The Optimal Path to Decarbonization for Puerto Rico

By Jussi Heikkinen

The path to 100 percent clean energy is practical, possible and even affordable for Puerto Rico by 2050. However, this decarbonization journey will require optimization of both short-term and long-term storage to achieve Puerto Rico’s ambitious goals. The thing to keep in mind is that just adding renewables will not do the job.

Weather and seasonal daylight patterns play a significant role in energy production with renewable power from sources such as solar, wind, and hydropower. As countries around the world make the transition towards 100 percent renewable power to mitigate the effects of climate change, it is important that these variables are considered in power systems planning to maintain reliability even during periods of extended little-to-no renewable generation, all while minimizing environmental impact and cost to consumers. Power systems need to be designed to operate reliably during all kinds of weather, not just your average weather patterns. Power-to-Gas technology with flexible generation can provide carbon neutral long term – even seasonal - storage capacity to create more resilient and reliable power systems while continuing on a path to rapidly decarbonize the utility sector. 

Long-Term Energy Storage Solution is the Missing Piece of the Zero-Carbon Puzzle

A hallmark of renewable energy production is that supply rarely coincides with demand. Solar and wind are both subject to daily and seasonal changes in generation. Solar output varies during the day and is zero at night. Of course, it is also greater during the summer months when the days are longer, and the sun is brighter. Wind generates throughout the day and night, but is subject to daily, seasonal and geographic variability. This results in a continuous variability of generation, and over or undersupply of wind and solar energy compared to the load - also known as consumption.

In the early phases of decarbonization, the typical practice is to keep adding renewables. But as power systems achieve higher penetrations of renewable energy sources, managing variability in supply and demand across days, weeks and months becomes crucial. Grid operators currently rely on dispatchable resources such as fossil-fuel plants and increasingly on battery storage, to balance renewables and to fill in the gaps. This approach works well in the early phases with low shares of renewables,  but at some point, the old plants reach their limits of flexibility and gradually become a hindrance as they stop the transition towards 100 percent carbon-free power. At this point they must be gradually replaced by truly flexible generation assets which can come online and go off-line quickly to avoid curtailment of wind and solar power which could become another obstacle for a reliable power system . What is needed is an integrated approach to balancing the renewables to maximize the utilization of renewable energy sources and minimize the curtailment of wind and solar energy during the transition to zero carbon power.

Battery storage, which keeps getting more competitive as costs are reduced, will solve the short-term storage needs and even daily shifting of solar elegantly. But battery storage is not an economical way to provide long term storage for unusual, longer weather patterns such as rainy seasons, winter, heat and cold waves. Actually long-term energy storage is the missing piece of the puzzle today. People are trying to use battery storage in this role, but quickly realize it is not an affordable solution.

Power-to-Gas (PtG) offers a highly competitive solution to long-term energy storage, even on a seasonal scale. PtG uses curtailed wind and solar energy to produce green hydrogen and other renewable fuels, such as synthetic methane, ammonia and synthetic methanol. Hydrogen as a fuel is carbon-free and synthetic methane, when produced using carbon captured from the air, is a carbon-neutral fuel, adding zero carbon to the atmosphere. 

These carbon neutral fuels can be stored indefinitely and used on demand to produce power for long periods of time, which provides balancing services to the grid during longer lasting weather patterns with too little wind, solar and hydro energy to serve the load. Synthetic methane can be transported and stored in existing natural gas storage infrastructure and used by flexible gas power plants to produce carbon-neutral power. Hydrogen can provide the same benefits as synthetic methane, but hydrogen infrastructure will need to be built. Ammonia and carbon neutral methanol can be transported by ship, rail and trucks and stored in fuel storage tanks at the sites of the flexible power plants. Scaling up is easy and cost efficient as the need to expand the fuel storage tank capacity increases.. 

Leading power plant suppliers are already focusing on developing gas engines and turbines that can be converted to use future fuels. Conversion to carbon neutral fuels will be possible sometime in the near future, therefore, Installing flexible generation capacity today to support and enable the transition and avoid investments in what will become stranded assets.

