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THE CASCADING PRINCIPLE

Transferring electricity from one place to another without transmitting it

Gerry Wolff

Introduction

Note: The 'cascading' idea described in this document appears to be valid although I have not yet discovered any published, peer-reviewed academic studies of it. Three well-qualified academics with specialised knowledge of power transmission, two in the UK and one in Germany, have confirmed that the principle is sound. A well-qualified person in the power transmission industry has confirmed that the principle is widely-recognised in the industry.

Given the monumental quantities of clean electricity that may be generated by CSP plants in North Africa, and given that CSP is likely to become one of the cheapest sources of electricity, it is natural to wonder whether it would be possible for the UK and other countries in northern Europe to buy that electricity from countries in North Africa and, if so, how? In what follows, we shall assume that Morocco is the supplier and the UK is the customer but the same principles apply to any other pairing where the supplier is in North Africa and the customer is in the north of Europe.

Since all the countries of Europe have well-developed HVAC electricity transmission grids, one might imagine that electricity could be transmitted directly from North Africa to the UK via existing transmission links. The difficulty here is that transmission of electricity over HVAC transmission lines becomes increasingly inefficient as distances increase. This problem can be overcome by building HVDC transmission lines for long-distance transmission of electricity. With 800 kv HVDC ‘classic’ transmission lines, carried overhead on pylons, it would indeed be feasible to transmit electricity all the way from Morocco to the UK with less than 10% loss of power.

However, there would be various problems to be overcome in creating transmission links like that, including objections on the grounds of visual intrusion (although there is scope for minimising this problem in various ways), the cost of the infrastructure and who would pay for it, regulatory problems, objections from electricity companies that might not welcome new competition, logistical problems in creating and installing the necessary equipment, and so on.

Solutions will be needed for those problems but, meanwhile, an alternative scheme that uses the existing infrastructure may facilitate imports of solar electricity to northern Europe on relatively short timescales. There are two aspects to the proposal—technical and regulatory—which will be discussed in the next two main sections.

Transferring power via an electricity ‘cascade’

Two key points are the basis for the scheme proposed here:

  • Since all electrons are identical, the UK may benefit from electricity generated in Morocco without it being necessary to receive electrons that originated in Morocco. Providing the UK receives an equivalent quantity of electricity from some other source, power generated in Morocco may, in effect, be transferred to the UK.
  • In addition to HVAC transmission lines between Morocco and the UK, there are power stations and consumers of electricity at many locations along the route.

Let us suppose that the HVAC transmission network between Morocco and the UK may be seen as a succession of local HVAC grids, each with its own power stations and each with its own consumers of electricity. In this discussion, the notion of a ‘local grid’ is a purely theoretical construct and has nothing to do with the administrative structures that may control the generation and distribution of electricity in any area.

If CSP electricity from Morocco is fed into the southern-most local grid, this will release generating capacity in the power stations there. That spare generating capacity may be used to feed electricity into the second local grid to the north. This in turn will free up generating capacity in that grid and that spare capacity may be used to supply the third grid to the north. In this way, generating capacity may be freed up in the succession of local grids, all the way to the north coast of France. At that point, the spare generating capacity may be used to supply the UK.

Since, in this scheme, electricity that is generated in Morocco is not literally transmitted from Morocco to the UK, it would be confusing and misleading to use the word ‘transmitted’ to describe what happens. Instead, we shall use the word ‘transferred’ to describe the notional or virtual moving of electricity from one place to another in the kind of scheme that has just been outlined.

Three analogies

There are at least three analogies, none of them perfect, for the idea that has just been outlined:

  • The lake. In some respects, a transmission grid is like a lake or pool of water. It is possible to add water at any point and take the same amount of water out at some other point and thus, in effect, transfer the water from one place to another without actually moving it between the start point and the end point. The main difference between a transmission grid and a lake is that the grid's storage capacity is relatively small.
  • The pipe. If a pipe is full of water, then pumping a given volume of water into one end will cause about the same amount of water to be ejected at the other end. As with the lake analogy, water is transferred from one place to another without it being necessary for the water to move the entire distance between the two points.
  • The cascade. As described in the following subsections, long distance transfers of electricity may be seen as being like the flow of water between a series of pools in a river.
In some respects, a transmission grid is like a lake. Water (electricity) may be added at one place and the same amount may be taken out at another place without it being necessary to move that quantity of water (electricity) all the way from one place to the other.

