AU2005274670B2 - Direct Heated Solar Collector - Google Patents

Abstract

The presented solar collector panel based on direct solar absorption offers significantly higher efficiency over the traditional tube and fins designs. The heat exchange is improved by increasing the heat dissipation area and coating of the absorber inner surface with a radiation enhancing layer. Meandering channel design enables higher volume of heat exchanging fluid, reducing the absorber temperature, the main condition for the high efficiency. It also ensures uniform temperature distribution. It has been demonstrated that uniform temperature distribution of the absorber enhances the efficiency. A system with individual panel control enables taking advantage of different roof slopes and sun movement throughout the day.

Description

Direct Heated Solar Collector Australian Patent Application no. 2005274670 By Bogdan Goczynski and Zbigniew Maderski 1 1. General description. The solar collector described below has been developed as a means to save hot water bills in a private household, without any prior knowledge of the existing designs. Apart from hot water, it can be used in a variety of applications including space heating and horticulture. In terms of the structure it is essentially a thin box filled with heat exchanging fluid, the top of the box coated with selective material to increase retention of the solar energy. The channels inside the collector improve its thermal performance and the mechanical stability. Above the absorber a glass pane is placed preventing heat losses through convection. The bottom and sides of the collector are lined with thermally insulating material. The collector is simple to manufacture and yet achieves the efficiency close to the theoretical maximum, surpassing existing designs. 1.1. Principal Features The principal design features of the presented collector system are as follows: 1. Direct heat transfer from the absorber plate to the heat exchanging fluid 2. Reduced overall temperature of the absorber 3. Uniform distribution of the absorber temperature through the meandering channel design 4. Improved dissipation of energy into the heat exchanging fluid 5. Coating of the internal absorber surface to enhance the heat radiation into the fluid 6. Individual panel control, enabling better utilisation of the solar energy throughout the day Figures 1 and 2 illustrate the operating principles of the presented solar panel. 1.2. Efficiency Improvement The theoretical analysis of the collector is presented in Appendix 1. It uses standard physics formulas for heat conduction and blackbody radiation. The blackbody radiation curve is supplemented by spectral characteristic of the selective coating of the absorber and the selective transmission of glass. In addition, typical measured efficiency curve for the flat panel (tube and fins) collector is shown. The presented analysis shows significant improvement over the traditional, tube and fins design. While basic theoretical but accurate formulas have been used for the analysis, the selective coating and conduction characteristics are empirical and obtained from publicly available sources. For the stated reasons as well as due to the much higher effective area, the described collector is also superior to vacuum tube type. 1.2.1. Direct Heat Transfer In the traditional, tube and fins panel, the solar energy is collected by the absorber and then passed to the tubes and to the fluid. For the process to occur there must be a temperature gradient on the absorber. In other words, the further away from the tubes the higher the temperature. This in turn causes higher energy losses through radiation and conduction. The optimum points from the thermal point of view are the tubes, where the temperature is the lowest and so are the losses.

2 In the collector described in this patent description, with the fluid being directly under the absorber, the heat transfer distance is negligible comparing to the tube and fins panel and the absorber temperature is optimal across its whole area. Lower, optimum absorber temperature results in the lower radiation and conduction losses. The more precise analysis of the efficiency is presented in Appendix 1. 1.2.2. Improved Heat Dissipation The heat transfer into the heat exchanging fluid is improved by increasing the effective heat dissipating area of the absorber, through corrugation or through the heat dissipating ribs as shown in Figures 4, 5, 8 and 9. 1.2.3. Uniform Temperature Distribution In areas free from the risk of freezing, the common view is the thermosiphon construction, whereby the hot water in the collector rises to be collected in the storage tank above the solar panel. It is shown in the Appendix 2, that the efficiency can be further improved by keeping the panel temperature uniform. This is achieved by horizontal arrangement of the meandering channels, preventing the heated fluid rising to the top as in the thermosiphon solutions. It is needless to say that this arrangement is applicable only to the pump configuration with the tank kept away from the panels. 1.3. Meandering Channels The clearance of the meandering channels under the absorber must be reasonably low to keep the fluid layer thin and to ensure good response to the changes in insolation. At the same time the clearance must be sufficient to allow unimpeded circulation of the fluid within the panel. Three-dimensional view of the panel base with the channels is shown in the Figure 3. 1.4. Preventing of Heat Loss The glass pane above the absorber prevents the air heated by the absorber from rising into the outside space, limiting heat loss through convection. In addition, the bottom and the sides of the collector are lined with the thermally insulating material. 2. Sequential and Complete Heat Transfer The presence of the temperature monitoring sensors and the existence of the meandering channels result in the heat being transferred the storage tank in a sequential manner, when the panel temperature reaches appropriate level. After the whole of the panel is filled with the cooler fluid, the circulation is stopped by the control system. 3. Absorber The absorber has multiple functions. Its purpose is not only to maximise the absorption of the solar energy and its transfer into the heat exchanging fluid, but also to contain the fluid, form the meandering channels and give the structure the necessary rigidity. The absorber should be durable. 3.1. Corrugation The cross-section A-A in Figure 1, apart from the meandering channels, shows corrugation of the absorber. The purpose of the corrugation is to increase the absorber area being in contact with the fluid, thus improving heat transfer and efficiency. Besides improving efficiency, the corrugation together with the channel structure, gives the absorber the additional rigidity.

