Economy
& Energy
Year VIII -No 49:
April-May
2005
ISSN 1518-2932
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Economy
& Energy |
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The Future of the Brazilian Electric System A “Destination Port” for the Brazilian Electric System New: No 48 in adobe pdf for download
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A “DestinATION PORT” FOR THE
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|
System |
Storage Capacity (GW month) |
Monthly Production (GW month) / month |
Storage/ production (months) |
Storage/ Production (years) |
|
SE |
176,6(*) |
25,8(*) |
6,8 |
0,57 |
|
S |
15,3 |
4,9 |
3,1 |
0,26 |
|
SE + S |
191,9 |
30,7 |
6,3 |
0,52 |
|
N |
11,8 |
3,1 |
3,8 |
0,31 |
|
NE |
49,6 |
4,7 |
10,6 |
0,89 |
|
N + NE |
61,4 |
7,8 |
7,9 |
0,66 |
|
Systems |
253,3 |
38,5 |
6,6 |
0,55 |
(*)Includes all Itaipu
It should be noticed the fact that the multi-annual character of the system has been gradually reduced, as shown in Figure 6 for the SE region.
Figure 6: Storage/ (thermal generation load) ratio, expressed in months, along time that shows the multi-annual storage reduction of the Southeast system.
Source: ABRAGET: Lecture of Antônio Gama Rocha of UTE Norte Fluminense at the 1º Continuous Forum on Energy– Brazilian Energy Agenda – Rio de Janeiro 9-10/12/2003 – FGV and COOPEFURNAS
In the description that follows, the systems with “exact regulation for a year with normal affluence” and “run-of-river” were included; even though not corresponding to any region, they are important from the conceptual point of view.
The main results for the types of the studied systems, described in Annex 1, are presented in what follows.
As an example, it is shown here the Southeast Region representation in year the 2003 as base[x]. The values used in the simulation are expressed relative to the average affluent energy (=100) and are shown in Table 2 that also shows data for the SE region that were used for the example case. The initial value of the stock was taken for simulating the 2001 “blackout”.
Table 2: Characteristics of the SE system and of the Simulation (Case 1)
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|
SE Region |
Simulation |
|
Average ANE |
27,4 GW month |
100(*) |
|
Maximum A NE |
42,2 GW month |
154 |
|
Production |
25,5 GW month |
93 |
|
Installed Capacity |
45,2 GW month |
336 |
|
Storage |
176,6 GW month |
640 |
|
Minimum Spilling |
|
7 |
(*)Reference value; the other
values are relative to the average Affluent
Natural Energy (ANE) in the Southeast system (27,4 GW)
In Figure 7 are presented, month after month, the affluence (affluent natural energy), the accumulated stock, the spillover volume and the production. There are options in the program for the bi-annual and multi-annual representation shown in the figure. In the multi-annual graphic it is indicated the annual average affluence (=100 in a normal year), pointing out the “dry year”. It is shown the expected evolution for a situation similar to the one that occurred in the Southeast Region where a low stock and a decrease of the annual affluence caused a production deficit in 2001
In the simulation (Figure 7) it was considered the demand to be satisfied plus the minimum flow (94 + 3) lower than the average affluent natural energy. In this case, the stored energy would tend to grow and after a sufficient time, to be spilled. However, with a low (as shown) atmospheric precipitation (20% lower than usually) there would not be sufficient energy stock to maintain the necessary production.
Example Case based on the Southeast Region
|
Average Affluence |
100 |
|
|
Minimum Stock |
10% |
|
Minimum Monthly Affluence |
46 |
Accumulation Capacity |
640 |
Maximum Stock |
100% |
|
Monthly Production |
94 |
Initial Stock |
60 |
|
|
|
Maximum Monthly Affluence |
154 |
Minimum Flow |
3 |
Loss in dry year |
20% |
Figure7: Expected evolution for a system with conditions analogous to those of the Southeast Region in 2003. The presentation of this figure is similar to the screen of the program, where it is possible to modify (white cells) the input data. Furthermore, it is possible to choose the type of graphic to be presented (bi- or multi-annual). The initial stock and the affluence decrease in the third year were considered so that they could simulate the “blackout” that occurred in 2001. Note: the % stock curves (scale on the right) practically coincide with those of the stock due to the scale adopted.
