1. Introduction

By the end of 2006, the solar thermal collector capacity, in operation worldwide, reached 127.8GWth, 182.5 million square meters of this capacity (corresponding to 102.1GWth) were accounted for by flat plate and evacuated tube collectors, which are mainly used for preparation of hot water. Another relatively more recognized application for low temperature heat is to provide space heating [1]. For these solar thermal applications, the low temperature level requirement(below 80°C) can be easily reached by using the commercial flat plate collectors and all glass vacuum tube collectors, while for other applications that need the lower-medium temperature level heat, solar thermal devices remain much less explored and exploited.

Currently, there exists a high demand for applications using heat in the lower-medium temperature, with a range of 90°C to 150°C. In US heat processing industrial market, steam between 100°C and 150°C accounts for 16.1% of the thermal energy consumption, while only 1.5% of this consumption is coming from applications using heat below 100°C. The demand of heat for processes below 160°C in special industrial sectors such as food, wine and beverage, paper, textile, transportation equipment is remarkably high. By the end of 2006, only 25MWth is reported for the operating capacity for industrial processes using lower-medium temperature heat, which represents only a small fraction (0.02%) of the total installed solar thermal capacity worldwide [2].

One typical application of low-medium temperature heat is solar cooling. In which the heat source of 90°C to 150°C can be used to drive the absorption cooling devices by using one single-effect absorption chillers with coefficient of performance (COP) of 0.7, or, as the heat source temperature increases to over 150°C, to drive a double-effect absorption chillers with COP of 1.1. Based on the observations of global weather and temperature distribution, without the consideration of the factor of humidity, areas that need air conditioning are oftentimes hot and sunny regions. This implies a high coefficient factor between the demand for cooling and sufficient sunshines geographically. With this fact in mind, several solar cooling projects have been successfully demonstrated in US, EU and China.

Current techniques in flat plate collectors are not sufficient for the system to work under a reasonable efficiency to meet the demand of the lower-medium temperature applications. The results based on the typical flat plate collectors SPF(collector test Nr C1139) test show that: the efficiency for them at 90°C and 150°C operating temperature is, specifically, around 41% and 6%, under the testing condition of ambient temperature being 25°C, solar irradiance being 800W/m² [3].

As till the day of this paper being written, the typical traditional all-glass vacuum tube consists of two glass tubes, between which vacuum is produced with certain processes. The outter surface of the inner glass tube is selectively coated to absorb the solar irradiance, and serves as the main part for thermal heat production. This type of collectors are not directly suitable for application of over 100°C without assisting modules, because the water inside the inner tube is boiled under 1 atm at this temperature. In order to transfer the heat outside of the glass tube, the modifications of this device to fulfill the application requirement can be achieved by inserting a U shape copper tube, or heat pipes into the inner glass tube. E.g. a cylindrical Al or Cu fin can be used for conducting the heat absorbed by the inner glass tube to the metal pipe, oftentimes this particular part is in contact with the inner glass tube. There are many different designs in China and Europe to improve the contact. The calculation based on typically modified all-glass vacuum tube collectors in the SPF test (collector test Nr C1152) shows that: the efficiency for them at 90°C and 150°C operating temperature is, specifically, around 45% and 24%, under the testing condition of ambient temperature being 25°C, solar irradiance being 800W/m².

In order to compete with other possible solar thermal devices for a share of the huge potential market of lower-medium temperature heat applications, analysis of the trade-off between price and efficiency shows that the economical 50% efficiency solar thermal collector in this case reaches the optimal balance as the key component of the system.

2. Overview of the metal absorber glass vacuum collector (MGVT) development

Since 1980’s, Philips, Dornier Prinz(Germany), Thermomax(UK), Sunda and Eurocon(China) have been focusing on the development and commercialization of metal absorber glass vacuum tube collectors (MGVT). In the last 20 years, a lot of progresses in this field have been made.

Figure 1 shows the basic structure of MGVT. The main components are: exterior glass tube as the envelop (1), the metal absorber sheet (2), and the metal absorber pipe which is wrapped around by it. The absorber pipe could be either a heat pipe (4) or a flow through pipe (4,5). The metal cover (3) on the end of the exterior glass tube is used to seal between the glass tube and the metal pipe. The whole glass tube is evacuated.

 

Fig. 1 Typical MGVT structures.

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2.1. Glass envelop

The glass tube commonly sizes in dia.65mm with 1, 800mm in length or dia. 100mm with 2, 000mm in length. The transmittance is approx. 90%.

2.2. Selective coating

In early 1980’s the electro-plated black chrome is used for the selective surface of the absorbers (Philips). Later on sputtering and PVD technologies are applied to coating the absorber. Tinox, Analod and Bluetec are currently the notable companies that hold commertially available selective coating techniques on the metal substrate. Tinox applied PVD in combination with the sputtering and electron beam technology. Tinox announced that its titanium oxnitride coating have a solar irradiance absorptance of 0.95, with lower emissivity (0.04 at 100°C) and good temperature stability. Their products are tested and proven efficient in several testing institutions [4].

