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Published on December 16th, 2015 | by Christopher Arcus

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How The Grid Works, & Why Renewables Can Dominate

December 16th, 2015 by  


Can we really generate most of our power from renewables in a few decades? In a word, yes.

But to understand further, we must understand how we produce and distribute power today. Part of the difficulty lies in the concepts we use to understand the electrical power system. A simple model might show just one power plant and one load, like a light bulb. Stepping up some, we might view a model showing several loads and distribution equipment, like transformers. These step up the generator voltage to the high-voltage transmission lines, and lower the voltage down to the 120V AC we generally use. The electric power is distributed to the commercial, industrial, and residential loads.

grid

Adding to the complexity, many generators contribute to generation, from many types of sources — gas, oil, nuclear, coal, wind, solar, and hydro, among others.

On the other side of things, demand varies daily, seasonally, monthly, and due to weather and other events. The demand curve has a daily peak in the middle of the day. The annual peak is in the summer due to air conditioning loads in most places. This is true in all US regions, even the Northeast. Here are some pictures that show how demand varies with season.

daily-demand-new-england-iso

This first one is for New England.

daily-demand-california-summer-caiso

Here is a summer day in California.

daily-demand-california-winter-caiso

And here is a California winter day demand.

Here is a video showing how demand varies over a year on the East Coast on NYISO:

To meet the peak annual demand, extra generators must be available. These are not used year round. In fact, the generators are used as a pool to meet demand all the time. On any given day, some units are idle and not needed, and some are out for repairs or refueling. Others are in reserve, waiting and expected to be used that day. Others are generating electricity to meet demand.

The generators and power lines are operated by utilities. The power system is run by independent system operators (ISOs), and operators in control centers dispatch the power on a daily basis, requesting power from the various generators. Some can vary their output quickly. Others can take days to be available. This requires planning. Because the power system is generally a monopoly, public utilities commissions (PUC) are tasked with regulating the utilities to provide the lowest cost power to users. The system is operated by utilities bidding to provide power from their generators on a day-ahead basis. So the operator sees which generators are available and looks at the predicted demand and plans and dispatches to meet the load at lowest cost accordingly.

A look at one system, CAISO, the California Independent System Operator, shows how this works. The graph below shows the difference between the predicted day-ahead demand and the actual demand.

CAISO+Supply+and+Demand+Graph+3

The bottom three curves show the predicted and actual demand. The upper yellow curve shows the reserves.

Lets look at how different sources provide energy to meet electricity demand. This is an approximate picture of how it was done in the past in California:

caloadshape-final

The amount of inflexible coal and nuclear is less than the lowest demand during the day so that it doesn’t have to change power output. Natural gas is used for most of the daily variation.

Now lets see what happens when we add renewables today:

California solar electricity curve

Here is the report, commissioned by then-Governor Arnold Schwarzenegger, that sources the graph (Fig. 15 page 35).

You can see that wind and solar displace the natural gas peak generation. And that, most of the year, reserves are available much greater than demand because they need to be there in case generation fails unexpectedly and because the power peak only happens during a short time of the year. The rest of the time, there is much excess generation available. That’s why up to 40% wind and solar can be integrated without much fuss. The grid is already set up to vary with load. Solar in particular works very well, because it happens when the daily demand is higher. And with so much flexible power available most of the year, wind and solar can be integrated even more often.

In California, we can see that large amounts of renewables are already integrated by exploiting the flexibility of natural gas. In fact, not just flexible generation, but flexible demand may be exploited. Right now, flexible demand is not used that much. One way to make demand flexible, is to set time of use (TOU) metering and pricing. This is not used yet in many areas of the US.

But how can we go higher than that? Well, we can look at the following video by Amory Lovins for guidance. As Lovins shows, even without storage, a high percentage of renewables can power the grid with a few techniques.

We don’t need storage breakthroughs, but they are welcome and can help.

For starters, all we need is flexible sources to meet the daily demand variation. Quite clearly, we could power the whole grid with natural gas or hydro. We wouldn’t do that everywhere for practical reasons, but it’s possible. In fact, some countries are predominantly powered by hydro or geothermal. They don’t require natural gas to meet demand. And some demand is met by importing excess power from other areas, as the next graph (shared above as well) from California shows.

California solar electricity curve

Next is one example scenario for meeting demand that was developed by NREL. It envisions having some excess renewables and curtailment to meet demand.

renewable energy curtailment

Keep in mind that every day in each location a different set of sources is used to meet the demand.

You can see that there is not one way, but many ways, and different ways on each day. In fact, since power is imported, it depends on how well operators are able to mix and predict power from many areas. This will be more important in the future as system operators in the Midwest dispatch wind and solar from the Midwest and Southwest to demand centers throughout the country, particularly the coasts. That’s why the electric energy regulation authority FERC has developed “Energy Imbalance Markets” as a solution. They provide quicker, more-up-to-date estimates of available power and demand so ISOs and utilities can better exchange power through the grid.

And meeting demand is done differently in each location, on each day, in different ways. Every town, in every location, on every day does not need to meet 80% renewables to get 80% renewables across the US as a yearly average.

The difference in regional resources is shown in this graph.

US renewable energy potential

And the mix by region is shown in this graph:

US renewable energy 80 percent

The same renewables are not used everywhere, but renewable resources are widespread. In fact, utilizing the wide distribution of renewables like wind acts to smooth and steady their output. Many seminal articles have been written by NREL’s Michael Milligan for example.

The combination of different sources acts to increase the ability to meet demand. The case for wind, solar, and other renewables being more than the sum of the parts is ably made by Ramez Naam here.

You can see how renewables work together to meet demand here:

Renewables-Symphony-Solar-Plus-Wind-Texas-ERCOT-region-800x603

And you can see how renewables output is smoothed by integration over wider areas here:

global irradiance

There are many other methods of integrating renewables besides storage.

But storage has become cheaper much faster than anticipated. The current wave of storage, Tesla’s Powerwall, can provide storage at prices low enough to displace gas peakers. And traditional sources like coal and combined cycle gas plants can also operate as flexible sources not just gas peakers. These flexible sources already exist, but operate using fuel today. Their use will be reduced, at first, making room for variable renewables. As time goes on, a mix of variable renewables like wind and solar, and flexible renewables like hydro, biomass, geothermal, and concentrating solar with thermal storage, as well as grid storage from flow batteries, lithium batteries, compressed air storage, and pumped hydro can make up the rest.

The combination of grid practices, transmission, flexible sources like CSP with thermal storage, geothermal, hydro, and biomass, plus 10% storage is all that is required to meet 80% renewables by 2050 economically by 2050.

The costs of storage and renewables are dropping quickly as volume increases:

Future-Solar-Cost-Projections-PPA-LCOE

Future-Wind-Price-Projections-Naam-14-Percent-Learning-Curve

But wait, there’s more. As wind capacity factor rises with increases in tower height and other changes, capacity factor increases. NREL recently found that wind power capacity factor improvements would have far-reaching implications allowing wind to displace more fossil fuels.

This will increase the amount of time wind is available, open more areas for wind farms, and bring some wind farms closer to load centers. Meanwhile, solar costs continue to plummet, soon to rival the already low costs of wind, and storage costs fall. As storage costs fall, utility storage enables more renewable integration, and more electric vehicles. The combination works as a positive feedback to increase the rapid deployment of all these technologies.


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About the Author

has studied wind, electric vehicles, and environmental issues. An electrical engineer familiar with power and electronics, he has participated in the Automotive X Prize contest. He is an avid writer, specializing in electric vehicles, batteries, and wind energy.



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