GAS-L: Re: [SOLAR] Fw: [energyresources] Low Heat solar tracker - addendum

["Kermit Schlansker" <kssustain@provide.net>]

        This discussion is about Rankine cycles and solar energy. However
the same material applies to using waste heat from cooling gases in a
gasifier.
         Butane is not only a good expandant, it is also a good refrigerant.
I have been collecting thermodynamic curves for years and have curves on
butane, propane, pentane, ammonia, CO2, most of the old refrigerants, SO2,
air, steam, and several others. A refrigeration chart is also the chart to
use for estimating power and a good refrigerant is always a good expandant.
I picked butane because it was in the right pressure range and is not as bad
as the freons as a greenhouse gas. A heavier vapor would give slower speeds
in a radial single stage expander.
        I don't have a scanner set up so I can't send a chart. This may
change. My computation was made in about 10 minutes so accuracy was not a
goal. I redid this and got a poorer number. expansion at constant entropy,
enthalpy, h out  340-312= 28 btus. The heat input at 350 psi to 250 deg f=
342--165=177. Ratio =.eff=16%. Condenser pressure=50 psi at 90f. I did not
compute pumping power. However usually it is small compared to output.The
efficiency in a system would be considerably lower.  According to the chart
the butane is at the saturation line both at start and finish. Frequently at
the low end the fluid is way out in the vapor region so you have to exchange
heat between the condensing vapor and the pressurized liquid in order to get
efficiency.
         I know nothing about scroll compressors but I think they are
chunky, rather than long and skinny. In refrigeration loss of heat is not so
important because you want to cool during compression. In contrast, the
piston and the multi stage  axial turbine expander, which are historically
the best have the hot end well separated from the cold end. I believe that
bigger cylinders are better and that long skinny cylinders are better. This
means slow speed operation which is not good because gears are needed to
drive an alternator. These remarks probably also apply to IC engines.
        The serial axial turbine expander would be good if you could build a
small one at a low price..My recommendation for a small Rankine expander
would probably be a piston expander with side port with a high speed radial
turbine in the exhaust to catch some more energy. The turbine of course has
to be geared down. Look at old thermo textbooks that show steam engines.
        There is an expensive but excellent book, Solar Energy Handbook" by
Kreider and Kreith which not only discusses expanders a little but has a lot
more good stuff.
        I have a book called "Thermodynamic properties in SI. (What does SI
mean) by WC Reynolds " which I purchased myself at the Stanford book store.
Inside the book it says " To order this book by mail send $12 to
Thermodynamic Properties, Dept of Mechanical Engineering, Stanford
University, Stanford CA 94305-3030T" The price may be higher now or the book
may be out of print. It is in metric units which I don't like but has good
charts for several refrigerants, including butane.

                                         Kermit Schlansker
-----Original Message-----
From: Perisho, Randal [COPE/LEB] <RJPerisho@Copeland-Corp.com>
To: 'kssustain@provide.net' <kssustain@provide.net>
Date: Thursday, January 31, 2002 10:43 AM
Subject: RE: [SOLAR] Fw: [energyresources] Low Heat solar tracker - addendum


>I do not have a butane chart than goes to 350F so I can't easily check your
>numbers.  Some questions I would ask are:
>
>Did you use the change in enthalpy to estimate the work done?
>The initial enthalpy is the saturated vapor at high pressure.  Easy
>The final enthalpy is derived.  Pressure is same as condensor, but you
>assume the entropy is constant.  This will tell you the saturation % the
mix
>is at.  Maybe 50% in your example.  Then you can use the formula h = hf + %
>x hfg.  Right?
>2.  Did you assume the entropy was constant during expansion to derive the
>final enthalpy at the low pressure?
>
>3.  The change in enthalpy will tell you the work per pound mass flow, but
I
>have been told by several people that most expanders only get 60% to 80% of
>this energy. I need to find out if this is because of all inefficiencies in
>the motor and mechanical or just from the expander.
>4.  Did you include the pump work as a loss?
>
>Then to get efficiency,  we take the net work divided by the total heat
>given to the system.
>Did you use all the heat as the divisor including that rejected in the
>condenser?
>
>I would be delighted if the cycle gave an 18% efficiency, but I admit I am
>very surprised.  Would you mind faxing the details to 417-588-8724?  I am
>trying to model the effect of using the scroll as an expander since we make
>them.  Maybe butane would be better than a refrigerant.  I do not
understand
>the thermal short circuit you describe.   The inlet and outlet are at
>different locations and the scroll functions similar to a turbine but at
>limited pressure ratio.
>
>As I do these calculations for a scroll with a pressure ratio of 3 to 4, I
>can only get a 5% efficiency at 270 psi inlet and 90 psi outlet.  It's a
>bummer.
>
>-----Original Message-----
>From: Kermit Schlansker [mailto:kssustain@provide.net]
>Sent: Wednesday, January 30, 2002 6:48 PM
>To: solar-concentrator@cichlid.com
>Subject: Re: [SOLAR] Fw: [energyresources] Low Heat solar tracker -
>addendum
>
>
>            You are mixed up. The numbers that I showed were taken from a
>thermodynamic chart for butane. The 18% efficiency agreed loosely with the
>Carnot efficiency. The actual efficiency would of course be lower because
of
>mechanical and heat exchanger inefficiencies. It is possible that I made a
>mistake but the procedure is straightforward and I have done it many times.
>         I am wondering what the temperature is that these PV cells can
>operate at. If high enough you could also generate power from a Rankine
>cycle.
>
>                             Kermit Schlansker
>
>      Using a thermodynamic chart for  butane as expandant, I came up with
>>the folowing numbers: boiler temp 250F, condenser temp 90F. Phigh 350 psi,
>>Plow 50 psi. Boiler density 5 lbs/cuft, condenser density.55 lbs/cuft
>>>Efficiency, 18%.



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