cloudy skies
cloudy skies

What is the variability problem of wind and solar energy, how serious of a problem is this and what can be done to fix it? This article examines this issue of variability, describing and defining it and listing some of the ways in which the unique challenges of wind and solar energy is being or can be addressed.

by Chris de Morsella, Green Economy Post. follow Chris on Twitter @greeneconpost

Renewable energy has a variability problem and pretending that it does not – at the very least on a public perception level – won’t make this issue go away. The wind does not always blow, nor does it always blow steadily and sunlight can be obstructed by clouds. Wind and solar are variable sources of energy and this poses certain unique challenges that need to be addressed. At times an entire region can experience periods of a day or sometimes even a week where winds or solar energy that is usually on average available is largely absent. Wind energy in particular is also variable over short time scales with gusting wind producing peaks and troughs in power output that can cause voltage problems, because of the unevenness of the power being put onto the grid.

Increasingly this variability problem is being seized upon by the opponents of wind and solar power as a cardinal argument against the very idea of renewable energy. By not addressing this issue, by not quantifying and, where needed, costing for it, proponents of renewable energy are allowing the issue of variability to be employed as an effective argument by those who are ideologically opposed to the very idea of renewable energy. The variability of wind and solar is an effective talking point because it does in fact represent a real problem with wind and solar power and this issue is increasingly being employed by opponents of renewable energy no matter what the underlying reason may be that is really motivating their opposition.

Keeping it in Context

There is an ongoing discussion as to just how much of a problem this intermittence represents to the industrial world’s big regionally connected grids. For example, according to a recent study issued by the New York Independent System Operator (NYISO), wind generation could be increased by more than five times the amount currently operating in New York. The study found that any operational requirements associated with integrating sufficient wind generation could be addressed, paving the way for the fulfillment of New York State’s goal of having 30 percent of the state’s electricity supplied by renewable resources by 2015. The full report is available on the NYISO website. This is good news for fans of renewable energy; it provides an authoritative counterweight that balances what are justifiable concerns about the intermittent nature of wind and solar and it indicates that the problems associated with variability are in fact manageable up to quite high degrees of penetration by renewable into the larger aggregate electric power market – much larger penetration than renewable energy has so far achieved.

Renewable Energy Is Variable Just Like the Weather

Wind, solar and wave energy all depend in different ways directly on the prevailing weather and weather is a chaotic and difficult to predict system with a lot of turbulent variability both over short durations and over longer durations as well. Although it is possible to get average or mean values for potential wind or solar energy for some geographic location and to predict on average what the potential energy yields will be for a given site it is pretty much impossible to predict or guarantee what the energy profile for a site (or region) will be in any given future instant. The accuracy of weather predictions decreases rapidly the further out in time that they are projected; so it is difficult to predict what a sites energy production will be in say… a week.

Wind, solar and wave energy vary over short duration time scales in addition to the longer more predictable seasonal and diurnal fluctuations. Wind gusts and swirls; wind is a chaotic laminar flow on the boundary of the earth/water to atmosphere interface. While preferred wind sites are selected in part because the wind blows steadily and does not gust and shift directions, all sites experience short duration peaks and troughs of power output in addition to longer duration swings between periods of stable weather patterns.

The solar energy output of a site will drop quite suddenly (especially for photovoltaic arrays) when a cloud wanders between the collectors and the sun and then surge back on when the cloud floats away and the sunshine returns to irradiate the mirrors or PV modules of the site. Solar energy also cycles with seasonal variations in the incident solar irradiance and of course the sun does not shine at night.

By far most waves are wind generated though not necessarily primarily by wind that is blowing where the waves are being harvested to produce electricity. Wave energy can also propagate for some time after wind has calmed. In fact some offshore deep water floating platform wind turbine designs integrate wave energy collection into the floating deep water energy harvesting platform


Something is variable when it is able or likely to vary or is subject to external variation or change. Variability or variance is also more formally defined in probability theory and statistics. Variance describes how far a value or data point lies from the mean distribution; it is a measure of the amount of variation within the set of values of the statistical distribution that is being measured.

For renewable energy variability measures the degree of variance of the energy potential at any given moment from the expected or predicted mean value.