The combination of renewable fuel and flexible generation capacity forms a large, distributed, long-term energy storage system that can provide large quantities of firm and clean electricity for days, or even weeks. When coupled with traditional, shorter-term storage technologies such as batteries, this integrated energy system can not only handle intraday balancing needs, but also meet seasonal demand such as moving excess generation from spring and autumn to the winter months when renewables are producing less energy as well as ensuring reliable supply of electricity during periods of extreme weather. 

The Pathway to Flexibility and Reliability  

Puerto Rico offers an example of how PtG technology can be leveraged to provide a clean, secure and affordable source of electricity. This U.S. territory is committed to decarbonizing its power system and has set an ambitious goal to achieve 100 percent clean power by 2050. Puerto Rico gets approximately 40 percent of its electricity from renewable sources.

In 2021, Wärtsilä conducted power-system modelling that compared Puerto Rico's current plan to an optimized pathway to decarbonization. This new pathway, the Optimal Path, introduced PtG technology, as a long-term storage alternative. Hydrogen-based carbon neutral fuel is produced using otherwise curtailed solar and wind power in the island, and stored for use in  flexible generation plants. By adding flexible generation to the mix, following the Optimal Path would lead to 19.6 GWh of battery storage. This would help Puerto Rico avoid overbuilding its renewable generation and battery storage infrastructure.

Solutions to Balance the Unpredictable and Uncertain 

As the Puerto Rico example demonstrates, hydrogen based fuels along with flexible generation play a key role as a viable long-term storage alternative and in reducing the need to overbuild both solar and  battery storage. Flexible generation with green fuels will play a key role in the worldwide transition to a 100 percent carbon-free future. Recent Wärtsilä research reveals that G20 countries will need over 3,500 GW of flexible generation to achieve 100 percent renewable energy. The United States alone will need 675 GW of flexible assets to ensure 100 percent carbon free electricity at the lowest cost. Being able to respond to any and all changes in the power grid with firm power will be critical to maintaining reliability and stability. However, not all thermal units are cut out for the job. The glory days of steam power plants such as combined cycle gas turbines are over because they are not capable of starting and stopping quickly and several times a day to balance wind and solar variations.  Such plants formed the core of the thermal power systems of the past but will not have a role to play in the future high renewable power systems. 

Balancing renewables efficiently requires flexible power plants which:

1. Can start and stop in just minutes, thereby never using fossil fuels when not needed
2. Have a high efficiency as especially the future fuels will cost more than fossil fuels
3. Have no starting cost. Such plants may have to start and stop more than 1000 times every year so technologies that would endure extensive wear on every start are not suitable for this application
4. Offer a reasonably low CapEx as operation hours are quite limited

According to extensive Plexos modelling, large medium speed reciprocating engines offer the best solution for this flexible generation application. They offer all four of the features described above. Heavy duty gas turbines offer the lowest CapEx, but high starting costs, long starting times and poor efficiency. Aeroderivative gas turbines offer 10 minute start, low starting costs, reasonable efficiency, but the CapEx is relatively high.

Reciprocating engines together with  battery storage offer an unbeatable combination of short and long term storage, providing lowest generation costs in most cases, to include Puerto Rico. Modelling confirms that flexible generation combined with PtG provides the missing piece of the decarbonization puzzle. 

To enable rapid decarbonization of the electric utility sector, Wärtsilä has launched an upgraded Balancer power plant that is designed to address the need to scale up power system flexibility. The plant is capable of ramping up from stand-still to full output in two minutes. The power plants range from 50 MW to 300 MW, and can run on natural gas, biogas, synthetic methane, and hydrogen blends. Wärtsilä is also actively developing a combustion process that will allow the use of 100% hydrogen and other future carbon neutral fuels like ammonia and methanol.  These solutions will offer reliability as well as flexibility for power systems of the future.