Like most electrical devices ThinkPads are commonly connected to a public power grid providing 100V to 240V of alternating current (AC). This alternating current is converted to direct current (DC) by an external AC power adapter. For example, 240V AC is converted to 20V DC. This direct current is the power input for a ThinkPad to run the device, to charge the battery or both at the same time.
But other power sources are possible. Most of them require a kind of converter to generate the required input of a ThinkPad model. Lenovo lists an adapter set for most models. With alternative power sources ThinkPads can operate long-term independet from a grid.
cigarette lighter receptacle
The cigarette lighter receptacle is a defacto standard DC connector in automobiles. It provides 6V or more commonly 12V, but this can fluctuate depending on the state of the car engine. A DC-to-DC converter can compensate these fluctuations and provide the required voltage for a particular ThinkPad.
The EmPower adapter is used by airlines and provides 15V DC. A DC-to-DC converter is required.
Descriptions of using solar panels to power a laptop are rare. Lenovo links to a reseller in the USA that advertises complete power backup sets. Most solar panel systems don't provide exactly the voltage used by a particular ThinkPad model. Then a DC-to-DC converter is required.
20V solar panels are available which could be used directly to charge actual ThinkPads. But no experience is available yet.
Fuel cells are available for the charge of smartphones but not for laptops yet.
There are a lot of power sources which have to be explored to be used with a ThinkPad. For example: wind, water, cranck-handle, bicycle, ...