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Dr Tomasz Śliwa, manager of the AGH University Geoenergetics Lab, demonstrates “power field A” between building A-3 and A-4, Photo by Marianna Cielecka
Not only do they serve for conducting research on optimal geoenergy solutions, but they also help in heating and cooling university buildings. Borehole heat exchangers with accompanying technical base, part of the Geoenergetics Lab at the AGH University Faculty of Drilling, Oil, and Gas, are a perfect example of novel, inspiring solutions for the efficient use of underground heat resources at a time of increasing demand for renewable energy.
Five borehole heat exchangers are located between buildings A-3 and A-4, making up “power field A.” Each is 78.5 metres deep and altogether they generate 26 kW of heating power. It is here that five solar collectors are located. They are used to regenerate heat resources in the rock mass, i.e. soils and rocks below the ground. Fourteen more boreholes, each 84.5 metres deep, and two 30 metre ones constitute “power field B,” between building A-4 and D-2, which delivers over 100 kW of heating power with its circulating pumps.
“Field A” is used for heating and cooling the auditorium for 150 people in building A-4 and sometimes also for melting snow on the 250 square metres large car park adjacent to the building in the winter. As a result, cars may leave the garages nearby without additional snow removal. The installation is also designed to act as a solar collector and may be used for regenerating underground heat resources. In turn, “field B” is the source of heat and cold for building D-2.
Dr Tomasz Śliwa, manager of the AGH University Geoenergetics Lab, demonstrates “power field B” in front of building D-2, Photo by Marianna Cielecka
Recently, two horizontal heat exchangers have been introduced to “power field B.” They were placed 2 metres below the ground and stretch for 114 and 40 metres along buildings A-4 and C-4 and from A-4 to D-2.
Subsurface heat resources
Gaining on popularity in the energy market, the borehole heat exchangers take advantage of the fact that the deeper the borehole, the higher temperature of the rock mass, on average 3°C each 100 metres. That is not the case when it comes to horizontal exchangers which are based on the phenomenon of subsurface urban heat islands, i.e. the rock mass gets warmer due to the urban infrastructure generating waste heat and solar radiation. That is why obtaining heat with this technology does not require creating deep vertical boreholes, which significantly reduces the costs of installation.
To receive the heat from the rock mass, pipes are introduced into the boreholes, and the heat carrier flows through them, usually in the form of glycol solution. Pipes create a closed circuit between the hole and the heat pump on the surface. The working medium is colder than the rock mass, so it receives its heat, and then it gives it back in the heat pump. The cooled working medium is then pumped back to the hole and the entire cycle starts again.
In the heat pump, the thermal energy is received in the so-called evaporator, where the heat delivered from the rock mass is used to evaporate another working medium, which flows in the pump in the next closed circuit. The resulting vapours are directed to the compressor, where their pressure increases due to compression, and thus the temperature also goes up. Received from the vapour of the working medium in the so-called condenser, the converted heat may be used to heat buildings and water. The operation of heat pumps is often compared to how fridges work à rebours, keeping in mind that the so-called reverse heat pumps allow to extract heat from a building to cool it down. In this type of systems, in the summer the heat is pumped in the opposite direction to that in the winter, and the rock mass functions as a heat or cold storage, depending on the season.
Extracting heat from ice
Due to the working medium used in the heat pumps having low evaporation temperature, a relatively low-temperature heat input is sufficient for the efficient operation of these devices. This allows heat pumps to operate even in the polar regions.
“Borehole heat exchangers may be located in ice as well. It the ice is -10°C and we cool it down to -15°C, we will then collect low-temperature heat that the heat pump is able to convert into usable heat. However, we must take into consideration the fact that the heat pump’s operation cost is higher the lower the temperature of the heat extracted from the environment. For equipment that extracts heat from boreholes, like the one we have at the AGH University, its cost is about a quarter of the cost of the electricity that powers the heat pump. If we obtained heat from boreholes made in ice, it would be half the cost of the power supply. In the permafrost in Siberia or in the north of Canada, this works perfectly well. The borehole exchangers used there may also apply the phase change heat, so from the change of water into ice or the other way round, which positively influences the energy efficiency of these systems (in the northern borderlands, the cold may also be of use). There is always the question of profitability; solutions that may be very useful in one environment, may be pointless in the other,” explained Dr Tomasz Śliwa, a professor at the Faculty of Drilling, Oil, and Gas and the manager of the Geoenergetics lab at the AGH University.
Great conditions for geoenergy at AGH University
The first boreholes for the heat exchangers were drilled in 2008, using the grant obtained for the purpose by Professor Śliwa from the state budget. It was then that the equipment necessary for the practical management of underground heat resources was purchased and installed. The questions on the profitability of the investment must have yielded a positive result, as the geological conditions on the university grounds are favourable for this type of investment.
“The operation of borehole heat exchangers is somewhat independent of geology, however depending on the prevailing conditions, more or less boreholes need to be drilled for a given facility. In our geographical area, the worse conditions occur where the rocks have low thermal conductivity and are dry. The best, however, are where they have a high thermal conductivity and contain water, especially when it flows through drilled aquifers. These are precisely the conditions that exist under the entire AGH University campus, so in this respect there are no obstacles to develop such an investment here,” describes Professor Śliwa.
