Air-conditioning with TBAB clathrate hydrate slurry as secondary loop refrigerant

Abstract

The demand of energy increases steadily due to the growing global population and the increasing living standard of emerging markets. The increasing energy demand and reduction of greenhouse gasses require a significant increase of renewable energy sources and energy efficiency of buildings. Also, the increasing degree of renewable energy sources increase the demand to store excess energy and to bridge the gap between energy production and consumption. This thesis focusses on the possible energy reduction for air-conditioning systems by the application of thermal energy storage and phase change materials (PCMs).

Tetra-n-butylammonium bromide (TBAB) is a promising PCM for air-conditioning applications. PCMs store thermal energy in the form of latent heat. PCMs are also promising cold storage media because of the capacity to store energy at constant or near constant temperature. TBAB has a high solubility in water and hydrate crystals are formed in a TBAB aqueous solution at temperatures from 0-12.5 °C depending on the concentration of TBAB in the solution. The solid mass fraction of the slurry is limited to maintain the slurry pumpable.

A pilot system to control the temperature of a 144 m² space is installed. The system is equipped with multiple sensors to monitor the system. The air-conditioning system consists out of a primary loop, which uses the conventional refrigerant R134a, and a secondary loop, which uses a TBAB solution as distribution fluid. An initial mass fraction of 36.5 wt% TBAB is used, resulting in a phase equilibrium temperature of approximately 12.5 °C. The hydrate crystals are produced in the generator and stored in a storage vessel of 300 l. The capacity of the air-conditioning system is around 3.5 kW.

The pilot system is modelled with both water and a TBAB solution as secondary distribution fluid by improving and extending an existing model. The crystals are produced in the generator by evaporation of the primary refrigerant. The crystals are removed from the heat transfer surface by the friction of the flow. The crystal layer thickness and flow velocity in the generator are calculated by comparing the friction losses to experimental determined scrapping forces required to remove the crystals. Experimental data of the pilot plant are used to validate the model.

The model simulations of the air-conditioning system show a COP increase from 3.27 to 4.30 when a 36.5 wt% TBAB solution is used as a secondary distribution fluid instead of water. Simulation results show also an increased performance when the initial mass fraction of TBAB is lowered to 35 wt%. The COP of the system increased to 4.50 mainly due to the reduction of the energy consumption of the compressor. The force required to remove the crystals from the heat transfer surface lowers by lowering the initial mass fraction of TBAB. Therefore, the steady state crystal layer thickness in the generator reduces, increasing the heat transfer rate by lowering the thermal resistance. However, reducing the initial TBAB mass fraction also lowers the evaporation temperature of the primary refrigerant and the total cooling capacity of the slurry.