A miniature concentrating photovoltaic and thermal system

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1 Energy Conversion and Management 47 (2006) A miniature concentrating photovoltaic and thermal system Abraham Kribus a, *, Daniel Kaftori b, Gur Mittelman a, Amir Hirshfeld a, Yuri Flitsanov a, Abraham Dayan a a School of Mechanical Engineering, Tel Aviv University, Tel Aviv 69978, Israel b DiSP Distributed Solar Power Ltd., Migdal Ha Emek, Israel Available online 17 April 2006 Abstract A novel miniature concentrating PV (MCPV) system is presented and analyzed. The system is producing both electrical and thermal energy, which is supplied to a nearby consumer. In contrast to PV/thermal (PV/T) flat collectors, the heat from an MCPV collector is not limited to low-temperature applications. The work reported here refers to the evaluation and preliminary design of the MCPV approach. The heat transport system, the electric and thermal performance, the manufacturing cost, and the resulting cost of energy in case of domestic water heating have been analyzed. The results show that the new approach has promising prospects. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Photovoltaics; Concentrator; CPV; Parabolic dish; Co-generation 1. Introduction Electric power from solar radiation by photovoltaic conversion is currently much more expensive than conventional power, and therefore it is used only where high government subsidies are available. Concentrating sunlight before its conversion greatly reduces the area of the expensive photovoltaic cells. This is well known and several attempts have been made to produce CPV (concentrating photovoltaic) systems. These use dish, trough, and Fresnel-lens concentrators and are usually in the m 2 scale. These relatively large devices are suitable for utility scale power plants in open areas, but are difficult to fit on rooftops and in an urban environment; much smaller units are needed for such applications. The value of the electricity produced at the power consumer site is the retail price that the consumer pays to the utility. For large solar plants located away from the consumer, the value is the utility conventional generation cost, which is much lower. Therefore, small systems that can be installed on rooftops can be more competitive than large plants located away from the consumer. This advantage benefits today s flat-plate PV systems, although it is not sufficient to make them competitive. * Corresponding author. Tel.: ; fax: address: kribus@eng.tau.ac.il (A. Kribus) /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.enconman

2 A. Kribus et al. / Energy Conversion and Management 47 (2006) Nomenclature A module area (m 2 ) h convection coefficient (W/m 2 K) I incident solar flux (W/m 2 ) k thermal conductivity (W/m K) l thickness of layer (m) Q power (W) q flux (W/m 2 ) T temperature (C) U heat transfer coefficient (W/m 2 K) Greek symbols e emissivity g efficiency r Stefan Bolzman constant Subscripts B back surface, back insulation layer C PV cell CHP combined heat and power COO coolant fluid EL electric F front surface GRO gross i inlet of coolant fluid IN incident on module INV inverter o outlet of coolant fluid OPT optical PL cooling plate PAR parasitic PV PV module REC receiver SUB substrate TH thermal 1 environment When the system is close to the consumer, it is possible to co-generate and provide to the consumer heat as a by-product of the power generation. Heat cannot be transported over large distances; therefore only distributed small systems that are installed close to the consumer can use co-generation. This is also known, but has been attempted so far only in flat (PV/T) collectors that do not take advantage of concentration. The low-temperature heat produced by PV/T collectors can be used for water heating and space heating, but not for more demanding processes such as cooling or industrial process heat. The conclusion from the observations above was to develop a miniature concentrating PV (MCPV) system that can be installed on any rooftop, as close as possible to the energy consumer. The design is based on a small parabolic dish, similar to a satellite dish, and very easy to handle, transport and install without specialized equipment. The conversion of the collected solar radiation to electricity is done with high-efficiency CPV cells. In principle, solar thermal conversion may also be possible, if small heat engines with reasonable performance become available [1]. This solar device concentrates sunlight about 500 times, so that the PV cell area is

