Earthtoair heat exchangers

Karsten Voss

12.9.1 Concept

An earth-to-air heat exchanger (eta-hx) consists of one or more air ducts buried in the ground. Ambient air is drawn through the ducts by free or forced ventilation, allowing the ground to temper the incoming air flow. At increasing depths, the ground temperature becomes nearly constant over the year, assuming the average annual ambient temperature. Accordingly, the air passing through a pipe buried 2 m deep will be warmed in the winter and cooled in the summer. An eta-hx as part of a building ventilation system is a simple construction and therefore reliable. The energy savings from an eta-hx are relatively small so that the construction must be inexpensive.

12.9.2 Eta-hx in high-performance housing

An eta-hx in high-performance housing is part of a strategy to increase the use of ambient energy. It can serve three possible functions:

1 Preheat intake ventilation air in winter.

2 Prevent freeze-ups of the heat recovery unit of the ventilation system.

3 Cool intake ventilation air in summer.

Function 1: Preheat intake ventilation air in winter

In a high-performance house with a highly efficient air-to-air heat recovery ventilation system, the benefit of preheating intake air with an eta-hx is small. The air temperature increase by the eta-hx reduces the temperature difference across the air-to-air heat exchanger of the ventilation system, reducing its heat recovery. The more efficient the air-to-air heat exchange, the lower the benefit of the eta-hx (see Table 12.9.1). Were the heat exchanger 100 per cent efficient, there would be no benefit from the eta-hx. Modern heat exchangers can operate with an overall efficiency of 80 per cent

Source: K. Voss, Wuppertal University

Figure 12.9.1 Basic system layout (left) and temperature profile (right) for a high-performance house application, winter mode

Source: K. Voss, Wuppertal University

Figure 12.9.1 Basic system layout (left) and temperature profile (right) for a high-performance house application, winter mode to 85 per cent. Practice suggests that it is more sensible to invest in a high-performance air-to-air heat exchanger, rather than in a less efficient heat exchanger combined with an eta-hx.

A slight improvement in comfort can occur, however. Comfort in winter is strongly affected by the temperature difference between the room air and entering ventilation air. The temperature increase shown in Table 12.9.1, though small, is nevertheless still a benefit. The importance of this benefit is greatest in the case of supply air heating.

Figure 12.9.2(a) shows an example of an eta-hx during construction for a terrace house in Neuenburg, Germany. The system consists of three plastic pipes (PPs) in parallel, each 20 m long, 110 mm internal in diameter at 1.5 m depth. The average air flow is 140 m3/h.

Figure 12.9.2(b) presents annual performance as monthly sums. Due to the simple operating strategy, there is a minor heat gain in summer, as well as cooling in winter. Nevertheless, the main yields occur during the appropriate season.

Source: K. Voss, Wuppertal University

Figure 12.9.2 An earth-to-air heat exchanger (eta-hx) during construction for a terrace house in Neuenburg, Germany (left),- annual performance of the terrace houses (right)

Source: K. Voss, Wuppertal University

Figure 12.9.2 An earth-to-air heat exchanger (eta-hx) during construction for a terrace house in Neuenburg, Germany (left),- annual performance of the terrace houses (right)

Ventilation heat demand and supply air temperatures for combinations of an air-to-air heat recovery system with an eta-hx are presented in Table 12.9.1. The example is for the following conditions: -10°C ambient temperature, 20°C indoor temperature and 10°C undisturbed ground temperature. The efficiency of the eta-hx is assumed to be 90 per cent with a balanced mass flow in the heat recovery system. The marked line emphasizes a typical heat recovery system for high-performance housing

Ventilation heat demand and supply air temperatures for combinations of an air-to-air heat recovery system with an eta-hx are presented in Table 12.9.1. The example is for the following conditions: -10°C ambient temperature, 20°C indoor temperature and 10°C undisturbed ground temperature. The efficiency of the eta-hx is assumed to be 90 per cent with a balanced mass flow in the heat recovery system. The marked line emphasizes a typical heat recovery system for high-performance housing

using conservation strategy.

