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.