Brackish Water RO plant as a variable load for Renewables based Hybrid Power System for increased power output
Nagaraj R
BARC, Kalpakkam, India
E-mail:rnagaraj@igcar.gov.in
Abstract: Water and energy are two inseparable commodities that govern the lives of humanity and promote civilization. Energy can be used to produce water in case of scarcity in water. Ironically most of the places that are water stressed are also energy stressed. The cost of extending grid power may be prohibitively high in those cases. Rural / Remote locations like hills and islands multiply the problem to a larger magnitude. Use of renewable sources like Solar, Wind, Biomass and other locally available energy sources is the only solution. But these renewable sources are of intermittent nature and have poor availability. Hence, it is practically difficult to produce water with single source of energy. Naturally, combining two or more sources of energy, known as hybrid power system, is the next available option. This paper carries out an analysis of various sizing combinations of systems with Solar Photo voltaic, Wind energy and stored energy in batteries for production of drinking water from a brackish water source. When the power produced is less than that required by the load, the generated power has to be stored in a battery and again discharged when required. We propose a BWRO plant that can take reduced power input also and reduced water output under such conditions and thus reduce the need of higher capacity of storage batteries. The system can operate the RO plant whenever the power is available, produce drinking water and store in a tank.This paper analyses the model of the entire hybrid power system in MATLAB to simulate the performance of the Hybrid power system for different combinations of capacities. The analysis under various input conditions and analyzed the results.
Keywords: Renewable energy; Hybrid power system; Desalination; RO; Solar; Wind; Battery
INTRODUCTION – DESALINATION AND ENERGY
Water, energy and environment are essential inputs for sustainable development of society [1]. The availability of fresh water is an important issue in many areas of the world. The ocean is the only perennial source of water. Their main problem is obviously its high salinity. The removal of salinity is accomplished by several desalination methods. But, all the Desalination processes require significant quantities of energy. It is a common phenomenon that certain packets of the country that are water stressed are also power stressed at the same time. These remote parts do not have conventional source of power and costs of extending the electricity grid to these places are very high. Fortunately, most of such locations have exploitable renewable sources of energy that could be used to drive desalination processes. [2]
Renewable energy systems utilize sources available locally and freely for production of energy. Production of fresh water using desalination technologies driven by renewable energy systems is thought to be a viable solution to the water scarcity at remote areas characterized by lack of potable water and conventional energy sources like heat and electricity grid. Also they are environmentally friendly [3]. Desalination systems cannot be compared with conventional systems in terms of cost without taking site specific factors into consideration. They are suitable for certain locations and will certainly emerge as widely feasible solutions in due course of time. [4]
Due to the intermittency nature of renewable sources, there is a need for storage batteries. The power generated has to be stored in batteries when the generated power is sufficient enough to feed the available load. Also the batteries will discharge to feed the load during low generation periods. The addition of large capacity of storage batteries not only increases the initial cost but also reduced power output due to its charging and discharging efficiencies.
This paper proposes to use a brackish water desalination plant as a variable load coupled with hybrid power system and attempts to analyze the effect in power output from the system.
MODELLING THE RENEWABLE ENERGY SYSTEMS
There are a variety of renewable energy sources identified and utilized in various levels. These cover solar energy which includes thermal collectors, solar ponds and photovoltaic, wind energy and geothermal energy. Major share being from Solar Photo voltaic and Wind energy, we shall discuss only these systems.
Solar Photovoltaic
Photovoltaic effect was discovered in selenium way back in 1839. The photovoltaic (PV) process converts sunlight directly into electricity. A PV cell consists of two or more thin layers of semiconducting material, most commonly silicon. When the silicon is exposed to light, electrical charges are generated and this can be conducted away by metal contacts as direct current (DC).
The Luque and Hegedus model of PV cell is given by the equation below and Table 1 gives the description of symbols used.
PV equipment has no moving parts and as a result requires minimal maintenance and has a long life. It generates electricity without producing emissions of greenhouse or any other gases, and its operation is virtually silent.
Table 1Symbols used in Solar PV model equations
Symbol Description
I_sc is the short circuit current
V_oc is the open circuit voltage
V_t is the thermal voltage
R_s is the series resistance
STC is standard test conditions
I_sc^* is the short circuit current of module at STC
V_oc^* is the open circuit voltage of module at STC
G^* is the Irradiance at STC
T_a is the ambient temperature
T_c is the operating temperature of module above ambient
T_c^* is the temperature of module at STC
NOCT is normal operating cell temperature
dI_SC/dT_c and dV_oc/dT_c are temperature coefficient of current and voltage
σ_oc is the empirically adjusted parameter equal to -0.04
G_oc is the empirically adjusted parameter taken as equal to the value of G^*
V_M^* is the maximum voltage of module at STC
I_M^* is the maximum current of module at STC.
