Enthalpie de changement d'état

En thermodynamique, l'enthalpie de changement d'état (anciennement chaleur latente de changement d'état) d'un corps pur est par définition la variation d'enthalpie qui accompagne un changement d'état du corps rapportée à la quantité de matière mise en jeu lors de cette transformation. Par exemple pour le passage de l'état liquide à l'état de vapeur on parlera denthalpie de vaporisation. L'enthalpie de changement d'état, notée \Delta_{1 \to 2} H, est exprimée en J/mol dans le Système international d'unités (unités SI), mais elle peut aussi être rapportée à l'unité de masse mise en jeu lors de la transformation : elle est alors exprimée en J/kg et est dite massique ou spécifique. À une température T donnée, l'entropie de changement d'état est égale à : \Delta_{1 \to 2} S = {\Delta_{1 \to 2} H \over T}. Elle s'exprime en J/(K·mol) en unités SI ou encore en J/(K·kg) pour l'entropie de changement d'état spécifique. L'enthalpie de changement d'état é

Key Energy and Technological Aspects of Three Innovative Concepts of District Energy Networks

Patrick Chatelan, Daniel Favrat, Samuel Henchoz, François Maréchal

Energy needs in urban areas are heterogeneous by their nature, spatial distribution and temperature level required. Three concepts of district energy networks that could provide the energy services efficiently to a city centre are compared. The focus is on the energy and technological aspects. These networks are characterized by similar temperature levels; between 9.5 °C and 18 °C, rely on free cooling for most of the cooling services and use a combination of centralized and decentralized heat pumps to provide the heating services. Two of these concepts exploit the latent heat of evaporation/condensation of CO2 and of the refrigerant R1234yf to store and transfer heat. The third concept is more conventional and uses the sensible heat of liquid water, however with a small temperature spread. The proposed networks allow the waste heat emitted by the users requiring cooling to be transferred and valourised by the users requiring heating, thus reducing the load on the central plant. For the area considered, where the annual heating and cooling demand are 53.1 GWh and 49.4 GWh respectively, the annual electricity required to supply the thermal services amounts to 10.87 GWh for CO2, 10.52 GWh for water and 9.60 GWh for R1234yf. © 2016 Elsevier Ltd. All rights reserved.

2016

Performance and Profitability Perspectives of a CO2 based District Energy Network in Geneva’s City Center

Daniel Favrat, Samuel Henchoz, Céline Weber

A new type of district energy network capable of providing simultaneously heating and cooling is being investigated. It is based on the use of CO2 as a heat transfer fluid by taking advantage of the latent heat of vaporization, to store and transfer heat across the network. The goal of the present study is to determine the performance of a CO2 network when applied to a real urban area. It focuses first on determining the requirements for the various thermal energy services for a part of Geneva’s city centre. The energy consumption is first computed for the energy conversion technologies now in place in this area - namely fuel boilers and vapour compression chillers. Then the new energy consumption is computed if a CO2 network were used instead of the existing technology. Finally a profitability analysis of the CO2 network variant is done accounting for investment, energy purchasing, equipments replacement, operation, and maintenance costs. For an interest rate of 6% and a price of the delivered heating/cooling energy at 0.13 CHF per kWh, a net present value of 82.8 million CHF after 40 years is achieved, while the break-even is reached after 5 years of operation.

2012

Multi-configuration evaluation of a megajoule-scale high-temperature latent thermal test-bed

Maurizio Barbato, Selmar Rudolf Binder, Sophia Haussener, Nithin Mallya, Alberto Ortona, Clemens Gregor Suter

Thermal energy storage via latent heat provides an energy dense solution and, during discharge, a hot stream at a well-defined temperature. Utilizing metal phase change materials (PCMs) and high-temperature operation might enable fast (dis)charging and application in previously unexplored areas (e.g. high-temperature processing industry). But detailed characterization and design guidelines of high-temperature metal latent heat storage systems with tailored characteristics are missing. Here, we introduce a combined experimental-numerical characterization and design tool for high-temperature heat storage units. We apply it to a megajoule-scale test-bed operated with steel encapsulated Al68.5Cu26.5Si5 (weight%) (melting temperature 791 K) as the PCM and air as heat transfer fluid (HTF) for charging and discharging. Thermocouples positioned at multiple locations throughout the storage unit are used to characterize the performance and heterogeneity. For the inaugural experimental campaign, three encapsulated tubes with a total of 1.83 kg PCM were vertically arranged in one of the modular stacks and several multi-hour charging/discharging cycles were performed with the HTF heated up to 30 - 70 K above and below the PCM melting point at a flow rate of 1500 lmin(-1). Specifically designed open-cell cellular Si-SiC lattices were wrapped around the PCM tubes to enhance the heat transfer. The experimental results showed a near-isothermal (+/- 10 K) exit temperature discharge maintained for 0.39 2.67 h. Temperature variations throughout the test-bed indicated that test-bed design is important to ensure homogeneous usage of the PCM and controlled phase change. A quasi-1D lumped parameter multi-mode heat transfer model was developed to quantify the thermal stratification within the PCM, heat losses, and phase change characteristics in the thermal storage, and provided explanation and quantification for the experimentally observed heterogeneities. The model was used to extrapolate the heat storage characteristics to larger scales: a 16.84 MJ and 50.51 MJ latent heat storage with the modular stacks filled with 25.7 kg (42 encapsulated tubes) and 77 kg (126 encapsulated tubes) of the PCM, respectively. The coupled experimental- numerical approach allowed for model validation, explanation of experimental variation, and to provide general design guidelines for practically relevant latent high-temperature (metal) thermal energy storage.

PERGAMON-ELSEVIER SCIENCE LTD2022

Multi-configuration evaluation of a megajoule-scale high-temperature latent thermal test-bed

Maurizio Barbato, Selmar Rudolf Binder, Sophia Haussener, Nithin Mallya, Alberto Ortona, Clemens Gregor Suter

PERGAMON-ELSEVIER SCIENCE LTD2022