Dr Tomasz Śliwa next to a mobile equipment for the thermal response tests (TRT) of the rock mass, Photo by Marianna Cielecka
Of greater concern in the design and execution of the works was the risk of damage to the technical infrastructure in the ground, especially in view of the paucity of documentation about it, which is not only a local problem.
Its amount is so high that we were afraid not to hit something, especially before drilling horizontal holes. Moreover, up to a depth of two metres, you can expect all sorts of things in the cities; if they were made before 1990, they were not entered on geodetic maps. That is also the case with the AGH University. So, in this aspect, the chosen location was not the best, but we still managed to do everything without damaging anything along the way,” emphasised the manager of the Geoenergetics Lab.
Research and demonstration of technical capabilities
Although borehole heat exchangers are reaching utilitarian goals with great success, the main motif for creating the system at the AGH University was to conduct research. By observing the operation of the “plant,” the researchers find out how different design solutions used in the exchangers and the configuration of the operating parameters, e.g. the flow rate of the working medium, glycol solution, affect the efficiency of the heat extraction (heating power per metre of a borehole). For research purposes, the exchangers are equipped with pipes of different types and thickness, also different materials were used to seal the boreholes. The boreholes next to D-2 were also drilled at different angles to the ground level.
“We conduct plenty of studies and, what is worth mentioning, they are almost always performed with the participation of students, as since 1998 I have also been the supervisor of GEOWIERT, a student research club that celebrates its 70 anniversary this year, which makes it one of the oldest student clubs at the AGH University. Students write their engineering and master’s theses on the basis of the research performed there. It has started years before we managed to establish a non-degree postgraduate programme and full-time studies in geothermal energy. As of late, we have also been focused on the interpretation of results and publication of our conclusions in prestigious scientific journals,” said Professor Śliwa.
The knowledge gained comes in handy when the lab gets requested to provide an expert opinion on the potential to use borehole heat exchangers in a specific location. The unit receives a dozen of such commercial requests a year. However, that is not the only contribution the lab makes to support the development of geoenergy. It is Professor Śliwa's intention for the solutions introduced at the AGH University to also serve as an example of good practices for contractors providing services on the market within this area.
“Every year, I organise a conference attended by more than 100 people, where I demonstrate our facilities and plants. Unaware of such possibilities, designers do their job in an old-fashioned manner, using mainly vertical boreholes, without any creative measures. I want to show them that it can be done differently while obtaining higher effectiveness of the boreholes, heating systems, and especially heating and air conditioning systems,” explains Śliwa.
Dr Tomasz Śliwa, the manager of the AGH University Geoenergetics Lab, demonstrates how to turn on the system in building D-2; the parameters may be read on site in the lab or on the lab’s website after signing in (login: student, password: student), Photo by Marianna Cielecka
To disseminate the knowledge gained by his team, the AGH University scholar has decided to found “Laboratory of Geoenergetics Book. Series.” Publications within the series are financed from various sources, also from sponsors, and distributed free of charge among the students of geothermal energy. Thus far, five books have been published, and two more are in the final stages of editing.
It was all about the management of disused boreholes
Professor Śliwa has been interested in geoenergy since the 90., as it aroused his interest during his university programme in drilling at the AGH University. The inspiration came from Professor Stanisła Jucha who provided the student with preliminary information about the possibility of using subsurface heat resources for energy purposes. It was then that Tomasz Śliwa decided on an individual course of study in said field under the supervision of Professor Andrzej Gonet, thus he selected additional classes related, for example, to heat engineering. For this purpose, he became certified as an energy auditor in 1999.
He began developing the interests from his university days as an assistant.
“Back then, it was mostly about not to decommission the disused oil and gas boreholes, or ones that were drilled with exploration in mind. There are millions of such drillings all over the world, if not tens of millions. Their decommission translates into enormous costs, so it’s better to try to somehow manage them. I was focused on the possibilities for their application in obtaining geothermal energy, but today, I also see other possibilities for their secondary use, such as gravity and pressure energy storage. The AGH University also has at its disposal three 30-metre boreholes for demonstration purposes.”
When Professor Śliwa began his adventure in geoenergetics, he was often met with the lack of understanding on the part of his scientific community for choosing a career path in this direction. It was suggested more than once that he should instead focus on the issues surrounding the extraction of traditional fossil fuels. The need to replace them with green energy sources, in connection with the fight against global warming, was not yet as strongly emphasised at the time as it is today.
Today, geoenergy is trendy due to the geopolitical situation and the drive to diversify energy sources associated with it as well as the need to protect the environment. The topic is being promoted in the EU, so the research we have conducted over many years now perfectly fits what is currently on top. However, it could have been very different. In science, you are not always this lucky. I may only be glad that I engaged in this field a long time ago, which at that time in Poland was of interest only to a very narrow group of specialists, and which is now bearing fruit, said Professor Śliwa.
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