Findings to improve the performance of a two-phase flat plate solar system, using acetone and methanol as working fluids

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Solar Energy 86 (2012) 1089–1098 www.elsevier.com/locate/solener

Findings to improve the performance of a two-phase flat plate solar system, using acetone and methanol as working fluids A. Ordaz-Flores a,⇑, O. Garcı´a-Valladares b, V.H. Go´mez b a b

Posgrado en Ingenierı´a (Energı´a), Universidad Nacional Auto´noma de Me´xico, Privada Xochicalco S/N, Centro, C.P. 62580 Temixco, Morelos, Mexico Centro de Investigacio´n en Energı´a, Universidad Nacional Auto´noma de Me´xico, Privada Xochicalco S/N, Centro, C.P. 62580 Temixco, Morelos, Mexico Available online 5 December 2011 Communicated by: Associate Editor C. Estrada-Gasca

Abstract An indirect two-phase water heating solar system was tested using acetone and methanol as working fluids. The working fluid circulates in a closed circuit that extends from the solar collector to a coil heat exchanger in the thermo tank. The working fluid evaporates in the solar collector and condensates in the thermo tank coil. This Phase Change System (PCS) prevents freezing, scaling, corrosion, and fouling; these advantages increase the lifetime of the system. The objective of this work is to characterise the performance of the PCS using different filled fractions of acetone and methanol, with two kind of initial pressures (atmospheric pressure, and partial vacuum), in order to find the appropriate conditions for a good performance of the system. For this purpose, the useful heat was determined, as well as the increment of temperature in the water of the thermo tank, and the experimental efficiency. Results are compared to a witness conventional Domestic Solar Water Heating System. The witness has the same characteristics (materials and dimensions) than the PCS, except for the coil heat exchanger presented in the PCS. The instrumentation set throughout the system includes temperature sensors, pressure transducers, a pyranometer and an anemometer, that permit to characterise and understand the performance of the system under different working conditions. By knowing the phenomenology of the working fluid in the closed circuit, the stratification profile of the water in the thermo tank, and the thermal performance of the solar collector, projections to improve the PCS can be formulated. The performance of the system is the result of several variables working together in combination: the working fluid, filled fraction, partial vacuum, coil length, as well as the working and ambient conditions. The appropriate combination of these variables is investigated to improve the performance of the PCS. Experimental results showed that the partial vacuum conditions at the beginning of the test led, as expected, to an improved performance of the PCS. Tests were carried out under the actual field conditions of Temixco, Me´xico. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: Phase change; Heat transfer; Indirect system; Solar collector; Thermosyphon; Water heating

1. Introduction In Me´xico, the most common Domestic Solar Water Heating Systems (DSWHS) used to produce hot water from sustainable sources of energy are the direct thermosyphon systems, in which the water is heated in a flat plate solar collector and stored in a thermo tank. Despite Me´xico has a wide solar potential, its use to heat water is ⇑ Corresponding author.

E-mail addresses: [email protected] (A. Ordaz-Flores), ogv@cie. unam.mx (O. Garcı´a-Valladares), [email protected] (V.H. Go´mez). 0038-092X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2011.10.031

not quite spread. However, recent Mexican Policies (CONAE, 2007) are set to encourage the extension of the current installed capacity from 1.16  106 m2 (SENER, 2008) to 1.8  106 m2 by 2012. In Me´xico, problems of these conventional direct thermosyphons come from the weather conditions and municipal raw water. For instance, seasonal freezing is common in some regions of the north of Me´xico, where anti-freezing devices have to be installed to protect the collectors, increasing the initial cost. Moreover, these anti-freezing valves have mechanical parts, and can fail after a few years of use. Also, hard waters constitute a primary issue in several regions of Me´xico, as the

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Nomenclature Aabs cp GWP hfg I m ODP Pabs Patm Pi

absorber area (m2) specific heat of water at constant pressure (kJ kg1 K1) Global Warming Potential (dimensionless) latent heat of vaporisation (kJ kg1) average solar irradiance on the plane of the collector (W m2) mass of water in the thermo tank (kg) Ozone Depletion Potential (dimensionless) absolute pressure (bar) atmospheric pressure (bar) pressure in the PCS’s closed circuit at the beginning of the tests (bar)

