Recent advances in the art of free piston Stirling engine design make possible the production of 1-10kW free piston Stirling linear alternator engine, hermetically sealed, efficient, durable and simple in construction and operation. Power output is in the form of single or three phase 60 Hz. AC, or DC. The three phase capability is available from single machines without need of external conditioning. Engine voltage control regains set voltage within 5 cycles in response to any load change. The existing SPIKE-2 design has an engine alternator efficiency of 25% at 650 C heater wall temperature and a service life of over three years in solar service. The same system can be scaled over a range of at least 100 watts to 25kW

Beale, W. T.

English

NASA Technical Reports Server

4 
SPTKE-2 - A P r a c t i c a l  S t i r l i n g  Engine For 
K i  l o w s t t  1,cwcl Solar  Power 
Wil i iam T. i<enle  
Sunpowc.r, Inc . 
Atlictns, Ohio L E A  
- ABSTRACT 
Recent  advances  i n  t h e  a r t  of f r e e  p i s t o n  S t i r i i n g  e n g i n e  d e s i g n  make 
p o s s i b l e  t h e  p roduc t ion  of  1-IOkR f ree  p i s t o n  S t i r l i n g - l i n e a r  a l t e r n a t o r  
e n g i n e s ,  h e r m e t i c a l l y  s e a l e d ,  e f f i c i e n t ,  d u r a b l e  and s i m p l e  i n  c o n s t r u c t i o n  
and o p e r a t i o n .  ‘i’hese machines  can  o p e r a t e  e i t h e r  a s  i ndependen t  u n i t s  or 
ganged t o g e t h e r  f o r  h i g h e r  power. They may a l s o  o p e r a t e  d i r e c t l y  connec ted  
t o  t h e  g r i d  wi thou t  need of  a n y  in t e rmed ia ry .  
Power o u t p u t  i s  i n  t h e  form of s i n g l e  o r  t h r e e  phase 60 Hz. AC, o r  DC. 
The t h r e e  phase  c a p a b i l i t y  is a v a i l a b l e  from s i n g l e  machines  wi thou t  need 
of e x t e r n a l  c o n d i t i o n i n g .  Engine v o l t a g e  c o n t r o l  r e g a i n s  se t  v o l t a g e  w i t h i n  
5 c y c l e s  i n  r e s p o n s e  t o  any  l o a d  change. 
The e x i s t i n g  SPIKE-2 d e s i g n  has  t h e  f o l l o w i n g  c h a r a c t e r i s t i c s  and mea- 
s u r e d  performance.  
Weight: - 38kg 
Length  - 50cm 
Diameter - 25cm 
Power - 1.5kW 
Working Gas - hel ium a t  7 b a r  p r e s s u r e  
Engine-A1 t e r n a t o r  E f f i c i e n c y  - 252 a t  650°C h e a t e r  w a l l  t empera tu re  
L i f e  - over  t h r e e  y e a r s  i n  s o l a r  se rv icc .  
Cost  i n  P roduc t ion  - less than  $1,000 
The same sys tem c a n  be  s c a l e d  ove r  a r ange  of a t  least  100 w a t t s  t o  25kW. 
For  l o n g e r  l i f e ,  non c o n t a c t  b e a r i n g s  may be  used w i t h o u t  adding  to  sys-  
t e m  complex i ty  or f a i l u r e  modes. 
INTRODUCTION 
The v e r y  r a p i d  advdnce i n  t h e  a r t  o f  f r e e  p i s t o n  S t i r l i n g  d e s i g n  h a s  re- 
c e n t l y  made p o s s i b l e  t h e  c o n s t r u c t i o n  of t h e s e  mzchines  a t  low c o s t  w i t h  
a t t r a c t i v e  performance c h a r a c t e r i s t i c s ,  making them s u i t a b l e  as  e lec t r ic  
power g e n e r a t o r s  i n  many a p p l i c a t i o n s ,  e s p e c i a l l y  w i t h  c o n c e n t r a t i n g  s o l a r  
c o l l e c t o r s .  The d i s t i n g u i s h i n g  f e a t u r e s  of t h i s  new g e n e r a t i o n  of S t i r l i n g  
machines  are  low p r e s s u r e ,  u s u a l l y  between 5 and 30 b a r ,  r e l a t i v e l y  s h o r t  
s t r o k e  and l a r g e  p i s t o n  d i a m e t e r ,  and r e l a t i v e l y  h igh  f r equency ,  50 t o  60 Hz. 
The d e s i g n  d e s c r i b e d  h e r e  was in t ended  from t h e  beg inn ing  as  a s imple ,  
low cost  and p r o d u c i b l e  free p i s t c n  S t i r l i n g  of  minimum complexi ty ,  b u t  
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wi th  reasonably good over a l l  thermal e f f i c i e n c y  a t  moderate temperatures.  
The  t a r g e t  was 25% a t  a h e a t e r  head t empera tu re  oE 650C. These design g o a l s  
were m e t ,  and t h e  engine is e n t e r i n g  t h e  preproduction prototype s t a g e  with 
