[thesis] investigation of the influence of the ground heat exchanger geometry on its thermal...

8/22/2019 [Thesis] Investigation of the influence of the ground heat exchanger geometry on its thermal performance by usin

1/17

Investigation of the influence of the ground heat exchanger

geometry on its thermal performance by using a water-to-water

ground-source heat pumpP-J Steenbeke

2013

Abstract

Ground-source heat pumps (GSHPs) are evolving into the most popular, reliable andcompetitive heating and cooling systems available. Yet, studies on the influence of the parametersof the horizontal ground heat exchangers (GHEs) are few. Consequently, the objective of thisstudy was to investigate the influence the depth and the length of the GHEs have on thethermodynamic performance while the other are kept constant. Therefore, year-roundsimulations were performed on a water-to-water GSHP with horizontal GHEs and a variable-speed compressor for a typical family house of approximately 190 m living space based on the

weather conditions in Brno and Ghent. Each parameter was given 3 different values resulting in 9combinations. It is concluded that with increasing depth, the coefficient of performance in theheating mode ( ) reduces while it increases in the cooling mode ( ). Moreover, thehigher the length, the higher the apart from the lower Keywords: Ground-source heat pump; Horizontal ground heat exchangers; Influence length and

depth of ground heat exchange; Thermodynamic performance

Abbreaviations:CalA, calculation area; COP, coefficient of performance (index H: heating; index

C: cooling); GHE, ground heat exchanger; GSHP, ground-source heat pump

Introduction

Worldwide, the awareness is enlarging of the threat climate change is to our future. Since fossilfuel consumption is the predominant origin, renewable energy technologies have been developed.

Wind, solar, biomass and biofuel are not the only clean, efficient and every saving renewablesources to furnish heat, light and electricity without polluting the air or disturbing large areas ofland or water. Temperature within the soil increases with depth as shown on Figure 1. This heatflows to the surface by conduction and is continuously replenished mainly by decay of naturallyradioactive elements. Hence, the earth possesses an enormous amount of heat. This heat can beused as an energy source, i.e. geothermal energy.

A new, potent, reliable and highly efficient instrument for space heating and/or cooling is aheat pump. It operates under the principle that heat can be moved from a warmer temperature to


2/17

2 |

a cooler temperature. In most cases, heat pumps use the air, ground or water as heat source.Even at temperatures we consider to be cold, these sources contain useful heat that iscontinuously replenished by solar energy. Special geologic conditions, such as hot springs or hotrocks (geothermal energy), are not required for successful application. Ground-source and water-source, referred as geothermal heat pumps (GHEs), can be designed as either a closed (ground-coupled) or open (water source) loop which itself can be either horizontal or vertical.

Figure 1 - Temperature at various depths of the earth [1]

A heat pump does not differ in principle from a refrigerator apart from its purpose: heatpumps provide heat whereas refrigerators obtain cold. In cooling operation it operates exactly asa refrigerator. A single efficient system can thus provide both heating and cooling. Thiseliminates the need for separate furnace and air-conditioning systems. In addition, a heat pumpcan also be used as a hot water system.

The thermodynamic performance of a heat pump is appraised by the coefficient ofperformance (COP). This is the ratio of the output energy divided by the input energy:

= (1)

= (2)where Q (Q) is the supplied (removed) heat and W is the energy required to power the systemor the compressor.

In general, the peak-load operation represents approximately 10% of the total system hours[2]. For maximum cost-effectiveness, heat pumps are generally sized to meet 60-70% of the totalmaximum dimension load. A supplementary heating system covers the occasional peak heatingdemand. Although the initial cost may reduce, the running cost will increase due to its lowperformance [3] [4]. Therefore, variable-speed compressors have been developed. Such systemscan eliminate the need for a supplementary heating system (see further) [5].


3/17

3 |

Heat pumps are energy efficient because they rely on the principles of vapour compressionrefrigeration. Via a refrigerant system it absorbs heat at one place and rejects it at another. Thisheat transfer is accomplished by cycle of evaporation, compression, condensation and expansionof a refrigerant liquid as seen on in Figure 2.

Despite all the advantages, the awareness and acceptance is unfortunately excessively little.The installation cost is higher than conventional systems. In addition, there is a lack of skilled andexperienced designers and installers of heat pumps. However, it is notably cheaper to run and themaintenance costs are lower. Furthermore, commercial heat pump technologies are currentlyrapidly enhancing. Thus, heat pumps are evolving into the most popular, reliable and competitiveheating and cooling systems available, especially due to present environmental consciousness andenforced governmental subsidy [6] [7] [8]. Though, heat pumps have been implemented for yearsin northern Europe and North America. The interest in them is augmenting in other countries,for example Japan and Turkey [6] [9].