Yucata´n Peninsula, and in several parts of the world (Xinjian et al., 2007; Solar Center Information, 2002; Arizona Solar Center, 2011), where the hard waters, with high concentrations of calcium oxide contribute to block the small water pipelines (for example, the risers) used in flat plate solar collectors. Other water characteristics, as the high levels of PH and minerals lead to corrosion, scaling or fouling. To prevent these problems, an indirect Phase Change System (PCS) thermosyphon is investigated. In the PCS, the working fluid is evaporated in the collector and condensed in a coil heat exchanger immersed in the thermo tank, transferring its latent heat of vaporisation to the water. Early investigations with PCSs, led by Soin et al. (1979) showed that the collector efficiency is linearly increased with the increment of liquid level, and that the energy collection is improved at low collector temperature (Schreyer, 1981). A detailed literature review was previously performed (Ordaz-Flores et al., 2011). Different working fluids have been investigated, such as R11 (Schreyer, 1981), R113 (Davidson et al., 1989, 1990) or R123 (Davidson and Walker, 1992,), as well as mixtures of refrigerants (Aziz et al., 1999). In addition, different geometries of two-phase thermosyphons have been studied (Ku¨rklu¨ et al., 2002; Farsi et al., 2003; Khodabandeh, 2004; Esen and Esen, 2005; Islam et al., 2005; Ordaz-Flores et al., 2011). The thermal performance of a two-phase thermosyphon flat plate solar collector, under sky clear conditions, using different refrigerants was investigated by Esen and Esen (2005). They constructed three small-scale solar water heating systems, and evaluated R134a, R407C and R410A as working fluids. They found that the latter offered the highest solar energy collection. Islam et al. (2005) reported two singular tests, with acetone and methanol, in a solar collector PCS, for a whole day in each case. They calculated the instantaneous hourly efficiency and heat removal factor, and found methanol to yield the best performance from their tests. Most of the working fluids used previously (like the refrigerants R134a or R410A) have low boiling points at atmospheric pressure (below 0 °C) (NIST,

Psat qr qu T amb Tf T0 DT Dt g k

saturation pressure (bar) energy received (MJ) useful heat (MJ) ambient temperature (°C) average final temperature (°C) average initial temperature (°C) difference of temperature, T f  T 0 (°C) time length of the test (s) efficiency (%) wavelength (lm)

2007), and low latent heat of vaporisation (below 250 kJ kg1 at 0 °C at best). See Table 1 to compare some thermophysical properties of these working fluids; both acetone and methanol have higher latent heat of vaporisation than R134a and R410A. Methanol and acetone work at pressures near the atmosphere, while R134a and R410A work at very high pressures, as seen in Table 1. The high operating pressures of R134a (13.2 bar@50°) and R410A (30.7 bar@50°) might compromise the technical feasibility and integrity of this system. Then, working with acetone and methanol converge on the safety of the equipment. Furthermore, the thickness of the tubes could be reduced and the construction could be easier; this, because the need to construct the systems to bear high pressure would become redundant, and ordinary welded joints could be used instead of special and more expensive ones. According to the Copper Development Association 2011a,b both acetone and methanol can be used with copper without corrosion problems. Although boiling points of acetone and methanol at atmospheric pressure are higher than those of R134a and R410A, they can be compensated by their higher latent heat of vaporisation (558.8 kJ kg1 at 0 °C for acetone, and 1205.1 kJ kg1 at 0 °C for methanol, as seen in Table 1). 1.1. Environmental aspects The Global Warming Potential (GWP) (United Nations Framework Convention on Climate Change, 2011; Houghton et al., 1996; Collins et al., 2002; Calm and Domanski, 2004; Solomon et al., 2007; Boucher et al., 2009) is a parameter widely used to express how much heat a greenhouse gas traps in the atmosphere. It is a relative measure that compares the quantity of heat trapped by CO2 (or another fluid of comparison) with the fluid in question for the same mass, for a time interval. GWP and lifetime of the fluids used in this work are summarised in Table 2, being the GWP of CO2 fixed to 1 for 100 years. Through the GWP, acetone (GWP = 0.5) and methanol (GWP = 2.8) have

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Table 1 Thermodynamic properties of the working fluids used (NIST, 2007). Working fluid

Tfreezing@Patm (°C)

Tboiling@Patm (°C)

hfg@0 °C (kJ kg1)

hfg@50 °C (kJ kg1)

Psat@50 °C (bar)

Acetone Methanol R134a R410A

94.7 97.7 101 (136, 103)a

56.5 64.4 26.1 52.7

558.8 1205.1 198.6 221.3

508.1 1127.9 151.8 135.1

0.8 0.55 13.2 30.7

hfg: latent heat of vaporisation; sat: saturation; atm: atmospheric. a The freezing points of R32 and R125, the components of R410A.

