Scaling Up Renewables in the Java-Bali Power System: A Case Study


One of the world’s largest floating solar photovoltaic (PV) power plants, Cirata, is under construction in Indonesia. It is an innovative design with floating PV arrays to provide power in association with an existing hydropower plant in West Java. The 145 MW floating PV installation on the Cirata Reservoir is expected to be completed by fourth-quarter 2022.

Indonesia plans to develop a further 60 floating PV installations to contribute to its target of 23% of power generation from renewables by 2025. Integrating rising levels of variable renewables into its power system is important for Indonesia.

In that regard, a system integration analysis of the Cirata PV project was conducted in mid-2020 by the International Energy Agency (IEA). This article highlights that analysis, its context and the findings.


Indonesia is an archipelago country that spans the equator. It is the world’s fourth most populated country with a significant role as an energy consumer and major coal exporter, as well as the global leader in biofuels production. By mid-century, Indonesia is forecast to be the world’s fourth-largest economy. Its government aims to modernise the energy sector and to achieve universal electricity access by 2024. Among its challenges, Indonesia is ranked in the top-third of countries in terms of climate change risk with high exposure to flooding and extreme heat.

Indonesia has a significant role in the global energy landscape. It faces a multitude of challenges to modernise and transition its energy system. As an IEA Association Member, Indonesia is tapping into the agency’s global expertise and technical capabilities, as exemplified by the analytical work conducted for this case study. 

In early 2021, the IEA Executive Director and Indonesia’s Minister of Energy and Mineral Resources agreed to establish the IEA-Indonesia Energy Transition Alliance, which was endorsed by Indonesia’s president. The Alliance will continue and expand the provision of the IEA’s leading analysis and practical solutions to real-world energy policy challenges to the government.

For example, upon request from Indonesia’s Ministry of Energy and Mineral Resources, the IEA undertook the system integration analysis presented here. It considers the planned project design and characteristics, provides an analysis of the impact of integrating the large-scale floating solar PV plant into the Java-Bali power system, and makes recommendations for the long term in view of further capacity additions of variable renewables. This was done in partnership with PT Pembangkitan Java-Bali (PT PJB), a subsidiary of the state-owned utility, Perusahaan Listrik Negara (PLN), which operates the transmission and distribution network in the country.

The analysis identifies how to maximise the benefits of the Cirata floating PV project, including taking account of potential seasonal variability in solar energy and maximising synergies with the associated hydroelectric plant. It illustrates ways to securely integrate the project into the Java-Bali grid and the relevant tools that would be most efficient for the entire system. Ongoing analysis is part of efforts to prepare for the planned integration of larger shares of variable renewable electricity.  

Electricity in Indonesia

Most electricity in Indonesia is generated from fossil fuels, about 83%. The power mix in 2020 was 63% coal, 18% natural gas, 2% oil, 7% hydroelectric, 10% non-hydro renewable energy (predominantly geothermal and biofuels).

Indonesia has significant renewable energy resource potential. Yet only a small percentage of it has been realised. Concerns about the variability of solar and wind generation have hindered development.

Dominated by fossil fuels with significant potential to expand renewables

Indonesia’s power system is expected to expand significantly to meet rapidly increasing demand and to provide electricity access to all. The national electricity supply business plan (RUPTL) for 2021-2030 projects an annual demand increase of at least 5%, which the government aims to meet with new capacity from renewables, striving to achieve 23% of total electricity generation from renewables by 2025. While PLN has indicated a commitment to stop building new coal power plants after 2023, a substantial amount of coal power projects are in the pipeline and due to be completed. 

Java-Bali power system

Indonesia’s power systems are fragmented in each island group. The Java-Bali power system is the largest in the country with 64% of Indonesia’s installed capacity. Its power mix was about 70% coal, 19% gas, 5% geothermal and 3% hydropower in 2020.

Power plants in the Java-Bali system are operated in traditional mode with coal and geothermal as base load while gas and hydro operate to meet changes in demand over a day and across seasons. Hydropower generation is dependent on water availability. With lower water inflow in the dry season (April to October), hydro plants generate less and gas-fired plants increase operations to meet demand.

A typical weekday load profile on the Java-Bali grid sees the lowest demand in the early morning with multiple peaks in the morning, afternoon and evening. On Sundays and public holidays, peak demand is typically in the evening.

Seasons slightly influence the demand curve: in the dry season, there is a higher peak demand than in the wet season, as well as faster ramping needs, which means that generators need to be able to increase or decrease generation faster. The figure below illustrates the typical daily power generation profile in both the wet and dry seasons.

While the impact on demand is minimal, the analysis has considered the solar PV project’s impact in both wet and dry seasons to provide a complete assessment.

