Evaluation of sustainable useful index (SUI) by fuzzy approach for energy producing processes

https://doi.org/10.1016/j.cherd.2015.11.006Get rights and content

Highlights

  • Multistep procedure for process energy sustainability is proposed.

  • All the energy terms were considered direct as well as the indirect by LCA approach.

  • Fuzzy modelling has been used to score different process using EROI and EPT values.

  • The Anaerobic Digestion using organic wastes and maize as feed has been analyzed.

  • Results shown that the use of organic wastes is two folds more sustainable than maize.

Abstract

The present paper proposes a fuzzy procedure for evaluating and scoring the energy sustainability of a process. The fuzzy procedure permits to handle Energy Return on Investment (EROI) and Energy Payback Time (EPT) values as characterizing parameters for the energy sustainability evaluation. The procedure is articulated in three steps: a first screening among many processes is performed by using the Energy Sustainability Index (ESI), which takes into account the total Produced Energy referred respect to the direct spent energy (heat and electricity). The second step is the Analogical Model, i.e. the quantification of each chemicals, materials and energy to estimate the Useful Energy with a Life Cycle approach, taking into account all the Indirect Energy terms. The third step is the evaluation of two parameters: EROI and EPT, considering all the involved energy flows. Therefore, a fuzzy combination of EROI and EPT permits to evaluate a Sustainable Useful Index (SUI). This procedure is useful for local planning, for allocating financing resources for new energy processes exploitation and for selecting the most sustainable choice among several research programs. Here two cases-studies are reported and compared to highlight the approach: the Anaerobic Digestion producing CH4 using Organic Refuses and maize as Energy Crop.

Introduction

The present energy crisis together with environmental issues, such as global warming, has persuaded men to search new energy sources (Balat, 2008). Different renewable sources are now being exploited. Lignocellulose including energy crops: wood, short rotation coppice, poplar, switch grass and miscanthus; Grass: leaves, green plant materials, grass silage, empty fruit bunch, immature cereals; Oily crops and seeds (e.g. Jatropha); Oily residues: waste cooking oils and animal fat; Aquatic: algae and seaweed; Organic residues: municipal waste, manure and sewage could offer sustainable energy solution and support bio-based economy (Sadhukhan et al., 2014, Angenent et al., 2004). Other renewable energy systems include solar photovoltaic, offshore wind, onshore wind, geothermal, tidal stream, tidal range, tidal wave and hydropower However, we believe that it is also important to introduce the concept of Energy Service, here intended as the amount of energy required by the end user as Useful Energy, i.e. the energy necessary to support human life, as outlined in Fig. 1. Surplus energy flowing from each block in Fig. 1 depends from the technology used, and it is of primary importance for society. Wealth, survival, art, army and even civilization itself is a product of surplus energy. The interplay of how much, what kind (quality), and at what rate the energy is delivered determines the Useful Energy. It gives the ability to the society to divert attention from life-sustaining needs towards luxuries, such as art and scholarships including research and innovation for the exploitation of different energy sources.

Among the primary energy sources organic waste material (Evans, 2001) is more than 70% of daily refuse production. The technology pellet to use organic waste ranges from biochemical processes (Sadhukhan et al., 2014, Pfeffer and Liebman, 1976) including microbial fuel cells (Tommasi et al., 2012, Logan, 2008, Aelterman et al., 2006) to thermochemical processes (Sadhukhan et al., 2014, Guéhenneux et al., 2005) as illustrated in Fig. 2.

Taking in mind Fig. 2, in order to select the most appropriate technology, it is necessary to establish which criteria should be used to valorise the sources (Sentimenti and Biorgi, 2006). In this context, economic criteria on their own appear to be inappropriate, because data can easily be manipulated according to the working hypothesis and the conclusions might not be completely reliable (Cleveland et al., 1984). Economists argue that the price of a technology or a fuel automatically captures all the relevant features, but in a finite resource scenario this at least, appears to be questionable. The Life Cycle Assessment (LCA), is an alternative to a conventional economic analysis because it takes into account all the aspects of such a technology (e.g. environmental impact, safety, toxicity, energy use and social issues) together with economics ones (Sadhukhan et al., 2014, Azapagic, 2010, ISO 14044, 2006, Baumann and Tillman, 2004, ISO 14041, 1998, ISO 14040, 1997, SETAC, 1993). New technologies have been developed in the last years, and others are at a developing and/or infancy stage right now. One of the difficulties of selecting a technology is the need to measure the sustainability level of it, in other words the necessity to score the technology towards energy sustainability.

