Труды Всемирного конгресса Международного общества солнечной энергии

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Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

672

curves were recorded for the two alloys in EdMPNTf

and compared with the pure nickel (99.99% goodfellow) in

the same conditions. All the systems reach an almost

constant value (AISI 304: -0.32÷-0.35 V; AISI 1018: -0.53

÷ -0.56V; Pure Nickel: -0.40÷-0.45V) after about 9 hours of

immersion. The highest voltage value corresponds to the

stainless steel AISI 304, confirming its ‘nobler’ behavior in

comparison to both pure Nickel and carbon steels.

Tafel Plots and Rpol measurements. These measurements

provide an insight on the corrosion current densities of the

alloys in ILs environment. After the registration of the

OCV curve, Polarization Resistance (R

pol

) measurements

were performed in the range of ± 25 mV respect to OCV. In

a few minutes after the R

pol

scan the potential returned to

OCV value. At this point, the anodic Tafel plot was

recorded starting from – 50 mV up to +250 mV respect

OCV. The cathodic Tafel plot was registered

independently, repeating the above described procedure

with fresh fluid, a new WE and performing first the OCV

scan. The scan rate for Rpol and Tafel plots was 0.166 mV

-1

The extrapolated quantities from Tafel plots are

summarized in Table 2 and they are of the same order

obtained for a similar IL

8,9

. The slopes of the linear portion

of the Tafel plots (β

and β

) were used to calculate

corrosion current densities from Polarization Resistance

measurements, which result 1,17 μA cm

-2

for AISI 1018

and 0,024 μA cm

-2

for AISI 304. Superimposed events

could influence the β values, such as different oxidation

state of the metals, the presence of several oxidants, etc.,

leading the observation of a mixed potential.

TABLE 2: TAFEL PLOTS EXTRAPOLATED PARAMET-

ERS

Alloy I

corr

(μA cm

-2

)

corr

(μA cm

-2

)

(mV d

-1

)

(mV d

-1

)

304 0,014 0,014 143 280

1018 1,22 0, 239 127 496

4. DISCUSSION

The results reported here show a complex behavior of the

different combinations of substrate and IL. In general, we

believe that the high reactivity of these systems can be at

least in part attributed to dissolved oxygen. It appears that

ILs can dissolve much more oxygen than water, as shown

by a recent work by Brennecke et al.

, which estimates the

dissolved oxygen in several ionic liquids, including

methyl-tri-butylammonium(Tf

N), similar to those

employed in this work. According to this study, the Henry

constant expressed in pressure units is about 1000 bar at 25

°C. In comparison, the Henry’s constant of oxygen in water,

expressed as function of gas concentration and partial

pressure (Eq. 1), is 1.28 10

-3

mol l

-1

bar

-1

at 25 °C:

1−

⋅= piCi

conc

H (1)

Thus, the oxygen concentration, obtained considering the

atmospheric partial pressure of the gas equal to 0.21 bar, is

2.7 10

-4

M. Since a dm

of water contains 55.51 moles, the

molar fraction of oxygen is something about 4.86 10

-6

, that

gives rise, according to the equation (2), to an Henry’s

constant of 43210 bar, a pressure 40 times higher to

dissolve in water the same amount of oxygen that dissolves

in IL.

For the application studied here the laws that govern the

electrochemical processes are expressed in terms of molar

concentrations. This is not only a formal question. The

electrochemistry is not concerned of the relative amount of

the solvent and the electroactive species since the solvents

only acts as a support for the reaction. The

electrochemical phenomena are only affected by the density

of the molecules of the electroactive species and of the

kinetic of the process. The number of active molecules

reaching the electrode surface is only related to the

volumetric concentration and also on some combined

properties which are function of both the electrolyte and the

electroactive species (e.g. the diffusion coefficient). In

terms of molar fraction, the ratio of the solubility of the

oxygen in the ionic liquid and in water is large:

