Picture
Picture
Picture
Picture
Picture
Picture

Stand alone PV/Diesel Hybrid Power Generation System on a Small Island

by the CADDET Japanese National Team

INTRODUCTION

A stand-alone PV/diesel hybrid power generation system has proved its ability to provide electrical power for a community of about 250 houses at the same level of service as a public grid. After a two-and-a-half year demonstration without grid connection, ending in March 1997, the system on Miyako Island, Japan is now in full, permanent operation.

Okinawa Electric Power Co Inc (OEPC) supplies electricity to all the islands in the Ryukyus, including many isolated islands for which electrical power must be supplied by small (<1 MW) diesel generators or through expensive undersea cables. To serve these small islands more efficiently, OEPC has worked for many years to develop photovoltaic (PV) power generation technology.

The project on Miyako Island aimed to demonstrate the practicality of a PV/diesel hybrid power generation system as a reliable stand-alone power source for an isolated rural community. So, a village of 250 homes was separated from the public grid of Miyako Island and supplied with electricity solely from this hybrid system. The village has a yearly average load of around 90 kW and peak demand of about 200 kW.


THE SYSTEM

Construction of the demonstration PV/diesel hybrid power generation plant began in February 1992. PV modules were added step by step while testing the system in grid-connected operation. In March 1994 the plant was completed as an independent power supply system in the village of Kusukube-cho.  It comprises a main PV power generation system (750 kW), a lead-acid battery power storage system (3,058 kWh) and a diesel generator (300 kW).

PV/diesel hybrid power generation technology has the following features:

  • by using supplementary diesel generators for power supply at times of insufficient solar radiation, PV facilities can operate at higher capacity factors;
     
  • with the smaller capacities of PV arrays and storage batteries, PV power generation systems become more cost-effective (the battery capacity can be reduced by a factor of 4 5);
     
  • the ability to cope flexibly with the fluctuations of solar radiation makes PV a very reliable power source. In addition, the hybrid plant installed on Miyako Island saves fuel by controlling the load factor of the
    diesel generator.

The system specifications and configuration are outlined in the table overleaf and the diagram below. When sunshine is abundant, the PV array provides electrical power for customers and uses any surplus electricity to charge the storage batteries which are used as a back-up. When the charge remaining in the batteries falls below 20% of the storage capacity owing to insufficient solar radiation, the supplementary diesel generator starts supplying power to customers and, at the same time, putting the batteries on charge.

By this simultaneous power supply and battery charging, the diesel generator can operate at full load, thus saving fuel. A simulation showed that if the hybrid system carried out these tasks separately, it would consume about 25 litres/day more than the average of 102 litres/day it currently uses. When electricity stored in the batteries rises above 30% of the storage capacity, the diesel generator stops operating.

The system does not require an operator. A remote monitoring and control system, located at the Miyako No 2 Power Station, both monitors the performance and controls the operation of the hybrid PV plant.

COMPONENTS

The PV array consists of about 12,000 silicon PV modules, of both single crystalline and multi-crystalline types, with an average maximum output of 62 W and an average conversion efficiency of 12%. Steps taken to safeguard the plant against typhoons and salt damage include a low tilt angle of 15 for each PV panel in the array, sealing with a silicone compound and an anti-pollution coating applied to the galvanised steel PV panel stands.

The power storage system has four sets of batteries set up in parallel, each consisting of 196 tubular-type lead-acid cells for PV power generation, connected in series (392 V), each with a capacity of 1,950 Ah (10 hours use). Tubular-type cells are suitable for cyclic use and have long service life. In PV systems, repeated and irregular charging and discharging of the PV cells tends to cause electrolyte imbalances in the cells, shortening their life. To address this problem, each cell in the system is equipped with an air-pump driven stirrer, which stirs the electrolyte for 30 minutes at regular intervals.

PWM (pulse width modulation) control by a IGBT (insulated gate bipolar transistor) power module is used to control the inverters. Power plants like this one, which mainly provide power for ordinary customers, operate at load for a large part of each day. Hence, the hybrid plant has three inverters and adjusts the number in operation at any one time to suit the load. This improves the system efficiency by enhancing the load factor of the inverter in operation.
 

Table: Outline of System Specifications

Scale

PV array       750 kW
Site area        about 18,000 m2
Array area     about 11,000 m2

Annual energy production

PV                671.2 MWh

 

Diesel            136.4 MWh

Annual power load

807.6 MWh (92.2 kW)

ac output

3 phase, 6.6 kV, 60 Hz

dc output

430 V at rated voltage (operational range: 350 - 600 V)

dc/ac inverter

Three 250 kVA units, self-induced PWM control
Rated conversion efficiency: 94 %
Effective conversion efficiency: 92%
(at load factor of 40%)

Harmonic voltage distortion factor

<5% overall

Control method

Constant voltage control with constant frequency

Lead acid battery capacity

3.058 kWh (10 hours)

Supporting diesel generator

300 kW

PERFORMANCE

Operation of the demonstration hybrid PV system started in October 1994. During the winter, solar radiation is low and not enough electrical energy is generated by the PV array to meet the whole electrical load on the system. Consequently, the amount of supplementary power supplied by the diesel generator increases. During the summer, in contrast, the PV array generates more electricity than the customers and the storage system can absorb. The surplus PV energy and supplementary diesel generator energy ratios of the system for these two years were 7% and 19% respectively. In other words, PV power generation satisfied 81% of the power demand on the plant.

THE FUTURE
Having operated smoothly over the demonstration period, the PV/diesel hybrid power plant has proved to be a very reliable stand-alone power source for remote isolated rural communities and can withstand
severe weather conditions. The durability of various cells was also verified. The system continues to serve the electricity needs of the village.

PV/diesel hybrid power generation
is expected to come into widespread use in isolated islands and villages as an alternative power source displacing diesel generators. Other suitable applications include non-electrified villages, remote from
public grids, in developing countries. To develop such markets, more effort is needed to improve the efficiency and reduce the costs.

For more information contact the CADDET Japanese National Team in Tokyo.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.