Category Archives: EuroSun2008-10

Comparison of the Thermal Performance of Different Working. Fluids in a Closed Two-phase Solar Water Heating Thermosyphon

A. Ordaz-Flores1, O. Garcfa-Valladares2*, V. H. Gomez2

1 Posgrado en Ingenieria (Energia), Universidad National Autonoma de Mexico, Privada Xochicalco s/n,

Temixco, Mor. 62580, Mexico

2 Centro de Investigation en Energia, Universidad National Autonoma de Mexico, Privada Xochicalco s/n,

Temixco, Mor. 62580, Mexico

* Corresponding author, ogv@cie. unam. mx

A closed two-phase thermosyphon solar system was designed and built to produce hot water for sanitary purposes. The aim of this work is to compare the thermal performance of a two — phase closed thermosyphon using different phase change working fluids (acetone, R134a and R410A). The choice of using a closed two-phase thermosyphon, instead of a conven­tional solar water heating thermosyphons obeys to the some advantages as the lower freez­ing point of the two-phase system compared to water, and elimination of fueling, scaling and corrosion. Disadvantages of these systems are the higher cost because of the working fluid used and the additional coil heat exchanger; moreover, refrigerants reach high pressures. A witness conventional solar water heating system has being installed to compare its perform­ance versus that of the two-phase closed system. The two-phase system consists of a flat plate solar collector coupled to a thermotank by a continuous copper tubing in which the working fluid circulates. The working fluid evaporates in the collector and condensates in the thermotank transferring its latent heat to the water through a coil heat exchanger. The conventional thermosyphon system has the same characteristics (materials and dimensions), with the exception that it lacks the coil presented in the two-phase system. Data were col­lected from the two kind of solar water heating systems, operating simultaneously, and com­parisons of performance were made. Results show that the performance of the two-phase systems is strongly dependent on the load of the working fluid: an optimum point should be found. R134a and R410A show better performance than acetone. The two-phase closed sys­tem shows hardly any difference in performance (when working with both R134a and R410A) compared to the conventional solar water heating thermosyphon.

Keywords: acetone, test, R134a, R410A, phase change.

1. Introduction

The increasing interest of preserving the non-renewable resources has led to focus on sustainable growing, based mainly on using renewable energy. The use of renewable sources helps to save economical expenses, as well as to prevent the inherent environmental impact of conventional sources. Renewable energy sources are the Sun, biomass, hydrogen, wind, etc. The Sun leads to thermosolar and photovoltaic technologies, mainly.

The current paper has special interest in Solar Domestic Water Heating Systems (SDWHS). SDWHS permit to diminish the consumption of liquid gas and electricity, helping to reduce the quantity of pollutants expelled to the atmosphere. In 2004, Kalogirou [1] studied the environmental impact of energy utilisation and the potential benefits to swap conventional for solar assisted sys-

tems. He estimated that, for the case of solar water heating (one of the two most widely used re­newable energy) the savings would reach up to 80%. Hence, the importance of solar water heating.

For instance, in Mexico, the use of flat plate solar collectors to heat 500 L of daily water would yield savings of 433 kg/year of LP gas [2].

The most common currently available solar equipments to heat water are the thermosyphons in which the water is heated in a flat plate solar collector and stored in a thermotank. Active systems use a pump to circulate the water, while in passive systems the water circulates by the thermosy­phon effect. The water presented in the flat plate solar collector is heated by the Sun energy, so its density diminishes; the lower density of the water in the collector, compared to that of the thermo­tank makes the water to circulate: that is the thermosyphon effect. In direct systems, the water is heated in the collector; in indirect systems, some fluid is heated in the collector, and it transfers the energy to the water by means of a heat exchanger; in a closed system, the working fluid is sealed from the atmosphere, in an open system, the heat transfer fluid is in contact with the atmosphere. If the fluid changes its phase in the collector, the system is called a two-phase or a phase-change sys­tem.

The system studied in this paper is a passive, indirect, closed, two-phase system. This kind of sys­tem prevents problems like freezing, corrosion, scaling and fouling [3], which are presented in the conventional systems, increasing the life of the system.

