GROUND SOURCE HEAT PUMPS Analysing the Brine Flow

in Boreholes, Mariehäll

M A R A L K A S S A B I A N

Master of Science Thesis

Stockholm, Sweden 2007

GROUND SOURCE HEAT PUMPS Analysing the Brine Flow in Boreholes,

Mariehäll

Maral Kassabian

Master of Science Thesis Refrigeration 2007:412 KTH School of Energy and Environmental Technology Division of Applied Thermodynamic and Refrigeration

SE-100 44 STOCKHOLM

Master of Science Thesis EGI 2007/ETT:412

GROUND SOURCE HEAT PUMPS Analysing the Brine Flow in Boreholes,

Mariehäll

Maral Kassabian

Approved

15 April 2007 Examiner

Björn Palm Supervisor

Peter Hill Commissioner

Contact person Tommy Nilsson

Abstract The brine flow through the boreholes of a 2 heat pump system in Stockholm, Sweden was examined. The objective was to identify the behaviour of the flow in order to optimise the use of the boreholes, heat pumps, and extra pump included in the system. The amount of energy extracted by the system was a function of the brine flow. Reynolds number and disturbances greatly affected the flow. It was seen to absorb the most energy during the first 15m of the borehole, and at the bottom of the borehole, after turning 180°. There were large amounts of energy lost in the last 15-40 m of the borehole. This was due to Re as well as the ground- and brine-temperatures. The flow appeared turbulent during the first 15m of the borehole, and seemed to transition to laminar as it travelled down the borehole. After passing the bend, it appeared to possess turbulent properties once again. An extra brine pump was used to increase the brine flow and thereby extract more energy from the boreholes. This was the case when two heat pumps were operational; net energy extraction with the extra pump was higher than without it. When a single heat pump was running, there was less energy extracted with the extra pump. This was attributed to the difference in ground- and brine-temperatures, as well as the number of boreholes open. Determining the optimal number of boreholes to operate was difficult as there were many variables such as dissimilar flows in boreholes, different borehole lengths, changing demand from the heat pump, varying ambient conditions. Some general conclusions have been drawn. Recommendations include insulating the pipes exiting and entering the house to the point where the boreholes begin. In addition, the last 30-40m of borehole on the up-flow side should be insulated to avoid energy dissipation to the surroundings. For more accurate studies, flow meters must be installed in each borehole. Heat pump manufacturers must label heat pumps with regards to the flow rate required to optimize energy extraction.

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TABLE OF CONTENTS

1 INTRODUCTION...................................................................................9 1.1 HEAT PUMPS................................................................................9 1.2 COEFFICIENT OF PERFORMANCE ..................................................10 1.3 BOREHOLE CONFIGURATIONS ......................................................11 1.4 LAMINAR AND TURBULENT FLOW ..................................................11 1.5 REYNOLDS NUMBER....................................................................12 1.6 EXTRACTED HEAT.......................................................................13 1.7 MARIEHÄLL.................................................................................13

2 OBJECTIVES .....................................................................................15

3 METHODOLOGY................................................................................16 3.1 STEP 1.......................................................................................16 3.2 STEP 2.......................................................................................16 3.3 STEP 3.......................................................................................16

4 EQUIPMENT.......................................................................................17 4.1 TECHNICAL DETAILS....................................................................17 4.2 PRACTICAL BACKGROUND ...........................................................19 4.3 PIPE DIMENSIONS .......................................................................20 4.4 TA METER ..................................................................................20 4.5 DATA ACQUISITION SYSTEM.........................................................20 4.6 HEAT PUMP CONTROL.................................................................20

5 EXPERIMENTAL UNCERTAINTIES ..................................................21 5.1 ERRORS IN MASS FLOW ..............................................................21

5.1.1 Flow Measurement Methods ....................................................21 5.1.2 Flow Comparison......................................................................21

5.2 TIME ERROR...............................................................................22 5.2.1 Time Error ................................................................................22 5.2.2 Averaging Temperature Values................................................24

5.3 OTHER ERRORS .........................................................................24

6 RESULTS ...........................................................................................26 6.1 BRINE PROPERTIES.....................................................................26 6.2 GROUND TEMPERATURES............................................................27

6.2.1 Ground Temperature ................................................................27 6.2.2 Stabilization Time .....................................................................28

6.3 FLOW REGIME IN BOREHOLE 7.....................................................29

6.4 EFFECTS OF THE EXTRA BRINE PUMP...........................................32 6.4.1 One Heat Pump........................................................................33 6.4.2 Two Heat Pumps ......................................................................34

6.5 EFFECT OF OPENING ONE EXTRA BOREHOLE................................34 6.6 TOTAL COP ...............................................................................36

7 DISCUSSION......................................................................................39 7.1 BRINE PROPERTIES.....................................................................39 7.2 GROUND TEMPERATURE..............................................................39 7.3 FLOW REGIME IN BOREHOLE 7.....................................................40

7.3.1 160 Meter Depth.......................................................................40 7.3.2 First 5 Meters............................................................................40

7.4 EFFECTS OF THE EXTRA BRINE PUMP...........................................41 7.5 EFFECT OF OPENING ONE EXTRA BOREHOLE................................42

8 CONCLUSIONS..................................................................................44 8.1 BRINE PROPERTIES.....................................................................44 8.2 GROUND TEMPERATURE..............................................................44 8.3 FLOW REGIME IN BOREHOLE 7.....................................................44 8.4 EFFECTS OF THE EXTRA BRINE PUMP...........................................45 8.5 EFFECT OF OPENING ONE EXTRA BOREHOLE................................45 8.6 TOTAL COP ...............................................................................45

9 RECOMMENDATIONS.......................................................................46

10 REFERENCES....................................................................................47

APPENDIX 1: BOREHOLE DIAGRAMS.......................................................49

APPENDIX 2: EFFECTS OF EXTRA BRINE PUMP.....................................59

APPENDIX 3: OPENING ONE EXTRA BOREHOLE....................................61

APPENDIX 4: SECONDARY APPENDICES ................................................64

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INDEX OF TABLES Table 4,1: Borehole depths Table 5,1: Error analysis of total brine flow calculation methods. Table 6,1: Viscosity comparison between brine and ethyl alcohol 23%weight. Table 6,2: Energy extracted from boreholes with 2 heat pumps. Table 6,3: Total COP, with and without extra brine pump. Table 6,4: Running time, with and without extra brine pump. Table 6,5: Total COP, with extra brine pump. Table 7,1: Effect of temperature difference and flow on total heat extraction. Table 7,2: Overall energy gained and lost. Table §B1: Total energy extracted from 7 boreholes. Table §B2: Effect of temperature and flow on energy extraction. Table §B3: Amount of extra energy extracted with extra brine pump on. Table §B4: Total energy extracted from 7 boreholes Table §B5: Effect of temperature and flow on energy extraction. Table §B6: Amount of extra energy extracted with extra brine pump on. Table §B7: Comparison of total energy extracted on different days. Table §C1: Amount of extra energy extracted opening one extra borehole. Table B1: Raw brine properties, given by Åke Melinder, 2006. Table B2: Ball and brine information for viscosity measurements. Table B3: Falling Ball Viscometer readings. Table B4: Detailed brine data Table C1: Brine flow measurement comparison, 06 March 2007. Table D1: Calculations to obtain performance curve for TOP-S 25/7,5 Table D2: Accurate flow values for ethyl alcohol, 23%-w, 5°C. Table E1: Effects of time averaging. Table G1: Sample excel spreadsheet including calculation, 11 Dec 2006. Table H1: Thermal Resistance calculations. Table I1: Trendline equations for detailed brine data Table I2: Detailed brine data Table J1: Recommended roughness values for commercial ducts [White, 2003].

