# Speed and Pressure Controls of Pumps-as-Turbines Installed in Branch of Water-Distribution Network Subjected to Highly Variable Flow Rates

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Case Study

## 3. PaT Performance

#### 3.1. Selection of PaT Operating at Rated Conditions in Turbine Mode

#### 3.2. Characteristic Curves in Turbine Mode

^{3}/s), $\omega $ in (rad/s), D in (m), ${P}_{\mathrm{m}}$ in (W), $\rho $ in (kg/m

^{3}), g in (m/s

^{2}) and H in (m).

#### 3.3. MATLAB©–Simulink Model

## 4. Results and Comments

#### 4.1. Case 1: Basic Flow-Rate Control Strategy

^{3}/h. If flow-rate demand was higher than $14.35$ m

^{3}/h, the head of each hydraulic machine would rise; thus, downstream pressure would drop below the value of 4 bar. Block B bypassed the excesses of flow rates in a secondary circuit to avoid an excessive drop of downstream pressure below the imposed limit. As shown in Figure 9, this operating strategy did not control PaT rotational speed, which was kept fixed to 2900 rpm. Hydraulic machines operate at their BEP only with ${Q}_{\mathrm{BEP}}$, while the operating point moves along the $Q-\eta $ curve when flow rate that is lower than that the BEP one is exploited. In this case, ${Q}_{\mathrm{BEP}}$ corresponds to the average flow rate of the analyzed WDN branch, as discussed in Section 2. When PaTs deal with off-design operating conditions, such as the ones indicated through Points 1 and 2 in Figure 9, mechanical efficiency decreases.

#### 4.2. Case 2: Basic Flow-Rate and Speed Control

^{3}/h, which would cause an excessive decrease of downstream pressure, was bypassed by following the same procedure as that explained in Section 4.1. When flow rates were below $14.35$ m

^{3}/h, PaT rotational speed was reduced according to the exploited flow-rate value by applying affinity laws to let PaTs always operate at their BEP. Due to this, the efficiency curve ($Q-\eta $) was shifted in order to match operative flow rate with the BEP condition, as shown in Figure 10. However, using this strategy, the amount of recovered energy increased by $1.5\%$ compared to Case 1; this result was due to small variations of flow rates with respect to the maximum exploitable one of $14.35$ m

^{3}/h.

#### 4.3. Case 3: Advanced Speed and Flow Control

^{3}/h, the rotational speed of PaTs automatically decreased in order to keep the downstream pressure fixed to 4 bar, following the same procedure explained in Section 4.2. Using this control strategy, the whole available flow rates were exploited at the expense of mechanical-efficiency reduction. The amount of recovered energy was $23\%$ higher than the one obtained in Cases 1 and 2 due to the higher exploitation of flow rates since, in the previous ones, values higher than $14.35$ m

^{3}/h were bypassed. However, neither PaT operated at its BEP when dealing with flow rates higher than the BEP one due to the constraint of downstream pressure fixed at 4 bar. Through analysis of the mechanical-efficiency trend when PaTs operate in off-design conditions, we can see how efficiency loss was limited when flow rates exceeded the one at BEP, while it dropped when flow rate was lower than the BEP one, especially at extreme part-load conditions.

#### 4.4. Case 4: Basic Flow-Rate and Speed Control with Reduced WDN Pressure to $3.5$ Bar

^{3}/h do not justify the use of a PaT speed-control strategy based on BEP tracking.

^{3}/h was obtained, which was higher than the one of previous cases ($14.35$ m

^{3}/h). Finally, results showed that this control strategy was the most effective since it led to energy recovery that was almost 11 % higher than the one obtained in Case 3, where all flow rates were exploited by the PaTs.

## 5. Energy and Economic Analysis

^{3}/h that would decrease downstream pressure below 4 bar. Even though Case 2 allowed PaTs to always operate at their BEP as shown in Figure 11, the high amount of bypassed flow rates significantly contributed to reducing energy recovery, thus reducing the effectiveness of the speed control applied to PaTs. The trend of energy recovery over the day and its ratio with respect to the wasted energy in the PRVs are shown in Figure 12 and Figure 13.

^{3}/h and it led to a significant bypass of flow rates during the day, thus affecting the effectiveness of energy recovery. The only advantage recorded in Case 2 was the possibility of having a speed-control strategy that helps PaTs to optimally exploit flow rates lower than $14.35$ m

^{3}/h, thus always operating at their BEP. However, if flow-rate variations are limited, advantages in terms of energy recovery are minimal.

