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Tuesday, June 22, 2021

Virtual Power Plant Performs Suboptimally During A Heatwave

UPDATE (6/27/2021): Title updated for accuracy. Details at the end.*

We had our first real virtual power plant (VPP) event and it didn't go as intended. 

The point of a pilot project is to "learn by doing" on a small scale. Lessons learned on a small scale can prevent problems in a bigger program later; so, from that perspective, this was a victory. 

Before getting too much further into this, I should explain what happened. 

On June 21st, one of the longest days of the year, we were having a heatwave here in the Northwest. In response, Portland General Electric decided to put its new VPP into action. Our batteries would be discharged to help offset the expected increase in air conditioner use. Perfect, this is why we signed up. If this helps the utility avoid using diesel generators and peaker plants, that's great.**

On the surface, this seems like a great plan. At 5PM PGE is going to take over the battery's operation. BUT two hours before, at 3PM, peak time starts. Our battery is configured to discharge during peak hours and remove our home's load from the grid. So at 3PM, the battery responds as expected and our home is off-grid (sometimes referred to as islanding). Actually, our home is better than off-grid. The battery is running our house and the solar panels are feeding the grid. 

Then at 5PM, PGE takes over operation of the battery. Up until this point, the battery had been discharging at a rate of about 7 to 10kW (adjusting up and down with our home's needs). When PGE took over, they had the battery discharging at a steady 2kW (see graph below).

Home Energy Flow: (grey is the grid, green is Powerwall, yellow is solar)

This was 5 to 8kW lower than it had been discharging. This increased the grid load, exactly the opposite of the intention of the program.

Looking at the graph, you can see that after ~9AM, when the battery was full, our home became a negative grid load. Our solar panels generated enough to run our home, air conditioning and all. In the times the AC cycled off, we were feeding the grid. Then starting at 3PM our battery took over and we continued to be a negative load. It was not until PGE took over at 5PM that we started to use energy from the grid and add to the demand. 

If our battery had been in standby/backup mode, just sitting at 100% charged up, waiting for an outage, then this VPP plan would have worked fine. However, that was not the case. 

The SmartBattery program needs to add another level of "smarts". For example, setting up the battery to discharge at least 2kW, that would have worked better. Alternatively, they could have requested that the battery discharge 2kW more than the home required, thereby guaranteeing some level of feed-in. Perhaps the simplest option would have been to have the battery discharge at a higher level, e.g., 8kW. This is well within the 15kW that our system can sustainably supply. 

It may be that such modes are not possible with the APIs available to VPP operators. Requesting 8kW works fine for our system, but if an owner only has a single Powerwall, 8kW is not an option. The VPP does not currently customize the request for each home. Tesla Powerwalls are not the only home battery system in the mix, so they may need to adhere to a lowest common denominator mode... 

It could be, that the net result was still positive, just sub-optimal. For example, say there are 500 homes in this pilot. 50 are in a state similar to mine. Each added an average of 4kW of load to the grid for a total of 200kW more load. The other 450 homes, however, added 900kW of relief to the grid. This means that the VPP added 700kW of net relief to the grid. Still a net gain, but not as good as adding 900kW or more grid relief.

It looks like it is going to be a hot one this summer, so you can expect that this will not be the last VPP call-to-arms. Perhaps they will make some improvements before the next event.


* UPDATE1: The initial title was "VPP Fails During A Heatwave". As I explained in the article, the VPP didn't perform as intended at my house, but that does not mean that the VPP as a whole failed. Some readers, rightfully so, called me out on this clickbait characterization and I've updated the title to be more accurate and less clickbaity. One other minor update: the original article referred to June 21st as solstice. June 21st is often the solstice, but this year, in N. America, the solstice occurred on June 20th.

** Sidebar1: Global warming is causing hotter summers, which increases energy demands, which (when energy is sourced from fossil fuels) increases emissions, which increases global warming... This feedback cycle can/must be broken. Summertime is when the sun shines and solar energy production scales well with AC usage. Combine this with just a few hours of energy storage and you can time-shift loads as needed to stabilize grid demand

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