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Aluminum heat pipes applied in solar collectors
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Solar Energy 94 (2013) 145–154 www.elsevier.com/locate/solener
Aluminum heat pipes applied in solar collectors Boris Rassamakin, Sergii Khairnasov ⇑, Vladilen Zaripov, Andrii Rassamakin, Olga Alforova Heat Pipes Laboratory, National Technical University of Ukraine, Kyiv Polytechnic Institute, Off. 709, 6 Polytechnichna Str., Kyiv-56 03056, Ukraine Received 7 June 2011; received in revised form 20 March 2013; accepted 28 April 2013 Available online 3 June 2013 Communicated by: Associate Editor Ursula Eicker
Abstract The previous researchers have developed a variety of liquid thermal solar collectors designs for water heating. It was reported by the other authors, that metal heat pipes applications to liquid solar collectors, especially to evacuated glass tube ones, is an efficient solution for water heating plants. However, the majority of thermal solar collectors do not meet the requirements on small weight, easy assembly and installation, versatility, scalability, and adaptability of the design, which are particularly important when they are facßade integrated. Very high hydraulic resistance, from 2000 Pa to 20,000 Pa, in liquid solar collectors and low thermal efficiency of some of them, less than 0.5, also are the problems to be solved by the developers. Current research is proposing to apply extruded aluminum alloy made heat pipes of original cross-sectional profile with wide fins and longitudinal grooves in order to avoid the above-mentioned drawbacks of liquid thermal collectors. Absorber plate of flat collectors could be composed of several fins. Fins at the opposite end of the heat pipe serve as a heat sink surface. Multiple tests proved that new lightweight and inexpensive heat pipes show high thermal performances. Maximum heat transfer power of one heat pipe is up to 210 W; and its thermal resistance is very low – from 0.02 to 0.07 °C/W. Hydraulic resistance of flat plate solar collector and evacuated one utilizing aluminum profiled heat pipes, could be reduced to less than 100 Pa, at the same time their thermal efficiency is rather high, up to 0.72. In the issue of authors study, the feasibility of the developed aluminum profiled heat pipes application to thermal solar collectors was proved; and they can be successfully integrated to building facßades and roofs. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Heat pipe; Solar collector; Absorber plate; Heat exchanger; Efficiency; Aluminum extrusion
1. Introduction Due to steady growth in energy sources costs, solar water heating systems become more economically advisable for use in residential, private, commercial, and industrial buildings, schools, hotels, swimming pools, industrial
Abbreviations: HP, heat pipe; SC, solar collector; PV, photovoltaic; SWHP, Solar Water Heating Plant; FPC, flat plate collector; ETC, evacuated tube collector. ⇑ Corresponding author. Tel./fax: +380 44 406 83 66. E-mail address: offi[email protected] (S. Khairnasov). 0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.04.031
processes, drying and distilling or autonomous boiler plants. Nevertheless solar water heating systems production technology is already well developed, it is being continuously perfected. Thermal solar collector (SC) is a key component of Solar Water Heating Plant (SWHP). Its thermal efficiency (g) mainly determines the efficiency of the whole SWHP (Burch, 2007). Copper or aluminum is most frequently used as the material for heat absorbing surface (absorber) in SC. Currently the most common types of liquid solar thermal collectors are the flat and evacuated ones. In general,
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Nomenclature RHP t Dtw Quse Qtotal Eirr. Fh.a.
thermal resistance of the heat pipe (K/W) temperature (°C) water heating rate in the storage tank useful solar heat (W) total solar heat (W) irradiative solar flux (W/m2) heat absorbing area (m2)
typical flat plate solar collector (FPC) (Philibert, 2007; Brunold et al., 2007; Riffat et al., 2000; Faca˜o and Oliveira et al., 2004) consists of glazed or unglazed absorber plate and variously configured pipeline for heat transfer (working) fluid circulation. All components are encapsulated into the casing with thermal insulation at rear and lateral sides. FPC has several advantages: reliability, manufacturability, durability, and low cost. In addition, they are highly efficient, up to 0.75 in summer time when ambient temperature and solar irradiance are high. However, their thermal performance is not high enough (less than 0.4) at low values of insolation and ambient temperature, for example, for Central and Northern Europe climatic conditions, especially in wintertime, when diffused (less than 300 W/m2) solar irradiation prevails. This disadvantage of FPC is mainly caused by rather high heat losses due to insufficient thermal insulation. Another considerable drawback of FPC is high hydraulic resistance in heat transfer circulation loop. Parallel piping of heat carrier in a shape of the harp a little bit lessens the latter disadvantage, but such FPC designs are rather complicated, expensive, not reliable and durable (Burch, 2007). Evacuated tube solar collectors (ETCs) (Brunold et al., 2007; Mahjouri, 2005) are of higher efficiency (nearly to 0.6) at low values of insolation and ambient temperature, even at below zero levels. Heat losses in ETC are small owing to high thermal insulation by means of evacuation inside the tubes. In general, such SC is a frame with several (usually from 10 to 30 units) evacuated glass tubes fixed to it, usually from 10 to 30 units. They may be of “glass– glass” type with double wall tube flask or “glass–metal” with single wall tube (Mahjouri, 2005). In the first case low pressure is being created between the walls of the glass flask, in the second one – the whole inner space of the tube is under low pressure. Heat transfer loop in the ETC may be done as U-type pipeline or copper made HP inserted into the glass tube (Chun, 1999; Esen, 2004). HPs application to SCs as passive heat transfer devices makes possible to avoid some disadvantages of SC. SCs with HPs for heat transfer has low hydraulic thermal resistance, uniform flow of heat transfer liquid, and almost isothermal heat absorbing surface. HP is reliable and durable device in operation. However, even such modern high-tech SC designs have some drawbacks. The main of them is high thermal
g
efficiency factor of solar collector
Subscripts out outlet in inlet
resistance of contact between the absorber and the HP, which is caused by method of joining of physical contact between the absorber plate and the pipeline or HP. Usually they are tightened, glued, soldered or welded together; and may be made of different materials, for example: copper heat pipe or pipeline and copper or aluminum absorber. One more disadvantage of conventional SC is its rather high thermal resistance in the manifold heat exchanger, i.e. high temperature gradient between heat transfer fluid and internal condenser part of the HP. It is caused by small contact area for heat transfer. Furthermore, condenser parts of the heat pipes are inserted into the manifold and obstruct heat transfer fluid flow and slow it down, thus increasing hydraulic resistance inside the manifold. Another aspect of solar collector designs taken into consideration by the authors in their research is observed tendency of SC integration into building facßade systems (Stadler, 2001). «Facßade-integrated collectors are becoming more and more popular. An advantage of facade-integrated collectors consists in a rather even irradiation of sunlight over the year, which is due to their vertical installation. This is very interesting for solar systems as a lot of irradiation can be used in winter, when the highest heat demand occurs for space heating. Further arguments for installing solar thermal collectors on the facßade are that there is often not enough space on the roof or no suitable oriented roof area is available. This is typically the case for multi-family buildings with a relative high number of floors.» (Eisker, 2003). Thus, facßade-integrated collectors should meet additional requirements such as: scalability, i.e. to be assembled from several identical modules; easy mounting and standard installation fittings to the exterior of the building; low hydraulic resistance of the loop; and, of course, low cost (Isaksson, 2005,2007). The majority of SCs in the market does not meet the most of those demands. ETCs are of not enough mechanical strength for this application. FPCs are more preferable for such a case. At the same time, FPCs after special upgrade could be used for thermal and moisture insulation of the wall or the roof of the building, as well as for their decoration. In order to avoid all the above-mentioned demerits of SCs, the article researchers have developed principally new thermal SC design made of HPs (Azad, 2008). It is intended to super highly effective heat removal from the solar heat absorber and heat transfer to compact manifold heat
