Elsevier

Energy and Buildings

Volume 49, June 2012, Pages 640-646
Energy and Buildings

Performance study of a constant temperature and humidity air-conditioning system with temperature and humidity independent control device

https://doi.org/10.1016/j.enbuild.2012.03.020Get rights and content

Abstract

To reduce the large energy penalty for constant temperature and humidity control in conventional air-conditioning systems, a novel temperature and humidity independent control (THIC) device is developed for typical cooling coils. The working principle of the THIC device is to adjust the chilled water flow rate in a cooling coil (CC) to maintain constant temperature and to adjust the apparatus dew point temperature of the CC to maintain constant humidity. A split-range control strategy is developed to control the cooling coil with the THIC device, the heater and the humidifier to achieve reliable control of indoor temperature and humidity while minimizing energy use.

Control performance of the THIC device was tested in an existing constant temperature and humidity air conditioning system serving a storeroom in a museum. The field test shows that the indoor temperature and humidity were well controlled in the required range. Energy performance of the THIC was tested using simulation tests on TRNSYS platform. Seven testing cases with different typical load conditions were conducted. The energy performance of the THIC is compared with conventional air conditioning systems. The results show that the THIC device can achieve power saving of 30–50%.

Highlights

► A novel temperature and humidity independent control (THIC) device is proposed. ► A split-range control strategy is developed to control the THIC device. ► Energy performance of the THIC was tested on TRNSYS platform. ► The results show that the THIC device can achieve power saving of 30–50%.

Introduction

With the development of economy and technology, built environment with constant temperature and humidity is required on many special occasions, such as industrial factories, scientific laboratories, archives, museums, etc. It is necessary for the air conditioning systems serving those spaces to fulfill the functions of cooling, heating, dehumidification and humidification and to provide reliable control of the thermal-hygrometric environment. Conventional air conditioning systems often fall into dilemmas in constant temperature and humidity applications due to the coupled cooling and dehumidification processes occurring on typical cooling coils. Under hot and humid conditions, the process air may be overcooled for the purpose of dehumidification and hence reheating is required. Under warm and dry conditions, the process air may be over-dehumidified to satisfy cooling demand and hence re-humidifying is required. In both situations, a large energy penalty is paid for independent and precise control of temperature and humidity.

A significant amount of research has been done on constant temperature and humidity air conditioning system. The methods developed in previous research can be categorized into two groups, i.e. utilizing energy efficient reheating sources and adopting independent dehumidification technologies. Yu et al. [1] presented the year-round experimental study on a constant temperature and humidity air conditioning system driven by ground source heat pump. Part of the heat rejected by the condenser was used to reheat the process air. The results showed that the indoor temperature and relative humidity can fulfill the national design code for archives in China. Compared with water cooled unit with boiler system, the heat pump system can save 48.4% of operating cost. Heat pipes were also widely adopted to recover heat for reheating the process air [2]. Independent dehumidification attracts increasing R&D interests in recent year. In an air conditioning system adopting independent dehumidification, the sensible load can be removed by high temperature coolants and the latent load can be separately removed by solid or liquid desiccants. Therefore, the indoor temperature and humidity can be controlled independently; moreover noticeable energy can be saved [3], [4], [5], [6]. According to the field test in a building adopting liquid desiccant based independent dehumidification system in Shenzhen [7], the energy consumption in unit net air conditioning area was 32.2 kWh/(m2 yr). However, the average energy consumption levels in similar buildings is 49 kWh/(m2 yr), which means the energy saving ratio is over 34.0%. Independent dehumidification technologies are promising; however control issues surrounding it are not sufficiently investigated [8]. They are super in independent control of temperature and humidity and energy saving, but not equally outstanding in constant temperature and humidity control with high precision at present due to the lack of reliable control. Wet cooling coil is still the most widely used dehumidification equipment in nowadays’ air conditioning applications.

This paper presents a novel THIC device which can be installed in typical cooling coils to achieve independent temperature and humidity control with satisfactory precision. A split-range control strategy is developed for the air handling units equipped with the THIC device to prevent the adverse concurrent cooling and reheating or dehumidification and re-humidification to save energy. Field test and simulation tests were conducted to study the control and energy performance of the constant temperature and humidity air conditioning system with the THIC device. Power saving potential of the THIC compared with conventional air conditioning systems under various load conditions is obtained.

Section snippets

THIC device and control logic

The three major air handling components in the proposed constant temperature and humidity air-conditioning system are cooling coil with the THIC device, heater and humidifier. The CC can cool and dehumidify the process air. Therefore, the three components can fulfill the four major air handling functions. Fig. 1 shows the configuration of the THIC device which includes two three-way control valves (CV1 and CV2), one small pump and several pipes. WCU is a water-cool unit which keeps the water

Field test on control performance

Control performance of the THIC was tested in an existing constant temperature and humidity air-conditioning system serving a museum storeroom (22 m × 16 m × 4 m). The air conditioning system is equipped with the THIC device to maintain strict thermal-hygrometric environment for cultural relic conservation while minimizing energy use. The split-range control strategy was adopted. The system schematic diagram is also presented by Fig. 4. The water temperature in the water tank is controlled by the

Simulation tests on control and energy performance

Energy performance of the THIC was tested using a simulator built on the TRNSYS platform. The above-mentioned storeroom and its associated air conditioning system as shown in Fig. 4 were used as the prototype for the simulator.

Conclusions

From the simulation analysis, it is concluded that by adjusting the mass flow rate and the temperature of the chilled water into the CC, and the power output of the heater or the humidifier, the temperature and the humidity of the process air can be automatically controlled independently by the THIC system with a high precision under different working conditions, and is suggested that when such CC is designed, the heat transfer area should be enlarged and calculated under a higher inlet

Acknowledgments

The research work presented in this paper was jointly supported by a research grant (A-PJ13) in the Hong Kong Polytechnic University, the Project 51176164 supported by National Natural Science Foundation of China, and the Major State Basic Research Development Program of China (973 Program) under contract no. 2011CB706501.

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