As energy costs continue to increase, there is an increased awareness of energy usage and a greater emphasis on methods to reduce the energy consumed by electronic equipment. The portion of electronic equipment power devoted to cooling can be significant. For example, the servers in a typical data center can require up to two times the useful computing power for cooling. The biggest of these data centers contain 400,000 servers and consume 250 megawatts of power [1]. It has been estimated that 20% of the total power supplied for a high end server is consumed by cooling fans [2]. This “non-value added” power required for fans has been a target for engineers attempting to reduce energy consumed by electronic equipment. Improving overall efficiency of the cooling fan has other cascading benefits for additional energy savings. With lower fan power consumption, there is lower current demanded from the equipment DC power supply and for all the power conversion equipment required between the fan and utility feed. Finally, a more efficient fan will require a smaller, less powerful, lower torque motor and lower current electronic drive saving both weight and space.
There are a number of methods of improving fan efficiency including speed control, optimized motor design, optimized electronic brushless motor drive design, and optimized system design. These are fairly well understood and have been deployed in modern forced convection cooling solutions. What is often not accounted for, even among sophisticated designs, is the optimization of the cooling fan for a specific flow pressure operating point. This Technical Brief will deal with the significance of fan selection and aerodynamic design optimization.
There is a surprising amount of power savings available by optimizing the fan design around a particular flow pressure point. Many of the fan manufacturers’ data sheets provide a flow pressure air performance curve but only provide power data during free delivery air flow conditions (zero back pressure). This is an unrealistic operating point within electronic equipment since there is always some restriction to air flow and resulting back pressure. Every cooling application produces a characteristic air resistance curve which intersects the fan air performance curve as shown on Figure 1. There is a trend in the electronics industry for applications to have greater air resistance as density of electronics within the enclosure increases. This is represented by the air resistance curve in Figure 1 with higher pressure for any given air flow value. When an electronic cooling fan application has a relatively high air resistance, choosing a fan that has been optimized for free delivery can result in unnecessary wasted power. Every fan has a peak aerodynamic efficiency point somewhere on its flow-pressure curve and there can be significant energy savings by ensuring the actual operating point of the application matches the peak efficiency point of the fan. In Figure 1 the flow pressure curve developed by fan “A” is one such design optimized for high flow but the fan performance of the flow pressure curve for fan “B” is optimized for the actual high resistance operating point. Many of today’s standard, commercially available fans produced in high volume are not easily modified without significant investment. These fans are often selected by engineers since they produce a flow-pressure curve that passes through the desired fan performance operating point — but often not at the peak efficiency possible.
A typical example of an actual fan application is shown below where a customized design significantly reduced power consumption when compared to the standard production catalog fan. Both fan designs delivered the same air performance with the same space claim, but fan power consumption was significantly different, in fact, cut in half!
Example
One of the most commonly used cooling fans in the industry is the 120mm tube axial fan. This fan has generally been optimized for low pressure applications with very little air resistance. An optimized version was developed for an airborne vapor cycle liquid chiller application where the density of heat exchanger fins requires the fan to operate in the high pressure portion of its flow-pressure curve. The flow and pressure performance point is: 38.6 mm H2O 23.6 l/s (50 CFM, 1.45 in H2O). The optimized fan required a new propeller design with more blades, lower pitch blades and a higher operating speed. Table 1 provides of comparison of these features along with test results showing input power cut in half when compared to the standard production fan operating at the same high pressure point. The basic motor design, electronic drive and venturi remained the same. The Computational Fluid Dynamics (CFD) simulation output showing velocity vectors for the two designs operating at the relatively high pressure, low flow point is provided in Figure 2. Note the airflow stall region in the existing production design just under the blade. Overall sound power was reduced by 2 dB even though the new design is operating with a significantly higher speed. In addition, an annoying pure tone was eliminated.
Conclusion
We have provided a convincing example of opportunities to save power by optimizing the fan aerodynamics for specific application operating points. For the application shown, the original catalog production fan selected by the customer achieved its desired flow pressure operating point. The problem was that the fan was not performing very efficiently. The technology to develop more efficient fans is in place. The precision of the latest CFD software, when used by a skilled engineer, has been shown to replicate actual test data within a few percentage points, greatly reducing engineering time and cost for custom designs. The barrier to more widespread fan design optimization is primarily an economic one due to the increased cost of a non-standard customized design produced in lower volume. However, the greater initial purchase price may be offset by the lifetime energy savings of the product.
Looking to the future, the increased life cycle cost of energy and social awareness of energy production by-products is surely going to move the threshold of justification in favor of high efficiency cooling fans [3].
References
[1] Hardy, Q., “Switchcraft,” Forbes, pp. 69 -73, September 29, 2008.
[2] “The Green Grid Opportunity- Decreasing Datacenter and other IT Energy Usage Patterns”, WP#2” February 16, 2007.
[3] Smith, N., “High Efficiency Electronic Cooling Fans”, in Proceedings of 25th Semi-Therm Symposium, 2009.
