Variability » Efficiency Tweaks for Your A/C
A/C systems can be set up to use 30-50% less power if variable control technologies such as variable speed drives (VSD), variable air volume (VAV), and variable refrigerant flow (VRF) are used. These technologies allow A/C systems to operate efficiently at part-load, lowering energy use and extending lifetimes. Find out more about how these can fit into your current site with our guidelines on the relative advantages for each type.
Variability Technologies & Strategies to Boost Efficiency
While some people might view variability as a bad thing that denotes a lack of consistency or fixed patterns, this can be far from the truth. In fact, variability actually implies flexibility and the ease of adapting effectively to a given environment. This is especially true when it comes to operating heating, ventilation, and air conditioning (HVAC) equipment as well as industrial machinery. Taking a variable approach means that equipment doesn’t have to run at full-load continuously, which significantly reduces the required energy input, while reducing associated wear and tear.
In this article we’ll look at some of the ways in which variability can be applied to air conditioning (A/C) systems to improve overall energy efficiency and extend equipment life. First we’ll elaborate on why this approach should even be considered at all, before moving on to explaining the options and how these can be incorporated across various situations. While we aim to provide a high level overview to these subjects, we also provide plenty of references for further detail on each topic.
Why Consider Using Variable Equipment?
Each of the different technologies available have a way of reducing power usage by 30-50% for the equipment they control. This power reduction potential applies regardless of whether they are HVAC-related such as motors, pumps, fans, and compressors, or more industrial, e.g. conveyor belts, hoists, and drilling machines. As a result, the power input modulates according to the need, leading to significant savings from running at part-load.
Accompanying the energy reductions, these technologies are flexible and adaptable and provide better control, improved performance and increased comfort and reliability in air conditioned spaces. Additionally, the lifespan of equipment increases from a reduction in stress due to work at lower part-loads.
What Devices Are Available?
Depending on your site and the current setup, it’s likely that two of the three technologies we’ll discuss can be utilised on site. The first (variable speed drives) can be employed almost anywhere that motors are found, while one of the other two (variable refrigerant flow or variable air volume) is applicable depending on the type of A/C on site.
You may even have some of these installed already, so it is important to have a solid preventative maintenance plan. This ensures that they continue to deliver the expected savings and lets you know when it’s time to update or replace appropriately. During a review of your equipment you may also note locations ripe for further improvements, such as additional motors in your HVAC system or industrial equipment being used in the conditioned space or elsewhere on site.
Variable Speed/Frequency Drives (VSD & VFD)
Practically speaking, the two terminologies of VSD and VFD are used interchangeably, and refer to a technology that manages the power input (current, voltage, and/or frequency) to control the speed and torque of a motor in line with what’s required. Technically speaking, VSD is the more generic category that can be used on AC or DC motor applications, while a VFD is AC specific. Other terms often used in the HVAC industry for the same thing are frequency inverters or simply inverters.
The motors that can be driven by these devices are used in a variety of applications, especially for variable torque loads which include pumps, fans, and compressors used in HVAC environments, but also for sewage and irrigation pumps. A VSD in conjunction with this type of load presents the highest savings potential, as the power varies with the cube of speed. If that seems confusing, it just means that as speed reduces, power reduces at an even quicker rate. For instance, a reduction of speed by 20% results in a power reduction of 49% (IDS shows how the cube law is applied).
VSDs also have a home with constant torque loads that are found in general industrial machines such as conveyors, hoists, rotary compressors, and printing presses. Likewise, VSDs can be applied to constant horsepower loads found in the machine-tool industry covering large drilling machines, lathes, and wheel-grinders. Although VSDs provide greater energy efficiency in all their applications, their improved control proves particularly useful in process manufacturing, where flow control is important. With soft-starting operation they also improve power quality usage on site. EC&M provide a further look into different applications for VSDs and their benefits.
According to equipment manufacturer, ABB, more than 65% of industrial electrical energy is consumed by motors, yet less than 10% of these motors are equipped with VSDs. This highlights a vast opportunity to reduce electricity consumption at a large scale. Furthermore, since the footprint of a VSD can vary from the size of a lunch box to a car depending on the application, their sizing makes them flexible enough to fit in almost anywhere. Returns are attractive too, often paying themselves off from the energy savings in a few months to a year in line with their individual purpose.
Variable Refrigerant Flow/Volume (VRF or VRV)
Used interchangeably, VRFs and VRVs refer to air conditioning systems that have a single outdoor unit and circulate refrigerant to different spaces for either heating or cooling. Basically, these systems circulate the minimum required amount of refrigerant to each area to achieve the desired level of heating/cooling. This variable flow rate is enabled by a VSD (a.k.a. inverter) connected to the compressor and allows for varying loads across conditioned spaces and even heating/cooling in neighbouring rooms. As a result, they are also referred to as heat recovery systems since they can simultaneously provide both heating and cooling. This system first designed by Daikin in 1982 and trademarked as VRV, has energy efficiency benefits and additional advantages in climate control and flexibility.
