Reverse Osmosis Desalination Plant Driven by Solar Photovoltaic System–Case Study

Several regions of the world, such as Arab counties in the Middle East, are facing a problem of drink water. While the heat is the main energy source for the thermal desalination processes, the electricity is commonly used energy bases for Reverse Osmosis and electrodialysis. Reverse Osmosis and electrodialysis are more efficient than thermal desalination processes for energy basis. Using diesel generators in arid areas can be considered a lower initial investment where the grid is not there. In such areas using of photovoltaic systems, PV as an energy source for desalination can be cheaper and renewable with low maintenance as well.

In osmosis process the water is physically moved from low salt concentration side to high salt concentration side. That is done by the difference in osmotic pressure between both two sides. In reverse osmosis an external pressure can force the water to flow in the reverse direction through a permeable membrane. A high-pressure pump is normally used to obtain that high pressure. On the other hand, the membrane is used to keep the flow in one direction from high salt concentration to low salt concentration.

Solar photovoltaic driven reverse osmosis desalination system is the most promising renewable energy that can replace the fossil fuels as conventional energy. Using of photovoltaic can reduce operational costs, and improve environmental sustainability. It was achieved efficient reverse osmosis salt water desalination without energy recovery or pressure exchanger devices. Comparison of the two processes; operated at similar permeate-water recovery ratios and throughputs. In the constant flow process, the permeate-water flow full at the first module and the flow drips at each successive module [1].

PV system is actuality worldwide direction. It was presented a practical method for PV sizing. PV sizing technique is still complicated as inventors and researchers are thinking. The calculations were developed for both PV stand alone and PV hybrid system. A limited photovoltaic (PV) powered reverse osmosis (RO) plant is intended to operate at variable flow and pressure conditions for stand-alone submissions in equatorial zones to desalinate salty water [2]. It was simulated and compared two operation plants. Plant 1 used two modules for the RO pumps where Plant 2 used three modules for both applications. Results shown that Plant 2 has an improved performance, such as: growth of 60% in the daily permeate output and of 32% in the daily operation time [3].

A valuation of a number of researches in relations of their technical and economic performance with feasibility was presented. Also, he discussed the several lessons that have been educated through the process and maintenance and considered the several reasons fundamental moreover their success and failure [4].

Reverse Osmosis (RO) plants driven by photovoltaic (PVRO) panels at public scale for remote areas are increasing in importance. Using of solar energy as a clean source of energy and minimum operating costs is advantageous. Moreover, continuing of the PV performance improvement make using of PV competitor. Therefore, a study aimed to determine the RO plant outline that enables the maximum recovery rate and principals to the lowest global cost for small PVRO plants. The objective was to develop an integrated method in terms of separation process, energy supply and efficiency [5].

It was demonstrated how saltwater reverse osmosis (SWRO) plants, required to meet growing future global water demand. Also he provided a global estimation of the water production costs for 2030 distillation demand with renewable electricity [6].

A paper presented a state of art in wind and solar-PV powered large scale RO plants. This review discussed and identified technical challenges and potential solutions [7]. Hybrid Optimization Model for Electric Renewable (HOMER) to find the greatest photovoltaic system PV in Oman’s conditions and to study the costs and the resulting contaminating emissions studied by Abdul-Wahab et al. [8]. Using HOMER was serving to determine the greatest location in Oman to exploit solar energy production. HOMER simulation grades showed that the greatest type of PV for Oman was 1164 kVA with generic PV.

PVROPRO Photovoltaic Reverse Osmosis Pressure Retarded Osmosis plant was developed and studied. His proposal is based on the hourly solar data of Australia (Perth) in a year. Annual production was amplified more than nine times related to the stand-alone PVRO plant. The water production rate is decreased in the range of 16% and the overall annual reduction is 18.07% [9].

Solar photovoltaic PV energy is a probable solution to decrease the energy costs of creating distilled water. A model was developed to simulate the membrane distillation unit attached with solar PV system in order to examine the steady-state performance of each component. The results showed the influence of external and internal parameters on the working characteristics of the membrane distillation unit attached with solar PV system [10].

It was established means of releasing water for domestic and industrial users. For most manufacturing RO applications, raising recovery, thus reducing waste brine generation, signifies the greatest cost-saving opportunities [11]. For saltwater RO, reducing energy consumptions has the highest prospect for dropping overall costs. Cost effectiveness of reverse osmosis (RO) desalination plant integrated with a solar photovoltaic (PV) source for Masdar Organization studied by [12]. The proposed scheme does not depend on expensive batteries and extremely decreases the government expenses to fund water production cost by $1.34 /m³.

A short review was presented showing efficiency and applicability of RO technology for some water desalination industries like distillery spent wash, ground water treatment, recovery of phenol compounds, and reclamation of wastewater and sea water reverse osmosis (SWRO) treatment [13].

