Proceedings of the
Second International Energy 2030 Conference,
November 4-5, 2008, Abu Dhabi, U.A.E.
Agitated Fluidized Bed Thermochemical Energy Storage for Industrial Waste Heat Recovery System
Jo Darkwa
Nottingham Trent University, UK
Abstract
Optimum utilization of residual heat is often hampered by economic and technical boundary
conditions. It is also affected by the mismatch between supply and demand with regard to quality,
quantity, time, and location. It is estimated that about 10% of current total UK energy consumption is
discharged into the atmosphere as waste [1-2]. With an appropriate technology a large proportion of this
waste heat could potentially be recovered for useful applications and thus minimize energy consumption
and CO2 emissions.
In recent years, thermal energy storage has emerged as a significant means by which residual heat may
be recovered for later utilization. However, thermal energy stores do not in their own right save energy, in
fact energy and thermodynamic availability is always lost in a storage process. In this regard various
methods of storing energy form an important set of “enabling” technologies which increase both the cost
effectiveness and energy conservation potential of storage systems. Close review of various storage
methods has shown that thermochemical energy storage process in inorganic oxides has the potential to
become probably the most effective and economic method of storing and utilizing waste heat. There are
however technical and scientific problems such as inadequate heat and mass transfers associated with
thermochemical energy reactors, which remain to be overcome. Various related studies have in the past
been carried out. For instance Fuji et al. [3] experimented on an integrated metal foam reaction bed as
supplement to retaining shape of reactant during reaction process, and ultimately to enhance the storage
performance. Goetz et al. [4] evaluated the impregnation of reactants in consolidated blocks of natural
graphite and observed dynamic limitations of heat transfer in the grains and in the bed. Groll [5] reviewed
the operational characteristics of different types of fixed reaction beds and found their performances to be
affected by unequal diffusivity of the chemical species and inadequate heat and mass transfers through the
reaction beds. Darkwa et al. [6-10] have also carried out analytical and experimental evaluation of an
integrated fixed bed thermochemical energy reactor but yet to overcome the associated problems.
Fluidized beds are considered to be favorable for rapid exothermal reactions due to the large specific
surface area available for reaction and short residences times for gas or solids. However, factors such as
minimum fluidization velocity (umf), i.e. the velocity at which fluidization begins, and the velocity at which
pneumatic transport begins (i.e. terminal velocity) do influence the performances of fluidized bed. There is
also the problem of bubbles associated with umf, which tend to affect the combined heat transfer coefficient
and thus the mode of heat and mass transfers in fluidized beds. Such parameters would therefore have to
be considered and incorporated into the system. To this end, a double-acting agitated fluidized bed
thermochemical energy storage system is proposed for an analytical investigation.
