# Hybrid Pumped Hydro Storage Energy Solutions towards Wind and PV Integration: Improvement on Flexibility, Reliability and Energy Costs

^{*}

## Abstract

**:**

^{3}, ensures 72% annual consumption satisfaction offering the best technical alternative at the lowest cost, with less return on the investment. The results demonstrate that technically the pumped hydro storage with wind and PV is an ideal solution to achieve energy autonomy and to increase its flexibility and reliability.

## 1. Introduction

## 2. Hybrid and Pumped Storage Technologies

#### 2.1. Characterization

#### 2.2. Optimal Design of a Hybrid System

#### 2.2.1. Dispatch Optimization Model

^{3}) in Equation (1) and the water pumping coefficient (m

^{3}/kWh) in Equation (2) are two key parameters of the PHS elements. According to [29,30] the following equation describes the total stored energy ${E}^{t}$ (in kWh) in the active volume of a reservoir:

^{3}), ${c}_{t}$ is the turbine generating coefficient (kWh/m

^{3}), $g$ is the acceleration due to gravity (m/s

^{2}), $\rho $ is the density of the water (kg/m

^{3}).

^{3}/k Wh).

#### 2.2.2. Operating Principles and Important Restrictions

#### 2.2.3. Daily Cycle for Electricity Supply

## 3. Case Study

#### 3.1. Modelling Assumptions

^{3}/kWh and 0.305 kWh/m

^{3}, respectively. The energy converter pump as turbine is of variable speed, which allows the exploitation of excess energy produced by PV arrays and wind turbines and also allows covering medium load by a remaining part of hydropower, to improve the overall energy system efficiency.

#### 3.2. Results

#### 3.2.1. Scenario 1

#### 3.2.2. Scenario 2

#### 3.2.3. Scenario 3

^{3}(2 times the initial one). The amount of energy that is satisfied by hydro is practically the same, comparing to scenario 2 since the volume used depends on the demand that was kept constant. With this scenario, it is possible to feel the influence or not in some parameters depending on how the intervenient variables are optimized and integrated in a more flexible solution (Figure 12).

#### 3.2.4. Scenario 4

## 4. Energy Balance

^{3}would ensure 72% annual consumption satisfaction. Figure 13 and Figure 14 show for each scenario the energy contribution of the three renewable sources for typical summer and winter days, respectively. Although it is considered a relatively low installed solar power (1/5 wind), this source can be very useful on summer days, especially in the middle of the day, when the wind slows and the solar radiation increases. However, although increasing the PV installed capacity ensures 65% of the consumption through wind + solar (Figure 14d and Figure 15d), comparing with scenario 2 (Figure 14b and Figure 15b), the hydropower can cover that difference with the pump/hydro power solution. For the majority of winter days, there is a surplus of wind production at the beginning of the day, taking advantage of reducing the energy costs for scheduled pumping in the morning. Additionally, for a storage capacity of 144 MWh and 288 MWh, there is practically no significant contribution when comparing to other scenarios (Figure 14c and Figure 15c).

## 5. Water–Energy Nexus

_{2}emission and water withdrawal, and a cost-optimal market solution also serves to improve the performance (Table 6). Regarding CO

_{2}and water withdrawal, it remains approximately the same between scenarios 3 and 4. This shows that storage actions taken in the water infrastructure between these scenarios can serve to improve the electric power infrastructure when the two or more sources are coupled together in a water–energy nexus.

_{2}emissions but exacerbates the night ramp in energy demand; in contrast, wind energy may bridge this gap, but it is usually intermittent, unpredictable and weather dependent. By employing an energy storage system, the surplus energy can be stored when power generation exceeds demand and then be released to cover the periods when net load exists, providing a robust back-up to intermittent renewable energy. Thus, water and energy storage presents a promising solution to these two problems, as it allows flattening demand curves and significantly reducing costs.

