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Condensation risk in a room with a high latent load and chilled ceiling panels and with air supplied from a liquid desiccant system.

Article Excerpt
INTRODUCTION

Radiant cooling has shown great advantages when it comes to improving thermal comfort. This is due to a reduced vertical temperature gradient in the occupied zone and to a low air speed (Behne 1999; Imanari et al. 1999, Hao et al. 2007). Radiant cooling can also reduce energy demand, because the energy used by its pump is lower than that used by fans in all air systems (Sodec 1999; Novoselac and Srebric 2002). With radiant cooling, the room air temperature in summer could also be higher than that in all air systems, so the cooling load would become smaller. As a result, the energy demand can be further reduced (Feustel and Stetiu 1995). Since radiant cooling alone cannot remove latent load and pollutants, such as volatile organic compounds, it should be supplemented with ventilation. In the 1990s, radiant cooling was used in combination with displacement ventilation in Europe. Experimental investigation and practical experiences in Europe (Givoni 1991; IEA 1998) have proved that radiantly cooled ceilings are able to remove high cooling loads with good thermal comfort. For example, Niu (1994) found that a cooled ceiling combined with displacement ventilation can produce a thermally comfortable environment at a cooling load of up to 50 W/[m.sup.2], compared with 40 W/[m.sup.2] with only displacement ventilation. However, Vangtook and Chirarattananon (2006) studied radiant cooling with natural ventilation in a hot and humid climate in Thailand. The temperature of radiantly cooled water was limited to 24[degrees]C to avoid condensation. They found that the low heat reception capacity of the panel would limit its use to only situations when loads were low. Obviously, the system is good for dry climates and could be problematic for humid climates. In the humid climates or high latent load spaces, such as operating theaters and certain hospital wards, a very high ventilation air requirement of a conventional system is essential or mandated, as pointed out by Ameen and Mahmud (2005), which would cause more energy consumption and cost, even discomfort.

Using desiccant dehumidified ventilation for the radiant cooling system could be adequate for that situation. Desiccant dehumidified ventilation can provide very dry air to decrease the supply airflow rate, because the humidity ratio is lower than that obtained with a conventional vapor compression chiller used for all-air systems. This makes it desirable for buildings in hot and humid regions, like the southeastern countries of Asia, where the risk of condensation is very high due to the low surface temperature in the radiant cooling panels. Another advantage is that the sensible and latent cooling load are decoupled and controlled separately, which means that control of humidity can be achieved better than with conventional vapor compression systems. Many investigations on using desiccant for radiant cooling have been conducted. Niu et al. (2002) proposed a hybrid air-conditioning system using desiccant rotors to dehumidify ventilated air. Their results show that up to 44% of primary energy demand could be reduced with the hybrid system, in comparison with a conventional all-air system. Ameen and Mahmud (2005) conducted a study on desiccant dehumidification with hydronic radiant cooling systems in humid tropical climates, and the trial run confirmed that the condensation problem was not insurmountable. Furthermore, the system could be easy to agree with the load changing. Tsay et al. (2006) experimentally studied a desiccant cooling system with a heat pump by using the condensation heat for the desiccant rotor regeneration to improve the system's thermal performance. Their results show that the coefficient of performance (COP) of the desiccant system was estimated to be 2.32, which was higher than the thermal performance of heat-driven desiccant system. However, energy consumed by the system is high-grade electricity.

Convectional radiant cooling uses chilled water from electricity-driven vapor compression chillers. The chillers have to use chlorofluorocarbon (CFC) or hydrochlorofluorocarbon (HCFC) refrigerants, at present, and electricity. The use of such refrigerants is bad for the environment. Electricity is very high-grade energy. Thus, it is necessary...

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