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Essay: Supply of energy to the desalination plant

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Energy is needed for the every stage of the desalination. Power required for the system includes power for seawater pumping, high-pressure pumping, and chemical treatment pumping. Considerable amount of energy is required pump. Renewable energy sources are one of the attractive options for the supply of energy to the desalination processes. In remote regions use of conventional energy is more costly. So the desalination plants combine with renewable energy are more attractive option rather than conventional energy sources.
There are different types of energy sources available for the supply of energy.
1. Nuclear energy (developing techniques)
2. Solar energy (commonly used for MSF and MED)
3. Wind Energy (RO, ED, mechanical Vapor compression, distillation process)
4. Geothermal energy (typically used for MED distillation)
5. Ocean Energy ‘Tidal Energy (RO {350 gal/day} (Kim 1997)
Membrane desalination (RO) requires only electricity and large RO plant consumes only 3.5 -5.0 kWhe/m3 of electricity (EU, 2008). The major renewable desalination plant is mostly based on RO process i.e. 62%. The dominant energy source for the desalination is photovoltaic’s which is used in 43% existing application. While considering the energy for desalination plant, we have to consider the locally available renewable resources. For example solar energy (concentrated solar power) for thermal desalination and electricity from photovoltaic for membrane desalination is key solution in remote regions, however the wind energy is of interest for membrane desalination in coastal area.
The power requirement of each pump can be calculated using Eqn. 1:
Pw = Q x Pr / (Ep x Em) , (1)
Pw [kW] = the power consumed by each pump,
Q [m3/s] = the feed water flow rates for each pump,
Pr [kPa] = the feed water pressure of each pump, and
Ep = pump efficiency
Em = motor efficiency
High-pressure pumps consume the large portion of energy, while other system pumps (such as the seawater and chemical dosing pumps) could be considered as only 20% of the total energy consumption.
The recovery ie. Ratio of fresh water production to seawater production, is around 30% in a seawater RO system and the remaining 70% is rejected as concentrate and at pressure below the feed water. In large RO systems, this waste energy can be recovered using the devices such as a Pelton turbine and returning the concentrate to the shaft of the high-pressure pump, thus reducing the motor size to be lower and dramatically improving the overall system efficiency.
For our RO process we are considering the wind energy as well as solar energy. These two options are the cheapest as compared to other option for energy supply. As for this process we can produce on site electricity to drive the pump and using solar energy we can run the plant when there is no electricity from wind and also can maintain the lightening of plant and control room. The wind based desalination is one of the most effective options for seawater desalination for coastal regions where the wind potential is high.
1. How is the electricity produced by wind?
The wind turbines are used to produce electricity from the wind. Wind passes over the blades which used to run the motor. The blades turn the low speed shaft. The low speed shaft and high speed shaft is connected by gears and that gears are used to drive the generator. Now here, slow speed of blades is increased to higher speed of generator. Then this generator is driven by rapidly spinning shaft to produce electricity.
The wind energy is a source of clean, non-polluting electricity. Also wind power plants impact very little on the environment. The major issue regarding this plant is high initial cost and intermittence of energy. But batteries and other storage systems for energy can provide the smooth and regular operation. Taking into account the average energy demand for RO based desalination is 3.5 kWh/m3. So the requirement of the electricity for 75MLD desalination plant is 11 MW. We need 15 700 kWhe wind turbines to produce 11 MW electricity for desalination. As one 700kwh turbine can produce 700kwh of energy. However wind power is variable so, during low wind period it need be replaced by another source.
2. Another method we considered for the electricity of desalination is solar energy.
PV systems use direct sunlight for the production of electricity. The typical Photovoltaic systems consist of
PV array. PV modules are used to made PV arrays, which are environmentally sealed and consist of collections of PV cells’the devices that convert sunlight to electricity.
Batteries. For stand-alone systems, a battery is used to provide energy storage. All PV batteries are deep-cycle, i.e., designed to be discharged down to 50% or more without damage so that they can supply power over a long period of time. The lifetime of a battery depends on factors such as how it is used, how it is maintained and charged, and temperature, but it is typically between 5 and 10 years.
Charge controllers. The primary function of a charge controller (or regulator) is to maintain the battery at the highest possible state of charge and provide the load with the required quantity of electricity, while protecting the battery from deep discharge (by the loads) or extended overcharge (by the PV array).
Balance-of-system (BOS) equipment. BOS includes mounting of system devices and system wiring used to integrate the solar modules into the structural and electrical systems of the RO unit. The wiring systems include disconnects for the DC and AC sides of the inverter, ground-fault protection, and over current protection for the solar modules. Most systems include a combiner board of some kind because most modules require fusing for each module source circuit.
DC-AC inverter. This is the device that takes the DC power from the PV array and converts it into standard AC power used by the different pumps.
The solar data for the UK were obtained from http://www.greenenergy.org.uk. The annual average radiation for the UK is 950 Kwh and it is equivalent to 3.42 GJ of solar radiation of per m2. The plant daily fresh water output is 75000 m3; therefore the total daily load is 10938 kwh. Due to the inverter sizes currently available in market, we can supply this energy by 2 PV- sub systems of 6 Mwh/day
Figure: annual solar radiation in kWh received in the British Isles per square metre.

Taken from: http://www.greenenergy.org.uk/sta/solarenergy/ukresource.htm
Water Desalination Using Renewable Energy (IEA-ETSAP and IRENA Technology Brief I12 ‘ March 2012),
ISSUE 132, PAGES 27-38, DECEMBER 2005, Tamim Younos, Kimberly E. Tulou



Perth Seawater Desalination Plant

M. Thomson and D. Infield, 2002, A photovoltaic-powered seawater reverse-osmosis system without batteries. Desalination 153, pp 1-8.
Desalination Economic Evaluation Program (DEEP), International Atomic Energy Agency, Vienna, Austria, 2006.
K.E. Thomas, Overview of village scale renewable energy powered desalination. NREL/TP-440-22083. UC Category:1210.DE97000240
M. Darwish, F. Al-Asfour, and N. Al-Najem. Energy Consumption in Equivalent Work by Different Desalting Methods: Case Study for Kuwait.

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