A predictable variance is less disruptive than chaotic variability; for example hydro power is actually quite variable – more water flows down watersheds in some periods of the year than in others and often this variance is quite pronounced between wet and dry seasons. However this variance is predictable and reservoirs act like gigantic batteries that act to smooth out the surpluses and deficits of flow into the reservoir system.

Chaotic variance, such as gusting wind or periods of little wind due to say a high pressure is more challenging, because it is harder to plan for and adjust other supplies to keep a satisfactory balance between supply and demand. This is an important consideration. Predictable variations in supply are easier to integrate because they can be planned for.

The Electric Grid Is Evolving to Better Match Renewable Energy

There was a time – and not so long ago too — when the railroad was king. There was no extended road or air transport system, no national highway network… no airports. Travel by road was slow and largely confined to regional/local road systems… and small biplanes landed on grassy fields. If one looks at the transportation infrastructure of one hundred years ago and compares it with the current transportation infrastructure one will see that entire new infrastructures have evolved over the years that simply did not exist before. Of course rail is still with us today and plays a vital role in our overall transportation system – and we here at the Green Economy Post certainly support the energy efficient rail transportation system we do have. The point being made is that the overall transportation infrastructure is not some static unchangeable thing, but rather evolves to fit with a changing reality.

Sometimes I sense that those who argue that the variable nature of many renewable energy systems is an insurmountable obstacle to their adoption are choosing to ignore the evolutionary nature of all systems including the electric power distribution networks that we simply call the grid. They adopt the unreasonable position that a snap shot of the electric grid as it exists today is how the electric grid must immutably remain and they then go on to highlight whenever there is a poor fit of renewable energy with this current grid and use this as an argument to bludgeon the very idea of renewable energy. To me that would be akin to someone arguing against the introduction of cars making the case that these because these vehicles could not be put onto the existing rail system of that time that there was no future for them.

Things evolve… and that is precisely what is now occurring with the electric grid. The insistence, by some, that wind and solar energy be held to some static snapshot of the grid as it exists today is unreasonable. In fact, as wind and solar energy become more important parts of our overall energy profile the grid will – and IS – adapting. As the grid evolves a better fit with these variable energy sources – and I want to get into how it seems to be evolving, the problems of variability will become diminished and become managed.

The Grid Is Evolving and Adapting to Renewable Energy

The grid is a vast and complex interconnected machine, a modern marvel really, however in many ways it has fallen behind the times. For the most part it is a simple unidirectional system that flows energy from a relatively few large power plants to a multitude of essentially dumb end points, but this is rapidly changing. This current and on-going evolution of our electric grids is widely known as the Smart Grid; so called because it is adding a networked parallel series of sensors and controllers throughout the grid nodes so that a near real time awareness of actual conditions is made available to the command and control grid management software platforms that keep our electric grid running day and night almost without interruption. In addition to these sensor networks the edge nodes of the grid – the meters that connect consumers to the grid are being upgraded with software and networking capabilities so that the grid can communicate real time conditions to these Smart Meters.

For a more in depth look at the Smart Grid and what is meant by this see our four part series on the Smart Grid, starting with: The Smart Grid Report: Part I – Overview

This added intelligence and almost real time situational awareness helps the grid adapt to the variable nature of renewable energy in several ways, which I will briefly enumerate. Each of these bullet points is an entire body of knowledge and discussion all by itself and there is not enough space in this article to get into the details.

• Real-time awareness enables the grid to respond and adapt in real time to changes brought about by variable supply. For example if voltage levels begin to spike, say because the wind is gusting other responsive power supplies can be throttled back in order to keep supply and demand in balance and ensure that high quality electric power is delivered.

• Grid connected electric energy storage nodes that can soak up surplus power and deliver energy onto the grid when there is a need – during peak demand periods – are also being added onto the existing grid and will help to smooth out the variable supplies from renewable sources. Rapid response electric energy storage systems range from existing hydro systems (or pumped storage), to various other energy storage systems such as molten salt thermal storage for solar thermal systems, to super capacitators (useful for frequency regulation), compressed air, flywheels, ice storage and the large grid scale batteries (such as Vanadium redox flow, sodium sulfur or liquid metal batteries) that are currently being deployed in order to provide a storage dimension to the grid that can manage variability.