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exchanger with low thermal and minimal hydraulic resistances. Moreover, it is presumed, that due to the proposed SC design, typical standard lightweight modules could be simply assembled together in situ. Therefore, they easily turn into integral part of building roof or facßade system. To the authors’ opinion, all these advantages are achievable thanks to application of the HPs made of aluminum alloy and originally shaped by means of extrusion method. Previously the technology of this kind of HPs application to space engineering and satellites had been perfectly developed by the authors. However HPs on-ground application has other peculiarities due to gravity force, they need to be thoroughly studied and verified by the field tests. The main objectives of the work described were to: create thermal liquid SC prototypes with low thermal and hydraulic resistance, where extruded originally shaped aluminum alloy made HPs (Baturkin et al., 2005; EN 12975-1, 2006a,b) are utilized, prove their applicability to SWHP, consider their facßade integration feasibility, assess their thermal performances in comparison with other ones. Results on original aluminum HPs developments and their experimental study as well as their application results to both kinds of thermal SCs–FPC with (213 100 8.5 cm) outer dimensions and 1.98 m2 absorber area, and «glass–glass» ETC with (210 135 12.5 cm) outer dimensions and 2.0 m2 absorber area are adduced in the article. 2. Description of the heat pipes Heat pipe is known as super highly efficient heat transfer device, where closed evaporation–condensation cycle is used for operation. Heat transfer fluid boils and changes its phase state from liquid to vapor and vice versa inside the sealed container. Evaporated working fluid, also called as a “heat carrier” or “cooling agent”, is then condensing and again returning from condensation (heat sink) zone to evaporation zone (heat source) due to gravitation or capillary forces. Heat transfer ability of the HP is 1000–10,000 times higher than of cylindrical copper bar of the same dimensions. The following additional design features are used for raising HP’s heat transfer performance. Flat plates, so called «fins» are made on the external surface of the HP for expanding its heat transfer area. Other special measures to enhance heat transfer fluid flow inside the HP are wick inserts into it, usually made of metal fiber, or longitudinally grooved inner walls. They increase the capillary forces. HPs applied to SC should meet the following requirements: – they should function at the tilt angles from 0° (horizontal collector attitude) up to 90° (vertical collector attitude). – heat transfer ability of one HP should be at least 150 W; – HP’s life time should be not less than 20 years. – the coolant should have low boiling point (30–50 °C), but
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– the should keep its operation ability up to 250 °C; – freezing point of the coolant should be not higher than minus 40 °C. Taking into consideration the above mentioned problems and requirements, originally designed aluminum alloy made longitudinally grooved HP (Fig. 1) was proposed and manufactured by the authors in order to apply it to thermal SC. There absorber plate, being the fin of the HP, is extruded as an entire whole with longitudinally grooved HP. Finned cylindrical HP’s container was extruded from the rod made of 6060/6063 aluminum alloy. Due to extrusion method not only heat transfer fin and cylindrical container are built as one entity, but also inner walls of the container are longitudinally grooved. Grooves are distinctively shaped (Baturkin et al., 2005). Therefore, there is almost no thermal resistance between the heat transfer surface and the container of the HP; and HP heat transfer ability is heightened. At the same time one part of wide HP’s fin could be an absorber of SC, thus the absorber and heat transfer device being one entity. Absorber plate of FPC could be formed of several HPs’ fins. The opposite end of the HP’s fin becomes a heat remover to the manifold heat exchanger.
3. Experimental study of the heat pipe performance Tests on overpressure outburst of aluminum cases of HPs’ prototypes had been carried out prior to their thermal engineering study. It was proved by the tests that cylindrical cases for HPs can endure up to 20 MPa of internal overpressure. Experimental study on thermal engineering performances of developed and manufactured aluminum profiled
Fig. 1. Aluminum finned HP proposed by the authors: (a) outer view of the HP’s fragment and (b) cross-sectional profile of the HP: 1 – absorber and 2 – grooved HP.
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HPs was carried out at a special experimental test-bench, which is shown in Fig. 2. Contact temperature sensors– copper–constantan thermocouples with diameters from 0.1 to 0.16 mm were used for temperature measurements. Thermal sensors were fixed at the outside of the HP’s container with the help of aluminized adhesive tape. Nichrome alloy heater coiled round the cylindrically shaped HP (without fins) was used as a heat source simulator instead of real fins. Heat losses were reduced with the help of highly porous basalt fiber thermal insulation of the heat source simulator together with HP’s evaporative
part. Special flat heat exchanger filled with fluid and fixed to the condenser part of the HP was used as a heat sink of the HP. Running water of constant temperature was removing heat from the heat exchanger. Thermal resistance RHP, measured in °C/W, is determined by following equation: RHP ¼
ðt1 þ t2 þ t3 Þ ðt5 þ t6 þ t7 Þ ; 3Q
ð1Þ
where t1, t2, t3, – temperature values at the heat source part of the HP (°C); t5, t6, t7 – temperature values at the heat
Fig. 2. Experimental test – bench for study on heat power engineering performances of the HPs. (a) Computer aided experimental data control and processing system. (b) Clean room with equipped and tooled prototypes for measuring their thermal performances; 1 – experimental HP prototype (its condenser section); 2 – experimental HP prototype (its evaporator section) with coiled heat source simulator; 3 – multi channel analogous data measuring unit; 4 – wattmeter; 5 – PC; 6 – RS485/RS232 AC3 adapter; 7 – flow rate meter; 8 – running water sink; 9 – hydraulic valve; 10 – autotransformer; and 11 – fuses.
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sink part of the HP (°C); Q – transferred heat power (W), measured by wattmeter. HPs thermal resistance curve vs. heat transfer power was obtained in the issue of the experiments. It is presented in Fig. 3 with indication of some selected points. Experimental study results on maximal heat transfer ability are the following: – maximal heat power transferred by the each HP is 210– 215 W at 0° tilt angle, which corresponds to its horizontal position, and it is rather high due to special capillary pump (longitudinal grooves) inside HP; – maximal heat power transferred by the each HP is higher-over 300 W at the tilt angles from 5° to 90° (when HP operates as a thermosiphon). The lower heat transfer ability at small tilt angles of the HPs can be explained from physics view point as follows: the working fluid (coolant) is returned to the evaporator part of the HP by the capillary forces predominantly to gravity, and at 0° angle – only by them. The experimental uncertainty of the thermal resistance of aluminum profiled HP was computed in accordance with the following equation (Anon., 1993): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2ffi U Rtt U DT UQ ð2Þ ¼ þ Rtt DT Q
The heat loss was accounted as 6% of the transferred heat power. This way, the estimated maximum uncertainty of HP’s thermal resistance for the 95 confidence interval is 8.1%, and for Q = 50 W thermal resistance of the HP is 0.071 ± 0.006 K/W. Respective error bars are indicated by horizontal dashes in Fig. 3. Cooling agent inside the HPs is frozen up at temperature values below minus 40 °C and its stagnation temperature is higher than plus 250 °C. Aluminum HPs prototypes applicable to SCs have been obtained as the result of R&D and fabrication. 4. Description of prototype solar collectors Experimental FPC prototype (see Table 1) has been fabricated in order to prove the feasibility of profiled aluminum HPs application to FPC and assessment of the concept on using them as facßade-integrated ones (Fig. 4). Solar collector panel for absorbing heat from solar irradiation consists of eight finned aluminum HPs made by means of extrusion. Solar heat is absorbed by flat longitudinal fins of the HPs and then transferred through condensation zones of the HPs to small heat exchanger installed on them. Proposed HPs design makes it possible to implement more efficient method of heat transfer in FPC owing to:
The temperature uncertainty of the thermocouple was 0.5 °C. The uncertainty caused by the wattmeter for measurements of transferred heat power was 2%.
Fig. 3. Thermal resistance of aluminum profiled HP vs. heat transfer power at the tilt angles from 0° up to 90°.