The represented system would be considered for the full use of the affluent energy in a year of normal precipitation (within the average value). It could include a storage considerably smaller than that of a system with multi-annual regulation.
For a system similar to the exemplified case (the same minimum and maximum flows relative to the average value), the stock could be twice the average monthly flow and only 30% higher than the month of largest affluence.
In this system, the stock of stored water would be annually “zeroed”, since the storage would coincide with that necessary to satisfy a normal year. All affluent energy can be used and it would be “the optimum system”, except for the predictable existence of years with precipitations lower than the average value, when supply is severely reduced. For a 20% reduction of affluence during a year, the electricity production would be affected for five months and in the most critical month it would drop to 40% of the demand.
It has also been simulated a system with no accumulation in which all generation would be made with the natural affluence. Depending on the expected rainfall regime for the region, an important fraction of the available energy would not be used. This fraction grows when the maximum natural flow / minimum flow ratio grows. As compensation, the intervention in the fluvial system would be minimal. It should be emphasized that the example case does not concern run-of-river plants using regulation with upstream dam but rather a system that was designed to operate entirely with the minimum annual flow, a run-of-river one. Obviously, the system could have been designed to better use the affluent energy: it would suffice to have the installed capacity higher than the minimum one. In this case, its contribution to the generation would be larger and its contribution to the stability of the system would be smaller or negative.
Since the system is designed to operate at minimum affluence conditions in a normal year, its production is quite stable. In this case, it was designed to operate using the typical minimum monthly affluence, 46% of the annual affluent energy would be used.
The possible use (of the total annual affluent energy) for a plant of this type was evaluated for the different regions using the average curves of ONS. It would be 52% in the Southeast Region, 58% in the South Region, 32% in the Northeast Region and 21% in the North Region. Since it is in the last region that it is expected the largest generation expansion for supplying the integrated systems, the installation of this type of plant could limit the usable potential of the region. However, it should be remembered that in a system like this one the really usable potential should be re-evaluated since the energy use conditions could vary because of minor environmental problems due to the adopted flooding pattern where, for example, could be included uses that would be presently unlikely.
Still concerning the North Region, it should be remembered that the present affluence values along the year are based on the Tocantins River flow. However, for the two largest projects under study (Belo Monte plant and the Mamoré River) the flows present dry months, with low affluence relative to the average value, much similar to that of the present plants of the region, as shown in a note at the end of the present study.[1].
The System with Partial Regulation is an intermediary type between that of regulation for one year and the run-of-river type. This type of system does not have the capacity of compensating the seasonal variations along the year but does not operate as a run-of-river system either. Water spillover is part of its normal procedure and only a fraction of energy is used. An example of this type of system is that operating in the North Region whose data, including those referring to the dry season, were the base for simulating a case studied in Annex 1.
Besides loosing production due to the uniform drop in the monthly affluence during the year, a new type of instability was detected in this type of system, caused by a variation of the monthly precipitation along the year (without reduction of the annual production), inducing an important production decrease. Therefore, this type of system presents a large instability regarding the rainfall regime, what indicates that the introduction of plants with strong seasonal character and low storage strongly requires complementation of other plants capable of sustaining the stability of the system.
As previously pointed out, the representation using the model should be the simplest possible and compatible with the correct description of the system. A good verification test of the equations used is to obtain by difference the value of the spillover volume + the evaporated volume. When coherent results are obtained it means that no important variable has been forgotten. Furthermore, the knowledge of the variables behavior of the model in the real situation is an important step for elaborating the scenario for the future. Comparing the results obtained is also a good intrinsic coherence test for the model. It was verified that in spite of the simplicity of the model, the reproduction of the real system is fairly good.
The simulation of the system with multi-regulation already shown (Figure 7) was made with characteristic data from the Southeast System. The situation previously simulated, as indicated in Figure 8, is much similar to the one that resulted in the 2001 “blackout”.
