Category Archives: The Experimental Analyze Of The Solar Energy Collector

Collectors with Higher Concentration Ratios that Require Tracking

To achieve working temperatures around 200°C with acceptable efficiency, the absorber area has to be further reduced. Two-dimensional concentrators create a focal line, in which the absorber is placed. These concentrators have the drawback that only direct radiation can be used. One approach to build these concentrators is the development of small and cost-effective parabolic trough systems.

Extensive research on a small parabolic trough collector was done by the institute AEE INTEC (Gleisdorf, Austria) for operating temperatures of 200°C [7]. Another example is the parabolic trough collector PTC 1800 from the company SOLITEM which was developed with research support of the DLR (Germany). Demonstration plants for solar air-conditioning with this collector were already installed in Turkey.

Currently other types of linear focussing collectors are under development [8]. The linear concentrating Fresnel collector of the PSE AG (Freiburg, Germany) is already available on the market and proved its feasibility in several demonstration plants.

3. Rough Comparison

The efficiency of a solar collector can be expressed by

where K (0) is the IAM (Incident Angle Modifier) of the collector, which expresses the ratio of the optical efficiency at a solar incident angle 0 to the optical efficiency at irradiance normal to the aperture of the collector. The IAM-characteristics of a collector type for all possible directions of incidence (3D-IAM) is a very important attribute, which highly influences the energy gain of a collector. Further information about the 3D-IAM can be found in the paper of Paolo Di Lauro et. al. within the EuroSun08 proceedings.

Подпись: Figure 3: Comparison of different collectors at 800 W/m2 for different solar air-conditioning techniques. a) Single glazed flat-plate with AR, b) Evacuated tube collector of the Sydney type, c) CPC flat-plate with Teflon foil, d) Flat-plate with double AR-glazing and inert gas filling, e) Small parabolic trough (under development; only the fraction of direct radiation = 600 W/m2 can be used). The values are for normal irradiance and refer to the aperture area.

The collector parameter n0 describes the efficiency of the light conversion into heat without thermal losses. These are expressed by the factors a1 and a2. Tav is the working temperature and Tamb is the ambient temperature. For a detailed description see [5] and [6].

4. Conclusion

Within the IEA-SHC Task 33 SHIP alternatives to standard flat-plate and evacuated tube collectors were constructed or are still under development. From the rough comparison in figure 3 it turns out that for solar air-conditioning technologies that require working temperatures below 110 °C improved flat-plate collectors or collectors of the CPC-type can be a cost-effective alternative to evacuated tube collectors. The currently developed small parabolic trough collectors are predestinated to support double effect absorption

chillers or steam ejection chillers if the fraction of direct radiation is high and the specific collector costs are comparable to those of evacuated tubes.

To select the suitable collector technology for a specific system properly, the incident angle modifier, the weather conditions and the industrial load profiles or the characteristics of the cooling machine always have to be taken into account by a detailed simulation. Simulations can only be performed properly, when the optical behaviour of a collector for all directions of the incident radiation (3D-IAM) can be well approximated.

Besides the specific costs of the collector field also aspects like the resulting collector area and building integration have an influence on the decision for a certain collector type.


[1] Henning, Hans-Martin: Solar-assisted air-conditioning in buildings: A handbook for planners. Wien; New York: Springer-Verlag 2004

[2] Henning, Hans-Martin: Auslegung von solaren Klimatisierungssystemen.

In: 13. Symposium Thermische Solarenergie. Tagungsband.

Bad Staffelstein, 14.-16. Mai 2003, S. 253 — 258

[3] Deutsche Gesellschaft fur Sonnenenergie: DGS-Leitfaden Solarthermische Anlagen. 7. Aufl. Berlin: Landesverband Berlin Brandenburg 2006

[4] Rommel, Matthias: Medium Temperature Collectors for Solar Process Heat up to 250°C. In: 2nd European Solar Thermal Energy Conference estec. Proceedings. Freiburg, Germany, June 21.-22., 2005,

P. 167 — 172

[5] Duffie, J. A.; Beckman, W. A.: Solar Engineering of Thermal Processes.

3rd ed. Hoboken, New Jersey: John Wiley and Sons 2006

[6] Rabl, Ari: Active Solar Collectors and their Applications.

