ENERGY CONSUMPTION OPTIMIZATION IN A LARGE SEAWATER DESALINATION PLANT (AGUILAS, 210,000 M3/day) Authors:

Fermin Lopez, Mario Araus, Ramon Gimenez, Eva Munoz, Graciano Romero, Jose Maria Bastida

Presenter:

Fermin Lopez Chief Operation Officer, Acuamed [email protected]

Abstract The Aguilas-Guadalentin Desalination Plant is one of the largest seawater installations in the Spanish AGUA program and in the Mediterranean region. The plant, with 210,000 m3/day at maximum capacity, supplies water both for human consumption and for agriculture. The plant includes an open intake, pumping station, intake and discharge pipes, chemical pretreatment, a physical pretreatment with 2-stage multimedia filtration (gravity and pressurized), cartridge filters, reverse osmosis trains with 2 passes and remineralization. It also includes a pumping station and around 25 km of distribution pipes with 2 large reservoirs for supplying agriculture consumers. The high pressure systems have been designed with high pressure pumps, pre-booster pumps to feed the high pressure pumps with variable frequency drive (VFD), energy recovery devices and intermediate booster pumps. The use of the pre-booster pumps with VFD allows for the operation of high pressure pumps at their optimum and most efficient working point, and this, combined with the energy recovery devices leads to very reduced energy consumption, close to 2.88 Kw-h/m3. The global energy consumption projected is around 5.4 kw-h/m3, including intake, desalination plant and product water distribution. On the other hand, an efficient management system and control permits the shutdown and startup of the plant within a few minutes, which facilitate production according to water demand, working when possible in periods with more reduced energy tariffs. Acuamed is also establishing a new energy efficiency control system focused in reducing energy consumption and operational costs. The development of such a system is the first step in the Energy Efficient Plan which recently received the support of the board of directors as one of the main goals for the future of the company. In this paper, the energy optimization and efficiency of this plant will be described, including the design and selection of equipment and the smart use of production management and control systems.

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I.

INTRODUCTION

The Aguilas-Guadalentin Desalination plant is one of the largest seawater installations in the Mediterranean region. With 210,000 m3/day at maximum capacity, it produces water both for human consumption and for agriculture irrigation. This plant is part of the different actions executed inside the AGUA program by the public company Acuamed, from the Spanish Environment Ministry. The program comprises the execution of 12 desalination plants able to supply up to 401 hm3/year (1.187.750 m3/day) (see Table 1). Table 1: Capacity of Company’s desalination plants Plant Name

Province

Oropesa Moncofar Sagunto Mutxamel Torrevieja Valdelentisco Águilas Bajo Almanzora Carboneras Campo de Dalias Atabal (brackish water pant) Marbella

Castellon Castellon Valencia Alicante Alicante Murcia Murcia Almería Almería Almería Málaga Málaga

Status Final testing Final testing Final testing Starting operation Final testing Operation Operation Under construction Operation Under construction Operation Operation

Capacity hm3/year m3/day 18 48.750 10 30.000 8 22.900 18 50.000 80 230.000 40 112.000 60 210.000 15 45.000 42 120.000 30 97.200 60 165.000 20 56.900 401 1.187.750

The installation was born from the need to relieve the water deficit suffered historically in the Guadalentin area, guaranteeing drinking water and with enough quality for the existing crops to be able to develop new agricultural, farming activities, industry, tourism, etc. In Figure 1, an aerial view of the plant is shown, surrounded by the town of Aguilas and greenhouses for agriculture production.

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Figure 1: Aerial view of the SWRO Desalination Plant Aguilas-Guadalentin Another important characteristic in water management in the area of influence of the desalination plant is the high seasonal variation of water consumption, reason why the desalination plant had to be modular and the most versatile possible for being viable. The basic treatment line of this installation is as follows: - Open intake by submerged tower - Intake pipes (920 m on-shore and 3,375 m off-shore) - Pumping station with submersible centrifugal pumps - Chemical dosing - 20 open gravity multimedia filters - Low pressure pumps - 44 pressurized multimedia filters - Cartridge microfiltration - High pressure trains (HP pumps, pre-booster pumps, Calder-Dweer energy recovery system, booster pump) - 1st pass of 12 x 16,200 m3/day RO trains, 45% recovery - 2nd pass of 5 x 27,100 m3/day RO trains, 90% recovery - Post treatment (remineralization and chlorination) - Product water tank and pumping station The International Desalination Association World Congress on Desalination and Water Reuse 2015/San Diego, CA, USA