Wind energy
Wind energy is basically by the pressure differences in atmosphere due to solar power. The wind turbine technology is highly mature and available in commercial scale. Small wind turbines play crucial role in distributed and decentralized energy systems. The production can be improved by using novel control strategies and better energy storage systems.
The Wind energy is modeled using the below relation. Table 2 gives the description of symbols used.
Table 2Symbols used in Wind model equations
Symbol Description
P_W (t) is the hourly output power of wind generator at wind speed v
v is wind speed at projected height
V_(in ) and V_out are cut-in and cut-off wind speed of the wind generator respectively
Reverse Osmosis (RO) desalination using Solar PV and Wind Energy
The photovoltaic technology can be connected directly to a RO system. The factors that determine economics are the plant capacity, cost of extending electricity grid and the concentration of the salt in raw water [5] [6]. RO is the desalination process with the minimum energy requirements. Wind power is abundant in coastal areas. Hence wind power desalination is a promising option [7] [8] [9]. The disadvantage of wind energy and solar energy is that they are intermittent (stochastically varying) source. This reduces the reliability of power output and hence the water output also. Hence a hybrid power system with combination of energy sources could be a possible solution. The RO plant is considered as a load because the plant can run as and when the enough power is available from any of these sources, produce water and stored in tanks. With this, we can keep the capacity of energy storage system like batteries to minimum and hence increase efficiency and reduce costs.
HYBRID SOLAR PV-WIND POWER SCHEME
The complementary features of wind and solar resources make the use of hybrid wind–solar systems to drive a desalination unit a promising alternative as usually when there is no sun the wind is stronger and vice versa [10]. It should be noted, however, that there will be conditions when both solar and wind energy is not available. This implies that the process operates only partially when the energy is available unless some storage device is used. Batteries are one such storage devices but are usually expensive. [2][4] The basic components of the hybrid system are the wind generator and the PV system. Others are additional/auxiliary components which help for full time functioning of the hybrid system. Figure 1 shows schematic of the hybrid system with the desalination plant as load.
A brackish water RO desalination plant is connected as a load to the hybrid power system. This plant produces drinking water when the power available (both generated and stored put together) is sufficient to operate the plant. The water produced will increase up to its rated capacity, if the power produced is also more. Hence the load can handle variation in the power generation due to the renewable sources. The RO plant indirectly stores power produced in the form of product water thereby eliminating the need for having higher capacities of expensive batteries.
Analysis of Hybrid Power System with RO Plant
The selection of capacities of Solar PV, Wind generator and battery greatly affects the total power generated in a particular year, capital cost and the Operation & Maintenance cost of the system. A hybrid power system can be designed with a particular combination of Solar PV capacity, Wind generator capacity and Battery capacity. Due to the intermittent nature of renewable sources, selection of capacity of any particular renewable energy resource based system may lead to component over-sizing and unnecessary operational and lifecycle costs. Also the total power generated may be reduced and hence the availability will reduce and cost of energy per unit will increase. Hence a proper combination of capacities of solar PV, wind and battery is crucial to achieve the required objective. The objective can be to achieve the required total power generation in the year and reasonable level of availability. Also the cost of energy per unit has to be minimum among other alternate combinations of capacities.
Hence an in-depth analysis of the combination of different renewables is necessary to obtain enough understanding of the system. We have modeled the entire hybrid power system in MATLAB using the equations (1) to (8). The main objective of the model is to analyze the various combinations of Solar PV, Wind Generator and Battery capacities with a variable desalination load to obtain the total power generation, availability.
Data
The hybrid power system design options are analyzed for the selected site, Kalpakkam. Kalpakkam is situated in South India and has the following latitude and longitude:
Latitude: 12o34’ North
Longitude: 80o 10’ East
The solar irradiation data is measured by using the ‘Online Solar Radiation Meter’ installed at the site and the wind data is extracted from the meteorological data available at IGCAR, Kalpakkam.
Simulation Methodology
We have used the hourly solar irradiance data measured in kWh/m2/day and wind speed data in m/s as inputs for simulation. The above data is used to calculate the power output available from PV and Wind sources using the models as described in Section 2. The simulation is performed for two cases:
Case 1: With constant load:
A constant load of 2 KW is chosen. The power generated by renewables is calculated for every hour of the year. When the power generated by the renewables is more than 2 KW, load will be fed and excess power will be used to charge the battery. When the power generated is less than 2 KW, load will be fed by both renewables and battery source, if available. Else, only batteries will keep charging and load will be disconnected.
Case 2: With desalination load:
A Brackish water Reverse Osmosis (BWRO) plant with a capacity of 24 m3 / day is chosen as the load to be the system. This plant can operate at full load with 2 KW of power input and with reduced output up to a power input of 0.5 KW. Again, the power generated by renewables is calculated for every hour of the year. The power generated is utilized as per the following logic. When the power generated by the renewables is more than 2 KW, the plant will operate at full load producing rated output and excess power will be used to charge the battery. When the power generated is less than 2 KW and more than 0.5 KW, the plant will operate at lesser capacity. If the power generated is less than 0.5 KW, only batteries will keep charging and the plant will operate at lesser load using batteries until the battery is discharged to it maximum level beyond which the plant will shut down and only batteries will keep charging.
