There is a growing demand for space cooling in hot climates as people seek to raise their standards of living and improve work performance, resulting in an increasing demand for energy during the day. The vapour compression refrigeration system dominates modern cooling technology. These systems consume around 40-50 per cent of the domestic power supply and contribute to local as well as global warming. As looking for substitutes, evaporative cooling is one of the oldest and eco-friendly technologies, which find applications in hot residential and industrial environments where the use of conventional air conditioning systems becomes prohibitively expensive. Evaporative cooling has been in use for many centuries in countries such as India for cooling water and for providing thermal comfort in hot and dry regions. The evaporative cooling (EC) technology is based on heat and mass transfer between air and cooling water. This system is based on the principle that the moist but unsaturated air loses the sensible heat (gets cooled and humidified) due to evaporation of water from the wetted surface and this cooling effect can be directly or indirectly used for providing thermal comfort. The cooling potential for evaporative cooling is dependent on the wet-bulb depression, the difference between dry-bulb temperature and wet-bulb temperature.
Advantages of evaporative cooling over modern air conditioning:
• Lower equipment and installation costs
• Lower operating and power costs (energy savings can be as high as 75 per cent)
• Ease of fabrication and installation
• Lower maintenance costs
• Ensures very good ventilation due to the large air flow rates involved
• Better air distribution in the conditioned space due to higher flow rates
• The infiltration of outside air is prevented
• Very environment-friendly as no harmful chemicals are used
• Disadvantages of evaporative cooling over modern air conditioning:
• Not applicable when the low humidity level in conditioned space is required
• Create high noise levels in conditioned space due to higher flow rates
• Precise control of temperature and humidity in conditioned space is not possible
• May lead to health problems due to micro-organisms
The classification of the evaporative cooler is presented in Figure 1. The evaporative cooler is in-general two types: direct evaporative cooling (DEC) and indirect evaporative cooling (IEC). A semi-indirect evaporative cooler is the modification of indirect evaporative cooler and the hybrid evaporative cooler is the combination of these two or/and combination with other technologies. In this article, various evaporative cooling technologies and its applicability or suitable selection for various climate zones in India are discussed in detail.
Direct Evaporative Cooler (DEC)
In the direct evaporative cooling, the process or conditioned air comes in direct contact with the wetted surface and gets cooled and humidified. Figure 2 shows the schematic of the direct evaporative cooling system and the process on a psychrometric chart. The unsaturated warm inlet air (1) enters in a pad that is sprayed with water and gets cooled and dehumidified due to the simultaneous transfer of sensible and latent heats between air and water. The heat is transferred by the air stream as sensible heat and is absorbed by the water as latent heat. The temperature of the outlet air (2) decreases due to the sensible heat transferred by the air, but the enthalpy of the outlet air will be the same with the enthalpy of inlet air as the effect of the latent heat recovered into the air as moisture. The working process (1-2) is realised at constant enthalpy as it can be observed on the chart. Again DEC may be active or passive. In the active system, the fan or blower is used for air flow and the pump is used for water flow. The most commonly used evaporative cooling system in north India is the active DEC (also swamp cooler, swamp box, desert cooler and wet air cooler) consisting of water, evaporative pads, a fan and a pump. In the passive system, no external energy is needed (air flow is natural). The use of wetted wick material or pad in window and pond around the buildings are some examples of passive DEC.
For buildings and areas that do not have a central air conditioning system, direct air evaporative cooling can be a very economical and achievable way to reduce the temperature. The main advantage of DEC is represented by the very simple construction of the equipment. If not properly designed direct type evaporative coolers may pose the following problems: The cooled air may be excessively humid, may result in discomfort; The high rate of air flow and a large number of air changes, which are necessary for effective cooling, cause large variation in the air speed and the associated thermal sensation within the cooled space. This results in a waste of energy, which has been used to cool the discharged air.
