Solar thermal utilization is generally divided into three areas based on
operating temperature: below 80°C is the low-temperature area, 80°C to 250°C is
the medium-temperature area, and 250°C to 800°C is the high-temperature area.
The heat utilization technology in the low-temperature field is relatively
mature at home and abroad. It mainly provides domestic water. Its products are
mainly flat-plate or vacuum tube solar collectors. The heat utilization
technology in the medium-temperature field can be used for regional building
heating, air conditioning and refrigeration, seawater desalination and For some
industrial heat applications, it can be said that solar thermal utilization is
currently the technology with the greatest development potential and the most
widely used applications. It is also one of the effective ways to effectively
solve the current energy shortage, environmental pollution, and global
warming.
In the high-temperature field, it is mainly solar thermal power generation
technology. Solar thermal power generation technology collects sunlight to a
solar collection device through reflectors, uses solar energy to heat the heat
transfer medium in the collection device, and then generates steam through heat
exchange to drive a steam turbine to drive a generator to generate electricity.
There are three types of solar thermal power generation systems: trough type,
tower type and dish type. Among them, the trough type has been commercially used
on a large scale abroad and the technology is relatively mature. The structure
of the trough system solar collector tube is a glass-metal sealed solar
collector tube. It uses solar energy to replace traditional fossil fuels and
reduce pollutant emissions. It is a clean energy power generation technology.
Table 1 shows the medium-temperature application fields of solar energy, and
Figure 2 shows the high-temperature thermal power generation technology.
1 Current research status in the field of medium-temperature solar
energy
In order to meet the needs of medium-temperature solar energy applications
and make greater contributions to the low-carbon economy, Linuo and Tsinghua
University jointly developed a new all-glass product for medium-temperature
applications—medium-temperature solar vacuum collector tubes. This product is
made of titanium that has reached the international advanced level. As a film
material, the current application temperature of gold coating is 80℃~160℃, and
special vacuum maintenance technology is used to solve the vacuum maintenance
problem of all-glass vacuum collector tubes working at high temperatures for a
long time. This product has the characteristics of high thermal efficiency, high
temperature resistance, anti-attenuation, long life, and high cost performance.
It marks the transition of solar energy from the era of low-temperature hot
water application to the era of thermal energy application, laying the
foundation for the development of medium-temperature heat utilization technology
at home and abroad.
In order to increase the operating temperature of solar energy, expand the
scope of solar energy applications, improve the thermal performance of solar
collectors, and extend the service life of solar collectors, Linuo has developed
a new product - medium-temperature solar collectors. This solar collector uses
medium-temperature solar vacuum collector tubes, inorganic heat pipes and
non-tracking concentrator technology. It can work in an environment of 150°C. It
has the characteristics of high heat collection efficiency, long service life
and low cost. Its technical level has reached the international leading level. ,
which has laid the foundation for the engineering and industrial scale of solar
energy in air conditioning and refrigeration, seawater desalination, industrial
heating, agricultural drying and other fields, and has very broad development
prospects.
Linuo has made great progress and breakthroughs in medium-temperature
applications in industry. The world's first CPC medium-temperature solar
industrial thermal system developed by Linuo can achieve more than 10% energy
replacement of the original 10t boiler. At the same time, Linuo achieved
breakthrough development in the low-carbon pavilion, China's first civil
demonstration project of solar medium-temperature thermal utilization, built by
Linuo. Linuo has also actively carried out the promotion of medium-temperature
applications of solar energy based on the original demonstration projects,
mainly targeting the fields of solar air conditioning and solar boiler systems,
among which solar boiler systems have been widely promoted.
1.1 Solar air conditioner
The system designed in the solar air conditioning system that combines
medium-temperature CPC solar collectors with single-effect lithium bromide
absorption refrigerators under construction can realize automatic operation of
the system in different seasons and different working conditions by switching
valves and water pumps. switch. During the year-round operation of the system,
it can be divided into summer, winter and transitional seasons according to the
building's different needs for cooling, heating and hot water: cooling is mainly
used in summer, heating is mainly used in winter, while the building's demand
for domestic hot water is Throughout the four seasons of the year. According to
the quality of the sun's rays, the system's working modes can be divided into
three types: full solar energy, solar energy + auxiliary electric heating, and
heat pump.
Solar radiation energy increases in summer, allowing air conditioning
systems driven by solar energy to generate more cooling capacity. The cooling
capacity of solar air conditioners increases with the increase of solar
radiation energy, which exactly matches people's urgent demand for air
conditioners in summer. This system can be used as heating in winter, reducing
the use of coal and environmental pollution. The system can provide hot water
for daily use such as showering. Figure 2 is the schematic diagram of the solar
air conditioning system, and Figure 3 is the installation effect diagram.
1.2 Solar boiler
Our country is the country that produces and uses the most boilers in the
world. The number of boilers is about 500,000, of which about 80% are coal-fired
boilers. Coal consumption accounts for 1/3 of the country's coal production. The
combustion efficiency is very low, only 45%~ 70%, more than half of which are
small boilers with lower efficiency. How to reduce the use of industrial fossil
fuels, which consume the most energy, has become an effective way to solve the
current situation. The medium-temperature solar collector developed by Linuo
fills this gap and makes solar energy applications possible in the industrial
field. Figures 4 and 5 both show solar boiler solutions.
