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Multi-functionality clean biomass cookstove for off-grid areas
Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 85–94
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Multi-functionality clean biomass cookstove for off-grid areas Risha Mal ∗ , Rajendra Prasad, Virendra K. Vijay Indian Institute of Technology, Centre for Rural Development and Technology, New Delhi 110016, India
a r t i c l e
i n f o
a b s t r a c t
Article history:
The burning of biomass in a traditional cookstove has led to significant particulate emissions
Received 24 November 2015
and does not utilize the biomass efficiently. A forced draft biomass cookstove can reduce
Received in revised form 11 July 2016
polluting emissions as compared to a traditional cookstove that is being used in developing
Accepted 3 August 2016
countries. But, to achieve a cleaner combustion, it is necessary for the household to be con-
Available online 12 August 2016
nected with an electric supply to run the small dc-fan of such forced draft cookstoves. Thus, a thermoelectric generator (TEG) in conjunction with a biomass cookstove has been devel-
Keywords:
oped and deployed for electricity generation. The TEG technology deployed in a cookstove
Renewable energy
helps to run a small dc-fan even in off-grid areas. Two novel electrical circuit topologies
Thermoelectric generator
with a TEG are presented here for the efficient power generation and clean cooking. Differ-
DC–DC converter
ent types of commercially available thermoelectric generators were tested during the design
Off-grid areas
& development process to obtain the required output. In addition to clean cooking, other
Biomass TEG cookstove
functions such as lighting and battery charging (or mobile phone battery charging) are also
In-door air pollution
available according to the user’s interest and price sensitivity. © 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Much research has been done during the past few years to integrate thermoelectric generators with cookstoves and room heaters. Most of the studies are concentrated on lighting (Mastbergen, 2008; Cedar and Drummond, 2012; O’Shaughnessy et al., 2013) and analysis of power delivery from wood stove (Mastbergen, 2008; Champier et al., 2011; O’Shaughnessy et al., 2013). But very few research studies have been done on development and analysis of TEG-integrated forced draft biomass cookstoves for clean and smokeless cooking along with other functions such as lighting, battery charging (or mobile phone battery charging) etc. The present research work highlights the multi-functionality of a TEG integrated cookstove (TEG stove), especially in off-grid areas. Most of the developing countries of Africa, Asia etc. still rely on biomass for cooking purposes. The burning of biomass in traditional cookstoves results significant pollution due to partial or incomplete combustion of biomass (Ezzati and
∗
Kammen, 2001). The major reasons for cooking with traditional biomass cookstove are: – (i) biomass (fuel) is usually available free of cost, (ii) biomass is a renewable source of energy, (iii) stove construction is usually free of cost, and (iv) such stoves are easily repairable. But, the poor (incomplete) combustion leads to harmful emissions that cause diseases and deaths (WHO, 2014). It increases the burden on almost exclusively female users due to biomass fuel gathering. Ultimately, the emissions also culminate to climate change and global warming. Most of the rural population is not aware of the potential for clean cooking by forced draft cookstoves. The concept suffers due to lack of affordability of liquefied petroleum gas (LPG) and no access to power supply (IEA, 2010). A forced draft biomass cookstove is equipped with a small dc-fan which drives excess air inside the cookstove to improve the thermal combustion and thus reduce polluting emissions. The only way to achieve clean and smokeless cooking from a biomass cookstove in an off-grid area is to use an alternative energy source to run the
Corresponding author. E-mail address: [email protected] (R. Mal). http://dx.doi.org/10.1016/j.psep.2016.08.003 0957-5820/© 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 85–94
small dc-fan connected with forced draft cookstove. The main aim of this research is to design and build a fully sustainable standalone forced draft cookstove where the alternative energy source can be used to drive the fan in off-grid areas. The required electricity can be sourced from technologies such as solar photovoltaic (SPV), biogas and thermoelectric (TE) sources etc. The major limitation of solar photovoltaic technology is that it is dependent on sunlight. The availability of sunlight is geographically limited and time variant. Within overcast areas with little or no sunlight, clean cooking may not be possible every day. Setting up of a biogas plant involves significant expenditure on infrastructure. Both of these renewable energy technologies are highly expensive. A compact solution that can be applied easily in cookstoves without changing the cooking technique is the thermoelectric generator (TEG). A TEG can be incorporated in any biomass cookstove for clean and smokeless cooking in off-grid areas. It is also confirmed by lab testing that a TEG stove can reduce fuel usage by 37–63% as compared to a traditional cookstove. The TEG stove has also the potential to cut down particulate emissions by 98% depending upon field conditions (World Bank, 2014).
2.
Principles of thermoelectric generation
When p and n type semiconductor is heated the charge carriers tend to flow from the hot junction to the cold junction. The non-uniform distribution of charge carriers helps to build charge across the junction that results in back e.m.f. and restricts further flow of charge. This phenomenon of conversion of heat energy to develop electrical power is known as the Seebeck effect. The effect was first observed by Thomas J. Seebeck in 1821. The junctions of the TEG are made up of dissimilar p-type and n-type materials and they are connected electrically in series and thermally in parallel (Goldsmid, 1960; Min and Rowe, 1995). The p-type junctions have a greater number of holes and n-type junctions have a greater number of electrons (Figs. 1 and 2). If we consider the hot side of the TEG module at temperature TH and cold side of the TEG module at temperature TC , the charge carriers will flow from TH to TC . The arrangement is such that alternate p and n junctions are connected with each other with the help of metal plates to allow flow of current. The resulting voltage that is developed by connecting two dissimilar materials of p-type and n-type whose junctions are a two different temperatures is given by: V = (˛p − ˛n ) (TH − TC ) where ˛ is the Seebeck coefficient V/◦ C, TH is the hot side temperature ◦ C, and TC is the cold side temperature ◦ C. A TE cell
Fig. 1 – Arrangement of junctions in a TEG module (Goldsmid, 1960; Lee, 2010).
Fig. 2 – Direction of flow of current with temperature gradient (Goldsmid, 1960; Lee, 2010). must comprise of two dissimilar materials, since the individual Seebeck voltages are added together owing to the opposite polarity. We assume that the external wires do not contribute to any TE voltage. The Rint of a TE couple is very low i.e. it has low voltage and low current output. Therefore, for most of the applications TE couples are connected in series forming a TE battery. Design, development and specifications
2.1.
Preparation of TEG
Most of the TEGs are commercially available in the market and need no preparation. However, for good thermal conduction, a thermal grease of high temperature tolerance (e.g. Boron nitrate, graphite, silicon etc.) is applied on two sides of the TE module. To isolate the junctions of the TE module from other metallic connections and to provide uniform thermal conductivity, a ceramic wafer is placed on the either sides of TE module. Two wires are taken out from TE module for external electrical connections.
2.2.
Integration of TEG setup
The TEG module is placed between a hot plate and a heat sink. The hot plate and the heat sink give sufficient contact surface to the TEG using screws to maximize pressure and thermal conductivity. The hot plate is attached to the wall of the cookstove where fuel is burnt. The heat sink is isolated from high temperature to keep it cool as much as possible. The only heat source to the heat sink is the cross section area of the TEG module and remainder is insulated using ceramic wool. The maximum temperature difference between the two sides of the TEG module will give the maximum power output. Most of the TE modules vary in thickness from 0.3 to 0.6 cm. It is necessary to reject heat from the cold side of the TEG. Two kind of cooling techniques (i) forced convection cooling, and (ii) water cooling (normal water, chilled water or ice) are used to remove the heat from heat sink to maintain the high temperature difference. In the design of TEG stove developed here, the forced convection cooling technique is adopted where the heat sink is covered with a duct and a fan is mounted on the top of the heat sink. The air from the fan hits the heat sink first and the same heated air is channelled inside the cookstove with the help of the duct. The hot plate of the TEG is made up of stainless steel owing to its reasonable thermal conductivity and cost. The material of the heat sink is aluminium. Aluminium
Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 85–94
87
is used so that more heat can be rejected from the cold side of the heat sink (Mal et al., 2015a).
2.3.
Design and thermal parameters
The heat source to the hot side of TEG module is the biomass that is burnt in the combustion chamber of cookstove. The temperature is highly unpredictable and susceptible to high fluctuations. So, the temperature variations inside the cookstove were examined when the stove is operated in a forced draft condition. As per the properties of Bi2 Te3 semiconductor, the hot side temperature of TEG should not exceed 250 ◦ C. There are few manufacturers who specify that hot side of TEG can tolerate continuous heat upto300 ◦ C and intermittently 400 ◦ C (e.g. Hi-Z Technology, USA). The temperature measurements on the hot side plate and the heat sink (cold side plate) of TEG module were verified during testing. The temperature measurements of the heat sink were also recorded in a forced draft condition. All the temperature readings were taken with the help of K-type thermocouple attached to a digital metre. The K-type thermocouples are thin wires which can be placed in the desired location of setup without disturbing the running process (Fig. 3). In the developed design of a TEG stove the hot plate is made of stainless steel with 5 probes. The problem of TE module degradation can be optimized by (i) selecting a high thermal conductivity hot plate so that it is not subject to subsequent power loss, and (ii) placing a mica plate on the face of hot side of the TEG module which can restrict temperature excursions beyond 250 ◦ C (Pickard et al., 2005). The 1.4 W dc-fan mounted on the heat sink helps to keep the temperature of the hot plate below 250 ◦ C (Mal et al., 2015a). The life span of the TEG module is a challenging topic and has to be evaluated carefully because the TEG module is subject to constant thermal cycling. The current results are presented after running the stove in the stove testing laboratory 90 times with 1.5 h per cycle. The inner combustion chamber of the stove was not blackened at all due to the complete combustion of the biomass burnt. Two novel circuits (electrical configurations) were designed for the TEG so that performance of the TEG and stove are not compromised.