Table 12.9.1 Performance of an air-to-air heat recovery system with an

earth-to-air

heat exchanger (eta-hx)

Heat

recovery without eta-hx

Heat recovery

with eta-hx

(90 per cent efficiency)

Efficiency of heat Effective

Supply air

Effective ventilation

Supply air

recovery by the ventilation

temperature

losses

temperature

air-to-air heat exchanger losses

per cent per cent

°C

per cent

°C

50 50

5.0

18

14.0

60 40

8.0

16

15.2

70 30

11.0

12

16.4

80 20

14.0

8

17.6

90 10

17.0

4

18.8

100 0

20.0

0

20.0

Function 2: Prevent freeze-ups of the heat recovery unit of the ventilation system

Due to the moisture sources in buildings, the extracted air frequently has higher water content than the ambient air. During typical winter indoor conditions with a room temperature of 20°C and a relative humidity of 40 per cent, water vapour will condense when the dew point of 6°C is reached. This can occur when the extracted room air passes through the heat recovery unit. Freezing of this condensed water can be expected when ambient air entering the heat exchanger is below -2°C. The higher the efficiency of the heat recovery, the more often freezing conditions will occur. Adding an eta-hx to keep the supply air above circa - 2°C is a simple, reliable and well-proven freeze-protection measure. This concept competes with other methods such as:

• heating the extract air; or

• adjusting the air flow to a defined misbalance for critical periods.

An eta-hx might be the right choice in cases where other benefits are valuable - that is, heating energy savings or thermal comfort in summer (functions 1 and 3).

Function 3: Cool intake ventilation air in summer

During summer, high-performance housing can be just as comfortable as conventional housing. An eta-hx can help to achieve summer comfort by 'cooling' the supply air. This application is illustrated in Figure 12.9.2. The air flow of 120 m3/h is cooled by 8K, yielding a cooling capacity of 317 W (specific heat cpair = 0.33 Wh/m3K). For comparison, 1 m2 of highly insulating triple glazing (total energy transmittance g = 42 per cent) fully irradiated by the sun (500 W/m2) produces solar heat gains of 210 W. This relation demonstrates that summer cooling by an eta-hx cannot compensate for large unshaded glass areas! On the other hand, this cooling power can lower indoor temperatures effectively if windows are shaded and closed during the hot part of the day.

12.9.3 Ground temperatures

The temperature of the ground is the result of:

• heat exchange at the surface via convection and shortwave and long-wave radiation;

• cooling through evaporation (dependent upon the vegetation); and

• heat conduction within the earth and groundwater flows.

A simplified but well-validated approach to estimate ground temperatures is to use air temperature data from typical meteorological data files (that is, test reference years, MeteoNorm data) to generate a time series of ground temperatures. Figure 12.9.3 shows results for an eta-hx in Freiburg, Germany, with an annual average ambient temperature of 10.4°C, amplitude 9.2°C and clay soil. This calculation requires the input of the soil properties, which are seldom known. Therefore, most calculation programs use a classification of general soil types. The most favourable conditions for heat transfer from the ground occur where there is groundwater. Unfortunately, excavation costs and sealing problems make the construction prohibitive, in most cases. As soil properties can influence the thermal system performance by up to 30 per cent, a careful determination of soil type and backfill around the pipes is necessary for accurate predictions. Of greater importance, however, is the layout of the eta-hx. Over-dimensioning can easily compensate for misjudgement of the soil properties.

--4 m

---

\ V

Mai Jun Jul month

Aug Sep Okt Nov Dcz

0 .Tan Feb Mrt Apr

Mai Jun Jul month

Aug Sep Okt Nov Dcz

Source: K. Voss, Wuppertal University

Figure 12.9.3 Calculated ground temperature as a function of depth of clay soil type in Freiburg, Germany

12.9.4 System configurations and sizing

The aim of system sizing is to minimize the costs of reaching a certain temperature level or energy gain under given circumstances of climate, available space, interaction with the building, etc. In the theoretical case of 100 per cent thermal efficiency (h), an eta-hx always supplies air at the soil temperature:

Due to the asymptotic decrease in the temperature difference between the soil and the air passing through the pipe, the first metre is the most effective and the last metre is the least effective. If the objective is to achieve high energy gain, a register of short pipes is a good solution and lengthy low yield segments should be avoided. If the objective is merely freeze protection of the heat recovery unit, then a long single pipe is adequate.

