GST Refund under Inverted Duty Structure - Goods and Services Tax - GST

Dear experts, One of my client is in the business of solar power plant. They are procuring raw materials such as solar panel, solar cell, structure etc. from vendors and assembles on project site. After completing solar power plant, they sold it to their customers as it is. Raw materials was purchased at 18/28%, however, sale of power plant at 5% earlier and currently 12%. Can we take refund of unutilised itc accumulated on account of inverted duty structure? Further, such ITC accumulated during the year 17-18 18-19. Can we file gst refund now for the said period anyhow? Thankyou