Air conditioning with TBAB clathrate hydrate slurry as distribution fluid

Abstract

At the moment, the worldwide demand for air conditioning is rapidly growing, and it is expected to exceed the demand for space heating by the 2060s. However, traditional refrigerants such as CFCs and HCFCs are regulated or phased out by the Montreal and Kyoto protocol. Secondary loop refrigerant systems use less of these harmful refrigerants since they make use of a distribution fluid (for example water) as transport medium between the chiller and the coolers. The efficiency of these systems can be improved by using a phase change material as a secondary refrigerant.

Tetra-n-butylammonium bromide (TBAB) is a promising phase change material for air conditioning applications. The phase change can take places at a temperature up to 12.5 ºC, which allows for an increase in the evaporation temperature. Furthermore, due to its phase change, the TBAB slurry has an up to 4 times larger cooling capacity than chilled water. This can be utilized in order to reduce the flow rates in the system.

At his moment, the TU Delft has in collaboration with Hollander Techniek installed a small pilot air conditioning system in the sports hall 'De Jachtlust' located in Twello, the Netherlands. The system has a capacity of approximately 3.5 kW. It has a single 300 L storage tank of and it is equipped with sensors to monitor the performance of the system. In this study, an existing model of a secondary air conditioning system is improved and extended taking into account the design parameters of the installation in Twello. This model is validated using the experimental data from the actual system.

The simulations predict that using a 36.5 wt% TBAB solution increases the COP of the system from 2.96 to 4.00, while the energy consumption reduces by 24.8 %. This reduction is mainly due to a 30.5 % decrease in the power consumption of the compressor. At the same time, the generation side pump consumes 204 % more electricity due to adhesion of the produced crystals to the heat transfer surface. The performance of the TBAB can still be improved by lowering the initial TBAB fraction to 35.0 wt% or by further optimizing the control strategy for the crystal production.