Abstract
Due to the rising need of energy, mankind is facing a great challenge in finding ways to reduce the energy demand. A big share of the total consumption is due to electricity and heating supply for buildings. This share also comes along with a wide field of opportunities to reduce the energy need. One of the fields is mechanical ventilation that becomes more and more important in order to overcome the lack of air changes due to the movement to air‐tighter buildings for insulation improvement purposes. Two different approaches to optimise ventilation systems are discussed in the following thesis: reduction of energy need by the use of liquid desiccant systems (LDS) and recovery of energy by the installation of heat recovery (HR) devices. The potential of the saving is depending on the devices choice as well as the applied control strategy. In order to understand this dependency, it is necessary to study the performance of the device for different conditions. To do so, as a further objective of this study was to establish a climate based comparison of the performance. Where the hourly weather data for 66 cities across Europe served as base for the calculations. These cities are located within the nine major Köppen‐Geiger climate classes (chapter 2). The performance of HR devices has been investigated taking both the influence of the indoor and outdoor air into account (chapter 3). A humidity source was introduced by making use of the definition of the specific latent load (Lazzarin et al., 2000) to assess the impact of excessive humidity and its control on the energy savings in the case of sensible (SHR) and total heat recovery (THR). The control strategies consider by‐passing the SHR or effectiveness‐partialisation for THR as well as – in order to enhance the saving potential – a temporarily higher indoor relative humidity. Further, an economic analysis has been performed including the cost savings, the payback time as well as the present value of the SHR and THR device respectively. The Köppen‐Geiger climate classification also served as tool to map the energy savings across Europe. The cost savings have been determined by taking the price for electricity and natural gas of chosen states into account. These tools and considerations shall help in decision making for the design process of a mechanical ventilation system and further to optimise the control strategy choice. The steady state performance of a LDS using LiCl‐ and KCOOH‐solution has been investigated by simulation (chapter 3). The performance of a LDS is described by the mass and heat transfer between the air and the liquid desiccant. The driving force of the mass transfer, meaning the vapour transfer, between the two fluids underlies the difference in vapour pressure. The given model used for the simulation consists of a system of partly nonlinear equations defining the heat and mass transfer that have been solved by the Newton and Broyden methods with the Matlab Software. It has been used to study the behaviour of the air and liquid temperature and humidity ratio through the packing height in the case of dehumidification and humidification. A parametric study has been conducted in order to investigate both the impact of the packing characteristics and the inlet condition of the liquid desiccant. Further, a heat exchanger‐like twin‐tower LDS has been introduced and its potential to reduce the energy demand has been studied and compared with the performance of the SHR and THR devices. A short chapter (5) is dedicated to the microscale analysis of the interface condition of a liquid desiccant by Molecular Dynamic simulation. First results as well as the potential and limitations of this method are discussed. In order to validate the theoretical consideration by experiments, the installation of a setup was started in the Building Physics Laboratory of the Free University of Bolzano. The installed facility consists of two separated duct systems each equipped with an AHU for the adjustment of the air condition in steady and dynamic state. Even though both LDS and HR devices can be tested, the start will be with the LDS and part of this work was its design, which is explained in chapter 6.