^{3}/h, as well as energy recovery, listed in Table 5. Indeed, these considerations can also be seen in Figure 11, where the trend of the mechanical efficiency in various cases is shown. Case 4 allowed PaTs to operate with higher variability of flow rates and, thanks to speed control, at their BEP. Compared to Case 3, the control strategy in Case 4 allows to recover a higher amount of energy from the WDN branch: a lower flow rate is exploited but higher head is available; thanks to speed control, PaTs always operate at their BEP.

## 6. Conclusions

^{3}/h. In Case 2, PaT speed control was implemented to let PaTs always operate at their BEP, and when flow rates lower than $14.35$ m

^{3}/h were available. In Case 3, an advanced pressure-control strategy was applied by varying PaT rotational speed in order to exploit all available flow rates. In this case, the aim of the speed-control strategy was to keep the downstream pressure constant and equal to 4 bar, also with flow rates higher than the BEP; this is done by reducing the rotational speed of the machine when operating off-design. The reduction of hydraulic efficiency is offset by the higher exploited flow rate. Finally, in Case 4, sensitivity analysis on the decrease of the downstream pressure of the WDN branch was performed, which was reduced from 4 to $3.5$ bar. The flow rate exploited by PaTs, as well as the net head, were increased compared to Cases 1 and 2, proving the advantages of lowering WDN operating pressures in terms of both energy recovery and economic savings.

^{3}/h; indeed, the amount of recovered energy was $1.5$% higher than that of Case 1. On the other hand, Case 3 showed significant improvement, leading to an increase of about 23 % in terms of energy recovery with respect to Case 1. Finally, Case 4 showed the effectiveness of a speed-control strategy that lets PaTs operate at their BEP in the case of strongly variable operating flow rates. In this case, energy recovery was $36\%$ higher than the one obtained in Case 1, which led to the best result among the analyzed control strategies.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

