Good question!

The surplus electricity from wind farms or solar PV farms if not used will have a negative value because it cost money to produce it but no one wants to buy it.

Ideally, this surplus electricity can be stored using one of the following technologies:

1) Store surplus electricity in the form of hydrogen gas. In this case, the surplus electricity is used for water electrolysis to generate hydrogen gas that can stored and when electricity is needed, then this hydrogen gas can be used as feedstock for the PEM fuel cells to produce electricity. Alternatively, the stored hydrogen gas can be burned in internal combustion engines (ICE) to generate mechanical or electrical energy.

2) The surplus electricity can be used to pump water to a higher elevation tanks and when electricity is need, water flow from the higher elevation can be used to run turbines to generate electricity. This concept is called hydropower generation where the potential energy of water stored in a higher elevation is converted into kinetic energy to run the turbine generators.

3) The surplus electricity can be stored in flywheels which can produce electricity when it is needed.

4) The surplus electricity can be used to compress air which can then be used to run turbine generators when electricity is needed. The concept is known as the compressed air energy storage (CAES).

Hope this helps answer your question!

Dear Saeed,

The amount of wind generation is proportional to the wind speed. Many wind generation forecast methods are based on the wind speed forecast. The amount of wind generation is also proportional to the area covered by the blades and the angles of the blades (Similar to aviation control mechanisms).  Therefore, from the technical perspective, wind generation can be curtailed by adjusting the angles of the blades. However, wind curtailment is usually the last option to consider. From the operational perspective, wind curtailment can be avoided, if dispatchable units are able to provide adequate ramping service. One method to handle excess renewable generation is formulating the day-ahead electricity market with multiple period ramping processes. Comparing sections IV.B.1 and IV.B.2 of the following paper will provide a detailed discussion on how wind curtailment can be avoided by taking advantage of the ramping capabilities of dispatchable units. 

http://dx.doi.org/10.1109/TSTE.2016.2570218

Regards,

Saeed

Article Multiple Period Ramping Processes in Day- Ahead Electricity Markets

Mohajeryami,

The excess generation with insufficient consumption is in fact, a fairly serious consideration in nations that plan to go for renewable based generation in a big way.  It can become a problem if not handled properly by planning, operation, and network management.

But in cases  where it doesn't go to the level of being a "problem", this supply-demand imbalance continues to be an issue in several ways. Some of these are:

• Overvoltages in PV fed distribution and MV transmission systems.
• Relay coordination problems due to reverse flow of power in healthy circumstances.
• Storage planning, by way of battery banks or pumped storage.
• Reserve regulation, and coordination of conventional units that operate with renewable sources simultaneously.
• Weather and time-of-day dependent short duration power variations that disturb the demand-supply balance in a transitory manner.

The last two essentially lead to ramping problems that Manshadi has already referred to.

With best wishes.

-Sanjay

Thanks Sanjoy and Saeed,

Adjusting the angles of the blades looks a plausible way for wind energy. Do you have know how solar power plants curtail the generation in case of excess generation?

Best regards,

Saeed

The simple answer to your question is: disconnect or store

Disconnect your wind or solar power plant and there is no harm to any equipment  

If you have a stand alone plant you cannot store in the grid. Either you have enough with batteries, or you install a pump power plant where water is pumped to a tank or reservoir from where the potential energy can be recovered later in a hydro turbine. Another option is Power-to-gas using excess electricity to generate hydrogen which is later used directly or in a fuel cell to convert to electricity. 

Thanks Ola,

That makes sense. I appreciate your answer.

Mohajeryami,

Ola has given you the most popular way (at least currently !) for curtailing renewable power.

Adjusting blade angles in wind turbines is more useful as a continuous power control measure, rather than a generation cut-down measure.

-Sanjay

In such case, generally the solution is saving energy in storage banks namely batteries or water tank, etc.. otherwise controlling the power generation it self : for example the wind turbine blades speed or pitch angle, and the MPPT or the tilted angle for PV panels

I agree with Ola.

