Design of an EAHE and Assessment of Its Impact on … · Type of days (weekdays, weekends,...

1

Design of an EAHE and Assessment of Its Impact on the

Air-Conditioning Needs of a New Building

Frederico Lopes

November 2012

Abstract The present work presents a model that was prepared to describe the passive air-conditioning

technology Earth-to-Air-Heat-Exchanger (EAHE) that is used to design a system for the ventilation

system of part of a restaurant. The values obtained with this model are compared to the results of the

Earthtube model of EnergyPlus. Simulations are then carried out with Energy Plus to analyze how the

system works on the heating and cooling project day. Control strategies are discussed based on the

model results to minimize energy consumption and peak power requirements. Finally an annual

simulation is carried out to assess the global efficiency of the EAHE proposed.

Keywords: Earth-to-Air-Heat-Exchanger, Buried Pipes, Passive Air-conditioning solutions, EnergyPlus Earthtube model, Ventilation Heat Gain.

1. Introduction

Earth-To-Air-Heat-Exchanger is a passive air-conditioning technology that consists of tubes

lying beneath the soil, through which ambient air is drawn. It operates on the fact that the temperature

of the subsoil is almost constant below a depth of 2 m, closely, matching the annual mean

temperature of the ambient air [1]. Generally, it has the effect of cooling the air in the Summer and

heating in the Winter, although inconvenient opposite situations might come up. Then, control

strategies are always recommended, which should use the ambient air and the soil temperature (if

gauged) as reference temperatures. Control strategies are also found in reference to indoor setpoint

temperature, as the ones implemented in the Earthtube model of EnergyPlus used throughout this

work.

In the specific case of the building understudy (a restaurant that seats 210 people), due to the

high heat gains imposed by the occupancy rates, it is not likely that this system might have capacity

to, solely, guarantee thermal comfort to the occupants. However, it might have an important

contribution on the pre-cooling or pre-heating of the outdoor air flow rates that have to be drawn by

law requirement. This is the potential that is assessed in the following study.

2

2. Modeling

The modeling section of this project may be divided in three main objects:

- Pre-Processor for EAHE sizing

- EnergyPlus Building Model

- EnergyPlus Earthtube Model

Firstly, a Pre-Processor calculation program for EAHE design was developed in Excel VBA, whereby

the whole thermal model of the heat transfer to air passing through the tubes was the one that is

implemented in Earthtube [1]. That thermal model was coupled with fluids dynamics calculations to

compute all the pressure drops across the system and calculate the fan power required to drive the

air. This program was born from two main limitations in the EarthTube Model:

- It allows one tube only (is not possible to setup an array of pipes)

- It does not compute the fan power needed for driving the air depending on the pressure

drops in the tube designed (which is the cost to pay for taking advantage of the soil

temperate condition)

So, the great capability of this program is to enable the sizing of an array of buried pipes depending on

the space constraints. As geometrical input data, this program receives the length and width of the

area available for the system deployment, and the Diameter of the buried pipes. Then, it sets a

spacing distance between the pipes’ axis equal to 3xDiameter in order to guarantee no thermal

interference among pipes. With that spacing distance defined, the model calculates the maximum

number of pipes that is possible to install knowing the available Width and the Diameter. The aim is to

maximize the number of pipes so that the design air flow rate may split into several channels and not

to pass as fast as it would be if there were a lower number of pipes. The application of the fluid

dynamics model (pressure drops due to the filter, pipes, and fittings) and the thermal model let one

know both the heat transfer and the fan power for each array of pipes. These two values allow the

calculation of the Coefficient of Performance of the Earth-To-Air heat transfer. This COP is one of the

variables available as criteria to choose the optimal arrange of pipes. This parameter was calculated

as the direct cooling or heating effect on the air divided by the ventilation power for specific conditions.

Later in this work a Mean COP is defined as the total cooling or heating energy saved by the EAHE in

the overall building consumptions divided by the energy in ventilation.

Secondly, the building model must be defined for the simulation in EnergyPlus may be run. The

envelope of the case study building was built in DesigBuilder and then exported to the EnergyPlus.