43210/1000=43, but in terms of volume concentration the

situation is different, because the number of molecules of

solvents per volume unit is different. We have earlier

calculated that the oxygen concentration in water is around

0.27 mM or 8.64 mg l

-1

. For the IL, considering that the

density of EdMPNTf

N is 1.41 g cm

-3

and a molar weight

is 396.37 g mol

-1

we can conclude that the number of mol/l

is around 3.5, much lower than the 55.5 for water. From the

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

673

inverse Henry’s law, expressed in term of molar fraction, X

and partial pressure, p

, of the i

gas:

1−

⋅= XipiH

(2)

and considering an oxygen partial pressure of 0.21 bar, we

get that the molar fraction of dissolved gas is 2.1 10

-4

. From

the definition of molar fraction and considering the number

of mol of IL per liter, the concentration of the dissolved

oxygen is around 7 10

-4

mol l

-1

or 22.4 mg l

-1

; approximately

three times larger than in water. Such a concentration is

sufficiently high to justify the occurrence of electrochemical

phenomena due to the oxygen. The β values could be also

affected by oxidant agents diffusion controlled mechanism at

the electrode interface, thus activation polarization

contributions seems to be not negligible. This fact probably

is also the cause of the high β values in the cathodic plots,

where the only reduced agents could be protons from

residual water. The determination of the corrosion rates by

such electrochemical approach needs the definition of the

charge number Z, that, at this moment, can only be a

hypothesis. Nevertheless, the comparison of the corrosion

current densities derived by the R

pol

method and to the

corrosion rates derived by Tafel plots extrapolation shows

good agreement. Such element provides a confirmation of

the reliability of the performed measurements.

5. CONCLUSIONS

The results reported in the present study show that the

resistance to corrosion of steel substrates in contact with

the Ionic Liquids investigated is still not satisfactory

enough for practical applications. Among the investigated

Ionic Liquids only the EdMPNTf

N does not decompose at

the working temperature of the trough collectors in

presence of air and even though the electrochemistry

estimated corrosion current densities at room temperature

are low, especially for AISI 304, the dissolved oxygen in

this ionic liquid is not negligible at 220°C, and it is the

probable cause of the observed localized corrosion after the

open vessels immersion tests. Considering the important

amount of oxygen that dissolves in ionic liquids, a better

resistance to corrosion is likely to be obtainable in absence

of air (i.e. nitrogen purged atmosphere), which is a

condition that can be obtained also in practical conditions

in a sealed heat transfer system in a solar concentration

plant. This will be the objective of further investigation in

this field.

6. ACKNOWLEDGMENTS

We gratefully acknowledge financial support provided by EC,

Project FP6-2003-INCO-MPC2 (STREP) Contract Number:

015434, REACt, Self-sufficient Renewable Energy Air-

Conditioning system for Mediterranean countries.

7. REFERENCES

(1) L. Moens, D.M. Blake, D.L. Rudnicki, M.J. Hale,

“Advanced thermal storage fluids for solar parabolic trough

systems”, J. Solar Energy Eng, 125, 112–116, 2003.

(2) C. P. Fredlake, J.M. Crosthwaite, D.G. Hert, S.N.V.K.

Aki, J.F. Brennecke, “Thermophysical properties of

Imidazolium-based Ionic Liquids”, J. Chem. Eng. Data,

49, 954–964, 2004

(3) M.E. Van Valkenburg, R.L. Vaughn, M. Williams, J.S.

Wilkes, “Thermochemistry of Ionic Liquid heat-transfer

fluids”, Thermochim. Acta, 425,181-188, 2005

(4) M. Kosmulski, J. Gustafsson, J.B. Rosenholm,

“Thermal stability of low temperature ionic liquids

revisited”, Thermochim. Acta, 412, 47–53, 2004

(5) U. Bardi, S.P. Chenakin, S. Caporali, A. Lavacchi,I.

