G. Rambaud*, N. Mazet, S. Mauran

1 CNRS-PROMES (Processes, materials and solar energy laboratory — UPR 8521) Rambla de la
thermodynamique Tecnosud 66100 Perpignan — France.

Corresponding Author, guillaume. rambaud@univ-perp. fr

Abstract

For safety and environmental reasons, thermochemical processes involving solid gas reactions with water as reactive gas are very promising, but the working pressure is rather low (103 — 104 Pa). Investigations have to focus on mass transfer coefficients (permeability, k, and the Klinkenberg’s coefficient, b) and heat transfer coefficients (effective thermal conductivity, X, and the heat transfer coefficient at the exchanger wall, hsw). The whole set of transfer coefficients will be identified on the same sample in a single characterisation apparatus. First, we analyse the transfer coefficients according to the characteristics of the composite. The mass transfer coefficients was analysed in transient state, by comparing the experimental pressure evolution versus time and simulations using Comsol®. In steady state, the heat transfer between the upper side of the composite and the heat exchanger is simulated. The experimentation shows that the reactive bed pressure is stabilized in less than 10 seconds, and depends on the transfer parameters k and b, on the bed parameters (p and wl), and on operating parameters as the initial pressure drop and the sample temperature. The ranges of heat and mass transfer parameters are consistent with previous experiments on similar porous samples: 10-11 to 10-16 m2 for the permeability, nearly 500 Pa for the Klinkenberg’s coefficient and about 1 W. m-1.K-1 for the effective thermal conductivity.

Keywords: thermochemical heat transformer, mass transfer, heat transfer, porous media

1. Introduction

Thermochemical heat transformers can be used for solar air-conditioning for individual dwelling. For safety and environmental reasons, solid gas reactions involving water are very interesting. Numerous solid reactants have been tested by the laboratory and only few of them have the thermochemical properties required. The selected solid reactant is strontium dibromide.

The salt reacts with 5 moles of water during synthesis phase and releases these 5 moles during the decomposition phase. The working pressure is between 1000 and 10000 Pa and that leads to a strong mass transfer limitation. The solution would be to increase the porous volume around the salt. That’s why the reactive salt is mixed with expanded natural graphite (ENG) which is a very porous material and a good heat conductor. The thermal conductivity increases with apparent density of ENG but on the other hand the permeability decreases. The accurate determination of these two parameters is very important for chemical reaction with steam at low pressure.

2. Experimentation

A single apparatus was developed to identify both mass and heat transfer coefficients on the same composite block. To reduce the handling of the composite, the compression and the reactions with steam were realised in a single apparatus as well as the characterisations.