Morahalom is a small town (pop. 5800) situated in the south of Hungary on the border with Serbia. It used to be listed among the 50 most disadvantaged communities of Hungary, but the investments of the past decade - upon some the current project also builds on - launched it into the top 10 most dynamically developing settlements. The installation of the geothermal district heating system can be considered the most influential and of highest impact of them all, which also presented a solid foundation to the local GEOCOM components. It is worth noting that there are two separate geothermal heating systems in place in the town. One of them was developed solely for balneological use at the local spa, the already mentioned district heating system has a much wider impact on the community. Some key specifications of this latter have to be highlighted.
The doublet configuration of one abstraction well (B-45) and one injection well (B-46) (1270m and 900 m respectively) allows the sustainable resource management of the 63°C thermal water produced on site from the Upper Pannonian sandstone reservoir with flow rates in the range of 25-30m3/hour in summer and 60m3/hour in winter. The annual thermal water production on this system is around 190.000m3. The full loop runs a total of 3,054 kms between the two wells serving with heat and domestic hot water (DHW) a total number of 12 municipal-owned public buildings mainly in the downtown area. By having the geothermal cascade system in place the proportion of renewable energy within the energy mix of public institutions has grown from 0% up to more than 80% - offsetting the use of 542.029 m3 natural gas annually, while providing 18.700GJ of heat per year. As a direct result annual heating-related emissions have also been reduced significantly (by 1590t of CO2, 585kg of NxOx and 1113kg of CO). The GEOCOM project aimed to improve the cascade system with a set of new elements to ensure total utilisation of geothermal energy and to demonstrate cutting edge energy efficiency/retrofitting measures that are currently lacking from geothermal projects in Eastern-Central Europe.
Mórahalom, as a result of its excellent setting is able to provide most of its public buildings with geothermal district heating from the local thermal wells. Via a series of individual heat exchangers at each of the served buildings the produced 62°C thermal water is cooled down to 40°C by the end of the cascade system. At this stage the 40°C water still contains considerable amount of energy which was not harvested earlier, but was sent to be re-injected. Within the GEOCOM project a heat-pump heating station was planned and delivered which can utilise the significant residual energy content of the already used thermal water (cooling it from 40 to the minimum of 20°C) to produce over 600kW thermal energy with an average COP=4,5-5,0 prior it gets re-injected.
This additional energy (418kW) can be easily used on site for the development plans include some higher profile buildings to be erected in the near future in the immediate vicinity of the new heat-pump station in addition to the recently completed Town Hall and Market Hall, thus creating a sufficiently dense heat market for such an investment. At the moment no such thermal water - heat pump coupled system with similar efficiency operates either in Hungary or elsewhere in Central-Eastern Europe.
The quantity of the thermal water (200-250 l/min) reaching the primary heat exchanger is self-regulated, making the system capable of adjusting the thermal output to meet the reduced need during the transitional periods (early spring, late fall). Since no power is required to operate the primary heat exchanger, heating energy produced via this way has priority over the heat pump application which has some power demand (approx. 90 kW). In case the outside temperature drops below a certain value (-5 C) this basic system alone can no longer provide sufficient thermal output and needs to be backed up by the heat-pumps. At this point a technical challenge had to be overcome related to the minimum flow rate the heat pumps require for a seamless operation, which is 15m3/hour. This means that an automatic system activates whenever the heat pumps are turned on to provide the sufficient flow (15m3/hour or more) of the thermal water to keep them running. Due to its mineral composition the received geothermal fluid cannot be directed straight to the heat pumps, but the installation of an intermediary heat exchanger was needed to protect the main condenser from scaling. In this separate and closed loop a dedicated pump circulating the 20% mixture of mono-ethylene-glycol and water ensures the heat transfer to the main condenser. In order to secure a continuous supply of hot water there was a 3000 litre buffer tank installed on the forward going side of the system to hold the heated medium of 52°C (max 55°C). The closer this heat is to the maximum the lower the COP of the system will be.
In the summer period the heating energy is used solely for domestic hot water (DHW) production since low enthalpy systems (less than 90°C) - similar to the one in Morahalom - cannot be employed economically for cooling purposes. DHW production however has proved to be a sound and economic utilization of thermal water in the summer as the reduced heat demand yield higher water temperatures in the whole system. During the summer DHW needs are met with the use of 20-200 kW DHW-dedicated heat exchangers at all the facilities, institutions and connection points that are linked up with the geothermal loop.
Schematics of the heat-pump heating station
Not having installed high demand energy sinks for summer use has an important geological reason. The local reservoir is Upper Pannonian sandstone which demands for a periodic pattern (high-low) of pumping intensity (abstraction and reinjection alike) in order to be able to regenarate, thus ensuring long term sustainable production of thermal water. Suspended particles present the main risk for well and formation integrity causing various issues such as wellbore narrowing (sandface bridging), wellbore fill-up, perforation plugging, and formation damage in case of inadequate periodic moderation of pumping intensity. Year-round abstraction can lead to the degradation of subsurface water levels and may yield decrement, while permanent reinjection is likely to cause capacity impairment and/or clogging of the reinjection well. Injection of heat-depleted brines into clastic sedimentary reservoirs with alternating clay, sand and sandstone sequences has long been reported as a sensitive matter among petroleum and geothermal operators.
Researchers at the University of Szeged have aimed at the elaboration of a uniform technological know-how of the economic, safe and standardized procedure of reinjection of used thermal water into Upper Pannonian sandstone. A basic element of this procedure in “summer-mode“ is the significant reduction of abstraction and reinjection intensity in order to allow reservoir recovery to happen. In accordance with these research results, the Mórahalom-system has been, is and will be operated in the summer period under the consideration of reservoir protection aspects and in a reduced mode favoring long-term sustainable abstraction and safe reinjection.