Where Solar Thermal Meets Photovoltaic for
High-Efficiency Power Conversion
MASSACHUSETS INSTfT1TE
OF TECHNOLOGY
by
David M. Bierman
OCT 16 201
B.S., Mechanical Engineering
University of Wisconsin, Madison (2012)
LIBR RIES
Submitted to the Department of Mechanical Engineering
in Partial Fulfillment of the Requirements for the Degree of
Master of Science in Mechanical Engineering
at the
Massachusetts Institute of Technology
September 2014
@2014 Massachusetts Institute of Technology. All rights reserved.
The author hereby grants to MIT permission to reproduce
and to distribute publicly paper and electronic
copies of this thesis document in whole or in part
in any medium now known or hereafter created.
Signature redacted
Signature of A uthor: ...................................................................
Department of Mechanical Engineering
August 8, 2014
Certified by: .....................................................................................
S ig n a tu re re d acted
K'\ 9vlyn N. Wang
Associate P6 ssor of Mechanical Engineering
4
Accepted by:
................................................................
S ig n atu re
_,)ksis
ippeVisor
.................
David E. Hardt
Professor of Mechanical Engineering
Chairman, Department Committee on Graduate Theses
Acknowledgements
It is difficult to see clearly how to acknowledge the various people in my life who contributed to
this thesis. The bottom line is that I am a lucky guy. When we are born, we are assigned a life.
Some are more difficult than others. The life I was given has presented opportunities that most
people will never see. While it was not earned, this life is appreciated.
It is important for me to begin by recognizing the people who made me who I am but are no longer
with us on Earth. Their energy remains strong in my life and keeps me going.
From an intellectual point of view, the mentorship of Andrej Lenert has been invaluable.
Developing the solar thermophotovoltaic experiments with him has simply defined my ability as a
researcher and has been an absolute privilege. Mostly everything I understand about solar thermal
and photovoltaic engineering, I have learned from Andrej.
I will forever be grateful for this
knowledge.
As will be discussed in the thesis, this work relied heavily on collaboration with material scientists
and physicists. The research groups of Professor Marin Soljacic and Dr. Ivan Celanovic played an
integral role in the development of this thesis. In particular, I would like to acknowledge Walker
Chan and Veronika Rinnerbauer who supplied us not only with innovative photonic crystal samples
but also many insights related to their operation. Their more specific contributions are discussed in
the text of the thesis.
The Device Research Lab is a unique environment to work in.
Through useful engineering
discussions to heated social debates, the people in our lab continue to challenge and help shape the
way I think about the world. It is an excellent place to learn to be a researcher.
3
One simply could not have a better thesis advisor than Professor Evelyn Wang. Her kindness,
patience, intelligence, diligence and leadership are unmatched. These qualities have allowed this
project, and every other project she is a part of to be successful. I feel lucky to be a part of her lab.
The people I choose to surround myself with outside of work have also contributed to this thesis.
They keep me humble and grounded but also thoroughly entertained. It takes a healthy brain to
write a thesis. But a healthy brain must be balanced. In my everyday life, they provide the balance.
They know who they are.
But all of these people would not have had the opportunity to provide what they do if it were not for
my family. My mom, dad, sister and brother are the reason I am alive. Anything I accomplish in
this world is a reflection of the love that they give to me unconditionally.
4
Where Solar Thermal Meets Photovoltaic
for High-efficiency Power Conversion
by
David M. Bierman
Submitted to the Department of Mechanical Engineering
on August
8 th,
2014 in Partial Fulfillment of the
Requirements for the Degree of Master of Science in
Mechanical Engineering
Abstract
To develop disruptive techniques which generate power from the Sun, one must understand the
aspects of existing technologies that limit performance. Solar thermal and solar photovoltaic
schemes dominate today's solar market but both bring intrinsic and practical constraints. What will
tomorrow's solar market look like? Third generation solar power generation techniques to utilize a
larger portion of the solar spectrum are a promising path for high efficiency power generation, but
experimental demonstrations remain limited.
In this work, the components of a solar thermophotovoltaic power converter are introduced and
discussed. While solar thermophotovoltaic devices have the potential to convert sunlight into
electricity at astronomically high efficiencies, there are a number of practical challenges that must
first be addressed. Novel photonic materials, design concepts, and both intrinsic and practical
limitations of solar thermophotovoltaic conversion are explored in this thesis. The conversion
mechanisms as well as a number of experimental implementations are presented. Finally, the device
performance is characterized and both geometrical and spectral improvements are discussed.
Thesis Supervisor: Evelyn N. Wang
Title: Associate Professor of Mechanical Engineering
5
6
TABLE OF CONTENTS
1.
2.
Introduction ............................................................................................................................................................................
9
1.1 Solar Th erm al Energy Conversion ..........................................................................................................................
10
1.2 Solar Ph otovoltaic Energy Conversion ..................................................................................................................
13
1.3 Solar Therm ophotovoltaic Energy Conversion............................................................................................
18
Components of Solar Thermophotovoltaic Conversion .............................................................................
21
2.1 The A bsorber...................................................................................................................................................................
3.
4.
2.1.1 Blackbody Absorption ..........................................................................................................................................
24
2.1.2 Spectrally Selective A bsorbers ........................................................................................................................
26
2.2 Sp ectrally Selective Em itters.....................................................................................................................................
27
2.3 Sub-Bandgap Filters ......................................................................................................................................................
29
2.4 C onclusions ......................................................................................................................................................................
30
M odelling A Solar Therm ophotovoltaic Device ..............................................................................................
31
3.1 M odel Form ulation ........................................................................................................................................................
32
3.2 Exploring Absorb er Characteristics .......................................................................................................................
34
3.3 Exploring Em itter Characteristics...........................................................................................................................
40
3.4 Conclusions ......................................................................................................................................................................
43
Experim ental D em onstration .......................................................................................................................................
45
4.1 Experim ental Setup.......................................................................................................................................................
45
4.2 Experim ental Procedure.............................................................................................................................................
50
4.3 Experim ental R esults ...................................................................................................................................................
51
4.3.1 Therm oph otovoltaic Conversion ....................................................................................................................
51
4.3.2 H igh Concentration R egim e ..............................................................................................................................
53
4.3.3 Low Concentration Regim e ...............................................................................................................................
57
4.4 Conclusions ......................................................................................................................................................................
5.
22
Efficiency Enhancement Through Spectral Conversion................................................................................
7
59
61
5.1 Concentrated Photovoltaic Characterization...............................................................................................
5.2 Solar Th erm ophotovoltaic D em on stration ...................................................................................................
62
. 63
5.3 Heat Generation and Therm al Managem ent................................................................................................
66
5.4 Perform ance C om parison ...........................................................................................................................................
67
5.5 Conclusion.........................................................................................................................................................................70
6.
B ibliography ........................................................................................................................................................................
8
71
1. Introduction
Solar energy is delivered to the Earth at a heat flux of approximately 1000 W/m 2. Thus, the average
amount of solar power incident on the land area of the Earth is on the order of 1016 W. If we
compare this number with the International Energy Agency's estimation of the energy consumed
per year by the human race (on the order of 1013 W) [1], we see over 3 orders of magnitude
discrepancy suggesting that the use of solar energy is the eventual solution to our burgeoning
energy consumption patterns. The solar resource is rich in nature - both thermodynamically, in the
energy content of sunlight, and geopolitically, as it sees no borders and provides little opportunity
for people to exploit others in the pursuit of energy sources. It is a local resource and is used as
such.
Since the beginning of human life, we have interacted in many complicated ways with the Sun.
However, with regard to directly utilizing the energy from the Sun in our current era to supplement
our energy demand, humans rely on two primary methods: solar thermal and solar photovoltaic
energy conversion. While progress in these two technologies has come a long way, the work is not
complete. To continue to develop disruptive techniques to generate power from the Sun, one must
understand the aspects of these common strategies that ultimately limit device performance.
9
Figure 1: a) Ivanpah Solar Power Facility. A 392 MW CSP plant with over 150,000 individual
mirrors that focus solar radiation on steam boilers that are located on the central tower [2]. b) 14
MW solar photovoltaic plant at the Nellis Air Force Base in Nevada.
