3.1. Thermogeology Results from Thermal Response Test Performed on the Test Piles
The TR testing was conducted during the summer period. The TRT apparatus (Precision Geothermal—model GeoCube500) has a maximum power of 9.0 kW (3 × 2.5 kW + 1 × 1.5 kW) at a 240 V voltage. Data recording was performed with the Hobo U30 Series Data Logger, HOBOware Pro Software 2.4.0. with intervals of 5 min. The measurement began with a circulation of 0.25 L/s with the heaters being turned off to determine the mean effective static ground temperature over the length of the pile, which was determined at 16.8 °C. Given that there are only 20 m of shallow energy piles, it should be noted that this is a seasonally variable value due to the climatic impact on the surface at the first 5–10 m of depth. Given the current experience in measuring ground temperature changes with depth and practical experiences with shallow drilling in the area, a temperature drop of 2.0 °C was expected at the site in winter conditions, i.e., the static value in the heating season was presumed to be 14.8 °C. It is also necessary to consider that above the piles, the heated part of the building is placed, so this value is not expected to be lower due to the formation of the so-called urban heat island.
After initial circulation, heaters of average power 3182 W (79.5 W/m) were turned on, along with voltage and electric current monitoring to determine the coefficient of thermal conductivity in the time interval that lasted for 100.7 hr. During this condition, 320 kWh of energy was rejected into the ground.
After the temperature measurement, the output/input temperature of TRT was +28.1/+31.3 °C, which is Δ11.3 °C more than the static initial temperature of the ground if the outlet temperature from the pile is observed. In the heating cycle (inverse curve on the diagram—sub-cooling the ground), an equivalent temperature difference would mean the temperature of the circulating medium with input to pile at +2.3 °C and pile output at +5.5 °C.
The rise in temperature during this period increases linearly in the logarithmic time unit ln(t), indicating the achievement of the semi-steady state of the heat transfer. It indicates the ability of the pile to operate over a longer period of time at this heat rejection rate, with a relatively small further rise in temperature in the logarithm of the time function.
Figure 5 shows the inlet and outlet fluid temperature data for the first interval of the pile test in the time function to determine the coefficient of effective thermal conductivity.
The diagram also shows the inverse temperature curve, for example, of heat energy extraction from the ground during the heating cycle (bottom lines). In order to determine the thermal conductivity coefficient of the ground, the data of the mean temperature of the circulating fluid from the first condition is drawn as a function of the natural logarithm of time ln(t)—upper right figure inside
Figure 5. After the electric heater is set to a certain power level, the temperature in the energy pile starts to grow due to the thermal conductivity of the ground. This diagram is used to determine the period of time when the rise in temperature becomes linear in the function of the time (semi-steady state) and when the radius of investigation surpasses the thermal resistances of the pile and near-drilling zone. Using derivation curve analysis, it was established that the period in which the semi-steady state heat transfer appears is 10 h, since after this period, the temperature change in the 5 min step does not exceed 0.25 °C, i.e., there is a logarithmic dependence of temperature rise in time function. The 5-min step was chosen because it corresponds approximately to one cycle of fluid flow in the piles.
In order to determine the thermal conductivity coefficient of ground from the part of the data with linear dependence of the temperature on the logarithm of the time, the slope of the interpolated line is determined (lower right figure inside
Figure 5), and using Equation (8) thermal conductivity is calculated. Given that the linear regression slope in
Figure 5. is 2.5421, with a mean thermal power of 79.55 W/m, effective thermal conductivity for a 40 m pile is then 2.49 W/m K according to Equation (8), which is a very good value in thermogeology.
With calculated thermal conductivity and known geometry of the pile (diameter and length), it is possible to calculate the pile’s thermal resistance. It is composed of thermal resistance to the heat energy transfer from the working fluid to the plastic wall of the pipe (convection), to the conduction of the heat energy through the wall of the PE pipe, and to the conduction of heat energy from the wall of PE pipes to concrete grout and ground. With Equation (5), thermal resistance is calculated for the 600 mm pile diameter and 2-h Standard Moving Average, giving a value of 0.109 m °C/W. This value in thermogeology presents efficient heat transfer inside the pile, which is expected due to the concrete filling of the pile.
3.2. Thermal Response Step Test Results to Determine Extraction Capacity of the Pile
After the first condition of 3182 W (79.5 W/m) for 100.7 h, a cycle of thermal power reduction was started with the additional three thermal steps to achieve the temperature stabilization points and the established steady-state mode of operation (
Figure 6).
