An Overall Systems View! - not just one efficient system at a time

BEYOND Cogeneration and Trigeneration
-- A brief description
-- A YouTube video

-- A US$65-mi, 65-mW(electric), 50-mW(thermal) cogeneration facility that I led, from its conception to its bidding in 1999

When I went to engineering school, Mr Mendoza, who was our Mechanical Engineering Dean, always gave us assignments that would force us to look at design jobs in "perspective."

There is no way I could forget one design assignment he gave us in 1955: "Design a compressed air plant on top of a mountain. You have three weeks to submit a complete engineering design, complete with feasibility studies, bill of materials, labor, costs, schedules, etc." That was all!

We had to make assumptions, such as, the end-user for the compressed air produced by my plant, our own electric power plant, the details of the road we would build, the number and kind of earth-moving equipment we would rent, etc. We also had to calculate the economics of the whole investment.

In the area of energy, for example, we have terms like enthalpy, and "specific enthalpy".  We know specific enthalpy is the energy content of a pound of air, or water, or steam, etc.  Thus, steam at the exit of a superheater is at a very high enthalpy.  After the steam does work in a turbine, it exits at a lower enthalpy.  It gave up some of its enthalpy doing work in a steam turbine.

Here is where the Engineering Practice needs updating. 

Steam that has done work is still at a high enthalpy.  We engineers remove the remaining energy that the steam carries and then discharge it into the environment as WASTE!  This is what the cycle calls for.

How much is this waste. 

The US Department of Energy's Energy Information Administration DOE/EIA at http://www.eia.gov/forecasts/steo/tables/pdf/2tab.pdf, says that the heating value of No. 6 Fuel Oil is 152,400 BTU/gallon and its estimated price for the year 2013 is 237 cents/gallon.

For the sake of simplicity, let us go to the example at Thermodynamic Analyses of Power Plants, by Rajaratnam Shanthini

http://www.rshanthini.com/tmp/ThermoBook/ThermoChap12.pdf.

After making calculations, resulting in a 39.5% efficiency, the author concludes, on page 294, that: ". . . for each MW of electric power generation there is at least about 1.5 MW of power is wasted" to the condenser.  Let us relate this to the costing of DOE/EIA.  If 39.5% of 237 cents/gallon is converted into electricity (0.395*237=93.6 cents), then the amount of energy rejected by the condenser is (93.6*1.5=140.4 cents).  This amounts to (140.4/237=59%).  For easy recall, I suggest just remembering that more than half of the energy of the fuel is wasted into the environment, through the condenser.

Now, why should the heat be rejected from the condenser, in the first place?

The cycle requires that the steam be condensed back into water so that it could be recycled back to the boiler to generate a continuous flow of steam.  This can happen only if the heat content of the steam is removed.  Way back in Physics, we said that we are removing the latent heat of condensation.

Of course, you really do not have to condense the steam back into water if you have an abundant supply of high quality, mineral-free water.  But you will not realize the vacuum that is developed at the condenser.  And your efficiency is severely affected adversely.

LET US USE A FRACTION OF THIS ENERGY TO DO MORE WORK FOR US. 

• Some send the steam to heat exchangers that send hot water to provide space heating for buildings.
• In addition, some send the steam to provide heating to a manufacturing process.
• Many engineers, these days, use the heat from the steam to "heat pumps."  

The magic is in the use of heat pumps.  We use the WASTE HEAT of the steam as the input energy for heat pumps.  The heat pumps gather energy from the environment and bring it to a higher energy level.  The air-conditioner is one example of a heat pump removing energy from the room to make it cooler, and releasing the heat to the environment at its exhaust.  You will notice that the air coming out of the air-conditioner is warm.  Another example is the household refrigerator.  There are many more applications of heat pumps in industry and commerce, but the majority of them use electricity.  And electricity COSTS MONEY!

The application of the heat pump that I am talking about uses the WASTE ENERGY as an input to heat pumps.  This energy is FREE!  All you need to do is put up the investment to recover this energy, and for the heat pumps, and, there you have it!

Wait a minute, you might say.  We are paying for the investment.  Yes, the investment costs money, but this cost has a PAYBACK, or an Internal Rate of Return (IRR).  In time, it pays for itself, and everything from then on is "gravy", not like electricity that is simply an entry in the expense stream of the profit-and-loss statement.

I want to repeat what the author Rajaratnam Shanthini said  ". . . for each MW of electric power generation there is at least about 1.5 MW of power is wasted" to the condenser   
I spearheaded a project and led a the team that developed a project to use this WASTE energy to drive a system of heat pumps.  I have described the concepts that apply, in the succeeding sections.

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NOTE:  I am still working on this blog.  I will be adding more details in the days and weeks to come.  In the meantime - - -  ENJOY!

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.