3 The absorber corrugation can be achieved by pressing a sheet of metal and should agree with the flow of the fluid. Pleas note that although the corrugation improves the heat transfer it does not result in absorbing more energy. The latter is determined by the solar radiation capture area, which in turn is defined by the absorber outline. 3.2. Heat Dissipating Ribs The corrugation shown in Figure 1, cross-section A-A, is only one of the ways to increase the heat dissipation area. Similar effect can be achieved by attaching perpendicular ribs to the absorber as shown in Figure 5. 3.3. Absorber Coating The sun facing surface of the absorber needs to be coated with a material with selective absorption and emission properties. Traditionally black chrome was such a material. Nowadays there is a range of commercially available coatings offering similar effects. A simpler solution is a non-selective black paint, although obviously it will not offer the same, high efficiency as the selective coating. It is also proposed to coat the inner surface of the absorber with a layer enhancing the infrared radiation. 3.4. Absorber Material The absorber needs to be reasonably thin to offer low thermal resistance, but at the same time, together with the corrugation, channel structure and the panel base, producing adequate rigidity. Another reason to keep the absorber thin is its weight and cost which are very important in the very competitive market. 4. Panel Base As mentioned in Section 1, the collector is essentially a thin box, with the absorber forming its top. The remainder of the "box", including the channel ridges, is to be made preferably from the same type of material as the absorber for the ease of bonding, avoiding corrosion and maintaining the same thermal expansion coefficient. Pressing from a sheet of metal e.g. stainless steel, meets these criteria and allows it to be produced in a single process. 5. Glass Cover The glass cover can be made from the standard solar, low iron, toughened glass available from glass manufacturers. It passes approximately 92% of the solar energy and ensures good incidence angles. In addition the glass cover acts as a filter blocking emission of the part of infrared radiation. 6. Complete Structure The absorber is to be firmly bonded to the panel base. The bonded structure is then inserted into the insulated case. The thermal insulation is to be preferably applied under both at the bottom and at the sides of the panel. The glass is to be mounted on supports forming part of the panel case. 7. Control System The control electronics monitors the temperature of the solar system to ensure optimal heat transfer to the storage tank. The typical solution is to compare the temperature of he collector with that of the tank. When the temperature of the collector exceeds the tank by a set difference, usually 6 2C to 10 C the system activates the collector and pumps the heat exchanging fluid to the tank heat exchanger.

4 8. Individual collector control Currently available pumped systems use only a single, common temperature sensor for all the collectors. Occasionally the orientation and construction of the roof is such that it is difficult to make the collectors face the equator (north or south depending on the hemisphere). The solution is to use separate temperature sensors and electrically controlled valves for the individual collectors and turn the collectors ON or OFF according to their temperature. It is also possible to use common sensor for the individual groups of collectors as shown in Figure 13. The schematic diagram of the described system is shown in the Figure 12.

5 Appendix 1 Analysis of Efficiency The efficiency analysis of the proposed collector has been calculated using basic physics formulas of blackbody radiation and heat conduction. The obtained results have been compared with the efficiency curve for a tube and fins collector, measured in a laboratory. The collector efficiency can be calculated as follows: A1.1 r7 = G Where: t : efficiency G: normalized solar power incident on the collector aperture and equal to 800 W/m 2 L : energy losses Energy losses include the following: e Loss of solar radiation due to the glass cover. * Losses due to the reflection of the solar radiation from the absorber. * Losses due to the imperfect cooling of the absorber as a result of the channel separations. e Losses due to the infrared emission. * Losses due to heat conduction across the collector bottom, through the sides and through the air gap between the absorber and the glass cover. Construction assumptions For the analysis of efficiency, the following panel construction has been assumed: Insulation under the panel base: 40mm mineral wool. Insulation of side walls under the glass pane: 40mm thick and 35 mm high layer of mineral wool. Air gap between absorber ad the glass cover: 30mm. Glass cover is assumed to be 3.2mm thick, low iron toughened glass. The glass spectral transmittance function in Fig A1.1 has been taken from the publically available literature. As a selective coating TiNOX has been assumed with the transfer function derived from the Fig. A1.2 Absorbed energy The distribution of the energy absorbed by the selective surface has been calculated using the solar radiation approximated by the Planck's blackbody radiation formula (A1.2) multiplied by the selective coating spectral transfer function and by the glass transmittance. The selective coating absorptance function has been derived from Fig. A1.2 in a following manner: absorptance = 1 - reflectance. The total of the absorbed energy is then obtained by scaling the solar radiation down to account for the nominal solar energy density at the Earth surface of 800W/m and numerically integrating it over the relevant wavelength range. This is the approach similar to that used for the purpose of calculating absorptance of the selective surfaces.

