Part-Load Efficiency

What is Part-Load Efficiency?

Part-load efficiency refers to the ability of an HVAC system to handle part-load energy use. Factors that impact the internal load of a building include heat generated by occupants, processes, equipment, and lighting; External load variables include insulation levels, glazing and window selection, and building orientation.[1] HVAC systems are designed to serve a wide range of loads. Higher efficiency standards set by ASHRAE 90.1 and advances in HVAC technology make modern systems highly efficient at full or maximum loads. However, efficiency rates decline at part-load or when loads fall below 90%, making it challenging for most commercial buildings that routinely experience variable loads throughout the day (or as much as 99 percent of the time) to maximize energy savings.[2] Coupled with the fact that most design engineers often oversize HVAC systems, selecting equipment and technologies designed for part-load operation offer the potential for significant energy savings (see Properly Sized HVAC Equipment).[3]

Figure 1 – Multiple Boilers (Source: Rutgers University).

How to Implement Part-Load Efficiency

First, consider all aspects of the building as part of an integrated design process and incorporate energy-efficient strategies that reduce heating and cooling loads. Install smart thermostats and HVAC systems that use multiple sensors and controls to monitor and ensure proper humidity levels and temperature settings, control energy consumption in unoccupied areas, and detect and diagnose issues (see Smart Sensors and Controls). Use energy modeling to understand the impacts of different energy-efficient design measures and building operations on peak heating and cooling loads and overall system loads as well as the expected average runtime of the HVAC system under part- load conditions (see Energy Modeling). For buildings with significant part-load profiles, select equipment and components that can operate efficiently at part-load or that can modify capacity at varying loads such as variable speed compressors, fans, and pumps; variable capacity boiler plants, cooling plants and cooling towers; and temperature reset controls for hot water, chilled water, and supply air.[4] For example, variable speed compressors use an inverter known as a variable frequency drive (VFD) to run a motor at different speeds to match the load and help reduce energy consumption.

For buildings with highly variable loads, consider multiple, small, modular boilers, which permit each boiler to operate around maximum rated load most of the time and reduce standby losses. Other options include condensing boilers and modulating boilers that can run at partial capacity rather than cycling on and off.[5] To reduce central air conditioning loads and provide zoning or heating and cooling for individual spaces, consider modular, mini split-system air-conditioners.[6]

Example 

GSA’s Emerging Building Technologies – HVAC Published Findings

The GSA’s Emerging Building Technologies program regularly publishes case study findings, including innovative and cost-effective HVAC technologies such as Variable-Speed Magnetic Bearing Chiller and Variable-Speed Direct-Drive Screw Chiller, to inform decision making on federal buildings.

Benefits

Designing for part-load efficiency optimizes energy-efficiency and cost-effectively reduces energy use, water consumption, and emissions, and manages occupant comfort and indoor air quality.

Costs

The cost of implementing part-load efficiency varies depending on the equipment and system selected. Considering part-load performance when designing and installing a system saves on first costs as well as operating costs.[7]

Resiliency

Part-load efficiency increases resiliency by decreasing energy and water consumption, including peak loads, which decreases reliance and stress on the electricity grid, natural gas, and water and wastewater infrastructure.

[1] Burdick, Alan. 2012. “Strategy Guideline: HVAC Equipment Sizing” Prepared for the US DOE Building America Building Technologies Program. https://www.nrel.gov/docs/fy12osti/52991.pdf (accessed November 27, 2018).

[2] Whole Building Design Guide. 2016. High-Performance HVAC. National Institute of Building Science. https://www.wbdg.org/resources/high-performance-hvac (accessed Nov 28, 2018).

[3] Whole Building Design Guide. 2016. High-Performance HVAC. National Institute of Building Science. https://www.wbdg.org/resources/high-performance-hvac (accessed Nov 28, 2018).

[4] Whole Building Design Guide. High-Performance HVAC. http://www.wbdg.org/resources/hvac.php (accessed June 5, 2018).

[5] United States Department of Energy Office of Energy Efficiency and Renewable Energy. Purchasing Energy-Efficient Commercial Boilers. https://www.energy.gov/eere/femp/purchasing-energy-efficient-commercial-boilers (accessed June 5, 2018).

[6] US DOE- Energy Savers. “Ductless, Mini-Split Air Conditioners.”

https://www.energy.gov/energysaver/ductless-mini-split-air-conditioners (accessed Nov 29 2018).

[7] Whole Building Design Guide. http://www.wbdg.org/resources/hvac.php (accessed March 20, 2011).