- Nassar, I.A.; Hossam, K.; Abdella, M.M. Economic and environmental benefits of increasing the renewable energy sources in the power system. Energy Rep.
**2019**, 5, 1082–1088. [Google Scholar] [CrossRef] - Silva, M.; Leal, V.; Oliveira, V.; Horta, I.M. A scenario-based approach for assessing the energy performance of urban development pathways. Sustain. Cities Soc.
**2018**, 40, 372–382. [Google Scholar] [CrossRef] - Balkhair, K.S.; Rahman, K.U. Sustainable and economical small-scale and low-head hydropower generation: A promising alternative potential solution for energy generation at local and regional scale. Appl. Energy
**2017**, 188, 378–391. [Google Scholar] [CrossRef] - Yuksel, I. Water management for sustainable and clean energy in Turkey. Energy Rep.
**2015**, 1, 129–133. [Google Scholar] [CrossRef] [Green Version] - Punys, P.; Dumbrauskas, A.; Kvaraciejus, A.; Vyciene, G. Tools for Small Hydropower Plant Resource Planning and Development: A Review of Technology and Applications. Energies
**2011**, 4, 1258–1277. [Google Scholar] [CrossRef] [Green Version] - Patsialis, T.; Kougias, I.; Kazakis, N.; Theodossiou, N.; Droege, P. Supporting Renewables’ Penetration in Remote Areas through the Transformation of Non-Powered Dams. Energies
**2016**, 9, 1054. [Google Scholar] [CrossRef] [Green Version] - Korkovelos, A.; Mentis, D.; Siyal, S.H.; Arderne, C.; Rogner, H.; Bazilian, M.; Howells, M.; Beck, H.; de Roo, A. A Geospatial Assessment of Small-Scale Hydropower Potential in Sub-Saharan Africa. Energies
**2018**, 11, 3100. [Google Scholar] [CrossRef] [Green Version] - Renzi, M.; Rudolf, P.; ¸Stefan, D.; Nigro, A.; Rossi, M. Installation of an axial Pump-as-Turbine (PaT) in a wastewater sewer of an oil refinery: A case study. Appl. Energy
**2019**, 250, 665–676. [Google Scholar] [CrossRef] - Rossi, M.; Nigro, A.; Pisaturo, G.R.; Renzi, M. Technical and economic analysis of Pumps-as-Turbines (PaTs) used in an Italian Water Distribution Network (WDN) for electrical energy production. Energy Procedia
**2019**, 158, 117–122. [Google Scholar] [CrossRef] - Samir, N.; Kansoh, R.; Elbarki, W.; Fleifle, A. Pressure control for minimizing leakage in water distribution systems. Alex. Eng. J.
**2017**, 56, 601–612. [Google Scholar] [CrossRef] - van Zyl, J.E.; Clayton, C. The effect of pressure on leakage in water distribution systems. Water Manag.
**2007**, 160, 109–114. [Google Scholar] [CrossRef] - Righetti, M.; Bort, C.; Bottazzi, M.; Menapace, A.; Zanfei, A. Optimal selection and monitoring of nodes aimed at supporting leakages identification in WDS. Water
**2019**, 11, 629. [Google Scholar] [CrossRef] [Green Version] - Saldarriaga, J.; Salcedo, C.A. Determination of Optimal Location and Settings of Pressure Reducing Valves in Water Distribution Networks for Minimizing Water Losses. Procedia Eng.
**2015**, 119, 973–983. [Google Scholar] [CrossRef] [Green Version] - Wright, R.; Abraham, E.; Parpas, P.; Stoianov, I. Optimized Control of Pressure Reducing Valves in Water Distribution Networks with Dynamic Topology. Procedia Eng.
**2015**, 119, 1003–1011. [Google Scholar] [CrossRef] [Green Version] - Yang, S.S.; Derakhshan, S.; Kong, F.Y. Theoretical, numerical and experimental prediction of pump as turbine performance. Renew. Energy
**2012**, 48, 507–513. [Google Scholar] [CrossRef] - Polák, M. The Influence of Changing Hydropower Potential on Performance Parameters of Pumps in Turbine Mode. Energies
**2019**, 12, 2103. [Google Scholar] [CrossRef] [Green Version] - Binama, M.; Su, W.T.; Li, X.B.; Li, F.C.; Wei, X.Z.; An, S. Investigation on pump as turbine (PAT) technical aspects for micro hydropower schemes: A state-of-the-art review. Renew. Sustain. Energy Rev.
**2017**, 79, 148–179. [Google Scholar] [CrossRef] - Arpe, J.; Prénant, J.; Dubas, M.; Biner, H.-P. Project Charactéristiques Des Pompes Fonctionnant en Turbines; Rapport Final du Project N° 100400/150 497; Ecole d’ingénieurs de Genève, Haute école valaisanne: Genève, Switzerland, 2006; p. 45. [Google Scholar]
- Rossi, M.; Renzi, M. A general methodology for performance prediction of pump-as-turbines using Artificial Neural Networks. Renew. Energy
