Performance of a linear Fresnel-based concentrating ...

Performance of a linear Fresnel-based concentrating hybrid PV/T collector using selective spectral beam splitting Felipe Crisostomo1, Robert A. Taylor1,2, Gary Rosengarten3, Vernie Everett4 and Evatt R. Hawkes1,2 1

School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, NSW 2052 Australia 2 School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052 Australia 3 School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, VIC 3001, Australia 4 College of Engineering and Computer Science, Australian National University, Canberra, ACT 0200, Australia Keywords: Solar energy, Si PV cell, dichroic mirrors, band-stop filters. Selective spectral beam splitting is a promising approach for hybrid photovoltaic/thermal (PV/T) technologies since it allows for the decoupling of receivers with different spectral absorption bands. PV cells operating in such systems may operate at lower temperature and thus with higher efficiency, while thermal receivers can still be designed to deliver a high temperature output. In this paper, the performance of a novel, hybrid PV/T collector is investigated through optical and heat loss analysis. Outdoor experimental testing of this design is carried out using a rig consisting of a set of 10 linear Fresnel mirrors, two band-stop dichroic mirrors (spectrally reflecting light between 800 and 1050 nm), two strips of 30 Si PV cells connected in series, and a thermal receiver. For comparison, the set-up also allows for measurements of a thermal-only configuration to obtain the heat produced with the same geometry and concentration ratio, respectively. This comparison revealed the added value of using beam splitting in the proposed collector, based on the price ratio between electricity and heat production. Finally, the thermal and electrical power outputs are measured to obtain the overall thermal efficiency curves. Contact author: Felipe Crisostomo – [email protected] 1

Introduction

In the constant effort to improve the efficiency conversion solar energy technologies, the development of hybrid Photovoltaic/Thermal (PV/T) collectors is a promising option, since it allows for an optimal utilisation of the whole solar spectrum [1–3]. However, the main conflict that arises from this approach is that the PV cells efficiency is deteriorated as their operation temperature increases [4,5]. In contrast, for solar thermal collectors, it is desirable to obtain a higher temperature thermal output – even for domestic water heating requires temperatures above 50 ºC [6]. One way to avoid this problem in PV/T systems is the use of optical beam splitting, which directs different spectral bands to different receivers [7–9]. This feature allows for a physical decoupling of the receivers, i.e. for operation at different temperatures. Despite the promise of this approach, it is still needed to demonstrate the reliability and cost-effectiveness of the solar PV/T technologies among the various splitting methodologies reported in the literature [10–13].

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

In this work, the performance of a linear Fresnel-based concentrated collector coupled with a hybrid PV/T receiver is theoretically and experimentally investigated. Here, the solar spectrum is split using thin-film filters custom-designed to reflect concentrated light only between 800-1050 nm to silicon PV cells. The rest (outside this range) is transmitted to a thermal receiver (based on the previous work done by the co-authors [10]). In the theoretical analysis, the optical and heat losses are identified to estimate the collector output. Experimental testing is carried out only for normal incident radiation at steady state operation, according to the AS/NZ Standard (ISO 9806-1) [14]. Under these conditions thermal and electrical outputs are measured in order to calculate the collector combined efficiency. The same set-up also allows for measurement without the PV cells and the filters devices. Thus, in addition to the hybrid system, we also conduct ‘thermal-only’ performance tests. This enables an estimate to be made of the added value of using beam splitting, and to compare the overall efficiency curves. 1.1

Proposed PV/T collector

As is shown in Figure 1b, the light that reaches the collector is concentrated by an array of 10 Fresnel mirrors that are capable of tracking the sun in one axis. The concentrated light is then split when it hits the two beam splitting mirrors that are placed in a “V” shape at the center line of the collector. The mirrors were design to reflect light only between 800 and 1050 nm, so this part of the spectrum is directed to two PV cell strips (see Figure 1b). On the other hand, the light outside this range is transmitted to the thermal receiver. Each PV cell strip corresponds to an array of 30 rear-contact silicon PV cells connected in series. The strips are attached to a flattened copper tube on the rear, with water flowing inside, to maintain them at a temperature below 30 ºC. The thermal receiver consists of a stainless steel tube with a Tinox selective coating on its outer surface, and is aided by a secondary receiver which reflects back any rays that miss the tube.

