Design of Advanced Solar Water Heaters

Transcription

1 N.K. Groenhout, G.L. Morrison and M.Behnia School of Mechanical and Manufacturing Engineering University of New South Wales Sydney, NSW 2052 AUSTRALIA Abstract In the past decade there has been a significant increase in the use of domestic solar water heaters. Whilst the range and quality of products on the market have increased to meet this demand, flat plate solar collectors of the type used in many modern domestic hot water systems have not changed significantly in the past twenty years. These types of absorbers typically have high heat losses and low efficiency, particular throughout winter months. A novel type of solar collector has been proposed which addresses a number of issues including heat loss characteristics and seasonal performance. The project is investigating both design and production aspects of the new collector to provide an optimised design for an advanced domestic solar water heater. An indoors experimental program has commenced and has shown that considerable reductions in the overall heat loss may be possible with the proposed design and further experimental work is being undertaken to confirm this. It is believed that further reductions can also be achieved by the introduction of convective suppression barriers on the underside of the upper leading edge of the double sided plate. Particle Imaging Velocimetry (PIV) will be used to visualise the flow inside the concentrator cavities and to experimentally determine the flow velocity profiles within the cavities. A computational fluid dynamics model has also been developed to assist in the design process. This model will be validated with experimental results. 1. PROJECT BACKGROUND 1.1. Introduction In the past decade there has been a significant increase in the use of domestic solar water heaters around the world, with solar water heater production now a major industry in China, Australia, Greece, Israel and the USA [1]. In recent years the global pressure to reduce greenhouse gas emissions and the quest for sustainable energy solutions has revitalised the domestic solar water market. The revitalisation has been aided by the strong commitment being made worldwide by governments to make solar water heaters a more attractive product to consumers. In Australia, government rebate schemes for consumers who install domestic solar water heating have helped boost the industry. Whilst consumers now have a wide range of products to choose from and the quality of products has improved, the technology has not changed significantly in the past twenty years. The continued growth of the domestic solar water market is dependent on an improvement not only in the quality of the products but also the performance. Domestic solar water heaters in the past have had a reputation for high initial cost and poor performance in winter when demand for hot water is typically highest. A novel design of solar collector has been proposed which addresses many of the issues associated with current

2 technology systems. This project aims to develop the new design using latest computer simulation tools and to develop computerised design optimisation tools for the design of advanced solar water heaters Thermosyphon Systems The most common type of system in use in Australia is the flat plate thermosyphon collector (see figure 1). This type of system has been manufactured in many variations by a number of companies for over forty years. The general collector design consists of two or more absorber plates connected to a Storage tank Tank inlet Upper header Absorber plates Lower header Tank outlet Risers Figure 1: Current technology thermosyphon system. horizontal hot water storage tank and are generally mounted flush on the roof with the tank located above the absorber plates. The absorber plate consists of a series of riser tubes connected to an upper and lower header tube, which operates on the thermosyphon principle. The upper header connects to the inlet of the storage tank and the lower to the outlet. Incident radiation on the absorber plate causes the fluid inside the plate to heat and flow up the risers into the storage tank. The cooler fluid is forced out of the tank and back down to the lower header. The system may work as a closed loop or open loop system. The closed loop system uses a mantle type heat exchanger, which is essentially a narrow annulus between the two shells of a storage tank. A heat transfer fluid such as Glycol (anti-freeze) is used as the working fluid, transferring heat to the water inside the inner shell. The use of the heat exchanger makes these collectors particularly suitable for use in cold climates or where water quality is a problem Heat Loss Reduction Flat plate collectors generally have high heat losses and low efficiency since only the upper side of the absorber plate is exposed to the sun. The reverse side of the absorber plate must be insulated to prevent heat loss through the back of the collector. Between half and three quarters of the heat loss is through convection and radiation losses from the absorber surface and up to twenty percent through the insulated back surface [2]. Losses through the top of the collector are influenced strongly by the collector design and orientation. Back losses are primarily related to insulation performance. To reduce heat losses in flat plate collectors, three aspects need to be addressed. The first is conduction through the back and sides of the absorber casing, which is minimised by insulating the rear and sides of the absorber plate. The second aspect is convective losses from the top of the plate. These are the most difficult to control in flat plate collectors. Convection is affected by the angle of inclination of the plate and the spacing between the absorber plate and the glass cover. Symons and Peck [3] have shown that the difference in heat transfer rates for varying inclination is related to the flow structure in the cavity. Buchberg et al [4] has shown that an optimum air gap of 20mm exists at which point the conductive/convective heat transfer is minimised. Honeycomb cellular type convection suppression has been shown to reduce the onset of natural convection [5]. However, these types of material have proved unsatisfactory in flat plate collector applications due to problems with the material. Historically the materials used for these types of Renewable Energy Transforming Business 295

