Spiral ground heat exchangers for heat pump applications

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Spiral ground heat exchangers for heat pump applications

R e f

T H 1

N 2 1 d

+r(

Natlonal Research

_{Council Canada}

ConseI

_{de racherches Canada}

national

n. 1685

1 9 9 0

'

Institute for

lnstitut de

B L D G .

Research in

recherche en

- -.+ . - -

Construction

construction

IKC PUB

Spiral Ground Heat Exchangers

- - -

for Heat Pump Applications

by Otto J. Svec

/I'

ANALYZED

Reprinted from

Proceedings of the 3rd International Energy Agency

Heat Pump Conference, Tokyo, Japan

12-15 March, 1990

pp. 443-453

(IRC Paper No. 1685)

NRCC 32355

N R C

-

ClSTl %+

I R C

- + % >

L I B R A R Y

-$

* > i

APR

25

1991

'.

B I B L I O T H ~ Q U E

I R C

CNRC

-

IClST

-

+ _

ResumC

Ce document dCait un nouveau systeme horizontal d'echange de chaleur sur

sol utilid dans une installation exp6rimentale de pompe B chaleur captage

au sol (PCCS).

Le

principe consiste

B

activer une grande masse de sol en

employant de gros 6changeurs de chaleur B spirale (pouvant atteindre

60 an)

faits de tubes de cuivre ou de polybthylene haute densite. Le systhme a Ct6

contr8lC au cours de l'hiver

1988-1989,

et sa performance s'est r6vCl&

excellente. Les rbultats obtenus alors et notre experience indiquent qu'il est

possible de faire des konomies substantielles sur les coots initiaux

&installation.

Il

se peut en outre que ces systemes horizontaux soient dans la

plupart des cas plus bnomiques que les systhmes verticaux de PCCS.

+ .

*

Spiral Ground Heat Exchangers for

Heat Pump Applications

Otto J. Svec

I n s t i t u t e f o r Research i n Construction National Research Council of Canada

Ottawa, Ontario, KIA OR6, Canada

ABSTRACT

T h i s paper describes a novel horizontal ground heat exchange system uaed i n

an

e x p e r h a n t a l Ground Source Beat Pump (GSES) i n s t a l l a t i o n , The b a s i c concept i a

t o

a c t i v a t e a large mass o f s o i l b y employing

"largen ( u p to 60 cm) s p i r a l heat exchangers made of copper and high density pclyethylene tubing. The results o f monitoring the system during the 198&/$9 winter season demonstrate its excellent performance. I n addition, t h e s e r e s u l t s together with o u r experience,

i n d i c a t e t h a t Large p o t e n t i a l savings on i n i t i a l i n s t a l l a t i o n c o a t s a r e possible. Moreover, s u c h horizontal systems may be

more

economical t h a n v e r t i c a l GSHP systems i n most p r a c t i c a l s i t u a t i o n s .

KEYWORDS

Heat pump, ground, heat exchanger, s p i r a l

,

1) Introduction

The GSBP system is recognized as the moat successful

of

all renewable

energy technologies i n most Northern European a n d North American

* c o u n t r i e s . The GSRPrs high initial cost i s t h e only factor slowing

-

down its wide-spread penetration i n t o the marketplace. A l a r g e p a r t o f t h i s cost, e . 30-40% land

i n

some casea sven more), i e associated with t h e i n s t a l l a t i o n of ground heat exchange systems.

The main

objective

of

t h e research program a t the Institute f o r

Research i n

Construction,

N a t i o n a l

Research Council Canada (IRC/NRC)

- has been t o develop a highly efficient ground heat exchanger. The

goal has bean t o reduce t h e xequired length of a ground c o l l e c t o r

through improved e f f i c i e n c y , and t h e r e f o r e

t o

lower t h e o v e r a l l coat of GSHPs. Efforts have been successful and t h e ground hest exchanga technology developed

in

IRC/NRC I s now ready t o be t r a n s f e r r e d t o t h e GSHP industry.