New York: Oxford University Press, Inc. 1985

[7] Jahnig, Dagmar: Development and Optimisation of a small-scale parabolic trough collector for production of process heat. Gleisdorf, Austria: AEE INTEC 2004. Available from: djaehnig@aee. at

[8] Weifi, Werner et. al.: Process Heat Collectors. IEA Task 33/IV: Solar Heat for Industrial Processes. Gleisdorf, Austria 2008. Internet: http://www. iea-shc. org/task33/publications/index. html

[9] Hefi, Stefan: Application of Medium Temperature Collectors for Solar Air-Conditioning. In: 2nd International Conference Solar Air-Conditioning. Tarragona, Spain, 18.-19. October 2007, Proceedings S. 118 — 123.

[1] Halogen lamp

2. Luxmeter

3. Flate plate collector

4. Temperature sensor 1-inlet

5. Temperature sensor 2-outlet

6. Temperature sensor 3-water tank

7. Hot water tank

8. Hot water tank overflow connection

9. Filler valve in primary circuit

10.Regulator valve for setting the volumetric flow rate

[2] Digital display

12. Air bleed valve

Fig. 1. The experimental stand and its components

Various incidence angles, measured using as reference the horizontal plane were investigated: 0o, 10o, 20o, 30o, 40o, 50o. For each measurement the light density, the flow rate, along with the inlet and outlet temperature were recorded after 15 minutes since the experiment was started. The thermal power and the system efficiency were calculated using the equations (1) and (2).

Measure of parameters

The test procedure include the measurement of: (a) climatic parameters such as global solar irradiance on the collector plane, ambient temperature, wind direction and speed; (b) energy consumption of pumps, electronic valves and controls included in the test system and (c) operational parameters such as orientation and inclination of solar collector, initial inlet water temperature, outlet water temperature, and flow rate.

These parameters allow evaluating increase or decrease of water temperature, available energy transferred to the water during the operation period, average thermal efficiency, steady-state thermal efficiency, instantaneous efficiency, etc.

1.1.1 Average solar irradiance

In order to evaluate the average solar irradiance on the collector plane, the following equation is used

n T

Подпись: (1)

.=1 n

Pool collectors

Unglazed absorbers in polymeric materials for (outdoor) swimming pool heating have been suc­cessfully in the market for more then 20 years. The heating demand for outdoor swimming pools is such that the operational temperature in the collector lies in the range of 15 °C — 30 °С; the ambient air temperature is approximately in the same range when pool heating is necessary, hence the heat loss is small and collector glazing is not required. Normally the pool water circulates as heat carrier in the pool absorbers,

Fig. 1 (a). The solar loop is not pressurized and the operational pressure in the absorber is deter­mined by the hydraulic design. For most pool collector types the maximum operational pressure lays in the range of 1.2 — 1.5 bar. The plastics should sustain temperatures up to approximately 100 °С (during stagnation), solar irradiation and water with swimming pool chemicals. In order to avoid damages due to freezing of the water in the absorbers and pipes, most pool absorbers should be emptied during the winter season. The low temperature level and small mechanical stress allows using low-cost commodity plastics.


Fig. 1. Solar pool collector system, thermosiphon system, integrated storage collector (ICS); Source: [5];

Performance measurements

Figure 2 shows the measurement data of the Bergamo collector on 2007/07/18, a sunny summer day. During this day only 10 out of 11 mirror rows were operated. Until 14:07 the collector power stays between 35 kW and 40 kW. On this day first tests of the automatic outlet temperature control at a set-temperature of 195°C were performed. By defocusing individual mirror rows, the mirror control software limits the maximum operation temperature. At 14:07 the set-temperature was reached and the collector power was reduced by turning several lines of primary mirrors out of focus. Five out of 10 primary mirror lines were automatically turned out of focus, i. e. instead of 120 m2 only 60 m2 primary mirror area were active, whereupon the collector power dropped to approx. 50% (green line in Figure 2). The collector efficiency refers to the active primary mirror area and therefore stays on the same level (red line in Figure 2). The outlet temperature dropped from approx. 196°C to 191°C, whereupon the inlet temperature dropped with a time delay from approx. 189°C to 186°C as soon as the temperature drop reached the collector inlet.