The main features of the building model can be found in the table below:

Floor Area (m2) 186.84

Ceiling Height (m) 4.0

Max. Occupancy 210

Design Flow Rate (m3/s) 2.042

Lighting (W/m2) 12.5

3

As important as the foregoing data for the thermal analysis of the zone is the scheduling of its

operating, which were defined as follows:

Occupancy

Ventilation HVAC

Thermostat

Heating Cooling

Annual Period

(Summer, Winter, )

01/01 through 12/31

01/01 through 12/31

01/01 through 12/31

01/01 through 12/31

Type of days

(weekdays, weekends, holidays) All days All days All days All days

Daily Operating schedule

0h – 11h 0 �� − −

11h – 12h 0 �� 22℃ 22℃

12h – 16h 210 �� 22℃ 22℃

16h – 18h 0 �� − −

18h – 19h 0 �� 22℃ 22℃

19h – 23h 210 �� 22℃ 22℃

23h – 24h 0 �� − −

The Envelope of the building is presented in the Figure 1. and 2. both in the architectural view and

DesignBuilder view, whereby it can be seen that it has a façade facing southwards meaning

reasonable heat gain through the glazing.

Figure 1. Building archithectural Project Figure 2. DesignBuilder building model

The glazing and opaque surface heat transfer coefficients are summarized below:

Lateral

façade Roof Floor Glazing

U(W/m�K) 2.339 0.620 1.162 1.563

Adding the foregoing data to the Climate data measured to Lisbon, all the crucial data that EnergyPlus

needs for running the simulation is given.

Finally, the last model that was used in this study was the Earthtube model of EnergyPlus because,

although this model has the limitations pointed out before, it is the only default way of integrating an

4

EAHE solution in EnergyPlus, so far. As the variation of the soil temperature in depth was not modeled

in the Pre-Processor developed, the soil temperatures used in that routine were obtained from

EnergyPlus. After the optimal array of tubes being found in the Pre-Processor, the methodology

consisted of establishing the same air flow rate and lengthening up the only tube implemented in

Earthtube (with the same diameter as the tubes in the array) as much as the conditions of the air at its

exit were the same of those found to the array of pipes. Naturally, the length of the equivalent tube in

Earthtube is not realistic but, for the result’s sake, the impact on the thermal zone is the same as the

array would have, if the air flow rate and the inlet temperature are the same.

Beyond geometric features and design air flow rate, the Earthtube model of EnergyPlus uses the

following data related to soil properties for computing the soil temperature at the depth of pipes

installation (3 meter-deep was used in every analysis):

Average ground surface temperature (℃) 18,06

Amplitude of ground surface temperature (℃) 4,77

Phase constant (days) 46

Soil Condition (Heavy soil, damp) �� 1.30�

�� ; �� 6.45

�!

"

3. Results

The following figure presents the deployed view of a generic array of tubes that constitutes the EAHE

and shows how the pressure drops behave across the system. As it can be seen in that graph, the

major contributions for the pressure drops are the air filter and the friction losses in the tubes.

Figure 3 Pressure drops along the EAHE

Dynamic pressure Total pressure (relative to atm) Static pressure (relative to atm)

Relative

pressure

(Pa)

Air filter Fan

5

2.1 Parametric analysis

The following sub-sections present the results of parametric studies that were carried out changing

each of the relevant variables alone, keeping the other ones with the values indicated in parenthesis.

2.1.1 – Air mean speed in the tubes (D=0.35 m, L=20 m and Tinlet=34ºC)

Figure 4. Parametric analysis to air flow mean speed

The parametric analysis to the mean air speed let one conclude that when the mean air speed

increases, both the rated cooling power of the coil and the fan power increase. However the later is

dominant and the COP tends to decrease. Nonetheless, for slightly low speeds there is a jump in the

heat transfer rate owing to the fact of the laminar air flow turn into turbulent. This change in the flow

regime leads to a higher value of Nu and, consequently, a higher coefficient heat transfer by

convection. The higher the mean air speed, the higher the air flow (as the diameter remains constant),

hence, although cooling power goes up, the absolute difference between inlet and outlet temperature

goes down.

0

5

10

15

20

25

30

0

50

100

150

200

250

300

350Power (kW)COP

vs Mean Air Speed(m/s)COP

Rated Cooling Power (kW)

Fan Power (kW)

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

0

5

10

15

20

25

30

0,0

10

0,1

10

0,2

09

0,3

09

0,4

08

0,5

08

0,6

07

0,7

07

0,8

06

0,9

06

1,0

05

1,1

05

1,2

04

1,3

04

1,4

03

1,5

03

1,6

02

1,7

02

1,8

01

1,9

01

2,0

00

Flow rate (m3/s)Temperature (ºC)

Mean air Speed(m/s)

vs Mean Air Speed (m/s)Outlet Temperature (ºC)

Volume air flow (m3/s)

6

2.1.2 – Diameter ( #$ =2,042m3/s L=20 m and Tinlet=34ºC)