To this aim, several approaches, ranging from a thermodynamic one (De Swaan Arons et al., 2004) to a more industrial oriented alternative (Azapagic and Perdan, 2000, De Simone and Popoff, 1997) have been put forward in recent years to evaluate the sustainability (Sadhukhan et al., 2014, Azapagic, 1999, Laws et al., 1984, Hall et al., 2009). Referring to energy sustainability, it has been recently proposed that the most appropriate way to judge the relative merits of different energy sources is to evaluate the ratio between the amount of energy produced and the energy needed to produce it, known as the Energy Return on Investment (EROI). EROI, in its simplest form, measures the output energy at the point of production or “mine mouth” (Murphy et al., 2011). The EROI evaluation in terms of energy source away from “mine mouth” needs the estimation of the energy consumed to deliver and to use it at the point of energy utilization, this causes obviously a decrease of EROI. In order to have some idea about this concept, it can be considered that the EROI for oil at “mine mouth” is about 20 this means that for 1 unit of energy consumed for extraction from reservoirs, well-head treatments and new exploration, 20 units of energy are available to society. Hall et al. (2009) estimated that at the end user level, EROI should be at least 10 to cover the needs of society/civilization to support an energy service. The EROI for ethanol derived from maize was instead estimated to be at best 1.3 (Cleveland and Costanza, 2010) and according to some authors (Patzek and Pimentel, 2006, Patzek, 2004) less than 1; this implies that maize-based ethanol requires some other energy source, subsidy for its production. Energy Payback Time (EPT) is a related concept to EROI. It permits to score a technology against the time parameter. It is the necessary time for a plant to produce the energy necessary to rebuild the plant itself. The higher the EPT value, the lower the annual rate of Useful Energy, hence the lower the sustainability of the technology. In other words EPT is the time of the operational lifetime of the plant necessary to reach the sustainability condition i.e. the time in which the technology starts to feed the society.

In the present paper a methodology to score energy sustainability of different technologies will be proposed. The procedure can help in local planning, in allocating financing resources for the exploitation of new energy processes as well as in selecting the most sustainable choice from an energetic point of view, among several research programs. Here the methodology will be applied to evaluate the energy sustainability of Anaerobic Digestion (AD) technology to produce methane (Bordoni et al., 2010) using two different types of substrates: Organic Waste Market (OWM), a local marked refuse, and maize as energy crop, which is one of the most used substrate as feed for AD (Bacenetti et al., 2015). To this end all the energy terms, directly consumed to produce biogas at technology level as well all the Indirect Energy (see following section), including the energy expenditure to produce and harvest the maize, or the energy saved, not necessary to dispose the waste, will be evaluated. The score of technologies towards energy sustainability, through Sustainability Useful Index (SUI), i.e. a parameter to evaluate the real energy useful for the society, will be obtained via a fuzzy procedure to combine the EROI and EPT values.

Section snippets

Methodology

According to the concept introduced by Röegen (1976), in order to have energy sustainability of an energy technology, it is necessary that the technology must be vital. Like a biological system, an energy technology must be able to produce a quantity of Useful Energy greater than the needed to sustain itself, in order to sustain “others” energy services. It necessarily needs to use only a part of the energy source for its operational necessities and reproduction, and the remaining part will be

Anaerobic digestion plant application

Anaerobic digestion (AD) is one of the most promising technology to synchronize human activities with natural cycles; in fact, it is able to digest organic matter to produce energy and an organic by-product with possible use in agriculture. AD is quite common in Europe and today it is able to produce 8.3 MToe of energy (Eurobserve’er, 2010), which represents approximately 0.4% of the 1703 MToe of primary energy consumption of EU 27 in 2009 (Capros et al., 2010). Here the sustainability of AD

Results and comments

The ESI tool, described in Section 2.1 was applied to the AD technology, considering the operative conditions defined in Table 1. Table 3 shows the ESI results for diameter values from 0.5 m till 10 m considering the OWM and maize as feed, taking into account the min and max productivities. In the case of maize at low diameter and in the situations of low productivities, ESI is lower than one or little higher, suggesting that in these situations the energy sustainability is questionable. It

Conclusions

In the present paper a fuzzy procedure has been proposed for evaluating the energy sustainability of a process using different sources, through the Sustainable Useful Index (SUI). Moreover SUI permits to score either different process using the same energy sources. The procedure is constituted by three steps: Energy Sustainability Index (ESI) evaluation, Analogical Model (AM) computation, Energy Return On Investment (EROI) and Energy Payback Time (EPT) determinations and finally EROI and EPT

Acknowledgements

The Authors wish to thank the Regione Piemonte for its financial support under the C16, 2004 and Eco-Food 2010 projects. The sponsor had no involvement in the study.

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