Study On Annual Control Strategy Of Outdoor Air Ventilation And Distribution For Multi-zone VAV Air Conditioning Systems

In order to obtain good indoor air quality, enough outdoor air is often necessary in VAV air-conditioning system, but plenty of fresh air also bring forth massive fresh air load. Due to the difference of occupancy, air-conditioning area and use of equipments, the air-conditioning load and outdoor air demand are not same in different air-conditioning zones. Nevertheless, the outdoor air ratio in the VAV air-conditioning system is unique, which may cause the outdoor air rates are insufficient in some zones while excessive in other zones. The traditional outdoor air control strategies for VAV air-conditioning system are often presented based on design condition, which can't adjust the outdoor air rate according to the change of occupancy in air-conditioning zone instantly. The CO2-based demand control ventilation (CO2-DCV) which is popular in recent years can control the outdoor rate effectively according to the change of indoor CO2 concentration. But the CO2-DCV method can't remove the pollutants which are not generated by human.This thesis selected a multi-zone VAV air-conditioning system to analyze the features of outdoor air ventilation and distribution in actual operation, and then presented an annual control strategy of outdoor air ventilation and distribution for multi-zone VAV air conditioning systems based on health and energy saving. The strategy estimated real-time occupancy by detecting the current outdoor air rate and CO2 concentration of outdoor air and return air, and then determined the set-point value of outdoor air rate and outdoor air ratio based on the real-time occupancy and American ASHRAE62 -2001. After that, using MSE equation, the strategy amended the system fresh air ratio according to the fresh air ratio in critical zone. At last, the strategy introduced the enthalpy difference method to adjust the fresh air rate reasonably according to the value of indoor and outdoor enthalpy, and then compared with minimum outdoor air rate to obtain the set point value. Considering all the pollutants generated by human and non-human synthetically, the method measures indoor air quality based on indoor CO2 concentration and TVOC concentration, and reduces the operational energy consumption of outdoor air control in multi-zone air-conditioning system under acceptable IAQ condition.In order to verify the rationality and reliability of the control strategy, the thesis established a multi-zone VAV air-conditioning system simulation model based on TRNSYS simulation software. The simulation study is under the meteorological data of the central plain region. The results showed that, compared with the traditional CO2-DCV, the strategy maintained the indoor CO2 concentration was below 1000 ppm, and TVOC concentration was below the upper limit of ASHRAE62-2001Standard comfort zone (300μg/m3) all day. Compared to the strategy of supply air in critical fresh air ratio, the optimal control strategy reduced the outdoor air energy consumption by 24.7% in cooling condition, and by 16.3% in heating condition. The simulation results indicate the strategy can reconcile the contradiction between the energy consumption of fresh air and IAQ, and resolve the problem of fresh air rates are insufficient in some zones while excessive in other zones. The strategy should be implemented in applications.