**2. Principle of GSHP systems**

460 Heat Exchangers – Basics Design Applications

average global temperature are likely to have risen by 4 to 6 oC by the end of 21st century (Gaterell, 2005). As climate change progresses, all the other environmental problems are becoming worse and harder to solve. Therefore, a sustainable future requires worldwide

Thanks to the awareness of the impact of global warming and its relationship with human activities, there has been a growing interest in reducing fossil energy consumptions. Specifically, more efficient use of energy and increased use of renewable energy seem to be

Heating and cooling in the industrial, commercial, and domestic sectors accounts for about 40-50 % of the world's total delivered energy consumption (IEA, 2007, Seyboth et al., 2008). Although, buildings regulations aim to reduce the thermal loads of buildings, as the economic growth improves standards of living, the energy demand for heating and cooling is projected to increase. For example, in non-OECD nations, as developing nations mature, the amount of energy used in buildings sector is rapidly increasing. Consequently, the implementation of more efficient heating/cooling systems is of clear potential to save energy and environment. However, the use of renewable energy systems for heating and cooling applications has received relatively little attention compared with other applications such as renewable electricity or biofuels for transportation. Yet, renewable energy sources supply only around 2-3% of annual global heating and cooling (EIA, 2010). It is worth mentioning that a century or more ago renewable energy accounted for almost 100%. In

other word, all current researches aim to approach what was the case in the past.

Nowadays, and due to its high thermal performance, the ground source heat pump (GSHP) has increasingly replaced conventional heating and cooling systems around the world. Such system extracts energy from a relatively cold source to be injected into the conditioned space in winter or alternatively, extracts energy from conditioned spaces to be injected into a

efforts to prepare for new energy sources and a more efficient use of energy.

Fig. 1. World oil consumption.

relatively warm sink in summer.

our main weapon against the ongoing global warming.

The ground source heat pump (GSHP) system are also known as ground coupled heat pump (GCHP), borehole systems or borehole thermal energy storage (BTES), and shallow geothermal system. Due to its high thermal performance, the ground source heat pump (GSHP) have increasingly replaced conventional heating and cooling systems around the world (IEA, 2007, Hepbasli, 2005, De Swardt and Meyer, 2001). Essentially GSHP systems refer to a combination of a heat pump and a system for exchanging heat from the ground. The GSHPs move heat from the ground to heat homes in the winter or alternatively, move heat from the homes to the ground to cool them in the summer. This heat transfer process is achieved by circulating a heat carrier (water or a water–antifreeze mixture) between a ground heat exchanger (GHE) and heat pump. The GHE is a pipe (usually of plastic) buried vertically or horizontally under the ground surface, Fig. 2 (Sanner et al., 2003). At the beginning of 2010 the totally installed GSHP capacity in the world was 50,583 MW producing 121,696 GWh/year with capacity factor and annual grow rate of 0.27 and 12.3%, respectively (Lund et al., 2010).

**Heating mode**: In this case, the GHE and the heat pump evaporator are connected together and the heat pump moves the heat from the ground into the conditioned space. The liquid of relatively low temperature is pumped through the GHE, collecting heat from the surrounding ground, and into the heat pump. Since the temperature of extracted liquid, which is around mean annual air temperature, is not suitable to be used directly for heating purpose, heat pump elevates the temperature to a suitable level (30-45 oC) before it is submitted to a distribution system.

**Cooling mode**: In this case, the GHE and the heat pump condenser are connected together and heat pump moves the heat from the conditioned space into the ground. The liquid of relatively high temperature is pumped through the GHE, dispersing heat into the surrounding ground, and into the heat pump.

As known, heat transfers from a warmer object to a colder one. Heat, as stated by the second law of thermodynamics, cannot spontaneously flow from a colder location to a hotter area unless work is done. The heat pump is simply a device for absorbing heat from one place and transporting it to another of relatively lower temperature. So, such device can be used to maintain a space temperature at desired level by removing unwanted heat (e.g. a fridge or air conditioning unit) or to transport heat to where it is wanted (space or water heating). In space conditioning application, heat pump system is composed of an indoor unite and an outdoor unite and the task of the heat pump is to transfer heat from one unite to the other. In order to keep inside temperature at comfort level in the winter, for example, the heat pump absorbs heat from outdoor and expels it into building. In the summer the reversed process occurs, i.e. the heat pump moves heat from indoor and expels it to outside.

