Early summers usually bring a pleasant relief from the bone chilling winters, but come April & May, the summer heat starts taking its toll, both physically as well as mentally. And when it’s a tropical climate, as in most parts of India, the hot days can be all together stressful. The energy levels of individuals go down and so does their productivity.
This is when the modern day Air conditioning comes as a God Gift. A very large no. of Offices, laboratories, hotels, public buildings are nowadays fitted with air conditioning systems. Centralized Air Conditioning Systems ensure cooling in every nook and corner of the working space.
While this all sound hunky dory, its not. Extensive air conditioning is synonymous to heavy power consumption (and wastage) leading to not just heavy electricity bills but also the dreaded frequent power outages. The next step is – of course – DG sets, to bring the desired comfort indoors, while puffing out clouds of black smoke with terribly high levels of sound pollution for the outside world and the environment to endure.
In the midst of saving ourselves from the fury of the nature, we are inadvertently attacking the nature more and more, which keeps returning even more furiously every now and then. One of the biggest points of concern thus is the impending danger due to the rising world temperatures (Global warming!!). So what shall we do to survive and survive with comfort?
Let’s not forget we have an immense source of energy in the very Sun that we’re shielding ourselves from. One answer is Generation of power using sun’s energy. It’s free and for now we can safely assume, unlimited. Yes, solar energy can light up our homes and cook our food. But can it also cool our houses? The answer again is yes. Solar air conditioning, a revolutionary and upcoming technology, provides the answer.
Solar air conditioningrefers to any air conditioning (cooling) system that uses solar power. A liquid or a gaseous substance either accumulates on the surface of a solid, porous substance (adsorption) or is taken up by a liquid or a solid substance (absorption for providing the necessary cooling). In some processes supply air is directly conditioned, i.e. treated in terms of temperature and humidity
This thermally driven cooling and air-conditioning process is at the heart of every solar cooling system. The main principle of Sorption Assisted Air Conditioning is shown in the graphic here. The solar energy is used to dehumidify the sorbent.
This is a 9 stage process. In Stage 1 to 2, Sorptive dehumidification of outside air takes place with simultaneous rise in temperature through the freed adsorption heat. Then the air is cooled in the heat reclamation rotor in the countercurrent to the exhaust air (stage 2 to 3). Air is further cooled (Stage 3 to 4) through evaporation-humidification; Thus the air inflow to the building has a lower temperature and less water vapor than the outside air. In Stage 4 to 5 Heating of the air takes place, and if necessary addition of water vapor. Then during Stage to 6 building’s exhaust air temperature is lowered through evaporative cooling in the humidifier. This exhaust air is heated (Stage 6 to 7) in the countercurrent to the air inflow in the heat reclamation rotor. The exhaust air is further heated (Stage 7 to 8) through external heat sources (i.e. solar thermal system). Finally in Stage 8 to 9 Sorption Rotor is Regenerated through the desorption of the bound water.
Open and Closed cycle Air Conditioning Systems:
Open-cycle systems are more beneficial in buildings with high humidity loads and high air exchange rates. The installation of ducts for the supply air and the return air, as well as a heat recovery unit is required. In this, the outside air is dehumidified through sorption and is then brought to the desired temperature by heat recovery as well as direct and indirect evaporative cooling. Usually dehumidifying is achieved in a rotor containing the desiccant material silica gel or lithium chloride, but increasingly liquid desiccants (desiccant material: lithium chloride) are being tried. In these systems the solar heat is required for the regeneration of the absorption/adsorption unit.
Closed-cycle desiccant evaporative cooling systems are based on the processes of Adsorption (based on solid desiccants like silica gel or Zeolites with water as refrigerant) or Absorption (where desiccants are liquid ex. material pairs of lithium bromide as desiccant and water as refrigerant, or water as desiccant and ammonia as refrigerant, where sub Zero temperatures may be required).
For cooling and acclimatizing of buildings, only solar-thermally driven processes are used.