Shortly later these five primary mirror rows were brought back into focus and subsequently fluid temperature and power were rising again. When half an hour later the outlet temperature reached 199°C two primary mirror lines were taken out of focus, so that the outlet temperature decreased slowly to 195°C. At 15:00 both mirror lines were brought back into focus — power and temperature rose again before the system was turned off completely at 15:10.












12:30 12:45 13:00 13:15 13:30 13:45 14:00 14:15 14:30 14:45 15:00 15:15 15:30 15:45 16:00

Time (GMT +1h)

Подпись:Date: 2008/07/16












8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Time (GMT +1h)

Figure 3. measurement data of the PSE Fresnel collector in Bergamo, Italy, on July 16th 2008;

Absorber system

The used absorber system is built of a standard sydney tube (see Fig. 4). This kind of absorber con­sists of a double walled glass pipe (1 and 2) with an evacuated annular gab. This reduces heat losses due to heat convection and heat conduction. On the outer surface of the inner glass wall (2), a selective layer is applied facing the evacuated gap, which allows the highly efficient absorption of the solar irradiation und avoids the heat losses by infrared radiation. A heat conducting system (3 to 5) is inserted over the whole length inside the inner glass pipe. It consists of a round bent heat conducting sheet of aluminium (3), which surrounds a U-bended copper pipe (4 and 5). The heat conducting sheet is attached to the inner side of the inner glass wall and transports the heat from the absorber layer to the copper pipe. The heat transfer medium streams through the U-bended copper pipe and transports the heat out of the absorber system. Inlet and outlet are located on the same side of the vacuum tube. The described kind of vacuum pipe is used in common evacuated tube collectors.


Fig. 4. Profile of a sydney tube

During the tests, it turned out that this kind of absorber is improper for the application in parabolic through collectors. The main reason is the temperature range, since the vacuum tube is supposed to work at temperatures between 100 °C and 150 °C at continuous operation and at a stagnation tem­perature around 200 °C, while the application of parabolic trough collectors starts at these tempera­ture ranges. Calculations have shown that at outlet fluid temperatures of 160 °C at continuous op­eration, the temperatures at the inner wall of the glass pipe already reach 210 °C.

The described effects increase because of the following reasons: the relatively high fabrication tol­erance leads to air gabs between the heat conducting sheet and the inner glass wall. The air acts like a heat insulation, which limits the transport of heat from the selective layer to the heat con­ducting sheet, with the result of an increase of temperature of the inner glass wall. For this reason the thermal losses increase and the efficiency of the system decreases.

During the measurements the glass pipe broke several times. Presumably the copper tube expanded due to the high temperatures in its length and U-turn, which resulted in a burst of the inner glass tube (see Fig. 5).


Fig. 5. Destroyed absorber system due to too high expansion of the heat conducting system

Planned optimization: The German manufacturer NARVA will develop a new absorber system with the required characteristics. A conceivable concept might be an evacuated one walled glass tube with a heat conduction system inside. The heat conduction system might consist of a U-tube combined with a bent metal sheet, which is applied with a selective layer on the surface. The inlet and outlet pipes are located on the same side of the absorber. This design is likewise the sydney tube but unlike the prototypes’ concept the U-tube does no longer implicate the danger of destroy­ing the absorber system in the new concept. It can expand inside the one-wall glass tube without colliding. Thus no critical tension is reached inside the glass.

Another opportunity would be to design a direct flow absorber with the inlet on the one end and the outlet on the other end of the absorber. Such kind of absorber would allow an easy piping of sev­eral collectors in one row as it is standard for large scale collector systems like the EuroTrough.

Solar absorbers’ testing

The system used for the solar absorbers’ testing is a laboratory scale system for domestic hot water heating (E 202, Gunt Germany). The stand image and its components are presented in Fig. 1. The commercial stand consists of a flat plate collector with the absorber plate fixed on the heating serpentine. The device was modified allowing the replacement of the absorber plate (1), while using the same serpentine during all the experiments. The solar absorber converts the simulated solar radiation, delivered by a halogen lamp (2), into heat which is transmitted to a heat transfer liquid (water). The light amount and density can be adjusted by modifying the lamp height or the solar collector inclination angle. A pump is used for water circulation through a warm water reservoir, equipped with a small heat exchanger. Adequate sensors are used for temperature (inlet, outlet and water tank), illumination and flow rate monitoring. Using a data acquisition card, the measured values are transferred to a PC for further processing. [1] [2]

Подпись: (1)

Подпись: n image081 Подпись: (2)

Pn = б-P-cpT — T1)


— PN: is the thermal power, [W];

— cp: the water specific heat capacity, cp = 4.18 kJ/kg’K;

— Ti: the inlet temperature, [oC];

— T2: the outlet temperature, [oC];

— n: the system efficiency, [%];

— Q: the volumetric flow, [L/h];

— p: the water density, [kg/m3];

— Ac: the collector area, [m2];

— E: the light density, [W/m2];

— 0: the incidence angle, [degree].

