Access trimmed description in module
The summary value will only be set if someone has actually filled out the summary textfield when editing the node. You'll need to do the trimming yourself in code. Drupal has functions such as https://api.drupal.org/api/drupal/modules%21field%21modules%21text%21text.module/function/text_summary/7.x or views has https://api.drupal.org/api/views/views.module/function/views_trim_text/7.x-3.x

So you'll need to do a check to see if the summary value exists, if it doesnt use the main text and trim it with the functions or something similar above