Study On The Energy Saving Control Of The Cooling Water System Of The Central Air Conditioning System

The building energy consumption accounts more than 30% of the total energy consumption. The building energy saving is so rather significant for reducing the total energy consumption. The central air conditioning system has the greatest potential as for energy saving in all the building energy consumption systems. So, the focus should first be paged on the central air conditioning system as for the building energy saving.The cooling water system is one of the most important parts of the central air conditioning system, energy consumption of which accounting 15～20% of the central air conditioning system. The cooling water system runs mostly with partial loads. The key for energy saving is how to run (say, how to control) the cooling water system with partial load.This paper first studied the energy saving potential of the cooling water system of the central air conditioning system. Based on studying the related literatures, this paper concluded that the energy saving potential of the cooling water system should be around 10%, not less than that of the chilled water system. A method to calculate the potential of the cooling water system was then put forward, which is rather simple and has rather good accuracy. There are mainly two kinds of methods to control the cooling water system presently: one to control the cooling water temperature difference between the inlet and the outlet of the chiller and another to control the cooling water temperature at the outlet of the chiller. The two methods could save energy compared with the method with fixed cooling water flow rate.But they are not the best one. This paper put forward a control method, which could save the energy consumption of the cooling water system to the greatest limit. This method requires to find the pump and the fan frequencies with the lowest energy consumption at all kinds of external conditions in the commissioning process. Then the pump and fan frequencies will be directly set to be the values according to the external conditions in the actual runs. The method to calculated the pump and fan frequencies with the cowest energy consumption at different external conditions is defined.To fulfill the above control method, this paper then design a control system (including hardware and software). The feature of hardware lied in a memory with high volume. A four- dimensional database was stored in the memory. The database told the optimized pump and fan frequencies at all kinds of external conditions. The feature of the software is to address the database. The test demonstrated the designed control system could implement the functions such as storing, addressing, inputting and outputting and fulfill the optimized control method.The control system was then used in a real building. The temperature differences between the inlet the outlet of the chiller became much greater than ever when the control system worked. The total energy saved accounted 4.9% of the sum of energy consumption of the chiller, pumps and the fans. It was principly demonstrated that the control method and the control system developed in this paper could be used practically and effectively.