Grid Efficiency: An Opportunity to Reduce Emissions
- Aug 10, 2017 9:00 am GMT
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Current efforts to conserve energy and reduce emissions are significant. Some governments in Europe have set targets of 50% for the energy conservation. But is this the whole story? The overall efficiency from primary energy to delivered work is about 33% for energy in the US. Almost 2/3 of the energy that is consumed as primary energy is released as waste.
There are two key areas where waste occurs. Fossil fuels are delivered to users with a relatively high efficiency, but when it is burned or used to create other forms of useful energy the waste grows rapidly. Electricity, which is a carrier or proxy for energy, is generally used efficiently, but the generation and delivery of electric power consumes almost 2/3 of the primary energy delivered to the grid, leaving les than 40% of the primary energy to be converted to useful work. These areas of loss are opposite for the two types of fuel. Electricity loses a large amount of energy in the production and delivery stages, while fossil fuel losses occur mostly at the end of the line; in user systems.
An example is the comparison between an electric vehicle (EV) and a fossil fuel powered car. The EV has an overall efficiency of about 60% while the fuel powered car is about 20% (or less) efficient. When one considers the total path from primary energy, the efficiency of the electric car is 60% x 40% (grid efficiency) for a total of 24% while the fuel powered car has an efficiency of about 20%. When the bigger picture is considered, the two forms of transportation are not all that different.
There must be ways to make the electric grid more efficient, and if this can be achieved, the savings in both costs and emissions may be significant. If this can be done, the conversion to EVs may make excellent sense, while there is only a marginal difference on an average basis, if the EVs are charged randomly with electrical energy from the existing grid.
There may be opportunities in the electric system to improve efficiency. One is to displace coal fired steam turbine generating plants with newer distributed technology, such as gas turbines or solar and wind power. Combined cycle gas turbines are capable of generating electricity at over 60% efficiency, while the maximum theoretical efficiency of the steam turbine alone is about 42%, and it is likely less because the turbines are not operated continuously at maximum efficiency. Combined Heat and Power systems (CHP) systems have the potential to increase efficiency to more than 80%; by using almost all waste heat for other purposes.
Solar and wind are another issue. While each may have inefficiencies, the fuel is free and delivery is not measured, and therefore the injection of power is often assumed to be delivered with 100% efficiency. Few people consider the primary energy supply to these systems.
There are a number of concepts that cam improve overall grid efficiency. Several of these are described below.
Utilities have long recognized that loss is proportional to the square of the current (I2). Utilities have used this fact extensively by raising voltage and reducing current, yet delivering the same energy. This is the key difference between transmission and distribution systems. The long-haul transmission systems operate at a high voltage, with the added cost justified by the value of the reduced loss and the added capacity. It is generally not practical to do this on a real-time basis. But the potential to reduce losses by reducing current flow enables at least two other opportunities to reduce loss. The customer receives his or her electric power as a combination of real and reactive power (Watts and VARs).
Watts, which deliver real energy, MUST come from a generator, but VARs can be created or used anywhere, using capacitors or inductors. Both watts and VARs require current to deliver them. Significant loss reductions can be achieved by minimizing the flow of VARs in the grid. By managing grid edge devices, such as smart inverters, one can reduce VAR flows in the distribution system. This approach has been shown to reduce losses by up to 5%.
Distributed generation located at a load centre eliminates transmission and perhaps much of the distribution loss. For more than 100 years, utilities have balanced supply and demand by adjusting generators. The total generation is adjusted continuously to be equal to the total load. This process has resulted in expensive side effects. First, there are always generators assigned to absorb load changes to maintain the balance, and these machines are not operated at their maximum efficiency. By fixing all central generation to run at its maximum efficiency, and absorbing all short-term load changes at the grid edge, where the changes occur, loss is reduced.
By managing loads, distributed generation and distributed storage devices located near the grid edge, the efficiency of central generation can be maximized and maintained. At the same time, because loss varies with I2,, the total delivery loss at peak load may be 4x the loss at the average (half) load.. Most power systems operate at an average capacity factor of about 50%. By smoothing demand peaks and valleys locally with distributed generation, the overall energy loss can be reduced. The addition of solar and wind, with their high perceived delivery efficiency, at the grid edge will almost eliminate all losses for the energy injected.
It is apparent that in addition to the use of conventional conservation methods, the electric grid can be optimized at a relatively low cost, simply by changing the operating method. An EV that is charged using locally generated solar or wind energy can actually achieve a 60% efficiency level making it a far better choice than a fuel powered car.
The rapid development of VPPs and DERMs platforms that will monitor, manage and oversee the operations at the grid edge appear to be a very timely addition, with a powerful future.
Photo Credit: TwistedFMJ via Flickr