Subtracting these two graph, we get the graph of the actual energy that went into each brick, in btu’s per pound:
The scale is btu’s per pound. The blue line is the glass bricks, the purple line is the grog bricks.
One of the interesting things to observe is that as the glass bricks reach their final state of fusion, they are actually consuming more energy than the grog bricks. This is because as the glass softens the mass conducts heat more efficiently, until it reaches maximum density.
Summarizing the graphs above, we have the following:
Grog bricks with kiln 3011 btu’s per pound
Glass bricks with kiln 2267 btu’s per pound
For a savings of 744 btu’s per pound, or 25 percent.
Subtracting the energy used by the empty kiln, which represents the heat loss through the sides of the kiln, we get:
Grog bricks 616 btu’s per pound
Glass bricks 478 btu’s per pound
For a savings of 138 btu’s per pound, or 22 percent.
The energy consumption of the bricks without the kiln is a more interesting figure because large-scale roller-hearth furnaces have economies of production that will make the wall losses per brick very small compared with these.
The brick manufacturing industry has been targeted by federal and state governments for decades as an industry that needs energy efficiency. But bricks are a wonderful building material. They are virtually permanent, sanitary, and structurally sound. Bricks manufactured hundreds of years ago are still salvaged, cleaned, and used again. When I was in Poland some years ago, I saw a Teutonic Knights’ castle for which the bricks were already old when they built the castle from older rubble in the 13th century.
Finally, this project begs the question of whether it’s worth using glass in brick manufacturing in monetary terms.
Comparing our actual energy invested in a grog brick of 616 btu’s per pound to the figures given by industry on earlier pages of this web site, it appears that industrial furnaces achieve about a 50 percent efficiency of total energy in vs. energy actually going into the brick.
A savings of 138 btu’s per pound of brick translates to 276 btu’s per pound of glass used to make the 50 percent glass brick, or 552,000 btu’s saved per ton of glass. Assuming 50 percent efficiency for an industrial-scale brick manufacturing plant would result in about 1,100,000 btu’s saved per ton of glass used. One cubic foot of natural gas contains about 1000 btu’s of energy. So the savings would be 1100 cubic feet of natural gas.
This is somewhat of a disappointing result, since natural gas costs about $10 per thousand cubic feet for industrial users.
So a case could be made that the value of glass in brick manufacturing would be $11 plus the cost of the grog currently being used. in many cases, the source of glass may be much closer than the source of the grog, but this effect would be local and dependent upon transportation.
Other considerations to a manufacturer would be that firing at lower temperatures lengthens the life of the refractories and reduces maintenance costs, but we are in no position to estimate those costs. And with California’s latest Greenhouse Gas reduction mandates, there may be other benefits to reducing energy consumption. Finally, wouldn’t architects think it was sexy to call for 50 percent recycled glass bricks on a LEED job.
So our conclusion is that substituting recycled glass for ceramic grog saves about 25 percent of the energy it takes to make a brick.