By setting the steady-state temperature in each of the conditions as a separate point, it is possible to construct the pilot yield diagram for both heat rejection and heat extraction rate (W/m) as the function of the desired outlet temperature (EST), as seen in
Figure 6. According to the EN14511 norm [
18], the temperature for the cooling mode is set at EST/LST 30/35 °C and in the heating mode at EST/LST 0/−3 °C, which is used as a standardized norm for a reliable coefficient of performance (COP) of the heat pump. In the first condition, no steady state has been achieved, and the extended interval by the linear dependence of the logarithm of time was chartered (dotted lines in
Figure 6) according to the general diffusivity equation.
With the graphically obtained equation of the steady-state heat transfer, the estimation of the heat exchanger steady-state yield seems to be 80 W/m, or 1.60 kW per 20 m long pile with the static temperature at 16.8 °C during the summer period. Therefore, it is possible to reject heat into the ground in the summer cooling mode at 30/35 °C for the mentioned 80 W/m (including waste compression heat). Therefore, the equation for estimating the cumulative cooling in W/m as the outlet temperature of the fluid is as follows:
According to norm EN14511, the EST of 0 °C is set in the heating cycle as the minimum permissible amount on the outlet of the pile during long-term operation. With constant heat load applied, it is also necessary to add the useful power of the heat pump compressor when operating in the heating mode. Since this is a shallow drilling, there is a climatic influence on soil temperature for the first 5–10 m; therefore, static temperature in the summer and winter seasons is not the same. To estimate the heat extraction potential of the piles, the static temperature values were corrected for an additional temperature drop of 2 °C (during the winter period). The static temperature from the measured summer 16.8 °C conditions was set to 14.8 °C in the winter conditions for the heating regime of the heat pump. Then, the equation for estimating the cumulative heating of the piles in W/m as the function of the outlet temperature of the fluid becomes as follows:
If a lower temperature limit of 0 °C is set as the minimum permissible on the heat pump evaporator, then by solving the trendline equation from
Figure 7, the heat extraction yield of the pile is 66 W/m for EST = 2.7 °C and LST = 0 °C, at the flow of 0.25 L/s. That is, the yield of every 20 m depth pile would be 1.32 kWt in the function of continuous long-term steady-state space heating without further ground sub-cooling. To this value, it is necessary to add the useful power of the heat pump compressor to obtain the total heat power of the geothermal system.
For example, in the system of 256 piles where they are connected as two in the series, a total of 338 kWth of heat power, plus compressor power, can be gained. However, it should be emphasized that it is a parallel bivalent system where the geothermal system carries the base load and runs continuously throughout the heating season, and the gas system complements the system at peak loads if needed.
Since field piles are spaced 4 m from each other, it is necessary to estimate thermal interference between individual piles (
Figure 8). The diffusion equation is solved with the line source theory of heat energy flow through the infinite medium. With the knowledge of thermal conductivity and thermal diffusion, it is possible to calculate the soil temperature at a certain radius from the pile for continuous and uniform heat load. If the energy pile is extracting heat for the determined maximum yield of 66 W/m, or 1.32 kWt per pile, it is apparent that the radius of the affected area expands to half the distance between two piles or 2 m (blue line). In this case, the value of ground temperature drop equals 1 °C for 2000 full load hours. It should be stressed that for the efficient and sustainable operation of the system, it is necessary to use geothermal cooling for the facility, i.e., to store heat energy in the ground during the summer months. In this way, the system is brought into balance, and the negative changes of sub-cooling are reversed on an annual basis.
3.3. Simulation of the Energy Pile Field and Optimization of the Borehole Grid
When modeling the BHE field and simulating the long-term operation of the heat pumps, a series of thermogeological parameters have to be considered, which, to a certain extent, affect the efficiency and economy of the entire system. Proper determination of the required length of the ground with adequate analytical and numerical models is the most important part of optimizing the system. Possible oversizing or undersizing of the heat exchanger will energetically and economically irreversibly affect the system as a whole. If the heat exchanger is undersized when modeling the system, it will directly reduce the thermodynamic efficiency of the heat pumps in the normal operating mode. In an undersized system, if the heat balance in the ground is not fully restored between the two seasons, the soil temperature will gradually sub-cool for many years to technically unacceptable working fluid temperature values in the pipes and ultimately cause the heat pump to stop working. Also, increasing the viscosity of the working fluid at lower temperatures by adding too much glycol mixture can cause laminar flow in the pipes, thereby increasing the equivalent pile resistance and reducing heat transfer. The correct dimensioning of the ground exchanger and the operating working temperature must, therefore, reduce a long-term negative change in soil temperature and working fluid temperature to a minimum. These changes are caused solely by imbalances in heat extraction and rejection between seasons, as well as by thermal interferences between individual piles in a particular geometric grid. The imbalance especially appears in locations where one of the heating or cooling cycles is dominant.
