What lies beneath

Engineers have the next best thing to a free source of heating and cooling right beneath their feet. Yet the uptake of techniques for actively harnessing the thermal storage capacity of the ground are often neglected.

Ground coupling using air is a case in point. The thermal storage capacity of soil and rock is such that ground temperatures at depths of 2 m and below are relatively consistent all year round. As such during the summer temperatures in the ground are always below those of the ambient air, and conversely in winter they are generally warmer. By passing air through pipes buried in the ground, designers can preheat or cool the incoming air to a building, realising significant reductions in heating and cooling loads.

There are a number of methods available for transferring this air within the ground. These range from labyrinths (consisting of a network or maze of interconnecting passages) to single earth tubes or pipes.

As a result of their complex construction, labyrinths are often constructed below a building to form a basement. In this case, only the sides and floor of the labyrinth are in contact with the ground and the labyrinth is directly coupled with the building. The dewatering process and cost of constructing deep basements often limits the depth of the labyrinth and hence limits the ability to utilise the heat transfer that increases with ground depth.

Earth tubes, on the other hand, can be decoupled from the building and installed far lower within the ground. Alternatively, earth tubes can be installed within made up ground, such as an earth berm or landscaping of the building.

Earth as a heat source

The rate of heat or coolth extraction of a single earth pipe will depend on the temperature of the air within the pipe and the conductivity of the soil surrounding the pipe. The higher the heat conductivity of the soil, the quicker the heat transfer from the soil in contact with the pipe will be replaced from the heat or coolth flowing from the soil further away.

Ground temperature records are only currently available from specific meteorological stations and, at best, usually only provide statistical data at relatively shallow depths of 300 mm or 1000 mm. These records can only provide a general indication of the range of ground temperatures that could be expected at a particular location and do not necessarily represent the many variables that affect ground temperature. This means that, for most projects, the likely temperature profile of the ground will need to be calculated.

Factors that determine the temperature of the ground fall into three categories: meteorological, terrain and subsurface variables. Large-scale regional differences in ground temperatures are determined by meteorological variables, such as air temperature, solar radiation and precipitation. Solar radiation and air temperature influence surface and subsurface temperatures by affecting the rate of heat transfer to and from the atmosphere and ground. Local variations in ground temperatures are usually a result of terrain surface and the thermal properties of the ground.

Temperature variations

Although the ground temperature is much more consistent than that of the air, it does vary slightly throughout the year. From figure 1 it can be seen that the difference between the maximum and minimum values of the subsurface temperature decreases as the depth increases: for depths below 5 m to 6 m, ground temperatures are essentially constant throughout the year. As would be expected, however, the temperature of the ground surface is closely related to that of the air.

As well as the annual cycle, the subsurface temperature undergoes a daily or diurnal cycle associated with changes in local weather conditions. These daily cycles generally take place at a depth of 0.5 m below the surface, while cycles resulting from the weather take place approximately 1 m below the surface. As a result, greater benefit can be obtained by utilising the earth at greater depths, which are more dependent on the annual weather cycle.

Below the surface of the soil, maximum or minimum temperatures occur much later than the corresponding air temperatures at the surface, although this will obviously be affected by the thermal diffusity of the soil (see the table below left). The time lag increases linearly with depth, as shown in figure 2: it takes about six months for maximum or minimum air temperatures to affect the soil at a depth of between 5 m and 6 m. The “penetration depth” is the point at which the amplitude of a temperature variation is reduced to 0.01 of its amplitude at the surface – in other words, the temperature is almost constant. The depth of penetration of the daily cycle is calculated using equation 3 (see panel, right).

The relationship between annual diurnal air temperature and the temperature of the ground can be seen in figure 2 (previous page). The results shown reflect those typically found in the UK and could be used as a basis for a computer modelling study using Fourier analysis. However, for most situations, equation 2 (right) should be adequate for calculating the mean annual ground temperature.

Heat transfer

When considering the use of air transfer techniques, it is not only the temperature of the soil that must be taken into account. The transfer of heat at the surfaces of the earth tube is fundamental to the performance of energy storage from the earth pipe structure.

Surface heat transfer, which determines the flow of heat into and out of the walls of the earth pipe, takes place by three modes: convection, conduction and radiation. Convection occurs as the air is circulated through the labyrinth or earth tube; conduction occurs as heat is transmitted from the ground through the labyrinth walls; and radiation comes from the internal surface of the pipe.

The thermal heat transfer between the labyrinth surfaces and surrounding earth provides significant thermal storage all year round, as a result of the very slow response between ambient air temperature and corresponding ground temperature. In contrast, the thermal storage of the building is subject to the diurnal cycle, which limits the storage and discharge capacity within the confines of this cycle.

Evaluation of the heat transfer mechanisms in the labyrinth indicates that turbulence will provide increased heat transfer as a result of convection from the surfaces of the labyrinth. Turbulence can be engineered into the earth tube by increasing the surface roughness and incorporating changes of direction, such as bends. However, this will also increase air resistance, which in turn will lead to higher fan static pressure and hence greater running costs. This may preclude the earth tube form being used as part of a passive or naturally ventilated scheme.

A good labyrinth design will take into account the optimum thermal storage without compromising the resistance of the structure. The primary driver of ground coupling will be the thermal storage and associated heat transfer between the tube structure and the surrounding earth.

To demonstrate the likely thermal performance of ground cooling using air, a model was created using the Tas thermal simulation software by EDSL. The model was designed with a below ground airway consisting of a 1 m high × 1 m deep concrete structure reaching a depth of 2 m below ground.

From this model, the temperature profile in figure 3 (previous page) was created. This profile indicates that the 2 m deep earth pipe can reduce the outside air temperature from 27.5°C to an exit temperature of 23.2°C at 4pm. This represents a free cooling load of 10.1 kW. It was also found that installing the same earth tube at a depth of 6 m provided a further 0.6°C of free cooling, resulting in a 4pm exit air temperature of 22.6°C. During the winter, the earth pipe has the capacity to preheat the air from -2.8°C to 0.8°C.

Air quality and health

The main risk to air quality associated with ground coupling using air is the potential of microbial growth in the airway. Once micro-organisms enter a ventilation system, they can be transported quickly throughout the building.

Fungal growth is potentially the biggest problem and is likely to occur where there is standing water. The main factors affecting fungal growth are temperature and relative humidity. In ventilation systems, the optimum conditions for microbial development exist when the temperature is above 21°C and the relative humidity is above 70%. However, some moulds can grow at subzero temperatures and in relative humidity as low as 45%.

While good design and specification of the earth pipe construction should avoid ingress of ground water, the risk of condensation within the pipe means it could act as a breeding ground for fungal growth. As a result, it is essential that this is assessed for each season.

Figure 4 (previous page) provides an assessment of the risk of condensation for the model earth tube of 1 m diameter installed at a depth of 2 m. It indicates that that there is a potential for some internal surface condensation, mainly during peak summer and autumn conditions. However, it also indicates that the risk will not be sustained for long periods and will be further reduced by evaporation from the transfer of air through the pipe.

Good quality filtration will provide the primary defence against fungal spores and micro organisms from entering the system. And it is essential that access is built in to the earth pipe to enable periodical inspections of the airway and thorough cleaning to be carried out as part of a planned maintenance regime.

Neville Rye CEng MCIBSE is a director of WSP Group.

References

¹ Williams and Gold, Canadian Building Digest, July 1976