UPS AND BACKUP POWER SYSTEMS

Maximizing Energy Cost Savings Using High Efficiency UPS

With today's rising energy costs, you can no longer ignore the costs savings from using a high efficiency Uninterruptible Power System (UPS).

The value of energy savings from using high efficiency UPS versus a conventional UPS often equals the value of the UPS in as little as three to five years. Even a slight advantage in energy efficiency can equate to thousands of dollars saved in a few years for a small UPS to hundreds of thousands of dollars for large UPS systems. While energy efficiency had little or no weight in past purchasing decisions, the cost savings resulting from today's high efficiency UPS has become so significant they can no longer be ignored.

What is UPS efficiency?
Energy efficiency of a UPS can be expressed as the difference between the amount of energy that goes into a UPS vs. the amount of useful energy the comes out of the UPS to power your loads. In all UPS systems, some amount of energy is lost as heat when it passes through the internal components of the UPS (transformers, rectifiers, inverters etc.).

Just how much energy is lost between the input and output can be significant when you consider how much the wasted energy is costing. Energy efficiency advantages of as little as 1 percent between one UPS and another can translate into thousands to tens of thousands of dollars saved per year depending on the size of the UPS.

The above diagram shows a typical UPS system. While the UPS loads are using 90 kW of power the UPS is demanding 100 kW of power. This means that 10 kW of power is being lost (as heat) as it goes through the UPS. This makes the UPS only 90 per cent efficient. Since the lost power is being dissipated as heat, this means that the air conditioner will have to use more energy to cool the facility.

Cost savings from using a high efficiency UPS often equals the value of the UPS in as little as three to five years.

Ten kW of lost energy may not seem like a lot of power, however, UPS loads operate continuously 365 days a year. 10 kW of lost energy now equates to 87,600 kWh of power each year. At a typical utility rate of $0.10/kWh this equates to $8,760 dollars of energy wasted by the UPS and an additional $2,600 in extra energy costs just to cool the heat rejected by the UPS -- resulting in a total of $11,360 in wasted energy!

No matter what UPS system you select, there will be some energy lost between the utility and the output, but high efficiency UPS systems can dramatically limit the energy loss, resulting in substantial cost savings. Consider the previous scenario. What if the same UPS was 95 per cent efficient as opposed to 90 per cent efficient, losing only 5 kW of energy as heat. The difference in energy savings would be over $5,500 per year. Considering that a typical 100 kW UPS costs approximately $25,000, the energy savings would pay for the UPS in five years!

Keeping your cool
The less efficient the UPS is, the more heat it will reject. This means air conditioning energy costs are also lower by using a high efficiency UPS. It takes an extra 3,400 BTU per kW of heat lost by the UPS to maintain the room temperature. A typical air conditioner requires 0.3 kW of energy to generate 3,400 BTU of cooling. As a rule, the air conditioning costs equal 33 per cent of the of the kW costs of lost energy of the UPS.

How do you know if you are getting a high efficiency UPS?
When comparing the energy efficiency of UPS vendors, published efficiency specifications from UPS vendors may all seem very similar, leaving you to wonder whether there is any difference between manufacturers. The only way to ensure that a UPS vendor is giving you straight facts about efficiency is to demand a witness test on your specific UPS prior to shipment from the factory. As a customer you have the right to know.

An efficiency test is like test-driving a car to measure fuel efficiency. Just as cars get radically different mileage when driving on the highway vs. a windy mountain road, UPS efficiency also can change with the type of load it powers. Today almost all UPS power 100 per cent non-linear loads (computers, servers, motors and electronic equipment).

When most manufacturers test their UPS, they use linear loads, which are not representative of customers' actual loads. This is very convenient for some manufacturers, as UPS efficiency will often be much higher when powering linear loads. Some manufacturers won't show the customer the true energy consumption of the UPS when installed at their site. Only by insisting that manufacturers demonstrate efficiency with non-linear loads representative of real world environments can you be sure of the UPS true efficiency.

The second major factor to influence UPS efficiency is at what power level the efficiency is measured. Just like a car will have its best mileage operating at around 60 mph, most UPS typically will have their best efficiency operating at 50 per cent to 100 per cent load level. Again in the real world, most UPS systems operate at 25 per cent to 60 per cent of their nominal load -- not fully loaded.

To determine accurate efficiency, the UPS should demonstrate efficiency at loads below 25-50 per cent where most UPSs will likely be operating, especially if operating in redundant configurations. Only by testing your UPS under actual expected load levels (25-50 per cent in most cases).and simulating an actual load profile in your facility will you know the real efficiency of your UPS.

How do UPS manufacturers achieve higher efficiency?
The principal area that energy is wasted in UPS is in switching losses in the inverter and transformers. To minimize switching losses a digitally based power management system optimizes the switching characteristics of the UPS inverter for specific load types and load levels by precisely controlling every pulse of the switching cycle. This results in the most efficient switching patterns with the least losses, outperforming older style systems with fixed switching patterns.

The system operates by creating the output waveform from hundreds of small but tightly controlled pulses. The inverter output is constantly compared to a computer generated theoretical sine wave, and corrective micro frequency adjustments are made by the pulses to keep the output waveform to within a few percent envelope of a perfect sine wave. By actively contouring the output to the load conditions the need for power hungry output filters are eliminated, maintaining a higher efficiency, and delivering a perfect sinewave output.

The inverter output waveform is created from hundreds of precisely controlled pulses with the switching constantly optimized for the load level and profile. This Digital Power Quality Management technology also keeps the output waveform within +/-1 per cent of a perfect sinewave.

Other energy saving features are high efficiency transformers that meet or exceed national standards for high-energy efficiency as required for the Energy Star certification. High efficiency transformers can offer a 2-3 per cent overall efficiency advantage over a generic lower efficiency transformer.

How much will a high efficiency UPS save you?
To calculate how much you will save using a high efficiency UPS, estimate your actual load and utility rate category. Your efficiency advantage will vary depending on your UPS load, but often will range from 2-4 per cent.

Alan Katz is with MGE UPS Systems. ET