Figure 5 Parametric aanalysis to the diameter

The change in the diameter leads to a variation in the number of pipes that is possible to burry

according to the minimum spacing distance established. However the increase of pipes is not

sufficient to compensate the decrease in the unitary cross section area and the speed tends to

increase when the diameter decreases. This has contradictory effect in the COP as both the heat

transfer and the fan power increase. The heat transfer increases due to the higher surface to volume

ratio and the fan power increases because of the higher friction losses. However, the Fan Power

outweighs the Heat transfer and the COP decreases. The higher COP value is found to a medium

value of diameter because larger diameters lead to a drop in the heat transfer (as the air speed goes

down). Finally, the largest absolute temperature difference between the inlet and outlet is achieved for

lower diameters because the Cooling rate is higher for the same air flow rate.

33 31 28 27 25 24 22 21 20 19 19 18 17 16 16 15 15 14 14 13 13

0

5

10

15

20

25

30

0

20

40

60

80

100

120

Number of pipesPowerCOP

vs Diameter(m)COP

Rated Cooling Power (kW)

Fan Power (x100 W)

0,0

0,5

1,0

1,5

2,0

2,5

0

5

10

15

20

25

30

35

0,2

00

0,2

15

0,2

30

0,2

45

0,2

60

0,2

75

0,2

90

0,3

05

0,3

20

0,3

35

0,3

50

0,3

65

0,3

80

0,3

95

0,4

10

0,4

25

0,4

40

0,4

55

0,4

70

0,4

85

0,5

00

Airflow

Speed (m/s)Temperature (ºC)

Pipes Diameter (m)

vs Diameter(m)Outlet Temperature (ºC)

Mean Air Speed (m/s)

7

2.1.3 – Length ( #$ =2,042 m3/s D=0,35m and Tinlet=34ºC)

As far as length parameter is concerned, the analysis is very straight forward because both Fan Power

and Cooling Power increase with the lengthening up of the pipes. The Fan Power increases because

the friction losses go up as long as the air travels longer paths. Longer pipes lead to higher cooling

capacity because the air gets more time for heat exchange. However as further the air goes, the lower

the difference between its temperature and soil temperature is, then, the heat transfer rate decreases.

Due to a slower increase in the cooling power, the constant increase of the Fan power gets dominancy

and the COP drops beyond the 50 m.

Figure 6. Parametric analyses to Length

2.2 Optimum EAHE

The fist criterion to decide on the optimum EAHE was to fulfill the minimum air flow rate imposed by

the legislation of 2.042 m3/s. For each of the diameters listed in the tables below, it was chosen the the

minimum air speed that fulfilled that airflow rate. Then, each pair was evaluated for 4 different Inlet

temperatures (34ºC, 26ºC, 4ºC and 11ºC) that represents two situations of cooling and two situations

of heating, each one for an extreme temperature and a usual temperature. The best candidates in

each relevant variable (Heat Exchange, COP, Fan Power and Outlet Temperature) were outlined in

bold. The final criterion relied on the operating point that would have more impact on heating or

cooling capacity. The higher heat exchange, was the pair D=0.2 m and v=2.0 m/s for all the

experiments, so, this one was chosen.

0

5

10

15

20

25

30

35

40

0

20

40

60

80

100

120

140

160

10,0

14,5

19,0

23,5

28,0

32,5

37,0

41,5

46,0

50,5

55,0

59,5

64,0

68,5

73,0

77,5

82,0

86,5

91,0

95,5

100,0

PowerCOP

Pipes length (m)

vs Length (m)COPRated Cooling Power (kW)Fan Power (x100 W)

8

1. Inlet Temperature 34ºC

Diameter (m)

Air flow mean speed (m/s)

Heat Exchange (kW)

Fan Power

(kW)

COP Air flow rate

(m'/s) Outlet

Temperature (℃)

0.2 2.000 25.26 0.59 42.84 2.072 24.16

0.3 1.353 19.40 0.26 74.51 2.103 26.48

0.4 1.055 15.47 0.20 75.87 2.120 28.05

0.5 0.806 12.70 0.18 70.40 2.056 28.97


Diameter (m)


Heat Exchange (kW)

Fan Power

(kW)

COP Air flow rate

(m'/s) Outlet

Temperature (℃)

0.2 2.000 12.61 0.58 21.78 2.072 21.19

0.3 1.353 9.68 0.26 37.44 2.103 22.29

0.4 1.055 7.71 0.20 37.86 2.120 23.07

0.5 0.806 6.33 0.18 35.02 2.056 23.52


Diameter (m)


Heat Exchange (kW)

Fan Power

(kW)

COP Air flow rate

(m'/s) Outlet

Temperature (℃)