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(1) significant increase of heat transfer area and, thus, decrease in temperature gradients between the condensation zone of the HP and coolant in the manifold heat exchanger; (2) very low hydraulic resistance of FPC due to “smart” external connection of the HPs condensation zones to the heat exchanger; (3) reliable “dry” connection of the HPs to the heat exchanger without risks of leakage. Thus highly efficient operation of SWHP, where many connected collectors are used, is possible as well as electric power consumption of circulation pumps could be reduced. The latter advantage of SC is of great importance for PV powered autonomous solar plants, where low power consumption circulation pumps are used. Such technical solution results in shorter payback period of solar heating systems.
Table 1 Specifications of FPC and ETC prototypes worked out by the authors. No
Collector’s feature
FPC
ETC
1 2 3 4 5 6 7
Length (m) Width (m) Absorber area (m2) Number of heat pipes Diameter of the heat pipe (mm) Absorber coating Glazing transmittance coefficient
2.13 1.00 1.98 8 14 Black anodized 0.83
2.10 1.35 2.0 15 8 Selective coating of evacuated “glass–glass” tube 0.91
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(shower, washing stands) (6). Data acquisition equipment of experimental plant consists of computer aided multichannel temperature and solar flux measuring system, which includes analog data input unit (7), signal adapter (8), and PC (9). Instruments for measurements while the experimental tests: – – – –
CMP3 pyranometer; resistance thermometers; timer with 0.1 s scale division; liquid flow rate meter.
Computer aided multi-channel temperature and solar flux measuring system is equipped with 8 channels for data measuring – 7 for temperature measurements and 1 – for incident solar flux. 5.1. Test procedure
Fig. 4. SC made of aluminum profiled HPs as an element of building facßade coating.
Besides that, facßade-integrated solar collecting systems could be composed easily and inexpensively, for example, in a way shown in Fig. 4. In such a case, there is no need to fabricate SC as a separate device. Only HPs and hydraulic loop, which contains the heat exchanger, should be made. Number of HPs and their length could be optional, and they could be replaced anytime. Solar heating systems with HPs could be upgraded easily by removing or adding HPs, because there is no need to discharge the working fluid from the loop, to stop the unit, and to recharge. Experimental “glass–glass” ETC prototype with inserted aluminum HPs has been fabricated in order to assess the expediency of aluminum HPs application to ETC. Evacuated double walled glass flask with selective coating on the outside surface of inner wall was used for solar heat collection. In this case, the fins made of aluminum foil and attached to aluminum HP absorb the heat. 5. Experimental study of prototype solar collectors Experimental study of SCs, having aluminum heat pipes as a core, was carried out at two independently functioning full-scale SWHPs: one of them with 1FPC (see Table 1) is located in Crimea (Ukraine) and the second one with 1 ETC (see Table 1) is in Kiev-city (Ukraine). Volume of the storage tank of both systems with FPC and ETC was 125 l. Schematic diagram of test SWHP is shown in Fig. 5. SWHP consists of: solar collector (1), water storage tank of 125 liters volume (2), with auxiliary heater, controller (3), pump unit (4), expansion tank (5), water running units
FPC was mounted in the outdoor test facility. Then the pump of circulation loop was switched on, and heat transfer fluid was circulating through it during 15 min while the collector was not irradiated (shadowed with the help of shielding). Flow rate of the heat transfer liquid was 2.5 l/ min. After that FPC was exposed to solar irradiation, and heat transfer liquid was preheated to defined temperature. Mean test temperature values of the heat transfer fluid were as follows: 15, 30; 45; 60; 75°C. Each temperature level was maintained constant for 15 min interval and after that measurements were carried out within 5 min. At first temperature levels were reached in ascending order, and FPC was kept at these levels until the collector came to the steady state, then, in the same manner – in descending order. The collector’s state was considered to be steady, if during 10 min interval the mean temperature values of the heat transfer fluid and ambient air varied not more than by 0.5 °C. The following parameters were measured while the experimental tests: time, heat transfer fluid flow rate, solar irradiance, ambient air temperature, and output and inlet temperature values. Data were averaged for the cycle of measurements. Instantaneous efficiency g and Dt/E values were computed from the measured data. Measurements were repeated for the cases when g values for the same Dt/E varied by more than 10%. Known test methods for determination of SC efficiency were taken as the basis for its estimation (Brunold et al., 2007; Certco, 2007a,b). Thermal efficiency g of SC prototypes was defined vs. X = Dt/Eirr, values, where Dt – is temperature difference between heat absorbing surface and ambient air, Eirr – is irradiative solar flux value. Also solar thermal efficiency factor g could be defined (Brunold et al., 2007) as a ratio of useful solar heat:
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Fig. 5. Experimental plant: (a) structure diagram; (b) FPC made of aluminum HPs; and (c) ETC with aluminum profiled HPs as a core in situ.
Quse ¼ GC p ðtout tin Þ
ð3Þ
to total solar heat, which falls to the collector’s absorber: Qtotal ¼ Eirr F h:a: ;
ð4Þ
where, Cp, G – heat capacity and flow rate of a coolant in the circulation loop; tin, tout – temperature values of the coolant at the inlet and outlet of the collector’s manifold, Fh.a. – effective heat absorbing area. The experimental uncertainties of the efficiency of SCs were estimated in accordance with the following equation (Anon., 1993): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 2ffi Ug UG U Cp U DT U Eirr UF ¼ þ þ þ þ g G Cp DT Eirr F
was calculated from measured data, against ETC with copper HPs (MT 58-1800 Certco, 2007a), and FPC without them (Vitosol 100-F, Certco, 2007b), both originating from Germany, are presented in Fig. 6. Data on SCs for comparison were taken from DIN CERTCO certification center for Eirr = 800 W/m2. On the results of experimental estimation study on SCs it became clear that SCs made of aluminum HPs are highly efficient. FPC made of aluminum HPs has slightly lower efficiency against curve 2 for X values within 0.04–0.06. From physics viewpoint it can be explained with the fact that
ð5Þ The maximum temperature uncertainty of the resistance thermometer was 0.4 °C. The maximum uncertainties of the Cp, G, Fh.a. were 0.5%, 2.5%, 0.5% respectively. The uncertainty caused by the solar flux was 10%. Therefore, the estimated maximum uncertainty of HP’s thermal efficiency for the 95 confidence interval is 12.3%, and for X = 0.02 thermal efficiency of the HP is 0.65 ± 0.08. Respective error bars are indicated by horizontal dashes in Fig. 6. High uncertainty of the efficiency results is mainly caused by large 10% total daily error of used CMP3 pyranometer. Some selected points of instantaneous efficiency of ETC and FPC prototypes with aluminum profiled HPs, which
Fig. 6. Thermal efficiency of compared SCs: 1 (curve) – ETC: MT 58-1800 for Eirr = 800 W/m2, 2 (curve) – FPC without HPs Vitosol 100-F for Eirr = 800 W/m2, 3 (dots) – experimental data on ETC with aluminum HPs, and 4 (circles) – experimental data on FPC with aluminum HPs.
