3- Pumped-storage hydropower - Autenticação · PDF file3- Pumped-storage hydropower 3.1 Brief considerations Since the electrical grid needs to assure the supply at any time, including

3- Pumped-storage hydropower

3.1 Brief considerations

Since the electrical grid needs to assure the supply at any time, including the time of

high demand, which is in the morning and late afternoon, the rest of the day, and

especially during the night, energy is not needed as much. The major goal is to try to

uniform this curve, getting the line as close to horizontal as possible, avoiding like that

peak demands, which make it necessary to have such quantities of energy available,

only for very short periods. In Figure 3.1, one can observe the individual household

electricity demand for a one-day period. As it was mentioned before, the peaks that can

be seen are the ones that should be minimized in order to facilitate the energy

efficiency, and avoid therewith the waste of energy.

Figure 3.1 – Individual Household Electricity Demand (University of Oxford).

In Figure 3.2 it is possible to learn about the aggregate energy consumption, where

again a less desirable peak can be verified. While the aggregate demand smoothes out

the individual peaks, the daytime load still doubles the night time load.

Figure 3.2 - Aggregate Energy Consumption (Berkeley,2010).

Concluding, a lot can be done in order to promote energy efficiency, and avoid waste of

energy. Once these measures are all taking place and in a hypothetical case, the demand

curve is horizontal, the electricity price could be constant as well, instead of having

several peaks, as Figure 3.3 shows.

Figure 3.3 - Electricity Tariff (€/kWh) (Vieira e Ramos,2009).

Considering that the goal of a horizontal curve of demand is close to impossible to

achieve, the option, besides not wasting the energy produced, is storage. By storing

available energy during low demand hours, the energy won’t be wasted and can

therefore be used at times of peak and high demand. Since the renewable energy sources

are intermittent, like the sun or the wind, it is not possible to count on it on hours of

shortage of energy, because they won’t guarantee the supply. The solution to this

problem is to store these produced energies, so that they can be used when they are

necessary and not when they are available. Unfortunately the storage of such energies is

not easy or cheap, which leads to the waste of it, if it is not necessary at the time it is

available. This major concern will be addressed in this document, presenting a solution

of storage of renewable energy through hydropower with pumped-storage plants.

Sustainability

“Sustainability is a concept that turned into a principle. It prescribes that the use of

natural resources to satisfy present needs should not compromise the satisfaction of

necessities of future generations.“

It is often considered a “fuzzy concept” that can be represented through three

overlapping circles as presented in Figure 3.4.

Figure 3.4 - The Three Spheres of Sustainability.

The three circles, hereby called spheres, represent the environmental, the social and the

economic sphere. The sphere that applies mostly to this work is the environmental one.

It’s in this area that the pumped-storage as well as the use of hydropower have a goal of

sustainability. The use of the natural resources, without abusing them or damaging

them, is the key to reach a sustainable development. With pumped-storage, renewable

energy from, for example sun or wind, can be stored with the help of water. All the

resources are natural and their use is harmless for future generations, because it doesn’t

affect their source.

In conclusion pumped-storage may contribute to energy efficiency as well as

sustainability of the use of natural resources. It is yet to be discussed if such a solution

can be possible, viable, and competitive.

3.2 Pumped-Storage Power Plants

Historical Review - In the 1890s the first pumped hydroelectric storage in the world

appeared in the Alpine regions of Switzerland, Austria and Italy. The earliest designs

use separate pump impellers and turbine generators. Since the 1950s, a single reversible

pump-turbine has become the dominant design for pumped hydroelectric storage. The

development of these systems remained relatively slow until the 1960s, when utilities in

many countries began to envision a dominant role for nuclear power. Many of these

facilities were intended to complement nuclear power providing peaking power.

