Aymen Achich
Geothermal energy is heat energy generated and stored in the Earth. With the awareness of the society about environmental issues, geothermal power is developing worldwide. It is cost-effective, reliable, sustainable, and environmentally friendly but has historically been extracted from the depth of the earth. Recent technological advances permits to use this energy from the shallow depth of the earth. Thanks to infrastructure foundations combined with heat pump technology, we are able to use this geothermal shallow energy. It have dramatically expanded the range and size of viable resources especially for applications such as home heating, cooling thus opening a potential for widespread exploitation. The renewable soil energy can be also used in transportation infrastructures such as the de-icing of bridges. Indeed, the icing of bridge decks in the winter is a real problem that potentially creates vehicules accidents. For example, in United states, more than 5’500’000 vehicle crashes occur per year and 23% of them are weather-related (data derived from Federal Highway Administration website). Instead of performing a mechanical de-icing combined with spraying de-icing salt, we can use geothermal energy to proceed to the de-icing by heating the bridge slab. This energy is extracted thanks to the foundations (piles) of the bridge. These foundations are used as heat exchangers to extract the heat from the ground. Snow melting system based on energy piles is a technology that combines the structural function as a foundation with geothermal energy extraction. This technology was already proposed in 1990s in Japan [35] but it is actually nearly applied. The concept is simple: during snowfall periods, hot fluid is injected inside the bridge’s deck through the pipes placed in the bridge’s slab. The heat is transmitted to the pavement and the ice melts down. The hot fluid is obtained thanks to the mechanical work of heat pumps which enhances the power extracted from the energy piles. This method overcomes the problems of prevalent methods (mechanical de-icing combined with spraying de-icing salt) which are associated with extensive corrosion of transportation infrastructure, huge amount of material consumption, and negative environmental impacts. In this technical report, we will see why such a melting system could be a good altenative to traditional de-icing methods ? How can we define the applicability and performance of such a snow melting system ? How this system could be designed and implemented in a real case ? What is the structural behaviour of a bridge deck when the system operates ? In the first chapter, we will explain the different methods used for bridge de-icing and we will focus in particular on the concept of deck de-icing using energy piles system explaining the physical phenomena of heat transfer involved in this system. In chapter 2, we will define a methodology that could determine the energy demand of a bridge deck to remove the ice in different weather conditions. Furthermore, we will investigate to what corresponds this required energy in terms of the fluid temperature injected below the pavement and in terms of the required number of energy piles. We will study the feasability of such a system through verifying whether the heat provided by energy piles will be sufficient to satisfy the energy demand. In the chapter 3, we will study a real case bridge located in the city of Jiangyin (China) where Hohai University geotechnical group is supervising the project. We will decribe first the main features of the project. Since I had the opportunity to assist to the construction phases of the bridge, we will explain in details the construction phases of the project (bridge+de-icing system). Then we will determine the thermal needs of the bridge deck considering the project data. We will conclude this report by dedicating a chapter for numerical simulations done using Comsol Multiphysics software. As a first part, a parametric analysis was performed considering the main elements forming the melting system of the bridge deck in order to understand more its thermal performance. In the next part of this chapter, a set of numerical simulations were done to compare different possible pipe layouts that can be installed on Jiangyin bridge deck. Finally, based on this real case bridge deck, we will concentrate on its structural behaviour during the de-icing process. Since the bridge was in construction during this study, iterative operations were done to optimise the efficiency of the system and predict the thermal and structural response of the bridge deck.
2018
