Energy efficiency evaluation method.

Grigoriu, Mircea ; Gheorghiu, Horia ; Crai, Alina 等

1. INTRODUCTION

A general applied solution is using variable speed driving pumps. Pumping equipments and installations energy efficiency improvement represents a key solution, taking into consideration the percentage of about 20% from the total energy consumption.

If for the maximum efficiency of the pumping equipments at the nominal working regime is hardly to obtain significant improvements, taking into consideration the actual performances of the devices, there is an important potential to be evaluated in pumping systems operation optimal adjustment.

There are different ways to operate adjustments of pumps working point, depending to the period of regime changing, the pumps power and dimensions, pumps and installation type, adjustment sharpness, etc.(Dragoi et al, 2007)

For definitive operation point changing, it is preferable to action on the pump itself, by using different types of impellers in the same casing, the same impeller in different casings, or providing a permanent changing to a standard pump type.

For operating point changing for a long period of time, it could be used different types of diaphragms at the pump outlet or in appropriate points of the network. (Grigoriu, 2006)

For short term changes, it is preferable to use special adjustable devices of the installation (vanes, by passes), of the driving engines (variable speed motors), or pump itself (adjustable impellers for axial flow pumps, or same diagonal pumps). (Lupea, 2007)

Generally, the challenge is between adjustable vanes and variable speed motors for short term changes in the majority types of installations.

2. THE PROPOSED ENERGY EFFIENCY METHOD

2.1 Operation point representation

Operation point could be obtain graphically as the intersection between the pump head-flow characteristic curve [H.sub.p](Q), equation (1), with the pipe head-flow resulting characteristic curve [H.sub.r](Q), equation (2), as it is presented in fig. 1, or analytically solving the equations system representing the polynomial approximation of the mentioned characteristics curves, as follows [2]:

[H.sub.p] = [a.sub.1][Q.sup.2] + [a.sub.2]Q + [a.sub.3] (1)

[H.sub.r](Q) = [b.sub.1][Q.sup.2] + [b.sub.2] (2)

It is to mention that the pump characteristic represent the resulting head-flow characteristic of one pump or of a whole pumping station, and the pipe characteristic represent the resulting head-flow characteristic of one pipe or a whole network. For variable speed driving motor, the pump polynomial form of the head-flow characteristic is (3), and for the pipe polynomial form of the head-flow characteristic is (4)

[H.sub.p](Q,n) = [a.sub.1][Q.sup.2] + [a.sub.2]Qn + [a.sub.3][n.sup.2] + [a.sub.4]Q + [a.sub.5]n + [a.sub.6] (3)

[H.sub.n](Q,x) = ([b.sub.0] + [b.sub.1]) Q2 + [b.sub.2] (4)

[FIGURE 1 OMITTED]

where:

Q--flow at the operating point;

n--rotation speed at the operating point,

[a.sub.1], [a.sub.2], ..., [a.sub.6]--polynomial coefficients for the pump head-flow characteristic,

[b.sub.0], [b.sub.1], [b.sub.2]--polynomial coefficients for the network head-flow characteristic.

In fig 2 is represented the operation adjustment of pumping system using the vane and a variable speed driving engine [3] from the initial flow ([Q.sub.1]), to the requested flow ([Q.sub.r]), where:

[eta](Q, n)--efficiency characteristic of the pumps, depending the flow and the rotation speed;

[WP.sub.1]--operation point of the system at the initial flow;

[WP.sub.2]--operation (working) point of the system at the requested flow, using vane adjustment;

[WP.sub.3]--operation (working) point of the system at the requested flow, using a variable speed engine adjustment;

[FIGURE 2 OMITTED]

Analytical representations of the pump efficiency at different rotational speeds are the equation (5) (Grigoriu, 1995).

[eta](Q,n) = [c.sub.1][Q.sup.2] + [c.sub.2]Qn + [c.sub.3][n.sup.2] + [c.sub.4]Q + [c.sub.5]n + [c.sub.6] (5)

where:

[c.sub.1], ..., [c.sub.6] represent polynomial coefficients for pump the efficiency characteristic.

For the initial and requested flows, the necessary head and efficiencies are:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

To see a procedure or equipment efficiency, it is compared the utile and consumed powers, or energy for a determined period of time (Grigoriu, et al., 2006).

2.2 Vane adjustment efficiency

The utile power is computed for the point WP3 and the consumed power, for WP2.

[P.sub.WP3] = [rho]g[Q.sub.r][H.sub.WP3], [P.sub.WP3] = [rho]g[Q.sub.r][H.sub.WP3]/[eta](Q,[n.sub.1]) (6)

2.3 Variable speed engine adjustment efficiency

Both the utile and the power are computed for the point WP3 and, defining head efficiency, as follows:

[[eta].sub.h] = [H.sub.WP3]/[H.sub.WP2] (7)

The vane adjustment efficiency is:

[[eta].sub.v] = [P.sub.WP3]/[P.sub.WP2] = [[eta].sub.h][eta] (8)

2.4 Variable speed engine adjustment efficiency compared to the vane adjustment

Both the utile and consumed power is computed for the point [WP.sub.3] [4]:

[P.sub.u] = [P.sub.WP3] = [rho]g[Q.sub.r][H.sub.WP3] (9)

[P.sub.c] = [P.sub.WP3]/[eta]([Q.sub.r],[n.sub.2]) = [rho]g[Q.sub.r][H.sub.WP3]/[eta]([Q.sub.r],[n.sub.2]) (10)

then, the variable speed adjustment efficiency is equal with the pump itself at [Q.sub.r]:

[n.sub.s] = n(Q,[n.sub.2]) (11)

2.5 Variable speed engine adjustment efficiency compared to the vane adjustment

They are compared the consumed power adjusting the flow with a vane ([P.sub.cv]), with the consumed power adjusting the flow with a variable speed device ([P.sub.cs]), by defining a specific efficiency ([[eta].sub.vs]) or consumed power difference

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (12)

[[eta].sub.vs] = [P.sub.cs]/[P.sub.cv] = [[eta].sub.h][eta](Q, [n.sub.l])/[eta](Q, [n.sub.2]) (13)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (14)

2.6 Energy savings using variable speed engine adjustment

Considering the period of operation time during one year or other period of time, it's easy to calculate the energy savings as follow:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (15)

3. CONCLUSIONS

Energy efficiency of the pumping systems is an important factor of municipal infrastructure investments evaluation and the proposed method is really effective, considering that the variation of system operation with vanes represents, in many situations the real solution that is modernized by using variable speed pumps.

5. REFERENCES

Dragoi, G., Funar, St., Solea, M., Cotet, C.E. (2007), Simulation and optimization of a waste processing flux, Mat. Plast., Vol. 44, No. 1, p.77-81

Grigoriu, M; (2006), Pumps and Pumping Systems. Ed. Printech, Bucharest, Romania.

Grigoriu, M. (1995), Pumps, Fans, Compressors. Ed. Printech, Bucharest, Romania

Grigoriu, M.; Guzun, B.; Barglazan, M. (2006), Optimal High Powerful Pumped Storage System. Rev Energetica, p.487 491, No. 11

Lupea, I., Cormier (2007), J., Size and Shape Optimization of a Polymeric Impact Energy Absorber by Simulation, Mat. Plast., Vol. 44, No. 4, p.339-344