By Erin Douglas

February 6, 2019 - Texas faces several challenges in replacing the electricity generated by coal with wind and solar sources alone, a study from consulting firm Wood Mackenzie found.

Due to the proximity coal plants have to big electricity markets and temporal patterns of renewable energy sources, large amounts of energy storage would be required to remove coal from the electricity mix, the report said.

A previous study by Rice University study suggested Texas could start to wean itself off coal by playing on the state's renewable resource mix. The study found that between wind energy from West Texas and the Gulf Coast and solar energy across the state, Texas could generate renewable power at different times of day and seasons. This, in coordination with greater investments in Gulf wind, could help replace generation from coal-fired plants.

While the Wood Mackenzie report states that the concept of a complimentary renewable mix is reasonable, it pushes back on the idea that it would be enough to wean the state off coal, suggesting the data used to make projections may be flawed.

The study found that the way ERCOT, the state's electricity grid operator, calculates reliability for renewable energy significantly overestimates the capacity of wind and solar sources. ERCOT uses a metric called peak average capacity percentage, which measures the performance of wind and solar energy sources during the top 20 demand hours of the year.

Wood Mackenzie researchers wrote that this method does not fully capture the nature of renewable energy because even on a low-demand electricity day there may not be enough power generated to meet demand if the wind stops blowing.

Because the top 20 demand days are used in ERCOT's forecasts, the data may not be enough to predict how renewables will behave, the report said. By using instead a metric that subtracts renewable generation from top demand days, the study found that if renewables made up a significantly larger portion of the grid the days Texas would worry about delivering enough power wouldn't be the hottest; it would be normal demand days when the wind doesn't blow.

"They would never worry about a day like that with their current methodology," said Wade Schauer, research director for power and renewables at Wood Mackenzie and author of the report. "It may not be the hottest day of the summer, but the day with no wind output."

Schauer said he believes ERCOT should update its methodology for renewables.

ERCOT said in a statement that it uses an approach that focuses on historical data from similar seasonal peak days in prior years.

"We continue to explore changes to the methodology to account for increasing amounts of wind on the ERCOT system," Leslie Sopko, ERCOT's communications manager, said in a statement.

The study also found that the physical location of electricity plants needs to be taken into consideration to see how feasible renewables are. Most remaining coal plants connected to Texas' electricity grid are located close to large cities that use a lot of power, meaning plants can provide local generation without extra transmission costs.

Coal plant capacity is better aligned with electricity loads based on location, according to the study. Houston, for example, accounted for 27 percent of the average power demand in the state in 2018. The city, which produced 17 percent of the state's coal capacity, is not expected to contribute any utility-scale wind or solar energy in 2019.

Higher electricity prices in the city can be attributed to this problem, analysts wrote, as getting power into Houston has proved difficult due to transmission congestion.

The Wood Mackenzie report suggested that ERCOT's renewable developers place more value on locations outside of West Texas -- where a large majority of the grid's wind power is produced -- so production is valued more highly.

Schauer said wind and solar eventually could make up the proportion of the grid currently powered by coal if more battery storage was available and if plants were located closer to major metro areas, but it will be very challenging.

"I'm not trying to say they can't be replaced, but it is not easy to do," Schauer said.