The cascading idea

We may imagine that each local HVAC transmission grid between Morocco and the UK is like a pool in a river, and we may suppose that the succession of local grids along the route is like a succession of pools, each one adjacent to the next one.

In this analogy, we suppose that if the local power stations in each grid are able to meet the local demand, then the ‘pool’ will be full to the brim. Further, we may suppose that there is a period of dry weather so that, although each pool is full, there is no water flowing down the river.

If now we imagine that some water (representing electricity) flows into the first (highest) pool, this will cause it to overflow into the next pool. Since that pool was already full, the water flowing into it from the pool above will cause it to overflow into the next pool below. This process will be repeated in the same way as a cascade down the succession of pools so that a quantity of water will flow out of the lowest pool which is about the same as the quantity of water that was fed in from the top—but it is not the same water that flowed in at the top and it has only traveled a very short distance.

The analogy is not perfect because it will only work in one direction. By contrast, the European network of HVAC transmission lines may be seen as a two-dimensional patchwork of local grids and the cascading principle may operate in any direction. If, for example, there is excess wind power from offshore wind farms in the North Sea but a shortage of power in Morocco, then excess power in the North Sea may be transferred indirectly to Morocco. If there was excess wave power off the west coast of France, that power may be transferred via the cascading mechanism to anywhere further east that may need it.

A schematic representation

Cascading image

Figure 1. A schematic representation of the way in which power may be cascaded through a succession of local transmission grids, as explained in the text. Key: Each blue square represents a consumer of electricity; each red circle represents a power station; broken lines represent transmission lines carrying little or no electricity; unbroken bold lines represent transmission lines carrying significant amounts of power; for the sake of discussion, the north-south axis is from the top to the bottom of the figure, the southern-most power station represents a CSP plant in Morocco and we may suppose that the northern-most two consumers are located in the UK.

Figure 1 is a schematic representation of the way in which electrical power may be cascaded through a succession of local transmission grids from Morocco in the south to the UK in the north. In (a), each power station except the CSP plant in Morocco is supplying significant amounts of power to nearby consumers a little to the south. In (b), the CSP plant in Morocco comes on stream and starts supplying consumers immediately to the north, in Spain. This means that the local power station can start supplying nearby consumers to the north instead of the consumers that it had been supplying before. Similar adjustments in all the other power stations mean that the two consumers in the UK can start receiving electrical power from the northern-most power station (in France), and the amount of electricity they receive will be much the same as the amount of electricity produced by the CSP plant in Morocco. In that way, Morocco may supply the UK with electricity without any need for long distance transmission of electricity.

Mechanisms

For this kind of system to work, it must be possible for each power station to redirect its output when any new source of supply comes on stream. Ideally, it should be possible to do this dynamically, from minute to minute, in response to changing conditions.

There would need to be some kind of control system to collate all the available information about conditions everywhere in the EUMENA-wide grid and calculate what changes are needed and where. There could, conceivably, be a single control centre at some single location but it would probably be more sensible to design the system as a collection of geographically-distributed nodes, each node controlling what happens in its own local area.

The whole system will depend on the gathering and distribution of information about conditions in each part of the network. The internet would provide a convenient means of distributing information throughout the network.

Discussion

Let us suppose that each of the local transmission grids has a boundary that is roughly circular and let us suppose that all the local grids are the same size, with a diameter D. Then in this ‘cascading’ scheme, the average distance that any one electron has to travel is D and the maximum distance that any one electron has to travel is D × 2. Assuming that D is small compared with the distance from Morocco to the UK, transmission losses will be much smaller than if one had attempted to transmit the electricity directly from Morocco to the UK via existing HVAC transmission lines. And, with important qualifications discussed in the next two sub-sections, there may be a reduced need or no need to build long-distance HVDC transmission lines.

