Knowledge about cold water risks in aquathermal energy

‘Look, there it goes!’ Experts from Deltares and energy specialists from Waternet, Rijkswaterstaat and STOWA stare intently at a dark blue stream of water in the IOS basin on the Deltares campus. This test setup simulates how cold water discharge from an aquathermal installation spreads under different conditions. Using thermal cameras, fibre optic sensors and lasers, the Deltares team records the movement of this so-called “cold plume” in detail. This gives water managers a better understanding of the possible effects of cold water discharge on aquatic life.

Insight needed

Lieke Noij, energy coordinator at STOWA, the Foundation for Applied Water Management Research, explains that this is one of the most important knowledge questions for water authorities. “The cold water from aquathermal installations is often discharged at a single point: the ecological effect of this cold plume is still unknown. A large, local concentration of cold water can, for example, form a ‘migration barrier’ for fish, which they may not be able to swim past.”

Previous research by STOWA and Deltares showed that cold plumes should mix with the rest of the water as much as possible so that it is gradually discharged. Lieke: “The likelihood of effects increases as the temperature differences become greater. On the other hand, the discharge of cold water may have a positive effect as the climate warms up.”

In view of the already under pressure water quality and the high ambitions of the 2027 Water Framework Directive (Kader Richtlijn Water), water managers are cautious and want more clarity about the ecological effects of aquathermal energy, according to Lieke. Following earlier studies in the WarmingUP programme, Deltares and STOWA have been working on mapping the impact of aquathermal energy on the environment for a number of years.

Mapping the cold plume

At Deltares, this includes Anton de Fockert, an expert in flow processes. Depending on the design of an aquathermal installation, the discharge and the flow in the surrounding area, the cold plume will mix with the surrounding water, he explains. “A stratified situation, in which the cold water remains below the warm water, is undesirable. In that case, the cold water cannot warm up further through interaction with the warm air or by solar radiation. This can have ecological consequences,” says Anton.

“Water managers currently find it difficult to assess whether the discharged cold water will mix. The current models they use to calculate this are unable to predict the behaviour of cold plumes sufficiently, which makes it difficult to obtain permits for aquathermal energy.”

Deltares is the initiator and financier of the laboratory tests involving cold water discharges. Anton: ‘Because the research is also of interest to organisations such as Rijkswaterstaat and STOWA, we are taking their wishes into account by adding extra measurements to the tests. Waternet has also joined in. We can now see how a cold plume behaves under different conditions. This provides us with a specific dataset that we can use to test and improve our models.’

Scaling up

Everyone present agrees that aquathermal energy can give an important boost to the energy transition. Aquathermal energy involves extracting heat (and cold) from surface water using a heat exchanger. This heat or cold can be used to heat or cool buildings via a collective heat network. The technology can be combined with heat or cold storage in the ground. Deltares and CE Delft have calculated that Thermal Energy from Surface Water can supply more than 40 per cent of the heat demand of our built environment.

Aquathermal energy is a sustainable, reliable and affordable heat source that is ideally suited to collective heat networks. It requires hardly any modifications to existing buildings and takes up little space, while also preventing further congestion of the electricity grid. Thanks to an open network infrastructure, aquathermal energy is also suitable for additional heat sources and is cheaper than individual sustainable heating solutions. The only condition is that the buildings to be heated or cooled are located near rivers, lakes or canals.

Proven

The technology has proven itself on a small scale, from a few houses to a small neighbourhood. Over the past fifty years, more than a hundred aquathermal installations have been successfully realised in the Netherlands. Deltares is working with other parties to build up knowledge and experience, enabling the application of aquathermal energy to be scaled up. This includes knowledge about potential, feasibility and impact, for example in the innovation programmes WarmingUP (now completed) and NieuweWarmteNu!

Water managers are currently unable to accurately predict the behaviour of cold plumes, which makes it difficult to grant licences for aquathermal energy.

Anton de Fockert, expert in flow processes

Responsibility

Arianne de Vries, aquathermal energy knowledge development coordinator at Rijkswaterstaat, emphasises that it is not only the water authorities that are responsible for this challenge: "Rijkswaterstaat manages many waters, including areas with potential for aquathermal energy. We see it as our social duty to facilitate this innovation and unlock this source for the energy transition." A legal framework is currently lacking, explains Arianne, because there is insufficient knowledge about the impact of discharges on water quality. “We currently assume that the effects of cold plumes are negative, but that may not be the case. That is why we are taking responsibility for developing knowledge about aquathermal energy, so that we can obtain as much certainty as possible about the effects of this energy source on aquatic life.”

Positive effects

Harry de Brauw, sustainable energy advisor at Waternet, points to the positive effects that the addition of cold water can have on ditches, ponds and canals. "The temperature of surface water can rise considerably during the warm months, which has all kinds of negative consequences, such as botulism, duckweed and algae growth. Cold water discharges may limit these consequences."

