Hybrid chillers and demand-response programs can help reduce operating costs, increase equipment payback
Buildings and their power and cooling systems are built to last for decades. However, they must respond to market conditions on an hourly basis to operate efficiently. This requires not only selecting the right equipment, but designing a strategy for it to operate at the lowest possible cost. Fuel prices can double in a month. Electricity prices can have even wider swings in a single day, and prices are highest just when air conditioning is most in demand. Further complicating the issue: A building owner can earn money by agreeing to participate in a utility's demand-response (DR) program, in which he or she agrees to curtail electricity use on the demand of the utility or wholesaler.
"The real gains are when you can automate your demand response, you can preprogram your strategies, and you can optimize which ones you want to use for certain conditions," Chuck Goldman, a staff scientist and group leader of the Electricity Markets and Policy Group at Lawrence Berkeley National Laboratory, said. "You can shave 5 to 15 percent off your peak demand and have the capacity to participate in emergency programs."
No nuclear plants have been built in decades. Coal plants already are hard to permit and will become expensive to operate if Congress passes a carbon tax. Natural-gas and oil prices are on the rise. Nevertheless, the demand for electricity keeps growing. To ensure reliability without adding new generating capacity, utilities are adopting various strategies to reduce peak electrical demand or shift it to other times of the day.
"A decrease in usage is just as valid as an increase in generation, and sometimes it is a lot faster," Ray Dotter, a spokesperson for PJM Interconnection, a regional transmission operator that manages the wholesale electricity market in 13 states and the District of Columbia, said. "You can open a circuit a lot faster than you can ramp up a generator."
According to the Federal Energy Regulatory Commission, the 274 entities that offered a DR program in 2008 had the combined ability to reduce peak load by 40,000 mw. Each region of the country has its own rules and incentives for participating in a DR program. In PJM's region, building owners typically signed up with one of more than 80 curtailment-service providers (CSPs), a company or organization that specializes in DR or offers it as a service. A CSP monitors power conditions and prices and sends a signal—which can be something sophisticated or as simple as a phone call or fax—to their customers requesting a usage adjustment.
"We would like to see that signaling become more sophisticated as some of the smart-grid technology gets deployed," Dotter said.
The Building Owners and Managers Association of Chicago (BOMA/Chicago), which represents more than 250 office buildings, is working on a $185 million smart-grid project that will link 262 buildings with a network operations center (NOC) at its CSP, Metropolitan Energy. The local electric utility, Commonwealth Edison, will install new electric meters to transmit usage data from the office buildings to the NOC every minute or less. Building-automation systems will be programmed to respond to the NOC's price signals, and, depending on price, reduce electrical usage through various methods, such as slowing down an HVAC system’s variable-speed drives.
"The system will allow them to rapidly change the power the building is consuming," Dotter said.
Additionally, hybrid cooling systems can cut costs while increasing reliability. Hybrid cooling systems can be set up in a variety of ways. For example, in 2009, testing firm ACT Inc.'s data center became the first in the United States to receive Leadership in Energy and Environmental Design (LEED) Platinum certification from the U.S. Green Building Council (USGBC). The data center's hybrid design uses outside air for cooling during winter and geothermal cooling during summer.
However, a more common approach is to use two different drivers, such as a steam turbine and an electric motor or gas engine, to power two centrifugal or absorption chillers. Depending on the price of electricity, gas, or steam, an energy manager could select which of the chillers to use at a particular time of day. In most cases, the motor-driven chiller would operate during the cooler hours of the day and be supplemented or replaced by the engine or steam turbine during peak hours.
The downside is a hybrid chiller system increases initial capital expenditures because two compressors, as well as piping and valving, are required to complete connections to an evaporator. However, this is offset quickly when operating costs decrease.
HVAC manufacturer York International Corp. compared the capital and operating costs of two 500-ton electric centrifugal chillers with the costs of six hybrid designs that could meet the same cooling requirements. York discovered that the hybrid systems could pay for themselves within six months to five years. (Note: Payback calculations were based on specific temperature ranges, as well as particular pricing structures for gas and electricity that may not be applicable to every building's conditions.)
