HVAC systems loads usually constitute the biggest energy expenditure in commercial buildings. Buildings in the extreme north or south of the planet often have substantial heating costs, whilst those in the tropics may require air cooling all year.
Commercial buildings, like residential structures, have a wide choice of heating and cooling choices, each with its own set of benefits and drawbacks. The following are three of the most often utilize commercial building HVAC systems:
- Variable-air-volume (VAV) systems with a rooftop unit.
- Systems for chillers, cooling towers, and boilers
- Water-source heat pumps with a cooling tower and boiler.
1) Packaged Rooftop Unit with VAV System
A condenser for air conditioning and a gas or electric boiler for space heating is basically included in package rooftop units (RTUs). In areas where the unit must offer air conditioning with low outside humidity, an economizer can be added to lower the condenser’s cooling demand. Fans are used to blast air into a duct system that distributes it throughout the various interior zones in all modes of operation.
- Each zone includes a variable-air-volume (VAV) box with a damper that opens and shuts based on cooling or heating requirements.
- The location of the damper is determined by the temperature setpoint of each zone. For example, if a certain zone demands maximum cooling or heating output, the damper will fully open.
When all building zones are under partial load with their dampers half-closed, duct pressure rises and the system may become loud, reducing the efficiency of traditional VAV systems. Furthermore, the excess pressure equates to wasted fan power. However, using automation and variable frequency drives, it is possible to get outstanding results:
- The condition of all VAV boxes is continually monitored by a control system. At least one of them should be fully open; otherwise, there is a waste of fan power.
- If none of the dampers are entirely open, the fan speed is lowered and all dampers are gradually opened until one is entirely open.
- The fan is now producing precisely the proper amount of airflow for the current HVAC systems load.
When fan speed is regulated using a variable frequency motor, significant energy savings can be realized. In general, fan power is related to speed cubed — a fan running at 90% speed uses only around 73% of the energy it would need at full speed. The reduction of noise is another advantage of speed control.
Given that air is the primary channel for transporting heat, VAV systems with packaged rooftop units are practicable in facilities with a significant rooftop area in relation to their inside floor size. Due to the restricted rooftop surface and great vertical distances involved, these systems are not practicable in multi-story structures; systems based on water-cooled chillers or water-source heat pumps are preferable in these situations.
2) Boiler and chiller with cooling tower
Water circuits travel via air-handling units (AHUs) that supply the appropriate airflow for each building zone in these systems, which employ water as a medium to transport or remove heat.
- The chiller collects heat from the building’s cold water circuit and rejects it into a secondary water loop linked to the cooling tower when it is in cooling mode. The heat is subsequently rejected outside by the cooling tower.
- The flowing water runs through a boiler in heating mode. The majority of boilers run on electricity, gas, or oil.
Heat is transferred between the flowing water and the interior air at the AHUs in both circumstances. When the chiller and boiler share a water circuit (two-pipe system), the entire building must run in either heating or cooling mode; however, when each mode of operation has its own water circuit (four-pipe system), various zones can receive simultaneous heating and cooling. A four-pipe system is, of course, more expensive due to the fact that the pipes and accessories are effectively doubled.
Control and automation, like VAV systems, can result in significant cost savings:
- Variable-speed compressors are used in modern chillers, allowing them to run effectively even when the chiller is only partially loaded. To increase efficiency, some versions integrate speed control with the staged operation.
- Cooling tower fans, water pumps, and air-handling units are all examples of system components that might benefit from variable speed drives.
- Economizers for water-cooled systems are also available, but they are only applicable in certain temperature zones. Where the system would offer air conditioning with low outside humidity.
Chiller-based systems are typically more efficient than VAV systems, and they are also more practical for multi-story buildings: instead of having multiple packaged rooftop units, the system can be consolidated into a single chiller and cooling tower, with only the cooling tower required to be located outdoors or on the rooftop.
3) Cooling Tower and Boiler with a Water-Source Heat Pump System
In terms of adaptability and energy efficiency, commercial HVAC systems based on water-source heat pumps are the better option. Heat pumps, like air conditioners, operate on the refrigeration cycle, but they are reversible; when numerous heat pumps are employed to service different regions of a business building, they may convert between cooling and heating modes as needed.
- All heat pumps in a building share a common water circuit, and depending on the demands of each zone, they will either reject or absorb heat.
- Equal heating and cooling loads balance each other out since the water circuit is share.
- A cooling tower is used to reject the additional heat if the cooling demand is larger; on the other hand, a boiler is used to compensate for the difference if the heating demand is larger.
It is possible to make the system even more efficient by adding speed control to all pumps and fans utilized. Just as it was in the two prior cases. Heat pumps are among the most energy-efficient heating and cooling systems on the market: in cooling mode. They may match or exceed the efficiency of a chiller, and in most situations. They can offer space heating with less than 40% of the energy used by a resistance heater.
The requirement to construct a specialized heat pump for each building zone raises the expense of these systems. But the higher energy efficiency compensates for this in the long run. When cooling and heating loads are equal, for example. This system can function with both the boiler and cooling tower off.
HVAC systems configuration is one of the most important design options in a commercial building. These HVAC systems account for a considerable amount of long-term ownership expenses. Low-rise buildings with plenty of rooftop space tend to use packaged rooftop units with VAV systems. Whereas multi-story structures prefer chillers or water-source heat pumps.
Of course, there are energy efficiency improvements that may be implemented in each situation. It is more energy efficient to modulate the speed of compressors, pumps, and fans than cycling them on and off. It also adds to longer service life and lower maintenance costs.