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MECH-PHD-05-301

Investigations on Liquid Desiccant based Energy Efficient Bus Air-conditioning Systems

Roshini Rebecca Easow, Ph.D, 05, 203 pp.

Department of Mechanical Engineering

Indian Institute of Technology Bombay, Powai, Mumbai 400 076.

Supervisor(s): Milind Rane

A 45 seat bus operating in humid environment like in the city of Mumbai uses a 7 TR

air-conditioning system. The fuel consumption increases by 82\% in summer and

60% in winter, when the air-conditioning system is in operation. The extra fuel fired

for driving the air-conditioning system alone adds 28 tonnes of CO2 per year per bus

to the already polluted environment of Mumbai (calculated at 2.82 kg CO2/l of diesel

fired). The aim of the present work is to investigate and recommend an improved air-

conditioning system for a bus that is energy efficient and reduces CO2 emissions.

Liquid desiccant based air-conditioning systems were considered for an improved

system since they are heat operated and can utilize the waste heat available from th

bus engine. The essential components of these systems are the absorbers and theregenerators. A liquid desiccant based dehumidification system incorporating a two

stage regenerator was fabricated. It used improved absorber and regenerator using

rotating contacting devices made of aluminium and/or plastic, which have surface

densities of the order of 465 to 600 m2/m3. Testing showed a moisture removal of

2.5 kg/kWh (thermal COP of 1.55) with recovery of potable water from the dilute

liquid desiccant. Different cycles were studied using the data obtained from these

experiments and Hybrid Air-conditioning System (HAS) was identified as suitable for

bus air-conditioning. A Hybrid Air-conditioning System was designed, fabricated and

tested. Use of liquid desiccants improves the Indoor Air Quality (IAQ) since they havethe ability to remove bacterial and other contaminants. The liquid desiccant used was

aqueous Calcium Chloride. The HAS operates with higher evaporator temperature

and lower condensing temperature as compared to the conventional air-cooled

systems. Experimentation on the HAS using a 1.5 TR compressor demonstrated that

the COP is enhanced by 34%.and moisture removal by 166%. For same cooling

load, the size of compressor reduced to 76% of its size when used in a vapor

compression system. Thus, a HAS equipped with a 4.5 TR compressor will be

capable of handling the 7 TR bus air-conditioning load. The HAS is therefore an

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energy efficient alternative to the conventional bus air-conditioning system with

35.7% reduction in CO2 emissions. Key words: bus air-conditioning, liquid

desiccants, dehumidification, two stage regeneration, hybrid air-conditioning