a d e l i v e r e d  e l e c t r i c a l  power output  of 1.5kW maximum. Its l i f e  i s  expected 
t o  be a t  least  t h r e e  years i n  s o l a r  s e r v i c e ,  and probably much longer  
ADVANTAGES OF LOW PRESSURE OVER H I G H  PRESSURE FPSE -- -- -
The u s e  of low charge p r e s s u r e ,  about one o rde r  of maknitude l o d e r  t han  
t y p i c a l l y  used i n  t h e  p a s t ,  permits  ve ry  l a r g e  c l ea rances  between s l i d i n g  
s u r f a c e s  and consequent ease of f a b r i c a t i o n  of components. Whereas a 70 bar  
30 hz. engine requires c l e a r a n c e s  on t h e  o rde r  of 10 microns t o  avoid ex- 
c e s s i v e  l o s s e s  from g a s  leakage, t h e  7 bar  engine can accep tab ly  use  over  
50 micron gaps without s e r i o u s  penalty.  T h i s  makes i t  much less succept- 
a b l e  t o  damage from f o r e i g n  par t ic les  i n s i d e , t h e  p re s su re  enclosure.  I n  
a d d i t i o n  t h e s e  loose  f i t s  nake repair and rework of bear ing s u r f a c e s  much 
easier. 
With low i n t e r n a l  p r e s s u r e ,  i t  is p o s s i b l e  t o  more r e a d i l y  use p l a s t i c  
and ceramic components. While t h e s e  are  not  used i n  t h e  e x i s t i n g  proto- 
types,  they may be worked i n t o  t h e  product ion v e r s i o n s  w i t h  consequent ad- 
vantages of lower weight and c o s t ,  and higher  e f f i c i e n c y .  
I n  a d d i t i o n  t o  t h e  s t r u c t u r a l  advant,ges, low p res su re  large diameter  
d e s i g n s  enjoy an a d d i t i o n a l  b e n e f i t  from t h e  f a c t  that t h e  working space 
s p r i n g  e f f e c t  is s t r o n g  enough t o  r e s o n a t e  t h e  p i s t o n  and a l t e r n a t o r  a t  t he  
d e s i r e d  frequency, t h u s  obv ia t ing  the  need f o r  an a d d i t i o n a l  sp r ing .  This  
considerably reduces t h e  system complexity and cos t .  
DESCRIPTION - OF E N G I N E  
Figure 1 shows t h e  e x t e r n a l  appearance of t h e  eng ine -a l t e rna to r ,  and 
Figure 2 t h e  i n t e r n a l  arrangement. 
t h e  d i s p l a c e r  and t h e  c o a x i a l  p i s t o n  w i t h  i t s  a t t a c h e d  a l t e r n a t o r  magnets. 
The hea te r  head i s  t n t e r n a l l y  and e x t e r n a l l y  f inned ,  and t h e  i.eat s i n k  i s  a 
l i q u i d  c i r c u l a t e d  around t h e  c o o l e r  s e c t i o n  which c o n t a i n s  a l a r g e  number 
of gas  flow passap,es. A c e n t r a l  rod provides  a m u l t i p l e  s e r v i c e  of a l i g n -  
ment of t h e  moving p a r t s  and bear ing s u r f a c e ,  as w e l l  as t h e  g a s  s p r i n ,  pis-  
ton f o r  t h e  d i s p l a c e r ,  Teflon based s o l i d  l u b r i c a n t s  are used on a l l  moving 
p a r t s .  
There are only two primary moving p a r t s ,  
A more d e t a i l e d  d e s c r i p t i o n  of t h e  engine is g iven  i n  Reference 1. 
ALTERNATOR C HARACT E R I  ST T C S 
The a l t e r n a t o r  is a very simple dev ice  us ing  a l i g h t  moving magnet 
(Samarium Cobal t )  d i r e c t l y  a t t ached  t o  t h e  pisto:,. Its s t r u c t u r e  a l lows  t h e  
use of convent ional  f l a t  t ransformer laminat ions and makes i ts  f a b r i c a t i o n  
no more complex than t h a t  o t  ord ina ry  r o t a t i n g  machines. 
mass ra t io  is approximately 7kg/kw. 
A l t e r n a t o r  power/ 
A t  t h e  c o s t  of nomfnal i n c r e a s e  i n  complexity,  t h e  a l t e r n a t o r  may be de- 
signed t o  produce t h r e e  phase power. 
i s  p r o p r i e t a r y  and cannot be revealed a t  t h i s  t i m e ,  but i t  results i n  power 
The method by which t h i s  is achieved 
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3 ' -  
o u t p u t  i n d i s t i n g u i s I i a l > l e  F r o m  t h a t  of conven t iona l  t h r e e  1)hdsc r o t a t i n g  ma- 
c h i n e r  y .  
The a l t e r n a t o r  r e sponds  to load  change r a p i d l y  w i t h  t h e  a i d  of a s i m p l e  
c o n t r o l  system. I t  w i l l  r ecover  set  vol tagc.  i n  3 to  4 c y c l e s  a f t e r  and 802 