In 1979, Soin et al. [3] described an experimental set up to evaluate the performance of a solar col­lector with a phase change working fluid. They used acetone and petroleum ether as working flu­ids, because of their high boiling and condensation heat transfer coefficient. They demonstrated that the collector efficiency increases linearly with liquid level.

In 1981, Schreyer [4] used a refrigerant, trichlorofluoromethane, to evaluate the energy recovery in a solar collector coupled to a heat exchanger, and the latter to a storage tank. The primary loop was passive and the secondary needed a recirculation pump. His system recovered up to 83% energy at low collector temperature difference.

Evaluation of R134a (among others) as replacing working fluids of ozone depletion promoting chlorofluorocarbons was made by Calm and Didion [5]. They concluded that there is no perfect fluid to prevent every environmental impact. R134a has a high latent heat of vaporization, does not contributes to ozone depletion but, yet low, does have impact on global warming.

Ong and Haider-E-Alahi [6] studied the performance of a heat pipe filled up with R134a, and found that the heat flux transferred increased with high refrigerant flow rates, high fill ratios and greater temperature difference between bath and condenser.

More recently, Hussein [7] studied a two-phase closed thermosyphon with the heat exchanger (condenser) in the solar collector; however, he did not mention the working fluid used. He carried out both experimental and numerical tests and set some dimensionless variables to determine ade­quate storage dimensions for the tank to improve the solar energy gain.

In 2005, Esen and Esen [8] studied a thermosyphon heat-pipe solar collector, to evaluate its ther­mal performance using three different working fluids, R134a, R407C and R410A. They found that the latter offered the highest solar energy collection.

In this work, refrigerants R134a and R410A were chosen due to their availability, low cost and small impact to environment. Acetone is also cheap and available, but it avoids the high pressures reached with the former ones; on the other hand, acetone is flammable.

2. Experiment

A water heating two-phase closed thermosyphon, using either R134a, R410A and acetone as work­ing fluids, and a conventional natural thermosyphon are compared simultaneously. Both systems have the same geometry, except for the coil presented in the two-phase system. The construction materials for the whole system are the same. Each collector has an absorption area of 1.62 m2 and the volume capacity of each thermotank is 160 L. The two-phase system consists of a flat plate solar collector coupled to a thermotank by a copper tubing circuit in which the working fluid circu­lates. A scheme of the systems is shown in Fig. 1.

Focusing on the fluid refrigerant behaviour, the solar collector is the evaporator of the system and the copper coil immersed in the thermotank is the condenser. The incoming solar radiation makes the temperature of the refrigerant in the collector to grow higher to reach the saturation liquid state. From this point, the working fluid starts to evaporate to reach the saturated vapour state and even the superheated vapour zone. As the refrigerant has a higher temperature than the water, the former donates its phase change latent heat to the latter and leaves the thermotank as sub-cooled liquid to come back to the solar collector to repeat the cycle.


Fig. 1. Two-phase closed thermosyphon and conventional thermosyphon.

Refrigerant R134a is one of the replacing working fluids of chlorofluorocarbons since it does not contribute to ozone depletion. R134a evaporates at -26.1 °C at atmospheric pressure [9] with an enthalpy of vaporisation of 216.98 kJ/kg; its freezing point at this pressure is -101 °С.

Acetone (also known as propanone) is a colourless liquid, used mainly as solvent, for cleaning, or as a drying agent; is flammable, and should not be inhaled. At atmospheric pressure, it evaporates at 56.05 °С [10] with an enthalpy of vaporisation of 501.03 kJ/kg and a freezing point of -94.7 °С.

R410A is a mixture of refrigerants R32 and R125 (50% of the volume of each one), it is used in air conditioning as substitute of R22; it is not toxic and does not contribute to ozone depletion; its boiling point at atmospheric pressure is -52.7 °С; its enthalpy of vaporisation is 275.93 kJ/kg. The freezing point of R410A is not determined yet, but the freezing points of its components are -103°C for R125 and -136°C for R32, at atmospheric pressure [9].

The combination of boiling point temperature (the lower, the better) and heat of vaporization (the higher, the better) will show which of the fluids is more suitable for these operating conditions; other parameters as viscosity and pressure must also be considered.

