More affordable solar energy using astronomical know-how. Roger Angel, University of Arizona and REhnu Inc

Transcription

1 More affordable solar energy using astronomical know-how Roger Angel, University of Arizona and REhnu Inc 1

2 Optics/Telescope heritage. University of Arizona Mirror Lab makes the world s largest astronomical mirrors, right on the campus Lab now branching out to making solar reflectors 2

3 Molten glass telescope disc 8.4 m in diameter in a spinning furnace

4

5 GMT with Chicago. 25 m, f/0.7 primary mirror made as a mosaic of m segments First three outer mirrors cast, one finished, center mirror up next, its mold started Secondary mirror to be adaptive, made of 7 deformable segments 5 Direct Gregorain focus diffraction limited at wavelengths down to 700 nm

6 Large Binocular Telescope in Arizona, has two 8.4 m mirrors on a single mounte 2-axis mount of unique design and stiffness.

7 planets around HR 8799 imaged at the LBT adaptive optics correction made with a deformable secondary mirror

8 Global warming convinced me to change research

9

10 The effect of this kick is unpredictable because the rise is so high and essentially instantaneous, and its going to get a lot bigger

11 meters

12 U.S. CO 2 emissions by sector Nonelectric Electric Transportation Industrial Residential Commercial Energy Information Administration:

13 Solar will displace fossil fuel for electric generation provided it can be made at lower cost How low does it need to get? US wholesale electricity, 0.05/kWh from gas/coal Suppose we have a solar generator rated at 1 kw In one year, 2000 hours of sunlight Annual yield is 2000 kwh, sell for $100 The bank needs to be repaid in 10 years, from $1000 income So cost of installed generator must be <$1/watt 13

14 Astronomical background is useful Problem is one of efficient photon management Difference between photons for energy and photons for information Information need to detect and record all astronomical photons with equal weight to realize maximum information Ideal detector has unit quantum efficiency over wavelength range of interest. Use telescope to collect light, Instruments to encode angle, wavelength and polarization as position on 2-d imaging detector. Solar energy need to convert energy of sunlight into electricity. A photon is not a 0 or 1, its an hν. Two generation paths: 1) Thermal: concentrate sunlight to make heat, use engines and electromagnetic machines (19 th century 2) direct photovoltaic conversion. Thermal efficiency potentially high, because focused sunlight is so hot. Best coal > 40% In practice, photovoltaic conversion dominates in current solar electric generation 14

15 new Solana 280 MW trough thermal plant in Arizona Sunlight focused onto black steam pipes by cylindrically curved glass mirrors 6 foot square 15

16 16

17 Unique for nighttime solar, but thermal expensive and not very efficient 15% of solar resource converted 17

18 No more of these planned in the US, advantage of nighttime generation not enough to justify high cost. Conversion efficiency is low, ~ 16% 18

19 The 800 lb gorilla of solar energy PV panels Reliable Mature technology, high volume production But still not competitive with natural gas. Is it possible to get solar cost lower? Possible if we can overcome the fundamental weakness of panels 8 out of 10 photons turned into heat, not electricity 19

20 Response to solar spectrum of ideal Si cell Shockley Queisser limit of 33% full spectrum efficiency In practice, little more than 20% full spectrum efficiency But >60% efficient at wavelengths around 1000 nm, near band gap 20

21 Multi-junction cells overcome limit cover full solar spectrum with each cell operating near its peak response 21

22 Technology improving Increasing efficiency 3 -> 4 junctions Champion cells: Sharp % Solar Junction 45% Commercial production cells today 1000x concentration. Improving at 1% per year Can project commercial 50% by ~

23 (Friedman et al IEEE October

24 Cost high per unit area, but not per watt if used at high concentration Standard 1.6 m 2 area panel of MJ cells Today s cost $80,000 Power ~ 500W $160/watt But if for cells illuminated at 1000 x concentration, cell cost $0.16/watt Cell cost likely to go < $0.1/watt given new much faster deposition Cheaper than silicon and more efficient! Provided we can build cheap concentrating optics and cheap dual axis trackers 24

25 Rest of talk how to get: 1000x concentrated sunlight + dual axis tracking + active cooling for total system < $0.9/watt installed, to keep total < $1/watt 25

26 Standard method array of Fresnel lens with an MJ cell behind each lens - ensures equal light and equal current, just as in PV module - but these modules are big and complex and go on big, heavy dual axis trackers. Most companies trying to commercialize this approach have failed 26

