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Tag Archives: variable renewables

Supergrid (2) – could it work?

By Dave Elliott

In my previous post I looked at the potential and problems of supergrids. The basic idea is that, since, in various parts of the EU, there will be times when there is excess electricity generated from wind etc. over and above local demand, this excess can be shunted to regions which are short and have high demand, using low-loss HVDC supergrids. Would it work on a large scale?


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Supergrids – a desert mirage?

By Dave Elliott

There have been reports that the Desertec Industrial Initiative (Dii) had abandoned its plan to help support the development of solar power in the Sahara and the export of some to Europe, since it looked as if the EU could meet most of its green energy needs indigenously, without significant imports. So is the desert CSP/supergrid idea dead? (more…)

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Balancing variable renewables- capacity markets, smart grids or super grids?

By Dave Elliott

The previous few posts have looked at the state of play with renewables in some key countries. In many cases an urgent issue is grid integration and balancing. The variable outputs from wind and PV solar outputs are balanced on some grid systems by using existing fossil-fueled plants, but the later are having a hard time competing, now that some of their peak market has been taken over by low marginal cost (zero fuel cost) wind and PV. To ensure that there is enough capacity still available capacity markets have been proposed, offering extra payments. Some critics don’t like the sound of that- it’s yet another subsidy, in effect for fossil fuel. However, the proposed UK version includes payment for energy storage and demand management options, as well as for gas-fired back up plants, and longer term, fossil gas might be replaced by green gas in the latter. There again there are other balancing options- supergrid links for example, which would open up a new multi-national balancing market.  Which option is best? (more…)

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Germany’s green energy

By Dave Elliott

Renewables have continued to grow in Germany, providing around 23% of total electrical generation from around 32GW of wind and 32GW of PV solar, most of this  being locally owned capacity, including  projects run by a growing number of local energy co-ops. And it works well: in bitterly cold March last year, the wind and PV were supplying about half of total electricity at one point:


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US green energy: store, curtail – or export?

by Dave Elliott

The US is pressing ahead with renewables, with around 60GW of wind and 10GW of PV solar already in place.  But that means some system operation issues are coming to the fore.  Since these sources vary, as does demand, when there is surplus output from wind of PV, should it be stored or just dumped?


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Isolation or interdependence?

By Dave Elliott

The Crown Estate has published a study by Garrad Hassan/Redpoint of the UK offshore wind market that reviews the existing programme, and identifies future opportunities and potential challenges. Among the key findings, the report concludes that the offshore wind sector is capable of meeting the government’s ambition of deploying up to 18 GW of capacity by 2020, subject to: regulatory certainty and the timely implementation of the Electricity Market Reform; achieving cost reductions in offshore wind; and securing a viable level of financial support. But the research revealed continuing concerns about a perceived paradox in which future political  commitment is contingent on cost reduction – which cannot be delivered without significant political support to enable the long term investments in the sector that can drive costs down.


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Storing energy

The Energy Futures Lab at Imperial College London has produced a ‘Strategic Assessment of the Role and Value of Energy Storage Systems in the UK Low Carbon Energy Future’ for the Carbon Trust, using a holistic system-wide modeling approach. It concludes that storage would allow significant savings to be made in generation capacity, interconnection, transmission and distribution networks and operating costs. In all it says that storage could provide up to £10 billion of added value in a 2050 high renewables scenario.

However, the relative level and share of the savings changes over time and between different assumptions. In the high renewables ‘Grassroots pathway’ used by the research team, the value of storage increases markedly towards 2030 and further towards 2050, so that carbon constraints for 2030 and 2050 can be met at reduced costs when storage is available. For bulk storage cost of £50/kW per year, the optimal volume deployed grows from 2 GW in 2020 to 15 and 25 GW in 2030 and 2050 respectively. The equivalent system savings increase from modest £0.12bn per year in 2020 to £2bn in 2030, and can reach over £10bn per year in 2050.

The value of storage is the highest in pathways with a large share of renewables, where storage can deliver significant operational savings through reducing renewable generation curtailment i.e. when there is excess wind output available. In addition, storage could lessen the even larger wind curtailment requirement that would result if there was also significant amount of inflexible nuclear capacity on the grid. However CCS scenarios yield the lowest value for storage: ‘adding storage increases the ability of the system to absorb intermittent sources and hence costly CCS plant can be displaced, which leads to very significant savings.’

Although it can be very useful in some situations, storage is not a magic solution for all our grid balancing problems: it is best used for specific purposes and durations. Crucially, Imperial say that ‘A few hours of storage are sufficient to reduce peak demand and thereby capture significant value. The marginal value for storage durations beyond 6 hours reduces sharply to less than £10/kWh year.’

