How Power Grids Work

How Power Grids Work

by Marshall Brain



Inside This Article

1.

Introduction to How Power Grids Work

2.

The Power Plant

3.

The Power Plant: Alternating Current

4.

The Power Plant: Three-phase Power

5.

Transmission Substation

6.

The Distribution Grid

7.

Distribution Bus

8.

Regulator Bank

9.

Taps

10.

At the House

11.

Safety Devices: Fuses

12.

Safety Devices: Circuit Breakers

13.

Lots More Information

14.

See all Energy Production articles

Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. Power is just "there," meeting your every need, constantly. It is only during a power failure, when you walk into a dark room and instinctively hit the useless light switch, that you realize how important power is in your daily life. You use it for heating, cooling, cooking, refrigeration, light, sound, computation, entertainment... Without it, life can get somewhat cumbersome.

Power travels from the power plant to your house through an amazing system called the power distribution grid.



Power grid distribution lines can be above or under ground. See more power grid pictures.


The grid is quite public -- if you live in a suburban or rural area, chances are it is right out in the open for all to see. It is so public, in fact, that you probably don't even notice it anymore. Your brain likely ignores all of the power lines because it has seen them so often. In this article, we will look at all of the equipment that brings electrical power to your home. The next time you look at the power grid, you will be able to really see it and understand what is going on!

The Power Plant

Electrical power starts at the power plant. In almost all cases, the power plant consists of a spinning electrical generator. Something has to spin that generator -- it might be a water wheel in a hydroelectric dam, a large diesel engine or a gas turbine. But in most cases, the thing spinning the generator is a steam turbine. The steam might be created by burning coal, oil or natural gas. Or the steam may come from a nuclear reactor like this one at the Shearon Harris nuclear power plant near Raleigh, North Carolina:




No matter what it is that spins the generator, commercial electrical generators of any size generate what is called 3-phase AC power. To understand 3-phase AC power, it is helpful to understand single-phase power first.


Photo courtesy U.S. Department of Energy
A breakdown of the major power plants in
the United States, by type




The Power Plant: Alternating Current

Single-phase power is what you have in your house. You generally talk about household electrical service as single-phase, 120-volt AC service. If you use an oscilloscope and look at the power found at a normal wall-plate outlet in your house, what you will find is that the power at the wall plate looks like a sine wave, and that wave oscillates between -170 volts and 170 volts (the peaks are indeed at 170 volts; it is the effective (rms) voltage that is 120 volts). The rate of oscillation for the sine wave is 60 cycles per second. Oscillating power like this is generally referred to as AC, or alternating current. The alternative to AC is DC, or direct current. Batteries produce DC: A steady stream of electrons flows in one direction only, from the negative to the positive terminal of the battery.

AC has at least three advantages over DC in a power distribution grid:

1. Large electrical generators happen to generate AC naturally, so conversion to DC would involve an extra step.

2. Transformers must have alternating current to operate, and we will see that the power distribution grid depends on transformers.

3. It is easy to convert AC to DC but expensive to convert DC to AC, so if you were going to pick one or the other AC would be the better choice.

The Power Plant: Three-phase Power

The power plant produces three different phases of AC power simultaneously, and the three phases are offset 120 degrees from each other. There are four wires coming out of every power plant: the three phases plus a neutral or ground common to all three. If you were to look at the three phases on a graph, they would look like this relative to ground:




There is nothing magical about three-phase power. It is simply three single phases synchronized and offset by 120 degrees.

Why three phases? Why not one or two or four? In 1-phase and 2-phase power, there are 120 moments per second when a sine wave is crossing zero volts. In 3-phase power, at any given moment one of the three phases is nearing a peak. High-power 3-phase motors (used in industrial applications) and things like 3-phase welding equipment therefore have even power output. Four phases would not significantly improve things but would add a fourth wire, so 3-phase is the natural settling point.

