Carl Wilhelm Siemens developed the Siemens regenerative furnace in the 1850s. Later, in 1865, French engineer Pierre-Émile Martin took out a license from Siemens and first applied his regenerative furnace for making steel(1). This technology was soon applied in a regenerative furnace to melt glass in a continuous process and since that time, many glass melting engineers have spent countless hours trying to make it as efficient as possible; and not without success.
Until the turn of the century, the industry continued to improve the fuel efficiency of regenerative end- and side-port furnaces. Starting from 12GJ/ton, the industry managed to reduce energy usage down to around 4-5GJ/ton. A huge improvement and a great achievement! However, as can be seen from figure 1, further improvements seem to have halted subsequently(2).
Even though there are a few technologies that still look promising and might squeeze a few more tons from a gigajoule, they are unfortunately rather complex and require specific technological and maintenance skills, which may not be available in a majority of commodity glass manufacturing plants.