2 ) The NRC Ground Heat Exchanaer

Research work a t IRC/NRC, which p r o g r e s s e d from l a b o r a t o r y s t u d i e s , Ref. 1, t h r o u g h t e s t i n g p r o t o t y p e s i n t h e f i e l d , R e f s . 2,3, and

.

f i n a l l y t o a f u l l s c a l e t e s t i n g of GSHP systems i n two e x p e r i m e n t a l houses, Ref. 4 , r e s u l t e d i n a new ground h e a t exchanger d e s i g n . I t s T- main f e a t u r e i s t h e s p i r a l shape of t h e h e a t exchanger i t s e l f and a s t r a i g h t t u b e r e t u r n , F i g 1.

BACKFILL

SAND

PLASTIC

33

mrn

D!A

150-1 80

m m

F i g . 1. H o r i z o n t a l S p i r a l Ground Heat Exchanger I n s t a l l e d I n A D i t c h

Such a heat exchanger can perfoxm well,

i n

both vertical and r h o r i z o n t a l c o n f i g u r a t i o n s . I n this paper, o n l y t h e performance of t h e h o r i z o n t a l c o p p e r and p l a s t i c s p i r a l heat e x c h a n g e r s i a d e s c r i b e d . T h i s t e c h n o l o g y has been d e v e l o p e d t o such a degree,

p a r t i c u l a r l y the plastic

60 cm

( 2 4 " ) s p i r a l , t h a t it can be r e a d i l y

manufactuxed a n d used

i n c o m e r c i a l

a p p l i c a t i o n s p r a c t i c a l l y w i t h o u t - modifications. The s i m p l i c i t y of i t s d e s i g n and i n s t a l l a t i o n t e c h n i q u e should make this ground h e a t c o l l e c t o r technique superior

.

t o any o t h e r c u r r e n t l y i n use.

There a r e s e v e r a l key a d v a n t a g e s o f t h e s p i r a l d e s i g n :

*

t h e t h e r m a l i n t e r a c t i o n between t h e " I N " l o o p ( s p i r a l ) and t h e

*

t h e h e a t exchange s u r f a c e i s always a t t h e o u t s i d e b o u n d a r i e s o f t h e d i t c h , s o t h a t t h e e f f e c t i v e s u r f a c e

i s

a c y l i n d e r 60 c m (24") i n diameter:

*

t h e s p i r a l a c t s a s a s p r i n g and t h u s can b e s t r e t c h e d t o any d e s i r e d s p a c i n g , i . e . a d e s i g n e r c a n choose t h e h e a t e x t r a c t i o n i n t e n s i t y :

*

it

i s

v e r y e a s y t o i n s t a l l : a l l t h a t

i s

needed i s t o s t r e t c h t h e c o l l a p s e d c o i l and t h e n t o b a c k f i l l :

*

t h e geometry of t h e s p i r a l i s a u t o m a t i c a l l y a s s u r e d by t h e c o i l i t s e l f :

*

t h e e n t i r e i n s t a l l a t i o n of t h e ground h e a t c o l l e c t o r c a n b e completed by o n l y one p e r s o n ( p l u s t h e back-hoe o p e r a t o r ) ;

*

b o t h t h e copper and p o l y e t h y l e n e s p i r a l s a r e s t r o n g enough t o a l l o w e a s y and e f f e c t i v e b a c k f i l l i n g ( p a r t i c u l a r l y w i t h s a n d ) ;

*

o n l y one c o n n e c t i o n i s needed a t t h e end of t h e d i t c h .

The o n l y d i s a d v a n t a g e s o f a r e x p e r i e n c e d ,

i s

t h e p o s s i b i l i t y o f a i r c o l l e c t i o n a t t h e t o p of t h e s p i r a l l o o p s . However, b y u s i n g an o v e r s i z e d pump a t t h e t i m e of f i l l i n g and c a r e f u l l y d e a e r a t i n g t h e system, t h i s problem can b e overcome.

