DEVELOPMENT AND OPERATION OF A ... - IEEE Xplore

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Michael F. Piszczor, Lisa L. Kohout & Michelle Manzo. NASA Lewis Research Center. Cleveland, OH 44135. Anthony J. Colozza. NYMA. Brook Park, OH 44142.
DEVELOPMENT AND OPERATION OF A PHOTOVOLTAIC POWER SYSTEM FOR USE AT REMOTE ANTARCTIC SITES Michael F. Piszczor, Lisa L. Kohout & Michelle Manzo NASA Lewis Research Center Cleveland, OH 44135 Anthony J. Colozza NYMA Brook Park, OH 44142

ABSTRACT A photovoltaic power system, designed and built at the NASA Lewis Research Center, has successfully operated over the past two summer seasons at a remote site in Antarctica, providing utility-type power for a six-person field team. The system was installed at the Lake Hoare site for approximatelyfive weeks during late 1992, put into storage for the Antarctic winter, and then used again during the 1993 season. The photovoltaic power system consists of three silicon photovoltaic sub-arrays delivering a total of 1.5 kWe peak power, three lead-acid gel battery modules supplying 2.4 kWh, and an electrical distribution system which delivers 120 Vac and 12 Vdc to the user. The system worked extremely well in providing quiet, reliable power. The experience gained from early system demonstrations such as this should be beneficial in accelerating the transition toward future PV systems in Antarctica and other similar areas.

INTRODUCTION In 1991, a small team at the NASA Lewis Research Center began to design and fabricate a photovoltaic system that could provide utility-type power to a six person field team conducting research at Lake Hoare in the Dry Valleys of Antarctica. Previously the camp had been powered by diesel generators, which were noisy and also polluted the pristine environment which was being studied. In an effort to reduce its dependence on diesel fuel, the National Science Foundation (NSF), which sponsored the research team, became interested in the use of alternate energy sources. The move to eventually utilize a solar powered system was motivated by environmental concerns as well as long-term cost issues. Design, fabrication and operation of the photovoltaic power system at Lake Hoare was initiated as part of the Antarctic Space Analog Program, a joint NASNNSF program designed to take advantage of the similarities

between Antarctica and the harsh environment of space. The Antarctic Space Analog Program was begun in 1990 to unlock the potential of the Antarctic environment for testing hardware and habitats that support human researchers. For NASA, the program provides opportunities to prepare for future missions in remote and hostile environments, while the NSF benefits from the application of NASAdeveloped technologies and systems that can increase the efficiency, reduce the cost, and minimize the environmental impact of the US Antarctic Program. NASA specifically benefited from the PV power system project by gaining valuable information with regard to system level deployment and operation in a remote location by a minimally trained crew, as well as validation of the initial integration concept.

DESIGN REQUIRMENTS The unique environment of Antarctica required an unconventional photovoltaic array design. A number of factors needed to be taken into account, such as the severe cold, the remote operational location and the circular path of the sun. From a human factors standpoint, the system had to be easy for the field team to deploy while wearing heavy gloves, which meant minimizing the number of bolted and hardwired connections. Also, the team would have minimal training so the system had to be easy to deploy and operate. Since the entire system could only be transported to the remote site by helicopter, strict volume and mass constraints had to be met. Power consumption at the site was largely unknown, therefore the system was designed to handle a varied load profile. The loads included the use of personal computers and printers, lab equipment, lighting and a small microwave oven. It was determined that the camp needed a baseline of 0.5 kW from the array and 2.4 kWh of energy storage. Provisions were made to supply both 12 Vdc and 110/120 Vac to accommodate all of the equipment.

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First WCPEC; Dec. 5-9, 1994; Hawaii

SYSTEM DESCRIPTION The photovoltaic power system consists of three silicon photovoltaic sub-arrays delivering a total of 1.5 kWe peak power, three lead-acid gel battery modules supplying 2.4 kWh, a field junction stand, an instrument and control rack, and two dc power supplies [l]. Figure 1 shows one of the sub-arrays with the balance of the system components. In order to minimize the number of hardwire connections to be made at the site, the modules were prewired prior to shipping and connections were made between modules with plug-type connectors. This design approach not only made for easier installation, but also reduced the risk of electric shock by protecting the field team from exposure to potentially hot wires. The only hardwired connection required during installation was between the power system and the existing ac distribution system at the site. This modular design approach also allowed for portability of the system and ease of relocation from the drop site to the installation point. The PV array design selected was a flat panel array capable of rotating 360'. The power system was sized to put out the minimal amount of required power based on an array panel positioned at 70' from the horizontal and realigned every 6 hours toward the sun. This design required 24 Siemens Solar Industries M55 solar modules to supply the needed power. These modules were divided into 3 sub-arrays of 8 series-connected modules each, with each sub-array being connected in parallel. To maximize the ease of installation, the sub-arrays were designed to be deployed and restowed without the use of any materials or tools. In the stowed configuration, the base legs are raised to a vertical position and two wings of two panels each are folded over the four central panels. This provides a compact stowed configuration and provides both protection for the PV cells and eliminates any electrical hazard since light can not reach the panels. (See Fig. 2). During installation, the array legs, which were hinged to the base, would be lowered and a bar along the side of the leg would slide forward, locking the legs in the horizontal position. The base legs could then be secured to the ground with either a ground stake or by weighing down the legs with rocks or sand. The two outer wings which were hinged to the central panels would then be unfolded and the outer edge of each would be secured to the structure with a movable brace connected to the main body of the sub-array. The brace would be attached to the outer panels with the use of a self-iocking pin. The rotation of the array is accomplished by a roller plate between the top and bottom portions of the base. A springactivated locking pin attached to a foot lever is used to lock the array into position every 90'. There is also a safety pin which could be inserted between the upper and lower portions of the base to secure the base from rotating when the array is exposed to high velocity winds. (The array structure was tested in a wind tunnel with wind