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Table 2 Field tests results on the SWHP with 4 ETC made of aluminum grooved HPs. Month
January
February
March
April
May
June
July
August
September
October
November
December
Total per year
Qdirect (kW h/m2) Qdiffused. (kW h/m2) Mean ambient air temperature (°C) QSWHP (kW h/m2)
5.21 22.1 5.6
7.59 32.5 4.2
48.66 51.2 0.7
77.18 59.9 8.7
121.7 69.94 15.1
99.5 78 18.2
126.5 70.8 19.3
104.2 63.7 18.6
83.95 46.2 13.9
33.7 36.5 8.1
7.2 21.8 2.1
1.86 16.2 2.3
717 569 7.7
7.9
12.8
39.7
57.2
81.8
94.2
105.2
97.7
61.9
30.6
11.5
6.3
606.9
selective coating was not applied to FPC. It was glazed with usual window glass with 0.84 transmittance coefficient. Performance of FPC type also could be increased for high X by some upgrading of SC with using low-ironed glass or selective coating to the glass pane. Regarding the ETC with aluminum HPs, its efficiency is close to the compared analogous prototype within 0.06. . .0.11 values of X. Our ETC performances could be increased by research on special improved shape of HPs cross-section profile with round fins for higher thermal contact of aluminum fin with grooved HP and with the inner wall of evacuated glass tube. It is worth to mention, that computed hydraulic resistance of solar SCs made of aluminum HPs is not higher than 70 Pa for ETC and not higher than 10 Pa for FTC at coolant flow rate about 130 l/h. Whereas hydraulic resistance of Vitosol 100-F FTC is 21 200 Pa at the same coolant flow rate (Certco, 2007b), and of Vitosol 300 evacuated tube one with HPs is 2 500 Pa (Anon., xxxx). 6. Results from field test of a system with prototype solar collectors Full-scale SWHP with four ETC made of aluminum HPs (see Fig. 5b and c) was assembled for the private house in Kiev suburb area (Ukraine) in order to prove its application expediency. ETCs are of the same design as previously described experimental one. The capacity of storage tank is 460 l. SWHP has been working successfully for 4 years. It endured cycled overheating many times, reaching almost stagnation temperature of 240 °C. Efficiency decrease of solar water heating system or HPs’ leaks or damages were not observed. Created SWHP provides up to 90% of seasonal demand in hot water due to solar energy. Field tests results on monthly QSWHP – heat output of the SWHP with 4 ETC having extruded aluminum HPs as a core, placed at the private house in Kiev region are shown in Table 2. Monthly mean values of solar irradiance to horizontal plane and ambient air temperatures for Kiev region are taken from http://soda-is.com. Data were averaged for three years. Thus, total annual heat energy output of SWHP with 4 ETC, each made of 15 extruded aluminum HPs is 4855.5 kW h. Diagram of Dtw water heating rate in the storage tank vs. irradiative solar flux variation during one year
operation is shown in Fig. 7. Data were taken at the time with zero hot water consumption. As it is evident from Fig. 7, the following trend was observed: the lower is the initial water temperature in the tank, the steeper is the slope line, that means the higher is water heating rate, and it corresponds to the thermal efficiency values of this ETC type shown in Fig. 6. In common, all thermal SC designs start water heating at some, their own, minimal value of Eirr – irradiative solar flux. Herewith, SCs of some modification start at lower minimal Eirr value – and they are considered to be higher efficient, while the other ones start at higher minimal Eirr values – and they are lower efficient. One could see in Fig. 7 that each of six slope lines starts at the point with diverse abscissas and ordinates. The beginning point of each line corresponds to the start-up of water heating by the SC. So, based on the analysis of the experimental data it is proved the following: minimal Eirr – irradiative solar flux values, which cause collector’s start-up, depend not only on the SC design but also on the initial water temperature inside of storage tank. Even more, as it is seen in Fig. 7, start-up values of the studied collector get into the band of low, so called, diffused irradiation – lower than 300 W/ m2, particularly they vary from 60 W/m2 at 10 °C initial water temperature to 250 W/m2 at 60 °C initial water temperature. In other words, it was confirmed by experimental measurements that ETC with aluminum HPs operates successfully at low values of solar irradiation, i.e. when it is diffused by the clouds that is especially important for winter and spring–autumn year periods.
Fig. 7. Water heating rate in the storage tank vs. irradiative solar flux for initial water temperature values: 1 – 10 °C, 2 – 20 °C, 3 – 30 °C, 4 – 40 °C, 5 – 50 °C, and 6 – 60 °C.
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Further developments of this topic could be targeted to: upgrade of FPC and ETC with extruded aluminum HPs, their further tests for industrial production and certification. Effective coating with selective properties to aluminum absorbers of FPC could improve its performance. Aluminum HPs with advanced cross-sectional profile would be developed for ETC thermal performances improving. Experimental introduction of FPC with aluminum profiled HPs as a core to facßade systems integration would be done. 7. Conclusions
1. The absorber of SC can be fabricated as the single entity with axially grooved cylindrical container of the aluminum alloy HP owing to the extrusion technology. Wide fin of the HP can function as an absorber plate and by its opposite end – as a heat release surface of SC; axial grooves enhance heat transfer inside the HP similar to a wick. 2. The developed SCs could be easily assembled and installed, are of high modularity and scalability due to utilization of standard separate HP units. SCs designs of various shapes and sizes could be assembled of them. The described collectors could be integrated into building structures. 3. The developed SCs are reliable and durable in operation. SWHP with such collectors could be easily maintained and upgraded without recharging coolant of circulation loop. 4. Developed HPs were fabricated and experimentally tested for meeting the requirements on their applications to FPC and ETC. Maximal heat power, which can be transferred by HPs experimental prototypes being in horizontal position, is not less than 210 W, and being positioned at tilt angles from 5° to 90° – is not less than 300 W. Thermal resistance of the developed profiled aluminum HPs is negligibly low and its values are within 0.02–0.07 °C/W. 5. Regarding test results for both SC types: FPC and ETC with originally profiled aluminum HPs as a core, it is claimed that their thermal efficiency factors are high: 0.45–0.72 for FPC and 0.4–0.6 for ETC. It was proved, that proposed SCs are operational, function according to the physical laws, which are common to the other thermal SCs, and their thermal performances are not worse than those of the compared ones without HPs, or made of copper with higher thermal conductivity. 6. Especially important advantage of the developed SCs is their very low hydraulic resistance: 10 Pa for FPC and 70 Pa for ETC (for Vitosol 100-F FPC hydraulic resistance is 21 200 Pa, for Vitosol 300 ETC – it is 2 500 Pa). So significant superiority of the developed SC
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over the other ones has been achieved due to reduced circulation loop with the help of HPs application and heat exchanger innovative designs. It is a great merit for using a lot of solar collectors in one system or PV controlled circulation pumps.
Acknowledgements Authors express their gratitude to Scientific and Technical Center of Ukraine for share granting of the research funding within the Project # 3984 and Professor Gennadiy Frolov, who extended all kinds of practical and administration assistance for carrying out the study on SCs at the “Heliocenter” in Crimea (Ukraine) of Material Science Institute of NASU. References Journal of Heat Transfer Policy on Reporting Uncertainties in Experimental Measurements and Results, Journal of Heat Transfer, vol. 115/5, February 1993.. Engineering VITOSOL 100, 200, 300, Viessmann #5829 135–8 GUS 4/ 2006. Azad, E., 2008. Theoretical and experimental investigation of heat pipe solar collector. Experimental Thermal and Fluid Science 32 (8), 1666– 1672. Baturkin, V., Rassamakin, B., Khayrnasov, S., Shevel, E., 2005. Grooved heat pipes with porous deposit to enhance heat transfer in the evaporator. In: Laplace, S., Pascal, W. (Eds.), International Conference “heat Pipes for Space Application”, Moscow, Russia, September 15–18, 2009, p. 11; Effects of magnetic field to the water droplets, Physics 20(3), 395–399. Brunold, S., Frey, R., Frei, U., 2007. A comparison of three different collectors for process heat applications. Solarenergie pru¨f – und forschungsstelle ingenieurschule ITR, 15p. Burch, Jay, 2007. An overview of one-sun solar thermal technology. In: SEET Solar Thermal Seminar, 26 July 2007, 96pp. DIN CERTCO. Summary of Collector Testing for MT 58-1800. Registration No 011-7S231R, March 2007. DIN CERTCO. Summary of collector testing for Vitosol 100-F Typ SV1. Registration No 011-7S329-F, October 2007. Wongee, Chun, Kang, YongHeack, Kwak, HeeYoul, Lee, Young Soo, 1999. An experimental study of the utilization of heat pipes for solar water heaters. Appl. Therm. Eng. 19 (8), 807–817. Eisker, 2003. Solar Technologies for Buildings. EN 12975-1, 2006. Thermal Solar Systems and Components. Solar Collectors. General Requirements. EN 12975-2, 2006. Thermal Solar Systems and Components. Solar Collectors. Test Methods. Esen, Mehmet, 2004. Thermal performance of a solar cooker integrated vacuum-tube collector with heat pipes containing different refrigerants. Solar Energy 76 (6), 751–757. Faca˜o, Jorge, Oliveira, Armando C., 2004. Analysis of a plate heat pipe solar collector. In: SET 2004 – International Conference on Sustainable Energy Technologies, Nottingham, UK, June 28–30, 2004. Isaksson, Charlotta, Ja¨hnig, Dagmar, 2005. In: Report: WP3.D5 Recommendations for Concepts for Easy Installation and Integration in Conventional Heating Appliances. Project “NEGST – New Generation of Solar Thermal Systems. .
