1、CH-06-6 Symposium on Thermal Modeling of Phase-Change Materials in Building Envelopes: Old Problem, New Developments CH-06-6-1 Use of Phase-Change Materials in Solar Domestic Hot Water Tanks Luisa F. Cabeza, Manuel Ibez, Cristian Sol, Joan Roca, Mique1 Nogus, Stefan Hiebler, and Harald Mehling 495 C
2、H-06-6-2 Diurnal Load Reduction Through Phase-Change Building Components Kelly Kissock and Sutrisna Limas 509 CH-06-6-3 Phase-Change Material Modeling within Whole Building Dynamic Simulation Dariusz Heim. . 5 18 CH-06-6-4 Research on Thermal Storage Using Rock Wool Phase-Change Material Ceiling Boa
3、rd Takeshi Kondo and Tadahiko Ibamoto . 526 CH-06-6-1 Use of Phase-Change Materials in Solar Domestic Hot Water Tanks Luisa F. Cabeza, PhD Manuel Ibiez, PhD Cristian Sol Joan Roca Miquel Nogus, PhD ABSTRACT Storage of heat is seen as a major issue for the large-scale and long-term development of sol
4、ar energy for house heating and cooling in all climates. Most of the storage systems avail- able on the market use water as the storage medium. Enhanc- ing storage performance is necessary in order to increase the performance of mostsystems. The idea studied here was to add a phase-change material (
5、PCM) module at the top ofu hot water storage tank with stratijcation. The advantages of the stratijication still remain in this new system, but the addition of a PCM module would give higher density in the top layer: For this work, an experimental solar pilot plant was constructed to test the PCMbeh
6、avior in real conditions, which could work continuously with the solar system or could also work with un electrical heater: The PCM module geometry adopted was to use several cylinders at the top of the water tank. Several experiments with two, fouc andsix PCMmodules were carried out in the real ins
7、tallation. A granular PCM- graphite compound of about 90% sodium acetate trihydrate and IO%graphite was chosen as the PCMfor the experiments presented here. This paper also describes the modeling ofthis new tech- nology. A new TRNSYS component based in the already exist- ing TYPE 60 was developed-TY
8、PE 60PCM. This new component wasjrst tuned with experimental results and after- wards validated with further experiments. Concordance between experimental und simulated data was very good. Since the new TRNSYS component was developed to simulate full solar systems, comparison of experimental results
9、 from a pilot plant solar system with simulations was performed and conjrmed that the TYPE 6OPCM is a powerful tool for eval- uating the performance of PCM modules in water tanks. Stefan Hiebler Harald Mehling, PhD INTRODUCTION Thermal energy storage has recently become of major interest due to the
10、concern about use of renewable energies, such as thermal solar energy, and waste heat. The mismatch between energy demand and energy availability can only be overcome by the use of an energy reservoir. The use of phase-change materials (PCMs) such as water in energy storage has the advantage of high
11、 energy storage vs. sensible heat storage. Another advantage of latent heat storage is its isothermal behavior during the charging and discharging process (Lane 1983). Thermally stratified storage tank systems are an effective technique widely used in energy conservation and load management applicat
12、ions. They are commonly used in solar energy systems but also have other applications such as waste heat reuse. If water of different temperatures is contained in a tank, thermal stratification arises because the temperature variation gives rise to a density variation in the water. The stratificatio
13、n phenomenon is employed to improve the e%- ciency of storage tanks as heat at an intermediate temperature (not high enough to heat the top layer) can still be used to heat the lower, colder layers (Wildin and Truman 1989; Wildin 1989; Wildin 1990; Nelson et al. 1999a; Nelson et al. 1999b). Stores f
14、or heat with different temperatures can also be designed using PCMs. In this case, two or more PCM modules with different melting temperatures would have to be used. The loading can be done separately in each module, with the right temperature level. The unloading of the storage can be carried out w
15、ith a pipe going through the modules (Figure 1). Luisa F. Cabeza is an associate professor, Manuel Ibiez is a lecturer, Cristian Sol is a student, and Joan Roca and Miquel Nogus are lecturers at the University of Lleida, Spain. Stefan Hiebler is a student at the Technical University of Munich and re
16、searcher at ZAE Bayern, Garching, Germany, and Harald Mehling is a researcher at ZAE Bayern. 02006 ASHRAE. 495 water based: PCM based: m unloading t I top layer with potential improvement Hot layer - I- - Transition layer Cold ayer - Figure 1 Diferent concepts for energy storage in tanks. From left
17、to right: hot water heut store with strutijcation, PCM store, hot water heut store with strutijcation and PCM. The advantage of this type of heat store is the good use of low-temperature heat andor waste heat. PCM is good for high energy density if there is a small temperature change, because then t
