Development of a Wireless Control and Monitoring System for Wave Energy Converters Ismail Sultan, Billy Wells, Stephen Wood Ph.D., P.E. Ocean Engineering Florida Institute of Technology Melbourne, Florida USA
[email protected] Abstract—The Ocean, inherently unstable and uncertain, is one of the harshest environments to test engineering designs; and with the increasing interest in ocean renewable energies, methods and tools must be developed for faster, reliable, low cost, and effective evaluation of the ocean energy technologies. This paper proposes one such tool/system, a small-scale, portable, wireless and universal Power Control and Monitoring Unit (PCMU) for the design and performance evaluation of wave energy converters (WECs). A prototype PCMU system was successfully deployed on June 8th, 2012 with wave energy convertor systems developed at Florida Institute of Technology (Florida Tech).
and aid in the rapid and successful transformation of ocean renewable energy technologies from prototype to full-scale matured systems. One aspect of any ocean energy system is to have a reliable, sophisticated and robust controlling mechanism to integrate offshore and onshore power systems [2]. This paper proposes one such tool, a small-scale, portable and universal Power Control and Monitoring Unit (PCMU) for the design and performance evaluation of wave energy converters (WECs).
Index Terms—ocean energy, wave energy, wave energy convertors, wireless, GPRS, power take-off, green energy, alternative energy, renewable energy.
The Ocean Engineering Department at Florida Institute of Technology under the direction of Stephen L. Wood, Ph.D., P.E., has been developing ocean energy systems both above and below the surface since 2007. Currently there are two prototypes WEC’s under development at FIT: Wing-Wave, a sea-bed mounted bottom hinged type WEC that captures energy from the horizontal component of wave orbitals at varying depths, and GECCO, a surface floating attenuation WEC for harnessing surface waves. The Wing-Wave system is designed to capture energy from ocean waves at a depth of 10 to 15 meters and produce electricity. This production of electricity is by harnessing the oscillatory motion of water particles. Particles follow an elliptical path under a shallow water wave. As depth increases the horizontal movement of the particles’ rotation remains the same while the vertical movement decreases in shallow water. This causes the motion close to the ocean floor to oscillate in an almost exclusively horizontal direction. The GECCO (Green Energy Coastal Collection Operation) floats atop the ocean surface and articulates about a fixed hinge and is freely moored to the ocean floor. GECCO uses the Salter Duck technology to capture the rotational motion of the waves. The Salter Duck is a teardrop shaped wave terminator system oriented perpendicular to the direction of the wave with the nose of the teardrop facing the oncoming wave. The device was designed to rotate and “bob” up and down as a wave passes [3]. The bobbing and rotating motion is used to pump hydraulics, which drives an electrical generator. So the GECCO is a floating heave/surge attenuator that works with a combination of a mechanical and a hydraulic system in unison. The GECCO harvests not only the energy of the big swells of the ocean, but the small capillary waves by the addition of Stephen Salters’ “Salters Ducks.” Different configurations of
I. INTRODUCTION The oceans, covering more than 70% of Earth, represent an enormous untapped energy resource containing potentially more energy than the combined output of all other resources on earth. Energy is stored partly in the form of kinetic energy from the motion of waves and currents and partly as thermal energy from the sun. It is estimated that the potential energy of the oceans is up to 2 Million Terawatt-hours [1]. Despite the enormous potential, ocean energy is still an emerging field that is lagging behind the renewable agenda of most countries. Ocean energy technology, being in the experimental and demonstrational phase of development, currently has a high cost for power generation, especially with extreme ocean weather conditions and growing “powerquality” requirements. These and other inherent factors in designing for the ocean create tough engineering problems when balancing efficiency, reliability, and economics with respect to ocean energy systems. The challenges, including the inherent unstable and uncertain nature of ocean energies, management of marine electric grids and stable integration of large amount of renewable energy, have imposed issues in the form of additional system requirements and corresponding costs. To ensure successful and smooth integration of the oceanic renewable energy generation into large power systems, it is necessary to develop methods and tools for faster, reliable, low cost and effective evaluation of the ocean energy conversion technologies. Such tools can be indispensable for concept verifications on a small-scale (e.g., research and development)