3 3584 A. Kribus et al. / Energy Conversion and Management 47 (2006) greatly reduced. A heat transfer fluid is pumped into the PV array to remove the heat rejected from the PV cells. The fluid is circulated to a heat exchanger and heat storage tank, providing hot water or another form of thermal energy to the nearby consumer. This additional thermal energy product is obtained at almost no additional cost: if the CPV receiver is actively cooled anyway, then delivering the heat to the end user should add no more than 5% to the system cost. If the receiver is passively cooled, then changing to active cooling should still add no more than 10% to the overall system cost. A major issue this system is the treatment of heat transport. Several requirements must be met. First, the heat removal from the PV cells must be effective and continuous, to prevent damage to the cells from overheating. Also, the cell efficiency is reduced when the temperature increases, therefore the cooling system must maintain the temperature below a prescribed value lower than the damage threshold. On the other hand, the removed heat is more valuable at higher temperature, and therefore a compromise must be found to increase the value of the heat to the consumer, while not degrading the PV performance. The electric energy needed to pump the heat transfer fluid through the cooling system is a parasitic load that must be subtracted from the net electricity produced by the system. The design of the cooling element and the other components in the heat transfer circuit must minimize this parasitic energy consumption, but without compromising the other requirements. The work reported here refers to the evaluation and preliminary design of the MCPV approach. The heat transport system, the electric and thermal performance, the manufacturing cost, and the resulting cost of energy have been analyzed. The results are discussed and compared to the state of the art in the PV field. 2. Miniature dish system The MCPV concentrator is a simple on-axis parabolic dish. The target is placed at the focal point and its aperture is perpendicular to the optical axis. This results in some shading of the primary concentrator, but this shading can be kept to a minimum if the converter size is not much larger than the receiver s aperture. For example, a PV dense array operating at concentration of about 500 should cause less than 1% shading. The reflector is made of a single piece of glass, thermally bent to shape and then back-coated with silver to produce the reflective surface. An external protective coating prevents exposure of the silver to the environment. The thickness of the glass will ensure that the dish is self-supporting, i.e., it does not need an external Fig. 1. The MCPV unit during solar testing.

4 structural support to maintain its curvature. It is connected to the drive mechanism by a support frame that attaches to the glass at the circumference. For the tracking mechanism we adopted a design approach that was originally developed for small satellite tracking stations. The design was adapted for solar application by modifying the control system and some of the components, and by replacing the metallic dish by a solar reflector. This design includes off-the-shelf components such as motors, bearings, etc., an innovative reduction gear, and simple metallic parts that can easily be mass-produced. The dish concentrator and its tracking mechanism are shown in Fig. 1. The system is under construction at Tel Aviv University in cooperation with Distributed Solar Power Ltd. of Migdal Ha Emek, Israel. The PV module is based on triple-junction cells with nominal conversion efficiency of 32%. Such cells are commercially available today, and more advanced cells are currently under development, which should reach about 40% in the near future. The cells are installed over a cooling plate that removes the surplus heat from the cells to a coolant fluid. The coolant is typically water, although other fluids may be used in a closed circuit. The hot coolant leaving the PV module is directed to a heat exchanger where the heat may be used as an additional energy product, for example to produce hot water for domestic or industrial applications, hot air for space heating, or to drive an absorption cooling machine. 3. Energy conversion efficiency The overall efficiency of the system can be analyzed by identification of the different loss mechanisms during the conversion from incident sunlight to the end energy products. The system produces both electrical and thermal energy, and each type of energy product is described by a separate efficiency. The light-to-electricity efficiency is a product of the subsystem efficiencies: g EL ¼ g OPT g PV A. Kribus et al. / Energy Conversion and Management 47 (2006) Q PAR Q GRO g INV g OPT, g PV and g INV are the efficiencies of the optics, the PV module, and the inverter subsystems. Q PAR and Q GRO are the parasitic power consumption (for tracking motors and coolant pump) and the gross electrical power produced by the module. For the thermal energy product, the conversion efficiency is: g TH ¼ g OPT ð1 g PV Þg REC ð2þ g REC is the receiver efficiency, accounting for thermal losses from the receiver module to the environment. The combined heat and power (CHP) efficiency is the sum of the two energy products: g CHP ¼ g EL þ g TH ð3þ ð1þ 3.1. Optical losses Imperfect reflection at the dish causes a loss of typically 6 12% of the incident sunlight, depending on the type of mirror used and its state of cleanliness. A clean mirror made of low-iron glass with a silver back-coat should provide a reflectivity of 90 94%. Imperfect transmission at the front face of the PV module can result due to the reflectivity of a glass or other transparent layer that protects the PV cells from the environment. This layer can be designed for low reflectivity, reducing this loss to about 2 4%. Spillage (radiation arriving outside the module aperture) may cause about 2 3% additional loss. The total optical efficiency used in the current analysis, including all of these losses, is then assumed as a representative value of g OPT = PV conversion losses The typical conversion efficiency of an individual cell under standard test conditions is 32% (data from Spectrolab Inc., Sylmar, California). Additional module losses are caused by some unavoidable spaces among the cells, by front contacts that shade the cell active area, and by current mismatch due to differences in the output of cells connected in series. We assume that these additional losses are 10% of the cells ideal output.