Variable Air Volume (VAV)
VAVs might be referred to as terminal units, VAV boxes, VAV units, or outlet boxes, and refer to devices that modulate the volume of airflow to a space while providing it at a constant temperature rather than supplying a constant volume of air at a varying temperature. They work in conjunction with an air-handling unit (AHU) from a central ducted A/C system providing either heating or cooling along with zoning. The dampers in the VAV system move through different positions and provide feedback to the AHU blower so that it can vary the fan speed according to the required demand.
While VAVs have high potential for efficient performance, they do have limitations. Included in the system are various controls working with numerous sensors and actuators. Due to the large number of sensors, a building management system (BMS) and regular maintenance are required to ensure failures and faults are identified and rectified quickly. Even minor issues not remedied have drastic effects on energy consumption and comfort.
These systems have been installed in shopping centres and large office buildings since the 1970s, and less commonly in some small buildings. For a further look into what VAVs are and how they work in conjunction with the rest of a central A/C system, High Performance HVAC has put together an informative overview.
How Can I Utilise These Technologies?
Hopefully by now you have a basic understanding of the options and can start to see why looking to variable control technology is beneficial. Now we’ll look further in depth at the advantages of each technology and provide recommendations for new equipment or maintaining your current savings with good practices.
There have been continual advancements in VSD technology, which translates into strong benefits whether you are replacing your decades-old drive or expanding the programme for new motors.
Maintenance and equipment reviews are vital to ensure that you continue to receive the benefits of the technology. While there is a fallback option that allows the motor to run on bypass if the VSD fails, it is only meant to be a temporary solution until it is fixed or replaced. Unless frequently monitored, such equipment can unknowingly be running in bypass mode for an extended period at +50% more energy consumption.
- Reduces motor wear for increased lifetime
- Power reductions as per the cube law: 20% speed reduction = 49% power reduction
- Quick paybacks: a few months to a year for many applications
- Flexible sizing: Small to large to match motor size, application, and space
- Review existing usage of VSDs for any faults and ensure a proper Preventative Maintenance Scheme
- Examine on-site equipment for additional motors used and consider upgrades with VSDs
- To get an idea of the savings possible for an application, try out the Energy Save Calculator tool from ABB
References for Further Reading
- You might be interested in the various designs and advantages for VSDs including pulse width modulation (PWM), current source inverter (CSI), voltage source inverter (VSI), and flux vector PWM.
- For a detailed understanding of the steps in the selection process of a VSD for your application, EC&M has provided sizing information, guidelines, and specific considerations involved.
- More information on specific similarities and differences between VSDs and VFDs
You may already have a VRF system on site, and understanding how they work will help you to see the various advantages they might have over another A/C system for your site. If you don’t have an existing A/C system, this section will help you decide whether a VRF might be the right fit for your needs. While the investment cost can be 2-2.5x greater than a traditional packaged A/C system, they can operate with up to 50% reductions in energy under the right conditions.
- Big efficiency gains (up to 50% reductions)
- Easier retrofit with less invasive refrigerant piping compared with ductwork or dedicated hot/chilled water piping
- Flexibility of variable climate control
- Flexible layout & adaptable design
- Variable flow of refrigerant to control the level of heating/cooling
- Best for smaller buildings although they can work with large buildings in certain circumstances
- Great when differing amounts of heating and cooling are needed throughout site
- Good complement with functional windows and natural ventilation for air changes, or may require a dedicated outdoor ventilation system
- Avoid applications with large spaces due to upper limit of individual cooling area at 3-5 tons of cooling
- Avoid use in industrial or manufacturing spaces
References for Further Reading
- JDB Engineering on When do Variable Refrigerant Flow Systems Make Sense?
- Flexible Solutions for Comfort with Variable Refrigerant Flow Systems by Carrier Corporation
- Example products from Daikin’s lineup of VRV solutions
As we saw earlier, VAVs work with the AHU of a centralised A/C system to modulate flow into the conditioned space. Here we further highlight some of their advantages and provide recommendations for their applications.
- Significant performance improvements over previous systems
- Potential for +30% energy reductions over conventional dual-duct systems
- Works well for large spaces and buildings with AHUs and central A/C systems
- A BMS is required for the level of controls involved
- Due to complexity of controls with sensors and actuators, proper maintenance is essential to avoid severe energy increases and occupant discomfort from small issues/errors
- At a minimum, annual maintenance and seasonal commissioning is necessary
- Review and optimise controls for existing VAV systems
- Consider refurbishing existing VAVs with upgrades such as pressure-independent VAVs over pressure-dependent ones
- The associated AHU systems servicing large multi-story buildings tend to be less efficient with supporting after-hours loads
References for Further Reading
- For more recommendations on improving existing VAV systems read through an article from Better Buildings Partnership
- How VAV Boxes Work
- A comprehensive, technical guide on VAV systems from ACHR News