engleski

Numerical Analysis of Variable Speed Capacity Modulation for Heat Pumps

Almost all heat pump and refrigeration systems operate in general on partial loads during the majority of their operating time. Adjusting the operation to different heating and cooling loads is performed through compressor control, such as on-off control, cylinder unloading, variable speed or variable clearance. Variable speed control is the most suitable for retrofit of existing heat pump systems. New developments in variable sped drives and their reduced prices compared with situation in the past, made those control systems competitive and increased their presence on a market. Depending on the type of used variable speed control system, the heat pump or refrigeration machine coefficient of performance (COP) increases significantly. That COP increase is caused by reduction of refrigerant mass flow and resultant decrease of temperature differences between the heat source fluid in evaporator and refrigerant, as well as a temperature difference between refrigerant and a heat sink fluid in a condenser. Increased COP leads to lower energy costs. Complex control systems have higher prices, and the choice of the control system depends on economic factors. The feasibility of retrofitting existing systems by implementation of variable speed drives is mostly empirically determined, but it can be obtained by numerical simulations for any new system. Numerical model of refrigeration machine or a heat pump which is suitable to be used for such analyses has to be able to predict dynamics of a refrigeration machine or a heat pump during transients caused by the action of a control system, but fast enough to be suitable for all year round simulations of system behavior. Development of such a model, as well as analyses of the feasibility of the capacity modulation applied to existing refrigeration machines and heat pumps are subjects of the presented paper.

heat pump system; variable speed capacity modulation; modeling

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