Small Decentralized Hydropower Program National ... - Cd3wd.com

flujo sin regulaci6n funcionaria bajo cargas parciales
por ciertos periodos de tiempo.
Caracteristicas Fisicas e
Hidr&ulicas de1 Sitio
Se debe entender que las caracteristicas hidrokgicas,
hidrzklicas y fkicas a que nos referimos en
este art&lo estin limitadas generalment a aquellas
~ir;e infk~yen ~~&23,TMTit~ la generaci6n de ia hidroekctricidad.
Se necesitak tambien una gran cantidad
de trabajo de ingenieria para el disefio de la represa
y crsllol n-n;deraciones de seguridad, incluyendo la
seguridad y economia de1 pasaje de1 caudal de1
desbordamiento. Cuando se considere una represa en
existencia es particularmen* importante que se lleve
a cabo una inspecci6n satisfactoria de la seguridad
hecha por un ingeniero competente.
La caida hidrkulica, y sus variaciones, disponible
para generar hidroenergia esti, kgicamente, relacionada
estrechamente al esquema de1 proyecto concebido.
Donde se esti desarrolando caidas altas las
variaciones son de importancia mimma. Sin embargo,
en 10s sistemas de caidas bajas, es importante
estudiar cuidadosamente el sitio y el esquema de1
proyecto to propuesto para determinar la relacicin de
la caida a la descarga tanto en el reservorio (cuando
se lo use) coma en el kea de salida de1 agtia. En este
case, la caida m&xima generalmente estar5 disponible
cuando 10s flujos Sean mtis bajos; asimismo, es muy
posible que la caida disponible sea tan pequefia en
tiempos cuando 10s flujos estkn a su altura extrema
de modo que la cantidad de energia producida sea
insignificante.
Aunque la mayoria de 10s proyectos para desarrollar
plantas hidroelktricas pequefias tienden a
aprovechar la correinte de1 150, es muy posible que un
reservorio consttido tenga volumen suficiente para
ofrecer alguna capacidad regulatoria. En todo case,
es necesario investigar las caractetisticas de1 reservorio
para determinar el Brea que ser5 inundada. Las
caractetisticas a determinarse incluiran las relaciones
entre las etapas de capacidad, y la capacidad de1
kea, extensi6n de1 kea, y 10s efectos de1 remanso.
Las caracterkticas fisicas de1 lugar para la
represa/reservorio debe incluir tambi6n las
caracteristicas de aguas subterraneas y
permeabilidad para asegurar el estado “hermetico”
de1 reservorio.
Hay numerosos factores en el lugar que pueden
controlar la eventual consideracibn de1 esquema para
el desarrollo potential. MU&OS de 6stos no tienen
tanto que ver con el lugar en si corn0 con las relaciones
de! mismo a otras considerciones. En el
53
energy basically assumes no storage for flow
regulation. It does permit, however, the incorpora-
tion of variable efficiencies of electric energy pro-
duction as well as the relationship between
effective head and discharge rate. A number of
alternative capacities can be analyzed and cost
comparisons made for eventual use in optimizing
the system design.
The typical development will probably consider
variable pitch propellers (“Kaplan turbines”) very
carefully because of the increased cost, in spite
of the vastly superior maintained efficiency. The
problem is that the efficiency curve for a fixed
blade system will show a peak at which point the
best use of the water occurs. A hydro plant with
a single non-adjustable turbine will then have on-
ly one flow with peak efficiency. For a situation
where there is some storage, it is possible that a
single unit may be acceptable.
Where no storage exists, better use of the flow
may sometimes be made if multiple turbine units
are incorporated. Multiple units may be of equal
size or, for greater overall efficiency, of unequal
size. As Purdy (1979) explains, “A plant with two
unequal size turbines has three peak efficiency
points, a plant with three unequal size units has
seven peak efficiency points. The ideal sizing is
approximately 70-30 and 57-28-15, respectively.”
As he notes, the important advantage is the
much improved operation during low flow. Also,
because a large portion of the flow duration
curve is used, the system can be operated much
closer to run-of-river with little reservoir draw-
down and, consequently, a high average head
Figures 7 and 8 show a simplified example
(Fischer, 1979) oi four equally sized turbines, the
operation of which is superimposed first on a
typical annual hydrograph, and second on a flow-
duration curve. In periods of high water flow, the
full capacity of all four units is exceeded - and
presumably excess water is being discharged via
spillways.
On the other hand, during the period of low
flows one unit is used, and then only partially.
Where the flow-duration curve is used (Figure 8),
it can be seen that (in this example) one unit will
operate 70 percent of the time at full capacity,
two units will operate 40 percent of the time at
full capacity, three units will operate 30 percent
of the time at full capacity, and four units will
operate only 20 percent of the time at full
capacity. In this example, for 20 percent of the