How to defossilize Switzerland: CO2 and renewable energy for an autonomous Switzerland

The 12th Conference on Sustainable Development of Energy, Water and Environment Systems – SDEWES Conference, held in Dubrovnik in from October 4. – 8. 2017, was dedicated to the improvement and dissemination of knowledge on methods, policies and technologies for increasing the sustainability of development by decoupling growth from natural resources and replacing them with knowledge based.

François Maréchal gave a keynote entitled CO2 and renewable energy for autonomous cities.

Considering that 75 % of the population will live in urban areas in the future, energy consumption in cities represent a major challenge for the energy transition. The density of the urban systems introduces the need and the opportunity to share resources and equipment, therefore district heating solutions have to be considered. An innovative concept of district heating using carbon dioxide is proposed to supply energy services in cities. Based on the concept of exergy, the idea is to divide heat pumps into 2 parts. The first part is harvesting heat at the best places while the second part is adapting the temperature of the heat to its requirement. In order to transfer heat from one part to the other, the idea is to use CO2 as a district heating fluid distributed in liquid and gas form at a temperature around 15 to 20 °C. Heating needs are then supplied by condensing CO2 while cooling needs are supplied by evaporation, the temperature levels being adjusted using decentralised heat pumps. The energy balance is closed by exchanging heat between the district heating fluid and the environment accessing therefore attractive sources by centralised heat pumps. The advantage of such system is the supply of the heating/cooling services at the temperature it requires and the possible synergies between heating and cooling demands. The use of the district heating system allows indeed to connect the most attractive environmental sources (geothermal, lake, river, waste water, industrial waste heat,…) with the users in order to maximise the efficiency of the system. Using this type of technology, it has been demonstrated that heating and cooling the city center of a European city can be reduced by a factor 5 to 6 (i.e. 80% energy savings) while having a pay back period of around 5 to 7 years . The concept can be extended to integrate the renewable energy resources in particular solar photovoltaics (PV). One of the difficulty of the PV systems is the excess of electricity available in the summer that needs to be stored to supply services in the winter. The idea here is to electro-chemically convert excess of electricity into methane by combining electrolysis of water and the Sabatier reaction that converts CO2 and H2 into methane. This can be done in co-electrolysis concepts using solid oxide electrochemical membranes. Methane can then be stored in liquid form in the summer. In the winter time, the methane is converted in decentralised solid oxide fuel cells into electricity and heat while separating the CO2. The CO2 is collected in the district CO2 network at 50 bar and stored in the liquid form for the summer season when it will again be converted into methane. The produced electricity is used to supply the needs of the households, to drive the heat-pumps of the CO2 based district network and even electrical cars. It is therefore possible to develop cities that are supplied only by renewable sources. Furthermore, the methanation concept can also be applied to convert the waste biomass produced in the city into methane making the cities autonomous.

Biomass for Swiss Energy Future