The main disadvantage of R134a and R410A is that they reach high pressures; for instance, the pressure of these fluids at 50°C is 13.18 bar for R134a and 30.71 bar for R410A; their main advan­tage of the refrigerants is their low boiling points; that means that the heat transfer will start soon after the beginning of the test. Acetone does not have problems of pressure: at 50°C, it only reaches 0.81 bar; and its enthalpy of vaporisation is higher related to the refrigerants, but it lacks of a low boiling point at atmospheric pressure: 56.5°C.

The two-phase system was loaded up to 91% when operating with R134a, up to 83% when operat­ing with acetone, and up to 62% when operating with R410A. The systems were loaded differently because of the characteristic of the fluids and the difficulty to load refrigerants. On the other hand, acetone is very easy to load and permits to have better control.

Energy input from the solar loop

The solar heat exchanger is a cupper coil immerged at the bottom of the tank. One or more elements contain this exchanger as the user can decide. We consider that the heat provided by the


solar loop is transferred in a time step to one or more elements depending of their temperature. For example, if the last three elements have a lower temperature than the hot water produced by the solar exchanger than only those three elements will be heated. No influence is considered for the fourth element, during that time step.

The solar panel temperature is computed depending on the operation of the pump: if the pump is not running than

Tpanel(i + 1) = Tpanel(i) + •


• Esun(i + [ • P-K1 • (Tpanel(i) — T0(i + 1))-K2 • (Tpanel(i) — T0(i + 1 ))2

image225 Подпись: (9)

And if the pump is running than

Where mpanel stands for the mass of water contained by the panels in kg, msolar stands for the water quantity (kg) flowing through the solar loop in a time step p, S panel is the active surface of the panels, Tho and Thi stand for the outlet and inlet temperatures of the exchanger, and T0 is the ambient temperature.

• (Tho(i + 1) — T0(i + 1))2 .• Spanel • P

Подпись: Esolar (i + 1) Подпись: Esun(i + 1) •P -K1 • (Tho(i + 1) - T0(i + V) -K2

Using the panel temperature, the control system can decide on the pump operation for the next time step, and finally we can compute the energy input of the solar loop if the pump is on.


Market situations and trends of small scale chillers

The first part of the project was devoted to an analysis of possible markets for Solar Combi+ systems. Since the competing technology are conventional (non-solar) air conditioning systems, the available technological solutions with small cooling capacity as well as their markets in Europe were analysed.

The European Air Conditioning (AC) market has grown rapidly during the last 5 years. The size of AC markets in the seven major European countries (France, Germany, Greece, Italy, Russia, Spain and the UK) expanded from some 2.4 million sold units in 2000 to 5 million sold units in 2004. A further breakdown of the European AC market reveals that Italy and Spain are holding the largest market of about 1.4 to 1.7 million units per year (after 2004), followed by France, Greece and UK at 300,000 to 500,000 units each [5, 6].

Подпись: Fig. 3. Evolution of the air conditioning market of individual air conditioners with a capacity below 17,5 kW in France. Sold units in the years 1998-2003 Подпись: Fig. 4. Share of air conditioners with different capacities on the overall Italian market in the years 2005 and 2006 [7]

For small cooling demands, typically room air-conditioning units or multi-split systems are used, as can be seen from a closer look at the France and the Italian markets can be stated that especially the monosplit units with small capacities are responsible for more than 50% of the overall sold units (see Figure 3 and 4). Application areas for these systems are mainly in smaller buildings such as the trade and residential sector and small office buildings where in the past mainly local solutions were used.

Подпись: Fig. 5. Number of systems sold until Feb. 2008, as reported by the SolarCombi+ industry partners Подпись: Fig. 6. Markets which are considered of high priority by the SolarCombi+ industry partners

However, small chiller systems have an increasing market share in many European countries. These smaller buildings are seen as the most promising target market for solar combi plus systems, which offer a central chiller system powered by solar heat (see Fig.5). The survey among the industrial participants of the SolarCombi+ project also showed that they see the most interesting markets for their Solar Combi+ systems in Spain, Italy and France (see Fig. 6).

