Property, plant and equipment (Summary of Property, Plant and Equipment) (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 26,018,879	$ 19,410,471
Less ----- accumulated depreciation	(7,621,242)	(6,149,328)
Property, plant and equipment, before contruction in progress	18,397,637	13,261,143
Construction in progress	3,533,652	3,992,505
Property, plant and equipment, net	21,931,289	17,253,648
Energy Systems
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	25,567,808	19,052,933
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	122,853	109,910
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	85,079	81,435
Vehicles
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 243,139	$ 166,193

The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.

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Gross amount of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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- Definition

Amount, net of accumulated depreciation, depletion and amortization, of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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