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Geothermal Energy in Bulgaria

Bulgaria has immense geothermal energy potential. A cheap, almost inexhaustible supply of environmentally friendly, reliable, renewable energy. Capable of powering the economy and heating and cooling homes and businesses

Why geothermal energy is important in Bulgaria now?

Increasing the use of geothermal energy in Bulgaria will reduce and could replace the need to burn fossil fuels, when used together with other renewable energy sources. It will accelerate the transition of the country to a modern and sustainable 21st century economy. It will create jobs and provide opportunities to develop new skills that could position Bulgaria as a future European leader in renewable technology applications. And, it will help the country reach the ultimate goal of carbon-neutrality by 2050.


The government of Bulgaria has stated a goal to oversee the development of 400 MW of geothermal energy capacity by 2026. The setting of this ambitious target highlights the important role geothermal energy will play in the transition of Bulgaria to a successful low-carbon economy.


Greater development of geothermal energy in Bulgaria is now a cornerstone in the plans of the country for the energy transition. This is because geothermal is the only source of renewable energy that can provide constant, clean and affordable base-load energy without additional needs for battery storage. Its development will reduce dependence on the mix of fossil-fuels and nuclear power that provide more than 80% of base-load energy capacity today. And bring about the eventual replacement of this base-load mix with reliable renewable energy sources. This is why increasing the capacity of geothermal energy use in Bulgaria is so important to the successful transition of the country to a sustainable, low-carbon economy.

What is geothermal energy and its uses?

Geothermal energy is heat (Greek therme) from the Earth (Greek geo). However, heat in the deep and shallow parts of Earth comes from two distinctly different sources. The first source is deep heat energy generated within the Earth itself by the radioactive decay of unstable isotopes and friction. Deep heat causes the temperature of rocks underground to increase with depth, to an estimate of over 5000°C at the Earth’s core. When the buried hot rock is permeable and contains water (termed an ‘aquifer’ or a ‘reservoir’), wells can produce the hot water to carry the heat-energy to the surface for use. The second source is shallow heat energy from the sun that is stored in the ground. This ambient shallow heat energy means the temperature of the Earth just below ground (from 2m to 250m depth) is a constant 10-15°C. The heat from both deep and shallow geothermal energy sources has many direct uses (‘Direct Heat Use’) and additionally, heat from deep geothermal energy sources at temperatures above 150°C can be used to power electricity generators (‘Power Generation Use’).


Deep geothermal energy in Bulgaria causes the underground temperature to rise, locally, from between 25°C per kilometer to possibly more than 40°C per kilometer, dependent upon the geology. This implies the temperature could exceed 150°C in parts of the country at depths between 4000m to 6000m below ground. Drilling wells to such depths is standard practice these days, especially in the oil and gas sector. Meaning the country potentially has abundant low (25 - 95°C), medium (95 - 150°C) and high (150 - 230°C) temperature sources of deep geothermal energy at accessible depths.

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Geothermal Energy – Temperature, Uses & Depth

In Bulgaria, low and medium temperature sources of deep geothermal energy, at 1km to 4km underground, have a wide-range of viable Direct-Heat-Use applications, so long as the resource is next to the end-user. Sources with temperature up to 75°C can be used to heat greenhouses, fisheries, balneology, and spa hotels. Whilst sources with temperatures between 75°C – 150°C are additionally usable for district heating and industrial processes. Higher temperature sources of deep geothermal energy above 150°C, at 4km to 6km underground, have both primary electric power generation and secondary direct-heat-use applications (using the residual heat left after power generation). However, for a deep geothermal energy resource to be technically viable for use depends not only on rock temperature and drilling depth, but also the rate at which the reservoir can produce hot water; commonly requiring thousands of cubic meters per day. Ultimately, for any deep geothermal resource in Bulgaria to be useful, the economic benefit of using it must exceed the economic cost of finding, developing and producing it.


Very low temperature (below 25°C) sources of shallow geothermal energy can also be economically exploited in Bulgaria, because of their shallow depth, using electric heat-pump systems. These systems are termed Ground Source Heat Pumps (GSHP) or Geothermal Heat Pumps. They use either groundwater (‘Open Loop’) or a liquid isolated in underground pipes (‘Closed Loop’) to transfer energy between the ground and buildings. They provide heating and hot water and can also extract heat from buildings and store it in the ground to provide cooling. The technology is simple and similar to that of an air conditioner, but it is much more energy efficient; saving up to 80% of the energy cost of heating and cooling buildings in comparison to traditional fuel heating systems. Geothermal heat pumps can be installed in buildings of all sizes, from individual homes to apartment blocks, offices, hotels, municipal buildings and sports centres (including swimming pools).

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Deep & Shallow Geothermal Energy Uses and Approximate Depth in Bulgaria

Where is the geothermal energy in Bulgaria?

There are three main uses for the deep and shallow geothermal energy resources in Bulgaria that could benefit the transition of the country to a sustainable, low-carbon economy. The first two uses harness the potential of deep geothermal energy. First, for electric power generation and, second, for direct-heat-use in large-scale district heating and industrial processes. The third utilises shallow geothermal energy via the use of geothermal heat pumps to heat and cool homes, larger residential, business and municipal buildings, which can be installed everywhere in Bulgaria.


