Development of portable geothermal property tester based on ground source heat pump

The ground source heat pump system has outstanding advantages compared with other air conditioning systems. Since the temperature deep in the stratum remains unchanged all year round, it is much higher than the outdoor temperature in winter, but significantly lower than the outdoor temperature in summer. Therefore, the ground source heat pump overcomes the technical barriers of the air source heat pump and has greatly improved its efficiency. In addition, it also has many advantages such as low noise, small footprint, no pollutant emissions, no need to extract groundwater, low operation and maintenance costs, and long life.

The design of the geothermal heat exchanger of the ground source heat pump system requires the thermal physical parameters of the underground rock and soil. If the thermal physical parameters are inaccurate, the designed system may not meet the load requirements; it may also be too large, thereby increasing the initial investment. The traditional method of determining the thermal physical parameters of the underground rock and soil is to first determine the geological composition around the borehole based on the samples taken out of the borehole, and then determine the thermal conductivity by checking the relevant manual. However, the underground geological composition is complex, and even for the same rock composition, the range of its thermal physical parameters is relatively large. Moreover, the thermal conductivity under different geological conditions of the stratum can differ by nearly ten times, resulting in the calculated buried pipe length also differing by several times, which makes the cost of the ground source heat pump system have a considerable deviation. In addition, different sealing materials and buried pipe methods have an impact on heat exchange, so only direct measurements on site can correctly obtain the thermal physical parameters of underground rock and soil. However, since such issues are rarely involved in previous engineering practices, there is a lack of data accumulation in this area and a lack of ready-made testing methods. In response to this problem, in-depth research has been carried out, and a portable rock and soil thermal physical property tester with independent intellectual property rights has been developed and applied to actual projects.

1 Principle and composition of the tester

The thermal conductivity of underground rock and soil cannot be measured directly, and can only be reversed by measuring relevant parameters such as temperature and heat flow. A conduit is buried in the drilled borehole and backfilled according to the design requirements. The conduit in the borehole can be used as a branch of the geothermal heat exchanger in the future. The loop is filled with water, and the water is allowed to circulate in the loop. From a certain moment, the water is continuously heated for a considerable period of time (several days), and the heating power, the flow rate of water in the loop, the temperature of the water and the corresponding time are measured. Finally, the average thermal physical parameters of the rock and soil around the borehole are calculated based on the known data.
The instrument consists of a flow sensor, a current sensor, a voltage sensor, a temperature sensor, a pump, an electric heater, a pipeline and a host computer. The structural diagram is shown in Figure 1.


In Figure 1, due to the action of the pump, the fluid enters from port A, the flow sensor collects the flow signal, and the temperature sensor collects the temperature signal (T1). After the fluid passes through the pump, it is heated by the electric heater, and the heated fluid temperature signal (T2) is collected by the sensor. Then the fluid flows out from port B and is input into the conduit buried in the deep rock and soil. After the heated fluid in the conduit exchanges heat with the deep rock, it returns to the instrument from port A to form a closed cycle. The heating power, temperature difference, and flow value collected continuously within a certain period of time are used as measurement data, and then the average thermal conductivity of the rock and soil is calculated using the parameter estimation method to achieve the purpose of detection. The current sensor and voltage sensor are used to measure the heating power of the heater in real time to ensure the detection accuracy.

1.1 Host hardware

As shown in Figure 2, the host consists of a CPU AT89C52 chip, an A/D conversion chip TLC2543, a serial communication chip MAX232, a program memory 27C128, a data memory AT24C64, a keyboard, an LCD display, a switch output, a printer, a power supply, etc. The main functions of each part are described as follows:


After filtering and I/V conversion, the current signals from each transmitter are converted into analog/digital signals by TLC2543. TLC2543 is a 12-bit analog/digital conversion chip with 11 channels. The conversion of signal channels is controlled by software.

The program memory 27C128 and the data memory AT24C64 are used to store some working programs and test data. The test data stored in AT24C64 will not be lost after the system is powered off.
MAX232 is a dedicated chip for serial communication and is used to transmit test data to the upper computer.

AT89C52 is a CPU with internal program memory, which controls the work of the entire system. The internal program memory stores the main working programs and parameters, while the internal RAM is used as the register area, flag area, print and display buffer area of ​​the system. The

auxiliary output of the switch quantity controls the power supply of the heater through a relay. When the heating temperature is too high for some reason, the power supply of the heater is disconnected to achieve the purpose of protecting the equipment. The printer is used to save permanent data. [page]

1.2 Host software

The system software adopts a mixed programming of assembly language and C language, and adopts a functional module and subroutine structure. The main program of the software consists of data acquisition, keyboard, display, clock, communication, printing, etc.

2 Test results

In order to calculate the thermophysical parameters of the surrounding rock and soil, the method of parameter estimation combined with the unsteady heat transfer model can be used. The results obtained by the heat transfer model are compared with the actual measured results, so that the variance sum f=Σ(Tcal, i -Texp, i)2 is minimized. The adjusted thermophysical parameter value is the desired result. Among them, Tcal, i is the average temperature of the fluid in the conduit calculated by the model at the i-th moment; Texp, i is the average temperature of the fluid in the conduit actually measured at the i-th moment; N is the number of groups of experimental measurement data.

The following is the test result of the underground geotechnical thermal property parameters at the site of the ground source heat pump air conditioning system project in the academic lecture hall of Shandong Institute of Architecture and Technology using the geotechnical thermal property tester and the developed software; the

borehole diameter is 115mm, the depth is 60m, the inner diameter of the buried pipe is 25mm, the outer diameter is 32mm, the pipe spacing is 70mm, the initial temperature of the underground geotechnical is 14.5℃, the thermal conductivity of the pipe wall is 0.33W/m℃, the thermal conductivity of the borehole backfill material is 1.5W/m℃, and the heating power

is 48W/m. The influence of the test time on the test results is shown in Figure 3. As can be seen from Figure 3, the average thermal conductivity of the underground geotechnical around the borehole is different with different test times. When the test time reaches about 50 hours, the measured thermal conductivity tends to be stable and maintains between 1.530 and 1.538 W/m℃. Usually the test time can be selected for about 60 hours, which can not only ensure the correct thermal conductivity, but also avoid too long a test time.

Keeping other conditions unchanged, only changing the spacing between the ascending pipe and the descending pipe, the effect on the thermal conductivity of the rock and soil is shown in Figure 4. When the pipe spacing changes by about 0.0 lm, the calculated thermal conductivity changes by about 4-8%. It can be seen from the figure that the larger the spacing, the smaller the calculated thermal conductivity: this is because the larger the spacing, the smaller the thermal resistance in the borehole, and when the total thermal resistance remains unchanged, the thermal resistance of the surrounding rock and soil is large, that is, the thermal conductivity is small. Therefore, how to determine the pipe spacing is an issue worthy of serious discussion in the design of ground source heat pump systems.


3 Application prospects

For many years, China has been in the theoretical discussion stage in the application of heat pump technology, and there is a lack of systematic research on ground source heat pumps. The main constraints on the application of heat pump technology in heating and air conditioning used to be insufficient power supply and low consumption level of the people, and the market demand for heat pump air conditioning systems had not yet formed. Since the reform and opening up, with the development of China's economy and the improvement of people's living standards, the above two constraints no longer exist. Air conditioning and heating have become the needs of ordinary people, and ground source heat pumps will become the best choice for heating and air conditioning systems due to their technical advantages and energy-saving advantages. Researching, developing and industrializing ground source heat pump air conditioning systems may become a new growth point for China's economic development.