Sunday, April 10, 2011

Ammonia Absorption Air Conditioning





The problem that is modern civilization


At the begin of the 20th century Mahatma Gandhi warned: The East can meet the West when the West threw overboard almost the whole of modern civilization; the East can also meet the West when the East embraced modern civilization; but that will be an armed truce.

The case for a normal civilization

A mall of a good size like the ones in Bangalore or Mumbai, spends 1300 to 1400 kWhrs(units) of electricity a day. 70 percent of that is for air conditioning and refrigeration- vapour compression type. If they used vapour absorption cooling(with associated lining up of low quality energy), 92 percent of the electricity consumption for air conditioning can be saved. The implications for climate change of this substitution are significant indeed. See an account of the forgotten technology below given by Dr MN Saha in his book on heat published 70 years ago. For a quantitative commercial application on a crash basis, reputed firms and technocrats and managers will surely come forward like Thermax and others. Especially when a power saving of some 50000 MW- fifty thousand megawatts can be effected by such means. Imagine the destructive and unsettling power stations that will not be built, if we have the wisdom to make wise choices.

This follows from the present electricity scene: 120000 MW capacity. Out of which, 80000 MW for air conditioning and refrigeration is the demand. With vapour absorption units instead of vapour compression systems in place, the electricity demand for this would be only 6400 MW! Thus 74600 MW would be transferred to other legitimate uses. The real need would come down to 120000-74000 or just 46000 MW. Six times this would be some 270000 MW by 2030, not 800000 MW! Note that conservation and substitution potentials in other areas have not been even considered in this estimate. Indian industries are pioneers in such applications. See for example how total energy systems are getting credit for demand substitutions in India (Examples: The Tata Chemicals and Bharat Petroleum).

There is an enormous thrust on meeting the shortage of power and energy on the supply side by making efforts to build climate changing, people displacing, death dealing power stations. Far more effective will be an immediate, mandatory and simultaneous prioritised thrust on substituting energy and power guzzling systems by superior methods of energy use and living (in this I include a drastic effort on a war footing on ecologically compatible analogous reforestation with active lead by people’s cooperatives who will also enjoy the fruits of their labour). See also the dramatic but partial cleansing of Delhi by CNG substitution by a mandatory approach.

Ammonia absorption machine.


See figure. The generator A contains a strong solution of ammonia(NH3) gas in water and is heated by the sun. Ammonia gas is expelled from the solution, and passes into the coils immersed in the condenser B, through which cold water continuously flows.

The gas condenses there under its own pressure into liquid ammonia. The liquid ammonia thus formed passes through a narrow regulating valve V to the spiral immersed in the refrigerator C, where on account of the low pressure it evaporates. The valve V is adjusted to maintain the desired difference of pressure on the two sides. Through the refrigerator flows a stream of brine water which becomes cooled by the evaporation of ammonia. The cool brine water is then taken to any place requiring cooling.

The ammonia gas formed in the coils C is absorbed by water or dilute ammonia solution contained in absorber D and thus the pressure is kept low. The solution in D becomes concentrated and is transferred by the pump P to the generator at the top. Thus the solution of concentrated ammonia is kept up. Dilute ammonia from the bottom of the generator may be run to the absorber and concentrated. Thus the cycle is repeated and the action continuous.

The ammonia that is used here could be replaced by other commercial working substances like a CO2 system, with implications for mitigating the unsettling effects of climate change and shortages.

The following properties of the working substance are useful in visualising the process:

Absorption coefficient in water of ammonia at 1 atm is 1305 at 0 C and 195 at 100 C. For CO2 the figures are 1.713 and 0.359 respectively.

The boiling point of ammonia at 1 atm is –33.4 C. Its latent heat of vapourisation is 327 kcal/kg, critical temperature 132.4 C, critical pressure 115.2 atm and density at 0 C and 1 atm is 0.771 g/dm3. For CO2, the corresponding figures beginning with boiling point are: -78.5 C, 137 kcal/kg, 31 C, 75 atm and 1.977 g/dm3.

A commercial machine is shown in the figure below:



References:

Saha M.N et al. 1936. A text book of heat. The Indian Press Ltd., Allahabad. P 142.

TNN and Agencies. 2006. PM on power: India needs 6-fold increase. The Times of India, Mumbai, Saturday, Sep 2 2006


1 comment:

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