2.4.
Electrical circuits (topologies)
Two novel electrical configurations are tested for generating the power from the TEG. In configuration 1 (battery operated 800
Temperature (ºC)
700
Fig. 4 – Performance measurement setup for TEG. electrical topology), the dc fan is connected with a battery and operated initially through the charged battery for generating the power from TEG. In configuration 2 (non-battery operated electrical topology), the dc fan is not connected with the battery, it takes time for start-up until the required temperature difference is achieved between both the sides of TEG for power generation. After 2–4 min it runs automatically after taking the necessary power generated from the TEG. These two configurations are evaluated on the basis of testing for thermal efficiency, emissions from the cookstove, power delivery from TEG, extra power utilization for lighting LED and battery charging etc. The following process may be followed to measure the power output from the TEG as shown in Fig. 4. The TEG has variable resistance at different T. When the switch is open and no resistance is connected across the TEG, it is possible to measure open circuit voltage Voc . To measure maximum power output, a load equal to Rint of the TEG is connected. When the switch is closed, as shown in Fig. 4, it is possible to measure VLoad and drop across the load as VR . The power output from the TEG will be maximal when it is connected with a matched load where internal resistance of the TEG is equal to the load resistance. Suppose, I is the current flowing through the circuit, VR is the voltage drop across resistor, RLoad is the resistance value of load R, Rint is the internal resistance of module, Voc is the voltage of module when no current is flowing (open circuit voltage), VLoad is the voltage of module when current is flowing (load voltage), Pactual is the actual power being generated by the module, then Pmax is the maximum power (Lee, 2010) of the TEG module produced at matched load, which may be evaluated by Eq. (4) as: I=
600 500
Flame temperature of Stove (Tf)
400
Hot plate Temperature (Th)
300
Heat Sink Temperature (Tc)
VR RLoad
Rint =
(Voc − VLoad ) I
Pactual = VLoad ∗
200 100
Pmax =
0 Initial
10
20
30
40
50
60
End
Time (minutes)
Fig. 3 – Temperature profile of the hot side plate, heat sink (cold side plate) of the TEG and flame temperature of the stove with time.
(1)
2 Voc (4 ∗ Rint )
2 I or VLoad
Rload
(2)
(3)
(4)
The maximum power flow is attained when VLoad = Voc /2 and ILoad = Isc /2. Here, Isc is short circuit current when the terminals of the TEG are short circuited. We assume that Rint is equivalent to RLoad . Rint is variable, depends on temperature difference and also the inverse of the V–I curve of the TEG.
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Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 85–94
12 HZ-9
Power (W)
10
TG1208-1LS
8
Two HZ-9 series
6 4 2 0 0.000
2.000
4.000
6.000
8.000
10.000
Voltage (V) Fig. 5 – Power versus voltage curve of different TEG. The current and voltage output from a TEG is variable and it depends on T of the TEG. As T rises, the voltage output rises. For a particular external load condition, the voltage and current exhibit a maximum. The maximum power output is dependent on the thermal conductivity along the TEG. The open circuit voltages of one HZ-9, two HZ-9 in series and one TG1208-1LS TEG modules were investigated with respect to temperature difference, time, voltage and power output. The two HZ-9 modules in series produces much greater power, but the cost also increases proportionally. The Fig. 5 shows the open circuit voltage of TEG on x-axis and maximum power output of the TEG across its voltage on y-axis.
2.5.
Performance analysis
2.5.1.
Battery operated electrical topology (configuration 1)
The Fig. 6 shows the functionality of the circuit in Configuration 1 where the output of the TEG is connected to the two DC-DC step-up converters of 12 V and 5 V ratings. A dc-fan is so chosen which can run on voltage as low as 2 V from its rated voltage of 12 V. The 12 V dc-fan used was purchased from M/s. Sunonwealth Electric Machine Industry Co. Ltd., Taiwan and its power consumption is 1.4 W. To run a 12 V dc-fan, it is necessary to step up the low voltage generated from the TEG to the required voltage level of dc-fan through a dc-dc converter. Hence, one DC-DC step up converter is used to step up
Fig. 6 – Configuration 1: Circuit diagram of battery operated TEG stove.
input voltage from 1 V to 12 V. The 1–12 V DC–DC step up converter has output voltage of 12.3–12.6 V and output current of 150 mA. This implies that the fan runs through the 12 V DC–DC converter when the TEG generates 1 V. Another DC–DC converter is used to step up the required voltage for charging of two 3.6 V lithium-ion batteries B1 and B2. In the designed circuit, the functionality of batteries B1 and B2 are interchangeable through a Single Pole Double Throw (SPDT) switch. One battery can be charged at a time if the DCDC step-up converter receives an input voltage of 3 V from the TEG and steps it up to 5 V. This DC–DC step-up converter is a ‘DC 3–5 V USB Output Charger Step-up Power Module Mini DCDC Boost Converter’ purchased from Chip World, China and its power consumption is 2 W. The function of the circuits in each step of operation is shown with different colours in Fig. 6. Initially, when the cookstove is ignited, the TEG does not produce any power. When the DPDT is pushed once, it is connected to terminals 1,2 and 4,5. The position of terminals 1 and 2 of the SPDT is towards B1. The function of the circuit is that B1 become the input to the 12 V converter and drives the fan. In the second step, about after 15 min the white LED glows and TEG produces sufficient power to drive the fan and charge B1 battery simultaneously. The quick rise in voltage is observed because T is high due to running of the fan and the forced draft of the air that reaches to the combustion chamber increasing the temperature of the flame. Further, the hot side of TEG heats up quickly and the cold side of TEG (heat sink) also rejects the heat simultaneously with the help of the dc-fan. In the third step, the SPST switch activates the 5 V DC–DC converter. In the fourth step, the DPDT switch is pushed again to connect terminal 2,3 and 5,6. The function of step three and step four should be done simultaneously. The fan still runs, but this time it runs from the generated power of the TEG. Now B1 gets disconnected as an input to 12 V DC–DC converter and starts charging from the output of the 5 V DC–DC converter. The voltmeter connected across the B1 indicates its fully charged state. At this point the power of the TEG is utilized for running of fan as well as charging of the discharged battery B1. This implies that the amount of charge that was wasted during running of fan initially is replenished by charging the battery. When B1 is fully charged, in step five we switch the SPDT to terminal 2 and 3 to start charging B2 from the power generated from the TEG. A lithium ion battery is fully charged at 4.3 V. So we noted the initial voltage of B1 as V1. On completion of feeding of 1 kg of wood in different batches in1 h in the cookstove, the heat still remains available on the hot side of the TEG and which is referred to as residual heat. This residual heat from the cookstove also contributes power output beyond an hour from the TEG for another 30–35 min. Hence, the total effective charging period for the battery becomes 70–75 min. The final voltage of B1 is noted as V2. At the end of the testing, the initial battery voltage remains equivalent to final battery voltage i.e. V1 = V2. This implies that the amount of charge that was discharged during initial running of fan though B1 is completely recharged when the cookstove is run for an hour in addition to its residual heat. It is also obvious from the test results that if the cookstove is run for longer hours the charging of B1 further increases. The voltage versus time graph is shown in Fig. 7. When the cookstove runs for longer hours, at a certain point battery B1 will be fully charged. The excessive power generated from TEG will be wasted if no other electrical energy storage device is put in the circuit. Hence, another battery B2 is added to store extra generated energy with the
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Load Voltage (V)
6 5 4 3
Fan run by TEG
2 1 0 0 7 12 14 20 24 31 38 45 50 57 60 66 72 78 84 90 96 Time (mins)
Fig. 7 – Load voltage (V) of the TEG as compared to time (min) in configuration1. help of terminals 2 and 3 of SPDT. B2 can be charged with the condition that battery B1 is fully charged and the dc-fan is drawing power from the TEG through the 12 V converter. The charged battery B2 can be further utilized for lighting an LED light or mobile phone battery charging. To visualize the voltage level, two voltage indicators are provided in the circuit. One voltage indicator is connected across the TEG output and other above diode D1. It helps the user to (i) visualize the output voltage of TEG, (ii) indicates when to disconnect battery B1 working as source to dc-fan through 12 V converter and thus TEG replaces B1, and (iii) when to connect TEG as source to charge B1 or B2 through 5 V converter. The other voltage indicator is connected across B1 (or B2) and it indicates (i) actual voltage of battery B1, (ii) charging voltage of B1 or the output voltage of 5 V converter. The operating voltage of voltage indicators is 4.17 V. The voltmeter across B1 only glows when B1 gets fully charged. The voltage level of TEG is maintained at 4 V by controlling the fuel feeding inside the cookstove and which is regularly monitored by digital voltmeter. To provide more accuracy of the voltage output from TEG, a white LED is also connected across B1 which glows at 3 V.
2.5.2. Non-battery operated electrical topology (Configuration 2) The Fig. 8 shows the functionality of the circuit in Configuration 2 where one TG1208-1LSmodule or two HZ-9 modules connected in series generates usable power in 2–5 min.