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INDEX OF FIGURES Fig 1,1: Temperature-Entropy diagram of the Carnot Cycle. Fig 1,2: Temperature-Entropy diagram of a refrigerant. Fig 1,3: Two main configurations of ground source heat pumps, horizontal, vertical. Fig 1,4: Fully developed velocity flow profiles, laminar and turbulent. Fig 4,1: Schematic of borehole depths. Fig 4,2: Schematic of thermocouple mounting. Fig 4,3: Schematic of heat pump side (“hot side”) of the system. Fig 4,4: Pipe dimensions, borehole pipe on left, total flow pipe on right. Fig 5,1: Difference in Reynolds number as a function of waiting time. Fig 5,2: Difference in heat extraction values as a function of waiting time. Fig 5,3: Difference in heat extraction as a function of waiting time. Fig 6,1: Borehole 7 temperature profile at various depths and flows. Fig 6,2: Temperature profile at varying depths in borehole 7. Fig 6,3: Borehole 7 temperature profile at 15 m depth, 29 Jan 2007. Fig 6,4: Borehole 7 diagram, Reynolds number and energy extraction. Fig 6,5: Borehole 7 diagram, XX44444, 2 heat pumps + extra, flow 0,65 L/s. Fig 6,6: High heat transfer regions in borehole. Fig 6,7: Total heat extracted opening extra boreholes, 16 Feb 2007. Fig 6,8: Total heat extracted opening extra boreholes, 19 Feb 2007. Fig 6,9: Total heat extracted opening extra boreholes, 06 March 2007. Fig 6,10: Total energy extracted from boreholes, 24 hours. Fig §A1: Re and energy extraction from BH7, 2HP+E, 444444V, 05 Feb 2007. Fig §A2: Re and energy extraction from BH7, 2HP+E, 444444V, 02 Feb 2007. Fig §A3: Re and energy extraction from BH7, 2HP+E, 444444V, 11 Dec 2006. Fig §A4: Re and energy extraction from BH7, 2HP+E, XX4444V, 05 Feb 2007. Fig §A5: Re and energy extraction from BH7, 2HP+E, XX4444V, 02 Feb 2007. Fig §A6: Re and energy extraction from BH7, 2HP+E, XXX444V, 05 Feb 2007. Fig §A7: Re and energy extraction from BH7, 2HP+E, XXX444V, 02 Feb 2007. Fig §A8: Re and energy extraction from BH7, 1HP+E, 444444V, 29 Jan 2007. Fig §A9: Re and energy extraction from BH7, 1HP, 444444V, 11 Dec 2006. Fig §A10: Re and energy extraction from BH7, HP, XX4444V, 11 Dec 2006. Fig §C1: Re and energy extraction from BH7, 2HP+E, 19 Feb 2007. Fig §C2: Re and energy extraction from BH7, 2HP+E, 19 Feb 2007. Fig A1: Nusselt number as a function of Reynolds number Fig B1: Data plot, trendline, and accuracy plot of trendline for density Fig B2: Data plot, trendline, and accuracy plot of trendline for thermal conductivity Fig B3: Data plot, trendline, and accuracy plot of trendline for specific heat Fig B4: Data plot, trendline, and accuracy plot of trendline for dynamic viscosity Fig B5: Data plot, trendline, and accuracy plot of trendline for kinematic viscosity Fig F1: Temperature profile at 5m depth, 29 January, 2007. Fig F2: Temperature profile at 15m depth, 29 January, 2007. Fig F3: Temperature profile at 75m depth, 29 January, 2007. Fig F4: Temperature profile at 160m depth, 29 January, 2007. Fig F5: Temperature profile at 5m depth, 31 October 2006. Fig F6: Temperature profile at 15m depth, 31 October 2006. Fig F7: Temperature profile at 75m depth, 31 October 2006. Fig F8: Temperature profile at 160m depth, 31 October 2006. Fig H1: Illustration of violation of 2nd law of thermodynamics. Fig J1: The Moody chart for pipe friction with smooth and rough pipe walls.

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NOMENCLATURE Notation: E energy [kW] V volume flow [m3/hr, m3/s, l/s] Nu Nusselt number [-] Q heat [kW] Re Reynolds number [-] ρ density [kg/m3] v kinematic viscosity [m2/s] μ dynamic viscosity [Pa*s] Units: Kg kilo gram kPa kilo pascals kW kilo watts l/s litres per second m3/hr meters cubed per hour MVP meter-vatten-pellare (pressure below 1m height of water) Measuring Tools: ARMATEC digital measuring instrument DA system data acquisition system TA meter Tour-Andersson measuring instrument TC thermocouple Other: BH borehole COP coefficient of performance

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1 INTRODUCTION

1.1 Heat Pumps A vapour compression heat pump is a system that is able to absorb heat from a lower temperature level and reject it to a higher temperature level. It consists of 4 components: the evaporator, compressor, condenser, and expansion valve. The vapour compression cycle is the same cycle used for refrigeration, the main difference being the purpose of the application, either heating or cooling [Boyle, 2005]. The ideal heat pump cycle is represented by the Carnot Cycle, shown in figure 1,1 below. Heat is absorbed at the lower temperature (TC), compressed, and rejected at the higher temperature (TH). There is a need for energy input, since the overall system is opposing the second law of thermodynamics which states that heat cannot naturally pass from a colder to a hotter place.

Fig 1,1: Temperature-Entropy diagram of the Carnot Cycle [Wikipedia, 2006]. .

Energy is represented by the areas between the plotted lines. The white and pink areas combined indicate the amount of heat rejected by the system, while the pink area indicates the amount of energy required by the system. As mentioned above, a vapour compression heat pump consists of an evaporator, compressor, condenser, and expansion valve. A refrigerant circulates through the heat pump, and experiences the following transformations (see figure 1,2). At point 1, the refrigerant is a saturated vapour at low pressure and low temperature. As it travels to point 2, it becomes compressed and obtains a higher pressure and temperature level. The superheated vapour is cooled and condensed by the condenser, as it moves to point 4. Through an expansion valve, the refrigerant decreases pressure and temperature to achieve a liquid-vapour state (point 5). The refrigerant runs through an evaporator and absorbs heat from the heat source to become a saturated vapour again.

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In real cycles, the refrigerant experiences an entropy increase between points 1 and 2, which results in a higher temperature superheated vapor.

Fig 1,2: Temperature-Entropy diagram of a refrigerant [Wikipedia, 2006].

Considering the second law of thermodynamics, energy input is required by the system shown in figure 1,2. The energy is used by the compressor, which compresses the vapour from low to high pressure and temperature (point 1 to 2). The amount of energy used by the compressor is important in relation to the amount of energy out of the system. This is defined by the coefficient of performance.

1.2 Coefficient of Performance The coefficient of performance (COP) is valuable to understand the efficiency of the system. The COP for the ideal Carnot Cycle is defined by equation 1,1. This efficiency is never attained in practice, as there are always irreversibilities encountered, such as the loss of heat.

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1

TTTCOPcarnot −

= EQ 1,1

For practical applications, COP1 can be defined as the ratio of the heat output from the system (Q1) to the operating energy of the system (E). [Granryd, 2005]. Components that consume energy include the compressor, as well as pumps to increase the flow in the system.

EQ

COP 11 = EQ 1,2

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1.3 Borehole Configurations Depending on the heat demand, a ground source heat pump system consists of one or more heat pumps and boreholes. There are two main configurations of boreholes; shallow coils arranged horizontally or vertical tubes placed in the rock. It has been noted that “both give in practice roughly the same heat source characteristics (roughly equal temperatures on the fluid) with the normally used dimensioning criteria.” [Granryd, 2005] However, horizontal ground coils require a much larger land area, and are practical for properties such as farmland. Vertical boreholes require much less surface area, and tend to be in the order of 100-150m deep for a single-family house [Granryd,2005]. Fig 1,3: Two main configurations of ground source heat

pumps; horizontal (left), vertical (right). [Natural Resources Canada, 2005]

Ground condition must also be taken into consideration. The type of rock affects the transfer of heat to the borehole. Thermal conductivity values are highest for hard rock and lower for unconsolidated rock, with a difference of 1.5 W/mK between the two [Boyle, 2005]. Clearly, the rock type with higher thermal conductivity will yield a higher energy value per meter.

1.4 Laminar and Turbulent Flow The amount of heat absorbed by borehole is very much dependant on the type of flow of the brine fluid through the boreholes. Turbulent flow, characterized by random and chaotic fluid motion, absorbs more heat from its surroundings than laminar flow, where the flow progresses in a smooth and orderly manner. [Caughey et al., 2001]. When operating a heat pump system where the flow regime is laminar, a higher temperature difference is required in the boreholes compared to the same case with turbulent flow. A fully developed, incompressible flow through a pipe will have a different velocity profile depending on whether it is laminar or turbulent. Profiles are shown in figure 1,4 below. The laminar flow profile is parabolic, while the turbulent flow profile is flatter at the centre of the pipe. According to the no-slip condition, zero velocity exists at the pipe walls. The flow develops as a function of the radius, where it is fastest in the centre of the pipe, the furthest point from the wall. In turbulent flow, the existence of eddies due to velocity and pressure changes disturb the natural flow and cause the profile not to achieve the parabolic characteristic [Sabersky, 1989].

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Fig 1,4: Fully developed velocity flow profiles, laminar (left), turbulent (right)

[Nuclear Power Fundamentals, 1998].

1.5 Reynolds Number Osborne Reynolds conducted experiments and found that in general, flow transitions from laminar to turbulent following a pattern. He defined a dimensionless parameter, the Reynolds Number (Re), which is used to determine whether a flow is laminar or turbulent. The Reynolds number is a ratio between dynamic and viscous forces, as defined in equation 1.3.

ν

du avg=Re EQ 1,3

where uavg = average velocity (m/s)

d = diameter of pipe (m) ν = kinematic viscosity (m2/s) It must be noted that the kinematic viscosity, ν, is directly related to the dynamic viscosity in the following manner.

ρ

μν = EQ 1,4

where μ = dynamic viscosity (kg/m-s)

ρ = density (kg/m3) While operating ground source heat pumps, it is desirable to have turbulent flow through the boreholes in order to achieve efficient heat transfer from the surroundings to the brine fluid. In current pipe flow theory, it is generally accepted that the transition from the former to the latter occurs at a Reynolds number of 2300. The Moody chart is a useful chart which displays Re as a function of pipe roughness. Based on relative roughness of the pipe and Re, it is possible to see in which regime the flow lies. According to Moody, the following is true.

1 Laminar flow occurs below Re 2300 2 Re 2300 the critical zone begins and remains until over Re 3500

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3 At Re 3500 the transition zone begins and remains until over Re 10000 4 Beyond Re 10000 occurs completely turbulent flow

All the types of flow considered in this report lie within the critical zone or the transition zone. The Moody chart can be seen in secondary appendix J. According to a study done by Björn Kyrk, the transition of flow regimes occurs at approximately Re 3000 for undisturbed flow and 2300 for disturbed flow. This value was found experimentally, by plotting the Re against the Nusselt number (Nu), a dimensionless parameter used to measure the rate of convective heat transfer. The change in gradient suggested the transition point of the flow from laminar to turbulent. This diagram can be seen in secondary appendix A.