Figure 8: Values for the Southeast of the storage, the affluent natural energy and of energy production that resulted in the 2001 “blackout”. It should be observed that the minimum level of the reservoirs reached 18% in December 1999, but a superior initial stock (22% at the end of 2000) lead to the 2001 “blackout”
The values of the spillover energy are calculated by difference and are rather reliable, showing that the adopted approximation, considering the Southeast systems as a single plant, supplies satisfactory results. The low value of the spillover energy relative to production reveals, on the other hand, that the system is well administered. It should mainly be considered that, taking into account the random characteristic of the rainfall and requirements imposed to the flows, it is not always possible to avoid, as it would be desirable, spilling water, concomitantly wasting generation from other sources (in thermal plants) or whenever its is still possible to accumulate water in other reservoirs of the region. The perfect administration of the system becomes more difficult when, as in 2003, the stocks approached the maximum level. It should be still pointed out the growing institutional complexity of the present system relative to the previous one that was almost exclusively state-owned. Until the system adapts itself to the new circumstances, it can be foreseen that the rigidity of the contracts prevents the optimal use of the available hydroelectric energy.
As in the simulation (Figure 7) previously shown, the year 2001 started with low water stocks in the reservoirs and it was known that an additional decrease in the average annual affluence could cause “blackout”. In fact, in the previous year this possibility already existed.[xi] . Therefore, for 2001 it was chosen the same tactics adopted in the previous year, namely not revealing the risk to the consumers. As the rainfall was below the normal level, the government was forced to adopt “blackout”, which could have been anticipated and perhaps partly attenuated.
It is shown in Annex 1 that should the stocks be in their maximum value it would be possible to face the 20% decrease of the affluent energy without problem and even a 35% decrease. Naturally it is not the aim of the system to reach every year the maximum stock since it would be anti-economic to use the thermal contribution to accumulate a water stock to be later possibly wasted by spilling the stored water. The procedure adopted consists in fixing a “risk aversion” curve and activate the thermal plants whenever the storage moves away from the desired level[xii].
For the approach of the next part of the present study, that will deal with the role of the thermal complementation (present and future), it is interesting to observe how the demand of each system was satisfied, including the inter-regional energy exchange and the thermal generation.
Figure 9: Generation and electric energy exchange in the Southeast System. Exchange has been represented as a negative value for export and positive for import, which permits obtaining the electric energy offer in the system. The total energy generated by Itaipu is shown.
In Figure 9 it is shown the electric energy offer (generation + exchange) in the Southeast region where it is included (coherently with which it has be done with the stock) all the production of Itaipú.
From the storage point of view, the NE System presents a situation similar to that of the SE System but, due to the fact that its demand is larger than the offer, it operates normally, except for special circumstances, by importing energy from other regions. The behavior of the NE system is shown in Figure 10.
Figure 10: The NE system has storage characteristics similar to those of the SE system. However, its larger dependency on the energy generated in other regions causes a lack of stability. Additionally, it calls attention the large participation of spilled energy, indicating the use of water for other purposes. As can be seen, the spilled energy presents a larger seasonal character in the dry season.
Regarding the transfer of energy and the thermal complementation, the historical values of the last years are presented in Figure 11. It should be noticed the dependency on imports from other regions and the near absence of thermal generation.
Figure 11: The NE System is characterized by the fact that it is an energy importer (from 1999 on), depending on the interconnection of the systems. The thermal generation share is still very small.
As has been pointed out, the North System is characterized by partial regulation. Its characteristics were used for one of the case studies of Annex 1 (Case 4). In the represented period (1996 to 2003) there has been considerable export for satisfying demand in other regions, notably the NE one. The system’s behavior (Figure 12) is quite similar to the simulated one, showing the two types of deficit caused in 2001 by a decrease of affluence along almost the entire year and in 2002 by a shift of affluence from the dry months to those of larger rainfall.
Figure 12: The North System has a small storage capacity and this fact makes it quite unstable. It should be noted that advance of the rainfall season (as it happened in 2002) can cause a collapse in supply which did not occur due to the existing interconnection, since thermal generation (in the interconnected part of the region) does not exist.
In Figure 13 one can note the exporting character of the system, with some importing episodes as it occurred at the end of 2002.