1.1 SOLAR THERMAL ENERGY CONVERSION
The first method converts light from the Sun into heat [3]. This type of conversion thermalizes
incoming photons (quantized light waves) from the entire solar spectrum using a solar collector and
uses that heat to run low or medium temperature processes such as space or water heating for
residential or commercial applications. For power generation, however, the cycle efficiency relies
heavily on its hottest temperature. Thus, it typically requires concentrating optics in order to run
high temperature processes for steam production and eventual electricity generation. A number of
solar thermal power plants have been constructed around the world [4], and while they require no
fuel other than the Sun to operate, it is still regarded as financially risky to invest in this technology.
All different solar thermal processes share many common aspects. Namely, incident photons are
converted to into heat via a process known as absorption. The amount of heat generated from the
incident light is commonly characterized by what is known as the absorber efficiency. Interestingly,
high absorber efficiencies will depend greatly on the operating conditions (i.e., temperature, energy
balance, etc.) of the particular thermal process. For example, low/medium temperature processes
(<400'C) might require spectral selectivity for high efficiency whereas high temperature solar
thermal designs might benefit from a broadband absorber (i.e., a black surface). In other words,
10
given a particular surface, it is difficult to understand its merit as a solar absorber without
understanding the thermal process it drives. These concepts will be further discussed and developed
in this thesis.
It is now appropriate to discuss the benefits and detriments of generating electricity using solar
thermal engineering. Like any thermal power generation, this scheme is ultimately limited by a
Carnot efficiency which dictates the maximum work that can be continuously extracted from a
stream of heat at a particular temperature [5]. The higher the absorber temperature, the higher the
Carnot efficiency will be. Practically speaking, engineers are usually limited in material set at these
elevated temperatures. What special types of materials are available for high temperature solar
thermal engineering? This is an important concept for this technology that is discussed in this work.
The most conventional way to generate power from the sun is to heat an absorber surface to high
temperatures (>350 'C) which exchanges heat with a working fluid - this could be the steam
directly or another heat transfer fluid (HTF, e.g., synthetic oil or molten salt) which would require
additional heat exchange with the working fluid of a power cycle (i.e., steam, refrigerant).
The
solar thermal device effectively replaces a combustor / boiler in a power cycle. This fact has
numerous implications regarding the benefits of solar thermal power generation. For one, it
suggests that so long as heat is present, power may be generated. In other words, the sun need not
be present for power to be generated. A solar thermal plant may be integrated with an auxiliary heat
source (e.g., geothermal, natural gas) in order to ensure a continuous stream of generated electricity.
In that way, solar thermal power generation is dispatchable - it may provide on-demand and
uninterrupted power regardless of the variability of the input heat source.
Since high temperature heat is required for continuous power generation, solar power plants may
actually go beyond simply integrating auxiliary heat sources: it is common to implement thermal
storage in order to greatly improve the dispatchability of the plant while still relying 100% on solar
11
energy. It has been shown that thermal storage reduces greatly the levelized cost of energy (LCOE),
the main economic metric used in solar thermal investing [3].
Tcollector
h
Heat Collection
Unit
Incident Light
ermalSystem
Input Heat
Iithermal
1collector
Solar-to-thermal
Useful Energy
_
0 Thermal-to-...
Figure 2: A general schematic of a solar thermal process made of solar-thermal conversion (left of
dashed line) followed by a thermally driven process which could be a variety of different schemes
depending on the applications (i.e., heating, cooling, power generation).
There are aspects to solar thermal energy conversion that are not ideal, however.
Any thermal-
mechanical conversion process is going to suffer greatly at the small scale. This can be attributed to
the intrinsic thermodynamic irreversibilities of small heat engines. Simply put, a small system that
is held at elevated temperatures will lose its heat in a variety of ways due to its parasitic surface
areas (i.e., surfaces that exchange heat with something other than the power cycle). Parasitic losses
typically do not scale with system output capacity. This fact suggests that solar thermal power
generation might be limited to the utility scale, where the size of the thermal equipment is large [6].
Another drawback of solar thermal power generation are the maintenance costs associated with the
moving parts of the system [7]. The necessary addition of a flowing working fluid which exchanges
heat with a hot and cold reservoir suggests both a pump and a turbine (pressurizer / de-pressurizer)
12
that are both large and expensive. Again, these components only really become sensible at a large
scale.
These downfalls motivate a discussion of a technology that can utilize solar energy in a more
scalable way. As mentioned, solar energy is inherently a local resource that should be able to be
accessed efficiently and cheaply for all inhabitants of this sunny world. One technology that has
attempted to do such a thing is called solar photovoltaic energy conversion.
1.2 SOLAR PHOTOVOLTAIc ENERGY CONVERSION
The second method is known as solar photovoltaic (PV) power conversion [8]. This is distinct from
the solar thermal scheme because it utilizes sufficiently energetic photons - a concept that does not
exist in solar thermal technologies. The light that reaches the Earth is made up of a range of energy
levels, and each energy level is present at a different intensity. As experienced on Earth, this solar
spectrum closely resembles a blackbody that is held at above 5500 K.
__.
2.5.
I
ww
a
Sunlight at Top of the Atmosphere
2
E
S*
5250'C Blackbody Spectrum
1.5.
ftcdiation at Sea Laval
.5.
tA0.J
250
500 750 1000 1250 1500 1750 2000 2250 2500
Wavelength (nm)
Figure 3: The measured solar spectrum compared with a blackbody spectrum above 5500 K. Also
depicted are the absorption bands of different gas molecules in Earth's atmosphere. Photo credits:
Wikipedia.com/sunlight
There are two main features to this spectrum: 1) it closely resembles a blackbody spectrum, so
photon energies received span a wide range (i.e., it is broad, ranging from the ultra-violet (UV) to
the near infrared (NIR)) and 2) the portion of the spectrum that arrives at the highest intensity is
13
visible light (between 400 and 700 nm wavelength) -- perhaps not a coincidence that many
creatures have evolved to make use of this peak. The solar spectrum is partially attenuated as it
radiates through the Earth's atmosphere. This is due to the scattering and absorption that occurs as
the photons interact with gas molecules and particulate matter that exists in our environment.
Why do energy levels (i.e., wavelengths) of this spectrum matter? For photovoltaic conversion, a
semiconductor device is used to absorb photons with sufficient energy in order to promote electrons
from the valence energy band to the conduction band. The energy bandgap between these two states
is what determines what "sufficient energy" means. This is a region where electrons are forbidden
to exist within. For a Silicon solar cell, this bandgap is about 1.1 eV, which corresponds to a photon
with a wavelength around 1 pm - a relatively good fit for our Sun from a photovoltaic point of
view. This will be discussed in greater detail.
Conduction band
Band gap
Figure 4: Simple schematic of a single junction solar cell. Electrons from the valence band are
promoted to the conduction band via the absorption of a photon with sufficient energy (i.e., the
bandgap energy). If not enough energy is present in the photon, the photon is not absorbed. If
excess energy is present then the available electrical energy is only that of the bandgap. The
remaining energy turns to heat.
However, the nature of a single junction (i.e., single bandgap) photovoltaic cell is exclusive.
Photons arriving at the semiconductor surface from the sun that have insufficient energy (about
20% of them for a Si cell), are useless to the photo-electric conversion process since energy is not
14
great enough to promote a valence electron to the conduction band. Photons that have energy above
the bandgap of the semiconductor typically only contribute the amount of energy equivalent to
promoting the electron to the conduction band, i.e., one electron per absorbed photon. Although
multiple exciton generation is an active area of research today [9]. The remaining energy is to be
dissipated as heat as the electron relaxes down to its equilibrium level. This makes the usefulness of
sufficiently high energy photons (photons above the electronic bandgap) inversely proportional to
their energy levels.
This discussion shows that a single junction photovoltaic cell is not optimally suited for a
blackbody spectrum light source. The breadth of the spectrum around the single bandgap will limit
the conversion process in a fundamental way, much like the restrictions that the Carnot limit
introduces for a heat engine. The fundamental limit for single junction solar cells is referred to as
the Shockley-Queisser limit [10]. Figure 5 shows the results of a detailed balance that describes the
highest efficiency that a semiconductor could exhibit while being illuminated by the solar spectrum
as a function of electronic bandgap energy. Notice the maximum of these curves falls around 1 eV,
close to the bandgap of Si.
