Evaluation frameworks are methodological proposals that allow elaboration of integral, quantifiable analyses of multi-factor systems. Nowadays, there are well-founded frameworks that evaluate the sustainability of agrosystems by incorporating general attributes as quantifiable indicators. Evaluation frameworks have been a focus of interest since the 1990s for several authors. The Framework for Evaluating Sustainable Land Management (FESLM), for example, is a framework for evaluating soils consideri environmental indicators (Smyth & Dumanski, 1995). Likewise, the Evaluation Framework for Natural Resource Management Systems incorporates indicators of sustainability (MESMIS) that cover a very broad range, from environmental indicators to socio-economic indicators (Masera, Astier & López-Ridaura, 1999).

Currently, a comparative study exists on evaluation frameworks for agro-systems (Sarandón & Flores, 2009); all these evaluation frameworks have the following characteristics: they evaluate systems on the basis of social, economic and environmental parameters or attributes that generate various quantifiable indicators, which is it allows to know the efficiency and functionality of the system. The data are integrated into radial graphs, adequate for both comparative and interpretative analyses (Brink, Hosper & Colin, 1991). Unfortunately, for energy systems that utilize renewable energy sources, ecotechnical systems and, especially, solar cookers, no such frameworks exist.

Solar cookers are thermoconverters that transform solar radiation into calorific power to cook food during varying time intervals. Today, a wide variety of solar cookers exists, and the number of models available makes it difficult to conduct comparative evaluations. Although several procedures for evaluating the thermal performance of these devices exist, even the most efficacious ones are limited to assessing cooking power, merit factors, and thermal performance. The work by Funk (2000) led to the creation of American Society of Agricultural Engineering Standard S580 (ASAE, 2003), which establishes a rigorous procedure for the thermal testing of solar cookers, based on determinations of standard cooking power.

The standard developed in 1992 by the European Research Committee on Solar Cooking (ECSCR), for example, is extensive and includes many qualitative factors, such as ease and safety of use (ECSCR, 1992). In India, the national standards uses a testing method based on research by Mullick, Kandal & Saxena (1987), which evaluates solar cookers with tools whit figures of merit.

As a result, the most common evaluation protocols for solar cookers do not offer integral assessments that take into account ergonomic and economic aspects, or environmental benefits. Therefore, efforts have been made to establish more integrated evaluation systems; in some cases by suggesting the incorporation of socio-economic parameters with indicators (i.e., quantifiable criteria) such as safety, ergonomics, and device quality (Fonseca, Abdala & Acosta, 2003). In their study of the evaluation of solar cookers, Kundapur & Sudhir (2009) propose consider not only thermal performance, but also parameters as stagnation capacity (Thermal accumulation), cost per watt delivered, weight and ease of use. Yahya (2013), suggests a new standard global procedure for testing solar cookers based on determining the thermal performance of parabolic concentrating solar cookers. This new standard sets limits for environmental conditions, specifies test procedures, and assesses performance in terms of cooking power. Kimambo (2007) carried out a study in Tanzania that included a bibliographic analysis; in the study, experimental testing and evaluations were made on 6 different types of solar cookers, it was intended to make a comparative tool for solar cookers. Pohekar & Ramachandran (2004), in turn, propose to formulate a policy to replace the energy used in cooking, with renewable energy, based on a multi-criteria approach. They evaluated nine cooking energy alternatives on the basis of 30 criteria that include technical, economic, environmental/social, behavioral and commercial elements. Unfortunately, these two evaluation frameworks, are attempts to elaborate more integral evaluations, lack quantitative indicators. They are qualitative, but they do not incorporate socio-economic indicators.

Solar cookers have been installed in rural areas to improve people’s quality of life and mitigate the extraction of forest resources by reducing biomass combustion (González-Avilés, López-Sosa & Servín, 2013). However, implementation of these thermal solar technologies often lacks appropriate systems that allow users to employ their solar cookers continuously. In order to determine the viability of inserting a specific model of solar cooker we must understand the elements that may propitiate its implementation, a process that requires evaluating possible economic, environmental and social benefits, that is, sustainable technology. For this reason, it is of importance to elaborate integral evaluation strategies that help us understand the potential advantages of certain solar cooking systems. The following sections present an Evaluation Framework for Small-Dimension Solar Cookers (EFSDSC).

The EFSDSC is an instrument proposed for evaluating solar cookers from a comparative perspective; thus, it cannot be used to assess individual models of such solar devices. However, the results of comparative analyses of two or more solar cooking systems can serve to demonstrate which device the best, or most favorable, energy, ergonomic, and economic features.

