Solar Photovoltaic Cell Thermal Measurement Issues

Transcription

1 Solar Photovoltaic Cell Thermal Measurement Issues Bernie Siegal Thermal Engineering Associates, Inc Copper Road Santa Clara, CA USA P: F: E:

2 Topics Introduction SPVC Type Diversity SPVC Equivalent Circuit SPVC Performance Limiters SPVC Temperature Characteristic Unique Power Dissipation Junction Temperature Calibration Thermal Measurement Setup Junction Cooling Charge Storage Thermal Transient Heating Curve Summary 2

3 Introduction Success of the Solar Photovoltaic Cell (SPVC) as a carbon-less source of global energy is dependent on: the amount of energy that it can generate, the efficiency of incident-to-output energy conversion, the capital investment required for SPVC installations, and the continuing output energy cost. All of these dependencies are based on the SPVC junction temperature. While much effort is being expended to address these dependencies, most of this effort is not specifically targeted to controlling the junction temperature. Thus, not much effort is being put into either the direct measurement of junction temperature or determination of the heat flow conditions from the cells to its environment. 3

4 SPVC Type Diversity SJ units typically subjected to 110 Suns MJ units typically subjected to Suns 4

5 SPVC Maximum Conversion Maximum theoretical conversion efficiency ( 30% and 68%, for SJ and MJ types, respectively) is determined by the Shockley Queisser limit that deals with the amount of photon energy that can actually be converted into electrical energy Reference: 5

6 SPVC Equivalent Circuit Single Junction SPVC Diode converts the solar energy into electricity All the resistive elements cause losses R Conductor and R Series are generally the same order of magnitude 6

7 SPVC Equivalent Circuit Diodes convert different segments of the solar energy into electricity. All the resistive elements cause losses R Conductor and R Series are generally the same order of magnitude. Multi-Junction SPVC 7

8 SPVC Performance Limiters A key measure of SPVC performance is the cells ability to convert incident solar energy (i.e., light) into electrical energy. Commercially available SPVC modules currently offer conversion efficiencies in the range of 12% and 28% from single junction and multi-junction units, respectively. Some of the most important performance limiters include: Semiconductor processing limitations Conductor series resistance Conductor shadowing Junction temperature 8

9 SPVC Performance Limiters Semiconductor processing limitations Large area junctions are more prone to wafer material defects, and/or processing defects and/or inter-cell variations Conductor series resistance Need to minimize R S to maximize deliverable energy and minimize internal heating Conductor shadowing Increasing conductor width to minimize RS will reduce incident light area Increasing conductor thickness leads to potential manufacturing issues Junction temperature 9

10 SPVC Temperature Characteristic Shown for a SJ-SPVC Conversion efficiency falls by about 20% for a T J rise of 35ºC 10

11 Unique Power Dissipation 1 Sun = 1,000W/m 2 = 0.1W/cm 2 P incident = 22.5W (150mmX150mm SJ-SPVC, 1 Sun) P incident = 62.5W (25mmX25mm MJ-SPVC, 100 Suns) P dissipation = P incident P conversion + P losses Thermal Pow er Thermal Pow er Incident Pow er Incident Pow er Power (Watts) mm X 150mm Single Junction 25mm X 25mm Multi-Junction Concentration Ratio 11

12 Junction Temperature Calibration Classic diode Electrical Test Method diode T J calibration setup, consistent with Mil-Std 750 and JEDEC JESD51 standards 12

13 13 Thermal Measurement Setup Classic diode Electrical Test Method setup, consistent with Mil-Std 750 and JEDEC JESD51 standards Ff Fi F V V V J V F K T J i J T T T H H F H J JX I V V K P T

14 Junction Cooling Junction starts to cool as soon as the power is removed. Must define when the V Ff measurement is actually made May need to correct for junction cooling 14

15 Junction Cooling Correction delta VF [mv] Junction Temperature Cooling Curve Reg Measurement Delay time (t MD) [µs] Raw t 0 is the instant the Heating Power is removed Initial region is due to nonthermal switching effects equipment switching speed limits, inductance between equipment and test sample, capacitance of test sample, etc. Linear portion on semi-log graph is due to actual junction cooling Linear portion is projected back to the Y-axis value a Actual measurement is made in linear portion value b Correction is the ratio of a / b Correction Factor CF = a/b 15

16 16 Applying Correction Factor b a V K ΔT F J J i J T T T H H F H J JX I V b a V K P T

17 Charge Storage High forward current applied to the diode junction(s) will cause charge to be stored in the diode junctions Charge must be dissipated either through recombination or reverse current in order to quickly measure V Ff Usually not too much of a problem in MJ- SPVC measurements. Usually is a problem in SJ-SPVC measurements Charge Dump is the process of reverse biasing the junction for a short time to remove the charge 17

18 Other Measurement Issues Incident light energy must be minimized during the calibration and measurement activities Typically, very high Heating Currents are required to generate sufficient ΔV F to improve signal-to-noise ratio Current crowding hot spots may occur during measurement and create hot spots These hot spots will distort measured junction temperature value Some of the applied heating power will be converted into optical energy (i.e., junctions glow during measurement 18

19 MJ-SPVC Assembly Stackup MJ-SPVC usually mounted on a heat spreader to better transfer heat into the package Each sequential bulk element in the heat flow path has an increased thermal time constant 19

20 Thermal Transient Heating Curve The difference in time constants produces a thermal transient heating curve that can be used to analyze the thermal interfaces 20

21 Summary The purpose of this presentation was to provide an overview of the issues associated with the thermal performance and thermal measurement of diode-type solar photovoltaic cells. The general measurement approach follows the procedure and circuitry for standard diodes. The specific requirements of SPVC have been mentioned to insure better thermal measurement results. The SPV industry needs a thermal measurement standard to generate comparable measurement results 21