Isı borusu prensibinin güneşli su ısıtıcılarına ve damıtmaya uygulanması

Abstract

ÖZET Takdim olunan tezde; ısı borusu kullanarak, güneş enerjisiyle sıcak su üretimi ve tatlı su üretimi (damıtma) ele `alınmıştır. Güneşli su ısıtıcılar, `don` olayı sebebiyle, sınırlı bölge ve iklimlerde kullanılabilmektedir. Kapalı Cçift) devre güneşli su ısıtıcıların ise verimleri, (eşanjör kaybı ve sıcak çalışmadan dolayı) oldukça düşüktür. Isı/ borulu tatbikatlar da; geniş toplayıcı alanında toplanan enerjinin, depoya teksif edilmesinde orta ya çıkan zorluk nedeniyle uygulama alanına girememiştir. Çalışmanın birinci kısmında; her iklim şartına uygun güneşli su ısıtıcısı geliştirilmiş, eşit şartlar altında açık ve kapalı devre güneşli su ısıtıcılarla beraber denenerek deney sonuçları karşılaştirilmiştır. Yeni sistemin; verimi diğerlerinden yüksek, cevap süresi daha kısa ve depo ortalama sıcaklıkları da ha yüksektir. Bu üstünlükler; ısı borusunun, gizli ısının çeyrimi esasına göre çalışmasından, ters akım ve dolayısıyla gece kaybı olmamasından, en önemlisi de, çalışma sıvısı cinsine göre çalışma sıcaklığının seçilebilmesi özelliklerinden kaynaklanmaktadır. Birçok buharlaştırıcı kol, depo içinde birtek helezon yoğuşturucuda birleştirilerek çok önemli bir problem halledilmiş, değişik sıvı ve geometriler denenmiştir. Damıtma işleminin en önemli problemi ise; yoğuşma bölgesinde, yoğuşmayan gazların mevcudiyetidir ?Birçok araştırmacı, konunun önemini vurgulayan araştırmalar yapmışlar, pratik bir çözüm elde edilememiştir. Damıtma ünitelerinde pekçok hassas cihaz yanında enerjinin yaklaşık %50 kadarı, yoğuşmayı güçleştiren bu gazların sistemden atılması için kullanılmaktadır. Çalışmanın ikinci kısmında; ısı borusunun yoğuşturucu bölgesi, damıtma işleminde yoğuşturma işi için kullanılmış, böylece yoğuşmayanlar problemine kolay bir çözüm getirilmiştir. İsı, borusu literatüründe ilk defa, açık ısı borusunun geçerliliği de-` neylerle gösterilmiş, açığa çıkan yoğuşma gizli ısının tekrar sisteme kazandırılmasının yolları araştırılmıştır. Son kısımda j açık ısı borusunda, besleme suyu giriş bölge sinde, su sıcaklığı ve buhar debisi değişim eğrileri elde edilmiş, günlük ürün debisi değişimi ve toplam ürün tatlı su miktarı üzerine tesir eden değişkenler ve aralarındaki korelasyonlar araştırılarak, katlı regresyon denklemleriyle en uygun eğriler tesbit edilmiştir. Bunlara ait bilgisayar programları ekte sunul muştur. Nihayet, bazı araştırıcıların amprik bulgularıyla deney sonuçları, bir model üzerinde teorik izahı yapılarak karşılaş tırılmıştır.