In the 1990s, the development of pumped storage power plants (PSPP) significantly

declined in many countries. Many factors may have contributed to the decline. Low

natural gas prices during this period make gas turbines more competitive in providing

peaking power than pumped-storage. Environmental concerns caused the cancellation of

several PSPP projects and significantly prolonged the licensing process. In some

countries, power sector restructure probably also contributed to this slowdown. During

the 1990s, several countries started to restructure the power sector by unbundling

generation and transmission. The nature of PSPP falls into the grey area between

generation and transmission. Because the net electricity output of PSPP operation is

negative, a PSPP facility usually cannot qualify as a power generator. Although PSPP

provides crucial load-balancing and ancillary services to the grid and reduces the needs

for transmission upgrades, PSPP do not typically qualify as transmission infrastructure.

For instance, in the United States, the Federal Energy Regulatory Commission denied a

request from a proposed PSPP project to be categorized as a transmission facility for

purposes of rate recovery. The regulations for PSPP vary from country to country. For

example, in China, PSPP is considered a transmission facility and the Chinese

government charges the state grid corporations with the primary responsibility for

developing PSPP and allows them to recover costs of PSPP through transmission tariffs

(Yang, 2011).

Nowadays, a very big interest in PSPP is taking place worldwide. Many new PSPP are

being built and older ones are being upgraded and renovated. Pumped hydroelectric

storage is at the present moment considered the most established technology for utility

scale electricity storage. Low-carbon electricity resource, like the wind, the sun, or even

nuclear power, can’t adjust their output to match fluctuation power demands, without a

storage component.

Importance, Potential and Technology of a Pumped-Storage Solution

- Importance - Global warming is an issue that concerns the world. The

major concerns are about the happenings over the past decades, namely:,

the world has been warming at a rate that is equally unprecedented, and

these circumstances have been made possible by the similarly unprecedented

magnitude of anthropogenic CO2 emissions, due to humanity’s ever-increasing

burning of fossil fuels such as coal, gas, and oil (McDaniel, 2011).

There is a big necessity of mitigating global warming, and reducing the burning of fossil

fuels is the major mean to achieve this final goal. This burning of fossil fuels may only

be reduced if some kind of energy exists to replace it. The option of renewable energy is

a very popular one, but in order to be competitive and available at all time, it needs to be

stored. Pumped-storage is therefore a piece of high importance in the puzzle of the

mitigation of the global warming.

- Potential - “Pumped-storage is the only viable, large scale resource that is

being broadly used for storing energy, and it offers the best option for

harnessing off-peak generation from renewable sources. With the ever-

increasing investment in variable generating sources, energy storage will be a

critical tool for using our clean energy resources effectively” (Miller and

Winters 2011).

An example of a pumped-storage power station can be seen in Figure 3.5, where the

upper reservoir and the lower water body can be seen.

Figure 3. 5 Pumped-storage Station in Germany.

These kind of pumped-storage facilities are being upgraded and constructed in several

countries in Europe. This fact is due to its capacity of storing electricity. Besides the

storage it still balances the load and can react quickly to change grid conditions (Züber,

2011).

Such facilities are different from normal hydroelectric power generation facilities.

Instead of only generating electricity it uses it as well. Energy gets stored in form of

water pumped from a lower elevation reservoir to a higher elevation reservoir. In hours

of low demand electricity gets consumed and water gets pumped into the higher

reservoir. When the peak hours arrive, and the demand is high, water gets turbined and

generates therefore electricity. This system buys electricity from the grid when this

electricity has a very low price, and can therefore be called a net consumer, but it sells

this same energy later in high demand hours, where electricity prices are high, and

creates therewith profit.

But generating this profit isn’t the only advantage of this system. This technology

provides the possibility of firming the variability of energy generated by intermittent

renewable sources. Since wind and sun are not constant and don’t guarantee they will be

available once they are needed, the excess of energy generated by these green sources

may be stored in pumped-storage systems instead of being wasted, during low demand

hours. These systems present an interesting option, since they have the possibility of

storage and provide significant flexibility regarding start-ups and shut-downs. The

necessary balance between generation and demand is facilitated by the existence of

these systems and are therefore being installed in all Europe. As explained before, the

European Union has a share of renewable energies that is continuously growing. Until

http://wdict.net/gallery/goldisthal+pumped+storage+station/

2020 the shares are supposed to increase until 20%. Most of this energy is and will be

generated by wind power. However the existing grids and power plants are not built or

prepared for such energy generation. In very windy times the grid might get overloaded,

but at the same time, when demand is high wind may not be available and some other

option has to replace it and generate quickly the needed energy for the supply. Pumped-

storage plants present a large-scale storage solution for this problem which compensates

the fluctuations of the loads. Although wind poses some challenges, as they were

described before, they can be highly valuable when working together with a storage

system. In several experts opinion pumped-storage is the best option for storage, and

this matter will be discussed further on in this chapter.

