Waste heat: Best green power source?

To better boost your green credentials, try to make use of heat thrown off by industrial equipment.

Trying to boost your green credentials? Back-burner your plans for the roof-top solar panels and on-site wind turbine. For many businesses, the most economical way of saving energy is to make use of the heat that industrial processes generate. No question that a lot of this heat gets wasted. Three years ago, researchers at UC Berkeley estimated the U. S. consumes 100 quadrillion Btus of energy annually and as much as 60% of that energy gets dissipated as waste heat.

Data centers and server farms are classic examples of wasted-energy problems. Experts say most of the energy dissipated at these facilities gets consumed by air conditioners needed to cool the electronics. Long term, data centers will generate less waste heat as power supplies and electronics become more power efficient. But servers and computer equipment will be throwing off excess heat for some time to come. They also illustrate a problem shared by other industrial facilities that house heat-generating equipment: Much of this heat is “low grade” — though it accounts for a lot of energy, there’s often not enough of it to make a heat-recovery system worthwhile.

It often takes an expert to figure out whether heat recovery is worth the effort and, if so, decide on the best way to approach the problem. The usual means is to configure a heat exchanger in an exhaust stack or as part of a cooling system. Makers of boilers, chillers, and other heating and cooling equipment frequently offer reclamation gear that works with their products. And numerous firms specialize in solving the problems that accompany reclaiming heat from buildings, smoke stacks, specific industrial processes, and refrigeration systems.

But those who might be tempted to grab a heat exchanger and take a stab at recycling some heat should be forewarned. The design of heat-transfer equipment is not a field for amateurs. Professionals with experience say a number of factors can make heat-harvesting equipment less than efficient and result in a disappointing payback.

As you might expect, one key is the ability to differentiate between low-grade heat and useful amounts of thermal energy. Advice comes from Tim Golden, director of aftermarket services at Megtec Systems Inc., De Pere, Wis. Megtec, among other things, makes heat-recovery systems that use heated air from process dryers, ovens, and air-pollution control systems. The majority of Megtec’s heat-reclamation work has been in capturing heat from oxidizers that oxidize pollutants out of air coming from a wide variety of processes. But the same principles apply to any situation that involves waste heat. “Applications under 250°F and volumes under 2,000 or 3,000 scfm are never going to make sense for heat recovery because of the equipment cost,” Golden says. “The sweet zone is 250 to 700°F, where you can use mild steels or low-grade stainless in the heat exchanger and you don’t have to get into super-insulated refractory-lined ductwork.”

A clean stream of air is also important. “You must be careful in environments where flux and acids are present, as in foundries. Air streams with a lot of moisture are problematic, and you should stay away from air with a lot of condensable material in it,” he says. Another caution: Recovery systems must not extract so much heat that the air stream falls below its dew point. Smoke stacks and similar exhausts pose problems because they contain particulates such as soot. The air must be cleaned before it hits a heat exchanger to prevent buildup that can degrade thermal efficiency.

The addition of heat-recovery equipment generally also puts a back pressure on the exhaust stack. If there is an exhaust fan involved, it will need to push air through the hot side of the recovery heat exchanger. The resulting pressure constitutes an extra load on the fan motor, so the motor must be sized for the additional load.

There are other factors in play when the airstream contains gases that can be hazardous, such as carbon monoxide. Such applications call for double-walled heat exchangers that isolate the exhaust gas from any fresh air that could be used to heat a room. Refrigerant introduced into the heat exchanger also typically circulates at a pressure higher than that of the exhaust gas, so any leak in the heat-transfer equipment puts refrigerant into the exhaust rather than the other way around.

It can also be a challenge to introduce the waste heat back into the facility economically. Energy-recovery system maker CMM Group LLC, De Pere, Wis., says site layout is one of the biggest factors determining whether a heat-recovery scheme is practical. Long runs of ductwork, for example, boost costs. To avoid this expense, the ideal situation is to use the waste heat in the same process that generates it, CMM says.