Fast Company: With 150 solar installations in seven countries, the retail giant is starting to make waves in the clean-tech world. And as it continues to invest in ways to lower its footprint, it has the potential to jump-start an entire industry. Given that it is the most massive retail store on the planet, Walmart is an unlikely champion for sustainable causes. But it has engaged in the past on issues ranging from local food and responsible sourcing to phasing out toxic chemicals. Every time Walmart makes...