How advancing standards and technologies can capture more part-load energy savings

This article is the conclusion of a two-part series on part-load efficiency. Part 1 examined how standards are evolving. Part 2 looks at how technologies are advancing to deliver more part-load energy savings.

By Oddgeir Gudmudsson and Jan Eric Thorsen, Danfoss April 6, 2017

Learning Objectives:      

  • Understand what district energy networks are, including the relationship to combined heat and power (CHP).
  • Recognize the potential of a district energy network as a solution for more efficient building heating and cooling.
  • See real-world examples of successful district energy network projects globally.

In recent years, the HVAC industry has enlarged its vision from focusing on equipment efficiency measured in terms of ratings points at specific conditions to include a whole-building perspective that uses models of yearlong, real-world conditions. Accordingly, energy standards have adopted new rating methods to evaluate equipment efficiency during part-load operation.

Part 1 of this article examined how these standards are evolving. This article will look at how technologies are advancing to deliver more part-load energy savings. 

How technology is advancing to improve part-load performance

The continuing push for more efficient HVAC systems is challenging original equipment manufacturers (OEMs), designers, and specifiers to find new ways to improve part-load efficiency.

In the past, the HVAC industry had it relatively easy when it came to achieving high energy efficiencies at full load. Selecting the optimum heat exchangers and compressors usually sufficed in designing equipment to meet the required load. When it was necessary to run the system at less than full load, various low-cost techniques were applied to turn down capacity, such as hot-gas bypass and compressor step modulation. But with systems optimized for full-load efficiency, this technique and others are inefficient, because the high compressor efficiency cannot be maintained at part load without capacity modulation to better match the load.

Designing systems to operate efficiently at part load, therefore, requires new thinking and a new set of technologies. The main idea is to apply technologies that can modulate capacity when encountering varying loads.

Advances in compressor configuration

Substantial advances have been made in compressors, which are the primary energy users in systems.

Driven by cost, designers typically use two approaches for the majority of residential and commercial systems in the U.S. market. The first is to employ a single fixed-speed compressor on a single circuit. The second is to use multiple circuits, each with a single fixed-speed compressor.

The first design approach will be phased out by ASHRAE 90.1 in 2016 for systems above 65,000 Btus/h, as regulators have recognized that this design is an inefficient way to meet the building load throughout the year. It also reduces the lifetime of the compressor by constantly short cycling on and off.

The second design improves the system’s capacity modulation by essentially creating two systems that are half the size of the total system. This approach allows the use of two uneven circuits, which enable the system to better match varying loads. Also, this configuration improves uptime, because one circuit can function independently while the other is down.

In such a system, technology that can assist fixed-speed compressors to handle varying load and pressure conditions—such as intermediate discharge valves (IDVs)—can further improve efficiency. Nevertheless, there are system designs that offer better capacity modulation and can capture more benefits from IDV technology, as well as other technology like variable-speed fans.

A more efficient configuration employs multiple compressors in one circuit to further improve system part-load performance. Multiple compressors may be configured in tandem or in so-called parallel configurations. When several compressors are installed in parallel, one or more compressors can be turned off and kept off for a longer period of time. Further capacity modulation can be achieved by using either unequally sized fixed-speed compressors in parallel or three compressors in parallel to turn down capacity from 100% to 66% to 33%.

Finally, variable-speed compressors take the industry to the energy efficiency limit of what today’s compression technology can provide. The majority of these applications are single compressors on a single circuit, but they can also be applied in parallel with fixed-speed compressors to further increase the turn-down ratio or achieve a specific capacity. Studies show this latter configuration provides superior capacity modulation while maximizing efficiency at partial loads.

Parallel configurations and variable-speed compressors are further enhanced by technologies that improve part-load efficiency, such as fixed-speed compressors with IDV and variable-speed fans. In fact, these technologies do more to enhance efficiency as more capacity modulation is used.

Advances in compressors

Today, advancements in nearly every type of compressor—scroll, reciprocating, screw, and centrifugal—provide efficient methods of handling varying loads.

New technologies have been developed to improve the part-load performance of fixed-speed scroll compressors. For example, the IDV helps the compressor respond to varying system load and pressure conditions, although the compressor itself doesn’t change speed as conditions change. IDV broadens the pressure ratio to allow a compressor to be optimized for an increased number of applications. The ability of the IDV to respond quickly to part-load conditions helps limit compression overshoot, which improves efficiency and also reduces mechanical stress on the scroll components.

For variable-speed compressors, an inverter, also known as a variable frequency drive (VFD), runs the motor at different speeds, thereby modulating refrigerant flow and cooling output.

Speed reductions that precisely match the load help reduce energy consumption—as much as 50% or more depending on the application. Less energy is wasted since the variable-speed compressor delivers precisely the capacity required by speeding the motor up and down.

There are operational benefits as well. A VFD uses an inverter switching circuit to change incoming ac power to dc, which is then output as an ac-like sine wave that can modulate in a controlled fashion. Consequently, a VFD eliminates the big inrush of line-voltage ac at start-up. The resulting "soft start" reduces amperage draw, which extends motor life and reduces costly peak electric usage.

