pump used as turbine in small hydropower stations

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Investigation of Pump as Turbine Performance in Small
Hydropower Stations
A.Nourbakhsh1 , Sh.Derakhshan2
1. Professor, Department of Mechanical Engineering, University of
Tehran, Tehran 14147, Iran
2. M.S. Student, Department of Mechanical Engineering, University of
Tehran, Tehran, Iran
Abstract:The major problem for installation of new small hydropower stations is
specific cost expressed in $/KW installed capacity. Part of the solution of this
economic problem is to substitute the conventional turbines by simple and
inexpensive machines such as reversible centrifugal pumps.
Based on our experimental tests described in this paper a centrifugal pump can
operate properly as a turbine, but of course with lower efficiency comparing to the
conventional turbines. From an economical point of view, using a pump instead of a
turbine can thus be recommended for power stations under approximately 500 KW
installed capacity.
Pump manufacturers do not normally provide the performance curves of their
pumps for working sturdiness. Therefore, establishing a correlation proper the
passage from the "pump" performance to "turbine" once is important for selecting
the proper machine in this study several empirical methods existing in the literature
for predicting the best efficiency point of pumps running as turbines are presented
and compared. However each of these of methods is applicable only for particular
cases. Therefore a great interest for future research in this field is to investigate a
theoretical way for predicting a pump's behavior operating as a turbine. This paper
describes the principles of a theoretical method which can be used in general cases.
Key-words: pump as turbines- best efficiency point-small hydropower-discharge- power-head
1 Introduction
Small hydroelectric power stations became
attractive for generating electrical energy after
the oil price crisis of the seventies. However
cost per KW energy produced by these
stations is higher than the hydroelectric power
plants with large capacity. Numerous
publications in recent years emphasize the
importance of using simple turbine in order to
reduce the cost of produced electrical energy.
There is need for installation small
hydroelectric power stations many developing
countries. Based on our literature survey,
approximately 200 known hydraulic potentials
are available in Iran and can be used in case
those projects will come feasible [1].
We considered the idea of using pumps as
hydraulic turbines an attractive and important
alternative. Pumps are relatively simple
machine, are easy to maintain and are readily
available in most developing countries. From
the economical point of view, it is often stated
that pumps working as turbines in the range of
5 to500 KW allow capital payback periods of
tow years or less which is considerably less
than that of a conventional turbine.
In this paper we report our experiments on
using pump as a turbine which was curried in
a complete laboratory model of mini
hydropower plant.
2 Classification
There are several method proposed by
different organization for classification of
hydropower stations according to their
powers, heads and flow rates. In table.1 the
classification according to Latin American
Energy Organization (OLADE) is presented.
Table.1.OLADE small hydropower classification
3 Applications fields
The application fields of conventional turbines
such as Pelton, Francis and Kaplan are well
known according to their heads and specific
speeds. Due to limited test data and
experimental results available for pumps
working as turbines the application fields of
these machines are not yet well defined.
However one can compare the application
Hydropower
Station
Power
Range
(KW)
Head (m)
Low
Mediu
m
High
H<50
15-50
H>50
Micro
Hydropower
Up to 50
Mini
Hydropower
50 to
500
H<20
20-100
H>100
Small
Hydropower
500 to
5000
H<25
25-130
H>130
field of multistage and axial pumps
respectively with Pelton, Francis and Kaplan
turbines (Figure.1).
4 Experiments and results
A complete laboratory model of a mini
hydropower plant was built in University of
Tehran [1]. The flow rate and head for pumps
working as turbines were generated with
several
pumps.
Serial
and
parallel
combinations of these pumps enabled us to
obtain a wide range of flow rates and heads.
When a pump is working as a turbine a control
system is needed for automatically regulating
the frequency. The classical governor used for
standard turbines are expensive and not
always recommended for small hydropower
plants. Since these types of plants are more
used in isolated areas we built and used an
electronic load controller with ballast loads for
keeping the constant frequency of generator in
our tests.
1000
Pelton
Francis
100
Cross-flow
Head (m)
In some countries and also in Iran, micro,
mini and small hydropower plants are
generally called under a common name "small
hydro power plants".
Kaplan
10
Single-impeller pumps
0
0
100
1000
Flow (m3/h)
10000
100000
Figure.1. application range for water turbines and
pumps as turbines
A conventional synchronous generator may be
installed for producing electricity. Induction
motors which are less expensive and more
available than convectional generators are
recently used in mini hydropower stations for
generating electricity. We coupled these types
of motors with our pumps working as
turbines.
We tested industrial centrifugal pumps
available in market without any modification
for operating as turbines at various rotational
speeds (1000 rpm, 1500 rpm, 2000 rpm and
\2500 rpm). In each rotational speed the head,
flow rate and power were measured. The
characteristic curves of centrifugal pump
operating as turbine with specific speed
(Ns=25) are illustrated as an example in
figure.3.
