Ayako Funabiki

Natural levees and human settlement in the Song Hong (Red River) delta, northern Vietnam

The Song Hong (Red River) delta, northern Vietnam, is characterized by huge natural levees in an area of the delta plain known as the West Floodplain where fluvial sedimentation predominates. The natural levees along the Day River, a major distributary of the Song Hong, are comparable in size with those of the main course of the Song Hong. The Day River levees are 3–8 km wide and rise 2–5 m above the adjacent backswamps and have played an important role in human settlements since the late Metal age. We discussed the relationships among the natural levees of the Day River, delta progradation and the distribution of archaeological sites on the delta plain. During the early Holocene, the accumulation of sediment discharged by the Song Hong enhanced both aggradation of the floodplain and river mouth progradation within the drowned valley of the Song Hong. Radiocarbon dates from cores, trench exposures, and archaeological sites record a dramatic slowing of aggradation when sea level stabilized during 6–4 cal. kyr BP (the Holocene sea-level highstand). As sea level fell to the present level during 4–0 cal. kyr BP, the river mouth prograded rapidly toward the Gulf of Bac Bo (Gulf of Tonkin) and the river channels extended seaward. In the West Floodplain, lateral accretion overtook vertical accretion to generate the present longitudinal profiles of the Song Hong and Day rivers. During this period, human settlements spread across the backswamp and Holocene terrace area, lagging around 2 kyr behind the shoreline migration.

The Effects of Groundwater Flow on Vertical-Borehole Ground Source Heat Pump Systems

Heat advection by groundwater flow is known to improve the performance of ground heat exchangers (GHEs), but the effect of groundwater advection on performance is not yet fully understood. This study examined how parameters related to groundwater flow, such as aquifer thickness, porosity, lithology, and groundwater flow velocity, affect the performance of a borehole GHE. Under the thin-aquifer condition (10 m, or 10% of the entire GHE length in this study), groundwater flow velocity had the greatest effect on heat flux. With a groundwater flow velocity of at least 10−4 m/s through a low-porosity aquifer filled with gravel with high thermal conductivity, the heat flux of a GHE was as much as 60% higher than that of a non-aquifer GHE. If the aquifer is as thick as 50 m (50% of the entire GHE length), the high thermal conductivity of gravel doubled the heat flux of the GHE with a groundwater flow velocity of at least 10−5 m/s. Thus, not only groundwater flow velocity, but also aquifer thickness and thermal conductivity were important factors. However, groundwater seldom flows at such high velocities, and porosity, gravel size, and aquifer thickness vary regionally. Thus, in the design of ground source heat pump systems, it is not appropriate to assume a large groundwater effect.Copyright

Numerical Solution of a Ground Source Heat Pump System Using Foundation Piles

A ground source heat pump (GSHP) system has higher cooling and heating performances than an air source heat pump system, so the GSHP system has attracted attention in the cold regions. Particularly after the 2011 earthquake off the Pacific coast of Tohoku, which damaged nuclear power plants, the GSHP utilizing renewable energy, has become attractive in heating system in the Tohoku region. However, it is necessary to install wells to collect ground source heat, and so, together with the cost of the heat pump itself, the installation cost is a barrier to widespread adoption of this technology in Japan.On the other hand, due to poor subsoil, foundation piles are often required in the construction of buildings in Japan. By using foundation piles as heat exchangers, which are commonly used in residential construction, the cost of using GSHP systems in houses may be reduced. However, since the placement of the piles depends on the floor layout of the residence, that is arbitrary sequence. Moreover, an arbitrary floor layout requires a complicated multi-dimensional numerical analysis to design the GSHP, and the analysis is burdensome for general designers. Therefore, the use of the model unit of the two-dimensional cylindrical heat exchange well is proposed. The use of this model, which includes an unused volume of soil, reduces the analysis burden for general design tools.On experience, the arrangement rate is 4 m2 per pile, and the well separation is narrow. And the foundation piles will form a group. So thermal interference between heat exchanger wells might be working hard. In addition, the foundation piles are very short, compared to the traditional borehole depth of 50 to 100 m. Therefore, the thermal performance of the well may degrade compared to that of the traditional GSHP system, although the initial costs are less. Therefore, we examined a GSHP system with heat exchanger piles by using the heat exchange well unit model for residences in the Tohoku region.As a result, the two-dimensional analysis and more is needed to predict the thermal performances of the heat exchanger piles so that the effect of the pitch of the heat exchange wells on the heat transfer in the axial direction will be large. In cold climates, the heating demand is large and, in the summer, the amount of waste heat to the ground is small, so a large amount of heat penetration into the ground from the atmospheric air is important for continuous GSHP operation in the Tohoku region, and in Fukushima Prefecture in particular.Copyright

Effect of sedimentary facies and geological properties on thermal conductivity of Pleistocene volcanic sediments in Tokyo, central Japan

When ground source heat pump systems are installed underground, an estimate of the thermal conductivity is required to determine the desired total length of the heat exchanger (U-tube). Many large cities in Asia are built on Quaternary sediments, but the thermal conductivity of these sediments is not well understood. To measure the thermal conductivity of Pleistocene volcanic sediments in Tokyo, Japan, we discuss methods of measuring thermal conductivity and factors influencing the thermal conductivity of volcanic sediment, which has low quartz content. The results obtained from experiments using a drill core, borehole data and artificial sediment samples are as follows: (1) values of thermal conductivity predicted using water content, porosity or sand content can be underestimated in volcanic sediment or sediments with large amounts of magnetic minerals; (2) magnetic minerals have a higher thermal conductivity than quartz, so there is a relationship between magnetic susceptibility and thermal conductivity: (3) comparison of thermal conductivity measurements performed using box- and needle-type probes showed that the values measured using the former are comparatively larger. This decrease in thermal conductivity is explained by formation of air-filled cracks when the needle penetrates the sediment, as air has a lower thermal conductivity than sediment.

