HOW A REVOLUTION IN DC POWER SYSTEMS CAN
REDUCE ELECTRICITY USAGE AND CARBON EMISSIONS
21.12.07
WWW.ELTEKVALERE.COM
- 1 -

TABLE OF CONTENT: ........................................................................................................ 2
HELPING CARRIERS GO GREEN ....................................................................................... 3
How a Revolution in DC Power Systems Can Reduce Electricity Usage and
Carbon Emissions ...................................................................................................... 3
INTRODUCTION: THE GLOBAL GREEN MOVEMENT.................................................... 3
A TELECOMMUNICATIONS INDUSTRY RESPONSE ..................................................... 4
Efficiency of DC Power Systems ............................................................................ 5
The Cost of Electricity .............................................................................................. 5
THE IMPACT OF RECTIFIER EFFICIENCY ........................................................................ 6
THE PAYOFF ....................................................................................................................... 8
TAKING EFFICIENCY TO A NEW LEVEL .......................................................................... 9
THE ECONOMICS OF THE HE RECTIFIER...................................................................... 10
HIGHER RECTIFIER EFFICIENCY: A DRIVER FOR UPGRADES.................................... 11
Case 1: Access Systems in the Outside Plant .................................................. 12
Case 2: Central Office Power Systems .............................................................. 14
Case 3: Existing Eltek Valere Systems ............................................................... 17
CONCLUSIONS.................................................................................................................. 18
REFERENCES..................................................................................................................... 20
A GREEN REVOLUTION IN DC POWER
- 2 -
AN ELTEK VALERE WHITE PAPER

Thinking green is becoming a global phenomenon. Countries, municipalities, businesses, and even consumers worldwide are focusing more attention on energy conservation and environmental responsibility. Service providers are also stepping up to this challenge, as they are significant energy users – consuming billions of kilowatt-hours of electricity per year. Many have adopted across-the-board reduction programs to ease the impact and these have yielded good progress. But more can be done to address the most significant area of power consumption in a telephony network - the DC power systems that power network equipment.
This paper will outline a revolution in DC power systems that dramatically improves power conversion efficiency allowing service providers to dramatically cut electricity use and reduce carbon emissions that contribute to global climate change. Adopting this new technology is an investment, and this paper will also explore how reduced operating expenses from these new power systems can provide a return on the investments – both for the planet and for the bottom line.
The green movement is driven by a number of factors. Energy demands are continuing to rise, despite energy prices that have reached historically high levels. There is growing concern over the security of the world’s energy supply and the increasing dependence upon it. The impact of air pollution and global climate change are becoming increasingly important topics throughout the world. And there is a growing recognition that the world’s non-renewable energy resources are being depleted.
In combination, these factors have created a heightened awareness of the global issue of energy conservation, which has generated a call to action around the world.
Governments have put in place incentive programs to encourage the deployment of alternative energy sources, and are creating mechanisms to regulate carbon dioxide (CO
2
) emissions, the principal greenhouse gas associated with pollution and global warming.
CO
2
is a byproduct of burning fossil fuels (such as oil, gas, or coal) to create energy.
About 65%
1
of the electricity produced worldwide comes from the use of these nonrenewable resources.
Formal energy reduction programs and goals are being established throughout the world.
In 2007, delegates from nearly 190 countries joined together in Bali, Indonesia, to develop the United Nations Framework Convention on Climate Change, a set of greenhouse gas emission goals for industrialized countries. The convention was acting
A GREEN REVOLUTION IN DC POWER
- 3 -
AN ELTEK VALERE WHITE PAPER

