What can Sandra Bullock teach us about microgrids?

29 May 2019



The Bass Strait islands are world famous for a number of reasons, beef, cheese and seafood amongst them. However, maybe less well known is the fact that King and Flinders are both also world leaders when it comes to grid-scale renewable energy deployment. Both island’s power systems now have the capability to be 100% renewable. Which is remarkable really when you consider that not only do both of these islands have their own, relatively speaking, large scale industries, but also that power on the islands wasn’t reticulated until the 1980s!  

 

But how does it all work?

 

Working at Hydro Tasmania the concept of a microgrid is something that comes up a lot. Whether in the context of the zero diesel systems on King and Flinders or as part of the work that our consultancy arm Entura does in bringing electricity to remote islands in the Pacific. But how they actually work has always been a bit complex to try to explain, until now.

 

Recently a very clever engineer came up with an explanation that actually made sense to us non-engineering folk. So if you are one of those people who thinks the ability to have power at the flick of a switch is akin to some sort of magic let's see if we can shed some light on the inner workings of a renewable microgrid. 

 

Like any good explanation this one starts with a film analogy…

 

You know the movie Speed? Sandra Bullock, Keanu Reeves in a bus that can't go under 50mph or it will explode? Well, let's say the microgrid systems on the Bass Strait islands are like that bus. Only unlike the Speed bus, the microgrid bus (system) must maintain a speed (frequency) of exactly 50mph (50Hz) otherwise there will be an explosion (blackout).  

 


 

So on a flat road, the bus is traveling along doing 50mph and using 100 units of energy to maintain that speed. The bus starts going uphill and to stop the bus slowing down the driver, Sandra, has to put her foot on the accelerator. The bus is still going at the same speed but it is now using more fuel. So instead of using 100 units of energy, the bus is now using 150 units of energy to travel at 50mph.  The reverse happens going downhill – less energy is required.


This is sort of what happens in a grid system over the course of a day. The load on the system (amount of energy being used) will go up and down as equipment (kettles, heat pumps, lights etc.) get turned on and off. To ensure that there isn’t a blackout the system must maintain a frequency of 50Hz. This means the system is constantly adjusting the amount of energy it is using to match demand and maintain that 50Hz frequency. 

 

Now, some bright spark realises that this bus is being driven in one of the windiest places in the world. So to reduce fuel costs they decide to put a sail on the bus. This sail can provide 100 units of energy. But, unlike the energy that comes from the bus's engine, the energy from the sail is hard to predict and control. Whilst you can get a bit of an idea you can't say with absolute certainty that if the wind is doing X now in 30 seconds it will be doing Y. Moreover, it depends entirely on how strong the wind is blowing as to how much energy the bus will get from the sail. The wind might be blowing 1 unit it might be blowing 300 units. Either way the bus can only catch as much wind energy as it can use and as the sail is only big enough to provide 100 units of energy that is the maximum about that can be caught. 

 

Back to the bus, imagine there are now 50 units of energy coming out of the wind. This means the bus’s diesel engine can reduce its workload as the bus only needs 50 units of diesel energy to have enough units of energy (100 units) to maintain its speed of 50mph. Now the wind increases and all 100 units of energy can be provided by the wind. Excellent thinks Sandra, let's turn this noisy engine off. 

 

 


But wait...

 

The wind is unpredictable. One second it will be blowing 100 units the next it might drop to 90. And it takes time to start up a diesel engine so if the engine has been turned off and the wind suddenly drops in the 30 seconds it takes to start the diesel engine the bus’s speed will drop below 50 mph and the bus will explode! A running engine however can easily increase its output to ensure the bus doesn’t slow down. For this reason, in this example at least, there is always a bit of diesel left running, let's say 10 units (running at any less would be bad for the engine).

 

 


This bus is not only being driven in one of the windiest climates in the world but it is also quite sunny. So, if the wind isn’t blowing strongly enough to meet the bus’s energy needs there is a fair chance that the excess energy required could be provided by the sun. Along comes the bright spark and installs a solar panel. The panel is capable of providing 100 units of energy. Like wind energy, energy from the sun is hard to predict and control.  So now we have a situation where the bus it traveling along at 50mph, getting 90 units of energy from the wind and 10 units of energy from the diesel engine. Suddenly the sun comes out and, because of the new solar panel, it is providing another 30 units of energy to the bus. Even if the diesel engine drops right back to 0 the bus will still be getting 120 units of energy (90 units of wind + 30 units of solar). With 120 units of energy the bus will be going too fast on the flat road and will explode. 

 

 


To absorb this excess energy brakes are added to the bus. The brakes are instantaneous and can be put on a little or a lot depending on how much extra energy is in the system. Similarly, in a microgrid, if there is too much energy in the system an artificial load can be added to the system to absorb that energy and prevent the system exceeding 50Hz. As the energy that goes into the brakes (artificial load) is not used to generate speed it can be thought of as lost energy. 

 

Enter the bright spark who decides instead of losing that energy it should be captured. A battery is added to the bus. Now any extra energy can be stored in the battery.

 

 

 

However, as batteries can only take so much charge when the battery gets full, the excess energy must go back into the brakes. 

 

 

 


Now that the bus has a battery if the wind drops off by 50 units and the sun disappears again there is no need to increase the amount of energy coming from the diesel engine as any shortfall can be made up with energy from the battery, at least until the battery is empty.  

 

The ultimate aim of the microgrid systems on the Bass Strait islands is zero diesel operation. In zero diesel operation, the wind and sun provide all of the energy and any leftover is goes into the battery or the brake. Because the battery can instantaneously provide varying levels of energy there is now no need to maintain the minimum standby diesel energy and the bus’s engine can finally be turned off.  When the battery is close to empty or the energy available from the wind and sun drops significantly the diesel engine can be started again. 

 

But the islands aren’t buses, and microgrids, which in theory can be explained using 90s film references, are in actual fact fairly complex, expensive systems. And whilst for now at least traditional large scale grid connection is still the best way for most of use to keep the lights on, systems such those on King and Flinders island show the exciting possibilities in combining traditional generation with variable renewables.

 
 

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