(a)Top view (b) Cross section Figure 1: Schematic of the proposed hybrid PV/T collector

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

1.2

Collector optical losses overview

As the proposed collector concentrates sunlight, it is only able to utilise the direct solar spectral irradiance (Idirect). If this term is multiplied by the collector harvesting area (A), the spectral power (SPap) reaching the collector can be obtained based on Eq. 1. This spectral power distribution is illustrated by the black area in the graphs of Figure 2. 𝑆𝑃!" (𝜆) = 𝐼!"#$%& (𝜆) ∙ 𝐴   (1)

Unfortunately, not all of the spectral power makes it to the receivers. The first optical loss that impacts the collector corresponds to the incoming rays which are blocked by the thermal receiver and PV cells. This shading loss can be simply estimated at normal incidence by the following relation, using the occluding width of these components: 𝑆ℎ𝑎𝑑𝑖𝑛𝑔 =

𝑊!! + 2 ∙ 𝑊!" = 13%   𝑊

(2)

Here, the terms W, Wth and WPV denote the occluding widths of the collector, thermal receiver and PV cells with the cooling tube included, respectively (as shown in Figure 1a). Additionally, when the light that is not blocked, it reaches the Fresnel mirrors on the bottom of the collector. From there, light is not perfectly reflected, so it must be considered as a second optical loss in the collector. To estimate this, the reflectivity of the Fresnel mirrors was measured in a Perkin Elmer spectrophotometer (Lambda 1050). In Eq. 3 these two losses are combined. Note that shading is a simple de-rating factor, while the measured reflectivity is a spectrally weighted loss. The resulting power corresponds to the spectral distribution of the light that reaches the thin film filters (SPmir), represented by the gray area in the Figures 2 and 3. 𝑆𝑃!"# (𝜆) = 𝑆𝑃!" (𝜆) ∙ 𝑅!"#$%#& (𝜆) ∙ (1 − 𝑆ℎ𝑎𝑑𝑖𝑛𝑔)   (3)

As mentioned before, when the light hits the thin film filters, it is split in order to send the desired portions of the solar spectrum to the thermal receiver and PV cells, respectively. To ensure our custom designed, thin film mirrors match our specification, their transmittance (Tmir) and reflectivity (Rmir) were measured using the Perkin Elmer spectrophotometer (Lambda 1050). These two optical properties are used to determine the actual spectral distribution of the light reaching the thermal receiver and PV cells, as is given in the following expressions of eqs. 4 and 5: 𝑆𝑃!! (𝜆) = 𝑆𝑃!"# (𝜆) ∙ 𝑇!"# (𝜆)   𝑆𝑃!" (𝜆) = 𝑆𝑃!"# (𝜆) ∙ 𝑅!"# (𝜆)  

(4) (5)

These two spectral power distributions are illustrated by the white areas in Figures 2a and 2b, respectively. Furthermore, to calculate the incident power on the thermal receiver (Pth) and PV cells strips (Pel), these terms can be integrated over the whole spectrum according to Eqs. 6 and 7. Note that in the calculation of incident power on the PV cells, the integral is multiplied by 0.5 (see Eq. 7), as two thin film filters divide the reflected light towards two PV cell strips located on the left and right sides of the “V” shape mirror.

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

!!!"""

𝑃!! =

!!!"#

𝑆𝑃!! (𝜆)  𝑑𝜆 = 79.4  [𝑊] (6)

!!!"""

