This article describes an overview about technologies of ground source heat pump and underground thermal energy storage systems. The history of the development and current technical trend as well as the benefits and problems are discussed. The evaluation of energy conservation and reduction of carbon dioxide emission for heating and cooling by using the ground source heat pump are conducted comparing with conventional systems.
In this paper, the effects of natural convection at the annulus of heat exchange wells in Ground-Coupled Heat Pump (GCHP) systems were investigated using analytical solutions and evaluated by field test results. For the analysis of natural convection effect, Rayleigh numbers were evaluated using experimental correlations for the determination of Nusselt numbers in the closed layer between the heat exchange pipe and the well wall. A computer program to predict temperature performances of heat exchange wells was developed using the above Nusselt number and cylindrical source functions. The interpretation method was verified using thermal response test results conducted at a test well drilled in Akita City, Japan. Sensitivity studies using the above procedure showed (i) effect of natural convection at the annulus is more pronounced in operating GCHP systems for cooling purposes than in operating them for heating purposes, (ii) larger well diameter is more advantageous in terms of heat transfer.
Evaluation of heat transfer due to groundwater flow is a central theme for obtaining a more reliable design of the ground-coupled heat pump with borehole (hereinafter referred to as GeoHP system) . In practical, the test site in Omachi, Nagano (Japan) showed the heat extraction rate of 209 W/m from GeoHP system with the borehole of 100 m in depth and double U-tubes through the heat recovery test in Feb. 2002. To evaluate such an advantage for GeoHP system, a relation of borehole temperature to heat extraction rate and groundwater flow velocity as been examined in this study, using two-dimensional numerical model. The calculated result clarified the upper bound of heat extraction rate (maximum heat load), which in turn roughly predicts borehole length required for the GeoHP system with groundwater flow. The constraint is the temperature of circulation fluid in heat exchanger (e.g., in U-tube), or the temperature of soil with groundwater around borehole, that is, the temperature must be maintained above the freezing point. The calculate result showed that for the Darcy fluid velocity of groundwater larger than 10-5 m/s at least, we can save the length of heat exchanger, compared to that of the GeoHP design assuming only the heat conduction as heat transfer in ground. Further, this study has applied the calculation to some data measured in the Omachi test site (in Omachi business office of Chubu Electric Power Co., Inc.). The calculation explained that the main distance of the heat extraction in this site was -30 m to -100 m in depth, assuming uniform flow velocity of groundwater in the order of 10-4 m/s. Its value agrees with the estimate from the thermal response in the observation well at this site.
The measurement of ground thermal property is important for the design of under ground heat facilities or the estimation of thermal impact to the geo-environment. The data from core samples obtained by drilling are commonly used for this purpose. However, for the thermal conductivity, the in-situ measurement is suitable because the property correlates well with geo-environmental properties such as water content or porosity of soil. On the other hand, the measurement of ground thermal property at each depth is also important, because the geological structure in Japan is complex and ground water level is shallow. As for the in-situ measurement of thermal conductivity, the needle-probe method is used commonly for the geothermal manifestation field or the mud at sea floor, but this method aims at the shallow depth measurement of very soft soil. In this study, we are interested in the penetrometer that is commonly used for the measurement of N value of soil. The instrument penetrates about 30m in the common soil. We applied this method to the measurement of thermal conductivity. The measurement probe consists of hollow rods, cable heater and optical fiber thermo-meter. In a field study, we used this thermal conductivity measurement method and we also measured the N value and the resistivity at the same time. The trends of measured conductivity, N values and resistivity along depth are consistent with each other.
The establishment of the horizontal ground heat exchanger has been done in Japan so far by excavator digging. However, there are many problems in the excavation in the Japanese city part, such as the small range and the noise. Accordingly, the horizontal ground heat exchanger that used new technology is reported about the way of installing it. This method of construction is used for the gas pipe install and so on by master from the ground. This technology is to dig it along the laying underground expected line to a position of arrival. Then, the ground heat exchanger is pulled from the arrival side to the starting side, and laid with reaming a pipe way by back reamer. The horizontal ground heat exchanger can be installed quickly even in the place where it can not be excavated by this technology.
We constructed combination piping system of 1 m depth horizontal line and 3 m depth wells (cascade type) for heat extraction from shallow underground. Total length is 180 m and expected heat extraction is 1.4 kW or more. This system was formed by use of popular civil engineering machines with very low cost and easy operation. Area need for this system is only 18 m2 and possible to construct in land spaces of average Japanese private houses. Underground temperatures of 16.2 and 9.3°C are maximum and minimum at 4 m depth for over one year record (from May 2001 to July 2002) . Based on these underground temperatures and high ground water level, our piping system have enough heat extraction capacity for small house cooling/heating. We just install heat pump and start long term experiment.
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