System Evaluation

Only in the middle of March 2008 the system was totally uncovered, the pumps fixed and the pressure reset. From that time on, the monitoring process has been more conclusive in the diagnosis of the performance of the STS.

Because the East-facing collector field is hydraulically unbalanced, it reaches a constant value of temperature at least 60° C higher than the totalizing channel. As an example, Fig. 3 shows this behaviour.

The West-facing collector field is well balanced and shows no sign of abnormal behaviour thus the temperature read in the temperature probe is real and representative of the whole west facing collectors.

In April 2008 the STS presented its highest performance level (52%). Since then and because of the leakages previously referred the performance has been steadily decreasing at a pace of around 6% per month (see Fig.4).

Temperature Profile in a Typical Day

—West Col. Output —Inlet Temperature —East Col. Output —Outlet


0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23


Fig. 3. Typical temperature profile for the STS


Fig. 4. Evolution of the performance.

But the system is operating satisfactorily with the right behaviour regarding the two azimuthal blocks of collectors. Fig 5 shows the time operation of both blocks through time of pump operation in a typical day of this last monitoring period. East sector begins first in the morning, then there is a period in the central part of the day with both sectors operating simultaneously and finally, in the last period of the day, only West sector is in operation.

Fig 5. Pump relays vs Flow

The solar DHW system


The simulated building is an apartment house of two storied with 10 housing units. The central

type of DHW supply system was assumed, which composed of solar collector units, a storage tank and a boiler on the roof as shown in Fig.1. As the simulation result of the past study [1-3] showed that the efficiency of the DHW supply system is not strongly affected by the tilt angle and the azimuth of the collector when the tilt angle is from 20 to 40 degrees and the azimuth is from -15 to 15degrees (0 degree means to face to south). In this study, it is designed that the azimuth of the collector is faced to south and the tilt angle is 30 degrees.

Collector area [m2]







































Table 1 Simulation cases of the solar DHW system for the apartment house.

Подпись: Simulation cases As shown in Table 1, four cases of collector area was assumed from 20m2 to 50m2, and six cases of the storage tank was assumed from 0.5m3 to 3.0m3. Furthermore, three cases of DHW supply rate was assumed taking account of the DHW supply rate, which may vary according to family and lifestyle. DHW supply profile data that extracted 10 housing units from measurements of the DHW supply rate of 30 housing units were arranged to the supply rate of 10 minutes. As shown in Fig.2, the DHW supply profile data are prepared for a week and the same data are repeated in the next week. Three cases of DHW supply rate are Case A (average of 241 liters/day for a housing unit), Case B (average of 152 liters /day for a housing unit) and Case C (average of 66 liters /day for a housing unit). In addition, DHW supply rate varies depending on the season as shown in Table 2. The DHW supply temperature is set to 60 degrees C using the auxiliary boiler. The boiler Подпись: Room101 ,1 , J, 1. ill. Room102 ,1 „ 1 1 1 „ . Room103 . . .1 1 ,„ll . . .1 1 Room104 і и. 1 Room105 . .її ,i 1 , II 1. 1 ,1 Room201 і. 1 и. .lllll nil.. .1, ill, .1. 1 .ll Room202 1 . 1 1 . . ,. i,. 1 , , ,1.1 ll. 1 1,1. J . Room203 1 .1 . Il 1. 1. 1. ll ,J. Room204 ...і. 1.1 1, ll . ll .ll., ,1 l.l J . .1 , Room205 ii 1. 1., .ll Lu 1 . LLJ 0 12 24 12 24 12 24 12 24 12 24 12 24 12 24 „ 30

.E 20 1 10 “ 30

.£ 20 1 10 “ 30

.£ 20 1 10 “ 30

.£ 20 1 10 “ 30

.E 20 1 10 “ 30

.E 20 1 10 “ 30

.E 20 1 10 “ 30

.E 20 S 10 “ 30

.E 20 1 10 “ 30

.E 20 1 10 “ 0

Fig.2 DHW supply profile of 10 housing units in the simulation (Case A DHW supply).

Table 2 Simulation cases of DHW supply rate for a housing unit.

