Initial Evaluation of a Heat Plate Solar Collector for ...

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consumption, followed by water. 2010 more than 3 million tones more than 64% of total final ener. Figure 1: Final Energy Consu a Heat Plate Solar Collector for.
4th International Conference on Renewable Energy Sources and Energy Efficiency - New Challenges Nicosia Cyprus 6 -7 June 2013 (Paper No. 1314)

Initial Evaluation of a Heat Plate Solar Collector for Solar Heating a and/or Cooling Systems G.. Martinopoulos1 and G. Tsilingiridis2 1

2

International Hellenic Uniiversity/School of Science and Technology, Thesssaloniki, Greece [email protected]

Thessaloniki, Greece Aristotle University of Thessaaloniki/Department of Mechanical Engineering, T [email protected]

KEYWORDS - solar energy, solar collectors, heat plate, solar cooling ABSTRACT

Solar collectors for heatingg and air conditioning systems have maturedd significant during the last decade providing a reliable alternative to fossil fuels or electricity andd research is currently aimed at improving solar collectoor efficiency, amongst others. One of the tecchniques that are under research in order to achieve this iis the use of liquids with low boiling point as heat transfer mediums mperatures, and utilize which minimize thermal losses tto the environment by operating in lower tem the enthalpy of evaporation. In tthis paper the experimental evaluation of a 2 m² novel heat plate collector is presented. The influeence of heat carrier volume to available colleector volume and from the collector’s inclination, as welll as of mass flow rate to the efficiency of thee system is assessed in u in solar heating and order to find the optimum combinnation of the above parameters for possible use cooling systems. 1

INTRODUCTION

Heating and cooling accounnts for a significant proportion of the world’ss total energy demand. Energy Agency, heat represented 47% of finaal energy consumption According to the International E in 2009 [1]. The building sector in particular is responsible for more thann 40% of final energy consumption in the European Unnion (EU) with space heating representing 68% of total household consumption, followed by water heating at 12% for 2009 (Figure 1) [2]. In the case of Greece, in 2010 more than 3 million tones of oil equivalent were consumed for spacee heating; representing more than 64% of total final enerrgy consumption, while water heating 6% (Figgure 2) [3].

Figure 1: Final Energy Consuumption by Energy Use in Households in EU [Source: Odyssee]



In an effort to reduce CO2 emissions and promote the use of renewable sources, the EU passed Directive 2009/28/EC [4]. The directive implied, that member states should increase their use of renewable energy sources along with energy efficiency and savings beyond 2012, by 20% until 2020. A further 80% reduction of energy consumption of the existing building stock, relative to 2010 levels by 2050, was recently passed in the European Parliament (in March of 2013). Employing solar collectors for solar heating and/or cooling systems is one of the solutions for achieving these goals.

Figure 2: Final Energy Consumption by Energy Use in Households in Greece In Greece, Domestic Solar Hot Water Systems (DSHWS) present a mature and widely available technology with an installed capacity of 4,1 million m² (2,8 MWth) at the end of 2011, that places Greece in the 3rd place in per capita installed capacity in the EU [5]. Although DSHWS is a widely accepted concept for hot water production with a high level of market penetration, solar space heating and/or cooling, although mature technologies, have rather low levels of market penetration and public acceptance. This is due to a number of nontechnological barriers such as the relatively higher initial investment costs compared to traditional heating and/or air-conditioning installations. Taking into account the increasing cost of energy as well as the demand for cooling during the summer, solar air conditioning presents a viable solution. Active solar systems use either liquid or air as the collector fluid with liquid systems being the dominant ones. A complete system includes solar collectors, energy storage devices, and pumps or fans for transferring energy to storage or to the load. The load can be space cooling, heating, or hot water. Although it is technically possible to construct a solar heating and cooling system to supply almost 100% of the design load, such a system would be oversized and uneconomical. Active solar energy systems have been combined with heat pumps for water and/or space heating. The most economical arrangement in residential heating is a solar system in parallel with a heat pump, which supplies auxiliary energy when the solar energy is not available [6]. For a solar thermal closed-loop air conditioning system to be as efficient as possible the coefficient of performance (COP) is of the outmost importance. Single, double or triple iterative absorption cooling cycles are used in different solar thermal cooling system designs. The more cycles, the more efficient they are. Efficient absorption chillers nominally require water of at least 85°C making the use of common, inexpensive flat-plate solar thermal collectors possible, for adsorption systems the driving temperature required is usually lower than 75°C, but their COP is



