In the conventional HVAC system that removes moisture by condensation, air is cooled and dehumidified simultaneously. In most cases, sensible load of building covers the majority part of the whole cooling load while the latent load (moisture load) takes only a small part. However, as the required cooling source temperature of dehumidification is much lower than that of cooling, the chilled water temperature has to be reduced to meet the demand for condensation dehumidification. Moreover, the ratio of sensible load to latent load varies largely due to the changes of outdoor climate, number variance of indoor occupants, indoor equipments and lighting utilization mode and so on. Therefore, the indoor temperature and humidity, the two key parameters, can hardly be satisfied with condensation by the cooling coil only. In practice, the common reaction to the increased humidity is to reduce the set-point temperature and then re-condition the air after passing the cooling coil to the proper temperature, which results in a plenty of energy wastefulness. To avoid the aforementioned problems, temperature and humidity independent control (THIC) air-conditioning system stands out as an appropriate pattern that temperature and humidity can be regulated independently with temperature control subsystem and humidity control subsystem respectively. Besides, the coil temperature for cooling in the temperature control subsystem can be considerably increased, e.g. from current 7 ◦C to 17◦C, so that improvement on the performance of chillers or even free cooling from ambient could be obtained. Many investigations have been carried out on the hybrid desiccant dehumidification and air-conditioning system, which integrates liquid/solid desiccant units with a conventional cooling system to avoid excess cooling. Liquid desiccant units developed quickly in recent years, for its advantages of dehumidifying at a temperature higher than the air’s dew-point to avoid reheat procedure in the system, and regenerating desiccant at a low temperature which can be driven by low-grade heat sources [5,6].Many studies focusing on improving its performance with process optimization have been conducted in depth, such as Yadav [7], DryKor Ltd. [8], and Liu et al. [9]. Chen et al. [10] designed an independent dehumidification air-conditioning system with a hot water-driven liquid desiccant and a chiller that provides 18–21 ◦C chilled water for an office building in Beijing, which saved about 30% cooling cost compared with conventional system. The performance of a hybrid system tested by Ma et al. [11] was 44.5% higher than conventional vapor compression system at a latent load of 30% and this improving could be achieved by 73.8% at a 42% latent load. Besides, the specific research on the feasibility and performance of the hybrid system in hot and humid regions is promoted.
This paper will investigate the real operating performance of a THIC air-conditioning system operated in an office building located in Shenzhen, a modern metropolis in southern China of hot and humid climate. In this THIC system, the liquid desiccant fresh air handling units driven by heat pumps are employed to handle
the outdoor air to remove the entire latent load and supply enough fresh air to the occupied spaces, and the high-temperature chiller that produces chilled water of 17.5 ◦C for the indoor terminal devices (radiant panels and dry fan coil units) is applied to control indoor temperature. The operating principle and performance test results of the THIC system will be shown in this paper, and suggestion for performance improvement will also be included.
The THIC system has been put into practice as a pilot project in an office building in Shenzhen, China. This system has been brought into operation in July 2008 and the basic information about the building and air-conditioning system goes as follows.
    The 5-story office building, as shown in Fig. 1, is located in Shenzhen, China, with total building area of 21,960m2 and the areas of 5940m2, 5045m2, 3876m2, 3908m2, 3191m2 for the 1st to 5th floor respectively. The main function of the 1st floor is restaurant, archive and carport, the 2nd to 4th floors are the office rooms, the 5th floor is the meeting room, and there is a vestibule vertically through up the 2nd to 4th floors in the north of the building. In the vestibule, curtain wall with ventilation shutters in the upper is applied on its north surface, as shown in Fig. 2. The outdoor condition in Shenzhen is rather hot and humid all through the year as shown in Fig. 3. The annual outdoor air relative humidity is about 80% and humidity ratio in summer is as high as 20 g/(kg dry air). The building requires cooling and dehumidification
in a long period of time, and no heating and humidification requirement in winter. Therefore, how to handle the moisture efficiently is a key issue in such a subtropical area.