At first sight, it might seem that, if the UK is buying electricity from France, then it is providing support for the nuclear industry in France (the main source of electricity there). In fact, the proposed scheme is completely independent of how electricity is generated in France or any other country in Europe. The proposed scheme would work in exactly the same way regardless of what kinds of sources of electricity there may be in France or Spain: nuclear, wave, wind, tidal stream etc.

How much power might be moved in this way?

The maximum capacity of a cascade of local grids, as described above, is the smaller of two things:

  • The size of most restrictive transmission bottleneck along the path. As an example, Chapter 1 of the TRANS-CSP report identifies the transmission link between Morocco and Spain as a major bottleneck with a capacity of only 400 MW.
  • The generating capacity in each local grid along the path. For simplicity in this discussion, we shall assume that all the local grids have the same amount of generating capacity, G.

With regard to the second point, G in each local grid will be roughly proportional to the area (A) that is served by the grid, so if there are no bottlenecks we may increase the transfer capacity of a cascade system by increasing the size of A for all the local grids in the cascade. But if, as we are assuming, the boundary of each grid is roughly circular, D will be a negatively accelerated function of A:

            Cascade image 3  

Larger local grids will mean longer transmission distances and, since this will result in greater transmission losses, there may ultimately be an impact on the overall efficiency of the system.

If there are no bottlenecks, it may be possible, as a rough guess, to transfer about 2 GW over a cascade of local grids with reasonable efficiency. Larger volumes of electricity may lead to unacceptably large losses.

There may, of course, be more than one path through a patchwork of local grids and this would allow larger amounts of electricity to be transferred than if there was just a single path. For example, there might be a 2 GW cascade from Morocco to the UK and Ireland via local grids in Spain and France, and another 2 GW cascade from Algeria to Switzerland, Austria and Germany via local grids in Italy.

Upgrading the transmission system

An HVAC transmission grid that operates as just described may be upgraded by removing bottlenecks, by the introduction of smart electronics (eg FACTS technologies), by the conversion of HVAC lines into HVDC lines and by the addition of new HVDC lines. This is described on the page about How to increase the capacity of an HVAC transmission grid.

If an HVAC transmission system were to evolve as just described, the last two steps would convert the HVAC grid into a hybrid HVDC/HVAC grid as proposed in the TRANS-CSP report. In that case, one might ask whether anything was gained by introducing the cascading principle in the first place? The answer to that question is “yes” on at least the following counts:

  • Given the electricity transmission systems of EUMENA as they are now, the use of the cascading principle can greatly expand the market for CSP electricity. For example, with the current arrangements, CSP plants in Morocco would be able to supply customers in Morocco and, possibly, a few customers in Spain or Algeria. But if the cascading principle was applied (with bottlenecks removed), it would then be possible for Morocco to sell electricity to many countries throughout Europe as well as customers at home.
  • A bigger market would give more confidence to investors and that should mean that CSP plants would be developed more quickly. This would mean that there would be more electricity available for local people in Morocco and more to sell abroad. It would also help to bring down prices via economies of scale and refinements in the technology. And this would mean cheaper electricity for everyone, including customers in Morocco.
  • There would be a psychological benefit, helping people in northern Europe to appreciate that "clean power from deserts" is not some remote possibility in the distant future but could be a useful source of electricity on relatively short timescales. This would provide an incentive to put the right policies in place (such as the creation of a single Europe-wide or EUMENA-wide market for electricity—see Regulation and administration, below) to enable the Desertec vision to become a reality.
  • In all scenarios, the cascading principle would reduce the need for long-distance transmission of electricity via HVDC transmission lines. This would mean a reduction in the costs of building and operating the HVDC transmission lines and it would mean less visual intrusion and fewer of  the other kinds of problems mentioned earlier.
  • The cascading principle has implications for the security of supplies to countries like the UK that are a long way from areas of desert. If the UK were to depend exclusively on long-distance transmission for its supplies of 'desert' electricity, then those supplies could be disrupted by a breakdown in transmission at any point between the desert source and the UK. But with the cascading principle, the UK would not be dependent on the integrity of long-distance transmission lines. Imports of electricity into the UK would come from a variety of sources that would be much closer to home.

