A couple of months back I was working on Installing and Configuring SharePoint 2010 with PowerPivot with SQL 2012. Everything was installed and working properly in our web applications, however, when opening up the PowerPivot Service Application in Central Administration we were getting an error loading the Excel workbook on the management page. After lots of searching and trying various things I finally came across this KB from Microsoft – http://support.microsoft.com/kb/981293. It applies to your SharePoint web applications hosting BI services, and is documented that it needs to be run on those web applications.
Well, one thing we didn’t think about was that the PowerPivot Service Application management page also uses BI Services, making Central Administration in SharePoint a BI Web Application. So, the following PowerShell (as mentioned in the KB Article) also must be run for Central Administration when using PowerPivot.
PS> $w = Get-SPWebApplication –Identity <URL of Central Admin>
PS> $w.GrantAccessToProcessIdentity(<span class="str">"<domainExcel Service Account>"</span>)
That should do it, when you open up your PowerPivot Service Application now all the PowerPivot analytics utilizing Excel Services should display without any errors.