0.2 2.000 21.95 0.55 39.87 2.072 12.94

0.3 1.353 16.82 0.25 66.24 2.103 10.69

0.4 1.055 13.39 0.20 65.73 2.120 9.29

0.5 0.806 10.98 0.18 60.32 2.056 8.48


Diameter (m)


Heat Exchange (kW)

Fan Power

(kW)

COP Air flow rate

(m'/s) Outlet

Temperature (℃)

0.2 2.000 10.99 0.56 19.65 2.072 15.58

0.3 1.353 8.43 0.26 33.02 2.103 14.40

0.4 1.055 6.71 0.20 32.96 2.120 13.69

0.5 0.806 5.51 0.18 30.32 2.056 13.28

The impact of this optimum EAHE in the overall thermal behavior of the building was evaluated

through an equivalent Earthtube of D=0,20 m but 325 m-long to draw the same air flow rate as the

ideal EAHE and achieving the same outlet temperatures.

The Heating Design Day Performance is illustrated in the Figure 7 that compares the use of direct

outdoor air or the use of the tubes. It can be observed that the maximum thermal capacity of the

heating system should be about 60 kW using direct outdoor air and is reduced to about 40 kW using

the air through the tubes. This maximum heating capacity however is only required during about one

hour, so one strategy that was tested successfully is to start heating the ambient before introducing

the ventilation that reduced the peak heating power to values below 20 kW.

Direct outdoor air

Figure

The Cooling Design Day Performance is illustrated in the

maximum thermal capacity of the cooling system is reduced

from the use of direct outside air to the underground tubes.

Direct outdoor air

Figure

The Figure 9 shows the breakdown of all the

simulation. Other Thermal Gains and Losses address the annual accumulated value of all the Heat

gains/losses that is similar independently from the ventilation system. It can be observed that the use

of the underground tubes reduces the ventilation thermal loads and as a consequence the required

fan coil heating or cooling.

9

his maximum heating capacity however is only required during about one


the ventilation that reduced the peak heating power to values below 20 kW.

EAHE

Figure 7. Heating Design day - Heating Rate

The Cooling Design Day Performance is illustrated in the Figure 8. It can be observed that the

maximum thermal capacity of the cooling system is reduced from 38 kW to 22 kW when switching

from the use of direct outside air to the underground tubes.

EAHE

Figure 8. Cooling Design day - Cooling Rate

The Figure 9 shows the breakdown of all the thermal loads according to their use for a full year



the underground tubes reduces the ventilation thermal loads and as a consequence the required

his maximum heating capacity however is only required during about one


. It can be observed that the

from 38 kW to 22 kW when switching

thermal loads according to their use for a full year



the underground tubes reduces the ventilation thermal loads and as a consequence the required

10

Figure 9 Thermal Loads Breakdown

This value corresponds to about 40% of savings in terms of energy consumption for air-conditioning

as indicated in the following table.

Annual Air-conditioning needs in case of usual

ventilation solution

Annual reduction of air conditioning needs due to

EAHE

Annual EAHE’s Fan Energy consumption

Mean COP Value

58.2 MWh 23.7 MWh (40%) 1.9MWh 12.5

4. Conclusions

The study on the viability of a EAHE system deployment for the building under analisys turns

out as a good solution due to the high values of the minimum outdoor air flow rate:

• In the heating design day, a reduction of 20kW of Rated Heating Power was observed. A

different schedule of ventilation would enable a reduction on the peak power.

• In the cooling design day, the reduction of the cooling rated power is about 18kW and the

performance of the cooling system is much steadier.

• In a annual basis the reduction on the air conditioning energy consumption achieved by the

EAHE was about 23,7MWh with a mean COP value of 12,5. The implementation of control

strategies would lead to better performance of the system.

References

[1] U. Eicker. M. Huber. P. Seeberger. C. Vorschulze (2005) Limits and potentials of office building climatisation with ambient air [2] Kwang Ho Lee. and Richard K. Strand (2006) Implementation of an Earth Tube System Into Energyplus Program

-60 -40 -20 0 20 40 60

Ventilation Heat Loss

Ventilation Heat Gain

Other Thermal Gains and Losses

Fan Coil Heating

Fan Coil Cooling

MWh

Air-Conditioning Needs Balance due to Ventilation (MWh/year)

Direct Outdoor Air

ETAHE

Design of an EAHE and Assessment of Its Impact on … · Type of days (weekdays, weekends,...

Documents

Transcript of Design of an EAHE and Assessment of Its Impact on … · Type of days (weekdays, weekends,...