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Summary of significant accounting policies - Revenue by Product Line and Services (Details) (USD $)
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3 Months Ended | 12 Months Ended | ||
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Mar. 31, 2014
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Mar. 31, 2013
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Dec. 31, 2013
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Dec. 31, 2012
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| Revenue by Producks and Services [Line Items] | ||||
| Products | $ 1,944,776 | $ 2,052,665 | $ 6,346,050 | $ 7,453,222 |
| Services | 2,270,981 | 1,993,653 | 9,503,819 | 7,800,750 |
| Total revenues | 4,215,757 | 4,046,318 | 15,849,869 | 15,253,972 |
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Cogeneration
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| Revenue by Producks and Services [Line Items] | ||||
| Products | 1,154,269 | 1,278,156 | ||
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Chiller
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| Revenue by Producks and Services [Line Items] | ||||
| Products | 790,507 | 774,509 | ||
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Service contracts
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| Revenue by Producks and Services [Line Items] | ||||
| Services | 1,772,981 | 1,745,946 | ||
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installations
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| Revenue by Producks and Services [Line Items] | ||||
| Services | $ 498,000 | $ 247,707 | ||