New York Times: John Hamilton paused from tinkering with the heat pumps, pipes and tanks arrayed before him, the mechanical heart of a small house assembled on the National Mall, to read the electric meter mounted off to the side. The digital display showed that over the previous two days, the pavilionlike structure, designed by architectural and engineering students from Virginia Tech, had drawn about 20 kilowatt-hours from the electric grid. But during the same period, double-sided photovoltaic …