Tri-generation is the generation of three energy outputs (namely Electricity, Heat, and Chilled Water) from one single fuel input. Typically, natural gas or diesel can be used as fuel in an on-site Combined Cooling, Heat and Power plant. A Tri-Generation plant would typically consist of three primary components:
- A CHP generation plant for the purposes of converting the fuel input to usable electricity and heat.
- An absorption chiller for the purposes of converting some of the CHP heat output to usable chilled water.
- A controls and distribution system for the purposes of delivering the above energies to the Client’s premises.
What are the benefits of CHP & Tri-Generation?
- Greater displacement of energy costs due to increased utilization of heat
- Reduced electricity costs and demand
- Reduced heating costs
- Reduced cooling cost
- Reduced CO2 emissions
- In-line with company environmental policy
- Security of supply
- Improved building energy rating
An ‘Absorption Chiller’ is a device that uses heat instead of mechanical energy to provide cooling. It does this via a thermal compressor, which consists of an absorber, a generator, a pump, and a throttling device. The thermal compressor of an absorption chiller replaces the mechanical vapour compressor used in electrical chillers. The two most common refrigerant/absorbent mixtures used in absorption chillers are water/lithium bromide and ammonia/water.
The absorption chilling process is as follows;
- In the chiller, refrigerant vapour from the evaporator is absorbed by a solution mixture in the absorber. This solution is then pumped to the generator.
- There the refrigerant re-vaporizes using a waste steam heat source.
- The refrigerant-depleted solution then returns to the absorber via a throttling device.
- The vaporized refrigerant passes to a condenser where it is cooled by circulating water and condenses. It is returned to the evaporator where it is once more evaporated.
Compared with mechanical chillers, absorption chillers have a low coefficient of performance (COP = chiller load/heat input). However, absorption chillers can substantially reduce operating costs because they are powered by low-grade waste heat. Vapour-compression chillers, by contrast, must be motor or engine-driven.
Absorption chillers come in two commercially available designs: single-effect and double-effect. Single-effect LiBr-absorption chillers are able to use hot water (90°C) as the energy source. Double-effect LiBr-absorption chillers need a higher temperature energy source (170°C), but can deliver a higher efficiency. Figure 12 shows a schematic of a single-effect absorption chiller.