Perissi, A. Tolstoguzov, “Surface modification of

industrial alloys induced by long-term interaction with

an ionic liquid”, Surf. Interface Anal. , 38, 1768-1772,

2006

(6) S. Chowdhury, R. S. Mohan and J. L. Scott, “Reactivity

of Ionic Liquids”, Tetrahedron, 63, 2363-2389, 2007

(7) K.J. Baranyai, G.B. Deacon, D.R. MacFarlane, J.M.

Pringle, J.L. Scott, “Thermal degradation of Ionic

Liquids at elevated temperature”, Aust. J. Chem,

57,145-147, 2004

(8) I. Perissi, U. Bardi, S. Caporali, A. Lavacchi, “High

temperature corrosion properties of Ionic Liquids”,

Corros. Sci., 48, 2349-2362, 2006

(9) M. F .Arenas, R.G. Reddy, “Corrosion of Steel in Ionic

Liquids”, J. Min. Met., 39, 81-91, 2003

(10) J.L. Anthony, J.L. Anderson, E.J. Maginn, J.F.

Brennecke, “Anion effects on gas solubility in Ionic

Liquids”, J. Chem. Phys. B, 109, 6366-6374, 2005

EXPERIMENTAL RESEARCH ON THE ALL-GLASS EVACUATED TUBE

SOLAR AIR COLLECTOR

Hong Liang

Institute of Electrical Engineering, CAS

Beijing 100080, China

Hongwei7600@vip.sina.com

ABSTRACT

This paper is focus on the research of the thermal

performance of all glass-evacuated tube solar air

collectors by experiment. Two sections composed of inlets

and outlets are formed by the inserted pipes with the

target to heat the forced convection air by heat exchange

with the vacuum pipes’ inside surface. Solar radiance,

work fluid flow rate, the static pressure difference

between inlet and outlet, the temperature of ambient and

inlet and outlet and ambient wind speed were measured in

this experiment, for they are the factors that can effect the

instance efficiency of solar air collector. The testing was

finished based on the methods introduced in

ASHRAE93-86 and ISO9806-2 standard.

1. INTRODUCTION

The requirement of energy in China is dramatically

increasing with the rapid development of its economy.

Fossil fuel consumption caused many widely known

problems like environmental pollution and climate warm

up. Nowadays, the entire worlds are discussing how to

protect the environment and to solve the energy lack by use

of renewable energy. Solar is one of the renewable energy

sources with most potential for its large amount and easy

collection.

In China, air conditioning energy demand accounts 65% of

the total architectural energy consumption. How to use

solar air heater instead of the conventional energy to meet

the heating and cooling demand in winter and summer now

becomes the main research field.

Furthermore, all kinds of solar air heater can be widely

utilized in the fields of agricultural crops drying, sea water

desalination and building heating and cooling. This paper

presented the experimental research results of the all glass

evacuated tube solar air collector inserted with metal pipes.

Those tests are based on the standards introduced in

ASHRAE 93-1986 and ISO 9806-1.

2. THEORETICAL ANALYSIS

2.1 Transient Efficient Test

Inlet air temperature from 4 points with equivalent interval

spaces was measured as follows under sunny weather:

(1) Total insulations on the evacuated tube solar air heater

surface G;

(2) Diffuse insolation on the vacuum tube solar air heater

surface G

;

(3) Incidence angle

θ ;

(4) Surrounding temperature

;

(5) Ambient wind speed u;

(6) Solar air heater inlet temperature

t ;

(7) Solar air heater outlet temperature

;

(8) Inlet air mass flow rate



;

(9) Outlet air flow rate



3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

675

Transient efficiency is calculated from the following

equations:

mc TΔ



()/TttG=−

;





2.2 Solar Air Collector Pressure Drop Test

Inlet and outlet air pressure drop which is crucial to the

solar collector’s characteristic is an important design

variable. The test is measured between flow rate of

0.01m

0.03 m

Variables including

and

PΔ

are

measured and the pressure drop

PΔ

vs. flow rate curves

are plotted.

