JPS6014276B2 - solar energy absorber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solar energy absorber that effectively absorbs heat from solar energy and dissipates little of the stored heat.

In general, for absorbers that absorb solar energy and convert it into heat, heat loss due to body flow and thermal conduction in the air layer in contact with the absorber is combined with heat loss due to radiation from the absorber surface into the atmosphere. Depending on the amount of total heat loss, the maximum temperature that can be reached,
That is, the maximum temperature that the absorber can reach while maintaining a balance between heat absorption and heat dissipation is determined.

In an absorber made of various substances with the same surface area, the amount of heat loss due to convection and heat conduction in the air layer does not depend much on the material or type of the absorber, but simply on the temperature of the absorber and the surrounding environment. It is only proportional to the difference in temperature.

On the other hand, the amount of heat loss due to radiation into the atmosphere is proportional to the difference between the fourth power of the temperature of the absorber and the fourth power of the ambient temperature, and largely depends on the material of which the absorber is made. Therefore, the difference in the amount of heat loss due to radiation not only becomes noticeable depending on the type of absorber, but also becomes even larger at high temperatures. Therefore, the limit temperature that an absorber can reach varies depending on the shape and material of the absorber, and is greatly influenced by its thermal radiation activity.

In other words, an absorber made of a material with low thermal radiation, that is, with low thermal radiation loss, absorbs solar energy well and reaches high temperatures in a short time. By the way, the thermal radiation energy of a substance is always smaller than the thermal energy due to radiation from an ideal black body at the same temperature, that is, an object that completely absorbs the emitted electromagnetic waves regardless of the wavelength. The ratio is called the thermal emissivity, and the magnitude of thermal radiation is expressed by the magnitude of this thermal emissivity. Furthermore, most of the electromagnetic waves in sunlight are in the visible and near-infrared regions, while electromagnetic waves thermally radiated from objects are distributed in the infrared region.

Therefore, a solar energy absorber is required that satisfactorily absorbs electromagnetic waves in the visible and near-infrared regions, does not emit electromagnetic waves in the infrared region, and easily reaches high temperatures.

BACKGROUND ART Conventionally, as solar energy absorbers applied to solar water heaters and the like, materials made of metal materials whose surfaces are simply painted black with pigments, or plastic molded products colored black have been used. However, these absorbers did not have sufficient durability such as peeling due to deterioration of the painted surface, film resistance, and heat resistance.

In addition, solar energy absorbers blackened by this method are similar to the so-called ideal black body in their thermal radiation, so they absorb the visible and near-infrared regions that make up the majority of sunlight. It absorbs electromagnetic waves well, but at the same time it has high thermal radiation in the infrared region, so there is a large amount of heat loss due to radiation as well as heat conduction to the air and other parts. Therefore, when a solar energy absorber as described above is used as a water heater, it is difficult to obtain high-temperature water efficiently and in large quantities.

On the other hand, materials that eliminate such drawbacks have been commonly referred to as selective absorbers.

There are several known solar energy absorbers that can effectively absorb heat from wavelength-dependent electromagnetic waves, which account for most of the solar energy, and have extremely low heat loss due to radiation from the surface of the absorber. For example, the surface of metal steel may be oxidized to black using a concentrated solution containing sodium chlorite and sodium hydroxide, or a nickel-subsalt alloy such as ammonium thiocyanate may be coated on a steel or aluminum substrate. There are some that are precipitated by electrodeposition and simultaneously converted into sulfides in solutions containing them.

However, the adjustment of the former selective absorber requires 130 pm.
This requires high-temperature treatment, and since unstable chlorite is used, side reactions are likely to occur, and it is difficult to form a uniform surface treatment film on the substrate, resulting in unstable electromagnetic wave absorption and thermal radiation. Furthermore, there were other drawbacks such as the generation of chlorine gas during waste treatment of the treatment liquid. In addition, in the case of the latter selective absorber, a film is generated by fin attachment, so in order to stably adhere to Motomura itself, it is necessary to perform the pretreatment process of the base material extremely strictly. The quality of the film affects the energy absorption and thermal radiation of the film, making it difficult to obtain uniform performance. The present invention aims to eliminate the drawbacks of conventional solar energy absorbers as described above, and to provide a selective solar energy absorber that can effectively absorb heat from solar energy and dissipate little of the stored heat. It has been done. The present inventors saw such a solar energy absorber, and as a result of intensive research on the base material and surface treatment method for producing it, the inventors decided to treat the surface of the base material made of copper or steel alloy with a specific treatment liquid. As a result, a black to dark brown colored film is formed with good adhesion, thereby effectively absorbing solar radiant energy and minimizing the dissipation of stored heat.A selective solar energy absorber is formed. I discovered that.