6 A1.2 I'(A,T)= 2hC 2 1 2hc eA-1 Where: I'(2,T) is the amount of energy per unit surface area per unit time per wavelength emitted by a black body at temperature T; h is the Planck constant = 6.62606896(33) x10-34 J*s; c is the speed of light in a vacuum = 2.99792458x10 8 m/s; k is the Boltzmann constant = 1.3806504(24)xl 0- 23 JK- ; A is the wavelength of the electromagnetic radiation; T is the temperature of the body in kelvins. Emitted energy The emitted infrared energy is calculated by substituting the temperature in the Planck's formula A1.2 with the temperature of the absorber ( 2 0 to 100 C), multiplying it by the selective coating transfer function derived from Fig. A1.2 as well as by the glass transfer function from Fig. A1.1 and numerically integrating it over the relevant wavelength spectrum. This, again, is the operation similar to that used for calculating emittance of the selective surfaces where the calculations are performed for T = 373K (100 2C). Please note that for a given wavelength, the absorptance is equal to the emittance. For technical reasons in Fig. A1.2, the reflectance rather than absorptance values are usually shown. The absorptance = 1 - reflectance. Conduction losses Thermal loss through the insulation is calculated using the formula: A1.3 Lc =cA " x Where: " Le: conduction loss e c: insulation heat transmission co-efficient. For mineral wool it is equal 0.035Wm/K and for the air: 0.023Wm/K. " A: area relevant to heat conduction " T: absorber temperature (K). " Ta: ambient temperature (K) = 293.16K (200C) " x : insulation thickness (m) Selective properties of glass Solar glass can be manufactured to have wavelength selective properties similar to the effect of the selective coatings further improving the collector performance.

7 1,0 0,5 0 0,2 1,0 2,0 3,0 A pm Figure A1.1 Transmittance of glass used in the simulation. Scaling At the collector temperature equal to the ambient, there are no energy losses and the two collector efficiencies (Application and tube and fins collector) should have a common value. Therefore a scaling factor of 3.23 % has been introduced to reduce and match the starting efficiency of the collector described in the application with the experimental tube and fins characteristic. Using the above data and formulas as well as the Excel spreadsheet, the efficiency chart in Fig. A1.3 has been produced. 1.0 0.8 0.6 0.4 0.2 0.0 0.3 0.4 0.6 0.8 1.0 1.5 2 3 4 6 8 10 15 20 Wavelength [pm] Figure A1.2. TiNOX selective coating reflectance characteristic.

8 collector efficiency comparison 90 80 70 >, 60 Application 50 .L 40A tube and fins .0 40 30 20 10 0 20 30 40 50 60 70 80 90 100 temperature Figure. A1.3. Efficiency comparison between the collector described in this Application and the tube and fins collector. Please note that while the curve for the collector described in the Application has been produced by simulation, the reference flat panel, tube and fins collector curve has been reproduced from a laboratory report. For the sake of comparison, the same TiNOX selective coating has been assumed for both collectors. Also the insulation and air gap were the same. The reference tube and fins collector has had the following construction parameters: Selective coating: TiNOX Glass cover: solar glass, 2.9mm Insulation: mineral wool, 40mm Number of tubes: 10 Air gap between the absorber and the glass cover: 30mm The graphs in Fig. A1.3 confirm the intuitive feeling that the heat exchange in the direct heated collector is superior to that of tube and fins. This is achieved by keeping the temperature of the absorber uniform and optimally low. In the tube and fins design on the other hand, to achieve the heat flow from the absorber to the tubes, a temperature gradient is necessary. This results in higher temperature away from the tubes and in higher losses both through the conduction and through the infrared radiation.

