Dynamic Modeling and Simulation of Two-phase Evaporators and Condensers in the context of a CO2 Based District Energy Network

A new type of district energy network has recently been proposed that is based on the use of CO2 as a heat transfer fluid. It uses the latent heat of vaporization, instead of sensible heat, to store and transfer heat. Previous studies have focused on demonstrating the potential of such a network in terms of energy efficiency and economy. In order to assess the technical feasibility of such a network, and to address some of the safety related issues, an analysis of the network’s dynamics is required. Such an analysis requires models to be developed for the network’s central plant, heating users’ substations, cooling users’ substations, as well as for the piping. Moreover these dynamic models should be developed in such a way that they can be calibrated with the help of a future experimental setup. In this context a dynamic model of a plate evaporator (also useable as a plate condenser) is presented, it is based on a one dimensional local formulation of the conservation laws for mass, momentum and energy. In this formulation, mass and energy in the components are allowed to vary, while non stationary terms in the momentum equation are neglected. On the water side, frictional pressure drops are computed with Churchill equation while heat transfer coefficient is computed with Gnielinski’s correlation. On the refrigerant side, a constant friction factor was implemented, while heat transfer coefficient is set to a reference value at reference conditions of mass flux and pressure. Variations of the heat transfer coefficient due to changes with respect to these two conditions are handled with exponents. These values are to be calibrated experimentally in a future work. The thermal capacity due to the mass of construction material was also accounted for in the model. In order to select an appropriate control scheme to be used in the experimental facility and, later on in a real network, the behavior of a cooling user substation including a control valve and an evaporator is discussed. A proportional integral controller showed its ability to stabilize the system to a desired degree of superheat at the evaporator outlet. Finally, perturbation rejection tests were made, and showed the ability of the system to operate under changing conditions of CO2 pressure and inlet enthalpy, as well as changes in the water mass flowrate, and inlet enthalpy.

Potential of refrigerant based district heating and cooling networks

Samuel Henchoz

Urban areas represent an ever increasing challenge in terms of energy use and environmental impact associated with it. Projections from the United Nation expect this number to reach 66% by 2050. In Europe 40% of the final energy consumption and 36% of the emissions of greenhouse gases are caused by buildings. The building sector is also one of the area where there is a great potential of reduction of the greenhouse gases emissions and of the dependence on fossil fuels. Among the measures that can help realise this potential, efficient energy conversion technologies supplying thermal energy services could play an important role. The present thesis aims at demonstrating the potential of a new type of district network, capable of delivering indistinctly cooling and heating services using a two-pipe network and in which the transfer of energy across the network is done by exploiting the evaporation/condensation of a refrigerant. The main research question is: "Is it possible to build and operate safe, reliable, energy efficient and economically profitable district heating and cooling networks that use a refrigerant as a transfer fluid?" The demonstration followsthree axes, the first being a thermoeconomic analysis. Thisanalysis focuses on a test case area in Genevaâs city centre where 5 variants of refrigerant based district heating and cooling networks, one cold water network and the mix of conversion technologies currently in use are compared on the basis of their energy and exergy performances and on the economic profitability. Considerations on economic uncertainty, safety and technical issues are also included in the analysis. The key findings are: â¢ All the variants of network can potentially reduce the final energy consumption of over 80% as compared to the current situation. â¢ All the variants of network have rather similar exergy efficiencies comprised between 39.5% and 45%. â¢ The most profitable variant uses CO2 as a transfer fluid and an open cycle CO2 heat pump at the central plant. It costs initially between 27 and 35 mio â¬ , reaches break-even in 4 to 6 years and the net present value after 40 years is comprised between 40 and 80 mio â¬. â¢ A cold water network is the second best option, although more expensive initially and thus less profitable, it has several advantages in terms of safety and availability of components. â¢ The CO2 variants exhibit a much better compactness than the cold water network. The second axis is the design, construction and testing of a lab scale refrigerant network. First a description of the design process and of the test facility is provided. It is followed by a presentation of the results of the test campaign. The tests aimed at demonstrating the practical feasibility of the concept, mostly by assessing the controllability of the network. Overall the good behaviour of the test facility and its ability to be smoothly and automatically controlled could be demonstrated, which further improved confidence in the practicality of the concept. The third axis is the development of dynamic models of simulation. These models are described in the present manuscript. They include, heat exchangers, pipes, pumps and valves. A short comparison between experimental and simulation results is also provided. The comparison between experiment and simulation showed that at their current stage of development the models cannot simulate accurately enough a refrigerant based network.

EPFL2016

Key energy and technological aspects of three innovative concepts of District energy networks

Daniel Favrat, Samuel Henchoz, François Maréchal

In urban areas, the use of thermal energy services is intensive and relatively heterogeneous by their nature, spatial distribution and temperature level required. In this study three concepts of district energy networks that could provide these services very efficiently to a specific area representative of mid Europe (Geneva) are compared. The focus is on the energy and technological aspects related to each of the proposed networks. 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 HFO refrigerant R1234yf to store and transfer heat across the network. The third concept is more conventional as it uses the sensible heat of liquid water, however with a small temperature spread (2 - 8 °C). The three proposed networks allow the waste heat emitted by the users requiring cooling to be collected, transferred and valorised 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 final energy (electricity) consumption required to supply the thermal services amounts to 10.87 GWh for the CO2 variant, 10.52 GWh for the water variant and 9.60 GWh for the R1234yf variant.

2015

Potential of refrigerant based district heating and cooling networks

Samuel Henchoz

EPFL2016