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Full Disclosure – Solar

Ed Reid's picture
Vice President, Marketing (Retired) / Executive Director (Retired) / President (Retired) Columbia Gas Distribution Companies / American Gas Cooling Center / Fire to Ice, Inc.

Industry Participation: Natural Gas Industry Research, Development and Demonstration Initiative Chair, Cooling Committee (1996-1999)   American Gas Association Marketing Section...

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  • Jun 21, 2022

Solar is intermittent. Therefore, solar generation is intermittent. Solar is unreliably available during the day and reliably unavailable at night. Solar conditions vary geographically and seasonally. Therefore, solar generation potential varies geographically and seasonally. Solar generation has been implemented initially in the best locations for solar generation potential. However, expansion of solar generation would require installations in less than ideal locations.

US Energy Information Administration reports a capacity factor of 25% for solar generation. Virtually all solar generation in the US and globally is redundant capacity, in that it cannot replace dispatchable conventional generation in a reliable grid, though it can displace the output of that conventional generation when solar generation operates.

Solar arrays are typically proposed and reported based on the rating plate capacity of the solar generators. However, since the solar collector capacity factors are in the 25 % range, their annual potential output is typically a quarter of the annual potential output of a conventional generator of the same rating plate capacity.

Solar generation must be supplied with full capacity backup to replace the solar generator output when the sun is not shining. This backup capacity is currently provided by the conventional generation fleet. This is also true of most rooftop solar installations. Some rooftop solar installations include storage plus excess solar collector capacity to permit them to operate independent of the utility grid.

Solar generation capacity cannot be permitted to increase beyond the capacity of the conventional generation fleet if the grid is to remain reliable. This situation could result from an increase in solar generation capacity or from a decrease in conventional generation capacity, or both.

The full cost of solar generation includes the capital, operating and maintenance costs of the solar array plus the capital and operating costs of the conventional backup generation required when the solar generation is unavailable. The solar industry typically ignores the real cost of conventional backup, so that it can claim that solar generation is cheaper than conventional generation.

As conventional generation is retired, either because of age and condition or because of environmental regulation or Executive Order, its capacity must be replaced by storage capable of storing the rating plate output of the conventional generators for the maximum period of time solar generation might be unavailable, plus additional solar generating capacity sufficient to recharge the storage in the shortest period of time between solar interruptions. In this case, the full cost of solar generation includes the capital, operating and maintenance costs of the solar array plus the capital, operating and maintenance costs of the storage and the extra solar generation capacity required to recharge storage.

Clearly, solar generation is not cheaper than conventional generation when its full costs are considered. Solar generation increases total generation investment in the short term, since it is redundant capacity. It also increases grid investment in the longer term, since it requires both storage capacity to backup the solar generation and additional solar generation capacity to recharge storage. The undepreciated investment in conventional generation retired by environmental regulation of Executive Order also remains as a grid investment, further increasing the cost of the solar generation plus storage which replaced it.

The expected service life of solar PV collectors is 20-25 years. This compares with the 40-year depreciation period for utility generation assets. This shorter service life expectancy increases the cost of ownership of the solar array.

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Matt Chester's picture
Matt Chester on Jun 21, 2022

Solar arrays are typically proposed and reported based on the rating plate capacity of the solar generators. However, since the solar collector capacity factors are in the 25 % range, their annual potential output is typically a quarter of the annual potential output of a conventional generator of the same rating plate capacity

I don't think this is unknown or overlooked in the grid interconnection process, right? Certainly the entities building these out recognize they are achieving revenue that reflects that 25%, not some higher number that isn't how solar works. 

Ed Reid's picture
Ed Reid on Jun 21, 2022

The grid interconnection must be capable of handling the maximum output of the solar array, though that does not occur continuously.  The gird operators certainly know this.

The entities building out solar arrays generally choose to ignore the need to store energy when available to be able to provide it when it is not being generated. The same is true of wind generators.