on a UN recommendation that emission be reduced by up to 40% of 1990 levels by the year 2020.
Some ways governments are reacting to this is by setting internal guidelines for reducing energy consumption. This is the case with the U.S. Energy Policy Act of 2005 (EPACT), which requires U.S. government agencies to reduce their energy intensity (measured in
BTUs per gross square foot) by 2% per year resulting in a cumulative 20% reduction by
2015. The overwhelming contributor to achieving this goal has been the reduction in energy use.
Emissions trading also have emerged as a technique for controlling pollution. The
European Union Emission Trading Scheme is the largest multi-national, greenhouse gas reduction initiative in the world. In this plan, the European Union establishes limits on the amount of CO
2
emissions that will be permitted in a in a country in a given year, and provides “allowances” (the total number of allowances equals the acceptable emissions level). The allowances are allocated among the companies or other groups that are large
CO
2
emitters. If a company is going to exceed its allowances for CO
2 emissions, it needs to buy additional allowances from companies that are polluting at a lower than allowed rate. In effect, reduced emissions are rewarded and excessive emissions are penalized.
Whether through regulations, incentive programs, or appeals to environmental responsibility businesses, institutions, and consumers are being asked to respond to the overall energy challenge.
The telecommunications industry is a major energy user. There is a growing commitment within the industry, to reduce energy consumption and the associated emission of greenhouse gases. Companies like AT&T, Deutsche Telecom, NTT, Telefonica,
Verizon, and other large service providers have explicitly stated their social and environmental responsibility for managing natural resources in their corporate policy.
Energy conservation has become a priority.
In 2006, Verizon consumed 8.9 billion Kilowatt hours (kWh) of electricity and generated more than 7.1 million metric tons of CO
2
2
. This represents not only a massive utilization of energy, but also an annual cost of almost $900 million (at $0.10 per kWh). The majority of this energy is associated with the DC power systems that support Verizon’s network. Verizon estimated in 2003 that 60% to 70% of the energy it uses is for DC power to run its core telecommunications switching network
3
. Telefonica estimates that
62% of its total electricity consumption is used to operate its networks
4
. It is reasonable to assume that DC power-based systems are the major energy user for most service providers.
Obviously, reducing the load on these DC power systems is a priority. Great emphasis has been placed on lower power consumption for the new generation of radio, transport,
A GREEN REVOLUTION IN DC POWER
- 4 -
AN ELTEK VALERE WHITE PAPER

access, switching and other equipment being placed into the network. But as wireless networks continue to expand and broadband capabilities grow, it is a challenge to achieve significant reductions in the overall need for DC power. In 2004, NTT projected that it would require 1.5 times its current energy usage level to support the widespread deployment of always-on broadband connections in 2010
5
.
One clear opportunity for energy reduction is within the DC power system itself.
The use of DC power systems has long been the standard in telecommunications networks. It was chosen for its reliability, safety, and ability to be paired with battery systems to provide continuous backup power. As with any power conversion technique, there are associated inefficiencies as electricity is converted to waste heat in the conversion process. The rectifiers used in DC power systems have improved dramatically with respect to efficiency.
Rectifiers in a DC power system take AC input from the public utility network and deliver a regulated DC voltage to operate telecommunications equipment and to charge the back up batteries. The typical DC power system voltage is 48V, although 12V, 24V,
130V and others are used in some applications.
Switch mode rectifier technology was introduced in the late 1970s, and was a revolution with respect to the power density and efficiency that it enabled. Typical efficiency for these first-generation switch-mode power supplies was in the 85% range, operating at 20 kHz switching frequency.
The switch mode technology has continued to advance since its introduction. During the
1990s, MOSFET technology, together with improved soft switching topologies and control solutions, enabled significantly higher switching frequency and partly lossless switching. This resulted in further improvements in power density and efficiency.
Today’s 48V rectifiers typically achieve efficiencies of 90% to 91%, with the best-inclass rectifiers approaching 93% or 94%.
Service providers have made vast investments in infrastructure over the last 10 to 15 years, particularly in the areas of wireless and broadband networks. These investments have been very capital intensive and rapid in their deployment. The operational and lifecycle cost for the installed systems has had less of a focus. This appears to be changing.
Recognizing both the environmental need to conserve energy and the increasing cost of electricity, service providers have started to give more attention to these operational considerations.
A GREEN REVOLUTION IN DC POWER
- 5 -
AN ELTEK VALERE WHITE PAPER

Figure 1 summarizes 2006 electricity prices in various countries around the world
6
.
International electricity prices
2006
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
Series1
Au st ra lia
Au st ria
Ch ina
De nma rk
Fi nl an d
Fra nc e
Ge rm an y
Gre ece
Hu ng ar y
Ita ly
Ja pa n
Ko re a
Me xi co
Ne the rla nd
Ne s w
Ze ala nd
No rwa y
Po la nd
Po rtu ga l
Sp ai n
Tu rk ey d
Ki ng do m
Un ite
USA
Figure 1
International Electricity Prices
The chart demonstrates that there is a wide variation in price from country to country, and from region to region. Electricity prices have risen during the past decade, and continuing increases are projected. Since 1998, the typical inflation rate for electricity prices has ranged from 2% to 9% per year (depending on country).
This paper will demonstrate that electricity cost and rectifier efficiency are significant factors which should be considered in the selection of a DC power system. For the examples presented, the price of electricity is assumed to be $0.10 per kWh during the entire study period. Please note that this is a conservative approach that depresses the savings that can be expected from increasing DC power system efficiency. Electricity prices are already higher than that level in many parts of the world, and for simplicity in the calculations there is no price inflation built into these models.
It is easy to underestimate the impact of a rectifier which offers a 1 or 2 percent advantage over an alternative unit. Rather than just focusing on the efficiency number, its more effective to think in terms of power losses. Let’s look at an example.
Consider the case of a power system that needs to deliver 8,000W of 48V power at a site, not an uncommon situation. To generate that 8,000W of output power using 90% efficient rectifiers, a DC power system will utilize 8,889W of AC input power. In that conversion, 889W is lost power in the form of heat.
A GREEN REVOLUTION IN DC POWER
- 6 -
AN ELTEK VALERE WHITE PAPER

