Step 3: Fan Load Reduction & Selection of Efficient HVAC Equipment

From a design standpoint, the next step is to meet these minimized loads with efficient air distribution systems and “best-in-class” heating and cooling equipment having the highest available efficiencies.

1. Efficient Air Distribution Systems with Low Fan Power Requirements: With the exception of some European countries where chilled ceilings + displacement ventilation (CC/DV) are predominant, most commercial air conditioning designs rely on all-air air handing systems with conventional overhead delivery of cold air that use either constant air volume (CAV), variable air volume (VAV) air handling equipment or distributed designs based on ductless mini-split A/C units typically mounted on walls at ceiling level. Such designs meet both the space sensible load, as well as, the outside air ventilation requirements.

Air handling units for these overhead delivery systems are typically sized to meet the entire space sensible cooling load using cool air delivered at typical supply air temperature (SAT) in the range of 55 to 60 ºF (13 to 16 ºC), air flows of 1 CFM/ft² (5.08 L/s.m² and cooling densities of 400 ft²/Ton. This is especially true of packaged systems which deviate little from these two metrics (i.e, a 10 Ton rooftop unit will have a typical air flow capacity of 4,000 CFM). These designs result in high fan power requirements and energy use because air is a less efficient medium to transport thermal energy compared to water. Fans as a result are one of the largest electrical end-uses in commercial buildings after lighting and plug loads and variously claimed to represent 40% of the electricity used by HVAC equipment or 23% of total world energy use. [1] [2]

The first system consists of a dedicated outdoor air system (DOAS) that is designed to provide only the required outside air to the space via a 100% OA make-up air unit (MAU) that would handle the space latent cooling load by cooling the air down to temperatures of 45 to 55 ºF (7 to 13 ºC). [3] [4] [5] The unit would also incorporate heat recovery for preheating outside air and a second heat recovery section to preheat the low SAT. Fan coils in the space would be responsible for meeting the space sensible cooling load. This system would still deliver conditioned air to the space via conventional ceiling diffusers but with a connected fan load design that is nearly half that of the conventional VAV system because the fan coils would exhibit a low pressure drop of 0.5 to 0.75 in (125 to 190 Pa). As a result, the fan coils only require 15 hp of fan motor power even after accounting for a low fan-motor combined efficiency of 20% that is typical for small fan coils with fractional hp motors. Overall fan energy use is only 6% less than the VAV system because both the MAU and fan coils operate as constant volume systems, although fan-coils with ECM motors are available and can function as VAV systems and achieve further reductions in fan energy use. There are a number of design implications for this system type relative to a VAV system, including the need to provide chilled water to the fan coils serving interior zones during the winter period since the system does not have the air flow capacity to operate as an air side economizer to provide winter free cooling. In cold climates this can be done with a fluid cooler, but mechanical cooling will likely be required in milder climates.

The second system reduces the fan energy use significantly more since it relies on radiant cooling panels mounted on the ceiling to passively meet the space sensible cooling load with chilled water. This technology is also known as chilled beams and chilled ceilings (CC) and also relies on a make-up air unit sized to provide the required outside air to the space. Air flows and fan energy use can be reduced anywhere between 30% to as much as 75% depending on the size and capacity of the CC to meet the space sensible load. Multiple options are available for deliver of the ventilation air including DOAS and displacement ventilation (DV). These combinations of systems are known as CC + DOAS and CC/DV.

There is currently considerable debate and a wide range of opinions in North America on the best system configuration and design approaches. This arises from the different air delivery and flow patterns between the two systems. The CC + DOAS delivers air via high mixing diffusers and results in a mixing of the room air. In contrast, the CC/DV relies on the DV thermal stratification to transport the air from the floor to the ceiling, removing contaminants on its upward path, resulting in superior indoor air quality. However, depending on its capacity and ceiling temperature, the CC can potentially work against the DV. A CC with a high surface area and low temperature will achieve a high degree of comfort, typically characterized as a “sensation of freshness”, but at the expense of a reduced temperature differential between the floor and the ceiling that must exist to maintain an effective thermal plume, which in turn will compromise indoor air quality or ventilation effectiveness. [6] Some studies however suggest that maintaining stratification and ventilation effectiveness is strongly tied to the volume of air and less on the temperature differential between the floor and ceiling. [7] In the end, there is significant agreement that properly designed CC/DV systems must balance the cooling capacity of the CC and the DV, which is typically referred as the ratio of the cooling capacity of the CC to the total space cooling load. A number of studies suggest that the maximum CC cooling capacity should not be higher than 65% - 75% in order to ensure thermal comfort and air quality. [8] [9] This is equivalent to a maximum load of 20 to 22 Btu/hr.ft² (63 to 69 W/m²) that can be handled by a properly designed CC compared to a reported maximum space cooling capacity of up 30 Btu/hr.ft² (95 W/m²) that can be handled by this type of system. [10] [11]

Interior space loads can be easily handled by a CC system, as these spaces have typical loads of only 6 to 8 Btu/hr.ft² (19 to 25 W/m²). Perimeter zones become more problematic due to solar loads, but a low energy building such as the hypothetical 5-storey office building example used throughout this website would display cooling loads below the maximum capacity of the CC thanks to the minimized perimeter cooling load. The available ceiling space is also a second important consideration that comes into play for a CCs ability to meet the space cooling load. In a typical building, lighting fixtures and return grilles can cover approximately 15% to 20% of the ceiling, leaving 80% to 85% for the CC panels. An efficient lighting design based on 64 ft² /fixture (6 m²/fixture) would leave more ceiling space to the CC, pointing to another design synergy needed for a CC to be a feasible design option.

From an energy savings standpoint both CC + DOAS and CC/DV achieve fan energy and cooling energy savings. The fan energy savings have already been illustrated above while the cooling energy savings arise as a result of a reduction in the fan heat gain seen by the cooling plant (smaller fans) and from a significantly higher COP of the cooling plant. The higher COP is achieved due to higher chilled water temperatures supplied to the CCs that is reported to range from 56 to 68 ºF (13 to 20 ºC) in order to prevent condensation. [12] [13] This in turn means a 25% to 50% higher COP. [14] There are additional savings mentioned in the literature such as lower ventilation rates needed with CC/DV systems due to the superior ventilation effectiveness possible with a properly designed DV system.