HVAC Systems

HVAC systems contribute to nearly 40% of the energy used by commercial buildings and over 50% of total energy consumption in IT buildings. After reducing cooling/heating loads through passive design strategies, enhancing the efficiency of HVAC systems should be the

top priority for any building energy efficiency. Apart from selecting energy eficient equipment, it is important to select the correct system type, size, and design for optimized energy efficiency. The system types are broadly categorized as follows:

1) Centralized system: Central chilled water system (Air cooled and water cooled)

2) Distributed system (DX system): VRF, Duct able system, split air conditioners, unitary

systems Energy saving potential in HVAC System Design is shown in Table

High efficiency chillers

Chiller is the highest energy consumer in the HVAC system. Chiller efficiency is rated in kW/

ton or coefficient of performance (COP). The efficiency is considered either in full peak load

or part load (IPLV). ECBC states the minimum requirement of COP for each chiller type and size. Today, water-cooled chillers are available in the efficiency (COP) range of 6.3 to 6.7. Air-cooled system is designed to achieve COP in the bandwidth of 3 to 3.3.

Water-cooled chillers

Water-cooled chillers reject heat to a condenser water system, in contrast to air-cooled chillers which reject heat directly to the atmosphere. Condenser water systems connected to cooling towers or ‘hybrid wet-dry coolers’ result in chillers running more efficiently in the majority of weather conditions. Where there is a substantial alternative water source (for example storm water harvesting) this can be utilized.

Chilled water storage

Chilled water storage allows chillers to operate at times of day that differ from when air conditioning is needed. Chilled water system is illustrated in Figure 8.16. Chilled water is typically created overnight (when chillers operate more efficiently due to cooler ambient temperature), stored in very large well-insulated tanks (designed to allow the coldest water to sink and warmer water to rise), and drawn upon as needed. In locations with a strong ‘diurnal swing’ in temperature, the additional energy for pumping is more than offset by the lower chiller energy. The system provides following benefits:

• Encourages consumers to operate the chillers during off-peak period when unit charges for electricity are lower thereby lowering the cost of electricity for cooling.

• By shifting the operation of chiller compressor to evening and night hours when ambient air temperature is cooler enables chiller to operate more efficiently and consume less power for cooling.

• Potential for negotiating for lower contract demand and thereby lower demand cost.

Ice Bank

This system is similar to chilled water system. Blocks of ice are created at night during off-peak periods, typically night time. Chiller cools an ethylene glycol solution to below 00C and the solution is circulated through tubes in a tank freezing the water held in the tank (Figure 8.17). During the day, the ice melts cooling the solution in the tubes. The chilled solution is moved through a heat exchange coils where it cools the air.

Radiant cooling system

Buildings designed with radiant cooling system offers energy savings, exceeding 30% over

an energy efficient building designed with conventional air-conditioning system. This is mainly due to supply of chilled water at higher temperatures such as 14-17oC. Pipes embedded in the structure cool the thermal mass of the building generally during the hours when it is unoccupied. For cooling, radiant systems use both thermal mass and nocturnal cooling. Chilled water in the pipes can be supplied through a conventional chiller (Figure 8.19).

Energy-efficient pumps and fans (HVAC system) Pumps and fans which are used in HVAC system are designed to achieve higher efficiency benchmarks with use of IE3 and IE4 (most energy efficient motors).

Air tightness

When the wind (or a ventilation system) causes a pressure difference between inside and outside, air tries to move from one to the otherincreasing air conditioning energy. ‘Blower door testing’ is used to measure how air tight a building is, and can be a useful diagnostic tool. Revolving doors perform much better than sliding doorsand where secondary swing doors are required they should be on push-button release to discourage their use.

Mixed mode ventilation System

Mixed mode ventilation systems combine mechanical ventilation (which uses fan energy) and natural ventilation. Some buildings have windows that open automatically, others turnoff their mechanical ventilation when someone opens a window. Transient spaces are often ideal candidates for such applications. Buildings designed for natural ventilation can also incorporate a ‘night-purge cycle’ easily, which flushes out hot air from the building overnight.

Demand-controlled ventilation (DCV)

Outside air is pushed into buildings by ventilation systems to dilute the carbon dioxide, odours and other chemicals produced by the people and materials inside. In conventional systems the amount of supply air is constant normally based on maximum occupancy levels or at predetermined ventilation rate, regardless of the occupancy level thus wasting energy due to fan operation as well as in conditioning and cooling the airthe energy is not only wasted due to the fan operation, but also in conditioning and cooling the air.

DCV operation at various modes: full occupancy and partial occupancy is illustrated in Figure 8.20. DCV ensures a building is ventilated, cost effectively, while maximizing indoor air quality. Sensors are used to continuously measure and monitor conditioned space and provide real time feed back to the space controls which adjust dampers or fan speed to modulate the ventilation rate to match with the occupancy of the building. Control technology used is a combination of VFDs, CO2 or volatile organic carbon (VOC) sensors, and exhaust

fan status monitoring. Sensor placement needs to be carefully considered during design  and periodic re-calibration of the sensors is important during operation. Potential energy savings with DCV is 1040%.

Electronically Commutated fans

Electronically commutated (‘EC’) fans use brushless motors with permanent magnets and DC voltage controlled by a microprocessor like the fans found in desktop computers. These motors are more energy efficient than conventional AC motors because they do not have the same copper wire windings. The speed of EC fans can be controlled without the needfor an external ‘variable frequency drives’ (‘VFD’). This measure is applicable for fans throughout the building (for example in fan coil units).

Low-temperature Variable Air Volume

Low-temperature ‘VAV’ (Variable Air Volume) air conditioning systems supply colder air (about 11°C as against conventional older (about 14°C), enabled by the development of ‘swirl diffusers’. As a result less air needs to be pushed through the system to provide the same amount of cooling, reducing fan energy.

Similarly, ‘Low-pressure’ ventilation systems are designed for air to be pushed through without applying as much pressure (measured as ‘Pa’ (Pascals), where lower is better), also

saving fan energy. Typical designs target a ‘pressure drop’ of about 0.8 Pa per metre of duct and aim to minimise the air speed across coils and filters.

Heat recovery ventilation (‘HRV’) systems

Heat recovery ventilation (‘HRV’) systems use the air-conditioned or heated air leaving a building to pre-cool or pre-heat the incoming outside air. ‘Run around’ pipe heat exchangers (which circulate a liquid between coils in two ducts) and most ‘plate’ heat exchangers transfer temperature only. ‘Enthalpy wheels’ and some plate heat exchangers transfer both temperature and humidity. Heat recovery is most beneficial during very hot and very cold weather.

Solar Cooling

Cooling loads in tropical countries is high during the hot summer season when solar radiation is available in abundance. Thus, application of solar cooling technology uses a renewable source of energy to reduce the cooling loads when air conditioning demand is at its annual high. Solar heat is used to re-generate the refrigerant in an absorption chiller (Figure 8.21).

Insulated roller doors

Where roller doors are required for access to a cooled space they can be a significant point of heat transfer, resulting in higher energy use. In these applications a product should be chosen that has insulated panels or slats and is well sealed around the edges. Ideally these roller doors should also operate automatically to limit the length of time they remain open. Industrial ‘air curtains’ (set to only operate in very hot or cold weather) can also be installed

to limit air movement into and out of the space while the doors are open.