This indicates that within 2–5 min of burning the biomass, sufficient temperature difference is achieved to run the dc-fan simply by the heat of the flame. The performance of the stove is not compromised because the fan starts running within 2–5 min and excess air from the fan assists in the reduction of emissions. For this particular operation, a single DC-DC step up converter (1 V input–12 V output) will suffice without the use of battery at initial start of stove and to improve stove performance. The fan can run directly from the heat of stove through one TG1208-1LS module and the extra power generated can be stored in batteries for lighting etc. As the temperature of combustion chamber increases due to burning of biomass it also increases the temperature of the hot plate of the TEG. The temperature of the cold side of the TEG does not increase a great deal as it is attached to a heat sink to dissipate the heat transferred from the hot plate. The 1.5–12 V step up converter becomes functional and starts running the fan as soon as minimum temperature difference is created. The speed of the fan becomes optimal as the temperature difference increases and as a result the power from the TEG is also boosted with time. A white LED indicator is connected across the TEG output and glows when extra power is generated from the TEG. This also indicates when to connect the LED lamp for lighting or battery for charging through the SPST switch without lowering the speed of the fan being the parallel load. This process takes about 27–30 min from the initial burning time of stove. The important parameter for configuration 2 is the open-circuit output voltage of the TEG. The TEG setup must be capable of delivering a high open-circuit voltage at a minimaltemperature difference at which itbecomes capable of producing workable voltage so that the dc-fan can run earlier and electronics involved would have become functional. The variation of open circuit voltage output with respect to temperature difference (T) and time is shown in Fig. 9. The HZ-9 module starts producing workable power within 10–15 min from the lighting of the stove. We observed that the time is high enough to create a very large quantity of emissions from the stove. For a single TG1208-1LS the temperature tolerance is not above 250 ◦ C. From the above graph it is clear that the Voc of single TG1208-1LS or two HZ-9 in series is greater than a single HZ-9 when the module operates simply with heat from the stove and the cold side is solely cooled by natural convection on the heat sink. The running of the fan starts at the point where the bold black line ends as shown in Fig. 9. This implies that the single HZ-9 module needs greater temperature difference as compared to singleTG1208-1LS module. 12 HZ-9
Voc (V)
10 8
TG1208-1LS
6
2 HZ-9 in series
4 2 0 Initial
2
5
10
20
30
40
50
60
125
136
158
End
Time (min) 0
30
35
40
54
107
ΔT (°C)
Fig. 8 – Configuration 2 for running fan and battery charging simultaneously with TEG power.
Fig. 9 – Open circuit voltage (V) versus time and temperature difference in the x-axis for different TEG.
170
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Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 85–94
Fig. 10 – (a) Schematic of the TEG stove and (b) final prototype of the TEG stove with lighting for configuration 2.
In case two HZ-9 connected in series, the output voltages of these two modules are additive so that the Voc is greater as compared to a single HZ-9. In both the cases, two HZ-9 modules in series and single TG1208-1LS module, the running of the fan starts 2–3 min after the kindling of fire. Hence, this configuration can enable the fan to run directly with the help of heat from the biomass burning in the initial phase without the need of any battery. The time taken to run the fan with the generated power of the TEG is very small. However, the fan is not usually switched on initially in any forced draft cookstove because it needs time for the fuel to start kindling and catch the fire properly. At this instant, if one injects the air inside the combustion chamber before the TEG kicks in by using an electrically operated dc-fan or by mechanically operated fan, then the presence of excess air may extinguish fire. After ignition, the fan should run at full speed so that the power of TEG may not be compromised. A digital multimeter was used to indicate the voltage from the TEG. When the TEG provided sufficient power, the LED indicator connected across the TEG glowed and indicated when to connect the battery for charging through the USB connecter. Charging of the battery is only possible when TEG provides adequate power. As per Kirchoff’s law of current distribution, if a fully discharged battery is connected in the initial phase of low power generation, the current supplied to the fan is reduced and consequently the speed of the fan is reduced causing the low power generation. This is because there is a significant feedback loop in operation i.e. max power is generated when T is maximal and T depends on the speed of the fan through enhancing the combustion within the stove. If the speed of the fan is higher, the combustion efficiency of the stove is also higher which result slower stove emissions. It is primarily necessary to maintain the running of the fan in its optimum speed. It is observed that when TG12081LS module allows running of the fan directly from heat, the fan achieves its optimum r.p.m. when output of the module is 2.8–3 V. Hence, the white LED is connected in the circuit and starts to glow at 2.5 V and gets brighter at 2.8 V. At this point, when the fan runs at a higher speed and the TEG starts generating extra power, an LED light can be connected or a battery can be connected to store the excess power. The time taken to produce extra power from the initial ignition time of fire is approximately 20–25 min. If the fuel feeding for cooking is one hour the extra power is generated for more than one hour. After the fuel feeding is stopped, the residual heat
from the biomass burnt inside the stove also aids in producing sufficient power for lighting or battery charging. Fig. 10(a) shows the schematic design of the TEG cookstove along with its design attributes. The final design of the stove has 8 primary holes of 5 mm diameter and two rows of 38 secondary holes of 4 mm diameter. Therefore, there are 76 secondary holes of 4 mm diameter. The final working prototype of the cookstove along with a lit LED light is shown in Fig. 10(b) with TG1208-1LS for configuration 2.
3.
Results and discussions
3.1.
Multi-functionality of TEG stove and comparisons
In configuration 1 the function of the charging and discharging of the batteries were observed and applicable to a single HZ-9, single TG1208-1LS and two HZ-9 in series. In this application it is recommended that the voltage of the battery that runs the fan should not be less than 3.8 V, since below 3.8 V the TEG will take more than 15 min to generate 3 V. We marked B1 as the battery which runs the fan. Suppose the voltage of B1 = 3.9 V. When the fan is initially run by B1 for 15 min, the voltage drops to 3.81 V. A power of maximum 3 W is consumed in the fan circuit. Once the voltage of the TEG reaches 3 V we change the position of the DPDT switch and allow running of fan and charging B1 simultaneously from TEG through converters. The fuel feeding in the stove is usually done for an hour. There is sufficient heat in the stove that allows running of the fan and charging B1 even after fuel feeding is finished beyond 1 h. The charging of B1 takes one hour and the final voltage is noted as 4.2 V. This implies that when B1 is discharged for 15 min it takes an hour to replenish the same charge. The battery B1 can gain extra charge when the stove is run for longer periods. When B1 gets fully charged B2 can be connected by changing the position of the SPDT switch. On one hour charging of B2 it can gain a rise in voltage of 0.3–0.4 V for a 2000 mAh battery. For example, if the voltage in B2 = 3.8 V, B1 = 4.2 V and we charge B2 for an hour, the final voltage that is noted for B2 = 4.2 V. Hence, we can achieve 0.8 Wh (0.4 V × 2 Ah) extra power. This extra power output is observed during the stove is fed with biomass for 1 h. The power output will be more than this if the stove is run for longer periods i.e. beyond one hour. After 40 min of the running of the stove, the power output from the TEG becomes stable at 5–6 W. The only factor that has to be controlled is the amount of fuel feeding which maintains the TH . The low
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7
Table 1 – Standard performance parameters.
Voltage (V)
6
Type of cookstove
Standard performance parameters
5 4 3 2
Thermal efficiency (%)
CO (g/MJd)
TSP (mg/MJd)
<25 <35
≤5 ≤5
≤350 ≤150
Natural draft type Forced draft type
Fan starts
1 0 0
2
5
10
20
30
40
50
60
90
120
Time (minutes) 0
30
45
70 92 130 149 165 Temperature difference (°C)
174
190
40
Fig. 11 – Power output from TEG stove with TG1208-1LS with configuration 2.
charging rate of the battery is due to the sharing of generated power of the TEG between fan and battery charging. The DC converters also have a power consumption of nearly 1 W each. Unfortunately, if B1 and B2 batteries both are discharged at initial stage of ignition of the stove, even then the fan may run directly through the generated power of TEG after 2–3 min in case of one TG1208-1LS module and two HZ-9 modules in series. But in case of single HZ-9 module it will take 10–15 min to run the fan and at this instance the performance of stove compromises as shown in Fig. 9. Configuration 2 is applicable to single a TG1208-1LS, and two HZ-9 in series. In a shorter interval of time the voltage output from the TEG enables the DC converter to become functional quickly to drive the fan and this results for greater power generation from the TEG. It is possible to provide an extra 2 W of lighting and battery charging from these single TG1208-1LS. The fan and other electronics consume a maximum of 3 W. The 3 W of power is generated within 23–27 min of cooking. The time varies according to the biomass combustion efficiency, which is a function of the fuel type. After 30 min of cooking the module produces extra energy. The extra energy is utilized for charging the battery during daytime cooking and lighting during the night for longer period. The TG1208-1LS module produces extra power of 2 W for one hour when the stove is fed with 1 kg of wood for one hour. It is observed with TG1208-1LS module that one hour of cooking produces one hour of lighting but the lighting starts only after 27 min of cooking and remains available even after the fuel feeding is stopped. Thus LED light remains for 30 min due to the residual heat within the cookstove. The longer period of lighting even after fuel feeding has ceased is obtained due to the continuous running of the fan which maintains the T of the module. It is also necessary to keep the fan running even after fuel feeding is stopped to reject the heat from the TEG module and to protect the TEG module from overheating and degradation. In the case of charging a lithium-ion rechargeable battery of 2 Ah, one and half hour cooking gives 40 min of lighting and 2 h of cooking gives 65 min of lighting (Fig. 11).
3.2.
for water boiling test (WBT) were followed for testing of the biomass stove (Mal et al., 2015b; Bureau of Indian Standards, 1991). The threshold values of the stoves under natural draft and forced draft conditions are given in Table 1. A stove has to meet the minimum requirements in terms of fuel savings and emissions to obtain approval from the Ministry of New and Renewable Energy, Government of India. The fuel savings can be measured by thermal efficiency (%) and the emissions can be determined by carbon monoxide (gram/mega joule delivered to pot) and total particulate matter (milligram/mega joule delivered to pot). The stove has been tested in Indian Institute of Technology, Delhi, India which is one of the biomass cookstove testing centres. As per information received from available resources this is the first forced draft TEG cookstove in India that was approved by the Govt. of India. Since the TEG stove has a dc-fan, it is regarded as a forced draft cookstove. The major equipment include multi-gas analyzer, total particulate matter measuring system, bomb calorimeter etc. (Mal et al., 2015b). The thermal efficiency of the cookstove is calculated as shown in Eqs. (5), (6) and (7) where is thermal efficiency (%), Hout is heat output of the stove (heat utilized) (kJ) and Hin heat input into the stove (heat produced) (kJ).
