A Real Case Study of Hybrid System for Enhancing the Energy Production

To equip the load with electrical power, the photovoltaic plant produces electrical power through the day, surplus electrical energy production is exploited to turn on storage pumping station to left water to upside tank during non-peak times and then uses it to operate the water pumping storage station as a generator to equip electrical energy to the load through period of peak load. Figure 2 shows proposed PV hydro pumped electricity storage system for first model.

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Figure 2. Proposed PV hydro pumped electricity storage system

Covering the station’s internal consumption is considered the first priority for energy produced, which is mainly represented by the sewage network to protect the station from floods because there is always a high percentage of water leakage in the station and it is discharged every hour through the pumps of the sewage system, and the hike energy is utilized charge the batteries.

Figure 3 shows the second model of the PV Batteries storage energy system is depicted.

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Figure 3. Proposed PV Batteries storage electricity system

NASA has determined the solar irradiance data for the site [12]. The site is located at 42°43'29.55" East Longitude and 36°30'50.77" North Latitude, respectively, as displayed by the HOMER Pro tool. Figure 4 shows the relation between monthly solar irradiance over a year, from this figure The average daily irradiance displays an annual average of 5.37 (kWh/m²/day), while Figure 5 shows the relation between the monthly average ambient temperature over a year.

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Figure 4. Monthly average solar irradiance

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Figure 5. Monthly average ambient temperature

PV arrays are made by connecting multiple solar cells in parallel and series to form solar modules. Short-wave radiation is captured by these cells, which convert it into direct current energy. The total annual energy contribution (Epv) of the solar array is calculated using Eq. (2) [2]:

Epv=Ypv. PSH. Fpv. 350 day/year     (2)

where,

Ypv: maximum capacity of PV arrays in kW.

PSH: peak sun hour (h).

Fpv: PV derating factors.

The derating factors are included loses of wire, air temperature, dust, and various elements on the energy output of the solar array. The relationship between anticipated and actual output, commonly referred to as PV efficiency, is the derating factor.

HOMER Pro tool is used to determine the total energy, number of PV modules, batteries, and pumped storage units in order to evaluate solar PV systems. Solar PV systems are available in a variety of configurations and power output ranges. It is necessary to establish the first stage's total energy consumption and daily sunlight hours in accordance with the location [7-11].

Upper tank capacity and average head are the very important factors for designing pumping systems [3]. In this test, the Tigris River is considered the bottom tank and its level is at an altitude of 300 metres.

A-Generator case

The turbo generator output during vacuum mode is written as follows:

$P_T(t)=\eta_T \rho \operatorname{gh} . q_T(t)$      (3)

where,

ƞ_T: the total efficiency.

q_T(t): the flow rate of water.

B-Pump case

When PV capacity exceeds the demand of energy. The rate of water pumping is represented by the battery charging rate. Since the pump receives its energy closely from the photovoltaic panels, it can be represent the water flow proportion drawn from the lower tank as follows [13]:

$Q_p(\mathrm{t})=\frac{\eta_p \cdot P_{P V p}(\mathrm{t})}{\rho \cdot g \cdot h}$          (4)

where,

$\mathrm{P}_{\mathrm{PVP}}$: input power.

$\rho$: water density (1000 kg/m³).

$\eta_\rho$: overall pumping efficiency.

C-Upper reservoir

The amount of water stored in the upper reservoir (UR) at time t is affected by:

$\operatorname{QUR}(\mathrm{t})=\operatorname{QUR}(\mathrm{t}-1)(1-\alpha)+\int_{t-1}^t q_P(t) d t-\int_{t-1}^t q_T(t) d t$         (5)

where,

$\alpha$: Evaporation and leakage loss, can be compared to the self-discharge of a battery.

These losses have been ignored in this analysis for simplicity. The amount of water in the higer reservoir can be represent the state of charge (SoC). Thus, the SoC of a storage system is given:

$\mathrm{SoC}(\mathrm{t})=\frac{\mathrm{Q}_{\mathrm{UR}}(\mathrm{t})}{\mathrm{Q}_{\mathrm{URmax}}}$           (6)

The following limitations also apply to the overhead tank water volume:

$Q_{U R \min } \leq U R \leq Q_{U R \max }=V_{U R}$        (7)

where,

Q_URmin : lower limit of U.

