solar heat injection into boreholes - … the system shown in figure 1 is simulated using trnsys...

SOLAR HEAT INJECTION INTO BOREHOLES

Parham Eslami-nejad1, Antoine Langlois1, Simon Chapuis1, Michel Bernier1, Wassim Faraj2

1Département de génie mécanique, École Polytechnique de Montréal

Case Postale 6079, succursale « centre-ville », Montréal, Québec, Canada

2Morrison Hershfield Limited, Mechanical Department 235 Yorkland Blvd., Ste 600, Toronto, ON M2J 1T1

ABSTRACT This paper focuses on solar heat injection into geothermal boreholes that are linked to ground-source heat pumps (GSHP) used for space conditioning of buildings. Single and multiple borehole configurations are examined. In the first case, the borehole is equipped with two independent circuits, one linked to a GSHP and the other to thermal solar collectors. With this arrangement simultaneous solar injection and heat retrieval is possible. The second example is a case study performed on an actual installation where a combined solar/GSHP system is considered to heat a 2,400 sq. meter building and three greenhouses. It consists of a square 5×5 bore field where heat from thermal solar collectors is injected throughout the year in the 9 center boreholes. The other 16 boreholes at the periphery are not linked to solar collectors as they are used to cool the building. Simulation results using TRNSYS indicate that solar injection into single boreholes has a negligible effect on heat pump energy consumption. As for the multiple borehole case, it is shown that solar recharging of the core is essential to limit the ground temperature decrease and to keep the return fluid temperature above the lower operating limit of the heat pumps.

INTRODUCTION As shown in the literature reviews provided by Chapuis and Bernier (2008, 2009), several solar/geothermal borehole configurations have been built worldwide. One of the most prominent example is the Drake Landing project in Canada. As was shown by Sibbit et al. (2007) and McDowell and Thornton (2008), this project will achieve almost a 100% solar fraction for space heating of 52 homes after several years of charging. The bore field is composed of 144 closely-packed boreholes arranged to promote radial stratification. The predicted end-of-summer storage temperature is approximately 80°C. Such a high storage temperature has two drawbacks. First, the return temperature to the solar collectors is relatively high which leads to relatively low solar collector efficiencies. Second, heat losses from the borehole storage are relatively high as they represent approximately 60% of the injected heat (Sibbitt et al., 2007). It has been suggested to lower the average borehole storage temperature to a level approximately equal to the annual ambient temperature and to use heat pumps for space heating (Chapuis and Bernier 2008, 2009). Had this approach been used in the Drake Landing project, the solar collector area would have been reduced by a factor of 4. However, heat pump energy consumption would limit the solar fraction to approximately 75%. This concept of lower borehole storage temperature combined with heat pumps is further examined in this paper. Furthermore, a novel concept is proposed whereby solar injection is performed in core boreholes while periphery boreholes act as a thermal guard to limit unwanted heat diffusion from the warm core to the cool undisturbed ground temperature. For single borehole configurations, Bernier and Salim Shirazi (2007) have looked at the effect of solar injection on the required borehole length. They concluded that solar injection has a minimum impact on the required borehole length because peak heating is done at night when solar heat is unavailable. However, they did not look at the reduction in heat pump energy consumption resulting from solar injection. This paper is divided into two main parts. In the first part, single boreholes are analyzed. The objective is to quantify, using multi-year hourly simulations, the impact of solar heat injection on the annual heat pump energy consumption. The second part is a case study performed on an actual installation where a combined solar/GSHP system is considered to heat and cool a 2,400 sq. meter building and three greenhouses. It consists of a square 5×5 bore field where heat from thermal solar collectors is injected throughout the year in the 9 center boreholes. The other 16 boreholes at the periphery are not linked to solar collectors as they are used to cool the building. Design and simulations studies are presented for this second part.

