On September 22, 2020, at the 75th session of the United Nations General Assembly, the mainland officially proposed the goal of achieving carbon peaking by 2030 and carbon neutrality by 2060.
The implementation of the dual-carbon policy has hit the traditional fossil energy, while clean renewable energy sources such as solar energy and wind energy have ushered in a wave of development opportunities.
Recently, some media said that Shandong found a "huge treasure" of new energy, with reserves equivalent to about 18 billion tons of coal, which can be used by China for 3800 years.
In fact, as early as 2016 and 2019, there were reports of this.
However, whether it will actually be available for 3800 years remains to be explored. In addition, although hot dry rock reserves are large and pollution-free, it is not easy to excavate and utilize it, and China is only in the exploration and basic research stage.
What exactly is hot dry rock?
Hot dry rock?
Hot dry rock, as the name suggests, is a dry, high temperature rock, mostly medium acidic intrusive rock, generally at temperatures greater than 180 ° C, buried thousands of meters deep underground, with no or only a small amount of dense fluid inside.
Unlike conventional new energy sources such as solar and wind energy, hot dry rocks are derived from the earth's interior (geothermal energy), so they are not affected by the climate and generate electricity more stably.
Due to the high temperature of hot dry rock, the tailwater after power generation can still reach 70 to 80 ° C, so it can also be used for heating, and it seems to be the most potential emerging energy source in the future.
It is estimated that the resources of hot dry rock in the world are nearly 30 times that of all fossil energy sources such as coal and oil, according to the law of geothermal science, the deeper the distance from the surface, the higher the temperature, so as long as it can be mined deep enough, the energy of hot dry rock is unlimited.
On the other hand, geothermal energy is extremely dangerous, and if it is out of control, it will trigger volcanic eruptions or earthquakes.
Although we can already "tame" a part of geothermal energy through technical means, for example, hot springs are the most common form of geothermal energy utilization in life, the whole process is very environmentally friendly, and can continuously obtain energy for a long time, but the use of hot dry rock and hot springs are very different.
How is hot dry rock developed?
Crust below 3 km from the surface, composed mainly of igneous and metamorphic rocks, is characterized by sufficient temperature but lack of pores, these rock masses are not really "dry", but the fluid content (water content) is not enough.
According to the different causes and output conditions, geothermal energy can be roughly divided into two categories: hydrothermal type and dry heat type. Hot springs belong to hydrothermal geothermal energy, which can obtain geothermal energy through the extraction of fluids in the hydrothermal system.
The hot dry rock structure is tight, the internal fissure is small, does not contain or only contains a small amount of fluid, belongs to the dry heat type geothermal energy, in fact, the vast majority of geothermal energy is also dry heat type, and through artificial fracturing to form an enhanced geothermal system, in order to mine and utilize this part of the geothermal energy.
The development principle of hot dry rock is not complicated, simply put, it is to install a "hot water faucet" on the earth.
First, a shaft is drilled on the ground in the area where the hot dry rock is stored, and when it reaches the hot dry rock layer, the wellhole is closed and water is injected into the well, so that high pressure is generated in the well. Due to the denseness of the rock mass, the high-pressure water will cause cracks in the rock mass, and with the continuous injection of low-temperature water, the cracks will continue to increase, expand, and communicate with each other, and finally form a surface-like heat storage space. These injected waters move along the fissures, and they exchange heat with the surrounding hot rocks to become a mixture of water vapor at high temperatures and pressures of up to 200-300 °C.
After that, the well is continued to be drilled in this heat storage space, and the high temperature water vapor is transmitted to the ground for the comprehensive utilization of geothermal power generation and thermal energy, and the warm water can be injected back into the well and recharged into the hot dry rock for recycling.
This process is to use geothermal heat to "boil water" and turn cool water into hot water, which is not complicated in principle, but the existing exploration technology cannot drill into the deep part of the earth, and can only mine shallow hot dry rock.
Of course, before mining, it is also necessary to make adequate preparations, understand the distribution of the entire resource and determine the direction of the rock crevices, which is conducive to achieving higher energy conversion rates and lower costs.
What are the trials and challenges of hot dry rock development?
Unconsciously, the research history of hot dry rock is about to span half a century, but most of them are limited to developed countries such as Europe, America, Japan, and Australia, which have established a number of experimental research bases for development and utilization. However, the development and research of geothermal heat in high-temperature rock masses in the mainland started late and is still in the stage of exploration and practice.
In 2012, the mainland launched the 863 program to officially carry out the thermal energy development and utilization project of hot dry rock.
After resource exploration and technical feasibility analysis, the Gonghe Basin in Qinghai is the best place for hot dry rock mining conditions, and the temperature of 4 kilometers underground can reach 200 °C, which is an excellent hot dry rock base. The mainland also laid the first hot dry rock well here, and the hot dry rock resources were explored.
Subsequently, well drilling was carried out in Fushan, Hainan and other places. However, due to core technologies such as high-temperature drilling and completion, the hot dry rock wells drilled by the mainland are exploration wells and have not been developed.
Although the hot dry rock reserves are large and pollution-free, due to the immature deep drilling technology and the high cost of fracturing, its mining is extremely difficult, and looking at the world, there are only a few hot dry rock projects that have really successfully operated.
What other problems can there be with the use of hot dry rock?
Although hot dry rock development is still in its infancy and is still a long way from large-scale utilization, on the other hand, we need to consider in advance what harm it will bring.
Among them, because the huge force required to be applied when fracturing rock masses, changing the natural stress field, coupled with artificial activities will activate the fractures, the development of hot dry rocks may induce earthquakes. In addition, there are theories that hot dry rock mining may overdraft the earth's insulation mechanism in advance and accelerate the cooling of the earth, perhaps this is just a concern.
"Our journey is the sea of stars", in the face of the vast space and the deep ocean, we are full of motivation and courage to seek knowledge and enterprising, but we have to admit that we know very little about the land under our feet, and perhaps bury more secrets deeper in the earth's crust.
Source: Popular Science China