2.3 Solar Air Collector Thermal Capacity Test

Variables including are measured G,

t , m



, based on the following equations:

Tt tΔ= −

CAGmC

=−



()

aina

TAUt tΔ− −



Two static state transient characteristics are calculated and

the two static equations are integrated coming to the following

thermal capacity:

()

AGdmCTdAUttd Td

ητ τ τ τ

⎡⎤

−Δ− −+Δ

⎢⎥

⎣⎦

−

∫∫ ∫ ∫



Curves are plotted based on the experimental results

including ˄t

̢t

˅,T and G. The space area between

two static stats are separately as:

()ttd

−

∫

The efficient thermal capacity can be calculated from the

experimental data.

The thermal capacity results from three experimental are:

=9733.6J/K; C2= 6317J/K; C3=10356J/K.

The solar air heater capacity is defined as the arithmetic

average of the three values above: C=8802.6J/K.

3. EXPERIMENTAL APPARATUS AND THE MEA-

SUREMENT

The all-glass evacuated tube solar air collector is composed

of vacuum tube, inserted metal tubes and the header box.

The vacuum tubes and header box are separated into two

sections with inlet and outlet by the inserted metal tubes. It

absorbs heat through air convection. The structural

schematic of the all –glass evacuated tube solar air heater

and the experimental apparatus schematic are as following

Fig. 1 and Fig. 2.

The test of this solar air heater has been finished in

Beijing during Jun-Aug 2006.Experimental results are

shown in Fig. 3.

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

676

Fig. 1: Schematic of the all-glass evacuated tube solar air collector.

Fig. 2: Schematic of the experimental apparatus.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

677

\ [























1 1/12 1/13 1/14 1/15 1/16 1/17

¨

Fig. 3: Transient efficiency equitation.

Fig. 4: Pressure drop V.S. mass flow rate.

4. CONCLUSION

From the tests finished, it can be found the thermal specific

of air is too small to establish static test state. Even the flow

rate tiny change can cause obvious difference of

temperature and pressure. Dust accumulated on vacuum

tubes’ surface also disturb the testing accuracy.

5. REFERENCES

(1) A. Ucar M. InallˈThermal and exergy analysis of solar

air collectors with passive augmentation techniques

International Communications in Heat and Mass

Transfer 33 (2006) 1281̢1290

(2) Performance analysis of new-design solar air collectors

for drying applications Suleyman Karsli ˈ

Renewable Energy

(3) Efficiency investigation of a new-design air solar plate

collector used in a humidification̢dehumidification

desalination process ˈ Mahmoud Ben-Amara, Imed

Houcine*,Aman-Allah Guizani, Mohammed Maalejˈ

Renewable Energy 30 (2005)

INTEGRAL-TYPE SOLAR WATER HEATER USING LOOP HEAT PIPE

B.J. Huang, P.E. Yang , J. H. Wang, J. H. Wu

New Energy Center, Department of Mechanical Engineering

Taiwan University, Taipei, Taiwanm, China

bjhuang@seed.net.tw

ABSTRACT

Taiwan University has been devoted to the development of

low-cost LHP (loop hat pipe). A new manufacturing

process for low-cost LHP has been developed that leads to

a cost down to less than 10 USD for a 100W LHP. The

present study utilizes the low-cost LHP to develop a new

type of solar water heater. A thermosyphon which is

thermally bonded with a solar absorber plate is used to

absorb solar energy and transfer the heat upward to the

evaporator of the LHP. From that, the heat is conducted

downward to the condenser of the LHP which is immersed

in a hot water tank beneath the solar collector. The solar

water heater thus can be designed in integral type with

streamline shape which is easy to install and has a good

outlook as well as high efficiency. A prototype was

designed and fabricated in the present study.