The likelihood evaluation of the heat exchanger unit model design method in the ground source heat pump system utilizing heat exchanger piles

A ground source heat pump system (GSHP) utilizing the foundation piles as heat exchangers is expected to reduce the initial cost compared with the borehole type of the conventional system because the heat exchanger piles is not needed to install anew. However, the arrangement of the piles is depend on the floor layout of a residence. Therefore, the thermal performance prediction of heat exchange piles has to strictly conduct three-dimensional analysis for every residence. Conducting such analysis, it is a heavy burden for a designer to perform performance prediction of the GSHP with satisfaction of the annual heating and cooling demand. Then, the two- dimensional heat unit model was proposed in order to ease a designers burden. This model has the characteristic of originally not using the soil portion where the ground heat source is available. And if it is seemed to be the design likelihood, it makes the designers load reduce. The introductory possibility of the GSHP, which uses the foundation pile in the semi-cold district of our country, is shown by the numerical analysis using this model. It is necessary to show clearly how much the analytical results calculated by use of this model has likelihood. Then, as compared with the higher-dimensional analysis results, the same heating demand conditions are employed at the thermal prediction. The rectangular and triangle arrangements of piles are inquired for the evaluations. The finite difference method is employed for the numerical analysis. It is shown that the evaluation using the proposed model has an average of 10 - 50% of likelihood compared with the higher dimensional analysis evaluation when single layer of piles surrounds the center piles. In the case of the arrangements in which multi layers of piles surround the center piles, the proposed thermal unit model leads the same analytical results by use of higher dimensional coordinates model.

Evaluation of Measurement Accuracy of Underground Thermometers Using Steel-Pipe Piles

For ground source heat utilization systems, pile heat exchangers are sometimes used. In order for these systems to achieve high performance, control of the system dynamics is important, and the underground temperature must be known. Typically, underground temperature is measured using a thermometer in a borehole. However, in the case of pile heat exchangers, a different method is required, making the system expensive to set up. To overcome this problem, the installation of underground thermometers in the heat exchanger piles themselves is proposed in the present study. The proposed thermometer system consists of thermocouples packed in grout such as silica sand within the piles. However, there is a possibility of measurement errors due to vertical thermal conduction in the steel pipes, and it is important to estimate the measurement accuracy before the development of this system. In the present study, the measurement accuracy is estimated using numerical simulations and then confirmed experimentally. The underground temperature profiles inside and outside the pile are compared. The results indicate that the proposed system offers sufficient accuracy for application to pile heat exchangers.

Optimum Heating Pattern of a Ground Source Heat Reference Map

A ground source heat reference map (GSHRM) shows the minimum necessary thermal performance of the ground heat exchanger (GHE) of a ground source heat pump (GSHP) system. Thermal performance depends on thermal properties of the ground, the ground temperature profile, heat advection by groundwater flow, and the GHE operating pattern. This study modeled optimum heating and cooling modes for a GSHRM. First, continuous and intermittent operation modes were compared, and a standard operation time was defined. In a standard household GSHP system, the quantity of heat transferred from the ground depends on household energy demand, which is relatively constant. Once the demand is known, an operation mode is selected that can meet it. Continuous operation increased the total amount of heat exchanged over a period of time but lowered the heat flux at the GHE, whereas intermittent operation with relatively long stopped periods decreased the total amount of heat but did not greatly decrease the heat flux at the GHE. Second, energy-saving efficiency and cost factors were compared among intermittent operation modes. Operation costs consist of the electrical energy supplied to the heat and circulation pumps. At a given operation time, the energy supplied to the heat pump depends on its coefficient of performance (COP), whereas that supplied to the circulation pump depends on its pressure loss, hence on the GHE length. A long GHE has a higher initial cost. Thus, the optimum heating pattern must consider the configuration of the GSHP system, including energy-saving efficiency and cost factors.Copyright

Effect of Thermal Change by Ground Source Heat Pumps on Groundwater and Geoenvironment in the Late Pleistocene Terrace Area of Tokyo, Japan

Soil and underground water pollution by waste heat in urban area is linked to the development of underground commercial facilities and subways and to the installation of ground source heat pump (GSHP) systems. In this study, we carried out laboratory tests of the dependence of dissolution elements on temperature, using boring core samples taken from the central Tokyo area in order to reduce the risks to underground water pollution by installed GSHP system. Our results demonstrate that we have to be careful not to risk contaminating shallow groundwater with hazardous elements, including marine and/or volcanic sediments, and by oxidizing the environment.

Research & development of the ground source heat reference map: The effect of the heat collecting pattern and the ground water flow

Ground source heat reference map (GSHRM) for designing the ground source heat pump system is proposed. By use of the map, designers of the ground source heat pump system are able to predict the thermal performance of the ground heat exchangers (GHEs) without the pre-evaluation test of the GHEs. In this map, the advection of the heat transfer due to the ground water flow is not expected, and is set to be the design margin. Also, the ground temperature is assumed to be the average atmospheric temperature. So, the map will show the minimum thermal performance of the GHEs with these conditions. To decide the thermal performance, the operation pattern of GHEs is necessary, and this pattern should not be any. In this paper, it is shown that the maximum cooling or heating operation pattern is existed, and this pattern is proposed to be the standard operation pattern. Also, the advection by groundwater flow is evaluated.