Including storage and the capacity required to recharge storage when used significantly increases the cost of renewable electricity per kWh, so that it is no longer cheaper than conventionally generated electricity.

Peter Farley's picture
Peter Farley on Jun 21, 2022

The grid connection is rarely sized for the full output of the solar panels. It is quite common for a 100MW connection to have 110-130MW of solar panels and sometimes up to 150 MW. This means that the system can still deliver 100MW on a hazy or very hot day and for an extra hour or two on a normal day or when there is a minor equipment fault. It is no different to designing a boiler for the poorest quality coal expected from local mines.
Wind farms are slightly different but again it is not uncommon for the whole wind farm to be rated slightly below the sum of the capacities.

Ed Reid's picture
Ed Reid on Jun 22, 2022

If the grid connection is undersized relative to the maximum solar field output, is it simply overloaded during periods when the field is producing at rated capacity?

Peter Farley's picture
Peter Farley on Jun 23, 2022

Not normally, that is the beauty of inverters, you can regulate Voltage and frequency independently in less than one cycle. If, however, there was excess demand on the grid, and not much load on the feeder most transformers can run at about 20% overcapacity for an hour or more. In a mixed technology grid that will allow hydro or gas to spool up safely and the solar farm can return to rated output   

Michael Keller's picture
Michael Keller on Jun 28, 2022

Fossil and nuclear plants run to support the grid’s demands  and can do that day-in and day-out. The machines are designed to be reliable.

Solar and wind energy are inherently not reliable. The grid must support solar energy’s demands. The full ramifications of this peculiar defect are not fully known. At some point the grid will become unstable, with frequency and voltage protective relaying tripping all or portions of the grid.

Should note that solar and wind energy must be in phase with the grid in order to properly synchronize (otherwise relays trip). When a grid is composed of massive installations of unreliable renewable energy, the reference for the grid’s frequency and voltage can becomes murky. That is a source of instability, particularly when major upsets in the grid occur, e.g. storms, fires.

Grids composed of numbers of rotating generators collectively create the grid’s reference for voltage and frequency. The inertia of the rotating generators stabilizes the grid’s reference characteristics - the machines tend to respond relatively slowly to upset conditions. 

The inverter controls of renewable energy constantly and rapidly match the grid’s frequency and voltage on every cycle of the alternating current. That can be a curse when the reference point of the grid is constantly bouncing about. The entire system can quickly become unstable and collapse.

This is a simple explanation of a complex subject. However, does highlight one part of the fundamental problems that emerge as more and more renewable energy fills the grid.

Xisto Vieira Filho's picture
Xisto Vieira Filho on Jun 29, 2022

Michael, these are excellent comments. In the great majority of material published about renewables, a lot of people simply ignore the basic concepts of power systems. And stability then, is ignored in a lot of publicatios. In fact, up to now, there is not anything being analysed that can substitute the atributtes os thermal and hydro conventional machines.

There are people so frenetic against thermal generators that they try to substitute them for syntethic inertia ( I still prefer real inertia) plus syncronous condensers, sometimes adding a lot of transmission lines , in a definite terrible economic solution.

In other words, sustainability and environmental issues are very important, but security and resilience continues to be of utmost importance.

Peter Farley's picture
Peter Farley on Jul 1, 2022

Syncons were invented to stabilise thermal grids as well as statcoms etc. There are plenty of ways of having better frequency, power factor, reactive power, and voltage control without rotating generators.

  • Eire Grid showed that a 10MW battery can provide as much fast frequency response as a 100 MW gas turbine.
  • A wind turbine fitted with synthetic inertia software can give up 25-30% of its mechanical inertia by slowing down, because it is asynchronous, a synchronous turbine can only give up 2-4% before the grid crashes.
  •  In the event of loss of load a wind or solar farm can ramp down or even disconnect in seconds or less and then when transmission is restored reconnect almost as fast, far faster than thermal plants can
Xisto Vieira Filho's picture
Xisto Vieira Filho on Jul 1, 2022

Interesting comments, Peter, but I would still like to emphasize two points

1) in a huge power system, we should talk about GW batteries .Those will still take time to be in service. A 100 MW gas turbine is not a good parameter for a big oscillation problem, caused by a major set of contingencies.

