Unlimited free energy of any sort is unsustainable. Our planet is a balanced system that has evolved over eons, simply adding energy upsets this balance and probably not in ways that will ultimately be beneficial for us. We can see many negative effects already from adding massive amounts of fossil energy to the system (besides the greenhouse effect and pollution) such as population growth beyond the bounds of the planet.
Solar panels are capturing energy that would otherwise just heat the ground. It won't upset the planet's energy balance. If anything, we can use the excess to capture CO2.
If we capture a non-trivial amount of the energy that would've returned to space and use it in industry, that adds energy to our system and causes it to heat up -- climate change. This is not an issue currently with intentional solar capture (greenhouse effect is unintentional solar capture).
I was simply refuting a claim that clearly violates the laws of thermodynamics - that unlimited free energy is without problems.
I want to be clear, I'm not anti-solar panel. I have them on my house and it's awesome.