In this guide
An absorption heat pump does something that sounds almost too good to be true: it captures low-grade waste heat — heat too cool to use directly — and delivers it at a useful temperature, driven mostly by heat rather than electricity. For district heating operators, energy-from-waste plants and process industries, it is one of the most powerful decarbonisation tools available today.
Most people have heard of electric heat pumps — the units appearing on the walls of homes across the country. Absorption heat pumps are their large, industrial cousins, and they work on a completely different principle. Instead of an electrically driven compressor, they use the same lithium bromide and water chemistry as an absorption chiller, plumbed to deliver heat rather than cooling. This guide explains how they work, the two distinct types, where they excel, and how they compare with electric heat pumps.
The same cycle as a chiller — used differently
Mechanically, an absorption heat pump is a close cousin of the absorption chiller. It has the same four functional sections — evaporator, absorber, generator and condenser — the same lithium bromide/water working pair, and the same deep vacuum. The difference is entirely in which streams you treat as the product.
In a chiller, the useful output is the chilled water leaving the evaporator, and the heat released at the absorber and condenser is rejected to a cooling tower as an unwanted by-product. In a heat pump, that "rejected" heat is the product: the machine is deliberately arranged to harvest heat at the absorber and condenser and deliver it to a heating circuit. Same physics, opposite intent.
Type I: the heat amplifier
A Type I absorption heat pump — sometimes called a heat amplifier — combines two inputs to produce one larger, more useful output:
- A high-temperature driving source — steam, hot water or direct firing — fed into the generator.
- A low-grade waste heat source — perhaps 20–45 °C from flue-gas condensate, cooling circuits, sewage or geothermal water — fed into the evaporator.
The output is a single, large flow of medium-temperature heat, typically up to about 90 °C, delivered at the absorber and condenser. The number that makes this compelling is the COP of roughly 1.7: for every 1 kW of high-grade driving heat you put in, you get about 1.7 kW of useful heat out. The extra 0.7 kW is harvested, for free, from the low-grade waste stream that was previously being thrown away.
Type II: the heat transformer
A Type II absorption heat pump — a heat transformer — solves a different and subtler problem. Suppose you have plenty of medium-temperature waste heat, say at 80–100 °C, but your process actually needs heat at 120–140 °C. A heat transformer takes that waste stream and splits it: roughly half is lifted higher in temperature by 30–50 °C, while the other half is rejected at a low temperature.
Remarkably, it achieves this temperature lift with almost no external energy input beyond small pumps — the waste heat itself drives the process. The COP is around 0.45–0.5: of the waste heat fed in, about half emerges upgraded to the higher temperature. That sounds modest until you remember the input was free waste heat that would otherwise have been discarded entirely. Half of something free, delivered exactly where you need it, is an excellent deal.
| Type | What it does | Typical COP | Output |
|---|---|---|---|
| Type I (amplifier) | High-grade heat + low-grade waste → large medium-temperature flow | ≈ 1.7 | Up to ~90 °C |
| Type II (transformer) | Medium-grade waste → smaller high-temperature flow | ≈ 0.45–0.5 | +30–50 °C lift |
Why district heating networks love absorption heat pumps
Across Denmark, China and increasingly the UK, absorption heat pumps have become standard equipment on district heating networks. They are installed to do three things:
- Recover flue-gas heat from biomass, energy-from-waste and gas plants — deep condensing recovery that cools the exhaust below its dew point and captures the latent heat in the water vapour.
- Lower network return temperatures, which improves the efficiency of every heat source connected to the network and frees up capacity in the existing pipes.
- Harvest large-scale waste and ambient sources — sewage works, industrial cooling water, data centres — and lift them to network flow temperatures at multi-megawatt scale.
Because the driving energy is heat rather than electricity, absorption heat pumps avoid the large electrical connections, grid-capacity charges and compressor maintenance of electric heat pumps at the multi-megawatt scale — and they use water as the refrigerant, so there is no F-gas compliance burden and no global warming potential. We explore this in detail in our guide to absorption heat pumps in district heating.
Absorption vs electric heat pumps
This is the question every energy manager eventually asks, and the honest answer is that they are complementary, not competing, technologies.
Electric (vapour-compression) heat pumps deliver higher COPs — 3 to 4 or more — per unit of electricity, and are the right answer where clean electricity is cheap and readily available. Absorption heat pumps win in a different set of circumstances:
- Where driving heat — steam, exhaust, burner gas — is cheap or effectively free.
- Where electrical capacity is constrained or expensive, and adding a large electrical load is difficult.
- Where very large capacities (5–50 MW) are needed in a single machine.
- Where low maintenance and long service life (25 years plus) matter, and a simple machine with few moving parts is preferred.
What absorption heat pumps need to thrive
Like their chiller cousins, absorption heat pumps depend on three disciplines: vacuum integrity, healthy lithium bromide chemistry, and clean heat-transfer surfaces. The service regime is essentially identical to that of an absorption chiller — 24-hour vacuum decay tests, annual solution analysis, tube cleaning and controls calibration — which is exactly why a specialist who lives with lithium bromide machines can look after both sides of a plant room that contains cooling and heating duties.
A well-maintained absorption heat pump will run reliably for decades. A neglected one will quietly lose performance as its vacuum degrades and its chemistry drifts, exactly as a neglected chiller does. The technology rewards discipline and punishes inattention.
Is an absorption heat pump right for your site?
The quick test is simple: if your site rejects heat above roughly 25 °C, and buys heat anywhere on site, an absorption heat pump feasibility check is nearly always worth an afternoon. The combination of a low-grade waste source and a demand for higher-grade heat is exactly what these machines are built to bridge, and the numbers are often startling — particularly once carbon costs and the value of freed electrical capacity are included.
Our Heat Recovery Solutions Explorer will show you what a given heat source can produce, and our engineers assess absorption heat pump schemes for sites across the UK and Europe. If you have a waste stream and a heat demand that do not currently talk to each other, they may be closer to a solution than you think.
Need help with your absorption plant or heat recovery project?
Catch Energy services absorption chillers and heat pumps across the UK & Europe — and we answer technical questions for free.
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