Mechanism of hysteresis for composite multi-halide and its superior performance for low grade energy recovery

Abstract

Sorption hysteresis commonly exists for different sorbents and has a great impact on the performance, and recently it was found that the multi-halide sorbents could reduce the hysteresis phenomena. Here we report the mechanism of the sorption hysteresis for multi-halide under equilibrium/non-equilibrium conditions and its superior performance for low grade energy recovery. We find that the inner reaction among different halides does not happen and contribute to sorption hysteresis in sorption/desorption phases under equilibrium conditions. While under non-equilibrium conditions, multi-halide sorbents reduce the hysteresis significantly (the average hysteresis temperature difference decreases from 23.4 °C to 7.8 °C at 4.41 bar). The phenomena is studied, and results show that the continuous reaction within different halides under heterothermic condition leads to an operable multi-stage reaction property, which corresponds to better flexibility and faster response to heat source. The utilization of solar energy as heat source for a cloudy day is analyzed, and multi-halide sorbent has much larger average refrigeration power (improved by 43%) and could work efficiently most of the time. Such characteristics are also prospective for other thermochemical reaction technologies, such as de-NOx and energy storage because of lower energy input and higher energy output features.

Figure 1

Solid chemisorption and resorption processes. (a) Solid chemisorption schematic diagram, showing the role of chemical potential and activation energy. (b) The precursor state of chemisorption, using CaCl₂-NH₃ as example. (c) Resorption hypothesis inside multi-halide sorbent, using CaCl₂ and MnCl₂ as example. (d) The Clapeyron figure of solid chemisorption. S represents solid halide, and G refers to the reaction gas.

Figure 2

Experimental results of single halide sorbents under equilibrium conditions. (a) Isobaric sorption/desorption processes of CaCl₂-NH₃ working pair tested by Rubotherm balance. (b) The typical sorption/desorption hysteresis phenomenon under 0.851 MPa of CaCl₂-NH₃ working pair. (c) Comparison among sorption, desorption, and the value in ref. from 0.199 MPa to 1.172 MPa for MnCl₂-NH₃ working pair.

Figure 3

Comparison between multi-halide sorbent (1:3:1.7) and CaCl₂ for isobaric sorption/desorption curves and hysteresis phenomenon. (a,c) For multi-halide sorbent, the sorption quantity is the sorbed mass of ammonia by multi-halide sorbent over the mass of multi-halide sorbent itself, at 0.622 MPa (a) and 0.851 MPa (c) respectively. (b,d) For the CaCl₂ inside multi-halide sorbent, the sorption quantity is the sorbed mass of ammonia by CaCl₂ over the mass of CaCl₂ itself, at 0.622 MPa (b) and 0.851 MPa (d) respectively.

Figure 4

Comparison between multi-halide sorbent (1:1:1) and CaCl₂ under non-equilibrium conditions. (a,b) Non-equilibrium sorption/desorption curves and hysteresis phenomenon at 0.441 MPa (a) and 0.622 MPa (b) respectively.

Figure 5

Relationship between spatial position of sorption bed (r) and reaction state of different halides inside multi-halide sorbent, during whole desorption stages under non-equilibrium conditions. (a) Temperature reaches desorption temperature of CaCl₂ and two halides (NH₄Cl and CaCl₂) react simultaneously. (b) Temperature reaches desorption temperature of MnCl₂ and two halides (CaCl₂ and MnCl₂) react simultaneously.

Figure 6

Non-equilibrium conditions for multi-halide sorbent and CaCl₂. (a) Energy and exergy efficiency under non-equilibrium conditions for multi-halide sorbent and CaCl₂, data from test unit with PDM. The uncertainty of the calculated data is shown in Fig. 8. (b) Refrigeration power on cloudy day, with solar energy as heat source, using multi-halide sorbent and CaCl₂.