4th Canadian Solar Buildings Conference Toronto, Ontario, June 25 - 27, 2009

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SINGLE BOREHOLE

Configuration

The single borehole configuration considered here is presented schematically in Figure 1. It consists of a single borehole GSHP system coupled to a solar domestic hot water (SDHW) system. The borehole is equipped with two independent circuits thus forming a four-pipe borehole (2 U-tubes). One of the circuits is linked to a GSHP which provides year-round space conditioning to the house. The other U-tube is connected to thermal solar collectors. The useful gain from the solar collectors, Quseful, can either be used directly by the solar domestic hot water system (Qin,DHW) or injected into the borehole (Qinjected) where it is either transferred to the other U-tube if the heat pump is running or to the adjoining ground when the heat pump is off. In this study, solar injection is either done only during the winter (from October 1st to January 31st) or over the whole year. Finally, auxiliary electric heat, Qaux , is used in the hot water tank in case solar energy is insufficient to meet the DHW needs. The objective of this part of the study is to quantify Qaux and the heat pump energy consumption (HPEC) for various scenarios.

Figure 1: Schematic representation of the single borehole configuration

H o u r o f th e y e a r

0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0

Bui

ldin

g he

atin

g/co

olin

g lo

ad (

kW)

- 8

- 6

- 4

- 2

0

2

4

6

Figure 2: Hourly load for the example building

I n le t t e m p e r a t u r e t o th e h e a t p u m p ( o C )- 8 0 8 1 6 2 4 3 2 4 0 4 8

CO

P

2

3

4

5

6

7

8

9

H e a t i n gC o o l i n g

Figure 3: Performance of the heat pump used in

this study

Methodology

The system shown in Figure 1 is simulated using TRNSYS (Klein et al, 2004) for the Toronto climate. Standard components from TRNSYS and from the TESS library (TESS, 2004) are used. The ground heat exchanger is modeled using the well-known DST model (i.e. TYPE 557a in TRNSYS). The borehole thermal resistance portion of the DST model was modified according to the recent work of Zeng et al. (2003). However, as pointed out by Bernier and Salim Shirazi (2007), even with this last modification the complex interaction between the solar and GSHP legs in the borehole cannot be accounted for. Therefore, the following assumptions had to be used: i) only the GSHP leg is considered. Thus, the borehole is approximated as one U-tube; ii) the heat transferred into the ground, Qground, is the sum of the heat collected/rejected by the heat pump in the ground, Qhp, and the injected solar energy in the ground, Qinjected (the value of Qground can either be positive or negative); iii) both return fluid temperatures from the borehole, Tin,hp and Tin,solar are equal. Solar collector efficiency, ηcoll , is given by: ,0.693 3.835( ) /

Coll in solar extT T G (1)

where Tin,solar (°C) is the inlet temperature to the collectors, Text is the ambient temperature (°C), and G (W/m2) is the solar radiation striking the collector array. Collectors are assumed to be facing south with a 45° slope. Simulation results are based on the hourly cooling/heating loads of a typical residence (Figure 2). Peak loads are around 6 and 4 kW in heating and cooling, respectively. The yearly space heating and cooling needs are 14,550 and 1535 kWh, in heating and cooling, respectively. This house is equipped with a commercially available 3-ton (10.5 kW) heat pump unit (Anon. A) whose performance is given in Figure 3 as a function of the entering fluid temperature (i.e. return fluid temperature from the ground). It should be noted that heat pump operation when the


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inlet fluid temperature is below 20°F (-6.7°C) is not recommended by the manufacturer. As for domestic hot water consumption, a typical daily water draw of 200 liters is used. The entering water mains temperature is assumed constant and equal to 10°C while the domestic hot water set point temperature is set at 60°C. This leads to an annual DHW consumption of approximately 4230 kWh if an all-electric hot water tank is used. Finally, the ground and borehole properties, which are summarized in Table 1, are the same as the ones used by Bernier and Salim Shirazi (2007).

