2010 12th Electronics Packaging Technology Conference

Thermal Challenges and Opportunities in Concentrated Photovoltaics

Seri Lee School of Materials Science and Engineering, Nanyang Technological University

50 Nanyang Avenue, Singapore 639798 serilee@ntu.edu.sg, (65) 6316 8976

Abstract Technical challenges and market opportunities for thermal

management of concentrated photovoltaics, CPV, are discussed. An overview of the current status and projected advancement of the CPV technology is provided, and many of the fundamental issues associated with employing advanced thermal solutions in the future CPV applications at higher concentrations are addressed. Using a case-example, the economics of cooling high-concentration photovoltaics is analyzed. A parametric study is carried out for a number of variables under different scenarios, and the sensitivity of the net gains that can be realized by employing an advanced thermal solution is presented. It is concluded that a proper thermal management plays a vital role in reducing the total cost of ownership per watt of electricity generated through improving two parameters: the efficiency and the reliability.

Introduction The world electricity consumption has increased tenfold in

the past five decades, [1] and it is projected that the needs will continue to skyrocket and triple in the next 20 years. Global oil reserves will be depleted in one generation: 54 years according to one estimate. [2] With the impending energy crisis and the exponential growth in the world electricity consumption, every corner of the globe is racing to secure alternative renewable energy sources and to develop enabling technologies. One such technology, rapidly raising intense competitions amongst developed, as well as developing nations, is in the area of concentrated photovoltaics which convert sunlight, a free and perpetual source of energy, to electricity.

CPV is one of the two main technologies used for producing electricity from concentrated solar power, CSP. CSP technologies use lenses or mirrors to focus large area sunlight onto a small area, concentrating the direct incoming solar energy to reach higher energy densities, thus making it more efficient for energy conversion. There are two different technologies being developed and utilized for converting CSP into electrical power: CPV and STP, Solar Thermal Power. The STP technology has been developed first since the 1970’s which use the heat of the concentrated solar energy to generate steam which, in turn, is used to power a conventional power plant to generate electricity. Alternatively, the CPV technology, one of the newest players in the field of solar energy, converts the high-density sunlight directly to electricity by projecting the concentrated light onto high-performance photovoltaic cells. Concentrating solar radiation allows use of smaller cells at a greater conversion efficiency, thereby offers an opportunity for minimizing the cost per watt, $/W, of electricity produced by the system.

Figure 1 shows the areas where the intensity of the solar irradiation is high enough and economically attractive for installation of CPV systems. [3] CPV’s sweet spots are where

the irradiation is greater than 5kWh/m2/day. A land area of approximately 200km x 200km in the Sahara, as indicated by a small white square-dot in the map, receives the same amount of energy consumed globally by the entire human activities. The earth receives the solar energy at the rate of 120,000TW while humans consume at the rate of 15TW.

Fig. 1. CPV’s sweet spots.

Fig. 2 shows a forecasted CPV installed power generation. [4] The growth will be helped initially by generous government incentives and, afterward, by technological advances and increased cost competitiveness. CPV system installations in the US will represent $70 million in 2010 and are expected to grow to more than $3 billion by 2015. [5]

Fig. 2. Cumulative CPV installations.

CPV System A CPV system consists of 1) PV cells, 2) modules which

contain the cells, primary and secondary optics and, if necessary, a heat sink, and 3) an assembly that has an array of modules mounted on a frame which in turn sits on a tracker complete with power controls and inverters.

Depending on the magnitude of concentration in units of suns, types of CPVs are categorized as low, medium, and high concentration systems. Low CPV systems have concentration

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2010 12th Electronics Packaging Technology Conference

factors of 2 to 100 suns and typically use low cost, less efficient silicon based solar cells. They usually do not need a dedicated thermal solution or sophisticated optical and active tracking mechanisms. Medium CPV systems range from 100 to 300 suns, and often require a certain degree of attention for heat sinking, whether passive or active, as well as for solar tracking. High concentrated photovoltaics, HCPV, systems concentrate sunlight to intensities greater than 300 suns. They employ sophisticated concentrating optics, such as dish reflectors or Fresnel lenses, and require high performance thermal solutions to prevent thermally induced failures and to maintain high conversion efficiency and long-term reliability of the system.