The use of 92% efficient rectifiers will require 8,696W of input power and will result in
696W of power losses. The use of the higher efficiency rectifiers reduces power losses by
193W, a reduction of 21.7%. This is illustrated in figure 2.
AC input
8,889 W
90% efficient
DC power system
8,000W
889 W
DC Output to load
Lost power (heat)
AC input
8,696 W
92% efficient
DC power system
8,000W
696 W
DC Output to load
Lost power (heat)
Reduction in lost power is 889W – 696W = 193W
Percent reduction in lost power is 193W / 889W = 21.7%
Figure 2
Rectifier Efficiency Comparison
The energy cost for the 8,000W of load power will occur in either case, but the use of the more efficient rectifier will save 193W of lost power, which equates to 1,693 kWh of energy annually. At $0.10 per kWh, that results in about $169 of savings per year.
There is an additional benefit associated with higher efficiency rectifiers. The power lost in the rectifier conversion from AC to DC takes the form of heat. This heat must be considered in the cooling requirements at a site. As an example, let us assume that the power system delivering the 8,000W of 48V power is in an outdoor cabinet for a cell site.
Power cabinets such as these are often air conditioned to provide a controlled environment and to extend the useful life of the batteries. Although it is more difficult to estimate the effect of the rectifier efficiency on the cooling system, there are some simplifications that can help provide a reasonable judgment.
In the example described above, the use of 92% rather than 90% efficient rectifiers resulted in a savings of 193W of lost power. That lost power equates to 659 BTU per hour of heat that no longer must be handled by the air conditioner. If in this application an air conditioner with an EER (Energy Efficiency Ratio) of 10 is used, then it follows that 65.9W of power is required to run the air conditioning associated with that 659
BTU/h of heat. Another factor needed to convert the power to energy usage, is an
A GREEN REVOLUTION IN DC POWER
- 7 -
AN ELTEK VALERE WHITE PAPER

assumption about how often the air conditioner is running. That number is certainly dependent on the location of the site. A conservative view is that the air conditioner runs
30% of the time during the year.
These calculations are summarized in Figure 3. The conclusion is that an additional
173.2 kWh of electricity is saved annually from the reduction in air conditioning needed when the higher efficiency rectifiers are used.
Note: A rule of thumb for assessing the impact on cooling energy of high efficiency rectifiers, in a cabinet, is to simply add 10% additional annual energy savings to that of the rectifier power savings .
Power lost in 90% efficient plant is 889W
Power lost in 92% efficient plant is 696W
Savings in lost power is 889W – 696W = 193W
Savings in heat is 193W x 3.413 = 659 BTU/hr
Assume EER of air conditioner is 10
10 = BTU capacity of air conditioner / Watts to run air conditioner
Power associated with the heat savings is 659 BTU/hr / 10 = 65.9 W
Assuming air conditioner runs 30% of the time
Annual energy savings is 65.9W x 7.2 hrs/day x 365 days/yr = 173.2 kWh
Figure 3
Impact on Cooling System
The overall annual energy savings associated with the use of the 92% efficient rectifiers is equal to 2116 kWh for the power to run the rectifiers, plus 173 kWh or the power to run the air conditioner. This is a total of 1866 kWh of annual savings. At $0.10 per kWh, the cost savings is about $187 per year.
The problem addressed in this paper is one of reducing energy usage, particularly electrical energy generated from fossil fuels. In the above example, the use of the higher efficiency rectifiers has reduced losses in the DC power system by 21.7%, which equates to a 2% overall reduction in energy usage. With this in mind, consider the Verizon usage statistics cited earlier. If Verizon could achieve a 2% reduction in overall energy usage, it would translate to a savings of 178 million kWh per year in electricity (worth $17.8M per year) and a reduction of more than 142,000 tons of CO
2
emissions.
A GREEN REVOLUTION IN DC POWER
- 8 -
AN ELTEK VALERE WHITE PAPER

