Servicios personalizados
texto en 
Inglés (pdf)
Articulo en XML
Referencias del artículo
Enviar articulo por email
Citado por SciELO 
Similares en
SciELO 
Permalink
INTRODUCTION
The exhaustion of fossil energy is an imperative to optimize the efficiency in the use of available energy, especially, when it conditions the quality of life of millions of people with limited access to this indispensable resource for diverse reasons (Montico et al., 2006).
In agroecosystems, besides solar energy, energy from other sources is used, mainly fossil energy derived from petroleum (Suárez et al., 2017). By definition, agriculture implies the modification of natural ecosystems and it requires to give additional energy to the solar one, in form of external inputs. The objective of agriculture is to manipulate the energy flows with the purpose of obtaining a certain net productivity as a product (grains, forage, meat, milk, etc.) and that necessarily implies an energy contribution (Flores & Sarandón, 2014).
At the present time, it is generally thought about energy necessities in terms of fuel, ignoring the contribution of nature and human beings, without noticing that the energy used in the services and obtaining of the material can be bigger than that of the fuels used in many processes (Izursa, 2011). Today’s economic development has been reached at the cost of the environment detriment, contaminating soil, water and air and exceeding their regeneration capacity. This has happened since it is the market which decides what will be produced, how to make it and how to distribute it, disregarding the services offered by the nature (to obtain the product) when defining the prices (Roque, 2016).
The combustion in excess of fossil hydrocarbons and other non-renewable resources, together with their intensive usage that diminishes the regeneration capacity of the renewable ones, threatens the life on Earth, evidenced in the global climate change that continues growing instead of attenuating. The concern of reverting this situation has taken some nations, among them Cuba, to commit in reaching objectives and goals for the sustainable development with date limit in the 2030.
In the achievement of the sustainability in food production and security, the efficient use of the energy that flows in the agricultural processes in form of natural resources, products and services, plays a fundamental part because it is the support of the ecosystems and is responsible for an important economic expense. The objective of the present revision is to establish the bases for the implementation of the methodology of emergetic synthesis, which offers different ways for the analysis of the energy efficiency of the agrarian production systems and, consequently, it allows increasing the efficiency in the management decision making on them.
DEVELOPMENT OF THE TOPIC
The term “emergy” was proposed by Odum (1988), in the face of the necessity of, not only quantifying the energy contained in the product or final service, but the whole energy invested in it; and he defined it as "the sum of the whole energy in a way, necessary to develop a flow of energy in another way, in a certain time”. More recently, Aguilar et al. (2015), define that emergy is "the quantity of energy that has been well utilized in a direct or indirect way in the generation of a certain good or service with the objective of analyzing the different contributions of energy flows (nature and economy) under a common unit, the solar emjoule (seJ) ".
In correspondence to that defined by these authors, the emergetic analysis would be the methodology of scientific base that counts the environmental and the economic values, making use of Economy, Theory of Systems, Thermodynamics, Biology and the new principles of open system operation (Ortega et al., 2002).
To increase the investigations about the monetary value of the ecosystem services contributes to attract the support of the political and market actors, to approach the environmental problems and to contribute in the conservation of the ecosystems; since there is where the outline of payments by environmental services is configured, and that is an instrument utilized in public policies in some countries and sectors that proposes to compensate the users of the land for the positive environmental outsources that generate through the adoption of sustainable agricultural practices (González y Serna, 2018).
The economic human systems produce materials and fuels to support the development of populations and cultures. However, human beings are only a small part of the great biosphere that includes forests, oceans, mountains, valleys, lands, rivers and the atmosphere. Ultimately, they are not only the human beings and their money those that determine what it is important, but it is the energy of the world. It would have then, much sense to measure this system by means of the energy flow, because it would be considered the contribution of nature (Izursa, 2011).
The starting point of emergy or emergetic synthesis like study field is that different energy types can be compared using conversion factors that show the quantity of equivalent energy types (Ortega et al., 2002; Flores y Sarandón, 2014; Stark et al., 2016). When connecting different energy types, several parts can be associated and the complexity can be visualized in a simple way, using diagrams, from which calculations of flows and deposits are made (Odum, 1996; Izursa, 2011). Many energy units diluted are needed to form a unit of concentrated energy. For example, 4 J of coal are required to produce 1 J of electricity, and 1 000 J of solar light to obtain 1 wooden J. The necessary total energy to obtain a product is the energy accumulated in that product (Odum y Odum, 2003; Izursa, 2011).
The emergetic synthesis is based on the study of biogeophysical and socio-economic flows of matter and energy that are exchanged among the constituent elements of the socio-ecological systems under the same base (Aguilar et al., 2015).
All the processes of self-organization of systems (example: ecosystems), like it has been mentioned, are governed by the second law of Thermodynamics, because the energy that passes from an inferior level to another superior of self-organization is smaller in each step, since there is no a hundred percent efficiency in the transformation process. However, the necessary energy for the construction of a higher level of self-organization is higher every time as the system becomes more complex, that is to say, as it advances in the organization chain (Bravo et al., 2018).