Simulation cases

Case A

Case B

Case C

Average of DHW supply rate for a

housing unit [liters/day]






(1/1-3/31, 11/1-12/31)




Spring, autumn (4/1-6/30,10/1-10/31)










Summary and outlook

The directive for optimising the solar collector area and the storage device capacity based on economical and ecological criteria leads to a comprehensive evaluation of the system and its components. The variation of parameters concerning the thermal behaviour as well as the costs of solar collectors has revealed a high potential for reducing both the solar heat generation costs and the primary energy savings while improving the cost/benefit ratio. In opposition to the impact of

collector variations on the dimensioning, the variation of rises in energy prices does not alter the resulting dimensions; however the additional costs are strongly dependent on the rise in energy prices. Bearing in mind that this study was performed on a theoretical MaxLean system concept, the general validity of the conclusions drawn have to be confirmed in further studies involving standard market solar systems and current trends within the energy market.

Подпись: be negligible. For consistency reasons in this case only the influence of a reduction of the insulation thickness in comparison to the base case is presented.

Table 3. Influence of the varied parameters on the cost/benefit ratio and the dimensioning of the system. The symbols ++, + and о indicate a strong, medium or low sensitivity. (1) range of the additional costs within the variation; (2) range of the primary energy savings within the variation; (3) resulting solar collector area within variation (4) resulting storage device capacity within variation


The financial support of the Swiss Federal Office of Energy is gratefully acknowledged. Parts of

the work presented have been contributed to the IEA SHC Task 32. The authors would especially

like to thank the members of Subtask D for the fruitful discussions.


[1] Haberl, R., Vogelsanger, P.: Technical Report of IEA SHC Task 32 Subtask D, Simulation and Optimization of the MaxLean System, http://www. iea-shc. org/task32/publications/task32- MaxLean_Concept. pdf, 2007.

[2] Antony, F. et. al., Solarthermische Anlagen, DGS, Deutsche Gesellschaft fur Sonnenenergie, 2004.

[3] Haberl, R., Vogelsanger, P., Frank, E.: Dimensionierung solarer Kombisysteme, OTTI Symposium Thermische Solarenergie, Tagungsband, Bad Staffelstein, 23. — 25. April 2008, S. 422 — 427.

[4] Institut fuer Solartechnik SPF: Collector data base on SPF InfoCD-ROM, 2008, Collector factsheets also available on http://www. spf. ch.

[5] Huttmann, M. et al.,: Marktubersicht Solarspeicher 2007, solid GmbH, Furth, 2007.

[6] Schulz, W. et al.: Energiereport IV, Die Entwicklung der Markte bis zum Jahr 2030, Prognos AG, Koln, Basel, http://www. prognos. ch/pdf/Energiereport%20IV_Kurzfassung_d. pdf, 2007.

[7] Statistisches Bundesamt: Daten zur Energiepreisentwicklung — Lange Reihen von Januar 2000 bis Mai 2008, Wiesbaden, http://www. destatis. de, publication date: 26.06.2008.

Test Procedure

During the development of the test, extractions of hot water are not carried out. The SDWHS is filled with water trying to maintain the inlet water (T0) at the same temperature. The test begins at 9:00 h (solar time) when the data acquisition system is switched on and the experimental data are registered every 60 seconds. At the end of the solar journey at 18:00 h, the valves of the solar loop are closed and the small recirculation pump is turned on in order to homogenize the temperature in the storage tank (Tf); after that the day test is finished.

In order to evaluate the thermal losses during the night period, the two valves of solar loop are opened again. The following day at 8:30 h, the temperature of the storage tank is homogenised again in order to obtain the final night temperature (Tf, 24h) and finally the system is completely empty. This test procedure is realised during several days in order to obtain the system characteri­sation under different weather and working conditions.