Indirect evaporative cooler (IEC)
In indirect evaporative cooling (IEC), the indoor air is cooled without any moisture addition (i.e. moisture contentment will remain constant; the temperature will decrease) and hence, the wet-bulb temperature of air decreases. Therefore, the IEC is more effective for humid climate and it is gaining popularity because it cools air more than DEC. IEC may be classified as passive and active. Roof pond is one example of passive IEC. Roof ponds provide cooling benefits through indirect evaporative cooling or radiant cooling. The roof acts as a heat exchanging element which is cooled by evaporation on its surface, long-wave radiation to the sky, or both. It then functions as a heat sink which absorbs indoor heat and the heat penetrating into the building. Since the ceiling is thermally coupled to the roof pond, the interior space is also cooled by radiation and convection. Driving forces behind evaporation and radiation are respectively, the difference between vapour pressure at water surface temperature and vapor pressure of surrounding air and difference between water surface temperature and effective sky temperature. Since the roof acts as a heat exchanging element, roof pond cooling does not elevate the indoor moisture content of the air.
The active IEC involves two air streams: primary air and secondary air as shown in Figure 3. The air directly cooled by water evaporation in the wet channel is called the secondary air. The cool and moist secondary air is used to cool the primary air (the air to be supplied to air-conditioned space) by a heat exchanger. At the outlet, the primary air will have a lower temperature as at inlet, due to the transferred heat. The secondary (working) air is flowing inside the wet channels together with the water. The behaviour of the air and water in the wet channel is similar to the DEC process. The water temperature is the wet bulb (WB) temperature of the secondary air. The heat transferred through the surface between the dry and wet channels is absorbed by the water as latent heat and a corresponding part of the water is evaporated being embedded by diffusion into the secondary air, increasing the moisture content of this air. If the secondary air arrives at the saturation state, after this stage forward the heat from the primary air is split as latent heat absorbed by the water and as sensible heat absorbed by the secondary air. Thus, the temperature of the secondary air at the outlet can be one of the following: (a) Lower than the wet-bulb temperature of the secondary air at the inlet (no saturation); (b) Equal with the wet-bulb temperature of the secondary air at the inlet (saturation is reached at the outlet); (c) Higher than the web-bulb temperature of the secondary air at the inlet (saturation before the outlet). The main advantage of the IEC is that primary air is cooled without modifying its moisture content. The main disadvantage of the IEC is that the cooling process of the primary air is limited by the wet-bulb temperature of the secondary air at the inlet. Because of this limitation, this type of equipment is also named wet-bulb IEC and the efficiency is also lower than DEC.
According to the types of heat exchanger used in IEC, there are tubular type IEC, plate type IEC and heat pipe IEC. In the plate and tubular type IEC, the first air and secondary air are separated by an air-to-air heat exchanger, while in the heat pipe IEC, the condenser section is used in the secondary air flow channel, and the evaporator section is used in the primary air flow channel. Flow arrangement between primary air and secondary air in the conventional IEC may by parallel flow, cross flow or counter flow. To avoid the wet-bulb temperature limitation of conventional active IEC, the sub-wet-bulb IEC was developed to decrease the primary air temperature at the outlet, below the WB temperature of the secondary air at the inlet. In this device, some fraction of primary cooled air is used as secondary air. This device is again two types: regenerative indirect evaporative cooler (R-IEC) and Maisotsenko indirect evaporative cooler (M-IEC). In R-IEC, some fraction of the outlet primary air stream (state 2 in Figure 3) is used as an inlet secondary air stream and there is no mixing in between. Whereas in M-IEC, there are numerous holes distributed regularly between dry and wet channels and hence, the primary air is cooled in the dry side and partially diverted to the wet side through the holes. The lowest possible temperature of the primary air at the outlet of the M-IEC is the dew point temperature of the entering primary air. Therefore, the saturation efficiency of M-IEC based on the inlet wet-bulb temperature can be higher than 100%, and also higher than that of the conventional IEC. The main advantage of the M-IEC is that primary air is cooled without modifying the moisture content almost near the DP temperature. The main disadvantage of the M-IEC is the complex construction and flow scheme inside the equipment.
