Research and Theory

Introduction

A fundamental goal for the Norwegian Ski Federation and the Norwegian Biathlon Association, together with their partners, is to contribute to a future where snow is easily accessible for the next generation of skiers, jumpers and boarders, regardless of if natural snow only exists in the high mountains.

To make this goal possbile, the project “Snow for the future” was started in 2017, and first marketed through the city of Trondheim’s Nordic World Skiing Championship bid. The project is led by SINTEF og NTNU, both World renown research institutions, and with financial backing by the Norwegian Ministry of Sport and Culture. Phase I of the project focused on mapping today’s existing technologies for snow production (especially temperature independent technology), and the potential for improvements.

More detailed information about phase I of “Snow for the future” can be found in it’s final report .

Phase II of the project was financed in 2019, with the goal of researching and developing technologies that can make snow production and snow preservation even more energy efficient, simpler and less expensive. It is also a goal to communicate the project’s knowledge, learnings and new technologies – and with this in mind the “Centre for Snow Competency” was initiated.

The Centre is however just one of the several original “Snow for the future” project goals.

• Develop a novel technology for efficient and environmenly friendly temperature independent snow production with solutions for interim storage in various cases
• Increase the number of skiing days in local communities and centralized facilities to aid in further development of the skiing tradition and culture in Norway and Europe
• Increase the predictability of organizing events, competitions, and activities pertaining to skiing in Norway and Europe
• Secure the future value creation for new technology manufacturers to sustain and further develop skiing destinations in Norway and elsewhere in Europe
• Establish a research platform and competency centre for snow technology and its practical applications, Center of Snow Competency, that will give lasting effects both nationally and internationally
• Generate new jobs and improve public health

For phase II, the main goal is to develop a novel technology for energy-efficient production of artificial snow, including snow production in plus degrees and production independent of the outdoor air temperature. The project focuses on systems and solutions that ensure a sustainable snow production with a limited environmental footprint.  Heatpump technology based on environmentally friendly natural freezing elements will be developed, with the focus on utilizing the cold side for snow production and the warm side for the purpose of using or storing heat.

• One possibility is to use the surplus heat from the snow production to heat adjacent buildings, swimming pools, etc. Contrary, it is also possible to use surplus heat from industrial processes or district heating during temperate months of the year for snow production. Such an integrated system also includes storage and reuse of snow.  
• In a combined system for snow production and utilization of surplus heat, the produced snow becomes a by-product with minimal use of extra energy. This enables cost efficient snow production in centrally located areas. An illustration of an integrated system with temperature indepedent snow production, storage and reuse of snow, and utilization and delivery of surplus heat is shown below:
  
   

The chapters below cover and summarize the reseach projects implemented through the “Snow for the Future” project.

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Utilization of excess heat for snow production

The text below is a summary of one of the “Snow for the future” research projects. The author is Ole Marius Moen – ole.moen@sintef.no.  The project can be categorized as a mapping task, where the focus has been to find:

• Which heat-driven cooling technologies exist and can be used for snowmaking?
• How much energy will heat-driven snow production require?
• What are the potential sources of heat?
• Which winter sport venues can potentially use this type of snow making technology?

Climate changes will lead to warmer temperatures and less natural snow.  A likely consequence is that it will become more difficult to provide good conditions for snow sports.  The ski season may even disappear in many typical “snow villages”.  Since the traditional snow production method using snow lances or fan-guns require temperature below zero degrees Celsius, many locations, especially at low elevation, may not be able to use this method in the future.

Temperature independent snow production

One alternative is to use technologies that can produce snow in warm temperatures, so called temperature independent snow production.  Today there are several providers of this technology, and it is used in several places around the World.  The systems are costly, and require that the snow is distributed since the production takes places in a central location.  Compared with traditional snow production, the technology demands up to 50 times more energy, which leads to high electric bills.

