You already have a reference to tileEntity, you don't need the world, but you do need to store the rotation in the tileEntity like Jordor suggested. Such as:
public class BlueSirenTileEntity extends TileEntity {
public float rotation = 0;
}
then in the Block, instead of changing the model, change the rotation of the TileEntity:
if (par1World.isBlockIndirectlyGettingPowered(par2, par3, par4)){
if (world.isRemote) ((BlueSirenTileEntity)world.getTileEntity(par2, par3, par4)).rotation += 0.05F;
}
then just use ((BlueSirenTileEntity)tileentity).rotation in your renderTileEntityAt method to determine how to render the model.