Transition to renewables increases winter reliability risk

By Karl Kohlrus, P.E.

The transition to renewable energy—solar, wind, and battery storage—is creating a cleaner generation portfolio but also adding much complexity to generation and dispatching flexibility. This creates increased electric system reliability risk, especially during winter when renewable resources are less available.

Unlike conventional power units, the output of renewable generators may be strongly influenced over a wide geographical area from natural phenomena. Common weather events, such as long winter nights, a string of calm, cloudy days, or even a snowstorm, which may only marginally affect conventional units, if at all, can cause large decreases in intermittent generation output.

This article examines some of the most important aspects of these issues, and it outlines some of the economic concerns as our society moves further from fossil fuels as a means of generating electricity.

Winter wind droughts could become catastrophic

A winter “Wind Drought” over a wide geographic area, such as the one that impacted the 15-state Midcontinent Independent System Operator (MISO) region on January 28-30, 2020, could lead to future capacity shortages. During the 2020 event, wind output was less than one percent of nameplate for 39 consecutive hours (Figure 1).

*Figure 1. Graph showing capacity loss during a wind drought in January 2020.*

At that time, the system had 20.2 GW of nameplate wind capacity but lost nearly all of it during the wind drought. This occurred during what is typically the coldest time of the year. Imagine a night during a future wind drought where there is no solar, little wind and depleted batteries.

Another example comes from Winter Storm Uri, as shown below.

*Figure 2. Effects of Winter Storm Uri on wind capacity.*

Reduced renewable capacity, combined with inadequate freeze protection in some locations, caused rolling blackouts and even long-term power outages during a period of extreme cold weather.

Winter brings decreased solar availability

As winter advances each year, the days grow shorter and nights longer with increasing latitude (Figure 3).

Figure 3. General illustration of illumination at the summer and winter solstice for the U.S. and Canada.

The combination of shorter days with widespread cloud cover and fog, as is often experienced during winter, greatly decreases solar output. Furthermore, snow cover, especially in the northern parts of the United States, can impact wide areas, reducing solar output for days or even weeks in some locations.

Combined wind and solar intermittency can bring substantial capacity risks. Below are some general statistics from the mid-latitudes of the United States:

Wind operates at roughly 40+% capacity factor.
Solar operates at roughly 20% capacity factor.
Wind is below 20% capacity ~20% of the time.
Solar is below 20% capacity ~73% of the time.
Combined wind and solar is below 20% capacity ~11% of the time.

Any calm night or calm cloudy day can create a potential capacity shortfall. This may become increasingly critical as we move away from conventional generation resources.

Increased winter load during extremely cold weather

During cold waves, the load curve both increases in magnitude and flattens due to increased heating demand. This is evidenced by the actual MISO load curve during Winter Storm Elliott shown in Figure 4.

*Figure 4. MISO load curve for December 23, 2022 during Winter Storm Elliot.*

Such uniform high load factors leave little room for charging Battery Energy Storage Systems (BESS) or electric vehicle batteries. The changeover from natural gas to electrical home heating systems, as is advocated by some, will further increase and flatten the winter peak load curve per the energy needed during nighttime hours.

Electrification impacts

Electrification, in the context of this article, is defined as electrifying home, commercial, and industrial building heating systems and providing the energy for automobiles and trucks. Continued conversion to electricity will result in very large increases in both annual energy use and peak demand, especially winter peak demand.

Three main choices exist for residential and small commercial electric heating systems:

Resistance heat: Resistance heat is 100% efficient; each kWH provides 3413 BTU of heat.
Air source heat pump: Air source heat pumps are usually twice as efficient as resistance heat. However, efficiency decreases with decreasing ambient temperature. Backup resistance heating is usually necessary at temperatures below ~20^oF.
Ground source (geothermal) heat pump: Ground source heat pumps are approximately three times as efficient as resistance heaters. However, they are also the most expensive configuration per the need for subterranean ground loops. Supplemental resistance heat is usually necessary when ambient temperatures reach 0^oF.

During the extreme cold of winter peak days, most electric heating systems will operate in resistance heat mode. This in turn will significantly increase the demand, creating a “needle peak” electric demand. The load curve shown in Figure 4 will become even flatter and grow in magnitude as electric heating systems run nearly continuously. For example, consider the changeover from a common 100,000 BTU, 95% efficient gas furnace to an air source heat pump with 25 kW of backup resistance electric heat. The gas system would have provided 95,000 net BTU of heat per hour to the house. On a winter peak day, the electric-resistance backup would only provide (25 kWH) (3413 BTU/kWH) = 85,325 BTU/H, much less than the 95,000 net BTU of the gas system. Thus, the resistance heat would likely run continuously, increasing the peak-day load and load factor.

