Improved test method for evaluation of bio-mass cook-stoves

Energy xxx (2014) 1e17

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Improved test method for evaluation of bio-mass cook-stoves P. Raman, N.K. Ram*, J. Murali The Energy and Resources Institute (TERI), Darbari Seth Block, India Habitat Centre, Lodhi Road, New Delhi 110003, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 July 2013 Received in revised form 21 April 2014 Accepted 26 April 2014 Available online xxx

More than two-thirds of the world’s population is relying on biomass fuel to meet their cooking and heating energy-requirements. Traditional biomass stoves operate at low efficiency and cause severe health problems and pollute the environment. Due to higher quantity of fuel use, these cookstoves increase the burden on fuel management. Several test protocols are being used across the world for evaluating the performance of cookstoves. One of the major challenges of existing protocols is to narrow down the gap between the test results obtained under lab conditions and actual cooking conditions. Hence, there is a need to evolve an improved test method that can reflect the stove performance under field conditions. This paper is aimed to reduce the gaps in test methodology in such a manner that the test results obtained in the lab are comparable with the results of actual cooking carried out in the kitchen. An improved test method, which includes a residual heat recovery phase, is proposed to evaluate the performance of cookstoves. Design parameters related to technical, social and economic aspects were identified. Common errors that occur during the water boiling test were identified and methods to minimize such errors were also proposed. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Biomass cookstove Test protocols and benchmarks Water boiling test Residual heat recovery Thermal efficiency

1. Introduction More than three billion people in the world are relying on solid fuel to meet their cooking energy needs [1]. About 1.3 million people die every year due to the exposure to smoke and other pollutants released, due to the burning of biomass fuels [2]. In India, out of 225 million households, 160 million households use biomass as a fuel to meet their cooking energy needs [3]. These cookstoves pollute the kitchen and atmosphere with significant emission of pollutants such as CO, CO2 and particulate matter [4]. Improved cookstoves with chimney were widely disseminated in India through the NPIC (National Program on Improved cookstoves) to reduce the indoor air pollution in rural households [5]. The program was met with limited success due to several reasons; however learning’s were well captured to improve the existing designs of cookstoves [3]. It is important to ensure that the cookstoves meet a certain standard and benchmarks to have a successful implementation of any program of cookstove dissemination [6]. Out of 50 improved cookstoves disseminated, 13 cookstoves did not meet the required standards [7]. QA & QC (Quality assurance and quality control) are a key requirement for large-scale dissemination of any product. For an effective implementation of improved cookstove

* Corresponding author. Tel.: þ91 11 24682100; fax: þ91 11 24682145. E-mail addresses: [email protected], [email protected] (N.K. Ram).

program, it is essential to provide a cookstove that is efficient and also meets the users’ requirements [8,9]. The research work reported in this paper is aimed to bring out an elaborate cookstove test methodology to ensure that QA & QC are in place. There are two types of cookstoves are promoted in many developing countries, as part of the current improved cookstove program. These cookstoves are natural draft and forced draft cookstoves [10]. In cookstoves working on natural draft mode, air required for combustion of biomass is supplied by the draft created by the chimney. On the other side, in cookstoves working on forced draft mode, air required for combustion is supplied using a small fan [10]. While the natural draft cookstoves has a single pot or a double pot cooking option, forced draft cookstoves are using single pot cooking at a time. Most of the biomass cookstoves with natural draft are operating with efficiency in the range of 10e20%, whereas the forced draft cookstoves works in the range of 40e43% [11]. Several research institutions across the world are working on developing clean combustion cookstoves to improve their efficiency and reduce emission levels [12]. Forced draft cookstoves reduce the use of fuel wood by 40% and emission levels by 90% [7]. The improved cookstove with chimney reduces the pollution to a large extent in the kitchen. The improved cookstoves reduce the particulate matters (PM2.5) to the order of 71e84% and CO by 98e 99% [7]. However the improved cookstoves (with chimney) reduce the pollution in the kitchen, but shift the problem outdoor [1]. Hence, in the improved cookstove program, it is essential to reduce

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the emissions through cleaner combustion of biomass fuel instead of shifting the emissions out of the kitchen. The chimney cookstoves reduce the pollution in the kitchen. But, cookstoves with chimney consume more fuel in the range of 61% and require 74% more time than the conventional cookstoves [7]. Many researchers were emphasized on the design principle for a wood burning cookstove and provided suggestions for improving the efficiency as well as meeting the user requirements [13,14]. Hence, it is essential to have a well-designed cookstove which operates at a higher efficiency and emits less pollution. Testing and evaluation of cookstoves are important to ensure that the new stoves meet the required standards and benchmarks. One of the key factors influencing the community to adopt a particular cookstove is effectively addressing their socio-cultural aspects, associated with cooking. Common socio-cultural parameter includes; type of food, cooking habits and availability of biomass fuels in that region. The improved cookstoves should meet the safety aspects of the stove, such as surface temperature and structural stability to avoid any accidents during and after cooking. A World Bank study and US State Department report prescribe a common set of parameters preferred by the users were [15,16]. A well designed cookstove should be economically viable, highly efficient and with a firepower level in accordance with the local food habit. It is equally important that the improved cookstove also meets the indoor air pollution standards set by the WHO (World Health Organization) or national standards in that country. 1.1. Issues and challenges Many traditional and improved cookstoves work at low efficiency and causes indoor air pollution. Over 95% of these stoves do not meet the prescribed benchmarks [7]. There is a need for an extensive test method to evaluate the performance of cookstoves to ensure that they meet the prescribed benchmarks and standards. There are reports indicating that test result of the same cookstove can vary between 5 and 25%, in the same laboratory [7]. Lack of performance monitoring and quality control is reported as barriers for the large-scale dissemination of improved cookstoves [17]. It was observed that WBT (Water Boiling Test) conducted in lab conditions gave little indication of the stove performance in field conditions [18]. Although, field measurements are able to get representative and reliable results, it requires relatively high labor intensity, high cost and usually time consuming [19]. Laboratory testing is intended to be reproducible and potentially suitable to compare the performance of different cookstoves [20]. Laboratory tests are useful for predicting field performance by simulating the field condition and test procedures [20]. There is a need for an improved test method, which is simple and provides test results with minimum human and instrument errors. Several protocols are in place to test the performance of cookstoves using water boiling test (WBT). Since 1985, these protocols and test methodologies are continuously modified according to the need in the cookstove sector. Important information of the cookstove testing protocols was compiled in Ref. [21]. This article also refers about a comprehensive water-boiling test, which can provide the test results closer to the field performance. Unmeasured factors can lead to erroneous conclusions which can affect the test results of the performance of the cookstoves [22]. The present study is aimed to review and analyze the existing test protocols used to evaluate the performance of the cookstoves. Based on the analysis an improved test method for conducting WBT is proposed. The steps adopted for strengthening the existing water boiling test (WBT) are:  A detailed study and analysis of selected WBT.

 A detailed study and analysis of the selected benchmarks prescribed to evaluate the cookstoves.  Design of a comprehensive water boiling test.  Identification of various performance indicators using the improved water boiling test.  Analyze and address the gaps in various evaluation methods to minimize the common errors. 2. Influence of cookstoves’ fabrication materials on its performance Efficiency of the cookstoves is largely influenced by the materials used for construction of the cookstove. Different types of materials are used for construction of the cookstoves, depending upon their design [23]. In the construction of three stone cookstoves, heavy stones are used to create the combustion space and support for the vessel [24]. The materials used for construction of cookstoves consume a large portion of the energy, which was generated from the combustion of biomass [25]. The stoves which consume more material work at a relatively lower efficiency, as most of the energy generated from biomass is consumed by the materials. In rural areas, many stoves are constructed using locally available clay [26]. There are also cookstoves, which are fabricated using only metal [27]. Some of the cookstoves are fabricated using a combination of locally available clay and metal [28]. Further, there are cookstoves which use material like vermiculate for construction of cookstoves. Portable forced draft cookstoves are made using a combination of metal and insulation materials [10]. The materials used for construction of a cookstove absorb a large amount of energy during the cold-start high-power phase. Hence, it is important to select the right combination of materials having low thermal mass and high insulation properties. A portion of the heat absorbed during the cold-start high-power phase is released during the simmering phase. The thermal properties of selected materials used in construction of cookstoves are presented in Table 1. The rate at which heat energy is released from the cookstoves’ fabrication material is an important factor for evaluating the performance of cookstoves. The energy released from the cookstove materials needs to be captured during the simmering phase itself. Hence, the estimation of the residual heat and its recovery rate is essential for evaluating the performance of cookstoves. In the proposed four phase WBT, the fourth phase consists of an appropriate method for estimation of residual heat and its recovery rate. 2.1. Existing cookstove models and issues Performance of twelve different models of cookstoves was tested and compared [11]. These cookstoves were classified in to three categories. They are “wood burning stove without chimney”, “wood burning stove with chimney” and “wood burning stoves with electric fan”. The cookstove with electric fan works on forced draft mode, whereas the other stoves work on the natural draft mode. According to the study results, the forced draft cookstoves

