Domestic and foreign geothermal power generation technology development status and trend

Global distribution of geothermal resources

Inside the Earth lies an unimaginable amount of energy. It is estimated that only within the outermost 10 kilometers of the Earth's crust, there are 125.4 billion coke heat, equivalent to the total heat of the world's current coal production 2000 times. If the total amount of geothermal energy is calculated, it is equivalent to 170 million times the total coal reserves. By some estimates, geothermal resources are 100 times greater than the potential for hydroelectric power. Even if the geothermal energy available is calculated at 1%, the heat energy that can be exploited within 3 kilometers underground is equivalent to the energy of 2.9 trillion tons of coal!

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Globally, the distribution of geothermal resources is uneven. The geothermal anomaly areas with obvious geothermal gradient greater than 30℃ per kilometer depth are mainly distributed in the regions of plate growth, rift-ocean spreading ridge, plate collision, declination-subduction zone. There are four major geotropics of global nature:

(1) Pacific Rim Geotropics: many famous geothermal fields in the world, such as Geisels, Long Valley and Roosevelt in the United States; Cerro, Prieto, Mexico; Wairakai, New Zealand; Taiwan manger in China; Japan's Matsukawa, Dayue and so on are in this area.

(2) Mediterranean-Himalayan geothermal zone: The world's first geothermal power station, Italy's Radero geothermal field, is located in this geothermal zone. China's Yangbajing geothermal field in Xizang Province and Tengchong geothermal field in Yunnan Province are also located in this geothermal zone.

(3) Mid-Atlantic Ridge Geotropics: Some geothermal fields in Iceland, such as Clafra, Namafiar and the Azores, are located in this geotropics.

(4) Red Sea - Gulf of Aden - East African Rift Valley geothermal field: including Djibouti, Ethiopia, Kenya and other countries.

3. Iceland is a country rich in geothermal resources. It is located near the Arctic Circle, and despite the cold climate, there is a huge amount of heat energy underground. Iceland accounts for almost a third of the world's rock flows, and in recent centuries, volcanic eruptions have occurred on average every five years, giving it a unique opportunity to generate geothermal energy. According to statistics, Iceland has more than 1,500 hot springs, hot springs, steam springs, geysers, etc.

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The United States is also rich in geothermal resources, according to geological surveys, the United States high temperature geothermal power generation potential equivalent to 755 to 729.7 billion tons of standard coal, or 60 to 475 billion barrels of oil; The medium and low temperature heat energy that can be directly used is equivalent to 1606 to 913.9 billion tons of standard coal. In addition, Japan, New Zealand, Italy, the former Soviet Union, India, the Philippines, France, Hungary, Mexico, Kenya and many other countries have geothermal resources.

China is also rich in geothermal resources. At present, more than 2,700 geothermal outcrops have been discovered (including natural and artificial outcrops), and there is a large amount of geothermal buried underground to be discovered.

Besides Tibet, Yunnan and Taiwan Province belong to high temperature geothermal areas. The coastal provinces such as Fujian and Guangdong belong to the medium and low temperature geotropics. There are low-temperature geothermal fields in some inland basins.

5. Distribution of conventional geothermal resources in China

Development of geothermal power generation technology

Geothermal power generation is a new power generation technology using underground hot water and steam as power source. The basic principle is similar to thermal power generation, and it is also based on the principle of energy conversion, first converting geothermal energy into mechanical energy, and then converting mechanical energy into electrical energy. Geothermal power generation is actually the conversion of underground thermal energy into mechanical energy, and then the mechanical energy into electrical energy, or geothermal power generation.

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After nearly a hundred years of development, geothermal power generation technology has a variety of types, including dry steam power generation, expansion steam power generation, double medium cycle power generation and Karina cycle power generation.

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1. Medium-low temperature geothermal energy

At present, the geothermal power generation projects are mainly developed by ordinary dry steam and flash steam. In the past 10 years, the dual-working medium medium and low temperature geothermal energy generation has developed rapidly. For specific geothermal resources, it is necessary to comprehensively consider the geothermal temperature, total geothermal reserves, geothermal water quality and other aspects, combined with power generation efficiency, operation and maintenance, equipment investment, environmental protection and other factors, and then determine the specific power generation technology route suitable for the geothermal resources.

Medium and low temperature (t<130℃) geothermal resources occupy a large proportion in the current proved geothermal resources, of which the temperature of about 90℃ geothermal resources account for about 90% of the total amount of such resources. For this type of geothermal resources, the dual-medium cycle power generation technology is more suitable. Dual Cycle power generation, also known as Organic Rankine Cycle (ORC), ORC power generation technology can use a wide variety of waste heat, coupled with the advantages in efficiency, process simplification, operation and maintenance costs, making this technology a development trend of low-grade heat recovery and utilization.

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In the process of exploitation and utilization of middle and low temperature geothermal resources, dual-working fluid cycle and Karina cycle technology have broad prospects for development.

10.A schematic diagram showing the basic concept of a low-temperature  geothermal binary ORC system for electrical power  generation.