Figure 2 - Refrigeration cycle

Many studies have been performed on GSHPs. Yet, studies on the influence of the parameters

of the horizontal ground heat exchangers (GHEs) are few. Tarnawski et al. [4] concluded that theCOP increases with increasing length. Moreover, they indicated that in the heating and cooling

CONDENSOR

EVAPORATOR

EXPANSIONVALVE

COMPRESSOR

W

Heated/HotArea

Cool/CooledArea

QH

QC

PT

PT


4/17

4 |

mode the COP is mainly affected by the pipes buried closer to the ground surface. Similar resultswere presented by ikula and Plek [10].

In these times of ever increasing prices and scarcity of ground, designers of heat pumps withhorizontal GHEs must understand the thermal interferences of the GHEs to be able to designsystems which take less surface area, with special emphasis on GSHPs, without reducing theCOP significantly. Consequently, the objective of this study was to investigate the influence thedepth and the length of the GHEs have on the thermodynamic performance while the other arekept constant.

Methodology

Computer simulations and analysis of water-to-water GSHP system with horizontal GHEswere carried out to two-year period meteorological conditions of Brno and Ghent for a typicalfamily house of 190 m living space. For Brno, the data of the year 2005 were obtained from astudy by ikula and Plek [10]. For the hometown of the researcher, the data of the year 2009

were acquired from Weather Underground [11]. Figure 3 and Figure 4 portray the heating andcooling demands during the year. The family house requires 33.1 and 0.184 GJ/year respectivelyfor heating and cooling in Brno whereas 37.2 and 0.0917 GJ/year in Ghent. As it is observed theGSHP in this study will mainly operate in the heating mode. The peak heating load is 2.9 kW and2.6 kW for Brno and Ghent, respectively, as the peak cooling load is 0.8 kW and 0.5 kW.

Figure 3 - Hourly heating and cooling demands during the year for Brno

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0 40 80 120 160 200 240 280 320 360

Heatingandcoolingload(kW)

2005


5/17

5 |

Figure 4 - Hourly heating and cooling demands during the year for Ghent

For these conditions 9 different GHE geometries with variable length and depth (Table 1) anduniform radius and spacing, were first simulated using the dynamic calculating softwareCalculation Area (CalA). This software simulates the temperature and heat flow of diversegeometries. As input, the thermal conductivity (), density () and specific heat capacity (c) of

the ground were considered as: = 2.30 W/m.K, = 2000 kg/m and c = 920 J/kg.Kcorresponding to water-saturated sand soil. Due to a symmetrical geometry of the GHEs, onlythe half of the geometry is simulated to save computation time and data storage. As output,hourly temperatures during the year of the control volumes assigned to the GHEs were used tocompare the different options (see T in Table 2).

Table 1 - Variables geometry GHE

Variable Option 1 Option 2 Option 3

depth (m) 0.50 0.75 1.00

length (m) 100 175 250

Tarnawski et al. [4] commented that after the third year of the GSHP operation, the groundthermal regime remains practically unchanged. Therefore, in order to save computational timeand data storage, all simulations should be carried up to three years and only the results from thethird year should be considered for the study. Due to inadequate time, only a two-year operationperiod has been simulated. Hence, in this study the results from the second year are used.

Forsmand concluded that the COP calculated using daily average data was within 3% from the

daily average obtained using hourly data [12], while Bose even suggested the use of monthly

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0 40 80 120 160 200 240 280 320 360

Heatingandcoolingload(kW)

2009


6/17

6 |

average climatic data for the GHE design [13]. However, to perform a purely theoreticalinvestigation, hourly data have been adopted.

During the heating operation, the condenser temperature was taken as constant as well as theevaporator temperature during the cooling operation. These and other temperatures in heatexchanging pipes are given in Table 2. Moreover, the refrigerant mass flow was adjusted to therequired capacity. Figure 5 depicts the schematic diagram and the heat exchanging pipe tempera-tures of the heating and cooling operation of the GSHP.