Table 1 Characteristics of the geothermal fields used in this study Parameter Single

borehole Multiple boreholes

Ground

Thermal conductivity (Wm-1K-1) Thermal diffusivity (m2/day) Undisturbed temperature (°C)

2.5

0.108 10

2.39 *

0.072 *

10.8 *

Borehole Total number (-) Spacing (m) Depth (m) Diameter (cm) Header depth (m) Thermal resistance (mKW-1)

1

N/A 100 15 1

0.12 **

25 8

146 10 1

0.10 *

* From a thermal response test (GeoEnergy, 2009) ** average for the year

Results

Twenty year simulations with a one hour time step have been performed. The main results are presented in Table 2. Four cases are examined and the results are presented for the first and 20th year of operation as well as for the 20 year average.

Table 2 Simulation results for the single borehole configuration Total energy consumption

Cases *collector

area Solar ** injection

Heating HPECH

Cooling HPECC

Total HPEC Qhp Quseful η coll Qinjected Qaux

Qaux + Total HPEC

m2kWh kWh kWh kWh kWh kWh kWh kWh

HP+SDHW 5 winter only 3385 197 3582 -9417 3407 45.6 942 2071 5653

1st year HP+SDHW 10 winter only 3361 197 3558 -9674 4954 33.1 1809 1682 5240

HP/ no SDHW 10 whole year 3296 248 3544 -9502 8842 59.2 8842 4231 7775

SDHW/HP 5 N/A 3412 197 3609 -9487 3124 41.8 0 1481 5090


20th year HP+SDHW 10 winter only 3416 194 3610 -9527 5034 33.7 1887 1686 5296

HP/ no SDHW 10 whole year 3266 225 3491 -9586 9003 60.2 9003 4237 7728 SDHW/HP 5 N/A 3519 193 3712 -9524 3125 41.8 0 1486 5198



HP/ no SDHW 10 whole year 3263 221 3484 -9604 9004 60.2 9004 4237 7720 SDHW/HP 5 N/A 3498 194 3692 -9450 3125 41.8 0 1485 5177

average over 20 years

Heat pump Solar DHW

*HP+SDHW: Set-up presented in Figure 1 HP/ no SDHW : All solar energy injected into the ground; DHW provided by resistance heating SDHW/HP : Two separate systems: HP for space conditioning and SDHW for DHW heating ** Winter is defined here as the period from October 1st to January 31st. The first thing to note in Table 2 is that there are slight differences between the energy consumption for the first and twentieth year. For example, the combined system (HP+SDHW with 5m2 of collector area) has a total energy consumption (HPEC + Qaux) of 5653 kWh in the first year and 5731 kWh in the 20th year. This represents a 1.4% difference. This increase is due to the fact that the outlet temperature from the borehole decreases with time. As indicated in Figure 4, the average outlet temperature during the heating season in the first year is approximately 4°C, while it is down to around 3°C for the 20th year. This decrease in outlet temperature increases the heat pump energy consumption in heating (HPECH) from 3385 to 3463 kWh. The decrease in outlet temperature is due to the fact that the injected solar energy, Qinjected, does not compensate for the annual amount of energy collected from the ground by


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the heat pump, Qhp. For example, for the first year, Qhp = -9417 kWh (the minus sign indicates that more energy is collected than rejected by the heat pump) and Qinjected = 942 kWh. On the cooling side, as indicated in Figure 5, the average outlet temperature during the cooling season decreases also by about 1°C. This outlet temperature decrease has a modest beneficial effect on the energy consumption of the heat pump in cooling with a drop from 197 to 193 kWh as indicated in Table 2. Given that there are only minor differences between the first and last year of operation, the analysis will now focus on the average values over the 20 year operation of the system, i.e. the last four lines of Table 2.

