Horizontal wastewater heat recovery heat exchanger, a model

Abstract: The residential and service sector amounts to approximately 40 percent of Sweden’s entire energy demand. In which 90 percent of that is used by households and non-residential buildings. All in all about 80 TWh are used for heating and the provision of hot water in households and non-residential buildings. Since heating has always been such a large part of the energy consumption for buildings in Sweden, it is only natural that there have been several improvements along the way. There’s a new facility just installed last year in the building Pennfäktaren 11, a horizontal wastewater heat recovery heat exchanger. This thesis study will be focused on creating a TRNSYS model of a waste water heat exchanger, where the crucial parameters such as water flow rate, temperature, and more can be used as inputs to assess the technical performance of the heat exchanger. The model developed in TRNSYS can simulate the performance of a single heat exchanger unit, with a few input parameters needed. The model was developed by using measurement data from the facility in Stockholm to get realistic results depending on time and actual measurements. From the measured data, there were a few parameters that needed to be calculated, first off the mass flow rate of the waste water flow, this was done by an energy balance over the heat exchanger. Following the mass flow rate the cold water set point had to be determined, so that the heat recovered was not larger than the heat that could be utilized by the building. Since data was available from a single site, there was not much else to do than accept the data as true, there were some data points that had to be sorted out however, such as negative flow rates and flow rates much higher than should be possible. The finished model uses all the data from the measurements as well as the calculated values, it provided heat transfer rate along with the outgoing temperatures of both waste water and the preheated water. The first reference scenario provided 25,3 MWh of recovered energy, but the best scenario with an increased waste water temperature as well as increased flow rate it could provide a total of 47,2 MWh, almost twice the original value. To conclude the model seems to simulate a waste water heat exchanger well and returns feasible data. It should be possible to use the model to see if a building is a good “candidate” to install a waste water heat exchanger in.