Pump Used As Turbine in Small Hydropower Stations

Investigation of Pump as TurbinePerformance inSmall Hydropower Stations

a.Nourbakhsh1, Sh.Derakhshan2

1. Professor, Department of Mechanical Engineering, University of Tehran, Tehran14147, 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 reversiblecentrifugal 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 importantalternative. Pumps are relatively simplemachine, 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.

In some countries and also in Iran, micro, mini and small hydropower plants are generally called under a common name "small hydro power plants".

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 field of multistage and axial pumps respectively with Pelton, Francis and Kaplanturbines (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.

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 indifferent 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.

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:

Based on these equations, a centrifugal pump with working as a turbine, in the same rotational speed should work under the Ht=1.33Hp and Qt=1.15Qp.

KR Sharma of Kirloskar Co. India [3] proposed the following equations:

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:

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 leakagelosses 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 turbinecan be defined in general from as the following:

Ht=a.Hp (7)

Qt=b.Qp (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.313-319. conference on Advances in Planning, Design and Management of Irrigation Systems as Related to SustainableLand 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.