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Accounts receivable, net (Details) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Trade accounts receivable $ 22,331,720 $ 13,341,438  
Receivable from OC-BVI 46,600 123,807  
Other accounts receivable 2,008,677 1,380,735  
Accounts Receivable, Gross, Current 24,386,997 14,845,980  
Allowance for doubtful accounts (158,902) (158,902) $ (193,338)
Accounts receivable, net $ 24,228,095 $ 14,687,078