Is solar-thermal power generation reliable?

Speaking of solar power generation, we must be no stranger to small street lights equipped with solar panels on the roadside, large to large solar power stations, there are solar power generation figures. However, most of these solar power projects are photovoltaic power generation, in addition to solar photovoltaic, there is a way to use solar energy into electricity - solar thermal power generation.

(A) What is solar thermal power generation?

Solar thermal power generation is a technology that converts relatively concentrated solar light energy into heat energy through the transmission medium, and then converts it into electric energy. General solar thermal power generation system can be divided into four parts: heat collection system, heat transmission system, heat storage and heat exchange system, power generation system.

The heat collecting system, as the name suggests, is to gather solar energy and convert solar energy into heat energy, in short, to use the sun to heat the working medium (editor's note: the working medium is to achieve the mutual conversion of thermal energy and mechanical energy). Heat transmission system, is through the pump and other equipment to transport the working medium to the heat storage system or heat exchange system, the transmission process is a word: "fast", so as to avoid the working medium "cool" down. The heat storage and heat exchange system is equivalent to a "big battery" and a "burning furnace".

The heat storage system stores the incoming heat, and the heat exchange system "boils" the working medium (usually water) into steam, which drives the turbine to spin. The power generation system is similar to the conventional thermal power generation system, the steam drives the turbine, and then drives the generator to generate electricity.

Therefore, solar solar thermal power generation has undergone a four-step energy conversion process: light energy - heat energy (stored or generated) - mechanical energy (turbine rotation) - electrical energy.

Four major systems of solar thermal power generation

Photothermal power generation is divided into high temperature power generation and low temperature power generation according to the temperature of the heating medium. High temperature solar thermal power generation is the use of water vapor as the medium of Rankine cycle. Low temperature solar power generation is a Rankine cycle with low boiling point organic matter as the working medium

Rankine cycle

At present, the mainstream form of solar thermal power generation in the world is high temperature solar thermal power generation, which can be divided into tower type, trough type, dish type and other types according to different forms of heat collection.

The tower system uses multiple flat mirrors (called fixators) to reflect sunlight to a receiver at the top of the central tower (the luminous part in the lower left image) and convert it into heat to transmit to the working medium.

The condenser of the trough system is a trough paraboloid, which is generally used in series, and the thin tubular collector is fixed on the focus line of the condenser, and the working medium is heated in the collector tube. At present, in the international solar thermal power stations that have been put into operation or under construction, there are many trough solar thermal power generation systems.

Dish photothermal power generation is a power generation method that uses rotating parabolic condenser to gather sunlight on the collector, and the working medium in the collector is heated to drive the generator to do work and generate electricity, which is the highest power generation efficiency at present, up to 30%.

Tower, trough, dish type photothermal power generation

(2) Tower type, trough type, dish type, three kinds of technical routes big PK

The three main forms of solar thermal power generation have been mentioned above - tower, trough and dish systems. Let's find out.

1, tower type photothermal power generation system

The concentrator of the tower system is generally a heliostat group (editor's note: heliostat is an optical device that reflects the light of the sun or other celestial bodies in a fixed direction), which concentrates the sunlight to a receiver fixed on the top of the receiving tower, and the heat absorber on the receiver absorbs the high heat flux radiant energy reflected by the heliostat system.

At present, most heliostats used at home and abroad are flat concave mirrors with small radian surfaces. Different from the other two, the tower system can store heat through molten salt, with high concentrating ratio, high working temperature, short heat transfer distance, less heat loss, and high comprehensive efficiency of the system, which can achieve high precision, large capacity, continuous power generation, suitable for large-scale grid-connected power generation.

2, trough type photothermal power generation system

Trough system Because the condenser is a trough paraboloid, the sunlight will be focused on a straight line, that is, the focal line. Tubular solar collectors are installed on this focus line to absorb the focused solar radiation energy.

The key technology lies in the production and manufacturing of the condenser, as well as two aspects of control, one is the automatic tracking control, so that the trough condenser is always aligned with the sun to ensure the maximum absorption of solar energy, according to statistics, the energy obtained by tracking is 37.7% higher than that of non-tracking. The other is the temperature and pressure control of the heat transfer liquid circuit.

The temperature of the trough system after concentrating light can reach about 400°C.

3, disc photothermal power generation system

The disk system is a point focus, and the solar receiver at the focal point collects high temperature heat energy, heats the working medium, drives the generator set, or places the solar Stirling power generation device directly at the focal point. This kind of system has the characteristics of long life, high efficiency, strong flexibility, can operate independently, and is very suitable for use as a small power supply in remote areas.

The general disk solar thermal power is 10.25kW, and the diameter of the condenser is 5.10 meters.

Small disc type photothermal power generation device

By comparing the three technical routes, the tower has the greatest development potential in large-scale power generation, but the initial unit investment is too large and it is difficult to reduce the cost, and there is a lack of practical experience in the operation of large-scale power generation equipment.