II. BACKGROUND
Fig. 1. Basic Power Take Off Concepts [5]
Fig. 2. Link between OPC software and automation hardware [8]
these two systems were deployed and tested during the summers of 2010, 2011 and 2012. In 2010 and 2011, major focus was on concept verification of the WEC’s designs without integrating any electrical part. In 2012, it was decided to develop a control and data acquisition system along with the power take-off system for the detailed performance evaluation of these prototypes. Inspiration for the system was ‘Power Analysis and Data Acquisition System’ being developed at Oregon State University for ocean wave energy device testing [6]. III. CONCEPTS Before moving to the technicalities of design, some fundamental concepts and terminologies regarding Ocean energy and electronics will be explained in this section. In terms of ocean energy, ‘Power Take-off’ mechanisms refer to the conversion of captured hydro-mechanical energy to electrical energy. ‘Power Take-off’ design depends on the working principle of the energy device and typically could be mechanical, hydraulic or a direct driven system [4]. Figure 1 compares different PTO schemes generally applied for ocean energy devices. The next terminologies are related to the control engineering. The Programmable Logic Controller (PLC) is generally an embedded system based control electronic device,
which interfaces signals in the physical world to a control system and has ability to transmit data to remote system for control and monitoring. These devices can be attached to Human Machine Interface (HMI) devices via web or other network technologies to present the data in readable formats. Remote Terminal Unit (RTU) is a similar device as the PLC with some subtle differences but with increasingly powerful hardware and capabilities, these terms are being used interchangeably in automation industry [7]. Another important concept for data connectivity is the Object Linking and Embedding (OLE) for Process Control (OPC), an open software interface standard. OPC standard has been adopted, by most of automation and control industry vendors, for communication and data exchange between Microsoft Windows programs and automation devices [8]. The OPC server software translates the data from the PLC or RTU into format compatible to the Microsoft Windows programs. The OPC client software is then used to connect the data stream from the hardware, save in databases and display the processed data on HMI. The OPC client uses the OPC server to retrieve data and/or send commands to the control hardware (Fig. 2). General Packet Radio Service (GPRS) is a packet oriented switching technology for data transfer in cellular networks. It is used for data communications (including mobile internet) on the 2G and 3G cellular communication system for GSM (global system for mobile communications) [9]. A charge controller is a device used to protect electrical batteries from overcharging and over-discharging by limiting the electric current inflow and outflow. It helps to prevent overvoltage, which may pose a safety risk. Charge controllers have also provision of diverting excess electricity to a secondary load to ensure that additional energy is safely dissipated [10]. IV. SYSTEM REQUIREMENTS AND DESCRIPTION The main task regarding the testing and performance evaluation of prototype WEC’s was to design and develop a modular portable tool capable of regulating output, providing an electrical load and logging the real-time system parameters (electrical and process) for analysis. Also based on experiences during previous testing, it was desired to have wireless connectivity for the control unit to view real-time operational status. Modularity in design was kept to facilitate troubleshooting and further development. To accomplish above, two modules and systems were designed for the Power Control and Monitoring Unit (PCMU): 1) Control and Data Acquisition Module: This module controls and monitors the PTO unit and sends data from PCMU to a remote system for logging ideally through wireless medium. This unit is capable of independently operating and logging. It is housed in a weather-proof housing to protect against the harsh marine environment. 2) Power Module: This module regulates the electrical output of PTO as per provided load requirements. This unit is capable of protecting the system load from undesired voltage levels. 3) Power Take-off (PTO) System: A small-scale PTO system up to 3 kW rating based on hydroelectric generator
Pelton Turbine
Power Module Fused Input; Rating 3KW
Electrical Energy
Wing Wave
DATA
GECCO Generator 2.0 kW
Main Control and Data Acquisition Module DATA/ Control
WEC Under Testing
POWER
Input: Variable AC from Generator Output: Rectifier Æ 12V DC Bus CONTROL
Hydro-Mech Energy
PTO
Test LOAD Battery And
Control Power Module (For all modules)
Fixed Resistive load
PTO Control and Monitoring Unit (PCMU)