Year5 10 15 20

Tem

pera

ture

(oC

)

2

3

4

5

6

7

HP+SDHW 5m2

HP+SDHW 10m2

HP/no SDHW 10m2

SDHW/HP

Year5 10 15 20

Tem

pera

ture

(oC

)

10

12

14

16

18

HP+SDHW 5m2

HP+SDHW 10m2

HP/no SDHW 10m2

SDHW/HP

Figure 4: Outlet temperature of the borehole in the heating mode

Figure 5: Outlet temperature of the borehole in the cooling mode

The operation of the system with an increased collector area from 5 to 10 m2 decreases the total energy consumption from 5715 to 5281 kWh. Most of this reduction comes from an increase in the amount of hot water heated with solar energy in the summer as Qaux decreases from 2075 to 1686 kWh. The total heat pump energy consumption goes down by only 45 kWh from 3640 kWh to 3595 kWh. Thus, doubling Qinjected (from 976 to 1876 kWh) has a small impact on the heat pump energy consumption as the outlet temperature for the 10 m2 case is only about 1°C higher than for the 5 m2 case (Figure 4). If solar injection is performed year-round and DHW is provided with electric heating (HP/no SDHW 10 m2 case), the total heat pump energy consumption decreases by 3.1% from 3595 to 3484 kWh. The heat pump energy consumption in heating decreases from 3401to 3263 kWh while it increases from 194 to 221 kWh in cooling. These behaviours can be explained by the increased outlet temperature which reaches a value close to 6°C in heating (Figure 4) and a value slightly above 16°C in cooling (Figure 5). Finally, if there is no solar injection in the borehole and solar energy is used only for DHW (SDHW/HP case), then the heat pump energy consumption increases to 3692 kWh. However, Qaux is now reduced to 1485 kWh. Overall, this arrangement gives the lowest total energy consumption at 5177 kWh. These results show that solar injection in a single borehole configuration does not reduce significantly the heat pump energy consumption in heating. From an energy point of view, it is preferable to use solar collectors for DHW than to inject solar heat in a borehole.

MULTIPLE BOREHOLES

Configuration

This section presents the results of calculations performed to design and simulate a solar/geothermal bore field that is planned to heat and cool a 2,400 sq. meter existing building and three new greenhouses located in Toronto, Canada. As shown in Figures 6 and 7, the bore field consists of 25 equally spaced boreholes arranged in a square pattern (5×5 with a 8 m borehole-to-borehole spacing) divided into two independent circuits. Base heating is provided by three 100 kW water-to-water heat pumps (Anon. B), denoted by HP-1, HP-2, and HP-3 in Figure 7, while peak heating is performed using two equally-sized high efficiency boilers with a total capacity of 328 kW. Cooling of the building is performed using HP-2 and HP-3.


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Figure 6 Geothermal field - 9/16 boreholes configuration - Overview

Figure 7 Schematic representation of the solar/geothermal field

As shown in Figure 6, the first circuit includes 9 boreholes centrally located in the bore field. They are connected in series to HP-1 and to a flat-plate collector array. The second circuit includes the 16 remaining boreholes located at the periphery. They are connected to HP-2 and HP-3. These boreholes are not linked to the solar collectors but they benefit indirectly from solar injection in the center boreholes through ground heat conduction from the warm core to the cool periphery. All 25 boreholes have single U-tubes. The characteristics of the bore field are given in Table 1.

Building and ground loads without solar injection

The peak and average monthly heating and cooling loads for HP-1-2-3 were estimated for the building and the three greenhouses. These loads are summarized in Table 3. The last three columns represent the peak and monthly ground loads which are calculated assuming a COP of 4 in both heating and cooling. As shown in Table 3, the annual ground thermal imbalance is 24.93 kW. This implies that the annual balance between the amount of energy collected from the ground and the amount of energy rejected into the ground is 218.4 MWh (24.93 kW × 8760 hours).

Table 3 : Monthly summary of building and ground loads(excluding boiler peak heating)

hours per month

monthly building cooling load

peak building cooling load

monthly building + greenhouse heating load

peak building + greenhouse heating load

peak hourly ground cooling load

peak hourly ground heating load (with greenhouses)

monthly ground load (with greenhouses)

