SUBCATEGORÍA SUELOS DE VALOR CULTURAL

Liquid desiccant cooling has been developed as an alternative and complementary method of cooling focusing on applications such as cooling of human dwellings, commercial buildings, hospitals (Dai et al., 2001, Gommed and Grossman, 2004, Jain et al., 2000, Mei and Dai, 2008, Oliveira et al., 2000). It is a solar thermal sorption refrigeration method. Sorption refrigeration can be divided into three main categories (see Fig. 2.1):

a) Desiccant cooling or open sorption cooling. The hygroscopic substance (or desiccant) in this process is used to dehumidify the air. The desiccant can be liquid (liquid desiccant cooling) or solid (solid desiccant cooling). The liquid desiccants utilised are aqueous solutions of lithium salts (LiCl, LiBr), aqueous solutions of calcium chloride (or mixtures of LiCl and CaCl2) and tri-ethylene glycol. The solid desiccants used are silica gel, activated alumina and zeolite.

b) Absorption. It is a closed cycle process using liquid desiccants such as LiBr- H20, NH3-H20.

c) Adsorption. Closed cycle process that can be subcategorised based on the nature of the process in:

i. Physical adsorption. High porosity substances such as zeolite, silica gel, activated carbon and alumina are used as adsorbents.

ii. Chemical adsorption. In this process the strength of the chemical bond between the adsorbent and the adsorbate drives the phenomenon. Usually CaCl2 is utilised with ammonia.

For comprehensive literature reviews in solar refrigeration the reader is referred to the studies of Kim and Infante Ferreira (2008) and Grossman (2002).

Fig. 2.1: Tree diagram showing the different types of sorption refrigeration.

Sorption Refrigeration

Desiccant Cooling or Open Sorption Cooling

Adsorption

Physical Chemical Absorption

G.Lychnos 51 As mentioned in Chapter 1, the use of liquid desiccants with solar regeneration in a fan-pad system coupled to a greenhouse was investigated theoretically by Davies (2005). The model presented used climatic data of Abu Dhabi and it was found that this system could lower summer maximum temperatures by 5oC when compared to conventional evaporative cooling.

2.7 Discussion

This review has differentiated among the various physical processes of greenhouse cooling and identified the technologies used to implement them. Ventilation (passive/forced) provides adequate cooling for greenhouses located in the temperate zone, especially in high latitudes. The need for cooling increases with decreasing latitudes, hence in lower latitudes only ventilation is not enough. In the subtropics, complementary methods to ventilation are used for cooling e.g. shading-reflection which adversely affects photosynthesis as it reduces PAR. However, even the combination of ventilation and shading-reflection is not enough in warmer climates. Evaporative cooling has proved to achieve better cooling than all the other mentioned methods in hot and dry climates. But even the latter method, as already shown by Ganguly and Ghosh (2007) and Kittas et al. (2003), fails to generate enough cooling when applied to hot and humid climates such as the tropics and the coastal area of the subtropics as also pointed by Kumar at al. (2009). In fact these studies concluded that a fan-pad evaporative cooling system can marginally decrease greenhouse air temperature by 2oC during 12-14hrs (high irradiance, high ambient relative humidity ~ 75%) or sustain temperatures at the ambient level if not increased (depending on the greenhouse length). Nevertheless, a solar liquid desiccant cooling system could address this problem as already discussed in Chapter 1, section 1.3. by lowering maximum summer temperatures by 5oC at least on the basis of theoretical predictions. This is the option explored further in this thesis.

Table 2.1 summarises studies published since the 80’s focusing on the type of cooling system, the geographical zone and whether the method is commercial or experimental.

Table 2.1: Brief summary of studies on greenhouse cooling published since 1980’s.

Year Authors Type of Cooling

System Geographical Zone Experimental / Commercial

1980 Kozai et al. Passive Ventilation subtropics commercial 1983 Cohen et al. Roof evaporative cooling subtropics experimental 1984 Sase et al. Passive Ventilation subtropics commercial 1989 Giacomelli et al. Fog-mist subtropics experimental 1989 Garzoli Roof evaporative cooling tropics/subtropics experimental

1990 Montero et al. Fog-Mist subtropics commercial

1992 Fernandez and Bailey Passive Ventilation temperate commercial 1992 Papadakis et al. Forced Ventilation subtropics commercial 1995 Boulard and Draoui Passive Ventilation subtropics commercial 1995 Kittas et al. Passive Ventilation subtropics commercial 1995 Shamim and McDonald Shading/Reflection temperate experimental 1995 Sutar and Tiwari Roof evaporative cooling tropics experimental 1995 Santamouris et al. Earth-Air Heat Exchanger subtropics experimental 1996 Papadakis et al. Passive Ventilation subtropics commercial 1996 Boulard et al. Passive Ventilation subtropics commercial 1997 Boulard et al. Passive Ventilation subtropics commercial 1997 Fuchs et al Passive/Forced Ventilation subtropics commercial 1999 Cohen and Fuchs Shading/Reflection subtropics commercial 1999 Kittas et al. Shading/Reflection subtropics commercial 1999 Arbel et al. Fan-Pad, Fog-Mist subtropics commercial 1999 Jirka et al. Fresnel Lenses covering

material temperate experimental 2001 Demrati et al. Passive Ventilation subtropics commercial 2001 Kittas et al. Passive and Forced

Ventilation subtropics experimental 2001 Baille et al. Shading/Reflection subtropics commercial

2002 Nielsen Passive Ventilation temperate commercial

2003 Willits Forced Ventilation, Fan-

Pad subtropics commercial

2003 Kittas et al. Fan-Pad, Shading subtropics commercial

2003 Ozturk Fog-Mist subtropics Commercial

2003 Arbel et al. Forced Ventilation, Fog-

Mist subtropics Commercial

2003 Ghosal et al. Roof evaporative cooling tropics Experimental 2004 Parra et al. Passive Ventilation subtropics commercial 2004 Sethi et al. Shading/Reflection tropics commercial

G.Lychnos 53 (Table 2.1 continued)

2004 Tripanagnostopoulos

et al.

Fresnel Lenses covering

material subtropics experimental 2004 Ghosal et al. Earth-Air Heat Exchanger tropics experimental 2005 Soni et al. Passive Ventilation tropics commercial 2005 Kittas et al Forced Ventilation subtropics commercial 2005 Davies solar liquid desiccant

cooling subtropics experimental 2006 Teitel et al. Passive Ventilation subtropics commercial 2006 Coelho et al. Passive Ventilation subtropics commercial 2006 Romero et al. Passive Ventilation temperate commercial 2006 Gazquez et al. Passive/Forced Ventilation,

Fogging, Shading subtropics commercial

2006 Fuchs et al. Fan-Pad subtropics commercial

2006 Sabeh et al. Fan-Pad subtropics commercial

2006 Toida et al. Fog-Mist subtropics experimental

2006 Hemming et al. NIR filtering methods temperate experimental 2006 Sonneveld et al. NIR reflective covering

material temperate experimental 2006 Garcia-Alonso et al. cool plastic films subtropics experimental 2006 Souliotis et al. Fresnel Lenses covering

material subtropics experimental 2006 Ghosal and Tiwari earth-air heat exchanger tropics experimental 2006 Yildiz and Stombaugh heat pump temperate experimental 2006 Hamer et al. various heat pump based

systems temperate experimental

2007 Ganguly and Ghosh Fan-Pad tropics commercial