Given these cost savings, how much could a carrier spend to get the higher efficiency rectifiers? The answer depends on the payback period that is targeted for this investment.
With a five-year payback, the calculations look like this: The present value of saving
$187 per year in energy costs for five years (assuming a 5% cost of money) is $810. That means up to $810 can be spent on high efficiency rectifiers because that up-front incremental investment will be recovered over the five-year period. For this simple example, assume the 8,000W plant is comprised of five 2000W rectifiers (providing N+1 redundancy). This translates into an incremental cost of $162 per rectifier to attain the
2% additional efficiency.
For a two year payback period, the present value of the energy savings is $348. In this case, an additional $70 can be justified for each of the five rectifiers to get the efficiency advantages. Our experience in the marketplace suggests that a payback of two years or less is certainly achievable for rectifiers used in this example. The energy cost savings in years three and beyond simply provide additional recurring benefits. This trade-off makes good economic sense and is a very effective way to address the energy reduction challenge.
Having demonstrated the very tangible value of deploying higher efficiency rectifiers in
DC power systems, let us now take that approach to the next plateau.
Eltek Valere has been the technology leader of the DC power systems industry, driving the development of rectifiers with the highest efficiency and power density. The company has, through intensive research, developed new technology for its next generation of rectifiers, labeled the HE series for High Efficiency. These rectifiers will offer efficiencies of greater than 96% , an unheard of level in the industry. The HE rectifiers will be offered as plug-in compatible alternatives to Eltek Valere’s existing, lower cost rectifiers, which will continue to be available.
As discussed above, the significant energy savings associated with high efficiency quickly offset the higher price. It should be noted that the incremental price paid for these HE rectifiers will have a payback time typically between one and three years, depending on the local electricity price level. Over a ten-year life cycle, the energy savings make the HE rectifier “nearly free” as will be demonstrated later in this paper.
The graph below (figure 4) shows efficiency as a function of load for the HE rectifier, compared to its sibling, the standard Flatpack 2, which is representative of the higher-end rectifiers on the market today. It should be noted that not only is the overall efficiency significantly improved with the HE rectifier, but it has been optimized to have a maximum efficiency in the most typical load range for real life applications, which is between 30%-70% of full load.
A GREEN REVOLUTION IN DC POWER
- 9 -
AN ELTEK VALERE WHITE PAPER

Figure 4
Efficiency Curves for HE and Standard Rectifiers
To examine the economics of the HE rectifier, let us consider a realistic example of a DC power system being placed in an air-conditioned, outdoor cabinet at a cell site. Such systems are typically in the range of 10-25kW, equivalent to 200-500Adc at 48V. The systems are designed to support the anticipated load (telecom equipment), have spare capacity for battery charging after an AC power outage, and have redundant capacity in case of a rectifier failure.
For this example a system equipped with14kW capacity is the configuration for comparing performance using the standard and the HE versions of Eltek Valere’s 2kW rectifier.
The 14kW system utilizes seven 2kW rectifiers. As in the previous example, the assumption is that the average load on the system is 8kW. Therefore, the rectifiers are operating at 57% of their capacity (approximately 1143W per rectifier). Note from the efficiency curves in figure 4 that the standard rectifiers will operate at 92% efficiency under these conditions, while the HE rectifiers are at 96%. The use of the HE rectifier reduces the power losses by 50% (from 8% down to 4%).
Using the calculation methodology described earlier in the paper, the annual energy savings associated with the HE rectifiers versus the standard models is 3490 kWh. This includes a 317 kWh reduction associated with the operation of the air conditioner. At
$0.10 per kWh, this represents an annual savings of $349 in overall energy charges for the site.
This example illustrates another economic benefit of the HE rectifiers. The use of HE rectifiers reduces the amount of heat generated in the cabinet by 1236 BTU/h. This heat
A GREEN REVOLUTION IN DC POWER
- 10 -
AN ELTEK VALERE WHITE PAPER

reduction will likely allow the use of a one-size smaller air conditioner. This would reduce the CAPEX (capital expense) cost of the cabinet by about $200.
Let us examine the economics of the HE rectifiers over a ten-year operational life for that site:
For the purposes of this analysis, let us hypothetically assume a $100 incremental price for the HE rectifier versus the standard unit. With seven rectifiers in the system, the additional purchase price for the power system is $700, which is partially offset by the
$200 savings on the cost of the cabinet (due to the smaller air conditioner). The net
CAPEX impact on the overall cost is $500.
The reduced OPEX (operating expense) due to energy cost savings is $349 per year, making the payback of the incremental $500 CAPEX outlay about 18 months. The Net
Present Value of the overall economics over the 10-year period (assuming a 5% cost of money) is $2,195. That means that the use of the HE rectifiers will generate $2195 of savings (in today’s dollars) compared with the standard rectifiers. That $2,195 will pay for a major portion of the DC power system.
The results are more dramatic in the example, when the results of the HE rectifier are compared to a 90% efficient unit (rather than the standard Eltek Valere unit of 92% efficiency). The annual energy cost savings would grow to $535. The payback period would be within 1 year and the net present value would grow to $3,631.
At 96% efficiency, the argument for deploying the HE rectifiers is quite compelling. The incremental price is projected to be well within the $100 hypothetical level used in the analysis, and the significant energy savings provide rapid payback.
The cases examined so far are those in which a new DC power system is required, and a comparison is made between alternative solutions of different efficiencies. However, it may still make sense for carriers to upgrade existing DC power systems with new high efficiency rectifiers.
Many older power systems can be characterized as approaching obsolescence. That generally means increasing rectifier failures, more difficulty in obtaining replacement units, and escalating prices for those replacements. Added to that is the fact that these older systems are significantly lower in efficiency than those purchased in the last decade. The result is a strong business case for upgrading the DC power systems at those sites.
A GREEN REVOLUTION IN DC POWER
- 11 -
AN ELTEK VALERE WHITE PAPER