    The THIC system serves from 1st to 4th floor with the net airconditioning area of 13,180m2 (total area of 18,769m2), and the 5th floor is served by several stand-alone air conditioners so that is not within the scope of our discussion. The schematic of the THIC system is shown in Fig. 4 with the main devices’ parameters listed in Table 1. The right side of Fig. 4 is the humidity control subsystem, including
9 liquid desiccant fresh air handling units that supply adequate dry fresh air into the occupied spaces. As the volume of the supplied fresh air is proportional to the number of people, the pollutants, CO2 and latent heat produced by human bodies can be removed by fresh air. The schematic of the fresh air processors using liquid
desiccant is illustrated in Fig. 5(a), which is composed of a twostage total heat recovery device and a two-stage air handling device coupled with refrigeration cycles. Lithium bromide (LiBr) aqueous solution is employed as liquid desiccant in these air processors. The total heat recovery device is used to recover the energy from indoor exhaust air (return air) to decrease the energy consumption in the fresh air handling process. And in the heat pump driven air handling device, the diluted solution from the dehumidification modules is heated by the exhaust heat from the condenser and concentrated in the regeneration modules, then the hot concentrated solution is cooled by passing through the heat exchanger and evaporator before it enters the dehumidification modules, and lastly used to remove moisture from the fresh air. To make it clear, the air-handling processes are displayed in the air psychrometric chart in Fig. 5(b) where the fresh air first passes the total heat recovery device to recovery the energy from the indoor exhaust air, and then flows into the dehumidification modules to be further dehumidified and cooled before it is supplied into the occupied spaces. In general, the COP of the liquid desiccant fresh air units (total heat removed from the fresh air divided by the power consumption of the heat pumps and solution pumps) can be as high as 5.0 with the following three main reasons: (1) the cooling capacity of the indoor exhaust air is fully exploited to remove heat from the fresh air by the total heat recovery device; (2) both the cooling capacity from evaporator and exhaust heat from condenser are utilized to enhance the air handling processes; and (3) the efficiency of the heat pump is significantly raised since the required evaporating temperature in this liquid desiccant device is much higher than that in the conventional condensing dehumidification system. Besides, as indicated in Fig. 5(b), the supplied air temperature is lower than the indoor air temperature, so the liquid desiccant system can remove some sensible load of the building as well as the entire latent load. The left side of Fig. 4 is the temperature control subsystem that takes up the rest sensible load to control indoor temperature, including a high-temperature chiller, cooling tower, cooling water pump, chilled water pump, and indoor terminal devices (radiant panels and dry fan coil units). The high-temperature chiller is a centrifugal chiller with the rated COP of 8.3 (designed condition: the inlet and outlet temperature of the chilled water and cooling water are 20.5 ◦C/17.5 ◦C and 30.0 ◦C/35.0 ◦C respectively), which is much higher than the conventional chiller operating at the chilled water temperature of 12 ◦C/7 ◦C. As for indoor terminal devices, as shown in Fig. 6, fan coil units (FCUs) operating in ‘dry condition’ are set up in the restaurant, archive and office regions which serve about 81% of the entire cooling load of the temperature control sub-system, while radiant floor and radiant ceiling panels are applied in vestibule and some office rooms which serve the rest 19%.
In the previous sections, the whole THIC system layout has been introduced briefly. Particularly, stratified air conditioning, a key design principle of large space, is selected in the air-conditioning design of the vestibule as shown in Fig. 2(b). Specifically, in the occupied zone (the height within 2m), chilled water with temperature of 17.5 ◦C is pumped and distributed into radiant floor for cooling, and the handled dry fresh air and indoor exhaust air are supplied and expelled in the bottom and in the middle of the space respectively, which forms a “dry air layer” to protect the cold floor surface from condensation; in the higher space that far from occupied zone, solar radiation that enters through glass curtain wall is absorbed by the ornamental decorations in the higher space, and the heat is then carried away by natural ventilation through the shutters directly. The temperature control subsystem and humidity control subsystem can be operated separately according to ambient condition and indoor requirement. The two subsystems operate together at hot and humid outdoor climate; Only the humidity control subsystem operates at cold but humid ambient condition; Outdoor air is directly introduced into occupied spaces after filtering when outdoor air is dry enough, such as 11 g/kg.
    According to our knowledge, cooling air can be realized more easily than dehumidification by condensation, since the latter one requires lower temperature of cooling source than the former. However, the COP of the tested temperature control subsystem is lower than or equal to that of the humidity control subsystem in present THIC system. Thus, this section will focus on how to improve the performance of the temperature control subsystem. According to the performance of each component in the temperature control subsystem shown in Table 4, three main improvements of the temperature control subsystem are recommended: (1) modifying the frequency of the chilled water pump; (2) improving the cooling tower performance by tightening the
strap; and (3) improving the performance of FCUs under dry working condition. The first two methods can be easily realized in the building, while the third one depends on the improvement of new FCU products.
    The operating performance of the THIC system in an office building in Shenzhen is presented in this paper. Liquid desiccant fresh air units are used to supply dry fresh air to control indoor humidity, and chilled water with the temperature of 17.5 ◦C is pumped and distributed into radiant panels and dry fan coil units to control indoor temperature. The followings are the conclusions based on
the tested results: (1) The THIC system can provide a comfortable indoor environment that indoor temperatures, humidity ratios as well as CO2 concentrations are all within the comfortable ranges. (2) The COP of the entire THIC system can reach 4.0, with the COP of the temperature control subsystem and humidity control
subsystem of 3.7–4.1 and 4.1 respectively. The energy consumption of the THIC system in the tested office building is 32.2kWh/(m2 yr) (net air-conditioning area), that is, the energy efficiency is much higher than that of the conventional airconditioning system available in literature. (3) Possible improvements of the temperature control subsystem are provided, including modification on the chilled water pump, cooling tower and FCUs. Thus, the expected system COP can be further increased to 4.4, which can save 9% compared to present air-conditioning system.