2) as far as the eventual potentialities of synth inertia, or during loss of loads, I will not even   discuss these issues , mainly  the one about the inertia. The first issue that I would raise before any technical discussion is what if there is no wind at the time of the contincy ? 

But anyway, these are really very good discussions. Thanks.

Peter Farley's picture
Peter Farley on Jul 7, 2022

I agree that we need more inertia/storage. If you extrapolate the Eire Grid analysis to the Texas big Freeze about 3-5 GW of batteries/syncons/flywheels/dry running hydro generators/VPPs would have provided the equivalent of the available inertia at the time. That is easily achievable in Texas, in fact I predict Texas will be there within 6-8 years at the rate they are building batteries.

Just as gas or steam turbines that are offline can provide no inertia, if there is zero wind, wind won't provide inertia, however it is economically feasible to motor wind turbines if there was excess solar/hydro/biomass, far cheaper than running a gas turbine at low load "just in case". Again, because a wind turbine can slow down, a 300 MW wind farm provides about eight times as much usable inertia as a 300 MW gas turbine.

Inertia is just a two way store of energy. If you suddenly lose 200 MW, you need to inject about 1-5,000 MWs before either load is reduced or other generation is ramped up. Apart from batteries and inertia in wind and hydro turbines, solar systems can also provide the same capability if they are deliberately run below their instantaneous capacity. So, it is a hazy day and the 200MW solar farm is producing 120 MW but only 100 MW is exported to the grid. That means that if another farm drops off, in a matter of milliseconds, the solar farm can inject 20 MW for maybe 600 seconds or 12,000 MWs 

Peter Farley's picture
Peter Farley on Jul 1, 2022

There are a number of points here where you misunderstand the current situation,


The whole concept of reliability is a mix of various characteristics, the sun and wind are reliable over a wide area and a prolonged period but vary widely in short time frames and small areas. With thermal power it is the opposite. For example, 2020 was the most productive year for renewables in Germany in the first half due to strong winds in winter they produced 132 TWh. The next year it dropped to 118 TWh (-10%) and this year it will just make 130 TWh.

On the other hand, France's nuclear power has dropped from a peak of 225 TWh in H1 2006 to 181 TWh in H1 2021 to 153 TWh 32% from peak and 15% in one year. One would expect with very high gas and power prices, nuclear plants would be going flat out and yet for the whole of May they averaged 48% capacity and fell as low as 35% for some hours.

Texas lost 40 GW of gas generation during its winter freeze so all forms of generation can be unreliable

Similarly, Australia hasn't closed a coal plant since 2017 and yet even though most of them have captive coal mines and can produce their coal needs for about A$12-20/MWh and power prices jumped from A$65 in 2020 to $350 in the latest quarter but coal output has dropped from 35 TWh in Q2 2019 to 25 TWh in Q2 2022.


A nuclear coal or gas plant can go offline in seconds taking many hundreds if not thousands of MW offline with it. For example, last year a 400 MW coal generator in Queensland Australia blew up. The resulting grid instability saw 2,700 MW of generation and 400,000 customers offline. That is why the concept of spinning reserve was developed, so you need spare capacity running across the system of two to three times the output of largest generator. So, if the largest generator is producing 750 MW, then if there are five other 500MW generators running on the grid, they must be producing no more than 1,000 MW between them. Even then as we saw in Queensland it doesn't always work although at the time only 3,500 of their 8,000 MW of dispatchable capacity was running