An access system is the general term given to the electronics that connect subscribers to the telecommunications network. In the 1970s, the concept of placing electronics in the outside plant network emerged as a technique for serving subscribers who were located at long distances from central office switching centers. Previously, those subscribers were connected via very long, expensive runs of copper cables. In the ensuing decades, that technique has evolved through the use of digital loop carrier systems, DSL access multiplexers and fiber to the node or fiber to the home systems. With the placement of electronics closer to the subscriber, the deployment of outdoor cabinets grew tremendously. These cabinets housed the access equipment and its supporting power system.
Unlike the outdoor cabinets that are used at cell sites, the majority of the access system cabinets do not include air conditioners. The equipment was designed to operate over a wide temperature range, with fans and/or heat exchangers as the cooling mechanism. The thermal design of these integrated cabinet systems was an important consideration and a challenge.
The access systems installed over the last 30 years have performed well and continue to do so. The exploding subscriber demand for higher speed Internet access has created a new challenge for service providers to upgrade these cabinets with broadband capability.
Equipment manufacturers have designed a number of solutions for providing the broadband service. However, there are three obstacles that must be overcome in implementing these broadband upgrades in the existing cabinets.
The first issue is the availability of space. The broadband solutions generally involve the installation of a new shelf of equipment (i.e. a DSLAM) into an already crowded cabinet.
The second obstacle relates to power. The broadband equipment will increase the DC power consumption, sometimes beyond the capacity of the existing power system. And the final hurdle relates to the thermal design of the cabinet. The additional broadband equipment (and its associated power demand) will increase the thermal load on the cabinet, often beyond its design limitations. A solution that addresses all of these obstacles is the replacement of the existing power system with one of higher efficiency and power density.
Eltek Valere has created a family of power systems targeted specifically at the upgrade of existing access systems. Many of the older power systems in these access cabinets are reaching the point of obsolescence. Failure rates are going up and replacement units are difficult and /or expensive to obtain. The design of these older systems did not include such beneficial capabilities as temperature compensation for battery charging, and in some instances did not allow for N+1 redundant operation. The replacement of the old power systems can often be justified based solely on the improvement of these two conditions.
A GREEN REVOLUTION IN DC POWER
- 12 -
AN ELTEK VALERE WHITE PAPER

However, the new systems offer additional benefits that are key to broadband upgrades.
The compact design of the power systems, due to a more digital design and higher efficiency, enables them to provide increased power and more fused distribution.
And, these systems have a footprint that ranges from 50% to as little as 10% of existing systems. This means more room to install the broadband electronics, and the availability of power to support it. This addresses the first two obstacles discussed earlier.
What is often overlooked is the impact of higher efficiency rectifiers in addressing the issue of the thermal design of the cabinet. Let us consider a specific example. Take the case of an 80 Type Cabinet (double-sided, 2 racks on each side) which is housing a SLC
Series 5 digital loop carrier system, a fiber multiplexer, and the associated power system.
This is a typical application used by service providers in the U.S. Let us assume that the power system has an average load of 500W (~10A on a 48V plant). The efficiency of the existing power system in that cabinet is about 80%. This means that 625W is required to generate that 500W of load power, and 125W is lost as heat (see figure 5 below).
AC input
625 W
500 W
80% efficient
DC power system
125 W
DC Output to load
Lost power (heat)
AC input
521 W
96% efficient
DC power system
500 W
21 W
DC Output to load
Lost power (heat)
Reduction in lost power is 125 W Š 21 W = 104 W
Percent reduction in lost power is 104 W / 125 W = 83.2%
Figure 5
Rectifier Efficiency Comparison: Access System
Consider now the replacement of the existing power system with one utilizing Eltek
Valere’s family of HE rectifiers. Operating at an efficiency of 96%, input power of
521W is required to deliver the 500W of load power. Only 21W is lost as heat, a savings of 104W. This is a reduction of 83.2% in those losses.
As stated earlier, the thermal design of the access cabinet can limit its use as a broadband system. This example however, has demonstrated that the use of HE rectifiers will eliminate 104W of lost power. That is power that can now be put to use to feed the additional broadband equipment that will be placed in the cabinet. This is demonstrated in figure 6.
A GREEN REVOLUTION IN DC POWER
- 13 -
AN ELTEK VALERE WHITE PAPER