Based on our experimental results (fig.3), we
observed that:
-A centrifugal pump can operate without any
mechanical problem as turbine in different
rotational speeds and various heads and flow
rates.
-The maximum efficiency value of the
machine working as a pump or as a turbine is
almost the same.
-When the machine runs as a turbine, the best
efficiency point is obtained for higher flow
rate than when it operates as a pump.
-Power produced by turbine at its best
efficiency point is higher than the power input
for the pump operating at its best efficiency
point.
-The reduced value of efficiency and high
efficiency region of performance curve of
centrifugal pumps operating as turbine
represents the main disadvantage of these
machines compared to conventional turbines.
Of course the pumps with higher specific
speed present better efficiency and the
propeller pumps with the adjustable impeller
blades enable wide working region with
higher efficiency.
Ht 
Hp
ht .hp

Hp
2h
(1)
(hp  ht ); ( N st  N sp )
Qt 
Qp
h
(2)
h : Hydraulic efficiency
h  0p.5
(p : pump overall efficiency ).
Based on these equations, a centrifugal pump
HEAD, EFFECIENCY, KW OUTPUT
RPM=CONSTAN
T
with   75% working as a turbine, in the
same rotational speed should work under the
Ht=1.33Hp and Qt=1.15Qp.
HEAD
EFFICIENCY
KR Sharma of Kirloskar Co. India [3]
proposed the following equations:
Qp
Q t  0.8
(3)
p
Ht 
KW OUTPUT
Figure. 2. performance curves of pumps and
turbines
5 Prediction of design point of a
pump operating as turbine
Pump manufactures, normally provide the
characteristic curves of their pumps.
Therefore, establishing a correlation enabling
the passage from the "pump" characteristics
the "turbine" characteristics is the main
challenge in using a pump as a turbine.
Several empirical methods and rules are
proposed in the literature for predicting the
best efficiency point for a pump running as
turbine.
Stepanoff [2] proposed the following
equations:
Hp
(4)
1p.2
According to experimental study by M.Gantar
[4], the accurate determination of best
efficiency point of a pump in turbine mode
operation is not possible from the pump
characteristics. However he proposed the
following relations:
Ht
 1.13 to 1.59
(5)
Hp
Qt
 1.23 to 1.44
Qp
(6)
Wong [5] proposed the following procedure
for predicting the performance curves of a
centrifugal pump working as turbine, figure.2.
1. Determination of best efficiency point
based on Stepanoff equations.
2. The shut off head of pump turbine is
equal to 1/3 of pump.
3. The performance curve of turbine is a
mirror reflection of pump.
4. The efficiency of turbine is zero on
0.4Q of design point.
Figure.3. experimental characteristic curves of pump running as turbine
We have predicted with a theoretical method,
efficiency, flow rate and head of a pump
working as a turbine. According to this
method, the hydraulic, disk friction, and
leakage losses and also slip factor are
calculated for pump mode and turbine mode
operations.
Of course more experimental data is required
for determining the accuracy of this method.
Based on our study the equation for prediction
of design point of a pump operating as turbine
can be defined in general from as the
following:
Ht=a.Hp
Qt=b.Qp
(7)
(8)
Were a>b>1
The values of (a) and (b) are dependent mainly
on specific speed and efficiency of pump.
Therefore it can be concluded that each of
above equations proposed by the mentioned
authors is valid only for some particular cases.
6 Conclusions
In the turbine mode of a pump the output of
turbine is more than the input power of pump.
And the pump in turbine mode works in
higher head and discharge. The pump operates
without problem in different rotational speed
and various head and discharge. The
efficiency is same in both of pump and turbine
mode and little more than the pump mode.
With slightly impeller and the other
component of pump modification, the
efficiency will increase. Of course the pumps
with higher specific speed present better
efficiency and the propeller pumps with the
adjustable impeller blades enable wide
working region with higher efficiency. There
doesn't exist the exactly formula or theory
method for prediction of the turbine
performance of pump. But with more try it is
possible.
References:
[1] Nourbakhsh, A, and Jahangiri, G.(1992).
Inexpressive small hydropower stations for
small areas of developing countries, pp.313319. conference on Advances in Planning,
Design and Management of Irrigation Systems
as Related to Sustainable Land use, Louvain,
Belgium.
[2] Stepanoff, A.J. (1957). Centrifugal and
axial flow pumps, John Wiley and Sons, New
York.
[3] Williams, A. (1995). Pumps as turbines a
user's guide, pp.34, intermediate Technology
publications, London.
[4] Gantar, M. (1988). Propeller pumps
running as turbines, pp 237-248, conference
on Hydraulic Machinery, Ljubljana, Slovenia.
[5] Wong, W. (1987). Application of
centrifugal pumps for power generation, pp.
381-348, World Pumps.
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