In short. Trigeneration extends CHP by using recovered engine heat to drive an absorption chiller, so one fuel input yields electricity, heat and chilled water. In India, where cooling often outweighs heating, it can make sense for hospitals, hotels, pharmaceutical plants and campuses with steady air conditioning or process cooling. The chiller’s low coefficient of performance means the case only works when the heat would otherwise be wasted and cooling demand runs for most of the year.
Why cooling changes the CHP picture
A CHP plant needs somewhere to put its heat. In many Indian facilities the thermal load is mostly cooling, not heating: air conditioning for wards, guest rooms, clean rooms and data halls, and chilled water for process cooling, which in Hyderabad runs for most of the year. A conventional site meets this demand with electric vapour compression chillers, which are often the largest single electrical load. Trigeneration recovers engine heat and feeds it to an absorption chiller, which produces chilled water using heat in place of most of the electricity a compressor would need. The engine’s electrical output then serves the rest of the site while its rejected heat displaces chiller power. Where there is also hot water demand, the heat is shared between the absorption chiller and direct heating according to the season and the hour.
How an absorption chiller uses engine heat
A lithium bromide absorption chiller uses water as the refrigerant and a lithium bromide solution as the absorbent. Heat drives water vapour out of the solution in the generator section; the vapour condenses, evaporates at low pressure in the evaporator to chill water, is absorbed back into the concentrated solution, and the cycle repeats. Electricity is needed only for small solution and refrigerant pumps, though the chiller rejects more heat to its cooling tower than an electric chiller does.
| Chiller type | Heat input | Typical COP | Fit with gas engines |
|---|---|---|---|
| Single effect, hot water fired | Hot water around 85 to 95 °C | About 0.65 to 0.75 | Matches jacket water directly; the most common choice |
| Double effect, steam fired | Steam at about 8 bar | About 1.2 to 1.4 | Needs a waste heat boiler on the exhaust; more cooling per unit of heat |
| Exhaust gas fired | Engine exhaust directly | About 1.1 to 1.4 | Avoids a separate boiler; jacket heat still needs a use |
| Multi-energy | Exhaust plus jacket water | Varies with mix | Uses both heat streams in one machine |
The COP values above are typical ranges and must be confirmed from the chiller manufacturer’s data at the actual heat source temperature and cooling water temperature. In a Hyderabad summer, cooling tower water may be warmer than the design condition and capacity falls accordingly.
Working out what the heat is worth
Q_cool = Q_heat × COP_abs and P_saved = Q_cool / COP_elec
Q_heat is the recovered heat delivered to the absorption chiller in kW, COP_abs is the absorption chiller’s coefficient of performance, Q_cool is the cooling produced in kW, COP_elec is the coefficient of performance of the electric chiller it displaces, and P_saved is the electrical power avoided. Worked example (illustrative): an 800 kW gas engine provides 420 kW of jacket water heat to a single effect chiller with a COP of 0.7. It produces 294 kW of cooling, about 84 tons of refrigeration. If the existing electric chillers deliver that cooling with a COP of 5, the electrical saving is 294 / 5 = about 59 kW, less the absorption chiller’s pumps and the extra cooling tower fan and pump power. That is a modest return on 420 kW of heat. The lesson is that absorption chilling turns waste heat into a small electrical saving and pays only where the heat really is free and the chiller runs long hours. If the site can use the hot water directly instead, that is usually worth more.
Matching three loads at once
Trigeneration has three demands to match: electrical, heating and cooling. Each must be profiled by hour, using real meter and chiller plant data, for representative days across the seasons. Peak figures alone mislead, because a plant sized for the hottest afternoon of May will run part loaded for the rest of the year. The usual approach sizes the engine to the electrical base load, sizes the absorption chiller to the cooling the recovered heat can support, and keeps electric chillers for the balance and as standby. Chilled water storage can help by letting the absorption chiller run steadily while demand varies through the day. The engine and its heat recovery follow the principles in CHP and cogeneration, and the fuel side those described for natural gas generators.
Equipment and complexity
A trigeneration plant adds an absorption chiller, its cooling tower and pumps, heat exchangers, three-way valves and a dump radiator to the CHP plant, plus controls that decide where the heat goes at each moment. Every subsystem adds capital, maintenance, failure modes and operator skill. The control system must handle engine start and stop, heat priority between direct heating and cooling, chiller sequencing with the electric chillers, and safe heat rejection when neither use is available. Absorption chillers also need careful water treatment and protection against crystallisation of the lithium bromide solution on shutdown or low cooling water temperature. These are manageable, but they mean trigeneration needs a stronger case than three outputs sound more efficient than one.
Where trigeneration fits
Good candidates have steady electrical demand, a gas supply, a long cooling season and existing chilled water systems: multi-specialty hospitals, large hotels, pharmaceutical plants with HVAC for clean areas, IT campuses and some food processing sites. Data centres are sometimes proposed but their reliability demands and the generator’s need for maintenance make trigeneration a secondary source rather than the primary one there. Where electricity is cheap relative to gas, the case weakens; where gas is cheap and grid supply poor, it strengthens. Behind-the-meter economics in general are discussed under behind-the-meter generation, and the system integration aspects under integrated power systems.
Common mistakes
- Sizing the absorption chiller to the site’s peak cooling load. The engine cannot supply that much heat and the chiller runs at part capacity.
- Assuming the absorption chiller saves as much electricity as its cooling output. Its low COP means each kW of heat saves only a fraction of a kW of electricity.
- Undersizing the cooling tower. Absorption chillers reject much more heat than electric chillers and capacity collapses on hot days.
- Removing the electric chillers. When the engine stops for service the building has no cooling.
- Neglecting crystallisation protection. A power cut or cold cooling water solidifies the solution and the chiller is out of service for days.
What to send us
Send twelve months of electricity bills, chiller plant data with tonnage, operating hours and chilled water temperatures, any hot water or steam demand records, and the gas connection details. Photographs of the chiller plant room, cooling towers and generator area help us see the space and interfaces. Use the application engineering enquiry and we will screen whether heat recovered from an engine can carry part of your cooling load economically.
Questions engineers ask
How much cooling does a gas engine trigeneration plant produce?
Roughly, each kW of recovered jacket water heat yields about 0.7 kW of cooling in a single effect chiller, and each kW of steam from exhaust heat yields about 1.2 to 1.4 kW in a double effect machine. An 800 kW engine might therefore give 80 to 150 tons of refrigeration depending on how much of its heat is recovered and at what temperature.
Is trigeneration better than CHP?
Only when cooling is the main use for the heat. If the site can use the recovered heat directly for hot water or process heating, that usually displaces more fuel than converting it to cooling. Trigeneration is the right extension when heat demand is small and cooling demand is large and steady.
Can an absorption chiller run on diesel generator heat?
Yes, the heat streams from a diesel engine are similar, and some sites recover heat from prime diesel sets. Diesel is rarely economic for the long running hours trigeneration needs, so most plants use gas engines. Standby diesel sets do not run enough hours to justify heat recovery.
What maintenance does an absorption chiller need?
Vacuum checks and purging of non-condensable gases, solution analysis and inhibitor top-up, tube cleaning, cooling water treatment and pump maintenance. Properly maintained machines have long lives, but neglect of vacuum or water treatment reduces capacity quickly.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.