Solar Air Conditioning (SAC) - Guilt-Free Comfort for Summers

 

Early summers usually bring a nice relief from the bone chilling winters, but come April & May, summer time heat starts taking its toll, both physically in addition to mentally. And when it's a tropical climate, as with most parts of India, the new days can be all together stressful. The energy levels of individuals go down and thus does their productivity.

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This is the time the modern day Air-con comes as a God Gift. A really large no. of Offices, laboratories, hotels, public buildings are nowadays fitted with air conditioning systems. Centralized Air Conditioning Systems ensure cooling in most nook and corner with the working space.

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Although this all sound hunky dory, it isn't. Extensive air conditioning is synonymous to heavy power consumption (and wastage) ultimately causing not just heavy power bills but also the dreaded frequent power outages. The next step is - of course - DG sets, to take the desired comfort indoors, while puffing out clouds of black smoke with terribly high degrees of sound pollution to the outside world and the environment to endure.

In the midst of saving ourselves from your fury of the nature, we're inadvertently attacking the more and more, which keeps returning even more furiously every now and then. One of the biggest points of concern thus will be the impending danger due to rising world temperatures (Global warming!!). So what shall we do to survive and survive with comfort?

Let's keep in mind we have an immense power supply in the very Sun that we are shielding ourselves from. One response is Generation of power using sun's energy. It's free as well as for now we can safely assume, unlimited. Yes, solar power can light up our homes and cook our food. But can it also cool our houses? The answer again is absolutely. Solar air conditioning, a new and upcoming technology, provides the answer.

Solar air conditioningrefers to your air conditioning (cooling) system that utilizes solar power. A liquid or even a gaseous substance either accumulates on the outside of a solid, porous substance (adsorption) or perhaps taken up by a liquid or possibly a solid substance (absorption for giving the necessary cooling). In some processes supply air is directly conditioned, i.e. treated with regards to temperature and humidity

This thermally driven cooling and air-conditioning process reaches the heart of every solar air conditioning system. The main principle of Sorption Assisted Ac is shown within the graphic here. The solar energy is used to dehumidify the sorbent.

This can be a 9 stage process. In Stage One or two, Sorptive dehumidification of outside air happens with simultaneous rise in temperature through the freed adsorption heat. Then this air is cooled from the heat reclamation rotor from the countercurrent to the exhaust air (stage 2-3). Air is further cooled (Stage 3 to 4) through evaporation-humidification; Thus mid-air inflow to the building features a lower temperature and less water vapor than the outside air. In Stage Four to five Heating of the air takes place, and if necessary inclusion of water vapor. Then during Stage to building's exhaust air climate is lowered through evaporative cooling in the humidifier. This exhaust air is heated (Stage 7) in the countercurrent to the air inflow inside 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 over 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 air exchange rates. Setting up ducts for the supply air as well as the return air, in addition to a heat recovery unit is necessary. In this, the outside air is dehumidified through sorption which is then brought to the required temperature by heat recovery in addition to direct and indirect evaporative cooling. Usually dehumidifying is achieved inside a rotor containing the desiccant material silica gel or lithium chloride, but increasingly liquid desiccants (desiccant material: lithium chloride) are tried. In these systems the solar heat is necessary for the regeneration with the absorption/adsorption unit.

Closed-cycle desiccant evaporative cooling systems are based on the processes of Adsorption (determined by 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 are usually necessary).

For cooling and acclimatizing of buildings, only solar-thermally driven processes are utilized.

In actual practice, solar collectors are widely-used to convert solar radiation to heat that is then fed in to a thermally driven cooling process or right into a direct air-conditioning process. Especially in special applications, by way of example for cooling medicines in remote areas that aren't 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.

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There has been considerable developments in 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 in all likelihood largest solar thermal based air-conditioning system went operational at Muni Seva Ashram (MSA) in Goraj, located in the Vadodara district of Gujarat state in western India. A total of 100 parabolic dishes (Scheffler type) with the Indian company Gadhia Solar power Systems (GSES) - each 12.5 m2 in space - now provide you with the already existing 100-ton air-conditioning system of the 160-bed hospital at Muni Seva Ashram. [i]

Performance of Solar Ac

While designing a SAC system as well as for judging its performance the important thing points to observe include the Load Profile, Outside temperatures & humidity.

Your choice whether solar cooling is practical at all is influenced strongly from the load profile. Solar irradiation and cooling demand should be correlated, since the usage of fossil fuels for the operation in the chiller is disadvantageous coming from a primary energy standpoint and should be avoided.

To have the highest possible degree of utilization, applications which also require heat for heating water and for the heating system outside the cooling season should be explored. Thus residential buildings and small buildings are most suitable. If cooling is needed throughout the year (process cold, server rooms) the solar air conditioning can be used merely as a fuel-saver for conventional air-conditioning. When this happens free cooling directly with the heat rejection unit may be an option in the cooler seasons.

Heat rejection rate applies to the performance and efficiency with the chiller. In most systems the waste heat is released into the environment by dry coolers or wet cooling towers. The first sort are suitable for moderate climate zones that just occasionally have high outside temperatures (>30°C). Wet cooling towers hold the advantage that cooling water temperatures below ambient temperature can be acquired. However, it only works if the relative high humidity enables evaporative cooling. In regions using a shortage of water it is also unsuitable. In addition, some countries have imposed strict hygiene regulations on wet cooling towers which can make the operation of small systems economically unviable.

It is very important be aware of the fact that boundary conditions besides the nominal conditions, can negatively modify the performance of the cooling machine. If the system is not designed for the nominal operation point, the relevant operating data should be requested from the manufacturer. Then, for example, a dry reverse cycle chiller could also be used in warm climates if the solar thermal strategy is adapted accordingly.

By using efficient EC motors inside the SAC set up helps to reduce the force consumption. To further reduce electricity consumption, these needs to be controlled according to operating conditions and cooling demand. Based on the respective local conditions waste heat can be rejected of in various ways, e.g. through boreholes, swimming pools or ground water. Ideally the warmth can be used in other places.

The main advantages of Solar Air Conditioning

In comparison to conventional, electrically driven compression cooling technology, solar cooling has a no. of advantages. Scalping strategies are most useful if the sun shines the most effective - in summers. The machines generally utilize beneficial to our environment refrigerants, leading to lower greenhouse gas emissions. Generally water is utilized which, in comparison to refrigerants used in compression cooling machines, doesn't have greenhouse potential. Significant power savings can happen from the fact that scalping systems merely require auxiliary energy for that operation of pumps, heat rejection, etc. in the event the systems are designed correctly. As well as cooling, the solar collector system can also provide thermal energy for domestic hot water preparation and heating support which results in 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 may offer a reasonable alternative to the typical ACs. In addition to drastically reducing and even eliminating current air conditioning bill, in many regions, it's possible to receive a huge cash rebate and a tax credit by powering his air conditioner with solar method. As of now, the capital investments are high nevertheless the system can justify its worth over time. It is just a matter of time if this technology would be mass produced, bringing down costs to affordable levels for your common man. Then, the days that pose the most cooling requirements shall be the very same days when maximum energy could possibly be extracted from sun while pocketing all the resulting benefits. The choice would really be ours!

s: Subhash Chandra Mathur is Thermal Power Generation Expert at Lloyd Registers and Quality Assurance UK. and sector Expert at Us Industrial Development Organization (UNIDO). He or she is currently Managing Director at Samarpit International Group (SIG). SIG works in clean and green energy.