But a renewable energy advocate , Amory Lovins of the Rocky Mountain Institute , says the nation 's car fleet could be converted to hydrogen in less than a decade , and a network of small hydrogen reformers -LRB- devices that produce hydrogen from natural gas or other sources -RRB- could be quickly installed in 10 to 20 percent of the nation 's 180,000 gas stations for $ 2 billion to $ 4 billion .