Many large building which are used for storing goods need clean air and controlled tempreture. This can be expensive. because it uses a lot of energy.1)_________________ save costs, some companies are now using a solar ventilation system. 2) ______________ a thin sheet of material is fixed to a south-facing wall 3) ______________ collect heat from the sun. 4)_________________ air passes through small holes in the metal panel. A space is left between the wall of the building and the metal panel 5) ____________ the air streams can mix together. 6)__________________ the hot air is sucked into the building 7) _____________ a fan. This fan is the only moving part in the whole process. at the next stage , by , so that, so as to , firstly, finally , in order to