Keywords: optimization, sustainability, geothermal energy, thermal water, cascade utilization, Hungary.

Geothermal energy is the most economical energy of the green energy resources such as solar energy or wind. It is a "baseload" type of power, which is constantly available, not just when the sun shines or the wind blows. Hungary has outstandingly good natural features considering its geology, geophysics and hydrogeology. Thermal water is available in more than 70% of the area of the country [1].

Geothermal utilization is commonly divided into two categories, i.e. electricity production and direct application. The main types of direct application are space heating, bathing and swimming (including balneology), horticulture (greenhouses and soil heating), industry and aquaculture [2]. Some of the uses may not promise an attractive return on investment because of the high initial capital cost. The use of geothermal fluid several times may have to be considered to maximize benefits. This multi-stage utilization, where lower and lower water temperatures are used in successive steps, is called cascading or waste heat utilization.

Bóly is a small, agricultural town with a population of 3,900, situated south of Pécs in South Transdanubia. In 2006, a new and successful project was initiated. A new 1,500m deep production well was drilled. The wellhead temperature is 84°C in this well, which yields 125,000m³ hot water annually. A 2.5 km long pipeline, which is used for district heating, has been working since October 2008. At the end of the pipeline, the thermal water is 76°C, which is reinjected under the ground. With the help of this system, the town gas consumption has been reduced by 800,000m³ per year.

The project is based on a one stage thermal water utilization, that is, the water is used only for heating buildings, so the exploitation of the thermal water capacity is low and the reinjection water is too hot.

New options for thermal water utilization should be found in Bóly, so the heat energy of thermal water can be exploited more intensively, which can lead to a decrease in the temperature of the reinjectional water, which can go below 30°C. In this way maximum benefits can be gained from geothermal energy with minimum energy use. Geothermal energy used in several forms has many benefits for society, including positive externalities of private investments, reduction of CO₂ and other emissions, security of energy supply and local economic development. Furthermore, geothermal energy can help to improve the competitiveness of industries and agricultures, at least in the long run, and can have a positive impact on regional development and employment. The use of geothermal resources appears to be particularly suitable for rural areas which are included in the target of the EU's regional policy instruments.

1

M. Árpási, Gy. Szabó, "Geothermal Development In Hungary Country Update Report 1995-1999", Hungarian Geothermal Association, Budapest, Hungary, 2000.

2

J. Szanyi, B. Kovács, "Experiences on reinjection at geothermal systems in South-East Hungary", Szeged, Hungary, 2009.

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