Accrued Liabilities and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accrued Liabilities and Other Current Liabilities
Employee compensation expense	$ 1,776	$ 1,535
Project asset costs	1,845	1,810
Accrued legal services	119	91
Other accrued liabilities	365	331
Total accrued liabilities and other current liabilities	4,105	3,767
Non current compensation liabilities related to engineering staff	700	1,100
Non current compensation liabilities related to executive staff	$ 1,300	$ 1,600