also somewhat lower at 0,59. Depending on the chiller utilized the nominal area (and type) of solar collectors required differs, ranging between 0,5 to 5,5 m²/kW of cooling power [7]. Furthermore depending on the system the driving temperature needed differs and in general COP increases as driving temperature increases. It is obvious that in order for solar heating and/or cooling systems to become widespread, low cost and more efficient solar collectors are necessary that can operate at various driving temperatures and mass flow rates. During the last few years a lot of effort is put towards the optimization of flat plate collector efficiency. One of the techniques employed is the use of liquids with low boiling point as the heat transfer fluid in solar collectors. The collectors that employ this technique are called phase change or heat plate collectors. In this paper an experimental evaluation of a full scale heat plate solar collector is presented as a continuation of previous work [8]. Furthermore, the influence of heat carrier volume to available volume inside the pipe, mass flow rate, and from the inclination to the efficiency of the system is reported. 2

HEAT PLATE COLLECTOR DESCRIPTION

In phase change collectors, the absorber comprises either of heat pipes engulfed in fins or of two plates welded at their extremities that can act as a heat plate. In its simplest form, a heat-pipe is a sealed tube containing a small quantity of a volatile liquid (such as water or ethanol) with no air or other "permanent" gas present. If such a pipe is placed vertically and heated, liquid will evaporate and the vapor formed will move to the upper part of the pipe where it can condense and give up its enthalpy of vaporization. The condensate will then flow back to the lower end where it can be reheated and re-evaporate (Figure 3).

Figure 3: Operation Principle of a Heat Plate The main useful characteristic of this type of collector is that it utilizes the enthalpy of vaporization, transporting heat at a smaller temperature difference with the environment reducing thermal losses. Furthermore the device acts as a thermal diode. That is, the conduction is very high in one direction (upwards) and very low in the other (downwards). Additionally, due to the low



pressure and subsequent low freeezing point of the heat carrier, destruction of o the heat pipes from low air temperatures is not possibble. Heat plate collectors were evaluated e for the first time by Soin et al [9] and a later by Ismail et al [10], Essen et al [11] and Maathioulakis et al [12] amongst others. Soinn et al [9] evaluated experimentally the performance of o a solar collector undergoing phase change employing an external condenser, with acetone and petrooleum as a working fluid, observing an increaase in efficiency as the liquid level increased. Ismael et al [10], presented a comparative studyy between theoretical predictions and experimental ressults of a flat-plate solar collector with methhanol filled heat pipes. The heat pipes were self-containned devices whose evaporators were insertedd under pressure in the flat plate solar collector and the heat h exchange was carried out at their condeensers. Essen et al [11] and Mathioulakis et al [12], studdied a two phase closed thermosyphon solarr water heater, in both cases the pipes were not interconnnected with each-other.

Figure 4: Heat plate collector’s absorber dimensions In the present work the colllector comprises of all the parts of a typical flat fl plate collector. The absorber consists of ten typical copper c pipes with a 10 mm inner diameter, 19900 mm length and 75 mm spacing between them, laserr welded onto thin aluminum foils painted bllack (with electrostatic painting) to increase solar radiattion absorption. As glazing a 4 mm low ironn glass was employed with 35 mm spacing from the abbsorber. Glass wool was employed in the baack of the absorber for insulation with a 40 mm thicknesss, while 40 mm rock wool was used for the sides. The novelty of the current work is that the copper pipes were welded inn the upper part with a double shell copper heat exchannger and at the bottom part with a simple copper 15 mm inner diameter tube with welded extrremities (which is the only difference witth a typical flat plate collector arrangement) providing an interconnected heat plate (Figure 4).



