dc.description.abstract |
The wide fluctuations that occur in the aggregate electrical demand of a generating
utility are punitive with respect to total system efficiency. Demand side management
techniques have been applied to reduce such fluctuations including the conversion of
electrical energy to thermal energy during periods of low demand for use during peak
demand periods. For thermal processes requiring energy above ambient temperature it
is feasible to use sensible heat due to the existence of stable storage mediums and
efficient methods of heating at the high temperatures required. However where energy
is required below ambient temperatures, efficiency of cooling limits the use of sensible
heat, hence latent heat storage has been adopted. Conventional cold storage systems use
ice banks to store cooling energy at 0°C in order to capture the high latent heat of
fusion of water. The rate of discharge for such stores is limited by thermal resistance in
the store and the thermal capacity of secondary coolants (such as glycol solutions).
This investigated the use of hydrophilic materials to overcome the limitations of
current cold-storage technology. Such materials have the capacity to absorb and retain
up to 95% by mass of water (or other aqueous solutions) regardless of how the
materials is subdivided. Furthermore the thermal properties of the polymers in their
hydrated state resemble those of the free hydration fluid, including any phase
transitions. By supporting the hydrated materials in a non-freeing, non-aqueous fluid
the resultant mixture provides a medium for cold storage that can be pumped and used
at the point of load, and is not limited by the thermal resistance of an encapsulating
material.
Three aspects concerning the utilisation of hydrophilic materials for thermal
engineering applications have been investigated; (i) the physical properties of the
materials
in their hydrated state, (ii) methods of fluidising material in a high density
store, and (iii) the heat transfer properties of hydrophilic based slurries while
undergoing phase transition.
Material tests have shown that currently available hydrophilic materials have thermo-
physical properties that depend principally upon the hydrating fluid, regardless of
particle size, and are stable over long periods (>3years). Suitable hydration fluids can
lower the temperature of the phase transition thus extending their potential as storage
mediums beyond those of ice-based technologies. Novel materials, of very high water
content (95%) have been produced and investigated. These appear to be very suitable
for thermal storage because they increase the maximum achievable energy densities of
a fluidised storage system and potentially reduce cost.
A number of thermal storage devices to utilise hydrophilic based slurries have been
designed and evaluated. The resultant devices has been shown to provide a means of
taking hydrophilic materials to, and from, a packed bed and feeding them at a
controlled rate into a fluid stream. The thermal charge/discharge rates of such a device
are limited only by the choice of external heat exchange systems. An experimental apparatus has been designed to investigate the effects of phase change
particles on the heat transfer properties of flowing mixtures. The results have shown
that (i) at temperatures above the phase transition temperature the presence of the
particles causes an increase in the measured heat transfer coefficient for concentrations
above 10% by volume, (ii) there is a significant interaction of particles at the heat
transfer surface, and (iii) that under high flow rate conditions, with phase change
occurring, heat transfer coefficients are considerably enhanced (ie 80%) above those of
the support fluid when used alone or with non-active particles.
Further work is recommended to extend this study to produce an engineering prototype
storage system for trial evaluation. |
en_UK |