3 ) E x p e r i m e n t a l S e t u p

I n t h e f a l l o f 1988 l a r g e h o r i z o n t a l copper and h i g h d e n s i t y p o l y e t h y l e n e s p i r a l h e a t e x c h a n g e r s were i n s t a l l e d a s p a r t of a

5.25 kW GSHP system. These s p i r a l c o i l s were f a b r i c a t e d s p e c i a l l y f o r t h i s p r o j e c t by copper and p l a s t i c t u b e m a n u f a c t u r e r s . There were no d i f f i c u l t problems e n c o u n t e r e d i n t h e f a b r i c a t i o n p r o c e s s . The s p e c i f i c a t i o n s f o r t h e s e h e a t e x c h a n g e r s a r e a s f o l l o w s : T a b l e I . S p e c i f i c a t i o n s Of Heat Exchangers M a t e r i a l Tubing S p i r a l S i z e S p i r a l P i t c h Copper 2 . 2 cm (7/811) 60 cm (24") 12

cm

(8") P o l y e t h y l e n e 3 . 3 cm (1.31") 45 cm (18) 1 2 cm ( 8 " )

Three h e a t e x c h a n g e r s were i n s t a l l e d : two copper s p i r a l s e a c h i n a 4.9 m ( 1 6 ' ) l o n g d i t c h and one p l a s t i c s p i r a l i n a 9.8 m (32' ) l o n g d i t c h . The t o t a l l e n g t h , t h e r e f o r e , was 19.5

m

( 6 4 ' ) i n t e r m s of t h e d i t c h . T h i s r e p r e s e n t s l / l O t h o r 1 / 6 t h of t h e s t a n d a r d d e s i g n depending on one o r two t u b e s i n a d i t c h , r e s p e c t i v e l y . A l l t h r e e h e a t e x c h a n g e r s were i n s t a l l e d i n a 1 . 8 m ( 6 ' ) deep d i t c h e x c a v a t e d i n l e d a c l a y s o i l and b a c k f i l l e d up t o t h e t o p of t h e s p i r a l s w i t h sand, F i g . 1. A m i x t u r e of water/methanol was u s e d a s t h e h e a t c a r r y i n g f l u i d . T h i s system was h e a t i n g a 90 m2 e x p e r i m e n t a l one

storey house, i.e.

90

m2 on the main floor and the same area in the

basement, during the

1988/89

winter season without any backup. The

indoor temperature was kept constant at

2Z°C

.

The main objective of

this experiment was to determine the heat extraction rates for these

-

coils operating at subfreezing temperatures.

w

4)

Monitorina Of Svstem Performance

The detsrmination of the

system performance

ia

baeed

on

monitoring

the

extracted energy

from

the

ground, energy used to run the heat

pump and the energy consumed by

t h e

circulation

pwrrp.

In addition,

the behavior

of the system during individual cycle8 is being

nbserved,

as

well

as

the

ground thermal depletion and its

recharging.

The

aystem has been monitored at approximately

15

second intervals

durkng

each running

cycle.

A

computer program

is

initiated

by the

impulse from the thermostat

as it demands heating

or

cooling. The

inlet

and

outlet

temperature

of

the

circulating

fluid

(water/methanol) is

measured

at

the distribution manifold

in the

basement of the house for each heat exchanger separately, and as

well, for the system as a whole. There are two thermistors in each

location. The overall accuracy of these sensors is

2

0.005°C.

The

accuracy of the fluid flow meters is about

1

-

2%.

After each cycle,

readinga are sufianarised

and the

average difference

o f

the inlet and the outlet

temperatures i s

computed. This

information, together with fluid

flow (accumulated volume), is used

to calculate gained

or dissipated

energy

by

each ground loop,

a n

well

as

by

the entire system.

Daily,

weekly,

and

monthly energy balances

are determined. In order

to

aassss

the overall performance

o f

individual

heat exchangezer

the

heat energy extraction

or

rejection

per

meter of

the

ditch

i s

aleo

calculated.

5)

Results

The average daily outside air temperatures from December to April are

shown in Fig.

2

for reference purposes. The data points represent

daily average temperatures measured at

20

minute intervals.

The most significant result of this study is that by using large

spiral heat exchangers, high rates of heat extraction, E,, from the

ground can be obtained, Table

11.

Table 11. Heat Extraction Rates

(Ex)

in Watts per meter of

the ditch, Coefficient of Performance (COP) and

water return temperatures (T,)

,

during

1988/89

winter season

*

...

E x

E"

E x E x

Month Plastic Copper

Copper

System

COP

T.

- -

Dec

.

140

170 165 158 2.55 -0.9

Jan.

130 155 155 148 2.45 -3.0

Feb

.

120 145 145 137 2.40 -3.8

Mar.

125 153 155 144 2.35 -3.8

Apr

.