Table 2 Lifetimes and global warming potentials of acetone, methanol, R134a and R410A. Working fluid

Lifetime

GWP (100 years)

Acetone Methanol R134a R410A

1–10 days 17.8 days 14.6 years –a

0.5 (Collins et al., 2002) 2.8 (Collins et al., 2002) 1300 (Houghton et al., 1996) 1730 (Devotta et al., 2001)

a As the components of R410A, R32 and R125, separate in the atmosphere, lifetime of this blend is not accounted, but the lifetimes of R32 and R125 are 4.9 years, and 29 years, respectively (Solomon et al., 2007).

negligible effect on global warming, compared with R134a (GWP = 1300) and R410A (GWP = 1730). Besides, lifetimes of acetone (up to 10 days) and methanol (17.8 days), are very short in the atmosphere. Another environmental parameter to consider is the Ozone Depletion Potential (ODP). The ODP of a chemical compound is the relative amount of degradation to the ozone layer it can cause. Being trichlorofluoromethane (R11) fixed at an ODP of 1.0, both R134a and R410A have an ODP ~0 (Calm and Domanski, 2004). For acetone and methanol there are not even ODP associated. Acetone evaporates rapidly and it is degraded by UV light in the short period mentioned earlier. Gathering all the reasons mentioned above (latent heat of vaporisation, working pressures, GWP, ODP), acetone and methanol were chosen as working fluids for the purposes of this work. 1.2. Objectives The objective of this work is to characterise a PCS using acetone and methanol, and find the most suitable of these working fluids for a better performance of the studied PCS. For this purpose, the PCS is characterised using acetone and methanol through several filled fractions in the PCS’s closed circuit, two kind of initial pressures (atmospheric pressure and partial vacuum), and two lengths of the coil heat exchanger. The filled fraction is considered as the percentage of the closed circuit filled with the working fluid in a liquid state. It is an objective of this work to find the filled fraction at which the PCS offers the best performance, for each working fluid, and for each coil length. The working fluid is initially charged into the PCS to a filled fraction shown in Table 3. For each filled fraction

Table 3 Filled fractions of the PCS tested, for both coil lengths 6 m and 10 m, for the two working fluids used, acetone and methanol. Acetone

Methanol

Coil length (m)

6

10

6

10

Pi = 0.46 bar (abs) Filled fraction

0.48–0.69

0.37–0.53

0.66–0.69

0.50–0.53

Pi = 0.87 bar (abs) Filled fraction

0.48–0.69

0.37–0.53

0.66–0.69

0.50–0.53

series of tests, two different initial pressures were investigated. One series of tests began at atmospheric pressure (the standard atmospheric pressure of Temixco, Me´xico is Pi = 0.87 bar (abs)). For the other series of tests, air was extracted from the PCS’s closed circuit, to reach a partial vacuum of Pi = 0.46 bar (abs). This process of air extraction was performed only once for each filled fraction. As the system is closed and sealed, the vacuum remained, and the tests with the PCS started at the same conditions every morning. The partial vacuum was useful to reduce the boiling point of acetone and methanol (in each case) during the first hours of operation of the system. Two prototype condensers (coil heat exchangers), of 6 and 10 m, are tested to investigate the effect of length for each working fluid used in the PCS. Each condenser is a vertical coiled tube of copper, that extends through the tank length, with the characteristics shown in Table 4. Summarising, the objective of this work is to find, for each working fluid, the appropriate combination of coil length, filled fraction, and initial pressure, that produce the best experimental result, for the investigated PCS. 2. Experimental procedure The experimental procedure was previously discussed in detail (Ordaz-Flores et al., 2011 and Garcı´a-Valladares et al., 2008). Briefly, the PCS consists of a solar collector, a thermo tank and an internal closed circuit that extends throughout the system, in which the phase change working fluid resides. In the primary circuit (see Fig. 1), the working fluid evaporates in the solar collector, moves by natural convection to the thermo tank, situated above the collector, where it is condensed in the immersed coil heat exchanger, transferring its latent heat of vaporisation to the water; then, the working fluid moves back by gravity to the solar

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Table 4 Geometric and technical characteristics of the PCS. Solar collector Type of collector Number of fins and tubes Material of fin and tubes Selective surface Risers and headers diameters Cover Absorber area Insulation

Flat plate 6 Copper Titanium oxide 3/800 and 3/400 6 mm Cellular polycarbonate 1.73 m2 (1.94 m length  0.89 m wide) 100 (25.4 mm) Polyurethane