A preliminary heat transfer test for a single unit of

thermosyphon-LHP combination shows that it is capable of

transferring the absorbed energy downward to the LHP

condenser immersed in a hot water tank. The overall

thermal resistance from the absorber plate to water is

0.34

C/W, and the thermal resistance of the LHP is

0.16

C/W. Two prototypes of the solar water heaters with

50 liters and 80 liters hot water tanks were designed

according to the results of the feasibility test. In addition,

an automatic monitoring system was designed and set up

for the performance test of these two solar water heaters

based on the test standard CNS B7277. The measured

overall thermal resistance from solar absorber plate to

water is 0.369

C/W. The daily solar water heater efficiency

is 0.503. The present study has shown that the application

of low-cost LHP in solar water heater is feasible and cost

competitive.

1. INTRODUCTION

Solar hot water heater is important in renewable energy

application. There are three types of commonly-used solar

water heaters: namely, forced-circulation system,

natural-circulation system (known as thermosyphon), and

constant temperature-intermittent output system (known as

once-through system). Among them, the thermosyphon

solar water heater is the most popular one since no

circulation pump or regulating valve is needed. However,

in order to prevent heat loss during night time or raining

days caused by reverse-flow phenomenon, the water tank

must be placed upon the collector [1]. In addition, the

whole solar water heater is manufactured at factory in three

separate parts: solar collector, tank, and supporting frame

(Figure 1). The solar water heater is then installed by

assembling the three separate parts together on site,

including frame fixing on roof and cold and hot water

piping. The installation cost is thus time consuming and at

high cost.

The design of integral-type solar water heater (ISWH) as

shown in Figure 2 can greatly simplify the installation since

the whole system can be manufactured in factory with good

quality control. The whole system is then moved to the

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

679

rooftop and just placed on the floor without additional

fixing work. A few solar water heaters of this type were in

market. However, the thermal efficiency is low since the

downward heat transfer design is poor, usually with daily

thermal efficiency < 0.4. The poor integral-type solar water

heater cannot survive in Taiwan solar market since the

government subsidy program asks all products to have

daily solar thermal efficiency larger than 0.5.

Fig. 1: Natural-circulation solar water heater.

Fig. 2: Integral-type solar water heater design.

Two companies in Taiwan had ever marketed the integral-

type product about 10 years ago, but failed (Figure 3).

Brand A was designed in forced-circulation type, with

water capacity 75 liters which is placed under the solar

collector. The water pump is driven by a solar PV to

circulate the water through solar collector to tank (Figure 4).

This product failed due to the reliability problem of

solar-pumping system.

Brand B is designed in thermosyphon type with water

capacity 150 liters. The solar collector is placed nearly on

floor in order to keep it below the tank (Figure 4). The

vertical distance between the tank inlet and the collector

outlet thus becomes negative and causes flow reversal at

night or cloudy days. The solar energy collection efficiency

is thus very low, < 0.40, and the product cannot survive on

the market. Since then, the integral-type solar water heater

disappeared from Taiwanese market. However, the

installation process of both brand A and B had been

simplified a great deal. Brand A (5 modules, 375 liters)

takes only two man-hour to install. Brand B (300 liters,

single module) is too heavy (>60kg) and took about 6

man-hour to install on the roof.

Fig. 3: Failed ISWH products.

Fig. 4: Failed ISWH design.

The present study intends to utilize the low-cost LHP to

develop a new integral-type solar water heater. Two

prototypes were designed and fabricated in the present

study.

Brand A

Brand B

Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement

680

2. PRELIMINARY TEST OF THERMOSYPHON-LHP

In order to prove that the low-cost LHP can achieve

downward heat transfer to the water tank from solar

collector, a thermosyphon-LHP experiment was first carried

out. A single unit of thermosyphon-LHP device was

designed. The device includes a single strip of tube-in-sheet

solar collector (0.12m

) with a thermosyphon, a LHP and a

water tank set beneath the collector. An electric film heater

is adhered on the solar collector to simulate solar heating in

both summer and winter. The thermosyphon is thermally

bonded with the solar absorber plate and transfers the heat

upward to the evaporator of the LHP. The heat is then

conducted downward to the condenser of the LHP which is

immersed in a hot water tank beneath the solar collector.