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-

Membrane CIP and effluent treatment Brine discharge Product water distribution (25 km pipes)

The location of the plant 3 km far away from the coast and at 38 meters altitude, which has caused important adjustments in the design and operation of the plant in order to reach the maximum level of energy efficiency for obtaining minimum operational costs, while being environmentally sustainable. The plant is so far the biggest of all the company`s facilities in operation. Table 2 shows the production and capacity of these plants. Table 2: Production of Company’s plants

Plant Name Aguilas Atabal Carboneras Marbella Valdelentisco II.

Capacity (m3/day) 210.000 165.000 120.000 56.900 112.000

Production (m3/year) 2013 2014 17.411.813 20.225.550 34.357.026 37.500.102 12.905.908 16.464.107 2.130.598 5.763.892 17.571.915 23.477.983

GENERAL DESIGN PHILOSOPHY

The present installation was designed according to the latest advances in the field of seawater desalination by reverse osmosis (RO), and always in agreement to the best practices in engineering. In the criteria for its design, particular attention has been paid to the following aspects: 1.

2.

3. 4.

5. 6. 7.

Flexibility of the installation: the SWRO plant can work only with any of its RO trains. It does not suppose in any case a reduction of the efficiency of the installation in any of its characteristic parameters: energy consumption, water quality, etc. Quality: all the selected equipment belong to the first brands and last models, with recognized experience in operation with seawater. The works and finishing are also of the maximum quality. All the operations of construction and exploitation are done in the frame of a system of management of the quality. Resistance and autonomy: the plant disposes of redundancy of the most important equipment of the installation: pumping, control system, filters, etc. Utmost attention has been paid to the protection of health and safety of employees during the construction and operation stages as well as to the operability of the plant, being these criteria key aspects for the selection of processes, installations and equipment. Absence of vibrations and noises: the equipment and facilities have been selected and designed to minimize these effects Interchangeability of the equipment: the choice of similar elements allows for their interchangeability as well as optimizing the stock of spare parts. Minimization of the environmental impact in construction and operation phases, paying special attention to the minimization of the discharges (brine, chemical cleaning wastes, sludges,

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packaging, etc. This was a very demanding task as the marine surroundings of the plant are a protected area included in the Red Natura 2000 (LIC Submerged Costal Strip of the Murcia Region) [1] 8. Energy efficiency: the plant is equipped with an advanced automatic control, and main pumps have variable frequency drive devices (VFD). III.

ENERGY EFFICIENCY. CRITICAL ANALYSIS AND DESIGN ACTIONS

In order to determine the critical points of energy consumption, the major consumers were ranked by its installed power. The main breakdown of installed power by stages is as follows; -

Seawater intake: 3,000 Kw 2nd step of Guadalentin pumping station: 1,691 Kw Desalination plant: 26,875 Kw TOTAL: 31.566 Kw

As mentioned before, main pumps were equipped with VFD. With this measure, the optimum operational point of the pump is established depending on the flow, which adapts to the different working conditions of the plant. Due to this measure an important global energy saving is obtained. As additional measure, all the process pumps have been designed with a nominal operation point corresponding to a flow 2% superior to the one foreseen for the same pressure. 3.1

Intake pumps

Intake pumping has been designed with submersible pumps with VFD, which starts and stops softly causing minimal pressure variations. The progressive management of pumping power allows adjusting the flow sent to the filtration stage minimizing energy consumption and avoiding energy losses due to flow control at the regulation valves. The pumping station is equipped with 7 (6+1) submersible pumps built in duplex steel with an unitary flow of 3,399 m3/hr at 48 mwc. All the pumps are equipped with VFD to face more than 100 scenarios of possible operation at the plant, optimizing the pump consumption and plant versatility. 3.2

Filtration stage

The design of the physical and chemical treatment was based on the optimization of the chemical reagents to prepare the seawater as well as the low filtration speeds in unitary processes [2]. Intake pumps feed directly the gravity filtration stage, and then this stage does not consume additional energy with the exception of backwash operations.