Rotating generators have inertia expressed in MWs, a 750 MW steam generator has about 6,300 MWs, a 150 MW aeroderivative gas turbine about 600. However, most of that inertia is unusable, if the frequency drops to 59.3 Hz only (1- (59.3/60)2) = 2.3% of the inertia is available so if you have 60 GW running with 250,000 MWs of inertia you can only use about 5,500 MWs, For the frequency on a 60GW system to drop to 59.3 Hz in less than 2 seconds would be a major catastrophe, 2-3 times as big as the Texas big freeze. So, the peak contribution rate of from inertia would be 5,500/2 MW = 3,800 MW. Batteries can contribute 100% power in 140 ms.

Thus, if a 70 GW renewable system with no inertia had 4,000 MW of batteries it would be just as stable as a 60 GW coal/gas system relying on inertia. However, wind turbines do have inertia and the inverters on solar farms can be programmed to emulate inertia.


Renewable systems clearly need more power over short periods 30 minutes to 4-5 hours where the combined output of wind and solar can be very low so a renewable system with a 80 GW peak demand may need 75 GW, whereas a thermal system such as France, Texas may only need 50 GW. However, the duration of the thermal shortfall can be much longer. So in terms of Energy required both France and Germany have shown that a balanced renewable system probably needs about half the total backup energy that a thermal system needs in extreme circumstances.


Michael Keller's picture
Michael Keller on Jul 1, 2022

Wind and solar are not reliable. Period. The output constantly fluctuates, including when in time how much energy will be available. That means the system’s inertia is also not constant.

Electrical inertia is the property of the system that opposes change. System inertia is expressed in gigavolt-amps. A single large generator has vastly more inertia (due to rotation of generator) than a solar panel or wind turbine. Attempting to match such inertia with unreliable green energy requires lots of green energy machines that are inherently unreliable. That creates instability.


Xisto Vieira Filho's picture
Xisto Vieira Filho on Jul 4, 2022

Sorry, but I suggest that you run some stability cases considering some of the concepts you wrote about dynamic stability, just to be sure . But that is really a very interesting discussion.

But I diagree of the first point. You said a big conventional plant could not be reliable , eventually, because we can have a contingency and loose the machine. Yes you can have. But with a renewable it is not eventually:  it is definetely unreliable. But again, I would like to complement by saying that this is NOT a competion between sources. It is a complementation , to ensure an optimal electric generation matrix

Peter Farley's picture
Peter Farley on Jul 7, 2022

I didn't say it could not be reliable but when it goes offline it can be offline for much longer. For example, one of the most modern coal plants in Australia has been offline for 100 days in the last 400 and even when online it has averaged 33% of its capacity. A few years ago, France's nuclear plants produced 220 TWh in the first half, in the first half of this year they produced 153 TWh. That is a far bigger fall than a good to bad renewable year 

Peter Farley's picture
Peter Farley on Jun 21, 2022

I am not sure what your point is, Germany has a far more stable grid than any US Region and it expects to have 200 GW of solar on a grid with about 90 GW of peak demand by 2030. South Australia already exceeds 100% solar at times and is building more, it is the most reliable sector of the Australian grid.

1. Inverters are orders of magnitude more controllable than turbo generators so if voltages are too high the output is regulated down in tenths of a second, An inverter dominated grid has far better frequency and voltage control than a conventional grid.

2. Most solar installations have more panels than they need so they can maintain output for longer in suboptimal conditions. In Australia the most common rooftop system is 6.2-6.6 kW of panels matched to a 5 kW inverter

3. Most loads, surprisingly enough are during daylight hours and many other loads such as water heating, municipal water transfer, ice making for cool stores, charging EVs at work or the train station, pool pumps etc can be moved to daylight where distributed solar will provide cheaper energy than overnight coal. In consequence solar can provide about 45-55% of annual energy needs with little or no additional storage.

