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Answer by Dale Blundell (Atkins)
Trigeneration is where some of the heat from a CHP system is used to operate an absorption chiller to generate cooling (or ‘coolth’), thus generating three (Tri) forms of useful energy –, heat, power and coolth.
The refrigeration cycle of an Absorption Chiller (sometimes referred to as an Abo or AC) is similar to the Vapour Compression (VC) cycle of electric chillers. The difference is that in an Abo, the electric compressor of the VC cycle is replaced by a chemical process (absorption), and a heat driven chemical regenerator to regenerate the working fluid, often lithium bromide (LiBr).
The AC requires both a refrigerant and an absorbent. For air conditioning applications (> 00C), the refrigerant is water and the absorbent is commonly lithium bromide (LiBr), which has a strong affinity for water.
The LiBr draws off the water vapour generated in the evaporator, and the water vapour condenses and dilutes the LiBr in the absorber. As the water vapour is absorbed the saturation temperature of the water in the evaporator is lowered and cools the chilled water. A small pump circulates the diluted water/ LiBr solution in the absorber to the generator where an external heat source (e.g. CHP heat) drives off the refrigerant (water), allowing the concentrated LiBr to return to the absorber. In this application, the temperature of the heat supply may range from 80 to 140 0C – the higher the temperature the higher the efficiency. The water vapour then passes through the condenser, where it is condensed into a liquid by giving up its heat and passing through an expansion valve, dropping the refrigerant pressure, before returning to the evaporator.
The coefficient of performance (COP) for an absorption system is defined by;
Energy Out / Energy In = Cooling Duty / Generator Heating
A typical COP for a VC chiller is about 3 to 4, whilst absorption chillers with LiBr/Water mix may typically be about 0.7, and up to 1.0 for double effect chillers. So around 5 times as much energy is required by an absorption chiller to generate the equivalent cooling by a VC system. Since heat is cheaper than electricity, or even ‘free’ where it is the waste heat from a process, then absorption refrigeration can still be beneficial. But where there is a choice between using heat or coolth from the CHP system, the economics normally favour heating.
Whilst capital costs for Abos are higher than VC chillers, their running costs are lower due to the fact that there is only a small pump and some control valves consuming power, and the main energy input is often waste or free heat.
Heat has to be removed from the absorption process, as well as the condenser, and this can be considerably more than for the equivalent VC cycle, maybe up to 2.5 times, requiring larger heat rejection equipment.