In regions without an interconnected grid, it is common to provide electricity from diesel power plants. Photovoltaics in particular could replace considerable parts of this in future. Under the coordination of SMA Solar Technology, the project partners in the PV Diesel project have developed suitable system solutions and components for this – the system is running successfully on the Caribbean island of St. Eustatius, which belongs to the Netherlands. The developed components also make it possible to operate electricity grids stably and safely in the future just using renewable energies. Engineer Brisa Ortiz from SMA Solar Technology coordinated the development work. She presents her system in the interview.

Ms Ortiz, briefly describe what makes the system so special?

In the project we focused on hybrid system solutions. The goal was to achieve an energy supply for an island grid system that can temporarily also be fed 100 per cent from renewable energies, and specifically in the megawatt power class. This requires a superordinate intelligent control system that can optimally regulate the entire island system. We have developed the SMA Hybrid Controller for this purpose.

What exactly does this controller do?

We have of course the following initial situation: there is a diesel network into which we would like to integrate electricity from photovoltaic installation. However, photovoltaic electricity represents a critical factor for the power grid. The larger its share is, the more fluctuations have to be compensated for by the other components in the grid. Diesel generators can still easily compensate for a 15 to 20 per cent share. But we ultimately want 100 per cent renewable energies. And this is what the new hybrid controllers are now doing.

What are their specific tasks?

You have to imagine that photovoltaic installation of this size can change their output by 50 per cent within seconds when there are interruptions in the solar radiation. And these are compensated for by the storage system. The hybrid controller takes over the complete control of the power, including starting and stopping the diesel generators and switching off the diesel completely. It also regulates the synchronisation with the grid and provides the functions required by the grid operator for system services. The aim is to reduce the consumption of fossil fuel, avoid CO2 emissions, protect all system components and increase the overall operating efficiency of the plant.

Ms Ortiz, what special requirements of the network operator does the hybrid controller meet? The thesis that too much photovoltaic power disrupts the supply network is still common.

The combination of hybrid controller and battery inverter provides one way for balancing the fluctuations between the generators and loads in the grid and thus enables a temporary full power supply with PV.

What were the particular research and development challenges in this project?

The greatest challenges were the control procedures for grid forming and controlling critical operating states. We first created a model for each component and then integrated it into a control simulation. We tested the simulation in the laboratory, then replaced the models with the physical components and repeated the test in the laboratory. The entire system was then shipped to the island of St Eustatius and rebuilt. The actual operation has been running without any problems since then and the system now supplies the 4000 inhabitants on the island around the clock without any problems.

The project has created the basis for implementing the largest battery system in an interconnected grid. What do you mean? That was actually, of course, a solution for an island.

Storage systems are also used in the interconnected grid. Last January (2019), there was almost a major blackout in Europe as a result of a strong frequency drop to below 50 Hertz. Thanks to the use of battery storage systems such as the one in Bennewitz in Saxony, it was possible to prevent the power supply from failing. In Pelham, England, SMA has already delivered a turnkey solution for a storage power plant that provides frequency control in the grid as well as necessary power reserves.

So this is something for the future?

If you go one step further and the share of renewable energies continues to grow – perhaps even one day achieving 100 per cent of the energy supply in good weather – then the conventional power plants that keep the grid stable and provide grid-forming will be lacking. All functions must be performed by inverters. This must not lead to mutually increased instability in the grid. In order for the supply network to operate stably, the power converters must meet all requirements for the safety and quality of the networks – as we know from conventional power plants.
This is how it already works today in small island grids or in larger island grids, such as on the Azores.

How much diesel can the new system save?

On St. Eustatius, 6.4 gigawatt hours of electricity are produced there from photovoltaics per year. This enables us to save 1.73 million litres of diesel each year. This corresponds to about 60 tank trucks. We currently have an energy share through photovoltaics of 46 to 47 per cent. This share can increase further in the future.
However, it is not easy to transfer this to the world market. According to market research companies, 40 gigawatts of new diesel power plants are being built every year. How much of it we can replace with wind or photovoltaics also depends on the location. Users must also be persuaded to use what they consider to be risky technology.

And now a very exciting question. A hybrid system consisting of a diesel power plant and a PV system leads to higher investment costs than a pure diesel power plant. Can the significantly lower operating costs compensate for these additional costs?

I firmly believe that. In recent years, the investment costs for PV power plants and storage technologies have fallen significantly. Let's take the example of St. Eustatius. Two million litres of diesel saved per year can amortise an investment of this magnitude within 4 to 6 years. The return on investment and the payback period vary depending on the location, the complexity of the system and its application.

Ms Ortiz, the energy supply of the future with renewable energies is an exciting topic. How long have you been dealing with this topic?

I have been dealing with the provision of energy with photovoltaics for more than 15 years, starting from a few kilowatts up to the megawatt power class for the most diverse applications.

And how did you arrive at this topic – sometimes it is pure coincidence?

My greatest motivation is to disseminate the freely available and clean electricity from solar energy in such a way that it is available worldwide and accessible and affordable for everyone.

Ms Ortiz is a graduate engineer for mechanical engineering. She graduated at the TU Berlin. She previously completed her studies in industrial engineering in her native Mexico. Since 2013 Ms Ortiz has been working for SMA Solar Technology AG in Niestetal as a project manager for funding projects with a focus on systems and storage integration.

Picture Brisa Ortiz
© SMA  Solar Technology AG

„A battery storage system combined with the newly developed grid-forming battery inverter is capable of forming electricity grids independently.“

PV-diesel hybrid system supplies the 4000 inhabitants on the Caribbean island of St. Eustatius.

Using PV electricity – saves costs for diesel fuel

All over the world, diesel generators supply regions without a grid connection with electricity. Photovoltaic electricity is now much cheaper to produce. With the help of suitable technology, photovoltaics could be added to the existing power supply.