3 convection suppression have had poor UV stability resulting in optical characteristics that deteriorate with age and exposure. Radiation losses can be minimized by the use of selective surfaces to increase the plate s absorbtance and reducing its emittance. The selective surface is designed to absorb short wave solar radiation with minimal emittance of long wave radiation. An absorber plate painted black will absorb up to 95% of the incident solar radiation, but have high long wave radiation losses. Absorber plates with special selective surfaces will absorb the same amount of solar radiation but radiate only around 6% of the long wave radiation that would be lost from a black surface The Reverse Flat Plate Collector A number of designs have been investigated that attempt to reduce heat losses in flat plate collectors. The reverse flat plate collector as shown in figure 2 is one such design and was introduced independently by Rabl (1976) and Sakuta et al (1977) [2,6,7 & 9]. The reverse flat plate collector has an inverted absorber plate with a stationary concentrating reflector underneath. Providing the absorber is mounted horizontally the collector cavity becomes thermally stratified and convection is suppressed. Whilst this design significantly reduces the convective heat losses, there are still heat loss paths through the insulation above the collector and conduction through the air cavity. Solar radiation Glass cover Absorber Mirror Figure 2: Schematic of the reverse flat plate collector (adapted from reference 9). This design allows the collector to be given a seasonal bias by modifying the shape of the concentrating mirror. The disadvantage from a commercial aspect is that the collector is bulky and would be difficult to mount on a typical roof The Bifacial Absorber Collector A further development of the reverse flat plat collector is the bifacial collector shown in figure 3. This design has been shown to achieve higher efficiencies than other flat plate collectors under low Stationary concentrators Double sided absorber plate Figure 3: Bifacial absorber collector (adapated from reference 8) Renewable Energy Transforming Business 296

4 irradiation conditions. The design incorporates two identical stationary concentrators with a flat plate absorber mounted above them. The plate is illuminated on both sides and hence the heat loss path through the thermal insulation on the rear of the absorber plate is eliminated. The rear of the stationary concentrators is insulated to reduce heat losses through the back of the collector system [8] Proposed Design The proposed design that incorporates features from both the reverse flat plate and the bifacial absorber collector is shown in figure 4. It utilises a standard 2 m x 1 m flat plate absorber placed across the slope rather than along the slope as in a conventional flat plate solar collector. A high absorptance, low emittance selective surface is applied to both sides of the absorber plate. The absorber plate is then mounted on two stationary concentrators aligned in the east-west direction, a larger one at the top and smaller one at the bottom. Traditional flat plate collectors have excess output during the summer months when the sun is high, however, during winter months performance drops due to the solar radiation striking the absorber at a more acute angle. This drop in performance occurs when demand tends to be highest. The different concentrator sizes in the proposed design increase the angle the absorber plate makes with the horizontal plane, thereby giving a seasonal bias towards low angle winter solar radiation. The concentrators reflect the sunlight onto the underside of the absorber plate utilising approximately 80 % of the effective surface area of the absorber plate compared with less than 50 % for a standard flat plate absorber. Ray tracing has been utilised in the design of the concentrators to maximise the amount of solar radiation incident on the underside of the collector throughout the day. The ends of the cavity are mirrored and whilst some shading will occur throughout the day, it is not expected to significantly effect the collector s performance. The stationary concentrators will be constructed from micro sheet glass. This material has a significantly lower weight than conventional mirror material with excellent toughness and impact resistance. Most importantly it is flexible enough to obtain the complex parabolic shapes required for the concentrators without the need for specialist forming processes. Low iron glass cover Low angle winter radiation Tank Double sided absorber plate Roof Stationary concentrators Thermal insulation and absorber plate support 30 Figure 4: Proposed design showing stationary concentrators and double sided absorber plate. Renewable Energy Transforming Business 297

