مساعدة:إضافة الصور

من ويكي مصدر، المكتبة الحرة
اذهب إلى: تصفح، ابحث
مقدمة عامة | قراءة | التحرير | إضافة نصوص جديدة | إضافة صور

لإضافة صورة ما، يفضل تحميلها في ويكيميديا كومونز، على أية حال، بإمكانك تحميل الصور هنا في ويكي مصدر عن طريق الرابط التالي تحميل ملف. بإمكانك أيضا الوصول إلى هذا الرابط عن طريق صندوق "أدوات" في يمين الصفحة. في جميع الأحوال، عليك التأكد من حقوق النشر للصورة المراد رفعها إلى الموقع. راجع سياسة استعمال الصور في ويكيبيديا وكيفية تحقيق الاستخدام العادل للصور للمزيد من المعلومات حول حقوق توزيع الصور ونشرها.

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4.1 System Components andAnalysis

Before going into the details of maintenance done, we will have a brief look at the system’s components, connections, andenergy analysis. Figure 4.1 is a schematic diagram thatshows the system components and connections.










Figure 4.1: Schematic diagram of systems’ components and connections


4.1.1 Solar Loop Circuit The solar loop consists essentially of solar collectors, which are DSCC25 selective copper absorber flat plate collectors by Derya Solar of an area equal to 59 m^2 installed at the roof of HMCSR, University of Jordan. The collectors are tilted at an angle of 25 degrees and placed so that the surface azimuth angle is zero. Figure 4.2 shows the system collectors







Figure 4.2: solar flat plate collectors The solar loop pump denoted in figure 4.1 as pump 1, supplies a flow rate of 1.5 m^3/hr and is connected to an on-off controller which turns it on when the temperature difference between the solar collectors’ outlet and the hot water storage exceeds 10 °C for effective heating of hot water storage. Moreover, the loop contains air relieve valves, safety valves against faulty high pressure, non-return valve to prevent backflow of water, expansion vessel to take in the expansion volume of steam as system overheats and a heat exchanger inside the insulated hot water storage tank. A PT 1000 resistance temperature sensor is used to measure the temperature directly at the collectors’ output [14]. Field solar and weather data is taken from the weather station installed at the roof of the Engineering Faculty, University of Jordan.

Figure 4.3 shows air relief valve releasing steam






Figure 4.3: air vent

4.1.2Chilled Water Circuit and SAC The SAC is of type ACS 08 water-silica gel chiller by SorTech with rated cooling power of 8 kW at given conditions of 55 °C hot water inlet, 27 °C cooling water temperature, and 18 °C chilled water set point. Figure 4.4 shows the SAC used and table 4.1 next shows further technical data of the chiller [12].




Figure 4.4: SorTech adsorption chiller Table 4.1: SorTech chiller technical data










Chilled water that flows from the adsorption chiller outlet after losing its heat to the evaporator of the SAC through a heat exchanger is stored in an isolated cold water storage tank. The chilled water is then heated by the air from the fan coil unit (which becomes cooler) creating therefrigeration effect required.Chilled water circuit pump is denoted by pump 4 on figure 4.1. The supplied flow rate by the pump is about 1.6m^3/hr. The volumetric flow rates were all measured using electronic flow sensors.

4.1.3 Heat Driving Circuit This circuit represents the thermal supply input to the desorption chamber of the SAC. The circuit consists of the hot water storage, which supplies the desorption chamber with the required thermal power input through a heat exchanger. The water is pumped using a WILO type Stratos PARA 25/1-7 T3 pumpdenoted by pump 2 in figure 4.1. This gives a hot water supply rate of about 1.4m^3/hr. Figure 4.5 shows the hot water storage.




Figure 4.5: hot water storage tank

4.1.4 Cooling Water Circuit After adsorption-desorption process silica gel becomes hot and its adsorption ability weakens, this circuit is used to cool the silica gel in the adsorption chiller. This is done by pumping cool water using a WILO type Stratos PARA 25/1-8 T3 pump denoted bypump 3in figure 4.1. The water absorbs heat from the silica gel through a heat exchanger, and then gets passedthrough a cooling tower to lose the heat it gained through a second heat exchanger.



Figure 4.6 shows the cooling tower used.





Figure 4.6: Cooling tower

4.2 System Energy Analysis The solar collectors heating power (SCHP), is the heat transfer rate by which the water gains thermal energy between the inlet and the outlet of the collectors and is given by equation 4.1 below. SCHP=m ̇_(1 )× C_(P,1 )×(T_2- T_1)……………………………………...…………………….4.1 Where SCHP is the solar collectors heating power in kW, m ̇_(1 )is the mass flow rate of the solar collectors loop in kg/s, C_(P,1 )is the specific heat capacity of water at average of T_1 and T_2 in kJ/(kg.k)andT_1 and T_2 are the temperatures in °C at the inlet and outlet of the solar collector respectively indicated on the diagram.



