Design and simulation of a new solar paraboloid dish collector

Transcription

1 Design and simulation of a new solar paraboloid dish collector E. Bellos a *, C. Tzivanidis a, K. A. Antonopoulos a a National Technical University of Athens, School of Mechanical Engineering, Thermal Department, Heroon Polytehniou 9, Zografou, Athens, Greece Abstract Solar energy utilization is vital for our society which faces the global warming problem and the increase in the prize of fossil fuels. Concentrating solar collectors are a promising technology for producing useful energy from the sun. In this study, an innovative concentrating collector with a paraboloid dish reflector is presented. The main idea of this collector is the use of a dish reflector for continuous heat production. For this reason, a receiver absorber with two linear insulated ducts is used. The cost of this system is relative low because there is not a cover but a tracking system is required. The final results show better performance than the typical collectors such as the flat plate collectors and the evacuated tube collectors, fact that makes this collector a promising solution. The design and the simulation are carried out with the commercial software Solidworks. Keywords: Solar dish collector, Solidworks, Simulation, thermal efficiency 1. Introduction The increasing demand of energy in the modern societies makes the energy production crucial for the new lifestyle. [1-2]. Solar energy utilization is vital for our society because this energy source has the highest availability and the lowest replenishment time among the renewable sources [3]. In the recent years, a lot of studies have been focused on the technological options for converting solar energy into useful energy by efficient ways. For applications with low temperature demand (<90 o C), flat plate collectors are the usual solution with a low cost. For higher temperature levels, up to 150 o C, evacuated tube collectors are used and for temperature level up to 300 o C, parabolic trough collectors are the most mature technology [2,4]. Solar dish collector uses a dish collector and usually is coupled with a Stirling machine producing electricity [4-6]. The advantage of the dish reflector is the high concentration ratio because the radiation is concentrated at a point, a fact that allows high temperature levels to be reached [7]. Other solar applications that use dish collectors are solar cookers where the solar energy is converted to household energy [8-9]. Applications that use parabolic dish reflectors for other heating purposes are seldom in literature. The main reason is the difficulty in the receiver design due to the demand of continuous tracking. In this study, an innovative collector with dish reflector is designed and simulated with commercial software Solidworks. Solidworks is a useful engineering tool and the last years many studies have been integrated with it [10, 11]. Other software that have * Corresponding author. Tel.: addresses: bellose@central.ntua.gr (Bellos Evangelos)

2 been used in similar studies are ANSYS [12-14], Comsol [15] and Fluent [16]. The goal of this analysis is the determination of the efficiency for various operating conditions and the explanation of its operation. As the model presents a new type of solar collector, emphasis is given in the design and in the explanation of its operation. 2. Examined model 2.1 Solar dish collector design and simulation The main parts of the examined collector are the paraboloid dish reflector, the spherical receiver and the tubes. Insulation layer covers the tubes and the upper part of the receiver, as figure 1 presents. Figure 1.Solar dish model design The reflector surface is the one with the grey color in figure 1 and has been created by revolving a parabolic shape for 180 o. The insulation layers are the yellow parts in this model. Insulation covers the inlet and the outlet tube of the model, while insulation layer has been located in the upper hemisphere of the absorber. The spherical absorber is the black one in figure 1. The working fluid flows through the inlet tube to the sphere and afterwards, the sphere continues in the outlet tube. The basic idea is to create a continuous flow which allows the direct use of the heat or the easy storage with a simple storage tank. Pressurized water is the working fluid of this analysis and is kept to liquid phase for all the examined cases. The use of spherical absorber leads the solar radiation to be distributed in an area and not in a point, the focal point of the parabola. More specifically, all the solar rays after their reflections are directed to the focus of the paraboloid dish. The center of the

