A proportional-integral (PI) law based variable speed technology for temperature control in indirect evaporative cooling system

The operation of indirect evaporative cooler (IEC) largely depends on the ambient temperature and humidity. To maintain stable indoor temperature, proper controller is essential. On-off control is a mature and stable control method used on constant speed fans. However, large fluctuation of indoor temperature can be observed because of limited control precision. To achieve better thermal comfort, a proportional-integral (PI) law based variable speed technology is proposed for accurate temperature control in an IEC system. This technology had been proved highly effective in central air-conditioning systems and direct expansion air-conditioners in terms of control precision and energy saving, but its techno-economic feasibility in IEC has not been investigated. In this study, annual dynamic simulation has been conducted to an IEC system based on the IEC model and control algorithm. Results show that indoor temperature can be controlled within +/- 0.5 degrees C around the setting point for 81.9% of time, while it is only 30.5% under on-off control. The PI based controller is well adapted to cooling loads in all seasons with good control precision, fast response speed and small overshoots. Response time of PI control is only 10 min in a disturbance rejection test, which is much shorter than 30 min under the on-off control. Annually, IEC with variable speed fans consume 50.0% less energy than that of on-off fans. At last, economic analysis shows that this technology is economically feasible only when the power of primary air fan is larger than 1.75 kW.