1.6 Extracted Heat The amount of heat extracted (Q) from the borehole is of primary importance. It is characterized by the type of rock the borehole is drilled into, the activity around the hole (such as water movement), the temperature of the ground, the depth of the borehole, as well as the type of brine fluid used. The amount of heat extracted can be estimated using the following equation.

TcVQ pΔ=•

ρ EQ 1,5

where Vdot = volume flow (m3/s) ρ = density (kg/m3) cp = specific heat capacity (J/kgK) ΔT = temperature difference (K)

The temperature difference is measured between two consecutive thermocouples.

1.7 Mariehäll Heat pumps are used in many applications; industrial, commercial, as well as residential. In the residential sector, heat pump systems are commonly used to heat water for domestic use such as tap water and space heating. Space heating is achieved through floor heating or installation of wall radiators. This report deals with an apartment complex located in Mariehäll, Sweden. There were 13 units in the building. The location included a ground source heat pump system that was used to heat domestic hot water and provide floor heating. The heat pump system at Mariehäll included 2 Viessmann heat pumps and 7 boreholes. The holes varied in depth from 90 m to 170 m, and the pumps had a rated capacity of 17.6 kW each. There was an extra brine pump which, under normal conditions, ran while both heat pumps were operating. The extra pump was used to increase the flow of the brine through the boreholes when necessary. The brine used was ethyl alcohol, containing a

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small percentage of additives isopropanol and n-butanol (less than 11% combined). Each borehole had a thermocouple at the top, which could read the ground temperature at the top of each borehole. Borehole 7 had seven additional thermocouples, at depths of 5 m, 15 m, 75 m, and 160 m, on both the down flow side and up flow side. The boreholes were drilled into rock that was primarily gneiss, with a small portion of silt and red granite, the first 6-12 m being silt. The rock that lay below the silt was thought to be unconsolidated, and according to borehole driller Brage Broberg, may once have been a lake [Broberg, 2006]. While drilling, ground water was met 2-4 meters deep.

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2 OBJECTIVES

The motivation for this research was to gain an understanding of what was occurring with the flow inside the boreholes. Once this was understood, the objective was to draw conclusions on how to run the system most effectively. The objectives of this report are clearly defined below.

1. Identify the type of flow in borehole 7 considering different flows and borehole configurations.

2. Determine the feasibility of running an extra pump loop in order to

increase the Re and heat extraction in the boreholes, with one and two heat pumps running.

3. Determine the optimal number of boreholes to operate by investigating

the extra heat extraction of opening one extra.

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3 METHODOLOGY

3.1 Step 1 The first objective was to identify the nature of the flow in borehole 7. This was done by investigating the power output of the boreholes at varying Re-values and pressure drops, with different borehole combinations. Both two and one heat pumps were running, in conjunction with the extra brine pump. The following steps were conducted in order to obtain required results.

a) Take a sample of brine for composition testing. b) Measure how the temperature in the boreholes changes, and

when it stabilizes after turning off the circulation. Measure ground temperatures with no borehole circulation.

c) Investigate the energy out of the boreholes (W/m) and how it changes with different Re-values, in order to estimate the spread of swirling, turbulent and laminar flow.

3.2 Step 2 The second step was to investigate the difference in total energy out of the borehole with and without the extra brine pump running, in the case of both one and two heat pumps in operation. This addressed the feasibility of running the extra pump loop to extend the length of turbulent flow in the boreholes. The following steps were taken.

d) Close different number of boreholes and compare how large the power output is with one vs. two heat pumps running, with and without the extra pump running.

e) Investigate the output energy from the borehole vs. the heat pumps, and estimate how many boreholes ought to be operating so that the extra pump circulation in the boreholes will be profitable.

3.3 Step 3 Next, it was necessary to optimize the number of boreholes operating. This was done by studying how large the energy contribution from opening one extra borehole became. The total COP of the system was examined.

f) Study the power out from one vs. two heat pumps, with different

numbers of boreholes open. Compare the difference when an additional borehole is opened.

g) Examine COPTOTAL with and without the extra brine pump running.

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4 EQUIPMENT

4.1 Technical Details The system at Mariehäll contained the following equipment at the noted specifications. 2 Viessmann Vitocal 300 heat pumps, rated power 13 - 17,6 kW 2 WILO TOP-S 25/7,5 brine pumps 1 WILO IPL 50/115-0,75/2 extra brine pump (750 W) 7 STA-D 40 valves 3 water tanks Note that the heat pumps were connected in parallel. The boreholes were also connected in parallel, and borehole 2 consisted of 2 boreholes. The boreholes were not drilled directly perpendicular into the ground, but rather on a slight angle of 10-15o. This was to avoid as much contact with each other as possible, as it could have affected the heat transfer and temperature difference in the boreholes [Hill, 2006]. The depths included in table 4,1 are the absolute lengths of the boreholes.

Borehole # Depth (m)1 1302 90+603 1304 1305 1306 1707 160

Fig 4,1: Schematic of borehole depths. Table 4,1: Borehole depths There were three water tanks that were kept at different temperatures, one used for floor heating, another for domestic hot water, and the third as an accumulator tank. The heating priority was to the floor heating first, and secondarily to the hot water tank. The floor heating tank was maintained up to 50oC, so that floor heating could be provided at 30oC (the difference was due to losses in the circulation system). The domestic hot water tank was kept at a maximum of 65 oC.

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The system contained 12 active thermocouples (TC), measuring temperature at the following locations. A comprehensive diagram is presented in figure 4,3. TC1 - 5m deep on the down-flow side of borehole 7 TC2 - 15 m deep on the up-flow side of borehole 7 TC3 - 75 m deep on the down-flow side of borehole 7 TC4 - 160 m deep in borehole 7 (AFTER the bend) TC5 - 75 m deep on the up-flow side of borehole 7 TC6 - 15 m deep on the down-flow side of borehole 7 TC7 - 5 m deep on the up-flow side of borehole 7 TC8 - exit of borehole 7 TC9 - just before the extra brine pump TC10 - just after the extra brine pump TC11 - at the entrance of borehole 7 TC12 - after the heat pumps, before entrance into the boreholes

Fig 4,2: Schematic of thermocouple mounting. A diagram of the thermocouple mounting is seen in figure 4,2 thermocouple. The thermocouples were mounted on the outside of the pipe (excluding thermocouples 8 and 11 which were on the inside). They were covered with a thin layer of aluminium foil, followed by 2 cm of insulation. The outermost layer was outer piping which had been shrunk to fit the form of the bulge, and to keep the thermocouple tightly placed. The distance from the centre of the thermocouple to the end of the insulation along the borehole wall was 20 cm. In other words, the pipe was insulated for 40 cm wherever there was a thermocouple. Since the thermocouple mounting took up space, the thermocouples of the same height level were offset by at least 40 cm. Measurements were taken by opening and closing certain boreholes, as well as adjusting the flows in the boreholes, focusing on borehole 7. Experiments were defined with a 7-digit notation, such as XXX444V. The letters and numbers represented boreholes 1 through 7 respectively, from left to right. An ‘X’ denoted that the borehole was fully closed. A number (1-4) denoted the STA-D 40 valve position of that borehole. A ‘V’ denoted ‘variable’, where the borehole valve position changed throughout the experiment. In the case mentioned, XXX444V, boreholes 1, 2, and 3 were closed, while boreholes 4, 5, and 6 were fully open. Borehole 7 had a varying valve position.

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4.2 Practical Background The first step was to learn about the system; the arrangement of the system and how to take measurements. A schematic of the borehole side of the system is presented below in figure 4,3.

Fig 4,3: Schematic of heat pump side (“hot side”) of the system.

The brine exited the boreholes into a common pipe of approximately 74 mm inner diameter. The extra brine pump was located before the heat pumps and allowed the flow to go through, whether it was on or off. The flow continued to the slave heat pump, then the master heat pump where the heat was extracted. Upon exiting the heat pumps, the brine continued along a common pipe, and flowed into the seven boreholes, with borehole 7 first. Since the boreholes were of different lengths, they had unique pressure drop and volume flow values. At the exit of each borehole, there was a STA-D 40 valve which was rotated from position 0 to position 4, where 4 meant fully open. The valve controlled the pressure in the borehole, and consequently, the flow. As the brine flowed up the pipe, it changed from high pressure to low pressure upon passing the valve. There were two measuring points located here, one which read high pressure flow (red) and the other reading low pressure flow (blue).

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4.3 Pipe Dimensions The piping was PVC, and pipe dimensions were as shown below. The first figure refers to the borehole piping, while the second refers to the pipe containing the total flow.

Fig 4,4: Pipe dimensions, borehole pipe on left, total flow pipe on right.

4.4 TA meter One method of measuring the pressure difference over the boreholes and the volume flow through each respective borehole was by using a TA-CMI meter, supplied by Tour and Andersson. It contained a high and a low pressure sensor which were hooked up to the system. The TA meter was calibrated with STA-D 40 type valve, brine concentration of 22.8% by weight, and varying valve positions and brine temperatures. It is important to note that the TA-CMI meter was inaccurate when the pressure drop is less than 3kPa (error of 30%). When the pressure drop was more than 3kPa, the error could have been greater than 10% [Sednert, 2006].