Set-up

At the edge of the basin, Arianne, Harry, Lieke and their colleagues ask the Deltares team lots of questions. What temperature are they maintaining this time, how quickly does the plume dilute and mix well with the surrounding water? “The average outflow from an aquathermal installation is between six and ten degrees colder than the water it enters. We built a cold water plant especially for this research. It cools the water from our facility reservoir by eight degrees,” explains Anton.

Anton says that in these tests, Deltares is looking at “how the plume behaves when it is discharged perpendicular to the ambient flow, at the effect of different discharge rates and different discharge temperatures. All of this has consequences for the mixing of the cold water plume.”

Flow

Water authorities currently use the Deltares Delft3D programme for licensing purposes. Anton: "This model works particularly well if you want to know the water temperature further away from the discharge point. We are less able to predict what happens close to the discharge point.” Lieke from STOWA is particularly interested in the spread of the cold plume when there is no flow: “Does the cold water sink and spread around the bottom?”

“We will provide insight into this with these tests”, Anton replies. He explains that customised assessments are often too costly, which is why water managers use models such as Delft3D to calculate effects. “Unfortunately, the existing models do not provide answers to the knowledge questions that are currently needed for granting permits. More tools are needed to arrive at a set of rules of thumb for water managers.”

Fibre optics as a sensor

Deltares is also investigating whether fibre optic cables can be used to record the spread of the cold plume. “The advantage of fibre optics is that they are easy to apply on a large scale and robust enough for use in the field. This allows the situation to be monitored in practice. Measurements with fibre optics provide better spatial coverage, but are less accurate”, says Pieter Doornenbal, expert in temperature measurement with fibre optic techniques at Deltares. “To obtain good measurements, a high resolution must be achieved.”

He looked at different cable thicknesses for this purpose. “We can lay a measuring instrument in a cable along the entire length of a channel, either straight or zigzagging, but we can also wrap it vertically around poles. This gives you more accurate results, because you can take multiple measurements in one place. After all, it can sometimes make a difference of a tenth of a degree.”

Arianne from Rijkswaterstaat is curious to know whether fibre optic technology is a good and less expensive method for taking practical measurements. Pieter: “The tests show the potential of fibre optics, but because the measurements have to be quite accurate, it is necessary to look at the results of the fibre optic measurements carefully and with expert knowledge.”

Houseboats and bridges

Waternet, the implementing organisation of the Amstel, Gooi and Vecht Water Board and the municipality of Amsterdam, has more detailed questions. “What does the cold plume do when there is a houseboat? And how does it behave when the water narrows, for example around bridges?” Harry asks model operator Nino Zuiderwijk. The Deltares team has created a pillar simulation and a scale model of a houseboat. This will later be placed in the basin to carry out the tests. The situation in the Amsterdam canals has also been simulated by narrowing the flume (as if there were bridge heads) and creating a shallow area (because there is no dredging under bridges).

Harry is also very curious about ‘the buoyancy effect’, which is what happens when cold water collides with a boat. “Our questions are indeed very specific to the Amsterdam situation,” laughs Harry. “We have the water on our doorstep, so we see many opportunities for aquathermal energy. Canal water could be used to heat as many as 200,000 homes, provided that concerns about water quality are addressed. We have to be careful, out of respect for aquatic life and because we have to comply with the Water Framework Directive. We are very pleased that Rijkswaterstaat and STOWA, together with Deltares, are making this kind of knowledge available.”

Reliability

After the tests, Harry, Lieke and Arianne unanimously conclude that no other party than Deltares can do this. Harry: “Deltares has unique and special facilities for conducting applied research”. He enjoys seeing how the test setup works with his own eyes: “This contributes to the reliability of the models we will be working with in the future.”

Balancing interests

Lieke agrees: “These are useful tests for validating the models used by ecologists, water managers and consultancies.” Deltares' knowledge is discussed at STOWA with the water authorities as a basis for adapting and, where necessary, expanding the licensing processes, guidelines and recommendations. Lieke emphasises that weighing up interests is an important part of this. “After all, in addition to ecology, there are other interests in our society, such as the energy transition and climate adaptation.”

“The scientific underpinning of this knowledge is necessary if we want to discuss the application of aquathermal energy at higher levels of government”, Lieke emphasises. “This knowledge gives us direction, which we currently lack. And yet the urgency is so great!”

Anton concludes the visit with a look to the future: “Once all the measurement results have been processed, we will know whether the current measurements provide sufficient insight into the behaviour of the cold plume close to the discharge point. In the meantime, we want to incorporate the results into the STOWA guide on cold discharges, which is updated from time to time.”