Figure 13: Energy generation in the North Region has been partially used for export. It should be noted the production deficits corresponding to the “blackout ” (2001) and to the rainfall shift (2002). Imports have permitted to face the 2002 deficit; there is no thermal production because all plants that integrate the system are hydroelectric plants.
The South System has also low storage capacity. Its particularity is that it is situated in a region with a rainfall regime different from the other ones. Furthermore, the rainfall cycle is not regular as in the other regions.
The irregularity of the rainfall regime makes it less attractive for applying the type of simulation used in the other regions. It is interesting to point out that the expected occurrence in the peak month (October) is that verified in a dry month in the Southeast region (rainy season only in the beginning) and would favor (whenever it occurred) a certain complementation relative to other regions. It also seems that there is some coincidence of dry years in the NE region and intense rainfall in the South and vice-versa[2]. This complementation enhances the interconnecting roles of the systems.
Examining the historical values of the last years in the South Region (Figure 14), it can be noticed that the adopted policy for the electric energy exploitation has adapted itself to the climatic reality. It should be also pointed out the more significant presence of thermal plants (operating at the base) that helps stabilizing the system (Figure 15). There has also been an intense energy exchange (between South and Southeast) that has permitted taking advantage of the differentiated rainfall.
Figure 14: Electric energy production in the South System has followed the availability of water. The system was able to have during years (including the “blackout” period) storage close to100%. There are transmission limitations that have prevented the use of the pointed out complementation.
Figure 15: Energy offer in the South Region that shows, besides an important variation in the generated hydroelectric energy, a participation of the thermal generation (mainly coal) in the base and a significant exchange with other regions. In this “exchange” is included the energy from Itaipu that, even though it is generated in the South Region , has its production included in the Southeast Region and reaches the South Region to be consumed through the interconnection between the two regions.
If the Integrated Systems were perfectly interconnected they could be treated as a single one. When the operating variables of the set of systems are observed (Figure 16) it is verified that they do not follow the logic of only spilling water when the maximum storage is attained. The limitations of a perfect use of the capacity of the set are due, on one hand, to the generation capacity limit which is designed to satisfy an assured average demand and, on the other hand, to transmission limitations.
In 2000 and 2001, for example, even with low stocks, the plants of the North and South Systems spilled a significant quantity of water. Besides those physical limitations, operation errors can occur in the system that can lead to shortage situations.
Figure 16: United operation of the integrated systems showing that when storage reaches the maximum value (or to satisfy other needs), the spillover logic is not followed by the set, as it was observed in each of the previously shown systems. This is fundamentally due to the limited capacity either of generation or of transmission among them.
In Figure 17 it is shown the generation in the integrated systems, including the participation of the nuclear generation and that of conventional plants. It should be observed the high predominance of hydroelectric generation and the low presence of import. Once the crisis is over, thermal generation tends to be reduced as long as hydroelectric energy is available (and the stocks are maintained to minimize the risk).
Figure 17: Service of the integrated system, showing the magnitude of the supply problem in 2001, only partially supplied by thermal generation . It should be noted the important participation of nuclear energy during the crisis, which can also be used to help restore the water stock.
The full integration of the existing systems and the increase of equipment in some plants in operation – within the economic limits of these investments – can favor the best use of the regulation capacity of the systems. However, it should be remembered that if the additional generation capacity becomes simply a part of normal operation, there would be a smaller generated energy/storage capacity ratio and, consequently, a smaller stability in the system.
In Annex 1 it is presented the computer simulation of a hydroelectric system and its behavior regarding different affluences and storage capacities. The regulation is carried out with the accumulation capacity of the reservoirs, either to face the predicted seasonal oscillations or to absorb annual variations of the rainfall regime.
The inclusion of thermal plants in the simulation of the systems is described in Annex 2 and aims at studying the role of these plants in the system regulation considering the expected reduction of the storage capacity/average affluent energy ratio.
In the simulation it was adopted the premise that the electric systems would be administered so that fuel consumption is minimized. This means that available hydroelectric energy would be used at its maximum. This also means that the stock of stored energy in the reservoirs would be close to its maximum at the end of the rainy season, defined here as the beginning of the month in which – in a typical year – the affluent natural energy (ANE)[xiii]<
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