2.3. Two possible structures: heat pipe and flow through pipe

There are two possible configurations for MGVT, heat pipe [Fig. 1(a)] which does not include the vacuum tube in the global fluid circulation, and the flow through pipe [Fig. 1(b)] which includes the vacuum tube in the global fluid circulation.

As shown in Fig. 1(a), heat pipe is used for the heat transfer from the absorber to system fluid loop outside. Eurocon use water mixture as the inside medium for heat pipe. The tested maximum heat transfer capacity with 8mm diameter evaporator is 200W. As for using the water medium for the heat pipe, the freezing damage has remained as a fatal problem untill solved by Sunda and Eurocon. Test at minus 40°C ambient temperature in South Pole of Earth shows no damage of the MGVT (Sunda). Compared to this technique, Thermomax uses organic medium for its heat pipes for similar anti-freezing purpose. Thermostop at 130°C has been developed to avoid the overheating of the solar loop system. Global loop fluid for the solar system is not circulated in the heat pipe in this case, Fig. 1(a).

Figure 1(b) illustrates the flow through tube configuration with concentric inlet pipe(5) and outlet pipe(4). The global loop fluid circulation for the solar system is directed in from the inlet and flows out from the outlet (5,4). The advantage for flow through MGVT is that the tube may be horizontal mounted. The two junctions, firstly between the absorber and the heat pipe evaporator, and secondly, between the outlet and the pipe of the flow through MGVT, are the key parts affecting the fin-efficiency. In order to correctly form these two juctions, several techniques have been compared and considered. The laser welding among these techiniques is drawing more and more attention for the welding process in flat plate collector manufacture, but not yet proven to be comapratively more efficient than the current ultrasonic welding techniques for the MGVT manufacture. The main reason is because of the junctions being in vacuum condition.

2.4. Metal-glass sealing

Because of the different heat expansion rates of glass and metal, caused by the temperature change during either the manufacture process or the real world application scenario, it took an unusual effort to find suitable metal alloy to join and seal the glass with the metal parts of MGVT. Thermomax applied a mature technology produced by the typical requirement of joining glass with metal in the lamp industries, to seal the glass tube with metal pipe. Dornier Prinz in 1980’s is the first company to develop thermcompression technique to seal the metal with the borosilicon glass tube, without the necessity of melting the glass. In 1990’s, Sunda, and later on in 2000’s Eurocon made one more step in the improvement for the thermocompression sealing used in MGVT with their own patents. The original Dornier-Prinz thermocompression MGVT has already been in real world operation for more than 25 years and proven to have no leakage problem up till now.

2.5. Vacuum

The vacuum tube collectors are oftentimes manufactured with a vacuum standard up to 10⁻³ Pa, the conventional heat loss is eliminated and the conduct heat loss is reduced to 0.1% at 1 atm. Since MGVT has the metal components inside the glass tube, the special gas elimination procedure of these metal parts is necessary during the manufacture process, before the sealing of the device. To maintain the vacuum inside of the glass tube, typical all-glass vacuum tubes implement getter for future gas elimination mainly targetting the glass emitted gas such as H₂O and CO₂. For the metal emitted gas during MGVT manufacture and the product lifetime, Eurocon successfully put additional getter targetting the absorption of H₂.

2.6. MGVT efficiency at lower-medium operating temperature

The test results of R/Z Solartechnik company’s MGVT collectors (made by Eurocon) in SPF(collector test Nr C953) are shown in Fig. 2.

 

Fig. 2 R/Z company’s MGVT collector efficiency curve tested by SPF

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Tested at the ambient temperature of 25°C, solar irradiance 800W/m² and operating temperature as 90°C and 150°C, the efficiency is, specifically, 68% and 54%.

At the end of year 2009, Eurocon and Dachang Viessmann together started mass manufacture of the promising high efficiency MGVT collector for the world market. Figure 3 shows the MGVT collector model manufactured.

 

Fig. 3 MGVT collector model

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3. Further development

Further possible technology improvements include:

3.1. Improvement on transmittance

The anti-reflection surfaces on both sides of the glass tube have a promising future of increasing the solar irradiance transmittance of the tube from 90% up to 95%. A special etching process is used for flat plate glass cover by Sunarc(Denmark). This technology has not yet been adopted for the glass tube manufacturing yet.

The SiO₂ is successfully used in the glass tubes produced by Shott (Germany) in the receiver tube for thermal power generation. The technology is still in development for the MGVT collector.

3.2. Improvement with reflectors

To further increase the efficiency at medium temperature up to 200°C, a cylinder metal absorber vacuum tube has been developed in Eurocon. This type of MGVT tube is used with stationary external compound parabolic concentrator(CPC) reflector designed by R. Winston. The model indicates that the collector efficiency is 52.6% at 200°C [5].

4. Conclusion

Through more than 20 years of continuous development and evolvement, the MGVT collector is currently commercially available to the lower-medium temperature applications. The further improvements are focused on raising the efficiency. In the mean time the cost will be lowered by the scale effect of the mass production. Equipped with this technology realized by continuous engineering effort, solar thermal application will jump one step significantly higher, from warm water preparation to the industrial processing heat generation, absorption cooling and water desalination in the near future.