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Isaksson, Charlotta, Ja¨hnig, Dagmar, 2007. In: Report: WP3 D7/D8 Recommendations Easily Installed Products. Project “NEGST – New Generation of Solar Thermal Systems.. Mahjouri, F., 2005. Vacuum tube liquid–vapor (heat-pipe) collectors ; Thermo technologies, Web. 16 September 2009 .
Philibert, Cedric, 2007. The Present and Future use of Solar Thermal Energy as a Primary Source of Energy. The Inter Academy Council, 16p. Riffat, S.B., Doherty, P.S., Abdel Aziz, E.I., 2000. Performance testing of different types of liquid flat plate collectors. Int. J. Energy Res. 24 and 25 (13), 1203–1215. Irene, Stadler, 2001. Facade integrated solar thermal collector. AEE Arbeidsgemeinschaft ERNEUERBARE ENERGIE. 10p.
Solar Energy 94 (2013) 145–154 www.elsevier.com/locate/solener
Aluminum heat pipes applied in solar collectors Boris Rassamakin, Sergii Khairnasov ⇑, Vladilen Zaripov, Andrii Rassamakin, Olga Alforova Heat Pipes Laboratory, National Technical University of Ukraine, Kyiv Polytechnic Institute, Off. 709, 6 Polytechnichna Str., Kyiv-56 03056, Ukraine Received 7 June 2011; received in revised form 20 March 2013; accepted 28 April 2013 Available online 3 June 2013 Communicated by: Associate Editor Ursula Eicker
Abstract The previous researchers have developed a variety of liquid thermal solar collectors designs for water heating. It was reported by the other authors, that metal heat pipes applications to liquid solar collectors, especially to evacuated glass tube ones, is an efficient solution for water heating plants. However, the majority of thermal solar collectors do not meet the requirements on small weight, easy assembly and installation, versatility, scalability, and adaptability of the design, which are particularly important when they are facßade integrated. Very high hydraulic resistance, from 2000 Pa to 20,000 Pa, in liquid solar collectors and low thermal efficiency of some of them, less than 0.5, also are the problems to be solved by the developers. Current research is proposing to apply extruded aluminum alloy made heat pipes of original cross-sectional profile with wide fins and longitudinal grooves in order to avoid the above-mentioned drawbacks of liquid thermal collectors. Absorber plate of flat collectors could be composed of several fins. Fins at the opposite end of the heat pipe serve as a heat sink surface. Multiple tests proved that new lightweight and inexpensive heat pipes show high thermal performances. Maximum heat transfer power of one heat pipe is up to 210 W; and its thermal resistance is very low – from 0.02 to 0.07 °C/W. Hydraulic resistance of flat plate solar collector and evacuated one utilizing aluminum profiled heat pipes, could be reduced to less than 100 Pa, at the same time their thermal efficiency is rather high, up to 0.72. In the issue of authors study, the feasibility of the developed aluminum profiled heat pipes application to thermal solar collectors was proved; and they can be successfully integrated to building facßades and roofs. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Heat pipe; Solar collector; Absorber plate; Heat exchanger; Efficiency; Aluminum extrusion
1. Introduction Due to steady growth in energy sources costs, solar water heating systems become more economically advisable for use in residential, private, commercial, and industrial buildings, schools, hotels, swimming pools, industrial
Abbreviations: HP, heat pipe; SC, solar collector; PV, photovoltaic; SWHP, Solar Water Heating Plant; FPC, flat plate collector; ETC, evacuated tube collector. ⇑ Corresponding author. Tel./fax: +380 44 406 83 66. E-mail address: offi[email protected] (S. Khairnasov). 0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.04.031
processes, drying and distilling or autonomous boiler plants. Nevertheless solar water heating systems production technology is already well developed, it is being continuously perfected. Thermal solar collector (SC) is a key component of Solar Water Heating Plant (SWHP). Its thermal efficiency (g) mainly determines the efficiency of the whole SWHP (Burch, 2007). Copper or aluminum is most frequently used as the material for heat absorbing surface (absorber) in SC. Currently the most common types of liquid solar thermal collectors are the flat and evacuated ones. In general,
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Nomenclature RHP t Dtw Quse Qtotal Eirr. Fh.a.
thermal resistance of the heat pipe (K/W) temperature (°C) water heating rate in the storage tank useful solar heat (W) total solar heat (W) irradiative solar flux (W/m2) heat absorbing area (m2)
typical flat plate solar collector (FPC) (Philibert, 2007; Brunold et al., 2007; Riffat et al., 2000; Faca˜o and Oliveira et al., 2004) consists of glazed or unglazed absorber plate and variously configured pipeline for heat transfer (working) fluid circulation. All components are encapsulated into the casing with thermal insulation at rear and lateral sides. FPC has several advantages: reliability, manufacturability, durability, and low cost. In addition, they are highly efficient, up to 0.75 in summer time when ambient temperature and solar irradiance are high. However, their thermal performance is not high enough (less than 0.4) at low values of insolation and ambient temperature, for example, for Central and Northern Europe climatic conditions, especially in wintertime, when diffused (less than 300 W/m2) solar irradiation prevails. This disadvantage of FPC is mainly caused by rather high heat losses due to insufficient thermal insulation. Another considerable drawback of FPC is high hydraulic resistance in heat transfer circulation loop. Parallel piping of heat carrier in a shape of the harp a little bit lessens the latter disadvantage, but such FPC designs are rather complicated, expensive, not reliable and durable (Burch, 2007). Evacuated tube solar collectors (ETCs) (Brunold et al., 2007; Mahjouri, 2005) are of higher efficiency (nearly to 0.6) at low values of insolation and ambient temperature, even at below zero levels. Heat losses in ETC are small owing to high thermal insulation by means of evacuation inside the tubes. In general, such SC is a frame with several (usually from 10 to 30 units) evacuated glass tubes fixed to it, usually from 10 to 30 units. They may be of “glass– glass” type with double wall tube flask or “glass–metal” with single wall tube (Mahjouri, 2005). In the first case low pressure is being created between the walls of the glass flask, in the second one – the whole inner space of the tube is under low pressure. Heat transfer loop in the ETC may be done as U-type pipeline or copper made HP inserted into the glass tube (Chun, 1999; Esen, 2004). HPs application to SCs as passive heat transfer devices makes possible to avoid some disadvantages of SC. SCs with HPs for heat transfer has low hydraulic thermal resistance, uniform flow of heat transfer liquid, and almost isothermal heat absorbing surface. HP is reliable and durable device in operation. However, even such modern high-tech SC designs have some drawbacks. The main of them is high thermal
g
efficiency factor of solar collector
Subscripts out outlet in inlet
