Economics of energy-efficient electrical systems

Energy efficiency is achievable in all building types, including mission critical environments.

12/18/2014


This article has been peer-reviewed.Learning objectives

  • Understand the types of electrical products and systems that can lose energy.
  • Learn how to measure energy loss.
  • Understand how to select and size equipment for the application.

Figure 1: The 500 kVA uninterruptible power supply (UPS) system shows input, bypass, and output switchgear. Today’s production, distribution, and consumption of electrical power have become a study in economics. With the ever-increasing efficiencies of electrical system products, energy losses from these products have decreased. This efficiency has a secondary benefit of reducing the cooling required due to the associated parasitic heat load. Energy efficiencies are achievable in all types of facilities such as health care and corporate buildings, but are most apparent in systems and products within data center electrical systems due to the power intensity of this industry. This includes power capacity products such as uninterruptible power supply (UPS) systems and power distribution unit (PDU) transformers, as well as cooling control products for compressor, pump, and fan components of HVAC equipment, such as computer room air conditioning (CRAC) units.

The cost of electrical power as measured in kilowatt-hour (kWh) units of energy, or the amount of energy delivered in 1 hour at a 1 kW power level, is the cost of overcoming the following losses and inefficiencies in delivering the required energy to power and cool a facility:

  • Losses due to power conversion from ac-ac, ac-dc, and dc-ac
  • Varied efficiencies due to technology differences of power conversion products
  • Varied efficiencies of power distribution transformers
  • Losses caused by inefficiencies due to lightly loaded equipment
  • Losses caused by inefficient sizing of power distribution equipment
  • Capital cost inefficiencies due to distribution of power at lower voltages rather than higher voltages.

Currently these energy loss and inefficiency factors may be overlooked on projects outside of the data center industry, but they will work on any project and efforts should be made to use them. Green industry guidelines such as the U.S. Green Building Council’s LEED program have indicated a desire to allow these as innovation credits if specific savings can be quantified. The reality is that modern-day projects do have data-intensive areas that require special consideration; therefore, consideration of these factors is warranted.

Power capacity products

Figure 3: Variable frequency drives (VFDs) are located in a mechanical room and are connected to pumps.The first example for improving energy efficiency in data centers is uninterruptible power supply (UPS) systems. UPSs are electrical apparatus that provide uninterrupted power to a load when the input power source fails. The first UPS systems used to support legacy data centers were very reliable, but were inefficient rotary-type UPS systems. They supported mostly inductive type mainframes having low power factors, lower than 80%. As computer system technology advanced, so did UPS technology.

The next generation of UPS was a double conversion, static type with battery backup. These UPS systems comprised both input and output transformers, and used silicon controlled rectifiers (SCRs) for high-power switching. They typically operated at an 80% power factor. Early static UPS systems tended to be less reliable than rotary, but because they were also less expensive than rotary technology, many UPS system designs included paralleled modules to increase reliability. This helped the mean time between failure (MTBF) rating of the static UPS come closer to that of the rotary, but would sacrifice efficiency because of the resulting low load conditions realized on each module of a paralleled system.

In a continued effort to increase reliability as well as energy efficiency, the UPS industry steered from the use of SCR transformer-based technology to transistor-based transformerless technology. Transistors ease of on/off switching as compared to SCRs resulted in a focus on the development of transistors with high switching speeds and high power-handling capabilities. Initially, at comparable power-handling capabilities to SCRs, this focused development resulted in the use of the isolated gate bipolar transistor (IGBT). IGBT UPS systems have very high full-load efficiencies, but their notoriety came in the higher efficiencies at part load.

As an example, the efficiency curve of a 500 kVA transformerless based design over a 500 kVA transformer design is on the order of 97% to 92%, respectively, at 40% load. The annual energy cost represented by this comparison at $0.10/kWh corresponds to $16,000 to $44,000, respectively, for an annual energy savings of $28,000 for the transformerless design.

A second example of power capacity products improving efficiency are PDUs. They are an electrical apparatus that consists of a voltage changing transformer and a means by which to distribute conditioned and continuous power from a UPS to the end-use computer loads of a data center. The PDU transformers are the same as a typical stand-alone transformer used in most facilities. They are energy-efficient TP-1 compliant transformers that meet or exceed the requirements of the Energy Policy Act of 2005.

The TP-1 standard dictates that power distribution transformers be more efficient at their typical loading level of 30% to 50%. The higher efficiency transformers required by the energy policy are 2% to 3% more efficient overall as compared to pre-TP-1 lower efficiency transformers. The improved efficiencies of power distribution transformers improve on the overall losses to the electrical system.

Standard pre-TP-1 transformers convert approximately 95% of the electricity received into usable output voltage. With a typical transformer being energized continuously, the 2% to 3% improvement in efficiency can provide significant energy savings. The TP-1 transformers use higher grade electrical steel to lower flux density and reduce losses, especially at the average 35% loading where TP-1 efficiency measurements apply. Beyond TP-1, TP-1S transformers are available to surpass the TP-1 standards to better accommodate nonlinear load profiles at K13 and K20 levels.

For reference and comparison purposes, the efficiency of a pre-TP-1 150 kVA transformer is 96.5%, as compared with 98.3% for TP-1 and 98.8% for TP-1S transformers. The annual energy cost represented by this comparison for a transformer loaded to 65%, at $0.10/kWh, corresponds to $9,000 for pre-TP-1 and $2,000 for TP-1S, for an annual energy savings of $7,000.


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