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Foreign ORC generator set product information

The Karina cycle is a new thermal cycle, which is different from the conventional Rankine cycle. The mixture of ammonia and water is used as the working medium, and the boiling point of the mixed working medium is changed with the change of the ratio of ammonia to water. When the heat source parameter changes, only the ratio of ammonia and water needs to be adjusted to achieve the best circulation effect. The temperature rise curve of the working medium is closer to the temperature drop curve of the heat source, so as to reduce the heat transfer temperature difference as much as possible, reduce the entropy increase of the system during the heat transfer process, and improve the cycle efficiency. Because of this remarkable feature of the Karina cycle, it has been widely used in the field of medium and low temperature geothermal power generation. The current industrial application shows that the cycle efficiency of the Karina cycle power generation technology is 20% ~ 50% higher than that of the Rankine cycle. However, due to the use of liquid ammonia as the circulating working medium, there are high requirements for the sealing of the system, and the storage and use of working medium will have a certain impact on the environment. Attention should be paid to strengthening the environmental assessment work in the construction process of the power station.

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The Kalina power cycle technology introduced from the United States is the main means for Shenghe Company to make breakthroughs in solar thermal power generation, geothermal power generation, cement waste heat power generation, float glass, iron and steel industry, thermal power plant to improve cycle efficiency, coke industry and ferroalloy furnace waste heat power generation and waste heat utilization. This technology could significantly increase the efficiency of Rankine cycles currently used in conventional thermal power generation and waste heat generation systems. This technology using ammonia water as the circulating working medium is derived from the conventional Rankine cycle and has great particularity.

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2. Exploitation of middle-deep geothermal resources and dry hot rock resources

In addition, after the large-scale development of shallow geothermal energy, middle-deep geothermal resources and dry hot rock resources will become new resources of geothermal power generation technology. In the next step of the development of geothermal power generation technology, attention should be paid to the exploitation of middle-deep geothermal resources and dry hot rock resources.

enhanced geothermal systems (EGS) are emerging for the development and utilization of hot dry rock (HDR) thermal energy 3 ~ 10 km deep underground. Enhanced geothermal power generation technology, by injecting water back into groundwater, To create new geothermal resources, geothermal resources can be obtained at higher temperatures, which can reach 175 ~ 225 ° C. Generally, dual-cycle power generation is used to maintain the pressure of geothermal water and reduce the energy consumption of recharge.

14. Fenton EGS in the United States, SoultzEGS in France, Rosemanowes EGS in the United Kingdom, Hijiori EGS in Japan, Cooper EGS in Australia, etc., after 40 years of field test and research, Technical achievements have been made in drilling exploration, hydraulic fracturing, artificial heat storage and heat recovery cycle. In the field of dry hot rock, China's initial investment is small, mainly funded academic exchanges, exploration and research, and has not formed a national level of dry hot rock technology research and development base and equipment conditions.

3. Development trend of geothermal power generation technology

However, the new combined cycle power generation technology is the development direction of geothermal power generation technology. The cycle efficiency of single steam Rankine cycle power generation technology is low, only less than 20%; The discharge temperature of tail water is relatively high, generally above 100℃, and the utilization of geothermal energy is insufficient. The dual-working fluid cycle and Karina cycle power generation technology systems are more complex, involving two sets of working fluid systems, but the cycle efficiency is high, and the tailwater discharge temperature can be reduced to below 60℃. In the future geothermal power generation technology, the way of combined cycle can be adopted. In the high temperature stage of geothermal water, the expansion type steam power generation system is used to utilize the high temperature part of geothermal energy. When the temperature of geothermal water cannot meet the operating conditions of expansion power generation, the dual-working medium cycle or Karina cycle technology is adopted to make full use of the low temperature part of geothermal energy and maximize the efficiency of geothermal power generation cycle. Based on the expansion system, the Kizildere geothermal power station in Turkey jointly uses the dual-working mass cycle technology to carry out the research of the test unit, the maximum power reaches 18.238kW, the cycle efficiency reaches 38.58%, and the performance of the combined cycle power generation system is stable.

Geothermal power generation can also be combined with solar thermal utilization. In the double working medium cycle or Karina cycle, the solar heat utilization method is introduced in the heat exchange stage of low temperature geothermal water to overcome the weakness of low temperature geothermal water and poor energy grade, and improve the cycle efficiency. At present, this technology has been carried out laboratory research in the United States, Chile and other countries.

15. Global installed geothermal power capacity

In recent years, the world's geothermal power generation has developed rapidly, and the global installed capacity of geothermal power generation has increased from 8594MW in 2000 to 13200MW in June 2016. The Asia-Pacific region and North America led the installed geothermal power capacity, with 4.8GW and 3.9GW respectively. The geothermal power market in the Americas is dominated by the United States, Mexico and Nicaragua. The major geothermal power markets in the Asia-Pacific region are Indonesia, Japan, the Philippines and New Zealand. The United States has the world's largest geothermal resources, estimated geothermal reserves of about 31,000 MW, the largest geothermal energy development scale, geothermal power generation ranks first in the world. Iceland in Northern Europe is a global model for geothermal development, with about one-third of its electricity coming from geothermal power, and geothermal accounting for 54% of its primary energy. The Philippines is a developed geothermal power generation country, geothermal power generation is second only to the United States, accounting for about 20% of the total domestic electricity generation. Indonesia has the second highest number of active volcanoes and geothermal potential in the world. Indonesia's installed geothermal power capacity ranks third in the world. At present, geothermal power generation accounts for 3% of Indonesia's total national consumption, and Indonesia's geothermal reserves are about 29,000 MW, accounting for 40% of the global total. Geothermal power development in Kenya is moving forward at a remarkable pace, with more than 600MW installed in 2016, and geothermal power generation in Kenya accounts for 49% of total electricity consumption.