Table 1 Thermal property of selected materials used for construction of cookstoves. Materials

Density, kg m3

Specific heat, J kg K1

Thermal conductivity, W m1 K1

Ceramic wool Clay Fire bricks Metal sheet (mild steel) Metal sheet (stainless steel) Vermiculate

84 2400 1280 7870 7900 90

1070 1381 1000 447 477 960

0.09 0.65 0.30 80.2 14.9 0.06

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P. Raman et al. / Energy xxx (2014) 1e17

work with higher efficiency and low emission as compared to natural draft cookstoves. The forced draft cookstoves reduce particulate matters and CO emissions upto 90% when compared to natural draft cookstoves [7]. Even though the forced draft cookstoves perform better than the natural draft cookstoves, the forced draft cookstoves are relatively expensive than the natural draft cookstove. The cost of the forced draft cookstove and its dependency on external power source are the key barriers for large scale adoption of these cookstoves. The cost of the cookstove should be within the affordability of its potential users. For a successful entrepreneurship development and commercialization of improved cookstoves, pricing and financial scheme are considered as key requirements [29]. There is a need for development of a cookstove which performs better, and less expensive. According to Bailis et al., the efficiency of traditional cookstoves and improved cookstoves was in the range of 15e16% and 17e19%, respectively [30]. The cookstoves working on natural draft were classified as open-fire (three stone fire), improved open-fire (traditional cookstove with an enclosure to the combustion chamber) and improved cookstoves (with chimney) [31]. The efficiency of these cookstoves was found to be in the range of 14e21%. A comparison of the performance of various cookstoves, along with their cost is presented in Table 2, based on the available test results in Ref. [11]. It may be noted from Table 2, that there is a large variation in the performance of cookstoves. Hence, there is a scope for further improvement of cookstove which works at higher efficiency and emits less CO and particulate matters. 3. Water boiling test (WBT) The purpose of designing and developing improved cookstoves is to achieve clean combustion and higher efficiency to meet the cooking requirements and provide a clean environment. Evaluating the performance of a cookstove in the kitchen is a complex process due to several constraints involved in carrying out the tests. Such constraints could cause several variations from the results expected during the actual cooking. Generally, a water boiling test (WBT) under certain lab conditions is proposed by several institutes to assess the performance of cookstoves. It is important to note that the WBT results are expected to be a true indicator of the stove’s performance during the actual cooking in the kitchen. Thus, it is essential to have in place a detailed test methodology to evaluate cookstoves and ensure the standards and benchmarks are met during the actual cooking conditions. 3.1. Test methods available for evaluation of cookstoves Several test protocols have been developed and recommended by different institutes to conduct WBT. In the present study, several Table 2 Existing cookstove models and their performance. Type of cookstove

Efficiency Emission SFCa (%) (g L1) CO Particulate 1 (g L ) matter (mg L1)

Cost (US $)

Natural draft open fire and without chimney (three stone fire) Natural draft open fire and without chimney (clay/metal stove) Natural draft with chimney (metal stove) Forced draft cookstove

16

11.1

0

19e54

3.1e10.1 258e858

16e36

3.9e9.6

136e1020 60e167 35e72

40e44

1.37

5.4

a

472.6

To boil 1.0 L of water and simmer it for 45 min.

139

57e110 0e5

49e52

40e229

3

WBTs evolved during 1985e2013 were studied and compared in detail. The popularly adopted protocols, selected for the analysis are listed below: i. Testing the efficiency of wood-burning cookstoves; international standards (1985) [32]. ii. Indian standard: solid biomass stove specifications (1991) [33]. iii. Testing method for the heating properties of firewood cookstoves (1993) [34]. iv. Efficiency test for a biomass cooking stove (2003) [35]. v. WBT Version 3 (2007) [36]. vi. Stove manufacturers’ emissions and performance test protocol (2009) [37]. vii. WBT version 4.1.2. (2009) [38]. viii. A heterogeneous testing protocol for certifying stove thermal and emissions performance for GHG (Greenhouse Gas) and air quality management accounting purpose (2010) [39]. ix. IWA (International Workshop Agreement) (2012) [40]. x. The Water Boiling Test Version 4.2.2 (2013) [41]. xi. Indian standard: Portable Solid Bio-Mass Cookstove (2013) [42]. A list of test protocol along with variations in test method is presented in Table 3. The WBT methods recommended in the test protocols (listed in Table 3) can be classified in to following three categories: Category-I. Two phase water boiling test: cold-start high-power phase, simmering phase [36] Category-II. Three phase water boiling test: cold-start high-power phase, hot-start high-power phase, simmering phase [38,41] Category-III. water boiling test with a fixed quantity of fuel wood and the high power phase with repetitive cycles [39] Two phase water boiling test (Category-I) was adopted during the period 1985e2009. The temperature of water during the simmering phase and the duration of the test were varied among the protocols. The three phase WBT (Category-II) was proposed during 2009 [38]. In WBT 4.1.2 a hot-start high-power phase was introduced between the cold-start high-power phase and simmering phase. The duration of the simmering phase was extended from 30 min to 45 min. The water temperature during the simmering phase was suggested at 6  C below the boiling point. During 2012, elaborate benchmarks were suggested to evaluate the different types of cookstoves at different power levels [40]. Based on the type of cookstoves, a total number of five tiers are proposed with different benchmarks. The benchmark for performance at high power and low power phases includes fuel use, emission of CO and particulate matter. A revised version of the test protocol of WBT 4.1.2 [38], is proposed as WBT 4.2.2 [41] in 2013. The WBT 4.2.2 prescribes a three phase WBT. When maintaining the same duration of 45 min for simmering phase, WBT 4.2.2 recommends the water temperature to be maintained at 5  C below the boiling point. This modification has been proposed to maintain the energy level at its maximum during the simmering phase. 3.2. The authorized test centers for testing and approving cookstoves In India, the MNRE (Ministry of New and Renewable Energy) relaunched a NBCP (National Biomass Cookstoves Program) during 2009e2010 [43]. There are three test centers authorized to test the

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Table 3 Test methods available for conducting water boiling test (WBT). Test protocols

Year

Ref.

Method

Testing the efficiency of wood-burning cookstoves VITA

1985

[33]

Indian Standard: solid biomass Chulha (cookstoves) specifications

1991

[34]

Testing method for the heat properties of civil firewood stoves

1993

[35]

Efficiency test for a biomass cooking stove WBT Version 3 Stove manufacturers emissions & performance test protocol (EPTP)

2003 2007 2009

[36] [37] [38]

The Water Boiling Test Version 4.1.2

2009

[39]

A heterogeneous testing protocol for certifying stove thermal and emissions performance for GHG and air quality management accounting purposes International Workshop Agreement (IWA)

2010

[40]

Two phase water boiling test. Cold-start high-power phase and simmering for 30 min at 5  C below boiling point. Two phase water boiling test. Cold-start high-power phase and simmering for 30 min at 5  C below boiling point. Two phase water boiling test. Partial estimation of residual heat recovery rate. The vessel is covered with a lid to avoid evaporation. Cold-start high-power phase and simmering for 30 min at 5  C below boiling point. Three phase water boiling test. Three phase water boiling test. Variation in water quantity to use used in WBT according to the starting temperature. Estimation of CO concentration with respect to the time. Particulate maters captured less than 10 microns. Three phase water boiling test. Variation in water quantity to use used in WBT according to the starting temperature. Simmering for 45 min at 6  C below boiling point. 3 different power levels of WBT are proposed (max, low stable and intermediate).

2012

[41]

The Water Boiling Test Version 4.2.2

2013

[42]

Portable solid bio-mass cookstove (chulha)

2013

[43]

cookstoves. For evaluation of the performance of the cookstove, a WBT along with a set of benchmarks was recommended by BIS (Bureau of Indian Standard) [33]. The cookstoves are tested using the test method prescribed by the (BIS). The details of the different types of approved cookstoves are published on the Ministry’s website [43]. The method of WBT and benchmarks for performance referred in Ref. [33] were revised in 2013 [42]. The revised test protocol has many modifications as compared to the earlier version [33] and other standard protocols. The modifications and its implications on the results of actual cooking conditions are presented in Table 4. The revised WBT [42] proposes a new method of water boiling test (Category-III), which excludes the simmering phase. A fixed quantity of fuel wood required to operate the stove in full power is considered as a fuel input to conduct the WBT. Thus, the duration of the WBT is about an hour. The stove is operated at its maximum power till the end of WBT. During the entire period of the WBT, the vessel is closed with lid, to minimize the energy loss due to evaporation of water. Temperature of water is raised up to 95  C at every batch. When a vessel reaches 95  C it is replaced with another vessel with fresh water. The WBT is continued till the fuel wood is finished and the charcoal remaining in the stove reduces to a negligible quantity. In this type of test (Category-III), the simmering

Focusing on defining benchmarks on efficiency and emission factors for different types of cookstoves, at different power levels. Three phase water boiling test. Simmering the water for 45 min at 3  C below boiling point. The vessel is not covered with a lid and allows evaporation. A fixed quantity of fuel is used to boil water. The stove is operated at high power phase only. The simmering phase is eliminated. CO and particulate matter are measured based on MJ of energy delivered. The vessel is covered with a lid to avoid evaporation.