Another serious issue with electrical heating is the high cost of electricity relative to the price of natural gas. Consider a natural gas cost of $1.00/therm and electricity cost of 10 cents per kWH, both very reasonable values. The gross cost of gas heat would be $10.00/MMBTU, while the cost of resistance electric heat would be approximately 3 times higher, $29.70/MMBTU. Not to be discounted are the high capital costs of service upgrades, new HVAC systems, new appliances, and wiring upgrades. The utilities’ distribution and transmission systems would also likely need to be upgraded which may result in higher electric rates. These costs may make electrification prohibitively expensive and uneconomical.

Beyond residential and commercial heating issues, the increase in the number of electric vehicles will require added electricity for charging. During cold weather, batteries are less efficient and require additional charging. This issue is greatly exacerbated by electric heaters (mostly resistance type) that rapidly consume EV battery capacity and reduce driving distance between recharging.

Battery energy storage has limitations

A major factor that still limits renewable energy development is electrical storage. For example, a typical lithium-ion battery energy storage system (BESS) has approximately 85% cumulative efficiency. Thus, a standard 100 MW, 4-hour BESS needs 400 MWh to charge but only returns 340 MWh to the grid upon discharging. Furthermore, a BESS requires auxiliary power, even in standby mode, to maintain temperature. Therefore, batteries are a net load on the system.

For comparison of BESS capacity to coal storage, consider a typical 500 MW coal unit burning Illinois coal with 30 days of coal storage. Suppose the unit operates at 70% capacity factor. It would produce (500 MW) (0.70) (720 H) = 252,000 MWh of energy per month. If the unit had a net heat rate of 10,000 BTU/kWh and burned coal with a heating value of 10,000 BTU/lb, the required coal storage becomes 126,000 tons. (252,000 MWh * 10,000 BTU/Kwh * 1000 kWh/MWh * 1 lb/10,000 BTU * 1 ton/2000 lb = 126,000 tons.) At $50/ton, the inventory value of this coal reserve would be $6.3 million.

Now let us calculate the number of batteries needed to store the same amount of energy. Assume 100 MW, 4-hour lithium-ion batteries with 85% efficiency and two charging/discharging cycles per day. Each battery would produce 680 MWh/day and 20,400 MWh per month. Thus, it would take 13 batteries (rounded up from 12.353) to store the same energy as available from the coal storage.

It is also important to point out that the battery system does not, in fact, produce the electricity. The production must come from another source. The 252,000 MWh supplied by the batteries would require 296,471 MWh for charging. The energy losses calculate to 44,471 MWh per month. Assuming a capital cost of $1000/kW for each BESS, the 13-100 MW batteries would cost $1.3 billion. This is 206 times the cost of coal energy!

So, while retiring fossil fuel plants is a primary goal in the efforts to slow down climate change, the economics are very problematic without significant improvement in energy storage technologies. Also critical, as the discussion below further illustrates, is grid reliability.

Future renewable risks

The combination of lower solar output due to shorter and cloudier days and snow-covered panels in certain locations, limited storage capacity and duration, and the possibility of extended wind droughts over large areas can put the electrical system at great risk during the winter. Conversely, renewable sources tend to create the most energy during low load periods. This leads to excess electricity that must be stored, transmitted, or curtailed.

Possible solutions

Build a more robust transmission system including interregional high-voltage direct current (HVDC) ties joining the Eastern Interconnection with the Western interconnection and Texas Interconnection.

–Opens new and diverse markets for sales and purchases.

-Geographic diversity lessens the impacts of winter storms.

2. Dedicate some renewable energy for electrolysis of water into green hydrogen and oxygen.

–Don’t connect these units to the electric grid.

–Solves interconnection queue backlogs.

–Solves congestion problems.

3. What to do with the hydrogen?

–Store it for later or even seasonal use.

–Use it at the point of production (Advanced hydrogen-fueled combined cycle plants, Reciprocating internal combustion engines, Fuel cell power plants, Fusion reactors, Fuel cell EVs)

-Transport it via pipelines.

Conclusions

The transition to renewables still requires a fleet of conventional, dispatchable resources for grid reliability. Keeping some coal plants around with their valuable but inexpensive coal storage reserves may not be a bad idea. Carbon capture and sequestration (CCS) may evolve into a viable method to continue fossil plant operation, but many questions exist regarding CCS technology and long-term influences of carbon sequestration.

References

MISO is the Midcontinent Independent System Operator www.misoenergy.org

Figure 3 comes from daylight hours map – Search Images (bing.com)

About the Author: Karl Kohlrus, P.E. graduated from the University of Illinois at Urbana-Champaign with B.S. degrees in Engineering Physics and Electrical Engineering and a master’s in business administration. Karl worked for 31 years in the Planning Department at City Water, Light and Power in Springfield, Illinois performing generation and transmission planning studies. He has since worked for over 12 years as Planning Engineer at Prairie Power, Inc.

Contact information:

Email kkohlrus@comcast.net

Phone 217-891-4870