Editor's note: In March 2006,1 Ben Erpelding, PE, CEM, discussed the optimization of entire central plants through utilization of variable-frequency drives (VFDs) and relational, demand-based control. A 0.5-kw-per-ton average wire-to-water central-plant efficiency was established and a case study presented. Over the last two years, Erpelding's firm has adapted and implemented that design and control philosophy for entire building HVAC systems. In this article, Erpelding establishes a new benchmark: average annual building cooling efficiency (Figure 1).

FIGURE 1. Average annual building cooling efficiency. Input energy includes all chillers, chilled-water pumps, condenser-water pumps, cooling-tower fans, supply- and return-air-handler fans, and exhaust fans.

How many of us have been in buildings in which the facility engineer set the chilled-water set point to a higher value in hopes of saving energy? While that may have resulted in improved chiller efficiency, it was at the cost of increased distribution-pump energy, decreased dehumidification, and even increased air-handler-fan energy.

Figure 2 is a diagram of a simplified all-variable-speed HVAC system. During design, balancing energy use is fairly easy, having led to low-temperature (both chilled water and air-handling-unit [AHU] supply air) designs, as well as the 2-gpm-to-3-gpm-per-ton condenser-selection debate. However, when it comes to operating all-variable-speed systems, the key question is how to avoid "robbing Peter to pay Paul."

FIGURE 2. All-variable-speed HVAC system. How do you control to avoid "fighting" and instability? 2

HVAC-SYSTEM ENERGY PERFORMANCE

How do you balance chilled-water-supply temperature and chiller, pump, and AHU-fan energy over all cooling loads and outdoor wet-bulb conditions? How do you provide a stable and energy-efficient balance between AHU supply-air temperature, chilled-water-plant energy, and fan energy? Recently, energy engineers have suggested resetting temperatures based on outside-air temperature, chilled-water-valve position, and variable-air-volume- (VAV-) box position. Similarly, static- and differential-pressure-reset strategies have been employed; however, these advanced "trim-and-response" methods are time-consuming, highly customized, and very difficult to implement and support.

To achieve optimum results, a control strategy must address both chilled-water-plant and air-handler-system efficiency.

Chilled-water-plant efficiency is analyzed best using kilowatt-per-ton bin analysis, in which plant efficiency is plotted against plant capacity, annual operating hours, and average outdoor wet-bulb temperature (Figure 3).

FIGURE 3. Kilowatt-per-ton bin analysis of chilled-water-plant efficiency.

Average annual efficiencies of central plants were presented in the article "Real Efficiencies of Central Plants."3 Depending on equipment selection, design, and climate, wire-to-water efficiency can vary from 0.4 to 1.5 (and, sometimes, more) kw per ton.

Air-handler efficiency can be determined by examining flow and fan-power draw. Some of the most common VAV systems are those utilizing mechanical inlet guide vanes to vary capacity. Also very common are pneumatic and direct-digital-control (DDC) VAV systems utilizing VFDs and a constant static-pressure set point measured two-thirds of the way down a duct. Typical operating static pressures at the duct-sensor point range from 1.0 to 2.5 in. wc. In Figure 4, these two system types are compared with theoretical fan-law operation.4

FIGURE 4. Percent power vs. percent design airflow for a VAV AHU fan.

RELATIONAL CONTROL

Figure 4 shows a power draw at 50-percent flow of 62 percent for a VAV system utilizing mechanical inlet guide vanes, 48 percent for a VAV system utilizing VFDs and a constant static-pressure set point, and 12.5 percent for the theoretical "best" fan law. This indicates a failure to realize full power-reduction potential at part-load operation. Thus, the potential for tremendous energy savings exists.

Use of stand-alone proportional-integral-derivative (PID) control techniques seriously undermines the performance, stability, efficiency, and reliability of modern building comfort systems.5 One alternative is relational control, which has been implemented in dozens of facilities around the country. Relational control involves:

ULTRAEFFICIENT-COOLING-PERFORMANCE EXAMPLE

1. Power-based speed control. Optimum HVAC efficiency is achieved when calculated relationships between cooling-tower-fan, condenser-pump, chiller, and AHU-fan energy are maintained. These nonlinear relationships can be developed mathematically based on design equipment selection, patented algorithms, and the Equal Marginal Performance Principle: 6

  • CoolingTowerkw is a function of Chillerkw.

  • CondenserWaterPumpkw is a function of Chillerkw.

  • Optimized chilled-water set point is a function of ChilledWaterPumpkw, Chillerkw, and AHUFANkw.

  • Optimized VAV supply-air temperature is a function of marginal chiller-plant kilowatts per ton vs. ÓMarginal AHUFAN kw .