In actual practice, solar collectors are used to convert solar radiation to heat that is then fed into a thermally driven cooling process or into a direct air-conditioning process. Particularly in special applications, for example for cooling medicines in remote areas that are not connected to the grid, a photovoltaic generator transforms solar radiation to solar power that then drives a refrigeration process – usually in the form of a compression chiller.
There have been considerable developments in the field of solar-assisted heating and cooling and world over this technology is being rapidly tried and adopted.
Recently Solar thermal World Magazine reported that India’s latest and probably largest solar thermal based air-conditioning system went operational at Muni Seva Ashram (MSA) in Goraj, situated in the Vadodara district of Gujarat state in western India. A total of 100 parabolic dishes (Scheffler type) by the Indian company Gadhia Solar Energy Systems (GSES) – each 12.5 m2 in size – now supply the already existing 100-ton air-conditioning system of the 160-bed hospital at Muni Seva Ashram. [i]
Performance of Solar Air Conditioning
While designing a SAC system and for judging its performance the key points to observe are the Load Profile, Outside temperatures & humidity.
The decision whether solar cooling is sensible at all is influenced strongly by the load profile. Solar irradiation and cooling demand must be correlated, since the use of fossil fuels for the operation of the chiller is disadvantageous from a primary energy point of view and should be avoided.
In order to achieve the highest possible degree of utilization, applications that also require heat for heating water or for the heating system outside the cooling season must be explored. Thus residential buildings and small office buildings are most suitable. If cooling is required throughout the year (process cold, server rooms) the solar cooling system can be used merely as a fuel-saver for conventional air-conditioning. In such a case free cooling directly through the heat rejection unit might be an option in the cooler seasons.
Heat rejection rate greatly affects the performance and efficiency of the chiller. In most systems the waste heat is released into the environment by dry coolers or wet cooling towers. The former are suitable for moderate climate zones that only occasionally have high outside temperatures (>30°C). Wet cooling towers have the advantage that cooling water temperatures below ambient temperature can be obtained. However, it only works if the relative high humidity allows for evaporative cooling. In regions with a shortage of water it is also unsuitable. In addition, some countries have imposed strict hygiene regulations on wet cooling towers which make the operation of small systems economically unviable.
It is essential to be aware of the fact that boundary conditions other than the nominal conditions, can negatively affect the performance of the cooling machine. If the system is not designed for the nominal operation point, the relevant operating data must be requested from the manufacturer. Then, for example, a dry reverse cycle chiller can also be used in warm climates if the solar thermal system is adapted accordingly.
Utilization of efficient EC motors in the SAC set up helps to reduce the energy consumption. To further reduce electricity consumption, these should be controlled according to operating conditions and cooling demand. Depending on the respective local conditions waste heat can also be rejected of in other ways, e.g. through boreholes, swimming pools or ground water. Ideally the heat can be used in other places.
The Advantages of Solar Air Conditioning
In comparison to conventional, electrically driven compression cooling technology, solar cooling has a no. of advantages. These systems are most useful when the sun shines the strongest – in summers. The machines generally utilize environmentally friendly refrigerants, leading to lower green house gas emissions. In most cases water is utilized which, compared to refrigerants used in compression cooling machines, has no greenhouse potential. Significant power savings can result from the fact that these systems merely require auxiliary energy for the operation of pumps, heat rejection, etc. if the systems are designed correctly. In addition to cooling, the solar collector system can also provide thermal energy for domestic hot water preparation and heating support which leads to a further reduction of emissions. SACs are also ideal for hotels, supermarkets, schools, factories, and large office premises etc. Furthermore, noise emissions are significantly lower since the machines work without compressors.
Solar based AC systems thus can offer a reasonable alternative to the conventional ACs. In addition to drastically reducing or even eliminating current air conditioning bill, in many regions, one can receive a huge cash rebate as well as a tax credit by powering his air conditioner with solar air conditioning systems. As of now, the capital investments are high but the system can justify its worth in the long run. It is just a matter of time when this technology would be mass produced, bringing down costs to affordable levels for the common man. And then, the days that pose the maximum cooling requirements shall be the very same days when maximum energy could be extracted from sun while pocketing all the resulting benefits. The choice would really be ours!