5 3586 A. Kribus et al. / Energy Conversion and Management 47 (2006) The cell efficiency can also vary depending on the incident flux and on the cell temperature. The module efficiency under incident flux of 1 kw/m 2 (one sun), accounting for the variation of the cell efficiency with the cell temperature T C is: g PV ¼ 0:9 ½0:32 0:00062 ðt C 25 CÞŠ ¼ 0:288 0: ðt C 25 CÞ ð4þ The dependence of the temperature coefficient on the concentration is not given by the manufacturer of the cell under consideration. However, it is known that the sensitivity of the cell efficiency to temperature (or temperature coefficient) becomes lower as the radiation flux increases [2]. Although the miniature dish is planned to deliver an incident fluxes of up to 400 kw/m 2 (400 suns), we used the one-sun temperature coefficient ( ) as a conservative calculation. Including the effect of increased concentration would improve the electrical performance, but the sensitivity to concentration is lower than the sensitivity to temperature Gross and parasitic power The gross DC power produced by the module is computed from the power incident on the dish reflector Q IN and the optical and module efficiency: Q GRO ¼ Q IN g OPT g PV ð5þ The parasitic power is the sum of the power consumption of the two tracking motors, the cooling pump, and the control electronics. The pumping power has been estimated using standard correlations for pressure drop in coolant flow through the cooling plate, and in all cases it was found that the required power is negligible. The consumption of the control electronics is also small, and the tracking motors then make the dominant contribution. We note that the tracking motors may operate intermittently, and therefore their average power consumption is lower than their peak power. For example, if the axes are arranged for polar tracking, then the seasonal motor may move only once or twice per day, and the parasitic power consumption will be significantly reduced. The estimate for the average power consumption for a unit with a reflector of 0.95 m 2 aperture area is Q PAR = 0.02Q IN + Q PUMP. The required pump power is: Q PUMP ¼ _mdp=qg p where _m is the coolant mass flow rate, DP is the pressure drop, q is the fluid density and g p is the pump efficiency. The pressure drop accounts for both friction and tube bending losses Inverter The inverter performs the conversion of the produced DC to grid-compatible AC, and synchronizes the output of the PV system to the grid frequency. The inverter efficiency depends on its size and other parameters; we use a typical value of g INV = Receiver (thermal) losses The part of the energy absorbed in the PV module but not converted to electricity contributes to heating of the module. Most of this thermal energy is removed by the coolant flow, but some is lost to the environment Fig. 2. The thermal model of the PV module includes heat transfer to the coolant and losses at the front and back surfaces.

6 through the front and back surfaces (Fig. 2). A simplified thermal balance of the module and the cooling system was used to find the efficiency of heat recovery from the module. The simple thermal model assumes steady state operation; the temperature in the metallic cooling plate is uniform due to its high thermal conductivity; and side losses are neglected. A more detailed heat transfer analysis such as [3] was not used in order to keep the system analysis simple. The module energy balance and the receiver efficiency are then: Q TH ¼ g OPT ð1 g PV ÞQ IN ¼ Q COO þ Q F þ Q B g REC ¼ Q COO Q TH The front loss equation assumes that the concentrator occupies about half of the PV module s front surface field of view. The convective loss includes a convection coefficient based on a representative wind of 5 m/s. Q F ¼ h F A F ðt C T 1 Þþe F A F r T 4 C 1T ð7þ Heat transfer from the cells to the cooling plate is by conduction through the substrate: k SUB A SUB ðt C T PL Þ ¼ Q l COO þ Q B ð8þ SUB The overall thermal resistance of the substrate is found from the thermal resistances of the individual layers (solder, substrate, and adhesive): k SUB ¼ l SOLDER þ l CERAMIC þ l 1 ADHESIVE ð9þ l SUB k SOLDER k CERAMIC k ADHESIVE The heat transfer from the cooling plate to the coolant assumes standard heat exchanger relations and a constant plate temperature. T o T i Q COO ¼ _mcðt o T i Þ¼h COO A COO ð10þ ln T PL T i T PL T o The estimation of the convective heat transfer coefficient assumes fully developed turbulent flow in the cooling plate, where the Nusselt number is: Nu d ¼ 0:023Re 4=5 d Pr 2=5 ð11þ The heat transfer coefficient is defined as: h COO = Nu d k/d. Re d is the Reynolds number based on the channel diameter, Re d ¼ 4 _m=lpd and Pr is the Prandtl number. The dependence of the thermophysical properties on temperature is accounted as: Pr 2/5 k/l 4/5 = T m where T m is the average fluid temperature. The back surface of the module is always aimed towards the sun, and therefore it is exposed to incident unconcentrated solar radiation, in addition to the other contributions of convective and IR losses to the environment, and conduction through the insulation. Q B ¼ k BA B ðt PL T B Þ ¼ h B A B ðt B T 1 Þþe B A B rðt 4 B l T 4 1 Þ e BA B I ð12þ B The heat transfer rates, as well as the unknown temperatures of the cell, the cooling plate, the back surface of the insulation, and the coolant outlet, are found from solution of Eqs. (6) (12) Performance analysis results A. Kribus et al. / Energy Conversion and Management 47 (2006) The results shown are for a miniature system with a reflector of 0.95 m 2 aperture area, under normal beam insolation of 900 W/m 2. The overall conversion efficiency of the miniature dish system for electricity, heat and CHP is shown in Fig. 3. The coolant outlet temperature T o is used as a free parameter. This temperature can be adjusted in a physical system by changing the coolant flow rate. The PV cell temperature is typically about 10 C higher than the coolant outlet temperature. PV cells are usually operated at temperatures below 100 C. ð6þ