=
Hout Hin
(5)
Hout = [(n − 1)(WCv + wCw )(t2 − t1 )] + [(WCv + wCw )(t3 − t1 )] (6) Hin = (Xfuel Hfuel ) + (Xk Hk )
(7)
where ‘n’ is the total number of vessels used, ‘w’ is the mass of water in vessel (kg), ‘W’ is the mass of vessel with lid (kg), ‘Cw ’ is specific heat of water (kJ/kg/◦ C), ‘Cv ’is specific heat of the material of the aluminium vessel (kJ/kg/◦ C), ‘t1 ’ is initial temperature of water (◦ C), ‘t2 ’ is final temperature of water (◦ C), ‘t3 ’ is final temperature of water in last vessel at the completion of test (◦ C), ‘Xfuel ’ is the mass of solid fuel consumed (kg) and ‘Hfuel ’ is the calorific value of wood (or solid fuel) (kJ/kg). The useful energy produced during an hour of burning fuel gives the measure of power output rating as shown in Eq. (8).
Po =
FHfuel 3600
(8)
TEG Stove testing protocol and standards
The two configurations, configurations 1 (battery operated) and configurations 2 (non-battery operated) were tested in laboratory conditions for power delivery from the TEG, thermal efficiency and emissions. The thermal efficiency and emission of the stove were tested to evaluate its performance. The protocols and standards of the Bureau of Indian Standard
The setup for stove testing equipment consists of fume hood where all the fumes or flue gases are collected and channelled to the gas duct (Fig. 12). The Eq. (9) is used in this conversion where COw is the weight of carbon monoxide gas in exhaust (g/MJd ), ‘p’ concentration of CO in the exhaust gas in ppm by volume or moles, ‘Q’ is the total volume of diluted exhaust gas (Litre), ‘T’
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Fig. 12 – Stove testing equipment setup (Mal et al., 2015b). absolute temperature of the gas in the duct (Kelvin), ‘Hd ’ total heat delivered to pot, (MJd ): COw = 3.411 × 10−4
TG1208-1LS
pQ THd
(9)
The TSP is calculated as in Eq. (10) where ‘V’ is the total volume of diluted exhaust gas (m3 /h), ‘G’ is the particulate matter (mg/m3 ) and ‘Hd ’ is the total heat delivered to pot in MJd . TSP =
3.3.
VG Hd
(10)
Test results and performance measurement
The stoves were tested under certain protocols with the overall objectives being increased efficiency and reduced emission. Fuel was fed continuously in accordance to the fuel burning rate. The cookstove runs for 1 h and burns 1 kg of standard eucalyptus wood. A weight of 6.1 l of water is boiled during the testing procedure. The vessel is changed when the temperature of water in vessel reaches 95 ◦ C. The standards of biomass cookstove testing are shown in Table 1. Each stove with different configurations is tested for six times to observe the consistency in results. The result of all the six tests is summarized by taking an average of the six results. The module HZ-9 does not provide enough power for lighting in the either configurations (Tables 2–4). It is observed from the testing results of cookstoves with either configurations 1 (battery operated) and configurations 2 (non-battery operated) that configuration 1 provides cleaner combustion as compare to configuration 2. Thus configuration1provides clean combustion in one hour cooking without providing the facility for 2 W lighting or battery charging. However, configuration 2comparatively provides less clean
Table 2 – Comparison between TEG stove (Config. 1 and Config. 2). HZ-9
Thermal efficiency (%) CO (g/MJd ) Total suspended particulates (mg/MJd ) Lighting led (min)
Table 3 – Comparison between TEG stove (Config. 1 and Config. 2).
TEG cookstove (config. 1)
TEG cookstove (config. 2)
37.4 2.3 109
30.21 4.77 230.56
–
–
Thermal efficiency (%) CO (g/MJd ) Total suspended particulates (mg/MJd ) Lighting LED (min)
TEG cookstove (config. 1)
TEG cookstove (config. 2)
37.4 1.7 108.78
35.78 1.79 129.73
–
60
combustion in one hour cooking but provides the facility for 2 W lighting for one hour or battery charging. But both the configuration meets the Indian WBT standards and protocols for biomass cooking of forced draft cooking. To compare the performances of the TEG cookstove, the results of natural draft and forced draft cookstoves (without TEG) are also shown in Table 5 for reference. In a forced draft cookstove the fan runs with the help of a battery or is directly connected to mains with the help of an adapter. Both these cookstoves have 1 kg/h burning rate similar to that of the TEG cookstove.
3.4.
Cost analysis
The cost analysis of the TEG setup is given in Table 6. The cost of the TEG is the bottleneck that has to be cracked to make the technology viable. The price of the TEG reduces when purchased in bulk as compared to single unit. For pricing of 1000 units, the cost reduces to 50$/unit for HZ-9 and 31$/unit for TG1208-1LS. The price of the electronics will be 3$/unit. The population of rural people using biomass cookstoves is huge, hence the cost for a single cookstove may reduce eventually. TEG Module TG1208-1LS used in configuration 2 is cheaper and provides lighting or battery charging facilities. A forced draft stove with solar technology and with
Table 4 – Comparison between TEG stove (Config. 1 and Config. 2). Two HZ-9 in series
Thermal efficiency (%) CO (g/MJd ) Total suspended particulates (mg/MJd ) Lighting led (min)
TEG cookstove (config. 1)
TEG cookstove (config. 2)
36.4 1.5 105.78
35.42 1.38 78.43
–
70
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Table 5 – Performance of a natural draft and a forced draft cookstove (Without TEG). Type of cookstove
Performance Thermal efficiency (%)
CO (g/MJd )
TSP (mg/MJd )
25.39 37.71
3.09 0.91
327.30 102
Natural draft type (Without TEG) Forced draft type (Without TEG)
Table 6 – The cost of different items for the TEG setup. Parts
HZ-9 ($)
TG1208-1LS ($)
Two HZ-9 ($)
TEG + ceramic wafer + grease Hot plate Heat sink Electronics Batteries Stove Miscellaneous
56 8 8 6 2.5 23 3.5
45 8 8 3 2.5 23 3.5
112 16 16 6 2.5 30 3.5
Total
107
93
186
these facilities costs about 146$ (The Energy and Resources Institute, 2015) in Indian market whereas the TEG stove costs about 79$.
4.
Conclusion
The technology of thermoelectric generator (TEG) has been successfully used for development of a forced draft biomass cookstove. The two novel electrical circuit topologies, configurations 1 (battery operated) and configurations 2 (non-battery operated) with TEG modules are proposed for the TEG stove. Different types of TEGs were tried and tested for power generation during design and development phase of TEG stove. A maximum power of 6 W was generated from the HZ-9 and TG1208-1LS modules. A maximum power of 10 W is generated when two HZ-9 modules were used in series. The developed design also shows that the residual heat from the stove contributes equally to the power generation even after fuel feeding is terminated. The overall system of a TEG stove has been tested and it is found that it can be implemented with most of the commercially available TEGs. Clean combustion is obtained throughout the cooking cycle, even in the initial stages of the burning of biomass fuel in the stove, with the assistance of a dc-fan from the battery when the TEG does not provide enough power as per configuration 1. For each cooking cycle of one hour, nearly 2 W can be achieved for lighting purposes from single TG1208-1LS module and 4 W from two HZ-9 modules in series without a battery as per configuration 2. The cost analysis indicates that the TEG stove is a costly technology for rural areas. But the cost of a TEG stove may likely be reduced considerably, if subsidized or manufactured in bulk at commercial level. Rural people may pay a substantial cost for a TEG stove, if they are intended to use the multi-functionality of TEG stove like clean cooking along with LED lighting and battery charging.
Acknowledgement We would like to thank Indian Institute of Technology, New Delhi, India to provide technical facilities and financial support for this research work.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/’j.psep. 2016.08.003.
