Power Generation: Sustainability

[jdahl]

Can someone please explain trigeneration? I understand combined heat & power (CHP) but how is it more efficient to take recovered heat from the engine and provide air-conditioning than traditional HVAC methods?

[Whisky · Accepted solution]

Trigeneration is a misnomer, you are really only converting the fuel to two forms of energy, electrical & thermal (Cogeneration or CHP). Putting waste heat into a Absorption chiller will at best give you a COP of about 1.4 for a double effect chiller and 0.95 for single effect. Compared to an efficent electric chiller with COP >6.  The only time you would consider a absorption chiller is if there is no other use for the Thermal energy this is the case in most commercial buildings and hence the populatity.

[Bilsko]

Not exactly - you are generating three useful outputs from a single fuel input: Electricity, Heating and Cooling - hence the term Trigeneration.   The waste heat that is captured from the production of electricity is then used for space conditioning --either heating or cooling-- and hot water.  

For the cooling (the third element, and the one that distinguishes trigeneration from cogeneration or Combined Heat and Power) you're correct on the absorption chillers - don't forget triple effect chillers that up your COP a bit more than the 1.2 or 1.4 that double-effect chillers get right now.  A COP of 6 for an electric chiller is also a bit unrealistic - most units in operation now run somewhere between 4 or 5 COP.   - Also as an answer to your question, the waste heat is what drives the cooling process in the absorption chillers - which typically employ a mixture of water and ammonia.  There are also small amounts of parasitic power that the absorption chillers require (to run pumps and the like).  As with the heating, the cooling is provided as a complete by-product of generating the kWh - this is why Trigeneration is so attractive:  you take the efficiency of grid delivered electricity (which at best is around 35%  after T&D losses) and bump it all the way up to 80-85% - much much better than grid power. 

You're correct that a given single  commercial building won't have a thermal load big enough to take advantage of all the usable thermal energy that is created while producing kWh - except in cases like Data Centers where the cooling loads for server rooms are a really nice fit for trigneration systems.  Where trigeneration can really shine and become not only financially viable but very advantageous, is when you find complementary loads - a commercial building sited near a hospital (which tend to have nice flat thermal loads) or a residential building, where both the electric load and thermal loads tend to balance the load in a commercial building very nicely.  Very good reasons to opt for a trigen setup - with 1.4 COP absorption chillers  - instead of a standard grid + boilers + electric chiller setup. 

Message Edited by Bilsko on 12-18-2008 01:07 PM