Technology - A pumped-storage hydroelectricity facility looks normally similar to

the typical one that can be seen in Figure 3.6.

The outlook can of course change depending on topography and specific characteristics

of the location of the facility. Further on in this paper, some different pumped-storage

facilities will be shown and discussed, and differences can be checked.

The average facility consists normally of an upper reservoir, which has a higher

elevation and a lower reservoir, which is located in a lower elevation. In addition it has

a reversible turbine and generator. These turbines act as a turbine and as a pump, and

are normally Francis turbine design. In one facility one can have several groups of

turbines, depending on how much power one wants to install in the system. Most of

these systems have a large variation of height between two natural bodies of water or

artificial reservoirs.

Figure 3. 6 - Pumped Hydroelectric Storage Scheme.

The next equation, namely Equation 3.1, explains how the power increases with the

increase of the variation of heights.

(3.1)

P: installed power (kW)

g: specific weight of fluid (N/m3)

Q: discharge (m3/s)

Hu: head (m)

h: turbine-generator efficiency

In the previous equation it is possible to understand that the installed power, P, depends

only on the discharge, Q, and the head, H, since the specific weight and the turbine

efficiency are constants, once the turbine has been chosen. The higher the head, H, and

the bigger the discharge Q, the bigger will be the power, P. Knowing this result one can

vary between bigger Hu and smaller Q, or vice versa. Normally the more economical

solution is linked to big Hu, and not so big Q. Bigger Qs need big equipment, big

turbines which increase very much the price of such an investment. The best option is

consequently two reservoirs with a big variation of heights of the lower and upper

reservoir. This means that the higher the installed power is, the smaller will be the ratio

€/MW. In summary, the most advantageous scenery is a big head, which leads to big

power. This results in a lower price of € per MW, which turns the system more viable.

Depending on the amount of power installed, one can apply one or more groups of

reversible turbines. Usually the turbines used in such a system are Francis turbines.

These are reaction turbines. Francis Turbines are typically used with heads between 20

and 500 m. Depending on this head, the size and the shape will differ. An example of a

Francis Turbine can be observed in Figure 3.7.

Figure 3. 7 – Francis Turbine.

These reaction turbines contain a runner that has water passages through it formed by

curved vanes or blades. As the water passes through the runner and over the curved

surfaces, it causes rotation of the runner. The rotational motion is transmitted by a shaft

to a generator. The largest Francis turbines have an output of 750 MW. In Figure 3.8,

one can see a cross sectional of another Francis turbine.

Figure 3. 8 - Francis Turbine from the Cahora-Bassa Central (Moçambique) (Quintela,

2000).

Pumped-Storage as a Solution to Firm Variability of Wind Capacity - The

pumped-storage solution is considered the best option by most experts. There are

several types of electricity generation technologies that can help to fix the problem

of variability of wind capacity.

To analyze the difference between possible options a study made by the Spanish

company Iberdrola was taken into account. The decisive factors were the start-up and

shut-down capacity, the regulation velocity and the technical minimum load. Table 3.1

presents the several electricity generation technologies.

Table 3. 1 – Electricity generation technologies to fix problem of variability of wind.

Advantages Disadvantages

Conventional

hydro

Very flexible in startups and shutdowns, since their continuous performance

doesn’t affect the equipment’s service

life.

Fuel cost is zero.

There are no CO2 emissions.

Connection to the hydraulic management of rivers.

Conditioned by the storage capacity and weather.

Conventional thermal

Control range is acceptable.