References Bureau of Indian Standards, New Delhi, India, on Portable Solid Biomass Cookstove(Chulha) – First Revision, IS 13152 (Part-1): 1991, ICS 97.040.20, http://www.bis.org.in/sf/med/MED04 1157C.pdf. Cedar, J.M., Drummond, A.H., 2012. Portable Combustion Device Utilizing Thermoelectrical Generation, 8297271, J.M. Cedar, A.H. Drummond, USA. Champier, D., Bedecarrats, J.P., Kousksou, T., Rivaletto, M., Strub, F., Pignolet, P., 2011. ‘Study of a TE generator incorporated in a multifunction wood stove’. Energy 36 (3), 1518–1526, ISSN: 0360-5442. Ezzati, M., Kammen, D., 2001. Indoor air pollution from biomass combustion and acute respiratory infections in Kenya: an exposure-response study. Lancet 358, 619–624. Goldsmid, H.J., 1960. Applications of Thermoelectricity, Methuen Monograph, London. 2010. World Energy Outlook. IEA, Paris, France, Available from: www.worldenergyoutlook.org/media/weo2010.pdf (cited 12.12.13). Lee, H.S., 2010. Thermal Design: Heat Sinks, Thermoelectrics, Heat Pipes, Compact Heat Exchangers, and Solar Cells. John Wiley & Sons, Inc. Mal, R., Prasad, R., Vijay, V.K., Verma, A.R., 2015a. The design, development and performance evaluation of thermoelectric generator (TEG) integrated forced draft biomass cookstove. Procedia Comput. Sci. 52, 723–729. Mal, R., Prasad, R., Vijay, V.K., 2015b. Design and testing of thermoelectric generator embedded clean forced draft biomass cookstove. In: 2015 IEEE 15th International Conference on Environment and Electrical Engineering (EEEIC), Rome, Italy, pp. 95–100. Mastbergen, D., 2008. Development and Optimization of a Stove-Powered Thermoelectric Generator. Colorado State University, ETHOS, Washington DC, USA. Min, G., Rowe, D.M., 1995. Peltier Devices as Generators, CRC Handbook of Thermoelectrics Chap. 38. CRC Press, London. O’Shaughnessy, S.M., Deasy, M.J., Kinsella, C.E., Doyle, J.V., Robinson, A.J., 2013. Small scale electricity generation from a
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portable biomass cookstove: prototype design and preliminary results. Appl. Energy 102 (February), 374–385, ISSN: 0306-2619. Pickard, D., DiLeo, F., Kushch, A., Hauerbach, M., LeVine, L., 2005. A Self-Powered Field Feeding System. The Energy and Resources Institute, 2015, June. Bundling Improved Cooking and Lighting Technology for Energy Access,
Available from: http://www.teriin.org/policybrief/files/ ImprovedCooking/files/downloads/Bundling Technology.pdf (cited 20.06.16). WHO, 2014. Household Air Pollution and Health, Fact sheet N◦ 292. 2014, May. Understanding the Differences Between Cookstoves. World Bank.
Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Multi-functionality clean biomass cookstove for off-grid areas Risha Mal ∗ , Rajendra Prasad, Virendra K. Vijay Indian Institute of Technology, Centre for Rural Development and Technology, New Delhi 110016, India
a r t i c l e
i n f o
a b s t r a c t
Article history:
The burning of biomass in a traditional cookstove has led to significant particulate emissions
Received 24 November 2015
and does not utilize the biomass efficiently. A forced draft biomass cookstove can reduce
Received in revised form 11 July 2016
polluting emissions as compared to a traditional cookstove that is being used in developing
Accepted 3 August 2016
countries. But, to achieve a cleaner combustion, it is necessary for the household to be con-
Available online 12 August 2016
nected with an electric supply to run the small dc-fan of such forced draft cookstoves. Thus, a thermoelectric generator (TEG) in conjunction with a biomass cookstove has been devel-
Keywords:
oped and deployed for electricity generation. The TEG technology deployed in a cookstove
Renewable energy
helps to run a small dc-fan even in off-grid areas. Two novel electrical circuit topologies
Thermoelectric generator
with a TEG are presented here for the efficient power generation and clean cooking. Differ-
DC–DC converter
ent types of commercially available thermoelectric generators were tested during the design
Off-grid areas
& development process to obtain the required output. In addition to clean cooking, other
Biomass TEG cookstove
functions such as lighting and battery charging (or mobile phone battery charging) are also
In-door air pollution
available according to the user’s interest and price sensitivity. © 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Much research has been done during the past few years to integrate thermoelectric generators with cookstoves and room heaters. Most of the studies are concentrated on lighting (Mastbergen, 2008; Cedar and Drummond, 2012; O’Shaughnessy et al., 2013) and analysis of power delivery from wood stove (Mastbergen, 2008; Champier et al., 2011; O’Shaughnessy et al., 2013). But very few research studies have been done on development and analysis of TEG-integrated forced draft biomass cookstoves for clean and smokeless cooking along with other functions such as lighting, battery charging (or mobile phone battery charging) etc. The present research work highlights the multi-functionality of a TEG integrated cookstove (TEG stove), especially in off-grid areas. Most of the developing countries of Africa, Asia etc. still rely on biomass for cooking purposes. The burning of biomass in traditional cookstoves results significant pollution due to partial or incomplete combustion of biomass (Ezzati and
∗
Kammen, 2001). The major reasons for cooking with traditional biomass cookstove are: – (i) biomass (fuel) is usually available free of cost, (ii) biomass is a renewable source of energy, (iii) stove construction is usually free of cost, and (iv) such stoves are easily repairable. But, the poor (incomplete) combustion leads to harmful emissions that cause diseases and deaths (WHO, 2014). It increases the burden on almost exclusively female users due to biomass fuel gathering. Ultimately, the emissions also culminate to climate change and global warming. Most of the rural population is not aware of the potential for clean cooking by forced draft cookstoves. The concept suffers due to lack of affordability of liquefied petroleum gas (LPG) and no access to power supply (IEA, 2010). A forced draft biomass cookstove is equipped with a small dc-fan which drives excess air inside the cookstove to improve the thermal combustion and thus reduce polluting emissions. The only way to achieve clean and smokeless cooking from a biomass cookstove in an off-grid area is to use an alternative energy source to run the
Corresponding author. E-mail address: [email protected] (R. Mal). http://dx.doi.org/10.1016/j.psep.2016.08.003 0957-5820/© 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 85–94
small dc-fan connected with forced draft cookstove. The main aim of this research is to design and build a fully sustainable standalone forced draft cookstove where the alternative energy source can be used to drive the fan in off-grid areas. The required electricity can be sourced from technologies such as solar photovoltaic (SPV), biogas and thermoelectric (TE) sources etc. The major limitation of solar photovoltaic technology is that it is dependent on sunlight. The availability of sunlight is geographically limited and time variant. Within overcast areas with little or no sunlight, clean cooking may not be possible every day. Setting up of a biogas plant involves significant expenditure on infrastructure. Both of these renewable energy technologies are highly expensive. A compact solution that can be applied easily in cookstoves without changing the cooking technique is the thermoelectric generator (TEG). A TEG can be incorporated in any biomass cookstove for clean and smokeless cooking in off-grid areas. It is also confirmed by lab testing that a TEG stove can reduce fuel usage by 37–63% as compared to a traditional cookstove. The TEG stove has also the potential to cut down particulate emissions by 98% depending upon field conditions (World Bank, 2014).
2.
Principles of thermoelectric generation
When p and n type semiconductor is heated the charge carriers tend to flow from the hot junction to the cold junction. The non-uniform distribution of charge carriers helps to build charge across the junction that results in back e.m.f. and restricts further flow of charge. This phenomenon of conversion of heat energy to develop electrical power is known as the Seebeck effect. The effect was first observed by Thomas J. Seebeck in 1821. The junctions of the TEG are made up of dissimilar p-type and n-type materials and they are connected electrically in series and thermally in parallel (Goldsmid, 1960; Min and Rowe, 1995). The p-type junctions have a greater number of holes and n-type junctions have a greater number of electrons (Figs. 1 and 2). If we consider the hot side of the TEG module at temperature TH and cold side of the TEG module at temperature TC , the charge carriers will flow from TH to TC . The arrangement is such that alternate p and n junctions are connected with each other with the help of metal plates to allow flow of current. The resulting voltage that is developed by connecting two dissimilar materials of p-type and n-type whose junctions are a two different temperatures is given by: V = (˛p − ˛n ) (TH − TC ) where ˛ is the Seebeck coefficient V/◦ C, TH is the hot side temperature ◦ C, and TC is the cold side temperature ◦ C. A TE cell
Fig. 1 – Arrangement of junctions in a TEG module (Goldsmid, 1960; Lee, 2010).
Fig. 2 – Direction of flow of current with temperature gradient (Goldsmid, 1960; Lee, 2010). must comprise of two dissimilar materials, since the individual Seebeck voltages are added together owing to the opposite polarity. We assume that the external wires do not contribute to any TE voltage. The Rint of a TE couple is very low i.e. it has low voltage and low current output. Therefore, for most of the applications TE couples are connected in series forming a TE battery. Design, development and specifications
2.1.
Preparation of TEG
Most of the TEGs are commercially available in the market and need no preparation. However, for good thermal conduction, a thermal grease of high temperature tolerance (e.g. Boron nitrate, graphite, silicon etc.) is applied on two sides of the TE module. To isolate the junctions of the TE module from other metallic connections and to provide uniform thermal conductivity, a ceramic wafer is placed on the either sides of TE module. Two wires are taken out from TE module for external electrical connections.
2.2.
Integration of TEG setup
The TEG module is placed between a hot plate and a heat sink. The hot plate and the heat sink give sufficient contact surface to the TEG using screws to maximize pressure and thermal conductivity. The hot plate is attached to the wall of the cookstove where fuel is burnt. The heat sink is isolated from high temperature to keep it cool as much as possible. The only heat source to the heat sink is the cross section area of the TEG module and remainder is insulated using ceramic wool. The maximum temperature difference between the two sides of the TEG module will give the maximum power output. Most of the TE modules vary in thickness from 0.3 to 0.6 cm. It is necessary to reject heat from the cold side of the TEG. Two kind of cooling techniques (i) forced convection cooling, and (ii) water cooling (normal water, chilled water or ice) are used to remove the heat from heat sink to maintain the high temperature difference. In the design of TEG stove developed here, the forced convection cooling technique is adopted where the heat sink is covered with a duct and a fan is mounted on the top of the heat sink. The air from the fan hits the heat sink first and the same heated air is channelled inside the cookstove with the help of the duct. The hot plate of the TEG is made up of stainless steel owing to its reasonable thermal conductivity and cost. The material of the heat sink is aluminium. Aluminium
Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 85–94
87
is used so that more heat can be rejected from the cold side of the heat sink (Mal et al., 2015a).
2.3.