- II - SUMMARY In this research, the prenciple of heat pipe was applied for both solar water heaters and solar desalination plants. Prototype heat pipes were designed, manufactured and tested on the rigs especially developed for these purposes. Firstly, prototype heat pipes were manufactured with the aim of developing a solar water heater that would not freeze in the winter (with capability of operation in the temperature range-50°C to + 100°C), that would heat water quickly and heat it to at least 70°C. For this reason some experiments were made to see how well the heat pipes would work. Copper pipes containing water, ethanol, methanol and acetone gave especially good results on small experimental solar water heaters. All pipes were about 3 m İn lenght and 3/4` in diameter. Some of them have had wicks of 100 and 400 mesh in their evapo?- rators. The heat pipe testing rig was consisting of a double glazed collector section (0,15x1,80 m ) and a tubular insulated water tank that was 8,5 liters in net volume. The evaporator section of the heat pipe was thermaly bounded to the collector plate while the condenser section (90 cm) protruded into the water tank in the form of a heat exchanger. The heat pipes were filled with working fluids (150 to 400 cm3) in the filling rig. All heat pipes worked without a difficulty. Showing no start up difficulties and gas generation problems and all met the design specification of less than 6°C temperature difference between the condenser and the evaporator. Tank temperatures fluctuated between a low of 60°C to a high of about 95°C. Collector temperatures fluctuated between 25°C at dawn to a maximum of about 100°C at noon. Comparison with an imported heat pipe (N0REN) of identical dimensions shows that local heat pipes are just as good in `** performance, and that some of them are more than 20 % better. The local heat pipes are more than an order of magnitude cheaper than the imported pipe due to the use of special manufacturing techniques developed during this research. Starting from amorning temperature of about 35°C at 10:30 a.m. in the water tank, all temperatures rose uniformly up to about 74°C at 2:30 p.m. considering that the tank volume was about 8,5 liters and the incident solar radiation during interval as obtained from a measuring facility in the Departments, was equivalent to 828 kcal for an insulation area of 0,15x1,8 m^ it is seen that the total efficiency of the prototype heat pipe solar water heater was about 40 %.Next, in the light of above experiments, it was decided to build a solar water heater (SWH) with heat pipes big enough to supply hot water for a whole family. An all weather heat pipe solar water heater, HITGI, comprising a collector of 4 m^ area and a low profile water tank of 170 liters was developed. A single heat pipe consisting of 30 risers (1/2` in diameter) and two inanrlf olds (2n ill diameter) in the evaporator and a s^±t&± ^oxiuensev^O/S^ in diameter, 7 m in lenght) was incorpo rated ??into the HITGI. Condensate metering to individual evaporator legs was done by olefin-f iber wicks. So far not a single competitive application can be cited in the heat pipe literature for the utilization of heat pipes in connection with flat plate collectors. The difficulty emanates mainly from the diffuse nature of solar energy; a multitude of evaporators need be used in the collector. No one has yet found a way to combine a multitude of evaporators into a single condenser which in turn can be introduced into a water tank. These difficulties were eliminated with HITGI. Assuming for HITGI a single piece collector of 4 m^ insulation area, an upper and a lower header of 4 m lenght, 2` diameter each, which would be joined by 30 risers, each of 1 meter lenght 1/2` diameter, the energy transported by each riser comes out to be about 80 W. The maximum flow rate of ethanol for transporting this power would be about 6 g/minute* The olefinf iber wick would then be expected to pump ethanol at about 90°C up a ramp of 2 cm into the riser at the same rate. The liquid ethanol would then trickle down the riser, wetting it along its entire lenght. The collector inclination of 10-15° would easily allow for this gravity assisted operation. Under these conditions flooding (entrainment of condensate to the condenser) would not happen, because entrainment limiting heat flux was about 5 kW. The HITGI then was charged with about 10 liters ethanol'. In effort to guarantee the satisfactory operation of the condenser without the danger of its being flooded by trapped noncondansables a copper buffer tank of 10 liters capacity was attached to the top end of the condenser. On the first day of testing 160 liters of water was heated from a temperature of 37°C to 77°C. On the second day the temperature was reached to 96°C. The average drop in water temperature overnight was about 10°C In Order to know how effective HITGI really was it was tested alongside