Thermo tank Material Internal diameter Insulation Capacity Position

Stainless steel 0.48 m 100 (25.4 mm) Polyurethane 160 L Horizontal

Condenser Material Diameter Length Arrange

Copper 1/200 (12.7 mm) 6 m, 10 m Vertical coil installed in the lower part of the thermo tank

Volumes of the whole closed circuit 6 m coil PCS 10 m coil PCS

3.78 L 2.89 L

collector to repeat the cycle. In the homogenisation loop, a pump was used to homogenise the water in the thermo tank, to obtain the average temperatures at beginning and end of the solar irradiation tests to calculate the temperature increment, useful heat and experimental efficiency of the system. In this work, two different fluids are used, acetone and methanol, to intent to obtain a good performance of the investigated PCS, and reduce the high pressures reached with the fluids used previously: R134a and R410A (Ordaz-Flores et al., 2011). As the new working fluids are in liquid state at Patm and Tamb, it is possible to extract air from the closed circuit and reach a partial vacuum just after each fluid charge. According to the initial pressure in the closed circuit, two kind of tests were performed: tests that began at atmospheric pressure (Pi = 0.87 bar (abs)), and test that began at a partial

vacuum (Pi = 0.46 bar (abs)). Reducing the pressure in the closed circuit was important to decrease the boiling point of the working fluid. Two prototype coil lengths were tested: 6 m and 10 m. Several filled fractions were tested to find the most appropriate for the evaluated PCS, and for each working fluid. RTD Pt-100 temperature sensors and pressure transducers were installed in the fluid circuit to verify the fluid’s working conditions at different points of the system: at the collector inlet, and the coil heat exchanger inlet and outlet. Another RTD Pt-100 was installed to monitor the ambient temperature. Three type “J” thermocouples were installed at different heights in the thermo tank, to assess the stratification profile and recognise when the water was homogenised both at the beginning and the end of the tests. A Class II Spectral Pyranometer was installed on the same plane of the solar collector to measure the instantaneous solar irradiance; an anemometer was also set up to obtain the wind speed and direction. Data on propagation of errors, developed from the uncertainties of the measuring instruments, are shown in Table 5. A conventional DSWHS with the same geometric and technical characteristics (shown in Table 4) was installed as witness. The DSWHS is a direct system that operates with the thermosyphon effect: the water density in the collector is reduced when the water is heated; the difference of density with the water in the thermo tank produces a natural circulation cycle in which the water is heated in the collector and stored in thermo tank. Two prototype heat exchanger coil lengths were investigated: 6 m and 10 m. For each one, the PCS was loaded with several filled fractions of acetone and methanol. For the 6 m coil PCS, acetone filled fractions were from 0.48, that corresponded to 1.4 L of the capacity of the two-phase closed circuit, to 0.69 (2.0 L). Methanol filled fractions were from 0.55 to 0.69 of the capacity of the closed circuit. For each filled fraction, tests beginning at atmospheric pressure, and tests beginning at partial vacuum were performed. Table 3 shows the filled fraction intervals tested for each system. After the initial charge of working fluid in the PCS air was extracted from the closed circuit, to reach a partial vacuum and reduce the boiling point of the fluid. A fluid’s lower boiling point presents some advantages for this system. First, the collector operates at lower temperatures, preventing higher heat losses. Second, the evaporationcondensation recirculation cycle starts earlier, and hence

Table 5 Uncertainties of the measuring instruments.

Fig. 1. Scheme and instrumentation of the PCS.

Instrument

Uncertainty

Range

Pyranometer Type “J” thermocouple RTD Pt-100 Pressure transducer

±1% of the measurement ±0.5 °C ±0.5 °C ±1% of the total range

k: 0.285–2.8 lm 0–723 °C 200–500 °C 0.0–34.5 bar

k: wavelength.

