The world population is currently of more than 7 billion and it is estimated that by 2030, there will be 8 billion people, population growth is closely related to the growth in world energy demand (^{Estrada, 2013}). The primary energy sources currently used are natural gas, coal, oil, nuclear energy and renewable sources (^{Blanco, 2004}). In the last decades the energy generation has been one of the great worries worldwide, since for its generation, great amounts of fossil origin fuels are used which has repercussions in severe damages to the environment by the emission of greenhouse gases (GHG). Renewable energies are characterized because, in their processes of transformation and use in useful energy, they are not consumed or depleted in a human scale of time (^{Jara, 2006}). In anaerobic decomposition processes high-energy compounds are obtained and have the potential to be used for producing power and heat energy (^{Magaña et al., 2006}).

The produced biogas can be burned in a gas turbine or internal combustion engines, which generate electricity and heat; the four-stroke engines were originally developed for natural gas and therefore are well adapted to the special characteristics of biogas (^{Alexopoulos, 2012}; ^{Deublein and Steinhauser, 2008}; ^{Meggyes and Nagy, 2012}). Several researchers have made adaptations to conventional motors to operate them with biogas to generate electricity, however, biogas power generators are more expensive today; energy is generated locally and is not centralized, it also plays an important role in accelerating socio-economic activities (^{Siripornakarachai and Sucharitakul, 2007}; ^{Nindhia et al., 2013}; ^{Ga et al., 2013}; ^{Sosa et al., 2014}; ^{Alam et al., 2015}).

In 2006, the Secretariat of Agriculture, Livestock, Rural Development, Fisheries and Food (SAGARPA), through the Shared Risk Trust (FIRCO), in collaboration with the Autonomous University of Mexico City, implemented a biogas project in 9 pig farms, with the installation of 60 kW capacity power generators operated with biogas for the generation of electric power (^{SAGARPA, 2007}). From 2008 to 2012, SAGARPA supported the acquisition of 137 power generators from biogas, to generate electric and thermal energy which is used for various productive purposes within the agricultural farms. Secretaría de Energía (^{SENER), 2012}). ^{Vera et al. (2014)} estimated the generation potential of biogas and electric energy, through the management of excreta of cattle and pigs in the Ciénega region of Michoacán state. The financial feasibility studies of power generation with biogas in livestock activities demonstrates the energy self-sufficiency of farms, also it helps producers to make the decision to incorporate into their farms a power generator to produce electricity (^{Cervi et al., 2011}; ^{Escalera et al., 2014}).

The study was conducted in three stages, the first consisting of the analysis of 18 projects supported by the Shared Risk Trust (FIRCO) from 2008 to 2012 on pig farms in Puebla. In a second phase, an estimate was made to determine the potential for power generation with the formula of United States Environmental Protection Agency (^{EPA, 2006}), in 19 municipalities with farms with more than 500 pigs, this formula was used by ^{Casas et al. (2009)}, for dairy stables. The third phase consisted of a proposal for five farm sizes, where a financial feasibility analysis for each proposed size was carried out with an integral biodigestor-power-generator system. The financial feasibility analysis was based on ^{Baca’s project evaluation methodology (2013)}, which analyzed indicators such as net present value (NPV), internal rate of return (IRR) and cost benefit ratio (B/C).

^{Escalera et al. (2014)}, evaluated clean development mechanism (CDM) projects of pig farms in Mexico, the financial assessment contemplated the acquisition, installation and maintenance costs of a 60 kW power generator with 15 hours of operation during 365 days, this study showed that the biogas power generator is profitable for the farms to generate their own energy. They obtained very high financial indicators, for example IRR with values between 30.34 and 152.58%, considering a update rate of 12%.

The FIRCO supported 18 projects from 2008 to 2012 with power generators, from 2012 the total savings derived from FIRCO projects is 6 590 163 pesos per year. Farms are saving an average of 71.99% of their electricity costs, farms can cover 100% of their electricity needs, but power generators are not operating at full capacity. On the other hand in Puebla there are 37 farms with great potential to implement complete biodigestor-power generator systems distributed in 19 municipalities. The estimates obtained for 37 farms distributed in 19 municipalities in Puebla represent a potential of 17 875 MW of electricity per year (Figure 1).

Figure 1 Potential power generation in 19 municipalities Puebla (MW year-1). Elaboration based on methodology ^{EPA (2006)}.  

Electricity consumption per capita in Mexico increased 1.5% compared to 2013, reaching 2 015.28 kW in 2014 (^{SENER, 2015}). Considering this data as a reference, the 37 farms of Puebla could supply the consumption of 8 869 people per year.

The financial analysis provided for a farm size of 500 pigs a power-generator of 10 kW, for farms of 1 000 and 2 000 pigs a power-generator of 30 kW and for farms of 3 000 and 5 000 pigs a power-generator of 60 kW, for the five proposed farm sizes, 312 days of operation were considered, with a variation in number of hours of operation. The analysis contemplates the acquisition, installation, maintenance of the biodigestor-power generador system and as income, the saving in electric energy payment and fertilizer sale. The evaluation was carried out with an update rate of 12%.

The results indicate that there is a direct relationship between size and profitability, the larger the size of the farm the better the financial indicator would be, the five proposed farm sizes showed favorable financial indicators, so that producers with similar size farms can make an investment to generate their own energy and make their production unit more efficient with cost savings, as well as contribute to environmental care (Table 1).

Table 1 Financial analysis for different farm sizes (heads). 

^¥= Elaboración con base en la metodología de evaluación de proyectos de ^{Baca (2013)}.