For the measurements, the water tank and the solar circuit were filled with deionised water. The lamp was fixed at constant height (70 cm) from the collector. The collector was inclined at the angles above mentioned.

3. Results and discussions

The experimental data for the (1) and (2) absorbers are presented in Table 1, while the thermal power and efficiency vs. incidence angle are illustrated in Fig. 2 and Fig. 3.

Table 2 Experimental data for solar absorber deposited on Al and Cu substrates

Aluminium substrate


[degree] To1 To2 Q

[oC] [oC] [l/h]

E cos 0* [kW/m2]

Copper substrate

T1 T2 Q [oC] [oC] [l/h]

E cos 0* [kW/m2]









2 .59














































*measured on the collector surface

1st International Congress on Heating, Cooling, and Buildings " ‘ 7th to 10th October, Lisbon — Portugal *


Fig. 2 Comparison between the efficiency of the Cu and Al solar absorbers


Fig. 3 Comparison between the thermal power of the Cu and Al solar absorbers

The efficiency (n) and the thermal power (PN) for the two solar absorbers mainly depend on the absorbers type, their optical properties (solar absorptance and thermal emittance) and on the irradiance incident angle.

As Fig. 2 shows, the efficiency of the solar absorber (2), is higher compared to solar absorber (1). The black nickel, on the top of the substrate has rather similar composition for (1) and (2), being obtained using the same materials and spraying parameters, except to the deposition temperature: 350oC for (1) and 300oC for (2). Thus, the higher values of the n (and also PN) can be explained, on the one hand, by the fact that the structure Cu/CuOx has a higher thermal conductivity comparing to Al/Al2O3. In this case, the energy transferred to the flow liquid is higher. On the other hand, the optical properties (solar absorptance and thermal emittance) for the solar absorber deposited on Cu

substrate are better (Table 1), thus having higher heat gain and lower heat loss capacity. Although deposition on Cu gives better results, the Al substrate has a lower price and weight.

Generally, when 0 = 0o, the solar radiation is perpendicular on the collector surface, maximum amount of energy is reaching the panel. The heat variation produced for different incidence angles is presented in Fig. 3. The results prove that the maximum heat gain is registered for incidence angles of 10…30°. These results can be the consequence of reduced reflections and/or radiation scattering for angles higher than 0. The almost 20% increased output of the copper based collector comparing to the aluminium one, at the maximum heat gain point can also be correlated with the heat conduction/storage of the two plates.

The results presented in Fig. 2 and in Fig. 3 shows that the heat gain and the efficiency are not following a similar trend. These observations could be further used in studies of the solar-thermal conversion efficiency at low radiation amounts (e. g. for cloudy days). Also, tracking systems could be recommended to maintain the most favourable inclination angles for the solar collectors, thus to get the maximum thermal power. An efficiency increase up to 11% can be registered in this case [12].

4. Conclusion

In the present study, two types of black nickel based solar absorbers deposited on copper and aluminium substrates were tested. The absorbers were incorporated in a laboratory scale domestic hot water system. These studies were developed in the research department of the centre: Product Design for Sustainable Development, in the Transilvania University of Brasov, Romania.

The objective was to obtain two low cost solar absorbers and to test their thermal performance, in laboratory conditions. The results proved that quality absorbers can be obtained by spray pyrolysis deposition technique. The CuOx/NiOx/TiO2 absorber proved to have a better efficiency and thermal power than the Al2O3/NiO/TiO2 absorber due to better optical properties and thermal conductivity. The study showed that maximum thermal power can be achieved, in both cases, for inclination angles in the range of 10-30o.


This work was supported by the Romanian Council for Research in High Education, CEEX 277/2008 grant.