The solar collectors’ circuitefficiencyη_c is given by equation 4.2. η_c= SCHP/(DNI ×A) ………………………………………………………………………………4.2 Where η_c is the solar collectors’ circuit efficiency, DNI is the direct normal irradiance in kW/m^2, and A is the total collectors’ area inm^2.

The heat power input used by adsorption chillerQ ̇_(in,ads) is calculated using equation 4.3. Q ̇_(in,ads)= m ̇_2×C_(p )×(T_5-T_4)………………………………………………………………4.3 Where Q ̇_(in,ads) is the thermal input to chiller in kW, m ̇_2 is the mass flow rate inkg/s of the heat driving circuit (thermal supply circuit to adsorption chiller),C_(p )is the average specific heat capacity in kJ/(kg.k) of water in the driving circuit, and T_5 and T_4are the inlet and outlet temperatures in °C to and from the heat exchanger of desorption chamber.Note that equation 4.3 assumes 100% efficiency of heat exchange between hot water and desorption chamber.

The cooling capacity of the chiller is given by chilled water thermal power transfer Q ̇_CHW found using equation 4.4. Q ̇_CHW= m ̇_3×C_p×(T_6-T_7)……………………………………………………………….4.4 Where Q ̇_CHW is the chiller’s cooling capacity in kW, m ̇_3 is the mass flow rate of chilled water in kg/s, C_p is the average specific heat capacity of chilled water inkJ/(kg.k), and T_6and T_7 are the temperature in °C of the chilled water inlet and outlet to and from the chiller’s evaporator.

The coefficient of performance of the adsorption chiller is found as the ratio of the cooling power of the chiller to the driving heat input. This is shown in equation 4.5. COP_chiller= Q ̇_CHW/Q ̇_(in,ads) ……………………………………………………………………4.5 Where COP_chilleris the coefficient of performance of chiller vessel alone, andQ ̇_(CHW )and( Q) ̇_(in,ads) are the cooling capacity of the chiller and driving heat power input in kW respectively.

An external coefficient of performance〖 COP〗_Ext, is defined as the ratio of the chillers cooling output to the total input power including pumps work. COP_Extis given by equation 4.6. COP_Ext = Q ̇_CHW/(Q ̇_(in,ads)+ W ̇_P )…………………………………….…………………………………4.6 Where COP_Extis the external coefficient of performance of the chiller vessel, andQ ̇_CHW,Q ̇_(in,ads), and W ̇_p are the cooling capacity of the chiller, the driving heat power input and the pumps power in kW respectively.

Solar fraction of cooling (SFC) is defined as the fraction of the total required cooling power that is covered by the solar cooling system, SFC is given by equation 4.7. SFC= (Total cooling Power of Chiller )/(Total Cooling Requirement) ………………………………………………………….4.7 Calculation of cooling power, input driving heat and factors above, for the SAC system at HMCSR based on experimental data collected after troubleshooting is presented in Chapter 6: “Results and Discussions.”





4.3 System’s Maintenance Many of the system components,including solar collectors, piping system, fittings, insulation, and pumps were not working properly or damaged. This is mainly due to lack of regular maintenance, high mineral content in mains water supply which leads to scaling and precipitation, and pipes and fittings corrupted as a result of water freezing inside; due to very low ambient winter temperatures in Amman. Details of each component malfunctioning, the expected causes, and methods of repair and maintenance are discussed inthis section.

4.3.1 Solar Collector’s Maintenance Solar collectors used for the Solar Adsorption Chiller (SAC) system at HMCSR are flat plate collectors (DSCC25 selective copper absorber) by Derya Solar. Collectors differed in their repair needs; some had their glass broken, others showed water condensation on the inner surface of the glass indicating cracks in the internal piping of the collector; most likely due to thermal stress fatigue. Some collectors had more than one problem while others just needed cleaning.Figure 4.7 shows some of the collectors before repair.





Figure 4.7: malfunctioning solar collectors


Collectors’ maintenance was done as follows: 1- Glass cover was de-assembled 2- By pressurizing water inside each collector with an air compressor, cracks in the copper internal piping of the collectors could be detected. 3- Cracks were repaired by welding the pipes using silver alloy rods. Silver alloy was chosen for welding for the following reasons: it binds well with the copper internal piping, also has a melting point lower than that of copper; hence, no damaging of pipes could occur. In addition to that, silver alloy used is unreactive; therefore, doesn’t corrode or rust and is relatively inexpensive. 4- Unions that connect the solar collectors with each other were replaced by welding-type copper unions. Silver alloy was again used for welding. 5- Collectors were assembled again fixing the glass using molten silicon and then cleaning it carefully.