The schematic representation of experimental apparatus for test procedure system is shown in Fig. 2. It has two different loops: the first one is the solar loop that includes the solar collector and stor­age tank. Three temperature sensors are inserted in the following positions: 1/4, 1/2 and 3/4 of the internal tank height. The three temperature sensors are used for two reasons: a) to obtain the strati­fication profile in the storage tank along the test and b) to determine when the homogenised tem­perature in the storage tank is reached. The second loop is the recirculation one, which is a closed circuit where a small pump permits the quick circulation of the water contained in the storage tank in order to homogenise the temperature. The homogenised temperature in the storage tank is reached (and the recirculation pump is turned off) when the three temperature sensors inside the tank vary less than 0.5 K (in the thermosyphon system tested, the time used in this procedure was lower than 5 minutes). Additionally the ambient temperature is measured, a global solar irradiance sensor is also integrated on the collector plane and an anemometer is also installed in order to measure the wind direction and speed [11].

The tests were carried out in the Solar Platform of the Centro de Investigation en Energia of the Universidad Nacional Autonoma de Mexico, located in Temixco, Morelos State, Mexico, at 18°50.36’ N latitude and at 99°14.07’ W longitude, with an altitude of 1219 m over sea level. The yearly average ambient temperature is 23.09 °C with a yearly average solar irradiance on the hori­zontal plane of 20.28 MJ/m2. The tests began from May 2007 to May 2008.

3. Results

Fig. 3 shows the radiation and the stratification profile of the water temperatures of the experimen­tal systems for one day of test; the two-phase system working with R134a. The morning and the early afternoon were sunny until around 3:00 pm with an average solar radiation on the collector plane of 546.44 W/m2.



Fig. 3. Radiation and temperature profile for one day of test, using R134a as working fluid.

The cloudy period seems to diminish the gain of heat in the conventional system more than in the phase change system. After the irradiation period (at 18:00 h), the temperature in the thermotank has increased around 21 °C to reach a temperature of 50.3 °C for both the conventional system and the two-phase working with R134a.

The efficiency and useful heat of the system are calculated from the equations:

r/ = 100 x — 4r


Подпись: Where n is the efficiency, qu is the useful heat [J] and qr is the energy received from the sun [J],

qr — (I)( AbXAt)

Where I is the irradiance on the collector plane [W/m2], Aabs of operation of the system [s],

qu — (mH2O )(Cp, H2O ‘)(ATH2O )

Where mH2O is the mass of the water [kg], cpH20 the specific heat at constant pressure [J/kgK] and ATmO=Tf-T0 [K].

On the other hand, the thermal losses during the night are calculated from the equation:

Us — mcp, n (Tf,24h ~ Tf )

T — T

f 1 amb, n (4)

The letter n stands for the night period; Tf is the available temperature at the end of the solar irradi­ance period [K], Us the thermal losses during the night [J/K] and Tambn the ambient temperature.

It can be seen from Table 1 that acetone has a lower performance; however more tests must be car­ried out to obtain a solid conclusion to this question; according to Soin et al. [3] the system must be loaded with a larger quantity of the working fluid to obtain a better performance.

Table 1. Performance of the two-phase (acetone) and the conventional system.


Water system








qu [MJ]


12.9± 1.8

Tf, d[K]



A T [K]






NOTE: Rounding of mean values was done according to standard deviation data.

Table 2 summarises the efficiencies and other parameters of the two-phase thermosyphon working with R134a and the conventional thermosyphon. Although the two-phase system exhibits a slightly better performance, a simple analysis indicates that the efficiencies are statistically equivalent.

Table 2. Performance of the two-phase (R134a) and the conventional system.


Water system








qu [MJ]


14.5± 1.3

Tf [K]



A T [K]


21.7± 1.9

Us [MJ/K]



NOTE: Rounding of mean values was done according to standard deviation data.

Comparison of the performance of the two-phase system using R410A, versus the performance of the conventional thermosyphon that uses water is shown in Table 3. As in the R134a case, both systems exhibit statistic equivalent performances, with an average increment of 21 °C of tempera­ture in each case, showing efficiencies of approximately 51 %.

Table 3. Performance of the two-phase (R410A) and the conventional system.


Water system








qu [MJ]



Tf, d[K]



A T [K]






NOTE: Rounding of mean values was done according to standard deviation data.

Although R134a and R410A were not tested at the same time, these data suggest that their per­formance may be equivalent. However, R134a works at significant lower pressures than R410A.

