Thursday, February 14, 2013

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





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






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

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!