And what about this "ground," as mentioned above? The power company essentially uses the earth as one of the wires in the power system. The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons. (Car manufacturers do something similar; they use the metal body of the car as one of the wires in the car's electrical system and attach the negative pole of the battery to the car's body.) "Ground" in the power distribution grid is literally "the ground" that's all around you when you are walking outside. It is the dirt, rocks, groundwater, etc., of the earth.

Transmission Substation

The three-phase power leaves the generator and enters a transmission substation at the power plant. This substation uses large transformers to convert the generator's voltage (which is at the thousands of volts level) up to extremely high voltages for long-distance transmission on the transmission grid.


A typical substation at a power plant


You can see at the back several three-wire towers leaving the substation. Typical voltages for long distance transmission are in the range of 155,000 to 765,000 volts in order to reduce line losses. A typical maximum transmission distance is about 300 miles (483 km). High-voltage transmission lines are quite obvious when you see them. They are normally made of huge steel towers like this:




All power towers like this have three wires for the three phases. Many towers, like the ones shown above, have extra wires running along the tops of the towers. These are ground wires and are there primarily in an attempt to attract lightning.

The Distribution Grid

For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid. This may happen in several phases. The place where the conversion from "transmission" to "distribution" occurs is in a power substation. A power substation typically does two or three things:

It has transformers that step transmission voltages (in the tens or hundreds of thousands of volts range) down to distribution voltages (typically less than 10,000 volts).
It has a "bus" that can split the distribution power off in multiple directions.
It often has circuit breakers and switches so that the substation can be disconnected from the transmission grid or separate distribution lines can be disconnected from the substation when necessary.

A typical small substation


The box in the foreground is a large transformer. To its left (and out of the frame but shown in the next shot) are the incoming power from the transmission grid and a set of switches for the incoming power. Toward the right is a distribution bus plus three voltage regulators.


The transmission lines entering the substation and passing through the switch tower





The switch tower and the main transformer


Now the distribution bus comes into the picture

Distribution Bus

The power goes from the transformer to the distribution bus:




In this case, the bus distributes power to two separate sets of distribution lines at two different voltages. The smaller transformers attached to the bus are stepping the power down to standard line voltage (usually 7,200 volts) for one set of lines, while power leaves in the other direction at the higher voltage of the main transformer. The power leaves this substation in two sets of three wires, each headed down the road in a different direction:


The wires between these two poles are "guy wires" for support. They carry no current.




The next time you are driving down the road, you can look at the power lines in a completely different light. In the typical scene pictured on the right, the three wires at the top of the poles are the three wires for the 3-phase power. The fourth wire lower on the poles is the ground wire. In some cases there will be additional wires, typically phone or cable TV lines riding on the same poles.

As mentioned above, this particular substation produces two different voltages. The wires at the higher voltage need to be stepped down again, which will often happen at another substation or in small transformers somewhere down the line. For example, you will often see a large green box (perhaps 6 feet/1.8 meters on a side) near the entrance to a subdivision. It is performing the step-down function for the subdivision.









Regulator Bank

You will also find regulator banks located along the line, either underground or in the air. They regulate the voltage on the line to prevent undervoltage and overvoltage conditions.


A typical regulator bank


Up toward the top are three switches that allow this regulator bank to be disconnected for maintenance when necessary:




At this point, we have typical line voltage at something like 7,200 volts running through the neighborhood on three wires (with a fourth ground wire lower on the pole):






Taps

A house needs only one of the three phases, so typically you will see three wires running down a main road, and taps for one or two of the phases running off on side streets. Pictured below is a 3-phase to 2-phase tap, with the two phases running off to the right:




Here is a 2-phase to 1-phase tap, with the single phase running out to the right:






At the House

And finally we are down to the wire that brings power to your house! Past a typical house runs a set of poles with one phase of power (at 7,200 volts) and a ground wire (although sometimes there will be two or three phases on the pole, depending on where the house is located in the distribution grid). At each house, there is a transformer drum attached to the pole, like this:




In many suburban neighborhoods, the distribution lines are underground and there are green transformer boxes at every house or two. Here is some detail on what is going on at the pole:




The transformer's job is to reduce the 7,200 volts down to the 240 volts that makes up normal household electrical service. Let's look at this pole one more time, from the bottom, to see what is going on:




There are two things to notice in this picture:

There is a bare wire running down the pole.
This is a grounding wire. Every utility pole on the planet has one. If you ever watch the power company install a new pole, you will see that the end of that bare wire is stapled in a coil to the base of the pole and therefore is in direct contact with the earth, running 6 to 10 feet (1.8 to 3 m) underground. It is a good, solid ground connection. If you examine a pole carefully, you will see that the ground wire running between poles (and often the guy wires) are attached to this direct connection to ground.
There are two wires running out of the transformer and three wires running to the house.
The two from the transformer are insulated, and the third one is bare. The bare wire is the ground wire. The two insulated wires each carry 120 volts, but they are 180 degrees out of phase so the difference between them is 240 volts. This arrangement allows a homeowner to use both 120-volt and 240-volt appliances. The transformer is wired in this sort of configuration:



The 240 volts enters your house through a typical watt-hour meter like this one:




The meter lets the power company charge you for putting up all of these wires.

Safety Devices: Fuses

Fuses and circuit breakers are safety devices. Let's say that you did not have fuses or circuit breakers in your house and something "went wrong." What could possibly go wrong? Here are some examples:

A fan motor burns out a bearing, seizes, overheats and melts, causing a direct connection between power and ground.
A wire comes loose in a lamp and directly connects power to ground.
A mouse chews through the insulation in a wire and directly connects power to ground.
Someone accidentally vacuums up a lamp wire with the vacuum cleaner, cutting it in the process and directly connecting power to ground.
A person is hanging a picture in the living room and the nail used for said picture happens to puncture a power line in the wall, directly connecting power to ground.


When a 120-volt power line connects directly to ground, its goal in life is to pump as much electricity as possible through the connection. Either the device or the wire in the wall will burst into flames in such a situation. (The wire in the wall will get hot like the element in an electric oven gets hot, which is to say very hot!). A fuse is a simple device designed to overheat and burn out extremely rapidly in such a situation. In a fuse, a thin piece of foil or wire quickly vaporizes when an overload of current runs through it. This kills the power to the wire immediately, protecting it from overheating. Fuses must be replaced each time they burn out. A circuit breaker uses the heat from an overload to trip a switch, and circuit breakers are therefore resettable.

The power then enters the home through a typical circuit breaker panel like the one above.









Safety Devices: Circuit Breakers



Inside the circuit breaker panel (right) you can see the two primary wires from the transformer entering the main circuit breaker at the top. The main breaker lets you cut power to the entire panel when necessary. Within this overall setup, all of the wires for the different outlets and lights in the house each have a separate circuit breaker or fuse:




If the circuit breaker is on, then power flows through the wire in the wall and makes its way eventually to its final destination, the outlet.




What an unbelievable story! It took all of that equipment to get power from the power plant to the light in your bedroom.




The next time you drive down the road and look at the power lines, or the next time you flip on a light, you'll hopefully have a much better understanding of what is going on. The power distribution grid is truly an incredible system.





4. Heat

Heat
Heat may be defined as energy in transit from a high temperature object to a lower temperature object. An object does not possess "heat"; the appropriate term for the microscopic energy in an object is internal energy. The internal energy may be increased by transferring energy to the object from a higher temperature (hotter) object - this is properly called heating.



Mechanical equivalent of heat




Heat and Work Example


This example of the interchangeability of heat and work as agents for adding energy to a system can help to dispel some misconceptions about heat. I found the idea in a little article by Mark Zemansky entitled "The Use and Misuse of the Word 'Heat' in Physics Teaching". One key idea from this example is that if you are presented with a high temperature gas, you cannot tell whether it reached that high temperature by being heated, or by having work done on it, or a combination of the two.