The double shell copper heat exchanger consists of two concentric copper pipes; the inner one with a 15 mm diameter was left open at its ends while the outer one with a 22 mm diameter was welded to the inner one, at its ends. The condenser was placed at the top of the absorber, encapsulating a flow through heat exchanger, but with a considerable smaller volume than that of previous studies [12], [13]. A brief overview of the technical specifications of the collector is presented in Table 1. Table 1 Collector Technical Specifications Collector Dimensions Copper Pipe (Risers) Copper Pipe (Bottom Header) Aluminum Foils Thickness Glazing Insulation (Back) Insulation (Side) Casing (Back) Casing (Sides) Absorber Painting Condenser

960 x1980 mm 10 x 10 mm 15 mm 1,2 mm 4 mm low iron Glass Glass Wool 40 mm, 20 kg/m³ Rock Wool 40 mm, 40 kg/ m³ Galvanized Steel Aluminum Semi Selective Black Flow through – 890 mm long

On the top of the collector’s hydraulic channeling, a valve was used in order to empty the collector’s hydraulic channels of all fluids, using a vacuum pump and afterwards to fill the collector with the heat carrier fluid, once the high vacuum had been established. A barometer was attached in a second tube, on top of the condenser in order to monitor pressure during operation. RTD Pt100 3 wire thermometers were used at the coolant inlet and outlet to measure the temperature gain in the condenser. The proposed collector provides ease of manufacturing, as it comprises of all the typical parts of a flat plate collector, and due to the interconnection of the heat pipes onto a heat plate the necessary vacuum and filling of the collector can be done in less time than with individual heat pipes. The integration of the condenser onto the absorber also minimizes costs, while making possible the connection of multiple collectors in series in order to reach through different mass flow rates the necessary, for various solar heating and/or cooling technologies, driving temperatures. 3

TEST BED DESCRIPTION

For the measurement of the boiling collector’s thermal performance a test bed, complying with the specifications of the ISO 9806-1[14], was used. A schematic diagram of the test bed configuration is shown in Figure 5.

Figure 5: Test bed diagram (The Test bed, all equipment used and the heat plate collector belong to the Process Equipment Design Laboratory from the Department of Mechanical Engineering, AUTh) 

A heating immersion circulator was used for the temperature regulation of the heat transfer liquid, in this case water, in order to maintain the temperature at the collector inlet within a range of ±0.1 K of the desired value. The flow was measured with a rototron flow meter and solar radiation was measured with a precision pyranometer. The signals from all the sensors were collected by a 24–bit data logger and were stored in a computer for further processing. As heat carrier, distilled water was used and testing was conducted for two different volumes of heat carrier. A volume corresponding to 50 and 80% of the total volume of the heat pipes was used, after vacuum (~50 mbar absolute) was achieved with a vacuum pump. The volumes selected conform well to those found in the literature [8]. The initial test matrix is shown in Table 2, flow rates selection based on the literature [15]. Table 2 Test Matrix Mass Flow Rate [kg/min]

Inclination 40° + + + + +

0,6 2,4 3,7 5,0 7,5

60° + + + + +

Heat Carrier Volume 50% 80% + + + + + + + + + -

The temperature of the heat carrier liquid at the inlet was appropriately set before every measurement, starting from ambient temperature and, before measuring, the system was left to reach steady state conditions. 4

RESULTS

All measurements were conducted at an ambient temperature of 20-28 °C and solar intensity of 600-900 W/m² at the plane of the collector (depending on inclination). The results are presented as average values of 10-minute measurement sets. Thermal efficiency is calculated with equation (1) as the ratio of total useful energy to total solar energy on the collector window.

   

   



      

  

(1)

In Figure 5 the average efficiency for the two different volumes and for various flow rates is presented for an inclination of 40°. The 50% volume filament, presents an overall better system efficiency which increases proportionally with the increase of mass flow rate, indicating that the increase in mass flow rate in the condenser minimizes heat losses to the environment and thus increases the heat exchanger efficiency, leading to an increase in overall efficiency, as expected [9].