5 1.7. Project Aims The project is investigating both design and production aspects of a new collector to provide an optimised design for advanced domestic solar water heaters. The design incorporates features from both the reverse flat plate and the bifacial absorber collector. The features being investigated in this project include: Stationary concentrators designed to give a winter bias to the annual performance cycle. Double-sided absorber plates with convection suppression to reduce heat loss while achieving lower production costs through the reduction of absorber plate area. New thermosyphon plumbing concepts that allow greater flexibility in the placement of the solar collector relative to the tank while minimising reverse circulation at night. New sputtering processes to produce high absorptance and low emittance selective surfaces with significantly reduced pollution in the production process compared to existing plating techniques. In conjunction with the investigation of the proposed features the project aims to develop a validated simulation model using a combination of computational fluid dynamics (CFD) and other simulation tools including TRYNSYS for design optimisation of solar water heaters. To achieve these aims an experimental program has been developed and is being run in conjunction with numerical modelling of the collector in a commercial CFD package, FLUENT v5.2. This paper discusses preliminary results from experimental heat loss measurements and 2 dimensional CFD analysis of the natural convection heat transfer within the collector cavity. 2. EXPERIMENTAL WORK 2.1. Experimental Program The experimental program is being undertaken in two stages. The first stage is being carried out indoors using a full scale model of the proposed collector. The purpose of the indoor testing is to undertake heat loss measurements of the collector, to determine operating parameters for the numerical work and to carry out flow visualisation of the internal convection process. The second stage of the experimental work involves the outdoor testing of a production prototype system and comparison of its performance with existing flat plate collectors Experimental Rig The experimental rig is a full scale model, located in a small temperature controlled room. The absorber plate has been replaced by three 1 kw element type heaters, which consist of an electric element sandwiched between two steel plates and the three heaters are then sandwiched between two sheets of 2 mm thick aluminium plate. The aluminium plates provide a more uniform temperature distribution across the heaters, since the elements inside the heater tend to produce localised heating on the steel plates. The underside of the heaters, between the two concentrators is insulated with 25 mm thick thermal insulation. The heaters are connected to variable voltage power supplies through digital power meters to control and measure the power input to the heaters. Sixteen thermocouples are used to measure the surface temperature on the heater plate and an average heater temperature for each of the three heaters is obtained. One thermocouple is used to measure the ambient temperature. Two sheets of low iron, anti-reflective glass, of the type used in a standard flat plate collector have been used for the Renewable Energy Transforming Business 298