Design and thermal parameters
The heat source to the hot side of TEG module is the biomass that is burnt in the combustion chamber of cookstove. The temperature is highly unpredictable and susceptible to high fluctuations. So, the temperature variations inside the cookstove were examined when the stove is operated in a forced draft condition. As per the properties of Bi2 Te3 semiconductor, the hot side temperature of TEG should not exceed 250 ◦ C. There are few manufacturers who specify that hot side of TEG can tolerate continuous heat upto300 ◦ C and intermittently 400 ◦ C (e.g. Hi-Z Technology, USA). The temperature measurements on the hot side plate and the heat sink (cold side plate) of TEG module were verified during testing. The temperature measurements of the heat sink were also recorded in a forced draft condition. All the temperature readings were taken with the help of K-type thermocouple attached to a digital metre. The K-type thermocouples are thin wires which can be placed in the desired location of setup without disturbing the running process (Fig. 3). In the developed design of a TEG stove the hot plate is made of stainless steel with 5 probes. The problem of TE module degradation can be optimized by (i) selecting a high thermal conductivity hot plate so that it is not subject to subsequent power loss, and (ii) placing a mica plate on the face of hot side of the TEG module which can restrict temperature excursions beyond 250 ◦ C (Pickard et al., 2005). The 1.4 W dc-fan mounted on the heat sink helps to keep the temperature of the hot plate below 250 ◦ C (Mal et al., 2015a). The life span of the TEG module is a challenging topic and has to be evaluated carefully because the TEG module is subject to constant thermal cycling. The current results are presented after running the stove in the stove testing laboratory 90 times with 1.5 h per cycle. The inner combustion chamber of the stove was not blackened at all due to the complete combustion of the biomass burnt. Two novel circuits (electrical configurations) were designed for the TEG so that performance of the TEG and stove are not compromised.
2.4.
Electrical circuits (topologies)
Two novel electrical configurations are tested for generating the power from the TEG. In configuration 1 (battery operated 800
Temperature (ºC)
700
Fig. 4 – Performance measurement setup for TEG. electrical topology), the dc fan is connected with a battery and operated initially through the charged battery for generating the power from TEG. In configuration 2 (non-battery operated electrical topology), the dc fan is not connected with the battery, it takes time for start-up until the required temperature difference is achieved between both the sides of TEG for power generation. After 2–4 min it runs automatically after taking the necessary power generated from the TEG. These two configurations are evaluated on the basis of testing for thermal efficiency, emissions from the cookstove, power delivery from TEG, extra power utilization for lighting LED and battery charging etc. The following process may be followed to measure the power output from the TEG as shown in Fig. 4. The TEG has variable resistance at different T. When the switch is open and no resistance is connected across the TEG, it is possible to measure open circuit voltage Voc . To measure maximum power output, a load equal to Rint of the TEG is connected. When the switch is closed, as shown in Fig. 4, it is possible to measure VLoad and drop across the load as VR . The power output from the TEG will be maximal when it is connected with a matched load where internal resistance of the TEG is equal to the load resistance. Suppose, I is the current flowing through the circuit, VR is the voltage drop across resistor, RLoad is the resistance value of load R, Rint is the internal resistance of module, Voc is the voltage of module when no current is flowing (open circuit voltage), VLoad is the voltage of module when current is flowing (load voltage), Pactual is the actual power being generated by the module, then Pmax is the maximum power (Lee, 2010) of the TEG module produced at matched load, which may be evaluated by Eq. (4) as: I=
600 500
Flame temperature of Stove (Tf)
400
Hot plate Temperature (Th)
300
Heat Sink Temperature (Tc)
VR RLoad
Rint =
(Voc − VLoad ) I
Pactual = VLoad ∗
200 100
Pmax =
0 Initial
10
20
30
40
50
60
End
Time (minutes)
Fig. 3 – Temperature profile of the hot side plate, heat sink (cold side plate) of the TEG and flame temperature of the stove with time.
(1)
2 Voc (4 ∗ Rint )
2 I or VLoad
Rload
(2)
(3)
(4)
The maximum power flow is attained when VLoad = Voc /2 and ILoad = Isc /2. Here, Isc is short circuit current when the terminals of the TEG are short circuited. We assume that Rint is equivalent to RLoad . Rint is variable, depends on temperature difference and also the inverse of the V–I curve of the TEG.
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Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 85–94
12 HZ-9
Power (W)
10
TG1208-1LS
8
Two HZ-9 series
6 4 2 0 0.000
2.000
4.000
6.000
8.000
10.000
Voltage (V) Fig. 5 – Power versus voltage curve of different TEG. The current and voltage output from a TEG is variable and it depends on T of the TEG. As T rises, the voltage output rises. For a particular external load condition, the voltage and current exhibit a maximum. The maximum power output is dependent on the thermal conductivity along the TEG. The open circuit voltages of one HZ-9, two HZ-9 in series and one TG1208-1LS TEG modules were investigated with respect to temperature difference, time, voltage and power output. The two HZ-9 modules in series produces much greater power, but the cost also increases proportionally. The Fig. 5 shows the open circuit voltage of TEG on x-axis and maximum power output of the TEG across its voltage on y-axis.
2.5.
Performance analysis
2.5.1.
Battery operated electrical topology (configuration 1)
The Fig. 6 shows the functionality of the circuit in Configuration 1 where the output of the TEG is connected to the two DC-DC step-up converters of 12 V and 5 V ratings. A dc-fan is so chosen which can run on voltage as low as 2 V from its rated voltage of 12 V. The 12 V dc-fan used was purchased from M/s. Sunonwealth Electric Machine Industry Co. Ltd., Taiwan and its power consumption is 1.4 W. To run a 12 V dc-fan, it is necessary to step up the low voltage generated from the TEG to the required voltage level of dc-fan through a dc-dc converter. Hence, one DC-DC step up converter is used to step up
Fig. 6 – Configuration 1: Circuit diagram of battery operated TEG stove.
input voltage from 1 V to 12 V. The 1–12 V DC–DC step up converter has output voltage of 12.3–12.6 V and output current of 150 mA. This implies that the fan runs through the 12 V DC–DC converter when the TEG generates 1 V. Another DC–DC converter is used to step up the required voltage for charging of two 3.6 V lithium-ion batteries B1 and B2. In the designed circuit, the functionality of batteries B1 and B2 are interchangeable through a Single Pole Double Throw (SPDT) switch. One battery can be charged at a time if the DCDC step-up converter receives an input voltage of 3 V from the TEG and steps it up to 5 V. This DC–DC step-up converter is a ‘DC 3–5 V USB Output Charger Step-up Power Module Mini DCDC Boost Converter’ purchased from Chip World, China and its power consumption is 2 W. The function of the circuits in each step of operation is shown with different colours in Fig. 6. Initially, when the cookstove is ignited, the TEG does not produce any power. When the DPDT is pushed once, it is connected to terminals 1,2 and 4,5. The position of terminals 1 and 2 of the SPDT is towards B1. The function of the circuit is that B1 become the input to the 12 V converter and drives the fan. In the second step, about after 15 min the white LED glows and TEG produces sufficient power to drive the fan and charge B1 battery simultaneously. The quick rise in voltage is observed because T is high due to running of the fan and the forced draft of the air that reaches to the combustion chamber increasing the temperature of the flame. Further, the hot side of TEG heats up quickly and the cold side of TEG (heat sink) also rejects the heat simultaneously with the help of the dc-fan. In the third step, the SPST switch activates the 5 V DC–DC converter. In the fourth step, the DPDT switch is pushed again to connect terminal 2,3 and 5,6. The function of step three and step four should be done simultaneously. The fan still runs, but this time it runs from the generated power of the TEG. Now B1 gets disconnected as an input to 12 V DC–DC converter and starts charging from the output of the 5 V DC–DC converter. The voltmeter connected across the B1 indicates its fully charged state. At this point the power of the TEG is utilized for running of fan as well as charging of the discharged battery B1. This implies that the amount of charge that was wasted during running of fan initially is replenished by charging the battery. When B1 is fully charged, in step five we switch the SPDT to terminal 2 and 3 to start charging B2 from the power generated from the TEG. A lithium ion battery is fully charged at 4.3 V. So we noted the initial voltage of B1 as V1. On completion of feeding of 1 kg of wood in different batches in1 h in the cookstove, the heat still remains available on the hot side of the TEG and which is referred to as residual heat. This residual heat from the cookstove also contributes power output beyond an hour from the TEG for another 30–35 min. Hence, the total effective charging period for the battery becomes 70–75 min. The final voltage of B1 is noted as V2. At the end of the testing, the initial battery voltage remains equivalent to final battery voltage i.e. V1 = V2. This implies that the amount of charge that was discharged during initial running of fan though B1 is completely recharged when the cookstove is run for an hour in addition to its residual heat. It is also obvious from the test results that if the cookstove is run for longer hours the charging of B1 further increases. The voltage versus time graph is shown in Fig. 7. When the cookstove runs for longer hours, at a certain point battery B1 will be fully charged. The excessive power generated from TEG will be wasted if no other electrical energy storage device is put in the circuit. Hence, another battery B2 is added to store extra generated energy with the
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Load Voltage (V)
6 5 4 3
Fan run by TEG
2 1 0 0 7 12 14 20 24 31 38 45 50 57 60 66 72 78 84 90 96 Time (mins)
Fig. 7 – Load voltage (V) of the TEG as compared to time (min) in configuration1. help of terminals 2 and 3 of SPDT. B2 can be charged with the condition that battery B1 is fully charged and the dc-fan is drawing power from the TEG through the 12 V converter. The charged battery B2 can be further utilized for lighting an LED light or mobile phone battery charging. To visualize the voltage level, two voltage indicators are provided in the circuit. One voltage indicator is connected across the TEG output and other above diode D1. It helps the user to (i) visualize the output voltage of TEG, (ii) indicates when to disconnect battery B1 working as source to dc-fan through 12 V converter and thus TEG replaces B1, and (iii) when to connect TEG as source to charge B1 or B2 through 5 V converter. The other voltage indicator is connected across B1 (or B2) and it indicates (i) actual voltage of battery B1, (ii) charging voltage of B1 or the output voltage of 5 V converter. The operating voltage of voltage indicators is 4.17 V. The voltmeter across B1 only glows when B1 gets fully charged. The voltage level of TEG is maintained at 4 V by controlling the fuel feeding inside the cookstove and which is regularly monitored by digital voltmeter. To provide more accuracy of the voltage output from TEG, a white LED is also connected across B1 which glows at 3 V.