However, variable-speed compressors require sophisticated engineering. The compressor itself must be rugged enough to handle repeated acceleration and deceleration from 110% to as low as 10% of its full-speed rating.

A major challenge in designing a variable-speed compressor is maintaining proper oil circulation. With scroll compressors operating at low rpm, for example, oil circulation is poor because the mass of refrigerant and oil flow is reduced at slower speeds. Consequently, compressor designers use dedicated internal systems to ensure adequate oil circulation as compressor capacity turns down. High rpm present other challenges including the higher speeds that place more stress on internal parts. To withstand the added stress, careful selection of the materials and the oil and wear is required to ensure compressor reliability.

For advanced centrifugal compressors, however, other techniques are used to handle changing refrigerant gas pressures and flows at part load. Variable-geometry inlet guide vanes and/or variable-geometry diffusers are used to maintain compressor stability when refrigerant flow is reduced. As far as lubrication is concerned, magnetic bearings have been developed that allow the entire oil system to be jettisoned. With magnetic bearings, the compressor shaft rotates in a magnetic field without physically contacting the bearing in normal operation. This eliminates the need for an oil system and all of its side effects—oil films on heat-exchanger surfaces and oil stacking in the evaporator at low loads.

Advances in fan motors

Technological advances have also boosted the efficiency of another major energy consumer—the fan motor. As with compressors, the main energy-saving strategy has been to use conditioned dc instead of the incoming line voltage of ac to modulate motor speed.

Reducing motor speed at partial loads produces immense energy savings. Fan motors obey affinity laws for turbomachinery, in that reducing speed exponentially reduces energy use. For example, cutting speed by 20% decreases power consumption by 50%.

In HVAC applications, significant fan-speed reduction is obtained with two types of motor technologies: electronically commutated motors (ECMs) and VFDs applied to ac induction motors.

ECMs, also known as brushless dc motors, have been used for decades. They employ permanent magnets rather than brushes and electricity to create a magnetic field in copper windings. That feature alone makes them about 30% more efficient than "squirrel cage" ac induction motors.

To control ECM fan motor speed, however, integrated electronics must be used to continuously adjust rpm based on control input. ECMs operate at relatively lower power (below 750 kW) and are often used as external-rotor fan and blower motors.

Three-phase ac induction motors, however, remain the workhorse for HVAC applications. They are found in applications below 750 kW as well as in huge 1500-kW motors for cooling tower fans.

While ac motors are available with optional external-speed controllers, they don’t always operate at optimum efficiency at slower RPM.

VFDs must be employed to efficiently control 3-phase ac motors at various speeds. Reducing the frequency of the conditioned sine signal reduces motor speed. Varying the applied voltage reduces torque.

For users, it is important to examine each application carefully to see what type of motor is most efficient—and what type of control delivers that efficiency in real-world conditions.

Advances in controls

According to a major motor-manufacturer association, approximately 10% of the potential savings in drive systems can be achieved by using motors with higher efficiency. By applying variable-speed technology, however, potential savings of approximately 30% can be obtained.

The best means of attaining maximum savings (approximately 60%) is by optimizing the overall system. Using components that improve overall system efficiency makes major energy reductions possible.

For example, capacity modulation of variable-speed compressors can only be obtained with an electronic expansion valve (EXV). An EXV allows condensing temperatures to be reduced to the lowest possible minimum, enabling capacity to be turned down. In contrast, thermostatic expansion valves (TXVs) do not offer a wide enough dynamic range to allow the system to remain stable at low pressures. As a result, higher condenser temperatures must be maintained, which wastes energy.

For fans, efficient operation of ECMs requires an optimal combination of frequency converter, motor, and fan impeller. Similarly, the algorithms in a VFD need to be tuned to match the individual application and motor to achieve optimum performance. Today, advanced frequency converters and VFDs are available with automatic tuning capabilities, programmable setpoints, and compressor/fan motor cycling to make system optimization at part-load conditions easy to achieve.

Bottom-line benefits

Today, contractors, consulting engineers, and building owners—in addition to HVAC OEMs and equipment designers—have a vested interest in technology optimized for part-load efficiency. Technologies that can turn down capacity to maximize energy savings at part load are financially attractive when energy costs are high. In addition, they provide further energy savings in milder climates, because these areas spend up to 99% of the year at part-load conditions. In those circumstances, when part-load technology is properly deployed, the actual energy efficiency obtained at real-world conditions exceeds the IEER or IPLV energy-efficiency rating printed on the equipment label. Regardless of energy prices, improvements in part-load efficiency reduce a utility carbon dioxide (CO2) emission, which addresses concerns about climate change.

New technologies are also being developed to improve whole-building efficiency. These trends include connectivity and electronic devices, more precise system control and monitoring, and peak-load management tools. These developments will, in turn, drive further development and adoption of variable-speed and other innovative technologies.

Advancing part-load efficiency in standards and equipment will significantly contribute to building performance, as well as nurture an energy-efficient ecosystem of technology, standards, and policies that will grow energy savings and reduce CO2 emissions for years to come.

Oddgeir Gudmundsson is director of projects at Danfoss. He is responsible for driving the technical development of new and renovation district heating projects. Jan Eric Thorsen, director of the Application Center at Danfoss, is responsible for consultancy on projects and technical development within district heating.