Q_URmax: higer limit of U.

V_UR: capacity of reservoir (m³).

The minimum storage capacity, QURmin is zero in this paper [14]. The Tigris River can be considered as a lower tank in this analyze, which can to a large extent reduce the cost of constructing a pumped storage system.

The number of days of autonomy can be calculated depending on the size of the UR and the probable power s in the UR:

$n_{\text {day }}=\frac{E_C}{E_{\text {load }}}=\frac{\eta_t \cdot g \cdot V_{U R} \cdot \rho \cdot h}{E_{\text {load }}}$               (8)

where,

E_C: upper tank capacity(J).

E_load: daily load consumption (kWh).

The first balance relationship for basic power is represented by the electricity that utilized by the solar panels being used directly for the load when the demand is equal to or less than the PV power or used for pumping water when the requestis greater than the PV power generation [18].

Considering the aforementioned limitations, Figure 9 illustrates the flowchart of overall system's operating approach.

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Figure 9. Flowchart of plant operation

The algorithm describes how to manage photovoltaic and hydro-turbine energy generation, respectively. The power produced by PV can be used to supply the load or to pump water to the overhead tank. The priority is to cover the load. When the PV power output exceeds the lower load value the PV power supply the load directly, if the PV power less than the load and the upper reservoir level greater than its minimum value, the turbine starts supplying the load. If the upper tank level is less than its minimum value, the pump starts pumping water to the upper reservoir exploiting the PV generated power which could not meet the load and the load must be supplied from the grid [19].

Figure 10 below provides a flowchart of the HOMER simulation.

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Figure 10. Flowchart of HOMER simulation process

The National Renewable Energy Laboratory in the United States (NREL) developed the hybrid optimization model for electric renewables (HOMER) as a computer model to assist designers in creating sustainable energy systems for grid connected and standalone projects and to make it simpler to evaluate methods for generating power through a variety of combinations [20]. Using the HOMER Pro software version 3.14.2, a comparison between to models of PV plant have been done to supply the load as follow. Homer pro. Software the optimal system configuration selected according to minimum excess electricity percentage, lowest net cost (NPC) and levelized cost of power (LCE). The optimization objectives were reliability maximization, cost minimization, energy production, environmental impact reduction, operation flexibility and load matching. In Homer pro, constrains included, load demand, minimum and maximum load generation, operational limits, storage technical and operational [21].

A. PV-Pumped Storage System

An off-grid PV power system with pumped storage system is designed to supply a load of 3000 kWh/day, PV panels of 500 watt was used and a pumped storage system of 245 kWh was used which has a reservoir of 1000 m³ and effective head of 100 m and discharge flow rate of 0.0231 m³/sec. Figure 11 shows the schematic of this system.

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Figure 11. Schematic system of PV-Pumped storage system

The simulation of the system as shown in Table 1 showed that for supplying the load a PV plant of 804 kW capacity (1608) panels produce 1,278,540 kWh/year or 3502 kWh/day and 20 units of pumped storage system with nominal capacity of 5083 kWh which has an autonomy of 40.7 hour are required.

Table 1. The main results of PV-Pumped storge system

where,

NPC: Net present cost

COE: Cost of energy

B. PV-Batteries System

An off-grid PV power system with batteries energy storage system is designed to supply a load of 3000 kWh/day, PV panels of 500 watt was used and a battery storage system of 1kWh of 12Vand 83.4 Ah. Figure 12 shows the schematic of this system. The results are shown in Table 2, and Table 3 shows a comparison between two types of storge system.

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Figure 12. Schematic system of PV-Batteries system

Table 2. The main results of PV-Battery storage system

Table 3. A comparison between two types of storage systems

The first model is more efficient and more economic than the second model due to the lower cost of power COE and net present cost NPC of the first model and the autonomy hours of the first model is greater than the second model by 16.2 hours. As well as the upper reservoir of Mosul dam is already constructed which is reducing the cost significantly.

The advantages of pumped storage systems are the facilitates in the integration of other renewable sources like hydro and PV are storing the excess energy from these sources and improving overall electricity generation efficiency. But the disadvantages of this type are some limitations like, environmental impacts, site suitability requirements and high initial costs. From overall above the advantages of PV pumped storge system is better than the battery storge system but the last has good feature and less limitation as compared with PV pumped storge system.