**2018**, 128, 265–274. [Google Scholar] [CrossRef] - Venturini, M.; Manservigi, L.; Alvisi, S.; Simani, S. Development of a physics-based model to predict the performance of pumps as turbines. Appl. Energy
**2018**, 231, 343–354. [Google Scholar] [CrossRef] - Rossi, M.; Nigro, A.; Renzi, M. Experimental and numerical assessment of a methodology for performance prediction of Pumps-as-Turbines (PaTs) operating in off-design conditions. Appl. Energy
**2019**, 248, 555–566. [Google Scholar] [CrossRef] - Frosina, E.; Buono, D.; Senatore, A. A Performance Prediction Method for Pumps as Turbines (PAT) Using a Computational Fluid Dynamics (CFD) Modeling Approach. Energies
**2017**, 10, 103. [Google Scholar] [CrossRef] [Green Version] - Carravetta, A.; Derakhshan, S.; Ramos, H.M. Pumps as Turbines; Springer Tracts in Mechanical Engineering; Springer: Berlin, Germany, 2018; p. 236. [Google Scholar]
- Wang, T.; Wang, C.; Kong, F.; Gou, Q.; Yang, S. Theoretical, experimental, and numerical study of special impeller used in turbine mode of centrifugal pump as turbine. Energy
**2017**, 130, 473–485. [Google Scholar] [CrossRef] - Carravetta, A.; del Giudice, G.; Fecarotta, O.; Ramos, H.M. PAT Design Strategy for Energy Recovery in Water Distribution Networks by Electrical Regulation. Energies
**2013**, 6, 411–424. [Google Scholar] [CrossRef] [Green Version] - Du, J.; Yang, H.; Shen, Z.; Chen, J. Micro hydro power generation from water supply system in high rise buildings using pump as turbines. Energy
**2017**, 137, 431–440. [Google Scholar] [CrossRef] - Lydon, T.; Coughlan, P.; McNabola, A. Pressure management and energy recovery in water distribution network: Development of design and selection methodologies using three pump-as-turbine case studies. Energy Procedia
**2017**, 114, 1038–1050. [Google Scholar] [CrossRef] - Lima, G.M.; Luvizotto, E.; Brentan, B.M. Selection and location of Pumps as Turbines substituting pressure reducing valves. Renew. Energy
**2017**, 109, 392–405. [Google Scholar] [CrossRef] - Lima, G.M.; Brentan, B.M.; Luvizotto, E. Optimal design of water supply networks using an energy recovery approach. Renew. Energy
**2018**, 117, 404–413. [Google Scholar] [CrossRef] - Kramer, M.; Terheiden, K.; Wieprecht, S. Pumps as turbines for efficient energy recovery in water supply networks. Renew. Energy
**2018**, 122, 17–25. [Google Scholar] [CrossRef] [Green Version] - de Marchis, M.; Milici, B.; Volpe, R.; Messineo, A. Energy Saving in Water Distribution Network through Pump as Turbine Generators: Economic and Environmental Analysis. Energies
**2016**, 9, 877. [Google Scholar] [CrossRef] [Green Version] - Carravetta, A.; Fecarotta, O.; del Giudice, G.; Ramos, H. Energy recovery in water systems by PATs: A comparisons among the different installation schemes. Procedia Eng.
**2014**, 70, 275–284. [Google Scholar] [CrossRef] [Green Version] - Delgado, J.; Ferreira, J.P.; Covas, D.I.C.; Avellan, F. Variable speed operation of centrifugal pumps running as turbines. Experimental investigation. Renew. Energy
**2019**, 142, 437–450. [Google Scholar] [CrossRef] - Mercier, T.; Hardy, C.; Tichelen, P.V.; Olivier, M.; de Jaeger, E. Control of variable-speed pumps used as turbines for flexible grid-connected power generation. Electr. Power Syst. Res.
**2019**, 176, 105962. [Google Scholar] [CrossRef] - Alberizzi, J.C.; Renzi, M.; Nigro, A.; Rossi, M. Study of a Pump as Turbine (PaT) speed control for a Water Distribution Network (WDN) in South-Tyrol subjected to high variable water flow rates. Energy Procedia
**2018**, 148, 226–233. [Google Scholar] [CrossRef] - Renzi, M.; Rossi, M. A generalized theoretical methodology to forecast flow coefficient, head coefficient and efficiency of Pumps-as-Turbines (PaTs). Energy Procedia
**2019**, 158, 129–134. [Google Scholar] [CrossRef] - SEAB, Servizi Energia Ambiente Bolzano. Available online: https://www.seab.bz.it/it/privati/lacqua-di-bolzano (accessed on 3 December 2019).
- Singh, P.; Nestmann, F. An optimization routine on a prediction and selection model for the turbine operation of centrifugal pumps. Exp. Therm. Fluid Sci.
**2010**, 34, 152–164. [Google Scholar] [CrossRef] - SNPA, XII Rapporto Qualità Dell’Ambiente Urbano—Edizione 2016. Available online: http://www.isprambiente.gov.it/it/pubblicazioni/stato-dellambiente/xii-rapporto-qualita-dell2019ambiente-urbano-edizione-2016 (accessed on 3 December 2019).
- AEEG, Relazione Annuale Sullo Stato Dei Servizi E Sull’Attività Svolta. Available online: https://www.arera.it/it/dati/eepcfr2.htm (accessed on 3 December 2019).
- Luo, J.; Wang, J.; Fang, Z.; Shao, J.; Li, J. Optimal design of a high efficiency LLC resonant converter with a narrow frequency range for voltage regulation. Energies
**2018**, 11, 1124. [Google Scholar] [CrossRef] [Green Version]

