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| 03/21/18

Widespread solar penetration in BC would impair flexibility of the grid


In recent years, both the province and its largest city, Vancouver, have adopted ambitious targets for eliminating emissions from new buildings by requiring that they use no more energy for things like heating and appliances than they can produce—otherwise known as ‘net-zero’ or ‘zero-energy’ buildings. The province wants this achieved by 2032, and the City of Vancouver wants to get there two years earlier.

In addition, Vancouver wants all energy consumed in the city to come from renewable sources by 2050, which includes the energy that is used to heat existing buildings, not just new ones.

It is widely envisaged that these targets will be achieved by the electrification of heating and increased on-site production of renewable electricity to service this new demand. Given how easy and cheap it is to stick solar photovoltaic (PV) panels on rooftops, this option is an attractive one.

Most of these PV systems would be integrated with the electricity grid as it is very difficult and expensive to be entirely self-sufficient with solar power without some sort of back-up. This however means that we now need to consider effects of integrating rooftop solar on the grid as a whole.

When the sun pops out from behind a cloud, there is a sudden and unpredictable sharp rise in the electricity generated by such PV systems. This increase in electricity is instantly added to the amount of electricity already being generated on the grid. If the amount of electricity produced across the province is greater than the amount of electricity needed, some generators of electricity must quickly reduce their own production of electricity so that supply and demand balance out.

It’s pretty easy to understand why insufficient electricity might be a problem, but why would too much also be a bad thing? Doesn’t that just make things cheaper?

Electricity operates slightly different to other commodities with respect to supply and demand. While the grid can tolerate a little bit of mismatch, if it gets too out of whack, this begins to negatively impact voltage and frequency. In the worst case, too much or too little electricity can damage generators, setting off a chain reaction that results in blackouts. To avoid an excess, some electricity production must be reduced or turned off. This is known as ‘curtailment’ by energy systems professionals. And when there is insufficient supply, generation needs to be quickly ramped up.

In sunny California, utility-scale solar PV penetration has reached 11 percent of capacity (potential to produce electricity) and six percent of actual generation. In the late afternoon when the sun starts to go down but people come home from work and turn on all their appliances, the state on average faces a rapid increase in net demand of about 11 GW between 1pm and 8pm. For comparison, that is equivalent to all the hydroelectric stations in BC ramping up from zero to their max output in an afternoon. Hydroelectric generators are famous for their flexibility, but even they would have trouble making this sort of jump, not just due to mechanical issues, but also due to what would happen to river quality.

Researchers with the Pacific Institute for Climate Solutions 2060 Energy Future Pathways team wanted to find out what sort of impacts there would be if BC provincial and municipal climate targets led to rooftop PV penetration that soared to levels approaching those in California, and beyond.

They considered what would happen if every single detached home in the province’s four major cities, Vancouver, Victoria, Kelowna and Prince George and their surrounding areas, installed PV systems on their roof on a typical August day (using 2016 data).

They also considered different sizes of rooftop PV system (ranging from a small one at 2.5kW up to a large one at 10kW), which corresponds to three different levels of capacity penetration. The bottom end scenario, 8.5 percent capacity penetration, is similar to California’s 11 percent, while the upper end scenario represents a 34 percent capacity penetration.

The good news is that when penetration is at the bottom end (8.5% penetration), the system retains a decent balance.

But at the upper end of penetration (34%), there is a substantial challenge from over generation. This excess would have to be curtailed, sold or stored.

In addition, there are seasonal complications in the province. We tend to think of our hydro system as the ultimate in flexible energy storage. But in fact, it does have its limits.

There is a peak in hydro generation in BC at the start of the summer. This is due to the ‘freshet’, or melting of the snowpack that boosts river flow. The hydro stations have a certain amount of ‘must-run’ generation—a minimum level of energy production that must be maintained at all times to ensure system reliability and stability. And the volume of must-run generation increases during the freshet to respond to this annual spike in river flow. This is because of the two types of hydroelectricity we have in BC: large-scale storage hydro and small-scale ‘run-of-river’. The former is flexible over the course of the day and year, but the latter depends on the flow of the river. Run-of-river cannot be controlled. There is no flexibility. Solar PV also peaks in the summer, so the greatest over generation occurs at the same time that hydro is least flexible.

BC 2016 balancing authority load data with solar penetration scenarios yearly generation stacked on top of average monthly minimum allowable hydro generation (must run generation) in BC

Depending on the time of day, demand in the summer in BC fluctuates between a 5000MW minimum and an 8000MW maximum. Meanwhile, at any one time, the excess produced under the 34 percent penetration scenario could hit almost 8000MW, although an overshoot of 6000MW would be more frequent.

Put another way, the amount of excess power the province would be producing in the summer as a result of solar PV, at times when home heating isn’t really needed, ranges from 75 percent to 160 percent of demand.

One response to this challenge frequently mentioned is storing the excess in batteries, but these generally cannot store much longer than a few hours or a day, and so cannot provide seasonal storage. Another option is to sell the excess to other jurisdictions, but these jurisdictions may also find that they have an excess to sell at the same time. Use of the excess to produce hydrogen or other synthetic fuels that can be used later is yet another possibility. The 2060 team researchers are currently assessing the efficiencies of this latter option.

As a result, the researchers conclude that at modest penetration rates (such as the low-end scenario of 8.5%), solar PV can potentially offer a real low-carbon benefit to BC. But at the high penetration rates, ensuring a balance between supply and demand becomes quite difficult for the grid, even for one that has ample hydroelectric infrastructure.

This is particularly challenging, as the solar PV penetration rate required to meet the net-zero building targets will likely be higher than 8.5 percent. The researchers are currently working through a massive amount of data to simulate what a province-wide net-zero scenario might look like and find out what that percentage will be.

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