Application Of Carbonaceous Adsorbents
APPLICATION OF CARBONACEOUS ADSORBENTS
IN ENERGY STORAGE SYSTEMS
L. Czepirski, B. Buczek
University of Mining and Metallurgy, Faculty of Fuels and Energy,
al. Mickiewicza 30, 30-059 Cracow, Poland
Much attention has been paid to storage methods of thermal energy. It is only recently that adsorption processes have been proposed for heat management (refrigeration, heat pumps, etc.). Adsorption systems involving a solid adsorbents and a gas or vapour have several attractive features for both heat pump and heat transformers applications. Adsorption cooling has attracted considerable interest for many years, and several studies, both theoretical and experimental, have been reported in literature.
Adsorption heat pumps are receiving increased attention for space heating and cooling since they offer an advantage over conventional heat pumps, which require a refrigerant and also demand a relatively large electrical input to the compressor. The adsorption heat pumps replace the compressor with a fan, which circulates the working fluid throughout the system. For domestic use, it is possible to envisage that an adsorption machine can be used both as a heat storage device with heat pump effect as well as a cooling system.
A number of choices are possible for the selection of an adsorbent - adsorbate pair, however for the domestic uses both components should be environmentally acceptable. Considering availability, cost, non-toxicity, non-flammability, latent heat of adsorption and evaporation and affinity for the adsorbents, water and methanol arise as the most suitable adsorbate for the most applications. Also, a number of suitable adsorbents (silica gel, alumina, zeolites, and active carbon) can be used to sorb substances from air with the simultaneous release of energy.
Among the gas – solid pairs that have been investigated for cooling applications, active carbon – methanol is one of the most promising. This pair takes the advantage of the high microporous structure and the large specific surface areas of carbonaceous materials in favouring adsorption of polar
molecules, like methanol or similar compounds. Besides, the use of methanol allows the system to work in cooling mode at temperature below 273K.
Scheme of experimental equipment used for testing of adsorbents in cooling systems is shown below.
The results obtained for system active carbon – methanol are listed in the table.
Adsorbent
Active carbon
Adsorbate
Methanol
Mass of adsorbent, [g]
153.15
Initial temperature in evaporator, [oC]
22.0
Initial mass of methanol in evaporator, [g]
158.34
Mass of methanol evaporated, [g]
20.46
29.43
Adsorption [g/g]
0.184
0.265
Final temperature in evaporator, [oC]
-0.3
-12.2
Active carbon adsorption cycles have great promise for small refrigeration systems. The porosity characteristics of available carbons give reasonable performance compared to the maximum that can be achieved with physical adsorption. Also, an improvement in the mechanical strength and conductivity of adsorbent is needed and the actual design of the total system, including heat exchangers, still needs to be considered in order to realise these systems.
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