The kinetics and mechanism of the sorption reaction between water vapour and silica gel have been investigated in the pressure range 1–50 Torr using thermogravimetry under controlled temperature and water-vapour pressure. Measurements at equilibrium were carried out in order to determine the pertinent equilibrium parameters and the extent of hysteresis. The influence of particle radius and temperature on the kinetics in the whole of the pressure range investigated are well described by theoretical curves fitted to experimental data by taking into account simultaneous mass- and heat-transfer. Calculations yield a diffusivity of 8 × 10–10 m2 s–1. The sorption kinetics in the pressure range 15–40 Torr are shown to be controlled by heat transfer.
An analysis of sensible and latent heat storage units is performed in both energy and exergy terms. The stores are compared, through variation of design parameters for both charging and discharging in order to find exergy extrema, i.e. operating conditions that maximize the exergy yield in the transfer of heat to and from the stores. Reference is made to the stratified sensible heat store.
The overall conversion efficiency for the thermal exergy supplied by the heat source to the heat load via the heat store is of the order of 10% at NTU = 1. The stratified store is the most efficient at high NTU values.
For the charging operation there exists exergy extrema for both the well-mixed sensible store and the PCM store. An energy analysis of the charging of the PCM store shows that the transition temperature should be low. From an exergetic point of view the transition temperature should be the geometric mean of the charging and the initial temperatures, if the charging temperature is fixed. When the end temperature of a store in a changing operation is fixed the transition temperature should be low. Charging with a constant heating rate is advantageous if the transition temperature is low. No optimal stopping time can be recognized for the discharging operation. The merit of a high exergy content is that of a high thermal power during discharging. This favours a PCM store with a high transition temperature.
A systematic study of the thermal conductivity of beds of moist silica gel is presented. The influence of porosity, water content, total gas pressure and temperature is determined through measurements under transient conditions with the transient hot-strip (THS) method and under static conditions in a bench-scale reactor. The predictions of the effective thermal conductivity of the beds from four different simple models (Russell, geometric mean value, unit-cell model and stochastic model) agree reasonably well with the experimental results. The unit-cell model is extended in order to account for the water sorbed in the micropores and describes satisfactorily the dependency of the effective thermal conductivity on the water content.
A total cost accounting approach was used to analyse the suitability of copper and aluminium as winding material for transformers, using available data from the Ecoinvent database. It could be concluded that the use of recycled metal is a necessary requisite for sustainability. Using cost data for energy and materials and reasonable assumptions about costs for labour and interest for the metal supplier and the product manufacturer, the copper alternative turns out to be the better choice, especially when the expected increase in the prices of energy, copper, and aluminium during life cycle is taken into account.
When considering environmental cost, useful indicators are those that can be expressed in cost terms. With the Ecoindicator 99 indicator as the basis for estimating environmental cost, the aluminium alternative is better than the copper alternative. However, the contribution of the environmental cost to the total cost has minor importance when compared with the effect you get from the negative cost contribution from the end-of-life phase. Therefore, the copper alternative is the better choice in terms of least total cost in the application considered.
From the study it could also be concluded that the total cost accounting approach would be a valuable design tool, when comparing two design alternatives of a product functional unit to decide which of the two is the more favourable from a sustainability point of view.