Thermal storage of solar energy-principle, components, and benefits

Principle of thermal storage of solar energy

Photovoltaic power generation and thermal storage of solar energy power generation are the two main forms of solar power generation. Photovoltaic power generation is a technology that uses the photovoltaic effect of semiconductor interface to directly convert light energy into electrical energy.

Thermal storage of solar energy power generation is a technology that converts light energy into heat energy, then into mechanical energy, and finally generates electric energy by using large-scale heat collecting mirrors and traditional steam generator thermal cycles to do work.

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Home energy storage is often used with household photovoltaic, and the installed capacity of pv battery system is experiencing rapid growth. Thermal storage of solar energy power generation gathers the collected solar radiation heat energy to the heat collection device through a heat collector such as a reflector and a condenser.

The heat transfer medium in the heating device, such as heat transfer oil and molten salt, heats water to high-temperature and high-pressure steam through the heat exchange device, and then drives the turbine to drive the generator to generate electricity.

Except for different heat sources used in power stations, the back-end technology path is not significantly different from thermal power generation, and the generated current is AC, which can directly realize grid connection.

Components of thermal storage of solar energy

Thermal storage of solar energy power generation can be roughly divided into four parts:
● Heat collection system
● Heat transmission system
● Heat storage and heat exchange system
● Power generation system

Among them, the heat collection system is the core of the thermal storage of solar energy system, which is mainly composed of the concentrator, receiver, tracking mechanism and other components. Among them, the concentrator is the core component of the heat collection system.

Under the control of the central control system, it can track the sun’s position, collect and reflect the largest amount of sunlight to the receiver. The reflectivity and focus deviation of the condenser and heliostat in the concentrator can affect the power generation efficiency, which requires high technical requirements for design, production and installation.

In addition, the technologies of the thermal storage of solar energy power generation system and the thermal power generation system are consistent, and the market maturity is high. Both of them use high-quality superheated steam to drive the turbine to do work, thus converting mechanical energy into electrical energy.

Classification of thermal storage of solar energy system

Thermal storage of solar energy power generation systems can be divided into four categories according to the way of light energy gathering:

● Tower type solar thermal power generation

The tower system uses many heliostats to reflect the solar heat radiation to the collector at the top of the high tower to heat the heat transfer fluid (mainly molten salt). The high-temperature heat transfer fluid generates superheated steam through the steam generation system to drive the turbine generator unit to generate electricity.

● Slot type solar thermal power generation

The trough system arranges multiple trough parabolic concentrators in series and parallel, continuously heats the heat transfer fluid (mainly heat transfer oil) at the focal line, and then generates superheated steam to drive the generator set to generate electricity.

● Disc solar thermal power generation

The disc system uses the drive device to automatically track the sun, and uses the disc concentrator to focus the sunlight on the heat receiver at the focus, and finally generates electricity through Stirling cycle or Brayton cycle. Because of its small size, it is often used in space solar stations.

● Linear Fresnel type photothermal power generation

Linear Fresnel type solar thermal power generation system: Fresnel system uses multiple strip plane mirrors that track the sun’s movement instead of parabolic mirrors to gather solar radiation on the heat absorption tube, heat the heat transfer fluid (usually water/steam), and generate electricity through the thermal cycle.

Benefits of thermal storage of solar energy

Improve stability

The energy storage system has the ability to smooth fluctuations, cut peak and fill valley, and is an important stabilizer of new energy power generation. Energy storage technology is an effective technology to deal with the volatility and intermittency of wind and light based new energy systems.

Mature energy storage technologies can smooth the fluctuation of wind and solar power system on the power generation side, so as to improve the power quality and stability of grid connected wind power and photovoltaic systems, and improve the volatility of new energy power generation and other weaknesses.

On the grid side and the user side, energy storage technology can well solve the problem of mismatch between power supply and demand, thus reducing the security and stability problems caused by the high short-time voltage bearing of the grid or insufficient peak time power consumption.

Compared with wind power pumped storage, thermal storage of solar energy power generation battery storage and other storage and generation separation systems, solar thermal power plants combine the two together, and can produce grid friendly dispatching power like traditional thermal power plants to meet the power demand in the morning and evening peak hours, peak hours and other scenarios.

By manually setting the energy storage duration and the load power of the generator, 24-hour continuous and stable power supply can be achieved.

Reduce energy consumption

The energy storage system can also reduce the energy consumption of peak regulation and frequency regulation, and serve as a backup power to ensure power safety.

In the traditional peak load regulation and frequency regulation of thermal power plants, coal power units often increase their generating capacity to meet the peak load regulation capacity, so as to have the ability to cope with peak load, but this makes thermal power units often fail to reach full power.

At the same time, thermal power generation lags behind the frequency modulation AGC signal response of the power grid, which seriously affects the unit operation economy.

The energy storage system is flexible in charging and discharging and fast in response, which can greatly reduce the capacity of the standby thermal power generator set and quickly respond to the peak shaving signal, greatly improving the operation efficiency of the power grid.

In addition, the energy storage system can also be used as a backup power supply to deal with power emergencies and meet emergency power use, with a certain energy security value.

Thermal storage of solar energy application

In recent years, in the context of the sharp rise in the price of fossil energy and the optimization of the energy structure, the cost advantage of burning fossil energy to prepare industrial steam has gradually lost.

Solar EOR using solar heat collectors to generate steam has broad prospects. In this way, the cost of heavy oil recovery can be greatly reduced and the carbon emission in the process of recovery can be reduced.

In addition, with the large cycle of thermal storage of solar energy as the main body, the generated heat energy is stored and released when the temperature is low, which can be used for clean heating in green towns and temperature maintenance in constant temperature vegetable greenhouses in winter.

In recent years, the economy and practicability of thermal storage of solar energy have become prominent; Its application in constant temperature vegetable greenhouse heating can greatly reduce the vegetable farmers’ cost of planting vegetables in winter.

Development of thermal storage of solar energy power plants in China

The development of thermal storage of solar energy power generation in China can be divided into four stages:

1)The first attempt of China’s thermal storage of solar energy power generation took place in Ordos, Inner Mongolia, more than ten years ago.

At that time, the government planned to build a 50MW solar thermal demonstration power station in the local area and developed it into a Sino German cooperation project in 2006. However, due to factors such as technical level and power generation cost, the project was gradually shelved.

2)In 2016, China determined the first batch of 20 thermal storage of solar energy power generation demonstration projects, followed by a series of projects such as the 10MW tower type of Delingha Project in Qinghai, China, and the 10MW tower type of the first flight in Dunhuang, driving the development of thermal storage of solar energy into the fast lane.

3)With the launch of the demonstration electricity price downgrade mechanism at the end of 2018, the construction of thermal storage of solar energy projects in 2019-2020 was stagnate again.

4)Since 2021, with the gradual deepening of China’s “dual carbon” strategy, thermal storage of solar energy has comparative advantages, and the industry’s attention is gradually rising, which is expected to usher in another development.

In recent years, the cost of renewable energy power generation has been declining, some of which are lower than the cost of traditional fossil energy power generation. The report shows that: from 2010 to 2020, the average cost of photovoltaic power generation in the world dropped the most, reaching 85%; Thermal storage of solar energy power generation system is the second, accounting for 68%.

In China, technological progress contributes about 42% to the cost reduction of thermal storage of solar energy power plants, 37% to large-scale production and 21% to mass production. According to the data, under ideal conditions, the overall decrease of the total investment in the power station brought about by large-scale development can reach 18.4%~27.6%.