Beyond Cogeneration and Trigeneration - What term to use?

Basic Cycle

In the basic cycle, only about a third of the energy content of the fuel is utilized.  The rest is thrown into the environment as waste.  The chart below shows how the cycle apportions the energy content of the fuel.
 


  
   
 
Cogeneration

This method of utilizing energy from the fuel is very popular.  A prime-mover, usually, gas turbines generate electricity.  Gas turbines are usually chosen because their exhaust gases carry a lot of energy.  The waste heat carried by the exhaust gases is sent to a boiler to generate steam, which is then fed to a steam turbine to generate electricity.
  

  




This system is capable of much higher levels of efficiency when compared to a system operating under a basic cycle.





  

Trigeneration

These systems produce three outputs, namely:

• Electricity
• Process Heat, and 
• Chilling

The US$65million project that I spearheaded operates along this technology.

There was one aspect of that project that advances the concept beyond "trigeneration."  The schematic of the project is given below.  The second stage of that project was conceived to incorporate the heating and drying of produce.  

        

 










Bidders for the project included big names in the energy sector, from Europe, Japan, Korea, and America.

The project was designed to burn very low-sulfur diesel fuel for the first five years, achieving an "overall thermal efficiency of 60%. At the proper time, it would shift to Natural Gas, to raise the overall thermal efficiency significantly.

Beyond these . . . . Part 1

Stage 2 would have been the PRIDE of Energy Engineering.  

• First:  The condenser of the steam turbine was going to be removed, as this sub-system rejects high-enthalpy heat into the environment, as waste.  

• Second:  Lithium-Bromide Absorption systems ("heat pumps") were to condense the steam.  Unnecessary waste of energy is avoided.  The absorption system uses the enthalpy of the steam as its energy source.  Let us keep in mind that the energy from the steam would have been wasted into the environment!  Now it is used as a FREE source of energy.

• Third:  The heat rejected by the heat pumps would be channeled to another process to heat and dry produce, such as, fruit, vegetables, meat, and fish.  Again all this energy would have been wasted into the environment.  Now it is used to perform a function.  This energy is also FREE!

(One side Note:  I say this energy is FREE, and it is!  Whatever expenses are incurred are for building the infrastructure to recover this energy.  These expenses are for systems that eventually pay for themselves.  They have a "payback" and an "internal rate of return" [IRR].  In calculating the financial viability of this kind of investment, the variable input energy cost is ZERO.  This cost would normally be a major cost item.  The investment is now going to be recovered through a "capital recovery fee".  The other elements of the fee structure will be negotiated with, and agreed upon with the end-user.  After the loans are all paid off, everything that comes in will be "gravy"!)

Heat Pumps.  

The improvement of the technology of heat pumps that use waste heat as their energy inputs has revolutionized the way an energy manager thinks.  Suddenly, there is "opportunity" in systems that are already operating very efficiently. For example, a one-megawatt gas-turbine-generator set could now be designed and installed on top of a building.  The GT-generator would supply electricity to the building.  The waste heat from the GT would be used by a "Lithium-Bromide Absorption Chiller", which is a "heat pump" to supply the chilled-water requirements for the building's air-conditioning system.  At the same time, it would be supplying steam to its laundry plant and to its kitchen.  

Heat pumps are evaluated, somewhat differently from motors, engines, and power plants. We might talk about efficiency, but most of the time, we talk of COP, or coefficient of performance.  Why is this so?

A heat pump "transfers energy" from one level to another.  The technical measure is how much energy can a heat pump transfer if it is supplied with one unit of energy?  Fortunately, the ratio of energy transferred to energy supplied is almost always higher than 1 in a mechanical system.  Nowadays, mechanical heat pumps might even deliver a COP of 6!

This is where the beauty of the heat pump lies: with an input of 1 kilowatt-hours, it can transfer 6 kilowatt-hours of energy, for a COP of 6.  Common systems that I saw had COP's of about 3.5.  Let us use 3.5.  If this mechanical heat pump is in an air-conditioning system of a building, 1 kWh removes 3.5 kWh of heat from the building and throws it as waste into the environment.  Take note: this waste is 1 kWh PLUS 3.5 kWh = 4.5 kWh being thrown into the environment.  

A mechanical heat pump still belongs to the traditional efficient system using electricity.  As an Energy Manager myself, I stay away from this system in favor of another option.

Going back to what I said earlier about our Mechanical Engineering Dean in Engineering school, I always try to see things in perspective.  In the example of the air-conditioning system above, I would use a Lithium-Bromide Absorption Chiller with a COP of 1.12.  

Why use a system with a low COP (1.12) as against one with a COP of 3.5?  

My answer runs as follows:

• The building needs electricity.  Generate it, rather than buy it.  
• Now that we are generating electricity, we have a prime-mover that wastes energy into the environment.
• GREAT OPPORTUNITY!  Use this waste energy as the energy input into the Absorption Chiller.  Even if it has a lower COP, the input is FREE!  
• In the case of the mechanical chiller, the cost of electricity will always be a factor, and as the forces of geopolitics change, so will the cost of electricity.  In contrast, after the investment for the energy recovery system is recovered, the operating cost is no longer as tightly tied to external forces.
• One might ask, what would the numbers be if you run them?  
• My answer: They are extremely GOOD!

Look at it another way - the view of the VP for Finance.

