Accrued Liabilities and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities and Other Current Liabilities
Employee compensation expense	$ 3,130	$ 1,851
Project asset costs	2,014	3,317
Valve installation incentive	2,000
Accrued legal services	313	141
Other accrued liabilities	896	547
Total accrued liabilities and other current liabilities	$ 8,353	$ 5,856