This article will analyze underground thermal energy storage from aspects such as its characteristics, usage scenarios, energy distribution, operating mechanism and principles.
Based on an overview of the current status of global underground thermal energy storage development and utilization, this article summarizes the development trends and new technology research and development directions of global underground thermal energy storage development and utilization
Starting from the distribution of geothermal resources, geothermal power generation utilization, direct utilization of underground thermal storage, and underground thermal energy storage .
1. What is underground thermal energy storage?
Underground thermal energy storage, derived from indigenous sources within the earth, is a clean, renewable energy source. Compared with wind energy, solar battery energy and other renewable energy sources, the biggest advantage of underground thermal energy storage is the stable and continuous output of energy. The International Renewable Energy Organization (IRENA) “Renewable Energy Power Generation Cost” report shows that from 2007 to 2021, the energy consumption of geothermal power generation The utilization coefficient is between 70% and 90%.
2. Development status of underground thermal energy storage
Since 2010, global underground thermal energy storage utilization has grown rapidly. The installed capacity and annual heat utilization of geothermal direct utilization are approximately 108GWt and 283,580GWh respectively.
China has long maintained the world’s first place in the direct utilization of underground thermal energy storage. Ground source heat pump systems account for approximately 72% and 60% of the global installed capacity and heat utilization of direct geothermal utilization respectively.
From 1995 to 2020, heat pump systems showed an exponential growth trend of approximately 16% per year, and this trend is expected to continue in the future.
As of 2020, the total installed capacity of global geothermal power generation is 15.9GWe, and the annual power generation is 73,550GWh. From 2010 to 2020, the average annual growth rate of geothermal power generation is approximately 4%.
3. Geothermal resource distribution
The global geothermal resources are abundant, but their spatial distribution is extremely uneven. High-temperature geothermal resources are mainly distributed at discrete plate boundaries and convergent plate boundaries. The distribution of high-temperature geothermal resources is generally controlled by the earth’s tectonic-thermal background.
The spatial distribution is related to global energy storage plate boundaries, seismic zones, and volcanic zones. The most significant features are There are high heat flows, strong high-temperature hydrothermal activities, active volcanoes and frequent earthquakes.
The four global high-temperature geothermal belts are: the Pacific Rim Belt, the Mid-Atlantic Ridge Belt, the East African Rift Valley Belt, and the Mediterranean-Himalayan Belt.
The circum-Pacific geothermal zone is the most widely distributed geothermal zone in the world and is distributed along the subduction/collision boundary between the Pacific plate, the American plate and the Eurasian plate.
The most notable features are high heat flow, young orogeny and frequent active volcanism. According to geographical distribution, it is further divided into three geothermal subzones: the East Pacific Ridge, the West Pacific Island Arc, and the Southeast Pacific Suture.
The reservoir temperature is generally between 250 and 300°C. Representative geothermal fields include Gaithers in the United States (288°C), Cerro Prieto in Mexico (388°C), Datun in Taiwan, China (293°C), and Matsukawa in Japan (250°C). ) and Philippine Buckman (300℃), etc.
4. Application of geothermal resources
Geothermal power generation is one of the most important ways to utilize underground thermal energy storage. At present, the mainstream geothermal power generation technologies include back-pressure power generation.
Condensing power generation, flash evaporation power generation, duplex power generation, full flow power generation, flash duplex power generation, etc. The power generation method used mainly depends on the local geothermal resources. endowment.
Geothermal power generation has a history of hundreds of years. In 1904, Ladrello in Tuscany, Italy, first used underground thermal energy storage to drive a small generator to generate electricity. The power generation was 0.55kW, which could provide lighting power for five 100W lamps.
5. Direct application of underground thermal energy storage
The direct utilization of underground thermal energy storage includes geothermal heating, geothermal cooling, geothermal drying, hot spring bathing and ground source heat pumps. After long-term development, the direct utilization of geothermal resources has evolved from small-scale single-purpose utilization to large-scale comprehensive utilization projects.
Currently, more than 80 countries around the world use underground thermal energy storage directly for building heating (or cooling) or for industry and agriculture. Among them, Iceland is a well-known example of efficient development and utilization of underground thermal energy storage.
Although it is close to the Arctic Circle and has low temperatures all year round, more than 90% of households across the country use geothermal heating due to its abundant geothermal resources.
6. underground thermal energy storage system
Underground thermal energy storage is an energy system that uses underground aquifers as a medium to store thermal energy. It injects and extracts groundwater from aquifers through groundwater wells to store and recover thermal energy.
Underground thermal energy storage can make up for the imbalance of energy supply and demand in time/space distribution, comprehensively utilize a variety of renewable energy forms, reduce dependence on fossil fuels, and provide a good solution for energy conservation, emission reduction and environmental protection. It is also A powerful means to help the world achieve the “double carbon” goal.
7. Classification of underground thermal energy storage
Due to its low temperature, shallow underground thermal energy storage is mainly used for heating and cooling of buildings. The actual international use of shallow underground thermal energy storage systems began in the mid-20th century.
China has started early in the practice of using shallow underground aquifers for heat storage. As of now, a total of 6 shallow underground thermal energy storage systems have been built and put into use in China.
In recent years, with the development of emerging industries, the utilization methods of shallow underground thermal energy storage have become more diversified. Currently, there are actual cases of using shallow underground thermal energy storage in greenhouse agriculture and large data centers.
Deep underground thermal energy storage can store high temperatures, some even exceeding 100°C, and can be mainly used for power generation and heating. The research and utilization of deep underground thermal energy storage began in the 1980s.
In recent years, with the increasing demand for energy, deep underground thermal energy storage technology has received renewed attention, and research and engineering practice on deep underground thermal energy storage technology have also become more and more important.
As a huge natural energy storage, geothermal storage is most suitable to assume the functions of energy storage and stable thermal energy output in a multi-energy complementary system. It can save wind energy, solar energy and other renewable energy storage sources to achieve efficient and large-scale cross-season energy storage utilization.
Improve the utilization efficiency of renewable energy and overcome the shortcomings of renewable energy such as intermittency and instability.
At the same time, under certain conditions, underground thermal energy storage systems have more significant economic benefits compared with ordinary geothermal systems. The return on investment time for a typical underground thermal energy storage system is only 2 to 10 years.
The payback time for underground thermal energy systems for cooling is even shorter because the stored cold energy can be used directly without the need for a heat pump. According to research reports, the average service life of underground thermal energy storage systems can reach 25 to 50 years.
8. Summary
Currently, shallow geothermal resources around the world are developed and utilized through geothermal pump technology. This technology is one of the fastest-growing renewable energy technologies in the world and the fastest-growing part of geothermal utilization technology.
Deep high-temperature hydrothermal systems only exist in specific geological structural environments and are relatively scarce, severely restricting the growth of high-temperature geothermal power generation.
Therefore, various geothermal direct utilization technologies should be developed in areas where conditions permit, and comprehensive cascade utilization should be adopted to improve the utilization efficiency of geothermal resources.
Although the earth is a huge heat reservoir, geothermal resources are extremely unevenly distributed. Often there are not enough exploitable geothermal resources in areas where there is demand, while geothermal resources are very abundant in areas where demand is small.
Therefore, how to store this waste energy and make full use of it is a big issue in the geothermal field and even the entire new and renewable energy field. In fact, large-scale cross-season energy storage is a major scientific issue in the energy industry. Currently, the main available method is pumped hydro storage.
However, pumped hydro storage faces problems such as low energy utilization efficiency and high site requirements. There is an urgent need to explore new large-scale storage methods, energy storage technologies, and underground thermal energy storage offers great possibilities.
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