1. Introduction
In Korea, interest in eco-friendly energy has increased remarkably since the country implemented a policy to reduce nuclear power in 2017. In particular, geothermal energy, which is attracting much attention as eco-friendly energy, is being used in combination with geothermal heat pumps, but it is difficult to find a use case for ventilation to remove (obtain) indoor heat. In the Middle East, earth tubes connected to wind towers have been used for a long time to decrease room temperature [
1,
2].
Domestic climatic conditions in Korea are characterized by a large variation in temperature during the year, with peak summer temperature exceeding 30 °C and minimum winter temperature of −10 °C or lower. The use of a ground heat–based earth-tube system could be sufficient to decrease room temperature in the summer and increase it in the winter. In general, an earth-tube system has a buried depth of 2–5 m. It is embedded in a shallower location than a vertical-type system and has lower or higher air temperature than the outdoor temperature entering the room, thereby reducing the cooling or heating load due to the indoor ventilation load. Lee et al. [
2] reported that the underground temperature at the buried depth is an important factor in the performance of an earth-tube system, depending on the climatic conditions in the area. Costa [
3] suggested that shallow soils, which utilize ground heat for annual heating and cooling, act as a heat source and heat sink according to ambient conditions.
At present, the Korea Meteorological Administration (KMA) measures outdoor temperature and surface temperature at 95 observation points. However, ground temperature, which is an important factor for using geothermal heat, is measured at only nine observation points, at depths of 0.05, 0.1, 0.2, 0.3, 0.5, 1.0, 1.5, 3.0, and 5.0 m, including Seoul, as shown in
Figure 1. For the other areas, it provides only outdoor and ground surface temperature. Ground temperature is greatly affected by ground surface temperature, which is often influenced by outdoor weather conditions [
4,
5]. In order to utilize ground temperature data in the field, it is measured in the vicinity of the target area. However, there are large variations not only in distance, but also in outer temperature and surface temperature depending on the target area; therefore, it is difficult to use the same ground temperature for different target areas.
Numerous studies are under way to predict ground temperature in various ways to utilize geothermal heat. Yener et al. [
6] developed a model that uses a sine function with the amplitude of annual and daily average outdoor temperature and the amplitude of underground depths, and they predicted ground temperature in Turkey. Furthermore, they suggested a correlation among the ground temperature amplitudes by depth, the constant value between amplitude and annual average outdoor temperature, and the daily outdoor temperature amplitude. The study assumed that the amplitude of temperature wave decays exponentially with depth, which requires measurement of the amplitude of daily mean air temperature.
Tsilingiridis et al. [
4] derived a regression equation according to ground depth by using the correlation with outdoor temperature to predict ground temperature at shallow depths in the northern Grecian region. They also proposed a quadratic equation to predict ground temperature at 1.0 and 1.5 m and reported that the predicted value had an accuracy of approximately 94.1% with respect to the measured value. However, the proposed six seasonal regression equations are only related to the local monthly average air temperature at one station in the northern Grecian region and cannot apply to predict ground temperature profiles of other regions.
Pouloupatis et al. [
7] measured ground temperature by drilling boreholes at one point in Athalassa in Nicosia, which is in the lowlands of Cyprus, and at two points on the southern coast of Cyprus, in addition to installing thermocouples at different depths. The shallow zones in the three measured areas were reported to show temperature distributions depending on the seasonal cycle.
Popiel et al. [
5] classified the ground into three zones: surface, shallow, and deep. The surface zone, located 1 m from the Earth’s surface, has a temperature distribution that is very sensitive to weather conditions, and the shallow zone, located 1–8 m (or up to 20 m) from the Earth’s surface, has a nearly constant temperature distribution that is closely related to the annual average air temperature. The deep zone is located 20 m or more from the surface of the Earth. The study reported that the amplitude indicating temperature change decreases as the depth from the surface increases. In addition, for the Poznan area, Popiel [
8] used a cosine function to estimate the empirical coefficient of the time lag of ground temperature, thermal diffusivity, and surface temperature in bare and short-grass-covered areas, in addition to predicting the ground temperature and verifying its validity. However, the equation was developed with limited measurements at two stations in Poznan City, Poland. In addition, the constant values of thermal diffusivity and the phase lag of ground surface temperature wave were applied.
Jacovides et al. [
9] analyzed a 74-year ground temperature measurement by using a Fourier technique to investigate surface and ground temperatures at different depths in Athens, Greece. They divided the study area into bare and short-grass-covered areas and estimated the ground temperature and the minimum/maximum values by depth and time through a statistical analysis. The study assumed homogeneous and constant physical properties of the ground. The Fourier coefficients of annual ground temperature and the amplitude of ground surface temperature wave were estimated from statistical fitting of the multiyear measurements.
Ouzzane [
10] proposed a correlation model of outdoor temperature, wind speed, horizontal solar irradiation, and sky temperature to predict undisturbed ground temperature. He suggested a correlation coefficient of 0.98 or higher in 17 regions from Canada to Saudi Arabia with a simple correlation based on outdoor temperature, which is the most influential factor among the correlation factors.
Previous studies were performed to develop a statistical model with multiyear measurements using Fourier technique and regression models with the measured data of a limited period or stations predicting ground temperature profiles at certain depths. In addition, physical properties of the ground are assumed to be homogeneous and constant, and several models require complex parameters and field measurements to estimate ground temperature. Therefore, the results of the study are limited in predicting ground temperature profiles applying to other regions.
The use of direct or indirect earth-coupling techniques for building engineering applications requires knowledge of the ground temperature profile. Although ground temperature is assumed to be constant at certain depths, it varies especially near the surface. Knowledge of the annual variation of ground temperature by depth is necessary to predict the performance of earth-integrated engineering applications. These include ground heat exchanger applications [
11,
12], horizontal ground-source heat pump systems [
13,
14], earth-coupling solar chimney systems [
15,
16], and earth-tube systems [
2,
16,
17,
18].
Therefore, outdoor temperature, ground surface temperature, and thermal diffusivity are key factors in predicting ground temperature in order to examine the performance of the engineering applications. This study develops a regression equation by using the amplitude ratios of surface temperature and outdoor temperature provided by KMA, compares the results of prediction with measured data from Korea’s representative areas, and examines the validity of the developed regression equation with the measured data from two areas in Japan and the field-measured data.
2. Theoretical Considerations
The temperature field of a one-dimensional semi-infinite solid has certain values of thermal properties if there is no internal heat generation, as expressed by Equation (1):
where
is the depth (m) below the surface of the Earth,
. is the time (s), and
is the thermal diffusivity (m
2/s) divided by the thermal conductivity and the volumetric thermal capacity of the soil:
. In Equation (1), ground temperature (
) depends on the variables of depth (
) and time (
), and if the boundary condition is
, it can be rewritten as follows:
where
is the annual average ground temperature (°C) and
is the ground surface temperature amplitude (°C);
is the wavelength of the surface temperature, which is
. Kusuda et al. [
19] corrected Equation (2) to Equation (3), assuming that the ground temperature was exposed to the periodically changing atmosphere over time:
where
represents the phase lag (day) of ground surface temperature.