Trough system structure is relatively simple, the technology is relatively mature, the commercial operation experience is rich, is still the mainstream route of the current solar thermal power generation, but its small concentrating ratio, the system operating temperature is low, the core component vacuum tube technology is not mature, the surface of the heat absorption tube selective coating performance is not stable and other problems still exist.

The disc has the highest thermal efficiency, compact structure and easy installation, which is very suitable for distributed small-scale energy systems, but the key technology of Stirling heat engine is difficult, and it is still in the experimental demonstration stage.

Comparison of three technical Routes (2013)

(3) are the use of solar energy, photovoltaic power generation and solar thermal power generation what is the difference?

The principle of photovoltaic power generation is called "photovoltaic", that is, when the sun shines on the solar cell, the battery absorbs light energy, and the abnormal charge accumulation occurs at both ends of the battery, that is, the voltage is generated, the electrode is led out and connected to the load, and the current is generated.

Photovoltaic cell power generation principle and photovoltaic power station system diagram

Therefore, in addition to the source of the sun, photovoltaic power generation and solar thermal power generation are completely different things. But the two are often compared together to see who is stronger. Let's also compare today, first look at photovoltaic power generation, the main advantages are:

(1) basically not affected by the region, theoretically as long as the solar energy can be installed;

(2) No fossil energy consumption, no pollution, zero noise;

(3) The power generation process is simple, directly from light energy to electricity, there is no intermediate link;

(4) Occupy less land, if installed on the roof, the area is basically zero;

(5) Simple structure, easy to build, low maintenance cost.

Of course, the disadvantages of photovoltaic power generation are also obvious:

(1) Cloudy days, no power output at night;

(2) Because there is no intermediate link, the cost of energy storage is high, which limits the scale of access to the grid;

(3) At present, the efficiency is still relatively low compared with thermal power, and the photovoltaic conversion efficiency is less than 20%.

Looking at solar thermal power generation, its biggest advantage is that there is an intermediate link, because there is heat as an intermediate energy source, it has three major advantages:

(1) The cost of energy storage is greatly reduced, and the maturity of thermal energy storage technology is much higher than that of electric energy storage;

(2) The resulting high schedulability of power generation, which is very similar to a thermal power station, can adjust the power generation according to the load at any time, smooth output power;

(3) Because the output can be smooth, it has the possibility of being used as a power grid rotation backup and peak filling output, which can be equivalent to "fast thermal power unit + pumped storage unit".

But the disadvantages are also obvious:

(1) Regional is hard, photothermal power generation has high working temperature requirements, and requires direct light, so it is generally built in the desert;

(2) The cost is high, the cost of solar thermal power generation is more than double the cost of conventional energy power generation, the investment cost of the power station is 4 times that of photovoltaic, the solar flow density is low, the need for a large area of optical reflectors and expensive receiving devices to directly convert solar energy into heat energy, the process of investment costs accounted for about half of the entire power station investment, This is the biggest reason for the high cost of solar thermal power generation;

(3) The technology is still immature, which can be known by the degree of commercialization.

(4) Although the solar thermal power generation is good, but it is still a long way to put into actual use

Since the birth of solar thermal power generation technology in the 1950s, it has experienced multiple stages of development, and as of the end of December 2015, the global solar thermal power station has been completed and put into operation close to 5GW.

Among them, the total installed capacity of Spain in the solar thermal power station is 2300MW, accounting for nearly half of the global total installed capacity, ranking first in the world, the total installed capacity of the United States is 1777MW, ranking second in the world, and the two together account for about 88% of the global solar thermal installed capacity.

In addition to these two major solar thermal countries, India, South Africa, the United Arab Emirates, Algeria, Morocco and other countries are also vigorously developing solar thermal technology, China is the world's eighth country to master large-scale solar thermal technology.

IRENA

Although the principle of photothermal power generation is simple, its energy conversion link is much more complex than photovoltaic power generation, involving optics, heat, electricity, materials science, thermal engineering and other disciplines of cross integration, for different technical routes, efficiency improvement obstacles and paths are also different, it can be said that the promotion and application is still a long way to go.

But we should see great prospects for its development:

1. Joint power generation with photovoltaic power stations and thermal power plants to form complementary effects. Balancing the power production and transmission between solar thermal and photovoltaic in the same power generation area can eliminate the intermittency problem of photovoltaic, and the combination of these two technologies can effectively reduce the power generation cost of the overall system.

2, the establishment of distributed power generation system, to solve the remote mountain power supply problem, the dish system is the most suitable, but because its power generation technology is not mature, the current use of trough power generation system.

Some professionals pointed out that domestic enterprises have entered the solar thermal power generation market, and the overall industrial chain has initially formed: five major power companies have followed up solar thermal power generation. Domestic enterprises are growing rapidly in the production of upstream and downstream components of the solar thermal industry chain, such as the capacity and capacity of large tower power stations with helioscopes, megawatt solar tower thermal power stations have been put into trial operation; In terms of trough solar thermal power generation, 300°C vacuum tubes have been developed, and they are now moving towards 450°C vacuum tubes.