Solarrelated air conditioning

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Some components of systems installed for the purpose of heating a building can also be used to cool it but without the direct use of solar energy. Examples of these systems can be heat pumps, rock bed regenerators, and alternative cooling technologies or passive systems. Heat pumps were examined in Section 6.3.5. The other two methods are briefly introduced here.

• Rock bed regenerator. Rock bed (or pebble bed) storage units of solar air heating systems can be night-cooled during summer to store "cold" for use the following day. This can be accomplished during the night when the temperatures and humidities are low by passing outside air through an optional evaporative cooler, through the pebble bed, and to the exhaust. During the day, the building can be cooled by passing room air through the pebble bed. A number of applications using pebble beds for solar energy storage are given by Hastings (1999). For such systems, airflow rates should be kept to a minimum so as to minimize fan power requirements without affecting the performance of the pebble bed. Therefore, an optimization process should be followed as part of the design.

• Alternative cooling technologies or passive systems. Passive cooling is based on the transfer of heat by natural means from a building to environmental sinks, such as clear skies, the atmosphere, the ground, and water. The transfer of heat can be by radiation, naturally occurring wind, airflow due to temperature differences, conduction to the ground, or conduction and convection to bodies of water. It is usually up to the designer to select the most appropriate type of technology for each application. The options depend on the climate type.

More details for the adsorption and absorption systems follow.

6.4.1 Adsorption units

Porous solids, called adsorbents, can physically and reversibly adsorb large volumes of vapor, called the adsórbate. Though this phenomenon, called solar adsorption, was recognized in 19th century, its practical application in the field of refrigeration is relatively recent. The concentration of adsorbate vapors in a solid adsorbent is a function of the temperature of the pair, i.e., the mixture of adsorbent and adsorbate and the vapor pressure of the latter. The dependence of adsorbate concentration on temperature, under constant pressure conditions, makes it possible to adsorb or desorb the adsorbate by varying the temperature of the mixture. This forms the basis of the application of this phenomenon in the solar-powered intermittent vapor sorption refrigeration cycle.

An adsorbent-refrigerant working pair for a solar refrigerator requires the following characteristics:

1. A refrigerant with a large latent heat of evaporation.

2. A working pair with high thermodynamic efficiency.

3. A low heat of desorption under the envisaged operating pressure and temperature conditions.

4. A low thermal capacity.

Water-ammonia has been the most widely used sorption refrigeration pair, and research has been undertaken to utilize the pair for solar-operated refrigerators. The efficiency of such systems is limited by the condensing temperature, which cannot be lowered without introduction of advanced and expensive technology. For example, cooling towers or desiccant beds have to be used to produce cold water to condensate ammonia at lower pressure. Among the other disadvantages inherent in using water and ammonia as the working pair are the heavy-gauge pipe and vessel walls required to withstand the high pressure, the corrosiveness of ammonia, and the problem of rectification, i.e., removing water vapor from ammonia during generation. A number of solid adsorption working pairs, such as zeolite-water, zeolite-methanol, and activated carbon-methanol, have been studied to find the one that performed best. The activated carbonmethanol working pair was found to perform the best (Norton, 1992).

Many cycles have been proposed for adsorption cooling and refrigeration (Dieng and Wang, 2001). The principle of operation of a typical system is indicated in Figure 6.19. The process followed at the points from 1-9 of Figure 6.19 is traced on the psychrometric chart depicted in Figure 6.20. Ambient air is heated and dried by a dehumidifier from point 1 to 2, regeneratively cooled by exhaust air in 2 to 3, evaporatively cooled in 3 to 4, and introduced into the building. Exhaust air from the building is evaporatively cooled from points 5 to 6, heated to 7 by the energy removed from the supply air in the regenerator, heated by solar energy or another source to 8, then passed through the dehu-midifier, where it regenerates the desiccant.

The selection of the adsorbing agent depends on the size of the moisture load and application.

Heat input





4. Cold-moist

^3. Cool-dry



Hot-damp y 9. Exhaust air

2. Hot-dry warm-wet

1. Ambient air warm-wet

Evaporative air Regenerator Dehumidifier coolers

FIGURE 6.19 Schematic of a solar adsorption system.

FIGURE 6.19 Schematic of a solar adsorption system.

Moisture content

Dry bulb temperature FiGURE 6.20 Psychrometric diagram of a solar adsorption process.

Air solar collector

Dry bulb temperature FiGURE 6.20 Psychrometric diagram of a solar adsorption process.

Air solar collector

Desiccant Wheel

Rotating desiccant wheel Humid air

Dried air /

Rotating desiccant wheel Humid air

Dried air /

FIGURE 6.21 Solar adsorption cooling system.

Rotary solid desiccant systems are the most common for continuous removal of moisture from the air. The desiccant wheel rotates through two separate air streams. In the first stream, the process air is dehumidified by adsorption, which does not change the physical characteristics of the desiccant; in the second stream the reactivation or regeneration air, which is first heated, dries the desiccant. Figure 6.21 is a schematic of a possible solar-powered adsorption system.

When the drying agent is a liquid, such as triethylene glycol, the agent is sprayed into an absorber, where it picks up moisture from the building air. Then, it is pumped through a sensible heat exchanger to a separation column, where it is sprayed into a stream of solar-heated air. The high-temperature air removes water from the glycol, which then returns to the heat exchanger and the absorber. Heat exchangers are provided to recover sensible heat, maximize the temperature in the separator, and minimize the temperature in the absorber. This type of cycle is marketed commercially and used in hospitals and large installations (Duffie and Beckman, 1991).

The energy performance of these systems depends on the system configuration, geometries of dehumidifiers, properties of adsorbent agent, and the like, but generally the COP of this technology is around 1.0. It should be noted, however, that in hot, dry climates the desiccant part of the system may not be required.

Because complete physical property data are available for only a few potential working pairs, the optimum performance remains unknown at the moment. In addition, the operating conditions of a solar-powered refrigerator, i.e., generator and condenser temperature, vary with its geographical location (Norton, 1992).

The development of three solar-biomass adsorption air-conditioning and refrigeration systems is presented by Critoph (2002). All systems use active carbon-ammonia adsorption cycles and the principle of operation and performance prediction of the systems are given.

Thorpe (2002) presented an adsorption heat pump system that uses ammonia with a granular active adsorbate. A high COP is achieved and the cycle is suitable for the use of heat from high-temperature (150-200°C) solar collectors for air conditioning.

6.4.2 Absorption units

Absorption is the process of attracting and holding moisture by substances called desiccants. Desiccants are sorbents, i.e., materials that have an ability to attract and hold other gases or liquids that have a particular affinity for water. During absorption, the desiccant undergoes a chemical change as it takes on moisture; an example we have seen before is table salt, which changes from a solid to a liquid as it absorbs moisture. The characteristic of the binding of des-iccants to moisture makes the desiccants very useful in chemical separation processes (ASHRAE, 2005).

Absorption machines are thermally activated, and they do not require high input shaft power. Therefore, where power is unavailable or expensive or where there is waste, geothermal, or solar heat available, absorption machines could provide reliable and quiet cooling. Absorption systems are similar to vapor compression air-conditioning systems but differ in the pressurization stage. In general, an absorbent, on the low-pressure side, absorbs an evaporating refrigerant. The most usual combinations of fluids include lithium bromide-water (LiBr-H2O), where water vapor is the refrigerant, and ammonia-water (NH3-H2O) systems, where ammonia is the refrigerant.

Rejected heat


Expansion valve



Input heat (cooling effect)

Input heat

(solar or other) Strong

' solution


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