This observation implies that, even when 1 joule of solar energy, 1 joule of coal or 1 joule of electricity, represent the same "quantity" of energy (1 joule), they do not represent the same "quality", in the sense of the potential these different types of energy sources have to act on the group of the system, that is to say, in the necessity that the system has of receiving bigger or smaller quantities of less concentrated energy to generate each of them (Bravo et al., 2018). It is concluded then, that a hierarchy of energy exists according to its quality or potential to influence in the system and it goes from not very concentrated energy sources (as the sun) to those very concentrated ones (as the petroleum) (Odum y Odum, 2003).
Solar energy is selected as the reference energy, because in the emergetic analysis, it is supposed that this is the main entrance of not very concentrated energy to the ecosphere. Therefore, the transformity would have units of energy seJ/unidad (solar equivalent joules / energy) (Bravo et al., 2018).
Besides emergy, diverse methodologies of environmental administration exist as Life Cycle Analysis (LCA) and Evaluation of Multicriterion (EMC), to mention some that allow analyzing the production with energy approaches to design more sustainable systems, in which the imports of inputs and the polluting emissions decrease (Odum y Odum, 1981). Opportunities can be identified in the different productive stages and to carry out an inventory of energy inputs used (Aguilar et al., 2015).
To achieve an integration of all the relationships man-nature in an ecosystem, the emergetic analysis separates the entrances of renewable and not renewable sources. These distinctions make possible to define the emergetic indexes that provide the tools for the sustainability decision making, especially when there are different alternatives (Brown et al., 2012; Bravo et al., 2018).
According to that outlined by Aguilar et al. (2015), the Emergetic Intensity is equal to the real value of the product, that is to say, all emergy used in the production of a certain quantity of the product. Three main types of emergetic intensity exist: Transformity (in seJ J-1), Specific Emergy (in seJ g-1) and Emergy per Monetary Unit (in seJ $-1).
The transformity of a product measures the energy quality and its hierarchical position in the universal energy. As bigger is the number of transformations of necessary energy for the elaboration of a product or the execution of a process, bigger will be the value of its transformity, being bigger also the importance of the resource for the ecosystems and for the human beings. This approach facilitates to visualize and to quantify, in a dynamic way, the flows of natural resources, environmental services coming from nature and of the impacts of anthropic activities, allowing the understanding of the limits in each ecosystem and the establishment of goals and objectives to guarantee the support capacity, that is to say, it determines the sustainability of the systems (Aguilar et al., 2015).
Methodology of Emergetic Synthesis
The Emergetic Methodology proposed by Odum (1996) is developed in four stages: Design of the Diagram of System, Chart of Emergetic Evaluation, Calculation of Emergetic Indexes and Interpretation of the results.
Design of the Diagram of System
This first stage is preceded by a diagnosis to identify the limits, components, entrances and exits of the system and how the materials and the energy fluctuate. With these data, the diagram is elaborated. Accordingly, all the complexity of the system is expressed and its elements and flows by means of the symbols defined by Odum (1996) (chart 1).
TABLE 1 Symbols used in the elaboration of the systemic diagram
| Symbol | Meaning o |
|---|---|
|
Energy flow: It connects the diverse components of the system. It reflexes the transferences of energy, materials or information among them. |
|
Interaction: Convergence of several types of flows that, by means of the performance of diverse processes, generates flows of more quality. |
|
Transaction: Exchange of a flow for another one. It generally refers to goods and services (continuous line) in exchange for money (discontinuous line). |
|
Deposit: It is a matter storage, energy, money, services, information whose use rates are bigger than those of their renovation. |
|
Consumer: Component that consumes more energy than that it produces, although it contributes services with bigger emergy. It uses the products of the producers. |
|
Producer: Component that, through certain processes, gathers and transforms energy of low quality, concentrating it. It makes products. |
|
Source: Resource that is outside the system limits, and matter flows and energy flow from it to the interior of the system. |
|
Energy drain: It represents the energy dissipation in heat that accompanies all the transformation or accumulation processes. |
|
Box: Limits of the system. |
Sources: Odum (1996).
The Systemic Diagrams facilitate to visualize in a holistic way the operation of the scenarios of interest and to know the reality in a detailed way. In spite of their precision, they should be the sufficiently simple as to facilitate their understanding. They constitute the base to organize the work and to develop the emergetic evaluation.
For a better understanding of each resource flow in the different stages of this methodology, the total emergy amount by the system (Y) is divided into resources of nature (I), as renewable (R) and not renewable (N); and the resources of the economy (F), as materials (M) and services (S), as it is represented in way simplified in the Figure 1.
Construction of the Table of Emergetic Evaluation
All the flows that cross the limits of the system (entrances), follow a sequence that go from the source of external energy to the internal component that uses it. These flows that were previously represented in the diagram, become a calculation line in the evaluation table of emergy (Table 2).