18、he latent heat is much larger than the sensible heat. On the other hand, the temperature change in the top layer of a hot water heat store with stratification is usually small, as it is held as close as possible at or above the temperature for usage. The idea presented here is to add a PCM module at
19、 the top of a hot water storage tank with stratification. The advantages of the stratification still remain in this new system, but the addition of a PCM module would give a much higher storage density in the top layer (Figure i). The advantages of such a system become obvious in the following examp
20、le. A PCM module with 50% of the radius of the store and 25% of its height (PCM is 1/16 or 6% of total volume) is inserted at the top of the storage tank. Therefore, in the top layer (25% of the store height), one fourth of the volume is PCM and three fourths are water. Let us assume the PCM used ha
21、s an enthalpy of 200 kJ/kg. The latent heat in the PCM module can then heat the water in the top layer, which has a volume three times that of the PCM by 25C. For the stor- age system, three advantages can therefore be identified immediately: 1. 2. 3. The energy density of the storage tank is increa
22、sed. A heat loss of 25C in the top layer can be compensated by the latent heat in the PCM module. It is possible to reheat the transition layer after partial unloading. The work presented here is a thorough study of this appli- cation of PCMs. The study has been conducted experimentally and by model
23、ing, taking into consideration many of the parameters that affect PCM applications. EXPERIMENTS Description of the Laboratory Installation To carry out the first experiments, a cylindrical vertical tank (Figure 2) was built at ZAE Bayern. The tank had a diam- eter of 20 cm and a height of.120 cm. Th
24、e material chosen for the construction was methacrylate. This had two advantages: the interior of the tank could be observed and the heat transfer through the container wall was minimized. For the experiments, the tank was insulated with two different insulation materials giving heat loss coefficien
25、ts of 5.0 W/m2.K and 1.5 W/m2.K. Ten thermocouples were set inside the tank at 10 cm distance from each other to record the temperature of the stratified layers. The temperatures of the water outlet and inlet, as well as the room temperature, were also recorded. All the outlets and inlets were inser
26、ted from the top of the tank. Cold water was conducted to the bottom of the tank with a rubber tube inside an acrylic-glass pipe. The bottom outlet was designed to minimize turbulence during operation, and a turbulence break was also included. The PCM module was fixed in the tank using a rack that a
27、llowed the module to be changed easily. The modules consisted of a brass cylinder with a diameter of 10 cm and a height of 30 cm filled with PCM (6% of the total volume of the tank was PCM). A PCM-graphite composite material with an improved thermal conductivity between 20 W/m.K and 30 W/m.K was use
28、d in the experiments (Mehling et al. 1999; Mehling et al. 2000; Mehling 2000). The temperature of the PCM was recorded in the top center of the module. 496 ASHRAE Transactions: Symposia 4 Unloading Diffusors I turbulence break b, I Figure 2 Experimental setup for the laboratory trials. 70 70 S0.07 0
29、.08-0.16 11.00 0.15-0.35 12.00 20.40 10.045 10.045 10.030 10.045 17.0-19.5 10.30 8.00-10.5 10.30 Five different common and cheap metals were used in the experiments. The metals, with their commercial identifica- tion, were aluminum (EN AW-2007 or AlCuMgPb), brass (Ms58 Flach or CuZn39Pb3), copper (E
30、-Cu 57), steel (St37 K or Mat. No. 1.0345), and stainless steel (Mat. No. 1.4301). Their chemical compositions are listed in Table 1. Metal samples with dimensions of 30 mm (length), 10 mm (width), and x mm (thickness-with x = 2 for brass, x = 3 for copper and stainless steel, and x = 5 for aluminum
31、 and steel) were cleaned with acetone in an ultrasonic bath. Three samples of each metal were immersed in melted PCM contained in glass test tubes, which were placed in a thermo- static water bath at 80C. The corrosion experiments were carried out with the metal pieces alone immersed in the PCM and
32、also with the metal pieces in contact with graphite and immersed in the PCM. Graphite is used in thermal storage systems to enhance heat transfer in the PCM (Mehling et al. 2000; Cabeza et al. 2002). The metal samples were removed from the test tubes after one, two, four, and ten weeks and were eval
33、uated with the following procedure: The pH of the solutions was tested. Changes in solution appearance and characteristics were evaluated to identi qualitatively the precipitate formed. The metal pieces were cleaned thoroughly with tap water and their change in appearance was evaluated visually. The