## 6. Conclusions

- (a)
- The optimization showed that in a hybrid solution, turbines and pumps can be used at the same time depending on the intermittency, availability and optimized variables, which include different renewable sources, the storage capacity and the load demand. The pumping system can be supplied by intermittent renewable sources when available, and at the same time, can be guaranteed a constant power production by hydraulic turbines. The only one pipe for the P/T solution requires different hours for each operation or the use of separate pipes, which can offer more operating flexibility, where one is kept running when the other is stopped or in operation, depending on the sources’ availability, constancy or intermittency, type of storage or type of grid connection;
- (b)
- Three sources were combined considering different pump/turbine installations, wind/solar powers and different water batteries as volume capacities. The analysis revealed that P/Ts with 4 MW are economically viable compared to 6 MW and 2 MW, with 70% to 85% satisfaction of consumption levels;
- (c)
- After selecting the best installation power for P/Ts, four scenarios were tested, changing the wind/solar powers and the water storage capacity. Three types of analyses were performed from the point of view of energy, economy and CO
_{2}emissions. The results obtained show the process of selecting the best scenario is not straightforward, depending on the final goal. Therefore, this analysis unfolds in important points:- i.
- Scenario 1 stands out from the point of view of reliability and flexibility, where there is a better use of hydropower (Figure 14 and Figure 15), specifically to accommodate the largest shares of other intermittent renewable (solar and wind) energies with a better bridge and compensation between these energy sources;
- ii.
- Scenario 2 showed a 4% increase in satisfied consumption from an operational point of view, maintaining the same characteristics as scenario 1 but requiring an increase in the installed wind power;
- iii.
- In scenario 3, increasing storage capacity to 288 MWh does not make a significant contribution to the best operation of the system. As a result, the reservoir is oversized to meet the satisfied consumption, i.e., there is a dependency not only on the maximum daily energy use of the system but also on the hydropower system;
- iv.
- In both scenarios 1 and 2, surplus energy from renewables produced at times of low demand (e.g., solar power in summer) can be stored and ready for release when demand rises;
- v.
- Scenario 1 offers more advantages and greater economic viability also in terms of CO
_{2}emissions. This hybrid solution is less expensive, with an interesting pay-back period of 7 years, considering the powers installed, with a lower initial capital cost than that of the other scenarios.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

AS | net annual saving (€) |

B | benefit (€) |

BPP | basic payback period (years) |

${c}_{p}$ | coefficient of the pump/motor unit (m^{3}/kWh) |

${c}_{t}$ | turbine generating coefficient (kWh/m^{3}) |

C | total investment cost (€) |

C_{P/T} | pump/turbine energy solution cost (€) |

C_{PHS} | Pump hydro system cost (€) |

Cw | wind energy solution cost (€) |

${E}_{i}^{p}$ | Pump power installed (MW) |

${E}_{i}^{t}$ | Turbine power installed (MW) |

G | acceleration due to gravity (m/s^{2}) |

H | hybrid power/energy available (MW) |

H | net head (m) |

IRR | internal rate of return (%) |

N | lifecycle of the project (years) |

NPV | net present value (€) |

p_{f} | peak factors |

Ps | solar power (MW) |

PHS | pumped hydro storage |

Pw | wind power (MW) |

Q^{p} | pump flow (m^{3}/s) |

Q^{t} | turbine flow (m^{3}/s) |

R | the discount rate (%) |

S | solar energy (MW) |

V | storage capacity (m^{3}) |

V^{p} | pump volume (m^{3}) |

V^{res} | volume of the reservoir (m^{3}) |

${V}_{max}^{res}$ | maximum volume of the reservoir (m^{3}) |

${V}_{min}^{res}$ | minimum volume of the reservoir (m^{3}) |

V^{t} | turbine volume (m^{3}) |

W | wind energy (MW) |

Greek letters | |

${\eta}_{p}$ | pumping efficiency (%) |

${\eta}_{t}$ | efficiency of the turbine/generator unit (%) |

$\rho $ | density of the water (kg/m^{3}) |

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**Figure 2.**Flowchart to find optimal hybrid system. V is for water volume, E is for energy, D is the demand, H is for hybrid power/energy available, S is the solar energy and W is for wind energy. The superscripts (p, t, res) are assigned for pump, turbine and reservoir.