To fairly assess the benefit of an eta-hx, the electricity consumed by the fan motor to pull the air through the earth pipes must be considered in relation to the heat gain:

COP = heat gain/electricity consumption. [12.3]

Typical observed coefficients of performance are, indeed, impressive, in the range of 20 to 60. The COP strongly depends on the air velocity within the eta-hx and its length. Typical pressure drops should be below 2 Pa per metre length. In total, an eta-hx may produce up to 10 per cent of the pressure drop of the whole ventilation system.

The following are a few design guidelines for engineering a system:

• Pipe profiles. In most application ducts, circular cross-sections are used. They are most economical and strongest in resisting earth pressure.

• Pipe materials. Most small systems use plastic pipes with smooth internal surfaces. Such piping is available in long, lightweight single segments (5 m) or in diameters up to 110 mm as flexible tubing from a roll. Such pipes or tubes are economical to lay, have low risk of leakage at joints and resist damage from inhomogeneous soil settlement. Ducts made of concrete are more economical for diameters greater than 150 mm. Due to higher installation costs, they are common only in large-scale applications. Such piping with their frequent joints has a higher risk of groundwater leakage. Concrete is also not tight against radon infiltration.

• Depth. Excavation costs typically limit laying depths to between 1.5 m and 3 m.

• Location. Pipes located below a building generate higher heat gains than pipes laid in open areas. This is partly the result of building heat loss down into the ground, which is then recovered by the eta-hx. On the other hand, the heated volume of high-performance housing is often insulated from the basement, minimizing this effect. Lastly, pipes buried under the basement can be difficult to access if leakage is problematic.

• Pipe or pipe register. In the case of small systems, single pipes are more economical than a register of parallel pipes. This is explained by the high investment costs (that is, plastic form parts) to connect the air inlets and outlets.

• Pipe distances. Parallel pipes perform worse than a single pipe due to the thermal interaction between the pipes. This interaction is small (less than 10 per cent difference to a single duct) if the pipes are separated by at least 2 m.

• Bypass. A bypass provides an alternative path for the air when the eta-hx is not needed. This reduces the fan electricity consumption of the ventilation system. As a result, the COP improves. Experiences from high-performance housing suggest that due to control complexity and investment costs, a bypass is not favourable in small applications.

12.9.5 Hygiene

During summer operation, the air may be cooled in the eta-hx below the dew point (30° air temperature, 80 per cent humidity, dew point 26.2°C). As a result, condensation will occur and then, with time, disappear as it evaporates. The question arises whether humid and warm conditions within the pipes promote bacterial or fungal growth. Hygienic investigations of 15 different systems in Switzerland have shown that, in general, the concentration of bacteria and fungal spores, in fact, decrease due to the use of the buried pipes since the source of mould is mainly external and the inlet air carries few organisms. Results have been underlined by further studies on demonstration housing projects of this International Energy Agency (IEA) activity. In the case of increased hygienic demands, fine filters can further reduce the concentration of bacteria and spores. It is good practice to locate the air inlet well above the ground (>2 m) and away from sources of contamination (such as compost, plants, car parking or sewage vents).

Cleaning can be done by flushing the pipes with water after some years. To accommodate the water runoff, the pipes should be sloped to a drain outlet of some kind (but not connected to the sanitary system). Generally, the air filter is the most critical part of the system. It must be protected from penetration by rainwater, snow, high humidity and condensation. Easy access is important so that regular changing or cleaning of the filter is possible at least once a year.

12.9.6 Examples

Table 12.9.2 lists the configurations of the eta-hx systems found in the demonstration houses of this IEA activity. These data and comments offer an initial overview on system types and relevant considerations for high-performance housing.

