Air System Thermal Storage

The most common storage media for air collectors are rocks. Other possible media include PCM, water, and the inherent building mass. Gravel is widely used as a storage medium because it is abundant and relatively inexpensive.

In cases where large interior temperature swings can be tolerated, the inherent structure of the building may be sufficient for thermal storage. Loads requiring no storage are usually the most cost-effective applications of air collectors, and heated air from the collectors can be distributed directly to the space. Generally, storage may be eliminated in cases where the array output seldom exceeds the thermal demand (ASHRAE, 2004).

Note: Unit designed for vertical air flow through the rock bed.

Pebbles

Pebbles

Hot Air Collector

Hot air opening

Insulating cover

Hot air opening

Concrete storage

Cold air outlet

Cold air outlet

Pebbles Bond beam block

FIGuRE 5.15 vertical flow packed rock bed.

The main requirements for gravel storage are good insulation, low air leakage, and low pressure drop. Many different designs can fulfill these requirements. The container is usually constructed from concrete, masonry, wood, or a combination of these materials. Airflow can be vertical or horizontal. A schematic diagram of a vertical flow bed is shown in Figure 5.15. In this arrangement, the solar-heated air enters at the top and exits from the bottom. This tank can work as effectively as a horizontal flow bed. In these systems, it is important to heat the bed with the hot air flow in one direction and to retrieve the heat with airflow in the opposite direction. In this way, pebble beds perform as effective counter-flow heat exchangers.

The size of rocks for pebble beds range from 35 to 100 mm in diameter, depending on airflow, bed geometry, and desired pressure drop. The volume of the rock needed depends on the fraction of collector output that must be stored. For residential systems, storage volume is typically in the range of 0.15-0.3 m3 per square meter of collector area. For large systems, pebble beds can be quite large and their large mass and volume may lead to location problems.

Other storage options for air systems include phase change materials and water. PCMs are functionally attractive because of their high volumetric heat storage capabilities, since they require only about one tenth the volume of a pebble bed (ASHRAE, 2004).

Water can also be used as a storage medium for air collectors through the use of a conventional water-to-air heat exchanger to transfer heat from the air to the water in the storage tank. This option has two advantages:

1. Water storage is compatible with hydronic heating systems.

2. It is relatively compact; the required storage water volume is roughly one third the pebble bed's volume.

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