resistance of contact between the absorber and the HP, which is caused by method of joining of physical contact between the absorber plate and the pipeline or HP. Usually they are tightened, glued, soldered or welded together; and may be made of different materials, for example: copper heat pipe or pipeline and copper or aluminum absorber. One more disadvantage of conventional SC is its rather high thermal resistance in the manifold heat exchanger, i.e. high temperature gradient between heat transfer fluid and internal condenser part of the HP. It is caused by small contact area for heat transfer. Furthermore, condenser parts of the heat pipes are inserted into the manifold and obstruct heat transfer fluid flow and slow it down, thus increasing hydraulic resistance inside the manifold. Another aspect of solar collector designs taken into consideration by the authors in their research is observed tendency of SC integration into building facßade systems (Stadler, 2001). «Facßade-integrated collectors are becoming more and more popular. An advantage of facade-integrated collectors consists in a rather even irradiation of sunlight over the year, which is due to their vertical installation. This is very interesting for solar systems as a lot of irradiation can be used in winter, when the highest heat demand occurs for space heating. Further arguments for installing solar thermal collectors on the facßade are that there is often not enough space on the roof or no suitable oriented roof area is available. This is typically the case for multi-family buildings with a relative high number of floors.» (Eisker, 2003). Thus, facßade-integrated collectors should meet additional requirements such as: scalability, i.e. to be assembled from several identical modules; easy mounting and standard installation fittings to the exterior of the building; low hydraulic resistance of the loop; and, of course, low cost (Isaksson, 2005,2007). The majority of SCs in the market does not meet the most of those demands. ETCs are of not enough mechanical strength for this application. FPCs are more preferable for such a case. At the same time, FPCs after special upgrade could be used for thermal and moisture insulation of the wall or the roof of the building, as well as for their decoration. In order to avoid all the above-mentioned demerits of SCs, the article researchers have developed principally new thermal SC design made of HPs (Azad, 2008). It is intended to super highly effective heat removal from the solar heat absorber and heat transfer to compact manifold heat
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exchanger with low thermal and minimal hydraulic resistances. Moreover, it is presumed, that due to the proposed SC design, typical standard lightweight modules could be simply assembled together in situ. Therefore, they easily turn into integral part of building roof or facßade system. To the authors’ opinion, all these advantages are achievable thanks to application of the HPs made of aluminum alloy and originally shaped by means of extrusion method. Previously the technology of this kind of HPs application to space engineering and satellites had been perfectly developed by the authors. However HPs on-ground application has other peculiarities due to gravity force, they need to be thoroughly studied and verified by the field tests. The main objectives of the work described were to: create thermal liquid SC prototypes with low thermal and hydraulic resistance, where extruded originally shaped aluminum alloy made HPs (Baturkin et al., 2005; EN 12975-1, 2006a,b) are utilized, prove their applicability to SWHP, consider their facßade integration feasibility, assess their thermal performances in comparison with other ones. Results on original aluminum HPs developments and their experimental study as well as their application results to both kinds of thermal SCs–FPC with (213 100 8.5 cm) outer dimensions and 1.98 m2 absorber area, and «glass–glass» ETC with (210 135 12.5 cm) outer dimensions and 2.0 m2 absorber area are adduced in the article. 2. Description of the heat pipes Heat pipe is known as super highly efficient heat transfer device, where closed evaporation–condensation cycle is used for operation. Heat transfer fluid boils and changes its phase state from liquid to vapor and vice versa inside the sealed container. Evaporated working fluid, also called as a “heat carrier” or “cooling agent”, is then condensing and again returning from condensation (heat sink) zone to evaporation zone (heat source) due to gravitation or capillary forces. Heat transfer ability of the HP is 1000–10,000 times higher than of cylindrical copper bar of the same dimensions. The following additional design features are used for raising HP’s heat transfer performance. Flat plates, so called «fins» are made on the external surface of the HP for expanding its heat transfer area. Other special measures to enhance heat transfer fluid flow inside the HP are wick inserts into it, usually made of metal fiber, or longitudinally grooved inner walls. They increase the capillary forces. HPs applied to SC should meet the following requirements: – they should function at the tilt angles from 0° (horizontal collector attitude) up to 90° (vertical collector attitude). – heat transfer ability of one HP should be at least 150 W; – HP’s life time should be not less than 20 years. – the coolant should have low boiling point (30–50 °C), but
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– the should keep its operation ability up to 250 °C; – freezing point of the coolant should be not higher than minus 40 °C. Taking into consideration the above mentioned problems and requirements, originally designed aluminum alloy made longitudinally grooved HP (Fig. 1) was proposed and manufactured by the authors in order to apply it to thermal SC. There absorber plate, being the fin of the HP, is extruded as an entire whole with longitudinally grooved HP. Finned cylindrical HP’s container was extruded from the rod made of 6060/6063 aluminum alloy. Due to extrusion method not only heat transfer fin and cylindrical container are built as one entity, but also inner walls of the container are longitudinally grooved. Grooves are distinctively shaped (Baturkin et al., 2005). Therefore, there is almost no thermal resistance between the heat transfer surface and the container of the HP; and HP heat transfer ability is heightened. At the same time one part of wide HP’s fin could be an absorber of SC, thus the absorber and heat transfer device being one entity. Absorber plate of FPC could be formed of several HPs’ fins. The opposite end of the HP’s fin becomes a heat remover to the manifold heat exchanger.
3. Experimental study of the heat pipe performance Tests on overpressure outburst of aluminum cases of HPs’ prototypes had been carried out prior to their thermal engineering study. It was proved by the tests that cylindrical cases for HPs can endure up to 20 MPa of internal overpressure. Experimental study on thermal engineering performances of developed and manufactured aluminum profiled
Fig. 1. Aluminum finned HP proposed by the authors: (a) outer view of the HP’s fragment and (b) cross-sectional profile of the HP: 1 – absorber and 2 – grooved HP.
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HPs was carried out at a special experimental test-bench, which is shown in Fig. 2. Contact temperature sensors– copper–constantan thermocouples with diameters from 0.1 to 0.16 mm were used for temperature measurements. Thermal sensors were fixed at the outside of the HP’s container with the help of aluminized adhesive tape. Nichrome alloy heater coiled round the cylindrically shaped HP (without fins) was used as a heat source simulator instead of real fins. Heat losses were reduced with the help of highly porous basalt fiber thermal insulation of the heat source simulator together with HP’s evaporative
part. Special flat heat exchanger filled with fluid and fixed to the condenser part of the HP was used as a heat sink of the HP. Running water of constant temperature was removing heat from the heat exchanger. Thermal resistance RHP, measured in °C/W, is determined by following equation: RHP ¼
ðt1 þ t2 þ t3 Þ ðt5 þ t6 þ t7 Þ ; 3Q
ð1Þ
where t1, t2, t3, – temperature values at the heat source part of the HP (°C); t5, t6, t7 – temperature values at the heat
Fig. 2. Experimental test – bench for study on heat power engineering performances of the HPs. (a) Computer aided experimental data control and processing system. (b) Clean room with equipped and tooled prototypes for measuring their thermal performances; 1 – experimental HP prototype (its condenser section); 2 – experimental HP prototype (its evaporator section) with coiled heat source simulator; 3 – multi channel analogous data measuring unit; 4 – wattmeter; 5 – PC; 6 – RS485/RS232 AC3 adapter; 7 – flow rate meter; 8 – running water sink; 9 – hydraulic valve; 10 – autotransformer; and 11 – fuses.