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the heat transfer to the water in the thermo tank. Third, the latent heat of vaporisation is greater at lower temperature and pressure of saturation, so more energy is transferred to the water. For instance, in Table 1, the latent heat of vaporisation of acetone at 0 °C is 558.8 kJ kg1, but it is reduced to 508.1 kJ kg1 at 50 °C. A similar analysis can be made for any of the other fluids in this table. For the tests with partial vacuum, air was extracted from the PCS’s closed circuit to reach an absolute pressure of Pi = 0.46 bar. At Pi = 0.46 bar (abs), the boiling point of acetone is reduced from 55.7 °C (at Pabs = 1 atm) to 31.9 °C; while the boiling point of methanol is reduced from 64.1 °C (at Pabs = 1 atm), to 43.1 °C at Pi = 0.46 bar (abs). To run the tests, the systems were exposed to the Sun from 9:00 to 18:00 h (solar time), without extractions during the development of the test. At 9:00 h (solar time), the data acquisition system was turned on; the experimental data were registered every 60 s. At the end of the solar irradiation day, at 18:00 h (solar time), the small recirculation pump was turned on in order to homogenise the temperature in the thermo tank. The homogenised temperature in the thermo tank was reached (and the recirculation pump was turned off) when the three type “J” thermocouples inside the thermo tank varied less than 0.5 C; this marked the end of the solar irradiation test. This procedure was repeated during several days in order to obtain the system characterisation curve under different weather and working conditions. The tests were performed in the Solar Platform of the Centro de Investigacio´n en Energı´a of the Universidad Nacional Auto´noma de Me´xico, located in Temixco, Morelos, Me´xico, at 18°50.360 N latitude and at 99°14.070 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 were carried out from April, 2010 to April, 2011.

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the thermo tank is reached. Either these stages take place or not depends on the configuration of the system and the working fluid used. Fig. 2 shows the temperature profile for the first hours of operation of the PCS working with acetone; tests beginning at atmospheric pressure. Three stages take place during these hours of operation: morning warm-up, pressurising stage and run stage. Morning warm-up takes place in the first 30 min of operation, approximately. The fluid reaches a temperature of 56.5 °C, while the pressure is barely affected. The pressurising stage takes another 40 min to complete and continues until the pressure reaches around 1.25 bar and the temperature around 66 °C. The high temperature needed to reach the saturation conditions makes the heat transfer to start long after the beginning of the test, delaying the major heat transfer of latent heat from the fluid to the water in the thermo tank. For this reason, air was extracted from the PCS’s closed circuit: if the partial vacuum reduces the boiling point of the working fluid, the evaporation–condensation cycle will be accelerated, and the PCS’s performance will be expected to improve. After the pressurising stage, the run stage starts. During the first part of the run stage, there is a period of stabilisation in which the pressure and temperature are struggling to reach the appropriate conditions to repeat the cycle continuously. Though, this period of stabilisation is not necessary for the fluid to completely evaporate in the collector and condensate in the thermo tank, transferring the latent heat of vaporisation to the water; it is just a short period in which the cycle is not continuous yet. In that period there is some minor increase in the water temperature. When the run stage is continuous, the major temperature increase takes place in the water of the thermo tank. In Fig. 3, the profile of a test with methanol as working fluid is shown. Prior to the tests, air was extracted from the PCS’s closed circuit, to reduce the pressure. Because of the partial vacuum, the evaporation–condensation cycle starts

3. Mode of operation of the PCS The mode of operation of the PCS closed circuit was previously discussed in more extension (Ordaz-Flores et al., 2011). Morning Warm-Up, Pressurising Stage, Run Stage and Evening Shutoff are the stages characteristic in the present system. The morning warm up is characterised by the heating of the system until the conditions for the evaporation of the working fluid are reached. During the pressurising stage, the pressure in the system is increased sufficiently above to reach the saturation conditions to start the condensation in the thermo tank and force the return of the condensed fluid to the solar collector to repeat the cycle. During the run stage, the system has the conditions to continuously repeat the evaporation–condensation cycle. The evening shutoff takes place when the solar radiation is attenuated; pressure and temperature of the system decrease until the equilibrium between the working fluid and the water in

Fig. 2. Close section of the temperature at the condenser inlet and outlet, the water temperature of the highest and lowest sensors in the thermo tank, and the pressure at the condenser inlet, for the first 3 h of operation of the PCS. Pi = 0.87 bar (abs).

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Fig. 3. Close section of the temperature at the condenser inlet and outlet, the water temperature of the highest and lowest sensors in the thermo tank, and the pressure at the condenser inlet, for the first 3;h of operation of the PCS. Methanol was used as working fluid. Pi = 0.46 bar (abs).

earlier and so the heat transfer to the water, that is, the system starts the run stage soon after the beginning of the test. 4. Results In order to monitor the experimental system, the increase of temperature in the thermo tank DT (°C), the useful heat in the water qu (MJ), and the experimental efficiency g, were chosen as the variables representative of its thermal performance. These variables are determined as follows: DT ¼ T f  T 0

ð1Þ

where T 0 and T f are the average temperatures of the three thermocouples in the thermo tank. T 0 is calculated at the beginning of the test, and T f at the end, after the solar irradiation period. The useful heat in the water is defined as qu ¼ mcp DT

ð2Þ

where m (kg) is the mass of the water in the thermo tank, and cp (kJ kg1 K1) is the specific heat of water at constant pressure; cp is calculated at T ¼ 0:5ðT 0 þ T f Þ. The experimental efficiency is defined as the ratio of the useful heat to the incoming solar energy qr (MJ): q g¼ u ð3Þ qr and qr ¼ IAabs Dt

ð4Þ

where I (Wm2) is the average solar irradiance on the plane of the collector, Aabs (m2) is the collector’s absorber area, and Dt (s) the time length of the test. 4.1. Acetone Out of the tests performed, and for each of the fluids tested, data were examined to find the highest experimental efficiency obtained; the results are shown in Tables 6 and 7.