Water distribution from the intake is carried out by means of open channels, which additionally reduces pressure losses. A large filtration area has been chosen, for reducing filtration velocity and maximum process optimization. The high number of filtration units also increases the efficiency in low production periods, adjusting the filtration area to the production level, saving in this way energy needed for operation and backwash.

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Designed solution includes 2 lines, divided in 10 filtration units each, with 195.5 m2 per unit (Figure 2). The large size of these gravity filters required a complex study of flow and velocity distribution, for both raw water and product water as well as water and air for backwash. The installation of prefab selfsupporting false bottom (Figure 3) instead of the most common strainers for water collection and backwash optimizes the operation of blowers and backwash pumps reducing the energy consumption of backwash.

Figure 2: Gravity filters

Figure 3: Self-supporting false bottom in gravity filters The first filtration stage (open gravity) is done over a multilayer bed with anthracite (0.8 m, 1.4-2.5 mm granulometry and 1.55 mm effective size), sand (0.4 m, 0.6-1 mm granulometry and 0.72 effective size), and supporting gravel (0.1 m). Filtration velocity is below 5 m/h and suspended solids reduction efficiency is more than 94%. This high efficiency filtration system results in an energy consumption reduction in the following stage of pressurized filtration (Figure 4), where the previous high rate elimination of thin solids contributes to the reduction of differential pressures in the filters. The installation of VFD in low pressure pumps allows also to get the consumption to a minimum by regulating the pressure to RO demands. The optimization of The International Desalination Association World Congress on Desalination and Water Reuse 2015/San Diego, CA, USA

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their operating curves using the VFD, allows getting the minimum electrical consumption in the second stage of filtration. As a result, the pressure losses in cartridge filtration are also reduced, saving in cartridge replacement as well.

Figure 4: Pressurized filters 3.3

High pressure pumps

The choice of high pressure pumps and energy recovery systems has been another energetic achievement. The design includes two pumps in series supplying the necessary pressure over the membranes as shown in Figure 5. The high pressure pump fed to 6 KV would raise a constant pressure at its maximum efficiency working point and a previous booster pump, operating with VFD is who supplies the different pressures required depending on the temperature, membrane aging and fouling, etc. An alternative solution with VFD installed in the high pressure pumps would be extremely expensive.

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Booster pump with VFD RO trains From pretreatment

VFD

Product  water

High pressure  pump VFD

Seawater from pretreatment

Booster pump

Pressurized  brine

Energy  recovery  device

Brine to discharge

Figure 5: High pressure and energy recovery configuration All RO trains were built under this scheme, also including the Calder-Dweer energy recovery device and the booster pump in the recovery loop. To manage pressure losses, pressure suction and safety margins, 13 groups (12+1) of HP pumps have been installed, with the following characteristics: Booster pumps (Figure 6) Units: 12 Type; horizontal centrifugal Flow: 687 m3/hr Pressure: 4.5-13 bar Installed Power: 355 Kw

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Figure 6: Pre-booster pump HP pumps (Figure 7) Units: 12+1 Type; horizontal centrifugal Flow: 687 m3/hr Pressure: 58 bar Installed Power: 1,500 Kw

Figure 7: High pressure pump Energy recovery devices (Figure 8) Units: 12 Type; isobaric chambers Calder-Dweer Flow: 276.4 m3/h Efficiency: >98%

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Figure 8: Energy recovery device Booster pumps (in recovery loop) (Figure 9) Units: 12 Type; horizontal centrifugal Flow: 807 m3/hr Pressure: 3.9-4.5 bar

Figure 9: Booster pump During the commissioning phase, key items of the equipment (so as HP pumps, Intake pumps and so forth) were required to repeat rigorous energy efficiency performance tests in order to guarantee specific energy efficiencies and provide assurance of design electrical consumption. [3]. Thanks to this, Aguilas plant has reached a very low energy consumption rate only comparable to the rest of new generation desalination plants. So it has been selected as a target for the update plan of the old generation desalination plants that the company has in operation. These energy saving targets are part of the company’s energy efficiency plan. To achieve them the company approved an 11M€ investment budget. The International Desalination Association World Congress on Desalination and Water Reuse 2015/San Diego, CA, USA

