Secondary Fluids Impact on Ice Rink Refrigeration System Performance

Abstract: Sweden has 350 ice rinks in operation which annually use approximately 1000 MWh each. Therefrigeration system usually accounts for about 43 % of the total energy consumption which is the largestshare of the major energy systems. Besides improving the facilities one-by-one, it is important todistinguish common features that will indicate the potential energy saving possibilities for all ice rinks.More than 97 % of the Swedish ice rinks use indirect refrigeration systems with a secondary fluid.Moreover, the thermo-physical properties of secondary fluids directly impact the heat transfer andpressure drop. Thus, assessing and quantifying their influence on the refrigeration system performance isimportant while estimating the energy saving potential for the ice rinks.A theoretical model as well as two case studies focusing on the importance of the secondary fluid choiceare investigated. The theoretical model calculations are performed assuming the steady-state conditionsand considering a fixed ice rink design independently on the secondary fluid type. Hence, they can becompared on the same basis. According to this theoretical model, the refrigeration efficiency rankingstarting from the best to the worst for secondary fluid is: ammonia; potassium formate; calcium chloride;potassium acetate; ethylene glycol; ethyl alcohol; and propylene glycol. Secondary fluids can be ranked inexactly the same order starting from the lowest to the highest value in terms of the dynamic viscosity. Itwas shown that potassium formate has the best heat transfer properties while ammonia leads to the lowestpressure drops and pumping power. Propylene glycol shows the worst features in both cases. Ammoniaand potassium formate show respectively 5% and 3% higher COP than calcium chloride for typical heatloads of 150 kW. When controlling the pump over a temperature difference ΔT, the existence of theoptimum pump control or optimum flow was highlighted. For common heat loads of 150 kW thisoptimum pump control ΔT is around 2,5 K for calcium chloride while it is around 2 K for ammonia. It isshown that the secondary fluids having laminar flow in the ice rink floor pipes have a larger share in theconvection heat transfer resistance (~20-25 %) than the secondary fluids experiencing turbulent flow (~3%).One of the case studies shows a potential energy saving of 12 % for the refrigeration system whenincreasing the freezing point of the secondary fluid. An energy saving of 10,8 MWh per year was foundfor each temperature degree increase in the secondary fluid freezing point.