An exergy analysis was established on cycle components. It is indicated that recuperator irreversibility has a significant influence on the cycle performance [14]. In work of Bilton et al. [15] it was focused on improving the feasibility Small-scale photovoltaic-powered reverse osmosis (PVRO), a methodology to estimate the economic feasibility was developed. He showed that growing the efficiency of PVRO systems can extend their feasibility to presently marginal or un- feasible locations.

It was gathered Photovoltaic thermal collector PV/T to desalinate water especially with reverse osmosis. It was developed a mathematical model with multi-inputs and multi outputs based on energy balance equations. He presented the performance of the system elements based on the parameters and correlations [16]. An economic study was established for photovoltaic-powered reverse osmosis for 79 global experimental and design systems. He showed that the unit can cost as low as 2–3 US$/m³ [17].

By the way if the overall efficiency of Photovoltaic Reverse Osmosis PVRO system is improved that can increase the usages of it [18]. While the system efficiency is dependent on the efficiency of individual system components, such as control system and reverse osmosis membranes improving of the efficiency of those components is a subject of the research [19]. However, Saudi Arabia as a leading oil manufacturer, it considers a special attention to utilization of solar energy in the most important field of water desalination.

To achieve these purposes of PVRO system models, the connection between solar power and desalination plant explained below. A large-scale investigational system was simulated to validate the system design characteristics. Details calculation of the system designing and gathering data are presented. Re-designing of RO system in local power plant by convert the electrical power supply consumed from auxiliary power in the power plant to solar photovoltaic system estimation in two conditions day and night time, mathematical modeling, numerical simulation and cost effectiveness.

The solar radiation data considered in that study are taken from the Photovoltaic Geographical Information System PVGIS [20] data sources. In that model it is assumed that the solar constant is approximate 1361-1362 W/m². That radiation incident on a rays perpendicular plane outside the atmosphere The model considers different processes of radiation scattering, absorption and dispersion during the atmosphere. That reduces the values of the solar radiation reaching the ground. The atmosphere components of ozone, CO₂, aerosols and water vapor are the reasons of radiation reduction. However, the water vapor is the cause of attenuation that affects the wavelength and spectral distribution of solar radiation.

3.1 Calculation of Solar Radiation from satellite

The solar radiation collected on the ground surface are three components, beam, diffuse and reflected radiation. The beam or direct radiation is the solar radiation reaching directly to the ground surface. The second is the diffuse radiation that is reflected or scattered by the atmosphere components. The reflected radiation is reflected solar radiation from the surroundings.

In calculation of solar radiation, the darkness of the sky is determined from the image of the sky taken along the day and year. The reflectivity of the clouds is calculated from the sky images. The clearest image during a month is taken as a clear sky and other images are estimated as a fraction of clear sky. By that method the clear sky radiation is the maximum radiation during a month where other values of radiation can be calculated as a percentage of that value depending on the darkness of the image taken [20].

Figure 2 explains the annual estimated hourly solar radiation (W/m²) from satellite for Shoaiba region 21 °N. As expected, the highest values of solar radiation shown as following is during summer days. That was the best time of high performance for photovoltaic system. The maximum solar radiation is ranged between 750 to 1000 W/m² and that is good enough to install a successful solar system in that area.

Figure 3 explains the measured solar irradiance for December 21 as the shortest day of the year for which the PV sizing is provided. Normal hourly variation can be found in the figure which presents a similar variation for both before and after noon hour. The maximum solar radiation at noon is found about 900 W/m² and it is good enough to produce a valuable amount of energy. The daily solar radiation can be collected in such day can be estimated as 5956 Wh/m².

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Figure 2. Hourly average solar radiation of KSA

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Figure 3. Average solar irradiance of December 21

5.1 RO plant technical data

A large-scale investigational PVRO system is designed to confirm the system need of power supply. Normally the auxiliary power of the plant covers the RO plant power consumption. In that study a PV solar system is used to cover the power needed to RO plant. The PV panels can be connected to the RO system as shown in Figure 7. The PV system is sizing to cover the power of the four pumps installed in the RO plant, intake pump, RO feed pump, 1^st and 2^nd HP pimps. Because the RO and its pumps are working 24 a day, storage is needed to the night work.

The technical specifications of the RO components are illustrated in Table 1. It is indicated the maximum pressure required, flow rate, power and voltage for each component.

Fortunately, the voltage required for all pumps are the same so one system can supply power to all pumps. Two PV systems are proposed to power the plant pumps, one for day operation and the other for the night operation. The system for the day operation is connected directly to the pumps and it does not need storage where the night system needs storage in batteries. Therefore, the electrical load depends on two situations day and night loads. The required energy load is estimated for the longest day (June 21) for the day load where it is estimated for the longest night (Dec. 21) for the night load.  The day length can be found for 21 °N for June 21and Dec. 21, it is found as 13.2 and 10.8 hours respectively. Therefore, the day length is 13.2 hours on June 21 and the night length is 13.2 hours at Dec. 21. They are equal as expected. The day and night loads can be simply calculated as the required power multiplied by operating hours. By summarizing the power from Table 1 the required load is found 6,824,400 Wh for both day and night.

