Memory use

I'm having memory problems writing several XLSX files with 500 columns and 200 rows. It seems PHPExcel is not releasing memory. After two files with memory_limit = 128M I get a white screen. Writing just one file succeeds. Tested with both PHPExcel 1.7.9. and 1.8.0.

Here are some numbers. This is what I use:

drupal_set_message('<pre>'. sprintf("%9s",  memory_get_usage()) . '   -     pre phpexcel</pre>');
  $result = phpexcel_export($headers, $data, $path, $options);
drupal_set_message('<pre>'. sprintf("%9s",  memory_get_usage()) . '   -     post phpexcel</pre>');

This results in:

 20871016   -     pre phpexcel
 74431992   -     post phpexcel

($data is about 15MB).

If I add some extra messages in phpexcel.inc I get this:

 20871016   -     pre phpexcel
 20871832   -       :start phpexcel_export
 21683144   -       :object created
 79609648   -       :columns set
 79717632   -       :writer created
 74452000   -       :end phpexcel_export
 74431992   -     post phpexcel

Looping through three files with memory_limit = 256M gives:

      3241880   - indicator 106 resource found
     21125888   -   set data
     21127336   -     pre phpexcel
     74537120   -     post phpexcel
     58790928   -   unset data
     58782832   - indicator 106 file created
     58818528   - indicator 295 resource found
     71658616   -   set data
     71659520   -     pre phpexcel
    115556344   -     post phpexcel
    102724992   -   unset data
    102716792   - indicator 295 file created
    102761176   - indicator 262 resource found
    128664744   -   set data
    128665672   -     pre phpexcel
    198014592   -     post phpexcel
    172095512   -   unset data
    172087304   - indicator 262 file created

What's going on? Am I doing something wrong? Is this due to an error in the module or the library, or just normal behavior? Could I manually free some memory?