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Abstract
Sunlight is being utilized to reduce energy crisis and it is a popular and reliable source of energy because of its pollution free nature. It is abundantly available on the Earth but difficult to achieve maximum efficiency during daytime without tracking. We have designed and developed a non-tracking solar concentrator system based on a combination of large Fresnel lens and segmented mirrors. Six segmented mirrors are mounted at different angles so that the concentrated light due to the Fresnel lens is redirected into a particular area throughout the day to achieve maximum concentration without tracking the sun. The entire system is mounted in a single mechanical mount with no moving parts. The concentrated sunlight was utilized for multipurpose solar applications. We have demonstrated simultaneous solar water heating and electricity generation by means of a thermoelectric generator (TEG) module using the developed system. The maximum temperature of water recorded at the outlet from the water heating system is 88°C and efficiency at this temperature is 0.47. The preliminary results of power generated from one TEG module is 40 mW. Experimental results of water heating throughout the day and electricity generation using TEG is reported. This type of system is unique, robust, and low cost for sunlight harvesting.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
At present, the consumption of energy mainly electricity has been increased and production of electricity through energy resources is less due to the limited availability of natural resources. Further, the electricity production using natural resources like coal and thermal plant is not good for environment because of the pollution [1]. To reduce the environmental pollution, it is necessary to generate electricity by means of new and renewable energy sources. One of the most promising green energy sources is the sunlight which is perpetual. Most of the sunlight harvesting system for energy generation has been mainly via photovoltaic solar cells. So far the photovoltaic based electricity generation has been very successful, but average efficiency of the energy conversion is low in case of polycrystalline PV solar module [2], however the efficiency of solar cells based on thin film multilayer structure has been reported to be around 40% [3]. This is due to the fact that most of the solar power plants are non-tracking the sun throughout the day and secondly only limited portion of the electromagnetic spectrum of the sun is utilized for the energy conversion. There has been lot of research and development for solar concentrators for improving the efficiency of the solar cells. Apart from generating electricity via photovoltaic there has been various alternating means to harvest the sunlight. These are parabolic solar concentrators [4] based systems and large Fresnel lens based [5–7] systems. But most of these systems have been utilizing a dual axis mechanical tracking system to concentrate the sunlight throughout the day. By means of concentrating the sunlight throughout the day efficiently various applications has been demonstrated such as transporting the sunlight inside the rooms, steam generation, solar cooking, solar heating, desalination etc. [4–11]. Also to get maximum sunlight at all times on particular area we need to remain in line of sight of sunlight throughout the day. For this purpose, different type of mechanical and programmable based single axis or dual axis tracking systems have been developed [12–16]. Single axis mechanical moving system tracks the azimuth angle only, while dual axis tracking system covers the movement of sun both in east-west direction and tilts the solar concentrator accordingly to utilize the maximum energy throughout the day. Abdallah et al. [12] designed and constructed an electromechanical based tracking system. With an open loop system of programming method, programmable logic controller is used for two axis sun tracking system. They have reported significantly larger collected solar energy as compared to fixed surface. Ali [13] presented movement of a photovoltaic module which was controlled by a programmable logic controller for sun tracking and reported 20% more output power in comparison to fixed module in daily calculations. In their theoretical approach Neville [14] presented a comparison of no tracking, East-West tracking and ideal tracking and reported that energy collected to ideal tracker is more than 50% and 5–10% in comparison to fixed collector and East-West tracker, respectively. Khalifa et al. [15] investigated the effect of two axes Sun tracking system on the performance of compound parabolic concentrator and reported that two axis tracking system gains energy up to 75% as compared to fixed collector system. Sun sensing photo transistor based tracking system is used by Hession et al. [16] in which they had shown sun’s position to be resolved to a precision of better than 0.1°. Researchers have also reported passive solar trackers based on thermal expansion of a matter or on shape memory alloys, like Clifford et al. [17] incorporated two bimetallic strips made of aluminum and steel positioned on a wooden frame, symmetrically of a central horizontal axis. They have reported that their designed solar tracker has the potential to increase the solar panel efficiency by up to 23%. Based on shape memory alloy Poulek [18] has developed a single axis passive solar tracker and reported that as compared to bimetallic actuators the efficiency of the shape memory alloy actuator is approximately two orders of higher magnitude. Use of four photo resistors with cylindrical shades, electro-optical unit as a sun sensor is studied by [19–21] and reported that the controllers used in their systems contained differential amplifiers, comparators and output components are highly accurate and well operated. A compact and semi-passive beam steering prism array for solar concentrator is proposed by Robert et al. [22]. They have used series of prism arrays that match different incident angles to enable the prism arrays. Celine et al. [23] has designed and studied about refractive Fresnel lens solar concentrator coupled with diffraction grating for spectral splitting and light focusing. This design spectrally splits the incident light into two parts which is then focused onto specific spatially separated photo voltaic cells for power generation.