**Figure 1.**Water Distribution Network (WDN) of Laives (South Tyrol, Italy, DMS: ${46}^{\circ}$${25}^{\prime}$$39.{42}^{\u2033}$ N, ${11}^{\circ}$${20}^{\prime}$$25.{72}^{\u2033}$ E) where analyzed branch is highlighted in red. Green dots are possible Pump-as-Turbine (PaT) positions. Red dot is proposed PaT position.

**Figure 7.**Flow-rate signal (Y-axis) expressed in (m

^{3}/s) generated by Block A of MATLAB©– Simulink model.

**Figure 11.**Comparison between mechanical-efficiency trends in different analyzed management strategies.

**Figure 12.**Trend of energy recovery over course of a day considering four different management strategies.

Hour | 6:00 | 7:00 | 8:00 | 9:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:30 |

$\mathbf{Q}({\mathbf{m}}^{\mathbf{3}}/\mathbf{h})$ | 14.67 | 19.57 | 16.40 | 14.94 | 13.24 | 11.17 | 13.56 | 13.56 | 12.18 | 10.24 | 12.10 | 11.51 | 18.58 | 19.34 | 14.25 |

Pump Mode | Turbine Mode | |
---|---|---|

Flow rate (m^{3}/h) | 13 | $14.35$ |

Head (m) | 20 | $22.8$ |

Mechanical efficiency (-) | $0.72$ | $0.69$ |

Rotational speed (rpm) | 2900 | 2900 |

Specific speed (rad/s) | $0.35$ | $0.29$ |

Impeller diameter (m) | $0.121$ | $0.121$ |

BEP Flow Rate Offset (%) | Flow Rate (m^{3}/h) | $\mathit{\varphi}$ (-) | Head (m) | $\mathit{\psi}$ (-) | Mechanical Efficiency (-) | Mechanical Power (KW) |
---|---|---|---|---|---|---|

$-40\%$ | $8.86$ | $0.0046$ | $12.50$ | $0.0908$ | $0.38$ | $0.12$ |

$-30\%$ | $10.33$ | $0.0053$ | $14.95$ | $0.1086$ | $0.50$ | $0.21$ |

$-20\%$ | $11.80$ | $0.0061$ | $17.52$ | $0.1273$ | $0.59$ | $0.33$ |

$-10\%$ | $13.28$ | $0.0069$ | $20.21$ | $0.1468$ | $0.64$ | $0.47$ |

$\mathbf{BEP}$ | 14.35 | $\mathbf{0}.\mathbf{0074}$ | $\mathbf{22}.\mathbf{80}$ | $\mathbf{0}.\mathbf{1656}$ | $\mathbf{0}.\mathbf{69}$ | $\mathbf{0}.\mathbf{62}$ |

$+10\%$ | $16.24$ | $0.0084$ | $25.90$ | $0.1882$ | $0.67$ | $0.77$ |

$+20\%$ | $17.71$ | $0.0091$ | $28.91$ | $0.2100$ | $0.66$ | $0.92$ |

$+30\%$ | $19.19$ | $0.0099$ | $32.03$ | $0.2327$ | $0.64$ | $1.10$ |

$+40\%$ | $20.66$ | $0.0107$ | $35.26$ | $0.2562$ | $0.64$ | $1.26$ |

Operating Strategy | Pressure Constraint (Bar) | |
---|---|---|

Case 1 | Flow-rate control | 4 |

Case 2 | Flow-rate and | 4 |

speed control | ||

Case 3 | Speed control | 4 |

Case 4 | Flow-rate and | 3.5 |

speed control |

**Table 5.**Energy recovery, economic savings, and percentage of recovered energy with respect to wasted energy in Pressure Reducing Valves (PRVs) obtained with four different operating strategies.

Energy Recovery | Economic Saving | % with Respect | Not Recovered | % of Recovered | |
---|---|---|---|---|---|

$(\mathbf{kWh}/\mathbf{Year})$ | $(\mathbf{\u20ac}/\mathbf{Year})$ | to Case 1 | $\mathbf{Energy}\text{}(\mathbf{kWh}/\mathbf{Year})$ | Energy | |

Case 1 | 4637 | 979 | - | 7833 | 37 |

Case 2 | 4706 | 994 | 1.5 | 7764 | 38 |

Case 3 | 5699 | 1204 | 23 | 6771 | 46 |

Case 4 | 6307 | 1332 | 36 | 6163 | 51 |

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## Share and Cite

**MDPI and ACS Style**

Alberizzi, J.C.; Renzi, M.; Righetti, M.; Pisaturo, G.R.; Rossi, M.
Speed and Pressure Controls of Pumps-as-Turbines Installed in Branch of Water-Distribution Network Subjected to Highly Variable Flow Rates. *Energies* **2019**, *12*, 4738.
https://doi.org/10.3390/en12244738

**AMA Style**

Alberizzi JC, Renzi M, Righetti M, Pisaturo GR, Rossi M.
Speed and Pressure Controls of Pumps-as-Turbines Installed in Branch of Water-Distribution Network Subjected to Highly Variable Flow Rates. *Energies*. 2019; 12(24):4738.
https://doi.org/10.3390/en12244738

**Chicago/Turabian Style**

Alberizzi, Jacopo Carlo, Massimiliano Renzi, Maurizio Righetti, Giuseppe Roberto Pisaturo, and Mosè Rossi.
2019. "Speed and Pressure Controls of Pumps-as-Turbines Installed in Branch of Water-Distribution Network Subjected to Highly Variable Flow Rates" *Energies* 12, no. 24: 4738.
https://doi.org/10.3390/en12244738