Limited regarding start ups and shut

downs, since process requires a substantial amount of energy, which

involves a substantial cost.

Repetition of this process reduces

significantly the service life of the

plant.

Gas

Significant flexibility for start-ups and

shutdowns,

Allows rapid power variations.

Minimum power of these plants is

usually about 60% of full load. This

fact limits the regulation capacity to 40% of rated power.

Combined cicle

More robust to perform the continuous

start-ups and shutdowns compared to

conventional thermal.

The regulation velocity is slightly lower than open cycle turbines, and

the minimum power of these plants

is nearly 50% of the power at full

load.

Pumped-storage

Technically same characteristics as

conventional hydro,

Solves the big limitation of conventional

hydro, which is the storage capacity

Availability of power at any time,

including dry periods

It is necessary to have fuel to pump

water into the upper reservoir.

Since conventional hydro is the lowest cost technology and the cleanest one, it is also

the most popular. Nevertheless most developed systems have their hydroelectric

potential almost completely harnessed, which makes it necessary to find other

technologies to provide balancing services.

After conventional hydro, pumped-storage is the best alternative to firm the variability

of wind. In these systems power is available without restrictions. The combined cycle

would be the third best option that is more likely to firm variability of wind

Briefly, pumped-storage presents various advantages. In summary these advantages are

the following:

to store energy

to generate profit

to firm variability of energy generated by intermittent renewable sources

to provide flexibility regarding start-ups and shut-downs

to compensate the fluctuations of the loads

Because of these positive aspects the market of pumped-storage power plants will, most

probably, grow like never before in the next 10 years. The prediction is that until 2020,

more than 60 new plants with an installed capacity of 27 GW will be constructed. These

numbers represent about the half of the current pumped-storage power plant stock and

an investment volume of almost 26 billion euros (Züber, 2011).

Wind Capacity and Pumped-Storage in Europe - Due to the growth of the share

of renewable energies in the total electricity generation, load fluctuations caused

by energy of wind, and photovoltaic have to be compensated by storage, which

lead to a boom of construction of such facilities. Figure 3.9 shows wind energy

capacities in some selected countries, and their development until 2020 is

presented as well. Countries with proper natural conditions for such systems are

the ones who will focus on this option. Countries like Austria, Switzerland, Spain or

Norway are big investors in this option because of their mountains.

Figure 3. 9 - Wind energy plant capacities in selected countries (Züber, 2011).

Table 3. 2 - Wind energy in countries in Europe ( Züber, 2011).

Country 2010 (MW) 2020 (MW) In percentage

Germany 27,526 45,750 + 68%

Spain 20,155 38,000 + 89%

Italy 5,800 12,680 + 119%

France 5,542 25,000 + 351%

United Kingdom 4,040 28,000 +593%

Portugal 4,256 6,875 +62%

Denmark 2,923 3,950 +35%

Total 70,242 125,150

As the previous Figure 3.9 and Table 3.2 illustrates, wind capacity is going to increase

very much in Europe. Not only onshore wind power generation will be included, but

also offshore wind farms. For example, in Germany which is a very big investor in wind

energy, offshore projects are supposed to generate 10.000 MW by 2010.

Considering Portugal, a large part of its electricity is generated in large Hydropower

plants like pumped-storage power plants, or simple hydropower plants. The share of

this generation is supposed to increase enormously in the future and the installed

capacity of pumped-storage power plants is supposed to double in the next years. In the

next Table 3.3, Portugal’s numbers can be checked and the pumped-storage power

plants percentages can be verified.

Table 3. 3 – Portugals Information.

Portugal

Inhabitants 10,707,000 Area [km2] 92,345

Number of pumped-storage

plants (PSP) 4 Capacity [MW]

1,089

Number of storage power

plants 36 Capacity [MW] 4,526

Share of PSP of renewable

energies [%] 14.2 Share of PSP of total

electricity gen. [%] 4.6

In this moment there are several new projects of hydro power plants under construction

or planned to be built or being upgraded in Portugal. Some of these include pumped-

storage. Most of them, and all of the following examples are under construction of EDP,

Energias de Portugal. The projects of EDP represent an investment of almost 3400

million euros in Portugal until 2020.

