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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 handlethe 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.
2.1. Basis information
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.
2.2. THIC air-conditioning system
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 pided 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.