AC input
625 W
500 W
80% efficient
DC power system
125 W
DC Output to load
Lost power (heat)
AC input
625 W
96% efficient
DC power system
600 W
DC Output to load
25 W
Lost power (heat)
Additional power available for load (without impacting cabinet thermal performance) is 600W Š 500 W = 100 W
Figure 6
Load Power Availability Comparison
It is assumed that the thermal design of the cabinet was adequate to support the 625W (or
2133 BTU/h) of power input to the existing system. By upgrading the power system to
96% efficiency, a 600W of load output (up from 500W) can be delivered while keeping the total input power at 625W. That additional 100W can support broadband service for about 75 subscribers, without affecting the overall thermal performance of the cabinet. In many instances this efficiency improvement will allow the cabinet to be equipped with broadband capability, without the need for an upgrade to the cooling system. This is a major benefit.
In summary, the replacement of the older power plant with the HE system has improved reliability; added beneficial features such as thermal compensation on battery charging; and has enabled the deployment of broadband capability without any impact on the cabinet or its cooling system.
Central office power upgrades are a prime application for higher efficiency rectifiers.
Many of the DC power systems in central offices have been in service for 15 - 20 years, sometimes even longer. As these plants continue to age, rectifier failures become more frequent. Obtaining replacement units becomes more difficult, and certainly more expensive, as suppliers are forced to raise the prices on these low-volume products.
Service providers are faced with the decision of upgrading the systems versus extending the life of the existing equipment. The economic advantages of higher efficiency rectifiers can make the decision a relatively easy one.
A GREEN REVOLUTION IN DC POWER
- 14 -
AN ELTEK VALERE WHITE PAPER

There are several factors that make the impact of high efficiency so dramatic in central offices. The first is that the DC loads are generally much higher in a central office than the levels needed for access system cabinets or cell sites. Many central offices are equipped with power systems of capacities ranging from 1200A through 10,000A, and the DC loads on these systems are hundreds or thousands of amps. At these power levels, energy savings resulting from higher efficiency rectifiers are quite significant.
A second factor relates to the rectifier technology used in the existing systems. Having been installed decades ago, many of the power systems found in central offices still incorporate ferroresonant rectifiers. While being an extremely robust technology, ferroresonant designs are much less efficient than the switched mode rectifiers available today. Qwest has developed a composite efficiency curve
7
for the collection of ferroresonant rectifiers it uses in central office applications.
That curve, which is shown in figure 7, is assumed to be a reasonable estimate for any service provider. From the curve it can be seen that the efficiency of ferroresonant rectifiers found in central offices will be much lower than the 92% or more that can be achieved with today’s switch mode rectifiers. This is particularly true of rectifiers operating under 80% of capacity (which is the usual case in central offices).
Ferroresonant Rectifier Efficien
90
87
80
82
85
80
77
70 75
60
66
57
50
48
40
39
30
30
20
0 10 20 30 40 50 60 70 80 90 100
Percent of Full Load
Composite Curve
Figure 7
Composite Ferroresonant Rectifier Efficiency Curve
A third consideration in central offices is the large impact of rectifier efficiency on cooling systems. The impact is different in the central office compared with the air conditioned remote cabinet, where the rule of thumb is that for every kWh of energy saved by the use of higher efficiency rectifiers, an additional 10% of energy savings is derived from the reduced operation of the air conditioning system. For central offices, that impact on cooling is even higher because the air conditioning systems essentially run continuously. Several service providers report that for every kWh of energy saved by the use of higher efficiency rectifiers in a central office, an additional 35% of energy savings is derived from the reduced operation of the air conditioning system.
A GREEN REVOLUTION IN DC POWER
- 15 -
AN ELTEK VALERE WHITE PAPER

