that with our efforts, cheap and ecological sustainable technology will be introduced to these villages.

3. Technical Analysis 3.1 TEG Technology The world wastes a lot of thermal energy. Technology to recycle waste heat into energy for use have been available for a long time, but the low efficiency of early thermoelectric generation systems (<5%) limits the practicality of such a process. TEGs are solid-state energy converters that combine thermal, electrical, and semiconductor properties to convert heat into electricity. It uses two sides held at different temperatures. The mobile charge carriers at the ‘hot’ side diffuse to the ‘cold’ side when a temperature difference is applied across. The build-up of charge carriers results in a net charge and produces voltage. The greater the gradient, the more power can be produced. In general, it is a useful and clean technology as it may result in better fuel economy with less CO2 emissions without being too expensive. A typical TEG is made of bismuth-telluride (Bi2Te3–PbTe) semiconductors sandwiched between two metallized ceramic plates. As these generators eliminate the need for wires and batteries, they are reliable and scalable, making them ideal for distributed power generation in remote places such as in offshore engineering operations and data collection in satellites and spaceships. Because of its advantages, it has had an increase in development and applications. The latest developments have increased the low efficient TEG from 5% to 20%. This has raised our interests in TEGs and interest in the research of TEG development. Research on its cost-effectiveness and practicality further confirms its feasibility for use in our proposed system. Firstly, since the temperature differences of the materials should be as large as possible to maximize power-generation efficiency, we have researched on the properties of different materials with respect to temperature. TEG requires materials that are good electrical conductors and poor thermal conductors so to maximise the Seebeck Effect [1]. But it is hard to find such ideal materials. We have concluded from our research that individual TE elements formed from two or three different TE materials laminated together achieves maximum efficiency. Fig.1 The Seebeck Effect describes a thermoelectric phenomenon by which temperature differences between two dissimilar metals in a circuit converts into an electric current.

Thermo electric emf E = αβ + 1/2 βt2 where a and b are thermo electric constants having units volt/°C and volt/°C2 respectively (t = temperature of hot junction).

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Moreover, we did some research on Phonon Drag Effect as well, a seemingly effective way of improving the efficiency of thermoelectric generation. It is an increase in effective mass of conduction electrons or valence holes due to interactions with crystal lattice. When an electron moves past atoms in a lattice, its charge distorts or polarises the nearby lattice and hence it suffers decreased mobility which leads to lower conductivity. However, TEG might benefit from it as the magnitude of thermo-power increases with phonon drag.

3.2 Maximum Power Point Tracking (MPPT) MPPT refers to a technique of varying applied load in order to achieve the appropriate voltage level for generation systems such as solar panels to generate most power. In a TEG system, the mismatch between load and the TEGâ&#x20AC;&#x2122;s internal resistance can significantly reduce the efficiency of generation. For our proposed design, it is essential to have a module controlling the operating power point because of the relatively unstable heat source (fuel burning). The mismatching power losses are also significantly dependent on the temperature distribution of heat source. In order to keep track of the system's operating point, we need to insert a power conditioner in between the TEG and the load. The conditioner is used to control the load impedance seen from the input/generation side of the whole circuit. In order for the impedance to be matched, the impedance of load has to equal the TEG module's internal impedance. The following graph describes the efficiency/temperature difference gradient with respect to load impedance for a Bi2Te3â&#x20AC;&#x201C;PbTe TEG system.

Fig. 2 Ratio of load (RL) to TEG (RTEG) electrical resistance as a function of the TEG temperature-dependent performance. Maximum Power Points are displayed.

