Donald James Highgate

Electrolyser-based energy management : A means for optimising the exploitation of variable renewable-energy resources in stand-alone applications

Abstract Electrolyser-based energy management (EBM) offers a versatile means for optimising the process of harnessing energy supplies derived from variable and/or intermittent renewable resources, e.g. solar (photo-voltaic), wind, wave and tidal. In general, EBM systems consist of an electrolyser, water and gas (hydrogen and, optimally, oxygen) storage and management systems and a means of (re-) generating electricity, e.g. a fuel cell. Such systems achieve their management via energy conversion and storage, this operational principle being referred to as electricity supply-and-demand management (ESDM). Implementation of this principle offers significant advantages in the utilisation of variable and/or intermittent renewable resources, as it permits electricity generated during periods of high-availability/low-demand to be “time-shifted” for subsequent re-supply during periods of low-availability/high-demand. Furthermore, EBM systems have the important advantage over other ESDM systems that the stored form of energy is readily utilisable as a pollution-free gas supply for thermal end-uses. This reconversion route significantly enhances the overall energy-conversion efficiency. Electrolyser and fuel cells based upon proton-exchange membrane technologies are preferred because these afford considerable operational advantages over any alternatives. In this paper these advantages are expanded upon and preliminary data based on these ideas are presented.

Electrolyser-based electricity management

A novel means for managing electricity, or energy supplies derived from electricity, is proposed. This is termed electrolyser-based management (EBM) and involves energy storage. The energy is stored as the chemical potential-energy of the hydrogen/oxygen-water reaction, with the input energy-conversion being performed by an electrolyser and subsequent regeneration to electricity via, preferably, a fuel cell. Proportions of the stored hydrogen and oxygen may be employed for combustion purposes, depending upon the application. Such a system has a unique energy-management capability in being able to provide a clean electricity-derived gas supply in addition to a managed electricity output. Furthermore, EBM systems, such as these, offer considerable scope for what can be broadly termed [`]hydrogen economy applications.

Anomalous [`]Freezing' of water in hydrophilic polymeric structures

Experiments, intended to establish the use of hydrophilic polymeric materials as vehicles for water in energy-storage devices, have revealed two anomalous effects during the freezing/melting phase-changes: 1. (1) the presence of a large difference between the temperatures at which latent heat is taken in, and at which the energy is evolved, i.e. a hysteresis effect in the phase changes exceeding 20°C under some circumstances; and 2. (2) a wide variation in the magnitude of the thermal energy stored per unit mass of the hydrated sample (i.e. of the apparent latent heat of the absorbed water), which varies with the type of hydrophilic polymeric material used, and depends in particular upon the [`]free moisture content of the structure. These effects suggest that the hydrophilic polymeric structure inhibits the normal freezing process, either by mechanically interfering with the formation of a conventional ice-crystal structure, or by chemically immobilising some of the water molecules present in the system by transiently bonding them to hydrophilic sites within the polymer.

Heat transfers through mechanically-stimulated particle beds

A particle bed can be effectively [`]fluidised by means of a vibrating diaphragm. Such beds of vibrating particles (whether in nominally-still air or even in a vacuum) exhibit high thermal-conductances, which are comparable with those of air-fludised beds. The mechanically-stimulated beds have potential applications as thermal switches.

Fluidised beds as ‘coolth’ stores

High energy-density thermal storage at low temperatures is feasible using phase-change materials. Thus an energy-storage device has been conceived which uses water-absorbing cross-linked polymers, known as hydrophilic materials. These were produced in the form of particles, which can be hydrated. Experiments have been carried out in which hydrophilic particles were fluidised in oil, using concentrations of hydrated particles up to 40% by weight. Energy storage, regularly well in excess of 50% of the theoretical capacity, has been achieved, although, for the maximum value, it was necessary to reduce the mixture temperature to about -8° C.

Electronics cooling using a simple gas-fluidised bed

The excellent prospects for the use of fluidised beds for the cooling of high packing-density electronic systems are considered.

Cheap effective thermal solar-energy collectors

A light-weight flexible solar-collector, with a wavelength-selective absorption surface and an insolation-transparent thermal-insulation protector for its aperture, was built and tested. Its cheapness and high performance, relative to a conventional flat-plate solar-collector, provide a prima-facie case for the more widespread adoption of its design.

Developments in fluidised-bed technologies

Conventional mobile-particle beds have been developed in order to store heat (or cold) for satisfying industrial and/or air-conditioning requirements by the use of thermally-active particles in which the principal storage mechanism is latent heat. Also fluidisation has been achieved, both in air and in high vacua, using ultrasonic stimulation. This phenomenon can be applied to the development of a controllable thermal-resistance link, as well as for temperature restraint, e.g. for cooling electronic systems in space environments.

Fluidised-bed combustion of bamboo-like cane

Fluidised-bed combustors achieve significant performance-benefits when burning conventional fuels, yet relatively little attention has been devoted to the use in them of natural bio-fuels. Such materials, as harvested, are characterised by highly-anisotropic shapes, low densities, and large moisture-contents. In the present investigation, the problems associated with the use of a bio-fuel, namely low-density hollow [`]bamboo-like cane have been assessed. It is concluded that appropriate bed and baffle designs will allow the cane to be entrained in the fluidising bed and to remain immersed therein for periods sufficient to enable complete combustion of the material to occur: no more elaborate pre-treatment than simply chopping the cane into short ( ~ 40 mm) lengths is necessary. Two operating processes have been examined--one in which the energy of the fuel is released within the bed primarily as heat, and a second in which the bed acts as a gasifier, producing both heat as well as a combustible gas for subsequent use.