Server/Operator Station
Fig 3. System Block Diagram showing PCMU
V. PROCEDURE The following sections describe the hardware and software design of the PCMU.
Fig. 4. Network Topology for the project
with Pelton turbine and hydraulics system is designed to harness energy from the wave energy converters. It is capable of being connected to the multiple WECs under testing. 4) Raft for PTO and PCMU: All of the above units are housed on the raft anchored near wave energy converters during testing. This raft is capable to protect the sensitive electrical and hydraulic components from the harsh marine environment. Figure 3 presents the block diagram of the PCMU, which also shows the energy flow through the overall system. Wave energy is converted to hydro-mechanical energy by the WEC. This is then converted to electrical energy through the Pelton turbine and coupling alternator based PTO. Finally the energy is fed into power module which then regulates and conditions it for the system load. At the same time, control and data acquisition module system controls the hardware, logs all instrumentation data, and sends it on the internet to the remote system.
A. Control System Design The biggest design issue for the control system was to fulfill the design criteria requirements of the PCMU while remaining within the project’s time frame and budget constraints: 1) Controller selection, reliability, flexibility, ease and cost requirements; 2) Protection from water-ingress and vibration; 3) Remote connectivity. Two options were considered for the controller selection. These were to develop a microcontroller based single-board computer or to obtain an off-shelf industrial grade control unit. Though the first option offered a great benefit in cost and customization, the downside was the time required for design and testing. The latter option, with all respective advantages, had a major issue of cost. The second option was finalized thanks to donation of the control units. The core of the control system (donated by Intech Automation) is a Cellular Micro RTU Controller ioLogik W5340 by Moxa®. In addition to analog and digital interfaces, the ioLogik W5340 supports GPRS technology for cellular remote monitoring and alarm systems. It can send the logged data on internet using a cell phone SIM. Moxa’s RTU controllers can log data through I/O and serial interface to a single, expandable SD card slot (up to 32-GB) and provide multiple methods to remotely retrieve data logs, whether through FTP, e-mail or OPC based software [11]. For additional analog inputs, ioLogik E1240 (Ethernet Remote I/O) was used. This unit also provides built-in 2-port Ethernet switch for daisy-chain topologies [12]. The control unit comprised of an RTU Controller W5340 and Ethernet I/O card E1240 along with two 12-V panel batteries (one for control units and one for the solenoid valves), signal conditioning card and system terminals. All these
Fig. 5. Control module internal components
Fig. 6. Hierarchy Diagram for MOXA® Automation Software ioAdmin and ActiveOPC [11]
Fig. 7. ActiveOPC Server software
components were boxed in an IP 67 rated weatherproof housing (Fig. 5). This controller was interfaced with the instrumentation including pressure for each WEC (Wing-Wave and GECCO), generator output voltage/current and the panel’s internal battery voltage, along with instrumentation controlling the solenoid valves installed at the inlet of the generator from Wing-Wave and GECCO. Industrial grade general-purpose pressure transmitters by WIKA® (Model 50398083, 4-20mA, stainless steel 316L, 0 to 1,379-kPa (0 to 200-psi) range, +/-0.5% Accuracy) were used for pressure measurement. To control the inlet flow at generator, normally closed 2-way 12-VDC 0.95cm (3/8-in) solenoid valves with a working pressure between 0 to 1,014-kPa (0 to 147-psi) were used. For measurement of
generator output voltage, battery voltage and load current, a simple voltage divider based signal conditioning card was used as input for the controller. The control unit in housing along with instrumentation was located on PTO raft. After storing the instrumentation data locally on a 16-GB memory card, the data was sent to the internet using the cellular network in real-time. The AT&T network was used due to good signal strength at deployment location (RSSI around 20), which was approximately 8-km (5mi) offshore. The network topology of the system is shown in Figure 4. The data from PTO is sent to the main server (AMD A8 1.5-GHz processor) located at the Florida Institute of Technology with a fixed IP address. The main server runs various software: IOAdmin™ (configuration and diagnostic software for Moxa RTUs), ActiveOPC™ (OPC server software for connectivity), DA-Center™ (OPC client software for datalogging), FTP server, and a web server. The data on the main server computer is accessible at the user end via a webpage on the internet updated each second in real time. This way, the end user is able to view the live data from PTO raft and understand its operational status. This wireless connectivity provision offers a great advantage as compared to hardwired connection for ocean energy applications. B. Power System Design Initial power system design considerations included converters and ac system but later it was decided to keep the system simple due to other project restrictions. The power module consisted of a charge controller NC25A, a 12-V/80-AH battery as primary load and a fixed power resistor as a secondary or diversion load. Charge controller NC25A by FlexCharge® is a weather proof, series regulator designed for alternative energy charging systems. The maximum current it can handle is 25-A. The NC25A also includes the charge divert feature and protects the load from voltage surge up to 140-Vdc [14]. The charge controller regulates the electrical output from PTO system and limits it to