To describe the energy that a high temperature object has, it is not a correct use of the word heat to say that the object "possesses heat" - it is better to say that it possesses internal energy as a result of its molecular motion. The word heat is better reserved to describe the process of transfer of energy from a high temperature object to a lower temperature one. Surely you can take an object at low internal energy and raise it to higher internal energy by heating it. But you can also increase its internal energy by doing work on it, and since the internal energy of a high temperature object resides in random motion of the molecules, you can't tell which mechanism was used to give it that energy.

In warning teachers and students alike about the pitfalls of misusing the word "heat", Mark Zemansky advises reflecting on the jingle:

"Teaching thermal physics
Is as easy as a song:
You think you make it simpler
When you make it slightly wrong.
Zemanzky's plea



Don't refer to the "heat in a body", or say "this object has twice as much heat as that body". He also objects to the use of the vague term "thermal energy" and to the use of the word "heat" as a verb, because they feed the misconceptions, but it is hard to avoid those terms. He would counsel the introduction and use of the concept of internal energy as quickly as possible.


Zemansky points to the First Law of Thermodynamics as a clarifying relationship. The First Law identifies both heat and work as methods of energy transfer which can bring about a change in the internal energy of a system. After that, neither the words work or heat have any usefulness in describing the final state of the sytem - we can speak only of the internal energy of the system.

Mechanical Equivalent of Heat
Heat flow and work are both ways of transferring energy. As illustrated in the heat and work example, the temperature of a gas can be raised either by heating it, by doing work on it, or a combination of the two.

In a classic experiment in 1843, James Joule showed the energy equivalence of heating and doing work by using the change in potential energy of falling masses to stir an insulated container of water with paddles. Careful measurements showed the increase in the temperature of the water to be proportional to the mechanical energy used to stir the water. At that time calories were the accepted unit of heat and joules became the accepted unit of mechanical energy. Their relationship is



First law of thermodynamics


Heat is Transfer of Thermal Energy
When you heat an object, you are transferring thermal energy to it from an another object that is at a higher temperature. Heat is the amount of thermal energy that is transferred between the two objects due to a temperature difference. Heat transfer between objects is done by conduction, convection and radiation. The standard unit of heat measurement is the calorie.

Questions you may have include:

What is heat?
How does heat get from one object to another?
What is the measure of heat?
Heat is energy in transit
Heating an object is when you are transferring thermal energy to the object from to another object that is at a higher temperature. Heat is often defined as energy in transit or the the flow of energy. Thermal energy is the energy itself.

Thermal energy is the amount of internal kinetic energy and potential energy of an object. It is also simply called internal energy. Temperature is a measure of the average kinetic energy of the particles in an object.

An object feels warm or hot if its temperature is higher than your skin. To say something is hot means its temperature is relatively high.

Cooling an object is when you are transferring thermal energy from the object from an another object that is at a lower temperature. You could say you are removing thermal energy from your object.

An object feels cool or cold if its temperature is lower than your skin. To say something is cold means its temperature is relatively low.

Whether heating or cooling, the end result is that the two objects become the same temperature after a period of time. This is called thermal equilibrium.

Heat transfer
Thermal energy is transferred from an object of high temperature to one of lower temperature by conduction, convention and radiation. This process is usually called heat transfer or heat flow, although it is the thermal energy that is really being transferred. Heat is the amount transferred.

(See Heat Transfer for more information on that subject.)

Conduction
Conduction is when materials are in physical contact and kinetic energy is transferred through collisions of their particles, according to the Kinetic Theory of Matter.

(See Kinetic Theory of Matter for more information on that subject.)

Convection
Convection is the movement of thermal energy from one area to another in a liquid or gas.

Radiation
Radiation is when warm or hot matter emits electromagnetic radiation--especially infrared--that is then absorbed by an object at a distance. The absorption heats the second object.

Units of heat
The amount of heat or thermal energy transferred from one object to another can be measured in joules, which is the unit of energy. But more often, you see heat measured in calories. A calorie (cal) is the amount of heat required to raise the temperature of 1 gram of of water by 1° C.

The relationship between joules and calories is: 1 cal = 4.186 J.