Often the energy in excess is simply lost, and (depending on the legislation of the Country) producers can be paid in any case (maybe a 30-50% of the market value) also for energy not dispatched.

The idea of producing H2 from not delivered electricity is good, but (fortunately) the electricity not dispatched at the end is only in the order of 2% (Italy, UK) so (for what I have seen) the production of H2 from lost electricity is not enough to repay the investment of (expensive) electrolyzers.

The most lucky countries are those who have hydropower, to store excess energy by pumping water in the reservoirs (and using it during the peak requests).

The coupling oh Hydro and Wind gives (like in Northern European Countries) enormous advantages!

Thanks Alberto, Imene, Sanjay, and Ola

Your answers were very helpful.

I, also, should thank David for sharing this article in telegram about utilities paying wind farms for switching off their generation. It blew my mind that in UK, on an annual basis, utilities pay 53 million pounds to wind farms to switch off their generation. I am wondering why they don't sell this production to the neighboring countries?

Best regards,

Saeed

Good question!

The surplus electricity from wind farms or solar PV farms if not used will have a negative value because it cost money to produce it but no one wants to buy it.

Ideally, this surplus electricity can be stored using one of the following technologies:

1) Store surplus electricity in the form of hydrogen gas. In this case, the surplus electricity is used for water electrolysis to generate hydrogen gas that can stored and when electricity is needed, then this hydrogen gas can be used as feedstock for the PEM fuel cells to produce electricity. Alternatively, the stored hydrogen gas can be burned in internal combustion engines (ICE) to generate mechanical or electrical energy.

2) The surplus electricity can be used to pump water to a higher elevation tanks and when electricity is need, water flow from the higher elevation can be used to run turbines to generate electricity. This concept is called hydropower generation where the potential energy of water stored in a higher elevation is converted into kinetic energy to run the turbine generators.

3) The surplus electricity can be stored in flywheels which can produce electricity when it is needed.

4) The surplus electricity can be used to compress air which can then be used to run turbine generators when electricity is needed. The concept is known as the compressed air energy storage (CAES).

Hope this helps answer your question!

Dear Dr. Yehia,

Thanks for your comprehensive answer. Your answer is definitely helpful. However, my main concern was the occasions that the storage is not available. In other words, assuming we switched off our wind farm, but the blades are still spinning, what would happen to the electricity? Is it harmful to devices? Fortunately, Ola answered this question, and he mentioned that switching off is not harmful to the devices.

Best regards,

Saeed

So, are we getting to the stage that fossil power plants pay renewable plants to go offline so they can make more mjoney? Wow.

On a more serious note, I think the hydrogen gas storage is good, but don't  get it from water. It's too difficult to electrolyse - it has  a high overvoltage. Instead use nickel oxyhydroxide, and you get what is called a nickel hydrogen battery - the most reliable type known and used in spacecraft. You can use the wind turbine tower as a gas storage tank.

Dear Tony,

The fact that the power grid is not ready to absorb all renewable energies' generation, should not impact the renewable energies' production.Because, investors invest on renewable energies based on the promise that they received from the FERC that are able to sell their generation in the wholesale market unconditionally.

By the way, I am sure,  back in the day, people paid the similar amount to conventional power generation before building the necessary infrastructure to deliver their generation to consumers. These power plants are not responsible for the power delivery, same as other conventional power generation resources.

Overall, the money that the wind farms receive for switching off their generation is not unfair. It is worth mentioning that the other conventional power plants are get paid if their power is not able to be consumed. However, no one pays attention to those monetary compensations due to the fact that conventional energy generation resources are not in the spotlight anymore.

Best regards,

Saeed

Renwables are good source of energy, however their main problem is intermittency-meaning the wind does not blow all the times during the day and the sun as well doesnot shines throughout the day. This gives a scenario whereby at one point in time you have peak/high power generation and at another point you have very little or no power generation. This situation contrasts with fossil power generation where you have continuous power generation 24h per day, regulated by input raw materials (fossils) and other control systems as you have mentioned.