• My "Income Statement" contains a row for Revenues and another row for Expenses.
• Electricity costs for the operation of the mechanical chiller is on the Expense row.  In some buildings, you might be talking about an expense of 40% of the electricity cost of the building.
• The VP for Finance would be very happy with me if I removed from the expense stream a portion, say, 60% of the 40%, or 24%!  Don't ask me, just yet, how I would operate the building without the 24%.
• He would exclaim "awesome!", if I told him that in doing so, I would add 84%, or more, to his revenue stream!  A fellow executive colleague of mine, in the past, would say "caramba!", Tris, now you are really stretching it.  No, no, no.  This is real!

Now my explanation:

• When I removed 24% from the expense stream, I removed the expense for the mechanical chiller.  The cooling function of this system would be 24%*3.5(COP), or 84%.  (Note that we are using 24%, which is the assumed value of the electricity cost for running this chiller.  There is some simplification here, but I am interested in showing "trends".)
• The cooling function that would have been supplied by the mechanical chiller will be provided by a Lithium-Bromide Absorption Chiller.
• The capacity of this LiBr Chiller has to be 84%, also, in order to provide the cooling function.  
• If the COP of the LiBr chiller is 1.12, the amount of energy input that it needs is 84%/1.12, or 75%.
• Now, 75% will be coming from the exhaust of a GT-Generator.  
• Let us assume that 75% is 1/4 of what is available from the GT.
• The total heat at the exhaust will therefore be 75%/(1/4), or 300%.
• If the efficiency of the GT is 30%, the 300% corresponds to (100%-30%), or 70% of the fuel input to the GT.
• The minimum capacity of my GT-Generator would be 128% (TotalExhaustOf300% / GTWastageOf70% * EfficiencyOf30% = GT-GeneratorCapacity128%).

What am I saying now, in summary:

• I will replace lighting and other electricity-consuming systems in the building to the extent of 128% (max), meaning I could operate my building independently of the grid.  But I will not do that, although I could, because I have 128%.
• I will negotiate a contract with the power utility for me to feed my excess electricity back to the grid, and he would supply me electricity when I need it.
• There are many models here.  Some are called "feed-in rate", some "net-metering", and others.  
• Some governments have passed legislation compelling power utilities to buy back excess electricity from cogeneration facilities.  Some specify that the buy-back rate would be equivalent to what the utility would sell, if it were to put up its own power plant for that purpose.  This is called the utility's "avoided cost".
• Let us go back to the Revenue stream.  If we are to displace Expense, and still perform the same function, don't we say we have "SAVED" something?  If we have saved something, doesn't this logically belong to the Revenue stream, since the financial viability of the building has already allocated Revenues and Expenses, when a decision was made to construct it?  And since we can even supply our lighting and other electrical needs from our own power plant, then more savings would be inputted into the Revenue stream.
• If the Expense stream has been reduced, and the Revenue stream increased, the net effect on the Internal Rate of Return (IRR) should be good.  Our VP Finance will say, Tris, this is a GORGEOUS project!  

But the story does not end there.

• When we discussed COP above, we said that the Heat Pump will release energy to the extent of "input Energy" PLUS "Energy Transfered", or in the case of a LiBr chiller, this is 1.12 + 1, or 2.12
• Would it make sense to release 2.12 units of energy into the environment?
• Of course, not!  We should find use for this heat.
• What about drying applications?
• What about producing distilled water?
• What about "space-heating" applications?
• What about combinations of these?
• And many more . . . . .

A Heat Pump Schematic Diagram

"Energy Amplifiers" - Heat pumps are sometimes referred to as energy amplifiers.  We have an energy input that enables us to move energy from one level to another - from a lower enthalpy state to a higher one.  

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NOTE TO MYSELF: EDITING TO BE CONTINUED FROM HERE
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Beyond these . . . . Part 2

Working on fuel-based systems leave something to be desired.  One must look to integrate fuel with renewable energy sources.  I will provide schematics, below, to achieve this objective.  

1. The first one has a heavy concentration on fuel.  
2. The second provides an "energy balance" for the first scheme.  
3. The third one is the converse - a heavy concentration on renewable sources.

Please take note of the "energy balance".  Our fuel input was 100 units of energy.  The theoretical work that the overall system does is 288!



This is right along the principles of Physics and Engineering.  By combining one system with another, we develop a more desirable functional system.  In this particular scheme, the amount of work is increased for the same amount fuel input, because we introduced a "heat pump" that took its fuel input from the waste energy of the GT.  By its nature, a heat pump transfers energy from one level to another.  In this scheme, we made it harvest energy from the environment to bring it into the system to do work there.  Since our heat pump takes in (harvests) energy from the environment, the whole system is an "open" NOT an "enclosed" one. 

If I can get more work from a system than my input fuel could give me, then it must be a project worth pursuing.  And that is why we pursued the US$65-million cogeneration facility, with a view of adding more energy-efficient features at its second stage.

Beyond these . . . . Part 3

The diagram above starts with fuel, then produces electricity, chilling, freezing, heating/drying.  At the very end of the process, the system produces distilled water.  At the same time, the system also integrates "Renewable Energy Resources". 

 

The diagram above is a "heat balance" for a desirable configuration.  

























The configuration above is for an "off-grid" or "decentralized energy system" (DES), consisting of a predominantly renewable energy resource base.