The Electric Reliability Council of Texas (ERCOT) grid is largely isolated from other grids in the USA and generates as much electricity as the UK. Here we take a look at whether ERCOT could eventually generate most or all of its electricity from intermittent renewables, specifically wind and solar. We find that the prospects for 100% renewables generation are effectively zero, and for the same old reason – the requirement for prohibitive amounts of energy storage. High percentages of renewables generation might be achievable with large amounts of dispatchable backup capacity, but the system would be highly inefficient and it is questionable whether this capacity could handle the high ramp rates that would be needed to balance the erratic wind and solar input.

Sources of data:

This post was prompted by a recent article by Peter Davies that was recently published on Judith Curry’s Climate etc. website. This article, entitled Electricity in Texas: is 100% renewables feasible? Part I presented the results of a detailed spreadsheet model analysis of ERCOT data that I don’t propose to discuss here. The analysis was, however, largely probabilistic – no plots of grid data were provided. And because such plots are important if one wants to gain a more complete understanding of how a future generation scenario might work in practice, here I provide some.

The data used are from two sources, both linked to in the Davies post:

The data are for the years 2010, 2011 and 2012, modified to simulate unspecified future years by factoring up demand relative to higher assumed peak loads and by assuming 75GW of wind and 80GW of solar capacity. (Currently ERCOT has about 18GW of wind capacity but only about 700MW of solar.) I then added one further adjustment by factoring wind and solar output upwards by about a third so that total generation was equal to total demand.

After this many manipulations these data cannot of course be considered real – they are no more than indicative. But any simulation of a future scenario can’t be considered any better than indicative anyway, so we are not losing much by using them.

ERCOT statistics:

The ERCOT grid is notable in that it exists to provide power only to Texas. It is at best poorly interconnected to the rest of the US grid, meaning that ERCOT can effectively be considered an island from the electricity standpoint. ERCOT covers about 70% of Texas and generates about 90% of all the electricity consumed in the state. It serves 24 million customers – less than half of the number served by the UK National Grid – but because annual per-capita consumption in Texas (~14,000 kWh) is more than twice that in the UK (~5,000 kWh) the ERCOT service area consumes as much electricity as the UK (352 vs. 347 TWh in 2015).

ERCOT’s installed capacity at the end of 2015 and generation by source in 2016 is summarized in the Table below. The 17,000 MW of wind capacity (up to 18,900 MW at the end of 2016) is the most installed in any US state. “Other” contains about 700MW of solar:

Figure 1 shows the extent of the ERCOT grid:

Figure 1: The ERCOT grid

In 1999 Texas adopted a target of 10,000MW of installed renewable capacity by 2025, which has already been comfortably exceeded. The target has not been revised since.

Data Review:

Figure 2 is a stacked bar chart that plots average daily wind and solar generation and average daily demand for the adjusted data set described earlier (the original hourly data are converted into daily averages to make the chart readable). Total wind plus solar generation is equal to total demand over the period considered. The 2010, 2011 and 2012 year designations represent the years from which the data were developed and are not representative of actual generation and demand in those years:

Figure 2: Wind and solar generation matched to demand, daily averages, 100% renewables model. Note that the month markings are not exact.

Combined wind and solar generation show considerable day-to-day fluctuations but no strong seasonal pattern. Demand, however, is significantly higher in the summer, with peak demand usually occurring in August (because the data are daily averages hourly fluctuations in demand, which can reach 40MW in the summer months, are not visible). An exception to this trend was the demand spike in February 2011, which coincided with the descent of an Arctic air mass into Texas that caught ERCOT with its pants down .

… extreme cold weather pushed power demand to very high winter levels. At the same time, fifty of the state’s power plants were offline due to the effects of the cold, and several others were undergoing planned maintenance. The combination of very high demand and reduced supply left the ERCOT grid perilously short of reserves. Rolling consumer outages were employed to protect the system from failing completely.

For ERCOT to be powered 100% by renewables combined wind & solar generation must be matched to the Figure 2 demand curve. The differences between the two, expressed as surpluses and deficits, are plotted in Figure 3. Daily deficits of up to 40GW occur in the late summer months and during winter cold spells. Surpluses usually occur in the autumn and late spring:

Figure 3: Surpluses and deficits derived from Figure 2 data, daily averages, 100% renewables model.

There are two ways of removing these surpluses and deficits and matching combined wind and solar generation to demand. The first is by storing the surpluses for re-use during deficit periods. To estimate how much storage would be needed to do this I accumulated the surplus and deficit values, obtaining the results shown in Figure 4:

Figure 4: Cumulative energy storage requirement derived from Figure 3 data, daily averages, 100% renewables model.

At this point I stopped, wondering if these numbers could possibly be correct. After running some checks I found that they were. As much as 50 terawatt-hours of storage would be needed to balance ERCOT’s wind and solar generation against ERCOT’s demand over the three-year period considered in this 100% renewable scenario. I varied the proportions of solar and wind generation to see if this might lower the storage requirement but found that it made little difference. And to put the 50TWh number in perspective, it’s equivalent to about 5,500 Dinorwig-sized pumped hydro plants or five billion Tesla 10kWh wall-mounted powerpack units.

The second way of matching combined wind and solar generation to the demand curve is to curtail surplus renewable generation and use backup dispatchable capacity (presumably CCGTs) rather than storage to cover the deficits. This of course results in less than 100% renewable electricity, but the case summarized in Figure 5 still achieves 86% renewables penetration with less than 15% curtailment:

Figure 5: Wind and solar generation sent to grid (light blue), curtailed (green) and backup generation requirement (dark blue) using backup generation model, daily averages.

The problem here, however, is the stress placed on the backup generation. Figure 6 plots the average daily backup generation requirement shown by the deep blue shaded areas in Figure 5. The backup CCGTs are clearly going to have to jump through hoops to keep up:

Figure 6: Backup generation requirement derived from Figure 5 data, daily averages.

And the problem is actually considerably worse than shown because Figure 6 is based on average daily generation and does not allow for fluctuations in demand during the day. Plotting the hourly generation for August 2011, the month which according to Figure 5 has the highest backup generation requirement, gives the results shown in Figure 7:

Figure 7: Wind and solar generation sent to grid (light blue), curtailed (green) and backup generation requirement (dark blue) using backup generation model, hourly data, August 2011.

And the backup generation requirements for the month are plotted separately in Figure 8:

Figure 8: Backup generation requirement derived from Figure 7 data, hourly data, August 2011.

The Figure 6 daily data indicate a requirement for maybe 50GW of backup capacity, but the hourly data indicate a requirement for at least 80GW, and were we to go to five-minute data it would increase even more. And so would ramp rates, which already reach 34.5GW/hour – a rate which makes the California Duck Curve look flat.

Conclusions:

Because of the liberties taken with the data the results discussed above can’t be considered particularly diagnostic, but they are diagnostic enough to allow us to conclude that running the ERCOT grid on 100% intermittent renewables will not be possible because of the prohibitive storage requirements, thereby adding yet another case to a growing list. The best ERCOT can hope to do is incrementally increase the percentage of renewables in its energy mix, filling in deficits with load-following backup generation, until some limiting economic or technical threshold is reached, much as discussed in this previous post. But this threshold will probably be reached at a fairly low level of renewables penetration. And while US natural gas prices stay low – and in the absence of any government-mandated renewables targets – there will be little incentive for ERCOT to increase its renewable energy share anyway.