An Evacuated PV/Thermal Hybrid Collector with the Tube/XCPC design

Transcription

1 An Evacuated PV/Thermal Hybrid Collector with the Tube/XCPC design Lun Jiang Chuanjin Lan Yong Sin Kim Yanbao Ma Roland Winston University of California, Merced 4200 N.Lake Rd, Merced CA ABSTRACT The current challenge for PV/Thermal (PV/T) systems is the reduction of radiation heat loss. Compared to solar thermal selective coating, the solar cells cannot be used as an efficient thermal absorber due to their large emissivity of the encapsulation material. Many commercial PV/T products therefore require a high concentration (more than 10x) to reach an acceptable thermal efficiency for their receivers. Such a concentration system inevitably has to track or semitrack, which induces additional cost and collects only the direct radiation from the sun. We propose a new PV/T design using a vacuum encapsulated thin film cell to solve this problem. The proposed design also collects the diffuse sun light efficiently by using an external compound parabolic concentrator (XCPC). Since the transparent electrode (TCO) of thin film cell is inherently transparent in visible light and reflective beyond infrared, this design uses this layer instead of the conventional solar cell encapsulation as the outmost heat loss surface. By integrating such a vacuum design with a tube shaped absorber, we reduce the complexity of conducting the heat energy and electricity out of the device. A low concentration standalone non-tracking solar collector is proposed in this paper. We also analyzed the thermosyphon system configuration using heat transfer and ray tracing models. The economics of such a receiver are presented. 1.INTRODUCTION The current biggest challenge for PV/T systems is the reduction of heat loss. Without the vacuum and the selective coating, the solar cells cannot be used as an effective thermal absorber, because the radiative heat loss goes up with T 4 and the esmissivity of most solar cells' encapsulations are high, as shown in figure 1. (1) Fig. 1: Emissivity comparison amongst different encapsulation materials. 1

2 Fig. 2:Comparison of radiative heat loss. Based on the standard testing condition of global normal irradiance 1000W/m 2, the heat loss at 80 C is above 350W/m 2, if glass is used as the encapsulation for the hybrid solar collector. Therefore the thermal efficiency under this condition is no more than 65%, requiring a PV/T system design to have a high concentration (more than 10X), in order to limit the radiating area. Otherwise the PV/T system has to suffer from the low thermal efficiency at a higher working temperature. With a vacuum tube design not only the heat exchange problem can be solved, the transparent electrode layer (TCO, or transparent conductive oxide) of the thin film PV can also be optimized so that it becomes a selectively transmissive coating to stop the radiative heat loss while allowing sun radiation to be efficiently absorbed by PIN or PN junction (2). A vacuum tube PVT design is both cost effective by using only glass as the raw material and thermally efficient by using vacuum as the insulation. It also exhibits the full benefits of the PV/T system which is compact, efficient, and low cost. 2. XCPC/PVT VACUUM TUBE COLLECTOR DESIGN 2.1 Thermodynamically Efficiency Concentrator Design any ray leaving from the absorber surface will end up within the acceptance angle of the concentrator. As proven in nonimaging optics (4), such a concentrator achieves the thermodynamically maximum concentration ratio theoretically. Second, the proposed XCPC concentrator has a wide acceptance angle as 60 (half angle), indicating a lower concentration ratio (1.1x, or 3.45 times compared to the diameter). The absorber can efficiently work for 8 hours during a day. Third, the proposed collector is stationary, which reduces the installation cost and eliminates the additional cost induced by tracking devices. The design of such a concentrator is shown in figure 3. Fig. 3 Design for the XCPC with an outer glass tube case. Even though the gap between the absorber and the reflector causes an optical loss, the proposed design maintains the acceptance angle for the backward ray tracing of the concentrator, resulting in thermodynamically maximum concentration. During the manufacturing process of the XCPC, the surface of the product will not be exactly the same as the theoretical calculation. Therefore we introduce optical inaccuracy into our optical model in an effort to quantify this additional loss, as shown in figure 4. To fully take advantage of the cylindrical shape absorber, we designed the back reflector using the external Compound Parabolic Concentrator(XCPC) based on the non-imaging optics. (4) Such a design has the following advantages. First, the XCPC is shaped according to the best concentration ratio by considering the outer glass tube casing for an evacuated tube. In another word, it is designed according to the rule that 2