The temperature difference between the indoor unite and outdoor unite is referred to as temperature lift. This temperature plays a major role in determining the coefficient of performance of heat pump (COP= delivered energy/driving energy). A smaller temperature

Ground-Source Heat Pumps and Energy Saving 463

As shown in the Fig. 3, the heat exchanging operations in the evaporator and the condenser occurs at constant pressure processes (isobar). The compression process in the compressor befall at isentropic process theoretically, while the expansion operation in the expansion valve occurs at adiabatic process. With these in mind, as per the thermodynamics rules, the

Where, *h* and *m* represent enthalpy and refrigerant mass flow rate, respectively (see Fig. 3).

Thermal efficiency of the heat exchanger, which expresses how efficient the heat

Heat loss factor of the compressor, i.e. ratio between heat loss of the compressor to the

 Regarding to the internal unite of the heat pump, in wintertime the condensation temperature was assumed 38 oC. In summertime, the evaporation temperature was

Heating/Cooling capacity assumed to be constant, thus a change in temperature will

The calculation results are illustrated in Fig.4. Apparently, the COP of heating machine increases as the evaporation temperature rises. Likewise, the performance of cooling

The ground temperature below a certain depth is constant over the year. This depth depends on the thermal properties of the ground, but it is in range of 10-15 m, see section 3 below. Thus, the ground is warmer than the air during wintertime and colder than the air during the summertime. Therefore, use the ground instead of the air as heat source or as a heat sink for the heat pump results in smaller lift temperature and, consequently, better thermal performance. In addition to improve the COP, the relatively stable ground temperature means that GSHP systems, unlike ASHP, operate close to optimal design temperature thereby operating at a relatively constant capacity. It is good to mention here that in outdoor unite fan, in ASHP case, consumes more energy than that of the water pump in the GSHP case (De Swardt and Meyer, 2001). Therefore, the comparison would be even more favorable for the GSHP, if the fan energy consumption is considered in the COP

In order to accomplish the calculations, the following assumptions were made:

Pressure drop at inlet and outlet of the compressor was assumed P8-P1=10

There is no sub-cooling in the condenser or useless superheat in suction line.

surroundings and the energy consumption of the compressor, is 15%.

*Q mh h <sup>h</sup>* 3 4 (3)

*Q mh h <sup>c</sup>* 7 6 (4)

*W mh h CP* 2 1 (5)

terms of Eq.1 and Eq.2 might be calculated as follows:

and P2-P3=23 KPa respectively, see Fig.3.

 The pressure drop through the pipe is negligible. The isentropic efficiency of the compressor is 80%.

exchanger utilizes the temperature difference, is 90%

affect the flow rate of refrigerant through the cycle.

machine increases as the condensation temperature decreases.

Refrigerant R22

assumed 8 oC.

calculation.

Fig. 2. Typical application of ground source heat pump system (Sanner et al., 2003).

lift results in a better COP. More specifically, extracting heat from a warmer medium during the heating season and injecting heat into a colder medium during cooling season leads to a better COP and, consequently, less energy use.

Fig.3 shows a schematic illustration of the components of assumed system as well as the thermodynamic cycle on diagrams temperature-entropy and pressure-enthalpy. Many techniques have been recently proposed in order to improve the cycle performance, more details are given by Wang, 2000, Chap.9 (Wang, 2000). In the current work, a heat exchanger has been added between the suction line and liquid line.

Like a heat engine but operating in reverse, the thermodynamics of the cycle can be analyzed on diagrams. In general COP is defined as the ratio between the delivered capacity and compressor capacity (Wang, 2000):

$$\text{COP}\_c = \frac{Q\_c}{\mathcal{W}cp} \tag{1}$$

$$\text{COP}\_h = \frac{Q\_h}{\mathcal{W}cp} \tag{2}$$

Where Qh, Qc, and Wcp represent the heating, cooling, and compressor capacity, respectively.

