taking the long view

Engineers are studying methods for controlling the world's ever-growing appetite for energy.

By Peggy Chalmers

Decisions by mechanical engineers influence every stage in a product's life span—from the time before it is manufactured through its obsolescence and disposal. The same decisions also determine how the product will affect the world in which it is used.

Urged on by economic demands, social concerns, or environmental considerations, a growing number of engineers have turned their attention to the way products use energy at every step, from manufacture to operation and recycling.

For example, taken together, computers make a hungry consumer of electricity, and they often are together, clustered in server hotels and corporate systems. Data centers of major corporations have voracious appetites for power, and what goes in as electricity comes out as heat. A large center may take 10 megawatts to operate and an additional 5 MW to keep cool.

But that isn't the only issue. When Avram Bar-Cohen, professor and chairman of mechanical engineering at the University of Maryland in College Park, considers a product's energy requirements, his timeline starts with the energy required to mine the raw materials used in the products.

According to Bar-Cohen, an ASME member, "If we are going to address sustainability, we have to look beyond just the immediate energy needs. We have to look at the total energy investment and find ways to minimize it."

Bar-Cohen's views developed when he became intrigued by the differences between the minimum-mass heat sinks he was designing and those based on minimizing entropy generation. "In a sense, both designs were trying to do the same thing: minimize energy consumption," he said, "but the two heat sinks looked so very different."

It dawned on him that the differences arose from different energy design goals. Minimum entropy adherents were trying to minimize the operating energy, such as that consumed by the cooling fan over the life of the heat sink, while he was trying to minimize the fabrication energy. It became obvious that to achieve minimum energy consumption engineers had to deal with both concerns.

"Engineers have to consider how energy is going to be allocated between the fabrication and operation over the life of the product," Bar-Cohen said. "Heat sinks are a convenient way to easily understand the issues, but the same approach applies when building heat exchangers, power plants, or furnaces."

Cool Data

Even the most sophisticated data centers are primitive when it comes to cooling. Air conditioning is sized by adding heat loads with no consideration for air movement within the room. Supply and return vents are distributed based on intuition and, in some cases, the air conditioners are fixed capacity. Typically, there is only one temperature sensor located at the air conditioner return, and adjustments are based strictly on the temperature of the return air.

"That is prohibitively expensive," said Chandrakant Patel, principal scientist at Hewlett-Packard Laboratories in Palo Alto, Calif. "Cooling air needs to be distributed based on the local heat load."

To understand what was actually happening to airflow and temperature distribution within a data center, Patel and his team created detailed computational fluid dynamics models. The models revealed that minor changes in rack arrangements have a major impact on the cooling system. Small changes in layout can produce positive pressure gradients at the ends of aisles triggering recirculation paths and creating hot spots. The hot air is then forced back into the servers.

The HP team can now model a data center that has a given distribution of heat load and air conditioning, and optimize its layout for minimum energy usage. The models verify the air conditioning requirements and identify optimum A/C and venting locations, producing an estimated energy savings of 25 percent. Assuming $77 per MWh, a 15-MW data center would save about half a million dollars annually, Patel said.

Dynamic Shifting

A data center is not a static environment. Equipment is frequently added or replaced, and workloads shift. To deal with these issues, the HP team is developing dynamically smart cooling in which the A/C capacity varies, and vents open and close based on local computer workload.

Farther down the pike, Patel plans to incorporate dynamic shifting of computer loads.

Most data center equipment is not used 100 percent, and computing loads would be transferred from underused equipment and concentrated on computers where local A/C would be used at its most energy-efficient rate. The extra servers and their associated air conditioners would go on standby. Load shifting could add an additional 10 percent to the 25 percent energy savings achieved with layout optimization modeling.

"When you move loads around, you have to know what will happen," Patel pointed out. "You just can't do it arbitrarily."
Knowledge takes the form of a 3-D thermal view of the data center volume. Key chips, components, and racks are instrumented with wireless sensors feeding thermal data from as many as 8,000 points in a 1,000-rack data center into an energy management computer.

A combined heating, cooling, and power system at the University of Maryland is the subject of study for increased energy efficiency on a small scale, up to 100 kw. The goal is a plug-and-play design.

The points relay only what is happening in the racks, not what is happening in the aisles. When an event occurs, such as an equipment failure, a robot is dispatched to that location to measure temperatures at various points in the aisles. This data is transmitted to the energy management computer, where it is used to fine-tune the air conditioning and computer resources.

"If a rack jumps, say, from 60 percent to full power, intuition says to open the vent in front of the rack," Patel said. "But if it is the end rack in a row, opening that vent would entrain hot air from the exhaust aisle. We are creating hot and cold zones that are fluidically separated with minimal infiltration from one to the other."

An interesting twist to Patel's thermal investigations has been the testing of the company's inkjet print head to dispense a cooling spray of inert dielectric fluid, or possibly water, on small high-heat-density devices.

"The beauty of the inkjet is that the amount of spray and the pattern can be controlled so that it operates in the continuous-phase-change regime," explains Patel, "and high heat flux is carried off by vaporization."

Free Heat and A/C

Getting something for nothing is an intriguing concept, and combined heating, cooling, and power systems might be just the place to find it.