4. You are just making things up. Coal plants in the US cost about four times as much to build per MW as solar and twice as much to maintain. At best they average 65% CF over their life so about 2.5 times the output of the solar system, So even if their design life is longer, the finance operation and depreciation costs per MWh are 30-50% higher than a solar plant - before you pay for fuel. 

Ed Reid's picture
Ed Reid on Jun 22, 2022

The US is not building coal power plants. NGCC power plants are ~30% the cost of coal plants. They are capable of 90+% CF operation and are dispatchable.

Solar and wind require overbuilding or storage, neither of which is free.

Peter Farley's picture
Peter Farley on Jun 23, 2022

They are capable of 90% operation, but they never do, mainly because demand fluctuates and fuel costs are higher than hydro, coal or nuclear even before wind and solar while transmission constraints are too high to transmit power from every generator to every load. If demand drops below about 50% of capacity, CC plants revert to open cycle operation and fuel costs per MWh jump about 30%.

It is clearly true that wind and solar need storage to cope with both demand and supply fluctuations, but it would be almost impossible to build a system based only on CC gas without a lot of storage. To cope with five-year peak demand corresponding with the worst recorded outage levels, and transmission constraints, you actually need capacity about 120-140% above annual peak demand or 140-180% of annual demand. You can see this in every grid in the world, where before renewables were available, peak demand was say 600 GW and registered capacity was 900-1,000 GW and average load, 460 GW. If you have 950 GW capacity and 460 GW average load, at least 60% of the time the CC plants would be operating in OC mode or turned off and average less than 50% CF.

Thus, if you had 600GW of CC capacity and 300GW of storage and 95% of plants available at any time then the operating plants would have an average capacity factor of a very satisfactory 80% and all plants 76%, but you could still meet peak demand with 75% of generator and storage capacity. I have not delved into how much duration storage would be needed, but you would probably need something like 20% of supply for the lowest two weeks demand or 20% of demand for the highest two weeks

Now I know the figures are out there for the US, but I can't find them. However, on the East Coast Australian grid renewables supplied 33% of demand over the last year and on the worst day about 22%. So what that tells you is that if you had enough renewables to supply 150% of annual demand, on the worst day you would get 0.22/0.33 x 150% = 100%. That does not mean you don't need storage, because there will be local shortages when some regions have more excess than they can export, and quite large hourly shortages and particularly in the Northern Hemisphere with Dunkelflaute, which is  weaker in Australia, you may need up to 10-15% of demand for two weeks. While the peak power of the storage in a renewable system is almost certainly higher than in a conventional grid, it is not clear the duration is any more than required for an optimised FF system

The question is, what is the trade-off between redundant generation and storage? and capital costs vs operating cost.? In the past the emphasis has clearly been on excess generation, but now that storage is much cheaper and operating costs for FF plants are higher the balance will change. The question is not quite decided but from the analysis I have done at the current state of the art, a 95% renewable grid with storage/demand response of about 60% of peak demand for 2-4 hours with gas backup running a few hundred hours per year, just like most peakers do now, would be the lowest cost system


Michael Keller's picture
Michael Keller on Jun 28, 2022

Your statement that a grid composed of CC plants is nearly impossible without storage is absolutely untrue and not supported by reality.

The CC plants are exceptionally operationally flexible and any grid demand can always be met by combinations of CC plants operating at various loads. Virtually zero storage is needed.

Peter Farley's picture
Peter Farley on Jul 1, 2022

Only if a large fraction of the CC plants run in OC mode at times of low demand.

Michael Keller's picture
Michael Keller on Jun 28, 2022

So how does solar energy work at night in Germany? Winter? Not at all and poorly.

Fact of the matter is renewable energy must be backed up and that costs money. End up with a lot more investment in all types of generation than otherwise needed. The cost of electricity skyrockets, as seen in Germany (and California).

Further, if your back-up relies on energy from say a hostile foreign country, then you have one hell of a problem. Once again, the pin-heads in Germany can take a bow.