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sink part of the HP (°C); Q – transferred heat power (W), measured by wattmeter. HPs thermal resistance curve vs. heat transfer power was obtained in the issue of the experiments. It is presented in Fig. 3 with indication of some selected points. Experimental study results on maximal heat transfer ability are the following: – maximal heat power transferred by the each HP is 210– 215 W at 0° tilt angle, which corresponds to its horizontal position, and it is rather high due to special capillary pump (longitudinal grooves) inside HP; – maximal heat power transferred by the each HP is higher-over 300 W at the tilt angles from 5° to 90° (when HP operates as a thermosiphon). The lower heat transfer ability at small tilt angles of the HPs can be explained from physics view point as follows: the working fluid (coolant) is returned to the evaporator part of the HP by the capillary forces predominantly to gravity, and at 0° angle – only by them. The experimental uncertainty of the thermal resistance of aluminum profiled HP was computed in accordance with the following equation (Anon., 1993): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2ffi U Rtt U DT UQ ð2Þ ¼ þ Rtt DT Q
The heat loss was accounted as 6% of the transferred heat power. This way, the estimated maximum uncertainty of HP’s thermal resistance for the 95 confidence interval is 8.1%, and for Q = 50 W thermal resistance of the HP is 0.071 ± 0.006 K/W. Respective error bars are indicated by horizontal dashes in Fig. 3. Cooling agent inside the HPs is frozen up at temperature values below minus 40 °C and its stagnation temperature is higher than plus 250 °C. Aluminum HPs prototypes applicable to SCs have been obtained as the result of R&D and fabrication. 4. Description of prototype solar collectors Experimental FPC prototype (see Table 1) has been fabricated in order to prove the feasibility of profiled aluminum HPs application to FPC and assessment of the concept on using them as facßade-integrated ones (Fig. 4). Solar collector panel for absorbing heat from solar irradiation consists of eight finned aluminum HPs made by means of extrusion. Solar heat is absorbed by flat longitudinal fins of the HPs and then transferred through condensation zones of the HPs to small heat exchanger installed on them. Proposed HPs design makes it possible to implement more efficient method of heat transfer in FPC owing to:
The temperature uncertainty of the thermocouple was 0.5 °C. The uncertainty caused by the wattmeter for measurements of transferred heat power was 2%.
Fig. 3. Thermal resistance of aluminum profiled HP vs. heat transfer power at the tilt angles from 0° up to 90°.
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(1) significant increase of heat transfer area and, thus, decrease in temperature gradients between the condensation zone of the HP and coolant in the manifold heat exchanger; (2) very low hydraulic resistance of FPC due to “smart” external connection of the HPs condensation zones to the heat exchanger; (3) reliable “dry” connection of the HPs to the heat exchanger without risks of leakage. Thus highly efficient operation of SWHP, where many connected collectors are used, is possible as well as electric power consumption of circulation pumps could be reduced. The latter advantage of SC is of great importance for PV powered autonomous solar plants, where low power consumption circulation pumps are used. Such technical solution results in shorter payback period of solar heating systems.
Table 1 Specifications of FPC and ETC prototypes worked out by the authors. No
Collector’s feature
FPC
ETC
1 2 3 4 5 6 7
Length (m) Width (m) Absorber area (m2) Number of heat pipes Diameter of the heat pipe (mm) Absorber coating Glazing transmittance coefficient
2.13 1.00 1.98 8 14 Black anodized 0.83
2.10 1.35 2.0 15 8 Selective coating of evacuated “glass–glass” tube 0.91
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(shower, washing stands) (6). Data acquisition equipment of experimental plant consists of computer aided multichannel temperature and solar flux measuring system, which includes analog data input unit (7), signal adapter (8), and PC (9). Instruments for measurements while the experimental tests: – – – –
CMP3 pyranometer; resistance thermometers; timer with 0.1 s scale division; liquid flow rate meter.
Computer aided multi-channel temperature and solar flux measuring system is equipped with 8 channels for data measuring – 7 for temperature measurements and 1 – for incident solar flux. 5.1. Test procedure
Fig. 4. SC made of aluminum profiled HPs as an element of building facßade coating.
Besides that, facßade-integrated solar collecting systems could be composed easily and inexpensively, for example, in a way shown in Fig. 4. In such a case, there is no need to fabricate SC as a separate device. Only HPs and hydraulic loop, which contains the heat exchanger, should be made. Number of HPs and their length could be optional, and they could be replaced anytime. Solar heating systems with HPs could be upgraded easily by removing or adding HPs, because there is no need to discharge the working fluid from the loop, to stop the unit, and to recharge. Experimental “glass–glass” ETC prototype with inserted aluminum HPs has been fabricated in order to assess the expediency of aluminum HPs application to ETC. Evacuated double walled glass flask with selective coating on the outside surface of inner wall was used for solar heat collection. In this case, the fins made of aluminum foil and attached to aluminum HP absorb the heat. 5. Experimental study of prototype solar collectors Experimental study of SCs, having aluminum heat pipes as a core, was carried out at two independently functioning full-scale SWHPs: one of them with 1FPC (see Table 1) is located in Crimea (Ukraine) and the second one with 1 ETC (see Table 1) is in Kiev-city (Ukraine). Volume of the storage tank of both systems with FPC and ETC was 125 l. Schematic diagram of test SWHP is shown in Fig. 5. SWHP consists of: solar collector (1), water storage tank of 125 liters volume (2), with auxiliary heater, controller (3), pump unit (4), expansion tank (5), water running units
FPC was mounted in the outdoor test facility. Then the pump of circulation loop was switched on, and heat transfer fluid was circulating through it during 15 min while the collector was not irradiated (shadowed with the help of shielding). Flow rate of the heat transfer liquid was 2.5 l/ min. After that FPC was exposed to solar irradiation, and heat transfer liquid was preheated to defined temperature. Mean test temperature values of the heat transfer fluid were as follows: 15, 30; 45; 60; 75°C. Each temperature level was maintained constant for 15 min interval and after that measurements were carried out within 5 min. At first temperature levels were reached in ascending order, and FPC was kept at these levels until the collector came to the steady state, then, in the same manner – in descending order. The collector’s state was considered to be steady, if during 10 min interval the mean temperature values of the heat transfer fluid and ambient air varied not more than by 0.5 °C. The following parameters were measured while the experimental tests: time, heat transfer fluid flow rate, solar irradiance, ambient air temperature, and output and inlet temperature values. Data were averaged for the cycle of measurements. Instantaneous efficiency g and Dt/E values were computed from the measured data. Measurements were repeated for the cases when g values for the same Dt/E varied by more than 10%. Known test methods for determination of SC efficiency were taken as the basis for its estimation (Brunold et al., 2007; Certco, 2007a,b). Thermal efficiency g of SC prototypes was defined vs. X = Dt/Eirr, values, where Dt – is temperature difference between heat absorbing surface and ambient air, Eirr – is irradiative solar flux value. Also solar thermal efficiency factor g could be defined (Brunold et al., 2007) as a ratio of useful solar heat:
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Fig. 5. Experimental plant: (a) structure diagram; (b) FPC made of aluminum HPs; and (c) ETC with aluminum profiled HPs as a core in situ.
Quse ¼ GC p ðtout tin Þ
ð3Þ
to total solar heat, which falls to the collector’s absorber: Qtotal ¼ Eirr F h:a: ;
ð4Þ
where, Cp, G – heat capacity and flow rate of a coolant in the circulation loop; tin, tout – temperature values of the coolant at the inlet and outlet of the collector’s manifold, Fh.a. – effective heat absorbing area. The experimental uncertainties of the efficiency of SCs were estimated in accordance with the following equation (Anon., 1993): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 2ffi Ug UG U Cp U DT U Eirr UF ¼ þ þ þ þ g G Cp DT Eirr F
was calculated from measured data, against ETC with copper HPs (MT 58-1800 Certco, 2007a), and FPC without them (Vitosol 100-F, Certco, 2007b), both originating from Germany, are presented in Fig. 6. Data on SCs for comparison were taken from DIN CERTCO certification center for Eirr = 800 W/m2. On the results of experimental estimation study on SCs it became clear that SCs made of aluminum HPs are highly efficient. FPC made of aluminum HPs has slightly lower efficiency against curve 2 for X values within 0.04–0.06. From physics viewpoint it can be explained with the fact that
ð5Þ The maximum temperature uncertainty of the resistance thermometer was 0.4 °C. The maximum uncertainties of the Cp, G, Fh.a. were 0.5%, 2.5%, 0.5% respectively. The uncertainty caused by the solar flux was 10%. Therefore, the estimated maximum uncertainty of HP’s thermal efficiency for the 95 confidence interval is 12.3%, and for X = 0.02 thermal efficiency of the HP is 0.65 ± 0.08. Respective error bars are indicated by horizontal dashes in Fig. 6. High uncertainty of the efficiency results is mainly caused by large 10% total daily error of used CMP3 pyranometer. Some selected points of instantaneous efficiency of ETC and FPC prototypes with aluminum profiled HPs, which
Fig. 6. Thermal efficiency of compared SCs: 1 (curve) – ETC: MT 58-1800 for Eirr = 800 W/m2, 2 (curve) – FPC without HPs Vitosol 100-F for Eirr = 800 W/m2, 3 (dots) – experimental data on ETC with aluminum HPs, and 4 (circles) – experimental data on FPC with aluminum HPs.
