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.

Designing heat exchangers
The heat exchangers inserted in industrial smokestacks must be designed with materials that can take high heat such as stainless steel. They are a far cry from ordinary aluminum and copper that typically go into applications that are less harsh. Aluminum-and-copper heat exchangers generally have a temperature limitation of about 200°C.

But the design of a heat exchanger, regardless of its make-up, is probably not a task for novices. In tube-and-fin heat exchangers, for example, “The copper tube expands to get the best contact with the fin for the heat to transfer effectively. That won’t happen without planning and it takes heat-exchanger design out of the realm of the do-it-yourselfer,” says Lytron Inc. Application Engineer John Miller. Lytron, in Woburn, Mass., makes cooling systems that can encompass seven cold-plate technologies and seven types of heat exchangers. “With a tube-and-fin heat exchanger, you must ensure contact between the tubes and fins. Manufacturers typically have proprietary software for the fin design that ensures transfer efficiency,” he says.

Makers of heat exchangers also typically make available software tools for selecting among standard products and for configuring such factors as the number of fins and their density, and the length of the tubes. Typical inputs include the mass of the fluid and of the hot gas, and the temperatures of the two fluids. The toughest part of the heat-exchanger design, says Miller, typically comes from staying within size constraints while still extracting the required amount of heat. And, of course, the smaller the temperature difference between the two fluids, the less the heat transfer. Miller says a 20°C temperature difference is generally the minimum needed to extract a useful amount of energy.

Harvesting refrigeration heat
One prime area for heat recovery is in large-scale refrigeration and cooling systems. The field is large enough that manufacturers of refrigeration and A/C equipment generally also offer heat-recovery gear. Nevertheless, heat harvesting from industrial-scale refrigeration is not always a no-brainer. Large-chain grocery stores frequently harvest the heat from their refrigeration compressors, but smaller establishments often don’t bother. “Major chains often have in-house engineering departments that will tack a heat-recovery exchanger onto a conventional chiller,” says Donald R. Wulfinghoff of Wulfinghoff Energy Services Inc., Wheaton, Md., an energy-efficiency consulting firm. “But there is a great deal of variability among grocery stores. Your small local stores probably don’t recover heat because it is a technical subject. If you are in the grocery business, what you know is groceries. Your life is too complicated and your margins are too small to be fussing with heat recovery.”

The issues surrounding the recovery of heat from chillers can become complicated. The terminology in the field sometimes doesn’t help clear things up. For example, the ratio of heat transfer to work input for a refrigeration cycle is not called the efficiency, but rather the coefficient of performance (COP) or advantage.

And there are complexities associated with even with the systems that are conceptually the simplest, those involving cooling towers. For one thing, cooling- tower water is dirty and often gets treated with toxic chemicals. So it may be necessary to use a heat exchanger to isolate the tower water rather than use its heat directly.

Some recovery systems for cooling towers might also employ an auxiliary condenser in which clean water circulates in a closed loop between it and the heat-recovery equipment. The compressor discharges to both the main and auxiliary condenser and refrigerant flows to both. In one type, called a double-bundle condenser, one of the tube bundles is cooled by cooling-tower water while the other tube bundle gets used for heat recovery.

There can also be trade-offs between the temperature of a chiller’s COP and the condensing temperature. Some facilities might depend on heat recovery for a specific amount of heat. To ensure they get it, they sometimes raise the condensing temperature above what it would be otherwise if refrigeration was the only goal. But boosting the condensing temperature can also reduce the chiller COP. Experts say experience plays a part in evaluating the trade-offs involved.

Different refrigeration setups use different compressor technologies that each have special considerations for reclaiming heat. Screw-compressor chillers are more frequently applied in smaller refrigeration applications, particularly in grocery stores, because they tend to operate quietly. Some models have variable-discharge ports that let them handle different pressure differentials more efficiently. They are particularly helpful when the cooling load and heat-recovery load vary widely and where the desired heat-recovery temperature varies as well.

Reciprocating chillers are also found in smaller applications, but have a lower COP than other types. Large heat-recovery applications also may use centrifugal heat-recovery chillers, which have the advantage of working efficiently at their design-condensing temperature, less so at condensing temperatures below the levels they were optimized for. Installing variable-speed drives on centrifugal compressors helps them better handle lower temperatures, but the application has to justify the extra expense of the drive.

Some heat-recovery situations may also call for a special kind of heat exchanger called a desuperheater. These devices get their name from the fact that compression often raises the refrigerant-gas temperature to a level higher than the saturation temperature in the condenser. The excess heating of the refrigerant over its saturation temperature is called superheat. A desuperheater installed between the compressor discharge and the condenser removes heat higher than the condensing temperature. But it doesn’t affect the performance of the chiller because it does not affect the condensing pressure or temperature.

© 2011 Penton Media, Inc.