3 sphere is located on this focus, which means that all the rays arrive in the down hemisphere of the absorber. In the case that all the solar energy was delivered to a smaller area, the concentrating ratio will be huge, leading to higher temperature level than the desired for heating proposes, increasing the heat losses. This design allows the water to be stored inside the spherical absorber for a time period and the solar energy delivered to the absorber heats a great amount of water. By this technique, the mean temperature inside the sphere is kept in desired levels. By insulating the system, the heat losses are diminished a lot. The only uncovered part is the down hemisphere of the absorber in order to absorb the delivered solar energy. The material of the absorber and the sphere is copper, while the insulation is glasswool. The system needs a full tracking system in order to operate efficiently because the sun rays have to be vertical to the aperture. Figure 2 gives the main dimensions of the examined model and table 1 presents the main parameters of the geometry and of the simulation. Figure 2. Basic model dimensions in mm Table 1. Main parameters of the model and of the simulation Geometry Simulation Insulation Values Values Parameters Parameters Parameters Values D to 56 mm ε r 0.1 α ins 0.8 D ti 50 mm ρ α 0.85 k ins 0.04 W/mK A a m 2 h air 10 W/m 2 K δ ins 20mm A r,net m 2 G b 500 W/m 2 ε ins 0.3 A r m 2 m 0.1kg/s D ins 96mm C T am 10 o C material glasswool

4 In order to simulate the examined problem with the right way, the suitable boundary conditions have to be determined properly. Thus, flow and radiation boundary conditions have been selected in Flow Simulations studio in order to describe the problem. Moreover, the materials of the separate parts have been selected and the convergence goals of the simulation. During the simulation tests of this model, the ambient temperature and the solar radiation are kept constant, while the water inlet temperature varies in order to examine the system efficiency for different operation cases. In every simulation, the main output results from Solidworks Flow Simulation are the temperature of the receiver and of the water in the outlet (bulk average). Also, the total thermal loss of the system is given from the Solidworks in order to determine the overall thermal loss coefficient. 2.3 Mathematical description In this paragraph, the main mathematical equations for the problem description are presented. Equation 1 determines the net concentration ratio of the collector. C a, (1) A A r, net This equation is the ratio of the total aperture to the area of the down hemisphere. The value 29.25, which is referred in table 1, is a high value which classifies this collector to imaging concentrating collectors. This kind of collectors has a specific image of the sun in the receiver and utilizes only the solar beam radiation. The solar energy in the aperture of the collector is presented in equation 2 and the useful energy gain is presented in equation 3. s u G A, (2) b p a out in m c T T, (3) Equation 3 gives the useful energy calculation by using the energy balance in the working fluid volume. The efficiency of the system is given in equation 4. u, (4) s The thermal losses of the system are separated to radiation and convection losses. The next equation (5) presents them: loss, (5) conv rad By knowing the thermal losses and the receiver mean temperature, the determination of the thermal losses coefficient is able to be done with equation 6.

5 η U L loss, (6) r T r T am 3. Simulation Results In this paragraph the results of the simulation are presented. In every case, the fluid inlet temperature was set by the user and the other parameters, such as outlet temperature, receiver mean temperature and overall thermal losses were calculated by the simulation tool. 3.1 Collector performance The first presented parameter is the collector efficiency. The following figure 3 presents this quantity for different operating conditions. The x-axis is the efficiency parameter (T in -T am )/G b which is usually used in order the efficiency of the collector to be presented (T in -T am )/G b Figure 3. Efficiency curve of the collector This curve of figure 1 seems to be near to 1 st approximation is presented: degree polynomial, so the next Tin Tam , (7) Gb The mean heat losses coefficient of the absorber from the above equation is W/m 2 K, which is a very low value. For usual flat plate collectors, this coefficient is about 6 W/m 2 K, for evacuated tubes 2 W/m 2 K and for linear parabolic trough collectors with evacuated tubes 0.25 W/m 2 K. For this reason, it is important to state that this collector is a very efficient collector which performs better than usual

6 U L (W/m 2 K) collectors and can compete with other efficient collectors, as parabolic trough collectors. Figure 4 presents the overall heat coefficient of the total system. The total system is determined by insulation layers, tubes and receiver. This agglomerate loses heat from radiation and convection (T in -T am )/G b Figure 4. Thermal losses coefficient of total system It is obvious that the correlation between the total thermal loss coefficient and the parameter (T in -T am )/G b is linear and equation 8 presents it: T U L in T G b am This coefficient is great because there is not cover and has great convection losses. The heat convection coefficient is 10 W/m 2 K, so the total heat loses coefficient is greater than this value, varying from to W/m 2 K. These great values are acceptable because the radiation losses are included. 3.2 Temperature distribution over the absorber and the flow In this section, the temperature distribution in the receiver and in the water is presented. Also, the flow inside the sphere is analyzed because the flow lines explain the way this collector operates. All the figures represent the case with water inlet temperature at 70 o C. Figure 5 presents the temperature over the absorber. More specifically, figure 5a shows that the temperature in the insulation layer is about 15 o C, while in the down part of the spherical absorber takes values over 100 o C. In figure 5b, only the temperature of the sphere is depicted. The upper part of the sphere has a temperature around 80 o C, while the down part is hotter with the maximum

