1. ELEG 620 Solar Electric Power Systems April 22, 2010 Systems and SunPower ELEG 620 Electrical and Computer Engineering University of Delaware April 22, 2010

2. ELEG 620 Solar Electric Power Systems April 22, 2010 ELEG 620 April 22 1.Richard Corkish, UNSW, April 23, 3 pm, 103 Gore 2. Michael Mackay, What Does Solar Energy Mean? 006 Kirkbride, April 29, 4 pm 3. System design 4. Design, construction and test of a solar power system 5. SunPower and other solar cells

3. ELEG 620 Solar Electric Power Systems April 22, 2010

4. ELEG 620 Outcomes 1.Understanding the nature of Solar Radiation 2. Design of a solar cell from first principles 3. Design of a top contact system 4. Design, construction and test of a solar power system

5. ELEG 620 Solar Electric Power Systems April 22, 2010

6. Typical Solar System XHighest reliability premium power solution XUnlimited backup time XNo fuel, no maintenance DC Output + Photons In + Storage Battery DC AC AC Output

7. Village System ELEG 620 Solar Electric Power Systems April 22, 2010

8. Pick Your Load (1-2 pages) 1.Pick a load. Available PV Power is 50W-800W (non full time graduate students can go as low as as 1W) 2.Identify what you will measure, starting with the oad. 3.Identify time intervals over which you will measure – i.e: # of days 4.Draw a diagram to show the energy flow and the components in the system for your specific load. 5.List the input, the output and the methods for your design part. (What information do you need, what information do you want, and how are you going to relate the two?) 6.List the methods and the tools you will use for your system test. (How to test whether the system is working as expected? How to identify the problems if it’s not?) ELEG 620 Solar Electric Power Systems April 22, 2010

9. Photovoltaic Systems System Design: –To make a successful system need to: Well-designed system. Reliable, appropriate and well-matched components. Suitable maintenance regimes. Conforming to legal, social, etc expectations, including relevant standards –Ensure that expectations and maintenance is realistic through education. –Well designed system: Appropriate choice of basic system topologies. Choice of array size, tilt angle, battery size and other components to give “best” performance ELEG 620 Solar Electric Power Systems April 22, 2010

10. Photovoltaic Systems Parameters to judge system performance –Availability: fraction of time that energy is available compared to time load is required. –Utilization of incident solar energy: Solar fraction: fraction of available solar energy which is utilized by the system. Array-to-load ratio: Has units of Wp /Wh per day. –If the (Wh per day) is from the load, then this is the hybrid indicator. –If the (Wh per day) is the net available to the load, it is a measure of the system location. ELEG 620 Solar Electric Power Systems April 22, 2010

11. Photovoltaic Systems Types of Systems –Direct-coupled PV system –DC PV system with storage –DC-AC PV systems –Hybrid PV systems –Grid-connected PV systems Increasing components in a system decreases reliability, decreases efficiency Adding additional power sources increases the availability (usually) and increases fraction of solar power used. Increasing components increases cost of systems, but not for same availability. ELEG 620 Solar Electric Power Systems April 22, 2010

12. Photovoltaic Systems Impact of variability in solar resource –A key element in renewable energy systems is the design of one component that has inherent variation (the solar resource) to drive another component (the load) in which the variation should be minimized as much as possible. –The larger the variation in the resource compared to the load, the more difficult the trade-offs. Some loads have a match to solar resources, but often higher loads are encountered in months with lower solar insolation. –Large variation in solar radiation means that in order to get higher availability, the system has: a lower solar fraction a substantial storage component. Higher cost. ELEG 620 Solar Electric Power Systems April 22, 2010

13. Photovoltaic Systems System Design: –Goal is to produce a system within specified cost and power specifications that has the highest availability and reliability –In addition to availability, cost and reliability, the fraction of solar used and the system losses are used to guide to the design process. –Key issues and trade-offs Theoretically, power from array over year = load over year + losses Needed availability = battery capacity to power load during periods without solar irradiance Not valid because storage can’t be large enough, so need to over design in one portion of year No way around problem of unused capacity, but can introduce a secondary system that is either more predictable, lower cost or well-matched to complement solar resource. ELEG 620 Solar Electric Power Systems April 22, 2010

14. Photovoltaic Systems Types of design procedures: –Several different types of design procedures, depending on availability of radiation data and time period over which calculations are performed. 1.Determine feasibility/select system topology: rough calculations, no specific location dependant parameters, and usually no comparison, iteration or checking 2.Indicative analysis: look at key trade-offs (tilt, battery size, array size) determine suitability system topology: location and load dependant parameters, usually averaged over a month. Different methods have different methods for choosing battery storage. ELEG 620 Solar Electric Power Systems April 22, 2010

15. Screen-Printed Silicon Solar Cell  This device structure is used by most manufacturers today The front contact is usually formed by POCl 3 diffusion The rear contact is formed by firing screen-printed Al to form a back-surface field  The cell efficiencies for screen-printed multicrystalline silicon cells are typically in the range of 14 – 16% ELEG 620 Solar Electric Power Systems April 22, 2010

16. Fabrication Process of Screen Printing Silicon Solar Cells POCl 3 DiffusionPECVD SiN x ARAl Screen-printingAg Screen-printing Belt Co-firing Texturing P-Si Senergen Devices February 26, 2009

17. ELEG 620 Solar Electric Power Systems April 22, 2010 Issues for High Efficiency SP Solar Cells Screen printed front contact - Broad and low conductivity Ag - High contact resistance Emitter diffusion - High Joe and low Jsc due to high surface concentration for low contact resistance Bulk : Conventional Multi-Si and CZ - Low lifetime New Structure (Back Contact Cell) Screen printed Al rear contact - High surface recombination velocity - Low reflectivity Senergen Devices February 26, 2009