For the last few years the solar concentrators with dual axis tracking system has been widely used for Solar thermal applications like water heating, steam generation and solar cooker. Kapurkar et al. [8] demonstrated heating of water using Fresnel lens concentrating system and reported overall efficiency of solar water heater 42.38 percent and 27.48 percent for distillation unit. Patil et al. [9] has shown design and optimization of solar water heating using Fresnel lens and two axis tracking system. Copper tube is used for water heating and water passes to storage tank for which they have reported temperature range of 60° to 70° in storage tank. Tsung et al. [10] has designed and demonstrated solar tracking system for solar thermal applications. They have reported best position for Stirling engine is at focal length of 345 mm and largest power of copper was acquired about 500 W/m2 at 12.00 noon. Aadesh Rajkrishna [11] theoretically programmed in MATLAB for design of solar geyser using spot Fresnel lens. Over conventional instantaneous efficiency of 43 percent and outlet temperature of 65°C they have achieved 63.26 percent efficiency and temperature about 82.89°C. But this work shows only theoretical approach.
In all the aforementioned mechanical tracking and programmable based designs for solar concentrating systems require an external source of electricity and also if at some stage tracking fails it could concentrate sunlight at undesired places eventually the system can damage or fire could spread out in the concentrated area. Although, such kind of systems are efficient and successful but the mechanical moving system becomes bulky, complicated and costly. Keeping all this in mind we have developed a non-tracking solar concentrator system based on a combination of large Fresnel lens and segmented mirrors [24]. Six Mirrors are arranged symmetrically to each other such that Fresnel lens will redirect sunlight rays in a specific small area of spot size 50cm2 so that this area can be utilized for many solar thermal applications. The system is utilized for solar water heating efficiently as well as electricity generation via thermo- electric generators. A square metallic box of size 50×50 cm2 and width 20 mm made up of copper with inlet and outlet for water flow and mounted on a thin copper sheet of area 50×50 cm2. Copper has high thermal conductivity which is most desirable part of heating water and sustains heat for a long time. It is also corrosion free. Both copper sheet and copper metallic box is black painted for good thermal absorption of sunlight on the copper surface. Further they can be folded or molded in any desired pattern. Concentrated Sunlight after reflected through segmented mirrors is focused on the metallic box which gets heated. Thermoelectric generator device is placed onto the back side of copper sheet to generate electricity for charging the battery. This type of system is unique, robust and low cost for sunlight harvesting. This design concentrates Sunlight throughout the day and for all seasons of the year.
2. Design of the system
Most of the Solar Concentrators like parabolic reflectors, trough and Compound parabolic concentrators requires a continuous dual axis solar tracking to concentrate Sunlight at the same spot. Use of Fresnel lens as a solar concentrator has been increased for its light weight and larger optical efficiency. Fresnel lens used as a concentrating system required tracking of sun during daytime. Azimuth angle of Sun moves from east to west during day hours shown in Fig. 1. Figure 1(a), (b) and (c) shows the schematic diagram of ray tracing using Zemax software for fixed Fresnel lens at three different angles of sun position i.e., (a) morning hours (b) at noon (c) evening hours.
Fig. 1. Schematic diagram of Ray Tracing in Zemax for sunlight rays at different angles with fixed Fresnel lens (a) Morning hours (b) at Noon (c) Evening hours
From Fig. 1 we can see that the focus shifts throughout the day with large amount. To concentrate sunlight in a particular area we designed and developed a segmented mirror system to focus sunlight in a limited area i.e., 50 cm2.
2.1 Design of segmented mirrors and single Fresnel lens based system
Sun moves from east to west in daytime covers azimuth and altitude angle. Sun rises from horizon and goes high in altitude at noon and return below horizon during night time. To cover Sunlight rays for both azimuth and altitude angle of sun during daytime we have designed a non-tracking solar concentrator based on large Fresnel lens and combination of six segmented mirror system to concentrate and redirect sunlight rays at specific area which can be used for many solar applications. Figure 2 shows the schematic diagram of the system, in which rays are plotted using Zemax.