For HCPV systems, multijunction thin-film photovoltaic cells are favored over silicon cells as the efficiencies of multijunction cells are much higher and increase with the concentration faster than those of silicon cells. Though the cost of multijunction cells is roughly 100 times that of silicon cells, the cell cost increasingly represents a smaller portion of the total system cost, and the overall economics of CPV, when measured in terms of the total system cost of ownership per watt of electricity produced over its lifetime, would make multijunction HCPVs more viable for large utility-scale implementations. The maximum efficiency of the multijunction cells has been improving rapidly in the past few years and is expected to reach 50% by 2015. It is at 41% today. The conversion efficiency of typical multijunction cells that are currently utilized in the market is about 35%.

A number of research institutions and R&D companies are also developing Xtreme Concentrated Photovoltaics, XCPV, with concentrations well exceeding 1,000 suns up to 2,000 suns with a path to many thousand suns in the future.

The modules may be categorized largely into two different types depending on the method of focusing the sunlight: 1) reflection type which uses mirrors, and 2) transmission type which uses Fresnel lenses. A system may use a combination of both along with a light guiding mechanism. The schematics of the two optical types are shown in Fig. 3. As will be discussed further in the latter sections, optics is a critical component of a CPV system as it contributes significantly to the overall conversion efficiency and to the system cost. The choice of which optical methods to be adopted depends on many attributes, including the concentration factor, the cell technology, weight of the system, etc. Again, the strategy is always to minimize the $/W of the system.

Fresnel lens

Optical rod

Primary mirror

Secondary mirror

Solar cell

Fig. 3. CPV optics: reflection type (L) and transmission type (R).

Each module may contain a single cell unit or, more often, an array of units, as shown in Fig. 4.

Fig. 4. CPV modules: reflection type (L) and transmission type (R).

Figure 5 shows photos of typical systems. The focusing type of CPV modules described herein can only utilize direct sunlight. As such, the conversion efficiency is very sensitive to the incident angle of the sunlight to the module, as well as the final distribution of the irradiance onto the surface of the cells. A precise tracker is a must and, being the only moving component in the system, the weight of the panel is a critical contributor to the overall system cost and reliability. A large panel complete with arrays of passive metal heat sinks, as shown at the bottom of Fig. 5, may weigh over many tons, and the tracking system for such a panel is massive and expensive. Providing light weight, high performance thermal solutions would contribute significantly to reducing the overall $/W.

Fig. 5. CPV systems.

System Cost Break-Down ($/W)2010 2011 2012 2015 Notes

System Installed

6.25 5.25 3.00 2.00 all inclusive

Power Module ~40% of System

2.50 2.10 1.20 0.80optics, die, heat sink

Rest of System ~60% of System

3.75 3.15 1.80 1.20tracker, inverter,

deployment

Fig. 6. System cost break-down.

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Figure 6 shows an approximate cost break-down of an example system and the forecast of a target cost reduction path. As can be seen, approximately 40% of the total system cost is budgeted for the power module and the same 40% is for the tracker alone. For passive heat sink solutions, the thermal budget which is part of the module cost typically ranges from about 2% to 4% of the total system cost. The industry target is to reach the system cost at 2$/W by 2015.

Thermal Challenges and Opportunities Basic un-concentrated PV systems do not need thermal

solutions. The maximum direct-normal solar irradiance, DNSI, at the surface of the Earth is approximately 0.1W/cm2

which normally never raises the ground temperatures to 100oC: the temperature commonly accepted as the maximum junction temperature allowed for reliable operation of PV cells. After all, no one witnessed water boiling off of a pavement even in the middle of a hot summer day in Arizona.

However, when sunlight is concentrated in an HCPV module, the magnified heat density raises the die temperatures and creates local hot-spots on the substrate and the heat sink. Assuming a conversion efficiency of 28% as an example, the amount of heat dissipated by a CPV cell is tabulated in Table 1 as a function of die size and concentration.

Table 1. CPV heat dissipation in watts at 28% efficiency.

200 400 600 800 1000 1200 1400

2 0.6 1.2 1.7 2.3 2.9 3.5 4.0

4 2.3 4.6 6.9 9.2 12 14 16

6 5.2 10 16 21 26 31 36

8 9.2 18 28 37 46 55 65

10 14 29 43 58 72 86 101

12 21 41 62 83 104 124 145

14 28 56 85 113 141 169 198

die size (mm)

suns

Note that the amount of heat dissipation from an individual PV cell exceeds 100W when the die size is around 12mm on a side and the concentration is about 1,000 suns. At this point, both the size and the amount of heat dissipation are comparable to those of the CPUs and GPUs of today’s typical high-performance personal computers. Without a proper thermal management, the magnified heat density can easily push the die temperatures to well north of the allowable maximum junction temperature, and the PV cells, much like the high-performance CPUs and GPUs, will experience instantaneous catastrophic failures.