This system is mainly used in low-pressure and low-temperature steam places
with discontinuous gas consumption and small gas consumption. It adopts two
closed pressure-bearing systems. The solar heat collection system is an
independent closed cycle system. When the working medium flows through the heat
collection system, it absorbs the heat of the solar energy to increase its
temperature, uses natural convection or uses a circulation pump to force
circulation into the steam generator, and releases the heat to the internal
water through the internal heat exchanger; temperature After being reduced, it
is recycled to the solar hot water system for secondary heating, and the heat is
transported back and forth to the steam generating package. The temperature of
the water inside continues to rise after receiving heat. When the temperature
reaches saturation, it vaporizes in an open manner. As the heat enters, the
amount of vaporization becomes larger and larger, achieving the purpose of
supplying steam.
The production cost of the system is low, the system installation is
simple, and there is no need to consider the system pressure problem. Due to the
high efficiency of direct heating, it can operate normally all year round and
has good thermal insulation effect.
2 Core technology of solar thermal power generation
Trough solar thermal power generation technology is to arrange multiple
trough-type parabolic concentrating collectors in series and parallel to focus
sunlight onto a high-temperature power generation tube located at the focus. The
heat transfer medium flowing through the high-temperature power generation tube
obtains energy. Directly or indirectly generate high-temperature steam to drive
a steam turbine to drive a generator to generate electricity. This technology
has the advantages of large scale, long life, and low cost, and is very suitable
for commercial grid-connected power generation. It is currently the most mature
large-scale commercial solar thermal power generation technology.
The core component of the trough solar thermal power generation system is
the high-temperature vacuum solar collector tube, which is composed of stainless
steel tubes covered with selective absorption coatings, cover glass tubes,
bellows and other components. Figure 6 shows the structure of the
high-temperature vacuum solar collector tube. A vacuum is drawn between the
cover glass tube and the stainless steel tube to reduce heat loss of the
collector tube and prevent oxidation of the selective absorption film outside
the stainless steel tube. The length of a single collector tube is about 4000mm,
the outer diameter of the stainless steel heat-absorbing tube is 70mm, and the
outer diameter of the cover glass tube is 125mm.
1—Metal heat absorber tube; 2—High temperature solar selective absorption
coating; 3—Vacuum interlayer; 4—Cover glass tube; 5—AR coating; 6—Getter;
7—Non-evaporable getter; 8—glass metal sealing ring; 9—corrugated pipe;
10—exhaust tail nozzle
Figure 6 Structural diagram of high temperature vacuum solar collector
tube
The key technologies of high-temperature vacuum solar collector tubes
mainly include: high-temperature solar selective absorption coating technology,
glass and metal sealing technology and vacuum maintenance technology.
2.1 High temperature film technology
The temperature of the working fluid in the trough solar power generation
system is as high as 390°C~400°C. Ordinary solar selective absorption coatings
cannot meet the high temperature usage requirements. Therefore, Linuo and
Tsinghua University jointly developed a high-temperature vacuum solar collector
tube suitable for use in high-temperature vacuum solar collectors. Solar
selective absorption coating, excellent coating performance: absorption ratio ≥
0.95, emission ratio ≤ 0.14 at 400°C.
2.2 Glass and metal sealing technology
High-temperature vacuum solar collector tube, the heat-absorbing tube is
made of stainless steel tube, and the outer tube is made of glass tube. Due to
this special structure, the sealing technology of glass and metal has always
been a key technology in its research and development. The sealing technology
directly affects the life of the collector tube, thermal performance, and even
the operational safety and operation of the entire solar thermal power plant.
cost.
There are two types of sealing between glass and metal, one is non-matching
sealing and the other is matching sealing. Matching sealing means that the
linear expansion coefficients of glass and metal are similar within a certain
temperature range (difference <10%); non-matching sealing means that the
linear expansion coefficients of glass and metal have a large difference within
a certain temperature range. (Difference>10%). The non-matching sealing
method requires special processing of the metal part, that is, the processing of
the metal cutting edge. The thinnest can be 0.03mm~0.04mm, which is difficult to
process. The glass and the metal are easy to deform during the sealing process,
and the finished product is sealed. It has low strength and is easily damaged
especially in high temperature environments.
Linuo has developed a new type of borosilicate glass, whose linear average
expansion coefficient is controlled within the range of (5.2±0.1)×10-6K-1, and
can be matched and sealed with Kovar alloy. The glass has strong weather
resistance, the critical value of mechanical impact strength is ≥8J, the solar
transmittance is ≥0.92 (AM1.5), the glass ring cutting grade is not lower than
grade B, it has good wettability with Kovar alloy, and the sealing point can
withstand It can withstand hot and cold shock temperature difference of 240℃
without damage, and the tensile strength is ≥2MPa.
2.3 Vacuum maintenance technology
Compared with ordinary solar collectors, high-temperature vacuum solar
collectors have undergone earth-shaking changes in terms of product components,
production processes and product working conditions. Linuo has more than ten
years of experience in R&D and production of ordinary collector tubes, and
began to do technical reserves and pre-research work on this difficult problem
many years ago, which gives Linuo a great advantage in the research and
development of high-temperature vacuum solar collector tubes.
Linuo has carried out in-depth cooperation with Tsinghua University and
other universities and scientific research institutions, devoted itself to
research and development, and tackled the technical difficulties of
high-temperature vacuum solar collector tubes. Currently, Linuo has produced
high-temperature vacuum solar collector tubes with excellent performance and
achieved the product's Mass production.
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