One solution for reducing the operational costs is to replace the electricity with heat as the energy source by using refrigeration technology driven by heat (for example absorption refrigerators).  To produce snow using heat requires temperatures of around 90 – 100°C.  Compared to using electricity, heat-driven cooling is relatively inefficient, since large parts of the heat can not be utilized.  To be economical, such a solution is therefore dependent on being able to use cheap or free excess heat.  This will also contribute to reducing the environmental footprint compared to snow making using electricity.

Potential heat sources

In Norway, studies have shown that one can find considerable amount of unused excess heat, especially in industry and waste incineration.  In the industry there is potentially up to 10 TWh of available heat at the right temperature range, while for incineration about 1 TWh is not utilized for district heating due to low demand in the summer.  Making use of heat from these sources is however not without challenges.  The low temperature of the district heating in the summer makes it difficult to use for snow production, and external use of excess heat from the industry is demanding and not common.  Both cases will require relatively expensive equipment for heat exchange both for user and provider, and pipes for transportation of the heat between the two.

• District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements.

Co-location of venues and heat sources

Snow production using heat as the energy source for snow production, it is advantageous that the heat source and the ski location is within a short distance; neither heat nor snow can be transported efficiently over long distances.  In Norway, the co-location of winter sport venues for Cross-Country biathlon, ski jumping and alpine, and potential heat sources has been mapped.  This shows that 76 out of 168 mapped venues are located in a municipality with district heating or available excess heat from industry. For many of these venues increased snow production would lead to increase skiing since they are located in densely populated areas or are already popular ski destination venues.

Based on calculations with Granåsen, Trondheim as model example, an annual consumption of 1,5 GWh for snow production would contribute to lengthening the ski season with more than 1 month by being able to snow cover the stadium, 3 km of courses and the ski jumps.  At the same time, the calculations showed that different factors can significantly alter the energy requirements.  The model calculations will therefore be unique for each venue.

Further work

It is suggested that further research focuses on large venues with existing nearby district heating.  Techno-economic analyses to concretize each individual venue’s potential may give us answer to if heat-driven snow production is a practical, economical and sustainable solution for future winter sport.

• Techno–economic assessment or Techno–economic analysis (abbreviated TEA) is a methodology framework to analyze the technical and economic performance of a process, product or service. TEA normally combines process modeling, engineering design and economic evaluation.

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Temperature independent snow production – what exists today?

The goal with this research project – mapping what exist today and how this can be improved – was to create a base for further work on energy efficient and environmentally friendly snow production. The reliability of natural snow, and the number of days it is possible to produce snow with traditional snow production equipment is decreasing due to the warmer climate. This results in increased use of temperature independent snow production equipment, and making this equipment more efficient is important for reducing the energy consumption.

Temperature dependent snow production

About 90% of all ski resorts produce artificial snow, and many resorts rely solely on artificial snow for parts of the winter season. Traditional temperature dependent snow production is based on nozzles spraying water droplets that freeze in the cold air.  This method requires temperatures of -2 °C or colder.  The lower the temperature is, the more efficient the production equipment is.  Artificial snow produced in this way has a density about four times higher than natural snow, which make it more durable.

Temperature independent snow production

Several ski resorts around the World have already installed temperature independent snow production equipment. This snow is produced by creating small ice particles/grains, and can be done in several ways. Flake ice, plate ice, scraped ice-slurry and vacuum ice are some of the methods, with flake ice being the most common.

Flake ice: Flake ice is produced by applying water to the surface of a cooled drum or tube.  The ice is normally removed by scaping, and will fall down as dry sub-cooled flakes.  The flake ice machines typically operate with temperatures at -20 to -25 °C, which is lower than the other methods. This makes the method more energy demanding, but gives a high yield.

Plate ice: In plate ice machines a water film runs across cooled vertical plates and freezes on the plate surface.  The temperature inside the plates is normally at -7 to -21 °C.  The ice is removed by warming up the plates in a defrosting cycle, such that the ice plates fall into a crusher. Plate ice machines normally have higher energy efficiency than flake ice machines due to the higher temperature on the cold side.