15
t
0.45t
"0.4
-
Full Concentration
1 Sun Illumination
0.35
0.3
G0.25[
0.2
-
UO
tt=
LU0.15C,
-
0.1
-
0.05
W.5
1
1.5
2
2.5
3
Bandgap Energy (eV)
Figure 5: Results from a Shockley-Queisser detailed balance on a single junction photovoltaic cell
illuminated by a blackbody source at 5700 K (AM1.5). Shown is the highest possible efficiency as a
function of the semiconductor bandgap energy.
Nevertheless, the production and installation of solar cells continues to rise. In fact, the world now
has the capacity to manufacture over 40 GW of solar panels (primarily Si based) each year [11].
While there is no doubt that solar installations will sustain this pattern in the near term, it must be
noted that the performance of single junction solar cells has remained relatively constant in the past
decade or so. In fact, recent studies indicate that while prices for Silicon solar cells have dropped,
efficiencies have stagnated [11] . This is due to the rapidly approaching Shockley-Queisser limit for
this technology (or the more practical Yablanovitch limit [12]).
In contrast to solar thermal power generation, solar PV power generation has complementary
benefits and detriments. Solar are highly scalable. The efficiency of the devices remains relatively
constant as a function of output power of the system. If 10 W are necessary, a solar panel should
cover about a tenth of a square meter. If 5 kW are necessary, a solar panel (or a number of them)
should cover about 5,000 square meters (1.2 acres). This allows the technology to be deployed both
16
at the residential / commercial scale as well as the utility scale with little change in performance if
properly engineered and electrically integrated.
Additionally, photovoltaics are solid-state devices. There are no moving parts (in the macroscopic
sense). Electricity can be generated without pumps, turbines, heat exchangers, working fluid, etc.
They operate quietly and near room temperature and can be cooled passively. These advantages,
along with impeccable timing from the computer transistor development, are primary reasons for
the success of PV cells over the last half of a century.
What happens when the sun is not shining bright? After sunset or in the presence of a cloud, there is
no way for a PV cell to continue to generate power as was the case with solar thermal technologies.
In this way, solar PV power generation is intermittent. This presents a huge problem in a society
where electrical power is required on demand. While it is possible to store electrical energy through
batteries, this is currently prohibitively expensive and impractical to implement at the large scale.
This inability to be dispatchable is one of the main aspects preventing PV cells from having a
substantial effect on our electric grid.
In summary, both solar thermal and solar photovoltaic technologies have great potential and should
be continued to be deployed in our society. However, their limitations both fundamental and
practical will ultimately restrict their penetration into our electricity generation portfolio. Thus,
there is a need for hybrid technologies that aim to leverage the benefits of each, while
simultaneously avoiding the pitfalls.
17
1.3 SOLAR THERMOPHOTOVOLTAIc ENERGY CONVERSION
The previous section indicated that a hybrid solar thermal / PV technology is needed for greater
penetration of solar power generation in the energy portfolio of our society. There are many
examples of technologies that seek to take advantage of the benefits of both solar thermal and PV
systems. For example, there are a wide variety of hybrid photovoltaic/thermal (PV/T) systems that
selectively transmit or reflect photons with energy around a particular semiconductor's bandgap
while collecting the other photons as heat that can be used to drive a thermodynamic cycle [13] [14].
Additionally, there are alternative technologies which try to harness electrical energy through a
temperature gradient and eliminate the photovoltaic limitations altogether [15].
Solar thermophotovoltaic (STPV) devices are unique. These machines seek to leverage the benefits
of both solar thermal and solar photovoltaic technologies in an inimitable way. The potential
benefits of an STPV system may not be completely obvious, but they are profound. A STPV device
converts light energy from the sun into electricity through a spectral conversion process - the broad
solar spectrum is converted to narrow band thermal emission at energies just above the photovoltaic
bandgap [16]. Before reaching the PV cell, incident solar energy undergoes a photo-thermal
conversion process at the absorbing surface of the intermediate absorber-emitter module. The heat
generated from the absorbed solar radiation drives the device to an elevated temperature where it
can begin a thermo-photonic conversion process and exchange a tailored emission spectrum with
the PV cell. The resultant photovoltaic conversion, through the extraction of excited charge carriers
in the semiconductor material, completes the solar-to-electric conversion.
The benefits of this strategy come from two main aspects, each one owing to the device's
resemblance of either a solar thermal or solar photovoltaic device. As a thermally driven process,
the device may harness the entire solar spectrum of light at the absorber surface and it is able to
integrate thermal storage for a high level of dispatchability. The second comes from the fact that
18
this device is solid-state. It has no moving parts and is highly scalable (though scalability may play
a role in a practical device design, this will be discussed in greater detail in later sections).
In principle, STPV conversion has the potential to overcome the well-known Shockley-Queisser for
photovoltaic conversion in a semiconductor [10]. In fact, by mitigating the intrinsic limitations of
PV conversion (namely, thermalization of high-energy photons and non-absorption of sub-bandgap
photons), STPV conversion efficiencies can theoretically be as high as the product of the maximum
absorption efficiency and the Carnot efficiency for a heat engine, reaching 85% at an emitter
temperature of 2200'C [16]-- also knows as the blackbody limit for solar energy conversion [17].
However, the actual efficiency of a STPV device depends on the efficiency of a number of
conversion steps. How well is incident sunlight captured and converted to heat? How much of that
heat contributes to parasitic heat loss in the system? How much of it is emitted towards the PV cell
and of that emitted power, how many photons carry sufficient energy to excite electron-hole pairs?
Is the PV cell kept cool? What are the thermal management requirements to keep this system
efficient?
Aperture
--
Sunlight
Vacuum
A bso rber
Supports
Thermal emisslon
Load
Figure 6: Schematic of an STPV device. Incident sunlight is thermalized at the absorber surface.
The generated heat conducts to the emitter surface where tailored thermal emission radiates towards
a photovoltaic cell. The maximum device efficiency could be as high as 85% [16]. Image credits:
Nature Publishing Group[13].
19
All of these critical questions aim to be addressed in this thesis. The integral components of a STPV
device are explored and discussed in great detail in the next chapter. Then, a radiative heat transfer
energy model is introduced and used to explore some of the design space that is critical for
understanding how the device operates. Next, we discuss how such a device is constructed,
including the experimental setup and fabrication of the surfaces tested. Then the experimental
procedure and results are provided for two different devices and we report the highest STPV
conversion efficiencies to date. This section also discusses device improvements to suggest realistic
pathways towards high-efficiency devices - power converters that could be competitive with both
solar thermal and photovoltaic technologies today. Finally, the concluding chapter of this thesis
makes a direct experimental comparison of a scaled up STPV device with a low-bandgap PV
converter and efficiency enhancements are presented. We show how mitigating unnecessary heat
generation in the PV cell both acts as a thermal management technique and also improves the
efficiency since these sub-bandgap photons may be recycled. This work helps to identify and
address some of the critical aspects of a STPV device. We believe it is an important step towards
high efficiency power generation through the solar resource - our ultimate fuel.
20
2. COMPONENTS OF SOLAR THERMOPHOTOVOLTAIC CONVERSION
When a solar thermophotovoltaic device is designed, it is convenient to let the width of the
electronic bandgap in the PV cell dictate the design of the individual components since it is
typically the hardest component to engineer. This information is used to select or design an emitter
which is appropriate both optically and thermally for the device. Appropriate optical properties can
be achieved through the use of selective emitters, which will be discussed in greater detail in this
chapter. Spectrally selective optical properties alone, however, are not sufficient for efficient STPV
conversion. For high efficiency from a selective thermal emitter at its operating temperature, the
spectral position of the allowed emission band must coincide with the peak of Planck's distribution.
These two must also coincide with the energy corresponding to the electronic bandgap in the PV
cell. In other words, in order to ensure that the maximum thermal modes above the bandgap are
excited, the emitter temperature should ideally be high enough such that Planck's blackbody peak
coincides with the bandgap. This temperature is given by Wien's displacement law:
K
Te i
2800
[
(4)
Eg
For a PV cell at 0.55 eV (such as an InGaAsSb cell previously used in [18]), the temperature of the
absorber/emitter device must be above 1000*C. The high temperature operation of the emitter poses
two key challenges to efficient STPV power conversion. The first is related to the photo-thermal
conversion process. Collecting sunlight to efficiently reach the optimum emitter temperature is not
21
trivial. In this thesis we present two different ways of doing so: high optical concentrations, and
spectral selectivity. The second main challenge is maintaining spectral selectivity at elevated
temperatures. Past STPV experiments have relied on the intrinsic properties of refractory materials
such as tungsten [19][20]. While thermally suitable, these intrinsic properties are insufficient for
this conversion process.