All of these factors contribute to a very favorable business case for upgrading central office power systems with higher efficiency rectifiers. Let us consider a typical example. Assume an existing central office power plant equipped with 2000A of ferroresonant rectifier capacity. The load on the plant is 1200A (approximately 60 kW).
Because of concerns over obsolescence, the service provider has determined to begin a power system upgrade program, and have chosen a “cap-and-grow” strategy.
In this plan, a new rectifier-only power system will be connected to the existing power plant (Eltek Valere has a methodology for operating the co-existing systems). Rectifiers will be added to the new plant over time, as the office load grows or as existing rectifiers need replacement. The load distribution (breakers/fuses, cabling etc) will be left on the old plant to minimize the cost and disruption of the upgrade. One objective of this strategy is to initiate the plant upgrade with a minimum number of new rectifiers, thereby deferring capital expenses until they are required. This is a sound strategy that balances
CAPEX costs with reduced OPEX costs when based on a standard efficiency rectifier.
However, the economics of high efficiency rectifiers may enable the service provider to accelerate their replacement strategy.
The rectifiers in the existing power system are operating at 60% of their capacity. Using the ferroresonant rectifier efficiency curve in figure 7, the overall plant efficiency is 77%.
The service provider has decided to install an Eltek Valere Flatpack2 Power System to upgrade the plant. The original plan was to equip the new plant with a minimal number of rectifiers initially. However, for this example, let us assume that enough rectifiers are purchased to enable the replacement of all of the ferroresonant units on day one. That is accomplished by providing 1500A of rectifier capacity (1200A for load and 300A for battery recharge/spare), utilizing the new Flatpack2 HE rectifier. The efficiency of this system, operating at 80% of capacity, is 95.8%.
Figure 8 compares the energy usage associated with the existing ferroresonant rectifiers with that of the new HE rectifiers from Eltek Valere. Using the 35% factor for cooling impact, the total energy savings associated with the plant upgrade is more than 180,000 kWh per year. This equates to $18,084 per year of energy cost savings at $0.10 per kWh.
Assuming that the service provider had already decided to install the new rectifier bay, the cost of all the rectifiers is recovered in approximately ten months by the energy
AC input
77,922W
60,000W
DC output to load
77% efficient
DC power system 17,922W
(61,168 BTU/h)
Lost power (heat)
AC input
62,630W 95.8% efficient
DC power system
60,000W
DC output to load
2,630W
(8,978 BTU/h)
Lost power (heat)
Annual energy savings: 133,954 kW/h (for rectifiers)
46,884 kW/h (for cooling*)
180,838 kW/h TOTAL
* Based on typical factor of 35% for additional
A GREEN REVOLUTION IN DC POWER
- 16 -
AN ELTEK VALERE WHITE PAPER
180,838 kW/h per year

savings.
Figure 8
Central Office Energy Savings
Using energy savings as a catalyst, the service provider can eliminate the concerns over obsolescence-related failures; implement state of the art controller capabilities; achieve recurring annual savings in energy expenses; and utilize the removed ferroresonant rectifiers as spares for other plants that use similar rectifiers. The result is a very compelling business case for replacing all of the old rectifiers.
Eltek Valere has shipped hundreds of thousands of their Flatpack2 and V Series rectifiers to locations throughout the world. These rectifiers are performing extremely well in the wide-range of applications and environments for which they are designed. While these are successful installations, the economics – and green benefits – of high efficiency rectifiers make many of these locations good candidates for upgrade to HE versions of the product.
The Flatpack2 and V Series of rectifiers have been selected as the initial products to be offered in the HE (high efficiency) design. As stated earlier, these products will achieve efficiencies of approximately 96% across their normal load range. And the products are designed to be plug-in replacements for the standard rectifiers from which they evolved.
That means that a service provider can replace their existing rectifiers with the HE version to achieve a 4% efficiency improvement, while maintaining all of the exisiting controller and plant capabilities.
A 4% efficiency improvement makes a significant contribution to corporate initiatives aimed at energy reduction. The question becomes whether the business case for replacing a relatively new rectifier is attractive.
Let us go back to the example of the air-conditioned cell site cabinet discussed earlier to demonstrate the value of the Flatpack2 HE rectifiers. This was a 14KW capacity system, utilizing seven Flatpack2 (2000W) rectifiers, with an average load of 8KW on the plant.
Assume that the plant was installed using the standard version of those rectifiers. From the curve in figure 4 it can be determined that the existing rectifiers will be operating at
92% efficiency. If those rectifiers are replaced with the new Flatpack2 HE version, the efficiency will improve to 96%.
The impact of the 4% efficiency improvement on this system is an annual savings of
3490 kWh. This includes a 10% impact factor for the cooling system. At $0.10 per kWh, this equates to a savings of $349 per year. The question now becomes how much one can afford to spend to achieve those savings.
A GREEN REVOLUTION IN DC POWER
- 17 -
AN ELTEK VALERE WHITE PAPER