Segmented mirrors are placed symmetrically to each other. Like Mirror ‘a’, in Fig. 2 is placed at a distance of 480 mm from center of Fresnel lens in x direction and 200 mm in y direction, tilted along −85° in y direction. Similarly all other mirrors are arranged accordingly as shown in Table 1.
Table 1. Details of data used for segmented mirror arrangement in Zemax software
2.2 Fresnel lens
There are two types of Fresnel lens. One is imaging Fresnel lens and other is non imaging Fresnel lens. These lenses are made up of Poly methyl meth acrylate (PMMA) material consists of concentric grooves on spot Fresnel lens and straight groves on linear Fresnel lens. For solar applications non imaging Fresnel lens are widely used because of its higher concentration ratio with less volume and shorter focal length, high numerical aperture, higher optical efficiency and larger acceptance angle. Main advantages of using Fresnel lens are they are very light weight as compared to conventional convex lenses and we can easily fabricate big curvature Fresnel lenses. They are easy to handle and portable in the fields. Fresnel lens has cuts on its edges in slopes and facet form so that light ray from air will bend due to different refractive index of PMMA material and it will redirect rays at focus due to refractive property of a lens. Hence light rays from an infinite can easily focus through Fresnel lens at particular area. Effective focal length of Fresnel lens can be calculated by
Where r is the radius and n is refractive index of the material used. Here material is PMMA which has a refractive index of 1.494. In our experimental set up we have used Fresnel lens of size 1 m2,thickness is 4 mm and focal length of 140 cm shown in Fig. 3 procured from YIWU RIFENG Optical Instruments Co. Ltd., China.2.3 Segmented mirror system
In this design we have developed segmented mirrors system arranged in different orientations. These mirrors are aluminum sheets of thickness 0.8 mm having mirror polishing on front side of it and pasted on Galvanized Iron (GI) sheets of 1.5 mm thickness. They can sustain high temperature easily without any bending and breaking unlike any silica based mirror. The size of each mirror sheet is 30 cm wide and 90 cm long. The mirrors orientated in different angles and redirect light in a common area. A mechanical mount is fabricated to mount all the mirrors in one with the angle control for each mirror attached with the main frame. The angles of individual mirrors are adjusted according to the position of focused spot of azimuth angle of sun using their controls mounted back side. The concentrated Sun light from Fresnel lens will be made incident on these mirrors at different orientation of Sun light shown in Fig. 2. For example in the morning the concentrated Sunlight from Fresnel lens will be incident on extreme left mirror ‘a’ and it would redirect the concentrated Sun light at the common area. After one hour the concentrated Sun light is incident on mirror placed adjacently to extreme left mirror, i.e., mirror ‘b’ and light will be reflected towards common area again. Similarly after each hour the concentrated Sunlight shifts and made incident onto the mirrors placed adjacent to each other and each mirror will redirect the concentrated Sunlight towards a common area. Thus make this complete system non-mechanical scanning [24].
2.4 Thermo-electric generator
Thermoelectric generators (TEG) are based on principle of Seebeck effect. It is a solid state device which converts heat flux directly into electrical energy. A thermoelectric (TE) module consists of units of n- and p- type semiconducting materials connected electrically in series and thermally in parallel [25]. Advantages of TEG are various like long life span, noiseless operation, no working fluids are inside TEG so it is maintenance free [26]. Widely used material for TEG module is Bismuth Telluride Modules (Bi2Te3) because of its low cost. We have procured TEG module (TG12-6) having dimension (44.7×40.1×3.91) mm (from Marlow Industries, Inc. USA). Researcher worldwide constantly has been working on finding inexpensive and highly efficient materials for TEG that can be produced commercially at larger scale.