4.3.2 System's Piping Maintenance Piping system components including bends, elbows, tees, and pipes had many problems. Most of the parts consisted of rust corroded steel; especially those associated with the hot water circuit, and had calcium deposits precipitated from mains supply increasing the pressure drop loss coefficient. Calcification takes place due to high mineral content in the mains, also hot water circuit is more likely to encounter corroded parts as high temperatures increase the rate of corrosion. Some pipes had cracks and water was leaking out through the insulation; not only this caused water mass loss, but rather had ruined the Rockwool insulation. Malfunctioning components of the piping system were all replaced by new ones.



Figure 4.8 below shows some of the pipes and fittings replaced





Figure 4.8: corroded pipes and fittings

4.3.3 Lift and Circulating Pumps Lift pump used to supply solar collector circuit, hot water storage, cold water storage, and the cooling tower was not working. This pump was replaced by a new SHIMGE-QB60 pump of 0.5 hp which has been chosen based on previous experience. Figure 4.9 shows the old and new pump installed.




(a) old malfunctioning pump (b) new pump installed Figure 4.9: System lift pumps

Solar loop circulating pump was not working, it was replaced by a new SXM32-45 pump by Salmson(see Appendix C1), which is of the same characteristics as the old one. This gave a low mass flow rate of 0.15 m^3/hrwhich caused very high temperatures at the collector outlet; therefore, the pump needed further replacement with a new A 80/180 XM pump by DAB giving a flow rate of about 1.5m^3/hr. This flow rate is recommended for good heat exchange rate between the solar circuit and the hot water storage tank. The pump by DAB was selected as follows: for a flow rate of 0.15m^3/hr, the SXM32-45 pump gave a head of 7.7 m at the highest speed according to its characteristic curve (see Appendix C1). This is the system’s head, using this head and the characteristic curve of the DAB pump (Appendix C2), the equivalent flow rate at the 3rd speed is 1.5m^3/hr; the required flow rate. When the new pump was installed, the flow rate given initially after reaching equilibrium had anaverage of about 0.6 m^3/hr which indicated circuit blockage. The pump was de-assembled, the heat meter filter which was partially closed by dirt was removed, cleaned, and thenthe circuit was assembled again. The system was operated for some time, and the cleaning procedure above was done again. The solar loop circuit was cleaned this way instead of emptying it which is recommended; as there was not enough water to refill the system. The other circulating pumps used in the driving circuit and in the cooling tower circuit were functioning properly.

4.3.4 Safety Valves and Sensors One of the safety valves was broken and needed repair. Maximum pressure endurance of the collectors is 6 bars; therefore, safety valve was chosen to open at 3 bars for a factor of safety of two. Figure 4.10 shows new 3 bar safety valve.



Figure 4.10: Safety valve Thermocouples were used to measure the temperature at the solar collectors’ outlet and the hot water storage temperature only, and no flowmeters were installed for water flowrate measurement. Three electronic compact heat meters with single jet flow sensor of type Zelsius C5-G3/4B by GIACOMINI were installed, where each contains two built in RTDs for temperature measurement in addition to their flowrate measurement facility. The heat meters were fixed at the chilled water inlet to SAC, solar collectors’ heating coil outlet, and the hot-water storage outlet to the desorption chamber. As these heat meters contain a built in RTDs in addition to flow sensor, temperature and flowrate measurements are available at the mentioned positions. The remaining three temperature probes of the heat meters were fixed at the chilled water circuit outlet from SAC, the hot water storage inlet from solar collectors, and the hot-water outlet from the desorption chamber. By connecting these heat meters to a data logger, record of the values read by the meters on regular time bases is validated. Figure 4.11 shows one of the heat meters installed




Figure 4.11: Heat meter





4.3.5 Insulation The insulation material used for pipes isolation to prevent heat loss to surrounding, was made of Rockwool which has a thermal conductivity of 0.044W/(m.K)equivalent to a thermal resistivity of 22.7 (m.K)/W. Many parts of the hotwater circuit were not well insulated; moreover, due to weather conditions and leakage in some pipes, the Rockwool insulation was damaged and needed replacement. Figure 4.12 shows the ruined Rockwool insulation.




 Figure 4.12: Damaged Rockwool insulation

The new material that was used for insulation is Armaflex with a thermal conductivity of 0.038W/(m.K)corresponding to a thermal resistivity of 26.3 (m.k)/Wwhich is about 16% higher than that of Rockwool insulation; hence, improving the system performance by reducing the heat loss. Figure 4.13 shows Armaflex insulation installed.






Figure 4.13: Armaflex insulation installed on pipes