The specific treatment liquid is an alkaline aqueous solution containing persulfuric acid or a salt thereof, and the solution is subjected to activation means.

The material of the base material of the absorber of the present invention is metallic steel or a steel alloy, and examples of the copper alloy include any copper alloy, such as copper and zinc, tin, nickel, silicon, chromium, iron, beryllium, titanium, antimony, It is an alloy with phosphorus, cadmium, zirconium, cobalt, etc.

Preferably, the copper content in the copper alloy is greater than or equal to 5% by weight. In the present invention, the treatment liquid used to treat the surface of the substrate is first an alkaline aqueous solution containing persulfuric acid or a salt thereof.

The alkali compound used for preparing this treatment liquid is an alkali metal hydroxide or carbonate, and for example, sodium hydroxide, potassium hydroxide, and sodium carbonate are preferably used. Examples of persulfates include alkali metal persulfates and ammonium persulfates, and among these, sodium persulfate and potassium persulfate are particularly preferably used. One kind of alkali compound is added to an aqueous solution containing at least one of the above-mentioned alkaline compounds.If persulfuric acid is used, it becomes an alkali metal salt in the alkaline aqueous solution.If the processing solution is not alkaline, persulfate However, since the copper does not act actively on the surface of the steel alloy base material and persulfates generally decompose in acidic solutions, the treatment solution should be alkaline, preferably strong. Alkaline is better.

Therefore, it is preferable that the alkaline aqueous solution contains 10 to 330 g of the above-mentioned alkaline compound per 1 g of water.
Particularly preferably, it contains 30 to 100 ta. If the amount of persulfuric acid or its salt contained in the treatment liquid is too small, the treatment capacity will be reduced, the required treatment time will be longer, and the required treatment liquid temperature will be higher. Alternatively, if the amount of the salt is too large, the reaction rate becomes too high, and as a result, the thickness of the formed film becomes thinner, and treatment unevenness and treatment time become too short, making it difficult to control.

On the other hand, the temperature of the treatment solution is low, close to room temperature, and in strong alkalinity, hydrogen peroxide, which is generated from persulfate and is a source of active oxygen, is stable, so the concentration of persulfate may be relatively low. The solubility is also about 20 μm/g.

Hydrogen peroxide is unstable when the treatment liquid is boiling at 80°C and is relatively weakly alkaline, so the concentration of persulfate must be relatively high; however, if the concentration is too high, as mentioned above, Care must be taken, as the reaction becomes too fast and the maximum thickness of the oxide film cannot be obtained. Thus, in an alkaline aqueous solution containing 10 to 330 tons of the above-mentioned alkaline compound per 1 hour of water, 1 to 50 tons of persulfate is added per 1 hour of water.
It is preferable that the persulfate is dissolved at 5 to 100 degrees. Furthermore, the present inventors have discovered that the treatment solution can be activated by adding a small amount of an oxidizing agent such as potassium chromate to the treatment solution, or by irradiating the treatment solution with ultraviolet rays or ultrasonic waves during the surface treatment process. It was discovered that the film formation rate increased and the treatment temperature was shifted to a lower temperature side.

The amount of additional oxidizing agent of about 5% of the weight of the persulfate is sufficient.

According to the activity of the treatment solution, detailed elucidation cannot be made, but A2S203 Toka 20 → 2AHSQ Toka 202 (A
In the reaction shown for (alkali metal), reaction intermediates are produced, and the presence of the reaction intermediates increases the oxidizing power,
It is thought that this control can be easily achieved.

The temperature of the treatment liquid is suitably from room temperature to 100°C, particularly preferably from 40°C to 80°C.

Since the treatment liquid itself is an aqueous solution, at temperatures exceeding 10,000° C., a large amount of water evaporates, resulting in poor workability, while at temperatures below 40° C., the required time becomes longer. When the temperature of the treatment liquid is between 40°C and 80qo, a short treatment time of about 1 minute to about 20 minutes is sufficient, and even if the treatment time is longer than that, the thickness of the film produced will be almost unchanged. It has no effect. When the processing solution is activated by adding a small amount of an oxidizing agent such as potassium chromate or by irradiating it with ultraviolet or ultrasonic waves, the temperature of the processing solution can be shifted to a lower temperature side, reducing the energy required for processing and making handling easier. It becomes easier. In order to treat the surface of a copper or steel alloy base material using the above treatment liquid, the treatment liquid at an appropriate temperature may be applied to the surface of the base material by means such as dipping, dipping, or sprinkling.

