The importance of saving energy is top of the agenda for many governments around the world as we face up to climate change. And heating, ventilation and air conditioning (HVAC) technology has a role to play.
HVAC systems, which can be found in most buildings, from schools to offices to supermarkets and manufacturing plants, account for around 40% of a commercial building’s total energy costs. These systems are rapidly becoming more energy efficient – saving energy costs and reducing carbon emissions.
But if you want to make significant energy savings, variable speed drives (VSDs) can be an excellent solution. In this article, you will find out more about VSDs and how they can save energy when used in HVAC system applications.
It’s all too common in the building service sector to encounter centrifugal fans and pumps connected to 3-phase motors that are running at full-rated speed and controlled only by a direct-on-line (DOL) or star-delta (Y/D) starter.
Under these conditions, consumed electrical power can be considered proportional to the cube of the speed of the motor shaft so that at any given time the power consumption will be at or near to maximum. No account is taken for the actual air flow or liquid demand from the process, and flow is normally controlled by throttling the ducting or pipework with dampers or valves, which is highly inefficient.
Installing VSDs gives the opportunity to electronically reduce flow by decreasing the speed of the motors that drive these fans and pumps, and run them according to demand from the process which results in significant energy savings. As the HVAC plant is not running at full speed all the time, there is less wear and tear to belts, pulleys and bearings. This means that maintenance requirements are lower, and the components last longer.
Flow can be controlled even more efficiently than by simply slowing the fan and pump motors down and leaving them operating at some reduced speed. Extending the use of VSDs and additional feedback systems throughout HVAC plant generates even greater energy and cost savings. For example, by actively monitoring duct pressure using a pressure sensor, a VSD can vary fan speed according to actual operating conditions rather than simply slowing it down.
Most modern digital VSDs can implement simple PID control by using a closed loop feedback signal from such a sensor so as pressure approaches the required target (set point), the VSD automatically reduces the motor speed and the fan slows down or, if not required, going in to ‘sleep’ mode whereby the VSD stops the fan while awaiting a change in system demand, saving energy. If demand increases and pressure starts to drop the inverter increases the motor speed and so increases duct pressure. Not running the pumps and fans at full speed reduces wear, with the accompanying benefits.
Depending on how the HVAC system has been designed, it’s possible to control more than one motor with a single VSD. As above, multiple fan and pump motors controlled by the single VSD run at the right speeds for the conditions and not at full capacity, reducing energy consumption.
This approach is not suitable for all HVAC systems. If used, each motor should have individual overload monitoring and protection. VSDs can only detect total current and not the current drawn from individual motors.
If this approach can be used, there are additional benefits. For example, fewer VSDs take up less space. The overall system control requirement is less, meaning reduced complexity.
The ultimate solution to energy efficiency is a network monitored and controlled smart HVAC system that operates the heating and air conditioning at optimal levels only when it is needed. By close monitoring and analytics of the HVAC system’s performance, the building manager can use the data to make further energy cost savings. The data can also be used to identify HVAC faults and optimise performance.
To do this, communications-card enabled VSDs communicate with the building management system (BMS). With temperature, flow and pressure sensors at key locations throughout the building, the HVAC plant and ducts, the BMS signals the VSD to adjust pump and fan motor speeds according to local temperatures. Rooms that are already at the correct temperature do not require further heating or cooling, which means no fan or pump motors are required, resulting in no wasteful energy use.
Using VSDs as part of a smart HVAC system provides higher levels of comfort for the building occupants. Not only can this have a significant impact on energy efficiency and the life of the plant, but it can also improve employee morale and productivity.
When VSDs are networked, the BMS can go a stage further and monitor VSD health and performance. Actively monitoring voltage and current readings enables the BMS to identify VSDs operating outside of normal tolerances. If highlighted early, it is possible to then replace HVAC components before they fail, minimising downtime. Post-failure data analysis also gives maintenance functions the opportunity to learn why HVAC outages occur so they can prevent similar failures from happening in the future.
Dalroad helps end users and electrical contractors in a wide range of industries to select the best VSD for their HVAC pumps and fans. Contact our team of experts today for no-obligation advice and recommendations.