18. ELEG 620 Solar Electric Power Systems April 22, 2010 High-Efficiency Cell Designs p-type FZ Si Al-BSF Al Contact n-type a-Si ITO Grid intrinsic a-Si SiN/SiO 2 p-Si Ag gridlines Al/Ag rear contact SiN/SiO 2 n + emitter SiN/SiO 2 p-Si rear contacts p+p+ n+n+ n + emitter ~100  /  p-Si Ag gridlines Al rear contact Al-BSF High-sheet-resistance emitter cell Interdigitated back contact cell Gridded back contact cell Si heterojunction cell (in collaboration with NREL)

19. BP Solar Saturn Solar Cell  The BP Solar Saturn solar cell utilizes a laser-grooved, buried front contact  The aluminum back contact is heated to form a back surface field, which reduces surface recombination  Best lab efficiency = 20.1% ELEG 620 Solar Electric Power Systems April 22, 2010

20. Localized Emitter Cell Using Semiconducting Fingers  This type of cell was developed at the University of New South Wales  Suntech may start production in the near future ELEG 620 Solar Electric Power Systems April 22, 2010

21. Sanyo HIT Solar Cell  The HIT cell utilizes amorphous Si intrinsic layers (~ 5 nm) as super-passivation layers. The cell is symmetric except for the a-Si p + emitter layer (~ 10 nm) on the front and the a-Si n + contact layer (~ 15 nm) on the rear. The transparent electrodes are sputter-deposited indium-tin-oxide (ITO)  Best lab efficiency = 22% (open-circuit voltages ~ 730 mV) ELEG 620 Solar Electric Power Systems April 22, 2010

22. SunPower Back Contact Solar Cell  The SunPower cell has all its electrical contacts on the rear surface of the cell  The diffusion lengths > twice the cell thickness  Best efficiencies ~ 23% (SunPower is now using CZ-Si) ELEG 620 Solar Electric Power Systems April 22, 2010

23. Advent Solar Emitter-Wrap-Through Cell  Advent Solar started selling EWT cells in the first quarter of 2007  They need to laser drill ~ 45,000 holes per wafer  They claim solar cell efficiencies of ~ 15% ELEG 620 Solar Electric Power Systems April 22, 2010

24. Metal-Wrap-Through Solar Cell  Photovoltech is commercializing the MWT solar cell; efficiencies ~ 15% ELEG 620 Solar Electric Power Systems April 22, 2010

25. The CSG Solar Cell  CSG Solar (Germany) is using laser patterning of thin polycrystalline silicon to construct a metal-wrap-through type of back-contact cell.  Their best cell efficiencies are ~ 10%. ELEG 620 Solar Electric Power Systems April 22, 2010

26. The Sliver ® Solar Cell  Origin Energy (Australia) is commercializing the Sliver® Solar Cell  They have demonstrated cell efficiencies > 20%

27. ELEG 620 Solar Electric Power Systems April 22, 2010 Senergen Devices February 26, 2009 Solar Cell Technologies Highest efficiencies are reached by making tandem solar cells, which consist of multiple solar cells stacked on top of one another. Each solar cell absorbs light with energy close to its band gap, allowing overall higher efficiency. Maximum thermodynamic efficiency is 86.8%, but material limitations give maximum efficiencies of just over 30%. Used primarily in space markets From Compound Semiconductor

28. 28 Physics of Solar Cells ELEG 620 Solar Electric Power Systems April 22, 2010

29. 29 Solar Cell Operation Boron-doped, p-type silicon Phosphorus doped n-type silicon Top metal contact grid Bottom metal contact Cell Cross-Section Anti-reflection coating ELEG 620 Solar Electric Power Systems April 22, 2010

30. 30 Solar Cell Operation (cont.) 1.Photon of sunlight knocks electron loose P-type silicon attracts holes 2. Free electron goes to top metal contact 3. Hole (broken bond) left behind goes to bottom metal contact N-type silicon Attracts electrons Top metal contact Bottom metal contact ELEG 620 Solar Electric Power Systems April 22, 2010

31. 31 1.8% 0.4% 1.4% 1.54% 3.8% 2.6% 2.0% 0.4% 0.3% I 2 R Loss Reflection Loss Conventional Solar Cell Loss Mechanisms Recombination Losses Back Light Absorption Limit Cell Efficiency29.0% Total Losses-14.3% Generic Cell Efficiency14.7%

32. ELEG 620 Solar Electric Power Systems April 22, 2010 32 High-Efficiency Back-Contact Loss Mechanisms Limit Cell Efficiency29.0% Total Losses-4.4% Enabled Cell Efficiency24.6% 0.5% 0.2% 0.8% 1.0% 0.2% 0.3% 0.2% I 2 R Loss 0.1%

33. Pick Your Load (1-2 pages) 1.Pick a load. Available PV Power is 50W-800W (non full time graduate students can go as low as as 1W) 2.Identify what you will measure, starting with the oad. 3.Identify time intervals over which you will measure – i.e: # of days 4.Draw a diagram to show the energy flow and the components in the system for your specific load. 5.List the input, the output and the methods for your design part. (What information do you need, what information do you want, and how are you going to relate the two?) 6.List the methods and the tools you will use for your system test. (How to test whether the system is working as expected? How to identify the problems if it’s not?) ELEG 620 Solar Electric Power Systems April 22, 2010