A cut-away view of a 4 x 4 cell module with a simulated heat-sink temperature distribution is shown in Fig. 7. The following assumptions are made in this simulation:

• 16 - 10mm x 10mm CPV cells with 28% efficiency • concentration at 1,000 suns • aluminum heat sink with average thermal resistance of

0.026oC/W • ambient air at 50oC The above parameters result in the minimum projected

module area of 127cm x 127cm, and the total heat dissipation of 16 x 72W = 1,152W. Further assuming that the heat-sink area is the same as the module size, the design has been optimized for natural convection cooling [6] with the resulting average thermal resistance of 0.026oC/W. For this heat sink,

the corresponding average heat-sink temperature is 80oC.However, due to the concentration of heat density, an additional thermal resistance, called the spreading resistance, [8] exists in the base plate of the heat sink. Including the additional temperature rise due to the spreading resistance, the maximum local heat-sink temperature is predicted to be 118oC. The weight of the heat sink in this example design turned out to be approximately 45kg.

Fig. 7. Heat-sink temperature distribution of an example module with 4 x 4 CPV cells at 1,000 suns.

As demonstrated in this example, utilization of large cells at high concentrations is only possible if the cell junction temperature is kept cooler and the reliability is improved with use of thermal solutions beyond a passive aluminum heat-sink. An approximate map of thermal solution space is provided in Fig. 8 which suggests the types of heat sink solutions depending on the cell size and suns. The passive solutions represent aluminum heat sinks with no active air movers. The advanced passive heat sinks would include those with local heat-spreaders of high thermal conductivity materials, such as copper or heat pipes, placed under the cells. The advanced active solutions would include those incorporate more than air cooling, such as pump driven liquid cooled cold plates.

Fig. 8. A map of thermal solution space.

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As the boundaries of each solution space may be pulled in or pushed out depending on many design variables, such as the size and weight of the heat sink, this map is obviously not definitive. It is intended to be used only as a reference guide for initial planning purposes.

Today’s sweet spots are generally in the neighborhood of where the word “Passive” is on the map: the die size of around 6mm and the concentration of approximately 600 suns. Continuing advancement in the CPV technology and cost reduction will drive the sweet spots toward the south-east corner of the map, and concurrent development of advanced thermal technologies for light weight, low cost and high performance heat-sink solutions is a must for future success of the CPV technology.

The discussion so far is based only on consideration of maintaining the maximum allowable junction temperature of the die at 100oC. The industry standard for the long-term reliability is typically 50kHrs. As the CPV technology is still at an early stage of development, not many long-term reliability studies have been carried out, and there exist little data that are “reliable.” However, based on common behaviors of other electronic devices, an approximate reliability performance, as the one predicted in Fig. 9, may be generated for discussion. According to this prediction, the lifetime that corresponds to the operating junction temperature of 100oC is under 20kHrs, and every 10oC cooling prolongs the life by about 40%. Note that the industry target of 50kHrs of reliability requires the junction temperature to be around 70oC.

Fig. 9. Long-term reliability of an example CPV.

It is well known that the efficiency of electronics improves with decrease in the operating temperatures. The conversion efficiency of CPV cells is no exception. The dependency of a CPV on the die temperature and the intensity of irradiance are shown in Fig. 10. [8] Note that:

• the output voltage decreases with increase in temperature • the output current decreases with decrease in irradiance Also, note that the power conversion efficiency increases

with cooling, but only at the rate of 0.16% per oC. Because of this weak dependency, it is often not compelling enough to cool the die temperature below 100oC for the sole purpose of improving the efficiency. Adding a higher performance heat sink would inevitably increase the total cost, volume and/or weight of the system and will most likely have an adverse effect on the overall $/W performance. However, as discussed above, cooling significantly improves the reliability and lifetime of a device. As will be seen in the later section,

producing more electricity through longer life can easily justify the economics of cooling, even below 100oC.

Fig. 10. CPV dependency.

Another area of importance is in improving the optical efficiency through thermal management. Taking a transmission type module for example, the primary Fresnel lens concentrates and focuses the sunlight onto the top surface of an optical rod (see Fig. 3). The function of the optical rod is to retain as much of the incoming-focused light as possible and disperse it as uniformly as possible through the bottom surface onto the PV cell. Optical rods are made of a glass-like material and encased within a reflective housing: Figure 11 shows the traces of simulated sunlight with a photo of the rod with the reflective housing removed for clarity.