kWh kW kWh kW kW kW kWjanuary 744 0 0.0 -112844 -219.8 0.0 164.9 113.8february 672 0 0.0 -105516 -219.8 0.0 164.9 117.8march 744 0 0.0 -48362 -219.8 0.0 164.9 48.8april 720 0 0.0 -20517 -219.8 0.0 164.9 21.4may 744 14655 87.9 -2931 -29.3 -109.9 22.0 -21.7june 720 23448 131.9 0 0.0 -164.9 0.0 -40.7july 744 29310 131.9 0 0.0 -164.9 0.0 -49.2august 744 26379 131.9 0 0.0 -164.9 0.0 -44.3september 720 23448 102.6 -2931 -29.3 -128.2 22.0 -37.7october 744 0 0.0 -23448 -219.8 0.0 164.9 23.6november 720 0 0.0 -63017 -219.8 0.0 164.9 65.6december 744 0 0.0 -106982 -219.8 0.0 164.9 107.8Totals 117240 -486546

annual ground imbalance (kW)Notes: 1- A COP of 4 is assumed in both heating and cooling 24.93

2- Peak loads are assumed to prevail for 6 hours3- Building loads are negative in heating; ground loads are positive in heating

Building Loads Ground Loads


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Preliminary design calculations (bore field sizing) As a point of reference it is useful to establish what would be the total required borehole length if they were no solar injection. This is accomplished here using the ASHRAE borehole sizing procedure (Bernier, 2006) where the length is determined according to equation 2:

, ,(2)

( ) 2

a a m mh b h h

out ground in groundg p

q R q R q R q RL

T TT T

where, qa is the yearly average ground load, qm is the monthly ground load, qh is the peak hourly load, Ra, Rm ,

and Rh represent effective ground thermal resistances for 5 years, 1 month, and 6 hours thermal pulses, Rb is the effective borehole thermal resistance ( = 0.10 mKW-1 – see Table 1), Tg is the far-field ground temperature (= 10.8°C – see Table 1), Tp is the temperature penalty (caused by borehole interference), and Tout,ground and Tin,ground are the outlet and inlet ground temperatures at design conditions. The value of Tout,ground, which is also the entering fluid temperature to the heat pump is assumed to be 0°C. This value is purposely selected higher than the safe operating limit of heat pumps (usually around – 6°C) to allow for a margin of safety. The value of Tin,ground (-3.7°C) is calculated based on the peak heating load and the peak fluid flow rate. A life span of 5 years was assumed as the greenhouses were to be dismantled after that period. Without solar injection In this first design length calculation all 25 boreholes are used for heating and cooling both the building and the greenhouses. There is no solar injection and all 25 boreholes are connected in parallel. Solving equation 2 for the loads given in Table 3 leads to a total required length of 5917 m (236.7 m per borehole):

(3)

Thus, in the worst conditions, i.e. a five year thermal pulse of qa followed by a monthly pulse of qm and a 6 hour pulse of qh, a length of 5917 m is required to have a Tout,ground = 0°C. This length is greater than the total planned borehole depth (146m x 25 = 3650 m). It is also worth noting that the temperature penalty is -3.5°C after 5 years. Thus, the 24.93 kW ground imbalance has the effect of decreasing the undisturbed ground temperature by -3.5°C after 5 years which decreases the effective temperature difference from the fluid in the borehole to the undisturbed ground temperature and, consequently, increases the heat exchanger area, i.e. the borehole length. With solar injection As was just shown, the ground thermal imbalance increases the borehole length. In this second set of design length calculations, the ground thermal imbalance, qa, is set to 0. This is accomplished by injecting an amount of solar energy equivalent to an annual average power injection of 24.93 kW. This injection is equally distributed in each of the 25 boreholes using the double U-tube set-up proposed by Chapuis and Bernier (2008, 2009) and presented in Figure 1. The required monthly injection rates are shown in the 5th column of Table 4. These rates were determined based on 245 m2 of solar collector area operating at an average efficiency of 60% in Toronto climate. The modified monthly ground loads are presented in the last column. The end result is that the annual ground thermal imbalance is 0 kW. Furthermore, a comparison between Tables 3 and 4 indicates that the monthly ground loads, qm, are reduced in winter. However, the peak ground loads are unchanged as these peaks occur at night when there is no available solar energy. With these new loads, the total required length is 3783 m (151.3 m per borehole). Calculation details are shown in equation 4.