Scraped ice-slurry: This is today the most used method for producing ice-slurry (a mix of small ice particles and cold water).  The process consists of water being cooled down or frozen on a surface, then scraped off with a rod or similar.  This creates a slurry, that can be separated further to wet snow. By using salt, it is possible to make this into a consistency close to dry snow.  The energy efficiency with this method is better than both plate ice and flake ice, since the operating temperatures are close to 0 °C.

Vacuum-ice: This is the most effective method of producing ice-slurry.  The technique consists of lowering the pressure inside a chamber such that the water freezes and ice-slurry is created.  These systems can be made in a large size, and are more energy efficient than the other methods mentioned above.  It is also possible to operate these systems using heat, such that for example excess municipal heat or heat from other industry can be used.

Potential improvements

Cooling systems are normally measured by their COP (coefficient of performance) which is a ratio of useful cooling provided to work (energy) required. Higher COPs equate to higher efficiency, lower energy (power) consumption and thus lower operating costs.  Carnot-COP says something about how much an ideal machine can deliver, and approximately 50% of Carnot-COP is normally achievable.  In the figure below, different existing temperature independent systems are drawn in a diagram with 50% Carnot-COP shown as a line.  The distance from the different systems to the line illustrates the potential improvements.  The figure shows that the energy efficiency of all the systems can be significantly improved.

Figure 1: COP vs. condenser temperature for the different snowmakers. Ice production technology is stated and capacity in m3/24 hrs as well as the condenser temperature/temperature lift is given inside the parenthesis (capacity m3/24 hrs – condenser temperature/temperature lift ℃)

Conclusion

Temperature independent snow production is a possible method for securing snow in above freezing temperatures.  Today’s systems require lots of electricity, and are therefore expensive to operate.  As an example, a temperature independent system will use approximately 22.8 kWh per m³ produced snow, while temperature dependent snow lances use approximately 1,42 kWh per m³.  The research project also showed that there are large differences between the different systems, and that they all can be significantly improved.

This research is part of the project “Snow for the future”. The research report was published in 2017. Author and contact person is Stian Trædal (stian.tradal@sintef.no)

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Framework for evaluating snow production technologies

This research work is presenting an estimation of the total environmental foot print for temperature independent and temperature dependent (snow lances/snow fans) technologies.  The model is a further development of an Excel-based model used for general planning of snow production.  The method for calculation includes greenhouse gas emission from consumption of electricity and heat, leaks from refrigerants, production of materials and transport of snow, as well as reduction of emission through export of excess heat.

The model is demonstrated through calculation of examples from the Granåsen ski arena in Trondheim, Norway (the venue for the 2025 Nordic World Skiing Championship), and will be further used to evaluate different snow production methods and their environmental impact, specifically greenhouse gas emissions.

Theme

The calculation of the total environmental impact for a specific snow production technology includes (in this project) five contributions as shown in the figure: 1) CO₂ intensity to consumed electricity, 2) CO₂ intensity to consumed heat, 3) leaks of refrigerants, 4) Production of materials, and 5) transportation of snow

The calculations are based on the CO₂ intensity for the electricity mix for the mid-Norway region, the consumption of electricity for temperature dependent snow production (traditional snow making), calculation of daily CO₂ emission from electricity consumption for both temperature independent and temperature dependent snow production, in addition to calculations of emission related to transportation in cases of off-site snow production.

The developed model and its possibilities have been demonstrated through examples from the Granåsen ski venue.  Five different scenarios, one being today’s situation, and where low, normal and high CO₂ intensity for electricity was given as input to the different scenarios.   

Conclusion

The results show that by exporting excess heat from snow production to a local external user or by using excess heat for heat-driven snow production at an external location, the CO₂ emission will be reduced compared with normal operations at the venue.

This research is part of the project “Snow for the future”. The research report was published in 2020. Author is Vidar Torarin Skjervold. Contact person is Ole Marius Moen (ole.moen@sintef.no)

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Like the ice cream machine: Snow production through direct expansion of CO₂

The existing temperature independent snow production technologies all have a high energy consumption and are very expensive to operate.  There are other ways to produce snow more efficient, also in temperatures above 0 degree Celsius.  CO₂ is getting more attention as an environmentally friendly cooling medium in heat pumps and cooling machines, and can also provide possibilities in snow production systems.