There are various ways of tracking the MPP. Most designs involve microcontrollers and DSPs (digital signal processors). There are also simple MPPT analogue circuits using low power op-amps. Here, we will only consider microcontrollers and DSPs as the analogue method still draws noticeable amounts of power even with low power analogue components. We looked into a paper by H. Nagayoshi, K. Tokumisu and T. Kajikawa on MPPT systems consisting of a buck-boost SMPS converter and a microcontroller. The control module draws up to 10mW of power which is relatively tiny. The referred load impedance is varied by the buck-boost switching circuit which steps up and down the output voltage. During buck operation, the circuit is used to step down output voltage. The load impedance is reduced according to (Vout/Iout). For boost mode, the opposite happens. 3


Input current and voltage can be measured with the microcontroller interfacing together with a simple resistor circuit from which we can compute virtual impedance. If impedance of load is smaller than that of the TEG, buck is switched on to step down voltage otherwise boost is on to step up voltage. Measurements of input voltage and current must be obtained before control is applied. Microcontroller is programmed to give a PWM output to control the switch circuit. The circuit can be further optimised with a synchronized switching circuit.

3.3 Product Efficiency

Fig 4. Simplified circuit diagram of the MPPT design

Fig.3 Buck/boost operation regions. Converters are used here to control output impedance so that a high power point is maintained.

Based on our research, thermoelectric generation can generate a small power output limited by the characteristics of the TEG module. Theoretically, 5W at voltages up to 12V can be generated in a single cooking stove alone. Later in this report, we will go further into details on experimenting with a cheap Peltier module and its characteristics displayed while a steady temperature difference is maintained. Combining multiple stoves will inevitably incur some losses. The efficiency of overall power generation regarding large electrical appliances will be largely determined by the efficiency of our mini-grid.

4. Product Design The system consists of three main modules, the combustion module, thermoelectric module and mini-grid module. Combustion module is where the biofuels (dried leaves, twigs, sticks) are burnt to generate heat for cooking and electricity. Good structural design will allow efficient, complete combustion without too much energy waste and pollution. There will be a fan powered by the TEGs included in the design to pump air (oxygen) in to improve combustion. In the thermo-generation module, the hot side of a thermoelectric module is mounted near the combustion chamber to obtain a relatively large temperature difference from the heat source. Ideally we would use a module with maximum temperature tolerances as close as possible to stove temperatures. A heat sink is then installed on the cold side to maintain a steady optimal temperature difference. For the power module, a low power MPPT and Switch-Mode PowerSupply (SMPS) circuit is needed to drive the load while keeping the operation of TEG in its optimal range. A buck/boost SMPS can be used for converting the voltage level to the nominal value of electrical 4


appliances. The circuit is connected with a simple 5V USB output port for charging mobile phones directly. Similarly, Lithium-Polymer batteries such as those used in mobile phones can also be directly connected to store energy. For larger electrical appliances, a battery bank could draw energy from multiple batteries combined in series and/or parallel.

Fig. 5. Battery Configurations

For example, a one cell Lithium Polymer battery such as that shown above contains 600mAH at 3.7V. An Asus EEE netbook battery contains 5000mAH at 7.4V. 600mAH*8=4800mAH. This means that with 8 parallel sources of 2 batteries in series (2S8P configuration, 16 batteries in total), one can charge the Asus EEE netbook to its full capacity, for 6 hours of good educational use.

4.1 Material Properties The combustion chamber material must be refractory in order to keep its shape under heat stress. Two choices are refractory ceramics and metallic materials. For finding the most appropriate material we considered if it could be produced locally which meant lower costs. On the other hand we need to plot the temperature profile inside the combustion chamber, so that we will be able to choose a material which can increase the life expectancy of our stoves. Heat insulation around the combustion chamber is important for safety and efficiency. Five most common thermal insulation materials are fiberglass, mineral wool, cellulose, polyurethane foam, and straw bales. As our project aims to help people living in rural areas such as Kenyans, we need to keep costs as low as possible. Thus cellulose might be the most eco-friendly method of insulation made from recycled paper, cardboard, and other cheap materials. Also as there is no oxygen within cellulose, fire hazard will be minimized. As temperatures in the fire will be too high for the TEG, we connect the two with a suitable heat probe, such as a long rod. It must have good thermal conductivity and high melting point. Metals are a good solution. Cast iron and steel could both be used. The diagram below shows this setup graphically.

