The real problem begins to bite when the percentage of the base load supplied by the alternative sources starts to become a significant percentage of the total power level. The actual percentage will vary depending on the local electrical grid an how it is managed. But for the existing electricity grids, there comes appoint where they just can’t handle the fluctuations.
For wind and solar, we have the issue that the energy output fluctuates over both short and long intervals during any 24 hour cycle. Solar goes completely away at night and varies during the day due to clouding, shading and the angle of the sun to the panels. Wind varies due to gusts and average wind speed but it still blows at night.
And of course, the energy put back into the grid is happening on the downstream side of the low end distribution transformers. So we have 2 separate problems here.
Reverse power flow
The first is that the control systems and distribution infrastructure for the electrical grid were designed with the idea that the generators would create the power and it would be distributed outward from them and into the grid. When we add many widely distributed energy sources to the grid, the are not operating according to this principle and we don’t have as a good a control of the grid as we used to. While the level of renewable energy sources is a very low percentage of the grid power level, this is not a big problem. But if we want renewable sources like wind and solar to provide a significant proportion of the total power then we have a big problem. The current infrastructure can’t handle that level of power flowing the wrong way. This is one of the reasons for limiting the solar power level you can install on your roof. Even if you had a very large roof and could fit 100KW of solar panels, you would not get permission to do so because the local distribution transformer couldn’t handle the reverse power flow.So part 1 of the problem require an infrastructure rethink and quite a bit of equipment might need to be swapped out of service well before its projected operating life is up.
Controlling power
Part 2 gets even more interesting. Assuming we can now feed a large amount of power back into the grid from distributed fluctuating power sources, no-one knows how to control this! So there is a multi-billion dollar problem waiting to be solved. I can see quite a few PHDs and start-ups getting rich when they do solve it.As a digression, this could be an excellent opportunity for “Open Innovation”. This is the new buzz term for companies going outside their 4 walls for new ideas and includes a well developed framework for how to go about it so that everyone wins. A big problem in Australia is harnessing the resources of the academic world, the research world and the commercial world to come up with solutions for problems like this. What we tend to do is to set up a PHD there, a CRC here, an internal project there and make very little progress toward a commercially viable and implementable solution. An overseas investor notices and reaps the commercial outcome because they do know how to do the commercialisation. What is really needed is large scale collaboration which meets the needs of all the stakeholders. Dr. Sarah Pearson at ANU is spearheading Australian efforts to establish this collaborative framework in Australia. It has taken off in the northern hemisphere for good reason – it makes companies more profitable.
So back to part 2, how do we handle a significant proportion of the grid power being distributed, unpredictably fluctuating energy sources?
The European approach to this problem is to have smart meters (real ones) that can be told by the grid to dial back PV output to match the grid demand. So the idea here is to dynamically throttle the fluctuating sources to limit the level of fluctuation. This seems both overly complex and unlikely to be an ultimate solution. In Victoria, the real purpose of smart meters will be to implement ‘time of use’ billing where you pay more for power that is used during the higher demand periods. This will help flatten the peak usage profile but only if the cost differential is large and the usage made conspicuous to the user at the time of use. I’m thinking ‘In Home Display’ of the smart meter tariffs as they are being charged.
Microgrids is another idea that tries to generate the local energy locally and only import the shortfall. This attempts to solve the problem by keeping it local rather than impacting the whole grid. This seems unlikely to have the same efficiencies a large scale grid has and is more likely to be an interim measure.
One American approach is to add damping to the grid to absorb the fluctuations. A good example was recently covered on IEEE Energy looking at flywheels as a stability source. The best measure of grid stability is control of the frequency of the grid. As load goes up. Generators must either increase power output or slow down. And the reverse is also true where falling load leads to reduction in generator capacity or increasing frequency. The flywheels act as short terms energy sources when the frequency tries to drop and absorb energy when it tries to speed up. The real problem with electricity is storing it efficiently so it can be supplied on demand and the emerging flywheel industry is one approach to solving that problem. So this is much more likely technology direction than just throwing away the energy capacity.
What is needed in the long term is:
- a better control system strategy for managing the grid with fluctuating loads and distributed fluctuating energy sources
- efficient and clean large scale energy storage mechanisms
- cleaner base load energy generation
- reduced energy use and wastage
- make energy use and cost conspicuous to the user at the time of use
Engineer's Perspective: What limits distributed renewable energy uptake? (Part 2) Electronics News
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