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Solar & Alternative Energy

The Hokuto mega-solar project aims to generate 2MW

A comparison of photovoltaic cells shows that high performance ratios and high energy densities do not always go together.
15 May 2009, SPIE Newsroom. DOI: 10.1117/2.1200904.1577

Solar-energy-generation systems represent a promising means to reduce global warming. They are also important future energy sources for Japan. The country's installed capacity target for solar-energy-generation systems in 2010 is 4.82GW, but in 2007 the total installed capacity was only 1.92GW. Approximately 80% of these 3 to 5kW systems, such as those on rooftops, are very small and mainly for residential use. However, some large-scale facilities are under construction, and intensive development of related technologies is an urgent need.

In 2006, we began to work on a project for Japan's New Energy and Industrial Technology Development Organization, ‘Verification of Grid Stabilization with Large-scale Photovoltaic (PV) Power Generation Systems.’ Verification tests are carried out at two sites, in Hokuto City (Yamanashi prefecture) and Wakkanai City (Hokkaido). A facility capable of generating approximately 2MW is constructed in Hokuto.1 During the first stage of development in 2006 and 2007, 24 kinds of PV modules were installed with a total capacity of 600kW. Their characteristics were measured and evaluated in preparation for the second stage, a 1.2MW installation begun in 2008, as well as for applicability in future mega-solar systems.

In the first stage, we developed large-scale power conditioners (PCSs) and their associated control and operational systems to connect the PV modules to existing AC systems. These control and operational systems include voltage-fluctuation suppression control to handle PV output fluctuations, low-order harmonics-suppression control, and a fault ride-through control module. We set the PCS capacity at 400kW based on cost, size, and future expansibility considerations (see Table 1 for development specifications).2

Analysis of a miniature model of the Hokuto system confirmed the PCS functions before manufacture and installation. In the tests, the voltage-suppression control reduced the power-system voltage fluctuations to 0.2%. The PCS operates continuously until the voltage drops below 0.4pu for 200ms or longer. Furthermore, the harmonics-suppression control reduced the power system's low-order harmonics currents to within 80% of the guideline. Harmonics analysis showed that the PCS does not always generate low-order harmonics currents. They are sometimes generated by voltage distortions in the power system instead. The PCS suppresses the low-order harmonics current flows by generating the same voltage waveforms as the power system through instantaneous voltage-waveform detection. The PCS in the miniature model used the same algorithm as the real system, and our confirmation tests demonstrated the suitability of these controls.

Figure 1. Measured performance ratio and generated energy density of 24 photovoltaic modules. One of the modules based on copper indium gallium selenide, CI(G)S, had the highest performance ratio (PR).
Table 1. Specifications and development targets for power conditioners.
 Capacity   420kVA/400kW
 Rated AC voltage (Vac) 420Vac±10%
 Rated DC voltage (Vdc) 400Vdc
 Input DC voltage 230-600Vdc
 Switching frequency 4-6kHz
 Conversion efficiency >95%
  Maximum power point tracking (by choppers)
 Control functions Suppression of ∆Vac
  Continuous operation
  Suppression of low-order harmonics

We constructed the PV support structures with ordinary pipes and buried screw pipes in view of the three Rs (reduce cost, recycle, and reuse). We developed special joints to connect the pipes, which do not use any concrete. This construction method saved energy and time, and is also ecofriendly. The standard fixed angle of the support structures is 30° (the latitude of Hokuto is 35°). For evaluation we also constructed PV modules at angles of 15 and 45°. The site features both one- and two-axis tracking systems.

Figure 1 shows the measured performance ratio (PR) and generated energy density of the 24 types of PV modules used in the first stage. The PR is defined as the ratio of the generated power to the designated rated power of a given PV module. We collected data between April and December 2008. The measured PRs range from approximately 75 to 95% because of losses to, e.g., shade and PCSs. PV modules with high PRs did not always have high energy densities.

We are evaluating the environmental effects and life cycle of the mega-solar system during and after construction. For example, we calculated the energy-payback time (EPT) with currently available data.3 The copper indium gallium selenide-based PV cells have the smallest EPTs, the lowest carbon-dioxide emissions, and the fewest environmental effects. These results will be recalculated and re-evaluated using accumulated measured site data, such as PV-generation characteristics, as construction of the second stage of the Hokuto mega-solar system continues.

We thank the Japanese New Energy and Industrial Technology Development Organization for supporting this research.

Hiroo Konishi, Mitsuru Kudou, Shinya Takagi, Ryou Tanaka
Solar Project Headquarters
NTT Facilities Inc.
Tokyo, Japan

Hiroo Konishi received a master's degree in electrical engineering from Osaka University in 1972. He joined Hitachi Research Laboratory in the same year. Since then, he has worked on research and development of high-voltage direct current (HVDC). In 2006, he began working on development of solar systems as a manager at Solar Project Headquarters. His research interests focus on power-electronics applications in power systems. He received a PhD from Osaka University in 1989 and is a senior member of the Institute of Electrical Engineers of Japan (IEEJ).