For the absorber, one common approach to effectively enhance the intrinsic solar absorptivity of
materials has been to use macro-scale cavity geometries. Because of the high aspect ratio of the
cavity needed to enhance absorption, this approach typically requires high levels of optical
concentration to reach the optimum emitter temperature (e.g. 3,183 times as used by Datas and
Algora, 4,600 times as used by Vlasov et al.). The downfall to this approach is that high optical
concentration in turn requires complex systems with relatively low optical efficiencies and high
installation costs [19] [21]. For the emitter, the intrinsic spectral selectivity of tungsten is poor at the
optimum emitter temperature since the emissivity at low photon energies (<Eg) increases with
temperature [22]. Ultimately, the reliance on the intrinsic spectral properties of materials for the
absorber-emitter has limited previously reported experimental STPVs to conversion efficiencies
around 1%[19][20][23].
2.1 THE ABSORBER
The absorber is a key component for any solar thermal device. The absorber serves as the interface
between the Sun and the thermal process as a photo-thermal converter. Its function is to convert the
incoming light from the Sun into heat as efficiently as possible. In this section, different solar
absorber concepts are explored. We show that for efficient photo-thermal conversion, the particular
operating conditions (e.g. optical concentration, process temperature, etc.) are critical for
determining an appropriate absorber design.
22
t aE
Intuitively, photo-thermal efficiency (or equivalently, absorber efficiency) is defined as the ratio of
the heat gained by the absorber to the incoming radiation that impinges that same surface. Consider
a general energy balance at the absorber surface:
dCG, = tYT 4 +
QuseJul
(4)
where C is the solar weighted absorptance, f is the thermally-weighted emittance, C is the optical
concentration, G, is the terrestrial heat flux of one Sun ( assumed to be 1000 [W/m2] ), o- is the
Stefan-Boltzmann constant (5.67x10-8 [W/m 2-K 4] ), T is the absorber surface temperature, and
finally ousefui is the heat flux that is not reflected or emitted-it represents the net heat flux gained
by the absorber and thus delivered to the thermal process.
We may re-arrange this expression in order to write the efficiency in terms of these variables:
T7abs
Ouseful
'
_
~
a incident
(aT4
(4)
CGs
This expression describes the photo-thermal efficiency as a function of the surface characteristics
(absorptance and emittance) as well as the operating temperature and the optical concentration.
Thus a particular solar weighted absorption or thermally weighted emission is not sufficient to
describe the absorber efficiency of a solar thermal device. The operating temperature and solar
concentration are also extremely useful parameters for the design of the device. This will be
discussed more thoroughly in the next section.
As mentioned, past studies have tried to incorporate macro-scale cavity designs to help trap incident
photons. The working principle of the cavity design is the increase the effective emittance of the
absorber aperture (often considered a virtual surface) relative to the intrinsic emittance of the inner
surface walls. This so-called cavity effect is extremely important if high solar concentrations are
available. Incident light enters the cavity aperture where it undergoes multiple bounces on the grey
23
surface. Depending on the aspect ratio of the cavity (l/w), much less light is reflected than would be
by a flat interface between the same material and air.
10.8
0.6
0.9
=...
......
= 0.4
0.8=
0.7
0.6
0.51
1
3
2
4
5
Aspect Ratio
Figure 7: The cavity effect showing how a virtual surface can have an effective emittance that is
beyond the intrinsic emissivity of the cavity wall owing to the multiple bounces undergone by the
photon upon entering the macro-cavity.
2.1.1 BLACKBODY ABSORPTION
In this work, we will utilize similar a concept as the cavity design but achieve it in a 2-D planar
device. Instead of achieving blackbody absorption with a macro-scale cavity, we look to the optical
properties of vertically aligned multi-walled carbon nanotube forests. Researchers have reported
that these nanotubes are almost indistinguishable from the highly-cited theoretical blackbody [24],
-
[25], [26]. We will see that this design will also suffer the same consequences as the cavity design
namely high required optical concentrations as we shift towards high aspect ratio devices. This is
perhaps a small price to pay, however, for improved efficiency.
The mechanism for extremely low broadband reflectance comes from both the morphology of the
carbon nanotube forests as well as the intrinsic emissivity of the carbon structure. The nanotubes
24
forests are extremely sparse, so that the effective index of refraction of the surface is very low, close
to that of air / vacuum (which is about 1). When light approaches this interface, it is able to pass
through almost unimpeded. Once inside the nano-structures, the wave is heavily attenuated at the
carbon surface as it penetrates deeper the forest. It is estimated that the wave is extinguished within
approximately 10 pm. The forests grown are about 50-100 pm in height.
The multi-walled carbon nanotube forests were grown using a conventional chemical vapor
deposition (CVD) process.
Before growth, a seed layer consisting of approximately 25 nm of
aluminum oxide (A1 20 3) and 1 nm of iron (Fe) is deposited using electron beam evaporation. The
samples are annealed to above 700'C before the carbon source - ethylene (C 2H 4)
-
is introduced.
Growth of CNT forests occurs on the Fe catalysts.
Figure 8: SEM images of the CNT absorbers. Their sparse nature gives rise to extremely high
optical absorption.
The optical properties of these surfaces were measured using both a UV-Vis spectrophotometer as
well as an FTIR spectrophotometer to cover the appropriate range of light. We measured above
99.9% solar weighted absorptivity and thermal emissivity. This is the first time that a vertically
aligned multi-walled carbon nanotube forest has been integrated into a solar thermal device to the
best of our knowledge.
25
CC.
3
3
4
6
7
)1
8
01
0~
Waveength (m)
400 600
800 1000 12DD 1400 1600
Wavelength (nm)
Figure 9: Optical properties of the multi-walled vertically aligned carbon nanotubes. The measured
properties are in agreement with those found in [24].
2.1.2 SPECTRALLY SELECTIVE ABSORBERS
In collaboration with the research group of Professor Marin Soljacic, we aimed to integrate
spectrally selective absorber surfaces to our device. A spectrally selective surface is one with
different optical properties depending on the wavelength of light that it is interacting with. For
efficient solar absorption, spectrally selective surfaces refer to surfaces with very high absorptance
(and thus, emittance) in the solar spectrum. They then undergo a transition to very low emittance in
the infrared, where thermal emission primarily exists. As will be discussed in the next section, by
suppressing thermal emission, these surfaces allow our device to operate at substantially lower
levels of optical concentration to reach the same equilibrium temperature.
One of the most straightforward ways to create a spectrally selective absorber is to use a thin
dielectric layer on top of a metal. If this dielectric is approximately one fourth in thickness of the
wavelength of light that is targeted for absorption, a resonance peak will be constructed. This is due
to the destructive interference that is established at this length scale. We integrated a surface that
takes advantage of this simple anti-reflective coating (ARC) phenomenon which was fabricated
using atomic layer deposition (ALD) of a 100 nm thin hafnium dioxide dielectric layer that was
26
deposited on flat tantalum.
The material set is common for high temperature applications since
tantalum and hafnium dioxide are refractory materials known for their thermal stability [27].
A more sophisticated absorber design that was explored was a two-dimensional photonic crystal
which was fabricated again by Professor Marin Soljacic's research group [27]. A photonic crystal is
a material that exhibits a spatially periodic variation in its index of refraction.
These materials
allow for an unprecedented control over the photon density of states and can therefore enhance or
suppress emission at particular wavelengths [28].
1
---
0.8-
-
MW-CNT
Ta (coated)
2dPhC
-7-*-*-* Norm. Solar
0.6
0.2~
0
500
1500
1000
Wavelength (nm)
2000
Figure 10: The three different absorber surfaces that were integrated into our STPV device. Also
plotted is a normalized blackbody spectrum at 5700 K which closely resembles the incoming light
from the sun.