6 glass cover and the two concentrating reflectors have been modelled using light colour laminex sheet with an emissivity similar to that of micro sheet glass. The end walls of the concentrator cavities are constructed from 12 mm thick plywood. The heaters are connected via solid state relays to a digital control card inside a personal computer. A control program turns the heaters on and off based on the variation of the average heater temperature. The instantaneous power input into the heaters is recorded every cycle of the control program, approximately every five seconds, and then averaged over a 30 second period. The heater and ambient temperatures are similarly collected and averaged over a 30 second period. The control program can maintain the average heater temperature to within ± 0.5 C of the required temperature. A fan coil unit located inside the experimental room is connected to an external chiller unit. The water flow through the unit is via a small solenoid valve located on the inlet side of the fan coil unit, which is controlled by the same computer program that collects the temperature and power data. The room can be maintained at an ambient temperature of 20 C ± 0.5 C Experimental Results A series of experiments have been carried out for average plate temperatures in the range 50 C to 80 C. Typical flat plate absorber systems used in domestic hot water systems operate over this range of temperatures. Table 1 shows a summary of experimental data. Table 1: Summary of experimental data Average Absorber Plate Temeperature ( C) Average Heat Flux (W/m 2 ) U overall (W/m K) Ambient Temperature: 20 C The heat flux is obtained by dividing the average power over a period of time by the total exposed area of the heater plate, i.e. q = Q/A total (1) The overall heat transfer co-efficient is then obtained by: U overall = q / (T w - T? ) (2) where T w is the average plate temperature and T? is the ambient air temperature. For these experiments the back of the stationary concentrators were uninsulated. The overall heat transfer co-efficient for the temperatures investigated for this collector model are, lower than for current Renewable Energy Transforming Business 299

7 flat plate collectors, which have overall heat transfer co-efficients ranging from 4.5 W/m K for very efficient collectors with selective surfaces applied to the absorber plate to over 8.0 W/m K for absorbers with a standard matt black finish. Direct comparison between designs is difficult and further experiments will be undertaken to determine the heat loss of the design for higher emissivity heater plates, and an insulated collector Future Experimental Work A ducted air supply has been fitted to the top surface of the indoor test rig allowing a forced convection boundary condition to be imposed on the glass surface to simulate outdoor conditions. Insulation has been fitted to the collector to reduce heat loss, and to be representative of an operating collector. The heat loss measurements will be repeated with these modifications and the parameters will be included in the CFD model. Current work involves flow visualisation in the larger cavity, using smoke illuminated by a light sheet, to verify the flow patterns obtained in the numerical simulation work. A prototype of the proposed design is currently being designed and constructed utilising a low emissivity selective surface on the absorber and its performance will be tested and compared over the next twelve to eighteen months with a flat plate absorber using the same selective surface. 3. SIMULATION WORK The aim of the simulation work is to develop models for design optimisation purposes. Experimental work can be expensive and time consuming even for the investigation of minor changes to a design. A CFD model is being developed with parameters determined from the indoor experiments. The model will be validated against the experimental results CFD Model The modelling of the collector is being done using a commercial CFD package, Fluent V5, to determine the heat loss characteristics and flow patterns of the air within the collector. The complex geometry of the collector requires the CFD approach, as standard heat transfer correlations cannot adequately describe the natural convection within the collector. The collector has been simplified initially to consider only a two dimensional geometry, taking a vertical plane through the middle of the collector as shown in figure 4. This approach assumes that the ends are sufficiently distant from the plane of interest to avoid edge effects in the solution. The backs of the reflector walls are assumed to be insulated with an overall heat loss coefficient boundary condition of 2 W/m 2 K. The glass has a much higher heat loss, the overall heat loss coefficient on the outside of the glass is assumed to be 15 W/m 2 K. The absorber plate has been modelled initially as an isothermal surface and the plate temperature is varied between 40 C and 80 C, however, it is proposed to further develop the model using a heat flux boundary condition based on experimental results. An ambient air temperature of 20 C has been specified Numerical Results Preliminary results for a variety of temperatures have been obtained. The streamline contours for a range of absorber plate temperatures is shown in figure 5. The flow pattern is very complex. It consists of a multicell circulation in the upper cavity with a very weak stratified flow in the lower one. In the Renewable Energy Transforming Business 300