2.5.2. Non-battery operated electrical topology (Configuration 2) The Fig. 8 shows the functionality of the circuit in Configuration 2 where one TG1208-1LSmodule or two HZ-9 modules connected in series generates usable power in 2–5 min.
This indicates that within 2–5 min of burning the biomass, sufficient temperature difference is achieved to run the dc-fan simply by the heat of the flame. The performance of the stove is not compromised because the fan starts running within 2–5 min and excess air from the fan assists in the reduction of emissions. For this particular operation, a single DC-DC step up converter (1 V input–12 V output) will suffice without the use of battery at initial start of stove and to improve stove performance. The fan can run directly from the heat of stove through one TG1208-1LS module and the extra power generated can be stored in batteries for lighting etc. As the temperature of combustion chamber increases due to burning of biomass it also increases the temperature of the hot plate of the TEG. The temperature of the cold side of the TEG does not increase a great deal as it is attached to a heat sink to dissipate the heat transferred from the hot plate. The 1.5–12 V step up converter becomes functional and starts running the fan as soon as minimum temperature difference is created. The speed of the fan becomes optimal as the temperature difference increases and as a result the power from the TEG is also boosted with time. A white LED indicator is connected across the TEG output and glows when extra power is generated from the TEG. This also indicates when to connect the LED lamp for lighting or battery for charging through the SPST switch without lowering the speed of the fan being the parallel load. This process takes about 27–30 min from the initial burning time of stove. The important parameter for configuration 2 is the open-circuit output voltage of the TEG. The TEG setup must be capable of delivering a high open-circuit voltage at a minimaltemperature difference at which itbecomes capable of producing workable voltage so that the dc-fan can run earlier and electronics involved would have become functional. The variation of open circuit voltage output with respect to temperature difference (T) and time is shown in Fig. 9. The HZ-9 module starts producing workable power within 10–15 min from the lighting of the stove. We observed that the time is high enough to create a very large quantity of emissions from the stove. For a single TG1208-1LS the temperature tolerance is not above 250 ◦ C. From the above graph it is clear that the Voc of single TG1208-1LS or two HZ-9 in series is greater than a single HZ-9 when the module operates simply with heat from the stove and the cold side is solely cooled by natural convection on the heat sink. The running of the fan starts at the point where the bold black line ends as shown in Fig. 9. This implies that the single HZ-9 module needs greater temperature difference as compared to singleTG1208-1LS module. 12 HZ-9
Voc (V)
10 8
TG1208-1LS
6
2 HZ-9 in series
4 2 0 Initial
2
5
10
20
30
40
50
60
125
136
158
End
Time (min) 0
30
35
40
54
107
ΔT (°C)
Fig. 8 – Configuration 2 for running fan and battery charging simultaneously with TEG power.
Fig. 9 – Open circuit voltage (V) versus time and temperature difference in the x-axis for different TEG.
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Fig. 10 – (a) Schematic of the TEG stove and (b) final prototype of the TEG stove with lighting for configuration 2.
In case two HZ-9 connected in series, the output voltages of these two modules are additive so that the Voc is greater as compared to a single HZ-9. In both the cases, two HZ-9 modules in series and single TG1208-1LS module, the running of the fan starts 2–3 min after the kindling of fire. Hence, this configuration can enable the fan to run directly with the help of heat from the biomass burning in the initial phase without the need of any battery. The time taken to run the fan with the generated power of the TEG is very small. However, the fan is not usually switched on initially in any forced draft cookstove because it needs time for the fuel to start kindling and catch the fire properly. At this instant, if one injects the air inside the combustion chamber before the TEG kicks in by using an electrically operated dc-fan or by mechanically operated fan, then the presence of excess air may extinguish fire. After ignition, the fan should run at full speed so that the power of TEG may not be compromised. A digital multimeter was used to indicate the voltage from the TEG. When the TEG provided sufficient power, the LED indicator connected across the TEG glowed and indicated when to connect the battery for charging through the USB connecter. Charging of the battery is only possible when TEG provides adequate power. As per Kirchoff’s law of current distribution, if a fully discharged battery is connected in the initial phase of low power generation, the current supplied to the fan is reduced and consequently the speed of the fan is reduced causing the low power generation. This is because there is a significant feedback loop in operation i.e. max power is generated when T is maximal and T depends on the speed of the fan through enhancing the combustion within the stove. If the speed of the fan is higher, the combustion efficiency of the stove is also higher which result slower stove emissions. It is primarily necessary to maintain the running of the fan in its optimum speed. It is observed that when TG12081LS module allows running of the fan directly from heat, the fan achieves its optimum r.p.m. when output of the module is 2.8–3 V. Hence, the white LED is connected in the circuit and starts to glow at 2.5 V and gets brighter at 2.8 V. At this point, when the fan runs at a higher speed and the TEG starts generating extra power, an LED light can be connected or a battery can be connected to store the excess power. The time taken to produce extra power from the initial ignition time of fire is approximately 20–25 min. If the fuel feeding for cooking is one hour the extra power is generated for more than one hour. After the fuel feeding is stopped, the residual heat
from the biomass burnt inside the stove also aids in producing sufficient power for lighting or battery charging. Fig. 10(a) shows the schematic design of the TEG cookstove along with its design attributes. The final design of the stove has 8 primary holes of 5 mm diameter and two rows of 38 secondary holes of 4 mm diameter. Therefore, there are 76 secondary holes of 4 mm diameter. The final working prototype of the cookstove along with a lit LED light is shown in Fig. 10(b) with TG1208-1LS for configuration 2.
3.
Results and discussions
3.1.
Multi-functionality of TEG stove and comparisons
In configuration 1 the function of the charging and discharging of the batteries were observed and applicable to a single HZ-9, single TG1208-1LS and two HZ-9 in series. In this application it is recommended that the voltage of the battery that runs the fan should not be less than 3.8 V, since below 3.8 V the TEG will take more than 15 min to generate 3 V. We marked B1 as the battery which runs the fan. Suppose the voltage of B1 = 3.9 V. When the fan is initially run by B1 for 15 min, the voltage drops to 3.81 V. A power of maximum 3 W is consumed in the fan circuit. Once the voltage of the TEG reaches 3 V we change the position of the DPDT switch and allow running of fan and charging B1 simultaneously from TEG through converters. The fuel feeding in the stove is usually done for an hour. There is sufficient heat in the stove that allows running of the fan and charging B1 even after fuel feeding is finished beyond 1 h. The charging of B1 takes one hour and the final voltage is noted as 4.2 V. This implies that when B1 is discharged for 15 min it takes an hour to replenish the same charge. The battery B1 can gain extra charge when the stove is run for longer periods. When B1 gets fully charged B2 can be connected by changing the position of the SPDT switch. On one hour charging of B2 it can gain a rise in voltage of 0.3–0.4 V for a 2000 mAh battery. For example, if the voltage in B2 = 3.8 V, B1 = 4.2 V and we charge B2 for an hour, the final voltage that is noted for B2 = 4.2 V. Hence, we can achieve 0.8 Wh (0.4 V × 2 Ah) extra power. This extra power output is observed during the stove is fed with biomass for 1 h. The power output will be more than this if the stove is run for longer periods i.e. beyond one hour. After 40 min of the running of the stove, the power output from the TEG becomes stable at 5–6 W. The only factor that has to be controlled is the amount of fuel feeding which maintains the TH . The low
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7
Table 1 – Standard performance parameters.
Voltage (V)
6
Type of cookstove
Standard performance parameters
5 4 3 2
Thermal efficiency (%)
CO (g/MJd)
TSP (mg/MJd)
<25 <35
≤5 ≤5
≤350 ≤150
Natural draft type Forced draft type
Fan starts
1 0 0
2
5
10
20
30
40
50
60
90
120
Time (minutes) 0
30
45
70 92 130 149 165 Temperature difference (°C)
174
190
40
Fig. 11 – Power output from TEG stove with TG1208-1LS with configuration 2.
charging rate of the battery is due to the sharing of generated power of the TEG between fan and battery charging. The DC converters also have a power consumption of nearly 1 W each. Unfortunately, if B1 and B2 batteries both are discharged at initial stage of ignition of the stove, even then the fan may run directly through the generated power of TEG after 2–3 min in case of one TG1208-1LS module and two HZ-9 modules in series. But in case of single HZ-9 module it will take 10–15 min to run the fan and at this instance the performance of stove compromises as shown in Fig. 9. Configuration 2 is applicable to single a TG1208-1LS, and two HZ-9 in series. In a shorter interval of time the voltage output from the TEG enables the DC converter to become functional quickly to drive the fan and this results for greater power generation from the TEG. It is possible to provide an extra 2 W of lighting and battery charging from these single TG1208-1LS. The fan and other electronics consume a maximum of 3 W. The 3 W of power is generated within 23–27 min of cooking. The time varies according to the biomass combustion efficiency, which is a function of the fuel type. After 30 min of cooking the module produces extra energy. The extra energy is utilized for charging the battery during daytime cooking and lighting during the night for longer period. The TG1208-1LS module produces extra power of 2 W for one hour when the stove is fed with 1 kg of wood for one hour. It is observed with TG1208-1LS module that one hour of cooking produces one hour of lighting but the lighting starts only after 27 min of cooking and remains available even after the fuel feeding is stopped. Thus LED light remains for 30 min due to the residual heat within the cookstove. The longer period of lighting even after fuel feeding has ceased is obtained due to the continuous running of the fan which maintains the T of the module. It is also necessary to keep the fan running even after fuel feeding is stopped to reject the heat from the TEG module and to protect the TEG module from overheating and degradation. In the case of charging a lithium-ion rechargeable battery of 2 Ah, one and half hour cooking gives 40 min of lighting and 2 h of cooking gives 65 min of lighting (Fig. 11).
