5. Intangible Assets

Intangible assets consisted of the following (in thousands):

 

 

 

Amortization expense for the intangible assets was $0.4 million, $0.5 million and $0.5 million for each of the years ended March 31, 2016, 2015 and 2014. Included in amortization expense for the three months ended March 31, 2016, is approximately $0.1 million of purchased TA100 backlog that was written off to align with management’s decision to limit the production of TA100 systems on a case-by-case basis for key customers.

Expected future amortization expense of intangible assets as of March 31, 2016 is as follows (in thousands):

 

The manufacturing license provides the Company with the ability to manufacture recuperator cores previously purchased from Solar Turbines Incorporated (“Solar”). The Company is required to pay a per‑unit royalty fee over a seventeen-year period for cores manufactured and sold by the Company using the technology. Royalties of approximately $35,000,  $71,100, and $83,000 were earned by Solar for the years ended March 31, 2016, 2015 and 2014, respectively. Earned royalties of approximately $35,000 and $19,300 were unpaid as of March 31, 2016 and 2015, respectively, and are included in accrued expenses in the accompanying balance sheets.