𝑃!" = 0.5   ·

!!!"#

𝑆𝑃!! (𝜆)  𝑑𝜆 = 20.6  [𝑊]   (7)

Even though the absorber tube of the thermal receiver is enclosed between the secondary reflector and the thin film filters (to reduce convective losses, see Figure 1b), the whole receiver is still exposed to the wind which may considerably affect the performance of the receiver – especially at high temperatures. Studies of a similar geometry rooftop micro-concentrator [15][16] have demonstrated a relatively flat overall efficiency curve, between 68% and 55% at mean temperatures of 30 ºC and 200 ºC. The key feature that allowed that is the use of a canopy that encloses all collector components, limiting the heat losses to internal natural convection and radiation emitted by the tube. In the case of the PV cells illuminated under the spectrum shown in the Figure 2b (which matches well with the region of high spectral response of the silicon cells [10]), there are still thermalisation losses due to the greater energy levels of the incident photons than the bandgap energy level [17]. As a reference, previous studies have reported theoretical [7] and measured efficiencies [12] up to 27% and 20%, respectively, for Si PV cells operating under similar split spectrums.

(a) Thermal receiver (b) PV cells Figure 2: Actual power spectral distribution of the light reaching the collector, before and after the beam splitting mirrors

Finally, in addition to the losses mentioned above, there are a few other miscellaneous optical losses that should be accounted for in a PV/T Fresnel concentrating system – Fresnel mirrors discontinuities, diffuse (scattering) reflections, and optical misalignments [18]. 2

Experiment

The experimental rig is composed of Fresnel mirrors, a thermal receiver, PV cells, and beam splitting mirrors. It was mounted on an external tracking system to perform on–sun testing under constant normal solar radiation (as is shown in Figure 3a). For this reason, the Fresnel mirrors are static during the testing. Also, two Pyranometers (one of them with a shading disc) are placed on the tracking system to log the total and diffuse radiation in real time.

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

Figure 3: Photos of the experimental rig and its components

2.1

Water circuit and thermal receiver

A 200 L heater-tank, in which the inside temperature can vary from 20 oC to 90 ºC, supplies water to the thermal receiver and is then returned to the tank. The thermal receiver itself consists in a stainless steel tube with a Tinox selective surface on its outer surface. This tube is located above the beam splitting mirrors and below the secondary receiver. Since the harvesting area is very small (0.55 m wide by 0.3 m long – 0.17 m2), a low mass flow rate of water (around 0.01 kg/sec) is set for all tests to achieve a measurable increase in water temperature across the thermal receiver. Also, based on the recommendation of the AS/NZ standard [14], the two RTDs are located immediately before and after the collection area, perpendicular to the flow, with an elbow fitting part. Furthermore, stainless steel wool is placed inside the tube to achieve a homogeneous fluid temperature due to the low mass flow rate. 2.2

Beam splitting mirrors

The beam splitting devices, dichroic mirrors, are multilayer thin film filters made of TiO2/SiO2 (high and low refractive index materials). The filters were located just underneath the thermal receiver at a relative angle of 60º. The dimensions of the filters are 3.5 cm x 34 cm. Note that the filters are longer than the PV cells strips to ensure that these are fully illuminated during testing. In order to obtain a simpler design than previous studies which used higher concentration ratios [12, 19], the filters layers were deposited just on one side of the borosilicate glass substrate. Consequently, the actual reflection window obtained (800-1050 nm) is not as wide as the theoretical optimal window (713-1067 nm) [10]. As a result, some of short wavelength light is still transmitted to the thermal receiver (as it can be seen in Figure 2b). Nevertheless, the thermal receiver can still use this short wavelength light band (713-800 nm), so it is expected that the collector performs very close to the optimal parameters [10]. 2.3

PV cells and I-V tester

The PV cells strips correspond to arrays of 30 single Si cells connected in series. The size of each PV cell is 1 cm x 3 cm, so the active area of the strip was 3 cm x 30 cm. To cool the PV strips, water was pumped through copper tubes (attached to the rear with “Artic Silver” model thermal adhesive) to keep the temperature of the PV cells below 30 ºC.

