Tri-Generation

TRI-GENERATION? WHAT IS IT?
Hardly had the public got used to the expression co-generation and had designers begun plans for equipping boiler rooms with co-generation units, than a new expression, which is far from being a household word, began to appear - tri-generation. Translated, it means the combined production of electricity, heat and cooling; technologically it is about connecting co-generation units with an absorptive cooling unit.

Tri-generation scheme
This is advantageous from the operational point of view of co-generation units, because it allows the use of heat even in summer outside the heating season, and in this way prolongs the annual running time of the unit. It is just this lowered possibility of using heat from co-generation units in the summer which leads to the use of smaller units than would otherwise be suitable. If we are then able to change heat into cooling, nothing stands in the way of co-generation units working to the full even through summer. The produced cooling can be used everywhere where air-conditioning is needed - in banks, hotels, business and administration centers, hospitals, sports halls etc. Air-conditioning equipment can be of two types:
- compressor - an electric motor runs the compressor
- absorptive - run on steam, gas, heat from hot water

The advantage of absorptive cooling (apart from the above mentioned possible connection with the co-generation unit) compared with compressor cooling is that it needs lower quality and even cheaper input heating energy than the more expensive input electric energy for compressor cooling. Absorptive cooling is also quiet, simple and reliable. The disadvantages are mainly the higher investment costs compared with compressor cooling, and bigger dimensions and weight.

The basic principal of the absorptive circuit is the replacement of compression by heat procession in which the cooler absorbs a suitable material (absorbant) at low pressure, then transfers to another exchanger working at higher pressure where the cooler by the input of heat into the liquid is released (expelled) again. The result is a cooler with higher pressure which resembles condensation. What happens in the condenser and evaporator is similar to a steam circuit.

How does it work in practice?

Absorptive cooling has three circuits among which a change of heat occurs. The first is the heating water circuit which is the drive medium for internal heat exchange. This circuit was connected to the heat source, in our case a co-generation unit. The second circuit is a cold water circuit which is directly connected to the cooling circuit - similarly as in central heating, but cold water flows instead of hot water, and which cools the air in rooms - and in it removes heat from the area. The third circuit is the cooling water circuit which removes water with heat for cooling. Cooling most often takes place with the help of cooling towers.

The deciding influence on the size and price of cooling equipment is held by the temperature of the hot water circuit. It is generally true that the higher the temperature of the hot water, the smaller and cheaper is the cooling equipment. Most industrially produced equipment works with a temperature of around 90 to 135°C. The cold water circuit works with temperatures necessary for the removal of heat from rooms, and this is around 7 to 15°C. The cooling water circuit which takes heat away from cooling equipment has temperatures of 20 to 45°C.

Reference:

Tri-generation in Tedom.

In order for test for ourselves the concrete possibilities of tri-generation we decided to air-condition the office area of the Tedom administration building. We installed a York International WFC 10 absorptive unit for cooling, and this was built to use hot water with a low temperature, e.g. connected to a co-generation unit. A Premi 22 co-generation unit with a max. heat output of 45.5 kW works in this case with a heat drop of 95/75°C; individual heating / cooling circuits have these parameters: hot water 95°C, cold water 8°C, cooling (tower) water 24°C. This equipment has been working in our firm since 1998 and proves that tri-generation can be used with low output co-generation units.