125 170 160 152 2.35 -1.7

As

expected, the copper apirala performed better than the plastic

ones due to

the

highex thermal conductivity of the copper. It should

be noted, however, that the contact (heat exchange) area between the

tube and

the

soil was

7.0

cmf/cm for copper and 10.5

cmi/em fox.

-

plastic, i.e.

L . 5 : l . O r a t i o

in favor of plastic tube. The linear

length of the tubes per meter of the ditch far copper and plastic

spirals was

approximately the

same.

Yet,

because

the.

copper spirals

had larger outside diameter than the plastic spirals, i.e.

60 cm

-

versus

45

cm, the former activated a larger soil maas. Therefore the

comparison between these two spiral

heat exchangers

is based

on

three

variables: material, (copper versus plastic), tube diameter

(2.2

cm

versus 3.34 cm)

and spiral

diameter

(45

crn

versus 60 cm), Table

I.

In spite of

these

differences, the comparison is realistic, since

two

practical designs based on coneiderations such

aa

cost, manufacturing

process, handling etc.

are compared.

Horeovex, it

is expected that

the future design

o f

either plastic or copper spiral

heat exchangers

will not significantly differ from those used in this project.

In surmnary, the significance of the results presented in Table I1 is

two-f

old:

a)

the heat extraction rates are very high and

b)

the difference between the heat extraction rates for copper and

plastic spirals is not as large as expected.

Economic and practical considerations will determine whether copper

or plastic spirals will be used in commercial installations. In the

author's opinion, large (60 cm) spirals made from high density

polyethylene tubing,

3.3

-

4.0 cm in outside diameter, will be the

most practical choice.

Variations in heat extraction rates during the winter season are

shown in Fig.

3.

These results for the months of December, February

and April represent typical behavior of the ground heat exchangers.

The important aspect here is the magnitude of energy withdrawn from

the ground.

Another

set

o f

data, presented

in Fig. 4 shows temperature

differences,

dT, between the " I N n

end

the "OUTw

tubes

of

all three

heat

exchangers during December, February and April.

It

can

be

observed that this diiference (except

i n

April) is

largest for the

plastic spiral.

This

result

can

be

attributed to the

f a c t

that the

length

of the plastic spiral is

t w i c e

as

long

as the copper

one.

The overall dT for the entire ground heat exchange system during

December, February and April is shown on Fig.

5.

The significance of

these results is that even though the absolute temperature of the

circulating fluid decreases with increasing heating demand and ground,

thermal depletion, the dT remains constant

-

approximately 2OC.

The Coefficient of Performance (COP), see Table 11, was lower than

would be required in a commercial installation, but, considering the

short length of the ground heat exchangers, the COP was still better

than expected. The objective

o f this

project was

to

d e t e d n e the

performance of prototypes of new ground heat exehangars,

i . e ,

large

horizontal spirals, and namely the rate of heat exchange during a

normal winter operation. The objective

was

not

to design sn efficient

GSHP symtem. In fact the

ground

heat collector was purposely

DEC. i88 JAN. /@a

1

o-o OUTSIDE AIR

-2sL-, ' , , , , , ,

l5

[

MAR. 109 APR. W8 M Y M

Fig. 2. Average Daily Temperatures During 1988/89 Winter

t h i s p e r s p e c t i v e . The COP i s , however, v e r y r e s p e c t a b l e f o r such a s m a l l ground h e a t exchange system. A s mentioned above, t h e t o t a l l e n g t h of t h e ground h e a t exchange s y s t e m ( i n

terms

of r e q u i r e d d i t c h ) was o n l y a f r a c t i o n of what i s c u r r e n t l y b e i n g u s e d by t h e

.

GSHP i n d u s t r y .

As c o u l d be e x p e c t e d , t h e COP d e c r e a s e d w i t h d e c r e a s i n g c i r c u l a t i n g f l u e d t e m p e r a t u r e , Table 11. The d e o r e a e e of t h e e n t e r i n g -

_

w a t e r h e t h a n a l t e m p e r a t u r e from O°C a t t h e b e g i n n i n g o f

December

t o

-5.5'C i n t h e m i d d l e

of

March, r e s u l t e d

i n

about a 1 0 % decrease of

!-

COP. Thia is o n l y a modest l o s s , one which might be lowered by

o p t i m i z i n g t h e e n t i r e system.