phase is eliminated. With this type of WBT the stove’s efficiency at high-power phase is estimated and the behavior of the stove can vary during the simmering phase. In BIS 2013 [42] the emission factors CO/CO2 and TSP (Total Suspended Particles) were replaced with CO and TPM (Total Particulate Matters). The concentration of CO/CO2 in flue gas is an important parameter, which can be used to determine the combustion efficiency of the cookstove. In the revised test protocol, TPM limits are proposed based on the type of the cookstoves. For the natural draft cookstoves the allowable limit of TPM is 350 mg MJ1 of energy delivered. For the forced draft cookstoves the allowable limit of the TPM is 150 mg MJ1 of energy delivered. In case of the natural draft cookstoves, the allowable limit for TPM is more than double as compared to forced draft cookstove. The forced draft cookstove may have higher velocity of the flue gas within the combustion chamber due to the forced draft and high temperature. Hence, the forced draft cookstoves need to be designed with more care to achieve a substantial reduction in TPM. The overall efficiency of the cookstove was calculated by estimating the total energy input from the fuel wood and the total energy captured in the process of repetitive water heating [43]. In this method, the efficiency of the cookstove during the cold-phase and hot-phase is not analyzed separately. The temperature of the

Table 4 Modifications in test methods and benchmarks of performance. Parameters

BIS 1991 [33]

BIS 2013 [42]

Significance/implications

CO CO/CO2 ratio Duration of the test

Not included Included No time limit prescribed

Provides more clarity in emission level CO/CO2 is a useful indicator to estimate the combustion efficiency Shortened the test duration.

Fuel wood quantity Simmering phase TPM

No fixed quantity is fixed Includes Referred to the volume of flue gas Included

Included Excluded/modified as CO and TPM One hour operation in full power mode Quantity is fixed Excluded Referred to the quantity of energy delivered Excluded

Turn-down ratio

Use of lid

With lid, during the first two phases of WBT

With lid (throughout the WBT)

Efficiency estimation with a fixed energy input Variable firepower in the kitchen, may bring difference in fuel saving Provides more clarity in emission level Turn down ratio is an important parameter to understand the fuel saving capacity and firepower required to meet the socio-cultural variation in food and cooking habit In actual cooking condition, most of the time the vessel is without lid

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water is raised up to 95  C, which is about 5  C bellow boiling point. The long duration, low power simmering phase is not a part of WBT in Ref. [43]. Hence, the specific energy consumption may not be comparable with other protocols as the water temperature is not raised till the boiling point and the simmering phase is eliminated. Evaluation of the controllability of firepower and turn-down ratio of a cookstove would not be possible without having the simmering phase. Controllability of a cookstove between its maximum and minimum firepower is a key factor to increase the adaptability of the cookstove according to the variation in the food and cooking habits of users. 3.3. Issues related to test methods and scope for improvements Factors affecting the performance results during the simmering phase are reported in WBT 4.2.2 [41]. The energy consumption and evaporation rate can be influenced by the variation of the water temperature within the prescribed band of 3  C below boiling point. The thermal efficiency of the stove is more controllable and accurate when the WBT is conducted at 2  C below the boiling point [44]. WBT 4.2.2 brings out the fact that fuel consumption rate during the simmering phase is influenced by the stored heat energy in the cookstove materials. Cookstoves with a high-thermal mass can store a large amount of heat energy during the high-power phases. This stored heat energy gives advantage during the simmering phase [41]. An alternative test sequence as “cold-start, simmer, hot-start, simmer cycle” was proposed to examine the difference in the results of the simmering phase [41]. As such a change in the test sequence is not proposed by any other test protocol. In this present study, the proposed four phase WBT brings more clarity on the heat energy stored in the cookstove materials. The fourth phase of the improved WBT estimates the quantity of energy stored in the stove and the heat recovery rate. These results can be used effectively used to estimate the stored energy and heat loss factor of the cookstove. Estimation of these parameters will contribute to improve the performance of the cookstove. There are variations among the test methodologies, benchmarks and standards recommended by various protocols. In reality there is a large gap between the assessment of performance of cookstoves using the WBT and the KPT (Kitchen Performance Test) [38,45]. Hence, it is important to have an improved WBT that represents the expected level of performance of cookstoves in actual cooking conditions. The proposed four phase WBT includes the three phase water boiling test and an extended phase, called the residual heat recovery phase. During the residual heat recovery phase the quantity of energy recoverable from the stove and rate of heat recovery is estimated. The quantity of the heat recovered from the cookstove, at the end of the third phase without adding fuel wood is proportional to the energy absorbed and stored in the materials of the cookstove. The quantity of heat energy stored in the cookstove materials influences the efficiency of the cold-start high-power phase and simmering phase. It also affects estimation of the actual energy input during the simmering phase. Appropriate selection of materials, which have minimum thermal mass and heat loss factor can improve the efficiency of the cookstoves. While the three phase WBT can evaluate the cookstove performance, the four phase WBT can help to improve the design for a better performance of the cookstoves. 3.4. Test methods and parameters considered for conducting the WBT WBT is a commonly used test for evaluating the performance of cookstoves in which defined quantity of water is boiled. VITA (Volunteers in Technical Assistance) came up with the first version of the WBT in 1982 and revised it in 1985 [38]. A detailed history of the

5

development of test protocols is reported in Refs. [38,45]. The objective of the protocols is to evaluate the performance of the cookstove at the lab and the results are comparable with field performance. The gaps in test parameters and the methodology need to be bridged in such a manner that the test results obtained at the lab are comparable with the actual cooking conditions. Generally cookstoves are evaluated based on its thermal efficiency and pollutants like CO, CO2 and particulate matter emitted by the cookstove. The testing parameters and methodology varies among the existing test protocols. The WBT version 4.1.2 highlights several limitations of the WBT [38]. Several reports and research papers [38,45,46] also highlight the demerits and concerns about the WBT. It has been identified that the test results are not necessarily a representative of field performance. In the present study, a set of protocols which are popularly used for WBT are selected for detailed analysis and identification of gaps. Various parameters used for estimation of cookstoves’ field performance using WBT were identified. A comparison of the test protocols along with the parameters considered for evaluation of cookstove is presented in Table 5. From Table 5, it may be noted that a large variation exists among the protocols with respect to the parameters considered and the methodology adopted. The international workshop-10, organized by the ISO (International Organization for Standardization) also recognizes that lab tests may not fully represent the field performance [47]. The present research work focuses on arriving a complete cycle WBT to minimize the gaps between the performance results of the lab and field test. 4. Test methodologies There are several protocols, which propose different methods for testing the performance of the cookstoves. Duration of 30 min for simmering phase and four time repetition of WBT was recommended [32,36]. Later this was modified with 45 min of simmering phase and three time repetition of WBT as recommended in Refs. [38,41]. The ambient conditions required for conducting WBT were specified in many of the test protocols. Ambient temperature has a large influence on the testing of cookstoves due to seasonal variations. For example, composite climatic conditions prevail in the northern region of India. In this region, the ambient temperature during summer reaches up to 47  C and drops down to 1  C during the winter season. Such variation can affect the performance of cookstoves to a large extent. The heat loss factor of a stove is a function of the temperature difference between the combustion chamber of the cookstove and the ambient temperature. The efficiency of the cookstove tends to reduce, when the temperature difference between the combustion chamber of the cookstove and ambient temperature increase. The residual heat recovery rate is considered for evaluation of the cookstove [35]. In this protocol the residual heat is considered as one of the parameters, although it has not been quantified. The test method proposed in Ref. [35] provides partial information about the heat recovery rate for a short duration. The improved test method proposed in this paper, recommends a methodology to estimate the residual heat recovery rate of a cookstove. As discussed in this paper the fourth phase of the WBT can provide information such as the total quantity of the heat recovered and the residual heat recovery rate. Residual heat recovery rate plays a crucial role in determining the efficiency of a cookstove. 4.1. Estimation of residual heat recovery The efficiency of a cookstove is influenced by several parameters. A significant portion of the energy from the biomass is carried away by the flue gas or absorbed by the materials used for construction. Residual heat is the heat energy absorbed by the fabrication material of a cookstove, which is not available for cooking.