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

To obtain the I-V curves and the maximum power point (MPP) from the PV arrays, a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) was in a custom-built I-V tester [20, 21] which sweeps across the operational voltage of the PV cells. A diagram of the circuit is shown in Figure 4, where the abbreviation NI corresponds to National Instruments voltmeter and Vv and Vc denote the measured voltages, which are used to estimate the voltage and current across the PV cell strips, respectively. To obtain the current (IPV), the voltage, Vc, is just divided by 1 Ω according the Ohm’s law, while Vv is multiplied by the factor 3 to obtain Vpv, due the fact that the voltage is divided by the 1.5 MΩ and 0.5 MΩ resistors (see figure 4). Note also that the terms IPV and VPV, correspond to the vertical and horizontal axis in the I-V curve shown in Figure 6.

MOSFET

NI  module

1  MΩ   Voltage  VV

NI  INDEPENDENT   VOLTAGE  SOURCE  

1  Ω   Si  PV  cell   array  1

Si  PV  cell   array  2

0.5  MΩ  

NI  module Voltage  VC

Figure 4: Diagram of the I-V tester circuit

3

Experimental results and discussion

Using the measurements taken in this experimental set-up, the thermal and electrical power outputs can be calculated. Also, the overall thermal efficiency curve for the collector can be obtained. According to the AS/NZ standard [14], only steady state data points are considered – defined by less than 0.1 and 1 ºC of absolute variation in the inlet and ambient temperatures, less than 1% of relative variation in the mass flow rate, and less than 50 W/m2 of absolute variation in the global radiation. Additionally, only data which meets these conditions for a steady state time of at least 10 minutes is reported in this paper. 3.1

Electrical output

The electrical output from each PV cell strip is obtained from the maximum value of the product between IPV and VPV in each voltage sweep, according to Eq. 8. Note that this product corresponds to the maximum power point. The logged current and voltage from both PV cell strips are shown in the Figure 5. It should be noted that the behavior in both PV cell strips is very similar due to the symmetry of the collector and the normal incidence testing. The voltage at the maximum power point of the strip is 16.3 V which is very close to that expected for an array of 30 Si PV cells, considering that for each individual cell the voltage at the maximum power point should be 0.56 V [18] – e.g. 30 x 0.56 V = 16.8 V. 𝑃!" = max{𝐼!" ·   𝑉!" } (8)

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

The data shown in Figure 5 give an average electrical power output for each PV cell strip of 2.8 W. Note that the incident power on each strip is 20.4 W (see Eq. 7), so the efficiency of each strip is 14%. This efficiency is significantly lower than the efficiency reported in a previous study for Si PV cells with a similar spectral split at 5 suns (incidence irradiance) [12]. The possible causes of this low efficiency may be related to misalignment of the PV cells with respect to the Fresnel and/or beam splitting mirrors during the test. Thus, further testing should include a sensitivity analysis of the Fresnel mirrors to estimate the individual contribution of each one to the electrical output in the PV cell strips to correct possible optical errors.

Figure 5: I-V curves experimentally obtained for both PV cell strips

3.2

Overall thermal efficiency curves

The overall thermal efficiency is obtained using Eq. 9, where Qu corresponds to the thermal output, calculated by the product between the mass flow rate (ṁ), the specific heat of water (Cp) and the difference of outlet and inlet temperatures (To and Ti). The mass flow rate is measured by extracting flow for two minutes using a watch and a mass balance, and the temperatures are measured with the two RTD sensors. In the denominator, Id represents the measured direct irradiance (from the subtraction of the measured global and diffuse components). 𝜼𝒕𝒉 =

𝑸𝒖 𝒎 ∗ 𝑪𝒑 ∗ 𝑻𝒐 − 𝑻𝒊 =       𝑰𝒅 · 𝑨 𝑰𝒅 · 𝑨

(9)

The overall thermal efficiency curves are shown in the Figure 6, where the diamonds and the solid line represent the measured efficiciency points and a linear regression through them for the hybrid configuration. Similarly, the circles and dashed line illustrate the results for the thermal-only configuration. Furthermore, the squares and dotted line represent the measured efficiency and linear regression for a larger scale, but similar geometry, Chromasun collector [16] that was tested in the same tracking rig. The nearly flat trend of this curve evidences how important the canopy is in this set-up, protecting the collector from the forced convection (wind) losses. Thus, in the present work, the thermal-only collector and the Chromasun collector curves are similar at low temperatures, where convective losses are expected to be similar.

