Thermal storage, auxiliary energy and turbogenerator subsystems

Thermal storage subsystems

In addition to the concentrating and heat collecting subsystems introduced in the previous article, the thermal storage subsystem is an essential part of the solar thermal power generation system. Because the solar thermal power generation system must rely on the stored solar energy to maintain normal operation during the morning, evening and daytime cloud cover. As for nighttime and rainy days, conventional fuels are generally considered as auxiliary energy. Otherwise, the initial investment of the entire solar thermal power generation system will be significantly increased due to the large demand for heat storage capacity.

The thermal storage subsystem of the solar thermal power generation system can be divided into the following four types.

① Low temperature heat storage. The low-temperature thermal storage system is a small-scale low-temperature solar thermal power generation system that collects solar heat by a flat-plate collector and uses a low-boiling-point working fluid as a power working fluid.

② Medium temperature heat storage. The medium temperature thermal storage refers to the thermal storage of 100~500℃, but usually refers to the thermal storage of about 300℃. This heat storage device is often used in low-power solar thermal power generation systems. Materials suitable for medium-temperature heat storage include high-pressure hot water, organic fluids (At around 300 °C, heat transfer oil, diphenyloxy-diphenyl fluid, stable and saturated petroleum fluid and fluid based on phenolic phenylmethane can be used, etc.) and heat-carrying fluids (such as caustic soda, etc.).

③ High temperature heat storage. High temperature heat storage refers to high temperature heat storage above 500℃, and its heat storage materials mainly include sodium and molten salt.

④ Extremely high temperature heat storage. Extremely high temperature heat storage refers to heat storage at about 1000℃, and aluminum or zirconium oxide refractory balls are often used as heat storage materials.

A typical solar thermal power plant operates as shown in Figure 1. At 8:00 am, the solar collector started to work, the steam turbine was started at 9:00 am, and the steam turbine entered a stable operation state at 10:00 am. Before 10 o’clock, the heat collector has been storing heat to the heat accumulator. After 10:00, the solar energy absorbed by the collector is directly used to supply the steam turbine generator set to generate electricity. From 3:00 p.m., the solar irradiance began to decrease, and the heat storage system began to release heat accordingly to ensure the normal operation of the steam turbine until the shutdown at 6:30 p.m. Therefore, the thermal storage system provides a buffer link between the collector and the turbine generator set to ensure the stable operation of the set.

Figure 1 - Example of how a typical solar thermal power plant operates
Figure 1 – Example of how a typical solar thermal power plant operates

A heat accumulator is a heat storage container that uses vacuum or thermal insulation materials for good thermal insulation. The heat storage material is stored in the heat accumulator, and the heat storage material is stored and extracted through a specially designed heat exchanger. At present, there are three heat storage methods that can be used: sensible heat storage, latent heat storage and chemical energy storage. Tycorun Battery provides you with chemical energy storage lithium batteries that can be used in conjunction with solar power generation systems.

(1) Sensible heat storage

Sensible heat storage media include water, oil, rock, sand, gravel, etc., as well as artificial alumina balls. These materials are inexpensive and readily available, but have a low heat capacity. Therefore, to store the same amount of heat, the required volume of the heat accumulator is very large. When using heat storage above 100℃, a special pressure vessel should be used for the heat accumulator.

(2) Latent heat storage

Using the latent heat of materials to store heat, the heat storage per unit volume is large, and the heat storage device is expected to be miniaturized.

The latent heat storage medium must have the following characteristics:

①It has the performance of thousands of reversible heat storage and release cycles (solid phase⇌liquid phase), and its phase transition temperature does not appear overheating or undercooling;

② The price is cheap;

③Do not corrode the container.

The study of latent heat storage using the heat of fusion of the medium has been carried out for many years. The main problems are that some latent heat storage media are decomposed during the melting process, the melting point is unstable, it is difficult to uniformly produce phase transitions during heat exchange, and there are concerns about toxicity and fire.

(3) Chemical energy storage

The basic concept of chemical energy storage is that after a certain substance A is heated by solar energy, it is converted into substance B+C, and when B+C is converted into A, heat is released. There are many such endothermic or exothermic chemical reactions in nature. The characteristics of chemical reaction energy storage are large heat storage, small volume and light weight per unit of energy storage, and chemical reaction products can be separated and stored, and exothermic reaction occurs when heat is needed, so the cycle time is long.

For chemical energy storage materials, it must have the following characteristics:

①The reaction of heat storage and heat release is reversible, and there is no side reaction;

②Fast response;

③ The reaction products are easy to separate and can be stored stably;

①The price is cheap;

⑤ The reactants and products are non-toxic, non-corrosive and non-flammable;

⑥ The reaction heat is large.

Auxiliary energy subsystem

In addition to the thermal storage subsystem, the solar thermal power generation system also needs to be equipped with an auxiliary energy subsystem to maintain the continuous operation of the power station. The auxiliary energy subsystem in the solar thermal power generation system is to add a conventional fuel boiler to the system for start-up in rainy days and at night. At this time, the continuous operation of the power station is maintained by conventional energy. Which conventional fuel is selected as auxiliary energy in the design depends on the local energy resources of the solar thermal power station, which can be natural gas, oil or coal. With the development of technology, the latest design concept of modern solar thermal power plants is to build solar and natural gas dual energy power plants.

Turbogenerator subsystem

The power generation devices used in the solar thermal power generation system can be selected as follows:

①Modern steam turbine;

②Gas turbine;

③ Low boiling point working fluid turbine;

④ Stirling engine.

The selection of the power generation device is mainly determined by the working medium parameters that the solar thermal collector system may provide. Modern steam and gas turbines have high operating parameters and are suitable for large tower or trough solar thermal power systems. The single-unit capacity of the Stirling engine is small, usually below tens of kilowatts, and is suitable for use in the disk parabolic mirror power generation system. The low boiling point working fluid steam turbine is suitable for the solar pond solar thermal power generation system.