MAKING Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/141/03/38/6385413/me-2019-mar2.pdf by guest on 01 October 2021 TEXAS

GREENIt is technically possible to run the state’s electrical grid using wind, solar, and nuclear power, but it will require a lot of energy storage. BY EFSTATHIOS E. MICHAELIDES

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ince the 1980s, the state tourism office has de- scribed Texas as “like a whole other country.” Texas definitely has one of the more distinc- tive characters of any of the United States, Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/141/03/38/6385413/me-2019-mar2.pdf by guest on 01 October 2021 known for its swagger, independent spirit, and economic vigor. If Texas actually were an independent country it would have the 10th largest economy in the world, just behind Italy. The state is also intensely identified with the energy industry. The oilman is a recognized Texas character, from fictional J.R. Ewing to real-life H.L. Hunt, Clint Murchison, and Sid SRichardson. For decades, one of the state’s NFL teams was known as the Oilers. While Texas is best known for its petroleum and gas, it is also rich in potential solar and wind power resources. According to the American Wind Energy Association, the state is first in the nation in installed wind power capacity, with 23,421 MW from 12,793 wind turbines that are now ubiquitous in the Texas landscape. This generates 62.2 TWh of electricity per year, or 17.4 percent of the power on the state’s elec- tric grid. The technically achievable wind power potential stands at a staggering 1,347,992 MW. The potential for in Texas is also enormous. In 2017, the solar farms in the state generated 2,119 GWh of electricity; accord- ing to a 2016 report from the National Renewable Energy Laboratory, Texas has more than 20,400,000 MW of potential utility-scale solar power, which could generate as much as 41,300 TWh per year. With all that potential, it is worth looking at whether it is technically possible to run the state’s electrical grid without emitting any carbon dioxide, using wind, solar, hydroelectric, and the existing nuclear power. Political writers like to classify Texas as a red state trending toward purple, but could it become in the future a green state? My colleagues and I have modeled the potential electricity supply and demand, and we have found that, yes, Texas could become a green state. It is not as simple as building out more wind turbines and solar panels. But because Texas is so large, the solutions for the reduction of CO2 emissions that may work here will also work without scaling in most of the developed economies.

ALL ITS OWN While Texas’s individual streak is famous, the state also goes its own way with electricity. Alone among the 48 contiguous states, most of Texas is on its own electric grid, separated from the large Eastern and Western Interconnections. The Electric Reliability Council of Texas (ERCOT) manages the production, distribution, and supply of electric power—357.3 TWh in 2017, more than Italy or the United Kingdom—to more than 92 percent of the state's inhabitants. In addition to wind and solar power contributions, nuclear power provides 11 percent of the annual electricity production, natural gas fuels about 38 percent, and coal another 31 percent.

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The lights of downtown Dallas reflect in the Trinity river. Texas consumes more electricity than Italy. Photo: Getty

That independent grid creates some problems for replac- consumers’ demand as the sunshine increases and decreases ing the carbon-generated electricity. Texas is big, but it isn’t on a daily basis. Once the market penetration of renewable so big that weather couldn’t affect most of the state at the wind or solar energy over the course of a year reaches levels same time, dimming solar power production with heavy higher than 30 percent, the supply of solar-powered elec- overcast skies or stilling wind turbines with calm air. Other tricity can encroach on the nuclear power output or even states faced with those conditions could draw wind power overwhelm the total demand during mid-day. Once the sun from the northern Great Plains or solar power from Nevada starts to go down and demand increases due to air condi- and Arizona. Texas will have to do without those out-of- tioners being turned on in the afternoon heat, the need for state resources. non-solar power ramps up extraordinarily fast. The situation is even worse with the intermittent wind power generation, which produces the rattlesnake curve The only way to achieve higher for the rest of the power-generating units in the area. It penetration of renewable energy is to would be impossible to operate nuclear units with this type mitigate the effects of the variability of electric power demand and without utility-level storage. In addition, the reliability of the entire electric power grid by storing energy. would be severely compromised. Similar problems with grid reliability arise when the penetration of renewables exceeds Being on its own grid presents another, almost opposite the range 20 to 25 percent of the annually produced elec- problem, too: oversupply. Increase wind and solar power tricity, not only in this region, but in most electric energy production enough, and on sunny or windy low-demand markets. days, the amount of power produced by nuclear and renew- It quickly becomes apparent that the only way to achieve ables could easily outpace the net demand. higher penetration of renewable energy in the ERCOT re- The swing from oversupply to undersupply of renewable gion is to mitigate the effects of the variability of the renew- electricity is captured in the so-called duck curve, which able energy sources by storing energy, when excess electric shows how the power production goes out of sync with the energy is produced, and using the stored energy at a later