As shown in the Fig. 3, the heat exchanging operations in the evaporator and the condenser occurs at constant pressure processes (isobar). The compression process in the compressor befall at isentropic process theoretically, while the expansion operation in the expansion valve occurs at adiabatic process. With these in mind, as per the thermodynamics rules, the terms of Eq.1 and Eq.2 might be calculated as follows:

$$Q\_{\rm li} = m \cdot \left(h\_3 - h\_4\right) \tag{3}$$

$$Q\_c = m \cdot \left(h\_7 - h\_6\right) \tag{4}$$

$$\mathcal{W}\_{\text{CP}} = m \cdot \left( h\_2 - h\_1 \right) \tag{5}$$

Where, *h* and *m* represent enthalpy and refrigerant mass flow rate, respectively (see Fig. 3).

In order to accomplish the calculations, the following assumptions were made:

Refrigerant R22

462 Heat Exchangers – Basics Design Applications

Fig. 2. Typical application of ground source heat pump system (Sanner et al., 2003).

better COP and, consequently, less energy use.

and compressor capacity (Wang, 2000):

has been added between the suction line and liquid line.

lift results in a better COP. More specifically, extracting heat from a warmer medium during the heating season and injecting heat into a colder medium during cooling season leads to a

Fig.3 shows a schematic illustration of the components of assumed system as well as the thermodynamic cycle on diagrams temperature-entropy and pressure-enthalpy. Many techniques have been recently proposed in order to improve the cycle performance, more details are given by Wang, 2000, Chap.9 (Wang, 2000). In the current work, a heat exchanger

Like a heat engine but operating in reverse, the thermodynamics of the cycle can be analyzed on diagrams. In general COP is defined as the ratio between the delivered capacity

> *<sup>c</sup> <sup>c</sup> <sup>Q</sup> COP*

*h <sup>Q</sup> COP*

Where Qh, Qc, and Wcp represent the heating, cooling, and compressor capacity, respectively.

*h*

*Wcp* (1)

*Wcp* (2)


The calculation results are illustrated in Fig.4. Apparently, the COP of heating machine increases as the evaporation temperature rises. Likewise, the performance of cooling machine increases as the condensation temperature decreases.

The ground temperature below a certain depth is constant over the year. This depth depends on the thermal properties of the ground, but it is in range of 10-15 m, see section 3 below. Thus, the ground is warmer than the air during wintertime and colder than the air during the summertime. Therefore, use the ground instead of the air as heat source or as a heat sink for the heat pump results in smaller lift temperature and, consequently, better thermal performance. In addition to improve the COP, the relatively stable ground temperature means that GSHP systems, unlike ASHP, operate close to optimal design temperature thereby operating at a relatively constant capacity. It is good to mention here that in outdoor unite fan, in ASHP case, consumes more energy than that of the water pump in the GSHP case (De Swardt and Meyer, 2001). Therefore, the comparison would be even more favorable for the GSHP, if the fan energy consumption is considered in the COP calculation.

Ground-Source Heat Pumps and Energy Saving 465

The ambient air temperature over the year or the day can be treated as a sinusoidal function

( ) cos 2 ) *a a*

Where T(t) is air temperature at given time t; Ta is average air temperature for the period to, Aa is the air temperature amplitude (half of the difference between the maximum and minimum temperatures for the period), to is the time for one complete cycle (day or year). Apparently, air temperature fluctuation generates variations in the ground temperature. In order to find out a mathematical expression of ground temperature, the equation to be solved is the one-dimensional heat conduction equation. The mathematical formulation of

*Tzt Tzt* (,) 1 (,)

*t*

Where α is the thermal diffusivity (m2/s), z depth below the surface (m), t is the time. Note that for oscillating temperature at the boundary, we do not need an initial condition The solution of Eq.12 can be found by Laplace transformation method (Carslaw and Jaeger,

> *z d*

Where do is the penetration depth (m), which is defined as the depth at which the temperature amplitude inside the material falls to 1/e (about 37%) of the air temperature at

> *<sup>o</sup> <sup>o</sup> <sup>t</sup> <sup>d</sup>*

Fig.5 shows the underground temperature as function of the depth at different seasons of the year. As shown, below a certain depth, which depends on the thermal properties of the ground, the seasonal temperature fluctuations at ground surface disappears and ground temperatures is essentially constant throughout the year. In other word, for depth below a few meters ground is warmer than air during the winter and colder than the air during the

Eq. 8 shows that ground temperature amplitude decreases exponentially with distance from

*<sup>d</sup> A Ae g a*

*o z*

the surface at a rate dictated by the periodic time, mathematically we can write:

Where Ag is ground temperature amplitude (oC).

