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Income Taxes - Company's net deferred tax assets (Details) - USD ($) | Dec. 31, 2020 | Dec. 31, 2019
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Deferred tax assets: |  |
Net-operating loss carryforward | $ (59,960,000) | $ (50,821,000)
Total gross deferred tax assets | 93,011,000 | 74,955,000
Valuation allowance | 91,315,000 | 73,930,000
Star Peak Energy Transition Corp [Member] |  |
Deferred tax assets: |  |
Net-operating loss carryforward | (13,291) | (176)
Start-up/Organization costs | (693,037) | (2,009)
Total gross deferred tax assets | 706,328 | 2,185
Valuation allowance | $ 706,328 | $ 2,185

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