A kilocalorie (kcal) equals 1000 calories. Transferring 1 kcal of heat to 1 kilogram of water will increase its temperature 1° C. A kilocalorie is also called a Calorie (with a capital "C") by those dealing with food and diets. When you hear that some food has 200 Cal, that means it has the potential of transferring 200 kilocalories of heat energy to the body.

In the United States, some use the BTU (British Thermal Unit) as a unit of heat transfer. A BTU is defined as the quantity of energy necessary to raise the temperature of 1 lb. of water 1° Fahrenheit. Often the BTU is used to indicate the heat capacity of a home furnace.

In conclusion
Heat is the amount of thermal energy that is transferred between the two objects due to a temperature difference. Heat transfer between objects is done by conduction, convection and radiation. The standard unit of heat measurement is the calorie.



WHAT IS HEAT

The Universe is made up of matter and energy. Matter is made up of atoms and molecules (groupings of atoms) and energy causes the atoms and molecules to always be in motion - either bumping into each other or vibrating back and forth. The motion of atoms and molecules creates a form of energy called heat or thermal energy which is present in all matter. Even in the coldest voids of space, matter still has a very small but still measurable amount of heat energy.



Energy can take on many forms and can change from one form to another. Many different types of energy can be converted into heat energy. Light, electrical, mechanical, chemical, nuclear, sound and thermal energy itself can each cause a substance to heat up by increasing the speed of its molecules. So, put energy into a system and it heats up, take energy away and it cools. For example, when we are cold, we can jump up and down to get warmer.

Here are just a few examples of various types of energy being converted into thermal energy (heat).

(1) Mechanical energy is converted into thermal energy whenever you bounce a ball. Each time the ball hits the ground, some of the energy of the ball's motion is converted into heating up the ball, causing it to slow down at each bounce. To see a demonstration of how this happens click here


A thermal infrared image of a ball before (left) and after (right) being bounced.

(2) Thermal energy can be transfered to other objects causing them to heat up. When you heat up a pan of water, the heat from the stove causes the molecules in the pan to vibrate faster causing the pan to heat up. The heat from the pan causes water molecules to move faster and heat up. So, when you heat something up, you are just making its molecules move faster.

(3) Electrical energy is converted into thermal energy when you use objects such as heating pads, electrical stove elements, toasters, hair dryers, or light bulbs.


A thermal infrared image of a hair dryer and a flourescent light bulb.

(4) Chemical energy from the foods we eat is converted into heating our bodies.

(5) Light from the sun is converted to heat as the sun's rays warm the earth's surface.

(6) Energy from friction creates heat. For example when you rub your hands, sharpen a pencil, make a skid mark with your bike, or use the brakes on your car, friction generates heat.


A thermal infrared image of a pencil after being sharpened (left) and of hot brakes in a car (right). Notice the hot tip of the pencil.























4. Buildings



Energy use in commercial buildings (from EIA)



Commercial buildings include a wide variety of building types—offices, hospitals, schools, police stations, places of worship, warehouses, hotels, barber shops, libraries, shopping malls—and that’s just the beginning of the list. These different commercial activities all have unique energy needs but, as a whole, commercial buildings use more than half their energy for heating and lighting.

How Energy is Used in Commercial Buildings





TYPES OF ENERGY USED IN COMMERCIAL BUILDINGS
Electricity and natural gas are the most common energy sources used in commercial buildings. Commercial buildings also use another source that you don’t usually find used in residential buildings—district energy. When there are many buildings close together, like on a college campus or in a big city, it is sometimes more efficient to have a central heating and cooling plant that distributes steam, hot water, or chilled water to all of the different buildings. A district system can reduce equipment and maintenance costs, as well as save energy.





