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

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

Erik Rossen wrote:
> 
> 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?

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