4.5 Data Acquisition System The brine flow was also measured with a flow meter which recorded the number of litres passing through the pipe in a set increment of time in borehole 7. This data was recorded by a data acquisition system (DA system), along with the thermocouple readings, the amount of time passed, and whether the heat pumps were on or off. This method was the most accurate and reliable way to measure volume flow.

4.6 Heat Pump Control For the purposes of this investigation, the heat pumps were manually controlled using the control panel. The primary control was of the master heat pump. The master and slave pumps, as well as their respective compressors, were turned on and off. The Viessman software, Vitocalc, was a good background to understand the workings of the heat pumps. With Vitocalc, the performance of the heat pumps could be observed under different conditions such as brine temperature or pump power.

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5 EXPERIMENTAL UNCERTAINTIES

5.1 Errors in Volume Flow The total volume flow running through the heat pumps was calculated using 3 different methods; the TA meter, data acquisition system, and the WILO CD. The three methods were compared to observe the errors between them.

5.1.1 Flow Measurement Methods The flow measured by the data acquisition (DA) system was the most accurate. It gave a pulse each time one litre passed through. It was situated at the exit of borehole 7, thus it was able to read only the volume flow of borehole 7. Values were collected and displayed on a computer. The TA meter directly read the volume flow through the STA-D valves on each borehole exit. Although it read the values to three decimal places, there was a lot of noise when trying to obtain the data. As much as 0,5 m3/hr of noise was present. The total flow was obtained by adding the separate volume flows together. By measuring the pressure drop over the extra brine pump, and the two small WILO pumps, the total flow was determined. The WILO CD provided performance curves for each pump, based on the properties of the liquid used (concentration and temperature). The liquid defined was ethyl alcohol with concentration of 23%-weight, and 5°C as the most commonly used temperature. The performance curve would read flow (m3/hr) on the x-axis, and pressure head (m) on the y-axis. Sven Franzén of WILO Sverige AB quoted an error of +/- 5% for the performance curves [Franzén, 2007]. Determining the total volume flow with the two small WILO pumps was more difficult than for the large extra pump. The performance curve available for the small pumps was for water at 20°C. Using another pump, values for water at 5°C were obtained, as well as ethyl alcohol at 5°C, 23%w. These values were used to estimate the performance curve of ethyl alcohol for the appropriate pump. Details are found in secondary appendix D. This was a large source of error.

5.1.2 Flow Comparison A comparison between the three flow methods was conducted, the results seen in table 5,1 below. Four cases were considered, with 7, 6, 5 and 4 boreholes open. The flow from each borehole was taken using the TA meter. Simultaneously, the DA system recorded the flow through borehole 7 (the most accurate flow reading). The total flow was found by applying the same difference between the DA system’s reading and the TA meter for borehole 7 to all the borehole readings from the TA meter. The WILO CD was used to obtain total flow from the pressure drop over the extra brine pump.

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4444444 BH1 BH2 BH3 BH4 BH5 BH6 BH7 TOTAL % AccuracyTA meter 2,31 2,37 2,42 2,35 2,36 2,11 2,07 16,00 0,98DA system 2,36 2,42 2,47 2,40 2,41 2,15 2,11 16,32 1,00WILO 21,10 1,29

X444444 1 2 3 4 5 6 7 m3/hrTA meter 2,54 2,65 2,50 2,36 2,19 2,21 14,46 0,99DA system 2,56 2,67 2,51 2,38 2,21 2,22 14,56 1,00WILO 17,00 1,17

XX44444 1 2 3 4 5 6 7 m3/hrTA meter 2,63 2,56 2,42 2,39 2,27 12,26 0,97DA system 2,71 2,64 2,49 2,47 2,34 12,65 1,00WILO 14,60 1,15

XXX4444 1 2 3 4 5 6 7 m3/hrTA meter 2,70 2,57 2,41 2,26 9,94 0,94DA system 2,87 2,73 2,56 2,40 10,55 1,00WILO 14,30 1,36

Flow (m3/hr)

Table 5,1: Error analysis of total brine flow calculation methods. The highlighted column shows the percent accuracy of the total flows, as compared with the DA system. The TA instrument was fairly close, with 94% - 99% accuracy. The results from the WILO CD had significantly more error, in the range of 15% - 36 % error. This error was very large, and consequently the calculations were performed using the other methods wherever possible. In the case of no extra pump running, the WILO CD had to be used because flows were too low to be read by the TA meter. Readings from the WILO CD could be the cause of some strange results found in further sections. The same investigation performed on another occasion can be seen in secondary appendix C.

5.2 Time Error

5.2.1 Time Error It took approximately 7 minutes for the brine to circulate once through the entire system [Hill, 2007]. After making a change to the system, the amount of time required to wait was important, in order for the system to stabilize and achieve a steady state once again. The case of 4444444 was considered. A measurement was taken 20 minutes after changing the system from 4444441. A second measurement was taken 55 minutes after the change. Figures 5,1 and 5,2 display the results.

22

Reynolds Number: 444444 4,0

50005050510051505200525053005350540054505500

0 1 2 3 4 5 6 7 8

Thermocouple Location

Re T=20 min

T=54.7 min

Fig 5,1: Difference in Reynolds number as a function of waiting time.

Heat Extracted per Meter: 444444 4,0

-40-30-20-10

01020304050

0 1 2 3 4 5 6 7

Sections along borehole

Q (W

/m)

T=20 minT=54,7 min

Fig 5,2: Difference in heat extraction values as a function of waiting time.

Re was found to have increased by approximately 250 for the second measurement. This change was very small, and did not influence the energy extracted energy per meter significantly, seen in figure 5,2. The total energy extracted was 783 W for the first case, and 803 W for the second case, a difference of only 20 watts. It could be concluded that a 20 minute waiting period between measurements was sufficient time for the system to stabilize, without significantly altering the data. All data was taken with at least 20 minutes waiting time. In contrast, the same study was conducted for a 15 minute waiting period. The energy extracted per meter varied significantly, especially for the first and last sections of the borehole. The difference was as high as 20 watts per meter, resulting in the total heat extracted to be 180 W higher for the longer waiting

23

period. This is viewed in figure 5.3 below. Re had no significant difference for the two cases. It was concluded that a 15 minute waiting period was too short.

Energy Extracted per Meter: 444444 4,0

-55

-35

-15

5

25

45

65

0 2 4 6 8

Section along borehole 7

Ene

rgy

Ext

ract

ed (W

/m)

T = 16 minT = 52 min

Fig 5,3: Difference in heat extraction as a function of waiting time.

5.2.2 Averaging Temperature Values To investigate the signal noise on the temperature readings, the average of 7 consecutive temperature readings was taken. It was repeated in several cases, and was found that the difference between logged values and averaged values was between 0,01°C to 0,03°C. Further values are included in secondary appendix E. The difference was insignificant for the purposes of this report.

5.3 Other Errors The amount of energy extracted from the boreholes was a function of how much heat the heat pumps require. This effected the data taken, and could not be controlled. The weather and habits of tenants affected the data collected. The ARMATEC instrument took power measurements (kWh) once every 2 minutes and 10 seconds. Having 20 minutes between readings meant that errors would be 10%. This was eliminated by taking kilowatt-hour values over a 24-hour period. Results can be seen in section 6.6. The temperature measurements taken by the ARMATEC would fluctuate minimally. The flow and energy measurements would fluctuate significantly, often as much as 3%, and a middle value was taken. The thermocouples were offset from each other along the pipes, so they could fit into the boreholes. The shortest possible distance between thermocouples

24

was 40 cm, while the longest could have been 1 m. This distance could have had a significant impact for the readings in the first 5 m of borehole 7. All thermocouples (except for TC8 and TC11) were placed on the outside of the pipe and covered with aluminium foil, 20 mm of insulation, and finally outer piping. Thermocouples 8 and 11 were placed inside the pipe, and thus gave the most accurate temperature readings. Thermocouple 4 was situated after the bend at the bottom of borehole 7. The velocity profile through the pipe varied depending on where the flow was. It could have been laminar and fully developed, not fully developed, heavily swirling, or perhaps turbulent. The temperatures read by the thermocouples were the temperature close to the wall, not the average temperature. The temperature profile of the flow was not considered. This may have had an effect on temperature readings. The uncertainties using the TA instrument were high. If the pressure was below 3kPa, the error was as high as 30% [Sednert, 2006]. The flow and pressure values would fluctuate by as much as 20%, and a middle value had to be taken. The time between each measurement was 20 minutes. The results of a having a longer waiting period are discussed in section 5.2.1 above, and concludes that 20 minutes was sufficient waiting time between measurements. While the flow may have stabilized, the ground may have needed a longer time to stabilize. This could have had an effect on data taken. It must be noted that all data was taken in different orders to eliminate coincidental errors. Data was taken in one direction, then the reverse direction, and sometimes in random order as well.