34、 metal pieces were polished with 150-grain abrasive paper. Gravimetric analyses prior to and following the corro- sion tests provided the mass loss, Am (mg), with respect to the initial mass: Am = m(fo)-rn(t) (1) Measurements prior to and following the tests provided reductions in sample thickness,
35、width, and length (mm). The corrosion rate was defined as mass loss per squared centimeter and year (mg/cm2yr). PCM Module Geometry For the solar installation, the first step was to analyze the correct geometry for the PCM module and the influence of the PCM module on the performance of the water ta
36、nk. The cool- ing-down process of the water from 65C to ambient temper- ature was simulated. The water cooled due to thermal losses to the ambient temperature, with an isolation of 1.5 W/m2.K. Moreover. numerical simulations of the svstem were conducted. 10.1 1 ASHRAE Transactions: Symposia 499 From
37、 the results of these simulations, the solution adopted was using several cylinders at the top of the water tank instead of only one, as is shown in Figure 3. Therefore, several experiments with two, four, and six PCM modules were carried out in the real installation. The modules used were commercia
38、l aluminium bottles filled with almost identical amounts of the PCM-graphite composite material. The dimensions of the PCM modules were 8.8 cm (diameter) and 3 1.5 cm (height), giving 1.5 L capacity. Tests Carried Out in the Solar System Several tests were carried out during 2005 in the real system.
39、 The tests were classified as cooldown process, reheat- ing process, or solar operation. The cooldown process tests consisted of heating the water tank to 80C and then cooling it with natural convection through the wall tank. This test was done without the use ofthe solar collectors. In the reheatin
40、g process tests, the water tank was heated to 80C to ensure the PCM was melted then the tank was emptied and refilled with cold water from the city network. Here the reheating of the top water by the PCM module could be observed (meaning the temperature increase and the time needed to heat up the wa
41、ter). The third set oftests was performed with the solar system. The installation operated with thc solar collectors, the pumps, and the tanks. When the system was used with the solar collec- tors, a five-minute morning shower was simulated to discharge the system. Then, new cold water from the city
42、 network came into the tank and the top of the tank was reheated by the energy stored in the PCM modules. SI MU LATI ON S First Simulation Model The first numerical method to simulate the performance of the storage with the PCM module was implemented using an explicit finite-difference method (Farlo
43、w 1993). The discretization of the model can be seen in Figure 5. The heat store of 120 cm (height) and 20 cm (diameter), as described in the experimental setup, is divided into 12 layers, each one 10 cm thick. Each layer is represented by one knot with one temperature. The top three layers contain
44、the PCM module, which is represented similarly to the water layers. The thermal effects taken into consideration in the model were: 1. heat conduction from one water layer to the next water layer, 2. heat loss from the water to the surroundings, 3. heat transfer PCM-water, and 4. heat loss of PCM. T
45、he temperatures were discretized as q,i, where j corre- sponds to the time knot and i to the space knot. The black dots Tamb i = N+1=13 N=12 11 T Figure 5 Modelizution of the cylindrical vertical tunk used in the laboratory experiments. in Figure 5 indicate knots where thermocouples recorded the tem
46、peratures in the experiments. TRNSYS Store Model Although the effort to simulate the performance of water storage tanks with PCM has previously been done (Mehling et al. 2003; Newton 1995), there are no references that modeled- such a tank, allowing easy implementation in a solar installa- tion with
47、 different elements. The analysis of the real possibilities of water tanks including a PCM module requires the simulation of the operation in different applications and installations. Due to its versatility for the complete simulation of a building and to the possibilities of connecting a simulation
48、 module with other installations such as solar and HVAC systems, the program TRNSYS was selected as a base tool with which to develop the new model of water-PCM tanks. 500 ASHRAE Transactions: Symposia The storage tank, including PCM modules, was based in a standard TRNSYS16 component, TYPE 60. This
49、 stratified fluid tank assumed that a water-filled sensible energy storage subject to stratification could be modeled considering that the tank consisted ofNfully mixed segments of equal volume. The degree of stratification was determined by the value of N. The stratification temperature in the tank was modeled one-dimen- sionally. Options of fixed or variable inlets, unequal size nodes, temperature deadband on heater thermostats, incre- mental loss coefficients, internal submersed heat exchangers, noncircular tanks, horizontal tanks, and losses to the flue of