• One additional grid storage technology that is worth mentioning in its own bullet is the intriguing idea to mate the growing fleet of plugin hybrid and all electric vehicles with the grid. These would function as a gigantic virtual battery and could, if it can be made to actually work, go a long way to providing the vast needed buffer to manage supply and demand.

• Smart Meters, especially when coupled with smart appliances that can respond to signaling from in home energy management software connected to the Smart Meters can help manage the demand side of the equation. For example if there is a transient power deficit on the grid certain energy intensive appliances could temporarily shut themselves off or at least lower their usage and in so doing help to close the transient imbalance between supply and demand.

• Grid inter-connects such as the Tres Amiga Superstation being built in Clovis, New Mexico will unite the Eastern, Western, and Texas Interconnects and be able to shuttle 5GW of electric power between them. This will help even out regional surpluses and deficits by transferring surplus power between the three grids that serve the US and Canadian markets.

• Deploying new High Voltage Direct Current (HVDC) point to point trunk lines, which can move large amounts of power over long distances with very little line loss are also being proposed and is some cases already have been deployed. One drawback of HVDC lines is that they are ill suited for providing power to points in between their end points because of the need to transform the DC into AC current; however because their line loss is inherently lower than AC lines of equivalent high voltage a HVDC super grid would make sense for shuttling available power over very long distances. Alternatively a very high voltage

In addition to an evolution of the grid and the build out of additional energy storage capacity better real time weather forecasting and weather forecasting with a higher level of granularity. Being able to forecast an impending surge of power from a uptick in wind speeds that is about to blow its way across a wind farm or to be able to forecast when cloud cover might impact a solar power installation can make the variability more manageable and minimize the impact that the variability has on the grid.

Demand Management the Other Side of the Equation

The other way to keep supply and demand in balance is of course to manage the demand side of the equation; in other words to use networked machine control software/hardware that is programmed to respond to signals about the current grid conditions to either ramp up power use or throttle it back. For example smart appliances such as air conditioning systems, washers, driers etc. that can shut down or minimize their power consumption based on current power availability. Conversely networked systems such as these could power up when supply conditions improved.

Another similar means of demand management is with time shifting systems that time shift their power needs from periods of high demand to off peak demand periods. An excellent example of this is the ice cooling systems that use available off peak power to freeze ice – in this manner storing up potential for use during the hot hours of the day when electricity demand also peaks.

The Grid Needs to be Upgraded Anyways

On a final note I think it bears mentioning that the electric grid is already extremely taxed in many places and something needs to be done in any case to keep it from reaching the point of collapse. Many of the same grid improvements – usually lumped under the rubric of Smart Grid — will be required even if there was no rapidly growing wind and solar energy harvesting energy inputs into it. Smart Grid enabled demand management is needed even if 100% of our electricity supply came from burning coal! The same argument can be made for the need for a very high voltage (either AC or DC) super grid and new grid interconnects. Similarly energy storage capacity needs to be seriously ramped up not only to store variable wind or solar energy, but because demand swings widely between peak and off peak periods while the big thermoelectric plants cannot easily vary their power output by very much.

In each of these cases these capital improvements to our electric energy systems will be needed regardless of where new electricity comes from. Perhaps wind and solar may require some incrementally greater investment, but to use the need for this investment as an argument against wind or solar energy ignores the fact that most of this capital expenditure will be required by the grid in any case.

Finally I am sure I missed some of the ideas and proposed solutions – this is a very big subject after all and I am continuously finding that I am being further educated in it. So, if anyone wants to weigh in, add to this or comment on this I (for one) will certainly appreciate the added depth that other voices bring.

Featured Resource: renewable energy worldRenewable Energy World -- Renewable Energy World effectively promotes all aspects of renewable energy technology in the worldwide marketplace. Renewable Energy World provides authoritative articles, case studies and essential news on global developments in the renewables sector. Every issue includes features on wind power, solar thermal, photovoltaics and biomass. Regular coverage is also devoted to geothermal, energy storage, small hydro, and hybrid systems. SUBSCRIBE FREE >>.

© 2010, Chris de Morsella. All rights reserved. Do not republish.