25

6 RESULTS

6.1 Brine Properties The specific properties of the brine were obtained by taking a sample of the brine, obtaining its density at a specific temperature using a mercury meter, and finally inputting this data into a spreadsheet composed by Åke Melinder, a lecturer at KTH researching property trends of fluids. The brine solution used was ethyl alcohol with ethanol concentration of 22,84% by weight, and freezing point at -13,5oC. Additives were n-butanol (2%) and isopropanol (7-9%), as specified by the supplier Masons Kem. Tekn. AB. The properties at seven different brine temperatures were determined, and graphs were composed of these properties as a function of the brine temperature. A trendline was fitted to each graph, and all were found to fit with greater than 99% accuracy. The trendline equations were used to interpolate more detailed brine properties, which were used for further calculations. The graphs, trendlines, equations, and specific brine properties can be found in secondary appendix B. The additives in the brine, namely isopropanol and n-butanol, composed as much as 11% of the solution. The brine properties described above were determined based on values that considered no additives. Viscosity tests were conducted to check the accuracy of the calculated brine properties. The dynamic viscosity of the brine was compared with the dynamic viscosity of an ethyl alcohol solution of 23 %-weight concentration, at 20 oC. The ethyl alcohol had a higher viscosity than the brine solution. The values are shown in Table 6,1. The similarity between the measurements was found to be 83%, making the error 17%.

Dynamic Viscosity μ (mPa*s)Ethyl Alc. Brine %difference

2,40 1,98 0,8261 Table 6,1: Viscosity comparison between brine and ethyl alcohol 23%weight.

The dynamic viscosity of the ethyl alcohol solution was 100% in accordance with the values given by Åke Melinder. It must be noted that the brine concentration was changed in November 2006. The concentration was reduced from 27%-weight to 23%-weight, in order to increase the Re values. The change resulted in a 20% decrease in kinematic viscosity, and a 2% increase in specific heat of the brine fluid in the temperature range considered. This lead to increased Re values.

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6.2 Ground Temperatures

6.2.1 Ground Temperature Plots of the temperature in borehole 7 at different height levels running at various flows can be seen in figure 6,1. Note that the legend refers to the valve rotation position of borehole 7, and there was only 1 heat pump running.

Temperature Profiles as a funtion of depth, BH7, XXXXXXv

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0 50 100 150 200 250 300 350

Length travelled in borehole (m)

Tem

pera

ture

(deg

C)

Position 3Position 2Position 1

Fig 6,1: Borehole 7 temperature profile at various depths and flows.

The temperature constantly rose until reaching the 15m thermocouple on the way up. There was an abrupt heat loss experienced at this point, which was unexpected. Concurrently, the first 15 m absorbed very much heat. As the valve was adjusted from position 1 to position 3, the temperature profile in figure 6,1 was seen to increase. Position 3 produced a lower pressure drop across the borehole and higher volume flow. Higher flow resulted in less heat being transferred from the ground to the borehole, thus the ground temperatures remained at higher values. Borehole 7 was closed for more than two days in order to obtain an accurate measure of the ground temperature at the different thermocouple levels. The temperature profile taken in October 2006 and again in January 2007 can be seen in figure 6,2 below. The x-axis denoted the length that the fluid had travelled in the pipe, and it must be remembered that the deepest length was 160m after which the flow turned and flowed back up. The total difference of temperatures thorough the length of the borehole was 1 oC to 2oC.

27

Ground Temperatures, Borehole 7

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

0 50 100 150 200 250 300 350

Distance along Borehole 7 (m)

Tem

pera

ture

(deg

C)

19-Feb24-Oct

Fig 6,2: Temperature profile at varying depths in borehole 7.

By comparing the plots, it was seen that the ambient temperature affected the borehole temperature as deep as 75 m into the ground. Snow and frozen ground also may have had an effect. In October, the borehole temperatures were much warmer for the first 75 m, exceeding the temperature of the deepest part of the borehole by almost 1,5 oC. In January, the temperature profile increased and decreased by less than 0,5oC from ground level to 75 m deep. From the 75 m point, the trend was the same for both cases, with temperature increasing until 160 m, and decreasing on the route back up.

6.2.2 Stabilization Time After turning off the circulation, it was noted how the temperatures at each level of borehole 7 behaved. In theory, the temperatures of the same levels should have met each other. At all ground depths, the temperatures of the down and up flow at the same level met each other within the first 10 – 30 minutes. Deeper levels took longer time to reach the same temperature. During operation, the ground temperatures on the down-flow side were lower than that on the up-flow side, this could have been due to the thermocouple offset. Within 30 minutes or less, the temperatures met each other, and the down-flow side obtained a higher temp than the up-flow side, about 0,1oC. The temperatures on both sides continued to rise, but the rate that they did so was approximately the same. The case at 15 meters depths can be seen in figure 6,3. Refer to secondary appendix F for corresponding plots at other depths.

28

Stabilization Time, 15m, T2 & T6

5.4

5.6

5.8

6

6.2

6.4

0 20 40 60 80 100 120

Time (min)

Tem

pera

ture

(deg

C)

T6T2

Fig 6,3: Borehole 7 temperature profile at 15 m depth, 29 Jan 2007. This comparison was a good method to check the accuracy of the thermocouples. The thermocouples of the same height should have tended towards the same temperature after some time, which they did do. This suggested that there was no break or error in the thermocouple readings.

6.3 Flow Regime in Borehole 7 The flow through borehole 7 was examined via 7 thermocouples placed in the borehole as described earlier. The objective was to examine the behaviour of the flow within the boreholes, in order to optimize the flow to extract the most energy from the hole with the least power input. Higher volume flow corresponded to higher Reynolds number. This in turn yielded more energy extracted from the boreholes. The pressure in the borehole was inversely proportional to the flow, thus when the flow increased the pressure consequently decreased. Consider the case XX44444 with 2 heat pumps and the extra pump running, shown in figure 6,4. The STA-D valve for borehole 7 was open to 0,5 in the first case (red) and 1,0 in the second case (green). The Re values increased as the flow travelled along the borehole. They continued to increase until they reached the 5m level on the up-flow side, where they decreased slightly. The temperatures followed the same trend, as Re values were directly proportional to the temperature readings. It was interesting to note the decrease of temperature in the last section of the borehole.

29

Fig 6,4: Borehole 7 diagram, Reynolds number and energy extraction.

30

Energy extracted from the borehole was directly related to the temperature difference between thermocouples. The temperature difference on the down-flow side was greater than the temperature difference on the up-flow side by a few tenths of a degree. This could be due to the thermocouple offset. The difference between temperatures is generally 0,2oC to 0,5 oC. It can be seen that the energy extracted decreased as the flow travelled down the borehole. After turning 180o to travel back up, the flow extracted much more energy. The energy again decreased on the path up the borehole, becoming negative for the final section. It is of interest to note that after the bend (160 m deep), the extracted energy was significantly higher. Consider the case in figure 6,5. The energy extraction values were average values taken over the length of the section. The bottom most section was 85 m long, which was a significant length to average over. If one considered the energy extraction in the previous section of borehole 7, assumptions could be made as to a probable heat extraction value (seen in red). Please note these were not calculated figures, rather rough estimates. On the down-flow side, Q must have been much higher than 4,7 W/m close to 75 m depth, and much lower closer to 160 m depth, following the decreasing trend. On the opposite side, the converse was true. Focusing on the bend, it was clear that there was significantly more energy extraction after the bend than before it. This suggested turbulence.

Fig 6,5: Borehole 7 diagram, XX44444, 2 heat pumps + extra, flow 0,65 L/s.

Following research by Björn Kyrk, for all cases that had Re over 2000, the trend described above was seen in regards to the behaviour of the flow. The remaining cases followed a trend where heat extraction continued to decrease as the flow moved along the borehole. It was difficult to note when precisely

31

the trend changed. Without the extra brine pump in operation, Re was less than 2000. With the extra brine pump in operation, Re was greater than 4500. Thus, it was unclear exactly when the transition in behaviour occurred. For the first fifteen meters of the borehole on the down-flow side, the amount of energy extracted per meter was very large. This could have been due to turbulent flow as a result of the bends in the pipe coming from the building. In the same region on the up-flow side, the energy extraction values were negative. Negative values denoted heat dispensed to the surroundings rather than being absorbed by the brine. This was seen in 100% of the cases. Why there was so much heat absorbed and dispensed within the first fifteen meters was an interesting point to consider, and will be further discussed in section 7.3. There was more energy lost with higher Reynolds numbers. This was due to increased flow.

The regions of highest heat transfer are detailed in the figure to the left. The inlet and exit of each borehole had the potential to loose energy due to the disturbed nature of flow in the pipes. The 85 m section after the bend gained much energy. This energy gain was continued up to the next section. How far it travelled was a function of Re. Fig 6,6: High heat transfer regions in borehole.

Following the flow on the down-flow side, the energy extraction value decreased. This suggested transition from turbulent to laminar flow. Cases considering the following variables can be seen in appendix 1.

1 varying STA-D valve positions (0,5 through to 4) 2 one or two heat pumps functioning 3 with and without the extra pump running

6.4 Effects of the Extra Brine Pump The effect of the extra brine pump was determined by investigating the amount of energy extracted from the boreholes while it was functioning and while it was off. The flow played a critical role as it was directly proportional to the amount of energy extracted. The temperature difference between thermocouples was also a large influence.

32

The total flow was obtained by noting the pressure drop across the extra brine pump. When this pump was off, the flow was found by the pressure drops over the small pumps at the entrance of each heat pump. The pressure-head versus mass flow curves provided by the WILO CD were used to estimate the total mass flow across the pumps. Errors in this method are discussed in section 5.1. Various cases were considered, with different numbers of boreholes in operation. Data was obtained in varying orders, to eliminate the possibility of systematic errors.