The system, when working with R410A reached consistently pressures near 38 bar. If water is re­moved from the thermotank, then the fluid cannot transfer its heat completely, and the pressure grows so high that the tubing resistant limit is reached, so the tubing can burst open at any mo­ment. Because of this, in our opinion, use of R410A is not recommended for this kind of system. To exemplify this, Fig. 4 shows the performance of the fluid temperature and the pressure for the two-phase system, working with R410A. Pressure reaches values near 36 bar.


4. Conclusions

A two-phase closed thermosyphon using R134a, acetone and R410A as working fluids was com­pared with a conventional natural solar collector thermosyphon. In tests, the two-phase system working with R134a showed statistically equivalent performance than the conventional system, though the former eliminates problems of freezing, fouling, scaling and corrosion. The two-phase system working with acetone showed a slightly lower performance; however, it is expected to im­prove after new tests with major loads of acetone in the closed circuit of the system and also, after trying vacuuming the system. The two-phase system working with R410A showed statistically equivalent performance than the conventional thermosyphon. Both R134a and R410A show good performance as working fluids, and in their respective comparisons with the thermosyphon system; however, the lower pressure reached by the R134a makes it more attractive as a working fluid.

5. Acknowledgements

This work has been partially financed by PAPIIT project (IN-111806-3) and CONACyT project U44764-Y and Modulo Solar S. A. de C. V. company. The authors thank to CONACyT for the sup­port provided to the student with the scholarship number 183846.


[1] S. A. Kalogirou, Environmental Benefits of Domestic Solar Water Heating Systems, Energy Conversion & Management 45 (18-19) (2004) 3075-3092.

[2] www. funtener. org

[3] R. Soin, K. Sangameswar Rao, D. Rao, K. Rao, Performance of a Flat Plate Solar Collector with Fluid undergoing Phase Change, Solar Energy 23 (1) (1979) 69-73.

[4] J. M. Schreyer, Residential Application of Refrigerant-charged Solar Collectors, Solar Energy 26 (4) (1981) 307-312.

[5] J. M. Calm, D. A. Didion, Trade-offs in refrigerant selections: past, present and future, International Journal of Refrigeration 21 (4) (1998) 308-321.

[6] K. S. Ong, M. Haider-E-Alahi, Performance of a R-134a-filled Thermosyphon, Applied Thermal Engi­neering 23 (18) (2003) 2373-2381.

[7] H. M. S. Hussein, Optimization of a Natural Circulation Two Phase Closed Thermosyphon Flat Plate Solar Water Heater, Energy Conversion & Management 44 (14) (2003) 2341-2352.

[8] M. Esen, H. Esen, Experimental Investigation of a Two-phase Closed Thermosyphon Solar Water Heater, Solar Energy 79 (5) (2005) 459-468.

[9] REFPROP version 8.0. Reference Fluid Thermodynamic and Transport Properties, NIST Standard Ref­erence Database 23, Lemon E. W., McLinden M. O., Huber M. L., USA (2008).

[10] N. v. Solms, M. L. Michelsen, G. M. Kontogeorgis, Applying Association Theories to Polar Fluids, Ind. Eng. Chem. Res., 43 (2004) 1803-1806.

[11] O. Garcia-Valladares, I. Pilatowsky, V. Ruiz, Outdoor Test Method to Determine the Thermal Behavior of Solar Domestic Water Heating Systems, 82 (2008) 613-622.

Energy transfer when pouring

The energy of the hot water poured by the customer is defined as:

Ethw(i) = mthw(i) • CP • (Tthw — Tcw) (1 1)

Подпись: E(i + 1,n) Подпись: 1 - Ethw(0 E(i n + a -1) + Ethw(l) • E(i,n + a) - E(i,n) E(i,a)) E(i,a) Подпись: (12)

Where thw stands for tap hot water and cw is cold water. The quantity of hot water poured from the tank during one time step can be higher or lower than the mass of water contained in one element. Never the less, the next relationship for the energy of an element n stands.

Where a is the number of elements providing the hot water for the pouring, 1 or higher. If n+a is larger then the total number of elements, N, than cold water is added to the bottom elements.

