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13 Plastic Pipe Requirements for Ground-Coupled Heat Pumps PHILIP D. METZ Brookhaven National Laboratory, Solar and Renewables Division, Upton,NY11973
This paper describes the temperature, pressure, fluid compatibility and mechanical strength requirements which must be met by plastic pipe materials for various ground coupled heat pump applications. Existing suitable materials are cited as well as areas where improvements are required. A ground coupled heat pump i s a heat pump which uses the e a r t h as a heat source o r s i n k , o f t e n by c i r c u l a t i n g a f l u i d through serpentine buried pipes o r v e r t i c a l heat exchangers. Ground coupled heat pumps can operate w i t h much higher e f f i c i e n c y than a i r source heat pumps because the ground i s warmer i n w i n t e r and c o o l e r i n summer than the ambient a i r , i s i n s e n s i t i v e t o short-term weather f l u c t u a t i o n s , and removes the need f o r a defrost cycle. A d d i t i o n a l l y , i t i s f e a s i b l e t o design ground coupled heat pump systems which r e q u i r e no a u x i l i a r y h e a t i n g , even i n very c o l d c l i m a t e s . The design of these systems has been discussed elsewhere.(1) Ground coupled heat pumps can a l s o be used t o provide backup heat f o r s o l a r heating systems, e s p e c i a l l y s o l a r heat pump systems. The use of ground coupled heat storage tanks i n s o l a r a s s i s t e d heat pump systems has been s t u d i e d e x t e n s i v e l y . ( 2 ) While long-term inground heat storage i s not p o s s i b l e f o r r e s i d e n t i a l s i z e s o l a r systems, i t may be f e a s i b l e f o r l a r g e r systems i n which the storage surface/volume r a t i o i s s m a l l e r . This paper considers only stand-alone ground-coupled heat pumps which can be considered s o l a r i n the sense t h a t t h e heat they draw was p r o v i d ed by t h e sun, and i s a v a i l a b l e i n w i n t e r due t o t h e thermal p r o p e r t i e s of the e a r t h .
0097-6156/83/0220-0211$06.00/0 © 1983 American Chemical Society Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Ground coupled heat pumps may now be f e a s i b l e , l a r g e l y due to the a v a i l a b i l i t y of durable and inexpensive p l a s t i c p i p e . Pipe m a t e r i a l s such as polyethylene, PVC, and polybutylene have been found to be adequate i n some i n s t a n c e s . However, no mater i a l standards or recommended design p r a c t i c e s e x i s t to f a c i l i t a t e the o p t i m i z a t i o n of the performance, cost and r e l i a b i l i t y of these systems. Temperature
Requirements
The range of f l u i d temperatures a n t i c i p a t e d f o r ground coupled heat pump systems i s approximately -10 to +50°C. The l a t e n t heat of f r e e z i n g of the ground source keeps f l u i d temper atures from dropping much below the f r e e z i n g temperature of the ground, even i n very c o l d c l i m a t e s . I t i s not f e a s i b l e to a l l o w ground s i n k temperature to r i s e much above 40°C as heat pump c o o l i n g e f f i c i e n c y decreases as sink temperature r i s e s . Systems i n which space c o o l i n g i s not r e q u i r e d w i l l not encounter f l u i d temperatures above about 25°C. Pressure Requirements L i q u i d source heat pumps t y p i c a l l y r e q u i r e a f l u i d flow r a t e of roughly 5.4 χ 10~ L/S per kW (3 GPM per ton) of h e a t i n g capacity. I n order to minimize c i r c u l a t i o n pump power, pipe (which o r d i n a r i l y would be considered o v e r s i z e d f o r these flow r a t e s ) of 4 to 6 cm inner diameter i s used. E a r t h c o i l lengths are t y p i c a l l y 26 m per kW (300 f t per ton) of heating c a p a c i t y . Thus, system operating pressures f o r a t y p i c a l 11 kW (3 ton) heating load do not exceed 7-14 χ 10* N/m^ (10-20 p s i ) , lower than most e x i s t i n g schedules f o r p l a s t i c pipe. However, the sys tem must a l s o withstand water l i n e pressure, i . e . 7 χ 10^ N/m (100 p s i ) . 2
