“On average, steam turbines reliably convert about 35 percent of a heat source into electricity, with about 60 percent representing the highest efficiency of any heat engine to date.”
Over 40 percent beats the average turbine, not the best one. The cell is one centimeter square and needs a heat source between 1,900 and 2,400 degrees Celsius. The grid-scale thermal battery it is meant to power exists so far as separate small experiments. “As efficient as a steam turbine” is a press office choosing its comparison.