Table 2 - Temperatures in the heat exchanging pipes in heating and cooling operation

HEATING OPERATION COOLING OPERATION

, = 27 (3) , = 22 (4), 35 = (5) , 14 = (6) = , 3 + 38 = (7) = , 3 11 = (8), = 3 + (9) , = 3 (10), = 3 (11) , = 3 + (12) = , 3 (13) = , 3 (14)

Figure 5 - Schematic diagram of the heating (a) and cooling operation (b) of a GSHP

EC

TC,in

TC,out

TE,in

TE,out

EC

TC,in

TC,out

TE,in

TE,out

(a)

(b)


7/17

7 |

With those assumptions the COP (COP) can be calculated as a function of the differencebetween condenser and evaporator (evaporator and condenser) temperature (T). Therefore, thesimulation software CoolPack was attained. CoolPack is a collection of simulation models for

refrigeration systems. Refrigerant 134a was used in the heat pump and the efficiency () waschosen 80%. For simplicity, the desuperheater option and the lag of the temperature cycles werenot considered in this study. Since it concerns a small house, DX evaporators were employed.Other inputs were considered constant. Figure 6 displays the COP and COP as function of theT. By using trendlines of the power type following equations were found:

= 220.81 . (15) = 205.08 . (16)allowing to calculate the COP during the year depending on the T. The average of these hourly

COPs was taken to be the overall COP of the GSHP system.

Figure 6 - COPH and COPC as function of T

Results

Brno

First, the influence of a GHE geometry with variable depth and uniform length was examined.Figure 7, Figure 8 and Figure 9 depict the temperature fluctuations in the GHE during secondyear for pipe lengths of 20 m, 35 m and 50 m, respectively. They show that the temperature ofthe GHEs closer to the surface are higher as well as they are more subject to temperaturefluctuations.

0

5

10

15

20

25

0 10 20 30 40 50 60

COP

(C)

Heating Cooling


8/17

8 |

Figure 7 - Temperature fluctuations in a GHE of 20 m length during the second year (Brno)

Figure 8 - Temperature fluctuations in a GHE of 35 m length during the second year (Brno

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

0.50 0.75 1.00

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

0.50 0.75 1.00


9/17

9 |

Figure 9 - Temperature fluctuations in a GHE of 50 m length during the second year (Brno

Second, the influence of a GHE geometry with variable length and uniform depth wasexamined. Figure 10, Figure 11 and Figure 12 depict the temperature fluctuations in the GHEduring second year for depths of 0.50 m, 0.75 m and 1.00 m, respectively. It can be seen that thegreater the length, the higher the temperatures, especially in the winter, while they are fewer

subject to temperature fluctuations. The latter is different from the results where the depth wasconsidered variable as length was kept constant.

Figure 10 - Temperature fluctuations in a GHE at 0.50 m depth during the second year (Brno)

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

0.50 0.75 1.00

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

20 35 50


10/17

10 |



Finally, the COP is calculated. Figure 13 and Figure 14 show the COP and COP, respectively.As observed, the COP is the highest for GHE with a length of 50 m at a depth of 0.50 m whilethe COP is the lowest. Since the heating operation predominates this combination seemed to be

optimal.

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

20 35 50

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

20 35 50


11/17

11 |

Figure 13 - COPH Brno

Figure 14 - COPC Brno

Ghent

Analogously to Brno, the influence of a GHE geometry with variable depth and uniform

length was first examined. Figure 15, Figure 16 and Figure 17 depict the temperature fluctuationsin the GHE during second year for pipe lengths of 20 m, 35 m and 50 m, respectively. It can beobserved that the temperature of the GHEs closer to the surface are higher as well as they aremore subject to temperature fluctuations.

05

06

07

08

0910

11

12

10 20 30 40 50 60

COPH

Length GHE (m)

0.50 0.75 1.00

05

06

07

08

10 20 30 40 50 60

COPC

Length GHE (m)

0.50 0.75 1.00


12/17

12 |

Figure 15 - Temperature fluctuations in a GHE of 20 m length during the second year (Ghent)


-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

0.50 0.75 1.00

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

0.50 0.75 1.00


13/17

13 |


Second, the influence of a GHE geometry with variable length and uniform depth wasexamined. Figure 18, Figure 19 and Figure 20 depict the temperature fluctuations in the GHEduring second year for depths of 0.50 m, 0.75 m and 1.00 m, respectively. It can be seen that thegreater the length, the higher the temperatures, especially in the winter, while they are fewer

subject to temperature fluctuations. The latter is once more different from the results where thedepth was considered variable as length was constant.

Figure 18 - Temperature fluctuations in a GHE at 0.50 m depth during the second year (Ghent)

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

0.50 0.75 1.00

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

20 35 50


14/17

14 |



Finally, the COP is calculated. Figure 21 and Figure 22 show the COP and COP, respectively.As observed, the COP is yet again the highest for GHE with a length of 50 m at a depth of 0.50m while the COP is the lowest. Since the heating operation predominates this combination

seemed to be optimal.