The test results (Figure 5) shows that the thermosyphon-

LHP configuration is capable of transferring the absorbed

energy downward to the LHP condenser immersed in a hot

water tank beneath the collector. The overall thermal

resistance from the absorber plate to water is 0.34

C/W, and

the thermal resistance of the LHP is 0.16

C/W.

0.1

0.2

0.3

0.4

0.5

0 50 100 150 200

Power(W)

Thermal resistance(

/W)

system thermal resistance(R +R )

LHP thermal resistance(R )

thermosyphon resistance(R )

Fig. 5: Thermal resistance measurement in a single

thermosyphon-LHP device.

3. LHP-BASED SOLAR WATER HEATER

Applying the low-cost LHP (Figure 6) [2] to solar water

heater, we can effectively transfer the heat downward to the

tank from the solar collector. A thermosyphon which is

thermally bonded with a solar absorber plate is used to

absorb solar energy and transfer the heat upward to the

evaporator of the LHP. From that, the heat is conducted

downward to the condenser of the LHP which is immersed

in a hot water tank beneath the solar collector. The solar

water heat thus can be designed in integral type with

streamline shape which is easy to install and has a good

outlook as well as high efficiency. Figure 7 shows the

design concept. Two models, 80L(Figure 8) and 50L

(Figure 9), were designed.

ISWH-1 was designed for feasibility study to validate the

downward heat transfer capability. ISWH-1 uses 1m

solar

collector and 4 sets of LHP. The tank is of closed-type

which can sustain the tape water pressure. ISWH-2 is

designed with compact and rectangular configuration with

open-type tank.

Fig. 6: Low-cost LHP.

Fig. 7: LHP-based solar water heater.

3 SOLAR COLLECTOR TECHNOLOGIES AND SYSTEMS

681

Fig. 8: ISWH-1.

The solar collector area of ISWH-2 is 0.5m

. ISWH-2 uses

2 sets of LHP. The daily solar energy collection efficiency

is measured according to the outdoor test standard

CNSB7277 [3].

Fig. 9: ISWH-2.

The daily solar energy collection efficiency is 0.503 (Fig.

10) and the heat releasing efficiency of hot water supply is

0.77 as shown in Figure 11. Both exceed the requirement of

the subsidy program. The measured overall thermal

resistance from solar absorber plate to water is 0.369

C/W.

ISWH has been proved at the first time to be able to utilize

low-cost LHP as the downward heat transfer element and

obtain a good efficiency.

In order to understand whether the LHP can perform

normally in the long run, we have carried out a long-term

outdoor experiment. The result indicates that LHP can

function normally; moreover, the daily solar energy

collection efficiency is 0.501 (Fig 12) for the test period of

3 months.

Fig. 10: ISWH-2 thermal efficiency.

Fig. 11: Heat releasing test of the tank.

̌ʳːʳˀ˃ˁ˄ˆˌˇ̋ʳʾʳ˃ˁˈ˃ˆ

˃ˁ˄

˃ˁ˅

˃ˁˆ

˃ˁˇ

˃ˁˈ

˃ˁˉ

˃ˁˊ

ˀ˃ˁˈ ˀ˃ˁˇ ˀ˃ˁˆ ˀ˃ˁ˅ ˀ˃ˁ˄ ˃ ˃ˁ˄ ˃ˁ˅

˫ːʻ˧˼ˀ˧˴ʼ˂˛̇ʳʳʻк̀ ˷˴̌˂ˠ˝ʼ

˘˹˹˼˶˼˸́˶̌ʳӯ

Fig. 12: ISWH-2 thermal efficiency in the long run.

Труды Всемирного конгресса Международного общества солнечной энергии - 2007. Том 2

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