Figure 5: Average system efficiency for 40° inclination for all cases examined In Figure 6, the average efficiency for the two different volumes and for various flow rates is presented for an inclination of 60°. As far as the influence of the heat carrier volume in the collector on the efficiency of the system is concerned, there are clear indications that high fillings (80%) result in overall inefficient operation which can be attributed to the fact that the overall available volume for the recirculation of the phase change fluid is minimum. That leads to restricted flow, causing the steam in the condenser to be superheated and thus, increasing heat losses to the environment.

Figure 6: Average system efficiency for 60° inclination for all cases examined The comparison of the cases examined, indicates that low (40°) inclined collectors with lower filament (50%) provide the best overall thermal efficiency. The overall better efficiency at an inclination of 40° is explained by less restrictions in the heat carrier recirculation, as the condensed water flows back to the bottom easier in small inclinations, while in 60° gravity increases the speed of the water droplets in turn causing strangulation to the upward flow of the steam due to the small diameter of the riser tubes. In order to evaluate if further increase in the mass flow rate would lead to further increase in the efficiency, another test was conducted at the optimum inclination and filament (40° and 50% respectively) with a mass flow rate of 7,5 kg/min. The 33% increase in mass flow rate, from 5 kg/min to 7,5 kg/min, resulted in a meagre 0,8% increase in thermal efficiency for a total of 33,07%, proving that the 5 kg/min flow rate presents the limit of the current heat exchanger which is apparent from figure 7, where the average pressure at the condenser of the heat plate collector is presented for the above case.



Figure 7: Average pressure in the condenser for 40° inclination and 50% volume 5

CONCLUSION

Generally speaking, the heat plate collector can be an attractive option in solar systems since it combines good energy behaviour with simplicity in manufacture, the use of common flat-plate solar collectors without significant alterations of the existing production line, excellent behaviour in freezing and overheating. The invariable system efficiency in the range of 30-35% achieved by this initial experimental evaluation, depending on the mass flow rate, makes heat plate collectors suitable candidates for solar heating and/or cooling systems, as they can provide a variety of driving temperatures. Research is currently under way for further tests with a larger variety for the evaluated parameters (inclination, flow rate, heat carrier volume and carrier type) that would help in a better understanding of the collectors operational characteristics and its efficiency as well as operation of an array of heat plate collectors in series. REFERENCES [1]

IEA, Key World Energy Statistics 2011.

[2]

EEA, Consumption and the Environment – State and Outlook, 2010.

[3]

CRES, Energy Efficiency Policies and Measures in Greece, 2012.

[4]

Directive 2009/28/EC On the promotion of the use of energy from renewable sources, 2009.

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ESTIF, Solar Thermal Markets in Europe 2011, 2012.

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SOLCO, Solar Cooling – Overview and Recommendations, 2009.

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SOLAIR, Requirements on the design and configuration of small and medium sized air conditioning applications, 2009.

[8]

Martinopoulos G., Tsilingiridis G., Kyriakis N., Evaluation of a boiling solar collector, Proceedings of Renewable Energy, Japan, 2006.

[9]

Soin R.S., Sangameswar Rao K., Rao D.P., Rao K.S., Performance of flat plate solar collector with fluid undergoing phase change, Solar Energy 23, 1979.



[10] Ismail K.A.R., Abogderh M.M., Performance of a heat pipe solar collector, Journal of Solar Energy Engineering 120, 1998. [11] Esen M., Esen H., Experimental investigation of a two-phase closed thermosyphon solar water heater, Solar Energy 79, 2005. [12] Mathioulakis E., Belessiotis V., A new heat-pipe type solar domestic hot water system, Solar Energy 72, 2002. [13] Nada S.A., El-Ghetany H.H., H.M.S. Hussein, Performance of a two-phase closed thermosyphon solar collector with a shell and tube heat exchanger, Applied Thermal Engineering 24, 2004. [14] ISO, ISO 9806-1: Test Methods for Solar Collectors. [15] Abramzon B., Yaron I., Borde I., An analysis of a flat plate collector with internal boiling, Transactions of the ASME 105, 1993.