The fossil trace of CO 2 emissions in multi-fuel energy systems Andrés Agudelo a, * , Antonio Valero b , Sergio Usón b a Department of Mechanical Engineering, Universidad de Antioquia, Calle 67 No. 53e108, Medellín, Colombia b Centre of Research for Energy Resources and Consumption e CIRCE, Universidad de Zaragoza, Mariano Esquillor,15, 50018 Zaragoza, Spain article info Article history: Received 15 September 2012 Received in revised form 17 April 2013 Accepted 16 June 2013 Available online 18 July 2013 Keywords: Thermoeconomic analysis Multi-fuel energy systems Hybrid renewable-fossil Carbon dioxide emissions abstract The search for sustainability in energy systems has increased the concern to reduce pollutant emissions and waste. Among the several strategies that help in this task are increased energy efficiency, carbon capture and storage, hybrid renewable-fossil systems, and system integration. All of them often result in complex multi-fuel multi-product systems. Conventional thermoeconomic analysis of such systems does not give information related to the type of energy source used, nor to the emissions generated. The aim of this work is to provide a method to reveal the fate of energy resources inside a system. We present a methodology to decompose exergy flows into as many parts as different types of external resources a system has. The proposed method was applied to a cogeneration system, showing to be a powerful tool to analyze multi-fuel systems, especially hybrid fossil-renewable plants, since the evolution of fossil resources can be tracked through the entire system. It also presents an answer to the unsolved problem of discriminated conversion efficiency, fuel impact and CO 2 emissions impact when different fuels are used, which allows an extended analysis of energy systems, by taking into account the existence of a carbon tax. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction In past decades, energy systems used to have a “linear” struc- ture, given that a single energy source was transformed into a single product. The resulting additional output streams, such as residual heat, were considered as inefficiencies or waste. Several energy, economical, environmental, and social phenomena have become a driving force for changing this paradigm. An example is the use of those additional outputs as in cogeneration or CHP (combined heat and power) systems, as well as in trigeneration and polygeneration systems, in which heat and refrigeration demands, as well as drinking water or chemical products are obtained in addition to electrical or mechanical power from bigger and more complex systems [1]. The same motivations have caused the combination of several energy sources into a production process to become a common practice. This allows to take advantage of locally available energy resources to overcome availability issues. In the same way, the integration of industrial processes and utilities often results in multiple energy inputs to a single system. Multi-resource energy systems are intended to reduce energy consumption and pollutant emissions while satisfying energy demands directly on site [1,2]. One of the strongest motivations to diversify the structure of production processes comes from environmental considerations. It is widely accepted that energy-related carbon emissions have a significant contribution to the anthropogenic global warming effect [3,4], since approximately 80% of the world’s energy comes from the combustion of coal, oil, and natural gas [5,6]. The greenhouse effect is a global pollution problem because carbon dioxide has a long lifetime in the atmosphere [7,8]. Its effects on climate and society can be devastating: if no GhG (greenhouse gas) control is exerted, global warming by the end of this century is likely to be between 3  C and 6  C [9,10]. Therefore, mitigation for diminishing the magnitude of the impacts is necessary, requiring large re- ductions in anthropogenic emissions of CO 2 [8,11]. The emission of GhG, and in particular net carbon emissions from an energy system, are accepted as an indicator of its sustainability [11e 13]. A convenient way to mitigate global warming is to follow the “polluter pays” principle by means of the internalization of power generation externalities [14]. This approach consists in the pricing of carbon emissions (and of many other important externalities) by means of taxes or permits, among other options [9,15]. It is believed that using the marketplace is the most efficient and cost-effective way to achieve this goal [9,14]. This alternative also has the advantage of making renewable technologies more competitive by rising the cost of electricity production from fossil resources [14,16]. Tax levels should be such that carbon capture and storage becomes a viable option for power producers [11]. * Corresponding author. E-mail addresses: afagudelo@gmail.com, afagudelo@udea.edu.co (A. Agudelo), valero@unizar.es (A. Valero), suson@unizar.es (S. Usón). Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.06.036 Energy 58 (2013) 236e246