This interesting paper, by a team of North American and Chinese academics, says that ‘extended and unexpected periods of extremely low wind and solar resources (i.e., wind and solar droughts) pose a threat to reliability’ and it adds that ‘the challenge is further exacerbated if shortages of the two occur simultaneously or if they affect neighboring grids simultaneously’. It propose three metrics to comprehensively assess renewable energy quality: resource availability, variability, and extremeness.
The paper, published in Nature, says that in China and many other countries, the resource availability ‘has traditionally been the decisive metric for renewable energy project development.’ But renewable availability can and does also vary over time, so ‘variability’ is a second key metric. And extreme wind and solar ‘droughts’ can sometimes coincide locally. So it also proposes a third metric, ‘extremeness’. Based on its mapping of China, it says that ‘at many sites in China, the availability, variability, and extremeness of renewable energy tend to manifest in undesirable directions. But not everywhere - and there are remedies.
For example, the paper notes that China already requires short term (1-4 hour) storage be installed, with a minimum power capacity of 10–30% of the solar or wind power capacity, depending on the location. However, it says developers should ‘also consider long-duration energy storage technologies or alternative solutions’. It also notes that ‘our findings on the spatial and temporal co-occurrence of energy droughts indicate that some regions could share their long-duration energy storage’. And, in addition, ‘regions where the droughts of solar and wind are not likely to coincide, could balance their approach with both resources’.
It does seem that renewable resource availability and quality are quite variable in China. The paper says ‘In some cases, wind and solar energy droughts coincide across large regions, causing severe impacts on energy systems’, and adding that these renewable energy droughts ‘may threaten efforts to accelerate renewable energy deployment by sapping investor, public, or policy support for these technologies’
As the study notes, the overall analysis can be quite complex with mixed solar and wind systems: ‘wind energy generally has higher resource availability than solar, but it also has higher variability and extremeness. However, there are more sites with an overall acceptable wind energy quality than solar. Most of the sites with high solar availability also have high variability, while more sites have high wind resource availability combined with low variability. This does not even consider the diurnal nature of solar energy, which is also an important disadvantage of this resource relative to wind.’
Moreover, in practice, it can be a quite hard to predict how all the various factors will interact (wind and solar droughts often occur at different points in the year) and exactly why, for example, ‘renewable energy droughts cluster in areas which generally have higher resource availability’. It also says it may be the same in Europe and elsewhere. For example ‘prior research has indicated a correlation between renewable energy droughts and the presence of extensive high-pressure systems over central Europe’. But it says more research on this is needed.
China is of course vast, covering multiple weather/climate zones, but so does Europe and the USA. There have been several studies of seasonal renewable variability and its impacts in the EU, US and elsewhere, as well as studies of technologies for dealing with the resultant intermittency- see my 2016 IoP book for an interim review. Options include batteries, smart grids and demand management for short term balancing; pumped hydro and compressed/liquid air for intermediate storage; and hydrogen production using green power surpluses and then its cavern storage for use for longer term balancing. Long distance power exchanges via HVDC supergrids are also possible, something China has been developing, along with some of the other options.
Grid balancing has become a key issue, with many scenarios now available. For example LUT University in Finland has been developing balanced 100% renewable scenarios for individual countries as well as globally, with its most recent study looking at inter annual balancing - this being a potentially larger and much tougher nut to crack than just dealing with seasonal variations. To help out, LUT looks at, amongst other things, the use of methane and methanol as storable e-fuels, but also possibly to ammonia.
Large scale storage of bulk energy between years would be expensive, as LUT noted, but they say might be viable with these liquid or gaseous e-fuels. If not, then, it says, recourse might be made to trading these fuels. In the Chinese case, in additional to internal power and gas trading between regions, it might mean importing surplus e-fuels from other countries and hopefully exporting its own occasional surpluses to offset the cost. Arguably, that would make more sense than building extra coal plants to meet very occasional annual shortfalls.
Apart from installing some local storage, building more coal plants seems, so far, to have been China’s main initial approach to seasonal or local shortfalls, since, unlike the EU/UK and USA, it has no fossil gas reserves. And balancing shortfalls is also sometimes said to be why more nuclear plants are needed. But quite apart from other issues, as renewables become even cheaper, running new coal or nuclear plants part time would make little economic sense. Far better to have a few cheap gas turbines in reserve using stored and/or imported green fuel or perhaps power, particularly in the case of responding to occasional inter-annual variations.
All of this though will still add to the costs, but some extra costs seem unavoidable, although new power management and trading systems may reduce overall system costs and improve energy-use efficiency once established. China has already been strengthening its long distance super grid transmission system to reduce its wasteful renewable supply curtailment problems, and as the new study of China suggests, it will have to do even more of that, as well a building much more long term storage, to deal with the coincidence of seasonal droughts. It warns that ‘power system planners and operators must consider the possibly disastrous consequences of spatially concurrent renewable energy droughts when deploying renewable facilities.’ And dealing with that, amongst other things, required ‘a robust and interconnected power grid with increased transmission capacity, flexible resource sharing at the national level, and a national integrated dispatch mechanism’.
Moreover, judging by the LUT study, China seems likely to have to do yet more, for example in terms of very long term storage and internal (or external) green fuel or power trading, to deal with inter-year balancing. The trading options could add even more to the overall cost of energy, depending on the value gained from the trade. So could very long-term storage. But, as also is the case with seasonal balancing, the use of surplus green power would mean there would be no extra fuel cost (it has already been produced), even if the storage/conversion technology has to be built.
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