For the case study in question, a simulation of the geothermal system was conducted to establish the expected consumption of the thermal and cooling energy of the facility since there are 13 separate dilatations, with central heating and cooling of all facilities (
Figure 9).
For the design outdoor temperature of −13 °C and an interior temperature of 20 °C, the thermal losses of the building at peak load were determined at 370 kWth, with the cooling requirements at 750 kWf. As previously determined, the maximum load of a single pile is 1.32 kWth, where temperatures at the inlet and outlet of the heat exchanger do not fall below the techno-economically set limit of EST = 0 °C (minimum operating regime 0.0/−2.5 °C at peak consumption). For a geothermal field of 256 pilots (128 pairs of piles connected by two in series), the installed heat power of the piles grid is then 336 kWth.
In the heating cycle, the cumulative delivered heat power is the sum of the installed heat power of the piles and the compressors of the heat pumps. If the work of the heat pump with COP = 4.0 is foreseen (for example, EST/LST = 0.0/−2.5 °C; LLT/ELT = 42/37 °C), then the compressor power will be 112 kWe and the system can total deliver 448 kWth of energy to the consumer in the peak period.
In the cooling cycle, the heat power of the compressors is waste heat and must be rejected to the ground together with the heat from the facility. For the same type of heat pump as in the heating cycle, the cooling effect for the operating regime EST/LST = 30/35 °C and LLT/ELT = 7/12 °C would be 477 kWf with the EER of 5.46. The power of the compressor would be 107 kWe, and the geothermal field of the piles would be operating with a total of 584 kWf.
Based on the analysis of the results of the step TR test and the equations of a steady-state heat transfer, it was seen that the field of 256 pilots could fully meet the heating energy needs of the building with a technically economical mode of operation. However, it was seen that the pile field could not fully meet the cooling needs of the building while respecting the technically economically acceptable EER on the heat pumps and critical limit temperature on condensers. As part of the mechanical engineering design, the commercial building was equipped with a backup heating system using gas boilers, as well as a dry cooling tower in a bivalent energy cooling system.
In order to use mostly renewable shallow geothermal energy, it was necessary to model the system with dedicated geothermal software in order to find a bivalent cooling point where the geothermal system will work efficiently and where the energy ratios from the renewable source will be favorable. Two scenarios were simulated. In the first one, 100% of the heating and cooling needs were covered by the geothermal system to set the basis, and in the second, the use of a bivalent cooling system is considered. The simulation was performed in the Earth Energy Designer (EED v4.19) program package, which achieved the optimum design of the pile grid and energy usage.
Scenario 1. Energy consumption of the facility in the heating and cooling was calculated for the situation where 100% of the heating and cooling demands are recovered from the geothermal resource and are determined at a total of 487.5 MWh
f for cooling and 555.0 MWh
th for heating (
Table 1). Given the peak values of the heating (370 kW
th) and cooling (750 kW
f) capacity, the total equivalent full load working hours of the heat pumps would be 1500 h in heating and 650 h in cooling. The simulation results showed that during cooling in peak conditions, pile fluid reaches an EST of 37.0 °C (LST 40.0 °C), which is technically and economically unacceptable as a condition at heat pump condensers (
Figure 10). Therefore, it was necessary to reduce the rejected energy values to the ground level using the auxiliary dry coolers during the warmest period of summer. However, the fluid temperatures during the first heating season in winter are quite high due to the substantial heat energy rejected into the ground during the summer, with mean EST at 6.5 °C (LST 3.5 °C)
Scenario 2. The second scenario implies the use of a bivalent cooling system by setting the bivalent point at an air temperature of 27 °C. Above this point, cooling capacity is also taken by the dry coolers so that the geothermal source remains in an effective temperature regime and avoids overheating of the pile fluid. By analyzing the frequency of occurrence of temperature bins for the City of Zagreb, it was shown that for average values of 15-year data, temperatures above 27 °C appear in 593 h. The energy analysis in
Table 2. shows the cooling needs of Scenario 2, with different energy needs and peak power in the cooling mode when compared to Scenario 1. In this case, the geothermal field takes over 70% of the total cooling needs of the facility, i.e., 343 MWh
f of cooling energy, with a peak cooling power capacity of heat pumps equal to 438 kW
f in July. The remaining cooling energy of around 140 MWh
f per year is taken over by dry coolers, with a total peak cooling power of the bivalent system of 750 kW
f in July.
Figure 11 shows the simulation results of monthly minimum, mean, and maximum heat exchanger fluid temperatures for the following 20 years of system operation.
The simulation results showed that during cooling in peak conditions, pile fluid reaches a mean temperature of EST 26.0 °C (LST 29.0 °C), while during heating in peak conditions, it reaches a mean temperature of EST 4.5 °C (LST 1.5 °C), which is technically and economically acceptable for the system operation.