Master of Science Thesis

KTH School of Industrial Engineering and Management Energy Technology

EGI-2013-156MSC

Division of Applied Thermodynamics and Refrigeration

SE-100 44 STOCKHOLM

Alternatives.PNG

ABSTRACT

Sweden has 350 ice rinks in operation which annually use approximately 1000 MWh each. The refrigeration system usually accounts for about 43 % of the total energy consumption which is the largest share of the major energy systems. Besides improving the facilities one-by-one, it is important to distinguish 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 and pressure drop. Thus, assessing and quantifying their influence on the refrigeration system performance is important 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 choice are investigated. The theoretical model calculations are performed assuming the steady-state conditions and considering a fixed ice rink design independently on the secondary fluid type. Hence, they can be compared on the same basis. According to this theoretical model, the refrigeration efficiency ranking starting 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 in exactly the same order starting from the lowest to the highest value in terms of the dynamic viscosity. It was shown that potassium formate has the best heat transfer properties while ammonia leads to the lowest pressure drops and pumping power. Propylene glycol shows the worst features in both cases. Ammonia and potassium formate show respectively 5% and 3% higher COP than calcium chloride for typical heat loads of 150 kW. When controlling the pump over a temperature difference ΔT, the existence of the optimum pump control or optimum flow was highlighted. For common heat loads of 150 kW this optimum pump control ΔT is around 2,5 K for calcium chloride while it is around 2 K for ammonia. It is shown that the secondary fluids having laminar flow in the ice rink floor pipes have a larger share in the convection 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 when increasing the freezing point of the secondary fluid. An energy saving of 10,8 MWh per year was found for each temperature degree increase in the secondary fluid freezing point.