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time, when the demand is high. Hourly demand versus generation Energy storage is a bit tricky in Texas. Pumped hydro total replacement of fossil fuel scenario storage requires a sort of terrain that is absent in Texas. typical week, mid-April And while battery storage may become an option in the 100 GW Electric demand future, the current generation of batteries is not suitable for Power surplus utility-level storage, especially for storing energy for several 80 Power deficit Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/141/03/38/6385413/me-2019-mar2.pdf by guest on 01 October 2021 months to better match the seasonal peaks of solar and wind 60 power with the seasonal electrical demand peaks.

Hydrogen storage is a more attractive option. The pro- 40 cesses of production, storage, and conversion of hydrogen are sufficiently advanced for hydrogen storage to be imple- 20 mented on a large scale: Production of hydrogen by elec- trolysis has been used for almost two centuries. Hydrogen storage tanks at pressures as high as 700 bar are currently 40 GW Wind power production

used by several commercial automotive companies that pro- 20 duce hydrogen-driven cars and buses. Fuel cells have also been used for almost two centuries to directly convert the 60 GW chemical energy of hydrogen to electricity. Solar power production To be sure, significant thermodynamic irreversibilities 40 occur during the processes of hydrogen production and hydrogen-to-electricity conversion in fuel cells. With cur- 20 rent technology, the round-trip efficiency of the electricity- to-hydrogen-to-electricity processes is in the range of 40 to Sunday Monday Tuesday Wednesday Thursday Friday Saturday 50 percent. Such losses would have to be accounted for. typical week, early August It is easy to envision a Texas-based grid that largely avoids 100 GW Electric demand the emission of carbon dioxide, relying on nuclear, solar, Power surplus wind, and hydropower for the generation of electricity, with 80 Power deficit mismatches between supply and demand mediated via the 60 production and consumption of hydrogen. In many respects,

electrolyzed hydrogen would replace natural gas. It could 40 be produced throughout the state, stored in local facilities, transported via pipeline as needed, with market prices used 20 to ensure the adequate supply of the gas in all the regions of the state during every season of the year. 40 GW Wind power production The question remains, though, whether such a system

would be practical. That’s where modeling comes in. 20

FOSSIL FUEL REPLACEMENT 60 GW Together with Matt Leonard and Dimitri Michaelides, Solar power production I calculated many of the main parameters needed for a 40 “green Texas” model. We started with the hourly electricity demand data of the year 2017 in the entire ERCOT system, 20 then added the hourly irradiance in the state (averaged over a period of five years) and the hourly available wind energy. Sunday Monday Tuesday Wednesday Thursday Friday Saturday The first, obvious question is, how can Texas forego the contribution of coal power plants that currently provide 31 Daily power demand and renewable power supply varies percent of the annual electric energy in the ERCOT region? hourly, daily, and seasonally. Here, the Texas-based Removing coal still leaves four nuclear reactors and a num- wind and solar production (plus nuclear and hydropower ber of gas-fired plants in the mix in addition to wind, solar, not shown) needed to replace all fossil fuel power is and a small amount of hydropower. Allowing the exist- matched against ERCOT demand, with hourly power ing nuclear and gas plants to operate as they do today, we surpluses and deficits identified. Data source courtesy author. looked at how much hydrogen storage would be needed to