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

Figure 6: Overall thermal efficiency for the hybrid, thermal-only and Chromasun collector.

It is logical that the efficiency regression line of the hybrid configuration is always below the efficiency regression line of the thermal-only configuration, due to the fact that some of the incident sunlight is split to the PV cells. Based on the results in Eqs. 6 and 7, the incident power on the thermal-only and hybrid configurations are aproximately 100 and 80 W, respectively. Also, from the numerator of in Eq. 9 (Qu), the thermal output values start at 80 and 65 W which represent a conversion efficiency in the thermal receiver of 80% in both configurations. Therefore, it can be concluded that the collector is performing very well at low mean temperatures (e.g. where Tm, the average between the inlet and outlet temperatues, is less than 30 oC). However, as the mean temperature increases, the thermal efficiency curves drop dramatically due to forced convection losses becoming more significant and consequently the thermal output is deteriorated. Moreover, the low mass flow rate of the fluid inside the tube in the testing intensifies this relative heat loss mechanism. 3.3

Total power output comparison

The total power output (PO) in the thermal-only configuration (sub-index “th”) simply corresponds to the numerator in Eq. 9 (Qu), while for the hybrid configuration (sub-index “hy”) the contribution of the PV cells is included too (see Eq. 10). This addition considers the electrical output delivered from both PV arrays multiplied by the worth factor w, according to the higher value of electricity versus heat production [10]. 𝑷𝑶𝒕𝒉 = 𝑸𝒖              𝒂𝒏𝒅          𝑷𝑶𝒉𝒚 = 𝑸𝒖 + 𝟐 · 𝒘 · 𝑷𝑷𝑽     (10)

As is shown in Figure 7, the combined power output of the hybrid configuration is always above power output of the thermal-only due to the contribution of the PV cells to the first one. In other words, it means that the sacrifice of thermal output in the hybrid configuration due to the light division- which can be inferred from the efficiency curves in Figure 6- can be compensated, and even more, exceeded by the weighted electrical output. This is a good indicator that the proposed hybrid collector could be a feasible technology to be developed, as the electricity generation could balance the increment in the collector capital cost due to the additional components (PV cells and beam splitting mirrors).

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

Figure 7: Measured power outputs in thermal-only and hybrid configurations.

4

Conclusion and future work

Preliminary results for the on-sun performance are obtained for the proposed hybrid PV/T collector. The thermal and electrical outputs are successfully measured at steady state conditions and the thermal efficiency curves are also obtained. The thermal performance of the collector agrees well with the larger-scale Chromasun collector’s at low mean temperatures (around 55% efficiency). However, since the thermal receiver is exposed in our test, the performance dramatically drops at mean absorber temperatures higher than 30 ºC. The sacrifice of thermal output in the hybrid configuration in relation to the output in the thermal-only collector is exceeded by the weighted electrical output contribution from the PV cells strips in the hybrid configuration. This fact indicates that the proposed collector can be a feasible solar technology. Furthermore, based on theoretical analysis for the PV cells, this electrical output could be further improved if the optical components are optimised further. Future work will consider testing with a canopy or wind block to reduce the forced heat losses at high temperatures and consequently prevent the dramatic drop in the thermal outputs and efficiency curves. For the electrical output, a sensitivity analysis will be carried out for the Fresnel mirrors and position of the beam splitting mirrors, in order to correct optical errors in the experimental rig and enhance the electrical output. Acknowledgment This Program has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government.