The first step of the comparative tool consists in conducting the evaluation of each indicator for each device. In this study, we analyzed the following three systems: (1) the Tolokatsin solar oven (Rincón & Osorio, 1999), a multi-component system of parabolic concentrators; (2) the HOT-POT solar cooking system, a commercial apparatus with an extensive chain of industrial production; and, (3) the Rural Solar Cooker (RSC) (López-Sosa & González, 2013), which is equipped with three dimension parabolic concentrators and has been implemented for use in various communities in the state of Michoacán, Mexico. These systems were selected based on the cost and size of the solar cooker. That is, high, intermediate and low cost; and small, large and medium size. Figure 1 shows the three models.

Figure 1. Source: Author´s own elaboration.

Direct data obtained in relation to the indicators are shown in table 3, while table 4 presents the values for each indicator after conversion to the EFSDSC evaluation scale (0 to 10).

Table 3

INDICATOR	TOLOKATZIN	HOT-POT	RSC
1- Thermal performance (%)	0.252	0.297	0.275
2- Standardized cooking power per area unit (W/m2)	113.3	138.0	203.1
3- Heating time (Minutes)	122.5	95.00	120.0
4- Mass of the solar cooker (Kilograms)	20.00	5.000	18.00
5- Load capacity of the pot (Liters)	6.000	5.000	6.000
6- Orientations per hour	1.000	1.000	1.000
7- Cost per Watt of standard cooking power ($USD)37	6.00	2.74	1.195
8- Reflector durability (years)	20.00	20.00	20.00

Source: Author´s own elaboration

Table 4

INDICATOR	TOLOKATSIN	HOT-POT	RSC
1- Thermal performance	5.04	5.94	5.50
2- Standardized cooking power per area unit	3.24	3.94	5.80
3- Heating time	5.92	6.83	6.00
4- Mass of the solar cooker	5.00	8.75	5.50
5- Load capacity in the pot	8.57	7.14	8.57
6- Orientations per hour	8.00	8.00	8.00
7- Cost per Watt of standard cooking power	0.00	5.43	8.00
8- Reflector durability	10.0	10.0	10.0

Source: Author´s own elaboration

Figure 2 shows the final stepof the EFSDSC with the radial graph that integrates the results of the comparative analysis of the three solar cookers: RSC, HOTPOT and Tolokatsin. A spreadsheet can be used to elaborate comparative graphs like this one for of solar cookers that one wishes to compare.

Figure 2. Source: Author´s own elaboration.

Data from the EFSDSC scale, together with the results of the evaluation, must present the quantities to two decimal. The analysis of the EFSDSC scale for the three solar cookers evaluated is shown in table 5, where it is clear that RSC is the device with the best characteristics. In addition to the sum of indicators, it is possible to conclude qualitatively that RSC is more energy and economic efficient. That is, if an efficient and economical system is desired, RSC has these characteristics, and in score of evaluation framework is better than other solar cookers analyzed.

Table 5

TOLOKATSIN	HOT-POT	RSC
45.77	56.03	57.37

Source: Author´s own elaboration

EFSDSC is a comparative tool for solar cookers of small dimensions. It seems that if the efficiency of the solar cookers increases, all the charges also increases, but it does not. EFSDSC limits charges when the cost of the solar cooker is high because the normalized value of the indicator from 0 to 10 is low. On the other hand, if the solar cooker has a large pot to cook food, the value of the indicator is high. This contributes to the economic parameter.

If the pot is large, it is possible that the device is large because it must have a large collector area to cook more food. But that does not always happen. There are solar cookers with small area and large pot. So EFSDSC represents a useful tool for all types of small solar cookers.

Therefore, the sum of the indicators is comparative value of solar cookers, because it suggests the highest values in the economic (low cost), environmental (efficiency) and social (Ease of use) parameters. These parameters are required for people and have been documented in the literature (González-Avilés, LópezSosa, Servín-Campuzano & González-Pérez, 2017).

When the sum of the values of the indicators is small, the higher parameters determine which solar cooker is best, according to the parameters considered most important for the user.

EFSDSC is a quantitative and qualitative tool that can be useful to evaluate solar cookers and to know the strengths of these systems to implement in the urban or rural areas (González-Avilés et al., 2017; González-Avilés et al., 2014b)

EFSDSC is proposed as an evaluation framework for solar cookers that incorporates social, economic and environmental parameters. Based on the assessment of a series of indicators, a data table was elaborated that makes possible to compare two or more solar cookers. On the basis of this proposal, the following conclusions can be reached:

EFSDSC incorporates evaluations of the thermal performance, cost-benefits, ease of use, and durability of the solar cookers assessed.