From: jtkare@ibm.net (Jordin Kare)
Newsgroups: sci.space.tech
Subject: Re: Solar powered Sterling
Date: Thu, 28 Oct 1999 23:39:02 -0700

In article <FJrBw0.1vC@spsystems.net>, henry@spsystems.net (Henry Spencer)
wrote:

> In article <7u878r$s2g@wiscnews.wiscnet.net>,
> CB Hughes <hughesc@msoe.edu> wrote:
> >    I was pondering sterling engines, and possible GOOD applications. I came
> >up with using them as an engine to produce electricity for a space station.
>
> Stirling engines have the problem, for almost any space application, that
> they are relatively heavy for their power output.  The use of reciprocating
> pistons also causes vibration problems; if you must generate power using
> moving parts, continuously-spinning turbines are preferred.

That depends, Henry.  At modest power levels (below a kW or so), Sterling
engines may be preferable, since Brayton-cycle (turbine) systems don't
scale down very well -- RPM's get too high.  There are balanced-piston
designs with damped mountings that generate essentially negligible
vibration.  There's a company that's been funded (not sure by who) to do a
Sterling generator for use with RTG heat sources, and has prototypes
built.  And Clementine took nice pictures despite having Stirling coolers
running on the IR cameras.

The biggest problem with Stirling engines (and Stirling coolers) in the
past has been that they had short operating lives due to sliding contact
between parts at the seals. Doesn't matter if you're cooling a missile
seeker head for a few minutes, but unacceptable for space use.  The recent
designs use very high quality flex bearings (think of the "spider" that
holds a loudspeaker voice coil) and clearance seals.  Don't know what the
current lifetime claims are; the Clementine coolers had 5000 hour MTBF and
I *think* the RTG-driven generator was supposed to run for 10 years.

But at Space Station power levels, yes, turbines will beat reciprocating*
engines every time...

Jordin (piston, or pissed off?) Kare

*I always wondered about those Toshiba laptop computer ads that read
"Turn them on, and they reciprocate."

Why would I want a laptop computer that bounces back and forth?