8 region between the heated plate and the glass, there exists a multicellular flow region as in standard single sided flat plate collectors. This flow resembles the flow observed in tall cavities with closed ends reported by Symons and Peck [3]. In a numerical study of natural convection in tall cavities with closed ends Le Quere [10] also observed a multicellular behaviour. He notes that there exist a number of solution branches with different numbers of cells. In the existing geometry the stability of the cellular flow pattern needs to be investigated. There is also a strong interaction between the flow in the larger cavity and the flow in the region between the heated plate and the glass cover. Figure 5: Streamlines for average plate temperatures of (a) 50 C, (b) 60 C, (c) 70 C and (d) 80 C. Boundary conditions for all cases are:? absorber =0.15,? glass =0.95,? wa ls =1.0, h absorber =15 W/m 2 K, h wa ls =2 W/m 2 K, Tambient = 20 C. 4. Concluding Remarks At present the heat transfer co-efficient of the glass cover has not been quantified. Once the convective boundary conditions for the glass surface over a range of temperatures have been quantified this will be included in the numerical modelling. Upon completion of the two dimensional study, the model will be extended to evaluate possible three dimensional circulation in the cavities. The preliminary evaluation of a design for an advanced solar water heater has been presented. This consists of numerically modelling the proposed design using a commercial CFD package, FLUENT. An experimental program has begun which will be used to validate the numerical model. The numerical model will then be used to optimise the design. Preliminary CFD results show that the proposed design has multicellular flows, however, considerable development of the two dimensional CFD work still needs to be completed prior to three dimensional analysis being undertaken. An experimental program has commenced and preliminary results show that considerable reduction in the overall heat loss may be possible with the proposed design. Further experimental work including outdoor testing with a prototype collector benchmarked against a standard flat plate collector will be undertaken to confirm this. It is believed that further reductions can be achieved by the introduction of convective suppression barriers on the top edge of the heater/absorber plate. Renewable Energy Transforming Business 301

9 5. References [1] Morrison, G.L. and Wood, B.D. (1999), Packaged solar water heating technology twenty years of progress, Solar Energy, in print. [2] Madhusudan, M. (1981), Optimization of heat losses in normal and reverse flat-plate collector configurations: Analysis and performance, Energy Con. & Mgmt, 21, [3] Symons, J.G. and Peck, M.K. (1984), Natural convection heat transfer through inclined longitudinal slots, J Heat Transfer, 106, [4] Buchberg, H., Catton, I. and Edwards, D.K. (1976), Natural convection in enclosed spaces a review of application to solar energy collection, J. Heat Transfer, 98, [5] Charters, W.W.S and Peterson, L.F. (1972), Free convection suppression using honeycomb cellular materials, Solar Energy, 13 (4), [6] Rabl, A. (1976), Comparison of solar concentrators, Solar Energy, 18, [7] Kienzlen, V., Gordon, J.M. and Kreider, J.F. (1988), The reverse flat plate collector: A stationary, nonevacuated, low-technology, medium-temperature solar collector, J. Solar Energy Engineering, 110, [8] Goetzberger, A., Dengler, J., Rommel, M. and Wittwer, V. (1991), The bifacial absorber collector: a new highly efficient flat plate collector,in Arden, M.E., Burley, S.M.A. and Coleman, M. (Eds) Proc of the biennial Congress of the International Solar Energy Society, Denver, Colorado, USA, August 1991, pp [9]McIntosh, K.R and Mills, D.R., (1994), The thermal losses from a reverse flat plate collector, [10]Le Quéré, P. (1990), A note on multiple and unsteady solutions in a tall cavity, J. Heat Transfer, 112, [11] Behnia, M., Reizes, J.A. & de Vahl Davis, G. (1985), Natural convection in a rectangular slot with convective radiatiave boundaries, in Dhir, V.K, Chen, J.C. and Jones, O.C. (Eds), National Heat Transfer Conf, Denver, Colorado,4-7 August [12] Morrision, G.L., Behnia, M., Cook, M., Groenhout, N.K. & Mills, D.R., (1999), Optimal design of advanced solar water heaters., in Farrow, D.E. (Ed), Pro. Sixth Australiasian Natural Convection Workshop, Murdoch University, Perth, 1-3 December [13] Fluent 5 Users Guide. (1998), Fluent Inc. Lebanon, New Hampshire, [14] Bejan, A. (1995), Convective Heat Transfer, Wiley Renewable Energy Transforming Business 302