In Bulgaria, the potential of deep geothermal energy for power generation and direct-heat-uses is strongly dependent upon the geology. The country can be divided into two geological provinces, north and south of the Stara Planina Mountains. The northern geological province is an ancient, large sedimentary basin that is prospective for medium to high temperature resources in hot-sedimentary aquifers. In this province widespread historic drilling for oil and gas has already proven the presence of reservoirs and high temperatures. In the deeper parts of the basin in northwest Bulgaria, at 4 km to 6 km, the temperature is commonly greater than 150°C and could even exceed 200°C in some places. The southern geological province is a complex patchwork (shown by the topography) of small sedimentary basins (lowlands) separated by hills and mountains made of older rocks (highlands). The small basins formed in places where tectonic movements have fractured, faulted and stretched the Earth’s crust. Shallow aquifers in these basins are prospective for low and possibly medium temperature resources. Whilst the prospect for higher temperature resources is most-likely in permeable fracture-systems within the older rocks that are now deeply buried underneath and adjacent to these basins. Thermal springs, commonplace throughout southern Bulgaria, fed by waters that percolate to the surface along the permeable fractures, provide indirect evidence that hotter geothermal resources may exist deeper underground. However, very limited drilling to more than 1 km or 2 km depth in the south of Bulgaria, unlike in the north, means the deeper medium to high temperature geothermal energy potential of the southern province remains unproven.


Both provinces have significant potential for deep geothermal energy for direct-heat-use (at 1km to 4km depth) and power generation (at 4km to 6km depth). The main difference between the two provinces is the level of knowledge of the deeper geology in the north is much higher (due to previous exploration for oil and gas) than it is in the south. Thus, to unlock their full geothermal energy potential different scientific challenges remain that must be overcome. In the north, the presence of reservoirs and high temperatures is proven, and the only remaining challenge is to prove wells can produce hot water at rates high enough for economic use. In the south, the deeper geology is much less explored and so, the challenge is to first identify the areas with the highest potential and then test these areas, by drilling exploration wells, to prove they are prospective.


Bulgaria is famous since antiquity for it thermal springs and bathing waters, which continue to play an important role in the economy today. Hot springs in Bulgaria are a positive indicator that much deeper underground medium to high temperature geothermal energy sources may be present, but the temperature and out-flow rates of the springs themselves are too low for larger-scale economic geothermal uses. Also, most shallow thermal waters in Bulgaria are potable (drinking water), a valuable hydrogeological resource that must be conserved and used wisely and sustainably for drinking, bottling, horticulture and spas. Deep geothermal resources mostly contain brine-waters (very salty, undrinkable) in geology not connected to the shallow hydrogeology. So, usage of these deep brines for large-scale geothermal applications has no impact on shallow, fresh-water resources.

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Map of Geothermal Energy Potential in Bulgaria. EU funded studies identify large areas of Bulgaria as prospective for geothermal energy exploration. The geothermal energy potential of the country can be split into two prospective geological provinces, north and south of the Stara Planina Mountains In the north deep heat potential is confirmed by widespread historic exploration and drilling for oil and gas. In the south, thermal springs infer deep heat potential is likely present, but this deep potential remains unconfirmed due to a lack of historic deep drilling

Why do we not use more geothermal energy in Bulgaria and when can we?

In the last 10 years (2010 – 2020) global geothermal capacity has grown by more than 100% for geothermal heat pump and direct heat use and nearly 50% for power generation. This rate of growth is set to accelerate in coming years as more countries invest in developing their geothermal energy potential as part of their transition to low-carbon economies.


Bulgaria has abundant potential for deep and shallow geothermal energy resources and should be benefitting from this growth in investment. However, growth in geothermal heat pump and direct heat use in Bulgaria over the last 30 years is only 16% and there is still no exploration for or development of geothermal power generation capacity (0 MW). Deficiencies in the current regulatory framework are the likely reason overall growth in geothermal energy use has been so poor and why there has been no significant investment in exploration and development activities.


To correct these deficiencies and create positive conditions for attracting investment for the exploration and development of geothermal energy in Bulgaria is straightforward. A reform of the regulatory framework to ensure procedures for the licensing and use of deep and shallow geothermal energy resources are appropriate. The reform should adopt the best practices of other countries to ensure the new legislation is ‘best in class’ and will probably be a relatively minor revision of existing legislation. Consequently, the timeframe for completion could be short, allowing large scale exploration and development of the geothermal resources of Bulgaria in the very near future.

How do we develop more geothermal resources in Bulgaria?

Growth in the development and use of geothermal energy can provide Bulgaria with reliable, clean and cheap base-load power and help replace the need to burn fossil fuels. It can also significantly increase the energy efficiency of homes, businesses and towns, by reducing power needs from greater use of direct-heat and geothermal heat pumps.


To become a reality, government, industry, academia and specialists in Bulgaria, Europe and beyond must collaborate to ensure decision-makers have the information needed to take knowledgeable decisions to help the development of geothermal energy in Bulgaria.


The purpose of the Bulgarian Association Geothermal Energy is to support and broaden this collaboration. By bringing together all companies, institutions and specialists with knowledge and experience to share and by creating an open forum for dialogue, proposals and work to encourage greater investment.


The government of Bulgaria has taken the first step on the road toward using deep geothermal energy for power generation by setting a strategic goal for 2026. The roadmap for achieving this goal begins with policy, legislation amendments and new scientific evaluations to identify the most prospective areas for development and to create positive conditions for investment. Pilot projects for power generation will follow, to demonstrate economic viability and to incentivise investors to take on larger scale exploration and development projects for geothermal energy across the country. This journey will require significant manpower and specialist expertise to complete. Successful exploration and development of deep geothermal energy requires the same skillsets and expertise that are current deployed in the oil and gas industry. Redeployment of this ready-made workforce to help find geothermal energy, instead of oil and gas, is the key to rapidly scaling up the development of geothermal energy in Bulgaria and will create jobs, training and many opportunities to develop new skills as the industry grows.


The next step on the road is for government, industry, academia and specialists in Bulgaria to take on the challenge and collaborate to make the everyday use of geothermal energy a reality. Throughout this journey the Bulgarian Association Geothermal Energy will be here to help.

Bulgarian Association Geothermal Energy

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