By Charlotte Glidden-Bush

In February 2001 Amonix Inc. installed the world’s first commercial high concentration photovoltaic (HCPV) solar arrays in Glendale, Arizona, USA.

The technology utilizes the sun’s powerful rays to supply clean, efficient and reliable power to Arizona Public Service’s grid.

In 1999 Arizona Public Service Co. (APS) contracted Amonix Inc. to build 300 kW of its integrated high concentration photovoltaic (IHCPV) solar systems, representing the world’s first commercial application of HCPV solar arrays. One project, located in Glendale, Arizona, USA, uses 100 kW of the Amonix technology to track the sun’s movements and employs special lenses to magnify the sun’s rays 250 times onto each solar cell. The HCPV plant supplies enough energy to power more than 160 homes.

The project is based on technology which was originally developed by the Electric Power Research Institute (EPRI) and was based on work by Stanford University. Amonix has modified and developed it so that it is now projected to be the lowest-cost photovoltaic technology when manufactured in volume.


Figure 1. Typical generated power for the array
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The HCPV system utilizes the renewable, sustainable energy resource provided by the sun to provide clean, reliable and efficient energy, and offers many other benefits.

“One of the main benefits is that it is a renewable, non-polluting source of energy,” said Vahan Garboushian, president and CEO of Amonix. “It is highly efficient and the plant will be very reliable. The units are also flexible in installation. You just need a place which has good sun. It’s modular so there isn’t a long lead time between the start of the plant and the start of electrical production, and it can start producing power before all the units are installed.”

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He added, “Another benefit is that when the demand for power grows, an extra ten or 100 units can be added to meet this demand, which you cannot do with conventional plants. The technology is also ideally suited for supplying power to remote locations.”

Providing a service

APS has been dedicated to researching, developing and using solar power for the last 25 years and understands the benefits that this technology can offer its customers. The IHCPV system allows more electricity to be generated from smaller and less expensive PV cells than used in other solar systems because the lenses concentrate the sun’s rays 250 times into each cell. By concentrating the solar energy, the specialized Amonix solar cell operates more efficiently than conventional ‘one sun’ or flat-plate cells. In fact, the same amount of electricity can be generated from only 1 cm2 of cell area under concentration as from 250 cm2 of non-concentrating cells. The solar cell is a significant cost factor in any photovoltaic system and the Amonix technology directly attacks this major cost driver. The Amonix system has the potential to produce solar electricity cheaper than any other solar generating system.

APS and its participating customers provided funding for the project through the APS Solar Partners programme. Under the scheme, participating APS customers pay $2.64 per month to have 15 kWh of their electricity needs generated by solar power. This money funds plants like this and enables research into developing solar power technology. The project also received financial assistance from the US Department of Energy through the Utility Photovoltaic Group. The land at Gendale’s Municipal Airport was provided by the city of Glendale through a low cost lease.

Glendale’s Mayor, Elaine Scruggs, explained the importance of a plant like this for the city of Glendale at the plant’s dedication. “The power plant represents a way to produce clean energy from a natural resource and do it in a way that fosters partnerships and contributes to the technology’s development.”

Installation

Construction at the site began in February 2001. After the foundations were dug and set, a pedestal was installed on top. The system’s drives and torque tubes were shipped to the site as a single unit where they were then lifted up and mounted on top of the pedestal.

The IHCPV system is delivered in modular units, known as MegaModules. Four MegaModules units, rated at 4.8 kW each and fitted together to make larger, more powerful units, were shipped two-to-a truck before being crane lifted and mounted on the array torque tube. It only takes three to four hours to mount the MegaModules.


Figure 2. Design of the Amonix IHCPV system
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Since the operation of each array is autonomous, each array can start producing power once it is installed and checked. The system does not need to wait until the last array is installed to start power production.

The Amonix patented integrated high concentration photovoltaic system uses a Fresnel lens to concentrate the sun’s energy on a silicon cell. Four of these silicon cells are mounted onto a strip of aluminium. The cells are then electrically connected by a copper strip which is placed on top of the aluminium strip, while being isolated from it. The solar cells are connected in series, referred to as a string.

Each solar cell has a secondary optical element (SOE) placed around the the cell to redirect any beam dispersion. Each cell on the strip also has a bypass diode which ensures that a single bad cell in the string will not interrupt the flow of current in the string.

Six strips are mounted on an aluminium plate with a heat sink mounted on the backside of the plate opposite the cell strip. The back plate and heat sink will transfer waste heat to the ambient air, preventing active cooling so that none of the generated power is used as parasitic power for cooling.

The Fresnel lens is a circular lens that has a 53 cm focus point. Twenty-four of these lenses are manufactured as a single unit of parquet. The parquet lens and cell plates are integrated with a support structure that aligns the Fresnel lens with the solar cell, positions the cells at the focus point of the Fresnel lens, and provides an enclosed space for environmental protection of the solar cells. This MegaModule unit is 13.4 m long, 3.3 m wide and 60 cm deep.

Maximizing efficiency

The MegaModule requires accurate pointing at the sun for maximum efficiency and is therefore mounted on a tracking structure. Multiple 5 kW units are used as the basic building block for a 25 kW power system. Different tracking structure configurations are used to mount a single MegaModule to seven MegaModules.

A 25 kW array has a total surface area of 224 m2. Amonix and APS developed a gearless hydraulic drive to position the array in the best position to receive the maximum rays and allows the array to move from any position to a face-up wind-stow position in less than 15 seconds.

The electronic controller is completely autonomous but has the capability to be monitored or controlled remotely. The controller has a built in GPS signal to obtain universal time. Using universal time and the array position, the controller calculates the sun’s position. As the sun’s irradiance increases, the inverter will connect to the grid-line and start producing power automatically. The controller ensures that the array is pointing at the sun all day. The units at Glendale are monitored from the APS STAR facility (the Solar Test and Research Centre) in Phoenix, which is a state-of-the-art research facility.

The array will automatically start operating in the early morning and start producing power when the sun’s irradiance reaches 300 to 400 W/m2. The power output will increase as the sun’ irradiance increases, and will decrease after solar noon when the sun’s irradiance decreases. The array will continue to operate until the sun’s irradiance reaches the operating limit in the evening.

The inverter will automatically disconnect from the grid when there is cloud cover but the array will continue to track the sun. When the cloud intensity decreases the inverter will reconnect with the grid and start producing power again.

The units have been producing power since the first unit was installed in February 2001. The final unit was installed in April of the same year, and since its installation the plant has been producing 100 kW of electricity for the APS grid.

Future plans

The future for solar power looks bright. Amonix is currently working with energy suppliers to promote this technology and its benefits.

“We are currently working with utilities and independent power producers to get the knowledge across. We are also sending out demo units with utilities so that they can get familiar with the technology,” said Garboushian.

APS recently completed two other solar powered projects, bringing its total capacity for solar powered generation to nearly 1 MW. The two projects comprise a 175 kW plant constructed with the Embry-Riddle Aeronautical University in Prescott, USA, and a 125 kW plant in Gilbert, USA. Both plants will supply power to the grid.

“We are pleased to reach the 1 MW milestone with the dedication of these three plants, but it is only the beginning,” said Ed Fox, APS vice president of communications environment and safety. “With the energy shortages faced across the country and especially in the west, every kW of power we can generate is important. These plants showcase our commitment to developing earth friendly, renewable energy sources.”