Abstract
M.Ing.
The research in this thesis focuses on the theoretical thermal modelling and design of a
Latent Heat Storage system (LHS) for an absorption refrigeration machine. A shell-and-tube
latent heat storage exchanger retaining any excess solar thermal energy is selected. Here,
solar thermal energy supplied by a collector is transferred to and stored by the LHS. During
low insolation, stored thermal energy is transferred by a Heat Transfer Fluid (HTF) into the
generator, a component of an Ammonia Absorption Refrigerator (AAAR), to ensure efficiency
of the cooling cycle.
The shell-and-tube LHS contains Phase Change Material (PCM) which fills space outside the
tube heat exchangers. The HTF flowing through the tubes exchanges thermal energy with
the PCM. The selection of a suitable PCM for a LHS is based on several factors. A primary
criterion for an efficient, reliable storage unit is the correct melting point of the PCM at a
desired operating temperature of the heating application.
An analytical model describing both the freezing process in the PCM and increased HTF
temperature in the tube heat exchangers is investigated. The model is developed using
energy balance equations. It is solved in terms of dimensionless parameters. The thermal
resistance of the tube heat exchangers is considered for this model.
From the result of the analytical model, the design approach to size the LHS is provided and
the different steps are given in order to determine the volume, mass, number of tube heat
exchangers, inner and outer radius of the tube heat exchangers and other parameters of the
LHS.
The dimensions of LHS are given as a function of a storage period, PCM properties, HTF
properties, inner and outer radius of the tube heat exchangers, material of construction of the
tube heat exchangers and the nature of load on the heating process.
Simulations from the analytical model developed are provided for the output thermal
parameters of the storage system. These thermal parameters of the shell-and-tube latent
exchanger are given in terms of the HTF outlet temperature, the front solidification of the
PCM and the heat transfer rate during the solidification process of the PCM.
A case study to demonstrate the application of the design approach with respect to the size
shell-and-tube latent heat exchanger is provided.The integration of the tube heat
exchangers thermal conductivity in the modelling of the LHS resulted in an increase of 2% in
mass of the storage material compared to an analytical model neglecting the thermal
conductivity of the tube heat exchangers.
The results of the model developed compared well with the results obtained from other
analytical models at similar operating conditions.