What type of additional storage system may be used in active solar air heating systems?

In active solar air heating systems, rock or pebble thermal storage is used effectively due to its ability to absorb and retain heat. This type of storage works by utilizing the thermal mass of rocks or pebbles, which can collect heat during sunny periods and release that heat during colder times, helping to maintain a stable indoor temperature.

This method is particularly advantageous in situations where the direct heating from the sun may not be continuous, such as during cloudy weather or at night. The thermal mass properties of rocks or pebbles allow for a gradual release of heat, making it an efficient way to prolong the benefits of solar energy. It is a simple, passive way to enhance the performance of active solar heating systems by providing a reservoir of thermal energy.

Other types of storage systems, such as water or liquid storage, can also be effective, but they may require additional plumbing and maintenance. Plastic thermal storage systems typically are less common and may not have the same thermal mass benefits as rock or pebble systems. Air thermal storage is less efficient, as air has a lower thermal capacity compared to solid materials like rocks or pebbles, making it less effective for long-term heat retention.