IntegrCiTy - D4.2 Report - Method towards key bottlenecks identification

The increasing use of renewable energy is a deep going trend [1] mainly supported by the sustained annual growth rate of solar photovoltaic (+45.5%), wind power (+24.0%) and Biogases (+12.8%) (Figure 1). In this context, decentralized power generation and heat pumps technologies are expected to play an increasing role [2]. This evolution is going to increase the stress on the electricity and gas distribution network while pushing the development of district heating and cooling (DHC) networks. At the grid level, the uncertainty in energy generation from renewable energy, the trend towards decentralisation and the emergence of new energy prosumers are going to increase bi‐directional energy interconnections [3], therefore challenging the energy networks to balance supply and demand. Therefore, there is a need for a method to identify actions and opportunities for the design and retrofit of future resilient urban energy systems. The IntegrCiTy project demonstrates the use of co‐simulation techniques to support the growing penetration of renewables systems through the interconnection of energy conversion technologies with multi‐energy urban networks supplying natural gas, electricity, heating and cooling. This report presents a method using co‐simulation models to identify and characterize the key bottlenecks (such as pipe sizes, heat exchangers or limiting transformers) of urban energy system from a multi‐energy perspective. Bottlenecks are defined and characterized at the level of the electricity, gas and heating/cooling distribution networks in Chapter 2. In Chapter 3, the selection of extreme operating conditions are presented to identify the bottleneck with a minimum set of operating points. Based on the definition of the bottlenecks, a stochastic hosting capacity approach is proposed in Chapter 4 to evaluate the penetration of renewable energy technologies resulting from the capacity limit of the existing infrastructure. Finally, the approach is demonstrated with the identification of the bottlenecks in the photovoltaic (PV) conversion of solar energy on the roofs of a district.

Optimisation of Urban Form by the Evaluation of the Solar Potential

Marylène Montavon

What will the cities of tomorrow look like? This is a crucial question because in the four and a half billion years of its existence, the Earth has never been so threatened, as it is today. Since 2008 half of the world's population has lived in towns – more than 70% in rich nations. Three quarters of CO2 emissions are produced in towns, 75% of the world's energy is consumed in towns. The battle for our climate, for the continuation of our species and all other life, will be largely waged in cities. Will it be possible to create the ideal ecological town? The objective of this study is to compare actual and theoretical urban forms in order to explore the diverse effects of daylighting and solar potential on densely built-up sites. Indeed, the search for ideal solar orientations has been a persistent problem from the 19th century until the beginning of the 20th. Even today, for all our scientific knowledge and empirical experience of solar energy, making the correct choice has still not crossed the boundary from an enigmatic, tricky art into the field of an exact science. As a first step, six existing urban sites in Switzerland and Brazil, as well as two projects in the United Kingdom, a few generic models and a utopian city, were modelled and analyzed. The goal was not merely the construction of new, exemplary districts but also the transformation of existing towns. The findings have been used to assess the potential of façades and roofs located in urban areas for active and passive solar heating, photovoltaic electricity generation and daylighting. The results obtained from these different case studies revealed large variations of the potential for solar energy collection on the buildings' façades and roofs. Ideally, these investigations are expected to help architects and town planners understand how to optimise solar collection on buildings design. In addition, a study was made of the Contemporary City of Three Million Inhabitants created by Le Corbusier. According to the latter, the Contemporary City could increase the urban capacity and at the same time improve the urban environment and the efficiency of the city. Therefore the present study extracted the solar potential of this utopian city in order to assess Le Corbusier's propositions. Since 1922, Le Corbusier's studies have influenced architects and town-planners in their choice of orientations and urban forms: it was therefore crucial to discover if this reliance on Le Corbusier was justified. Solar irradiation and illuminance values obtained through numerical simulations form the core part of the method. The method adopted for the entire set of computer simulations consisted in modelling — via a computer-aided drawing piece of software, namely AutoCAD — the digital model of each building or group of buildings prior to exporting them to an urban performance analysis software (PPF/RADIANCE software package). Spatial distributions of solar irradiation and daylight fluxes over the overall building façades and roofs were calculated using ray-tracing simulation techniques to determine the appropriate placement of different solar technologies (passive and active solar, photovoltaic and daylighting). Performance indicators were used to assess the solar utilisation potential of the urban sites (e.g. statistics of sky view factors and daylight factors) in order to determine the optimal solar strategies for a given urban context. In addition, various works and sun-related recommendations that have appeared mainly since the end of the nineteenth century, together with the various scientific models and software tools pertaining to this topic, are presented in this study.

EPFL2010

Energy integration of CO2 networks and Power to Gas for emerging energy autonomous cities in Europe

Luc Girardin, François Maréchal, Raluca-Ancuta Suciu

The concept of urban CO2 networks has been developed to deploy heat pump based district heating and cooling systems in dense urban areas. The use of the CO2 phase change reduces the cost of the heat distribution while allowing to recover waste heat that is typically rejected to the environment. The use of heat pumps to harvest heat from the environment and to supply heat to buildings allows one to propose district systems with COP as high as 6. Heat pumps can use the electricity produced by photovoltaics already providing up to 60 % of the total consumption. This paper studies the integration of fuel cell based power to gas for the seasonal storage of the excess electricity produced in the summer by PV panels. The methane stored in liquid form is used in the winter to balance the electrical needs by fuel cell based co-generation, making therefore the city 100% supplied by renewable energy. The present work evaluates the integration of CO2 district energy network including power to gas systems on a compact urban block considering heating, cooling, electricity, e-mobility and waste management for different European climatic zones. In order to reach fully autonomous blocks using solar PV and municipal and industrial waste heat, a PV area of 10 to 35 m2/cap would be needed. The rooftop area available appears to be sufficient in areas like Southern Europe, while more area or alternative renewable sources such as wind or hydro are needed for other climatic zones. Regarding the economic feasibility of the system, the results show that an investment of 900 to 1300 euro/cap would be needed, with a payback time between 11 and 14 years, depending on the different climate zones in Europe.

2018

Conventional and advanced district energy systems

Daniel Favrat, Céline Weber

District energy systems have the potential to decrease the CO2 emissions linked to energy services (heating, hot water, cooling and electricity), thanks to the implementation of large polygeneration energy conversion technologies, connected to a group of buildings over a network. To transfer the energy from the large polygeneration energy conversion technologies to the users, conventional district energy systems use water as energy transfer medium with often two independent supply and return piping systems for heat and cold. However, sharing energy or interacting with decentralized heat pump units often results in relatively large heat transfer exergy losses due to the large temperature differences that are economically required from the water network. Using refrigerants as a district heating or cooling fluid at an intermediate temperature could alleviate some of these drawbacks. Because of the environmental concerns about conventional refrigerants, CO2, which is a natural refrigerant, used under its critical point, could be an interesting candidate. Pipe sizing of a multiservices superstructure, based on a two pipe CO2 network at 18°C is compared with a standard 4 pipes water network for heating and cooling.

Universita degli Studi di Padova2007

Optimisation of Urban Form by the Evaluation of the Solar Potential

Marylène Montavon

EPFL2010