3 the absorber. The light source is the sun which subtends a half angle of The sun has both seasonal change and day time change (defined as θ and φ specifically, as shown in figure 5). As shown in figure 6, under the normal insolation (θ=0, φ=0, the nonimaging concentrator output has stripe like hot spots. Fig. 4: Simulation considering manufacturing inaccuracy Fig. 6 The angle set up for ray tracing Fig. 5: Ray trace on 30 degree day angle, 25 degree seasonal angle Figure 5 shows the ray tracing of 30 degrees, a small portion of the total rays pass through the gap and escape the concentrator. The tradeoff between the optical efficiency and concentration ratio can be optimized based on the emissivity of the solar cell and the working temperature. 2.2 Optical Simulation And Its Results The ray tracing program is setup based on a configuration used in a thermosyphon system. According to the latitude of Merced, California, the angle between the ground and the collector plane is set to 37 facing south. The collector troughs are northsouth positioned. The receiver is setup as a 2D plane and wrapped around the cylindrical absorber. Its x axis is the polar angle and its y axis is along the long axis of Fig. 7 Ray tracing result on normal radiation The theoretical optical efficiency varies according to the time of a day as shown in figure 7. With higher φ, i.e. early morning and late afternoon, the lower efficiency is caused mainly by the inaccuracy of the reflector, as shown in figure 8. With lower φ, i.e. around noon, the efficiency dip shown in figure 9 is mainly caused by the gap loss. The optical efficiency, 94% on average, is not greatly reduced by the gap loss or the reflector inaccuracy. 3

4 the efficiency of the solar module. The solution to such problem points to a series connection along the y axis of figure 6, using rings of solar cells on the absorber. However, even if each solar cell is receiving same sunlight along the y axis, the solar cells on the two ends of the collector are subject to shadowing during the seasonal changes, as shown in Figure 10, which implies a more complicated circuit design within the solar cell module itself. Fig. 8 Incident Angle Modifier (IAM ) result, the theoretical optical efficiency according to φ Fig. 11: Simulation result from 25 seasonal angle. Fig. 9 The ray tracing for lower sun elevation angle 3.THERMOSYPHON SIMULATION To understand the temperature distribution of the absorber under different radiations distribution, we set up a numerical simulation of steady natural convection heat transfer in a 3-dimensional single-ended tube in solar water heaters. Fig. 10 Ray tracing for higher sun elevation angle 2.3 Shadowing Problem and Solar Cell Design Based on The Hotspot Because of series connection of individual cells, the minimum cell current dictates the module current. Therefore the non-uniform distribution of illumination on the inner absorber tube, or hot spots will reduce 3.1 Problem definition and numerical methods Numerical simulation was performed to study the steady natural convection heat transfer in one threedimensional single-ended tube. The governing equations of heat transfer and fluid flow considered have been formulated considering steady-stated threedimensional flow configuration and taking into account the following assumptions (7) : Incompressible flow; Newtonian behavior; Viscous dissipation in the energy equation is negligible; 4