Primary Lens

Optical Rod

Irradiance distribution over rod entry aperture (11 mm sq.)

Irradiance distribution over cell (5.5 mm sq.)

Fig. 11. Irradiance distributions at the entry and exit of an optical rod.

Table 2 shows the modeled and measured efficiencies of typical optics of a transmission-type module. Due to the surface reflections, scatterings, absorptions, edge losses, etc., the amount of irradiation that successfully lands on and utilized by a CPV cell, known as the optical efficiency of a module, is typically about 80% of the incoming sunlight.

Table 2. Optical efficiency.Tracing Model Measured

Geometric Concentration 476 476 Primary Lens Efficiency 89% 84% Optical Rod Efficiency 96% 95% System Efficiency 85% 80% Optical Concentration 405 380

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Furthermore, this module efficiency is achievable only when the structural geometry of the module is intact as designed. As illustrated in Fig. 7, severe temperature gradients may exist across the heat sink which not only cause thermally induced component failures due to mismatches in thermal expansions but also increase thermally induced structural distortions. The effective power conversion efficiency is extremely sensitive to the alignment of the concentrated sunlight with the optical rod. When the focal point of the sunlight over the rod entry surface moves off from the center position, the optical tip-loss increases, and the power production from the cell falls off dramatically. As can be seen in Fig. 12, a 1o deviation of the focused sunlight from the center of the rod entry aperture results in an approximately 10% reduction in power conversion. The industry standard for the maximum acceptable deviation, named Acceptance Half-Angle, is 1o.

Fig. 12. Power vs. Panel Angle: irradiance distribution over rod entry aperture with no deviation (L) and 1o deviation (R).

Recall that the output voltage of a cell decreases with increase in temperature, and the output current decreases with decrease in irradiance. Thermally induced distortions would cause optics to be misaligned and throw them out-of-focus, resulting in a non-uniform irradiance distribution over the cell which in turn cause creation of more hotspots on the die and reduction in the output voltage. The die-junctions are connected in series, and the maximum output current of a die is therefore limited by the worst current generating junction receiving the least amount of irradiance. Hence, the same non-uniform irradiance distribution which caused a reduction in output voltage is also responsible for the reduction in the maximum output current of the die. Assuming the typical multijunction cell efficiency of 35%, the module efficiency often becomes around 28% in practice.

It is emphasized again that the strategy should always be reducing $/W. The needs and opportunities for thermal management of CPVs exist mainly through improving the efficiency and the reliability. It can be summarized that:

• maintaining the die temperature not to exceed its maximum allowable junction temperature ensures the high conversion efficiency and long-term reliability.

• additional cooling further improves the efficiency and extends the lifetime.

• maintaining uniform temperatures through proper thermal management minimizes thermally induced failures and optical misalignments, further improving the reliability and the system efficiency.

• use of advanced materials and light weight thermal solutions, such as heat spreaders and liquid cooling with remote heat-exchangers, offers opportunities for improving the thermal performance of the module and reducing the weight and cost of the tracking system.

Economics of Thermal Management The strategic metric $/W depends on a countless number

of variables. In this section we will take a case example and examine it in most simplistic way. Though it may be too rudimentary to some readers, a number of high-level conclusions can be drawn, offering useful insights for choosing different design and implementation options during the initial planning stage.

Electricity prices vary widely by type of customer, locality and over time. It ranges from $0.06/kWh to more than $0.50/kWh with the high side corresponding to a residential use during the normal working hours when the total demand, including commercial and industrial uses, is high with the peak demand occurring between noon and 2pm. Per kWh, it is currently at around $0.28 in metropolises of New York and California, $0.42 in Hawaii, and S$0.25 in Singapore.

Table 3 shows the simple payback period for the initial system installation cost at different points of electricity price. This simple break-even period is based on the fixed dollar value with no other cost included. It can be seen from this simple calculation that for a CPV system to have any hope to become profitable, the lifetime should be longer than 50kHrs and the system cost be lower than about 3$/W. A better CPV efficiency reduces the system $/W which in turn reduces the payback period for the initial capital cost.

Table 3. Simple payback period.

