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 year-long, real-world conditions. Accordingly, energy standards have adopted new rating methods to evaluate equipment efficiency during part-load operation.
In Part 1 of this two-part article series we examined how these standards are evolving. The article can be found at https://coolingbestpractices.com/technology/refrigeration-compressors/advancing-standards-and-equipment-improve-building-energy. This article looks at how technologies are advancing to deliver more part-load energy savings.
Improving Part-load Performance
The continuing push for more efficient HVAC systems is challenging 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.
Danfoss VLT® HVAC Drives are designed to improve part-load performance of fixed-speed scroll compressors.
Advances in Compressor Configuration
Substantial advances have been made in compressors, which are the primary energy users in chiller 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 was to be phased out by ASHRAE 90.1 in 2016 for systems above 65,000 Btu/hr., as regulators have recognized 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.
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