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Fluid Compatibility In systems where subfreezing temperatures are not expected, water can be used as a heat t r a n s f e r f l u i d ( c o r r o s i o n i n h i b i t o r s may be necessary). A l l common e x i s t i n g p l a s t i c pipes are compat i b l e w i t h water. In systems where f r e e z i n g i s expected (roughly the n o r t h e r n h a l f of the U n i t e d S t a t e s ) , a n t i f r e e z e i s r e q u i r e d . S o l u t i o n s which have been used i n c l u d e ethylene g l y c o l - water, propylene g l y c o l - water, and CaCl2 - water. Organic a n t i f r e e z e s such as g l y c o l s are expensive and may enhance s t r e e s c r a c k i n g i n some pipe m a t e r i a l s such as i n the lower d e n s i t y polyethylenes, p a r t i c u l a r l y at h i g h e r temperatures. The extent of t h i s problem over long times i s not known. Ethylene g l y c o l i s poisonous, and t h e r e f o r e a danger to underground water s u p p l i e s . Inorganic a n t i f r e e z e s o l u t i o n s , such as CaCl2 ~ water, are c o r r o s i v e and require i n h i b i t o r s . Otherwise, because these s o l u t i o n s are
Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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i o n i c , g a l v a n i c c o r r o s i o n between d i s s i m i l a r metals not even i n p h y s i c a l contact i s p o s s i b l e . Mechanical Strength Requirements P l a s t i c pipe f o r ground coupled heat pumps must withstand i n s t a l l a t i o n at depths of approximately 1-2 m f o r h o r i z o n t a l systems o r i n v e r t i c a l s h a f t s as deep as 100 m i n v e r t i c a l systems without e x t r a o r d i n a r y excavation p r e c a u t i o n s , as w e l l as ground s e t t l i n g a f t e r i n s t a l l a t i o n , and thermal s t r e s s e s during operat i o n without f a i l u r e . Pipe Materials P o l y e t h y l e n e , PVC, and more r e c e n t l y polybutylene have been used f o r pipes i n ground coupled heat pump systems. The use o f r i g i d pipe m a t e r i a l s such as PVC has diminished due t o the l a b o r i n v o l v e d i n connecting j o i n t s and the r i s k of l e a k s . Low d e n s i t y polyethylene pipe has been used w i d e l y i n heating-only ( i . e . , low temperature f l u i d ) systems i n Sweden (3) w i t h an ethylene glycol-water solution. This pipe i s u n s u i t a b l e f o r higher temperatures. Medium d e n s i t y (PE 2306) polyethylene pipe has been used i n experiments i n the United States a t temperatures between -10 and 50°C using water and ethylene g l y c o l - w a t e r s o l u t i o n s . (1) This pipe i s pressure r a t e d a t about 25°C, and has no r a t i n g a t higher temperatures. Some l o n g i t u d i n a l cracks caused by mechanical breakage were found i n pipe a f t e r heating t o about 50°C. High d e n s i t y polyethylene and polybutylene have b e t t e r pressure r e s i s t a n c e and d u r a b i l i t y at elevated temperatures. Conclusion S u i t a b l e pipe m a t e r i a l s f o r ground coupled heat pumps may already e x i s t . However, no m a t e r i a l standards, recommended des i g n p r a c t i c e s , i n s t a l l a t i o n procedures o r s a f e t y precautions have been developed. The operating c h a r a c t e r i s t i c s of these systems are now known w e l l enough f o r these standards t o be developed. This w i l l f a c i l i t a t e the o p t i m i z a t i o n s of the p e r f o r mance, cost and r e l i a b i l i t y of these systems. Acknowle dgment s Work performed under the auspices o f t h e U.S. Department o f Energy under Contract No. DE-AC02-76CH00016.
Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Literature Cited 1.
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Metz, P. D., Design, Operation and Performance of a Ground Coupled Heat Pump System in a Cold Climate, Proc. 1981 Intersoc. Energy Conv. Conf., ASME, Atlanta, August 9-14, 1981, BNL 29625. Metz, P. D., The Use of Ground Coupled Tanks in Solar Assisted Heat Pump Systems, I. Comparison of Experimental and Computer Model Results, Proc. ASME Solar Energy Div. Fourth Ann. Tech. Conf., ASME, Albuquerque, April 1982, BNL 30913. Mogensen, P., Experiences from Earth Heat Pumps in Sweden, Proc. Nordic Symposium on Earth Heat Pump Systems, Goteborg, Sweden, October 15-16, 1979, Chalmers Univ. of Technology.
RECEIVED
November 22, 1982
Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