In case os renewbles, power generation depends soley on resource (wind, sunshine etc) availability and they are actually what regulates power production together with the efficiency of technology used and the system installed capacity. This is what brings about the idea of storage in renewable energy systems as a means of dealing with excess power generation over consumption demands. The excess power produced during peak production are stored and used when there is limited supply.

Renewable energy systems and technologies are design in such a way that power generations are rarely regulated by the system components or parts. I know of a case of wind turbines which are adapted in a way that when the wind speed exceeds the maximum expected/designed blade speeds, some systems automatically pitch the blades (reduce contact angle with the wind), so as to maintained the blades at its maximum operating speeds. This is only a safety measure to prevent overspeeding of the rotor blades that might destroyed the system. It however doesnot control power generation regarding your question.

So, according to me, the only solution to your situation is storage using any of the methods highlighted by Yehia F. Khalil.

Thank you and hope this helps.

Thanks dear Erasmus,

Your answer was very informative and helpful. I appreciate it.

Best regards,

Saeed

Hello Saeed,

I have read all the discussion. I have same thought as others. Apart from the technology I may suggest grid tied systems with Feed In Tarrif. If your govt. implemented this policy you may sell your extra without shutting down or investing more on a same technology.

Hi Saeed,

Letting the blades of the wind turbine after the power is switched off may not be desirable as it cause overheating of the power electronics in this system. Typically, there is more than one method to apply shut off the blades using mechanical or hydraulic braking systems.

Applying the brakes in wind turbines can happen for other reasons such as high wind conditions in excess of 20 m/s.

Hope this helps!

Dear Gour,

Thanks for your answer. Feed-in-Tariff is a standard form of contract in most of regions. This contractual form reduce many risks associated to the investment on renewable energies. However, besides the contractual obligation, and monetary concerns of such renewable resources, there are many occasions that the grid is unable to take any generations from renewable sources for some reliability reasons (There are many of such instances occurring in Germany and other European countries). In such moments, the resource owners need to bring down their generation to zero, or manage it the other ways.

The other scholars, in response to this query, suggested that it is better to divert such extra generation to the storage units (different battery technologies, or making hydrogen using electrolysis, and etc. ). Some others, also, suggested that adjusting blades to reduce the wind exposure, or literally shutting down PV solars are the other harmless methods (Some, also, disagree about these approaches being harmless to devices). Nonetheless, it turns out there are many approaches to manage the extra generated energy. 

I am very thankful to all your answers. Your answers helped me to grasp how the extra generation can be managed.

Best regards,

Saeed

I would like to add a few points to answers above, First of all , the case of grid faults , also we need to disconnect the wind / solar supply from the grid or get ride through the fault based on the grid code in order to let the wind farm be connected all the time. Also, there are two ways to control this situation, first by controlling the wind speed (pitch angle or stall control)  or by controlling the conversion system like in DFIG by using the brake chopper by discharging the extra generated DC voltage in the Rb chopper resistance. And finally, we can use this to improve the power quality like reactive power compensation or improving the voltage profile. 

Assuming that  all connected customers get enough and all  connected storing device are full, the simplest way is to open the electric ciruit linking the generator or PV cells to the customers and storing devices (usually the electric grid). This does not damage anything, except that, in windmills, the rotor is no more braked and may get a high rotation speed if the blades are not put to a pitch that rediuces the force of the wind.

Surplus energy can be stored through storing devices as

DC electric energy (Battery)

Hydrogen (Electrolysis)

Compressed air                and

Hydro / Mechanical energy

for grid connected system habitually, the surplus of energy is injected into the grid but in one  case when the faulty grid is disconnected from the solar/wind farm, the surplus of electricty can be  stored using one of many technologies indicated by Mr khalil.

in other case when there is no storage system, in this case the command (maximal power point tracking) must degraded (change) and ensures only the demand.