TABLE 2 Structures of the Table of Emergetic Synthesis
| Note | Names of Contributions | Quantity | Unit (/ha/year) | Transformity (seJ/unit) | Solar emergy (seJ/ha/year) |
|---|---|---|---|---|---|
| R1,2,3...n | |||||
| N1,2,3...n | |||||
| M1,2,3…n | |||||
| S1,2,3...n |
The first column Note refers to the order in that each flow is placed (1, 2, 3…n) and to the order of the note on foot of the table in which the origin of the fact is expressed (R, N, M, S). The second, Name of the contributions refers the name of the evaluated flow. The Quantity (numeric value or flow) specifies the quantity or proportion in that each flow enters. For a system in stationary state the values corresponding to the flows annual means are placed. In column four, flows are expressed in their respective Units (grams, kilograms, joules, $, etc.), considering the area (ha, m2, m) and the time (year). The Transformity (emergy per unit or specific energy) of each flow should be obtained of a source of information mentioned in previous studies.
The Solar Emergy (flows of emergy) is calculated multiplying the quantity in that each flow enters (column 3) by the corresponding transformity (column 5). This value is interpreted as the necessary solar energy to produce a service or a product. To multiply by the transformity allows having all the flows expressed in some common and comparable units, solar Joules (seJ); since the different quality of each energy is pondered.
This table offers an accounting of all the components of the analyzed environmental system.
Calculation of Emergetic Indexes
Starting from the data obtained in the Table of Emergetic Evaluation, emergetic indexes related in table 3 are calculated and interpreted. These indexes offer certain information on the system; it allows knowing its efficiency in the use of the resources, to evaluate the sustainability, to establish comparisons among several scenarios and, therefore, they serve as support for the management.
TABLE 3 Emergetic Indexes
| Emergetic Indexes | Formula | Concept |
|---|---|---|
| Solar Transformity | Tr = Y/EP…(1) | Total Emergy / Resource energy |
| Renewability | %R = (R/Y) x100… (2) | Renewable inputs of nature/ Total emergy |
| Emergetic Investment Ratio | EIR = F/ I…. (3) | Resources of the economy / Resources of nature |
| Emergetic Yield Ratio | EYR = Y/F… (4) | Total Emergy/ Resources of the economy |
| Environmental Load Ratio | ELR = (F+N)/R… (5) | (Resources of the economy +non-renewable resources) / Renewable resources |
| Emergy Exchange Ratio | EER = Y/ Em sales….(6) | Total emergy/ Emergy received from selling |
Interpretation of the Emergetic Indexes
Transformity (Tr) It is the relationship among total emergy that enters in the system (Y) and emergy of the products that come out (Ep). This index reveals a quality of the system, as bigger Tr is more emergy is required to generate products. It can be interpreted as the inverse value of the efficiency of an agroecosistema.
Renewability (% R) it is the relationship between the renewable entrances of nature (R) and total emergy that enters in the system (Y). Expressed the percentage that renewable energies represent inside the system.
Emergetic Investment Ratio (EIR) it is the relationship between the contribution of the economy (F) and the nature (I), it is dimensionless. It is an indicator to understand the intensity of emergy "bought" used in the agroindustrial systems.
Emergetic Yield Ratio (EYR) it is the relationship between total emergy that enters to the system (Y) and the contribution of the economy (F). This index is dimensionless and allows knowing in a general way, the net profit that the system offers to the global economy.
Environmental Load Ratio (ELR) it is the relationship between the sum of the non-renewable resources of nature (N) and those of the economy (F) for the renewable resources of nature (R), it is dimensionless. When the value of the index is high, greater will be the environmental impact of the system. It also indicates that the production costs are higher, and because of that the final price will be increased, making that the product or areas producers are less competitive in the market with a relationship of lower environmental load.
Emergy Exchange Ratio (EER) it is the relationship between total emergy (Y) and emergy received from the sales of the products (Em sales). The Em sales, is obtained of the multiplication of the price of the product by the Emergy money ratio (relationship emergy-money or emergetic exchange). The last one is the quantity of emergy that can be bought in a certain country for a unit of money (a dollar) in a specific year. It is dimensionless. It represents if the producer is receiving in the sale of the products the necessary emergy for the production. If the value of the index is bigger than the unit, it represents that the expenses are bigger than the sales.
CONCLUSIONS
When the energy flows of different qualities that intervene in agriculture are expressed in common and comparable units, they can be counted in monetary terms.
The Methodology of Emergetic Synthesis reveals the energy efficiency, the grade of use of the renewable resources and the measure in which non-renewable are used. It also shows the contribution of the economic and natural resources and the environmental impact of the agricultural systems.
The efficiency of the agrarian processes in the use of energy, allows diminishing the polluting loads, to conserve the non-renewable resources and to increase the resilience, the self-sufficiency and the economic sovereignty. Therefore, to regulate those processes, it is necessary to quantify their flows.