2.2 SPECTRALLY SELECTIVE EMITTERS
For efficient (S)TPV conversion, it is crucial to introduce spectral selectivity of the emitted light
which reaches the PV cell. This can be done in a few ways such as a selective emitter or a subbandgap reflector. With recent development in photonic crystal design and fabrication, efficient
27
selective emitters have become a reality. Since the emitter is at a far lower temperature than the
sun, the transition between high emissivity and low emissivity must occur at substantially higher
wavelengths than it did for the selective absorbers. In fact, the transition should occur at
wavelengths corresponding to the energy bandgap that exists in the semiconductor device that is to
receive the thermal emission.
For our device, our semiconductor is an InGaAsSb quaternary cell
with a bandgap of about 0.55eV [18] . This corresponds to a wavelength of about 2.2 pm.
The emitters that were studied in our device were photonic crystal designs. The first one was a
multi-layer stack of Si/Si0 2 that was designed to exhibit a sharp transition from high to low
emissivity at the designed cutoff wavelength. This one-dimensional photonic crystal design is stable
at high temperatures and has an extremely high emittance at wavelengths smaller than that of the
bandgap.
a)
b)
Ie
x
0.5
IQE
1
2
3
4
5
X ([pm)
Figure 11: a) SEM of the one dimensional photonic crystal fabricated by Prof. Marin Soljacic's
research group. b) Spectral emissivity tuned to match the spectral response of the PV cell.
The next emitter studied was very similar in nature to the two-dimensional photonic crystal
absorber that was previously discussed. The difference was that the geometric properties were
tweaked in order to provide an appropriate cutoff wavelength around the bandgap of the PV cell.
This is accomplished by shifting the cavity modes (geometrical arrangement that produces a
standing wave cavity resonator for light waves [29]) to lower energies as the length-scale of the
nano- features was enlarged.
28
ib) 0.6
0.5-
0.4
0.3-
02
0.12
_
absorbermesred
_
_
absorbe sirm~saed
emi tter measuredemitter simulated
05
1
_
_
15
_
2
_
_
2!5
3
Wavelength (n)
Figure 12: a) SEM image of the two dimensional photonic crystals. b) Simulated and measured
optical properties of both the two dimensional photonic crystal absorber and emitter surfaces.
Notice the red-shift in the emitter transitional wavelength due to the increased length scale of the
cavities. Image credit: Veronika Rinnerbauer
2.3 SUB-BANDGAP FILTERS
Another method to enhance the spectral control of the emitter/PV pair is to introduce an optical
filter between these two components. These have been integrated in previous TPV systems and
have shown to substantially boost the performance of the device [18]. Filters aim to recycle subbandgap photons back to the emitter, keeping it hot. But they must do so without substantially
affecting how many useful photons transmit towards the PV cell. This is achieved using another
surface that undergoes a sharp optical transition - high transmission at wavelengths smaller than
that of the bandgap energy and high reflectance at longer wavelengths. Below are the optical
properties measured from an optical filter that was integrated in our setup. The near-IR properties
were recorded using a KBr detector and beam splitter with an IR light source while the mid-IR
properties were measured with a KBr detector, a CaF 2 beam splitter and a white light source.
29
100
80
S60
C
40
---
20
MIR
NIR
---- TPV6-1
0
1
2
3
4
5
6
7
Wavelength (microns)
8
Figure 13: Optical properties measured using an FTIR of the samples that were obtained from
Rugate Technologies, Inc. The dashed line are the properties given by the manufacturer while the
solid lines show the measured properties.
These optical properties were achieved using two different phenomena. The first is an interference
short pass filter is tuned to the electronic bandgap of the PV (2.2 Pm in this case). This also
provides the high reflectivity up to about 6.5 pm. The remaining reflection comes from a plasma
filter: -1
Im thick of InPAs provides high reflection above 6 Pim while it is transmissive below 3
pm.
2.4 CONCLUSIONS
The materials discussed in this chapter will serve as the building blocks for both the theoretical
analysis as well as the experimental characterization of an STPV system that records the highest
efficiency to date.
These state-of-the-art materials exhibit tunable spectral selectivity, high
temperature chemical stability, and feasible fabrication techniques. While a deep understanding of
STPV conversion is required for high efficiencies, it must be re-iterated that it is material
innovation which pushes the performance of our STPV device. Hopefully this chapter highlights
the importance of the collaborative effort of this project with physicists, material scientists, and
engineers.
30
3. MODELLING A SOLAR THERMOPHOTOVOLTAIC DEVICE
A solar thermophotovoltaic (STPV) device converts light energy from the sun into electricity
through a spectral conversion process - the broad solar spectrum is converted to narrow band
thermal emission at energies just above the photovoltaic bandgap [16]. Incident solar energy
undergoes a photo-thermal conversion process at the absorbing surface of the intermediate
absorber-emitter module. The heat generated from the absorbed solar radiation drives the device to
an elevated temperature where it can begin a thermo-photonic conversion process and radiate a
tailored emission spectrum towards the PV cell. The resultant photovoltaic conversion, through the
extraction of excited charge carriers in the semiconductor material, completes the solar-to-electric
conversion.
In principle, STPV conversion has the potential to overcome the well-known Shockley-Queisser for
photovoltaic conversion in a semiconductor [10]. In fact, by mitigating the intrinsic limitations of
PV conversion (namely, thermalization of high-energy photons and non-absorption of sub-bandgap
photons), STPV conversion efficiencies can theoretically be as high as the product of the maximum
absorption efficiency and the Carnot efficiency for a heat engine, reaching 85% at an emitter
temperature of 2100'C [16]. This upper bound, while important, does not lend particular insight
towards realistic STPV operation, however, since the simultaneous constraint of the high operating
31
temperature and ideal selective emission is impractical. For this reason, numerous models have
been constructed to predict realistic STPV performance [19], [30], [31].
In this work, we developed a model for a planar STPV device shown in Figure 16. This model
incorporates spectral and temperature-dependent radiative properties. Unlike previous modeling
efforts, the complete energy transport in our model has been experimentally validated in our
previous work [13] and here it is used to explore the effects of various components and design
aspects of an STPV converter.
g
a
Shield
1
Inactive Surface
Aperture
Shield
Inactive Surface
Figure 14: a) Schematic representation of a planar STPV device consisting of an absorber, emitter,
radiation shield, PV cell, and inactive surface. The planes in the device are separated by hundreds of
micron gaps. b) Optical image of the STPV device used to validate the radiative transfer model.
3.1 MODEL FORMULATION
A system level model was developed to predict the performance of our planar STPV device for
different geometries. The radiative transfer between participating surfaces is determined using the
concepts of radiosity and irradiance. Namely, the radiosity and irradiance are solved for each
surface on a spectral basis.
Jai = EXAEbXi +
(1
- EX)Hji
(1)
n
HAL = YJajFi;
j=1
32
(2)
As shown in equation 1, the spectral radiosity (JAj) is the net heat flux that leaves surface i. This heat
flux has two contributions: 1) the thermal emission from the surface based on the blackbody
spectral irradiance (EbAi) and 2) the reflection of the irradiance (HAi) on surface i. The irradiance is
defined as the incident radiation on surface i. It is determined by summing the product of the
radiosity of all other surfaces and the diffuse view factors between surface i and the other surfaces.
The diffuse view factors between all surfaces in the network can be determined using concepts of
summation, reciprocity, and enclosure.
To avoid iterating, an equilibrium temperature of all surfaces is prescribed. By setting these
temperatures, the spectral blackbody irradiance is specified, as well as all temperature dependent
radiative properties. The PV cell temperature is fixed at 20'C and the aperture / shield temperature
is fixed at 150 'C. Both temperatures are consistent with experiments.
As mentioned, the performance of an STPV device is strongly dependent on both the absorber
efficiency (photo-thermal) and the emitter efficiency (thermo-photonic)--in other words, the
devices ability to convert the solar spectrum into a tailored, narrow band emission spectrum. The
model solves two different sets of systems of equations to isolate the performance of the absorber
and the emitter. The absorber-side network includes the absorber surface, the inactive surfaces, the
shield, and the aperture /environment. The emitter-side network includes the emitter surface, the PV
surface, and the environment.
