Following the pattern of sunshine for sustainable air conditioning

“The thing which makes us to sweat can be used to fulfill the climbing demands of human thermal comfort.”

The increase in population, technology advancement, and living standards has significantly increased the energy demand for cooling devices in last few decades. Almost 15% of world’s total energy is consumed by airconditioning systems. The human thermal comfort conditions are mentioned in term of efficient control of sensible and latent load. The basic function of an air conditioner is to simultaneously control temperature, humidity, and quality of supply air. Generally, in order to provide thermal comfort conditions to the occupants, an air conditioning system should maintain indoor air temperature of 18 – 26oC and relative humidity of 40 – 70%. The accurate and effective control of humidity becomes more crucial for applications where less moist environment is required.

Figure 1: The basic functions of an air conditioner.

The term sensible heat ratio is used to determine the performance of an air conditioner in terms of its ability to control sensible and latent load. Smaller the value of sensible heat ratio larger the value of latent cooling loads. The value of sensible heat ratio is about 0.75 for the commonly used conventional vapour compression air conditioning system. The vapour compression air conditioning system controls the latent load by process of condensation. The air is cooled below its dew point temperature to remove the moisture and then reheated again to desired supply temperature. A considerable amount of energy is wasted during this process of overcooling and reheating which lowers the system overall coefficient of performance. Moreover process of condensation creates an environment for the growth of harmful fungi and bacteria.

Figure 2: Working principle of a vapor compression air conditioner.
Source: Arch Media Group LLC

“The cooling needs for thermal comfort and sunshine follow the same pattern. The demand of air conditioning is higher in summer because sun shines with higher intensity; what if this intensity of sun can be used as an input source of energy for a cooling device?”

The issues related to conventional air conditioning technology can be addressed using a new technology called as desiccant based evaporative cooling. This technology is a combination of a desiccant dehumidifier and evaporative cooler. The only energy used in this system is to drive the fans, water pump, and to regenerate the desiccant dehumidifier. The required energy can be provided using solar thermal collector for desiccant dehumidifier and photovoltaic modules to drive the fans and water pump according to the load requirements. The desiccant dehumidifier controls the latent load whereas evaporative cooler controls the sensible load. Heat recovery medium is used to make the system more energy efficient. For the continuous operation of the system, the regeneration air is heated up to the required regeneration temperature using a solar thermal collector to regenerate the desiccant dehumidifier. The load demand and sunshine follows the same profile which makes this system an effective alternative to conventional air conditioning system. The energy demands for continuous operation of this system can be fulfilled using solar heat according to the load profile.

The use of desiccant cooling technology reduces the energy consumption substantially because of no overcooling and reheating of supply air for moisture removal. More research should be conducted on innovative design of this technology taking the associated investment costs into consideration. The development of technology is in progress and it is attaining stability in the market. It appears to be reliable, safe, and environmental friendly system according to the needs of our society. This technology needs to be developed and more attention is required for its implementation and promotion.

Figure 3: Principle of desiccant based evaporative cooling technology.
Source: https://doi.org/10.1016/j.rser.2015.01.051

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