An absorption heat pump is a heat pump driven by heat instead of a compressor. Rather than compressing the refrigerant mechanically, it lets a liquid absorb it, carries the resulting solution to high pressure with a pump, and then heats it to separate the refrigerant from the solution again. What it mainly needs to run is not electricity but waste heat, steam, hot water or a gas flame. That is why it makes sense in industrial plants, cogeneration plants and district heating networks, where heat is cheap or cannot be used any other way. In homes and most commercial buildings, an electric compression heat pump is far more efficient and practical.
What replaces the compressor?
In a compression heat pump, refrigerant that evaporates at low pressure is compressed to high pressure in the compressor. We describe the four steps of that cycle in our article on what a heat pump is and how it works. In an absorption system the same job is handed to a "thermal compressor" made up of three parts: the absorber, the solution pump and the generator.
The core idea is this: pumping a liquid takes far less work than compressing the same mass of vapour. The vapour is first dissolved in a liquid, pumped as a liquid, and turned back into vapour by heat on the high-pressure side. The solution pump therefore runs on a small electric motor; the system's real energy input is the heat supplied to the generator.
The cycle step by step
- Evaporator: the low-pressure refrigerant evaporates, taking heat from a low-temperature source. If the system is used for cooling, chilled water is produced here.
- Absorber: the vapour is absorbed by the strong solution. Absorption releases heat, which is carried away by cooling water or by the heating circuit.
- Solution pump and heat exchanger: the solution, now rich in refrigerant, is pumped to high pressure. On the way it exchanges heat with the hot solution returning from the generator and is preheated; this heat exchanger noticeably improves the efficiency of the cycle.
- Generator: heat supplied from outside boils the solution and the refrigerant leaves as vapour. The solution, having given up its refrigerant and become strong again, returns to the absorber.
- Condenser: the high-pressure vapour condenses and gives off its heat. In heating mode this heat goes to the heating system.
- Expansion: the condensed liquid refrigerant passes through a throttling device back to the low pressure of the evaporator, and the cycle starts again.
In heating mode, the heating water receives the sum of the heat released in the absorber and the condenser. That sum is greater than the heat supplied to the generator; the difference is the heat collected from the low-temperature source in the evaporator. This is what makes an absorption machine a heat pump.
Two working pairs
Lithium bromide and water
In this pair the refrigerant is water and the absorbent is a lithium bromide solution. Under a deep vacuum, water evaporates at a few degrees Celsius. Because water freezes, the machine cannot go below 0 °C; LiBr–water machines therefore produce chilled water at around 5–7 °C for air conditioning and processes, or hot water at medium temperatures for heating. If the solution becomes too strong it can crystallise, so the control system keeps the cooling water and generator temperatures within that limit. Most absorption chillers used in large buildings and district energy systems are of this type.
Ammonia and water
Here the refrigerant is ammonia and the absorbent is water. Because ammonia can evaporate below zero, this pair suits low-temperature applications such as cold storage and industrial refrigeration, as well as gas-fired absorption heat pumps that take heat from the outdoor air. Since some water evaporates in the generator together with the ammonia, a rectification stage is needed to purify the vapour. Operating pressures are higher than in LiBr–water systems, and because ammonia is toxic, design and placement take this into account. The small, silent, compressor-free refrigerators used in caravans and hotel rooms are a variant of this pair.
The first absorption machine working with ammonia and water was developed in the late 1850s by the French engineer Ferdinand Carré. You can find the wider history of the technology in our article on the history of the heat pump.
Heat pump and heat transformer
The absorption cycle can be set up for two different purposes:
- Absorption heat pump (type I): high-temperature heat (steam, pressurised hot water or a gas flame) is supplied to the generator, heat is also collected from a low-temperature source, and the sum of the two is used at a medium temperature. The heating COP is typically in the 1.6–1.8 range: 1.6–1.8 kWh of useful heat for 1 kWh of driving heat.
- Heat transformer (type II): part of a medium-temperature waste heat is raised to a higher temperature and the rest is rejected at a low temperature. The COP is typically around 0.5, but since the input is heat that would be thrown away anyway, lifting the temperature of that waste heat by a few tens of degrees is worthwhile.
In absorption chillers built for cooling, single-effect machines typically have a COP of around 0.7, and double-effect machines one in the 1.2–1.4 range. Double-effect machines need a higher driving temperature, usually steam or a direct gas flame.
Where does it make sense?
The efficiency of the absorption cycle is lower than that of a compression heat pump; its advantage lies in the kind of energy it uses. Instead of electricity, it uses heat that often cannot be put to any other use. Typical applications are:
- Industrial waste heat: furnaces, drying lines, process steam condensate and hot exhaust gases.
- Cogeneration and trigeneration: the exhaust and jacket water heat of a gas engine is turned into cooling in summer with an absorption chiller.
- District heating networks: when heat demand drops in summer, the network's hot water can be used for cooling buildings.
- Solar energy: with evacuated-tube collectors that can deliver high temperatures; flat-plate collectors often do not reach a high enough temperature.
- Sites with limited electrical capacity: where the grid connection cannot be increased, it shifts the cooling load from electricity to heat.
Absorption machines are usually installed at large capacities. They are heavy, and because they also reject their driving heat, for the same cooling capacity they send roughly twice as much heat to the cooling tower as an electric chiller. The tower grows accordingly; we explain how a tower is chosen in our article on cooling tower selection.
At a plant with waste heat, the first question is at what temperature, in what quantity and at which hours that heat is released. In the Çetin Civata project we installed and handed over the system that recovers the waste heat from the bolt production furnaces and uses it to heat the factory's social areas.
Compared with an electric heat pump
At AD we manufacture electric compression heat pumps. To weigh the two technologies on the same scale, you need to look at the energy per unit:
- Electric heat pump: typically 3–5 kWh of heat from 1 kWh of electricity. Our technical tables, for example, list a COP of 4.38 for the V20 outdoor unit, 4.39 for the W28, and 4.20–5.60 for the Mega central system, depending on operating conditions.
- Absorption heat pump: typically 1.6–1.8 kWh of heat from 1 kWh of heat, plus a small amount of electricity for the solution pump.
If the driving heat is not free, for example if it comes from burning natural gas, the numbers favour the electric heat pump. Even if the electricity is generated at about 50% efficiency in a gas-fired power station, a heat pump with a COP of 4 gets about 2 kWh of heat out of 1 kWh of gas. Absorption heat pumps fired directly with gas, with their combustion losses counted, typically stay at around 1.4–1.6.
When the heat is free, the result reverses: an absorption machine running on heat that would be thrown away anyway produces heat or cooling while using almost no electricity. In short, the choice is about the source more than about efficiency. If there is unused high-temperature heat, absorption; if not, an electric heat pump.
Scale matters too. Absorption systems are not practical in homes and small commercial buildings. AD heat pumps start at 12–16 kW in the climate series and extend to 56 kW with the commercial outdoor units and to 1,200 kW with the Mega central system. The same logic applies to cooling: our electric chiller units can be selected from 5 kW to 2,000 kW. For a comparison of air-, water- and ground-source systems, see our article on types of heat pumps.
For your project
If your plant has waste heat, tell us its temperature, flow rate and the hours at which it is released; if not, tell us the building's heating and cooling loads. We will work out together which electric heat pump or chiller can meet the need. All models are on our heat pump page; for a quote, use the contact form.
