Chiller Boiler Systems
HVAC (Heating, Ventilation and Air Conditioning) systems are also known as comfort systems -- while some people are willing to wear a sweater or use a fan indoors, most aren't game to really sweat it out in the summer or freeze in the winter in the name of saving energy. There's no shame in that, but HVAC systems use more energy than anything else in a commercial building. Air conditioning also results in tons of carbon dioxide emissions, so making HVAC systems as green as possible is a huge concern. There are plenty of ways to minimize how much they're used (like passive solar design or improved insulation), but there's no way to ditch them entirely.
While some traditional HVAC systems are more efficient than others, there are also alternatives like a chiller boiler system. Typical HVAC systems use forced air to heat and cool, but a chiller boiler system is hydronic -- meaning that it uses water -- which is more efficient because it provides even temperatures. These systems are attached to the water supply in the building. If this sounds familiar, you may be thinking of radiators, an older hydronic system that uses steam to heat. A chiller boiler warms you up using radiant heat instead of hot water. It's sort of like the heat you feel if you hold your hand near a flame -- it's not actively blowing hot air on you but giving off heat that warms the room from water-filled copper and aluminum pipes. Putting in a chiller boiler isn't really cost-efficient for a really large commercial building, but they can work well in smaller ones.