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Table 5 Comparison of the test methods based on the parameters considered. Parameter

BIS IS13152 (1991) [35]

Testing methods (1993) [34]

WBT Version 3 (2007) [36]

EPTP (2009) [37]

Version 4.1.2 (2009) [38]

Version 4.2.2 (2013) [41]

BIS IS13152 (2013) [42]

CO CO2 CO/CO2 Efficiency Estimation of residual heat Fire-bed temperature Firepower Flame temperature Flue gas flow rate Heat elevation rate Simmeringa Specific fuel consumption Specified fuel thicknessa Specified humidity rangea Specified Initial temperature of watera Specified room temperature Specified surface temperature Temperature while simmeringa

N N Y Y N N Y Y N N 45 min N 3  3 cm2 N 23  2  C 25  5  C Y 5  C below boiling point Y Y

N N N Y Y N N N N Y N N N <80% N 10e30  C N At boiling point

N N N Y N N Y N N N 45 min Y Diameter: 3e4 cm N N Y N 3  C below boiling point N Y

Y Y N Y N N Y N N N 45 min N 1.5  1.5 cm2 N 4e30  C N N At or above 90  C

Y Y N N N N Y N Y N 45 min Y 1.5  1.5 cm2 N 15  C Y N 6  C below boiling point Y Y

Y Y Y Y N N Y N Y N 45 min Y 1.5  1.5 cm2 N Corrected to 25  C N N 3  C below boiling point Y Y

Y Y N Y N N Y N N N N N 3  3 cm2 N 23  5  C 25  5  C 60  C N

Total Particulate Matters Turn down ratio

N N

Y Y

Y N

Y: parameter considered. N: parameter not considered. a Parameters considered and specified are captured in the table.

The quantity of residual heat in a cookstove depends on the type and quantity of the fabrication materials used in the construction of the stove. Thus, a heavy cookstove will have more residual heat than a lightweight stove. A cookstove, which has a more residual heat will consume more energy and shall take more time to cook. Hence, the residual heat directly influences the overall performance of cookstoves. A portion of this energy may be used during the simmering phase while most of the residual energy remains in the stove, even after completion of the cooking process. The quantity of the residual heat is estimated based on the total thermal energy recovered from the cookstove after the simmering phase. The heat recovery rate is estimated using the quantity of residual heat recovered over a period of time. The quantity of residual heat as a fraction of the total energy input can be used as an indicator for evaluation of cookstoves. Therefore, a cookstove absorbs more heat in its construction materials and works with poor efficiency. However, by estimating the residual heat factor the efficiency of the cookstove can be improved, by minimizing the energy absorbed in the construction materials. 5. Benchmarks for performance of cookstoves 5.1. Design parameters and benchmarks It is important to quantify the improvement in the performance of the cookstove developed with reference to several technical and non-technical parameters. Test protocols and benchmarks should be in line with the parameters relating to technical, economic and social aspects. Several design parameters, which are required to meet the users’ requirements, are presented in Table 6. The test protocol should be in a position to evaluate such parameters for identification of cookstoves which meets the user requirements. 5.2. Benchmarks for evaluation of cookstoves The benchmarks identified as the minimum performance requirements for evaluation of cookstoves are reported by many studies [33,42,48]. Among them, three benchmarks have many variations with respect to the parameters and test methods. A set of

benchmarks for estimation of the thermal efficiency and emissions of the cookstove are prescribed [33]. A comparison of the benchmarks used for evaluation of cookstoves is presented in Table 7. The thermal efficiency is defined by a direct method of estimation, as a percentage of input energy [33]. The thermal efficiency is defined by use of fuel wood to boil and simmer a fixed quantity of water [49]. There is a large variation in the representation of CO 1 ðg MJ1 d ; ppm; gÞ and particulate matters ðMJd ; mgÞ. From Table 7, it may be noted that different protocols propose different units for measurement of the same parameter, which may create complications in comparing the results of one method to another. Based on the type and the level of performance, the cookstoves were classified in five tiers. The benchmark for the performance of the cookstoves was recommended according to the tier level and firepower [40]. These benchmarks are much more elaborated in comparison with the benchmarks provided in Refs. [33,42,48]. Benchmarks of performance recommended based on efficiency and emission levels of cookstoves at different tier level and firepower are presented in Table 8.

Table 6 Design parameters related to technical, economic and social aspects. Design parameters

Nature of parameter

Emissions (CO, CO2 and TPM) Flame temperature Performance efficiency Surface temperature Stability and ruggedness Turn down ratio Cooking time and duration Ease of operations Kitchen configurations Physical structure of the traditional stove Safety aspects Service availability Type of food Type of fuel Type of vessel Cost of the stove Cost involved in operation and maintenance Life of the stove

Technical aspects

Social aspects

Economical aspects

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7

Table 7 Benchmarks considered for evaluation of biomass cookstoves. Parameters

Unit

Performance benchmark ARC [49]

CO emission CO emission (30 cm above the stove) CO emissiona Energy to complete WBT Emission: CO/CO2 Fuel to complete WBT Particulate matter Particulate matter Thermal efficiency a

g MJ1 d ppm gm MJ Mass fraction gm mg MJ1 d mg Energy fraction %

IS-13152 [33]

IS-13152 [42]

Stove without chimney

Stove with chimney

Natural draft

Forced draft

e e 20.0 16.5 e 850.0 e 1500 e

e 50.0 e 29.0 e 1500.0 e 1500 e

e e e e 0.04 e e 2.0 25

e e e e 0.04 e e 2.0 40.0

Natural draft 5.0 e e e e e 350 2.0 25

Forced draft 5.0 e e e e e 150 2.0 35.0

To boil 5 L of 25  C water and then simmer it for 45 min.

6. Combustion and heat transfer In the test protocols discussed in Table 2, the parameters like temperature of the flame and the fire-bed of the cookstove were not considered to evaluate the performance of the cookstoves. Increased flame temperature and fire bed temperatures are indicators of clean combustion of biomass. The temperature of the flame and the fire-bed of the cookstove is at their maximum when the air-supply for combustion is adequate and distributed uniformly. Thus, the temperature of the flame and fire-bed directly influences combustion, efficiency and emission factors of the cookstoves. The difference between the flame temperature, firebed temperature, material and thickness of the cooking pot influences the heat transfer rate. When the convection heat transfer rate is obtained by a linear function of the temperature difference, the heat transfer rate of radiation is obtained by fourth order exponential function of the temperature difference. Hence, the heat transferred from the flame through radiation is an important factor for increasing the efficiency of the cookstove. The standard equations used for estimation of heat transferred by radiation and heat transferred by convection are given below; The heat transferred from the combustion chamber to the vessel by convection can be estimated by the standard equation as given in Eq. (1).

  Q ¼ h  A  Tf  Tv

(1)

The heat transferred from the combustion chamber to the vessel by radiation can be estimated by the standard equation as given in Eq. (2).

  P ¼ ε  s  A Tf4  Tv4

(2)

The importance of the temperature of the flame and the fire-bed in the context of heat transfer rate may be noted from the Eqs. (1) and (2). The existing test protocols do not consider flame temperature and the fire-bed temperature. These are the key factors influencing the heat transfer rate and thermal efficiency of the

cookstoves. The energy emitted by the fire-bed corresponding to its temperature is theoretically calculated [49]. It is important to include the flame temperature and fire-bed temperature measurements in the test protocol to have an estimation of the heat transfer rate and the performance efficiency of the cookstove. Most of the test protocols do not discuss the material and thickness of the vessel, which can significantly influence the heat transfer rate and residual heat. A diagram depicting the various paths of energy flow with respect to cookstoves is shown in Fig. 1. The quantity of water used for WBT should be proportional to the firepower. The test protocols [33,37,41] provide indicators of the mass of water to be used according to the firepower. The test protocols [36,38] provide specification for the quantity of water to be used in two categories. The quantity of the water to be used is recommended based on the firepower of the cookstove. 7. Need for a complete cycle water boiling test A set of common errors and gaps which can occur during various phases of WBT were identified. The gaps in conducting the water boiling tests were identified based on the study and analysis of the existing test protocols. The existing test protocols have not considered the estimation of heat gained by the construction material of a cookstove. A part of the heat absorbed is available during simmering phase and a major portion of the energy is retained within the construction materials of the cookstove. The heat gained by the construction material, the heat released during the simmering phase and the heat retained by the materials (after the simmering phase) are three major factors which influence the efficiency of cookstoves. There is a need to estimate these values for evaluation of the cookstoves and for further improvement. 7.1. Factors which influence the performance of the cookstove By using the existing test methods and protocols, the cookstoves are evaluated to ensure that they meet three basic parameters. These parameters are the efficiency of the cookstove, emission of

Table 8 Recommended benchmarks for performance of cookstove by their type and firepower. Stove classification

Tier Tier Tier Tier Tier

o 1 2 3 4

Evaluating parameters prescribed in Ref. [40] Particulate emission (PM2.5)

Fuel use

High power (g MJ1 delivered)

Low power (g min1 L1)

High power (mg/MJ delivered)

Low power (mg min1 L1)

High power (Efficiency %)

Low power: (specific energy consumption) (MJ min1 L1)

>16 <16 <11 <9 <8

>0.20 <0.20 <0.13 <0.10 <0.09

>979 <979 <386 <168 <41

>8 <8 <4 <2 <1

<15 >15 >25 >35 >45

>0.050 0.050 0.039 0.028 0.017

CO emission

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P. Raman et al. / Energy xxx (2014) 1e17 Table 10 Efficiency of the cookstove and energy absorbed in the cookstove materials.

Fig. 1. Various components of input materials and energy flow of cookstove.

