Ground Coupled Heat Pump Systems a Key for a Sustainable Development of Heating and Cooling Buildings

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1 Ground Coupled Heat Pump Systems a Key for a Sustainable Development of Heating and Cooling Buildings GABRIEL V. BENDEA*, MARCELA F. PRADA**, CODRUTA C. BENDEA*, CALIN D.SECUI* Department of Energy Engineering *, Department of Civil Engineering **, University of Oradea, 1, Universitatii Street, ROMANIA Abstract: - It is very important that for new technologies, such as ground coupled heat pump systems are for Romania, to have besides the theoretical knowledge also the possibility to make computer simulations, and more important to be able to validate their results with real data obtained from a monitored installation. At University of Oradea, we have demonstrated that ground coupled heat pumps may represent a long term solution for heating and cooling buildings, being a sustainable technology for the future. We have theoretically analyzed the influence of different parameters on the average temperature of brine and simulate the variation of brine temperature in 8 Cases. Future work focuses on recording system, since we need long term logging data to validate the simulation Key-Words: - ground coupled heat pump, brine temperature, energy efficiency 1 Introduction Heat pumps are reversed cycle machines that can extract heat from a cold source and deliver it to a warm source, at higher temperature. This may be done by means of electricity consumption at the compressor. When cold source is the ground and the heat is carried by a fluid (usually, brine) that circulated through pipes buried either horizontally, vertically or any other way (Fig. 1), the heat pump system is called ground coupled. As the heat comes a). from the core of the Earth, it is the geothermal energy the one responsible for allowing the groundcoupled heat pump system run. vertical spiral Fig. 1 Type of pipes for heat extraction from the ground [1] The system that heats a building or cools it down by using a ground-coupled heat pump comprises of three subsystems: the underground subsystem; the heat pump subsystem; the building heating and cooling subsystem. All three components are equally important and a correct design is necessary in order to achieve an energy efficient building. horizontal b). 2 The experimental system a). Oradea is situated in the Western part of Romania, close to the border with Hungary. The Pannonian Basin has its influence and, therefore, this area is blessed with geothermal water. All the buildings belonging to the University of Oradea Campus are heated with geothermal water extracted from a nearby well, from a depth of 3,000 m. Researchers from University of Oradea carried out a lot of work in the field of geothermal energy, starting from the 80s a). ISBN:

2 [2,3], but it was only in the recent years that the shallow geothermal energy - extracted by groundcoupled heat pumps - has started to be studied. In , a ground-coupled heat pump (GCHP) system was installed, having two types of ground heat exchangers (SCS) (Fig. 2): a vertical one - SCSV - (borehole heat exchanger) consisting of a double-u pipe inserted into a borehole of 75 m depth, and a horizontal one - SCSO - consisting of 4 pipes laid down into a 75 m long trench, on two layers (1.8 m, respectively 1.2 m depth). Because of manifold headers the system is capable to separate the individual brine circuits. This system is used for heating a laboratory belonging to Energy Engineering Department. The maximum heat flow required is 2025 W and the annual energy requirement is 8636 kwh when considering an indoor temperature of 20 C and variable outdoor temperatures. [4] The characteristics of the borehole heat exchanger are: borehole diameter: 210 mm shank space: 140 mm pipe diameter: 32 mm pipe wall thickness: 3 mm pipe conductivity: 0.42 W/mK brine type: ethylene glycol, 25 % thermal resistance between grouting material and borehole heat exchanger pipe: 0.1 mk/w 3 Temperature Simulation To supply the energy requirements of the building, the amount of heat extracted from the ground is very important. Furthermore, this depends on the temperature of the brine coming out from the borehole heat exchanger. Therefore this parameter must be thoroughly analyzed. First, we have to identify the factors that influence the temperature of the brine that enters into the heat pump evaporator. These factors are: the borehole diameter (DF), the shank space (DA), type and conductivity of grouting material (CU), Cp PC 1 PC 2 V PC 3 V Cd E H VR SPC Co SC R 1 R 12 SCSO 1,8 m SCSV 75 m SST Fig. 2 Layout of the experimental system SST underground subsystem; SPC HP subsystem; SC building subsystem; SCSO, SCSV horizontal, respectively vertical ground heat exchanger; R1 R12 valves; PC1, PC2, PC3 circulation pumps; V evaporator; Cp compressor; Cd condenser; VR expansion valve; VEH header; Co heat consumer ISBN:

