Simple suggestion really. Add power shards as a form of currency at the market. It would be something lesser then an iron ingot for items everyone can easily obtain (i.e. cobblestone).
In terms of making shards more like a real currency I'd be interested in seeing them take the place of iron ingots and other ores in the trading system. It hasn't really bothered me too much yet, but I see why you'd want to go one way or the other. As of right now shards only mean three things to me, leaf cutters, ancient ingots, and town generator.