Baixo Sabor:

Baixo sabor is a power plant under construction, and is located in the Sabor river, in the

Douro river basin. It has a storage strategic role added to the electricity generation and

is equipped with reversible units. The main indicators relating to this project can be

found in Table 3.4.

Table 3. 4 - Main indicators of Baixo Sabor power plant.

Main indicators

Construction works (strated) 2008

Commissioning year (estimated) 2014

Normal operating capacity 642 hm3

Number of units 4 (reversible)

Power 171 MW

Annual average capacity 444 GWh (with contributory pumping)

Reduction in CO2, equivalent per year 1037 kt

Estimated investment (ref. 2009) 491 M€

National contribuition 75% to 80%

Ribeiradio-Ermida:

This power plant, also under construction, is located in the Vouga river and includes

two dams and two power houses equipped, the upstream, with one unit, and the

downstream, with two units. This project has the indicators one can see in the next

Table 3.5, but, unlike the previous one, it does not include reversible units.

Table 3. 5 - Main indicators of Ribeiradio-Ermida power plant.

Main indicators



Normal operating capacity 87 hm3

Number of units 3

Power 77 MW

Annual average capacity 134 GWh




Carvão-Ribeira:

This project is, unlike the previous two, not yet in construction, but only planned to be

built. It is supposed to be located in the Távora river, and includes an underground

powerhouse in cavern equipped with two reversible units and an hydraulic circuit by

tunnel. The main indicators are shown in the following Table 3.6.

Table 3. 6 - Main indicators of Carvão-Ribeira power plant.

Main indicators




Power 555 MW





It is possible to notice in this table how big the installed power will be in comparison to

the first two examples. With the big power comes a bigger reduction of CO2, a bigger

national contribution, but of course a bigger investment as well. This power plant

includes reversible units, and will therefore be working with pumped-storage. The

particular characteristic of this example is that it will be the first project in Portugal with

pure reversibility cycle. The other existing systems that include storage, like Alqueva,

Vanda-Nova or Aguieira are not pure, because they have two functions. Besides storage

they are also required to regulate the flows

The two next and last examples consist in an upgrading of two already existing hydro

power plants, namely Alqueva II and Venda Nova III. These are only two examples of

the several others that are being upgraded as well, like Salamonde II, Paradela II,

Bemposta II or Picote II.

Alqueva II:

The particular upgrading of the Alqueva I will be discussed more extensively further on,

since it will be one of the included case studies. This project is located on the right

bank of the Guadiana River. The additional powerhouse will be equipped with two

reversible units and a hydraulic circuit with independent tunnels. The main factors of

this project are in Table 3.7.

Table 3. 7 - Main indicators of Alqueva II power plant.

Main indicators




Power 256 MW





In Figure 3.10 one can observe the longitudinal profile of the hydraulic circuit.

This example presents a lower value of the power, in comparison to Carvão-Ribeira.

However, as it was explained before, this system regulates its own flows, and doesn’t

work only as a pumped-storage system. It complements its duties of regulating the

flows with the advantages of pumped-storage.

Figure 3. 10 - Longitudinal profile of the hydraulic circuit.

Venda Nova III:

The upgrading of Venda-Nova, that is now under construction is positioned on the left

bank of Cávado River. An additional underground powerhouse is included, and it’s

equipped with two reversible units, and a hydraulic circuit. This example was chosen

because it has the particularity of becoming, once it’s ready, the biggest hydro power

plant existing in Portugal. The main indicators are the following, represented in Table

3.8:

Table 3. 8 Main indicators of Venda Nova III power plant.

Main indicators




Power 736 MW





The cases mentioned above justify the conclusion that hydro-power and pumped-

storage have a lot of positive aspects on their side, and are therefore, a sector where big

investments are being done. In the next Figure 3.11 it is possible to observe a market

forecast for Portugal.

referring the number of pumped-storage power plants and the installed capacity. The

cases mentioned earlier are only a few of the ones that are planned to contribute to the

installed capacity of the country.

