Kohana has a hard time handling the rendered file using the send_file() method, so write the file to disk using the built-in method from PHPExcel. Then use Kohana to deliver the saved file as a download.
$objWriter = new PHPExcel_Writer_Excel2007($objPHPExcel);
$objWriter->save($filename, 'EXCEL2007');
$this->response->send_file( DOCROOT.$filename );