Energy Storage By Sand in Fluidized BED

Renewable energy such as wind or solar energy is becoming more and more important. However, electricity generation by renewable energy is usually unsteady and cannot match the demand of electricity consumption, especially, in regions where wind or solar power covers a high percentage of power generation. Parts of wind or solar power have to be abandoned sometimes to maintain stability of the electricity grid. Therefore, it appears reasonable to store electricity when it is in excess. There are several approaches to store electricity, such as direct storage in secondary batteries or storage in form of heat, mechanical energy or chemical energy. It is believed that transformation of electricity into heat is the cheapest method for large scale storage (for example, 100MWh).

Energy storage by sand in fluidized bed (referred to as “ESFB” below) is proposed in this paper. ESFB is one kind of thermal energy storage. ESFB is used to store heat generated by electricity in heating mode and to release heat when necessary in heat-release mode.

Heating mode is executed, when more electricity is generated than needed. Electrical heaters are set in wind box and wall of the riser. Cold particles are fed from a bubbling fluidized bed of low temperature, heated in the riser and then stored in a hopper. The hot air used for particle transportation is ducted from the cyclone to the low temperature fluidized bed in order to recover heat from the air.

The heat-release mode is executed, when heat (or electric power) is needed. The hot particles are fed from the high-temperature hopper to the riser in order to heat air supplied from the bottom. The hot air is then exported to working equipment, which can be a power generator or a furnace.

Advantages of the ESFB are low cost of the storage material, high energy density and system flexibility. The ESFB is cheap, because that the material used to store thermal energy is cheap sand. It has a high energy density, because the temperature can be as high as 1000℃. It provides high flexibility both, in heating mode and heat-release mode, because load and temperature can be adjusted by changing feeding rate of sand and inlet velocity of air into the riser.

Numerical simulations are performed for the electric heater in the riser, because that it is an important component of the ESFB system. Equivalent heat transfer coefficient, pressure drop, erosion, residence time distribution and segregation of sand particles are assessed for several design parameters of the heater. Optimization of the heater is outlined.