(4)


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Table 4: Monthly summary of ground loads resulting from qa = 0

peak hourly ground cooling load

peak hourly ground heating load (with greenhouses)

Total radiation on tilted surface(Toronto)slope=45° azimuth=0°

monthly mean solar injection

monthly ground load (with injection)

kW kW W/m2kW kW

january 0.0 164.9 111.7 -16.4 97.3february 0.0 164.9 145.1 -21.3 96.4march 0.0 164.9 167.3 -24.6 24.2april 0.0 164.9 194.5 -28.6 -7.2may -109.9 22.0 217.7 -32.0 -53.7june -164.9 0.0 225.2 -33.1 -73.8july -164.9 0.0 226.8 -33.3 -82.6august -164.9 0.0 216.7 -31.9 -76.2september -128.2 22.0 207.9 -30.6 -68.2october 0.0 164.9 153.8 -22.6 1.0november 0.0 164.9 83.2 -12.2 53.4december 0.0 164.9 82.8 -12.2 95.7

annual ground imbalance (kW)0

As expected Tp =0°C which increases the value of the denominator and, consequently, decreases the required length. The final length is close to the planned borehole length of 3650 m. Although this solution was considered, the construction of the double U-tube boreholes was estimated at a cost equivalent to about 2.5 times the cost of regular single U-tube boreholes. As an alternate less costly solution the system presented in Figures 6 and 7 has been proposed. However, with this system, the ASHRAE borehole sizing procedure cannot be used as boreholes do not experience the same loads. Instead, hourly system simulations have to be performed. System simulation

Control sequence The proposed system presented earlier in Figures 6 and 7 is simulated using TRNSYS. The system operation flow chart is presented in Figure 8. During the heating season, there are two main controlling factors. First, if the entering fluid temperature in HP-1 (Tewt,HP) is less than 1°C, then this heat pump is deactivated. Heating is provided by HP-2 and HP-3 and solar charging is activated if solar energy is available. If the entering fluid temperature in HP-1 is greater than 1°C than HP-1 is activated. The activation of HP-2 and HP-3 (and eventually of the backup boilers) will depend on whether HP-1 can handle the building load; solar charging takes place when solar energy is available. In the cooling season, HP-1 is always deactivated, HP-2 and HP-3 provide cooling to the building and solar charging is performed whenever solar energy is available. This flow chart was implemented in TRNSYS in the form of a control sequence. Ground loads The heating and cooling ground loads are the same as the ones presented earlier in Table 3. Hourly loads would have been preferable for system simulation. However, they were not calculated due to time constraints. Instead, the average monthly ground load was assumed to prevail each hour of the month except for a six hour peak period. These peak periods were set such that they would occur at night in the winter and during the day in the summer. Borehole model The 9/16 configuration cannot be simulated with the DST model used for the single borehole analysis. Instead, another borehole model has to be used. Space does not permit to describe this model. Therefore, only a brief description is provided here. The heat transfer process in the ground is modeled using the analytical solution for transient average temperature over the borehole length developed by Lamarche and Beauchamp (2007) based on the finite line source model of Zeng et al. (2002). The multiple load aggregation algorithm (MLAA) of Bernier et al. (2004) is used to account for the load variation effects with time. For more details concerning the finite line source model and the borehole thermal interactions, the reader is referred to the work of Sheriff and Bernier (2008).


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Figure 8: Operation flowchart for the system presented in Figures 6 and 7 Solar collectors Simulations are performed with a 200 m2 solar collector area. This area was determined based on preliminary simulations which indicated that such an area was required to maintain the inlet temperature to HP-2 and HP-3 above 0°C. The collectors used for the single borehole analysis are used (Equation 1). They are facing south with a 45° slope. As indicated in Figure 7, the solar collector array has been placed downstream of HP-1. Therefore, as it exits the bore field, the fluid is first used by HP-1 before being sent to the solar collectors. Preliminary simulations indicated that a reverse order (i.e. borehole, followed by the solar collectors then by HP-1) gives similar results. A series of valves are used to either send flow only to the solar collectors or only to HP-1.