For this experimental efficiency, a filled fraction and a coil length are associated. Those are the parameters at which the PCS offers its best performance (for each working fluid) for the tests carried out. Table 6 shows the results at the highest experimental efficiency obtained in the PCS, working with acetone, and the corresponding result of the conventional DSWHS. The highest experimental efficiency for acetone was 47.9 ± 1.0% at a filled fraction of 0.55 of the 6 m coil PCS, which resulted in an increment of 25.1 ± 2.0 °C of the water temperature in the thermo tank. Fig. 4 shows the increment of temperature in the 6 m coil PCS, at every filled fraction tested. The highest increment of temperature was 27 °C, found at a filled fraction of 0.52. Also, tests with two initial pressure conditions in the closed circuit are compared in Fig. 4: Pi = 0.87 bar (abs) corresponds to the atmospheric pressure of Temixco, Me´xico, while Pi = 0.46 bar (abs) is the partial vacuum reached. Tests with partial vacuum definitely improve the performance of the system, increasing the DT up to 19 °C more than the case that began at atmospheric pressure. Fig. 5 shows the useful heat in the water of the thermo tank for every filled fraction using acetone as working fluid. Tests were made in the 6 m coil PCS, with two initial pressure conditions (Pi = 0.46 and 0.87 bar (abs)) in the closed circuit. The highest energy collected in the tests that began with partial vacuum was 18 MJ at a filled fraction of 0.52.

Table 6 Comparison of the PCS with acetone and the DSWHS. Acetone, partial vacuum

PCS

DSWHS

PCS

DSWHS

Coil length (m) Filled fraction I (W m2) T amb (°C) T 0 (°C) DT (°C) qu (MJ) qr (MJ) g (%)

6 0.55 716 ± 40 35.1 ± 1.7 25.5 ± 2.5 25.1 ± 2.0 16.8 ± 1.3 35.1 ± 3.1 47.9 ± 1.0

– – 716 ± 40 35.1 ± 1.7 26.3 ± 1.3 27.4 ± 2.1 18.4 ± 1.4 35.1 ± 3.1 52.4 ± 1.0

10 0.48 656 ± 59 38.0 ± 1.8 25.5 ± 1.1 21.4 ± 2.7 14.3 ± 1.8 34.1 ± 3.8 41.9 ± 1.5

– – 656 ± 59 38.0 ± 1.8 27.1 ± 1.0 25.3 ± 2.3 16.9 ± 1.6 34.1 ± 3.8 49.6 ± 1.1

Rounding of mean values are according to standard deviation data.

Table 7 Comparison of the PCS with methanol and the DSWHS. Methanol, partial vacuum

PCS

DSWHS

PCS

DSWHS

Coil length (m) Filled fraction I (W m2) T amb (°C) T 0 (°C) DT (°C) qu (MJ) qr (MJ) g (%)

6 0.59 774 ± 8 28.0 ± 0.7 29.8 ± 0.7 22.2 ± 0.7 14.9 ± 0.5 36.07 ± 0.33 41.2 ± 0.5

– – 774 ± 8 28.0 ± 0.7 32.4 ± 0.7 25.4 ± 0.7 17.0 ± 0.5 36.07 ± 0.33 47.1 ± 0.5

10 0.53 710 ± 7 30.8 ± 0.7 23.2 ± 0.7 23.8 ± 0.7 15.9 ± 0.5 36.40 ± 0.36 43.7 ± 0.5

– – 710 ± 7 30.8 ± 0.7 28.3 ± 0.7 25.9 ± 0.7 17.3 ± 0.5 36.40 ± 0.36 47.5 ± 0.5

Rounding of mean values are according to uncertainty data.

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Fig. 4. Increment of temperature of the water in the thermo tank of the 6 m coil PCS working with different filled fractions of acetone in the primary circuit. Pi is the absolute pressure in the PCS’s closed circuit at the beginning of the test. The average temperature increment of the conventional DSWHS, for all the days of test, was 24.7 °C.