"When you consider that a building needs electricity anyhow, the heating or cooling can essentially be free by making good use of waste heat from electrical power production," said Reinhard Radermacher, professor of mechanical engineering at the University of Maryland.

CHP technology uses waste heat to extract free heating or cooling from systems in the 30-kW to 100-kW range. They are suitable for small commercial applications, such as small buildings, fast food restaurants, and the like. While these units are practical today with certain utility rate structures, their market penetration will increase as they become more efficient and of lower cost as systems are more effectively packaged, Radermacher said.

While the actual energy saved will depend on climate, location, type of building, and building operating schedule, Radermacher, an ASME member, believes these combined systems can achieve a 30 percent reduction in energy consumption. To reach that point, much work will be required in component development, control algorithms, and packaging—all areas Radermacher is addressing.

Smaller systems have poorer efficiencies than their larger counterparts, partly because leakages don't decrease with system size so that their relative impact becomes worse. Parasitic power is also a problem. This is the power consumed by minor, but necessary, system components, such as the pumps and fans. Often this drain is overlooked, but it can significantly decrease system efficiency.

"Control strategies don't exist to handle all the numerous unanticipated operating states," Radermacher said. "For example, you sometimes may want to run a power source alone. There's a damper that cuts off the exhaust from the adsorption chiller and it has a leakage rate of one-half percent. That is enough to potentially overheat an adsorption chiller that is not operating."

Eventually, the goal is plug-and-play systems with all the pieces packaged as a single unit. Until this happens, costs will remain high. One stumbling block has been that the existing pieces are optimized for their particular functions, not for a cooperative effort.

"We throw them together in a building and expect them to work as an optimized system, and they don't," said Richard S. Sweetser, president of Exergy Partners Corp., a Herndon, Va., energy consulting firm under contract to the U.S. Department of Energy. "We need combined functions in a system that is easy to install and simple to operate."

The DOE is trying to address this issue and has funded seven teams to work on packaged and modular systems.

A Better Refrigerant

Radermacher is also working on environmentally safe refrigerants and applies this work to combined systems.

Hydrofluorocarbon refrigerants that are currently used in the United States are safe for the ozone layer, but may contribute to global warming. Hydrocarbon and CO₂ alternatives are more environmentally friendly, but suffer in performance and present other problems. Hydrocarbons can burn, and CO₂ systems require higher operating pressures and may cause problems for those with breathing impairments, if there is a refrigerant leak into a confined space.

Currently, hydrocarbons are used throughout Europe and Asia, where the average refrigerator size is much smaller than in the U.S. Safety is not an issue because the amount of fluid is very low—approximately 20 grams of butane, equivalent to a cigarette lighter. A refrigerator suitable for the U.S. market would probably take about 80 grams, far above the 50-gram UL limit on refrigerators.

Using CFD, Hewlett-Packard researchers learned that poorly placed vents (modeled at top) can cause undesirable mixing of air (shown in green).

Radermacher feels that both hydrocarbons and carbon dioxide could serve as alternatives to the R-22 (HCFC) and R-134A (HFC) presently used in residential air conditioners, vending machines, food display cabinets, and hot-water heat pumps. Heat pump water heaters using CO₂ are just now appearing on the market in Japan.

"For CO₂ applications, it may be necessary to use a two-stage refrigeration cycle or an expander," Radermacher said. "We are working on both. If the high-temperature waste heat is used to heat hot water, the system may outperform a conventional refrigerator."

By melding an environmentally safe refrigerant with the energy savings of a combined cooling, heating, and power system, Radermacher will be taking two steps toward improving the environment instead of one. The improvements are targeted for small systems with a potentially sizable market.

End of LiFE

Hewlett-Packard operates a huge recycling program, with plants that handle as many as three million pounds a month of its own and other computer vendors' products. With the mountain of obsolete computer equipment growing, the company knew it had to alter its design approaches if it were to slow that growth.

"If we could reduce the amount of material used in products, it would reduce the energy needed to build and recycle them," said Renee St. Denis, manager of HP's product recycling solutions organization in Roseville, Calif. "So we started working closely with our product divisions to help them understand the impact their design decisions made on our ability to recycle the product."

Input from the recyclers, along with efforts by the company's "design-for-the-environment" program, have produced dramatic paybacks, notably in printer production. The inkjet print head and cartridge have been redesigned into two pieces instead of one, so the head is no longer discarded along with the old ink cartridge.

The change in design reduced raw material consumption by 40 percent and solid waste generation by 93 percent. Based on estimates of gathering, transporting, and refining materials, and manufacturing parts, HP says the design reduces wastewater production by 92 percent and air emissions by 67 percent per page printed.

Even the product packaging has shrunk. A redesign of packaging for inkjets sold with printers saved 1,300 tons of paper and paperboard in the first 15 months, and significantly reduced production costs.

While significant environmental strides have been made, there is still a lot to be done. For the engineer who wants to make a difference, the timing may be perfect.

"There is more sensitivity to environmental issues in the boardroom and among executives than there has been in the past," Bar-Cohen said. "I think mechanical engineers have an opportunity to take the lead and do something positive, instead of being defensive about their work. It's a great way to get their work aligned with their values."

Peggy Chalmers, a frequent contributor to Mechanical Engineering, is a freelance technical writer based in Sunapee, N.H. She holds an M.S. degree in mechanical engineering from Drexel University.

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