CO and particulate matters. The parameters used to evaluate the cookstove and their associated functions along with their influencing factors are presented in Table 9. Though the objective of the evaluation of cookstoves is based on these three parameters there are several variables associated with them. The influencing parameters were studied for a better understanding of the factors leading to variability and uncertainty in cookstove test results [50]. A cookstove performance standard should classify different parameters that influence the performance of the cookstove in field conditions [51]. It is essential to understand the complete process involved in the cookstove operation and to study the factors influencing its performance. Since, several variables are associated with the parameters considered for evaluation of the performance of the cookstove, testing of the cookstove is a complex process. Evaluation of cookstove in the lab to represent the field condition has several difficulties in simulating the firepower and the ambience to match the actual cooking conditions in the field. Hence, the test methods and protocols are being revised continuously, since 1985. Evolving of test protocols since 1985, to improve the test methods and selection of parameters are presented in Table 3. It is essential to analyze the gap in test methods to evaluate the cookstove in the lab such that it is able to predict the firepower, fuel saving and emission factor in the field conditions. The key parameters of the WBT should be focused for evaluation of the performance of the cookstove and identify components that need to be improved. 7.1.1. Thermal mass and efficiency of the cookstove The thermal mass of the cookstove plays an important function in determining the efficiency of the cookstove. It was estimated that when a cookstove works at 33% efficiency, the energy absorbed

Component

Unit

Ref. [53]

Ref. [54].

Ref. [55]

Efficiency of the Cookstove Energy absorbed in the cookstove materials

Energy fraction in%

33.0

32.1

16.5

Energy fraction in%

16.0

20.2

30.9

by the stove material is 16% [52]. When the efficiency of the cookstove was at 32.1%, the energy lost to the stove materials was 20.2% [53]. When the efficiency of a cookstove was 16.5%, the energy absorbed in the cookstove was 30.9% [54]. The relation between the efficiency of the cookstove and the fraction of energy absorbed by the cookstove materials is presented in Table 10. From Table 10, it may be noted that for a stove working at an efficiency of 30%, about 16e20% of the energy produced from the fuel wood is lost to the cookstove materials. Whereas about 31% of energy produced from the fuel wood is lost in the cookstove materials for the stove working at an efficiency of 17%. These values confirm that a major portion of the heat energy produced from the fuel wood is absorbed by the cookstove materials. 7.2. Improved test method Evaluation of the parameters related to estimation of the firepower and efficiency of the cookstove are key objectives of the proposed improved test method. In the proposed test method, efforts were made to estimate the recoverable heat energy absorbed in the cookstove materials, by introducing a fourth phase in WBT. The parameters to be measured are similar to the parameters measured in the three phase WBT [41]. The estimation of the residual heat adds value to the WBT to optimize the cookstove materials and to improve the overall performance efficiency of the cookstove. The improved water boiling test has the following four phases to evaluate the performance of the cookstove. The improved test method is having an additional phase to estimate the residual heat in the materials used for fabrication of the cookstove. Estimation of residual heat and heat release rate can be used an indicator for optimizing the construction material and to improve the efficiency of the cookstove. Also, in the proposed water boiling test, the water temperature is kept just close to the boiling point. This is to avoid the complications arising in maintaining the water at a specified temperature. Any correction for initial temperature of water is not proposed, since the efficiency of the cookstove is a function of rise in water temperature. The improved test method is focused for a detailed analysis of the cookstove performance in each phase of the WBT. The details of the four phases of the proposed improved water boiling test are:

Table 9 Parameters used for evaluation of cookstoves and its influencing factors. Parameter

Functions

Influencing factors

Efficiency

Combustion Heat transfer

Level of CO, CO2, O2 in flue gas Flame temperature (which is a function of combustion efficiency, firepower and appropriate air supply for combustion) Flue gas temperature and excess air supply (Can be estimated by CO2 and O2 concentration in flue gas) Mass and thermal property of the materials used in cookstove Size of opening through fuel feeding port and materials used for construction of the cookstove Adequate supply of air for efficient and clean combustion of fuel wood Design of the combustion chamber, air supply rate and flue gas velocity exiting the combustion chamber

Heat loss through flue gas Heat absorption by the cookstove materials Heat loss from combustion chamber by radiation Gaseous emission Particulate emission

Partial combustion of fuel wood and formation of CO Particulate maters carried away through flue gas (PM10, PM2.5)

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Phase I: Cold start high power phase. This phase involves rising of water temperature from the initial temperature to the local boiling point, when the cookstove is at room temperature. Phase II: Hot start high power phase. This phase involves rising of water temperature from initial temperature to the local boiling point, when the stove is hot. Phase III: Simmering phase. This phase involves maintaining the water temperature close to the local boiling point with a minimum evaporation of water. Phase IV: Estimation of the residual heat and the rate of residual heat recovery. This phase involves raising the water temperature by recovering the residual heat stored in the stove materials. Details of various phases of the proposed improved water boiling test are shown in Fig. 2. In Fig. 2, T0 represents the ambient temperature and T1 represents the initial temperature of water. Only the initial temperature of the water is considered by the majority of test protocols. The ambient temperature has a strong influence on the efficiency of the cookstove irrespective of the design and the type of cookstove. The initial temperature of water depends upon the ambient conditions and is responsible for the duration and power consumption during the cold-start and hot-start high power phase. T2 refers to the maximum temperature reached during heat recovery phase and T3 represents the boiling point at local conditions. Fuel wood is added only during high-power and simmering phase. Fuel wood is not added during the heat recovery phase. The cold start high power phase duration is represented by t0 to t1, hot start high power phase duration is represented by t1 to t2 and simmering phase duration is represented by t2 to t3. The duration of the residual heat recovery phase is represented by t3 to t4. The period from t4 to t5 is the stagnation stage where the temperature remains unchanged. It is important to conduct the experiment till the stagnation stage to ensure that the recoverable heat estimated is the maximum of the residual heat. After t5 water temperature will drop from the peak temperature of the heat recovery phase, which is the cooling phase represented by t5 to t6. Heat recovery rate and quantity of heat recovered are calculated based on the temperature rise during the period t3 to t4. The slope of the temperature rise from T1 to T2 and during t3 to t4 represents the residual heat recovery rate. The heat energy recovered from t3 to t4 represents the recoverable heat energy absorbed by the stove material. The four phase complete cycle WBT provides detailed information about the performance of a cookstove for its evaluation.

9

Table 11 Estimation of various parameters at different phases of the improved test method. Parameter

Phase of the water boiling test

Energy input

Complete cycle of water boiling test Cold-start high-power Hot-start high-power Simmering First three phases of the water boiling test

Emission factors: CO, CO2 and TPM Firepower

Rate of Residual heat recovery Residual heat recovered Turn down ratio Useful energy

Complete cycle of water boiling test Cold-start high-power Hot-start high-power Simmering Residual heat recovery phase Residual heat recovery phase Hot-start high-power versus simmering phase Complete cycle of water boiling test Cold-start high-power Hot-start high-power Simmering

7.3. Estimation of various parameters Various parameters proposed for estimation, at different phases of complete cycle WBT of a cookstove are presented in Table 11. From Table 11, it may be noted that the improved WBT enables estimation of nine types of design parameters. The performance of the cookstoves in field conditions can be predicted by a detailed evaluation of the estimated parameters of the improved test method. Estimation of various parameters (presented in Table 5) for evaluation of cookstove, as per the proposed WBT, (shown in Fig. 2) is presented below. The thermal efficiency of a cookstove during the complete cycle of WBT can be estimated using the ratio of input and output energy. The input energy represents the energy produced by the fuel wood used during the experiment, whereas the output energy represents the useful energy which is gained by water and the vessel. The useful energy transferred into the vessel and water during the complete cycle WBT can be estimated by using Eq. (3).

Ue ¼


 ðWc þ Wh Þ þ Mv  Cpv  ðT1  T3 Þ þ ðWEch þ WEhs Þ  Lew

(3)

The energy input during the four phases of the complete cycle WBT can be estimated by Eq. (4).

Fig. 2. A complete cycle of water boiling test including estimation of residual heat.

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P. Raman et al. / Energy xxx (2014) 1e17

Ie ¼ WF  Cv  Mc

(4)

The energy input during the cold start high power phase can be estimated by Eq. (5).

Ic ¼ WFC  Cv  Mc

(5)

The energy input during the hot start high power phase can be estimated by Eq. (6).

Ih ¼ WFH  Cv  Mc

(6)

The energy input during the simmering phase can be estimated by Eq. (7).

Is ¼ WFS  Cv  Mc

(7)

The thermal efficiency of the cookstove during the complete cycle of WBT can be estimated by Eq. (8).

ht ¼ Ue=I e

(8)

The useful energy during cold start high power phase can be estimated by Eq. (9).

Uc ¼


 Wc þ Mv  Cpv  ðT2  T3 Þ þ ðWEc  Lew Þ

(9)

The thermal efficiency of the cookstove during cold start high power phase can be estimated by Eq. (10).

hc ¼ Uc=I

(10)

c

The useful energy during hot start high power phase can be estimated by Eq. (11).