[1] A. B.Ceedy, (1999). Handbook of Solar System Layouts, SciPress, Lisbon.

[2] G. Katumba, L. Olumekor, A. Forbes, G. Makiwa, B. Mwakikunga, J. Lu, E. Wackelgard, Solar Energy Materials and Solar Cells, 92 (2008), 1285-1292.

[3] T. Tesfamichael, PhD thesis, Uppasla, Sweeden, ISBN 91-554-4772-4.

[4] A. Biswas, D. Bhattacharyya, H. C. Barshilia, N. Selvakumar, K. S. Rajam, Applied Surface Science 254 (2008), 1694-1699.

[5] W. E. J. Neal, Surface Technology 20 (1983), 295-299.

[6] C. E.Kennedy, Technical Report NREL/TP (2002) 520-31267.

[7] M. Voinea, C. Bogatu, G. C.Chitanu, A. Duta, Rev. Chim. (Bucuresti) 59 (2008), 659.

[8] E. Purghel, M. Voinea, L. Isac, A. Duta, Rev. Chim. (Bucuresti) 59 (2008), 469.

Comparative performance with natural circulation type solar water heater

Performance of ICS solar water heater was compared with natural circulation type solar water heater. Natural circulation type solar water heater consist of a flat-plate collector made of copper fin having absorber area 1.9 m2 and doubled walled storage tank of 100 litres capacity. The detailed design has been described by Nahar [16]. Both the solar water heaters were filled in the morning and hot water temperatures were recorded regularly. The average hot water temperature was 57.3o C and 62.0oC at 1600 hours same day that was retained to 43.0o C and 50.4o C till next day 0800 hours when tap water temperature was 17.0o C in ICS and natural circulation type solar water heater respectively. This suggest that performance of ICS solar water heater is as good as of natural circulation type solar water heater while cost of ICS solar water heater is Rs 8000.00 where as cost of natural circulation type solar water heater is Rs. 12000.00 ( 1.0 Euro = Rs 60.00).

Heat loss coefficient. UL determined from thermal stagnation measurements (set-up A)

The heat loss coefficient UL is determined according to Section 2.3 for the measurements with the reference collector (not ventilated) and a collector tilt angle of 45° (data of June 10, 2006). The slope U2 and the interception with the y-axis U are found by linear regression, see Fig. 6 (a, b). In the case of (a) and (b) we find: UL= 3.8 + 0.030 AT (red line).

image156 image157

The collector’s heat capacity is not accounted for in (a), hence the derived UL-values of the measurements in the morning and afternoon do not coincide. By choosing the heat capacity Ce=9 kJ/(m2 K) in (b) the AT-dependent heat loss coefficient from the morning and afternoon match. The chosen heat capacity Ce does not have a significant influence on the UL-values as long as the measurements before and after noon cover about the same AT interval. The U-values were determined for different tilt angles and the resulting collector efficiencies have been compared with earlier efficiency tests of the same collector. The presentation and discussion of these results would exceed the frame of the present work; it is referred to [19].

Fig. 6. U-value UL as a function of the temperature difference AT between the mean absorber temperature
and the ambient temperature if the collector’s heat capacity Ce is set to zero (a) and to 9 kJ/(m2 K). The line
represents the least square fit; results based on raw data of measurements performed at set-up A.

Effect of mirror deformations

image036After having introduced the procedure for mirror deformation reproduction, the first study examines the consequences produced by the deformations of a solar trough collector. The study is based on the use of ray tracing analyses that allow controlling all the optical parameters. Considering the application of sunlight exploitation, the most important features probably are collection efficiency, angular aperture and acceptance angle. The collection efficiency E is the ratio between the light focused on the absorber and the light received by the collector entrance aperture. The angular aperture of a collector represents the total aperture angle receiving the sunlight rays and it is usually expressed as Field Of View (FOV). While the acceptance angle is the limit aperture for which the collection efficiency keeps its maximum value. The angular analyses of the solar trough collector are discussed in Sections 4-5.

The collection efficiency of solar trough has been monitored in order to evidence how much it is affected by the geometrical deformation of mirror profile.

The configuration considered in this study is characterised by the following parameters: f=780mm, D=50mm,

G=70mm, T=2mm. The absorber centre is located in the focal point of the parabolic mirror.