7 3588 A. Kribus et al. / Energy Conversion and Management 47 (2006) Fig. 3. The variation of the overall system efficiency for production of electricity, heat and CHP with the coolant outlet temperature is shown. Temperature ranges for potential thermal applications are shown as well. However, recent evidence shows that operation at higher temperatures is possible [4]. This may be desirable in order to increase the value of the thermal energy and feed it to a variety of industrial processes. At coolant outlet temperature of 58 C the electrical output of the system is 172 W and thermal output is 530 W. The CHP efficiency is high, nearly 80%, and not sensitive to the temperature. The electric efficiency is about 20% at low temperature and is gradually reduced at elevated temperature, but the lost energy is mostly recaptured as thermal energy, which is available as an additional product. More than 60% of the incident radiation is captured as thermal energy in the coolant that can be supplied to the consumer. This does not account for any thermal losses in piping between the MCPV collector system and the consumer. Also shown in Fig. 3 are the temperature ranges of common thermal applications. The low end is domestic water heating which is currently done in dedicated solar collectors. The same hot water can be obtained from the proposed MCPV system as a by-product of generating electricity with little or no additional cost. Operation at higher temperatures near 100 C enables refrigeration with single effect absorption chillers, or desalination with a thermal process such as distillation. Even higher temperature operation can provide double effect (higher performance) absorption cooling, steam generation and other industrial process heat applications. The choice of an optimal operating temperature for the MCPV system depends on the application, on the ability of the specific consumer to use thermal energy at elevated temperature, and on the value of the thermal energy as compared to alternative sources such as electric or gas heating. 4. Cost analysis The manufacturing cost of the MCPV system was estimated based on quotes from manufacturers for the major components [5]. The total cost was normalized by the net produced power (electricity only) to yield the cost of $2.5 per peak electric Watt. This is a very competitive cost, which is below the stated EU medium-range target of $3 W 1 p. The usual market price for PV systems is in the range of $6 8 W 1 p, depending on system size. Therefore the MCPV has the potential for a significant cost reduction even when compared based on electricity generation only, before considering the additional benefit of the thermal energy. The overall cost effectiveness of the system, taking into account the contribution of the thermal energy, can be measured by an overall monetary value such as the payback time, or the overall gain after the system s nominal lifetime (future value). A simplified financial analysis has been performed to provide a rough comparison between the MCPV providing electricity and heat at C, a conventional Flat-Plate PV (FPPV) collector producing electricity only, and a PV/T flat collector similar to the FPPV collector but also capturing thermal energy. The main assumptions used in the analysis are: interest rate 5%; no inflation; the solar conditions of Tel Aviv, Israel (2014 kwh/m 2 year beam radiation, and 2141 kwh/m 2 year global radiation on an optimally tilted surface); the collector installed price is $6 W 1 p for the FPPV and PV/T systems and $4 W 1 p for