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Fig. 6. Experimental efficiency obtained in the 6 m coil PCS working with different filled fractions of acetone in the primary circuit. Pi is the absolute pressure in the PCS’s closed circuit at the beginning of the test. The average experimental efficiency of the conventional DSWHS, for the days of test, was 48%.

Data from Figs. 4–6 show a tendency that a maximum value can be found. According to Soin et al. (1979), the performance of a PCS improves with the increment of the load level. However, it was found that, for this case, the highest filled fraction did not produce the best results; the highest experimental efficiency was found at an intermediate filled fraction. For the case of acetone, the best performance was found in the 6 m coil PCS, at an intermediate filled fraction of 0.55, where an experimental efficiency of 47.9% was reached. 4.2. Methanol Fig. 5. Useful heat collected in the 6 m coil PCS working with different filled fractions of acetone in the primary circuit. Pi is the absolute pressure in the PCS’s closed circuit at the beginning of the test. The average useful heat of the conventional DSWHS, for all the days of test, was 16.5 MJ.

In tests beginning at atmospheric pressure, no more than 9 MJ of useful heat could be collected. In addition to DT and qu, the experimental efficiency g, Fig. 6, completes the description of the system. The experimental efficiency is important because it incorporates both the energy input (solar energy) and the energy collection (useful heat), normalising the data from different environmental conditions as the weather, irradiation, or the raw water from the municipal network. For this reason, the experimental efficiency, that includes the incoming solar energy and the energy recovered, was considered like the most important parameter for the evaluation of these tests. Fig. 6 shows the experimental efficiency of the 6 m coil PCS, at every filled fraction tested, with initial pressures Pi = 0.46 and 0.87 bar (abs). The best result was 48% of efficiency, found at a filled fraction of 0.55. Tests beginning with partial vacuum resulted in a definite improvement of the performance of the system in, at least, near 10%, and up to 27% in the best case.

Methanol is an attractive working fluid for the PCS, for its high latent heat of vaporisation, in comparison with the other working fluids tested (see Table 1). Although methanol has a high boiling point at Patm (64.4°), the partial vacuum in the closed circuit had it reduced to 43.1 °C at Pi = 0.46 bar (abs). Fig. 7 shows the comparison of the increment of water temperature in the thermo tank of the 6 m coil PCS, working with different filled fractions of methanol in the primary circuit. Evaluation of the PCS with initial pressures Pi = 0.46 and 0.87 bar (abs) is shown in Fig. 7. As seen previously, tests with partial vacuum definitely improve the performance of the system, with major increments in water temperature DT, from 4 to 6 °C higher, for the days of test. The highest increment with the 6 m coil PCS was DT = 22.2 °C, found at a filled fraction of 0.59. Fig. 8 shows the useful heat in the 6 m coil PCS working with different filled fractions of methanol in the primary circuit. The initial pressures are Pi = 0.46 and 0.87 bar (abs). In tests with initial partial vacuum at the beginning of the test, the collection of energy increased to an extra 3–6 MJ, approximately. The best result with the 6 m coil PCS, qu = 14.9 MJ, was found at a filled fraction of 0.59.

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Fig. 7. Increment of temperature of the water in the thermo tank of the 6 m coil PCS working with different filled fraction of methanol in the primary circuit. Pi is the absolute pressure in the PCS’s closed circuit at the beginning of the test. The average temperature increment of the conventional DSWHS, for all the days of test, was 25 °C.

Fig. 8. Useful heat collected in the 6 m coil PCS working with different filled fractions of methanol in the primary circuit. Pi is the absolute pressure in the PCS’s closed circuit at the beginning of the test. The average useful heat of the conventional DSWHS, for all the days of test, was 16.7 MJ.

To complement the information on methanol, Fig. 9 shows the experimental efficiency obtained in the 6 m coil PCS working with different filled fractions in the primary circuit. Tests with two initial pressures (absolute pressure and partial vacuum) are shown in the figure. Fig. 9 confirms the importance of the initial partial vacuum in the PCS’s closed circuit: results are up to 15% higher than the case that began at atmospheric pressure. The best result found with the 6 m coil was g = 41.2%, at a filled fraction of 0.59. Tests with the highest experimental efficiency obtained with methanol, for each coil length, are shown in Table 7. The best result obtained with methanol was with the 10 m coil PCS, reaching an experimental efficiency of 43.7 ± 1.4% with an increment of the temperature of the water of 25.1 ± 2.0 °C. The increment of temperature was only 2 °C lower than the conventional DSWHS.