two conventional solar water heaters of identical- IV dimensions, an open loop system and a closed loop system. One of these heaters (open loop system) was purchased from a broad and the other one was manufactured at the University. All three heaters having the same collector area, same tank size and other details were compared. Extensive tests showed that HITGI always heated the water fastest and to the highest temperature. The peak value of mean tank temperature in HITGI was 23°C higher than the corresponding value in the closed system and 15°C in the open system during normal mode of operation. During a warm mode of operation this gap widened to 33°C and 20°C for the closed and open systems, respectively. The open system closed thisgap partially to 12°C during stagnation conditions thanks to onset of boiling in the HITGI. The corresponding gap in the closed system remained to be 33°C. It must be remembered that both the open and closed loop systems rely on sensible heat transfer, whereas the HITGI operates by latent heat cycle. The thermal mass of the HITGI heating system is much smaller than those of the open and clos ed systems since the former is largely vapor space. Thus HITGI has shorter response times than the other two. In addition, reverse flow is not possible and heat losses are minimized. As the results of theses HITGI was found to be 50 % more effective than the better of the other two heaters. The HITGI had a lower thank than the other two heaters such that it would be architecturally more attractive. The cost of HITGI was comparable to the cost of the other two heaters. It is expected that the HITGI will find ready acceptance in all climates of the world, and help use free solar energy for meeting the needs of people. In the second section of this research, the principle of heat pipe was applied for desalination. For this purpose an `open heat pipe` prototype was developed. Principle of opera tion of the heat pipe was utilized for enhancing phase change and recycling of latent heats in the second stage of the protoype to be developed for this purpose. The expected benefits of the research included the development of an efficient solar desali nation technology that promises cost effectiveness and portability, Since in the conventional solar still almost all of the heat of condensation as well as the sensible heats of distillate and the brine are lost, the yield of fresh water is low (commonly less than 3 1/m^.day). There is, however, an additional andformidable obstacle to efficient condensation in the still, even if the recycling of the latent heat were possible. This is the presence of air in the form of a noncondansable gas. Because in the still the noncondansable gases form a stagnant'blanket of film adjacent to the glass, the evolved water vapor has to diffuse and the heat of condensation has to be conducted through this film in order to get to condensing surface. This is a very inefficient process. Water distillation industry, on the other hand, using non- solar energy sources, has seriously tackled the problem of condensation and hence that of noncondansables by investing hea vily in deaeration equipment, chemical additives and the use of ejectors and vacuum pumps. The phenomenon of the heat pipe is proposed below to be utilized for suppressing and entraining the noncondansables such that efficient condensation and heat transfer can take place. Interestingly, this will be achieved without the use of deaerati on columns, ejectors or vacuum pumps. During operation of gravity assisted heat pipe, the energy applied on the evaporator region raises the fluid temperature such that some of the fluid vaporizes. Soon the whole heat pipe is filled by the saturated pure vapor. As there will be tempera ture difference between the heated evaporator and the cooler condenser section, condensation starts on the condenser wall. The condensed droplets trickle back to the evaporator, only to be re-, evaporated and rushed again to the condenser. This cycle of events causes the vapor to storm into the condenser region with relatively high velocities, such that in effect the resulting momentum of vapor flow develops a pumping action toward the condenser. Conse quently any noncondansables trapped inside the heat pipe are at first consentrated at the condenser, and then pushed farther and farther toward the condenser end under the action of this vapor bombardment. In practice there is a fairly sharp dividing line between the region of trapped noncondansables, which remains essentially at ambient temperature, and the active part of the condenser, which is practically isothermal with the evaporator. It is this very remarkable property of the heat pipe. In this research this advantage was taken into consideration over the condensation process. To this end it is proposed to develop an open ended heat pipe desalination device which effectively sweeps away the noncondansable gas