Spectral properties are used for the absorber surface in order to determine the solar absorptivity. For
absorption in flat metals this is assumed to be independent of temperature whereas other surfaces
use properties taken at a representative operating temperature. For the thermal emission from this
surface, a temperature-dependent total hemispherical emission is used. Similarly, the inactive
surfaces are considered diffuse emitters with a temperature-dependent total emissivity. The emitter
33
optical properties are not temperature-dependent. The properties used are measured properties at the'
TPV operating temperature (1000'C).
Once the heat flux at all surfaces is determined, the results are post-processed in order to determine
the useful radiation delivered to the cell. This information is used, along with measured PV cell
parameters (internal quantum efficiency and reflectivity) to calculate the photocurrent generated in
the conversion process. The generated photocurrent is described as:
'ph = -e AVf 00A IQE QepvdA
(3)
where e is the charge of an electron, h is Planck's constant, co is the speed of light in a vacuum, A,
is the area of the PV cell, IQEA is the spectral internal quantum efficiency of the PV cell, and
Qe-,
is the spectral radiative flux between the emitter and the PV cell. This photocurrent, along with
empirically determined PV cell characteristics, is used to determine the maximum power point
(MPP) of the I-V curve for the diode. That value is the predicted electrical power generation.
However, in the following discussion, we will use photocurrent as the primary method of evaluating
performance as it is a completely predictive measure.
3.2 EXPLORING ABSORBER CHARACTERISTICS
The ability of the STPV device to efficiently deliver sunlight as heat to the emitter is determined by
the absorber-network model.
The key parameters that affect this thermal transfer are the
geometrical and the radiative properties of the absorbing surface, the inactive surface, and the
radiation shield. As mentioned, we will use the predictive measure from our model, the generated
photocurrent, as our figure of merit for most of the discussion. While it does not quantify efficiency,
34
it provides useful information about the performance of the device as higher photocurrent results in
higher output power. Additionally, when considering the absorber, note that a given generated
photocurrent corresponds to a particular absorber/emitter temperature if the emitter, the emitter-PV
cell gap (g2), and the PV cell remain constant. However, in this model the effect of radiative
recombination on the energy balance is neglected.
Before discussing absorber performance in detail, something of a disclaimer must be provided.
When comparing different absorber surfaces, it is important to understand that at least some part of
the thermal performance is related to the rest of the system. For example, if one selective absorber
surface that was delivering heat to a blackbody emitter, it would "outperform" the same surface in
the same system that was delivering heat to a metallic emitter. That said, so long as the system
behind the emitter stays constant, a fair comparison can be made within that operating regime.
In Figure 17, three different absorber surfaces were investigated: vertically aligned multi-walled
carbon nanotube forest (MWCNTs), 2-dimensional photonic crystals (2D PhC), and bare tungsten
(see the inset for the optical properties). MWCNTs are investigated due to their nearly-blackbody
optical properties. The extremely high absorption (and thus low reflection) is due to the apparent
index of refraction of the sparse MWCNT forest nearly matching that of the vacuum, as well as the
added roughness due to the non-uniform tip heights. When light reaches this interface, it passes
through and is ultimately absorbed by the carbon material. For this surface, the solar weighted
absorptance as well as the emittance at all temperatures is approximately 0.996 [24].
The 2D PhC optical properties are from a 2D array of high aspect ratio cavities which have been
optimized for selective solar absorption [30]. These photonic crystals have great potential for
spectrally selective solar applications, with the ability to control the photon density of states [3]. To
include directional characteristics of this absorber, we assume a relationship between incident light
angle and the level of solar concentration. At high solar concentrations (>1 00x), we assume diffuse
light incident on the absorber surface. In this regime, the spectral hemispherical emissivity is used
35
to determine the absorber characteristics (see inset of Figure 17a) resulting in a solar weighted
absorptance of 0.7. For low concentrations (<100x), we use the optical properties at normal
incidence (see inset of Figure 17b), resulting in a solar weighted absorptance of 0.8. Thermal
emission from the 2D PhC surface is always determined using the full hemispherical properties. For
example, the total hemispherical emittance at 1000 *C is 0.25.
The tungsten absorber surface considered has no coating or structure added to the surface. This
surface has a solar weighted absorptivity at normal incidence of 0.46 [32] and a total hemispherical
emissivity at 1000 *C of 0.13 [33]. As a refractory metal, tungsten is thermally suitable for the high
temperature STPV operation.
Figures 17a,b show the photocurrent as a function of input power for two different geometries.
Since the emitter surface, the emitter-to-cell gap spacing, and the PV cell remain fixed for this
analysis, we can directly compare absorber performance based on this generated photocurrent. In
Figure 17a, the absorber is 7x smaller than the emitter while in Figure 17b, the absorber and emitter
surface areas are equal. For the reduced absorber area, the "black" MWCNT surface outperforms
the spectrally selective 2D PhC surface, which in turn outperforms the flat tungsten surface.
However, for the equal absorber/emitter areas, the trend is completely different, as the spectrally
selective 2D PhC is the best performing, followed by the flat tungsten. To understand the reason the
best performing absorber in Figure 17a is the worst performing absorber in Figure 17b, we examine
the particular operating point.
For efficient photo-thermal conversion, recall the following expression for absorber efficiency is
considered:
77abs
(aT
\CGS
where C is the solar weighted absorptance, a is the Stefan-Boltzmann constant,
(4)
T is the
temperature of the collector, C is the solar concentration, Gs is the solar constant (1000 W/m 2), and
36
i is the total hemispherical emittance of the surface at a given temperature. The relative importance
)
of the surface absorptivity and emissivity is thus governed by the ratio of the emissive power (c-T 4
and the incident radiation (CG,).
In Figure 17a, the solar concentration (i.e., input power divided by the aperture area) is in the range
of 500x to 700x for temperatures above 850"C for this device with an absorber area 7x smaller than
the emitter area (the merit of de-coupling of these areas is discussed below). For these conditions,
the ratio of emissive power to incident heat flux is between 0.09 and 0.2 (depending on the absorber
surface). This result indicates that the STPV device is operating in a regime where absorption of the
highly concentrated light is far more important than suppressing emission. As such, the black
surface (e.g., MWCNTs) outperforms other surfaces such as a 2D PhC selective surface or a bare
tungsten metal surface.
b 15
a 1.
1
Z0.6
10.5
0-4
01
0
05
1
15
wavglenoh
2
(pm)
0.5
5
0.5
1.5
2
2.5
0.5
0
1
wavelength (pm)
5
10
-- CNT
-- 2D PhC
Bare W
A
15
05
10
i
Input Power (W)
Input Power (W)
Figure 15: Generated photocurrent as a function of input power for different absorber surfaces for a
device with an a) absorber surface 7x smaller than the emitter surface and one with b) equal
absorber and emitter surface areas. The enhanced spectral selectivity introduced by the 2D PhC
design provides substantial improvements from the bare metal in both cases, but for the highly
concentrated radiation (Figure a), the black MWCNT surface is the most efficient. The inset of the
figure shows the hemispherical spectral emissivity of the three surfaces considered.
For the device shown in Figure 17b at temperatures above 850*C, the required solar concentration is
below 100x. Note that due to the lower optical concentrations, normal emissive properties are used
37
Elabs
the absorber surfaces (see inset). For these conditions, the ratio of emissive power to incident heat
flux is between 0.65 and 1. Compared to the geometry in Figure 17a, this ratio is much greater such
that no longer is high absorption much more important than low thermal emission, spectral
selectivity is more beneficial to the efficiency of the photo-thermal conversion process. See Table 1,
below which summarizes the results.
Table 1: Summary of the different operating points for the two different geometries discussed. The
operating temperature has been fixed to 1000'C (the optimum TPV temperature).
Absorber
-
(T = 1300K)
MWCNT
2D PhC
W
CGs
1
1
0.18
0.80
1
1
0.65
0.34
0.71
0.24
0.16
0.67
0.79
0.24
1.02
0.55
0.467
0.13
0.09
0.42
0.46
0.13
0.72
0.38
Note: this value of solar weighted absorptance is an overprediction of the actual value for high
concentrations as it assumes normal incidence. The actual performance of this absorber for these
conditions is expected to be worse.