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

20 35 50

-10

0

10

20

30

40

0 40 80 120 160 200 240 280 320 360

Temperature(C)

Day of the year

20 35 50


15/17

15 |

Figure 21 - COPH Ghent

Figure 22 - COPC Ghent

Conclusions and recommendations

Computer simulations and analysis of water-to-water GSHP system with horizontal GHEs

were carried out to meteorological conditions of Brno and Ghent for a typical family house ofapproximately 190 m living space, 2.9 kW of design heating load and 0.8 kW of design coolingload for Brno while 2.6 kW and 0.5 for Ghent, respectively. Moreover, the family house requires33.1 and 0.184 GJ/year respectively for heating and cooling in Brno 37.2 and 0.0917 GJ/year inGhent. For these conditions, the optimal GHE geometry for both regions comprises a length of50 m at depth of 0.50 m. Though, this combination provides the lowest , the GSHP in thisstudy will as mentioned above mainly operate in the heating mode. Therefore, this combination

was concluded to be the most optimal.The results seems to indicate that with increasing depth, the reduces while the

increases. Moreover, the higher the length, the higher the apart from the lower . Theobservation that the increases with length is supported by Tarnawski et al. [4]. However,

06

07

08

09

1011

12

13

10 20 30 40 50 60

COPH

Length GHE (m)

0.50 0.75 1.00

05

06

07

08

10 20 30 40 50 60

COPC

Length GHE (m)

0.50 0.75 1.00


16/17

16 |

that the reduces with increasing length conflicts with their results. This is due to an error inthe simulation.

Due to inadequate time, only a two-year operation period has been simulated while Tarnawskiet al. [4] recommended to carry out all simulations up to three years and consider only the resultsfrom the third year for the study, since the ground thermal regime remains practically unchangedafter the third year. Therefore, to achieve more accurate results, the third year should besimulated and the results analysed.

Length and depth of the GHE are only two of the several parameters which influence theCOP of a heat pump. Therefore, the other parameters should be investigated as well as well asthe amount of options should be larger.

References

[1] VITO - subsites - geothermal power - what is geothermal energy?, VITO, [Online]. Available:

http://www.vito.be/VITO/EN/HomepageAdmin/Home/Subsites/Geothermie/geothermie_wa

t/. [Accessed 17 April 2013].

[2] C. Lee, Dynamic performance of ground-source heat pumps fitted with frequency inverters for

part-load control,Applied Energy 87, pp. 3507-3513, 2010.

[3] . Lund, B. Sanner, L. Rybach, R. Curtis and G. Hellstrm, Geothermal (Ground-Source) Heat

Pumps: A World Overview, 2004.

[4] V. Tarnawski, W. Leong, T. Momose and Y. Hamada, Analysis of ground source heat pumps

with horizontal ground heat exchangers for northern Japan, Renewable Energy 34, pp. 127-134,

2009.

[5] F. Karlsson and P. Fahln, Capacity-controlled ground source heat pumps in hydronic heating

systems, Refrigeration 30, pp. 221-229, 2007.

[6] . Lund, Ground source (Geothermal) heat pumps. Course on heating with geothermal energy:

conventional and new schemes, Convener Paul J Lienau, WGC 2000 Short Courses Kazuno,

Thoku District/Japan, 2000, pp. 209-36.

[7] . Lund, Direct use of geothermal energy in the USA. Appl Energy 2003;74:33-42.

[8] . Lund, Geothermal heat pump utilization in the United States. Geo-Heat Center Q Bull

1988;11(1):507.

[9] . Lund, Geothermal heat pumps - trends and comparisons. Geo-Heat Center Q Bull 1989;12(1):1-

6.

[10] O. ikula and J. Plek, Tepeln erpadla - simulace celoronho provozu zemnho kapalinovho

kolektoru (in Czech), TZB-info, 2011. [Online]. Available: http://oze.tzb-info.cz/geotermalni-

energie/7152-tepelna-cerpadla-simulace-celorocniho-provozu-zemniho-kapalinoveho-kolektoru.


17/17

17 |

[Accessed 25 February 2013].

[11] Weather History for Weather Station IWEST-VL3 | Weather Underground, Wunderground,

[Online]. Available:

http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=IWEST-VL3&month=1&day=1&year=2009. [Accessed 24 May 2013].

[12] M. Fordsmand and A. Eggers-Lura, Analysis of the factors which determine the COP of a heat

pump and a feasibility study on ways and means of increasing same., Charlottenlund, 1981.

[13] . Bose, J. Parker and F. McQuiston, Design/data manual for closed loop ground coupled heat

pump systems, Atlanta: ASHRAE, 1985.

[thesis] investigation of the influence of the ground heat exchanger geometry on its thermal...

Documents