Denver wants to do it, as do Fort Collins, Pueblo and Breckenridge. Aspen has already done it, and Colorado Gov.-elect Jared Polis wants the whole state to do it by 2040: run on 100 percent renewable energy.

There is even a push by incoming congressional Democrats, led by U.S. Rep.-elect Alexandria Ocasio-Cortez, of New York, to set a national 100 percent renewable energy goal as part of a “Green New Deal.” U.S. Rep.-elect Joe Neguse, who will take Polis’ seat in the House, has signed on to the plan.

Polis says it’s not a mandate for Colorado, but an aspiration that would help reduce emissions of the greenhouse gas carbon dioxide. And that’s a good thing because even towns already on the quest aren’t sure how to meet the goal or what it might cost.

Renewable fever is sweeping Colorado even without a national or statewide target. Summit County and eight municipalities — Aspen, Breckenridge, Pueblo, Longmont, Lafayette, Nederland, Fort Collins and Denver — have adopted the energy standard. And there are moves for Estes Park, Loveland and Frisco to join.

Xcel Energy and Platte River Power Authority, two of the biggest power generators in the state, say they are willing to help the communities they serve. Platte River is set to adopt its own resolution to get to 100 percent noncarbon resources by 2030.

“We are having conversations with communities to define what is important,” said Alice Jackson, CEO of Xcel’s Colorado subsidiary. “What is clear in Colorado is that the sustainability piece has become more and more important.”

The aim is to reach the goal by 2030, or by 2035 in some towns. Of course, no one is sure exactly how to do it.

“How do we get to 100 percent community wide by 2035? We don’t have an answer,” Breckenridge’s sustainability coordinator Jessie Burley said. “We think we can get to 70 percent.”

Elizabeth Babcock, manager of air, water and climate for Denver, said: “We are less interested in debating what 2030 will look like than doing everything we can to get on that glide path.”

MORE: Read more environmental coverage from The Colorado Sun.

Can the electric grid run only on renewable energy?

The electric grid depends on stable, instantaneous electricity flowing through it, so when you flip the switch at 2 p.m. or 2 a.m., the lights go on.

If the grid were a network of water pipes, generators would be pumps pushing water into the system, water molecules would be electrons and the pressure to move those molecules along the voltage.

It isn’t quite so simple because to move large amounts of electricity over long distances this direct way, through direct current, the pressure, or voltage, would have to increase to dangerous levels, and the pressure losses would be great farther from the pumps.

So, the grid depends upon alternating current, which handles high voltage with greater ease and where the direction of the flow periodically reverses. Before it arrives at your house, the electric current is stepped down to a lower voltage that runs your refrigerator and hair dryer.

If wind farms and photovoltaic solar arrays are the pumps, one problem is they aren’t always pumping since the sun doesn’t always shine and the wind doesn’t always blow. Wind tends to be a better nighttime resource, while solar is better in afternoon and early evening.

And even when they are working, they may underperform due to clouds or weak breezes. One solution is to build extra generating capacity, a cushion of reserve generation.

“There are some issues around stability,” said Paul Denholm, a member of the grid analysis group at the federal National Renewable Energy Laboratory (NREL). “Can you balance the system? Can you carry enough reserve?”

The grid still runs on “baseload generation”– coal, natural gas, nuclear and in some cases, hydropower– which can run all the time. Natural gas-fired generation has been growing and works well with renewables since it can easily be turned on and off.

That constant baseload has to be replaced with something if fossil-fuel generation, even natural gas, is abandoned. The most ready answers are extra renewable generation or batteries.

“If you go above about 80 percent you have to overbuild your generation or build massive storage,” said Steven Davis, University of California, Irvine Earth-system sciences professor and a member of a research team that looked at what it would take to get to 100 percent with just solar and wind.

Wind turbines near Matheson, Colorado, are part of Xcel Energy’s new 600 megawatt Rush Creek Wind Project. Power generated by the project is carried along 83 miles of transmission lines to Colorado customers. Xcel says sustainable power generation is increasingly important to the customers it serves. (John Leyba, Special to The Colorado Sun)

They calculated twice the generating capacity actually needed would have to be built and found that at least 12-hours of storage was required on solar-heavy. Long-term storage of 32 days would reduce the needed generating reserve to 10 percent.