Fig.6 Stove Design

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4.2 Battery Factors such as battery’s capacity, memory effect and recharge cycle are relevant to our project. A battery should have no memory effect. Memory effect is the process in which the battery’s capacity will drop over time if it is charged before it is completely empty. Batteries with memory effects should not be chosen. Having higher capacity is important to eliminate the need for frequent charging. The number of recharge cycles - number of times the battery can be charged and discharged before rendered unusable should be high. For our experimental purposes, an available Lithium Polymer battery (EFLB5001S) was used. As for the choice of a battery to be used when implementing our design, ‘Fuji NP-60 replacement battery’ (capacity: 1,100 mAh; output voltage: 3.7V) made by Duracell was chosen. A lithium-ion battery is generally used in smartphone, as it has great number of recharge cycles.

4.3 Heat Sink Fin arrangement plays a heavy role in ensuring heat is dissipated properly. The three types of fins are pin, straight and flared. According to Kordyban, a straight-finned heat sink is comparable, if not better, to a pinned one. Naturally, the flared arrangement is the best because it is a straight-finned heat sink with fins bent outwards. This improves airflow around the fins. The second factor is the thermal resistance. This is the ratio of the temperature difference with respect to the heat power. Having a lower thermal resistance would mean that the heat sink’s base is closer to the ambient temperature than one with a higher thermal resistance. The heat sink (ML33G) manufactured by AAVID THERMALLOY, which has thermal resistance of 8.5K/W and straight fin arrangement, is chosen for our experiments.

4.4 TEG Our choice of a cooler module instead of the purpose-built Seebeck module is based on cost, and its performance will lack on the voltage generation and operating temperature fronts, but functionally it is identical to a TEG. The parameter of interest for a TEC (Thermo-Electric Cooler) is Qcmax, the cooling capacity. It is the amount of heat pumped from the cold side to the hot side in Watts. After considering many available choices, we decided on module TEC1-12706. It can operate at temperatures of up to 100ºC.

5. Tests and Experimentation In preliminary tests, a TEC1-12706 module was placed at the vent of a fan heater. Ice was used in place of the more expensive heat sink. Temperatures averaged 70 °C on the hot side and 10 °C on the cold side. For this Peltier cooler module these temperatures were well within its tolerances and noticeable power was being generated for different loads. The first test conducted determined the optimum period of time to warm one side up while keeping the other relatively cool. Inadvertently both sides heat up over time as the ice melts. A quick conclusion was reached at 40 seconds. Generally, the voltage induced started to drop off between 35 to 50 seconds, when outlet air has reached maximum temperature and the temperature difference starts to decline. Fig. 7 Preliminary Tests with TEC module

With this kept constant, we then measured power generated across different loads. I-V characteristics for the TEG turned out to be quite 6


similar to a solar panel’s. The internal impedance of the TEC module we used was 2Ω, varying slightly with temperature. We found that by matching this impedance, we obtained the MPP. The following table displays different the I-V characteristics for 4 different load resistors, and reinforces the fact that MPP was obtained when internal impedance was matched. For this temperature difference it is about 2Ω. Load Resistance/ Ω Voltage across Load/mV Current across Load/mA Power Generated/mW 100 460 3 1.38 10 450 20 9.0 2 430 135 58.05 (maximum) 1.8 405 142 57.5 With the more expensive Seebeck TEG module, this impedance would be much smaller, and generated power would increase as less power is lost to the internal impedance. Furthermore, fan heater output temperature is also fairly constant compared to the stove, reducing the need for active MPPT with voltage readings and varying loads. Constant impedance matching has sufficed in this case. However we do hope to garner more funds to begin more relevant testing. With the MPP obtained through impedance matching, it was now necessary to step up the generated voltage to be able to charge the 3.7V LiPo. LiPo chargers generally output 4.2V. This voltage difference of 0.5V is enough to cause a charge current between the converter and the battery. When the battery has reached 4.2V (its maximum resting voltage), the charge current naturally disappears. This has also been tested and proven. A last test to confirm the validity of our concept was to ensure that multiple batteries could in turn charge a much bigger one, found in a netbook. Wiring them up in a proper 3s1p configuration, attaching an adapter cable consisting of crocodile hook inputs and a DC jack output, we confirmed that the small batteries, when brought up to the right voltage by connecting them in series, could provide sufficient power to charge a bigger one. With all these “proof of concepts” secured, it is our hope that by merging with a larger company such as BioLite we will be able to make this a reality, and provide even more electrical capabilities to rural Kenya and other third world countries.

