Electricity for heating and storage: geothermal energy development

According to the statistics of the 2020 World Geothermal Congress, at the end of 2019, the total installed capacity of direct utilization of geothermal resources in 88 countries worldwide was 107727MWt (Figure 3), and the energy use was about 1020887TJ/ year (283580GWh/ year). The variation trend and distribution of installed capacity of various geothermal direct utilization modes are shown in Table 1. It can be seen that the installed capacity of various geothermal resources direct utilization modes has increased significantly, especially the rapid growth of ground source heat pump.

With more and more attention paid to ground source heat pumps, the application area and scale of geothermal resources continue to expand.

The comprehensive geothermal cascade utilization undoubtedly improves the utilization rate and economic benefits of geothermal resources. Many countries have begun to adopt this technology one after another. Kenya has developed rapidly in geothermal power generation in recent years, and its direct utilization of geothermal energy is also very distinctive. Kenya's Eburru project utilizes geothermal resources produced in two shallow Wells for drying agricultural products, heating greenhouse, and poultry incubation, etc. In addition, there are also geothermal cascade utilization projects in Iceland, Austria, and Germany.

Since the first World Geothermal Congress in 1995, the heat directly utilized by geothermal energy in China has always ranked first in the world. At the end of 2014, the direct use of geothermal energy in China took place gratifying changes, and the proportion of geothermal heating exceeded that of hot spring bathing for the first time. By the end of 2019, China's geothermal direct utilization capacity was 40,610MW, accounting for 37.7% of the world's total installed capacity, and the annual utilized energy was 443,492TJ/year, accounting for 43.4% of the world's total utilized energy. Geothermal heating is the most important geothermal utilization mode in China besides ground source heat pump, accounting for 55.6% of the world's total annual heat utilization.

In 1990, the country's geothermal heating area was only 1.9 million square meters, 11 million square meters in 2000, and 478 million square meters in 2019. In 2019, the area of hydrothermal geothermal heating in Hebei Province reached 160 million m2, and Xiongxian County was also built into a "smoke-free city" of geothermal heating. Under the guidance of the "double carbon" goal, it can be predicted that the speed of geothermal resource development and utilization will continue to accelerate.

Geothermal energy storage utilization

Geothermal energy storage is a kind of energy storage system that uses underground aquifers as the medium to store heat energy. It injects and draws groundwater from aquifers through underground Wells, enabling heat energy storage and recovery (Figure 4). Geothermal energy storage can make up for the imbalance in the time/space distribution of energy supply and demand, can comprehensively utilize a variety of renewable energy forms, reduce dependence on fossil fuels, provide a good solution for energy conservation, emission reduction and environmental protection, and is also a powerful means to help China achieve the goal of "dual carbon".

According to the depth of the aquifer, geothermal energy storage systems can be divided into the following two categories:

1. Shallow geothermal energy storage, aquifer depth is shallow at 500m, storage hot water temperature is generally lower than 50℃;

2. Deep geothermal energy storage, aquifer depth is usually 500m deep, storage hot water temperature is generally 50~150℃.

Because of the low temperature of shallow geothermal energy storage, its main use is the heating and cooling of buildings. The international application of shallow geothermal energy storage systems began in the middle of the 20th century. In China, the practice of using shallow underground aquifers for heat storage was developed earlier. In the 1960s, Shanghai carried out underground aquifer heat storage technologies of "winter irrigation for summer use" and "summer irrigation for winter use".

Up to now, a total of six shallow geothermal energy storage systems have been built and put into use in China. In recent years, with the development of emerging industries, the utilization of shallow geothermal energy storage has become more diversified. There are practical examples of using shallow geothermal energy storage in both greenhouse agriculture and large data centers.

Deep geothermal energy storage can be stored at a higher temperature, some even exceeding 100 ° C, and can mainly be used for power generation and heating. The research and utilization of deep geothermal energy storage began in the 1980s. In recent years, with the increasing demand for energy, deep geothermal energy storage technology has received renewed attention, and more and more research and engineering practice on deep geothermal energy storage technology have been carried out.

In 2018, the EU funded the underground heat storage project HEATSTORE, with a total investment of 50 million euros, 9 countries participated (Germany, France, the Netherlands, Switzerland, Belgium, Denmark, Iceland, etc.), the project lasted for 5 years (2018-2022), and funded a total of 6 underground heat storage demonstration projects, including 3 deep geothermal energy storage projects. In addition, the National Science Foundation (NSF) funded the heat storage project Geothermal Battery, a total investment of $10 million, led by the University of Utah, Idaho National Laboratory and a number of enterprises to participate.