Peter Farley's picture
Peter Farley on Jul 1, 2022

Hydro, wind, waste to energy, and yes storage.

Have you had a look at the cost of power in Northern Italy, Spain, the UK, all higher than Germany and all dependent on gas. In fact the day ahead price in France has been 20% higher than Germany this year. 

Michael Keller's picture
Michael Keller on Jul 1, 2022

The Europeans placed their energy security with Russia's natural gas. Really dumb idea. Trotting out unreliable green energy as their savior is not technically or economically rational. Europe's problems are deep seated, self-inflicted, and cannot be solved with a short-range mentality. Nor can they be solved with a mindless zero-emissions mindset.

You really need to stop throwing out little snippets of information to justify your positions. The issues are broad and complex in nature and require more than a sound-bite approach. 

Xisto Vieira Filho's picture
Xisto Vieira Filho on Jul 2, 2022

Michael, again I fully agree with you. People have to understand that those green plants have their importance , due to environmemental issues, but they definetely are unreliable. It would be very interesting to see these guys that fight against reality to sit in a huge power system control room and see how they would deal without controllable reserves, how they would prepare the system to withstand contingencies and so forth. Or even sitting in a chair to really perform dynamic stability cases, studies,  and see actually how the system perforns. And again , congratulations for your comments.

Peter Farley's picture
Peter Farley on Jul 7, 2022

Not entirely correct. Spain, the Netherlands, France and Britain get almost zero gas from Russia and yet their power prices have also spiked.

Pro nuclear France and Britain's reduction in nuclear output over the past few years, more than 100 TWh/y is as much as the entire gas-powered electricity from France Germany and Belgium. Just imagine how much more secure Europe's energy system would be if nuclear had lived up to the hype.

At today's prices and even with the low productivity of wind and solar in Europe they can make energy from renewables for about US$60/MWh including interest, depreciation and operation. The fuel alone for a high efficiency coal plant is US$140/MWh and a CC gas plant using US LNG $260, so who in their right mind would not build as much wind and solar as they can to minimise the use of expensive fuels.

You really need to provide some actual information rather than specious throw away lines and deprecations. I would be happy for you to dispute any facts I have thrown out, but you won't. You sit in your own little cave of misinformation and misunderstanding, and you can't be bothered getting off your seat of biases and misconceptions to provide any facts at all.

Anyway, I am grateful: every time you make a ridiculous claim, I check the facts and I learn something new; you should try it.  


Joe Deely's picture
Joe Deely on Jul 3, 2022

These arguments remind me of some I hear concerning the Golden State Warriors basketball team. Commentators say - "Warriors are too small, no good big man", or "They are just a 3 point shooting team - not real basketball".  Yet the Warriors keep winning championships - 4 in the last 8 years - more to come. Most other teams are now trying to emulate the Warriors.

Commentators on here keep  disparaging wind and solar - yet they keep winning. In fact, their growth is accelerating. Here are the latest WW numbers from Annual BP Statistical Review - 2022.

Renewables WW have increased by an average of 337 TWh over the last 6 years. Even better, the average generation growth has been 428 TWh/yr over the last 2 years. 2021 was a massive year for installation of wind and solar so 2022 will again show large growth in generation.

It's obvious that renewables are winning and more folks are jumping onboard. Renewables will be No 1 source of WW generation by 2030.

Facts. Stubborn, are they not?  

Xisto Vieira Filho's picture
Xisto Vieira Filho on Jul 4, 2022

Joe, your Table is interesting. We can see that in 2021,we had 87% of " conventional " generation and 13% of REN's, that is a Penetration Level of only 13%. The big issue is to determine , for each different system, what is the maximum Penetration Level, to ensure a reliable and resilient operation. In other words, with enough conditions of maintaining stability and controllable reserverves even for unexpected contingencies. So, it is not a matter of competition , but of complementation. In fact, REN's can only expand if there is an adequate expansion os conventional plants. 