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

Chow, T.T., "A review on photovoltaic/thermal hybrid solar technology," Applied Energy, 87, 365–379 (2010). Charalambous, P.G., Maidment, G.G., Kalogirou, S. a. and Yiakoumetti, K., "Photovoltaic thermal (PV/T) collectors: A review", Applied Thermal Engineering, 27, 275–286 (2007). Tyagi, V.V., Kaushik, S.C.and Tyagi, S.K., "Advancement in solar photovoltaic/thermal (PV/T) hybrid collector technology", Renewable and Sustainable Energy Reviews, 16, 1383–1398 (2012). Feteha, M.Y. and Eldallal, G.M., "The effects of temperature and light concentration on the GaInP/GaAs multijunction solar cell’s performance", Renewable Energy, 28, 1097–1104 (2003). Green, M.A., "General temperature dependence of solar cell performance and implications for device modelling", Progress in Photovoltaics: Research and Applications, 11, 333–340 (2003). Tian, Y. and Zhao, C.Y., "A review of solar collectors and thermal energy storage in solar thermal applications", Applied Energy, 104, 538–553 (2013). Hamdy, M.A. and Osborn, D.E.: "The potential for increasing the efficiency of solar cells in hybrid photovoltaic/thermal concentrating systems by using beam splitting", Solar & Wind Technology, 7, 147–153 (1990). Mojiri, A., Taylor, R., Thomsen, E. and Rosengarten, G., "Spectral beam splitting for efficient conversion of solar energy—A review"' Renewable and Sustainable Energy Reviews, 28, 654– 663 (2013). Imenes, A.G. and Mills, D.R., "Spectral beam splitting technology for increased conversion efficiency in solar concentrating systems: a review", Solar Energy Materials and Solar Cells, 84, 19–69 (2004). Crisostomo, F., Taylor, R.A., Mojori, A., Hawkes, E.R., Surjadi and D., Rosengarten, G., "Beam splitting system for the development of a concentrating linear fresnel solar hybrid PV/T collector", ASME 2013 Summer Heat Transfer Conference (2013). Kraemer, D., Hu, L., Muto, a., Chen, X., Chen, G. and Chiesa, M., "Photovoltaic-thermoelectric hybrid systems: A general optimization methodology", Applied Physics Letters, 92, 243503 (2008). Shou, C., Luo, Z., Wang, T., Shen, W., Rosengarten, G., Wei, W., Wang, C., Ni, M. and Cen, K., "Investigation of a broadband TiO2/SiO2 optical thin-film filter for hybrid solar power systems", Applied Energy, 92, 298–306 (2012). Aste, N., Del Pero, C. and Leonforte, F., "Optimization of Solar Thermal Fraction in PVT Systems", Energy Procedia, 30, 8–18 (2012). TM "Test methods for solar collectors Part 1  : Thermal Australian / New Zealand Standard performance of glazed liquid heating collectors including pressure drop" ( ISO 9806-1  : 1994 , MOD ), (2007). Sultana, T., Morrison, G.L. and Rosengarten, G., "A Numerical and Experimental Study of a Novel Roof Integrated Solar Micro-concentrating Collector", Solar2011, the 49th AuSES Annual Conference (2011). Rooftop solar thermal micro concentrating collector (Chromasun), http://chromasun.com/MCT.html. Green, M. a, "Third generation photovoltaics: solar cells for 2020 and beyond", Physica E: Low-dimensional Systems and Nanostructures, 14, 65–70 (2002). Vivar, M., Everett, V. and Fuentes, M., "Initial field performance of a hybrid CPV-T microconcentrator system", Progress in Photovoltaics: Research and Applications, 2, (2012). Imenes, A.G., Buie, D., Mills, D.R., Schramek, P. and Bosi, S.G., "A new strategy for improved spectral performance in solar power plants", Solar Energy, 80, 1263–1269 (2006). Durán, E., "Different methods to obtain the I–V curve of PV modules: a review", Photovoltaic Specialists Conference, 33rd IEEE. pp. 1–6 (2008). Kuai, Y. and Yuvarajan, S., "An electronic load for testing photovoltaic panels", Journal of Power Sources, 154, 308–313 (2006).

Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne May 2014 ISBN: 948-0-646-92219-5