Aqua Ammonia as Secondary Fluid in Ice Rink Applications

University essay from KTH/Skolan för industriell teknik och management (ITM)

Abstract: Refrigerant management is crucial in the attempts to slow climate change. Emissions from the refrigeration sector are primarily due to poor management and unsafe destruction of refrigerants currently in circulation. Safe refrigerant management and improving system operating efficiency can result in a reduction of emissions. Ice rinks are some of the most energy-intensive public buildings, providing both heating and cooling. The major share of energy in an ice rink is the refrigeration system, which consumes about 43%. There are more than 360 ice rinks in Sweden as of 2018 and the most common type of refrigeration system is an indirect system. With the push for natural fluids, aqua ammonia is becoming a more appealing option as a secondary fluid in ice rinks because of its minimal negative impact on the environment and favorable thermophysical properties. The main drawbacks of the fluid are its toxic characteristics and material compatibility. However, since the first use in 2007, there has been an increase to 34 of the total ice rinks in Sweden that have aqua ammonia as a secondary fluid.  Thermophysical properties are used to calculate refrigeration design parameters, including secondary fluid concentration and pumping power required. The properties of aqua ammonia have not been experimentally tested within this century to the extent presented in this thesis. Existing data is either derived from measured values taken several decades ago or has been calculated. The novelty of this thesis project stems from the unique and more accurate results measured through laboratory work and from the ability to determine the impact of the newly measured values in ice rink refrigeration design. A total of 11 varying concentrations of aqua ammonia were tested for density, dynamic viscosity, specific heat capacity, thermal conductivity, and corrosion of 7 metal specimens. The solutions tested ranged from 2 wt-% to 30 wt-%, correlating to freezing points from -2C to -84C. The measurements for density resulted in values similar to reference values, ranging in a difference of only 0.3% to 1.7%. Dynamic viscosity results followed nearly the same trend as references with changing temperature and solution concentration, with values varying from 0.8% to 17% different than references. Specific heat capacity measurements proved significantly different than reference values. The trend is opposite of the reference, leading to drastically different values, especially at lower temperatures and higher solution concentrations. The difference in values ranges from 0.1% to 28%. Thermal conductivity results show similar trends, but higher values than expected. The difference between measured values and reference values range from 0.1% to 13%. Corrosion results show that copper and brass have the highest corrosion rates of 16.2 mm/yr and 1.84 mm/yr, respectively. The most compatible specimen was stainless steel, followed by carbon steel, with maximum corrosion rates of 0.041 mm/yr and 0.11 mm/yr, respectively. Brass connections commonly used in industry were also tested and resulted in corrosion rates ranging from 69.6 g/yr to 112 g/yr, which accounts for about 1% and 1.5% of the connections’ total weight lost per year. Compiling the laboratory measurements taken during the completion of this thesis project results in a more complete and accurate list of thermophysical properties for aqua ammonia that has never existed before.  These updated thermophysical properties for aqua ammonia, along with measured properties for other secondary fluids, were used to calculate operational parameters in a hypothetical ice rink refrigeration system. The results show that aqua ammonia is favorable with high COP and low pumping power, and therefore low pressure drop. Ammonia is most comparable to CaCl2 and K-formate for most results. The changes in calculated COP between old reference data and new measured data were less than a 1% decrease when plotting versus the temperature of the ice surface and with a set pump control (T) for cooling capacities of 200kW and 300kW. The change in heat transfer coefficients was more significant, with a range of about a 9% to 27% decrease in either the U-pipe under the rink floor or in a plate of the heat exchanger. Even though these heat transfer coefficient values are lower than previously calculated, the required pumping power is also lower using updated properties: 40% lower at a secondary fluid temperature of -10C. Even though the change in heat transfer coefficients is larger with experimental values, the impact on COP is minimal.  The takeaway from this project is that aqua ammonia is a favorable secondary fluid compared to calcium chloride and ethylene glycol, the two most commonly used secondary fluids in ice rink refrigeration. A system using aqua ammonia would have a 45% and 47% lower pumping power requirement compared to calcium chloride and ethylene glycol, respectively. The system would also have a 4.7% and 11.6% higher COP when compared to systems with calcium chloride and ethylene glycol, respectively. The significantly lower pumping power will lower total energy demand of the ice rink, thus decreasing operation costs.

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