Semi-Indirect Evaporative Cooler
Semi-indirect evaporative systems with porous media are a successful attempt to improve the efficiency of indirect systems. The semi-indirect evaporative cooler (Figure 4) has two independent air flow supplies, one used for cooling, together with a second, the return air flow, in direct contact with water to favor heat and mass transfer. Water is forced against the return air flow and it is constantly circulating. The cooling effect of the impulsed air would thus be the addition of two processes: the heat exchange between the two air flows (supply and return) plus the heat exchange process, through evaporation, between the air supply and the external wall. The semi-indirect evaporative cooler works with the following mechanisms: heat and mass transfer in the return air flow, the spread of mass due to porosity and heat transport through the solid wall and evaporation or condensation as well as heat and mass exchange in the air flow supply. All of these features are presented together, thus combining heat and mass transfer, increasing the cooling effect of the air to be conditioned and achieving optimisation of the thermal process. Depending on the permeability of the wall of the solid porous cooler which separates the two air flows, there is greater or lower liquid diffusion (water) towards the air flow supply from the external pores, in all cases. The partial pressure of water vapour in the supply air is the controlling factor in this mass transport process.
Hybrid Evaporative Cooler
Several modifications are possible by combining DEC, IEC and other systems (hybridisation), which can improve the efficiency or applicability of the evaporative cooling systems significantly. Hence, the various hybrid evaporative coolers are (i) combined or multi-stage DEC and IEC, (ii) combined DEC and desiccant system, (iii) combined IEC and desiccant system, (iv) combined DEC and refrigeration system, (v) combined DEC, IEC and refrigeration system, etc. IEC yields lower cooling capacity but higher effectiveness as compared to DEC and hence, both devices are combined in a series to get both advantages as shown in Figure 5. This two-stage device is available in the market. Both DEC and IEC are not suitable for high humid conditions. For the high humid conditions, the moisture in the air needs to be removed and the desiccant system or conventional refrigeration system can be used for this purpose. In the combined DEC or IEC and desiccant systems, the solid or liquid desiccant systems driven by solar thermal, waste heat or other heat sources are used to remove the moisture. In the combined DEC and refrigeration system, the water used in DEC is cooled below the dew point temperature by using the refrigeration system, so that the air moisture will condense and hence, the supply air gets cooler and dehumidified. DEC is also used to pre-cool the condenser air of the conventional air-conditioning system.
Climate Zone and Cooler Selection
India possesses a large variation in climate and generally falls under five climatic zones i.e. hot-dry, warm-humid, composite, temperate and cold. Out of these, major areas undergo composite, hot-dry and warm-humid conditions. In the warm-humid climatic zone, the direct evaporative cooler cannot be applied successfully because of the high relative humidity in the summer season of about 80–90 per cent. The indirect evaporative cooler is also not so effective for this zone due to high specific humidity. Thermal comfort in this climate is possible only by using the air conditioner. In the hot-dry climatic zone, the direct evaporative cooling is more applied because of less relative humidity in the range i.e. 30–50 per cent and it has the added benefit of conditioning the air with more moisture for the comfort of building occupants. The major area of India undergoes a composite climate zone. For this zone, DEC is best applied in the dry season (March-May) and it is widely used; however, the IEC can be applicable in relatively humid season (June-July). For the high humid season (August-September), the hybrid system (with desiccant or refrigeration system) has to be used. Hence, we can recommend the cooling option for various climatic conditions as follows: (i) Dry or low wet-bulb temperature – DEC, (ii) Medium humid or medium wet-bulb temperature – IEC, semi-indirect or hybrid and (iii) High wet-bulb temperature – hybrid system. To avoid the use of different options in different seasons of composite climate, a single multi-mode evaporative system can be used, which can operate in a required mode based on the ambient humidity level or wet-bulb temperature. We have developed such a system (dual-mode evaporative cooler) in our laboratory. Now, we can summarise the climate-zone-wise recommendation of cooling options: (i) Hot-dry climate zone (ex. Jodhpur) – DEC or indirect-direct evaporative cooler, (ii) Composite climate zone (ex. Delhi, Varanasi) – Multi-mode system, and (iii) Warm-humid climate zone (ex. Mumbai, Kolkata, Chennai) – hybrid (with desiccant or refrigeration/air-conditioning) system. DEC is the cheapest, followed by IEC, semi-indirect and hybrid systems.
Dr. Jahar Sarkar,
Department of Mechanical Engineering,
Indian Institute of Technology, Varanasi