3.2 Model validation

To valid our numerical model a comparison is done with a commercial solar thermal software TSol and with experimental data. The computation time step p is one hour. Identical systems are compared in both cases located in two cities: Paris and Grenoble. The present model over estimates the performance of the system with 1.4% in Paris and 1.3% in Grenoble when comparing with TSol, and with less than 3% in both cities when comparing with experimental data.

4. Results and comments

Solar thermal markets in Europe

image177 image178 image179

Currently, solar thermal markets are growing all over Europe (see Fig. 7), even if with different path and different focus: Germany is still the largest market; Austria and Greece are among countries with the highest per capita collector area, but while in the first solar combi systems become increasingly important (35% of installed area), in the latter dominate thermosiphonic systems for DHW; other southern European countries as e. g. Spain and France are catching up now. Solar combi plus systems have a large potential here because systems can be used year around for DHW, pool heating, space heating and last but not least cooling. Although small solar combi plus systems are relatively new to the market, sales are rapidly increasing. The industrial partners involved in this project have already installed more than 130 systems all over Europe.

Fig. 8. Answers to the question, whether consumers ask
retailers about energy efficiency

Owners satisfaction

As a result of system follow-up it was possible to visit the system and talk with some of the owners which lead to the detection of some problems:

Tenants refer to be happy with the general performance of the system since spring 2008 and to have no need to use the back up boiler for domestic hot water.

But there are some complaints regarding the information on how to correctly use the STS.

In some cases there is also high gas consumption for tenants that have the boiler regulated for higher consumption temperatures. We noticed that the reason comes from the fact that the three way valve that should prevent heat transfer in the wrong direction was not installed.

The lack of a maintenance contract has brought problems in the relation between the contractor and the owner of the installation, based on claims of violation of the terms of guarantee. This aspect is also affecting the actual dwelling owners because they will have to support now the costs of such contract, which is absolutely necessary and mandatory, nowadays, in accordance with the new building code.

In the process, a questionnaire was decided to do to the dwellings owners. The questionnaire focused the establishment of the consumption profile of the tenants (including number of people per apartment, usage schedule, central heating use, total gas consumption and gas fired utilities), to assess their satisfaction level and suggestions for improvement of systems performance.

Unfortunately most of them did not answer until the moment and because of that is not possible to present the results. Anyway this aspect is becoming more and more important and it should be taken into account, since the first beginning of the monitoring period, in order to have detailed qualitative information in parallel with the quantitative one.

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

3. Conclusion

Although some problems are not satisfactorily solved, the solar thermal system has been delivering enough energy to have an interesting overall solar fraction which makes the dwelling owners happy with its performance since spring 2008. Some of them refer to have no need to use the boiler. This qualitative result matches with the 40% efficiency collection that we are measuring now.

Most of the problems found in this installation would have been avoided if the commissioning process of the total heating system had been executed by the same team, avoiding by this way the misunderstandings and problems between EPUL and installer, that occurred later and contributed to the delay on finding and executing the right decisions, proposed by INETI.

Another important source of problems was the bad solution implemented by the promoter related with the maintenance contract, which was not part of initial contract with the installer and it was supposed to be done between installer and the dwelling owners grouped in a condominium. This solution delayed the execution of such contract for system maintenance, contributing to the actual situation. So, the experience of this installation shows the importance of a warranty maintenance contract that the actual RCCTE code imposes in connection with the actual solar obligation.


[1] J. A. Duffie, W. A. Beckman (1980). Solar Engineering of Thermal Processes, John Wiley & Sons, New York.

[2] NetPlan (March 2007), Manual do Sistema Solar Termico, Empreendimento EPUL Telheiras XXI.

[3] INETI (January 2004), Estudo de Viabilidade Tecnico-Economica de Instalagao Solar Termica para produgao de Aguas Quentes Sanitarias em fracgoes autonomas de habitagao Lote 1 — Telheiras Norte III.

[4] RESOL DeltaSol® ES, Mounting, Connection, Handling, Fault localization, Examples.

[5] ServiceCenter Software Suite for controller configuration and visualization: Installation, Operation.

[6] RESOL Data logger DL1: Mounting, Connection, Operation, Fault localization.