One way to cut a hybrid system's capital costs while achieving the same amount of energy savings is to connect multiple drivers to a single compressor (Figure 1). While new to building HVAC systems, this approach has been used successfully with natural-gas pipeline compressors for years, allowing them to operate with natural gas or electricity, depending on cost.
Driving a chiller compressor with an electric motor and reciprocating engine or steam turbine can provide demand-side management capability, emergency or standby electricity, and/or chiller power while saving energy costs. First-cost savings can be greater with this method than with separate standby engine generators and electric-driven chillers.
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Hospitals, campus facilities, and large office complexes often experience huge differences in day-to-night energy charges. An electric utility often will pay a fee or provide lower power rates if electric demand can be reduced on-call. Many of these facilities require emergency electric-power capability, such as keeping at least some of their chillers running to provide cooling for critical computer equipment.
The mechanical-system arrangement for a dual-driven chiller is engine-clutch-motor-clutch-compressor (Figure 1). The motor should be synchronous, designed for motor and generator operation. For peak-demand-shaving operation in which the motor already is driving the chiller compressor, the engine should be started and brought up to speed under governor control. A synchronous self-shifting gear-type overrunning clutch then mechanically synchronizes the engine to the motor shaft automatically. The engine load is adjusted to pick up as much of the motor load as desired. Or, if the engine is sized appropriately—and if allowed by the local utility—the engine could handle the entire chiller load and operate the motor/generator in generator mode, feeding power back to the grid. This mode of operation could be used if the price of engine fuel is less than the cost of the motor’s electric power, which often occurs during peak load times on the power grid.
In the event of a power failure, this system could operate in one of three modes:
- The engine driving the motor/generator generates emergency electrical power with the clutch to the chiller in the locked-out position.
- The engine powers just the chiller with both clutches engaged and the motor/generator breaker open.
- The engine drives the motor/generator and the chiller, producing chilled water and electrical power.
A considerably smaller engine can be installed if it is used to drive a chiller compressor directly during emergency operation, rather than drive a generator sized to start an electric-motor-driven chiller. In this manner, starting reliability also is enhanced.
In peak-shaving mode, the motor/generator does not have to act as a generator paralleled with the grid and instead can remain in motor mode. Engine load can be adjusted automatically to pick up the greater part of the chiller load, but not pump power back onto the grid, avoiding the complex engineering studies and special electrical equipment required by a utility if a generator is going to be paralleled with its system. (A reverse current relay would be needed.) The system still would be able to generate emergency power for other loads if a transfer switch is included, but a separate engine generator would need to be paralleled with the grid for peak shaving or demand management.
A dual-driven chiller can be incorporated into a combined-heat-and-power system, particularly if a facility needs heat during winter, but not during warmer months. The engine then would be used continuously during winter, but only operate for peak shaving and demand management during warmer months, using the motor most of the time. If an engine is used, it must meet any environmental permits and regulations.
Hemant Mehta, PE, president of WM Group Engineers in New York City, said this arrangement also could work with steam turbines and electric motors, particularly when factoring in DR payments or funding from government energy-reduction programs.
"Much of the cooling in the city is done by steam-driven equipment, but the cost of steam in New York is extremely high," he said. "If you put in a motor and clutch, you can use either the utility power or the steam to run the equipment, depending on what is cheaper."
Flexible cooling systems can help building owners and managers be more confident about their facilities' energy costs. Smart investors diversify their portfolios to make money no matter what is going on in the market, and building owners can hedge against high operating costs in a similar manner. Optimum solutions vary depending on building design, steam availability, climate, and the cost of integrating a flexible cooling system with existing equipment. By examining cooling strategies that were not available only a few years ago, many building owners will find they can cut their budgets substantially.
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A technical consultant for SSS Clutch (www.sssclutch.com), a supplier of automatic over-running clutches for high-power/high-speed applications, Jim Berry has more than 40 years of experience in the sales, marketing, application engineering, and service of centrifugal and reciprocating compressors, pumps, gas turbines, and diesel and gas engines. He has a bachelor's degree in mechanical engineering from Drexel University. He can be contacted at firstname.lastname@example.org.