 Uh ¼ Wh þ Mv  Cpv  ðT3  T1 Þ þ ðWEh  Lew Þ

(11)

The thermal efficiency of the cookstove during hot start high power phase can be estimated by Eq. (12).

hh ¼ Uh=I h

Us ¼ ðWEhs  Lew Þ

(13)

The thermal efficiency of the cookstove during the simmering phase can be estimated by Eq. (14).

hs ¼ Us=I s

(14)

Firepower is the rate of energy input to the cookstove. The firepower during the cold start high power phase can be estimated by Eq. (15).

  Ic=ðt  t Þ  60 1 0

(15)

The firepower during hot start high power phase can be estimated by Eq. (16).

Fph

  ¼ Ih=ðt  t Þ  60 2 1

(16)

The firepower during the simmering phase can be estimated by Eq. (17).

 Fps ¼

 Is=ðt  t Þ 3 2

Apart from the heat loss from the cookstove certain amount of heat is consumed by the materials of the cookstove, during the high-power phases and simmering phase. The term “heat recovery” refers to recovery of heat stored (residual heat) in the materials of the cookstove. The quantity of energy absorbed by the fabrication materials is directly proportional to the type and weight of materials used to construct a cookstove. Also, the rate of heat transfer from the cookstoves’ fabrication material will be at maximum during the cold-start high-power phase. The loss of energy to ambient is also influenced by the construction materials of the cookstove. Estimation of heat recovery rate is discussed in Ref. [34]. For a better understanding of the cookstoves performance, it is essential to estimate the heat recovery rate and quantity of residual energy. To increase the overall efficiency of the stove, the ratio of the energy absorbed by the cookstove materials to the energy input should be minimized. Analysis of the residual heat recovery rate of can be used to estimate the amount of energy absorbed by the cookstove materials. Thus, the quantity of the residual heat energy plays an important role in optimizing the performance efficiency of cookstove. The recoverable residual thermal energy of a cookstove can be estimated by Eq. (18).

Rte ¼ Wr  ðT2  T1 Þ

(18)

The ratio of residual heat recovery of a cookstove can be estimated by Eq. (19).

dr ¼ fWr  ðT2  T1 Þg=I

e

(19)

The residual heat recovery rate during the heat recovery phase can be estimated by Eq. (20).

  Rhr ¼ fWr  ðT2  T1 Þg=ðt  t Þ  60 4 3

(20)

(12)

Useful energy during the simmering phase can be estimated by Eq. (13).

Fpc ¼

8. Ratio of residual heat recovery

 60

(17)

9. Firepower and turn-down ratio Turn-down ratio is the ratio of the firepower during hot-start high-power phase and simmering phase. During the high-power phase the heat energy is absorbed by the construction material of the cookstove. A part of this energy is released during the simmering phase. Hence, a cookstove which has more energy absorbed in the materials will consume less energy during the simmering phase. Fuel consumption during the simmering phase is also influenced by the thermal efficiency of cookstoves. Since the duration of simmering is more in comparison with high-power phase, the fuel consumption during the simmering phase plays a crucial role in determining the overall efficiency of a cookstove. The higher turn-down ratio will result in higher efficiency and will reflect in actual fuel saving in field conditions. The turn-down ratio of the firepower of a cookstove can be estimated by Eq. (21).

sr ¼ Fph=F ps

(21)

Performance of a natural gas cookstove is considered to have an optimized turn-down ratio. This can be used as a benchmark for designing biomass cookstoves. The firepower and turn-down ratio of a natural gas cookstove are presented in Table 12. From Table 12, it may be noted that natural gas cookstove works at 25% of firepower when compared with the high power phase of WBT. Hence, the turn-down ratio of the natural gas stove is 4.0. A forced draft

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P. Raman et al. / Energy xxx (2014) 1e17 Table 12 Firepower and turn-down ratio of a natural gas stove. Power level

Firepower (kW)

Gas flow rate (L h1)

Useful power (kW)

Turn-down ratio (TDR)

High power Medium power Simmering

2.53 1.14 0.63

200 90 50

1.52 0.68 0.38

1.00 2.22 4.02

11

complete cycle four phase water boiling test of an improved cookstove consumes about 20 min in addition to the recommended duration of the WBT, in Ref. [41]. During the fourth phase, the cookstove made of clay takes 36 min to reach the peak temperature of 55.5  C. Hence, the fourth phase of a traditional cookstove will consume another 40 min in addition to the recommended duration of the WBT in Ref. [41]. 11. Addressing the gaps of the WBT and the way forward

biomass cookstove works at 46% of firepower when compared to the high-power phase of WBT. Hence, the turn-down ratio of the biomass cookstove is 2.2. These values indicate a need for further improvement of biomass cookstoves to increase the turn-down ratio closer to 4.0. 10. Test results Performance of a traditional cookstove and an improved cookstove were evaluated using the complete cycle, four phase water boiling test. The traditional cookstove is made of clay and works on natural draft mode. The Improved cookstove is a metal stove, which works on forced draft mode. Both the cookstoves are portable models and use single pot. The test results obtained by the improved four phase water boiling test are presented in Table 13. From Table 13, it may be noted that the overall performance efficiency of the forced draft cookstove was 47%. The overall performance efficiency of the traditional cookstove was 16.9%. Thus, the efficiency of the forced draft cookstove was about 30% higher than the natural draft cookstove. Due to the reduction in efficiency, the traditional cookstove consumes more fuel and time to boil the water. The residual heat recovery rate of the traditional cookstove was 164 W for a traditional cookstove, made with clay. The residual heat recovery rate of the traditional cookstove was 299 W for an improved cookstove, made with metal and insulation materials. The heat release rate of the improved cookstove is 82% higher than the traditional cookstove. Hence, the efficiency of the improved cookstove was 1.8 times of the traditional cookstove. The weight of the traditional stove made of clay was 8.170 kg and the forced draft cookstove made of SS (Stainless Steel) was 6.575 kg. From the thermal properties of materials are presented in Table 1. It may be noted that the a clay stove can hold 3.6 time of heat energy as compared to metal stove made of stainless steel. The heat energy absorbed by the cookstove material in the clay stove was 1941 kJ and the metal stove was 539 MJ. Since the heat transfer rate of metal is higher than clay, the heat recovery rate is faster in the metal stove than the clay stove. The temperature profile of water during the complete cycle, four phase water boiling test conducted in traditional cookstove is shown in Fig. 3. The temperature profile of water during the complete cycle, four phase water boiling test conducted in forced draft cookstove is shown in Fig. 4. A quantity of 5 L of water was used during the high power and simmering phases. A quantity of 3 L of water was used during the heat recovery phase. The metal stove takes 17 min to reach the peak temperature of 53.8  C, during the fourth phase. Hence, the

As discussed in Table 1, for evaluating the performance of cookstoves, it was observed that the test methodologies need to be simple, practical and error free. If the test methodology has any undefined or complex procedure, it may lead to several types of errors during the WBT. It is important to have an authentic test result with a factual assessment of cookstoves’ performance under lab conditions. The cookstoves’ performance results obtained under lab conditions need to be closer to the results obtained under field conditions. Errors could occur due to various reasons, but they need to be eliminated or minimized. A set of common errors and gaps which can occur during various phases of WBT are discussed in detail along with recommendations to minimize the same. 11.1. Additional key parameters which can be included in performance evaluation of biomass cookstoves Several key parameters, which need to be included in the test method, were identified for a better evaluation of cookstoves. Inclusion of these parameters can minimize the gap in performance results obtained during the laboratory test and field conditions. The parameters which can be included in WBT and their impact are presented in Table 14. 11.2. Combustion efficiency Combustion efficiency of a cookstove is an important parameter, which influences thermal efficiency and emission factors. Combustion efficiency of a cookstove can be estimated by the measurement of CO, CO2 and O2 in flue gas. These measurements and estimation of combustion efficiency are not considered in most of the protocols. During the evaluation of the performance of cookstoves, it is essential to estimate the parameters which influence its performance. The results of the additional parameters obtained during the water boiling test can be used to improve the design and to improve the performance efficiency of the cookstove. 11.2.1. Oxygen content in flue gas The ratio of the quantity of air required to achieve complete combustion of one unit mass of fuel is known as the SR (Stoichiometric Ratio). The ER (Equivalence Ratio) is the ratio of the quantity of air supplied during combustion and the quantity of air required for complete combustion. When the equivalence ratio is less than one the combustion efficiency will be poor. In such a case, CO concentration will be more and the CO2 concentration will be less.

Table 13 Performance comparison of two types of cookstoves tested by the proposed four phase WBT. Stove type

Conventional natural draft Improved forced draft

Cold-start high-power phase

Hot-start high-power phase

Simmering phase

Duration (min)

Power (kW)

Eff. (%)

Duration (min)

Power (kW)

Eff. (%)

Power (kW)

TDR (ratio)

Eff. (%)

Residual heat recovery phase Power (W)

Energy (kJ)

Overall efficiency (%)

68

4.5

11.4

32

6.4

16.2

3.9

1.6

27.2

164

354

16.9

18

5.4

32.0

15

5.3

46.0

2.5

2.2

61.2

299

305

47.0

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P. Raman et al. / Energy xxx (2014) 1e17

110.0

Phase II

Phase I

Phase III

Phase IV

100.0 90.0 Temperature in °C

80.0 70.0 60.0

50.0 40.0 30.0 20.0 10.0 0.0 0

20

40

60

80

100

120

140

160

180

200

Time in minutes Fig. 3. Temperature of water during different phases of WBT of the traditional cookstove.

