Heat and mass transfer

 

Thermal energy storage with phase change material

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 is being tested.

With this system, we combine the main advantages of both technologies: the high heat transfer coefficients of bubbling fluidized beds and the high thermal energy storage capacity in a reduced volume of phase change materials.

 
 

Simulation of heat and mass transfer in absorption refrigeration and other gas-liquid mixtures

Absorption chillers and heat pumps constitute a promising technology for energy-saving policies and have received a growing attention over the last decades. These machines allow the use of low-grade heat to produce a cooling effect, or a heat transformation to higher temperature levels, with no (or minimal) mechanical power consumption. However, the coefficient of performance (COP) of the absorption refrigeration machines is still far from that of conventional vapour compression systems. For this reason the world research activity in the field of absorption refrigeration is intense.

We have developed models and robust numerical simulation techniques to understand and optimise the absorber module in absorption machines. The absorber module is a key element in absorption refrigeration, the COP of the absorption machine being highly dependent on the local efficiency of this module. In the absorber, a concentrated solution (normally salt or ammonia solutions) absorbs the vapour of refrigerant (typically water or ammonia) that comes from the evaporator of the machine. In particular we have focused our attention to adiabatic absorbers containing radially expanding liquid films of solution (e.g. fan, conical and disk sheets). This flow configuration is capable of producing high absorption ratios of vapour in small volumes, making it a very attractive option.

The figures on the right show: i) a diagram of a fan sheet adiabatic absorber; ii) numerically simulated maps of normalized mass fraction of refrigerant and temperature in the fan sheet.

 
 

Moving Bed Heat Exchangers

Moving Bed Heat Exchangers (MBHE) are used in industry for applications involving heat recovery, solids drying or filtering. Compared with other systems, they provide a large heat transfer area in a reduced volume and, concerning filtering, they avoid common operational problems that are typical of fixed-bed or ceramic filters, such as the pressure drop increase during operation. The MBHE consists of a flow of solid particles moving down that recovers heat from a gas flow percolating the solids in cross-flow. In order to describe the heat transfer in the system, two solutions for the MBHE energy equations can be studied: an analytical solution considering only convection heat transfer (and neglecting solid conduction) and a numerical solution with the solid conductivity retained in the equations. The MBHE performance can be improved by determining the appropriate design parameters with an exergy optimization analysis.

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