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Table 2: Energy saving targets in old generation desalination plants Plant SWRO Aguilas SWRO Valdelentisco SWRO Carboneras 3.4

Specific energy consumption (kwh/m3)

Difference (kwh/m3)

2.88 3.40 3.86

0.52 0.98

Improvement improvement target target (%) (kwh/m3) 0,40 0,80

11,8% 20,7%

Management of production and control software and systems

The supervisory control and data acquisition system allows the shutdown and startup of the plant within a few minutes, adjusting production levels to almost real time water demands. The production plan of the plant can select periods with the cheapest energy if there is water storage available in the reservoirs. Electricity costs account for almost 40%-70% of all costs. In fact, the construction of another reservoir with capacity for 16 hours production is under study. It will allow working mainly in P6 period (the cheapest electricity tariff period). IV.

IMPROVEMENTS AND MANAGEMENT OF PLANT OPERATION

As the plant was designed for cost optimization, all main operations are driven by the control system, according to prior calculations and designs. Anyway, there is remaining space for operation optimization, as is described below. Current electrical consumption of the RO trains is 2.3 kwh/m3 and no membrane chemical cleaning has been needed in the last 2 years with no significant negative evolution in membranes performance. Only 1 cartridge filter was replaced during the startup of the operation due to the extensive use of chemicals such as ferric chloride no longer in use. The plant operating strategy has reduced the chemical dosing globally, reducing so the costs and fouling problems.

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Figure 10: Cartridge filter replacement during startup 4.1

Optimization in pretreatment

Depending on the incoming seawater flow, a different and particular number of gravity and pressurized filters are enabled for maintaining filtration velocity below 4 m/s. This measure allows separating in time backwashes, limiting the maintenance interventions on the active filters. In fact, if the frequency of backwash determined by the filter fouling was over 4 weeks, a maintenance backwash would have to be done to avoid the clogging of the filtration beds which might be solidified, impeding the correct operation and backwash. For this reason, a balance has been achieved between low filtration velocity and number of filters in operation, getting an energy consumption in the gravity filtration almost irrelevant compared with other processes. 4.2

Optimization in pumps operation

Depending on demanded production, the velocity curve of each pump is adjusted at startup by means of the VFDs for limiting the consumption at this operating point. 4.3

Modification in plant automation

The synchronizations of startups with simultaneous high amperage have been avoided to the extent permitted by the process, avoiding as far as possible peaks in energy consumption. For example, backwash of filters is done sequentially, and startups of pumps are also cascaded. Other small modifications have been done to the electrical installation such as optimization of the filters of harmonics. Due to the large number of VFDs, active compensator filters have been implemented, working in all the spectrum of electrical disturbances (inductive and capacitive). Identifying the usual deviation of the plant, these equipment can be optimized for acting only over it, reducing consumption and activating only the necessary equipment for correction of the specific deviation. Some of these compensators can consume, at full capacity, more than 150 A, each.

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V. ENERGY EFFICIENCY CONTROL SYSTEM BY MEANS OF MONTORIZATION AND CONTINUOUS REGISTER OF ENERGY CONSUMPTION The optimization of the energy consumption of the Aguilas Plant is only one example of the energy efficiency plan that Acuamed is implementing in all the installations. This plan is based on a system for permanent controlling the energy efficiency by continuously measuring the energy consumption of main consumers. This continuous measure will allow: -

To establish comparative measures of efficiency between homogenous processes in different installations To evaluate the efficiency in energy savings of new implementations To detect deviations in electrical consumption from standards for any process and specific equipment To detect new improvement areas

To do so a software is under development that will obtain measures from each plant’s SCADA and send them for its centralized treatment. The procedure used for this task can be divided in the following stages: 1. Definition and characterization of unit processes It contemplates the definition and characterization of the unit processes in which it is possible to divide the seawater treatment. The follow-up of the energy efficiency of each desalination plant will be done over these processes. a. Processes must be common and able to be compared either to other installations or to standards. b. Processes must be related to electrical-mechanical equipment where the electrical consumption will be evaluated c. On each process the electrical-mechanical equipment is detailed and all main nominal data are registered for obtaining standards. d. A centralized codification of all equipment will be done for facilitating the comparative analysis between similar equipment. The same codification will be used for easy and precise location of any equipment and maintenance. e. The measured variable is the unit energy consumption per unit of production. 2. Identification of measuring devices Measuring devices on each installation are identified and a diagnosis of its suitability to the monitoring plan is done. The objectives of this stage are: a. To check if all processes can be measured in homogeneous conditions and compatible with the centralized information system. b. To check that the above mentioned measures can reach the central management system.

