You can't change the default location.
But, whenever you save your project file, you can direct it to be saved wherever you'd like.
BTW, .prel files are actually very small. A megabyte or two. It would take 500-1000 project files to even take up a gigabyte of space!
Are you sure it's the .prel files that are filling up your hard drive?
And if your hard drive is that full, it might lead to many other problems. How old is your computer?
I would look at the location of the preview files and conformed audio files in particular.
How much space is there on an alternative drive? If enough space to accept them, why not go to Edit Menu/Preferences/Scratch Disks and direct the choices there to this alternative hard drive? The Video Preview files setting there would direct the preview files to the Adobe Premiere Elements Preview Files Folder location of your choice. Those preview files pile up automatically as you render the Timeline content, and they grab a lot of hard drive space. For SD project, these preview files are DV AVI and for HD project they are MPEG2.mpg (whether the rendered content is video or still).Conformed audio files also pile up quickly. See the Media Cache in Edit Menu/Preferences/Scratch Disks.
As for the project.prel, its save location is not set in Edit Menu/Preferences/Scratch Disks. It is set in the new project dialog which you can get at via File Menu/New/Project. This project file has a very small file size. I try to keep the Scratch Disks files together but my emphasis is on the preview files especially and conformed audio, and conformed video files.
Because of its regeneration into the same location, I would not focus on trying to move the conformed video from its default location. See Edit Menu/Preferences/Media and its Media Cache Database.
The following is an Adobe document which offers its Scratch Disks information.