Control systems for liquid desiccant air conditioning systems

Abstract

Methods and control systems are disclosed for operating a liquid desiccant air-conditioning system to efficiently maintain a target temperature and humidity level in a space.

1 . A method of operating a liquid desiccant air-conditioning system to maintain target temperature and humidity level in a space, the liquid desiccant air conditioning system comprising:
 a conditioner for treating a first air stream flowing therethrough and provided to the space as a supplied air stream, said conditioner using a heat transfer fluid and a liquid desiccant to treat the first air stream;   a device for measuring temperature and a device for measuring humidity in the supplied air stream;   a regenerator connected to the conditioner such that the liquid desiccant can be circulated between the regenerator and the conditioner, the regenerator causing the liquid desiccant to desorb water vapor to a second air stream or to absorb water vapor from the second air stream depending on a selected mode of operation of the system;   a refrigerant system;   a first refrigerant-to-heat transfer fluid heat exchanger connected to the conditioner and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the conditioner;   a second refrigerant-to-heat transfer fluid heat exchanger connected to the regenerator and the refrigerant system for exchanging heat between the refrigerant heated or cooled by the refrigerant system and the heat transfer fluid used in the regenerator; and   a system controller for controlling operation of the system;   wherein the method comprising the steps of:   (a) measuring the temperature and humidity level in the supplied air stream;   (b) comparing the temperature measured in (a) to a target temperature to determining a temperature error, and comparing the humidity level measured in (a) to a target humidity level to determine a humidity error;   (c) comparing the humidity error and the temperature error on a common scale to determine the greater error;   (d) using the greater error to drive the system controller to control operation of the system to reduce the greater error;   (e) repeat (a) through (d) a plurality of times.   
     
     
         2 . The method of  claim 1 , wherein the temperature measured in (a) is a dry bulb temperature and the humidity level measured in (a) is a relative humidity level, and wherein the target temperature is a dry bulb temperature and the humidity target is a dew point target;
 and the step (c) is based on errors in dry bulb and dew point.   
     
     
         3 . The method of  claim 2 , wherein when the system operates as a dehumidifier, the system controller is operated to prevent the dew point based on measurements from being lower than the target dew point. 
     
     
         4 . The method of  claim 2 , wherein the system controller controls the setting of a compressor in the refrigerant system to control the cooling capacity of the system, and
 the system controller controls the liquid desiccant and heat transfer fluid flow rates in the regenerator, wherein higher liquid desiccant and heat transfer fluid flow rates in the regenerator increase the sensible cooling rate of the supply air stream.   
     
     
         5 . The method of  claim 4 , wherein the liquid desiccant air conditioning system further comprises a liquid desiccant dilution device, and the method further comprises controlling the sensible cooling rate of the supply air stream by diluting the liquid desiccant using the liquid desiccant dilution device. 
     
     
         6 . The method of  claim 5 , wherein the liquid desiccant dilution device is used to maintain minimum level of liquid desiccant in a liquid desiccant tank of the liquid desiccant air conditioning system to maintain a minimum relative humidity level in the supply air stream. 
     
     
         7 . The method of  claim 4 , wherein the liquid desiccant air conditioning system further comprises an air-cooled coil associated with the second refrigerant-to-heat transfer fluid heat exchanger, the method further comprises increasing the sensible heat ratio by increasing fluid flow rates through the air-cooled coil. 
     
     
         8 . The method of  claim 4 , wherein the liquid desiccant air conditioning system further comprises an air-cooled coil associated with the first refrigerant-to-heat transfer fluid heat exchanger, the method further comprises reducing the sensible heat ratio by increasing fluid flow rates through the air-cooled coil. 
     
     
         9 . The method of  claim 1 , wherein the temperature measured in (a) is a dry bulb temperature and the humidity level measured in (a) is a relative humidity level, and wherein the target temperature is a dry bulb temperature and the humidity target is a dew point target;
 and the step (c) is based on errors in wet bulb and relative humidity.   
     
     
         10 . The method of  claim 9 , wherein the wet bulb error controls the setting of a compressor in the refrigerant system to control the cooling capacity of the system, and
 the relative humidity error controls the liquid desiccant and heat transfer fluid flow rates in the regenerator, wherein higher liquid desiccant and heat transfer fluid flow rates in the regenerator increase the sensible cooling rate of the supply air stream.   
     
     
         11 . The method of  claim 10 , wherein the liquid desiccant air conditioning system further comprises a liquid desiccant dilution device, and the method further comprises controlling the sensible cooling rate of the supply air stream by diluting the liquid desiccant using the liquid desiccant dilution device. 
     
     
         12 . The method of  claim 11 , wherein the liquid desiccant dilution device is used to maintain minimum level of liquid desiccant in a liquid desiccant tank of the liquid desiccant air conditioning system to maintain a minimum relative humidity level in the supply air stream. 
     
     
         13 . The method of  claim 9 , wherein the liquid desiccant air conditioning system further comprises an air-cooled coil associated with the second refrigerant-to-heat transfer fluid heat exchanger, the method further comprises increasing the sensible heat ratio by increasing fluid flow rates through the air-cooled coil. 
     
     
         14 . The method of  claim 9 , wherein the liquid desiccant air conditioning system further comprises an air-cooled coil associated with the first refrigerant-to-heat transfer fluid heat exchanger, the method further comprises reducing the sensible heat ratio by increasing fluid flow rates through the air-cooled coil.