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c. To identify those processes or elements in which it is necessary to adapt the measuring devices to the expected objectives. d. To budget the measures and to begin the necessary actions for the adaptation of measuring devices. 3. Creation of a structured database by processes and equipment This database is structured by processes and equipment and registers the measures of the monitoring plan. 4. Development of specific applications for data treatment The software will help: -

To make comparisons between similar processes in different installations To evaluate the efficiency of new developments aimed for energy savings To detect deviations in energy consumption from defined standards To detect new development areas

The setting of this continuous monitoring system is part of a broader “Internal procedure for the development of energy revisions”, also approved by the board of directors. The procedure establishes the scope, methodology, criteria, periodicity, and the responsible staff for the elaboration and supervision of energy revisions. Energy revisions must be updated either annually or in response to major changes in installations, equipment, systems or processes. The revision has to be wide enough to analyze the main activities and to suggest and to quantify possible improvements if needed. Final report includes: ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐

Consumption, type and sources of energy Industrial process Horizontal technologies as energy consumers Main variables affecting energy consumption Energy uses Significate energy rates Baselines Identification of improvement opportunities

The baseline is the quantitative reference for comparison of energy consumption for an specified period. It establishes the relationship between energy use and business activity. This line can apply to the whole consumption in the installation or to those processes or equipment considered as significate. This line should be normalized using variables which consider production level versus full capacity in a desalination plant. The first baseline for the Aguilas SWRO desalination Plant was established in May. The International Desalination Association World Congress on Desalination and Water Reuse 2015/San Diego, CA, USA

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VI.

NEW ACTIONS OF OPTIMIZATION UNDER STUDY

Some new measures for consumption reduction are under study: ‐

‐ VII.

Construction of a new product water tank with 16 hours capacity for supplying in cheap energy periods. As the plant is fed by a mix of conventional + renewable energies, it could be encouraged the operation of the pumping stations in periods with higher percentage of energy proceeding from renewable sources, if the plant had enough capacity for storage in tanks and reservoirs. Installation of VFD in all the product water pumps in order to better adapt the operation of the plant to the consumer’s needs, reducing the consumption in pumps and globally. CONCLUSIONS

Energy costs are the higher contribution to the water production costs in seawater desalination. In plants such as Aguilas, where part of product water is destined to agriculture irrigation, the energy consumption reduction is a key factor for the success of the installation. The innovative use of the pre-booster pumps with VFD allows the operation of high pressure pumps at their optimum and most efficient working point, and this, combined with the energy recovery devices leads to very reduced energy consumption, close to 2.88 Kw-h/m3. On the other hand, an efficient management system and control permits the shutdown and startup of the plant within a few minutes, which facilitate production according to water demand, working when possible in periods with more reduced energy tariffs. The most inexpensive energy is the energy not consumed in the first place. It is of the utmost priority that the design and construction of desalination follows this criterion. During the operational phase, a system to permanently keeping track of the energy consumed by the desalination processes must be set up in order to be able to react and correct inefficiencies as soon as possible. This will not only keep desalination costs down but also will help to improve the environment. VIII. REFERENCES Arconada, B., Delgado, P., García, A. 2013. “Minimizing environmental risks on constructing marine pipelines: Aguilas desalination plant”. Desalination and Water Treatment Volume 51, Issue 1-3, 2013 2. Buendía Candel, R., García Soto, CG., Zarzo Martínez, D., Díaz Pérez, A., Mañueco, G., García A., 2009. “Design of the Pretreatment at the New Águilas-Guadalentín Desalination Plant”. DB09-287. DB09 Proceedings 3. Mañueco G., Garcia, A., Buendia Candel, R., Galdos Aller, J. 2011. “Aguilas seawater desalination plant commissioning: procedure and problems solving”. PER11-158. PER11 Proceeding. 1.

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