*t z T(t,z) T A e ( <sup>π</sup>*

*<sup>t</sup> Tt T A ( <sup>π</sup>*

*o*

cos 2 ) *<sup>o</sup>*

*o o*

(8)

(9)

(10)

*t d*

*<sup>t</sup>* (6)

(7)

around its average value Ta . This fluctuation might be expressed by:

2 2

*z*

*a a*

**3. Ground temperature** 

this problem is given as:

1959):

the surface:

summer.

Fig. 3. Illustration of heat pump and the thermodynamic cycle on the LnP-h and T-S diagram.

Fig. 4. Actual COP as a function of condensation /evaporation temperature.

#### **3. Ground temperature**

464 Heat Exchangers – Basics Design Applications

1

8

3 2

Ln(P) 2

T,<sup>o</sup> K

h, kJ/kg

Fig. 4. Actual COP as a function of condensation /evaporation temperature.

Fig. 3. Illustration of heat pump and the thermodynamic cycle on the LnP-h and T-S

S, kJ/kg <sup>o</sup>

1

3 `

> 7 8

4

6

5

K

3

7

4 3̀ 5

6

diagram.

The ambient air temperature over the year or the day can be treated as a sinusoidal function around its average value Ta . This fluctuation might be expressed by:

$$T(t) = T\_a + A\_a \cdot \cos(2\pi \cdot \frac{t}{t\_o})\tag{6}$$

Where T(t) is air temperature at given time t; Ta is average air temperature for the period to, Aa is the air temperature amplitude (half of the difference between the maximum and minimum temperatures for the period), to is the time for one complete cycle (day or year).

Apparently, air temperature fluctuation generates variations in the ground temperature. In order to find out a mathematical expression of ground temperature, the equation to be solved is the one-dimensional heat conduction equation. The mathematical formulation of this problem is given as:

$$\frac{\partial^2 T(z,t)}{\partial z^2} = \frac{1}{a} \cdot \frac{\partial T(z,t)}{\partial t} \tag{7}$$

Where α is the thermal diffusivity (m2/s), z depth below the surface (m), t is the time. Note that for oscillating temperature at the boundary, we do not need an initial condition The solution of Eq.12 can be found by Laplace transformation method (Carslaw and Jaeger, 1959):

$$T(t,z) = T\_a + A\_a \cdot e^{-\frac{z}{d\_o}} \cdot \cos(2\pi \cdot \frac{t}{t\_o} - \frac{z}{d\_o}) \tag{8}$$

Where do is the penetration depth (m), which is defined as the depth at which the temperature amplitude inside the material falls to 1/e (about 37%) of the air temperature at the surface:

$$d\_o = \sqrt{\frac{\alpha \cdot t\_o}{\pi}} \tag{9}$$

Fig.5 shows the underground temperature as function of the depth at different seasons of the year. As shown, below a certain depth, which depends on the thermal properties of the ground, the seasonal temperature fluctuations at ground surface disappears and ground temperatures is essentially constant throughout the year. In other word, for depth below a few meters ground is warmer than air during the winter and colder than the air during the summer.

Eq. 8 shows that ground temperature amplitude decreases exponentially with distance from the surface at a rate dictated by the periodic time, mathematically we can write:

$$A\_{\mathcal{g}} = A\_a \cdot e^{\frac{-z}{d\_o}} \tag{10}$$

Where Ag is ground temperature amplitude (oC).