Eltek Valere recognizes that the replacement of relatively new power equipment is not something that is routinely done in the industry.
For the purpose of this analysis, let us assume that the seven new rectifiers will be purchased for a net total price of $1,750. This assumes purchase volume discounts and potential rebates for returned rectifiers or other promotions. With an annual savings of
$349 in energy, the simple cash flow for a 10-year life cycle is depicted in figure 9.
SAVINGS
$349 $349 $349 $349 $349 $349 $349 $349 $349 $349
Year
0
Year
1
Year
2
Year
3
Year
4
Year
5
Year
6
Year
7
Year
8
Year
9
Year
10
Rate of Return: 15%
EXPENSES
$1750
Figure 9
Rate of Return on Flatpack2 Replacement
In this example, the payback on the rectifier investment will occur in approximately 5 years. Over the 10 year period, the energy savings will provide an overall 15% rate of return on the investment in the new HE rectifiers. This is an attractive business case for service providers who are committed to reducing energy usage. Obviously, the five-year payback period must be weighed against other uses of capital.
The case for replacement of existing Eltek Valere rectifiers is the most challenging one since the installed units operate at the highest efficiency in the industry today (prior to the
HE products). The analysis is presented here to demonstrate that, even in this extreme case, the HE rectifiers provide significant energy reduction and a favorable rate of return.
Eltek Valere estimates that the global telecommunications industry uses nearly 1% of all the electricity consumed in the world. That estimate is based on an extrapolation of energy usage statistics from some of the largest service providers in the world, such as
Telefonica, Deutsche Telekom, NTT and Verizon. The data suggests that about
2
/
3 of that electricity is used by the DC power systems that supply the telecommunications network
That projects to 115 billion kWh of electricity in 2007, associated with DC power systems for telecommunications.
A GREEN REVOLUTION IN DC POWER
- 18 -
AN ELTEK VALERE WHITE PAPER

A major focus of service providers throughout the globe is the reduction of energy usage and the emission of greenhouse gases. This is both an environmental and social responsibility, as well as an economic opportunity. A broad-range of energy conservation initiatives is being evaluated, and some are being implemented. One of the most logical and attractive opportunities for energy reduction is found in the DC power system itself.
Eltek Valere has unquestionably been the technology leader in rectifier design, advancing efficiencies from an industry standard near 88% to the current benchmark of 92% or higher. Even at 92% efficiency, 8% of the AC power is lost as heat in the power conversion process. But the technology advancement continues. Eltek Valere is introducing its new HE series of rectifiers, which cuts the lost power in half by achieving unprecedented efficiency of 96% or more.
As demonstrated through numerous examples in this paper, the HE rectifiers enable service providers to achieve both significant reductions in energy usage and a lower overall cost than the alternative solutions provide. A life cycle analysis of these typical cases clearly shows that the energy savings from the use of the HE rectifiers will quickly payback any existing first-cost premium. These cases are further improved by the positive impact of the HE rectifiers on cooling systems, or on the thermal performance of cabinets that do not have air conditioning. For the replacement of older systems, there is the added benefit of improved reliability and operational features.
The specific payback period for the use of the HE rectifier is dependent on a number of factors including capacity and load on the system, efficiency of the alternative product being compared, the cost of electricity at the site, etc. For new systems, the payback period is generally in the range of two years or less. For retrofit applications, particularly those involving older rectifiers, the payback period is often closer to one year. In nearly all instances, a 10-year life cycle analysis will result in an extremely favorable rate of return on the initial rectifier investment.
What impact could the HE rectifier have on energy usage? As stated earlier, the telecommunications industry used an estimated 115 billion kWh of electricity to run DC power systems for its services in 2007. If all of those systems could achieve a 4% efficiency improvement, which is the minimum offered by the HE rectifiers, then the overall result would be a savings of 4.8 billion kWh of electricity and a reduction of 3.5 million tons of CO
2
emissions. Those annual reductions are equivalent to the electricity used by 480,000 homes, and the emissions from 580,000 automobiles. In addition to improving the environment, this 4% efficiency increase would result in an annual savings of $480 million (at $0.10 per kWh for electricity).
The introduction of the HE rectifiers has given service providers an outstanding vehicle to help accomplish both their environmental and financial objectives. This product has reinforced Eltek Valere’s technology leadership in the industry, and it has enabled the start of a Green Revolution in DC Power Systems.
A GREEN REVOLUTION IN DC POWER
- 19 -
AN ELTEK VALERE WHITE PAPER

1
Electric Power Monthly, November 2007 Edition (Energy Information Administration)
2
Verizon Corporate Responsibility Report 2006
3
Verizon Communications – Green House Gas Emission Reduction Initiatives (on Clean
Air, Cool Planet web site)
4
Telefonica CR Report 2006
5
NTT Group Environmental Protection Activity Report 2004
6
Energy Information Administration
7
Qwest Power Engineering Organization - Central Office Rectifier Replacement Study
A GREEN REVOLUTION IN DC POWER
- 20 -
AN ELTEK VALERE WHITE PAPER