0319_MEM_FEA_Green Texas.indd 39 2/6/19 1:40 PM Modeling shows that storing pressure 500 bar. That storage volume, 45,800 m3, is equal enough hydrogen is possible and not to about 1.6 million cubic feet, which is a rounding error compared to the 853,043 million cubic feet of natural gas even technically challenging. storage in Texas, as reported by the Energy Information Administration. account for the inevitable lulls in wind and overcast days. The intersection of weather and demand means that stor- Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/141/03/38/6385413/me-2019-mar2.pdf by guest on 01 October 2021 Modeling the minimum storage requirements is not age and depletion follows a regular cycle. Energy is stored straightforward. We had to account for thermodynamic primarily during the spring season and the energy is used in irreversibilities and the large fraction of lost electric energy the late summer, when the sun and wind supply less electric associated with the storage process, mentioned above. A power, but the demand is still high because of air-condition- second constraint is that at the annual minimum storage ing. (High temperatures in Fort Worth peak in early August, level, the storage systems should contain sufficient hydro- but that month has about as much daily solar insolation as gen to provide the grid during the next 10 days with the the much more temperate May.) surplus power that was taken from storage in the previ- We also calculated what it would take to substitute all ous ten days. That constraint allows the grid operators to fossil fuels—both coal and gas—with solar and wind to produce additional hydrogen or to purchase hydrogen in the power the ERCOT system, leaving only nuclear as the non- market in cases of spurious demand or system malfunction. renewable source of electricity. Obtaining agreement on The constraint also implies that the stored energy in the such a complete switch to renewables would be a formi- entire system does not reach zero at any time in the year, but dable challenge, as natural gas is produced in Texas and its maintains a minimum value. consumption has become a patriotic task for many Texans. Our hourly simulations of the demand and supply showed But modeling such a system is useful to determine the scale that, for the complete elimination of coal from the electric of energy storage needed to accomplish it. power production mix, the electric utilities will need to add To replace the 69 percent of electricity generation now 22,130 MW (rated) wind power and 9,320 MW (rated) PV produced by coal and gas power, utilities would need to capacity. In addition, approximately 45,800 m3 of hydro- add approximately 30,800 MW (rated) of wind power and gen storage capacity is needed to be developed in order 52,200 MW (rated) PV capacity, in addition to the existing to store an annual maximum of approximately 700 Mmol capacity. The needed hydrogen storage capacity is approxi- 3 H2 (equivalent to 46 GWh of electricity) at the maximum mately 15.9 million m , or 562 million cubic feet, that would store a maximum of 243,000 Mmol H2 (equivalent to 16,070 GWh) at the maxi- ourly energy storage level mum pressure of 500 bar. needed for total replacement of fossil fuel electric generation by As with the more limited scenario, windsolar in T region in this complete replacement scenario thousand h would see stored hydrogen accumulate in the spring—or more precisely, from Feb- ruary to mid-June—with another small bump up in autumn. A small fraction of the stored energy is used during the win- ter months when solar input is minimal, but most of the stored energy is spent in the summer, when the air-conditioning demand is high. From mid-June to late September, the storage system would dis- charge the full 16,070 GWh, or a bit less than 12 percent of the electricity demand during that season in 2017. MORE LIKE TEXAS An electricity system dominated by wind, solar, and storage would look quite a bit different from the current one. One an eb Mar pr May un ul ug ep ct ov ec major change would be the size and dis-

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Wind and solar power plants, such as this facility outside tribution of the facilities. Currently, coal and nuclear power Austin, Texas, can be more widely distributed than large, plants are scaled in many hundreds of megawatts, so only a central coal and nuclear stations. Photo: Getty handful are needed. There are less than two dozen coal and nuclear power stations scattered across Texas. Solar and wind power are diffuse, and to effectively Our modeling shows that storing enough hydrogen to capture that power, facilities need to be widely distributed. buffer a wind- and solar-dominated electrical system in Some places are windier and some are sunnier, so the largest Texas is possible and not even technically challenging. We wind and solar farms will be located there. But rooftop solar, did not calculate the cost of adding the green generating and for instance, will bring electricity production to nearly every storage capacity because prices drop precipitously when neighborhood, and will likely feed power into small districts such systems are widely used as household items—as the served by microgrids. history of the dramatic price reductions of refrigerators, To match this , hydrogen pro- microwaves, and personal computers has shown. We also duction and storage may also be distributed throughout did not consider any political resistance that might be raised the ERCOT system area. The estimated 15.9 million m3 of by owners of coal and gas power assets. hydrogen storage capacity needed to support wind and solar But if converting the electrical system to non-carbon power corresponds to approximately 1.5 m3 or 53 gallons per sources can work in Texas—and I believe it can—then it can household. One can imagine the electricity system shored be a model for most places on Earth. Texas may be a whole up with relatively small hydrogen tanks in each household. other country, but in terms of energy, most countries can be Texas is just one state of 50, but it plays a large and out- like Texas. ME sized role in the energy industry. And it consumes a lot of electricity—so much so that converting the state to “green” EFSTATHIOS E. (STATHIS) MICHAELIDES is the Tex Moncrief Chair of Engineering power production would reduce global CO2 emissions by at Texas Christian University in Fort Worth and the editor of The Journal of Non- 0.71 percent. Equilibrium Thermodynamics.

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