A look at optimizing HVAC systemsDnyanesh Joshi | March 05, 2020
HVAC systems are often the most energy hungry systems in buildings. In 2018, residential and commercial sectors accounted for about 40% of total U.S. energy consumption. Heating and cooling is responsible for most of that energy usage.
Reducing commercial HVAC system energy consumption has been a focus point of the engineering community over the past two decades. Large commercial facilities, such as hospitals, schools, department stores, shopping malls and offices, are under constant pressure to reduce energy consumption of building HVAC systems.
HVAC systems are complex, with components, equipment, interfaces and subsystems interconnected with each other. To gain insight about system performance and efficiency, data collection is key. There could be number of sensors used in this regard, chief among them are temperature, humidity, flow, pressure, air quality, smoke detection and occupancy. All these sensors must accurately measure their respective parameter, and this data must be aggregated and centralized for analysis.
Take the example of a hypothetical grocery store, which has a central condensing unit that delivers refrigerant to a number of display cases and walk-in freezers, and at the same time cools, heats and ventilates the interior using ducted systems. The intricacies of such an HVAC system results in a highly variable load that changes according to shopper and employee behavior, weather and climate, and ambient needs, as the produce aisle requires different local humidity than dry goods. Hospitals are another facility with complex heating, cooling and ventilation requirement, where there could be dozens of zones, each with distinct temperature, humidity, ambient atmosphere and refrigeration needs.
Considering the example of a grocery store, various questions can be asked regarding performance of the HVAC system. Are the components suitable to meet the intended functions? Does the system maintain suitable temperature and humidity in the display cases? Does the system provide comfort to customers in the store?
If the answer to these questions is no, then it is necessary to revise components in the system and decide on the replacements or modifications. If the answer is yes, then it is essential to evaluate the operating conditions at which various components are performing. Are these components operating at optimum conditions to have lowest possible energy consumption? Such a question-answer approach can help to arrive at the decision.
Ductwork, peak loading and digitalization
Another consideration for air conditioning and heating is duct design. A proper duct design and selection of blowers can help to reduce energy consumption. An improper duct design may cause high pressure drops, and will cause improper air distribution without oversizing blowers to make up for the difference. Ducted systems thus provide significant opportunity for optimization. Using only visual observations, experts can envisage potential improvements, and computational fluid dynamics can also help to optimize the duct systems.
In many commercial buildings, air conditioning is provided with fan coil units. Secondary coolant is circulated through the coils of these units and air is passed over the coil by the blower. These systems have a heat exchanger between the refrigeration loop and the coolant loop. The system is nothing but a chiller. Secondary coolant is then stored in the tank and circulated to the fan coil units with the help of pumps. A proper pump selection and the piping design is of utmost importance.
HVAC systems are typically sized for peak load. Cooling systems are selected for maximum summer temperature and highest possible occupancy; heating equipment is selected for the coolest winter temperature. At any given time in between the peak load periods, HVAC systems are operating at less than their full potential.
Variable frequency drives (VFDs) are the obvious choice when designing an efficient HVAC system. In conventional systems, dampers in the duct system and bypass valves or three-way diverting or mixing valves are used in secondary coolant systems. These accessories provide good control of the variables (e.g., temperature, flow), yet pumps or blowers continue to run and consume high power. VFDs reduce the speed of induction motors in proportion to the demand, thus reducing power consumption of the prime movers. In other words, VFDs help the induction motors run at the optimum operating point. Though the initial costs are high, they can be fruitful over longer periods.
Digitization is further helping HVAC systems become more efficient. Linking the mechanical HVAC system to a digital network enables remote monitoring and adjustment, and alerts staff to a malfunction or unusual HVAC parameter. This allows personnel to remotely adjust settings to maintain optimum efficiency and performance, or helps identify issues and solves them quickly.
In a world where competition, energy efficiency and stability are everlasting goals, optimizing HVAC systems is an unavoidable task for facility managers and contractors. Though the systems are getting more complex, so are the tools and techniques to make the best HVAC decisions. Optimization of HVAC systems is not a one-time procedure, but rather a process.