110.0 Phase I

100.0

Phase III

Phase II

Phase IV

90.0 Temperature in °C

80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0

20

40

60

80

100

120

Time in minutes Fig. 4. Temperature of water during different phases of WBT of the forced draft cookstove.

When the equivalence ratio is above the required stoichiometric ratio, it will result into complete combustion but the excess air will carry away more heat. The O2 content in the flue gas is an important factor which can be used to estimate the air supply rate and its impact on the efficiency of the cookstove. Almost all the test protocols available so far have not included the estimation of O2 in flue gas. Estimation O2 in flue gas can facilitate identification of the

Table 14 The parameters which can be included in WBT and their impact. Parameters to be included in WBT

Impact

O2 % in flue gas Useful power

Combustion efficiency and excess air supply Minimum requirement of energy into the pot during Hot start, high power phase Performance efficiency Performance efficiency and Reduction of pollution from improved combustion Reduction of pollution, clean combustion and dilution of flue gas Influence the efficiency during cold start, can be used for material optimization and to improve the efficiency Bridge the gap in estimation of fuel saving between WBT and KPT

Fire bed temperature Flame temperature Flu gas flow rate (dilution rate) Rate of residual heat recovery Turn Down Ratio (TDR)

scope for improvement in the thermal efficiency of the cookstove, by optimizing the air supply. 11.2.2. Ratio of CO and CO2 The efficiency of a cookstove is influenced by the combustion efficiency of the fuel. Inefficient combustion of fuel will produce more CO and less CO2. When the carbon in the fuel wood is converted into CO it reduces the energy conversion rate of the fuel wood and the thermal efficiency of the cookstove. The ratio of CO to CO2 is an important parameter which can indicate the combustion efficiency of cookstove. The combustion efficiency is poor when the value of CO/CO2 is high and it is better when the CO/CO2 is minimized. For a complete combustion the value of CO/CO2 should be zero. The estimation of CO/CO2 and O2 content in flue gas can be used to evaluate the combustion efficiency and air supply rate. Estimated result of these parameters can be used to improve the cookstove design and to increase its performance efficiency. 11.3. Firepower measurement The firepower of a cookstove influences the efficiency and the level of emission. Each cookstove will have a threshold limit for the minimum and maximum for the firepower, in which it works at a

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better efficiency. The maximum firepower is limited by the primary and secondary air supply. During the simmering phase, the minimum power input required to maintain the water at a given temperature is influenced by three factors. They are: thermal mass of the cookstove, heat loss factor and controllability over the firepower. The range of firepower during the different test phases of WBT is a critical information for evaluation of cookstoves. The protocol version 4.1.2 [38] reports a maximum energy input for a complete WBT. In the above mentioned protocol the firepower during hot start high power phase and simmering phase are not defined. According to the firepower, various vessel diameters are recommended for conducting WBT [33]. This protocol recommends the firepower for the hot start-high power phase only. The firepower during simmering phase is not defined, which influences the fuel saving in field conditions. Meals and cooking habits are influenced by the socio-cultural factors. Cooking of different type of meals will demand a different level of firepower. Hence, it is essential to define the range of operating firepower for each phase of WBT. 11.4. Duration of individual phases of WBT Different phases of WBT are recommended in the test protocols [32,36e39]. Along with the firepower level, a defined maximum duration of the individual phases of WBT will make the evaluation more reliable. According to the protocols referred in Refs. [33,35,38,41] duration for simmering phase is recommended, but the duration required for the high power phases is not defined. One of the options to minimize the error level in WBT is by setting up of a limit for the duration of individual phases and defining the quantity of water, according to the firepower of the cookstove. The other option could be, the protocol can recommend the total duration of the WBT instead of defining the duration only for simmering phase. This will avoid prolonged and arbitrary duration for conducting the WBT. An efficient stove which takes a long duration to complete WBT may take more time for cooking and might not be acceptable for the users in field conditions. 11.5. Water quality and boiling point Generally any test protocol should be able to provide a consistent result at a particular locality. However, due to differences in ambient temperature, altitude and water quality the energy required to boil a given quantity of water also differs. The maximum allowable salinity in drinking water is 1000 ppm [55]. The effect of salinity on boiling point of water is reported in Ref. [56]. The boiling point of water will be affected by 0.1  C when the salinity level is 1.0 g L1. The altitude of the location also is a key factor, affecting the boiling point of water. A method to estimate the local boiling point of water using the altitude is provided in the protocol [41]. According to this method, every 100 m rise in altitude reduces the boiling point of water by 0.34  C. Hence, the altitude and water quality can affect the SFC (specific fuel consumption) rate while conducting WBT. 11.6. Variation in feedstock and design of the cookstove Fuel wood from the same species and from the same lot of feedstock (without variation physico-chemical property) is preferred to use during the WBT, as in case of protocols [41]. When there is a variety of fuel wood being used in a particular region, individual tests of WBT need to be performed with each species, which has a similar physico-chemical property. The test method and monitoring parameters are same for the stoves irrespective of

13

the type of fuel they use. The performance results of the cookstoves may vary when there is a variation either in the design of cookstove or variation in the physico-chemical property of the fuel. To have a consistent and reliable result, it is preferred to eliminate the variation in the physico-chemical property of the fuel used in the field and in the lab during WBT. 11.7. Simmering phase The recommended water temperature to be maintained during the simmering phase is 5  C below the boiling point [33,35]. The recommended water temperature to be maintained during the simmering phase is 3  C below the boiling point [38,41]. The temperature of water during the simmering phase is recommended to be at boiling point [34]. Controlling the water temperature by controlling the fuel input may vary from person to person. Hence, a recommendation to maintain the water at a specified temperature below the boiling point may lead to erroneous results. At high altitude places, water boils much below 100  C even in the range of 92e94  C. A large variation in the boiling point can affect the specific fuel consumption rate. Maintaining the water at a particular temperature may need continuous adjustment to firepower which will lead to variation in thermal efficiency. Hence, maintaining the simmering phase at boiling point or just close to the boiling point will eliminate the error in maintaining a prescribed temperature. The useful energy during the simmering phase is an independent function of the water temperature. Hence, the temperature of water during the simmering phase is not used in the estimation of the useful energy or efficiency. Useful energy during the simmering phase is estimated only based on the quantity of water evaporated. It was observed that in actual cooking conditions the stove has never been operated below the boiling point. 11.8. Human error Though the test protocol defines methods and guidelines for testing the cookstoves, the result varies from person to person while conducting the test, on a same stove. This point was brought out in the protocol referred in Ref. [38], which emphasizes the requirement of trained personnel to conduct the test. Hence, there is a need to minimize human error. A test protocol should be capable to predict the fuel saving and emission reduction in field conditions, based on the results obtained in lab conditions. The difference in the WBT result and its influence on fuel saving and performance in the kitchen is reported in Refs. [38,45,48]. The skillset of the person who conducts the water boiling test in the lab and the skill-set of the person operating the stove in the kitchen need to be considered as part of the test protocol to minimize human error. 11.9. Instrument error Evaluation of cookstove performance involves use of different types of instruments. These instruments are different in terms of use and technology. Most of these instruments are equipped with electronic sensors and a digital display. Some of the advanced equipment used in cookstove testing is incorporated with signal processing and data logging features. The equipment used in the process of evaluation of cookstove must be calibrated periodically to obtain actual performance results. The accuracy and calibration frequency become crucial for the equipment which measures the pollutants in part per million levels. Any error in data collected from these tests may lead to unrealistic representation of field conditions. It may be ideal to have a periodic check on instrument accuracy to ensure a realistic assessment of cookstoves’ performance.

