3.1. Active building wall
Active building walls are kind of walls which can actively control the heat flux of the walls with solar energy [21] and [22]. Fig. 2 depicts the sketch diagram of an active building wall, which is integrated with a photovoltaic (PV) unit and a thermoelectric (TE) unit. The PV forms the external surface of the wall with an airflow channel between it and the inside thermoelectric panel. In the active building wall, the PV unit changes the solar radiation energy into electrical energy while the TE unit changes the electrical energy into thermal energy, as shown in Fig. 3. The TE unit can operate in a heating or a cooling mode, depending on the direction of the current supplied by the PV unit. This feature allows for the active building envelope to be used for heating as well as cooling applications. Compared with conventional thermal technologies, the active building walls have advantages as follows: (1) operating with solar energy, (2) consisting of solid state devices and operating silently with no moving parts, (3) using little or no fossil energy sources, and (4) beneficial to environment in the long term.