Although the best absorber efficiency is reported for the black MWCNT surface (80%), this
performance can only be achieved when the level of solar concentration exceeds 700 times the
intensity of the sun (i.e., 700x). In other words, there is another cost associated with this improved
performance. In practice this type of irradiance is achieved through 2-axis tracking strategies such
as a power tower or a dish concentrator. As the required solar concentration for a solar thermal
application increases, so does the cost of the optical components relative to the total installed cost of
the device [34]. While the absorber performance is reduced for the 2D PhC selective absorber, it
38
allows the STPV device to be implemented at relatively low concentrations (<100x) where linear
Fresnel lenses or parabolic troughs sufficiently provide the necessary power. These single axis
tracking concentrators would substantially reduce the optical costs for the device. Thus we see an
interesting coupling between the complexity of the absorber surface (e.g., black vs. selective) and
that of the concentrating optics (e.g., single vs dual axis tracking).
In the design of any solar thermal device, this concept is important for efficient photo-thermal
conversion and low cost. For example, small scale solar thermal applications such as solar hot water
heaters typically implement coatings to suppress IR emission (temperatures below 100*C with no
solar concentration), as the ratio of thermal emission to incident radiation is approximately unity.
Conversely, for solar power towers (temperatures above 800'C, solar concentrations around
1 000x), black absorbers are used, since the ratio falls to about 0.1.
In our previous work, we exploit this concept to show a dramatic improvement in this thermal
transfer by reducing the area of the absorber relative to the area of the emitter [13]. We showed that
by tuning the energy balance such that a higher optical concentration is required to reach the same
emitter temperature, the emissive loss from the black absorbing surface is reduced. However, since
our absorber/emitter samples are fabricated on the same substrate, this decoupling of their areas
exposes a non-absorbing, inactive surface in the absorber-network (Figure 16). To explore the effect
of this inactive surface, we first considered a device, which has an emitting area that is 7x larger
than the absorbing surface. This geometry will remain fixed as the absorber surfaces are explored.
In order to understand the effect of the inactive area on the performance of the device, we
considered the radiative properties of various surfaces. As shown in Figure 18a, metallization of the
surface is necessary in order to approach the ideal, adiabatic case. Figure 18b shows the same
results with a silver coated glass reflector placed 300 microns away from the inactive surface in
order to recycle the emitted photons back towards the device. This reduces the apparent emissivity
of the inactive surface area, and is an important aspect for high thermal transfer to the TPV
39
converter. As expected, the improvement provided by the reflector is far more dramatic for highly
emissive inactive areas than for the tungsten metal, for example.
a
1.5
b 1.5
w
Si
--- W
-Si
CNT
-
I
CNT
I
11-0.5
0.5
05
5
10
15
0
Input Power (W)
1
15
Input Power (W)
Figure 16: Generated photocurrent as a function of incident power for a) three different inactive
surface areas without a radiation shield and b) the same inactive surfaces with a radiation shield to
recycle lost radiation. The enhancement is far greater for highly emitting surfaces. For both figures,
the device has a MWCNT absorber with an area 7x smaller than the emitter.
3.3 EXPLORING EMITTER CHARACTERISTICS
Once heat is delivered from the absorber to the emitter, tuned thermal emission is directed towards
the PV cell. The thermophotovoltaic (TPV) conversion process largely depends on the ability of the
emitter to suppress sub bandgap photon emission, as well as the view factor between the emitter and
the PV cell. To evaluate the TPV performance, we will present the results by showing the
photocurrent as a function of temperature, as well as the TPV efficiency.
The development of 1D[35] and 2D PhC [30] designs for spectrally selective surfaces extends
beyond that of solar absorbers. The application of photonic crystals for emitters in a TPV converter
[3] provides an ability to engineer the suppression of useless sub-bandgap emission without the
need for long-wave optical filters [18], which in turn reduces the amount of components and
therefore the parasitic losses in the device. Figure 19a shows the effect of different emitter surfaces.
40
The plot of photocurrent density as a function of temperature allows for the de-coupling of the
emitter performance from the previously discussed absorber performance.
a
b 0.15
.
1.6
1.41 00-5
'
1
E_
\
I
~' 1 w
o9.5
2
o 0.8
j0.1
2.5
.
C
Wavelength (pm)
4'
C>
0.F-
0.05
- d PhC
0.
0 0.2
-CNT
0
1000
1100
1200
8 00
1300
Emitter Temperature (K)
1400
1000
1200
800
Emitter Temperature (K)
Figure 17: a) Generated photocurrent density as a function of the emitter temperature for various emitter
surfaces. The spectral emissivity in the appropriate range of wavelengths is shown in the inset. b) TPV
conversion efficiency for the same emitter surfaces.
These results again seem counter intuitive, with the blackbody emitter outperforming both the 1 D
and 2D PhC designs. In fact, if photocurrent density (and therefore power density) of a particular
TPV device is to be maximized, then blackbody emission towards the PV cell will lead to the
highest performance; the generated photocurrent for different emitters is largely a measure of the
emission above the bandgap. Power density can be an important aspect when considering the
normalized cost of the PV cell. While conventional PV conversion is achieved through highly
available, low cost silicon, in practical realizations of TPV converters, low bandgap semiconductors
are often implemented to reduce the optimum operating temperature. These materials do not share
the industrial maturity of silicon, and therefore have significant costs associated with them. Thus by
increasing the power density for a given TPV temperature, a reduction in total cell area to achieve
the same desired output power could prove economical depending on its effect on the overall
conversion efficiency.
41
It is important that the emitter and the PV cell in a planar STPV device are fixed as close as
possible in order to ensure high geometric view factors (See inset of figure 20). The degraded
photocurrent in figure 6 is a result of nearly diffuse emission from the emitter surface escaping from
the sides of the parallel plate configuration to the environment. Since the environment acts as a
blackbody, this useful emission cannot be recovered.
The photocurrent for the 2mm gap is
approximately 0.45 A at 9 W of input power, while the photocurrent at 0.35 mm is up to 0.7 A. This
shows that the dominant mechanism for the enhanced photocurrent is the improved view factor
which increases from 0.6 to 0.99 between these two gaps. One notices, however, that a twofold
increase in the current is not quite accompanied by a two fold increase in the view factor. This is
due to higher order effects such as the reflections off the PV cell as well as the metallic fingers
which take up approximately 10% of the cell area.
While Figure 4a lends insight into the energy conversion process, it does not directly give any
information about the efficiency of the TPV converter. Thus, in Figure 4b, we show the TPV
efficiency defined as the ratio of electrical power generated in the cell to the total thermal emission
by the emitter surface. For different emitter surfaces held at a constant distance away from the same
PV cell, this allows us to evaluate both the emission above the bandgap as well as the suppression
of emission below the bandgap. We can see that in fact the 2D PhC design outperforms the other
surfaces, showing over a 100% improvement from a blackbody emitter.
42
0.9
0
0
0.8
--.
0.7
-
0.6-
-2
measured, .35 nm
measured,.65 mm
35 rm
.65 mm
-- 1 MM
mmr
0.5.
-.0'o
.5
6
6.5
7
7.5
8
8.5
3
2
Emitber I PV Gap,
9
g2(mm)
9.5
10
Input Power (W)
Figure 18: Generated photocurrent as a function of input power for different gap spacings between
the emitter and the PV cell. The generated photocurrent is decreased by approximately 40% as the
diffuse view factor is decreased. Experimental results will be discussed in the experimental
characterization chapter of this thesis.
Again, a competing trend of upfront cost versus overall performance is revealed. Blackbody
emitters can ensure a high generated power density at the expense of the full emission of subbandgap photons. Conversely, selective emitters can ensure a high TPV efficiency at the expense of
maximizing the emission above the bandgap. The blackbody emitter then would reduce the installed
cost of a device by reducing the normalized cost of the semiconductor material as well as by
avoiding the complex manufacturing associated with the fabrication of photonic crystal designs. As
the design and fabrication methods for selective emitters improves, however, the efficiency of the
device relative to the blackbody emitter will begin to overwhelm any advantage associated with the
reduced normalized cost of the PV cell.
3.4 CONCLUSIONS
We have investigated the effects of various geometric and optical parameters on the performance of
a planar STPV device and discussed a few key design aspects. For high solar concentration
43
operation, black absorbers provide substantially higher absorber efficiency than selective photonic
crystal designs under diffuse illumination. To achieve these high concentrations, however, the
absorber surface area must be reduced relative to that of the emitter.