7 temperature at 130 o C. This maximum value is observed in a peripheral region in the down hemisphere and is the red one in the figure 5a & 5b. Figure 5. Temperature distribution a) Over the insulation and in the sphere, b) Over the sphere The next presented image is in figure 6 where the flow inside the absorber is shown. Figure 6a presents the temperature level with colors and the vectors show the flow direction. The upper part of the sphere is hotter with temperature about 78 o C, while the inlet temperature is at 70 o C. The water entering into the sphere moves in the down part of the sphere due to gravity and continues tangent to the geometry up to outlet. By observing the figure 6a, the blue colors become green and afterwards become yellow before mixing with the red of the over part. Figure 6b presents the same situation with flow lines. The extra information is a vortex in the bottom part of the sphere which is created by the flow conditions inside.

8 Moreover, figures 6a & 6b show that the upper part of the sphere operates as a storage tank, containing hot and approximately motionless water. The reasons for hot water storage in the upper part are two. The first one is the lower density of the hot water and the second is the flow of the cold entering water in the down part due to gravity. Figure 6. Temperature inside the sphere in the middle cross section. a) The vectors shows the flow direction, b) Flow lines Figure 7 presents the temperature distribution of hot water in the outlet of the tube. The upper part of the fluid is hotter and the coldest region is the down one. This can be explained because the hotter water enters in the tube from the upper part of the sphere. Its lower density captures it in the upper part of the tube for all the length.

9 4. Conclusions Figure 7. Temperature profile in the outlet of the tube In this paper, a new solar dish collector is presented and analyzed in order to predict its efficiency and to explain its operations. The design and the simulation of this model have been integrated with commercial software Solidworks. The paraboloid dish has a diameter of 3.2m which leads to an aperture of 8m 2. The concentration ratio is about 29, a high value which allows the utilization of solar beam radiation. The efficiency of the system is greater than 70% for water inlet temperature lower than 90 o C. This makes this collector more efficient than conventional collectors as flat plate collectors. The overall thermal loss coefficient is about 0.64W/m 2 K when it is referred to the net receiver area and about 12 W/m 2 K when it is determined for all the agglomerate. This great difference is created because the outer face of the agglomerate is insulated and is colder than the hot receiver (down hemisphere). The receiver temperature is greater in the down hemisphere, where the solar radiation is concentrated. More specifically, when the water inlet temperature is 70 o C, the maximum temperature in the receiver is 130 o C and the mean 93 o C. The water entering in the sphere moves to the down part of this due to gravity (higher density as colder) and it is getting warmer while moving in the sphere. In the outlet of the sphere a mixing between the water of the upper and the down part takes place. It is essential to state that the water temperature in the upper part of the sphere is greater and the fluid in this part is approximately motionless, making the upper part to operate as storage tank of hot water. The temperature profile of the fluid in the outlet shows that the upper part is warmer than the down one. This can be explained as the warmer water goes in the upper part due to its lower density.

10 Nomenclature A area, m 2 C concentration ratio c p specific heat capacity, kj/kg K D diameter, m G b solar beam radiation, W/m 2 h air Air-receiver convection coefficient, W/m 2 K m mass flow rate, kg/s heat flux, W T temperature, K U L heat transfer losses coefficient, W/m 2 K Greek symbols α absorptance δ Thickness, mm ε emittance η efficiency κ Conductivity, W/mK ρ α Reflectance-absorptance product Subscripts and superscripts a aperture am ambient in inlet ins insulation loss thermal losses out outlet r receiver rad radiation r,net receiver net S Solar ti tube inner to tube outer u useful

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