Fig. 9. Experimental efficiency obtained in the 6 m coil PCS working with different filled fractions of methanol in the primary circuit. Pi is the absolute pressure in the PCS’s closed circuit at the beginning of the test. The average experimental efficiency of the conventional DSWHS, for the days of test, was 50%.

The three variables analysed, DT, qu and g show that beginning with partial vacuum is necessary for the PCS to operate with an improved performance. Also, they show that the best result does not come from the highest filled fraction, but from some intermediate filled fraction. This intermediate level depends on several factors such as the total capacity of the closed circuit, the working fluid used, tubing diameters or the coil length: the best result has to be found for each case. In the case of methanol, the best filled fraction was 0.53, found in the 10 m coil PCS, where an experimental efficiency of 43.7% was reached. In both cases, with acetone and methanol in the PCS, the conventional DSWHS obtained slightly higher efficiencies, resulting in an average extra 2 °C increment of the water temperature in the thermo tank. Regarding coil length, results show that they have less influence than other parameters tested (as the partial vacuum, or the filled fraction) to be determinant in the performance of the system. For acetone, the best result was obtained with the 6 m coil, but for methanol, the best result was obtained with the 10 m coil. For the investigated PCS, the overall best result was found with acetone in the 6 m coil PCS, at a filled fraction of 0.55, with an increment in temperature of 25.1 °C, and an experimental efficiency of 47.9%. These results differ from the work of Islam et al. (2005), that found methanol to be the most suitable fluid in a similar phase change system. They made singular tests with each of the fluids, acetone and methanol, and calculated the hourly useful heat, instantaneous collector efficiency, among other parameters, to establish which of these fluids work with better efficiency. They concluded that the higher latent heat of vaporisation of the methanol was responsible of their result. However, the methanol has a higher boiling point than acetone, at any pressure. For the experiment of this work, the higher boiling point of methanol resulted in a delayed beginning of the evaporation-condensation cycle (with respect to acetone); so, actually, less heat was trans-

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ferred to the water during the solar irradiation period, despite the methanol’s higher latent heat of vaporisation.

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 The conventional DSWHS still has slightly better performance than the PCS.

5. Conclusions Acknowledgments An Indirect Solar Collector Phase Change System (PCS) to heat water was investigated in daylight tests to determine the useful heat, temperature increment of the water in the thermo tank, and the experimental efficiency. A conventional Domestic Solar Water Heating System was also installed to compare with the PCS. The PCS aims to prevent problems like freezing (as in the north of Me´xico), corrosion, fouling, scaling and blocking of the tubing (as in the Yucata´n Peninsula). The working fluids for the PCS were acetone and methanol. Tests were performed with two initial pressure conditions in the PCS’s closed circuit: atmospheric pressure and partial vacuum, with two prototype coil lengths, 6 m and 10 m, and the results were compared. In order to reduce the pressure and get a partial vacuum, air was extracted from the closed circuit to reach an absolute pressure of 0.46 bar (abs), being 0.87 bar (abs) the atmospheric pressure of Temixco, Me´xico. Different filled fractions of acetone and methanol were tested. Tests were performed under the actual field and environmental conditions of Temixco, Me´xico. The temperature increment of the water, useful heat, and the experimental efficiency, improved significantly with partial vacuum at the beginning of the test in the PCS’s closed circuit. The increment of temperature DT, the useful heat qu, and the experimental efficiency g, were the parameters chosen to describe the behaviour of the system. In both cases, with acetone and methanol, it was found that it was not the highest filled fraction, but and intermediate filled fraction, 0.55 for acetone and 0.53 for methanol, that produced the best result. For the tests with methanol, the highest experimental efficiency obtained was 43.7%, in the 10 m coil PCS, at a filled fraction of 0.53. According to the experimental tests, the highest experimental efficiency obtained in all the tests was 49.7%, with acetone working in the 6 m coil PCS, at a filled fraction of 0.55. Comparisons with the conventional DSWHS showed that the PCS still exhibits slightly lower performance, However, the PCS presents advantages of no freezing, corrosion, scaling, fouling, nor blocking of the tubing, that improve the lifetime of the system. The investigated PCS is a promising device that has still room for improvement. Stressing:  Partial vacuum for the beginning of the test improves the performance of the PCS.  A intermediate filled fraction could be found in each case at which the PCS showed its best experimental efficiency.  Results with the two coil lengths showed, for this experiment, than the influence of this parameter was not determinant in the results behaviour, as they were the partial vacuum and the filled fraction.

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