Typical weekday generation profile in the Java-Bali power system in the wet season


Typical weekday generation profile in the Java-Bali power system in the dry season


Cirata: An innovative project to pair a large floating PV facility with an existing hydropower facility

Today the power generation mix in Indonesia has very low shares of solar PV. However, it has strong solar potential that can provide clear benefits in terms of economic and environmental considerations. The 145 MW Cirata floating solar PV project that is under construction is a key milestone in Indonesia’s clean energy transition. It will be the country’s largest solar PV plant when completed in late 2022 and one of the biggest floating solar PV plants in the world. The project is being built on a 225 ha section of the Cirata Reservoir in West Java. It will be integrated into the Java-Bali grid which is the backbone of Indonesia’s electricity supply providing power to more than half of the nation’s population.

The location of the Cirata floating PV facility offers a number of advantages:

  • Constant high electricity demand in the industrialised West Java region.
  • Additional demand needs foreseen with the construction of the city of Kota.
  • System services available from the Cirata Hydropower Plant.
  • Absence of land use issues due to its installation on an existing reservoir.
  • Cooling effect of the reservoir water on the solar panels that enables a higher capacity factor.

Tests at the project site indicate that the Cirata solar PV plant will have an annual average capacity factor of 20% and will generate electricity from early morning to evening year round, based on geographic and weather data. Annual electricity production is estimated to be around 250 GWh.

Further, the testing shows that while the average solar PV production profile is similar in May and November, there is a significant difference between sunny and cloudy days: cloudy days in the wet season have a more variable solar PV production pattern than sunny days in the dry season. The figure below illustrates this difference.  

Solar PV production from the test panel at Cirata on a sunny day in the dry season and a cloudy day in the wet season


The Cirata hydroelectric plant operates in non-uniform generation mode and serves as a peaking resource in the current system. It produces around 1 200 GWh per year with an annual capacity factor of 16-18%. Its operation is highly dependent on the season.

In order to maintain minimum water levels for irrigation, aquaculture and other uses in the dry season, only three to four of its eight turbines are operated and not for the entire day. Its highest output is in the mid-day hours on a typical weekday and production is shut down in the evening in the dry season.  

In the wet season, most of the turbines at the Cirata hydro facility operate more than ten hours a day, except Sundays. Water availability and demand levels determine its operations in the wet season.

Development of the floating PV plant will complement the electricity production from the Cirata hydroelectric plant in a hybrid fashion. The hydro plant will produce more power in the wet season while the PV plant will produce more power, and in a more uniform pattern, in the dry season. This is a key advantage of the design of the Cirata project: the higher capacity factor of the hydro plant during the wet season allows the system to manage lower PV production, while the additional PV production in the dry season can help with water availability issues, especially during peak demand periods. 

Integrating the solar PV facility will benefit the Java-Bali grid through effective resource sharing

Complementarity between the Cirata hydro and solar PV facilities provides a boost to smooth variability of solar PV output and to accommodate limitations of hydro generation in the dry season. The figure below shows the hydro generation pattern and simulated output from the PV project for a day in the dry season in 2019. Water availability was scarce so the hydro plant was used mainly to meet afternoon peak demand on weekdays and evening peak demand on Sundays. Integration with the floating PV plant will provide solar output to displace some of the afternoon hydro generation in the dry season and shift it to meet the evening peak. Solar output thus offsets water use in the hydro plant to meet peak demand when availability is low in the dry season. 

Hydropower and solar PV generation profiles at Cirata on a Sunday in the dry season


Hydropower and solar PV generation profiles at Cirata on a weekday in the dry season


The hydro-solar complementarity at Cirata offers a distinct advantage. Nevertheless, every variable renewable plant does not need to be equipped with a specific flexibility pairing. Indeed, if system-level considerations are taken into account effectively during the design of a project, flexibility can be mutualised from a range of resources. Planning at system level allows flexibility options to be shared on a wider level by utilising system-smoothing approaches; thereby enabling the secure and economically efficient incorporation of additional variable renewable generation sources.

To assess the contribution of the Cirata floating PV plant at the Java-Bali grid level, we assessed the correlation between the demand and solar PV output profiles, as well as the availability of other resources in the system. Looking at the actual load and the simulated solar PV generation profiles, there appears to be a strong correlation, especially for weekdays, as illustrated in the figure below. 

Actual load profile in the Java-Bali grid and simulated Cirata PV output on a Sunday in the dry season


Actual load profile in the Java-Bali grid and simulated Cirata PV output on a weekday in the dry season


The profiles indicate that as demand picks up in the morning it correlates well with the ramp up of solar output and that maximum solar output coincides well with high demand in the mid-morning to afternoon period on a weekday. This correlation is less visible on a Sunday as peak demand is in the evening and there is no generation from the solar PV at that time. 

