Cost of revenues and general and administrative expenses (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cost of revenues	$ 43,458,260	$ 36,132,425	$ 34,098,059
Electricity [Member]
Cost of revenues	14,473,583	12,622,788	10,132,956
Depreciation [Member]
Cost of revenues	6,959,912	5,537,098	5,854,297
Fuel Oil [Member]
Cost of revenues	10,078,724	7,195,692	4,928,409
Employee Costs [Member]
Cost of revenues	4,423,899	4,276,682	4,444,814
Cost Of Plant Sales [Member]
Cost of revenues	0	61,843	1,833,835
Maintenance [Member]
Cost of revenues	2,849,453	2,140,776	2,541,927
Royalties [Member]
Cost of revenues	1,451,672	1,406,160	1,368,181
Insurance [Member]
Cost of revenues	1,625,733	1,545,771	1,490,121
Other Credit Derivatives [Member]
Cost of revenues	$ 1,595,284	$ 1,345,615	$ 1,503,519