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
Abstract

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.

image159

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.

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