As mentioned, the Java-Bali grid is a mainstay of electricity supply in Indonesia. In that respect, the 145 MW Cirata PV installation to which it will be integrated is rather small. To analyse the operational impact of the Cirata solar PV plant on the Java-Bali grid, we have analysed the grid’s typical ramping requirements by comparing the maximum daily ramp of the existing system with a simulation that includes the 145 MW Cirata solar PV power plant. The analysis included three timescales for ramping: 30 minutes, 1 hour and 3 hours, both upward (increasing demand) and downward ramping (decreasing demand).

The results show that the potential operational impacts related to output from the Cirata PV facility would be minimal. Indeed the ramping requirements of the Java-Bali system would be relatively unchanged, with the maximum daily ramp ranging 2-13% of daily peak demand. The highest increase in ramping requirements is at the 3-hour timescale on weekdays where the increase would be 50 MW, or 1% of the daily peak.

Thanks to the planned location of the floating PV installation in the Cirata Reservoir and complementarity with the existing hydropower plant, variability can be minimised. This highlights the key role hydropower can play in accommodating the rising share of solar PV generation in the Java-Bali power system.

System-wide approaches to integrate renewables are essential to ensure long-term, cost-effective and secure integration

Analysis indicates that the Cirata PV project will be beneficial to the Java-Bali grid and its integration can be accomplished securely with the current level of renewables in the system. Three approaches for integration are summarised in the table.  

Integration approaches for the Cirata PV project





Smart controller for Cirata hydropower plant

Optimise operation of the Cirata hydro plant with an artificial intelligence (AI) smart controller. It would take into account real-time data from the solar PV facility and weather data to balance PV generation.

No active needs for variability management.  

System could be configured to provide system services.

Narrow focus on the specific hydro-solar PV operation.

Does not consider the wider system implications or opportunities to integrate additional variable renewables generation.

Smart controls for both the hydro and PV assets at Cirata

AI smart controllers installed at the PV and hydro generation assets. Communication between the operations helps signal when balancing services are needed.

Creates synergies for the Java-Bali system and takes into account the principle of system-wide flexibility by providing the ability to support voltage and frequency requirements of the grid.


Upgrade the Cirata hydroelectric plant

Several upgrade options are possible:

  • increase plant efficiency;
  • create a reservoir for pumped hydro;
  • install a smart controller;
  • install a mini hydro plant to recover lost efficiency of the main hydro plant.

Boost planning for long-term operation, and planning and integration for system-wide approaches.

Pumped hydro correlates well with the PV plant and enhances the flexibility of the grid.

High upfront costs of some upgrades with benefits to be realised over the medium to long term.

The second and third approaches in the table offer opportunities to enhance the capacity of the Java-Bali electricity system to incorporate larger shares of variable renewables in the power generation mix over time. In order to expand the contribution of additional variable renewables, project designs need to take full account of the balancing needs of the grid to ensure cost-effective integration and system operation.

Java-Bali grid will benefit from the integration of the Cirata PV project

The analysis indicates that the Java-Bali grid is ready and able to integrate the 145 MW Cirata floating PV project, due to be completed in 2022, with minimal impact on power system operations regardless of the season. Further, the analysis shows that the Java-Bali power system is capable of accommodating higher shares of variable renewables. The innovative design of the Cirata PV project underpins beneficial synergies with the hydropower plant that allow a smooth integration of the PV generation. In addition, it benefits the system by providing supplemental advantages such as conserving water use in the dry season.  

Cirata PV integration favourably foresees expansion of renewables generation to achieve Indonesia’s goals

This analysis of the Cirata floating PV project is an initial activity in IEA support for the energy policy modernisation agenda of Indonesia. The IEA is committed to co-operate with the government on its clean energy transition, including substantial work on renewables integration. A system-wide perspective on integrating variable renewable energy in secure and cost-effective ways is extremely important, particularly with the rising share of clean energy in Indonesia’s electricity mix. As climate change becomes an increasingly severe problem, Indonesia’s pivot towards power generation from renewables can be instructive to other major emerging economies. Power system planning which considers flexibility and sustainability can considerably reduce future investment needs while lowering greenhouse gas emissions. Mutualising knowledge and expertise from country experiences is key to successful energy transitions. The IEA will continue to support Indonesia in meeting its clean energy targets. 

This article has been produced with the financial assistance of the European Union as part of the Clean Energy Transitions in Emerging Economies programme. This article reflects the views of the International Energy Agency (IEA) Secretariat but does not necessarily reflect those of individual IEA member countries or the European Union (EU). Neither the IEA nor the EU make any representation of warranty, express or implied, in respect to the article's content (including its completeness or accuracy) and shall not be responsible for any use of, or reliance on, the publication.

The Clean Energy Transitions in Emerging Economies programme has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 952363.

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