Ground-Source Heat Pumps and Energy Saving 467

The Kharseh chicken farm in Hama, Syria, was selected as a study case to show the contribution of ground source heat pumps in saving energy consumption of heating and cooling system. Even though the annual mean temperature in Syria is 15-18 oC, heating of such farm consumes considerable amounts of energy. The reason is that the air temperature is close to freezing during three winter months and that chickens require a relatively high

The chicken hangar is placed parallel to the main wind direction has a floor area of 500 m2

The mean heating load composed of heat losses through the external walls and ventilation, while cooling load composed of heat gained through external walls, ventilation, solar radiation, and heat released by chickens. In current work the degree-hour method was used to estimate the thermal demand of the hangar (Durmayaz et al., 2000) using following

Floor and ceiling area, of thermal resistance 5 and 0.45 K.m2/W, respectively, is 500 m2

Ventilation rate 20 m3/m2,h (ventilated area of chicken farm varies with chicken age)

The capacity of the hangar is 5 cycles/year of 55 days the period of each cycle life. This

 During their first day, the chickens occupy about 85 m2 of the building. This area is increased 14 m2 per day until they occupy the entire area of the hangar after about one month. This mean the average occupied are during one cycle is 77% out of whole

**4. Ground source heat pump systems and energy saving** 

temperature, 21-35 oC, depending on chickens' age as seen in Table. 1.

Table 1. Appropriate indoor temperature in chicken farms.

**4.2 Heating/cooling demand** 

assumptions:

hangar's area.

(50 m x 10 m) in E–W direction. The total window area is 24 m2.

External wall's area, of thermal resistance 0.45 K.m2/W, is 336 m2

mean that the hangar will be occupied 75% out of the entire year.

Windows's area, of thermal resistance 0.2 K.m2/W, is 24 m2

Heat release from chickens: 50 W/m2 (varies with age)

**4.1 Case study – the Kharseh chicken farm** 

In addition, Eq. 8 shows that there is a time lag between the ground and air oscillating temperature. In other words, the maximum or minimum ground temperature occurs later than the corresponding values at the surface. From the cosine term in Eq. 8 one can conclude that the time lag increasing linearly with depth. The shifting time, , between surface and the ground at a given depth z is:

$$
\varphi = t\_2 - t\_1 = \frac{\pi}{2} \cdot \sqrt{\frac{C \cdot t\_0}{\pi \cdot \lambda}} \tag{11}
$$

Indeed, change in temperature of ambient air results in change in the undisturbed ground temperature. Measurements of borehole temperature depth profile (BTDP) evidently show that there are temperature deviations from the linear steady-state ground temperature in the upper sections of boreholes (Goto, 2010, Harris and Chapman, 1997, Lachenbruch and Marshall, 1986, Guillou-Frottier et al., 1998). Mathematical models have been used to simulate the change in ground temperature due to GW. Kharseh derived a new equation that gives the ground temperature increase in areas where the surface warming is known (Kharseh, 2011). The suggested solution is more user-friendly than other solutions. The derived equation was used to determine the average change of ground temperature over a certain depth and therefore the heat retained by a column of earth during the warming period. This average change of ground temperature is of great importance in the borehole system.

Fig. 5. Temperature profile through the ground.

In addition, Eq. 8 shows that there is a time lag between the ground and air oscillating temperature. In other words, the maximum or minimum ground temperature occurs later than the corresponding values at the surface. From the cosine term in Eq. 8 one can conclude that the time lag increasing linearly with depth. The shifting time, , between surface and

> *tt* <sup>0</sup> <sup>12</sup> 2

Indeed, change in temperature of ambient air results in change in the undisturbed ground temperature. Measurements of borehole temperature depth profile (BTDP) evidently show that there are temperature deviations from the linear steady-state ground temperature in the upper sections of boreholes (Goto, 2010, Harris and Chapman, 1997, Lachenbruch and Marshall, 1986, Guillou-Frottier et al., 1998). Mathematical models have been used to simulate the change in ground temperature due to GW. Kharseh derived a new equation that gives the ground temperature increase in areas where the surface warming is known (Kharseh, 2011). The suggested solution is more user-friendly than other solutions. The derived equation was used to determine the average change of ground temperature over a certain depth and therefore the heat retained by a column of earth during the warming period. This average

change of ground temperature is of great importance in the borehole system.

Fig. 5. Temperature profile through the ground.

*λπ z tC*

(11)

the ground at a given depth z is:
