# On the use of CO2 for the integration of Renewable energy in the energy system.

Abstract

In the context of the energy transition, efficiency and renewable energy integration are identified as having the highest potential for mitigating the $$CO_{2}$$ emissions . From the energy system engineering point of view, this means that not only one has to convert renewable energy resources into distributed energy but also to make it available to supply the energy services when they are needed. In this paper, we demonstrate the role of process system engineering for designing the future energy systems and we explore the possible role that $$CO_{2}$$ could play as a material to help the energy transition. Considering $$CO_{2}$$ as a carbon support, we show on the one hand the possible role of $$CO_{2}$$ in the energy system : as an energy carrier in district energy systems or as a possible carbon source for long term renewable energy storage when considering power to gas concept. We also highlight the importance of understanding the carbon cycle, considering the $$CO_{2}$$ in the atmosphere and the possible ways to replace fossil fuel produced $$CO_{2}$$ by the one harvested in the atmosphere.

## Introduction

In the context of the energy transition, the energy efficiency and the integration of renewable energy resources are identified as having the highest potential for mitigating the $$CO_{2}$$ emissions. According to the energy technology perspective report of the International Energy Agency (Agency 2014), it corresponds to more then 65% of the contribution to the $$CO_{2}$$ mitigation effort needed to limit the temperature increase to 2°C. This leads to a considerable effort of the research community to propose new advances for the energy usage and conversion using bio-chemical, thermo-chemical or photo-chemical or physical concepts. For the engineers who will be responsible of designing the future energy system, the goal is today to integrate these new technologies in the energy system and to create innovative new smart multi-energy infrastructures that will allow the penetration of these technologies and to operate them with a maximum of efficiency. The role of the process and energy system design is to calculate the equipments to be considered to supply the products and the services required by the community (being a household, a district, a city or a country). This means defining the most attractive technologies to be used and their corresponding sizes and location, the way they are interconnected and the way they will be operated over their life time. It is also important to consider how they will be manufactured and dismantled or recycled at the end of their lifetime. The systems have therefore to be assessed with respect to different criteria like thermodynamic, economic or environmental. They have also to be assessed considering not only the existing technologies but also to show the potential role of the future technologies. It comes from this analysis a definition of the possible role of the energy policy as well as of the need for infrastructure development in particular considering multi-energy networks like electrical, gas or heat and information. For the engineers and the policymakers it is therefore important to define in a systematic way the most important competing system design scenarios and to define in a comprehensive way the performance indicators that will allow to compare them.

## Energy and resource efficiency

Industrial processes convert raw materials in products and by products. Solvents like water are typically used as a support for the conversion process. The driving force of the conversion is the energy typically in the form of work or heat. This energy is typically obtained by conversion processes that convert the process energy resources (e.g. fuel, electricity) into useful process energy forms. By the expression of first principle of thermodynamics mass and energy is conserved in the system. This means that the materials that is not leaving the system as a product will leave the system as a waste or has to be recycled. The same stands for the energy that will leave the system in form of waste (typically heat). The goal of the process system engineering approach is to better understand the mass and energy conversion in the system in order to maximise the conversion efficiency of raw materials and energy, maximising the reuse of production support materials and energy which in turns means minimizing the production of waste streams and limit the amount of waste heat. In this field, the industrial process and energy systems engineering group develops computer aided methods to realize the rational use of materials and energy in the industrial processes. The approach starts with an energy audit that aim at identifying the major energy drivers in the process and understanding the energy usage in the process(Muller 2007). For the analysis of the energy usage, the thermodynamic methods based on the exergy analysis and particularly useful to understand the quality of the energy usage in the process unit operations(Kotas 1985). The development of pinch analysis (Flower 1979) has been a major advance in the understanding of the heat usage and recovery in industrial processes. The method has later been extended to consider the combined heat and power integration(Townsend 1983) and to use mathematical programming techniques(Papoulias 1983). The group of prof Marechal in EPFL has further developed the concept by especially considering the mathematical programing methods for the combined integration of heat recovery and and energy conversion (Marechal 2003, Maréchal 1998). The integration of exergy analysis and process integration techniques has been studied by (Staine 1996) in EPFL followed by the integration of such concepts with mathematical programing techniques by (Maréchal 2005). The mathematical programming methods allows for a combined approach between materials and energy usage(Kermani 2014). This leads to a comprehensive approach of the resource and energy efficiency in industrial processes that has been applied successfully in different industrial sectors like the petro-chemical and the chemical industry (Kalitventzeff 2001, Pouransari 2014), the cement industry (Mian 2013), the food industry (Muller 2007), the milk industry (Becker 2012), the beer production(Marechal 2013), the steel, glass industry as well as the metal treatment industry. The level of energy savings goes from 45% to 90% with a high level of combined heat and power and therefore considerable impact on the $$CO_{2}$$ mitigation. It has to be mentioned that combining process integration techniques and exergy analysis naturally leads to the large scale integration of industrial clusters where the waste materials and waste heat are considered as resources for new processes integrated on the same site(Maréchal 1998)(Pouransari 2014). In particular, it allows to study the integration of heat pumps (Becker 2011) and of waste heat to electricity conversion processes (Bendig 2013).