Vapor Absorption Technology

Knowledge Centre – Passive Strategies

Low-energy cooling systems with environment-friendly refrigerants
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Cooling accounts for almost 50% (or more) of the energy consumption in commercial buildings. Further, refrigerants used in conventional HVAC systems are often ozone depleting substances that are also greenhouse gases. Thus, low energy cooling solutions using environment friendly refrigerants are essential for achieving energy efficiency while providing comfort. This approach is essential for NZEBs.

This webinar discusses one such low energy cooling technology as an alternative to conventional electric chillers – Vapor Absorption Machines (VAM). Mr. Vaidyanathan from Thermax, walks us through the working principle and application of VAMs along with insightful case studies.

Just like a conventional vapor compression-based HVAC system, VAM consists of evaporator, condenser and a thermal compressor. The difference is that instead of the compressor, we have an absorber. There are two combinations of ‘refrigerant:absorber’ that are used – ‘demineralized water:Lithium Bromide (Li-Br) solution’ and ‘ammonia:water’.

The vapor absorption technology relies on 2 properties of fluids:

  1. Evaporation and cooling at low temperatures in low pressure/vacuum environment
  2. Hygroscopy of materials
Fig 2. basic principle 1 of 2

Evaporator and Absorber stages in VAM

Let’s now look at the working mechanism. Cooled refrigerant (demineralised water) is introduced into the evaporator which is maintained at very low pressure/vacuum. It absorbs the heat from the chilled water supply of the HVAC system and evaporates. These vapors are absorbed by the concentrated absorber (Li-Br solution), thereby maintaining the pressure levels in the chamber. The heat that is released from the conversion of vapor to liquid is absorbed by the cooling water pipes that is connected to the cooling tower.

Fig 3. basic principle 2 of 2

Generator and Condenser stages

This diluted absorber solution is then pumped to the generator where it is heated using the driving heat source. This leads to the evaporation of the refrigerant (in this case water) vapors which flows towards the condenser. There, the vapor is condensed back to liquid and the cycle repeats.

A vapor absorption machine (VAM) utilizes this principle along with heat recovery for cooling in a HVAC system. Heat Recovery is the utilization of waste heat energy for the heating purposes of the system. This energy utilization improves the efficiency of the system

Fig 4. CHP System Efficiency

Heat recovery

The reason why vapor absorption machines (VAM) are more energy efficient than the vapor compression refrigeration machines can be mainly contributed to:

  • Utilization of heat recovery for the heating processes
  • Power consumption of the pumps used in VAM being around 1/10th as that of the motors in the compression cycle.

The overall performance of VAM is best understood in the context of ‘net COP’ when compared to electrical chiller. The net COP of electrical chiller, which is a measure of the cooling load generated for the source energy input, is around 1.3, while it is around 1.5 for VAM. Using a natural refrigerant with 0 ozone depleting potential (ODP) and global warming potential (GWP) in this technology, we have a cooling system which is energy efficient and more sustainable.

Fig 1. COP of chiilers

Understanding net COP for electrical chillers

VAMs can be classified on the basis of the heat recovery mechanism used. The different types of VAMs are:

  • Water driven vapor absorption chiller: This chiller uses the hot water as its heat driving fuel. It has a COP of 0.8 and is usually used for smaller cooling capacities.
  • Direct fired absorption chiller: This is the most commonly used VAM. It is usually found in commercial establishments like hotels, hospitals, malls, etc. Fuels like Natural gas, LPG, propane, etc. are used as sources of heat. This system typically has a COP of 1.5.
  • Exhaust gas driven absorption chiller: This VAM utilizes the heat from the exhaust gases. It has a COP of about 1.5 and is used for capacities as high as 3500 TR.
  • Multi Energy Absorption Chiller: This VAM can utilise the heat from multiple sources like exhaust gas, hot water or even a combination of both.
Fig 5. VAMs

Different VAMs from Thermax

Although VAM has a lower COP than an electric chiller, it has several advantages:

  • Works with any thermal source and is not reliant on electrical sources.
  • Utilizes heat recovery for better efficiency.
  • Less power requirements for pumps.
  • Lesser moving parts which translates to quiet operation and no vibration.
  • Less maintenance costs.
  • Has a higher net COP than electric chillers (heat source to cooling energy)
  • Can work on lower loads without affecting COP.
  • Natural refrigerants which have 0 ODP and GWP.

Considering its advantages, VAM is being considered as a viable alternative to conventional electric chillers. Several examples for applications have been discussed in the webinar.

This webinar was conducted on 15th November, 2019

Speaker profiles

Vaidyanathan K S | Product Manager for Absorption Cooling Business, Thermax Ltd

Vaidyanathan K S has been associated with Thermax Ltd for over 15 years and has extensive experience in design, testing, marketing, selling and troubleshooting of absorption chillers. Vaidyanathan holds a bachelor’s degree in mechanical engineering from NIT, Calicut, India and a Postgraduate Diploma in Management from SIBM, Pune, India.

Q&A

Q. Does the COP of VAM chiller include energy consumption of pumps?
Yes, it is including the pumps. The power consumption of the pump is negligible. Even if excluded the COP figure changes by not more than the second or the third decimal point.

Q. Is there a cost to sustaining the vacuum in the evaporator?
No, the vacuum inside the material is sustained by the absorption process. The evaporator vessel is hermetically sealed and has a life of around 20-25 years.

Q. What could be payback if a screw chiller of hundred tons capacity is replaced by the same capacity of VAM chiller?
So the payback depends upon the cost of electricity and the cost of the heat source. If the heat source is waste heat like the jacket water or the exhaust gas, the payback period is usually less than 1 year. But, it depends on case to case.

Q. Does the quantity of the lithium bromide solution depend on the cooling capacity of the system?
Yes, the size of the equipment and the quantity of the absorbent and the refrigerant depends on the size of the equipment.

Q. What are the challenges other than cost for making smaller capacity VAM for domestic applications?
The main challenge is the availability and the cost of the heat source. Another challenge is availability of components like the control system, the pumps etc is small sizes. Due to lack of demand in domestic applications, not much research has been done in miniaturization of these products.

Q. Is it economic to connect VAM to district cooling network in the summer to produce cooling?
Yes. Using smaller cooling solutions (split AC, VRF,etc) is much more expensive than using district cooling using VAM, which can be achieved by using underground pipelines. There are district heating mechanisms using absorption heat plants that have been used in countries like Denmark and Germany for the last 30 years. A similar solution is possible in the cooling field as well.

Q. You were able to achieve 300 kilowatts of savings of connected load by using an absorption chiller plus a heat generator. Was there a whole life cycle study like a Capex versus Opex done for this?
Yes. Investments in Capex model will be higher, but the payback is tremendous. This model already exists in the West as many countries have started moving towards the CHPC systems. It is well established, and the paybacks are well quantified. Another thing to keep in mind is that the electricity provided by the government today is subsidized. Hence, we can only expect the payback period to reduce once the subsidies have been lifted. Also, it releases pressure on the government infrastructure funding as they can do away with power plants and supply gas directly to the consumer.

Q. What is the cost per ton for a VAM system?
Cost per ton of VAM systems depends on the size. It depends on case to case basis and needs to be discussed in detail. But a rough rule of thumb is: 100 TR costs around 25-35 lakhs and 1000 TR will cost around 2-2.5 crores.

Webinar presentation

View webinar presentation – VAM final ppt

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View the recorded webinar here