Figure 3. 11 – Market forecast: Portugal (Ecoprog, 2011).

The prognosis shown in Figure 3.11, was made in 2011. It shows that the pumped-

storage installed capacity will be double in the next years. These plants undoubtedly

contribute to the development of countries and local communities, and the investment is

also being done, taking into account all the positive aspects of such a solution. Dams are

considered a winning solution for the environment. They promote regional

development; improvement in roads; support for entrepreneurship; promotion of

cultural and scientific events; between lots of others. But the crucial benefits are:

tackle global warming, contributing to a reduction in CO2 emissions by replac-

ing fossil fuels;

reduction in external energy dependency;

establishing conditions for regional development and stabilization of local popu-

lations;

irrigation and agricultural improvement;

guarantee of supply for the electricity system in situation of normal variations in

load, even in periods of low water availability;

hydroelectric power plants equipped with pumped-storage can use surplus wind

power production, since they can store this power at off-peak times for subse-

quent use at times of greater demand;

water supply reserves for domestic and industrial use;

management of floods and droughts;

development of inland navigation;

support in forest fire fighting;

development of tourism and recreational activities.

Viablity

In Europe, pumped-storage is being installed rapidly, and one of this papers goal is to

find a pattern for the restrictions. The installed capacity in Europe can be observed in

Figure 3.12.

Figure 3. 12 – Pumped-Storage capacity in Europe (Züber,2011).

This brings this study to its main issue. Which are the restrictions that turn pumped-

storage into a viable solution? Both technical and economical viability have to be

achieved to guarantee a systems success.

Most pumped-storage power plants that are successful and working have a minimum

limit of head and power, H and P. The suggested examples that include pumped-storage

follow these rules as well as many others all over the world. However these values are

not fixed and other solutions are possible, although a big part of pumped-storage

projects find themselves between these limits.

In Table 3.10 are simply estimates to give some guidance referring the viability of

pumped-storage. Most projects, especially the ones that consist of a pure reversible

cycle, respond to these levels, and these values can therefore be a first evaluation or a

first thought about the viability of such a project.

Table 3. 10 – Guidelines for pumped-storage for large systems.

Variables Limits

Power (MW) >100-400

Head (m) >50-500

Price of the energy (M€/MW) >0,2 and <0,8

Discharge (m3/s) >2

3.3. Case studies

Upgrade Project Alqueva II: The hydroelectric power station of Alqueva I is located at

the Guadiana river, in the districts of Beja and Évora. It started to operate as the

Alqueva I pumped-storage scheme in 2004. The structure creates a very large reservoir,

with 4.150x106 m

3 at full water level (152m), the largest in Western Europe.

EDP made it possible to accompany part of the studies, namely the economical

component aswell as the environmental benefits of this project.

The Alqueva I scheme consists of a 96m high concrete double arch dam, and it was

conceived to be a development to benefit the region of Alentejo. Its main objectives are

the following:

electricity supply;

public water supply;

irrigation of 115 000 ha of agricultural land;

implementation of leisure and tourism infrastructures.

A plan of the Alqueva I power plant can be seen in Figure 3.13.

Figure 3.13 - Plan of Alqueva I powerplant (EDP).

The Alqueva I powerhouse is equipped with two Francis pump-turbine units, which

have a total capacity of 2 x 128 = 256 MW in turbine mode. The rated flow is 200 m3/s

and the rated head is 72m.

P = 256 MW

H = 72 m

Q = 200 m3/s

As a first approach, these values, especially the head, 72m, are lower than the ones

suggested in chapter 3. However, as it was mentioned in the same chapter, those values

are mainly for pure reversible cycles, which is not the case of Alqueva I. Besides

storage and regulation of inflows, irrigation or public water supply are also very

important tasks of this dam, and they were strong reasons that justified building this

scheme. Because of its various functions, the limiting values of power and head need a

sensitivity analysis, and can’t be directly applied.