2. Power-based sequencing of equipment. Is it more efficient to run two variable-speed chillers at 40 percent or one at 80 percent? Sequencing is a function of equipment selection, Equipment kw , and load.

3. Iterative network control of AHU fans and chilled-water distribution pumping. As shown in Figure 4, the use of minimum pressure set points can contribute to substantial fan- (and pumping-) energy waste, especially during periods of low loads, which are frequent in the United States. Optimization of AHUs and chilled-water distribution pumping can be accomplished without the use of static or differential pressure:

  • VAV AHU-fan speed (AHUFANRPM) is a function of ÓVAV-box cfm and damper position.

  • Chilled-water-pump speed (CHWPRPM) is a function of ÓAHU valve position.

4. Demand-controlled ventilation. Carbon-dioxide, enthalpy, and return-air relative-humidity sensors are used to control outside-air dampers, economizers, and minimum and maximum supply-air temperature. These sensors not only reduce energy use, they ensure adequate indoor-air quality.

5. Iterative automatic balancing of terminal units. With relational control, there is no need to manually balance VAV systems. Comfort is maintained, while complaints, rogue zones, and maintenance costs are reduced.

6. Continuous performance verification. With relational control, the overall efficiency of a system is monitored remotely on a real-time basis. Initially, the resulting data can be used to verify system performance and for utility-incentive documentation. Over the long term, it can be used by service and operations staff to determine when maintenance or other corrective measures are necessary to keep performance at commissioned levels.

The impact of "whole-system" relational-control strategies encompassing both chilled-water and air-handling systems can be seen in the following example.

An office and laboratory facility in Southern California was upgraded to all-variable-speed operation. The system is served by a 500-ton primary-only chilled-water plant (VFDs on all chillers, cooling-tower fans, chilled-water pumps, and condenser-water pumps) and nine VAV air handlers. Two of the air handlers serve biology laboratories equipped with a variable-control-valve system with face-velocity and occupancy sensors on fume hoods. All systems operate 24 hr a day, seven days a week. Figures 5 through 8 show different performance parameters for the all-variable-speed facility.

FIGURE 5. Percent power vs. percent design airflow for AHU with relational controls (2008 data).
FIGURE 6. Percent power vs. percent design airflow.
FIGURE 7. Static pressure measured two-thirds of the way down a duct in a facility with demand-based controls. Though not a control point, static pressure is measured and used as a high-pressure limit (safety).
FIGURE 8. Total efficiency for the building cooling system. August 2008 average: 0.85 kw per ton.

Conclusion

Over the last several years, measurement and verification data have shown many "green" office facilities operating well above the 1.4-kw-per-ton benchmark in Figure 1. Research has shown tremendous opportunity to significantly reduce building energy use by taking advantage of new relational-control theories. We have learned that:

  • The single-variable feature of PID control makes realizing the full power-reduction potential of part-load operation of VFD components difficult.

  • The independent nature of PID control modules makes coordinating components to achieve optimal operation under all operating conditions difficult.

  • Eliminating direct static-pressure- and differential-pressure-control strategies (fan and pump) creates new opportunities for optimization.

  • Eliminating direct temperature-control strategies (cooling-tower fan, chilled water, and AHU supply air) creates new opportunities for high-performance operation.

Control strategies based on relationships between subsystems enable the energy use of various pieces of equipment to be balanced and allow you to efficiently rob Peter to pay Paul.

References

1. Erpelding, B. (2006, March). Ultraefficient all-variable-speed chilled-water plants. HPAC Engineering, pp. 35-43.

2. Hartman, T., & Erpelding, B. (2006, September). Relational control: Simple, ultraefficient all-variable-speed HVAC. Engineering Green Buildings Conference and Expo, Austin, TX.

3 Erpelding, B. (2007, May). Real efficiencies of central plants. HPAC Engineering, pp. 14-24.

4. Hartman, T. (1993). Terminal regulated air volume (TRAV) systems. ASHRAE Transactions, 791-800.

5. Hartman, T. (2002, July). PID control: May it rest in peace. HPAC Engineering, p. 9.

6. Hartman, T. (2005). Designing efficient systems with the Equal Marginal Performance Principle. ASHRAE Journal, 47, 64-70.

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The director of engineering for Optimum Energy LLC and a member of HPAC Engineering's Editorial Advisory Board, Ben Erpelding, PE, CEM, has more than 12 years of experience related to energy efficiency, distributed generation, renewable energy, and demand response. To date, he has performed more than 500 detailed HVAC-energy assessments. For six years, as part of the non-profit San Diego Regional Energy Office, he measured and verified actual performance and cost savings for energy-efficiency retrofits, photovoltaic installations, demand-response audits, and combined-heat-and-power projects.