speeds up to 60 mph.) The array can be turned up to 360' clockwise and then must be turned back 360" counter clockwise to begin the next cycle. A stop was placed inside the base to prevent the continual rotation of the array. This feature prevented the power wire from coming off the array during rotation of the array. Batteries are required on the electrical bus to provide stabilization during current surges and also to provide power during shadow periods. The system is designed to provide 2.4 kWh of storage. At room temperature it is capable of delivering 9.6 kWh at a 20 hour discharge rate. However, operational battery capacity was significantly derated because of the extremely low temperatures encountered. The battery storage system consists of 10 sealed lead-acid gel-electrolyte batteries connected in series to provide a nominal 120 Vdc on discharge. The cell capacity is rated at 80 AH at the 20 hour discharge rate at room temperature. Dynasty GC12V80 batteries, manufactured by Johnson Controls, were chosen for the system. The batteries are configured into three modules: two consisting of three batteries each and one consisting of four batteries. The batteries within each module are connected in series to provide a nominal 36 Vdc and 48 Vdc, respectively. The three modules are further connected in series to achieve the 120 Vdc battery subsystem. Each battery module enclosure is made of aluminum to minimize weight. A layer of Styrofoam between the enclosure walls and the batteries provides thermal insulation. Connections between modules are made via Supercon connectors installed through the walls of the enclosures. These connectors were chosen for the inherent safety feature of shrouded contacts so as to minimize the risk of electrical shock. The field junction stand serves as the electrical junction point for the sub-arrays. At this junction, the individual sub-arrays are electrically connected in parallel and the array output power is routed to the instrument and control rack. For each sub-array, there is an associated nonfused disconnect box mounted on the stand, which isolates the sub-array from the rest of the bus for servicing. The instrument and control rack provides the main power bus interconnects, circuit protection, battery charge/ discharge control, ac/dc supply connections, and data display and acquistion. Battery state-of-charge is regulated through the use of a battery charge controller, manufactured by Photocomm, Inc. The battery charge controller uses a series regulator to control the battery charge/discharge state. A utility-type ac inverter is also built into the rack. It generates a quasi-sine output, 117 Vac(trms), 60 Hz from the main bus high vottage dc. In order to provide the user with 12 Vdc power, dc-to-dc converters were designed into a power supply package which provides dual outputs for a total of 72 W.

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REMOTE SITE OPERATION The power system was modularized for ease of deployment and operation. The crated units were transported to the remote site by helicopter, with final assembly and placement of the system being done by hand under demanding Antarctic conditions. The system was first installed at a field camp at Lake Hoare in the Dry Valleys of Antarctica in October 1992. (See Fig. 3). The power system supported a team of 6 researchers during the austral summer, providing power to both their living area (a Jamesway hut) and a small laboratory facility. The system was restowed at the end of the five week period, wintered-over at the site and deployed again the following season (October 1993). The photovoltaic power system was officially transferred to NSF in August 1993. Analysis of the system data showed that the array output voltage approached 180 Vdc, which was higher than expected, most likely due to lower than anticipated ambient temperatures (approximately -22 C). This array output voltage exceeded the rated voltage of a gas discharge tube and metal oxide varistor, causing them to fail. The failure of these devices, which are not necessary for Antarctic applications, did not affect the operation or performance of the system. The battery charge controller was modified to accommodate the higher voltages. Very favorable comments were received from the remote

site team on the simplicity of the design and the reliability of the system. Another important feature was the quiet operation of the system, compared to the diesel generators used before. The significant increase in power also prompted the research team to start planning for the use of additional equipment the following season.

SUMMARY The photovoltaic power system was installed at the Lake Hoare site for approximately five weeks during late 1992, put into storage for the Antarctic winter, and then used again during the 1993 season. Significantly more power than expected was generated by the system, due to lower average temperatures. Favorable results were received from the field team with regard to the simplicity, reliability and quiet operation of the system. While it is expected that the use of photovoltaics, and other alternate energy systems, will increase in Antarctica, the lessons learned from these early systems should be extremely beneficial in accelerating the transition toward these future systems.

REFERENCES [ l ] L.L. Kohout, A. Merolla and A.J. Colozza, “A Solar Photovoltaic Power System for Use in Antarctica”, NASA Technical Memorandum 106417, December 1993.

Fig. 1. Antarctic photovoltaic power system showing (left to right) the instrument 8, control rack, field junction stand, one PV sub-array and the battery modules.

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Fig. 2. One photovoltaic sub-array in the stowed configuration (front view).

Fig. 3. Photovoltaic power system in operation at Lake Hoare site in Antarctica.

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