Direct expansion of carbon dioxide

This research work evaluated a flash cooling system for snow production.  It utilized expansion of CO₂ under high pressure, originally used to produce ice cream.  The system was compared with existing temperature independent snow production systems.  An alternative system was also presented, with the purpose of further lowering the energy consumption by using waste heat from other processes.

Conclusion

The presented solutions had higher COP (coefficient of performance) values and lower energy consumption than the existing technologies.  The main solution achieved a theoretical COP value of 5, while the existing technologies at the same temperature had values around 2.  The energy consumption for the main model was about 50% of the existing technologies, with even lower values for the alternate model using waste heat.

This research is part of the project “Snow for the future”. The research report was published in 2019. Author is Lasse Borg Anderson. Contact person is Cecilia Gabrielli (cecilia.gabrielli@sintef.no)

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Plate ice machines for snow production: Freezing of plates and possible improvements

Plate ice technology is found to be a good candidate for temperature independent snow production by SINTEF’s analyses, and this research project includes an in-depth study of this technology.  Plate ice technology works by freezing a water film that flows down vertical plates which are cooled from the inside. The plates of ice are released into a crusher in a cyclic process that warms up the plates.

An energy calculation identifies the consumption for each of the components in the plate ice machine, based on thermodynamic calculations and operating data from the machine provider.  A freezing model was created and estimates the thickness of the ice layer over time, based on a variable evaporation temperature.

Defrosting

Defrosting was identified as a «low hanging fruit», both for reduction of energy and for increased capacity. The method used is defrosting by heated gas, where the gas from the compressor outlet is led directly into the freezer plates. The duration is timed without any feedback from the process.  Defrosting makes up a reduction of capacity of 8% and uses 6% of the consumed energy.  Different measures were suggested to alter the strategy for defrosting, such that the speed of production could be increased and the energy consumption lowered.

This research is part of the project “Snow for the future”. The research report was published in 2021. Author is Espen Halvorsen Verpe. Contact person is Cecilia Gabrielli (cecilia.gabrielli@sintef.no)

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Optimization of defrosting system for plate ice freezing machine

Global temperatures are increasing more and more. During winter it leads to less natural snow. This is why it becomes necessary to use snow machines to provide enough snow for winter ports, especially for the FIS Nordic World Ski Championships hosted by Trondheim in 2025.

Over the years in the field of refrigeration, the focus has been on saving energy and protecting the environment. Efforts are made in this regard by researchers who have investigated the impact of many variables on refrigeration equipment, system performance, and energy consumption while still being able to produce ice/snow to meet the market demand.

Today, plate ice machines are used in many fields for different purposes such as desalinization, concentration, purification, etc. However, such machines can be used to produce ice and crushed into snow. In the scientific report, the plate ice technique is investigated and particularly a machine from the company PTG currently using the ice chunks to freeze salmons in fisheries in Norway. The machine is run with ammonia and the defrosting is made by hot gas from the compressor’s discharge line.

The main issue is the defrosting cycle. It is crucial in the process to release ice from the plates but dwindles the capacity. It ends with a timer which is set to a slightly longer time than needed to prevent any extra ice growth on the plates and damage. This extra time leads to a loss of time, capacity, and thus money.

Different solutions are proposed to optimize the defrosting cycle. The solutions introduced are ultrasound ice detection or sound detection of falling ice as well as vibrations. Ultrasound ice detection is based on measuring the distance between the plate and the sensor using the speed of sound in the air. If this distance matches the distance without ice, the cycle is stopped otherwise it keeps going. Moreover, a microphone could be used to detect the sound of the plates falling and stop the system as soon as all the signals are received. A speeding up technique can also be to apply vibrations to the plate to make the ice fall faster.