**Figure 3.**Average wind power distribution during an average year [28].

**Figure 4.**Average solar power distribution during an average year [28].

**Figure 5.**Hybrid solution: shematic diagram of different combinations used (

**a**); satisfied demand (wind + hydro; solar + hydro; hydro) for different pump as turbine (PAT)’s power (

**b**).

**Figure 7.**Electricity generation (Wind, PV and Pumped-Storage Hydro) between February (

**a**) and March (

**b**).

**Figure 8.**Electricity generation (Wind, PV and Pumped-Storage Hydro) between August (

**a**) and September (

**b**).

**Figure 11.**Total amount of hydropower when using wind energy with 5 MW, between February and March (

**a**) and August-September (

**b**).

**Figure 12.**Electricity generation and stored in scenario 3, between February and March (

**a**) and August and September (

**b**).

**Figure 13.**Total amount of energy stored vs demand in scenario 4, between February and March (

**a**) and August and September (

**b**).

**Figure 14.**Energy contribution on summer day: (

**a**) scenario 1; (

**b**) scenario 2; (

**c**) scenario 3; (

**d**) scenario 4.

**Figure 15.**Energy contribution on a winter day: (

**a**) scenario 1; (

**b**) scenario 2; (

**c**) scenario 3; (

**d**) scenario 4.

**Figure 16.**Satisfied consumption by different energy sources and unsatisfied consumption in (

**a**) scenario 1; (

**b**) scenario 2; (

**c**) scenario 3; (

**d**) scenario 4.

**Figure 18.**Electricity generated from renewables (GWh) in each scenario (

**a**); total avoided emissions by each scenario (

**b**).

Days | Tarif Period | Winter | Summer |
---|---|---|---|

Workdays | Peak | 5 h/day | 3 h/day |

Half-peak | 12 h/day | 14 h/day | |

Normal off-peak | 3 h/day | 3 h/day | |

Super off-peak | 4 h/day | 4 h/day | |

Saturdays | Half-peak | 7 h/day | |

Normal off-peak | 13 h/day | ||

Super off-peak | 4 h/day | ||

Sundays | Normal off-peak | 20 h/day | |

Super off-peak | 4 h/day |

V (m^{3}) | Pw (MW) | P_{P/T} (MW) | Cw (€) | C_{P/T} (€) | C_{PHS} (€) | Total (€) | Profit (€) | Return (Years) |
---|---|---|---|---|---|---|---|---|

378,000 | 4 | 6 | 2,333,320 | 303,834 | 835,544 | 3,168,864 | 282,336 | 11.2 |

378,000 | 5 | 2,916,650 | 3,752,194 | 299,948 | 12.5 | |||

756,000 | 4 | 2,333,320 | 3,168,864 | 281,997 | 11.2 | |||

756,000 | 5 | 2,916,650 | 3,752,194 | 299,601 | 12.5 | |||

378,000 | 4 | 4 | 2,333,320 | 200,034 | 550,094 | 2,883,414 | 357,336 | 8.1 |

378,000 | 5 | 2,916,650 | 3,466,744 | 365,525 | 9.5 | |||

756,000 | 4 | 2,333,320 | 2,883,414 | 360,530 | 8.0 | |||

756,000 | 5 | 2,916,650 | 3,466,744 | 366,695 | 9.5 |

_{P/T}—Pump/Turbine Power.

Peak | Half-Peak | Off-Peak | Super Off-Peak |
---|---|---|---|

0.097 | 0.0406 | 0.0115 | 0.0115 |

Scenarios | Peak Consumption | Wind Power | Solar Power | Pump/ Turbine | Storage |
---|---|---|---|---|---|

Scenario 1 | 4 MW | 4 MW | 0.54 MW | 4 MW | 144 MWh (378,000 m^{3}) |

Scenario 2 | 5 MW | 0.54 MW | 4 MW | 144 MWh (378,000 m^{3}) | |

Scenario 3 | 5 MW | 0.54 MW | 4 MW | 240 MWh (755,685 m^{3}) | |

Scenario 4 | 5 MW | 1.60 MW | 4 MW | 144 MWh (755,685 m^{3}) |

Parameters | Scenario 1 | Scenario 2 | Scenario 3 | Scenario 4 |
---|---|---|---|---|

Investment Cost (€) | −3,198,412 | −3,781,742 | −3,781,742 | −4,400,072 |

Total annual cost savings and income (€) | 479,762 | 415,992 | 415,992 | 352,006 |

NPV (€) | 4,611,564 | 5,452,627 | 5,452,627 | 6,344,153 |

IRR (%) | 30.63 | 25.51 | 25.51 | 20.24 |

BPP (years) | 6.7 | 9.1 | 9.1 | 12.5 |

Scenario | Cost (M€) | CO_{2} (Mtonnes) | Water Withdrawal (m^{3}) |
---|---|---|---|

1 | 3.20 | 2.01 | 169,920 |

2 | 3.78 | 2.72 | 165,304 |

3 | 3.78 | 2.76 | 354,304 |

4 | 4.40 | 2.74 | 357,831 |

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## Share and Cite

**MDPI and ACS Style**

Simão, M.; Ramos, H.M.
Hybrid Pumped Hydro Storage Energy Solutions towards Wind and PV Integration: Improvement on Flexibility, Reliability and Energy Costs. *Water* **2020**, *12*, 2457.
https://doi.org/10.3390/w12092457

**AMA Style**

Simão M, Ramos HM.
Hybrid Pumped Hydro Storage Energy Solutions towards Wind and PV Integration: Improvement on Flexibility, Reliability and Energy Costs. *Water*. 2020; 12(9):2457.
https://doi.org/10.3390/w12092457

**Chicago/Turabian Style**

Simão, Mariana, and Helena M. Ramos.
2020. "Hybrid Pumped Hydro Storage Energy Solutions towards Wind and PV Integration: Improvement on Flexibility, Reliability and Energy Costs" *Water* 12, no. 9: 2457.
https://doi.org/10.3390/w12092457