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Ten key parameters of measurements are compiled from Refs. [32,41] along with their purpose are presented in Table 15. It may be noted from Table 15, that there are ten parameters were measured for estimation of several variables. Several types of instruments were used to measure the selected parameters. These instruments are required to meet certain standard, to minimize the error while conducting WBT. The test protocols referred in Refs. [41,42] provide a list of equipment to be used during WBT. The accuracy and tolerance limit of the equipment, used during WBT, are provided in Refs. [41,42]. Hence, it is essential to use the appropriate instruments prescribed in the protocols to minimize errors which can occur during WBT. In addition to the procurement of appropriate instruments, it is essential to calibrate the equipment as per the recommendations by the manufacturer of the equipment. 11.10. Socio-cultural factors The acceptability of an improved cookstove is influenced by its adoption into local cooking habits, locally available fuel wood and affordability. Socio-cultural aspects were discussed in Ref. [57]. Importance of simplistic, meeting of basic needs and addressing issues relating to socio-cultural aspects of a cookstove are emphasized in Ref. [57]. Depending on their need, the households make decisions and choice of cooking equipment [58]. Economic factors of the community for whom the stove is developed need to be considered for large scale adoption and dissemination. A cookstove which performs well, but not affordable will be limited in use. Socio-cultural parameters include traditional cooking habits such as type of food, quantity, type of fuel, firepower and duration of cooking. These parameters, related to socio-cultural aspects, should be addressed in the process of evaluation of cookstove. 11.11. Addressing the socio-cultural aspects and human errors In the past 30 years the focus was shifted on socio-cultural aspect of a stove [59]. Same cookstove cannot meet the sociocultural requirements of all regions [60]. The socio-cultural aspects need to be an integral part of the cookstoves. For example India is having many segments of food habit. A clear understanding of user expectations and cooking practices is an important factor in the process of development and testing of improved cookstoves. Table 15 List of parameters monitored when conducting WBT. Measurements

Purpose

Atmospheric pressure and boiling temperature

To estimate the local boiling temperature and corrected specific energy consumption, for a specified temperature difference To eliminate errors due to variation in energy input To avoid in consistency due to ambient temperature, wind conditions and relative humidity To estimate the emission factor of CO To eliminate the variation in firepower and duration of the cold start high power phase To estimate the energy input To estimate the energy input

Check on fuel wood variation Climatic conditions

CO concentration in flue gas Ignition

Mass of fuel wood Moisture content of the fuel wood Particulate matter in flue gas Vessel description Volume of water used for WBT

To estimate the emission factor of particulate matter To estimate the energy observed in the vessel material To estimate the mass of water for estimation of useful energy and efficiency

The firepower of the cookstove varies according to the food habit of the local population and the socio-cultural factors. In India, diversity in type of food and cooking habit creates a need for multiple stove design, in order to cater the variation in preferences [61]. It is important to design the improved cookstoves in accordance with the local cooking preferences and the type of food [62]. To accommodate the need of users, it is essential to study their cooking practice and firepower required during the high-power and simmering phase. Variation in energy consumption rate during these phases was in the order of 6.8e8.7 GJ per capita per year [63]. In Sub-Saharan Africa region, per capita energy consumption between lunch and dinner varies about 65% and day-to-day energy consumption varies about 44% [64]. Firepower and turn down ratio of a cookstove provides flexibility in varying the energy output according to the need of the people. Emission factors with variation in the type of cookstoves and fuel are discussed in Ref. [65]. A large variation is observed in emission factors of improved cookstove and traditional cookstoves. In Cambodia, five types of household categories were identified on the basis of the size of the family and their energy requirement [66]. The family member per household varies in the range of 3e10. The cookstove must be able to deliver the required firepower to meet the cooking energy need of different size of families. Variation in cooking vessels and meals cooked is studied in detail [67]. The type of vessel affects the heat transfer rate. A cookstove needs to be designed to operate at a wide range of firepower to match the demand of the local cooking habits. In countries like South Africa, the gap between the access to affordable energy and demand for clean energy is large [68]. Also the energy efficiency can accomplish the multiple social and economic objectives of the people [68]. Hence, a cookstove has to be designed considering the need of its users and should be tested in the lab to ensure that it meets the design requirements. Trained local cooks can be engaged to use the traditional and improved cookstoves for verification of results obtained in the lab and in the field [32]. The initial three phases of the WBT can be used to check the firepower at different levels. The results obtained during the fourth phase of the WBT can be used to fine tune the design and to improve the cookstoves’ performance in such a way that it meets the users’ requirements in field conditions. 12. Conclusions Most of the improved cookstove programs in the past did not achieve its objectives due to several reasons. One of the reasons is the gap between the expectations of the users and the characteristics of the cookstoves promoted by the program. An improved test method is proposed to eliminate various types of errors that occur during the testing of a cookstove. The proposed test method includes key parameters which will enable the lab test results to represent the actual performance in field conditions. The proposed complete cycle WBT focuses on establishing an improved test methodology for a detailed performance analysis of the cookstoves. The complete cycle WBT can be used for estimating the overall performance of the cookstove as well as performance during the different phases of WBT. The proposed WBT comprises of four phases. The fourth phase of the WBT enables estimation of residual heat and the rate of residual heat recovery. Estimation of the residual heat can be used to improve the performance efficiency by understanding the energy flow and optimizing the design of the cookstove to. The improved test method includes estimation of firepower and efficiency in each phase of the WBT along with the turn-down ratio. While observing the performance result of the natural gas cookstoves, the preferred turn-down ratio for the biomass cookstoves was found to be 4.0. Since, maximum duration of cooking is to be done at the simmering phase, the efficiency of

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the cookstove increases proportionally with an increase in turndown ratio. The efficiency of the cookstove during the simmering phase and the turn-down ratio are key factors influencing the quantity of fuel saved during actual cooking conditions. It is important to assess the cookstoves with respect to different parameters that are relevant for clean combustion and increased efficiency. The test method should provide test results with minimum human error and instrument errors, while conducting the WBT. The thermal properties of the materials used in construction of a cookstove, the weight of the cookstove and its cost are equally important for large scale dissemination of cookstoves. Hence, well designed cookstove should be able to meet the performance standards and benchmarks for a large scale adoption. The proposed four phase WBT is based on a holistic approach, which addresses several important parameters for a detailed evaluation of improved cookstoves. Performance of a traditional cookstove and an improved cookstove was compared using the improved test method. Several parameters proposed in the four phase WBT were analyzed. The overall performance of the improved cookstove and the traditional cookstove was 47 and 16.9%, respectively. The heat release rate of the improved cookstove (metal stove with insulation) is 82% higher than the traditional cookstove. Hence, the efficiency of the improved cookstove was 1.8 times higher than the traditional cookstove. A clay stove can hold 3.6 times of heat energy as compared to metal cookstove made of stainless steel. A total number of 11 factors were identified to minimize the variations and to enhance the reliability of the test results.

WEhs Lew Ie WF Cv Mc Ic WFc Ih WFH Is WFS

ht Uc

hc Uh

hh Acknowledgment

Us

We are grateful to Dr. R K Pachauri, Director General, TERI for his continuous encouragement and support. We would also like to thank Mr. Amit Kumar, Director, Energy Environment Technology Development Division of TERI for providing valuable support to conduct the study.

Fpc Fph Fps

hs

sr

Rte Wr

Nomenclatures T1 Q h A Tft Tv P ε

s Ue Wc Wh Mv Cpv T1 T3 t0 t1 t2 t3 WEch

thermal energy transferred by convection (MJ) coefficient of heat transfer by convection (W m2 K1) area of heat transfer (m2) flame temperature ( C) surface temperature of the vessel ( C) thermal energy transferred by radiation (MJ) emissivity coefficient StefaneBoltzmann constant (W m2 K4) useful energy transferred into the vessel (MJ) mass of the water used during cold start high power phase (kg) mass of the water used during hot start high power phase (kg) mass of the vessel used for water boiling test (kg) specific heat of the material used for vessel (J kg1 K1) initial temperature of water during the high power phases ( C) finaltemperatureofwaterduringthehighpowerphases( C) starting time of the cold start high power phase (min) closing time of the cold start high power phase and starting time of the hot start high power phase (min) closing time of the cold start high power phase and starting time of the simmering phase (min) closing time of the simmering phase and starting time of the residual heat recovery phase (min) mass of water evaporated during cold start high power phase (kg)

T2

dr Rhr t3 t4

15

mass of water evaporated during hot start high power and simmering phase (kg) latent heat of evaporation of water (J kg1) total energy input during the complete cycle of water boiling test (MJ) mass of the fuel wood used during the entire test period (kg) calorific value of the fuel wood on dry basis (MJ kg1) correction factor for the moisture content of the fuel wood (mass fraction) energy input during the cold start high power phase of the water boiling test (MJ) mass of the fuel wood used during the cold start high power phase (kg) energy input during the hot start high power phase of the water boiling test (MJ) mass of the fuel wood used during the hot start high power phase (kg) energy input during the simmering phase of the water boiling test (MJ) mass of the fuel wood used during the simmering phase (kg) thermal efficiency of the stove during the complete cycle of water boiling test (%) useful energy during cold start high power phase (MJ) thermal efficiency during cold start high power phase (%) useful energy during hot start high power phase (MJ) thermal efficiency during hot start high power phase (%) useful energy during the simmering phase (MJ) thermal efficiency during the simmering phase (%) firepower during cold start high power phase (kW) firepower during cold start high power phase (kW) firepower during cold start high power phase (kW) turn down ratio of the stove recoverable residual thermal energy (MJ) mass of the water used during the residual heat recovery (kg) initial temperature of the water used during the residual heat recovery ( C) final temperature of the water used during the residual heat recovery ( C) ratio of the residual heat recovery residual heat recovery rate (kW) starting time of the heat recovery phase (min) closing time of the heat recovery phase, when the T5 reaches to the maximum (min)

Abbreviations BIS Bureau of Indian Standard ER Equivalence Ratio MNRE Ministry of New and Renewable Energy NBCP National Biomass Cookstoves Program NPIC National Program on Improved Cookstoves KPT Kitchen Performance Test QA & QC quality assurance and quality control SR Stoichiometric Ratio TPM Total Particulate Matters TSP Total Suspended Particles WBT water boiling test WHO World Health Organization References [1] World Health Organization. Fuel for life: household energy and health, fuel for life; 2006.

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