As the US electrical grid incorporates more renewable energy production, the ways in which technologies such as wind and solar photovoltaics (PV) differ from thermal generators, becomes more and more apparent.

 

The obvious, and often commented upon difference, is that wind and solar are intermittent electrical generators.  There is no power generated "when the wind does not blow, or the sun does not shine".  Battery technology is often trotted out as the solution to storing renewable energy when the sun is shining and the wind is blowing.

 

Unfortunately the battery solution is very costly in at least four ways:

1) there are the direct initial costs of the massive battery farms,

2) there are the annual maintenance costs of the massive battery farms,

3) there is a doubling of the initial costs of the wind turbines and solar panels, since the batteries must also be charged when the sun is shining and the wind is blowing, (and that power must come from the extra renewable sources that are not just feeding the currently required power into the grid), and

4) there are the annual maintenance costs of the 2-fold extra wind turbines and 2-fold extra solar panels needed to charge the batteries, for the periods when the intermittent generation is not available.

 

Many studies have examined the variability and uncertainty of wind and solar generators, and have described how generation and load can be balanced for a wide variety of annual renewal energy penetrations of the grid, at timescales from seconds to years.  Solving these issues is only a matter of how much the consumer is willing to pay for their electricity consumption, 2X, 3X, 5X, 10X.  In Germany it is currently 3X and headed higher, causing a "temporary return" to abundant coal fired power generation, when the Russians turned off the natural gas running the cleaner, base load power plants.

 

Ignoring all of that, there is a more basic issue with renewables as they are scaled up on the grid - inverters.  Inverters are the electrical devices necessary to convert the direct current (DC) power generated by the renewables, to the alternating current (AC) power fed into and distributed by the electrical grid.

 

An often overlooked characteristic of these DC resources is their asynchronicity.

Synchronous generators, whose frequency of alternating current (AC) injection is physically coupled to the rotation of the generating machine itself, have been used since the dawn of the electric era.  Coal, natural gas, nuclear and hydroelectric generators are all synchronous generators.  They produce their energy with the same frequency, or in lock step with the grid frequency.  This is why clocks, and computers, and motors work as accurately as they do.  They get a 60 hertz frequency signal from the grid.  If that 60.000 hertz signal deviates by 0.001 hertz, then lots of our devices no longer work properly.

 

Inverter based asynchronous generators (wind and solar) do not share the same physical coupling with the generated 60 hertz grid frequency.  They generate power in different amounts when gusts of wind or clouds dictate.  Hence DC inverters must be used to interface the asynchronous energy sources with the power system grid, as opposed to synchronous generators which match the grid frequency as they come online and go offline when demand changes.

 

These subtle differences greatly impact the operations of power systems developed around the characteristics of synchronous generators.  Batteries only add to the inverter issues, as the DC power stored in the battery must be converted to synchronous AC power before it can flow back into the grid.

 

Can an inverter be designed to deliver its power in synchronous with the grid?

Yes of course, that is what wind farms and solar farms do.

 

The DC energy from the wind turbines or the solar panels is collected and by using a single massive inverter, it is made synchronous with the grid, then it is fed into the grid.

 

But what happens when the single massive inverter fails or misbehaves due to operating outside of its design limits?

That is the essence of the grid stability problem...

 

The more renewables on the grid, the more inverters, the more inverter failures, the more unpredictable modes of inverter failure, and the more potential instability of the grid.

 

You can think of this as if we are designing and building an airplane, while it is in flight.

 

It took over 100 years to get equipment designed and installed to run on a synchronous grid.  There were numerous failures, but as with any new technology, customers were then tolerant of those failures, because the benefits outweighed the harms.

 

Now that we have a highly reliable and stable electrical grid, customers are no longer tolerant of any outages or harms, including electricity price increases due to technology changes.  The harms, however slight, have become highly intolerable...

 

The Texas grid failures in February 2022 may be a portend of things to come.

There were many small items that contributed to the failure.  No single item was that significant, but collectively they cascaded into an unpredictable and uncontrollable outage.

 

- unusually cold weather,

- unusually high demand for electricity,

- the natural gas pipelines and natural gas pumps and compressors had been converted to run on electricity,

    (with no electricity, pumps could not supply natural gas to natural gas powered generating plants),

- poor winterizing of various system components,

- deferred maintenance of some components,

- scheduled maintenance of some base units at the worst opportune time (a winter lull, vs. summer AC peak),

- the Texas grid was independent of, and not connected to, the other major US grids.

 

There have been numerous analyses of the Texas grid failure, and many academic papers published with suggestions of how to prevent a re-occurrence. 

 

But unfortunately the 800 lb gorilla in the room, that everyone is avoiding (hoping it will go away or be appeased), is the synchronous design of the current grid, and the asynchronous nature of renewables. 

The current bandaid solutions will only go so far.

And politicians are inclined to just kick the can down the road, until ...