Subject: New (?) solar thermal / hydro-electric hybrid

From: Erik Rossen <[email-masked]>
Date: Tue, 21 Oct 1997 01:54:04 +0200

As some of you may know, I've been looking around for a cheap motor
system to
use in high-concentration solar thermal applications.  I haven't had too
much
luck thus far, but I haven't stopped looking.
Anyway, I want to know if anyone has ever heard of the following system
configuration:
1.  A high temperature solar concentrator (dish, heliostats, whatever),
2.  Producing steam at about 6 bars pressure,
3.  Connected by pipe to a filled high pressure water tank, say 2 cubic
meters
in volume,
4.  With a pipe out of the bottom of the tank that feeds a Harris
micro-hydro
electric turbine,
5.  The splash is fed back into a twin high pressure water tank,
6.  When the water in the first tank is exhausted, steam is fed to the
twin and
the water in the twin is used to propel the turbine,
ad infinitum.

Just a few bits of info:  a "high output" 4 nozzle Harris turbine will
do 1500
watts under 200 feet of head with 100 gallons per minute of flow.  The
turbine
costs $995 and a high output DC alternator to go with it costs $200.
This is according to the Solar Living Source Book, 9th ed. by John
Schaeffer
et al.  According to the book, such a system is 40% efficient in
converting
gravitational power (i.e. head x flow) into electrical power.

I guess this is a bit like a water ram, but without the tank on a tower.
Anybody?

Erik.

---

From: David Wells <[email-masked]>
Date: Tue, 21 Oct 1997 06:38:24 -0700

> I guess this is a bit like a water ram, but without the tank on a tower.

Erik-

An interesting idea.  Others may laugh, but I think these sorts of low
cost designs might actually win out over the higher technology ones.

A moment of thought here about the analysis.

When you go take a thermodynamics class, the usual way of dealing with
Rankine cycles in text books is to compute the output work from the
changes in enthalpy of the gas going in versus going out.  This is
correct, of course, but as someone else pointed out here on the
newsgroup, turbines don't usually recover the pv work that occurs during
the boiling of the gas.  They take gas at pressure and volume 1), expand
it through a nozzel to produce a gas with momentum, reduce the momentum
with a turbine blade, which then produces work.

Piston engines and your engine concept do, instead, try to recover the
work done in the boiling process.  It just so happens that the work
produced when a liquid boils is exactly equal to the work that the gas
does on the container volume.  So, your proces starts out delivering to
the system an amount of work equal to the enthalpy change of water
boiling.

The trick to computing things in your design is, I think, in computing
the work INPUT which is required.

I assume that the best usage of the compressed steam in the chamber
after it has expanded would be NOT to feed into a turbine (which would
not produce that much work/lb of steam anyway), but instead, its best
use would be as a means of pre-heating the incoming water going to the
boiling process.  The preheater could be a simple counter-flow shell and
tube heat exchanger.

One final note:

You list the efficiency of the hydraulic turbine/generator as being
around 40%.  This is the MECHANICAL EFFICIENCY, not the THERMODYNAMIC
EFFICIENCY of the combination.  I'm sure you probably all ready know
this.

I recently looked into the mechanical efficiency of gear pumps. 
According to the manufacturer I talked to, the mechancial efficiency of
a typical hydraulic gear motor or pump is around 70%.

Motor efficiency is typically between 60% and 95% for motors in the
sub-horsepower range to a few horsepower.  Three phase motors are
usually about 10% higher in efficiency than single phase motors.

So, all in all, the turbine/motor operating at 40% seems to be a bit low
to me.  The advantage is, of course, that the turbine can take the
water.  But can it take the hot water?

Just some thoughts...

By the way:

I think the U.S. Jet Propulsion Laboratory (JPL) should get off of the
goddamn planet MARS and come back here to good ole EARTH and help save
us from all this techno spam!!

JPL.  Just think about it.  Jet Propulsion.  Not rocket propulsion.  Jet
pumps.  Solar energy.  Makes sense.

-Dave

---

From: Gene Alan Townsend <[email-masked]>
Date: 21 Oct 1997 04:13:00 -0700

> I guess this is a bit like a water ram, but without the tank on a tower.

Eric:

Yes, this will work, but will yield a very low efficiency system.

The main problem is how do you react high temperature steam with low
temperature water without most of the steam condensing?  Answer; a thick
insulating piston.  It all gets very complicated.

No matter what you do, a large fraction of the steam will condense out
in the large water pumping cylinder, reducing efficiency greatly.  This
process is called "initial condensation" in steam engines.

I would look for a water turbine with MUCH better performance (like >80%
>from  small Francis turbines, 70% from converted outboard motor lower
ends).  

Most of the early steam engines were used for water pumping.

Small steam engines tend to have around 5% thermodynamic efficiency.  5%
of 40% is only 2% net efficiency.

I would suggesting sticking with a standard small piston steam engine,
and eliminating the expensive hydraulic transmission.

Regards,

Gene A. Townsend