Accounts receivable, net (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts receivable, net
Trade accounts receivable	$ 23,138,736	$ 22,331,720
Receivable from OC-BVI	10,808	46,600
Other accounts receivable	222,381	242,799
Accounts Receivable, Gross, Current	23,371,925	22,621,119
Allowance for doubtful accounts	(142,236)	(158,902)	$ (158,902)
Accounts receivable, net	$ 23,229,689	$ 22,462,217