l oad  v a r i a t i o n .  Newer c o n t r o l  schemes g i v e  promise of r ecove ry  i n  iwo c y c l e s  
from a 100% load  change. The c o n t r o l  system dc 3s n o t  add s i g n i f i c a n i l y  t o  
t h e  e n g i n e - a l t e r n a t o r  complexi ty  or number of f a i l u r e  modes. I n  t h e  reduced 
load  c o n d i t i o n ,  sys tem e f f i c i e n c y  is reduced o n l y  a modera te  amount. 
OPERATINS CHARACTER1 S'TI CS 
The eng ine  is s t a r t e d  by a small e l e c t r i c  impulse  t o  t h e  a l t e r n a t o r  a f t e r  
t h e  h e a t e r  head has  reached  about  450C. A t  t h i s  t empera tu re  i t s  power is 
o n l y  abou t  300 watts, uhLch r a p i d l y  rLses t o  t h e  s e t  power and v o l t a g e  as  t h e  
h e a t e r  head t empera tu re  rises t o  about  550C. As t e m p e r a t u r e  r ises abovc 
t h i s ,  power rises r a p i d l y ,  but  v o l t a g e  is he ld  c o n s t a n t  by t h e  c o n t r o l l e r ,  o r  
i f  t h e  eng ine  is on t h e  e l e c t r i c  g r i d ,  t h e  u n c o n t r o l l e d  c u r r e n t  i n c r e a s e s  
w h i l e  v o l t a g e  remains  a t  t h e  g r i d  va lue .  
Changes i n  s o l a r  i n p u t  s imply change eng ine  power w i t h  o n l y  r e l a t i v e l y  
small changes  of h e a t e r  head t empera tu re .  Engine e f f i c i e n c y  remains  rela- 
t i v e l y  c o n s t a n t  as  d e l i v e r e d  power v a r i e s  between maximum and . ~ K W .  T h i s  
o p e r a t i o n  d o e s  n o t  r e q u i r e  any change i n  eng ine  p r e s s u r e  and hence needs  a 
minimal  c o n t r o l ,  o r  none excep t  a d i s c o n n e c t  i n  t h e  case of  a g r i d  c o n n e c t i o n  
( F i g u r e  3 ) .  
When t h e  s o l a r  i n p u t  is below t h e  minimum, e n g i n e  v o l t a g e  d r o p s  below t h e  
set ( o r  g r i d )  v a l u e ,  and t h e  e n g i n e  i d l e s ,  p roducing  no power u n t i l  t h e  h e a t  
i n p u t  is once a g a i n  adequa te .  The eng ine  w i l l  r u n  down t o  a v e r y  l o w  h e a t e r  
head v a l u e ,  about  80 d e g r e e s  above coo ' an t  t empera tu re ,  b e f o r e  i t  a c t u a l l y  
s t o p s  o s c i l l a t i n g .  T h i s  is  because  of t h e  ex t r eme ly  low mechanica l  f r i c t i o n  
i n  t h e  f r e e  p i s t o n  machine. 
Opera t ion  is ve ry  q u i e t ,  and c y l i n d e r  o s c i l l a t i o n  i s  v e r y  low i n  ampli-  
tude ,  pbout  l m m  excur s ion .  T h i s  low a m p l i t u d e  v i b r a t i o n  can  e a s i l y  be iso- 
l a t e d  by mounting s p r i n g s .  
The c o o l i n g  sys tem may be  mounted on t h e  eng ine ,  o r  on t h e  ground as pre-  
f e r r e d .  Engine e f f i c i e n c y  i s  of c o u r s e  a f f e c t e d  by c o o l a n t  t empera tu re ,  
which should  be  as l o w  as reasonab ly  a c h i e v a b l e ,  u n l e s s  i t  i s  d e s i r e d  t o  u s e  
t h e  ho t  c o o l a n t  f o r  some o t h e r  purpose.  
SPIKE-2 FOR SOLAR US& --- 
I.. 
A SPIKE-2 s o l a r  sys tem can be expec ted  t o  have a n  o v e r a l l  c o n v e r s i o n  
e f f i c i e n c y  of  about  17% when used  w i t h  a good q u a l i t y  c o n c e n t r c t o r  and ab- 
s o r b e r  and a w e l l  de s igned  h e a t  r e j e c t i o n  system. T h i s  i s  h i g h  enough to  
w a r r a n t  c o n s i d e r a t i o n  f o r  u se  i n  some a p p l i c a t i o n s ,  and is a l s o  a t t r a c t i v e  
f o r  expe r imen ta l  o r  e v a l u a t i o n  purposes .  FGL t h i s  sys tem a d i s h  between 
3.5 and 4 meters would be  r e q u i r e d  t o  produce a n e t  1.5kb1 t o  a load .  
Expected c o s t  o f  t h e  e n g i n e - a l t e r n a t o r  i n  r easonab ly  l a r g e  scale  produc- 
t i o n  is about  $800, b u t  t h e  p r  w n z l y  a v a i l a t l e  p r o t o t y p e  c o s t s  much more. 
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SPIKE-2: a Practical Stirling Engine for Kilowatt Level Solar Power

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