6.4.1 One Heat Pump The results using 1 heat pump with and without the extra brine pump can be seen in table 6,1. Three separate cases were considered. The energy extracted from the boreholes with the extra pump was less that that extracted without the extra pump, as seen by the highlighted values. This result was curious, and will be further investigated in the discussion section. With the extra pump running, the flow was higher (approximately twice as fast), while the temperature drop across the system was substantially lower (2 to 5 times in magnitude). It must be noted that the energy values with and without the extra pump were very close to one another. Errors in flow estimates could have played a significant role in calculated energy values. The energy extracted from borehole 7 was greater with the extra heat pump operational, as can be seen in the third column in table 6,1. This gave an indication that perhaps flow values had a large effect on Q values, a larger effect than the temperature difference. Diagrams of the flow and energy extracted in borehole 7 for this case can be seen in appendix 2.

pumps case ΔT (t9-t12) Total Flow TC 5 BH 7 BH total(deg C) (L/s) (deg C) P (kW) P (kW)

4444444 0.43 5.78 5.20 0.88 10.49XX44444 0.59 4.69 5.10 1.99 11.72XXX4444 0.78 3.25 4.96 2.58 10.644444444 2.16 1.39 5.31 0.79 12.60XX44444 2.12 1.39 5.37 1.75 12.40XXX4444 2.14 1.39 5.16 2.12 12.47

1HP+extra

1 HP

Table 6,1: Energy extracted from boreholes with 1 heat pump.

The results seen are a function of the temperature difference between the ground and the brine, as well as the number of boreholes open. The total flow values for 1 heat pump are the same due to the pressure drop (MVP) over the small pumps on each heat pump. The same pressure drop yielded the same total flow value based on the pump’s performance curve.

33

6.4.2 Two Heat Pumps When two heat pumps were operating, the result was different. Highlighted values in table 6,2 showed that more energy was extracted when the extra brine pump was operational. With the extra pump running, the flow was approximately 1,5 to 2,5 times faster than without the extra pump. The temperature difference across the system was approximately the same in magnitude, but slightly lower. This indicated that while the flow was higher with the extra brine pump on, the temperature difference caused by the flow was the important factor to be considered. The temperature difference would help to conclude whether the extra pump should be operated.

pumps case ΔT (t9-t12) Total Flow TC 5 BH 7 BH total(deg C) (L/s) (deg C) P (kW) P (kW)

4444444 1.08 5.81 5.02 3.03 26.35XX44444 1.45 4.00 4.65 4.54 24.30XXX4444 1.68 3.25 4.15 5.41 22.924444444 2.57 2.28 5.32 2.38 24.63XX44444 2.29 2.28 4.85 1.95 21.91XXX4444 2.96 1.78 4.75 5.11 22.15

2HP+extra

2HP

Table 6,2: Energy extracted from boreholes with 2 heat pumps.

In the case seen by table 6,2, the faster flow induced by the extra brine pump increased energy extraction by 3 -10%. In other words, 1 – 2,5 kW extra were extracted with the extra pump on. The extra pump operated at 750 W, indicating that the net energy extracted was larger with the extra pump on rather than without. A few days were allowed to pass between taking data with the extra brine pump on and with it off. The objective was to note if there was any error due to ground temperature instability. Perhaps the ground temperatures had not had enough time to stabilize between readings when there was only 20 minutes between measurements. The results can be seen in appendix 2 and show that there was no significant difference between the two readings.

6.5 Effect of Opening One Extra Borehole There was more energy extracted from the system when more boreholes were open. The most energy should have been extracted when all 7 boreholes were open. The aim was to see how much more heat was extracted by consecutive boreholes being opened. In order to reduce errors, the data was taken starting with 2 boreholes open and opening 1 more consecutively, as well as the reverse direction (7 boreholes open, closing 1 consecutively). Consider the case shown in figure 6,7. The amount of energy extracted increased as more boreholes were open, as was logical. It can be seen that the most drastic change occurred between having two and three boreholes open. The amount of extra energy extracted in this case was 13 kW. The energy extracted in the other cases ranged between 1,5 – 5 kW. There was

34

no pattern where opening a consecutive hole yielded a lower or higher amount of extra energy than opening the next one. This was perhaps due to the varying depths and flows of the boreholes. Energy extraction differences are shown in appendix 3. The reasons for such high energy extraction opening 2-3 boreholes were as follows. There was almost 50% more borehole surface area to absorb energy. In addition, the pressure head over the extra pump was high for this region. The performance curve was flat in regions of high pressure head, and dropped significantly thereafter, following a fairly consistent trend. The pressure heads recorded in these cases fell in this region of the performance curve.

Heat Out as a funtion of number of boreholes open

0

5

10

15

20

25

30

1 2 3 4 5 6 7

Number of Boreholes OPEN

Heat

Out

(kW

)

Q

TC5 (oC): 3,24 3,80 3,81 4,46 4,53

Fig 6,7: Total heat extracted opening extra boreholes, 16 Feb 2007. The same trend can be seen in figure 6,8 below. It is curious to note the temperature drop when a 6th borehole is opened. Referring to the borehole diagram in appendix 3, it can be seen that Re and the heat extracted from borehole 7 were higher with 5 boreholes open than with 6. This meant that borehole 7 was consistent with the trend (total energy out increased when more boreholes are open). The other boreholes must have been contributing to the loss of energy in this case. A similar case can be seen in the second figure below.

35

Heat Out as a function of number of boreholes open

0

5

10

15

20

25

30

1 2 3 4 5 6 7

Number of Boreholes OPEN

Hea

t Out

(kW

)

Q

TC5 (oC): 4,30 3,97 4,59 4,48 5,11 4,98

Fig 6,8: Total heat extracted opening extra boreholes, 19 Feb 2007.

Heat Out as a function of number of Boreholes open(2HP + E)

0

10

20

30

1 2 3 4 5 6 7

Number of Boreholes Open

Hea

t Out

(kW

)

Q

TC5 (oC): 4,35 4,70 4,95 5,09

Fig 6,9: Total heat extracted opening extra boreholes, 06 March 2007. Although there are some exceptions, the general trend was that the total energy out increased when more boreholes were open. Surely, the two graphs above must be exceptions. Perhaps the system did not reach equilibrium between measurements which yielded inconsistent results. Consecutively opening (or closing) one more boreholes requires more research to be done, with more time in between measurements to assure the system obtains equilibrium.

6.6 Total COP In order to obtain an accurate COPtotal, measurements were taken with a 24 hour waiting period. The difference in kWh recorded by the ARMATEC corresponded to the amount of energy output from the heat pump system. The difference in kWh recorded by the electrical meter corresponded to the amount of energy input into the system as well as the lights and computer.

36

The first law of thermodynamics states the conservation of energy for a closed system. In other words, EQ 6,1

EQQ inout += The values obtained from the ARMATEC should have equalled the amount of energy from the heat pumps plus the energy used to run the compressor. Losses were considered insignificant and were ignored. The results were as shown in table 6,3 below. The COPtotal was interesting to note. The energy values (in kW) were an hourly average over a 24-hour period. The actual energy fluctuated between 25-28 kW with 2 heat pumps running, and 11-13 kW with one heat pump running. There were 2 cases considered. The system was not running continuously, rather, the heat pumps turned on and off to meet the demand. There could have been 2 heat pumps running, or 1 heat pump, or no heat pumps at any given time. The cases examined differed as follows. In the ‘Normal’ case, the extra pump was on when 2 heat pumps were in operation. In the ‘No Extra’ case, the extra pump was off when 2 heat pumps were in operation, in other words, the extra pump was never on.

NORMAL NO EXTRAEl Meter 137 119 kWhARMATEC 386 370 kWhCOP 2.82 3.11

Table 6,3: Total COP, with and without extra brine pump. In table 6,4 below, the amount of time that each heat pump ran over 24 hours can be seen. It is clear that both heat pumps ran for longer time in the ‘Normal’ case. This means that they also extracted more energy as is seen in table 6,3 above. The electrical meter also used more energy. The total COP was lower in the normal case. This is curious, since it was established earlier that when 2 heat pumps run with the extra pump, they extracted more energy than without the extra pump. The amount of extra energy was more than the energy required to run the extra pump. It would be logical that the ‘Normal’ case would have a higher COP than the ‘No Extra’ case. This is not what was seen, and further testing must be done. The results could be due to the fact that the brine temperature was higher with the extra pump operational, which meant that less total energy would be extracted from the ground, since the difference between brine and ground temperatures would be smaller. NORMAL NO EXTRA

Right HP 770 744 minutesLeft HP 785 765 minutes

Table 6,4: Running time, with and without extra brine pump.

Time intervals of 15 minutes were taken to construct the graph seen below. The system was running under normal conditions, with 7 boreholes fully open.

37

The extra pump ran when 2 heat pumps were operating, and was off when only 1 heat pump was in operation. Data was recorded every minute. From this data, it was seen that the system was on for 25-40 minutes at a time (generally between 32-35 minutes), and off for roughly the same amount of time. When the system was running, both heat pumps were in operation; however the slave heat pump usually turned off approximately 5 minutes before the master.

Total Energy Out, 24 hours, Normal Operation

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hours)

Ener

gy O

ut (k

W)

Q

Fig 6,10: Total energy extracted from boreholes, 24 hours.