6) Conclusion

*

Large d i a m e t e r s p i r a l h o r i z o n t a l copper and p l a s t i c h e a t e x c h a n g e r s have been developed, c o n s t r u c t e d and t e s t e d i n a r e a l f i e l d i n s t a l l a t i o n . No d i f f i c u l t i e s were e n c o u n t e r e d i n t h e f a b r i c a t i o n p r o c e s s and no problems were e x p e r i e n c e d i n t h e i n s t a l l a t i o n . The i n s t a l l a t i o n p r o c e s s proved t o b e s i m p l e , f a s t and i n e x p e n s i v e . I n f a c t , t h e e n t i r e ground h e a t exchange system u s i n g e i t h e r copper o r p l a s t i c s p i r a l s c a n b e i n s t a l l e d e a s i l y by o n l y one p e r s o n ( p l u s an o p e r a t o r f o r t h e e x c a v a t i o n machine).

Beat e x t r a c t i o n r e s u l t s a r e v e r y good. Due

t o

t h e expected low c o s t

of t h e h e a t exchangers, the s i m p l i c i t y o f t h e i r i n s t a l l a t i o n , a n d t h e i r h i g h performance, t h i a

new

ground h e a t exchange t e c h n o l o g y w i l l

decrease t h e initial cost of GSBPrs i n g e n e r a l . It i s expected that

GSHP'B u t i l i z i n g t h e s e

new

ground h e a t e x c h a n g e r s will become

c o m p e t i t i v e i n t h e h e a t i n g / c o e l i n q market.

T h i s new t e c h n o l o g y ( p a r t i c u l a r l y t h e p l a s t i c s p i r a l s )

is

a t t h e s t a g e where

it

c a n be adopted e a s i l y by t h e GSHP i n d u s t r y . Manufacturers of p l a s t i c tubing have t o be found, who are p r e p a r e d t o

produce s t a n d a r d 6Ocm/3.34cm (24"/1.31") s p i r a l / t u b i n g coils. T h e I n i t i a l r e s p o n s e from a few m a n u f a c t u r e r s i n Canada was p o s i t i v e : no major problems associated w i t h t h e m a n u f a c t u r i n g p r o c e s s are

e x p e c t e d .

REFERENCES

Svec, O . J . , L.E. Eoodrlch a n d J.A.L. Palmer (1983).

Heat

t r a n s f e r c h a r a c t e r i s t i c 0 of in-ground heat e x c h a n g e r s . Energy Research,

V O ~ . 7, p.265-278. (NRCC 2 2 6 7 4 )

Svec, O . J . I L 9 B 5 ) . P o t e n t i a l f o r improvement between ground and h e a t pump energy exchange, Proceedings, Second Workshop on Solar

A s s i s t e d Heat Pumps w i t h Ground Coupled S t o r a g e , Vienna, A u s t r i a ,

July 1985, p 4 3 1 - 4 4 0 . (NRCC 27413)

Svec, O.J. and Palmer J.H.L. (1989). Performance of a s p i r a l ground

heat-exchanger f o r heat pump a p p l i c a t i o n .

Int.

Journal o f E n e r g y ' -

Research, V o l 13, p . 503-510.. d

Svec, 0. J. (1988 1

.

Spiral h e a t exchangers

-

Demonstration o f

a

new

.

ground s o u r c e h e a t pump t e c h n a l o g y . Proceedings, Jfgastock 88,

ACKNOWLEDGEMENTS

The author wishes to express his gratitude to Dr. J.H.L. Palmer of

.

IRC/NRC for many inspiring discussions, to Messrs. D. Eldred and

D. MacMillan for their excellent technical assistance and to

Mr.

Mike

?

Wiggin of the Department of Energy Mines and Resources, CANMET

This

paper is being distributed in reprint form by the Institute for Research in Construction. A list of building practice and research publications available from the Institute may be obtained by writing to Publications Section, Institute for Research in Construction, National Research Council of Canada, Ottawa, Ontario,

KIA

0R6.

Ce document est distribue sous forme de tir&&-part par I'Institut de recherche en construction. On peut obtenir une liste des publications de I'Institut portant sur les techniques ou les recherches en mati* de bgtiment en krivant B la Section des publications, Institut de recherche en construction, Conseil national de