Primer: Wind Power vs Wind Energy

In discussing the merits of wind energy many people  fail to comprehend the difference between wind energy and wind Power, and use the terms interchangeably. Governor Patrick has set a goal for 2,000 megawatts of wind generation capacity to power 800,000 homes by the year 2020. This paper will try to explain the difference between Power and Energy and put the governor’s claim into perspective.

Power is defined as the rate at which energy is generated or consumed and is measured in watts. Energy is defined as the amount of electricity that is consumed over a period of time and it is measured in watt-hours. Mathematically they are related as follows:

Energy = Power x Time

1,000 watts is called a kilowatt (kW), and 1,000 kilowatts is known as a megawatt (mW).

In energy units, 1,000 watt-hours is referred to as a kilowatt-hour (kWh), and 1,000 kilowatt-hours make up one megawatt-hour (mWh).

Fossil Fuel Power Plants

To illustrate how fossil fuel power plants operate, Figure 1 shows a fossil fuel power plant connected to a 100 watt light bulb through a switch. Even though fossil fuel plants generally can provide power measured in megawatts, for simplicity we can assume that the nameplate rating here will be 100 watts. With the switch off and the power plant running, we can say that 100 watts of power is available to power the light bulb as soon as the switch is turned on. In this state the power plant is producing no energy. However, it is consuming fuel that goes into generating waste heat. As soon as the switch is turned on, electric energy begins to flow from the power plant to the light bulb causing the light bulb to produce the rated amount of light. If we leave the light on for an hour, then we can say that the light bulb has consumed 100 watt-hours of energy which is equal to the amount of energy that was produced by the power plant. The amount of fuel used is dependent on how long the plant is running with the switch off and how long the plant runs with the switch on.

If we now change out the bulb, and replace it with a 50 watt unit, and repeat the experiment, we can say that 50 watt-hours were consumed, and supplied by the plant. If we leave the light on for 2 hours, then the amount of energy used would be 100 watt-hours.

A couple of points to remember. When the power plant is running it is producing 100 watts of power. However, it is not producing any energy until it is connected to the load (light bulb). This scenario can be scaled up without violating any of the principles involved. Let’s say that the average house with all of its electrical things turned on needs 10,000 kilowatts, then, a one megawatt power plant can supply power to 100 houses.

Fossil fuel power plants continue to burn fuel when placed on standby, and their consumption of fuel per energy unit produced increases when the plant is run below its rated capacity.

Wind Farm

Now let’s look at how a wind turbine or a wind farm would behave under the same circumstances. In figure 2 we have a wind turbine with name plate capacity equal to 100 watts. However the only time we can turn the switch on to light the bulb is only when the wind speed is between about 26 mph and 56 mph. This is because that is the range of wind speed that wind turbines generally produce their name plate capacity. Beyond 56 mph, the turbine is turned off to avoid wind damage, and bellow 26 mph the turbines will produce less than 100 watts. When the wind drops to 13 mph power output drops to about 12.5 watts. In this range the amount of energy consumed by the bulb is highly variable and the bulb would be flickering . If we leave the switch on for an hour the amount of watt-hours used will always be less than 100.The exact amount is unpredictable and can only be measured after the fact which makes scheduling wind turbines and wind farms next to impossible.

If we scale up to a wind farm, the problem stays about the same, and because there are prolonged periods of no wind, the number of houses that can be powered by wind is always zero.

Wind Farm with Backup

To overcome the intermittent operation of wind turbines and wind farms, backup with fossil fuel is a necessity. Figure 3 shows a 100 watt wind turbine with a 100 watt backup power plant. The two are designed to work in synchronism so that 100 watts of power is always available for the light bulb. When the wind is between 26 and 56 mph the fossil fuel plant is running in what is called spinning reserve, ready to compensate for any fluctuation of wind power including total shut down. At grid level this is known as fortifying wind to maintain grid balance.

When the switch is turned on for an hour 100 watt-hours of energy will be consumed by the bulb. However, how much energy comes from wind and how much from fossil fuel is dependent on what the wind did during that hour. In practice an energy meter is used to measure the energy produced by wind and used for billing and Renewable Energy Certificate (REC) calculations.

Conclusion

Claims as to the number of homes powered by wind are misleading. Wind power without backup is worthless.

For each megawatt of wind turbine name plate capacity an equal capacity fossil fuel plant is needed. Adding wind turbines to the grid does not reduce the grid’s need for fossil fuel capacity. The Capital cost of wind farms and the transmission lines to connect them is an additional cost to the production of electricity.

The wind industry’s claim that each megawatt-hour supplied by wind is one megawatt-hour less supplied by fossil fuel fails to account for the fuel wasted by the necessity to provide backup for wind. Fossil fuel plants that are backed down to make room for wind energy on the grid, continue to run, burning fuel that must be attributed to wind energy production.

The need to fluctuate fossil fuel energy output to balance wind energy fluctuations increases both fuel consumption and emissions which need to be attributed to the generation of clean wind energy.

Studies are available from power grids with significant levels of wind penetration that show the fuel saved and the emissions displaced is small to non-existent. Under some circumstances the addition of wind results in more fuel usage, and more emissions released into the atmosphere.