They have fixed energy values.

bcoz the energy of bound system is always negative

There are some energy sources, such as wind energy, solar energy, and geothermal energy, that are available in abundance - basically for free. But please note that there is always a cost to convert such energy types into something useful. The energy is free; converting it into useful energy to run machinery is not free.

chlorophyll that captures light energy.

In any circumstance where a threshold of energy is required to free an electron from a bound state, an incoming photon must have at least that energy to do the job. The energy of a photon is proportional to the frequency of the light, so the minimum energy corresponds to a minimum frequency of the light, or maximum wavelength necessary to free an electron. This observation was a major step in the development of radiation theory (Einstein).