A new thermal response method for ground heat exchangers and parametric investigation of their performance

Nearly %20 of annual energy consumption of our country is used for heating of spaces. In our country and other countries that have similar rates there are great interest to efficient heating and cooling systems because of increasing of energy prices and public awareness to increasing negative effect of energy consumption on nature. Ground source heat pumps (GSHP) that using ground for space heating and cooling and easily integrated to other renewable energy sources are the best economically systems for heating and cooling. Despite the ground source heat pumps are one of the best economic systems for space heating and cooling they could not spread inside the country adequately. One of the reasons of not spreading inside the country is higher initial cost of the system build up. The other reason is loss of confidence caused from improper applications that also caused from insufficiency of information and trained staff. In GSHP, because of the most of the heat (70-80%) transferred to spaces comes from the ground, designing of ground side is crucially important. Underground structure can varies widely depending on different ground strata, ground waters, different moisture contents, fractures, and gaps. Therefore in situ thermal response tests are necessary and used to obtain information about ground side. With thermal response tests thermal conductivity of the ground and borehole resistance can be obtained. In conventional thermal response tests constant heat flux is injected in the ground for about 48-50 hours. Thermal response test have nearly 20 years background, improvements on it still continuing. Before the start this thesis, working plan is prepared, and to do studies a laboratory was built up and different kind of boreholes are drilled, horizontal ground heat exchangers are applied and and test setup is built in laboratory. 6 boreholes having different properties like; pipe diameter, distance between pipes, number of U-tubes, depths are prepared. In horizontal ground heat exchangers, slinky, snail, and helix types are applied. To see yearly temperature variations inside the ground a 20 m borehole is drilled and 15 unit temperature sensors located to different depths of borehole and 3 year data are obtained with this system up to today. In the application of boreholes we developed our special spacers and to fill the gaps in borehole one of the best grout recipe Mix111 is used and grout is pumped to the borehole from bottom to top. Furthermore for lowering of ground heat exchanger we prepared a lowering reel and used in the application processes. For numerical analysis Comsol computer program which uses the finite element method is used. With the numerical model, experimental results are validated. Mainly this thesis focused on two topics: Firstly as an alternative to conventional constant heat flux thermal response test, constant temperature thermal response test which has some specific advantages is proposed and applied to boreholes which have different properties. For proposed thermal response test, an analytical model is developed for modeling and interpreting at injecting constant temperature tests. In this method similar to conventional thermal response test, constant temperature fluid is sent to the borehole during the test. After a period of time in the test steady state conditions can be assumed in the borehole. In a borehole there are more pipes and analytical solution is more difficult to use than numerical ones. To solve the problem analytically equivalent diameter approximations is used. In the equivalent pipe constant temperature boundary conditions is assumed and then by knowing grout properties, effective thermal conductivity and borehole resistance are estimated. A test station is set up in the laboratory for the constant temperature thermal response test. By using the analytical model, effective thermal conductivity of ground obtained for four different boreholes. For a borehole long term performance prediction is calculated assuming the worst case. Furthermore optimum test duration is investigated for constant temperature thermal response test and found that even 24-36 h test duration is found to be enough to obtain reliable results. Because of the length of boreholes in a GSHP system, application is very important, it is better to consider the worst case during the determination. Therefore the method used in this work can be used for determining the thermal conductivity of ground and required borehole length and can predict long term heat transfer rate of boreholes in GSHP applications. On the other hand in this study 1U-tube boreholes considered with suitable equivalent radius approximation to simplify the problem. Secondly heat transfer mechanisms between circulating fluid and ground is analyzed, studies are aimed increasing the heat transfer rate. Therefore effects of number of U-tubes, distance between inlet and outlet pipes, diameter of pipe, depth of borehole on the heat transfer rate are examined. Effects of number of U-tubes on both time variation of borehole performance (unit HTR value) and initial cost are experimentally and computationally examined. Results show that 2U and 3U-tube configurations have 14% and 25% better performance in comparison with that of 1U-tube configuration for the averaged values of 16 weeks non-stop operation (as a worst case). The computational model is calibrated by considering the experimental results and then 4U-tube and 5U-tube configurations are computationally investigated. Performance increments are remarkable for 2U-tube and 3U-tube configurations while it is nearly insignificant for 4U and 5U ones. When the initial cost per thermal power of a borehole is considered, the optimal configuration is 2U-tube configuration if the prices in Turkey are taken into account. 2U-tube configuration is 8% cheaper than 1U-tube one for the same heat load. On the other hand, this percentage depends on the ratio of cost of polyethylene pipes to the total cost. If the cost of polyethylene pipes decreases, then 3U-tube or even 4U–tube configuration can be the cheapest solution. When the application area is considered, 3U-tube configuration constitutes an optimum solution since increments in unit HTR values are too small for 4U and 5U-tube configurations. Furthermore, it is seen that advantage of multi-U tube configurations decreases with increasing operational time period due to faster change in temperature of ground surrounding borehole. Therefore, it is necessary to keep longer distance between boreholes in case of multi U-tube configurations to allow recovering undisturbed ground temperature during non-operational time period. Effects of depths to performance in deep boreholes are also examined by using two boreholes having 50 m and 100 m. During the test fluid stays longer time inside the pipe in a 100 m deep borehole than a 50 m deep borehole. Therefore temperature difference between the inlet and outlet pipes increases and this causes higher thermal shortcuts between the pipes. For these reasons the unit heat transfer rate in the 100 m borehole will be lower than that in 50m borehole. However when the heat pump runs in heating mode, cold fluid is sent to borehole, and the average ground temperature will be higher in deeper borehole (>200m) therefore in heating mode a high performance is obtainable. Furthermore increasing of flowrate increases performance partially, however response to increasing of velocity, pumping energy also increases 3 times, then increasing of flowrate will be ineffective for the overall performance of system. Because of pump power increases 3 times with flowrate and consequently total power consumption increases, therefore this partial improvement does not effect on COP of overall system. To see the effect of pipe diameter on performance of borehole two boreholes which have Ø32 mm and Ø40 mm diameter U-tubes are used. In the borehole which has Ø40 mm U-tube shows 4% better performance. However size of increment may count as insignificant. At the same time increasing the distance between pipes effects the performance positively. Hence increasing pipe diameter without changing borehole diameter decreases also distance between the pipes, then positive effect of increasing of pipe is curtained by the decreasing of distance between the pipes. Furthermore shallow ground heat exchangers are investigated experimentally. The performances of different shallow ground heat exchangers (snail, slinky and helix) are compared. Results show that snail ground heat exchanger is the best in terms of heat transfer rate per unit pipe length, while vertical helix gives the best results for heat transfer rate per unit application area. In the cost analysis, vertical slinky gives the best result in terms of heat transfer rate per unit investment cost. These costs are taken from the costs in experimental study in the application area. Since costs may change from region to region the results may also be changed. If we comparing these results with vertical boreholes we see that because vertical boreholes with U-tube do not affected seasonal temperature fluctuations they have better heat transfer rate. Also vertical boreholes need very small application area and are better in terms of unit application area, but their cost is higher than those of all horizontal ground heat exchangers.