3 conductivity (CC), diameter (DC) and wall thickness (GP) of borehole heat exchanger pipe, type (CF) and flow rate (V) of the heat carrier fluid (brine), etc. For each of these influence factors a simulation was made, in order to determine the variation of the minimum value of monthly average temperature (that occurs at the end of January in the last year of simulation). For instance, various types of grouting material (all having different conductivities shown in Table 1) were analyzed. Table 1 Thermal conductivities of different grouting materials [W/mK] Conductivity Code Grouting material u CU1 Thermally grouting enhanced 1,47 CU2 Dry clay, dry gravel 0,4 CU3 Wet clay 1,6 CU4 Water saturated gravel 1,8 CU5 Wet sand 1,0 CU6 Water saturated sand 2,4 CU7 Bentonite (40 %) and water 0,6 CU8 Bentonite/cement/sand (9,5/9,5/15 %) 0,7 Figure 3 shows how the brine temperature varies over different types of grouting material. Fig. 3 Brine temperature at the end of January, in the 20 th year of simulation Graphics and simulations, like the above-one, have been made for each of the influence factors individually, considering that all other input data is constant. But, in order to come closer to the reality, the simultaneous variation of more than one factor must be considered. Therefore, a set of several case studies were settled (Table 2). Table 2 Input data for brine temperature simulation Influence Factor DF DC GP CC DA CU CF V [mm] [mm] [mm] [W/mK] [mm] [W/mK] [W/mK] [l/s] Real Case Case Case Case Case Case Case Case Case The simulation program Earth Energy Designer, version 3.15, needs some more input data that will be considered as having the same values in all studied cases: monthly energy consumption; mean annual temperature at ground level; double-u type borehole heat exchanger; depth of borehole heat exchanger; seasonal performance factor; simulation period. ISBN:

After running the simulation, besides the brine minimum temperature last year of simulation in January, other parameters can be determined: heat flow density over a year, ground coupled resistances

4 After running the simulation, besides the brine minimum temperature last year of simulation in January, other parameters can be determined: heat flow density over a year, ground coupled resistances (Figure 4), flow type inside the borehole heat exchanger (Figure 5) and monthly variation of brine temperature in the 1 st year (Figure 6) and in the last year of operation (Figure 7). Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Real Case Internal borehole thermal resistance Brine/pipe thermal resistance Pipe thermal resistance Borehole heat exchanger thermal resistance Pipe/grout contact resistance Fig. 4 Thermal resistances in the ground Effective thermal resistance of borehole heat exchanger Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Real Case Fig. 5 Reynolds values ISBN:

5 Case 1 Case 4 Case 7 Case 2 Case 5 Case 8 Case 3 Case 6 Real Case Brine Temperature [C] Brine Temperature [C] JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Fig. 6 Brine temperature variation in the 1 st year of operation Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Real Case JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Fig. 7 Brine temperature variation in the 20 th year of operation The flow inside the borehole heat exchanger is strongly influenced by the flow rate, the nature of the fluid (pure water or brine) as well as the its concentration (25% or 33%). It can be seen that when brine has a small flow rate (regardless its concentration) the flow is laminar and, therefore, the heat transfer between ground and brine is poor. Even if we would change the brine with pure water, for Case 4 and Case 8, the flow will not become turbulent (Reynolds would be only 1129). For these cases, an increase of flow rate should be considered. As concerning the brine temperature, in the beginning it has the same value as the undisturbed ground temperature, but as heat is extracted, the temperature will decrease having a minimum value at the end of January (3.6 7C, depending of the case studied). In the next months, the ground temperature regenerates to (11, C), but doesn t reach the initial value. So, ISBN:

6 the brine temperature has a cyclic variation over the years, at the end of simulation period (20 years of uninterrupted operation on heating mode) its value varies between 2.5 C and 6 C. All these simulations helped with the design of the Real Case and showed the weak point of each Case. After the commissioning of the ground coupled heat pump system, temperature measurements were made, but unfortunately, data logging equipment was missing. It was only last year that a data recorder was installed and now is working properly. 4 Conclusion Because in Romania this technology - that uses ground coupled heat pumps for heating and cooling buildings - is quite recent a lot of research activity is necessary, in order to prove the efficiency of the system. The main difference between ground coupled heat pump systems and any other heat pump system is the way heat is collected. The ground subsystem requires a large initial investment if comparing to air source or even water source heat pump systems, and any mistakes done in the engineering design stage can have major consequences on the performances of the system. So, a thorough investigation of all the factors that might influence the monthly average temperature of the brine is really important As it could be seen from the simulation values, an improper mixing between bentonite, sand and water, or drillers failure in following percentages of the recipe for preparing the grouting material, or even a wrongly chosen grouting material by the project designer can have a significant importance on the mean temperature of the brine. The difference may be more than 1.5 C. There are some private companies that install this type of systems, but they don t measure, nor monitor the efficiency. It is very hard to improve something when you don t have a starting point. Therefore, University of Oradea has built its own system for testing on real-life the computer simulation that had been done. Since the simulated period is relatively long, the results will be seen in several years, but what is the most important is that the monitoring equipment is in place and records data. References: [1] J. Lund, Ground-Source (Geothermal) Heat Pumps, Text-book of the European Summer School on Geothermal Energy Applications, Oradea, [2] C. Bendea, M. Roşca, Industrial Uses of Geothermal Energy in Romania, Transactions of the 1999 Geothermal Resources Council Annual Meeting, Reno, NE., USA, 1999, pag [3] I. Cohut, C. Bendea, Geothermal Development Opportunities in Romania, Geothermische Energie, nr. 24/25 Marz/September 1999 [4] C. Bendea, I. Felea, G. Bendea, Energy Performance Analysis of the First Research- Only Ground Coupled Heat Pump in Romania, Journal Of Sustainable Energy, Vol. 1, No. 4, December, 2010, pp ISBN:

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