9 Figure A1.4. Illustration of the flat panel, tube and fins collector design.

10 Appendix 2 Comparison of efficiency between panel with a uniform temperature distribution and a thermosiphon (vertical temperature rise). It has been asserted in the section 1.2.3, that the uniform temperature distribution of the absorber improves the efficiency over the thermosiphon designs. This can be demonstrated using the Stefan-Boltzmann formula (eq. A2.1) describing the power radiated from a surface of a body at a given temperature. Equation A2.1 dE - AeOT4 dt Where: T is the body temperature in kelvins. A is the area of the body. a is the Stefan-Boltzmann constant equal 5.67 10-8 (W/(m 2

K

4 ). e is the emissivity determined by the body's surface, with the values between 0 and 1. Consider two objects, one at the temperature T, emitting energy, the other at the temperature T 2 receiving the radiation and also emitting energy, the balance of the absorbed power absorbed by the second object can be described as: Equation A2.2 Pa = AeoT' 4 -Ae 2 dT24 = A -JeTii - e 2 T24I Imagine now an object radiating power equal to the solar power at the Earth surface e.g. 800 W/m. This way the equivalent solar temperature can be calculated. Equation A2.3 TS = s e cAP Where Ps is solar power density at the Earth's surface power and Ts is the equivalent solar temperature. Assuming A = 1 M 2 , Ps = 800 W/m, the equivalent solar temperature is 71.50C. The emissivity of such an object would be equal to 1 by definition. Assuming now uniform temperature across the absorber and the absorber dimensions as w and h, (Fig. A2.1) the equation A2.2 can be rewritten as: Equation A2.4 Pu = whea(T S- TU Where: Pu is the absorbed power for panel with uniform temperature. w and h are defined in Figure A2.1. T, is the equivalent solar temperature as defined in equation A2.2. Tu is the absorber temperature, in this case uniform across its whole area. h w Figure A2.1. Panel dimensions for the comparison of efficiency.

11 Equation A2.3 describes the balance of power absorbed by the collector. In case of a thermosiphon panel, the temperature distribution can be described as in the Figure A2.2. Ry Thermosiphon distribution Uniform ------------ distribution TUi 0 h Figure A2.2. Temperature distribution in a thermosiphon panel. Mathematically it is represented by the formula: Equation A2.5 T(y) = Tm. + AT Y h Where: AT = T. - Tm. Assume Prh, is the total power absorbed by a thermosiphon panel and Ps is the local power density. h Equation A2.6 P,, =wf P, (y)dy Based on Equation A2.2, the power absorbed by a thermosiphon panel can be described as: Equation A2.7 P, = weFT -T +AT- dy Substitute Equation A2.8 T.in +AT Y = g h Thus: Equation A2.9 dy = hdg AT and Equation A2.1 0 P, = we 7 hT4 _ AT Jgdg AT and Equation A2.11 Pa =hwe- cT - 5 - T 12 Assuming the uniform temperature of a panel as the average temperature of a thermosiphon panel, Tvcan be expressed as Equation A2.12 TU = "'i" "'ax 2 The efficiency improvement of the panel with uniform temperature distribution over the thermosiphon panel, can be calculated as Equation A2.13 I= U ""Th Where Pu and Prhs have the meaning described earlier. Using equations A2.4 and A2.11 the efficiency improvement can now be represented as: T' -T Equation A2.14 I= U I T' -. 5AT The efficiency improvement becomes more visible with the increase in temperature difference between the bottom and top of the thermosiphon panel. For example: assume Ts = 361 K(882C), for Tnan = 283K(1 02C) and Tnm = 351 K(889C), the efficiency improvement is 5.34%. Please note that the Stefan-Boltzmann law applies to non-selective (wavelength independent) surfaces like the black paint. In this context the analysis is precise. For the selective surfaces the analysis is more complex, however, the above reasoning enables the intuitive understanding of the phenomenon.

Claims (5)

1. Solar panels whose principle is described in this Application and shown in the Figures 1, 3, 4, 5 and 6, maximise the solar absorption through the direct heat transfer from the absorber into the heat exchanging fluid.

2. The solar panels as referred to in Claim 1, achieve higher efficiency by lowering the temperature of the absorber through cooling of the whole absorber area with the heat exchanging fluid contained directly under the absorber.

3. The panels as described in the Figures 1 and 3, and with regard to Claim 1, transfer the heat from the collector panel to the storage tank by sequential flow of the heat exchanging fluid.

4. Increasing the efficiency of the panels, referred to in Claims 1, 2 and 3, is, in addition to the earlier claims, achieved through uniform distribution of temperature by avoiding the vertical temperature gradient.

5. In the solar collector panel described in this Application, the heat dissipation area of the solar energy absorber is increased, thus improving the energy transfer into the fluid and improving the collector efficiency.