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This post highlights three interesting insights from the 2017 IEA report Getting Wind & Sun onto the Grid.
1 – decorrelation of risks to grid stability
An electricity system operator faces different sources of uncertainty when managing a grid. Three of the major sources of uncertainty are:
1 – risk of losing generators or transmission infrastructure – either a large power plant tripping out or loss of part of the transmission system
2 – demand forecasting – electricity supply must be balanced with demand in real time. Backup plant will balance the grid in case demand is higher than forecast
3 – intermittent renewables – the weather dependent nature of wind & solar means that future generation is uncertain. Sudden changes in the weather can lead to dramatic changes in power generated by wind & solar plants. Backup plant must be kept spinning to make up for the lost renewables generation.
The IEA make the point that these three sources of uncertainty are not correlated. I would argue there is a degree of correlation between demand forecasting and the intermittent nature of wind & solar. Generation by low voltage plant like residential solar appears to the system operator as a reduction in demand.
I love the idea of optimizing over all three sources of uncertainty together rather than in isolation. By being smarter about how de-risk these uncertainties we can reduce the grid’s reliance on backup plant.
End consumers of electricity ultimately pay for all grid costs. Avoiding the installation, operation & maintenance of backup plant will mean cheaper & cleaner electricity for us all.
2 – dilemma of resource strength, co-location with grid and geographical smoothing
Optimizing the location of wind & solar plants requires balancing three factors:
1 – renewable resource strength
2 – cost of connecting to the grid
3 – geographical smoothing
Optimizing for resource strength means placing renewables where the wind is strongest or the sun shines brightest. A stronger or more consistent resource availability improves economics by increasing electricity generated. Unfortunately the best wind & solar resources often in remote areas. The cost of connecting to remote resources to the grid is higher.
Co-location with the grid minimizes the investment cost for renewables projects. The IEA quote a study that the cost of expanding the grid had a median value of around 15% of the cost of the generation capacity. Co-location with the grid will keep this cost to a minimum. It will also reduce electricity lost as heat during transmission.
Geographical smoothing is about spreading renewable generation over different areas. Short term variations in renewable generation in different locations tend to cancel out. Concentrating wind turbines in one place means that when the wind stops blowing there it has a major effect on the grid. By spreading the generation geographically, the output is more consistent.
Another concept related to geographical smoothing is technological diversification. This is the smoothing effect gained by diversifying between wind and solar. The IEA make the point that generation from wind & solar is often negatively correlated. This means that changes in solar generation are often balanced by changes in wind generation.
Optimizing the location of a renewables plant requires optimizing all three of these factors together. At first glance we should put wind turbines where it’s windiest – but perhaps a less windy location closer to the grid would be a better use of capital.
3 – technical tools to strengthen the grid
One of the major costs of integrating renewables is grid infrastructure. Renewables plants are often located where the resource is strongest – which can be far away from the transmission system.
The IEA highlight a few techniques to strengthen the grid at minimum investment cost. I’m going to detail two of them.
1 – dynamic line ratings
The capacity of cables to carry electricity is limited by temperature – if the line gets too hot you’re gonna have a bad time. This capacity is also known as the line rating.
Currently the rating of cables are often based on a fixed temperature. Fixing the temperature limits the capacity of the line to carry electricity. A dynamic line rating takes into account the changing ambient temperature. When it gets cold a dynamic line rating would increase the capacity of the cable to carry electricity.
The IEA quote a Swedish project where dynamic line ratings were used to identify a 60% increase in colder periods. These colder periods correlate with windier periods – meaning the increased capacity often coincides with lots of available wind power. I’m not sure if ‘identify’ means these savings were actually realized or just identified as potential savings. Nevertheless – 60% is a significant increase.
Interestingly enough it seems that forecasting for dynamic line rating is an active area of research. It’s another potential application of machine learning – similar to the forecasting of electricity demands or prices I discuss here.
2 – flexible AC transmission systems
This is the use of power electronic devices to improve grid power factor.
Power factor is the ratio of active to reactive power. Active power is useful – reactive power is not useful. But the cables in our electricity systems need to carry both simultaneously.
Reducing reactive power means more space for active power in the cables that make up the grid. The investment required is the solenoids or condensers that are installed midway on the line to absorb or inject reactive power. This is a lot cheaper than building new transmission lines.