Dynamic optimization of heat exchanger using entropy generation minimization

International audienceIn order to reduce fossil fuel consumption for heat and cold generation, different lowtemperature heat sources have been considered for heat pump operations such as sewagesystems, sea water, ground water and drinking water. Raw water systems can also beused as a heat source for heat pump, heat sink for chiller, or for direct cooling. These systemsrepresents a high thermal potential for some clients, according to their activity. In this study,the thermal potential of a raw water system (made up of 5000 km of piping and carrying 200million m3 annually in the south of France) is investigated. To this day, this source has notbeen used. The main problem is the optimisation of the equipment sizing according to thetemporal variability of water flow, temperature and heat (or cold) demand. The dynamicbehaviour of heat exchanger between network and user is the key parameter to enhance theperformance of the system.For the raw water source, temperature, flow, and heat demand are highly time dependent.The method is based on the minimization of entropy generation in the heat exchangersbetween the water pipes and the users. The optimisation of the sizing and the location indicatewhere to install direct heat exchange systems. In order to calculate entropy generation in theheat exchanger, a dynamic model of a simple heat exchanger (concentric tube) is developed.It determines the pressure and temperature profile along the heat exchanger according to time.Temperature is calculated for water and for the heat exchanger partition wall. By integratingthe terms of entropy generation due to temperature difference and to pressure drop along theheat exchanger, the value of total entropy generation is obtained under transient operation.Total entropy generation is used as an objective function to optimize heat exchanger geometry(length and diameters) and control mechanism.The method has been used to model direct cooling of a datacenter using a raw waternetwork as a heat sink. Optimization is done considering a year of operation; seasonal anddaily fluctuations for water flow and temperature are considered. Results show that entropygain is important when the heat exchanger optimal size is chosen. The use of the raw waternetwork connected to reversible heat pump for building heating and cooling have also beenstudied and achieves high gain compared to air-source heat pump