“Although they may seem unremarkable at first glance, water slides involve a combination of many thermodynamic effects that can lead to significant heat losses. It is important to investigate this interplay as accurately as possible in order to obtain meaningful results regarding the expected heat losses and potential for optimisation,” explains Sebastian Dragosits, project manager at Forschung Burgenland.
Invisible energy flows: loss mechanisms and their interaction
Crucially, it is not only sensible heat (temperature differences) that is transferred, but also latent heat, which is generated during the phase change between water and water vapour:
- Evaporation: This extracts a great deal of energy from the water, causing it to cool down. The air absorbs this energy as latent heat.
- Condensation: When the warm, humid air stream encounters colder sections of the wall, it cools down. As cold air can hold less water vapour, the air quickly becomes saturated locally. Excess vapour condenses on the wall and releases the latent heat previously absorbed.
- Heat transfer to the outside: This released heat travels through the slide wall by thermal conduction and is transferred to the environment on the outside via convection and thermal radiation.
The interplay of evaporation and condensation shifts the heat losses towards the outer wall. The air flow acts as a conveyor belt for the heat. The characteristically high heat transfer from the water flow to the slide wall is a further significant factor. Taken together, these individual heat flows therefore result in high heat losses for the slides.
Case study: Sonnentherme Lutzmannsburg
Sonnentherme Lutzmannsburg is planning to build a new, large-scale water slide complex. This expansion comprises seven ‘mega-slides’ with a combined total length of 1,284 metres. This currently makes it the largest water slide project in the DACH region.
Forschung Burgenland is supporting the planning process with calculations to estimate heat losses and assess the impact of slide insulation on heat demand. The aim is to ensure efficient operation and to optimise the connected load of the heating supply.
To what extent can heat losses be reduced?
Both the water flow and the air circulation within the chute cause heat losses to the surroundings. The exact breakdown varies significantly from chute to chute. In the chutes under consideration, the proportion of losses on the water side was just over 50 per cent, although the wetted surface area usually accounts for only a small proportion of the total chute area (on average around 10 to 15 per cent). The main reason for this, apart from evaporation, is the high rate of heat transfer from the water to the slide wall. Insulating the surface in contact with the water therefore offers the greatest potential for savings, as only a comparatively small surface area needs to be insulated. The calculations revealed the following potential savings on the water side:
- Reduction in peak power of up to 90 per cent
- Reduction in annual energy demand of up to 60 per cent
With full insulation, the total heat demand can be reduced further, as around half of the total heat losses still occur at the surfaces in contact with the air. Insulation would allow higher internal wall temperatures to be maintained along the slide, thereby significantly reducing condensation and thus minimising the most energy-intensive pathway of air-related heat loss. Whilst this measure would also reduce air-side heat losses in a similar way to the water-side losses described above, the insulation required for this is considerably greater.
Sustainable heating solutions
Reducing peak loads and annual energy demand through insulation measures creates key conditions for sustainable and cost-effective energy systems – both during installation and in operation. Lower flow temperatures and reduced power requirements facilitate the use of heat pumps, solar thermal systems and waste heat recovery. They also reduce the need for generation and storage capacity, improve system efficiency and facilitate the integration of renewable energy sources and load management. The Sonnentherme is also a partner in the GEO.MAT research project, which is investigating the efficiency potential of pool operations. Initial optimisations have already been successfully tested. The Sonnentherme is therefore not only a flagship facility for family fun, but also for efforts towards sustainable, CO₂-free spa operations.