This machine is modelled in Matlab coupled with Refprop to estimate the freezing and defrosting duration. The cycle time found is 9.4 minutes including 39.4 seconds of defrosting. The cycle given by the manufacturer is 13.5 minutes including 30-40 seconds of defrosting. There is a difference of around 4 minutes which is consequent. The simplifications made in the calculations as well as the difficulty to estimate some parameters are discussed in the discussion part of this report. However, the defrosting time is 39.6 seconds which is in the range of the manufacturer.

If used properly, the new techniques to end the defrosting cycle would be able to reduce the defrosting time by saving the extra time on the timer set currently to 60 seconds. Indeed, compared with the real defrosting time, 20.4 seconds would be saved every cycle. The capacity of the machine would increase by 4.6 % producing 750kg more of ice every day.

Scientific Research Report, Norwegian University of Faculty of Engineering

Baptiste Flohic, June 2022

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Theory – about snow

Physical properties

From the mechanical point of view, snow is a complex material. Its behavior depends on a number of parameters. The various types of snow are characterized by their mechanical properties. For example, the resistance to pressure of newly fallen snow is lower than that of mechanically processed (groomed) snow. Snow deforms under its own weight, depending on its temperature and density. Its viscosity increases at lower temperature and with greater density.

From an optical point of view, the snow absorbs only part of the short-wave radiation received from the sun. For example, up to 95% of the sun’s radiation is reflected by new snow. The albedo (the reflectivity of an object) depends on the condition of the snow surface (grain shape and size, contamination and water content). Dirty snow has low albedo and will melt faster.

The snow’s thermal properties appears clearly when the temperature difference between the various layers of snow strongly influences the properties and the metamorphism (change of form) of the snow layers. Temperature changes are greatest at the surface and smallest close to the ground.

The properties of machine-made snow are distinctly different from those of natural snow. The basic difference is that natural snow freezes from water vapor while machine-made snow freezes from water droplets. In the latter case, water droplets freeze on the outside before the core does. The thermal differences contribute to the fact that compacted natural snow on the surface stays colder than equivalent artificial snow (which melts easier and thereafter freezes)

• Compacted natural snow stays ca. – 3 to -5 °C colder than artificial snow during equal condition in the the winter.

Meteorological factors

Solar radiation varies throughout the year. It reaches it’s maximum level in the summer and it’s minimum level in the winter. It depends on the time of the day, the slope of the terrain and the altitude of the location. In December, a 30 degree gradient slope facing south will receive almost two and a half times as much radiation as a horizontal surface.

Wind causes a rapid thermal exchange between the surface of the snow and the surrounding air. The stronger the wind, the faster the exchange. A warm wind accelerates the melting process on the surface rapidly and a cold wind accelerates its cooling.

When the surrounding air is warmer than the snow surface, the temperature of the snow rises. With strong emission or pollution, a stable layer of air can form at the bottom of valleys, since cold air is heavier than warm air. Air exchange progresses at a much slower rate in this case.

When air humidity is high, water vapor condenses on the snow surface. Water then accumulates on the surface. With very low air humidity, air can absorb more water vapor from the snow surface. Snow cools down during evaporation. Low air humidity is therefore beneficial for the freezing of the snow surface. In dry air, ice/snow also sublimates (evaporates without turning to water first) and the snow surface cools down.

Rain or snow transfers energy to the snow surface, depending on the temperature of the precipitation. Due to its higher temperature and thermal exchange effect, free water (wet snow or rain) raises the temperature of the surface of the snow and causes it to partially melt. Rain has however not as much melting effect as wind. If it rains 10 mm and this rain is cooled to 5 °C in the snow, it will only cause 0,6 mm of snow to melt (from Wikipedia).

As energy is absorbed when the snow is at 0℃, the snow crystals/grains start to melt at the edges and corners. They become rounder and a thin layer of water forms around the grains. As melting progresses, pores keep filling with water. When water content is high, the bond between the grains will also dissolve causing the snow to soften. Heat intake and melting may frequently be interrupted by colder night temperatures, and water freezes again. Consequently, the water present between snow grains freezes, forming strong bonds again.

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