Your example of the warriors is also great. Imagine if all the remaining teams give up because the Warriors have been winning. The championship would be unstable , because the Warriors alone cannot carry out a championship.Not even with one or two more teams. There is number of teams besides the Warriors that woul allow a competitive and good championship. 

Joe Deely's picture
Joe Deely on Jul 4, 2022

We can see that in 2021,we had 87% of " conventional " generation and 13% of REN's, that is a Penetration Level of only 13%

Not how I see it - in fact I added a line break to split fossil vs non-fossil. I see 38% of ZC resources vs 33% in 2021 for ZC back in 2015. Share growth will continue.

In fact, REN's can only expand if there is an adequate expansion os conventional plants. 

Wrong.  In many places we have more than enough existing capacity to complement renewables. One small example, Sweden will be adding 50TWh of wind generation over the coming decade without any of your "conventional" capacity. We do however have to be careful about closing existing capacity without have replacements.

Imagine if all the remaining teams give up because the Warriors have been winning. The championship would be unstable

Agree, we need more than just renewables. Improved storage, more nuclear and even a little more hydro will also help.

Xisto Vieira Filho's picture
Xisto Vieira Filho on Jul 6, 2022

1) it is a matter of definition. For me " conventional generation " is a plant that has rotor with a considerable inertia, frequency and voltage adequate controllers , and , in the majority of times that can provide controllable reserves to the system. We can include as a conventional: hydros , nukes, thermal generators. And the renewables when they started to appear were called alternative generation. But anyhow, we can also call, for the porpose of this discussion, firm plants, which are those whose fuel can be controlled and non firm for thosr whose fuel depends on external factors, like wind and solar ( hydros only those that do not have reservoirs).. Then what i wrote is correct. In other words, we have 87% of reliable generation ( as far as fuel supply and enhancement for the system are concerned).

2) Sorry, but it is right again The sweden system has something like 38% of hydros and about 37,7 % of nukes, and also interconnections with other system. That is to say 78% of reliable plants, and a penetration level of 22%. This penetration level is ok, as far as system dynamic performance is concerned.

3) excellent, in this item at least we both agree.Hydros and nukes will definetely enhance system performance. In the long time picture, you can also include other types of thermal generation using CCUS, CCS, or even green H2

Joe Deely's picture
Joe Deely on Jul 6, 2022

2) Sorry, but it is right again The sweden system has something like 38% of hydros and about 37,7 % of nukes, and also interconnections with other system. That is to say 78% of reliable plants, and a penetration level of 22%. This penetration level is ok, as far as system dynamic performance is concerned.

Yeah but you said: "REN's can only expand if there is an adequate expansion os conventional plants. "Sounds like now you are agreeing with me that if there is already adequate capacity then "conventional" plant capacity  doesn't have to expand.  

Even though Sweden is rapidly increasing its wind share there is no further expansion of hydro or nuclear. In fact, they have recently closed two nuclear plants. By the end of this decade, wind penetration will pass nuclear and match hydro and their will be no further expansion of "conventional" plants.

Plus of course, there are many, many more examples of this around the world. The SPP grid in Central US is an another good example . The below is from their annual report. They added 8GW of new wind in the last couple of years with no expansion of their "conventional" plants. They have plans to add 16GW more of wind with no plans to add NG or coal. In fact some coal , will be retiring. Currently wind is tied with coal for top spot in generation - at about 35% share. See below.

Xisto Vieira Filho's picture
Xisto Vieira Filho on Jul 6, 2022

This is right, and it seems that we are both almost in the same page. The only thing that I would emphasize is that the load grows, an then the " adequate capacity " and the maximum penetration level of REN's can also change. Many other effects can change this type of adequacy (e.g.: what is the reliability level that each system accepts: what is the maximum recovery time accetable , after a rhuge disturbance, which type of load shedding, or even load managemet the system is prepared for, etc) The actual problem is that you still cannot replace conventional plants with REN's due to security, reliability and resilience factors.