3.2.
for water boiling test (WBT) were followed for testing of the biomass stove (Mal et al., 2015b; Bureau of Indian Standards, 1991). The threshold values of the stoves under natural draft and forced draft conditions are given in Table 1. A stove has to meet the minimum requirements in terms of fuel savings and emissions to obtain approval from the Ministry of New and Renewable Energy, Government of India. The fuel savings can be measured by thermal efficiency (%) and the emissions can be determined by carbon monoxide (gram/mega joule delivered to pot) and total particulate matter (milligram/mega joule delivered to pot). The stove has been tested in Indian Institute of Technology, Delhi, India which is one of the biomass cookstove testing centres. As per information received from available resources this is the first forced draft TEG cookstove in India that was approved by the Govt. of India. Since the TEG stove has a dc-fan, it is regarded as a forced draft cookstove. The major equipment include multi-gas analyzer, total particulate matter measuring system, bomb calorimeter etc. (Mal et al., 2015b). The thermal efficiency of the cookstove is calculated as shown in Eqs. (5), (6) and (7) where is thermal efficiency (%), Hout is heat output of the stove (heat utilized) (kJ) and Hin heat input into the stove (heat produced) (kJ).
=
Hout Hin
(5)
Hout = [(n − 1)(WCv + wCw )(t2 − t1 )] + [(WCv + wCw )(t3 − t1 )] (6) Hin = (Xfuel Hfuel ) + (Xk Hk )
(7)
where ‘n’ is the total number of vessels used, ‘w’ is the mass of water in vessel (kg), ‘W’ is the mass of vessel with lid (kg), ‘Cw ’ is specific heat of water (kJ/kg/◦ C), ‘Cv ’is specific heat of the material of the aluminium vessel (kJ/kg/◦ C), ‘t1 ’ is initial temperature of water (◦ C), ‘t2 ’ is final temperature of water (◦ C), ‘t3 ’ is final temperature of water in last vessel at the completion of test (◦ C), ‘Xfuel ’ is the mass of solid fuel consumed (kg) and ‘Hfuel ’ is the calorific value of wood (or solid fuel) (kJ/kg). The useful energy produced during an hour of burning fuel gives the measure of power output rating as shown in Eq. (8).
Po =
FHfuel 3600
(8)
TEG Stove testing protocol and standards
The two configurations, configurations 1 (battery operated) and configurations 2 (non-battery operated) were tested in laboratory conditions for power delivery from the TEG, thermal efficiency and emissions. The thermal efficiency and emission of the stove were tested to evaluate its performance. The protocols and standards of the Bureau of Indian Standard
The setup for stove testing equipment consists of fume hood where all the fumes or flue gases are collected and channelled to the gas duct (Fig. 12). The Eq. (9) is used in this conversion where COw is the weight of carbon monoxide gas in exhaust (g/MJd ), ‘p’ concentration of CO in the exhaust gas in ppm by volume or moles, ‘Q’ is the total volume of diluted exhaust gas (Litre), ‘T’
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Fig. 12 – Stove testing equipment setup (Mal et al., 2015b). absolute temperature of the gas in the duct (Kelvin), ‘Hd ’ total heat delivered to pot, (MJd ): COw = 3.411 × 10−4
TG1208-1LS
pQ THd
(9)
The TSP is calculated as in Eq. (10) where ‘V’ is the total volume of diluted exhaust gas (m3 /h), ‘G’ is the particulate matter (mg/m3 ) and ‘Hd ’ is the total heat delivered to pot in MJd . TSP =
3.3.
VG Hd
(10)
Test results and performance measurement
The stoves were tested under certain protocols with the overall objectives being increased efficiency and reduced emission. Fuel was fed continuously in accordance to the fuel burning rate. The cookstove runs for 1 h and burns 1 kg of standard eucalyptus wood. A weight of 6.1 l of water is boiled during the testing procedure. The vessel is changed when the temperature of water in vessel reaches 95 ◦ C. The standards of biomass cookstove testing are shown in Table 1. Each stove with different configurations is tested for six times to observe the consistency in results. The result of all the six tests is summarized by taking an average of the six results. The module HZ-9 does not provide enough power for lighting in the either configurations (Tables 2–4). It is observed from the testing results of cookstoves with either configurations 1 (battery operated) and configurations 2 (non-battery operated) that configuration 1 provides cleaner combustion as compare to configuration 2. Thus configuration1provides clean combustion in one hour cooking without providing the facility for 2 W lighting or battery charging. However, configuration 2comparatively provides less clean
Table 2 – Comparison between TEG stove (Config. 1 and Config. 2). HZ-9
Thermal efficiency (%) CO (g/MJd ) Total suspended particulates (mg/MJd ) Lighting led (min)
Table 3 – Comparison between TEG stove (Config. 1 and Config. 2).
TEG cookstove (config. 1)
TEG cookstove (config. 2)
37.4 2.3 109
30.21 4.77 230.56
–
–
Thermal efficiency (%) CO (g/MJd ) Total suspended particulates (mg/MJd ) Lighting LED (min)
TEG cookstove (config. 1)
TEG cookstove (config. 2)
37.4 1.7 108.78
35.78 1.79 129.73
–
60
combustion in one hour cooking but provides the facility for 2 W lighting for one hour or battery charging. But both the configuration meets the Indian WBT standards and protocols for biomass cooking of forced draft cooking. To compare the performances of the TEG cookstove, the results of natural draft and forced draft cookstoves (without TEG) are also shown in Table 5 for reference. In a forced draft cookstove the fan runs with the help of a battery or is directly connected to mains with the help of an adapter. Both these cookstoves have 1 kg/h burning rate similar to that of the TEG cookstove.
3.4.
Cost analysis
The cost analysis of the TEG setup is given in Table 6. The cost of the TEG is the bottleneck that has to be cracked to make the technology viable. The price of the TEG reduces when purchased in bulk as compared to single unit. For pricing of 1000 units, the cost reduces to 50$/unit for HZ-9 and 31$/unit for TG1208-1LS. The price of the electronics will be 3$/unit. The population of rural people using biomass cookstoves is huge, hence the cost for a single cookstove may reduce eventually. TEG Module TG1208-1LS used in configuration 2 is cheaper and provides lighting or battery charging facilities. A forced draft stove with solar technology and with
Table 4 – Comparison between TEG stove (Config. 1 and Config. 2). Two HZ-9 in series
Thermal efficiency (%) CO (g/MJd ) Total suspended particulates (mg/MJd ) Lighting led (min)
TEG cookstove (config. 1)
TEG cookstove (config. 2)
36.4 1.5 105.78
35.42 1.38 78.43
–
70
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Table 5 – Performance of a natural draft and a forced draft cookstove (Without TEG). Type of cookstove
Performance Thermal efficiency (%)
CO (g/MJd )
TSP (mg/MJd )
25.39 37.71
3.09 0.91
327.30 102
Natural draft type (Without TEG) Forced draft type (Without TEG)
Table 6 – The cost of different items for the TEG setup. Parts
HZ-9 ($)
TG1208-1LS ($)
Two HZ-9 ($)
TEG + ceramic wafer + grease Hot plate Heat sink Electronics Batteries Stove Miscellaneous
56 8 8 6 2.5 23 3.5
45 8 8 3 2.5 23 3.5
112 16 16 6 2.5 30 3.5
Total
107
93
186
these facilities costs about 146$ (The Energy and Resources Institute, 2015) in Indian market whereas the TEG stove costs about 79$.
4.
Conclusion
The technology of thermoelectric generator (TEG) has been successfully used for development of a forced draft biomass cookstove. The two novel electrical circuit topologies, configurations 1 (battery operated) and configurations 2 (non-battery operated) with TEG modules are proposed for the TEG stove. Different types of TEGs were tried and tested for power generation during design and development phase of TEG stove. A maximum power of 6 W was generated from the HZ-9 and TG1208-1LS modules. A maximum power of 10 W is generated when two HZ-9 modules were used in series. The developed design also shows that the residual heat from the stove contributes equally to the power generation even after fuel feeding is terminated. The overall system of a TEG stove has been tested and it is found that it can be implemented with most of the commercially available TEGs. Clean combustion is obtained throughout the cooking cycle, even in the initial stages of the burning of biomass fuel in the stove, with the assistance of a dc-fan from the battery when the TEG does not provide enough power as per configuration 1. For each cooking cycle of one hour, nearly 2 W can be achieved for lighting purposes from single TG1208-1LS module and 4 W from two HZ-9 modules in series without a battery as per configuration 2. The cost analysis indicates that the TEG stove is a costly technology for rural areas. But the cost of a TEG stove may likely be reduced considerably, if subsidized or manufactured in bulk at commercial level. Rural people may pay a substantial cost for a TEG stove, if they are intended to use the multi-functionality of TEG stove like clean cooking along with LED lighting and battery charging.
Acknowledgement We would like to thank Indian Institute of Technology, New Delhi, India to provide technical facilities and financial support for this research work.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/’j.psep. 2016.08.003.
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