the voltage level of 12 or 24-V depending on mode selected. The controller unit then directs the electrical energy to the primary load (battery) and and then secondary load (resistor) in case battery has been fully charged. C. Power Take-off Design The main design goal for the power take-off system was ability to integrate with both WEC`s being tested. Two types of PTO mechanisms were considered: mechanical and hydraulic. Hydraulic system was finalized as it was cost-effective and relatively easy to interface and test different types of WECs on the same PTO. The components inside the PTO system consist of two accumulators, one generator (Harris Hydroelectric Pelton wheel style, received through donation to the university), and a reservoir to collect the hydraulic fluid after it turns the turbine. A system of valves and fitting was also implemented into the system. Using these valves, flow could be diverted away from the generator in a separate closed loop to provide a means for testing the WECs individually. Figure 8 shows the overall process diagram of the PTO. Electrical system of the PTO is based on Harris® hydrokinetic Pelton wheel style impulse turbine with 2-kW rated PM brushless alternator [13]. Hydro-Mechanical energy
Controller
G
PI-302
MV-311
CV-303
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M
MV-312 MV-314
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MV-322
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PTO
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Starting Hydraulic Pump MV-202 MV-309
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PTO RAFT
CV-203
CV-206 CV-205
HC-210
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GECCO
CV-105
HC-110
WING WAVE
Fig. 8. Process diagram for the PTO
Fig. 9. Harris® Hydroelectric generator [13]
Fig. 10. PTO raft
from WECs is converted to electrical energy by the Pelton turbine and coupled alternator. It is actually designed for low flow applications on land but it was successfully interfaced with hydraulics system of the PTO.
For the purpose of the wave energy system, kinetic energy within propagating wave swell is harnessed by the energy converters (Wing-Wave and/or GECCO), and transferred via a working fluid (freshwater) through a hydraulic line into a bladder style diaphragm tank. The energy is stored there under a specific pressure until a solenoid valve is opened, releasing the energy into a Pelton wheel style generator. The fluid under pressure spins the wheel, generating electricity. The working fluid is then collected in a reservoir, and returned through low pressure lines to the wave energy converters to begin the cycle again. The alternator generates electrical output that is regulated by the charge controller and is then directed to the primary and secondary diversion loads. D. PTO Raft Design The purpose of raft is to house and protect the sensitive electrical and hydraulic components from the harsh marine environment in which they operate. The PTO raft is designed as a floating vessel, shaped like a simple box, constructed of 0.64-cm (¼-in) plywood with a fiberglass coating. The finishing process is similar to a wood boat hull, in order to prevent water ingress into the vessel body. Angle 6061 aluminum is used along the edges of the housing to add structural strength. This allows two people to stand on or in the vessel while still maintain a minimal draft of about 10-cm (4in). VI. LABORATORY TESTING Laboratory testing included mechanical and process testing of the Harris generator with the PCMU. The lab data had to be
B. Process Testing: For process related parameters, the generator was setup in a wave tank and a centrifugal pump 1.12-kW (1.5-hp, 3450-rpm; 115/230 FLA 15.6/738A; 60-Hz) was used as input source. Figure 12 shows the recorded results that with approximately 90-kPa (13-psi), inlet pressure the generator started to operate with output of approximately 22-V (no load). VII. DEPLOYMENT
Fig. 11. Results for the mechanical testing of Harris Hydroelectric Generator
Fig. 12. Results for the process testing of Harris Hydroelectric Generator
The entire wave energy system, Wing-Wave, GECCO, and universal PTO systems (Fig.13), were deployed off the central east coast of Florida, approximately 2.4-km (1.5-mi) east of Ft Pierce Inlet from June 8-10, 2012 by the research vessel M/V Thunderforce, owned by American Vibracore Services. Each section of the wave energy system had significant results and many lessons were learned by the design team over the course of the deployment. The Wing-Wave system was successfully deployed and worked as per design. However GECCO didn’t work due to mechanical failure and no data was recorded from the system. The PCMU successfully logged instrument data and established remote connectivity. However, there was a fault in the control algorithm for the solenoid valve operation and electrical output could not be produced despite sufficient hydraulic pressure differences generated by the Wing-Wave system. An attempt was made to update the program of the PCMU, however due to rough seas and limited maintenance access; it was not possible during deployment. Except for this issue, the PCMU worked as per plan for all the deployment period. VIII. ANALYSIS AND FINDINGS A summary of two sample datasets that were collected for the hydraulic pressures of the Wing-Wave system (Table 1) and their time-series plots are shown in figures 14 and 15. TABLE I. WING WAVE DATA ANALYSIS Wing wave Data Description
Fig. 13. Wing-Wave, GECCO, PTO system
used as the base data for generator due to missing data (nameplate, datasheets, maximum current/speed limitations, etc.). A. Mechanical Testing: The generator was coupled with a mechanical rotational source (max speed 1500 revolutions-per-minute or rpm) using tachometer and voltmeter to determine rpm and voltage relationship. Figure 11 shows the findings.