It might be a game, what will come first for reasonable costs: REN's ( and accessories ) to ensure security , or NG thermal plants to eliminate CO2 , with CCS or H2 ?  I would be extremely happy if both things happen.

I am going to attend the CIGRÉ meeting next August , when we will discuss the state of the art of those issues concerning the energy transition all over the world, and the big disturbances that have been occuring in several countries. 

I will be glad to divide some of the conclusions with you.

Peter Farley's picture
Peter Farley on Jul 7, 2022


You need to understand that a renewable grid is different to a conventional grid, so saying we need X amount of spinning reserves or Y of inertia is meaningless or in some cases just completely backwards.

For example, inertia. If you look at an early steam or diesel engine, they had huge flywheels to stabilise the output. Even a vehicle engine from 30 years ago had a substantial flywheel. Now engines have much smaller flywheels because the speed sensors detect speed variations well within one revolution and can control the rate of fuel injection from 0-100% within milliseconds. Because the control system is orders of magnitude faster, combustion smoother etc. a modern diesel engine can be both more stable at load and respond 5-10 times as fast to significant load changes than older generation machines. The same is true of a renewable/storage grid, the voltage/frequency variations in the case of a sudden load change will be smaller and of shorter duration therefore, it too needs far less inertia, probably about 1/10th of what was acceptable in a coal/nuclear grid.

Another advantage of renewables is the many small units more evenly distributed throughout the grid. This makes it less vulnerable to storms and surprisingly reduces the average transmission distance. It also tends to limit the effect of an outage to smaller areas and reduces the chance of cascading failure. So ERCOT has discovered that with increasing wind penetration they have reduced their spinning reserves, Germany has reduced its thermal capacity from about 102 GW to 80GW while at the same time reducing lost time per customer from around 20 minutes to 12 minutes.

 You don't need rotating inertia if your energy sources can go from 0-100% of power and back again in tenths of a second. You don't need spinning reserves if a battery can go from charging at 100% to discharging at 100% in 140ms. You don't need so called Base Load power plants if the base load is near zero which it is in some grids with high rooftop solar penetration    

Xisto Vieira Filho's picture
Xisto Vieira Filho on Jul 8, 2022

I suppose that Peter`s comments do not take into account actual simulations of power system stability, transient and dynamic behaviours included , nor even actual system operation with a huge amount of all kinds of generation.

Again , it would be of paramount importance if some more people could attend the meeting I refered above , and see large and regular big disturbances that have been occuring in several systems, where the causes are analysed . Again, in the future we can introduce large amounts of bateries, together wich several specific synhrous condensers, and a very large number of transmission lines and transformers to assure opertaion and stability. By the way one is going to introce also a large amount of reactors to absorb reactive power , because in light load we woul have to open several lines, but even though with reactive power being injected into the grid. And how about thesse costs ? are they adequate or can we implement new techonologies fot NG and nuclear plants ? In EU now they are implementing a new regulation to consider NG plants and nukes as " transition green plants " This has been approved there and why ? because nukes are green ,even considering the issue of atomic trash. And NG will be soon green throuh the use of H2 or CCS.

Then , finaly, let us not be radical in such an important matter. And let us face the truth:Like Michael said, renewables are nor reliable sources, because they are intermittent, their intermittence cannot be predictable, they do not have inertia, they do not contribute to controllaballe reserves ( neither in Mars a sysyem can operate without controllable reserves). But REN"s are definetely important, so we have to expand them carefully, always with thr help of natrural relible plants ( for the system).

PS: this is an answer to the interesting points raised by Peter, but there was not space to reply  his comments. 

And this is a real great technical discussin . We should have a specific place to go on discussing these issues.These conversations with Michael, Joe and Peter has been a pleasure for me. 

And also, congratulations to Eid for raising these points. focusing in solar plants.




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