27 For collection and concentration, better to use back-silvered glass mirrors as developed for trough thermal, but adapted for point focus for higher concentration Extensive field experience 10 km 2 of mirrors installed Long lived > 20 years Also very simple to support on tracker Mirrors are cheap - $35/m 2 Works out at $0.1/watt 27

28 28

29 Steel mold for glass dish reflector

30 Manufacture of point focus version at the Mirror Lab slumped in 150 sec heating burst Mounts on simple 4-point support, like trough reflector segment area 2.6 m 2, 1.5 m focal length

31

32 Advantage of Ball Lens + Secondary Optics Large Tolerance to Tracking Error for Dish-Based System Uniform Light Distribution Between Cells * Images from US Patent Application PCT/US13/ Filed November 26, 2013 Solar Generator with Large Reflector Dishes and Concentrator PV Cells in Flat Arrays

33 Gen 2 U Arizona prototype, 2012 Gen 2 prototype on-sun 10 m 2, 4 panel glass dish 28.6% system efficiency using 36% cells Spaceframe holds 200 lb unit at focus

34

35 Gen 2 spherical secondary optics for 1250x * Image from Dish- Based High Concentration PV System with Kohler Optics B. Coughenour et. al. - published in OSA Optics Express, January 2014

36

37 Active cooling: cells operate only 20 C above T ambient Low thermal cycling fatigue Cool cells efficiency boost > 2% parasitic loss for active cooling

38 Gen 3, 2014, redesigned for commercial manufacture by REhnu Inc Single 2.6 m 2 dish to feed 800 W receiver

39 REhnu Gen W Power Conversion unit Matched to 2.5 m 2 dish Dish focus at center of fused quartz ball 800 W for 1000 DNI Weighs 6 lb Separated architecture - simple installation and simple swap-out First on-sun test March 2014 Development supported by REhnu DOE Incubator award 39

40 HCPV Module with Single Large Dish and Compact Multi-Cell Receiver

41

42 Dish image formed by the ball is diced into 12 cell slices by origami optics. No cell gap loss 42

43 HCPV Cell Geometry Gen Flat Circuit Cards Cheaper, better Blake Coughenour PhD thesis 9 x 8.8 mm triple junction cells Four 200 W cards in 800W receiver Black rays on-axis Red rays off-axis

44 Gen 3 System Performance Current (A) Power output Initial commissioning was performed at an effective DNI of 198 W/m 2 by covering the mirror with a partially transmitting screen. Existing tracker with older, prototype mirrors that have a reflectivity that is 2-3% lower than current commercially available mirrors. The IV curve obtained during this testing is shown below. AT 996 W/m 2, One cell card produced 201 W, for a DC efficiency of 31% at an ambient temperature of 26C Corresponds to a power of 804 W for a full PCU. Cell Operating Temperatures were 20-25C Measured acceptance angle for 90% MP is ± Voltage (V) 100% 80% 60% 40% 20% 0% Pointing Error (deg)

45 Based on this design, complete receiver for $0.2/watt looks doable 45

46 Blue electrical output Red - thermal output (No thermal output) Module cost, mirror plus receiver $0.30/W

47 Low-cost dual axis tracker now the focus of attention 47

48 Model: Brightsource heliostat. 15 m2 back silvered glass, 0.1 degree tracking, 140,000 alone at Ivanpah thermal power tower

49 REhnu initial 2 x 4 layout 21 m2 total area 6.4 kw 2:1 panoramic aspect minimizes self shadowing Total steel mass with foundations 500 kg = 24 kg/m2

50 Mirrors held as in CSP troughs, 4 supports/mirror Receivers held from behind by four T supports

51 Hexapod of 3 tubing Torsion elevation tube behind is the optical bench Gravity and op. wind loads < 0.1 with 8 schedule 20 tube

52 Screw anchor foundation Quick to install No concrete Very efficient use of steel

53 Dual-axis mount to carry 21 m 2 total module area built around hub with bearings Hub base connects to hexapod support Two flanges support wings, each carrying 4 modules Dual axis mount cost: az bearing plus worm drive, motor and encoder $750 linear actuator $300 steel, 1,000 lb $1,000 total = $2,050 for 8 kw (in 2020) $0.25/watt 53

54