Surplus electricity from wind and solar PV can be stored in several ways. The most known is using batteries, but it is too expensive. Other ways of energy store are being studied, for example,: compressing air in caverns, pumping water to high reservoirs, producing hydrogen, etc.

Of course, the last option is curtailing wind and solar resources. Wind turbines have two kinds of "breaks": pitch and stall. Then, an option is decreasing electricity production from wind turbines or not producing anything, just stopping the turbines. In the case of solar PV, some systems have defocusing systems, then you can defocus the pv panels to produce less electricity.

The best scenario is decreasing production from thermal power plants and maximize the use of wind and PV. If it is not possible due to tehnical restrictions, export electricity to neighour regions is a good option.

Att

Rafael

It can be stored in storage devices like battery, hydrogen compressed air, etc. The use of MPPT techniques controls the power flow to load. But it don't help to control the panel output .

In brief,  there are several methods have been developed over times (like Compressed air 60 to 90% Efficient, Liquid air efficiency of up to 70%, Batteries over 95%, Flow batteries, Flywheel, Hydrogen, Power to gas, The power to ammonia Concept, Pumped water, Superconducting magnetic energy, molten salts, molten silicon and gravity base etc) and these methods are being used to store surplus energy from wind turbines  as well but all these solutions are not suitable to all situations and there is a continuous efforts to store surplus energy efficiently, suitable to most of the situations.

Since wind flow subject to frequent fluctuations, so stored energy mostly, should use to offset losses and to stabilize power output during transient wind fluctuation period to meet the grid codes.

Hope this of help.

Surplus wind or solar power could be used to replace other energy sources to produce heat.

In Germany this senseful and easy use is prevented because the consumers would have to pay the "EEG tax" as for any other electric power consumed. So it is easier to take the wind mill or the solar panel of the grid and pay the operator even for the not produced energy.

https://www.synergy.net.au/Our-energy/Electricity/Electricity-generation/Wind-energy

http://www.solarpoweristhefuture.com/combination-solar-wind-power.shtml

https://spectrum.ieee.org/energy/renewables/wind-water-and-solar-power-for-the-world

Visit below link for Matlab Code

https://dg-algorithm.blogspot.com/2018/02/ess-allocation-and-load-shedding.html

simply it can be stored as the above answers or at the worst case curtailed.

I don't see why unwanted power has a negative value; if there was nothing generated it would have zero value. This is a common error in calculating the 'cost' of something, by working out the costs of all the components and amortising them. The answer doesn't mean much. Ask instead what you would save if you didn't do it. That is the true 'cost' of something. In the case of energy, what would be the cost of not generating the power - just the depreciation cost of the equipment. What is the cost of generating? Just the depreciation cost of the equipment. The difference is zero, so the cost of generating unwanted power is zero. The same with ALL research and development. It costs far less than most calculations suggest.

I'm answering with repeating the initial question, as nobody has actually answered it:

• My question is how a wind farm/solar power plant does manage the extra generation? assuming a scenario that the generation can not be sold to the customers, or consumed in any other ways .

So - the question is not how to store or use the energy, the question is if there are technical issues in closing down production from a solar panel or a windmill.

As far as I'm aware, there are none. For windmills I'm pretty sure they can always be stopped by angling the blades away from the wind, without any detrimental effects. For PV solar, I believe it is the same.

In markets with Feed-in-tariffs renewables producers won't stop producing even if electricity prices are negative, as they get paid a guaranteed minimum to supply electricity anyway. This again gives thermal power plants (coal, typically) problems as they can't get anyone to take their electricity that they may be forced to produce as they can't shut down or increase production immediately.

This again mean that "renewables percentages" in power production is misleading, as renewable power does not replace emissions from fossil power plants 1:1.

Are you asking if there is a way to circumvent the law of conservation of energy? Surely not. In which case you have answered your own question! Excess energy can't just be lost - the wind farm just has to reduce its generation. Perhaps wind farms should partner with storage companies to provide on-site or nearly-on-site energy storage.