Model Prediction Controlled Refrigeration System

• US 20080000241A1
• Filed: 09/30/2005
• Published: 01/03/2008
• Est. Priority Date: 09/30/2004
• Status: Active Grant

• Current Assignee: Danfoss A/S
• Original Assignee: Danfoss A/S
• Inventors: Larsen, Lars, Thybo, Claus
• Granted Patent: US 7,905,103 B2
• Time in Patent Office: Days
• US Class Current: 62/117
• CPC Class Codes: F25B 2400/075 with parallel compressors
• CPC Class Codes: F25B 2400/22 Refrigeration systems for supermarkets
• CPC Class Codes: F25B 2500/05 Cost reduction
• CPC Class Codes: F25B 2500/19 Calculation of parameters
• CPC Class Codes: F25B 49/02 for compression type machines, plants or systems

Abstract

The invention provides a refrigeration system with a compressing unit, and a method of controlling a refrigeration system. To facilitate a better control, the capacity of the compressing unit is controlled based on a predicted future cooling demand rather than an actually determined cooling demand. The invention further provides a system wherein a cost value for changing the cooling capacity of the system is taken into consideration.

Claims

1-21. -21. (canceled)

22. A refrigeration system comprising a closed-loop system for circulation of a refrigerant between

a compressing unit comprising a variable capacity element to provide a variable volumetric compressing capacity for compressing the refrigerant, and

at least one evaporator for evaporating the compressed refrigerant and thus for providing a cooling capacity to meet a cooling demand to refrigerate a secondary fluid of a refrigerated space,

a control system adapted:

to establish an estimate of a future cooling demand, and

to control the cooling capacity to adapt to the estimate,

wherein the control system is adapted to determine a cost value representing costs of operating the system by identifying a set of compressor capacities that minimizes a cost function by use of;

a model of the system,

said cooling demand predictions, and

actual system measurements.

23. The system according to claim 22, wherein the first compressor capacity of the set of compressor capacities is used as the control action, and wherein the procedure is repeated in a subsequent time step using new system measurement and updated demand predictions.

24. The system according to claim 22, wherein the cooling capacity is controlled by controlling the compressing capacity.

25. The system according to claim 22, wherein the cooling capacity is controlled by controlling a mass flow of the refrigerant through the evaporator.

26. The system according to claim 22, wherein the compressing capacity is controlled in discrete steps.

27. The system according to claim 22, adapted to control the compressing capacities based on the cost value.

28. The system according to claim 22, wherein the control system comprises a data set representing future external operating conditions, and wherein the control system is adapted to establish the estimate of the future cooling demand from the external operating conditions.

29. The system according to claim 28, being adapted to record external operating conditions and corresponding cooling demands during operation.

30. A method for controlling a refrigeration system comprising a closed-loop system for circulation of a refrigerant between

a compressing unit with a variable compressor capacity for compressing the refrigerant, and

at least one evaporator for evaporating the refrigerant,

the method comprises the steps of:

estimating of a future cooling demand, and

controlling the cooling capacity to adapt to the estimate

wherein the step of controlling the cooling capacity comprises;

within a time horizon, calculating a cost function for a number of trajectories of sequences of possible compressor capacities within the time horizon,

selecting from the trajectories a trajectory with a lowest cost value,

selecting an initial cooling capacity of the selected trajectory and controlling the compressing unit or the mass flow of the refrigerant to provide that capacity.

31. The method according to claim 30, wherein the estimated future cooling demand is comprised in at least one of a mathematical model and a table.

32. The method according to claim 30, wherein the cooling capacity is controlled by controlling the compressing capacity.

33. The method according to claim 30, wherein the cooling capacity is controlled by controlling a mass flow of the refrigerant through the evaporator.

34. The method according to claim 30, wherein the cost value depends on a predicted cooling demand and an estimated cooling capacity.

35. The method according to claim 34, wherein the cost value further depends on a number of changes in the cooling capacity within the time horizon.

36. The method according to claim 30, wherein the cost value is established as an accumulation of cost contributions of time frames within the time horizon.

37. The method according to claim 36, wherein a trajectory of subsequent cooling capacities of each of the time frames is selected based on cost contributions of the time frames.

38. The method according to claim 30, wherein the estimate is expressed as a product of a mass flow of the refrigerant through the evaporator and a change in specific enthalpy of the refrigerant through the evaporator.

39. The method according to claim 38, wherein the estimate is proportional with the mass flow.

40. The method according to claim 38, wherein the estimate is proportional with the compressing capacity.

41. The method according to claim 30, wherein the steps are repeated for a subsequent time horizon.

42. The method according to claim 30, wherein the cost value is established as an accumulation of cost contributions of time steps within the time horizon.