So it seems we are talking about short storage cycles, ready for the next demand peak- not long term grid balancing to deal with long lulls in wind availability. That makes sense: storage is expensive, so you want to use the hardware regularly to capture excess energy (when it’s cheap) and sell it soon after to meet peaks, when energy prices are high.

This may be fine for short cycles. But how then do you deal with longer lulls? Especially in areas where there is a lot of wind capacity? Imperial say ‘Bulk storage should predominantly be located in Scotland to integrate wind and reduce transmission costs, while distributed storage is best placed in England and Wales to reduce peak loads and support distribution network management.’

The report also offers some other valuable insight into the interactive nature of the overall system options and operation. For example, one option for balancing grids in the short term is the use of flexible demand – e.g. reducing peaks by time-shifting demand. Imperial say that ‘Flexible demand is the most direct competitor to storage and it could reduce the market for storage by 50%.’ So with that, you would not need so much storage.

Another option, which might also help with longer-term grid balancing, is the use of interconnectors. While pumped hydro is the cheapest large scale bulk electricity storage option, the UK does not have much potential for large amounts, and some have argued that it would be cheaper to get access to the large pumped hydro storage capacity on the continent, in Norway for example, using ‘supergrid’ interconnector links. That could also allow the UK to import power when there was a long lull in wind availability.

Interconnectors are expensive, but Imperial say that cross-channel links (maybe 12GW or more) could be ‘beneficial for the system because it significantly reduces the amount of curtailed renewable electricity generation in the UK from 29.4 TWh to 15.1 TWh annually’. They add ‘this also suggests there will be less scope for storage to be used to reduce the system operating cost through reductions in renewable curtailment. The operating cost savings component is indeed lower in cases with increased interconnection capacity, by about 50% compared to the baseline (Grassroots) case.’ So we would not need so much storage.

Nevertheless, Imperial do see a need for perhaps 15GW of storage, given that ‘in the Grassroots Pathway, storage has a consistently high value across a wide range of scenarios that include interconnection and flexible generation.’

While there is a good overview of some storage technologies, beyond the points as above about the relative merits of bulk and distributed storage, the report doesn’t specify what sort of storage is best. Moreover, it is primarily about electricity storage. But how about rival modes of storage/ transmission e.g. heat or gas (including green gas). That would open up even more interactivity and may also improve the overall efficiency of the system and perhaps even reduce costs. After all, it is much easier to store heat or gas than electricity. Imperial admit that there are technical limits to conventional storage: the round-trip efficiency of storage can be low, and trying to increase it might not actually be worthwhile: ‘higher storage efficiencies only add moderate value of storage’ although ‘with higher levels of deployment efficiency becomes more relevant’. They also warn that ‘operation patterns and duty cycles imposed on the energy storage technology are found to vary considerably, and it is likely that a portfolio of different energy storage technologies will be required, suited to a range of applications.’

Fair enough: clearly more research is needed! Imperial do make a good job of promoting the benefits of storage and defending it against some critics. They say that ‘by providing reserve capacity and the resulting improved scheduling of plant, storage enables more wind energy to be delivered at the time of generation. In such instances the round trip efficiency of storage does not directly affect the amount of avoided curtailed that displaces other plant.’

However they add that ‘there remain a number of important unknowns with respect to the technologies involved in grid-scale energy storage, in particular relating to the cost and lifetime of storage technologies when applied to real duty cycles within the electricity network.’ While it is useful to get some idea of the possible interactions and their impacts, these technological and operational uncertainties do make you wonder how useful high-level modeling is: we are some way from being able to optimize the design of the emergent new grid systems, especially given the advent of novel storage technologies. So perhaps its not surprising that, on policy, Imperial ends up saying, ‘it is not clear whether government policies should incentivise the development and deployment of novel storage technologies, and if so, what sort of mechanisms should be considered, e.g. ranging from subsidies to direct procurement.’

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Dealing with variable supply and demand

Energy systems will increasingly have to cope with variable supply inputs, as more renewables come on the grid. The grid system already has to deal with variable demand patterns, and it is usually argued that the strategies used to do that can be extended to also deal with variable supply. So, for example, in addition to short term frequency adjustments, variable wind can be backed up using power from the flexible fast start up gas turbines, or plants kept on spinning reserves, that normally deal with demand peaks or sudden plant shut downs. But there are limits (maybe above a 20% input) to how much variable supply can be accommodated in this and similar ways, and it is often suggested that more energy storage will be needed.

There are some new short to medium term storage options emerging, which can buffer variable outputs from wind etc, for example various types of flow battery, liquid air storage and even conversion to hydrogen gas via electrolysis, but the problems with storage is that it’s inefficient and expensive. Much more so than just adding more cheap backup capacity. So some say the best type of storage is natural gas, stored ready for use when power is needed from back up plants. Using that of course incurs an emission penalty. The next best option, and one that is 100% carbon free, is pumped hydro storage, but there is not much of that available in the UK.