Table 12.9.2 Typical eta-hx configurations applied in the International Energy Agency (IEA)

task demonstration buildings

Table 12.9.2 Typical eta-hx configurations applied in the International Energy Agency (IEA)

task demonstration buildings

Location

Function

Air flow

Earth-to-air heat exchanger

d

l

0

A

Material

Bypass

m3/h

m

m

mm

m2

Neuenburg, Germany

1,2,3

120

1.0/2.0

60

110

20.7

PE

no

Büchenau, Germany

1,2,3

150

1.5

30

150

14.1

PVC

no

Stuttgart, Germany

1,2,3

100

2.0

30

200

18.8

PE

no

Rottweil, Germany

1,2,3

88

1.0

34

200

21.4

PVC

yes

Wenden-Hilmicke, Germany 1,2

255

1.2

99

126

39.2

PVC

no

Horn, Austria

1,2

150

1.7

50

160

25.1

PE

no

Klagenfurt, Austria

1,2

200

2.0

60

150

28.3

PVC

no

Dornbirn, Austria

1,2

200

2.0

60

150

28.3

PVC

no

Nebikon, Switzerland

1,2,3

127

1.6

30

200

18.8

PVC

no

Wallisen, Switzerland

1,2,3

450

0.8

25

150

11.8

PE

no

Winterthur, Switzerland

1,2,3

800

2.0

180

170

96.1

PE

no

Notes: d: laying depth; l: single duct length; 0: internal diameter; A: total surface area.

Notes: d: laying depth; l: single duct length; 0: internal diameter; A: total surface area.

12.9.7 Conclusions

An earth-to-air heat exchanger in high-performance housing is part of a strategy to increase the use of ambient or renewable energy. However, the eta-hx competes with the air-to-air heat exchanger of the ventilation system. With the prewarming of inlet air by the eta-hx, there is less temperature difference in the air-to-air heat exchanger to drive the heat recovery. The main arguments for an eta-hx are preventing freezing in the air-to-air heat exchanger and providing modest cooling of the supply air in summer.

Table 12.9.3 Simulation tools

Name Source Website Comments

GAEA Universität Siegen, Germany www.nesa1.uni-siegen.de Time-step simulation of eta-hx using an analytical form factor model.

PHLuft Passivhaus Institut, Germany www.passiv.de Time-step simulation of eta-hx using a capacity model (without consideration of interaction of earth-register tubes).

WKM Huber Energietechnik, www.igjzh.com Time-step simulation of eta-hx using a capacity

Switzerland model. The user interface of WKM is based on MS-

Excel and therefore requires this software. WKM is capable of variable air flow rates, and calculates sensible and latent heat flows and the spell of condense water.

References

Blümel, E., Fink, A. and Reise, C. (2001) Luftdurchströmte Erdreichwärmetauscher - Handbuch zur

Planung und Ausführung, AEE-INTEC, Gleisdorf, Austria, and Freiburg, Germany Dibowsky, G. and Wortmann, R. (2003) Luft-Erdwärmetauscher, Teil 1 - Systeme für Wohngebäude, Luft, Ministerium für Schule, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, Düsseldorf, www.ag-solar.de/de/service/downloads.asp Flückinger, B., Wanner, H. IP and Lüthy, IP (1997) Mikrobielle Untersuchungen von Luft-Ansaugregistern, ETH, Zürich

Gieseler, U. D. J., Bier, W. and Heidt, F D. (2002) Cost Efficiency of Ventilation Systems for Low-energy Buildings with Earth-to-Air Heat Exchange and Heat Recovery, Proceedings of the International Conference on Passive and Low Energy Architecture (PLEA), Toulouse, France, pp577-583 Hollmuller, P and Lachal, B. (2001) 'Cooling and preheating with buried pipe systems - monitoring, simulation and economic aspects', Energy and Buildings, vol 33, issue 5, pp509-518 Pfafferott, J., Gerber, A. and Herkel, S. (1998) 'Erdwärmetauscher zur Luftkonditionierung',

Gesundheitsingenieur, vol 119, no 4, pp201-213 Sedlbauer, K., Lindauer, E. and Werner, H. (1994) 'Erdreich/Luft-Wärmetauscher zur

Wohnungslüftung', Bauphysik, vol 16, no 2, pp33-34 Zimmermann, M. (ed) (1999) 'Luftansaug-Erdregister', in Handbuch der Passiven Kühlung, EMPA, Dubendorf

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