5.2 Photovoltaic PV panel sizing

A photovoltaic panel is selected as a famous manufacturer of Canadian Solar in the worlds [22]. The panel model HiKu has a high-power output, 30% more power from back side, 24% more front side power than conventional modules, low temperature coefficient (P_max): -0.37%/℃, efficient under high temperature.

As shown in Figure 8 the photovoltaic panel I–V curve in left explains the PV performance under different conditions of solar radiation where the left curve illustrates the effect of ambient temperatures on the I-V curve.

Power required at day time as per calculated = 6,824,400 Wh.

The number of PV panels can be estimated as follows [23].

- Corrected power output/ panel/ day = 415 W * $\frac{5.956 \mathrm{kWh} / \mathrm{m}^{2}-\mathrm{day}}{1 \mathrm{~kW} / \mathrm{m}^{2}}$= 2471.7 Wh/ day

No. of PV panel required at daytime =$\begin{gathered}\frac{\text { day time load }\left(\frac{W h}{\text { day }}\right)}{\text { panel output }\left(\frac{W h}{\text { day }}\right)} =\frac{6,824,400\left(\frac{W h}{d a y}\right)}{2471.7\left(\frac{W h}{d a y}\right)}=2,769\end{gathered}$                    (1)

panels No. of PV panel required at night time

PV arrays area at day time = 2108 mm * 1048 mm * 2769 panels= 6,118 m² = PV array area at night time         (2)

Table 1. Reverse osmosis equipment specification

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Figure 7. PV system power supply to RO reverse osmosis desalination plant

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Figure 8. PV panel performance with standard test condition

Table 2. Specifications of PV Panel

Due to system voltage is 480 V and PV panels voltage is 39.3 as in Table 2, thirteen panels should be connected in series and 213 panels in parallel. Then the number of PV panels required is 2769 panels for each system.

5.3 Inverter selection

It is proposed to use the inverter of Canadian Solar to convert the panel DC current into Pumps AC current the technical specifications are indicated in Table 3.

Table 3. Specification of inverter selected [24]

5.4 Battery sizing

The solar battery storage is selected as a more reliable manufacturer, Greensun Solar in the worlds [25]. The battery model and type (LFB12-300), (LiFePO4) has a good capacity rate of 300 Ah where it has long cycle life by 100% DOD after 800 cycles 80% capacity remain, small size, light weight than Lead acid battery and low self- discharge rate less than 3% per month as shown in Table 4.

Table 4. Specification battery

As per the longest night time in the year (Dec. 21) its length is 13.2 as operating hours. Power storage required as per calculated = 6,876.1 kWh. For battery charging efficiency of 75% and Maximum depth 70% dependent on Greensun Solar batteries [25].

5.5 Battery storage sizing

Assuming of three days cloudy days so three nights load is assumed to be stored in batteries. It can be calculated the number of batter ties as follows.

Storage load $=\frac{6,824,400 \mathrm{Wh} * 3 \text { days }}{07 \times 075}=38,996,571.4 \mathrm{Wh}$          (3)

No. Of Battery $=\frac{\text { Storage load }}{A h * V}=\frac{38,996,571.4 W h}{300 A h * 12.8 V}=10,155.4$ batteries, round to 10,156 batteries       (4)

The voltage of the RO pumps is 480 Volt so it is needed 38 batteries connected in series. Therefore, the total number of batteries required is 10,184 batteries.

5.6 Approximate initial cost and pay back

In any project study, capital cost becomes as an important factor foe decision maker in top management. For this study, the capital cost is very high but also the price of PV panels is not fixed. The price can be changed when dealing with manufacturer vendors due to the huge numbers of panels and batteries. The following calculations demonstrate initial cost and pay back.

The initial cost is estimated in Table 5 the price of the system components is considered for 2019 manufacturer prices.

Table 5. Initial cost of photovoltaic panel and battery

With neglecting of any other economic parameter like depreciation, interest, operating costs, taxes, worth etc. the payback period can be estimated as follows. It is Noted that as Saudi Electricity Company prices the price of kWh is SR0.30/ kWh.

• Power consumption of RO system is 13 MWh/ day as calculated before.

• Per day = 12000 kWh/day * SR0.30/ kWh = SR3600/ day

• Per year = SR3600 * 365 = SR1,314,000/ year

• Pay back = $\frac{\$ 10,515,493 * 3.75}{S R 1,314,000}$ = 30 years