Line Break

Author: Chris de Morsella (146 Articles)

After a decade performing as a lead guitarist for rock bands, Chris de Morsella decided to return to the career his uncle mentored him in as a youth....Software Engineering. Since that time he has thrown himself into his work. He has designed a compound document publishing architecture for regulatory submissions capable of handling very large multi-document FDA regulatory drug approval submissions, for Liquent, a division of Thompson Publishing. At the Associated Press, Chris worked with senior editors at facilities around the world, to develop a solution for replacing existing editorial systems with an integrated international content management solution. He lead the design effort at Microsoft for a help system for mobile devices designed to provide contextual help for users. Chris also helped to develop the web assisted installer for LifeCam2.0, the software for Microsoft’s web cam and developed late breaking features for the product He also served with the Rhapsody client team to redesign and build a major new release of Real Networks Rhapsody client product. His most recent assignment has been Working with the Outlook Mobile Time Management team for the next release of Outlook Mobile for the SmartPhone. Chris' interests are in green building and architecture, smart grid, the cloud, geo-thermal energy, solar energy, smart growth, organic farming and permaculture. Follow Chris on Twitter.

  • bravoltio

    You say: “proponents of renewable energy are allowing the issue of variability to be employed as an effective argument by those who are ideologically opposed to the very idea of renewable energy” ¿¿IDEOLOGICALLY?? You are speaking of rotating inertias, and I ask myself what is the ideology of the rotating inertias of a power system. Has the wind ideology? Is intermittency an ideology?

    • Chris de Morsella

      Not sure what you are trying to say here.

      However let me point out that ideology is a uniquely human cultural and mental/psychological phenomena. A wind mill or a nuclear power plant has no ideology per se. I think it is abundantly clear that there are many who are vehemently opposed to the very idea of renewable energy based on their ideological affiliation and I will stand by that statement.

  • bravoltio

    …And you have forget to speak about the “human being ideology” regarding to be in the habit of getting up early to workunder the sun light and going to bed to sleep by night. Has the dayly load curve ideology as well? Any variability study has to take into account the pattern loads.

  • Dick Bonnet

    While I support renewable power of any sort I wonder if the driving force is not tax credits and not improving our electic infrastructure. For every KW/MW of solar or wind e have to have an equivalent coal or nuke KW/MW on line for when the wind doesn’t blow or the sun doesn’t shine. I would much rather see the money spend on increasing the efficiency of coal to the 40-45% that is possible or build nuclear. All the renewables put together barely keeps up with the new demand from all our new electronics. And BTW if you think you are ever going to get energy back from using electric cars as an energy sink good luck.

    • Chris de Morsella

      The world is running out of easily recoverable fossil energy supplies — including as is becoming more and more evident coal as well. In fact there is more and more evidence that existing reserve figures may be vastly overstated as has been made evident by for example: In 2004, without explanation, Germany downgraded its estimate of proven hard coal reserves from 23 billion tons to 0.183 billion tons, a 99 percent reduction, as did the UK (by 99%) and South Africa as well. In many cases putative reserves treat low BTU marginal brown coal as if it were high grade rock hard Anthracite. The coal sector needs to come clean with reserve figures that are more rooted in actual reality.

      This is why we need to develop a renewable wind/solar based energy harvesting infrastructure. Even if you regard it as an insurance policy on our energy future it is a vital strategic element of any sane energy plan.

      In fact our largest economic and strategic competitor, China is forging ahead with its own development of both wind and solar. Perhaps that should give you some pause to ask yourself why the central authorities of the People’s Republic of China are putting so much resources into making China a world leader in cleantech.

      The grid as the NYISO study (amongst others) has made clear can absorb much more wind and solar energy than it currently does — up to around 25% or even 30% — without major overhauls and need for storage. The grid needs to make the capital investments in information awareness and real time responsiveness that will better match it to renewable supplies such as wind and solar in any case because it is seriously overtaxed as it currently is. Distributed energy storage nodes — such as pumped storage and utility scale batteries are needed by the grid in any case to help it meet peak demand loads.

      Coal is the dirtiest energy source there is period and if it was not backed by a very powerful and politically connected lobby and if coal was forced to bear the true cost of it being mined and burned it would wither away as an energy source. Coal is dirty and it will become increasingly expensive as the true global reserve picture becomes more clearly evident in the coming years.