column, giving rise to very high rates of heat tarânsfer and hence condensation. As there would be no pumps, ejectors, arid deaeration columns in such a- VI - converter, the desalination process would be much simplified. To test the validity of the above line of thinking a simple test set up was built. This set up consisted of a heat pipe with suitable openings for water inlet, outlet, and a sink for conden sate and noncondensables. The evaporator section of the heat pipe was clamped to the solar collector plate in a conventional flat plate arragement (same as before). It was observed that the heat pipe converter indeed behaved as expected, and that more and more of the condenser became active as the column of noncondansables was pushed farther and farther back until the entire condenser was essentially isothermal with the evaporator, making it a true heat pipe.Thus very high rates of heat transfer and condensation were possible in the active region of this modified heat pipe, whereas the inactive region was characterized by a temperature lag of about 50°C and very low rates of heat transfer and conden sation. Then, this prototype was improved. Inlets and outlets were made in the form of U-tube to prevent outside air to escape into the pipe. The condenser was bended down along the evaporator in order to take out condensate easily. Effects of superheater region located between the evaporator and adiabatic section, and olefin fiber wick inserted inside the evaporator on daily amount of product distillate water were investigated. The solar collector portion of the heat pipe was boosted in energy possibility by plane reflectors of Selçuk type. Various condenser configurations and lenghts (60 cm to 6 m) were examined. All pipes were made of Copper tube of 3/4`. Some start up difficul ties were seen in the condenser formed spiral and zigzaging. Becau se vapor flow was forced and locked at sharp turnings; stability in heat pipe working could not establish in these cases. Thus, entire condenser could not work as a heat pipe. Consequently, the TOKA type des tiller, that has the simplest design and has stable and smooth working as a heat pipe, was preferred. Flow rates was adjusted such that some of the incoming water would live the system unprocessed. The condenser was insula ted and the condensate yields were measured for various configura tions and lenghts. Optimum feeding rate was found as about 5 g/ minute, and distillate rate was 2-2,5 g/minute. Thus, for the collector of 0,15 x 1,80 m^ area daily product distillate water was about 480 - 520 g/day for single colec tor and about 620-650 g/day for boosted in energy by plane mirrors. The time period in which condenser worked as heat pipe was called effectiveworking time. During these tests, ef fee tiye working time was about 4-5 hours, and this working time become 6-6,5 hours when the flat reflectors were used. Finally, the results of experiments were correlated using multiple regression analysis and following expressions that indicated the product distillate water performance of the TOKA type distiller were presented: n /AQfl 0,1174 v 0,8546 x-^ s 0,4688 X2 x5 where, x-i : daily total distillate water (g/day) X2 : variation of collector temperature with the time (°C/hoürs) x^ : total solar radiation collected during effective working time (kJ/day) This expression is valid for (Lnxi -0,02123) with 0,997 probability. In addition, corresponding formulations were founded out for distillate rate during a day and total product variation with other independent variables. When the flat mirror s iver e used, and selectively coated tracking surface collectors were used instead of conventional flat plate collectors and the latent heat of condensing could be recycled product water would be about 850-950 g/day (which was equivalent to 3,5 liters /m2. day) during effective working time of 4-6 hours a day. This result was found out from above expression. Next, entering region of feeding salted water was investigat ed using energy and mass balance equations. As a result of this study following relationships were yielded : nip = 10`3 / 60 (2-1,4268 x3 + 10,0875 x2) ' 30,49+2, 735x-0,326x2 Tf« 20,l(x3-0,06x +0,032) {100- --- 3,5+0,06x2-x3 n (x2+l,47x+2,28)°>4 6c+0,745) ~J-ri -;?. 1,49 Arctg}. (l,532-x)°>8 lj32- VIII - where : bu : vapor flow rate (kg/s) Tf : temperature of feeding water ( C) x : dimension measured from the begining of introduction region (m) Consequently, the TOKA type desalination unit has effectiv< condenser, because there is pratically no noncondans able gases inside the tube. It does not need any special device for elimi nation these gasses> The TOKA can recycle latent heat of condense t ion and sensible heats also can be reused as pre-heater or economizer. The system is suitable for recovery of latent and sensible heats. The TOKA works quietly» it has no rotating part, it is portable and it does not need energy source except sun.