Up until now, solar generation on the electricity grid operated in most places in what is known as a “must-take” scenario. That means that all solar that is generated must be used on the grid. When renewable energy was a fledgling new technology, this scenario made sense.
However, as solar penetration has increased, this “must-take” scenario is beginning to cause big problems for other generation technologies because solar can be slightly higher or lower than forecast and therefore requires a flexible source of generation to back it up.
Today, that flexibility is provided by natural gas combined cycle generation plants, which can ramp up and down to accommodate for the fluctuations of solar generation, leaving some gas power plant owners worried that the extra ramping is degrading their assets quicker than anticipated.
But a new study by Energy and Environment Economics (E3), First Solar, and Florida integrated utility Tampa Electric Company (TECO) and sponsored by First Solar shows that at high penetration levels solar generation can provide the same flexibility that natural gas plants have been providing and in doing so, not only does it lower emissions for the entire system but it also saves money for the utility and ultimately the customers.
Mahesh Morjaria, report author and VP, PV Systems Development at First Solar explained why other generating technologies are not happy with the “must-take” scenario.
“It’s more or less like [solar is] saying ‘hey I can generate however much electricity I want and the rest of you become flexible…I can be a nuisance to you but you be flexible,’” he said, adding “but if I will also be flexible, the relationship works better, don’t you think?”
The study models an actual utility system – TECO – and its generation portfolio to investigate the economic value of using solar as a flexible resource, exploring four solar operating modes: “Must-Take,” “Curtailable,” “Downward Dispatch,” and “Full Flexibility.”
Morjaria explained some of the key findings in the 55-page study, which you can download here.
Curtailable and Downward Dispatch
At penetration levels exceeding 20 percent on the TECO system, solar curtailment can be reduced by more than half by moving from the Curtailable to the Full Flexibility solar operating mode. This results in significant additional value due to reduced fuel costs, operations and maintenance costs, and air emissions.
Utilities with high solar penetration are already curtailing solar, which means just cutting it off when the grid doesn’t need it. When solar is forecast to be generating more electricity than the grid can handle, say for example on a spring day when the load is low, many utilities will just plan to curtail it.
“And that’s wasting all that energy,” said Morjaria.
Instead the study shows that utilities could be using solar for downward dispatch. (see figure below.)
“If I need to do something where I need to reduce the amount of solar I can push it down and so the downward dispatch actually accommodates more solar because now you are not making a commitment of the thermal generation,” explained Morjaria, pointing out that thermal generation in the figure above is reduced in the middle of the day.
“Then in the evening time, there is a sharp ramp up of the generation and that is all accommodated because the thermal generation can handle that and solar is helping as much as it can,” he said.
In the full-flexibility scenario, the study shows that the utility could even eliminate the need for gas steam generation, making better use of the solar generation so “you can get more value out of it,” said Morjaria.
“If you look at these two pictures, you would say the one on the right is a better way of using solar,” said Morjaria, because “you are reducing the burden on thermal generation to provide headroom and footroom.” (Headroom and footroom are needed in the case where utilities experience higher or lower load than expected.)
There are economic benefits of working this way, said Morjaia.
“If you are an integrated utility you can clearly see if I operate the plants this way, I actually not only reduce CO2 emissions but I also increase the benefit to my customer base…I cycle the thermal plants less and I have more ability to increase the penetration of solar.”
Markets are the Problem
Morjaria said that the purpose of the study is to show utilities and grid operators that we have the technology to let variable renewable energy plants (wind can do this, too, according to Morjaria) provide the flexible resource that the grid needs to accommodate them.
“The other way to approach [the grid] is to say if we add solar and solar is flexible then sure it can get curtailed but on the other hand it can provide the flexibility so you don’t need as much thermal generation to provide flexibility and hence you can accommodate more solar, which is a virtuous cycle that enables the system to be more cost effective.”
What we don’t have are market mechanisms to provide the signals needed to operate solar plants this way. Morjaria explained that all the different ISOs have different market mechanisms in place and will need to adjust in order to use solar energy in this fashion.
“That’s the whole purpose of this report to say technically this is not an issue. Ensuring that we can get it into operational practice and overcome all the market frameworks and regulatory environment is the challenge,” he said.