But the group only looked at wind and solar. “You could fill in the gaps with hydro, biogas or some other generation and that would change the calculations,” Davis said. Hydropower does not count as renewable generation under Colorado’s renewable-energy standard.

A second issue is assuring inertia in the grid so that if there is a disruption and the alternating current’s back-and-forth cycle begins to degrade, the few moments needed to adjust are there. All those big generators spun by steam or water provide that inertia.

In an all-renewable world, some of that inertia can be provided by geothermal, biofuel or concentrating solar plants, which all still spin, Denholm said. Hardware, such as a synchronous condenser, which is just a spinning motor, or batteries could also do the trick.

Denholm said a more exotic solution could be something like sensors in every refrigerator, they all have motors, which kick down their operation when a degraded cycle is detected.

“Your beer would not get warm,” he said.

“We haven’t completely figured it out how much inertia is needed,” Denholm said. “We don’t know, but we’ll figure it out.”

Batteries are such an attractive complement to renewable generation because they can address both variability and inertia. “With 12-hour batteries, you could get to 98-percent, 100-percent renewable most of the time,” Davis said. Most storage being installed now is four-hour batteries.

“Batteries are the biggest hammer in the toolbox” Denholm said. “Batteries can do a lot, but we have to be careful of just jumping to the assumption that batteries are the only tool in the toolbox. We need to consider a variety of flexibility options for improving grid efficiency and our ability to integrate renewables.”

The shift to a highly computerized grid where energy management can be done in fractions of a second is speeding along the shift to renewables. Xcel Energy, for example, has developed sophisticated modeling and forecasting tools that enable it to forecast wind and solar generation and manage it in 15-minute increments, enhancing grid stability.

Getting to 100 percent? “We see it as more of an economic issue than a technical issue,” Denholm said.

What will it cost to go to 100 percent renewable energy?

The short answer is no one knows right now.

What is known is this — renewable-energygeneration is getting cheaper. The cost of onshore wind generation has fallen 23 percent and the cost of photovoltaic solar is down 73 percent since 2010,according to the International Renewable Energy Agency.

The cost of lithium-ion storage batteries, still an expensive component, has fallen 80 percent since 2010, according to Bloomberg New Energy Finance.

Under its Colorado Energy Plan, Xcel is going to close two coal-fired units at its Comanche Generating Station, near Pueblo, and add about 2,000 megawatts of wind, solar and storage. When the utility sought bids for new renewable generation, it received some of the lowest prices ever quoted.

The median price for the wind projects was $18 a megawatt-hour (MWh) and for the solar projects, $29.50 a MWh. Wind plus storage was $21 a MWh and for storage alone, $11.30. By comparison, Xcel’s estimate for the all-in cost of operating the two Comanche units is $31 a MWh.

Xcel’s experience isn’t unique. The Northern Indiana Public Service Co. recently did an analysis that showed it could save its customers more than $4 billion over 30 years by replacing coal, which provides 65 percent of its generation today, with a combination of wind, solar and some natural gas by 2028.

“Renewable generation is competitive,” said Kenneth Gillingham, an economist at the Yale School of Forestry & Environmental Studies. “But by far the cheapest dispatchable generation is still natural gas.”

Even without storage, Gillingham said a high level of renewables could be put on the grid.

“The challenge is reducing those last emissions,” he said. “There is a lot of technological innovation that has to take place.”

MORE: Read more politics and government coverage from The Colorado Sun.

The Platte River authority did a study in December (2017) that estimated going to net zero carbon emissions by 2030 could add 10 percent to 20 percent to rates. The authority is already set to have 50 percent renewable generation by 2020.

“If someone asked me are you going to be 100 percent renewable by 2030, I’d say I don’t know,” Jason Frisbie, Platte River CEO, said in a panel discussion at the Colorado Rural Electric Association (CREA) conference Oct. 29. “We will keep working our way there.”

Another element that can boost the level of renewables and reduce costs is large, interstate wholesale electricity markets.

They offer more efficient pricing and lessen the variability because if the wind and sun aren’t blowing and shining in one state, they might be in another. A larger grid can also handle larger amounts of renewables.

Colorado doesn’t have such a market, which, Frisbie said, “is necessary for us to have any chance of getting to 100 percent … It is a more efficient way to run a system.”

