Setting the start dir when calling Powershell from .NET?

I'm using the System.Management.Automation API to call PowerShell scripts a C# WPF app. In the following example, how would you change the start directory ($PWD) so it executes foo.ps1 from C:\scripts\ instead of the location of the .exe it was called from?

using (Runspace runspace = RunspaceFactory.CreateRunspace())
{
    runspace.Open();
    using (Pipeline pipeline = runspace.CreatePipeline())
    {
        pipeline.Commands.Add(@"C:\scripts\foo.ps1");
        pipeline.Invoke();
    }
    runspace.Close();
}