3. Experimental results
Sunlight radiations reached on the earth surface contains 40 to 45 percent visible region in the wavelength range of 380 nm to 780 nm, 50 to 55 percent infrared radiation in the range of 800 nm to 2500 nm and 3 to 5 percent ultraviolet radiation [27,28] shown in Fig. 4. Experimental setup contains Fresnel lens which concentrates sunlight and redirected through segmented mirror system on specific area shown in Fig. 5 throughout a day [24]. In Fig. 5, the entire system of non-mechanical scanning solar concentrator with segmented mirrors, water heating system and TEG system is demonstrated. As already mention above that the concentrated sunlight is redirected only within the common area of 50×50 cm2 throughout the day, as shown in Fig. 5. A square metallic box of size 50×50 cm2 and width 20 mm made up of copper with inlet and outlet for water flow and mounted on a thin copper sheet of area 50×50 cm2. Sunlight is focused on the above device and water gets heated. Thermoelectric generator device is placed onto the back side of copper plate to generate electricity for charging the battery. Hence we utilize sunlight for water heating and electricity generation simultaneously. Area of the receiver spot size should be taken as the actual size of the spot incident on the receiver. Therefore for calculating the efficiency, the actual spot size of 50 cm2 was taken into the account. We have measured periodically temperature gain of outlet water flowing through tube in an interval of half an hour from morning 9 A.M to 3 P.M in the month of October in Delhi, India shown in Table 2. Latitude and longitude of Delhi is 28.7041° N, 77.1025° E respectively. Mass of flowing water at output is 1500 gram per minute. Aperture area of the Fresnel lens is 1 m2 and specific heat of water is 4.187 J/g °C. We have recorded readings for solar irradiance with digital solar power meter (TenmarsTM-207) on the same location (longitude, latitude) with same height and position on a clear sky day in regular interval of 30 minutes near to the experimental setup.
Table 2. Data collection for water heating
Heat gained by the water [29] is given by
where, Q is the heat gain by water (in Joules)m = mass of water (in grams)
${C_p}$ = specific heat of water (J/g °C)
${T_{3}}$ = Storage Temperature (°C)
${T_{2}}$ = Outlet temperature of water (°C)
${T_{1}}$ = Inlet temperature of water (°C)
3.1 Thermal efficiency
Efficiency of the system is evaluated using an Eq. (3) [29] shown below
where, ${\eta _{Th}}$ = Thermal Efficiency$Q$ = Heat gained from the collector
${A_{p}}$ = Aperture area of the Fresnel lens
${I_b}$ = Intensity of solar radiation
$\Delta t$ = Time interval in minute
Effect of Solar Radiations on metallic box increases temperature inside the box eventually it would give hot water at outlet. Variation of solar intensity and water temperature rises periodically with respect to time variation is shown in Fig. 6, curve in red dot depicts the temperature rises at outlet of metallic box while black curve depicts about inlet temperature of water entering the box and solar radiations are incident on box is shown by blue curve. We have found out that maximum temperature rises is at noon.
Heat gain by water and thermal efficiency is measured along with the variation of time and solar irradiations are shown in Fig. 7. Heat gain is represented by red square curve and efficiency is calculated using Eq. (3) is shown by blue dot curve. Maximum efficiency is observed at peak time of 11.30 A.M in month of October in Delhi, India.
3.2 Electrical efficiency
Thermo electric generator module fixed at back side of metallic sheet is used for electricity generation. The Preliminary result of one TEG module recorded is 2 V and 20 mA. Hence power generated is 40 mW.
4. Discussion
We have calculated the geometrical concentration ratio of the solar collector used in the system with the relation shown in Eq. (4) [29,30]. Aperture area of the Fresnel lens is 1 m2 which is 10000 cm2 and focused concentrated area of spot size is 50 cm2. Therefore geometrical concentration ratio comes out to be 200. It also signifies high heat accumulation on copper box which is required for heating the water.
5. Conclusion
We have developed a non-tracking solar concentrator system based on a combination of large Fresnel lens and segmented mirrors. The combination of Fresnel lens and mirror system is a unique device to concentrate sunlight in particular area throughout the day. We have demonstrated simultaneous solar water heating and electricity generation using the developed system. Sunlight is focused onto the top of the metallic box which gets heated and thereby heating the water inside the box instantly. Thermoelectric generator device is placed onto the back side of metallic plate to generate electricity for charging the battery. Maximum temperature of water is recorded at outlet is 88°C and efficiency at this temperature is 0.47. The preliminary results of power generated from one TEG module is 40 mW. This type of system is unique, robust and low cost for sunlight harvesting. This designed and fabricated system is useful for many solar thermal and electrical applications without tracking sunlight throughout a day and for an entire year.
Funding
Department of Science and Technology, Ministry of Science and Technology (DST) Govt. of India (DST/TMD/CERI/C24(G)).
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