Gas Absorption Heat Pump: Overview, Construction, Working, Types, Advantages, Disadvantages, and Applications
We all know that heat travels from high to low temperatures. But the heat pump works just the opposite of it. It transfers heat from a low-temperature source to a high-temperature source when a certain amount of energy is supplied.
The heat pump draws heat from the outside air and pumps it inside the room so that you feel warm. Hence it is also known as the winter air conditioner.
The heat pump mainly comprises the compressor, condenser, throttle valve, and evaporator. The evaporator here will act as an outdoor unit, and the condenser will be an indoor one. The compressor’s low-pressure and low-temperature refrigerant is compressed into high-pressure and high-temperature refrigerant.
It then enters into the condenser, where it exchanges with the air surrounding your room and is condensed. The condensed refrigerant enters the expansion valve, where its pressure and temperature are drastically reduced. It then enters into the evaporator.
The refrigerant absorbs heat from the surrounding air, gets heated, and converts it into vapor. It finally makes its way to the compressor. Thus the cycle continues, and your room gets heated up.
The gas absorption heat pump uses almost the exact mechanism as the heat pump. But here compressor is not used. It means the gas absorption heat pump does not run on electricity. It is driven by a heat source like natural gas or waste gas to pump heat from low-temperature to high-temperature sources.
Gas absorption heat pumps are more efficient in comparison to vapor compression heat pumps. They are even quiet to operate as there is no compressor. These heat pumps can be used domestically and for industrial purposes.
Figure 1:-
Construction Of Gas Absorption Heat Pump
The gas absorption heat pump usually consists of the following components.
1. Evaporator
The refrigerant which flows inside the evaporator absorbs heat from the ambient air and converts it into vapor.
2. Absorber
There is an absorbent in the absorber, which absorbs the refrigerant.
3. Condenser
The condenser condenses the refrigerant into liquid form. It is a heat exchanger where the refrigerant flowing in the condenser tubes will exchange heat with the air inside the room.
4. Receiver
The condensed liquid collects in the receiver.
5. Expansion valve
The expansion valve drastically reduces the temperature and pressure of the refrigerant.
6. Generator or Desorber:
Here the absorbed refrigerant is heated by an external source. The heating or external source includes natural gas or waste gas.
Working Of Gas Absorption Heat Pump
The vapor compression heat pump uses a compressor to heat your room. The gas absorption heat pump can also heat your room, but there is no use for mechanical power.
Here the compressor is replaced by an absorber, generator, pump, and pressure-reducing valve. The gas absorption heat pump includes other components like the evaporator, condenser, and expansion valve. The refrigerant commonly used here is either ammonia or water.
Ammonia is not an ozone-depleting gas, i.e., it will not deplete the ozone layer on release to the environment. So it is perfectly safe for the environment. But remember, this gas is toxic and flammable; hence, the units are hermetically sealed and present outside.
Let’s discuss the working principle of an gas absorption heat pump.
Residential gas absorption heat pumps mainly use ammonia as refrigerant and water as the absorber.
The refrigerant (ammonia)in the evaporator absorbs heat from the ambient air and gets heated. The low-pressure ammonia vapors move into the absorber, where they will be absorbed by the absorbent(water) and form aqueous ammonia.
A strong solution of ammonia vapors and water is thus formed in the absorbed. It is then pumped into the generator. An external source in the generator heats the strong solution of ammonia vapors and water.
The external source can be natural gas or steam. The ammonia vapors will be released to the condenser at high pressure and cooled to liquid ammonia. The condenser is a heat exchanger where the ammonia vapors exchange heat with the air inside your room.
Now the liquid ammonia will be collected in the receiver.
The weak ammonia solution(more water and fewer ammonia vapors) from the generator will reach the absorber by pressure reducing valve.
The high-pressure liquid ammonia collected in the receiver passes into the expansion valve, where its pressure and temperature are reduced drastically.
So finally, the low-pressure and low-temperature liquid ammonia moves into the evaporator, where it absorbs the heat from the outside air, changes its phase, and then moves to the absorber.
The process continues repeatedly and helps to heat our room continuously.