      Nuclear energy has its own set of problems not least of which is a looming peak in the global uranium reserves and the unresolved and unresolvable high radioactive waste disposal problem. So called Gen IV reactors only exist on paper and the plutonium economy that these fast neutron breeder reactors would bring about is troublesome on so many levels.

      There is no easy energy future for us… and anyone who says otherwise either is lying or they do not know of what they speak. We are facing an era of increasingly expensive and difficult to get energy… the era of easy oil has run out.

      While wind and solar have problems they represent the best long term investment we can make in our energy future and will provide our country with a much needed energy security cushion as fossil fuel sources begin to decline — as they in fact are doing.

      It is quite clear that we disagree. I urge you to look at the larger picture and perhaps then you will see why developing wind and solar is in our nations long term strategic interest.

  • Sean

    Hello All,

    While I mostly agree with adding the renewable distributed generation (DG), I think the concern is the reliability of the DG. If the system relies on the DG too much, then when the DG goes away the system could go down also. DG needs to be added carefully. Maybe a good plan would be to force any DG over a certain size to have an energy storage source available on demand as well?

    Although I am a proponent of renewable DG, I also think we should still develop nuclear. Nuclear, I think, would ‘get us over the hump’ of transitioning to cleaner sources. Containment of nuclear waste has developed greatly recently. Also, nuclear plants are very clean when operating, producing essentially only water vapor and waste heat.

    I think like anything, a little DG in moderation, but we can’t jump all the way into it because of some non-scientific political pushing. If we just look at the facts and ask how reliable we want the system to be, I think some agreement could be reached.

    Does any on else have the same opinion or any other thoughts, comments, etc…?


    • Chris de Morsella

      The penetration of solar and wind renewable energy into the total electricity market is still so small that the types of concerns you raised will remain negligible for quite some time. Renewable energy can reach 25%-30% penetration and the electric grid system can handle the variability at this level of penetration with relatively easy to achieve power management policies and procedures. Wind and solar without attendant distributed storage cannot act like baseload power sources; however some new solar thermal installations are using molten salt to store the energy enabling them to put electricity up onto the grid when needed and to act both as peak load power sources and as baseload power sources.

      Nuclear power has many problems — most critically the mind boggling long term waste management issue. Nuclear power plants — including the small modular reactor types being proposed — all produce extremely long lasting high level radioactive waste that we just do not know what to do with. While it is certainly an intriguing idea to burn up these actinides in a breeder reactor system — such Gen IV reactors only exist on paper now and would be thirty to forty years off into the future before they could begin to be deployed — even assuming all the technological problems with their exotic coolant (such as liquid lead for example) can be worked out.

      Our world requires energy solutions that are ready today. Wind and solar are ready today and are rapidly moving towards grid parity in terms of their cost structure. Distributed generation combined with adequate distributed storage and a more widely linked up power distribution system could effectively wean our world off of its unhealthy and unsustainable dependency on diminishing supplies of fossil energy.

  • Daisy Gentry

    Not sure what you are trying to say here. However let me point out that ideology is a uniquely human cultural and mental/psychological phenomena. A wind mill or a nuclear power plant has no ideology per se. I think it is abundantly clear that there are many who are vehemently opposed to the very idea of renewable energy based on their ideological affiliation and I will stand by that statement.

    • Chris de Morsella

      Daisy I agree with your statement that “there are many who are vehemently opposed to the very idea of renewable energy based on their ideological affiliation” — I run into them a lot and it can be frustrating at times because it is hard to have a discussion based on a factual and calm headed exchange of ideas when one party in the discussion is already fixed by an ideologically rooted perspective which they will defend to the bitter end. There is a battle for public perception going on right now — and like all such battles what matters is not what is right, but rather what the “public” perception ends up being (or not being). I have noticed that the variability issues that exist in wind and solar power systems are being used increasingly as a “talking point” by those who are for whatever reason — be it ideological or profit motive based — opposed to the very idea of renewable energy. There is a variability problem, but as I hope the article illustrated — it is a problem that can and is being addressed AND it is an issue that — I believe — is being blown out of proportion by opponents of renewable energy. I wrote this piece because I believe it is important to be ready to immediately address this issue whenever it is raised. To have an effective response ready when someone says “yeah, but the wind doesn’t always blow… so wind energy is … I want to suck the oxygen out of using this argument as an effective tool to bludgeon the idea of renewable energy itself.