Daily cron job to download and email Webform submissions file
Using a custom module, you can implement hook_cron() to export the csv and send an email:

function MODULE_cron()
{
    //this is the function that creates the csv, you'll need to research how to use it properly.
    webform_results_download($node, $export_info);
    //send email
}


As far as sending email, you can use php mail or drupal_mail(), but phpMailer might be the easiest to use.