Específicos

High heat transfer coefficients can be achieved between an encapsulated PCM and a gas flow thanks to the high exchange surface. There are two different concepts: in packed beds, the PCM is not moving and is used only for heat energy storage; in moving beds, the encapsulated PCM is used for the transport and storage of the heat energy.

Powder beds are of 4 types: fixed, moving, bubbling and circulating fluidized beds. When a gas is passed through a bed of powders, the bed will remain fixed at very low gas flow rates. Since there is no particle movement, the heat transfer coefficient is mostly determined by the convection heat transfer of the gas flow only (order of magnitude < 20 to 50 W/m²K). When the drag force of the gas flow exceeds the bed weight per unit cross sectional area, the bed will fluidize and bubbles will occur. The bubbling activity will increase at increased gas flow rates. Due to the turbulent movement of the particles, induced by the bubble flow, the heat trtansfer coefficient will significantly increase, with measured values of 100 W/m²K for coarse particles, to 700 W/m²K for fine (~100 µm) particles. In circulating fluidized beds, obtained at a gas flow rate that carries all particles out of the bed, hence requiring a recycle to keep the bed inventory,

the heat transfer coefficient is again high due to the intensive particle mixing. Values between 150 to 500 W/m²K are achieved, as further demonstrated in Chapter 5.

4.4.1

Previous works in carried out on packed beds.

The following example of encapsulated PCMs packed bed is quoted several times in this thesis as a pioneer for heat waste recovery at very high temperature (Maruoka and Akiyama, 2003, 2002; Maruoka et al., 2002).

The steam reforming of methane is an essential endothermic reaction in the chemical and metallurgy industries, involving the direct reduction in the production of hydrogen and carbon monoxide. The hydrogen and carbon monoxide generated are thus made available to synthesize methanol. Figure 42 illustrates the system developed by Maruoka and Akiyama to recycle waste heat energy. A packed bed of Nickel coated beads of copper accumulates the excess heat from the LD converter in the steel production (Linz Donawitz process). The latent heat energy stored is further used to produce hydrogen and carbon monoxide.

Figure 42: Schematic diagram of the proposed process consisting of the LD converter and the PCM heat storage reactor (Maruoka and Akiyama, 2002).

The high temperature encapsulated PCMs are demonstrating their full potential with that particular application.

4.4.2

Previous studies on using fluidized beds.

The circulating fluidized bed (CFB) is increasingly used in chemical reactors and in physical gas-solid processes (e.g. drying). New concepts of thermal energy storage systems are being developed for concentrated solar energy capture/storage systems or industrial waste heat recovery. They use a fluidized bed as transfer and storage medium to replace thermal fluids or molten salts (Chen et al., 2005).

In the first case, an air-to-sand exchanger is installed in the receiver tower in order to minimize the losses. The sand is fluidized in the exchanger and air is passed through it. The exchange of heat between the air and the moving sand is excellent thanks to the fluidization of the sand. The air circulating reaches very high temperatures in the receiver, and carries the heat to the storage and the production unit. Such a system helps to improve the efficiency and storage rates of the plant (Figure 5 a). The typical set-up of Figure 5 b, applicable in solar towers and in waste heat recovery, applies a dense particle bed conveyed within a tube bundle to capture the heat at bed temperatures between 500 and 750 °C, thereafter using the stored heat in a bubbling fluidized bed steam boiler (European Union, 2011). A CFB shares many of its advantages with traditional bubbling fluidized beds (BFB), including temperature uniformity and excellent heat transfer. The continuous carry-over of particles implies solids’ collection and return equipment. In solar energy capture systems, solar heat will be captured at the outside tube wall, and subsequently transferred to the circulating solids.

At present, this specific solution is still in its development stages and several problems are still, as yet, to be resolved (Medrano et al., 2010).

4.4.3

Encapsulated PCM in fluidised beds.

A new system for the storage of low temperature thermal solar energy, which consists of a bubbling fluidized bed filled with granular phase change materials, has been undergoing tests in Madrid since 2010. In this system, the main advantages of both technologies are combined: the high heat transfer coefficients of bubbling fluidized beds and the high thermal energy storage capacity in the reduced volume of phase change materials. The project is focused on storage at specifically low temperatures.

Such a technology combined with SiC encapsulated molten salt PCM would be capable of combining the benefits of both technologies. The thermal conductivity of the particles, which are the result of their ceramic coating, would optimise the heat exchange. As in HT concrete, the latent heat would increase the amount of energy stored. The life cycle of the system and aging of the particles would require a careful study. If the shells of the PCM were to break or leak, the result would be leakage of the molten salt from the shell and consequent damage to the fluidised bed, by corrosion, defluidization and possible sintering of the bed material if low melting eutectics can be formed.