Several other counties in the EU, do however have very large hydro capacities (there is 30GW in Norway alone) and some of these reservoirs are now being converted to pumped storage; e.g. Germany’s Thuringia State has identified 13 potential sites, including 3 existing dams, for constructing pumped- storage plants that would total 5,130 MW.

However, even without pumped storage, hydro reservoirs can be used to help balance variable renewables by simply delaying their output when there is plenty of power – a head of water in kept back until extra power is needed. But that does not help with what is probably going to be a bigger issue than occasional low power availability from wind – the regular excess power that wind, and also solar, wave and tidal power, will produce when demand is low.

A possible solution to that is transmission to locations where there are shortages: even if it means long distance transmission. You would need that anyway to link to hydro around the EU. As well as being used to balance variable renewables around the EU and for liking to offshore wind in the North Sea and hydro across the continent, High Voltage Direct Current (HVDC) links could also be used for importing solar derived power from the desert areas of North Africa and the Middle East. HVDC supergrid grids can do that with low losses (2-3% /1000km) compared with conventional HVAC grid (up to 10%/1000km).

HVDC has its disadvantages- you have to have expensive transformers/invertors to upload AC from generators and to download DC to AC end-users (assuming that generation and use can’t actually be done with DC), and so it’s best for long distance interconnections. AC is best locally and perhaps also regionally. Interestingly, while backing HVDC for longer distance links, the SRU, Germany’s Advisory Council on the Environment, has suggested that, to avoid the problem with local uplinks, there could be a lower frequency 16.7 Hz overlay AC grid in Europe, at 500 kV. They say that would ‘reduce the ratio between line length resistance and frequency, which would represent a threefold reduction relative today’s 50 Hz frequency’.

All the foregoing is on the supply side. What about managing demand? There is much interest in ‘smart demand’ system , e.g. time-shifting electricity use by incentivising customers to run their energy consumption appliances off-peak, through time-of-use tariffs delivered through smart meters. More aggressively, some loads than are not sensitive to short interruptions can be turned off remotely for a while.

A study by Delta says that shaving system ‘peaks’ reduces the need for peaking plants which are often less efficient and/or use more polluting fuels, to run; it can also enable delay or prevention of the need for investment in new network capacity. And it can fill system ‘valleys’ – helping to increase and optimise the operation of lower carbon baseload plant. Based on studies by the Brattle Group they suggest that demand response across all sectors (through the use of time-of-use tariffs) with a 43% take up, can shift 5% of the system peak to off-peak times.

Smart grids can also offer other benefits, including diagnostic checks on the performance of heating & ventilation systems, heat pumps, refrigerators, etc. with savings of up to 20% being claimed in some cases. In addition, providing on line ‘smart meter’ information to consumers about their energy use, could help them cut out waste, change their lifestyles and/ or lead them to invest in more efficient systems.

So there are a range options on both the supply and demand side. They are not the only ones. For example, the emphasis above has been on electricity production and management, but there are also heat production and storage options. Heat is much easier to store than electricity and heat stores could be used as a way to balance variable renewables.

Whichever route is taken some of these options will take longer to deploy than others, and all will be relativelycostly. But then so will continuing to relay on fossil fuels- in economic and environmental terms. We have to shift away from them.

The key issue then is – will the new supply and demand management options be enough to allow variable renewables to expand to the 50% and even 100% that some say is possible at some point?

Optimists, including NGOs like Greenpeace and WWF, but also many energy analysts, say yes. For example see National Grids recent scenarios, which include a version with up to 67 GW of installed offshore wind generation capacity by 2030. and on a wider scale, the new roadmap to 2030 from the European Climate Foundation:

Pessimists, including some from traditional engineering and/or conservative political backgrounds, say no, renewables can’t be used on a wide scale, but offer few alternatives except maybe nuclear and /or Shale Gas with Carbon Capture and Storage. For a recent example see the report published by the Adam Smith Institute:

It will be interesting to see which view prevails.

Both the optimist and pessimists seem to be listened to by government and both are also being backed by corporate money to some extent, whether its EDF and EON being willing to invest in nuclear, or Siemens, Vestas, GE and Mitsubishi investing in off shore wind turbine manufacturing plants in the UK . Some think you can and should do both, and also CCS, but given limited resources, it may be hard to do that well, and there are operational conflicts between basically inflexible nuclear and variable renewables. We may need to make choices.

In my next Blog I’ll look in a bit more detail at some off the arguments on grid balancing in relation to wind.

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