Dataset 1
Dataset 2
1
Avg time for a sample (seconds)
Overall
10.846
3.219
2
Average PSI
11.802
21.524
3
Maximum Value PSI
38.879
33.261
59.678
4
Minimum Value PSI
0
2.990
0
5
Std Dev PSI
9.261
7.104
6
Max Frequency (cyc/min)
6.000
9.000
7
Min Frequency (cyc/min)
0.500
6.000
8
Avg Frequency (cyc/min)
2.316
7.055
Data recorded from Wing-Wave was very encouraging, showing a maximum recorded peak of 407-kPa (59-psi) and moving average between 138 to 172-kPa (20 to 25-psi). As per earlier lab testing of Harris hydro-electric generator, the minimum pressure required for the electrical output generation is 103-kPa (15-psi), thus implying there was sufficient pressure available for generator operation. After careful evaluation of all
A summarized list of major findings is as follows: • Failure of the control algorithm kept the generator from starting despite sufficient pressure. The fact that full scale integrated testing could not be executed with PCMU and WECs before deployment, contributed to the issue. • Additional and redundant instrumentation would have made the process of root cause identification easier (like tachometer or flow meter etc.). • Due to experience and time, all maintenance aspects for the PTO raft were not considered. The maintenance activity for reprogramming the PCMU during deployment was very difficult and hazardous, and could not be completed. • Since there was no external power supply charging the system, the PCMU’s pre-charged battery discharged at the end of deployment.
Fig. 14. Results recorded for Wing Wave Dataset 1
IX. CONCLUSIONS
Fig. 15. Results recorded for Wing Wave Dataset 2
Fig. 16. Moving average for Wing Wave Dataset 2
parameters in the system, a faulty control algorithm was found to keep the generator from starting despite sufficient pressure. The lab testing implies that there was sufficient pressure to generate output voltage around 30 Volts. Figure 16 shows the time series plot for the wing wave pressure with the moving average between approximately 124-kPa and 159-kPa (18 and 23-psi). The PCMU performed successfully. There was establishment and smooth operation of the remote link between the work station on the research vessel and the PCMU. The PCMU was able to record and send real-time values from the PTO successfully. This was helpful in understanding the operational status of the Wing-Wave and the generator. The housing of the unit provided sea-worthy protection and there were no signs of water ingress in spite of rough weather and rains.
The overall performance of the PCMU system was successfully demonstrated. The unit was able to log and successfully transmit data over the cellular network smoothly without any failure, which was very helpful in understanding the system’s operation. However, there were some operational issues including a faulty control algorithm, battery, software issues, and maintenance aspects. Future work will include additional process instrumentation along with wave instrumentation to establish the relationship between environmental and energy data. Furthermore, an independent power source would be added in the design to ensure continuous control supply availability. Operational interface would be further enhanced for the end user. In the future, advance power electronics would be used for power conditioning and integration with grids. The current state of ocean power technology requires various tools to harvest ocean energy. Such tools can be indispensable in the rapid and successful transformation of ocean renewable energy technologies from prototype to full-scale matured systems. REFERENCES [1]
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