Hybrid systems: mixing things up

20 June 2019

Aliki van Heek from the IAEA’s planning and economic studies section speaks to NEI about the potential for combining nuclear and renewables in a hybrid energy system, as well as non-electric applications of the technology.

There is increasing interest in hybrid systems involving different energy sources. Can nuclear contribute to such systems and what are the issues?

There is interest among IAEA Member States.

Last October, 24 experts from 15 Member States took part in a week-long IAEA Technical Meeting to review and discuss innovative concepts and research on the coordinated use of nuclear and hybrid energy systems. Combining nuclear and renewables in a hybrid system can significantly reduce greenhouse gas emissions compared to conventional fossil fuels and also foster cogeneration for seawater desalination, hydrogen production, district heating, cooling and other applications.

While further R&D and the introduction of appropriate policies and market incentives would be important next steps in the development of hybrid energy systems, some countries have already successfully adapted nuclear power plants to be load following — that is, to provide flexible operation based on energy demand and fill the gaps in output left by intermittent sources such as wind and solar. By performing this sort of balancing act, nuclear power can enhance the efficiency of renewables while ensuring the overall system is reliable and low carbon.

Load following is more an economic than a technical issue. When you have invested a lot in plant, you want it to run for as many hours as possible. It is not economic for a nuclear plant to stand idle for a period of time just because the wind happens to be blowing. It generates no income during that period. However, there are different situations — it depends on the market and national legislation. In some cases there may be payment for a plant to stand idle in order to ensure the stability of the grid.

On the consumer side, as mentioned, such hybrid systems may also offer different products including electricity, heat, or fuel such as hydrogen. In the event of a coupling of renewables and a dispatchable power plant, the plant may have to step back production when renewable energy is plentiful. But if no electricity is needed then, for example, heat could be delivered directly to a user such as a nearby hydrogen plant.

However, this is still mostly for the future and is not yet realised. Participants at our recent Technical Meeting agreed that for the integration of nuclear and renewables to be efficient, possible market incentives, such as a CO2 emissions price, would be needed, along with additional studies to better determine competitiveness and costs.

Are any hybrid systems operating today?

Flexible operation, or load following, is already practiced in some countries such as France and Germany — countries with a high share of nuclear. In France, they were already load following because with a 75% nuclear share, sometimes not all of the plants are needed. Now, with the increasing share of variable renewables on the grid, this practice is becoming even more useful.

Germany is interesting. When they started with nuclear energy, there was an expectation that they would follow France in their nuclear share, so load following was thought to be necessary and they therefore configured their plants appropriately.

In the end Germany’s nuclear share didn’t grow as expected, but now they have a high share of renewables, they can do load following without any of the adaptations that were necessary in other countries.

What types of plant are best adapted to load following?

From the technical viewpoint, gas-fired plants are the most suitable for load following because they are the most flexible and can change output in a short space of time. It is also the most economic option in most electricity markets. Construction costs are low but fuel costs are high, so if you keep it just for peak loads or a few hours a year, it’s the best thing to do because you save fuel.

With a plant that has a high investment cost, such as nuclear, you have this expensive facility which still needs to be depreciated, and which shouldn’t stand idle for a long time. So it is best for base load.

However, from the environmental viewpoint gas-fired plants may not be the best option for load following, as they emit about 30 times as much CO2 as nuclear plants, measured over the entire life cycle. In addition, the operation of gas-fired plants requires gas (methane) transport, which is always to a certain extent associated with leaks and losses, and methane is a gas with 30 times the greenhouse effect of CO2.

What about small modular reactors (SMRs)?

SMRs may be a better solution and not only because of their size. It may be useful to have a smart plant consisting of a number of modules. If only a few are needed then some could be shut down completely and others run at 80%. That is better than running a big plant at 50%, which is more technically challenging and wasteful of investment.

Different modules could be assigned to different functions. For example, if the electricity is not needed some modules could be used to provide heat – high temperature reactors, for example, are well suited for this. The heat could be used for hydrogen production or desalination. These are products that can be stored, in contrast to electricity.

What are the prospects for hybrid systems using SMRs?

We usually speak of tightly and loosely coupled nuclear- renewable hybrid energy systems.

Loosely coupled means coupled over the electrical grid only, and this already exists in various countries as discussed above.

With tight coupling, meaning also heat use and storage components are involved, a much higher system efficiency could be reached. Experiments are under way, for example in the USA, but these test installations are mostly without a nuclear component yet. There have been simulations and various different options involving nuclear, but a nuclear demonstration is still needed.

NuScale Power’s 60MWe SMR is to be built at the Idaho National Laboratory and will be a basis for demonstration. The results can then be shown to future investors.

What about SMRs as part of a hybrid system for non-power uses?

We have been talking for decades about co-generation for non-electrical applications, but this is still applied at only few plants in the world. For nuclear desalination, there is considerable experience in certain niche markets, which is not yet the case for hydrogen production. However, there is more interest now because of a number of policy initiatives concerning hydrogen as a fuel, including one from the European Union.

What is different from the past is that there is a new environment, with an increasing number of electric cars on the road. Oil companies are growing concerned that there will be fewer people coming to their petrol stations. So they are considering hydrogen, and this could be a new driver for SMRs.

When it comes to SMRs for the chemical industry, HTRs could be perfect technically, but there are regulatory issues. There are already a large number of regulations relating to chemical production without adding regulations for nuclear plants. However, for nuclear district heating good experience exists, like the Swiss Refuna project, which has been providing district heating to buildings and industry since 1985.

Are there any other possible uses for SMRs in a cogeneration system?

Recently we had a technical co-operation workshop with Member States from the European region where we discussed SMRs. One interesting development related to a Member State with several remote locations that are not connected to a centralised grid. Some depend on diesel generators for their electricity, and this could be a niche market for SMRs.

They could also be used for desalination, especially in warmer areas of the world. This may be a possibility for some Middle Eastern countries, for example, saving the fossil fuels for export.

Have climate change concerns affected the situation?

Because of climate change, nuclear is being considered again in some places. The climate issue is urgent, so nuclear can contribute most to mitigating climate change by using existing plants — keeping them in operation as much as possible. Advanced reactors, with their huge potential for design and materials innovations, should join in as soon as possible, addressing challenges like waste.

However, it is all about how to best optimise energy production to minimise CO2 production. Some regions in the world use a lot of solar during the day, but the setting of the sun coincides with everyone coming home from work and turning their lights on, so that electricity has to come from somewhere else. This is when dispatchable sources have to come in. The challenge is to fit these together on the grid.

Theoretically a nuclear plant could be switched to use its heat for desalination, for example, but again it is a question of economics. This would not be financially viable if water is cheaper from other sources. The same applies to hydrogen production. If it is more expensive than the traditional methods for producing hydrogen, then it will not be done using nuclear, unless the government intervenes with subsidies because of climate concerns. In the end, it is up to each country to decide which technologies to include in its energy mix.

Aliki van Heek joined the International Atomic Energy Agency (IAEA) in 2016 as Unit Head 3E Analysis (Energy, Economics, Environment) with the Planning and Economics Studies Section. She studied Applied Physics in Delft, Netherlands, and got her PhD in Nuclear Engineering in Aachen, Germany. Afterwards she joined the Dutch energy research centre ECN, later NRG as the nuclear department became an independent organisation. Here she worked on high temperature reactors and later on led the national research programme on Generation IV Nuclear Energy Systems, which included participation in EU research on advanced reactors. 

Solar panels near Slovakia’s Mochovce nuclear plant (Photo: A. Evrensel/IAEA)

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