ENERGY USE BY TYPE OF BUILDING
Retail and service buildings use the most total energy of all the commercial building types. This isn’t too surprising when you think of all the stores and service businesses in most towns. Offices use a large share of energy, too. Education buildings, like your school, use 13 percent of all total energy, which is even more than all hospitals and other medical buildings combined! Lodging buildings (like hotels or dormitories) use 8 percent of all energy. Warehouses and food service (like restaurants) each use 7 percent. Public assembly buildings, which can be anything from libraries to sports arenas, use 6 percent; food sales buildings (like grocery stores and convenience stores) use 4 percent. All other types of buildings, like places of worship, fire stations, police stations, and laboratories, account for the remaining 10 percent of commercial building energy.


















The Energy Efficient Commercial Buildings Deduction
EPAct 2005’s new incentive for lighting and building efficiency
The Energy Policy Act of 2005 created the Energy Efficient Commercial Buildings Deduction, which allows building owners to deduct the entire cost of a lighting or building upgrade in the year the equipment is placed in service, subject to a cap. This website, developed by the Lighting Systems Division of the National Electrical Manufacturers Association (NEMA) in cooperation with the Commercial Building Tax Deduction Coalition, provides education about the lighting aspects of the Deduction and resources to help with its implementation. It was created as the first of a series of lighting education initiatives by the lighting industry addressing lighting quality and efficiency.



More than one-third of the energy consumed in the United States is used in buildings.

EETD addresses building energy efficiency issues, including

· building technologies,

· the indoor environment,

· building codes and standards, and

· end-use energy efficiency issues,

through multidisciplinary research and analysis.

The Building Technologies Department works closely with the building industry to develop, test and deploy advanced technologies, integrated systems and new tools for design and operations that reduce energy bills while improving the comfort, health and safety of building occupants. Research and development efforts focus on windows and daylighting, lighting systems, building simulation tools, commercial building systems, demand response and high-tech buildings.

Indoor Environment Department researchers working in the buildings area focus on infiltration and mechanical ventilation systems, and on human health and productivity in buildings, with an emphasis on indoor chemistry and exposure and on air flow and air quality modeling.

Analysts working in the Energy Analysis Department gather and interpret information to examine the feasibility of different approaches to designing energy-efficient appliance standards and building codes in the U.S., and have worked with developing nations to create programs, codes and standards to reduce greenhouse gas emissions and encourage efficiency.

This website is a portal to more than fifty current and recent projects in commercial buildings. We have organized these projects into two broad areas:

· Technologies & Systems

Windows/Facades









Integrated Façade-Daylighting
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Dynamic Window System Performance
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Daylighting Quality
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Lighting


Advanced Lighting Controls
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HVAC/CHP


Low Energy Cooling Systems
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Thermal Distribution Systems



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Distributed Energy Site Simulation
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Demand Response


Demand Response Automation



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Ventilation/IAQ


Reducing Ventilation Energy with Air Cleaning



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Establishing Control of Building Ventilation



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Ventilation Performance of UFAD



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IAQ, Health, and Productivity



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Plug and Process Equipment


ENERGY STAR® Office Equipment



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Power Control User Interfaces



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· Tools & Process

Modeling and Simulation









Interoperability and Virtual Building Models
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Life Cycle Building Information Models
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EnergyPlus Development and Deployment
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Building Controls Virtual Test Bed



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Daylighting Modeling
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Commissioning/Performance Monitoring


Costs and Benefits of Commissioning


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Commissioning Persistence


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Commissioning Tools and Guides


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Functional Test Analysis Tool for AHUs


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Monitoring Based Retro-Commissioning



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Performance Metrics Tracking
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Energy Information Systems


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Automated Diagnostics and Prognostics



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Performance Monitoring Specification



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Benchmarking


Benchmarking



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Design Assistance and Assessment


Design Assistance for Federal Buildings
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Public Sector Procurement



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Sustainable Design for Federal Buildings
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Energy Use in Federal LEED Buildings



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IPMVP



































Last Revised: December 2006
Source: Energy Information Administration, 2003 Commercial Buildings Energy Consumption Survey.







I know I have sent you numerous emails with articles about efficiency, I hope it is helpful. It is not in any organized fashion.

If I can be of any additional help, let me know.

Jay Draiman, Energy analyst