When 1 heat pump was in operation the flow was 1,3 l/s, while with 2 heat pumps and the extra pump running the flow was 5,6 l/s. It must be noted that the flow changed with the changing brine temperature, but the effects of this were so small that they were negligible (less than 0.01 m3/hr for 5 degrees change). Since the temperature difference was less than 2 degrees, the flow value was assumed to be constant while the pumps were working. The COPtotal was calculated again based on average hourly values from the graph constructed (table 6,5). The COPtotal found in this case was lower than that seen in the previous section. This can be attributed to averaging errors. The graph uses one data point each 15 minutes within the 24 hour period.

Graph Avg 14.25 kWElec meter 5.71 kWDifference: 8.54 kWCOP: 2.50

Table 6,5: Total COP, with extra brine pump. Although the system was running under normal conditions, the outdoor temperature greatly effected the operation of the system. During measurements, the ambient temperature was between -2 oC to +4 oC. Under colder conditions such as -10 oC or less, the system would operate for longer periods off time, and consequently extract more energy from the ground.

38

7 DISCUSSION

7.1 Brine Properties The brine solution used was ethyl alcohol with ethanol concentration of 22,84% by weight, and freezing point at -13,5oC. Additives were n-butanol (2%) and isopropanol (7-9%), as specified by the supplier Masons Kem. Tekn. AB. Detailed brine properties were interpolated using values for ethyl alcohol 23%-weight concentration, given by Åke Melinder. Additives were not considered. Viscosity tests were conducted to observe the effect of the additives on the brine properties. The dynamic viscosity of the brine was compared with the dynamic viscosity of an ethyl alcohol solution of 23 %-weight concentration. The error was found to be 17%. Calculated values were altered accordingly.

7.2 Ground Temperature The ground temperature was determined by analyzing temperatures in borehole 7 with no circulation running through the borehole for more than 48 hours. Two sets of data (taken in October 2006 and January 2007) corresponded with each other from 75 m and deeper. The highest temperature was at the bottom of borehole 7, 160 m deep. Above 75 m, the ground temperatures increased in the case of October, and decreased in the case of January. This was due to ambient conditions. With flow circulation, the temperatures in borehole 7 were considered. When flow values were higher, the borehole temperatures were higher. The same trend was seen for various flows. The borehole temperature increased rapidly for the first 15 m, dropped slightly at 160 m deep, and dropped quite drastically for the last 15 m. These results are consistent with the energy extraction seen in sections above. The time it took for the ground temperatures to stabilize after circulation was turned off was 10-30 minutes. The amount of time to stabilize was proportional to the depth. The deeper parts took longer to stabilize. After stabilization, the ground temperatures on the up-flow side were lower than those on the down-flow side by approximately 0,1°C. This could have indicated errors in the thermocouple mounting. It could also have been the result of the thermocouple location, as the thermocouples of the same level were offset by 0,4 - 1 meters.

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7.3 Flow Regime in Borehole 7

7.3.1 160 Meter Depth In the majority of cases, there was much more energy extracted by the brine after the bend at the bottom of the borehole, 160 m deep. The absorption of extra heat suggested a change in the type of flow existing in the pipe. It was suggested that the bend caused the flow to become turbulent, and thus through its random and sporadic motion absorbed more heat [Beaumont, 2006]. It can be agreed that something happened to the flow after the bend. It experienced disturbance which caused it to lose some of its laminar properties, such as the laminar profile. The amount of energy extraction suggested turbulent flow. Alternatively, it perhaps had begun a swirling motion, and absorbed more heat from the surroundings due to heavy swirling. Palne Mogensen, an experienced heat energy consultant suggested that the flow was heavily swirling but somewhat predictable, rejecting it as turbulent. The behaviour described in the paragraph above was not seen for flows with Re lower than 2000. The flows in this Re range were considered laminar. Although the 180° bend caused disturbance in the flow, the flow mostly maintained its laminar properties. The brine was not travelling fast enough to cause significant disturbances while passing through the bend, and consequently the energy absorption trend was consistent with the rest of the borehole. While the Re range where transition occurred was not precisely seen, in can be concluded that it occurred somewhere between Re 2000 and 4500.

7.3.2 First 15 Meters The first fifteen meters of borehole 7 were puzzling. There was a lot of energy absorbed by the brine on the down-flow side, and much energy dissipated on the up-flow side. The energy values per meter were much higher in this section compared with any other section, often more than four times in magnitude. Consider the down-flow side. Since the flow was coming from the heat pumps, it experienced some disturbances in the form of pipe bends, which may have caused flow disturbance and extra energy uptake as a result. Although a factor, this was not the main contributing cause. The thermocouples were not placed exactly 10 meters apart. As described in section 5, the thermocouples could be offset as much as 1 meter. Considering a total length of 10 meters between thermocouples, this offset could have a significant effect on the energy values. Looking at the up-flow side, there was much energy dissipated from the brine to the borehole and surroundings. Some heat may have been absorbed by the down-side, resulting in a thermal shortcut; however this was not where most of the energy went. Calculations of heat transfer and thermal resistances suggested that the energy could not possibly be lost through a thermal shortcut, as this would violate the second law of thermodynamics. Refer to

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secondary appendix H for detailed calculations. Most of the energy seemed to be lost to the surroundings. The amount of energy lost was directly proportional to the brine flow. Higher Re yielded more energy loss. The ground temperature was close to the brine temperature, which facilitated energy dissipation. The precise length over which energy loss occurred was unknown. Higher Re resulted in a longer length of energy loss. It was known that energy was lost within the first 15 meters, after that the length of energy loss was a function of Re.

7.3.3 Turbulent to laminar transition The energy extraction on the down-flow side of the borehole consistently decreased, even though Re was increasing. This occurred with Re as high as 6000. After the bend, there was a significant increase in energy extraction. This suggested transition from turbulent to laminar flow on the down-flow side. Although the difference between ground and brine temperatures had an effect on the amount of energy absorbed by the system, having turbulent flow yielded a larger effect. Turbulent flow is desired through the boreholes.

7.4 Effects of the Extra Brine Pump In the case of a single heat pump, there was less energy extracted from the system without the extra brine pump running compared to with it. The amount of extra heat extracted was in the range of 1,4 - 4 kW. Errors in flow could have had a significant effect with energy calculations. The flow was obtained by reading pressure drops over the small pumps for each heat pump, and using the WILO CD. The errors in flow calculation can be considered a large contributing factor to the results found. Shown in table 7,1 are the ratios with and without the extra brine pump for temperature difference and flow. As seen in the highlighted column, the temperature difference in the borehole had a larger effect on heat extraction than the increase in flow for the case of a single heat pump.

pumps case HP / HP+E HP+E / HP Temp/FlowTemp Flow

4444444 5.00 4.16 1.20XX44444 3.58 3.38 1.06XXX4444 2.74 2.34 1.174444444 2.38 2.55 0.94XX44444 1.58 1.76 0.90XXX4444 1.77 1.83 0.97

1 HP

2HP

Table 7,1: Effect of temperature difference and flow on total heat extraction.

As a result of the temperature difference, there was less heat extracted when the extra pump was operational, as seen in the highlighted section in table

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7,2. These values denote the amount of energy gained (positive) or lost (negative) in the cases mentioned. The most energy was lost when there were 7 boreholes functioning, with less heat lost as fewer boreholes were used. Further experiments to support this are included in appendix 2.The loss of energy in the case of 1 heat pump was attributed to the very small temperature difference, along with errors in flow values. It can be generally stated that the results seen in table 7,1 are a function of the difference in temperature between the ground and the brine, as well as the number of boreholes open. With less boreholes open, there could be a feasible use for the extra pump.

pumps case(%) (kW)

4444444 0.83 -2.12XX44444 0.95 -0.68XXX4444 0.85 -1.834444444 1.07 1.72XX44444 1.11 2.39XXX4444 1.03 0.77

1 HP

2HP

Power Difference

Table 7,2: Overall energy gained and lost.

When 2 heat pumps and the extra pump were operational, the increased flow had a larger effect, and the temperature drop decreased (table 7,2 above). There was more heat extracted while the extra brine pump was running. Extra energy extracted from the boreholes was in the range of 1-2,5 kW. Since the extra brine pump required 700 W to operate, it was a net gain to run the extra brine pump. The extra pump extracted the most amount of energy in the cases of all 7 boreholes open and 5 boreholes open. These experiments were repeated, and the same trends were found. Further results can be seen in appendix 2.

7.5 Effect of Opening One Extra Borehole Opening one extra borehole yielded extra energy out from the system as a whole. While the system extracted more energy from the ground, borehole seven extracted less energy as was expected. The highest amount of extra energy extracted was by opening a third borehole. This was due to the percentage of extra area available to absorb energy, as well as the pressure drop over the extra pump. Thirteen kilowatts of extra energy was obtained. Opening the other boreholes yielded 1,5 - 5 kW of extra energy per hole. There was no trend that showed decreasing amounts of extra energy extracted when more boreholes were opened, or the reverse. This could be due to the varying lengths of the boreholes, as well as the flow within each hole. As one more borehole was opened, the amount of energy extracted from each borehole decreased slightly, while the total energy extracted increased. This resulted in higher ground temperatures in the boreholes. Having many

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boreholes open and opening one more (ie: 6 to 7 boreholes open) had less effect than having few boreholes open and opening one more (ie: 3 to 4 boreholes open). This was because the energy out of each borehole was less with many boreholes open.