Y2010 Y2011 Y2012 Y20156.25 5.25 3.00 2.00

0.28 68 kHr 57 kHr 33 kHr 22 kHr0.42 45 kHr 38 kHr 22 kHr 15 kHr

Target System Cost ($/W) in:Electricity Price

($/kWh)

Table 4 presents an interesting situation. Taking the example heat-sink discussed in Fig. 7 and using today’s aluminum price of approximately $2,400 per ton, the table shows the material cost of the 45kg heat-sink which alone accounts for 4 to 12% of the total system cost. Of course, as discussed earlier in Fig.8, this example case with a 10mm die at 1,000 suns is not in today’s sweet spot. But this clearly highlights the fact that the cost of thermal solutions must be reduced significantly in parallel with the cost of other components in the system.

Table 4. Percentage material-cost of the example aluminum heat-sink shown in Fig. 7.

System Budget ($/W) 6.25 5.25 3.00 2.00Heat-sink mat'l cost ($/W) 0.24 0.24 0.24 0.24

Heat-sink mat'l cost (%) 4% 5% 8% 12%

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For evaluating the economic impact of providing a high performance thermal solution, the baseline case-example examined throughout this paper is chosen again with the additional assumptions included below:

• 10mm die size at 1,000 suns • 28% system efficiency and 50kHr lifetime • electricity price, EP, of $0.28/kWh and an operating cost

at 70% of EP Let us further assume that, while it costs $10 more per cell,

employing a high performance heat sink provided an additional cooling of 10oC and extended lifetime by 10kHrs as compared to the baseline case. The dependency of cell efficiency on cooling is assumed at the rate of 0.16%/oC.Under this scenario as the default-case, it can be determined that the net gain per cell in fixed dollar value over the lifetime is $20.17.

Figures 13-15 provide the parametric sensitivity of the net gains in $ per cell over the lifetime of the above baseline case as a function of the following variables:

• concentration (suns) and die size (mm) • efficiency (%) and extended life-time, XLT (kHr) • electricity price, EP ($/kWh) and operating cost, OC (%EP)

In the figures, the default case is indicated as the yellow-point. As can be seen, the potential for a greater net-gain increases significantly with the die-size, concentration factor, CPV efficiency, and the price of electricity.

0

10

20

30

40

50

60

0 200 400 600 800 1000 1200 1400

Net

Gai

n ($

)

Suns

6

8

10

12

14

Die Size (mm)

Fig. 13. Net gains as a function of suns for different die size.

0

10

20

30

40

50

60

20 25 30 35 40

Net

Gai

n ($

)

Efficiency (%)

5

10

15

20

XLT (kHrs)

Fig. 14. Net gains as a function of efficiency for different extended life-time, XLT.

0

10

20

30

40

50

60

0.00 0.10 0.20 0.30 0.40 0.50 0.60

Net

Gai

n ($

)

Electricity Price, EP ($/kWh)

50

60

70

80

90

OC (%EP)

Fig. 15. Net gains as a function of electricity price, EP, for different operating cost, OC, given as percentage of EP.

Conclusions The current status and projected advancement of the CPV

technology have been reviewed. Providing a low cost, light weight, high performance heat-sink solution plays a vital role in enabling CPV to become grid parity. It has been concluded that utilizing larger cells at higher concentrations would provide greater opportunities for reducing the net cost of ownership and increasing the production of electricity. Using an example case, the economics of cooling high concentrated photovoltaics has been quantitatively examined. Thermal management of CPV lowers the cost of ownership per watt of electricity generated mainly through improving two parameters: 1) efficiency and 2) reliability.

Acknowledgments The financial support of Pipeline Micro Pte Ltd under the

Research Collaboration Agreement of 01-07-2010 is acknowledged. The author also extends his thanks to Professors Freddy Boey and Ma Jan for their personal support.

References 1. International Energy Agency, IEA Database and Analysis,

Historocal World Electricity Consumption, 2010. 2. PennWell Corporation, Oil & Gas Journal, Vol. 105.48,

December 24, 2007. 3. Pharabod and Philibert, 1991. 4. “CPV Industry Growth Forecast & Strategic Landscape,”

The CPV Challenge Report, Part II, July 2009. 5. CPV Industry Report, 2010. 6. S. Lee, “Optimum Design and Selection of Heat Sinks,”

IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part A, Vol. 18, No. 4, pp. 812-817, December 1995.

7. S. Lee, S. Song, V. Au, and K.P. Moran, “Constriction/Spreading Resistance Model for Electronic Packaging,” Proceedings of the 4th ASME/JSME Thermal Engineering Joint Conference, Vol. 4, pp. 199-206, Maui, Hawaii, March 19 - 24, 1995.

8. 25kW CPV Solar Array Fact Sheet, Time One Solar Power, March 2008.

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