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Table 2 Field tests results on the SWHP with 4 ETC made of aluminum grooved HPs. Month
January
February
March
April
May
June
July
August
September
October
November
December
Total per year
Qdirect (kW h/m2) Qdiffused. (kW h/m2) Mean ambient air temperature (°C) QSWHP (kW h/m2)
5.21 22.1 5.6
7.59 32.5 4.2
48.66 51.2 0.7
77.18 59.9 8.7
121.7 69.94 15.1
99.5 78 18.2
126.5 70.8 19.3
104.2 63.7 18.6
83.95 46.2 13.9
33.7 36.5 8.1
7.2 21.8 2.1
1.86 16.2 2.3
717 569 7.7
7.9
12.8
39.7
57.2
81.8
94.2
105.2
97.7
61.9
30.6
11.5
6.3
606.9
selective coating was not applied to FPC. It was glazed with usual window glass with 0.84 transmittance coefficient. Performance of FPC type also could be increased for high X by some upgrading of SC with using low-ironed glass or selective coating to the glass pane. Regarding the ETC with aluminum HPs, its efficiency is close to the compared analogous prototype within 0.06. . .0.11 values of X. Our ETC performances could be increased by research on special improved shape of HPs cross-section profile with round fins for higher thermal contact of aluminum fin with grooved HP and with the inner wall of evacuated glass tube. It is worth to mention, that computed hydraulic resistance of solar SCs made of aluminum HPs is not higher than 70 Pa for ETC and not higher than 10 Pa for FTC at coolant flow rate about 130 l/h. Whereas hydraulic resistance of Vitosol 100-F FTC is 21 200 Pa at the same coolant flow rate (Certco, 2007b), and of Vitosol 300 evacuated tube one with HPs is 2 500 Pa (Anon., xxxx). 6. Results from field test of a system with prototype solar collectors Full-scale SWHP with four ETC made of aluminum HPs (see Fig. 5b and c) was assembled for the private house in Kiev suburb area (Ukraine) in order to prove its application expediency. ETCs are of the same design as previously described experimental one. The capacity of storage tank is 460 l. SWHP has been working successfully for 4 years. It endured cycled overheating many times, reaching almost stagnation temperature of 240 °C. Efficiency decrease of solar water heating system or HPs’ leaks or damages were not observed. Created SWHP provides up to 90% of seasonal demand in hot water due to solar energy. Field tests results on monthly QSWHP – heat output of the SWHP with 4 ETC having extruded aluminum HPs as a core, placed at the private house in Kiev region are shown in Table 2. Monthly mean values of solar irradiance to horizontal plane and ambient air temperatures for Kiev region are taken from http://soda-is.com. Data were averaged for three years. Thus, total annual heat energy output of SWHP with 4 ETC, each made of 15 extruded aluminum HPs is 4855.5 kW h. Diagram of Dtw water heating rate in the storage tank vs. irradiative solar flux variation during one year
operation is shown in Fig. 7. Data were taken at the time with zero hot water consumption. As it is evident from Fig. 7, the following trend was observed: the lower is the initial water temperature in the tank, the steeper is the slope line, that means the higher is water heating rate, and it corresponds to the thermal efficiency values of this ETC type shown in Fig. 6. In common, all thermal SC designs start water heating at some, their own, minimal value of Eirr – irradiative solar flux. Herewith, SCs of some modification start at lower minimal Eirr value – and they are considered to be higher efficient, while the other ones start at higher minimal Eirr values – and they are lower efficient. One could see in Fig. 7 that each of six slope lines starts at the point with diverse abscissas and ordinates. The beginning point of each line corresponds to the start-up of water heating by the SC. So, based on the analysis of the experimental data it is proved the following: minimal Eirr – irradiative solar flux values, which cause collector’s start-up, depend not only on the SC design but also on the initial water temperature inside of storage tank. Even more, as it is seen in Fig. 7, start-up values of the studied collector get into the band of low, so called, diffused irradiation – lower than 300 W/ m2, particularly they vary from 60 W/m2 at 10 °C initial water temperature to 250 W/m2 at 60 °C initial water temperature. In other words, it was confirmed by experimental measurements that ETC with aluminum HPs operates successfully at low values of solar irradiation, i.e. when it is diffused by the clouds that is especially important for winter and spring–autumn year periods.
Fig. 7. Water heating rate in the storage tank vs. irradiative solar flux for initial water temperature values: 1 – 10 °C, 2 – 20 °C, 3 – 30 °C, 4 – 40 °C, 5 – 50 °C, and 6 – 60 °C.
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Further developments of this topic could be targeted to: upgrade of FPC and ETC with extruded aluminum HPs, their further tests for industrial production and certification. Effective coating with selective properties to aluminum absorbers of FPC could improve its performance. Aluminum HPs with advanced cross-sectional profile would be developed for ETC thermal performances improving. Experimental introduction of FPC with aluminum profiled HPs as a core to facßade systems integration would be done. 7. Conclusions
1. The absorber of SC can be fabricated as the single entity with axially grooved cylindrical container of the aluminum alloy HP owing to the extrusion technology. Wide fin of the HP can function as an absorber plate and by its opposite end – as a heat release surface of SC; axial grooves enhance heat transfer inside the HP similar to a wick. 2. The developed SCs could be easily assembled and installed, are of high modularity and scalability due to utilization of standard separate HP units. SCs designs of various shapes and sizes could be assembled of them. The described collectors could be integrated into building structures. 3. The developed SCs are reliable and durable in operation. SWHP with such collectors could be easily maintained and upgraded without recharging coolant of circulation loop. 4. Developed HPs were fabricated and experimentally tested for meeting the requirements on their applications to FPC and ETC. Maximal heat power, which can be transferred by HPs experimental prototypes being in horizontal position, is not less than 210 W, and being positioned at tilt angles from 5° to 90° – is not less than 300 W. Thermal resistance of the developed profiled aluminum HPs is negligibly low and its values are within 0.02–0.07 °C/W. 5. Regarding test results for both SC types: FPC and ETC with originally profiled aluminum HPs as a core, it is claimed that their thermal efficiency factors are high: 0.45–0.72 for FPC and 0.4–0.6 for ETC. It was proved, that proposed SCs are operational, function according to the physical laws, which are common to the other thermal SCs, and their thermal performances are not worse than those of the compared ones without HPs, or made of copper with higher thermal conductivity. 6. Especially important advantage of the developed SCs is their very low hydraulic resistance: 10 Pa for FPC and 70 Pa for ETC (for Vitosol 100-F FPC hydraulic resistance is 21 200 Pa, for Vitosol 300 ETC – it is 2 500 Pa). So significant superiority of the developed SC
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over the other ones has been achieved due to reduced circulation loop with the help of HPs application and heat exchanger innovative designs. It is a great merit for using a lot of solar collectors in one system or PV controlled circulation pumps.
Acknowledgements Authors express their gratitude to Scientific and Technical Center of Ukraine for share granting of the research funding within the Project # 3984 and Professor Gennadiy Frolov, who extended all kinds of practical and administration assistance for carrying out the study on SCs at the “Heliocenter” in Crimea (Ukraine) of Material Science Institute of NASU. References Journal of Heat Transfer Policy on Reporting Uncertainties in Experimental Measurements and Results, Journal of Heat Transfer, vol. 115/5, February 1993.
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