5 Except for the density, the properties of the fluid are taken to be constant; Thermal expansion coefficient is chosen as different constants for different cases. This model uses a segregated solver with implicit formulation. The flow is buoyancy driven natural convection and the Boussinesq approximation has been used for the density term. The Rayleigh Number, which is a measure of the strength of the natural convective forces, is in the range of 10 6 ~10 7, which means laminar flow model could be applied here. Non-slip boundary and heat flux was added to the wall of the inner glass tube, which was constant but nonuniform from optical simulation. The opening of the tube was set as pressureoutlet. The main heat loss is due to the radiation from the tube surface to the environment, which is estimated with emittance as 20%. The fluid was modelled initially as an isothermal bulk. With heating from the radiation, heated water will go upward due to its decreased density by buoyancy force near the wall. In order to determine the independence of each solution on the size of the grid, solutions for two different grid resolutions have been conducted. The results show that the grid system of 200,000 cells, with 2000 faces on cross section as shown below in figure 12 and 100 uniform intervals in the axial direction, is fine enough to capture the accurate results. The comparision results with denser mesh of 480,000 cells are shown in figure 13. The set-up of the test case is uniform heat flux on the tube surface at 300W/m 2 and initial temperature is 300K. Fig. 12: The coarse mesh configuration on the crosssection of the tube Fig. 13: Comparison of temperature distribution along the Y-axis of the top opening surface between two meshes, tin = 300K 3.2 Results and discussion Natural convection in a single-ended tube with nonuniform heat flux input was numerically investigated and temperature distribution and velocity vectors were presented. Flow T high (K) ΔT(K) θ φ rate(kg/s) Case1 4.41e Case2 4.76e Case3 4.78e Case4 4.71e Case5 4.57e Case6 4.79e Case7 4.99e Case8 4.91e Table.1. The flow rate caused by natural convection, highest temperature in the tube and averaged temperature increase for different cases 5

6 Case 1 Fig. 14: The temperature (contours) and velocity (lines) distribution at the opening of the tube for four cases, T in = 333K From figure 14 we can see that the main cold flow going down is in the Y-negative domain, even though this side of tube wall is still heated up by the relatively larger heat flux from the reflector. Fig. 16 The temperature distribution of the YZ crosssection plane at x=0, with enlarged near the top opening and bottom for case1,. Because of the heat flux from the back surface, the whole surface of the tube wall will be heated up. However, the hottest point is still on the side facing the sun due to the natural convection, not on the other side, where experiences the largest heat flux. Fig. 15: The velocity vectors of the YZ cross-section plane at x=0 near the top opening and bottom for case1, Due to the driven buoyancy force, the velocity of the heat flow gradually increases as it goes up near the wall. And the velocity of cold flow will gradually decrease to zero when it reaches the bottom. 4. CONCLUSION To effectively reduce the radiative heat loss of a hybrid solar collector, the high emissivity encapsulation layer and the low emissivity TCO layer must be separated with a vacuum layer. However, most vacuum collectors prefer a tubular shape for pressure and mass production reasons. A nonimaging wide angle concentrator can efficiently exploit the back of the absorber efficiently. The hot spots produced by such a concentrator have both thermal and PV efficiency effects. The themosyphon system is good enough to control them temperature within several degrees, therefore the negative effect of the hot spot on the thermal efficiency can be neglected. The PV efficiency, however, will be greatly impacted if the design of the circuit is not correct. 8. REFERENCES: (1) IEA task 35 report: New Generation of Hybrid Solar PV_T Collectors 6

7 (2) Pei, Sun, Tan, Wen et al, Optical and electrical properties of direct-currrent magnetron sputtered ZnO:Al films. Journal of Applied Physics, (3) Poiry, Balkoski, Jiang, Winston, Efficient Solar Heating, WREF 2012 (4) Winston, Thermodynamically efficient solar concentrators, Journal of Photonics for energy, SPIE, (5 Jiang, Kim, Winston, A vacuum PV/Thermal Hybrid Collector with CPC-tube Design, Poster, UC Solar symposium (6) Kim, Kang, Winston, Modeling of a Concentrating Photovoltaic System for Optimum Land Use, Progress in Photovoltaic: Research and Applications. doi: /pip.1176, 2011 (7) Shahi, Mahmoudi, Talebi, Numerical simulation of steady natural convection heat transfer in a 3- dimensional single-ended tube subjected to a nanofluid, International Communications in Heat and Mass Transfer,