Utilities want to move at a measured pace

Utility executives are also wary of moving too fast.

“Jumping in too early can be detrimental,” Xcel’s Jackson said at the same CREA panel. Early commitments to present technology could preclude adopting future technological innovations, adding to cost, she said.

The wind contracts Xcel purchased 10 years ago charged 6.9 cents per kilowatt-hour. The new contracts will be around 1.5 cents. Those more expensive contracts will remain on the system for the next 10 years. Making commitments to technologies too early, Jackson said, “may have some cost impacts that can come back to bite you.”

The key question remains what is the cost of closing the gap. “Right now, we don’t know what it is,” said James Bushnell, an energy economist at the University of California, Davis. “What if going from 90 to 100 percent renewables triples the cost?”

While the wind and solar prices quoted to Xcel look good, as you get closer to 100 percent, there are additional costs, and the value of those wind and solar contracts can diminish, Bushnell said.

The costs of retiring not only old coal plants, but newer natural gas plants can also add to the tab, as would the cost of upgrading the grid and adding technologies, such as Xcel’s wind and solar computer modeling.

A study done for the libertarian Competitive Enterprise Institute put the price tag for the switch statewide at $45 billion. This rough analysis simply closed all fossil-fuel plants and replaced them with wind, solar and battery storage based on assumed prices.

Power generated by the Rush Creek Wind Project is some of the cheapest wind-generated electricity in Xcel Energy’s Colorado system. Gov.-elect Jared Polis has set a goal to get Colorado powered by 100 percent renewable sources by 2040. But even towns that have set out to hit that mark aren’t sure the goal is attainable, or what it might cost. (John Leyba, Special to The Colorado Sun)

The price really shot up in the final years of the transition as more expensive battery storage was added to get to 100 percent, according to Dustin Meyer, an analyst with Energy Ventures Analysis, the consultant who did the study.

“We can build a lot of renewable capacity at remarkably low prices, but if that 100 percent is enforced rigidly, it could be expensive,” Bushnell said. “Then you have to ask what the cost benefit is.”

David Hornbacher, Aspen’s director of utilities and environmental initiatives, knows all about the difficulty and cost of getting that last bit of renewable energy.

More than 80 municipalities, five counties and two states — Hawaii and California — have a 100 percent renewable goal, according to the Sierra Club’s “Ready for 100 Campaign,” which is promoting the target.

Aspen is one of only six communities that can claim to have reached it. Getting to 80 percent was relatively easy, Hornbacher said. “The last 20 percent was difficult.” Trying to do it locally was too expensive.

Hydropower played a big role as the city already had two hydro facilities. It added additional hydro by purchasing power from a new hydro project 90 miles away in Ridgeway. To get that last increment, it added wind and biogas.

But the wind project is in Nebraska and the biogas plant in Iowa. The electrons from those two facilities never make it to Aspen.

Still Aspen claims to be 100 percent renewable because it matches its electricity demand to renewable generation. “We’ve ensured that what we take out of the grid is what we put into the grid in renewable energy,” Hornbacher said.

Bushnell said, “That is how the stuff will be done with contracts for a whole lot of renewables, but some of the juice will come from a fossil plant.”

Indeed, coal remains very much in Colorado’s future and in the future of three of the biggest electricity generators — Xcel, Platte River and the Tri-State Generation and Transmission Association, which serves 18 rural electric cooperatives in the state.

Tri-State, which has a total of 43 member co-ops in four states, gets about half its electricity from coal-fired generation and about a third from renewables. Tri-State officials say they will add renewable resources where it is economically feasible.

While Xcel is closing two coal units at Comanche, the 750-MW unit 3, which came on line in 2010 at a cost of $1 billion, is slated to operate through 2070. Jackson called it “a wonderful baseload unit” and “cost effective for our customers.”

New technologies that remove carbon dioxide from the smokestack for reuse or storage may be on the horizon and could make Comanche 3 carbon neutral, Jackson said.

Platte River’s Frisbie called the 280-MW Rawhide Energy Station, in Wellington, “our Comanche.” It is the largest generating unit in the system.

“Our view is that it is going to run for a very long time,” Frisbie said. “How that squares with 100 percent, I don’t know.”

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