Boxes containing uniformly sized pebbles or stones are typically used as
the basic component of thermal storage subsystems of solar space
heating systems. During storage of excess thermal energy, heated air is
routed to the top of the pebble bed. Air passes downward through
spaces between pebbles and transfers thermal energy to the pebbles.
Because of the large exposed surface area of pebbles and limited
physical contact, pebble beds are progressively heated from top to
bottom. This provides good vertical temperature stratification within the
bed. Exit air temperatures from the bottom of the bed are usually at or
near room temperature. The low air temperature returning to the coliectors
is ideal because collectors will operate at near-optimum thermal
efficiency.