Integrating Renewable Resources Onto The Grid

Posted on January 5th, 2011 by

This is a new game.For decades, utility operations have been built around managing “conventional” generation such as natural gas or coal, where the operator has control over generation output.Renewable generation (which is mostly wind and solar [1]) is mainly non –dispatchable which means that energy is generated when Mother Nature says so.Historically, in the equation of supply and demand, operators have primarily had to deal with the demand variable.Now with more renewable generation coming online, operators have to balance variables on both sides of the equation.

One of the most challenging aspects of integrating renewable generation resources (renewables) from an operations standpoint, is dealing with their inherit intermittent generation profile.This includes both the non-dispatchable production, as well as the quick generation ramping rates. Wind parks can ramp from zero megawatts (0MW) to full capacity in a matter of hours.Solar photovoltaic (PV) plants can ramp from full capacity to 0MW and back to full capacity in a matter of minutes when a cloud passes overhead.[2]

This can be a huge operational challenge, especially in California where the goal is to have 33 percent of all generation by 2020 come from renewables.With this much renewable generation, days with low load (such as Thanksgiving Day) could theoretically be supplied by 100 percent renewable generation. [3]When people expect a constant supply of electricity to their home, managing a high amount of supply that comesfrom a resource that can drop to 0MW in a matter of minutes can be challenging to say the least.

With these daunting challenges in play, it can be overwhelming to think about how to integrate renewable resources onto the grid.However, there are solutions that can be implemented now as well as longer-term solutions that will allow us to accomplish the task of integrating renewables with conventional generation resources.

The first solution is to take advantage of liquid trading hubs on the wholesale energy markets.By purchasing and selling electricity on both a day-ahead and hour-ahead basis, operators can sell off excess generation or buy up needed generation to match forecasted demand (load) with forecasted supply (generation).This solution is in place in most areas, however operators have to depend heavily on the hour-ahead markets because of the large errors when forecasting wind or sun more than a day in advance.

The second solution is to develop more sophisticated wind and cloud prediction models so that operators can be more accurate in predicting renewable production further in advance.Currently, a one day out wind forecast can be dramatically different than the actual weather.The main challenge is refining their prediction models for when, how much, and how quickly the wind will pick up.

Third, the adoption of intra-hourly scheduling will allow operators to schedule the intermittent renewable production on a more granular scale.This solution will help alleviate the quick ramp up and down rates of wind and solar.By scheduling a resource 15 or 30 minutes prior to the flow hour, versus the current hour ahead scheduling, the smaller scheduling blocks will more closely follow the variable generation profile.Additionally, the forecast used for the schedule can be a more accurate predictor of actual production since it can be refreshed closer to the flow hour.

A fourth solution is to build renewable resources in geographically diverse locations.This is more difficult for wind facilities, as there are a few prime locations to place wind turbines.Solar is easier to build in diverse locations because there are more areas where sun and open sky are available.Geographic diversity allows for the generation imbalance of different renewable resources to “net” against each other over the system.This can dramatically reduce the amount of balancing reserves needed. [4]

Although not an immediate need, a fifth solution is to build more balancing reserves from quick ramping thermal units. Some older thermal resources that are near the end of their useful life have startup times that exceed a day. This is obviously too long of a ramp rate to effectively balance intermittent resources. When they are decommissioned, replacing them with quicker ramping thermal units (such as a CCGT) could boost the necessary balancing reserves for wind or solar. [5] This solution may prove to be more critical as more renewable resources are brought online, since a greater percentage of the generation portfolio will be from intermittent resources.

Finally, a sixth solution is to take advantage of the up and coming smart grid to balance load pockets with intermittent renewable generation. For example, if an area experiences a spike in load, a wind resource is producing excess generation, then theoretically that wind supply could be scheduled to directly offset that load. Also, if distributed generation proliferates, then possible the smart grid could balance load pockets with localized renewable generation, and use conventional generation from the grid as a balancing or contingency reserve. Both of these speculations on the future implementation and use of the smart grid, but could prove to be viable solutions.

No single one of the aforementioned solutions will solve the challenges of integrating the increasing amount renewable resources onto our grid.It is rather the combination of these solutions that will allow operators to effectively manage both the supply and demand variables of the energy equation.The country is at all different stages of integrating renewables into their generation portfolio, and likewise the adoption of these solutions will happen at various times.California is leading the pack as it pushes towards a 33 percent Renewable Portfolio Standard, however more Midwest and Southwest states will likely follow suit as more wind and solar resources are built in those regions.

Editor Endnotes

[1] Although there are some dispatchable renewable resources, such as geothermal, wind and solar generation have proliferated in new generation contracts.Large hydro, such as Hoover Dam, is not considered a renewable resource.

[2] I’ve seen a wind facility go from full capacity to 0 MW and back up to full capacity over a period of 6 hours.I’ve also seen a solar PV facility go from near full capacity to 0 MW and back to full capacity over a 3 minute span- likely a cloud passing overhead.

[3] Assuming a system’s average load is 10,000 MW (for a single hour), and assuming the capacity factor of an average renewable plant is 33% (very optimistic), then reaching 33% integration would require 10,000 MW of renewable generation to be built.That means if the system’s load is average, and all renewable generation is at full capacity, then all load would be met with renewable generation.In practice this is impossible for a plethora of reasons, such as maintaining system reliability, voltage support, having to curtail baseload units, etc.

[4] For example, if all of a system’s wind is built in a single location, then all of the production is proportional to a single, possibly volatile, wind pattern.However if the system’s wind parks are built in areas that have opposite wind patterns, then the production profile will smooth out.

[5] New Combined Cycle Gas Turbine (CCGT) power plants can ramp from 0 MW to full capacity (sometimes 1000 MW) in a matter of hours.Some of the old thermal plants, which used to run on fuel oil but have been converted to natural gas, can take over a day and a half to reach full capacity.A large CCGT can often run at “idle”, then ramp up several hundred MW’s in an hour.This may prove to be quick enough to provide effective balancing reserves if intra-hourly scheduling is developed and renewable are built in geographically diverse locations.

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Written by Kai Stephan, Energy Analyst in Renewable Resource Integration at Southern California Edison.

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