Power Systems - It's a Renewable Thing

Part 2

Continuing our conversation about the technical issues with renewable energy as the only resource on our power systems. Hopefully this will generate some conversation going forward.

The main reason lithium-ion batteries have made such a strong market penetration is they are easy, and they are the lowest upfront cost battery solution on the market. A utility can connect a battery park to an existing substation bus in a matter of months, where it takes ten years or more to construct a pumped-storage hydro plant, so that makes them attractive. On the alternative side, the batteries have issues.

• Batteries upfront are very costly on a dollar per MW of storage comparison, so they require a lot of capital. As mentioned earlier, the service capacity starts to decline with age, and that decline is expediential as the battery ages, so the ROI and depreciation cycle is pretty short. It remains to be seen if once subsidies end if this is a sustainable technology cost wise.

• Despite claims that the issue has been resolved, troublesome reports of battery fires continue to surface. Unfortunately, once a lithium-ion battery enters a thermal runaway event, there is little that can be done other than let it burn out. If gases are vented from a thermal event, they are toxic and can result in evacuations and shelter in place orders, a PR nightmare for a utility. Reports of injured personnel or emergency responders are also still surfacing.

• Batteries have limited capacity; the present fleet has a 4-hour (when new) discharge time at full load. This is perfect for peaking when there are no issues with supply on the system, but they lack the performance needed for a long duration event. A 10-hour battery is in development which is an improvement, but the 4-hour fleet is already in service. Even ten hours is insufficient for a major weather event.

• Batteries are limited to nameplate; batteries cannot be overloaded even momentarily; this is also true of the inverters that work with them. Most traditional rotating equipment can absorb a significant amount of overload for short periods and some extreme overloads for momentary events. These overload capabilities are key to restoring system stability after a “contingency” on the Interconnection. Batteries are only able to help within their nameplate. NERC had to implement new requirements because initially they were designed to just trip.

Even with these issues I don’t see batteries going anywhere, they have proven to be too useful. I do think we are on the wrong battery technology track. I believe flow batteries are a better long-term solution.

• Flow batteries are not a fire hazard

• Flow batteries have a 25 year + life without losing capacity.

• Flow batteries can be rebuilt and components recycled.

• Battery capacity in storage is determined by the amount of liquid electrolyte stored, the energy the battery can produce in energy flow is based on the reaction membrane. These are separate independent components and do not need to be the same capacity.

However

• Flow batteries are still developing.

• Flow batteries have a lower energy density.

• Flow batteries at present are considerably more expensive than their lithium-ion counterparts.

• Flow batteries are complex, they have pumps, tanks, and sensors. Hoses are a potential source of leaks.

• Flow batteries will freeze if not heated

Only time will tell if flow batteries make it to the forefront, but they do represent a more typical long term utility investment.

I wanted to cover two more things; the first one is the thought that we can run the grid off renewables with batteries. Batteries need to be charged, and the load needs to be served. The sun only shines during the day when the load is high. This means you need even more solar capacity to charge the batteries and serve the load during daylight hours. You are also going to always need the battery to carry part of the load because it is the only frequency responsive, dispatchable source you have (you could part load solar, but then it’s not available to charge batteries). The wind blows when it blows, and when it doesn’t, it doesn’t.

Let’s also discuss inertia and renewables a little more. If you remember Power Systems – Big Iron Rolling, we talked about inertia, and why it’s important. The issue with wind and solar is they do not provide inertia to the system. For solar the reasons are pretty obvious. With wind it’s the way wind turbines are built. They are made to be extremely light. The generator is not a traditional synchronous generator, it is a small very high-speed machine with a permanent magnet rotor. The speed of the generator is not governed, and the output of the generator is rectified to DC. The inverter takes power off the DC bus and inverts it to AC power to feed to the Interconnection based on several sensors on the wind turbine. The intent is to pull 100% of the available energy out of the wind turbine all the time, if you don’t it will overspeed and trip. So, no inertia is available there. This leaves batteries, we are sure putting a lot of eggs in the battery basket. The battery inverters are programed to deliver synthetic inertia, basically go to full output when a rapid drop in frequency is detected. The issue is the battery has a finite amount of energy it can deliver, and it must wait for a control circuit to detect a problem, then react, which takes time.

3.2MW inverter

Poor inertia means larger excursions in frequency for the same size event. If inertia gets poor enough, we will start shedding load every time an energy source trips. Hawaii is already there.

I do want to remind everyone that one of the ways the renewable community wants to deal with the issues put forth in this article is by removing load when there is a capacity shortfall. Please refer to my publications “What is Demand Response and How Are Virtual Power Plants Connected Part 1” and “Part 2” regarding how load control might be handled. In addition underfrequency load shedding is considered a viable operational solution with a renewable only interconnection in some circles.

This completes this series, let me know if there are other rabbit holes you want to to go down like voltage control, and power flow control, just leave me a comment on what is your point of interest. I hope you enjoyed this information.