# Does a vapour absorption chiller run on waste heat cut emissions compared with an electric chiller?

Canonical: https://senseible.earth/answers/vapour-absorption-chiller-waste-heat-emissions

Yes, when the heat is genuine waste that would otherwise be lost: you save most of the electric chiller's kWh and pay only for pumps and a bigger cooling tower. If a boiler burns fuel to make that heat, a good electric chiller usually emits less: absorption gives only 0.7 to 1.2 units of cooling per unit of heat.

## Why the heat source decides everything

An electric chiller at 0.65 kW per TR delivers about 5.4 units of cooling per unit of electricity (3.517 / 0.65). A vapour absorption machine (VAM) delivers about 0.65 to 0.7 units of cooling per unit of heat in a single effect machine and about 1.2 in a double effect one, which needs higher grade heat (CIBSE Datasheet 7, absorption cooling). That trade only pays in CO2 when the heat costs no extra fuel: DG or gas engine jacket water and exhaust, excess low pressure steam, hot condensate or process waste heat.

## The test for genuine waste heat

- The heat was being vented, dumped to a cooling tower or lost before the VAM came in.
- No boiler fires harder because of the VAM, in any season.
- If the waste heat runs short and a backup burner tops it up, that fuel counts against the VAM.

## Costs people forget

- The cooling tower rejects the cooling load plus the driving heat, so a VAM needs a larger tower, more pump power and more make-up water than an electric chiller of the same TR.
- Solution pumps and vacuum pumps draw a few kW.
- Part load COP falls, and a VAM needs steady heat to hold its rating.

## How to report the change

Electricity avoided lowers location-based Scope 2 at 0.710 kg CO2 per kWh. Any fuel burned for the VAM is Scope 1. Report both lines rather than a single net saving, so a reviewer can see the heat source assumption.

## Illustrative example: 200 TR for 5,000 hours (1,000,000 TR hours)

- Electric chiller at 0.65 kW per TR: 650,000 kWh; CO2: 650,000 x 0.710 = 461,500 kg
- VAM auxiliaries, pumps and extra tower power at 0.15 kW per TR (assumed): 150,000 kWh; CO2: 106,500 kg
- On genuine waste heat: saving = 461,500 minus 106,500 = 355,000 kg, about 355 t a year
- On heat from a furnace oil boiler: cooling 1,000,000 x 3.517 = 3,517,000 kWh; heat at COP 1.2 = 2,930,833 kWh; fuel at 85% boiler efficiency = 3,448,039 kWh = 12.41 TJ
- Fuel CO2 at 77.4 t per TJ (IPCC default, residual fuel oil): 12.41 x 77.4 = 961 t, plus 106.5 t for auxiliaries, against 461.5 t for the electric chiller

kW per TR, COP and boiler efficiency are assumptions; use your data sheets and measured hours.

## What Senseible does here

Senseible reads electricity and fuel bills into one monthly record, so the shift from chiller kWh to heat, and any extra fuel, shows on the same page.

- [Chiller efficiency in kW per TR](https://senseible.earth/answers/chiller-efficiency-kw-per-tr)
- [Emissions saved by waste heat recovery](https://senseible.earth/answers/emissions-saved-by-waste-heat-recovery)
- [Chiller plant emissions per TR hour](https://senseible.earth/answers/chiller-plant-emissions-per-tr-hour)

Sources: [GHG Protocol Corporate Standard](https://ghgprotocol.org/corporate-standard), [2006 IPCC Guidelines, Volume 2 Energy (residual fuel oil default)](https://www.ipcc-nggip.iges.or.jp/public/2006gl/vol2.html), [CEA CO2 Baseline Database, version 21.0](https://cea.nic.in/cdm-co2-baseline-database/?lang=en)

Last reviewed 2026-10-18.