6. Business Aspects 6.1 Market Prospectus (Rural Areas) TEG technology can be used for energy optimisation in several markets such as the automotive, maritime, biofuels and combined heat and power industries. It can increase the efficiency of petrol-based hybrid cars up to 10%. Other cars can drive up to 10% longer per litre. Since our project’s target is mainly to connect the TEGs efficiently, we can have many potential markets in any of these industries. In this project, we find it important to provide utility to rural areas. The economy in rural areas is highly dependent on natural resources; however, they will finally see the over-use and destruction of resources which resists their development even more. There is also a need to ensure that environmental concerns are integrated into national development plans. According to statistics we obtained, more than 90% of rural households use firewood for cooking. Improved biomass stoves enhance lives by providing easy heating, lighting and cooking.

6.2 Contribution to Sustainable Development This project contributes to the sustainable development of rural areas: i.

Social Development 7


a. Improved Biomass Stoves will use less fuels which is economically beneficial for the communities. b. Reduced amount of pollutant emissions from burning biomass will benefit the country as a whole and improve health standards. c. Lowers the workload required for collecting wood fuel as well as fire risks related to old and unsafe stove designs ii.

iii.

Economic Development a. During project implementation, it will help generate a section of the rural economy during the installation, maintenance and monitoring phases. Employment opportunities are therefore created which will help the countryâ&#x20AC;&#x2122;s economic situation. b. Because of the increase in thermal efficiency, costs related in the purchase of fuel can be reduced. Protection of Environment a. There is no doubt that significant reductions in CO2 emission will result. Biomass Stoves use waste agricultural products which are normally carbon neutral. This reduces deforestation rates and assists in the maintenance of existing ecosystems. b. The displacement of wood fuel means that unsustainable harvesting of wood is halted and the watersheds that prevent disasters such as flash flooding have a chance to regenerate. c. Reducing the use of coal in inefficient stoves will lead to the reduction of acid rain problem as well as health effects caused by the air pollution.

6.3 Future Market Potential (Limitations) Although there are a few specific policies in Asian and African rural areas, non-governmental organisations and small enterprises have promoted and marketed the idea of improved biomass stoves. Despite the many technological advances, there are still several barriers before the technology can be fully implemented and utilised: i. ii. iii. iv.

The high initial costs of the biomass stove; Lack of local availability of electronic devices; Lack of product standards such as a battery system; Lack of coordination between government and private agencies

However, we can indeed come up with different methods to solve the potential problems. Take the costs issue as an example, government can provide financial support and identify suitable incentives, community fund and soft loans for the widespread adoption of the technology. To change long established customs, the diffusion of the biomass stoves required a strong promotion strategy and mobilization among the public to attract more funds and recognitions. We can achieve much with skills in marketing strategy.

7. Conclusion With a small but definite presence in the form of companies such as BioLite, the idea of thermoelectric generation while reducing hazardous pollution for the rural third world is not new. But our group has created a system, whether through batteries or through a mini-grid, to increase these contributions manifold. With proper field tests and implementation, it is not hard to imagine an alternate world where everyone has access to a computer. Even those living without an electrical grid. With proper support and initiative, it is not hard to realise this. Working alongside other start-ups such as Aurora-Wimax (Imperial College) bringing wireless internet to the same target areas, we can help connect everyone in the world to the efficiency, convenience, and unbound knowledge and technology of the modern world-wide-web.

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EE2 Project Report