7.6 Total COP The COPTOTAL lay between 2,5 and 3,1. It was curious that COPTOTAL had a lower value when the extra pump was running in conjunction with 2 heat pumps, compared to the case when it was not running with the 2 heat pumps. Further investigations must be done in this regard.

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8 CONCLUSIONS

8.1 Brine Properties The brine solution had the following specifications:

1 Ethyl alcohol, C2H5OH 2 22,84 %-weight concentration 3 Freezing point: -13,5 oC 4 Additives: n-butanol (2%), isopropanol (7-9%)

Detailed brine properties were interpolated using values for ethyl alcohol 23%-weight concentration, given by Åke Melinder. Additives were not considered. Viscosity tests were conducted to observe the effect of the additives on the brine properties at 20 oC. The ethyl alcohol solution was found to be more viscous than the brine solution, and falling ball viscometer tests demonstrated a difference of 83% between the two.

8.2 Ground Temperature The highest temperature was at the bottom of the borehole, at 160 m. The coldest temperatures were at 75 m depth. Temperatures above 75 m increased in data taken in October 2006, and decreased in data taken in January 2007. This can be attributed to ambient conditions. Stabilization time, the time for thermocouples at the same height to reach the same value when circulation was turned off, was 10- 30 minutes. Time required to stabilize increased with depth. After stabilization the up-flow side temperatures were 0,1oC lower than the down-flow side. This can be attributed to an offset in the thermocouple locations, or an error in thermocouple readings.

8.3 Flow Regime in Borehole 7 With Reynolds numbers greater than 3000 there was more energy extracted after the bend 160 m deep in borehole 7. This was due to disturbance of the flow by the bend. The flow became heavily swirling, perhaps turbulent, and extracted larger amounts of energy. There are two possibilities. The flow may have transitioned from laminar to turbulent due to the bend. Alternatively, it may have been heavily swirling yet still predictable. Transition occurred between Re 2000 and 4500. There were unusually high amounts of heat transfer in the first 15 meters of borehole 7, for every case considered. On the down-flow side, the high amounts of energy absorbed by the brine could have been due to a heavily swirling flow, as well errors in thermocouple placement. On the up-flow side, energy was dissipated to the surroundings. This was a function of Re, which had a greater effect on extracted energy than the ground temperature, as

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ground temperatures were close to brine temperatures. This section of borehole should be insulated to increase energy gains. Regarding the decreasing energy extraction on the down-flow side, it can be suggested that flow transitions from turbulent to laminar in this region.

8.4 Effects of the Extra Brine Pump In the case of a single heat pump running, there was less energy extracted when the extra pump was running. This was due to the low ground to brine temperature difference in the boreholes. The number of boreholes was too many to make the extra pump feasible. With fewer boreholes open there would be use for an extra pump. With two heat pumps and extra pump operational, there was more energy extracted from the boreholes. Considering the energy required by the extra brine pump, the net energy gain was higher with the extra pump on. The extra brine pump should be in operation when both heat pumps are running.

8.5 Effect of Opening One Extra Borehole Opening one extra borehole yielded more energy extracted from the boreholes. Consequently, each borehole extracted less energy, and the ground temperatures around the boreholes were higher. The highest amount of extra energy extracted was by opening a third borehole. This was due to the percentage of extra surface area available to absorb energy, as well as the pressure drop over the extra pump. Thirteen kilowatts of extra energy was obtained. Opening the other boreholes yielded 1,5-5 kW of extra energy per hole. The amount of energy extracted from the boreholes is a function of how much energy the heat pumps require, demanded by the domestic hot water and floor heating tanks. This varies with time. The optimum number of boreholes open is a function of this demand.

8.6 Total COP The COPtotal for the whole system, under normal operation was between 2,5 and 2,8. When the extra brine pump was turned off, COPtotal was 3,11.

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9 RECOMMENDATIONS

The extra brine pump should operate only when two heat pumps are running. To extract more energy, flow should be maintained with Re 4500 or higher when two heat pumps are operating. To avoid energy losses, it is recommended to insulate sections of borehole piping at Mariehäll. All piping from the building to the borehole site should be insulated, as well as the piping from the boreholes back to the building. Furthermore 30 – 40 meters of the piping on the up-flow side should be insulated. This is to eliminate the large heat dissipation that is occurring in this section. To obtain more accurate readings and results, water meters should be placed for each borehole. An accurate total flow reading can thus be taken, and the behaviour inside each borehole can be better examined. It would be good to have the water meters a size larger so the pressure drop will decrease. Heat pump manufacturers must label the heat pumps in regards to the best flow conditions for operating the heat pump. In this way, the heat pump owner can optimize the amount of energy extracted from the ground by using the optimal flow rate. An extra pump may have to be purchased.

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10 REFERENCES

Beaumont, Sam; 2006 “In site measurements regarding laminar and turbulent flow in heat pump boreholes” Department of Energy, KTH, Stockholm. Sweden. Boyle, G.; 2004 “Renewable Energy: Power for a Sustainable Future” Oxford University Press, Oxford, UK, ISBN 0-19-926178-4 Broberg, B.; 2006 Private communication, 12 October 2006. Caughey, D.A.; Liggett, J.A.; 2001 "Fluid mechanics" McGraw-Hill, New York, USA, DOI 10.1036/1097-8542.262300 Franzén, Sven; 2007 Private communication, 06 February 2007 Gnielinski, V.; 1976 “New Equations for Heat and Mass Transfer in Turbulent Pipe and Channel Flow” Int. Chem Eng., Vol 16, pp. 359-368. Granryd, E.; 2005 “Refrigeration Engineering, Part II” Division of Applied Thermodynamics and Refrigeration, Royal Institute of Technology, Stockholm, Sweden. Hill, Peter; 2006, 2007 Private Communication, 03 November 2006 Private Communication, 31 January 2007 Kyrk, Björn; 1989 Forskningsrapport R18:1989 BFR Natural Resources Canada, 2005 “What is a heat pump and how does it work?” http://oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump12 Oct 2006 Nowacki, J.E.; 2006 Private communication, 05 October 2006 Nuclear Power Fundamentals, 1998 “Velocity Profiles” http://www.tpub.com/content/doe/h1012v3/css/h1012v3_40.htm, 14 Dec 2006

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Sabersky, R.H. et al.; 1989 ”Fluid Flow, A First Course in Fluid Mechanics” Macmillan Publishing Company, New York, USA, ISBN 0-02-404960-3 Sednert, Ove; 2006 Private communication, 09 November 2006 and 14 November 2006 Sibulkin, M.; 1962 “Transition from turbulent to laminar pipe flow” Phys.of Fluids 5, 280-284 White, Frank M.; 2003 “Fluid Mechanics, fifth edition” McGraw Hill, New York, NY, USA, ISBN 0-07-119911-X Wikipedia, 2006 “Carnot Cycle” http://en.wikipedia.org/wiki/Carnot_cycle, 12 Oct 2006 Wikipedia, 2006 ”Refrigeration” http://en.wikipedia.org/w/index.php?title=Image%3ARefrigerationTS.png, 10 March 2007

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APPENDIX 1: BOREHOLE DIAGRAMS

Diagrams are arranged in order number of pumps and boreholes operating. Refer to top right corner for borehole specifics. 2 Heat Pumps + Extra Pump, 444444V :

Fig §A1: Re and energy extraction from BH7, 2HP+E, 444444V, 05 Feb 2007.

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Fig §A2: Re and energy extraction from BH7, 2HP+E, 444444V, 02 Feb 2007.

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Fig §A3: Re and energy extraction from BH7, 2HP+E, 444444V, 11 Dec 2006.

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2 Heat Pumps + Extra Pump, XX4444V :

Fig §A4: Re and energy extraction from BH7, 2HP+E, XX4444V, 05 Feb 2007.

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Fig §A5: Re and energy extraction from BH7, 2HP+E, XX4444V, 02 Feb 2007.

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2 Heat Pumps + Extra Pump, XXX444V :

Fig §A6: Re and energy extraction from BH7, 2HP+E, XXX444V, 05 Feb 2007.

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Fig §A7: Re and energy extraction from BH7, 2HP+E, XXX444V, 02 Feb 2007.

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1 Heat Pump + Extra Pump, 444444V :

Fig §A8: Re and energy extraction from BH7, 1HP+E, 444444V, 29 Jan 2007.

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1 Heat Pump, 444444V & XX4444V :

Fig §A9: Re and energy extraction from BH7, 1HP, 444444V, 11 Dec 2006.

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Fig §A10: Re and energy extraction from BH7, HP, XX4444V, 11 Dec 2006.

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APPENDIX 2: EFFECTS OF EXTRA BRINE PUMP

This appendix includes data which suggests the effect of using the extra brine pump. Experimental trials were repeated, data is seen below. 22 Feb 2007. It is seen that there is less energy out with the extra pump running. Temperature difference has much larger effect than flow increase. Negative kW extracted out with extra pump running. BH 7 extracted more heat (in general) with extra pump.

pumps case ΔT (t9-t12) Total Flow BH 7 BH total(deg C) (L/s) P (kW) P (kW)

4444444 0.33 5.42 0.69 7.56XX44444 0.60 4.22 2.08 10.63XXX4444 0.7