As a result, a black to dark brown oxidized steel film is formed on the surface of Motomura in a relatively short period of time, and after that, the treatment liquid adhering to the surface is removed by washing with water, and then it is dried. The coating consists mostly of cupric oxide with a small amount of cuprous acid. The thickness of the oxidized steel coating thus formed on the substrate surface is about 0.2 to 0.6 microns, and solar energy absorbers with coatings in this range have the desired excellent properties. It was found that this material exhibits various properties.

In other words, the surface of the copper or copper alloy substrate is coated with the above-mentioned components, concentration ranges, and activated treatment solution, particularly at a temperature of about 40°C.
It has been found that if the contact is carried out for a relatively short time at 0 to 80 qC, an oxidized steel film with a thickness of about 0.2 to 0.6 microns is formed on the Motomura surface, resulting in a favorable solar energy absorber. Served.

If the thickness of the coating is less than 0.2 microns, the absorption coefficient of solar energy is low, and
When the diameter is larger than microns, the amount of absorbed energy emitted, for example, the emissivity increases. The thickness of the oxidized steel coating is particularly preferably at or near 0.4 microns. The thickness of the film can be measured optically, so if you want to obtain a film with a specific desired thickness, preliminary tests will show that the concentration of the processing solution, its temperature, and It is also possible to predetermine an appropriate processing time. According to experiments conducted by the present inventors, the solar energy absorber obtained as a result of the surface treatment described above has a high absorption rate of electromagnetic waves in the visible and near-infrared regions, and has good heat absorption of solar energy. In addition, the emissivity in the infrared region is low, so there is little heat loss due to radiation.

Thermal energy can be accumulated in a short period of time to obtain a high temperature, and the high temperature can be maintained for a long time, allowing the absorbed solar energy to be used to the maximum extent possible. It has been confirmed that this material can be suitably used as a solar energy absorber.

Surface treatment is performed only on the necessary surface of the base material,
Unnecessary surfaces can be appropriately covered in advance so that they do not come into contact with the processing liquid. The surface-treated solar energy absorber is suitable for use as a solar energy absorber because the surface treatment layer does not peel off and has excellent durability. Next, the present invention will be explained in more detail by giving Examples and Comparative Examples of the present invention.

Example 1 1 part water, 50 parts sodium hydroxide, 20 parts potassium persulfate
A treatment solution was prepared by dissolving the solution.

This treatment solution was kept at 40'0, and 10 pure steel plates (purity 99.9%) with a width of 5 skins, a length of 6 sections, and a thickness of 0.3 degrees were placed in the solution.
It was immersed for 5 minutes while being irradiated with a 0W mercury lamp, thoroughly washed with water, and dried to obtain a solar energy absorber. The thickness of the oxidized steel coating formed was approximately 0.25 mm. Example 2
The same treatment solution as in Example 1 was maintained at 40q0, and pure steel plates (purity 99.9%) of the same size as in Example 1 were heated to 2 HZ.
After soaking for 5 minutes while irradiating with ultrasonic waves, the solar energy absorber was thoroughly washed with water and dried to obtain a solar energy absorber.

Example 3 1 part water, 50 parts sodium hydroxide, 20 parts potassium persulfate
In the evening, potassium chromate was dissolved to prepare a treatment liquid.

This treatment solution was maintained at 40 qo, and a pure steel plate (purity 99.9%) of the same size as in Example 1 was immersed in 3/7 minutes of inkstone sand, thoroughly washed with water, and dried to form a solar energy absorber. did.

Comparative Example 1 A treatment liquid was prepared by dissolving 1,002 parts of sodium hydroxide and 50 parts of sodium chlorite in 1 part of water.

This treatment solution was maintained at 130 qo, and a pure steel plate (purity 99.9%) of the same size as in Example 1 was immersed in it for 15 minutes, thoroughly washed with water, and dried to obtain a solar energy absorber. Comparative Example 2 The same treatment solution as in Comparative Example 1 was maintained at 90q0, and a pure steel plate (purity 99.9%) of the same size as in Example 1 was immersed in it for 45 minutes, thoroughly washed with water, dried, and left in the sun. It was used as an energy absorber.