Results

Figures 9 to 12 present the results of the TRNSYS simulations. These figures show the hourly outlet temperatures for both the 9 “solar” and the 16 “regular” borehole circuits over a 5 year period starting on May 1st. It should be noted that even though the results for both circuits are presented separately, the model accounts for the thermal interactions between the core and the periphery. Figures 9 and 10 show what would happen without solar injection for the 16 and the 9 boreholes, respectively. As expected, the outlet temperature from the 16 boreholes rises in the first summer with peaks corresponding to the peak building cooling loads. Then, the outlet temperature drops to a value close to -1°C at the end of the first winter. This cycle is repeated each year but with a downward trend caused by the thermal imbalance. At the peak of the 5th winter, the outlet temperature is around -6.5°C. Most heat pumps would cease to operate at such low temperatures. The 9 borehole circuit (Figure 10) experiences a similar pattern. In the summer, HP-1 is not operating and the outlet temperature is somewhat stable as no energy is rejected in these 9 center boreholes. Figures 11 and 12 show results obtained with solar injection. A comparison between Figures 9 and 11 shows the beneficial effect of solar injection as the minimum outlet temperature is raised from -6.5°C to about of 0°C. Thus, solar heat injected in the core boreholes has diffused to the periphery boreholes helping raised the ground temperature and, consequently, the outlet temperature. However, this heating up of the ground in the periphery could be detrimental in summer when the heat pumps are in cooling mode.


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Figure 9 Outlet temperatures from the 16 boreholes circuit over a 5 year period - 0 m2 collector area

Figure 10 Outlet temperatures from the 9 boreholes circuit over a 5 year period - 0 m2 collector area

As shown in Figure 11, this is not the case here as the summer outlet temperature reaches a maximum of approximately 25oC, a temperature well below the high temperature limit of heat pumps in cooling (40-45°C). As for the 9 solar boreholes, Figure 12 shows that the outlet temperature fluctuates from about 28°C down to 0°C. These relatively low temperatures are favorable for solar collector operation as the annual average solar collector efficiency is 64 %. Finally, the total annual amount of solar energy injected in the core boreholes is 190 MWh. This value is close to the value of 218.4 MWh indicated earlier for the case of solar injection in all 25 boreholes to offset the ground thermal imbalance.

Figure 11 Outlet temperatures from the 16 boreholes circuit over a 5 year period - 200 m2 coll. area

Figure 12 Outlet temperatures from the 9 boreholes circuit over a 5 year period - 200 m2 coll. area

CONCLUSION This paper focuses on solar heat injection into geothermal boreholes that are linked to ground-source heat pumps (GSHP) used for space conditioning of buildings. Single and multiple borehole configurations are examined. In the first case, the borehole is equipped with two independent circuits, one linked to a ground-source heat pump and the other to thermal solar collectors. With this arrangement simultaneous solar injection and heat retrieval is possible. The second example is a case study performed on an actual installation where solar heat injection is considered to heat a 2,400 sq. meter building and three greenhouses. It consists of a square 5×5 bore field where heat from thermal solar collectors is injected throughout the year in the 9 center boreholes. The other 16 boreholes at the periphery are not linked to solar collectors as they are used to cool the building. Simulation results using TRNSYS show that solar injection in a single borehole configuration does not reduce significantly the heat pump energy consumption in heating. From an energy point of view, it is preferable to use solar collectors for DHW than to inject solar heat in a borehole. As for the multiple borehole case, it is shown, for a particular case, that solar recharging of the core is essential to limit the ground temperature decrease over time and to keep the return fluid temperature above the lower operating limit of the heat pumps.

ACKNOWLEDGMENTS The authors would like to express their gratitude to Howard Lee, Executive director of Parc Downsview Park inc. This work was funded in part by the Solar Buildings Research Network under the Strategic Network Grants Program of the Natural Sciences and Engineering Research Council of Canada.


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solar heat injection into boreholes - … the system shown in figure 1 is simulated using trnsys...

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