Alqueva I operates together with the Pedrogão reservoir. The Pedrogão reservoir is a

small hydro scheme, located about 18km downstream of the Alqueva dam. This scheme

has an elevation of the water level of 84,80m and consists of a 39 m high RCC gravity

dam and a powerhouse equipped with a 10MW tubular turbine. A picture of Alqueva

and Pedrogão schemes can be seen in Figure 3.14.

Figure 3.14 – Scheme of Alqueva I (left), before the upgrade project, and Pedrogão

(right) (EDP).

Alqueva II

The reasons for the upgrade of the Alqueva II, were mainly the impressive boom in the

wind energy sector, as mentioned before. Most of the new hydro power projects, not

only this one, include the possibility of pumped-storage to act as a key instrument in the

optimum exploitation of large existing reservoirs, storing the excess of wind energy

generated in periods of lower electricity demand and to increase hydropower generation

in more valuable periods.

The upgrade project of Alqueva II was already predicted by the time Alqueva I was

being constructed. However the upgrade came much sooner than it was expected. In the

following Figure 3.15 the structure that was left in order to facilitate the construction of

Alqueva II once it was needed, can be observed.

Figure 3.15 - Structure that was left, for the future construction of Alqueva II (Efacec).

By the end of the construction of the Alqueva II the scheme will look like the following

Figure 3.16, where Alqueva II is shown in pink.

Figure 3.16 - General layout – plan of Alqueva I and II (EDP).

The Alqueva II scheme is very similar to Alqueva I. It will be equipped with two

pump-turbines with very similar characteristics to those of Alqueva I. The new

powerplant will be located on the right bank of the Guadiana river. Two separate

underground upstream hydraulic circuits will connect the water intake structures in the

Alqueva reservoir to the surface powerhouse. The water intake structures are founded at

elevation 121.65 m, and will have an outside platform at elevation 154 m.

Hydromechanical equipment includes steel trash racks, sluice stoplogs and fixed wheel

gates. The stoplogs and gates will be operated by a gantry crane installed on the outside

platform. Each of the two tunnels of the upstream hydraulic circuits, with an inner

diameter of 8.5 m, and 360/387 m long, will have an initial short sub-horizontal stretch

after the water intake, followed by a short vertical shaft which will lead to a new and

longer sub-horizontal stretch extending to the powerhouse. The powerhouse will be

39.70 x79.10 m2

in plan. The setting of the machines is at elevation 62 m, and the main

powerhouse floor at el. 73 m. Each unit will be isolated downstream by two gates. The

outlet conduits will end in a downstream 70 m-long trapezoidal tailrace channel, with its

invert at elevation 74 m.

The powerhouse roof will be integrated in the final outside platform, where the

substation and the control building will be built. The substation structures will be partly

founded on the powerhouse roof slab. Any interferences to the operation of the

Pedrógão reservoir will be restricted to short time periods (for example, during

excavations for the complementary concrete cofferdam and execution of the final

stretch of the outlet channel). The connection of Alqueva II to the nearby Alqueva

Substation of the National Grid Company (Rede Nacional de Transporte) will be by a

1.5 km 400 kV line, parallel to the existing line which serves Alqueva I and using the

same infrastructures.

Concluding, all this upgrade of the Alqueva dam has the goal of doubling the installed

reversible power, in order to use the conditions created by the Alqueva dam and the

Pedrógão dam, which have the possibility of creating week-cycles of pumping and

turbining. This means that with the new Central II the cycles are much easier to rule.

While one is turbining the other one can be ready to pump, when necessary, and the

changes between turbine and pump in the same system start to decrease, which

improves the maintenance of the system. Since there are differences in the energy

cost during one day the system has to pump some hours and turbine some other

hours. With two powerhouses available, the efficiency will be higher and the

deterioration of the equipment will be lower.

The installation of the new central and the two new reversible groups brings advantages.

However it is necessary to be aware of the limiting factor, which is the storage capacity

of the Pedrógão reservoir, which forbids the installation of higher capacities. As much

as a higher capacity would be able to pump a higher quantity of water to the Alqueva

reservoir, the limiting size of Pedrogão reservoir won’t have enough water.