Incentives

Why should France, Spain or Italy disturb their current arrangements for the sourcing and distribution of electricity so that, in effect, they become conduits for the supply of electricity to countries further north? Here are some possible incentives:

  • Since, as predicted in the MED-CSP report, CSP electricity is likely to become much cheaper than electricity generated from fossil fuels, and since it is free from the high costs and many other headaches of nuclear power (see www.mng.org.uk/green_house/no_nukes.htm), there is a clear incentive for all the countries of EUMENA to take advantage of it. France, Spain and Italy would of course share in those benefits and that would provide a good reason for them to participate.
  • The system may operate in reverse. Solar power in North Africa will be less during the winter but at that time of year there will be relatively large amounts of wind energy available in northern Europe. So in principle, southern parts of Europe may benefit indirectly from surplus wind power from Scotland, Denmark, Germany or the North Sea. Likewise for wave power off the west coasts of Europe.
  • The recent agreement made by EU heads of state that the EU would cut its greenhouse gas emissions by at least 20% by 2020, entails obligations on each country to make its own contribution to those cuts, with some countries making a larger contribution than others. The import of clean electricity from North Africa is an effective means for Europe to reduce its CO2 emissions from electricity generation and it should not be too difficult to share out the ‘credit’ for those reductions in an equitable manner amongst the countries that are either paying for the electricity or providing the support that is needed for transfers of electrical power via a cascading system.
  • With sensible arrangements for the regulation of electricity distribution (see next), it should be possible to ensure that all the costs of running a cascading system are paid and that no country or organisation loses money from the scheme.

Regulation and administration

To take full advantage of the cascading principle, we really need a single market for electricity throughout EUMENA or, at least, throughout Europe. Here is an outline of what is required:

  • It should be possible for any consumer of electricity, anywhere in EUMENA, to buy electricity from any supplier throughout the region, without worrying about how the electricity would be transmitted or transferred. This is the kind of system that operates in the UK, where any householder or business may buy electricity from any supplier without worrying about how the electricity will be delivered. The transmission grid is owned jointly by all the suppliers of electricity.

This system works well, although the electrons that pass through the wires in our house may have come from a supplier that is quite different from the company that we buy our electricity from.

The system is similar to the “through ticketing” system on the UK railways which makes it possible to buy a ticket between any two stations without worrying about which company or companies would be providing the train or trains for the journey.

  • Whenever a customer, anywhere in EUMENA, buys electricity from any supplier, some of the money will be transferred to a central fund. That fund will be used to pay for the cost of administering the EUMENA-wide grid, including the cost of administering transfers of electrical power via a cascading system. It will also be used to pay for additional transmission links, FACTS technologies, or HVDC transmission lines that may be needed to upgrade the system.

But even without a single market for electricity throughout EUMENA or Europe, countries towards the north of Europe (like the UK) can, via the cascading principle, benefit indirectly from "clean power from deserts" because solar electricity that is imported into the southern parts of Europe would free up generating capacity that may be used to supply countries further north.

Conclusion

The cascading scheme described here appears to be valid. It should provide a useful means of transferring electrical power over long distances between suppliers and customers without the need for long-distance transmission of electricity.

Potential advantages of the scheme include rapidly expanded markets for CSP (and other renewable sources of electricity) leading to a more rapid uptake of those technologies and reductions in cost from economies of scale and refinements in technologies. A cascading system would have a psychological benefit, helping people to appreciate that "clean power from deserts" can become a reality on relatively short timescales. It may also reduce the need for long-distance HVDC transmission lines with corresponding savings in cost and fewer of the problems associated with overhead transmission lines, such as visual intrusion. And it would increase the security of supplies because countries like the UK would not be dependent on the integrity of long-distance transmission lines but could receive their 'desert' imports from a variety of sources much closer to home.


Last updated: 2009-10-09 (ISO 8601)