The simulations are carried out for various values of the conic constant K,

corresponding to elliptic and hyperbolic Fig. 3. Deformation effects versus wrtiral

edge displacement.

deformations. The results are graphically reported in Fig. 3, whose calculation parameters are detailed in Table 1. The sampling for the conic constant K (Column 2) is not linear because we preferred to choose as reference parameter the deformation at the mirror extreme. The reference quantity chosen to represent the mirror deformation is the vertical displacement of mirror extreme Delta Z, reported in Column 6 of Table 1.

Column 1 indicates the deformation type (specifying the curvature direction of the deformation with reference to Fig. 2): elliptic (internal/up) or hyperbolic (external/down). Then Column 2 reports the value of each corresponding conic constant K.

The successive three columns of the table refer to the geometrical parameters of every deformed profile. Referring to Fig. 2, the point of mirror extreme has as coordinate (Ymax; Zmax), respectively reported in Column 3 and Column 5. Still referring to the semi-profile, half of the length of each deformed profile is in Column 4, confirming that the rigid deformation is calculated keeping almost unchanged the total length of the mirror. The calculation is approximated considering only integer values of mirror semi-aperture Ymax.

Table 1 — Effect of mirror deformations.






Vertical edge



constant K








Ymax (mm)


Zmax (mm)

Delta Z (mm)

E (%)












































































































Finally the effect of mirror deformations is reported in Column 7 as collection efficiency values; the calculation has been carried out with linear sampling in Delta Z. Figure 3 visualises the variations of collection efficiency as a function of the mirror deformation parameter Delta Z.

The sign of the Delta Z values reported in Table 1 is in agreement with Fig. 2: the vertical edge displacement is positive in the elliptic case (int./up in Fig. 2) and is negative in the hyperbolic case (ext./down in Fig. 2).

In conclusion for the examined trough collector the effect of elliptic deformations is significant only for vertical edge displacement Delta Z > 2.5 mm.

While in the hyperbolic deformation case the effect of solar trough deformations is considerable only for absolute values of Delta Z > 3 mm.

Thermotropic hydrogels

Thermotropic hydrogels are chemically or physically cross-linked polymer networks which are poured with appropriate water content. At low temperatures the aqueous solution is dissolved homogeneously on the molecular level, so that a clear state is achieved. Above the switching temperature scattering domains are formed by an aggregation of the polymers and/or by separation of free water from the polymer network [2,3,7,8].

Numerous thermosensitive hydrogels exhibiting a cloud point are discussed in the literature. Examples are aqueous solutions of polyvinylethers [9], polyvinylalcohol [10], poly(N-substituted acrylamide) [11], poly(N-vinyl alkylamide) [12], ethyleneoxide-polypropyleneoxide [13], copolymers of N-vinyl — 2-pyrrolidone and hydroxyethylmethacrylate [14], poly(methyl-2-acetamidoacrylate) [15],

poly(methyl-2-acetamidoacrylate-co-methyl acrylate) [16], polyvinylacetone [17], poly(N-oxazolines) [18] or cellulose derivatives [4].

As to performance properties thermotropic hydrogels possess a high potential for solar collector applications. The materials are characterized by a high transparency in the clear state (>82%) along with low haze and a change in solar transmittance by 77% at temperatures adjustable between 5 and 100°C. The materials exhibit an excellent switching performance with a steep switching gradient, a high reversibility, and low hysteresis [4,19,20,21]. But as the transition is based on physical interaction between the components the materials have several problems with long-term stability and ageing. Furthermore the materials have to be UV protected [4]. The water makes the thermotropic hydrogel susceptible to freezing and limits the operation temperature range [21]. Thermotropic hydrogels filled into the intervening space of a double glazing, place high demands on sealing of the glazing. If not sealed properly the layer will dry out [2]. If synthetic materials are used thermotropic hydrogels are quite expensive. If biopolymers are used costs decrease significantly. However, the use of biopolymers requires a thorough protection against microorganisms [22].

A thermotropic hydrogel glazing, a sandwich of two glass panes and the encapsulated hydrogel, is announced by Affinity Ltd. (Japan) [2]. However, no thermotropic hydrogel is described currently that exhibits the working temperature range and switching temperatures required to provide adequate overheating protection for solar collectors. Further developments should focus on the adjustment of adequate switching temperatures (55-80°C) and the improvement of the long-term stability.