8 A. Kribus et al. / Energy Conversion and Management 47 (2006) MCPV system; FPPV and PV/T electric efficiency is 12%, and PV/T thermal efficiency is 48%; system lifetime is 20 years. The results depend on the conventional alternative used for heating, for example an electric heater or a natural gas burner. Results are presented for both these options. Fig. 4 shows the results of this simple analysis as a function of the retail price of electricity (assuming that the price of gas is linearly linked to the price of electricity). The FPPV that produces electricity only does not show a reasonable payback time even if the electricity price is 20 kwh 1. The two co-generation systems show much better results, and the payback is less than the system lifetime (20 years) for a wide range of electricity prices. In the case of competition against natural gas heating, the PV/T system has a payback time of 10 years for electricity costs of 18 kwh 1, while the MCPV is more competitive and can achieve the same payback time in a 13 kwh 1 market. When the competition is heating with electricity, both systems are much more competitive, but the advantage of the MCPV remains: a payback of 10 years is achieved by the MCPV already at electricity cost of 5 kwh 1, while the PV/T requires at least 8 kwh 1. The same results can be seen from a different point of view, as shown in Fig. 4(b). When computing the future value of the saved energy over the system lifetime, and subtracting the future value of the initial investment of the solar collector, the result is the net monetary gain (or loss) to the owner. The FPPV system shows a negative gain for the entire range of electricity costs, corresponding to the previous result of a payback longer than 20 years. The case of the highest gain is when the solar heat replaces electric heating. In this case, a positive gain is realized even if the cost of electricity is only 5 kwh 1 : about $200 m 2 and $900 m 2 for the PV/T and MCPV, respectively. For a higher electricity cost, for example 15 kwh 1, the corresponding gains Payback Time (years) FPPV MCPV, electric heating MCPV, gas heating PVT, electric heating PVT, gas heating (a) Electricity Price ( /kwh) Gain After 20 Years ($/m 2 ) 15,000 10,000 5,000 0 FPPV MCPV, electric heating MCPV, gas heating PVT, electric heating PVT, gas heating (b) -5, Electricity Price ( /kwh) Fig. 4. (a) Payback period, and (b) Gain after 20 years lifetime, for the MCPV system and reference flat-plate PV and PV/T systems.

9 3590 A. Kribus et al. / Energy Conversion and Management 47 (2006) are $4500 m 2 and $6300 m 2. Even for the more difficult competition against gas heating, the gain becomes positive if the electricity cost is higher than 8 kwh 1 (for the MCPV) and 11 kwh 1 (for PV/T). 5. Discussion A miniature concentrator PV/thermal system is under development, producing about W of electricity and an additional W of heat. This system can operate over a wide range of temperatures, and provide thermal energy not only for water heating, but also for cooling, desalination, and industrial process heat. An analysis of the system performance has shown that at elevated temperatures, the electrical efficiency is somewhat lower, but most of the lost electricity is recovered as thermal energy. The MCPV system is suitable for installation close to a consumer that needs multiple forms of energy, electricity and heat or its derivative such as cooling. According to current cost estimates, the MCPV system can be produced at a low and competitive cost relative to conventional FPPV systems that produce only electricity. The result of the competitive cost and the higher combined efficiency of electricity and heat, is that the net savings to the consumer is significantly better than FPPV systems, as shown by a shorter payback time and higher gain over the lifetime of the system. A comparison to a similar co-generation approach but using a flat collector has also shown an advantage to the MCPV, due to lower estimated installed cost and higher efficiency. Ongoing work in this project is aimed to provide experimental validation of the small concentrator technology. A model unit is currently under construction, where the concept, components, and performance can be tested and validated. A start-up company has been established to develop, promote and commercialize systems based on the miniature concentrator approach: Distributed Solar Power Ltd. (Di.S.P.) at Migdal Ha Emek, Israel. Di.S.P. is developing a first unit that includes a miniature concentrator, a concentrating PV cell array, and related components. According to current plans, a first full system demonstration should be completed and tested during Acknowledgments Support for this research was provided by the Israel Ministry of National Infrastructure, and by the Israel Ministry of Industry, Trade and Employment. References [1] Kribus A. Thermal integral micro-generation systems for solar and conventional use. J Solar Energy Eng 2002;124: [2] Nishioka K, Takamoto T, Agui T, Kaneiwa M, Uraoka Y, Fuyuki T. Annual output estimation of concentrator photovoltaic systems using high-efficiency InGaP/InGaAs/Ge triple-junction solar cells. Solar Energy Mater Solar Cells 2005;90: [3] Mahderekal I, Boehm RF. Thermal analysis of a concentrator receiver. Solar 2004, Portland. [4] Meneses-Rodriguez D, Horley PP, Gonzalez-Hernandez J, Vorobiev YV, Gorley PN. Photovoltaic solar cells performance at elevated temperatures. Solar Energy 2005;78: [5] Kribus A, Kaftori D, Levy N. A miniature concentrating collector system for distributed generation. In: 12th Int sympos solar thermal concentrating technologies, Oaxaca (Mexico), 2004.