Concluding it is predicted that the central Alqueva II will be equipped with two

reversible groups similar to the ones existing in Alqueva I, with the following

characteristics:

number of groups: 2;

type: reversible Francis turbines – One of them can be seen in the next Figure 3.17 which was

taken during the construction;

available net head: 65m;

turbine average discharge: 192 m3/s;

pump average discharge: 162 m3/s;

power (turbining): 120 MW;

power (pumping): 110 MW.

Figure 3.17 - Francis turbine to be installed in the central Alqueva II.

Alqueva II is still in construction. A simulation of how it is supposed to look when it’s

finished can be seen in Figure 3.18.

Figure 3.18 - Simulation of how the central Alqueva II will look like when finished

(EDP).

Concluding, the Alqueva II project shows how pumped-storage is viable and a good

solution for energy storage. Using the variation of the energy price during the day and

the week shows that such a project, even with a need of a huge investment, is viable and

can generate profit, by taking advantage of this variation.

Seawater Pumped-Storage Power Stations (SPSPS)

A SPSPS consists, as a normal pumped-storage system of two water reservoirs, which

are separated vertically. During off-peak hours water is pumped from the lower

reservoir to the upper reservoir. When required, the water flow is reversed to generate

electricity. The difference in a SPSPS is that the lower reservoir is the sea. This means

that the turbines will pump seawater up to an artificial upper reservoir. An example of

how such a structure would look can be seen in Figure 3.19.

Figure 3.11 - Sectional view of Waterway of the SPSPS Okinawa in Japan (Fujihara,

Imano, Oshima, 1998).

Figure 3.20 – Okinawa Pumped-Storage Power Station.

Figure 4. 2 - View of Pilot Seawater Pumped Storage Power Plant Upper Octagonal

Reservoir (left). The sea is the lower reservoir, the outlet of the tailrace is surrounded by

tetra-pods for protection from waves (right) (Hitachi, 1998).

Figure 4. 3 - Schematic of an energy island using renewable energy and pumped-

storage. The island created in the sea contains a lake in the middle. Wind energy or

energy from the grid is used to pump water out of the lake at off-peak times. The water

can be allowed back into the lake, generating electricity.

Innovation: Seawater Pumped-Storage in Arid Regions - Most islands present

favorable conditions in the matter of renewable energy sources. A good example is the

islands of Cape-Verde. Wind, solar, hydro, and wave energy are available, and in big

quantity, especially wind and solar. Renewable energies are considered a good option

for islands, since most of these are highly dependent on fossil fuels regarding energy

production and supply.

Cape Verde presents a big scarcity in fresh water. Rivers almost don’t exist, and long

drought periods happen very often. Rain only comes in torrential events, and the water

only lasts a few days as superficial runoff after the storm. Most of the islands are

included in the desert area categorization, and only three are in the semi-arid area

categorization. These three have an average annual rainfall lower than 500mm. This

aspect will represent a first big challenge to this project.

http://climatetechwiki.org/sites/climatetechwiki.org/files/images/extra/pumpstorageisland.jpg

Concluding

Japan is not only the country who holds the biggest installed capacity, but it was also

the first country trying the Seawater Pumped Storage Power Station. Also Ireland was

one of the first considering the SPSPS, and has recently a capacity of 292 MW. These

countries were able to see the significance of electricity storage in a “carbon-

constrained” world. They realized the non existing flexibility of low-carbon electricity

resources, to adjust their output to match fluctuating power demands. The search for this

system was done, because nuclear power operates best continuously and can’t adjust to

peak and off-peak situations, and wind and sun are intermittent and have therefore little

control over the schedule of electricity output. The ability of stabilizing the electricity

grid through peak shaving, load balancing, frequency control and reserve generation

makes the pumped storage the most proven and commercially viable solution for the

problems and obstacles mentioned before.

Documents

3- Pumped-storage hydropower - Autenticação · PDF file3- Pumped-storage hydropower 3.1 Brief considerations Since the electrical grid needs to assure the supply at any time, including