Comparative Example 3 The outer surface of a pure copper plate (purity 99.9%) having the same size as in Example 1 was painted with standard black paint (trade name "Nextel Black" manufactured by 3M ■) to prepare a solar energy absorber.

Next, the results of measuring various performances of the absorbers of the above examples and comparative examples will be shown.

The absorption rate in the visible and near-infrared regions of each solar energy absorber described above was determined by measuring the reflectance using an integrating sphere photometer using magnesium oxide as a standard.

Table 1 The absorption rates of the solar energy absorbers of the present invention are comparable or superior to selective absorbers treated with chlorite solutions known from the field and comparable to absorbers painted black.

Next, each solar energy absorber mentioned above was heated to 50℃ and 100℃.
Table 2 shows the results of measuring the thermal emissivity of the sample with , 150 qo. To measure the emissivity of heat, use an emissometer (emissometer) and install a temperature detector between the standard heat source on the high temperature side and the standard heat source on the low temperature side. The surfaces of the standard heat source are treated with black paint so that absorption rate = emissivity = 1, a sample at a specified temperature is attached to the surface of the high temperature heat source, and these are placed in a high vacuum where there is almost no heat conduction. Two hours after installation, measure the temperatures of the high-temperature side standard heat source, temperature detector, and low-temperature side standard heat source, that is, T, , T2, and T3, and calculate the emissivity of the sample using the following formula. Ta.

T24-T24 Go: TkoF Table 2 As is clear from Table 2, the value of the heat emissivity of the solar energy absorber of the present invention in the range of 50qo to 15qo is significantly higher than that of other absorbers. 4. Therefore, the solar energy absorber of the present invention can maintain high temperature for a long time.

Next, we measured the spectral reflectance in the infrared region of some of the solar energy absorbers mentioned above in the wavelength range of 2.5 to 15〆 using an infrared spectrophotometer equipped with an attachment for measuring the reflection spectrum. Measurement was made at an angle of 30o.

FIG. 1 is a graph of the above measurement results. The numerical numbers attached to the curves in the figure are the numbers of the examples, and the numerical numbers enclosed in parentheses are the numbers of the comparative examples.

According to Kirchhoff's law, for an opaque material, when absorption rate Q, emissivity is equal to reflectance y at a certain wavelength, Q is
There is a relationship: =go=1-y.

In other words, a substance with high reflectance has low emissivity. Further, electromagnetic waves thermally radiated from the surface of a material with a wavelength of around 10 mm are generally distributed in the infrared region with a wavelength of 5 μm to 8 μm. 1st
As is clear from the figure, all of the solar energy absorbers of the present invention exhibit a reflectance of approximately 0.9 or more at a wavelength of around 6 m, and therefore there is almost no heat radiation from the absorber at a wavelength of around 10,000 m. It shows that it is extremely small.

Therefore, it is clear that the solar energy absorber of the present invention can absorb solar energy well and emit less heat, so that it has better heat absorption efficiency than conventional absorbers. As described above, since the solar energy absorber of the present invention has the above-mentioned structure, the adhesion between the surface treatment layer and the base material is extremely good, and the manufacturing method is easy and highly safe, and is superior to the conventional method. Heat resistant compared to plastic ones
It has excellent weather resistance and durability, and has high heat exchange efficiency with the heat medium.

In addition, since the solar energy absorber of the present invention is treated with a treatment solution that has been subjected to activation means, the rate of formation of the treated film increases, and the treatment temperature shifts to a lower temperature side. This makes it possible to reduce the energy required to maintain the processing liquid at high temperatures, and makes handling easier.

The solar energy absorber of the present invention has a large solar energy heat absorption hall and little heat radiation, so thermal energy can be accumulated in a short time and a high temperature can be obtained. It can maintain high temperatures and make the most effective use of solar energy.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the measured spectral reflectance of the solar energy absorber of the present invention and a conventional absorber.

Claims (1)

[Scope of Claims] 1. The surface of a metallic copper alloy or copper alloy base material is treated with a treatment solution that is an alkaline aqueous solution containing persulfuric acid or a salt thereof, and the solution is subjected to activation means. A solar energy absorber featuring: 2. The solar energy absorber according to claim 1, wherein the activation means is irradiation with ultraviolet light. 3. The solar energy absorber according to claim 1, wherein the activation means is ultrasonic irradiation. 4. The solar energy absorber according to claim 1, wherein the activation means is the addition of an oxidizing agent. 5. The solar energy absorber according to claim 4, wherein the oxidizing agent is potassium chromate.