Types Of Gas Absorption Heat Pump:
There are mainly two types of gas absorption heat pumps. We are listing them below.
1. Single Effect Gas Absorption Heat Pump
Image:- Same as figure 1.
In a single-effect gas absorption heat pump, there is only one cycle of absorption and desorption. It is mainly used in places where high temperature is not required, like space heating.
Let’s talk about a lithium bromide gas absorption heat pump. Here water is used as a refrigerant and lithium bromide as an absorber.
The water in the evaporator absorbs heat from the ambient air and turns into vapor. It is further absorbed by lithium bromide present in the absorber.
Now the lithium bromide solution is pumped to the generator(desorber), where any external source heats it. Water gets heated and converted into vapor, whereas lithium bromide settles down. The generator sends the lithium bromide back into the absorber.
The water vapor generated in the evaporator makes its way into the condenser, giving off its heat to the air inside your room and condensing into liquid form.
The water(refrigerant) enters the throttle or expansion valve, where its pressure and temperature decrease considerably.
Now the low-temperature and low-pressure water will move to the evaporator, where it will absorb heat from the ambient air, get heated up, and then move to the absorber.
So the cycle continues repeatedly, and continuous heating is achieved inside the room.
2. Double Effect Gas Absorption Heat pump
Double-effect gas absorption heat pumps are more efficient than single-effect gas absorption heat pumps. They are mainly used in places where high-temperature requirement is necessary.
Here there is the use two generators (high-temperature and low-temperature generators) and two heat exchangers. Water will also act as a refrigerant and lithium bromide as an absorber.
The working principle is as follows.
The water(refrigerant) in the evaporator will absorb heat from the ambient air and convert it into vapor form. The lithium bromide absorbs the water vapors, and thus a strong solution of lithium bromide and water. Here strong solution means a high percentage of water(refrigerant) and a low percentage of lithium bromide.
Two heat exchangers pump The strong solution into a high-temperature generator. An external heat source will heat the strong solution in the high-temperature generator to create water vapors.
The weak solution from the high-temperature generator enters the low-temperature generator by passing through a heat exchanger and expansion valve.
The strong solution(more water vapor and a very less percentage of lithium bromide) will also make its way to the low-pressure generator.
The hot water vapor will be condensed in the low-pressure generator, and its released latent heat is enough to heat the weak solution. This stream will finally move into the condenser. The weak solution from the low-pressure generator will move into the absorber through the low-temperature heat exchanger.
The strong solution from the low-pressure generator will move towards the condenser and condense into liquid form. It then moves into the expansion valve, where its pressure and temperature reduce drastically. Low-temperature and low-pressure refrigerant(water) will move toward the evaporator. Here, the water will again absorb the heat from the ambient air, heated up, and then move into the absorber.
Thus the cycle continues repeatedly, and your room is heated continuously.
It is a very complex process and is quite expensive to install.
Advantages Of Gas Absorption Heat Pump:
1) In a gas absorption heat pump, the heating and cooling input temperature can be managed better.
2) No electrical demand which makes it a good option for areas where electricity is not available or have disrupted electricity supply.
3) It has a much quieter operation as compared to the electrical heat pump.
4) It has much longer life span.
Disadvantages Of Gas Absorption Heat Pump:
1) it is expensive to use as natural gas is not cheap.
2) Initial installment cost is also high.
3) It has lower efficiency at higher temperatures.
4) It has a lesser cooling capacity than a traditional electric air conditioning system.
5) Requires large space to be installed.
Applications Of Gas Absorption Heat Pump:
1) It is used for various residential and commercial heating and cooling.
2) It is also used in various industrial processes.
3) It can be used for waste heat recovery. These heat pumps can recover waste heat from various sources like exhaust gases from industrial processes and use it to generate useful heating and cooling.
Conclusion:
The gas absorption heat pump has many benefits like they are energy efficient and environmentally friendly compared to traditional compression heat pumps. They have few moving parts, leading to a long life span and reduced maintenance.
We have discussed construction, working principles, and types of gas absorption heat pumps and hope the information is helpful.