The above simulation is carried out for various combinations of Solar PV capacities (1 KW to 10 KW), Wind generator capacities (1 KW to 10 KW) and battery capacities (0.5 KW to 2 KW).
RESULTS AND DISCUSSIONS
The output of the simulation gives data on Total KWHrgenerationthroughout the year for 400 combinations of Solar PV, Wind generator and Battery capacity for the ranges mentioned in Section 3.3. Table 3 lists the partial results of different configurations of hybrid power systems with constant load and with BWRO plant load.
Table 3Partial list of different configurations of hybrid power systems and KWHr/year generated with constant load and BWRO plant load
Combination no. of each configuration of PV, Wind and Battery capacities PV Wind Battery
KWHr/year produced
With constant load KWHr/year produced with desal load
1 1 1 2 58 2557
5 1 2 0.5 778 3901
10 2 1 1.5 1226 4773
12 2 1 0.5 1368 5667
13 1 3 0.5 1712 4928
50 1 7 1 3732 7673
75 4 2 1 4480 7167
80 1 9 2 4600 8652
81 3 4 0.5 4816 7862
92 2 7 2 4940 8716
100 5 2 1 5142 7520
101 4 3 0.5 5174 7774
108 3 5 2 5262 8396
116 2 8 2 5364 9153
124 4 4 2 5564 8302
150 4 5 2 5980 8799
160 2 10 2 6106 9918
200 10 1 0.5 6478 8660
225 5 6 0.5 6806 9483
250 7 5 1 7082 9421
275 4 9 1.5 7362 10360
300 8 6 2 7618 9921
325 8 7 0.5 7918 10283
350 8 8 1 8172 10609
375 10 8 1.5 8454 10759
385 8 10 0.5 8626 11200
393 9 10 0.5 8794 11270
394 9 10 1 8794 11270
395 9 10 1.5 8794 11270
396 9 10 2 8794 11270
397 10 10 0.5 8912 11332
398 10 10 1 8912 11332
399 10 10 1.5 8912 11332
400 10 10 2 8912 11332
From the results, we can see that there is a clear increase in the total KWHr produced throughout the year. The increase is mainly due to direct utilization of renewables power generation to the maximum extent particularly when the power generated in low. In other case, this has to be used for charging the battery or will be wasted if the battery charge is already full. Hence the BWRO plant indirectly acts as a storage device tapping the generated energy to the maximum possible extent.
For example, in combination number 100, a Solar PV of 5 KW, Wind of 2 KW and Battery of 1 KW capacities produces 5142 KWHr/year with a constant load and 7520 KWHr/year with BWRO plant as load. The average increase in KWHr/year is around 25%.
Figure 2 Plot showing increase in KWHr/year produced with BWRO plant as load
Figure 2 shows the plot of KWHr/year produced for various combinations of Solar PV, Wind and Battery capacities. We can observe that there is a clear increase in the power produced with BWRO plant. Also we can see that the increase in power production is more predominant when the capacities of Solar PV, Wind and Batteries are in lower ranges. This is because of effective utilization of power during low power periods. At higher ranges, the increase becomes more or less steady.
CONCLUSION
In this paper, modelling of renewable energy based desalination systems with Solar PV and Wind turbines was presented. The above configuration was subjected to detailed simulation to analyze the performance of the system under constant load of 2 KW and a BWRO plant load of 2 KW maximum. From the results obtained, we observe that there is an average increase of KWHr/year production by around 25%. Also this increase is very predominant in lower capacity ranges of Solar PV, Wind and Batteries. In high capacity ranges, the increase in power produced more or less saturates to around 25%.
The increase in KWHr/year is primarily because of better utilization of power generated in the low power period which is either used to store in battery or wasted if battery is also full. Additional batteries are required to be employed to tap full power. The battery introduces an efficiency loss of 80% in its charging and discharging cycle. Also the life of battery is limited after which the entire battery system needs to be changed.
The BWRO plant directly utilizes the power generated to produce water. This reduces the battery requirement for the system for a given KWHr/year production. Thus the BWRO plant indirectly serves as an energy storage device. The typical values shown fairly hold well for scaling up to larger systems as Solar PV and battery systems are completely modular in nature. As for as wind turbines are concerned, more economy can be achieved at higher capacities depending upon the prevailing wind data. However, an analysis like the one presented here will enable the designers to take decision on selection of appropriate configuration.
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Nagaraj, R., "Renewable energy based small hybrid power system for desalination applications in remote locations," IEEE Xplore Power Electronics (IICPE), vol., no., pp.1-5, ISSN :2160-3162 , Print ISBN: 978-1-4673-0931-8 doi: 10.1109/IICPE.2012.6450437