In a planar design, this
exposes an inactive non-absorbing surface area. We show the importance of metallizing this surface
to greatly reduce radiative losses, especially when the absorber surface is much smaller than the
emitter surface. Additionally, by incorporating a radiation shield the apparent emittance of the
inactive surface can be decreased, which is more pronounced if metallization is not possible.
Conversely, an appropriately design selective surface can shift optimum operation towards low
solar concentrations, greatly reducing the cost and complexity of the device. We also modeled
different emitter surfaces and showed the importance not only of high emission above the bandgap
for increased power density, but also of suppressing sub bandgap emission for increased conversion
efficiency. As is the case absorber selectivity, TPV performance and cost/complexity may not
necessarily go hand in hand. By discussing these important aspects, we hope to provide design
guidelines to help improve the overall conversion efficiencies of real STPV devices in the nearterm.
44
4. EXPERIMENTAL DEMONSTRATION
While solar thermophotovoltaic devices show great promise as a result of their fundamental ability
to harness the entire solar spectrum, still experimental demonstrations of this technology are
limited. As discussed in previous chapters of this thesis, there are a number of critical components
/
of solar thermophotovoltaic conversion - and an efficient device hinges on an efficient absorber
emitter / PV cell. For our experimental demonstration, we chose to investigate planar devices. That
is, the absorber and emitter are integrated on the same chip (i.e., monolithic) allowing for ease of
fabrication and characterization. As discussed in further detail in previous sections, our devices
integrated both spectrally selective (using quarter wavelength interference and two dimensional
arrays of nano-cavities) as well as broadband (using vertically aligned CNTs) absorbers.
4.1 EXPERIMENTAL SETUP
The PV cell, emitter, absorber, and aperture / radiation shield were incorporated into an
experimental setup to ensure repeatability and control over the conversion process. The PV cell is
mounted to a heat sink (122-0101, Opto Sigma Corporation)which allows temperature control and
thermal load measurements during operation. As mentioned, the PV cell is an InGaAsSb quaternary
semi-conductor which has a bandgap of approximately 0.55 eV. This corresponds to a wavelength
of 2.2 pm. The PV cell was fabricated at Lincoln Labs [18].
45
a)
Polished
metal
reflectors
Absorber/Emltter module
Photovoltolc cel
Figure 19: a) Schematic of our planar STPV experimental setup. b) Optical image of the STPV
device with a bonded thermocouple at high temperature.
We prepared absorber / emitter samples that matched the dimension of the PV cell using a die saw.
This allowed us to align the edges of the sample with the PV cell by placing the absorber / emitter
chip directly on top of the PV cell and matching the edges using a vertical straightedge. Once the
surfaces are aligned in the horizontal plane, mechanical support needles are moved in towards the
sample's edge. Since the sample has a thickness of 550 pm, these needles could secure the sample's
edges, maintaining its position without interfering with the absorption / re-emission process. Three
mechanical supports were used in the experiment: two hypodermic needles (27 gage x 1.25
and one spring-loaded pogo pin (POGO-72U-S, ECT) opposite from the needles.
46
",
B-D)
L-
0
U- 0.8
>0.6
S S0.410
1
2
3
Emitter I PV Gap, g2 (mm)
Figure 20: Diffuse view factor between parallel plates that are Icm 2 as a function of the gap spacing
between them. In order to ensure above 95% of the radiation to be intercepted by the PV cell, the
gap spacing should be about 300 pm.
The spring-loaded pin was crucial as it ensured a sufficient force on the absorber / emitter sample
minimizing pitch errors due to thermal expansion of the sample during operation.These pin supports
were also selected to minimize parasitic conduction losses that would take heat away from the
conversion process.
Once the sample was aligned with the PV cell and secured with the mechanical supports, a 300 pm
gap between the emitter and the PV cell was introduced using the z-stage to lower the PV cell / heat
sink with respect to the supported absorber / emitter sample. This fixture was then aligned with the
silver coated glass aperture in a similar way, and a 400 pm gap was introduced. These gap sizes
were chosen in order to ensure high diffuse view factors (>95%) for efficient radiative transfer.
47
Figure 21: a) Solar simulator, primary concentrator, and vacuum chamber that sits on an optical
table. b) Experimental setup inside the vacuum chamber showing the secondary concentrator and
the aperture / shield assembly.
The experimental setup was then placed in a vacuum chamber and the environment was evacuated
to about 0.3 Pa. This pressure is sufficiently low to effectively suppress all conductive and
convective losses between the hot emitter surface and the cold PV cell. Thus all heat transfer across
the micron gap is assumed to be radiative. This can be quantitatively supported by calculating the
mean free path of the particles in the gap and comparing that to the gap size. From kinetic theory,
the mean free path of the gas in the chamber is calculated with the following equation:
kbT
V27iPd2
(1l
)
1
Where kb, T, P, and d are the Boltzmann constant in J/K, the temperature in K, the pressure in Pa
and the diameter of the gas in meters. Comparing this value to the gap spacing gives the Knudsen
number (Kn). At a pressure of 0.3 Pa the Kn is nearly 300. This indicates that any transport that is
not radiative in this gap is almost entirely suppressed.
Once the setup is aligned and the chamber is evacuated, solar simulated light is supplied to the
absorber surface. This light is simulated from a Xe-arc lamp in a solar simulator (92129, Newport
48
Oriel Inc.). While the generated spectrum is not exactly that of the Sun, optical filters are integrated
in order to output an AMI.5 spectrum. This is done primarily through suppressing the high
emission just below 1 pm.
4
AM 1.5 Direct Circumsolar
3.5
C'4
E
---
efed Solar SirriLor
3
2
1.5
a.
0.5
U)0
0.5
1
1.5
2
2.5
Wavelength (tan)
Figure 24: Comparison between the AM1.5 direct spectrum used as a standard for CSP applications
and the spectrum provided by the Xe-arc lamp in our experiments.
The deviation from the solar spectrum is not crucial for the spectrally independent, black, carbon
nanotube absorber. However, when spectrally selective absorbers are tested, the AM 1.5 filters are in
place. The light from the lamp is first concentrated using a converging lens that focuses the light to
down to a focal plane giving a boost in intensity to about 50x that of the sun. The light is further
concentrated using a secondary concentrator which was constructed with silver coated glass slides
(250 nm silver layer was sputtered on the glass slide with a thin titanium adhesion layer and
protected with a transparent alumina film) that were cut into a converging geometry. The pieces
were assembled as a frustum and allow for light intensities as high as 1000x that of the sun (~1000
kW/m 2).
With these different optical components, our concentrator system can be used with 4
different configurations allowing for a very great range of input solar concentrations to our device.
Following each experiment, the input power was determined by measuring the radiative power that
comes through the aperture fixture. We measured this value using a thermopile detector (919P-04049
50, Newport Oriel Inc.) that is able to sense the total radiative power incident on its surface, which
is placed in the same plane as the absorber surface. The total power is divided by the aperture area
to determine the irradiance.
4.2 EXPERIMENTAL PROCEDURE
When the solar simulator is turned on, the STPV device undergoes a transient process as its
temperature climbs. Given the high input heat fluxes, the temperature of the sample can typically
rise hundreds of degrees Celsius per second.
1400
1.4
)'
* 1200
E
U 1000
E
E
W
1.2
b)
0.8
800
0.6
0.4
600
CJ 0.2
400
20
40
60
80
100
120
time (s)
n
20
40
60
80
100
120
time (s)
Figure 22: a) The absorber / emitter temperature as a function of time since a step change in input
heat flux was applied. b) The corresponding photocurrent that is generated as a result of the thermal
radiation at the emitter surface. Quasi-steady state is determined by a flat photocurrent measured on
the PV cell.
After approximately 30 seconds the system is said to be quasi-steady state.
Once steady state
operation of the STPV device was established, we used a precision source-meter (2440, Keithley
Instruments Inc.) to perform several current-voltage (I-V) sweeps. The sweep was conducted in
a 4wire configuration and would acquire 50 data points in the range of 0 - 0.7 V. From the sweep
we
can determine a number of performance metrics for the conversion process such as open-circuit
voltage, short-circuit current, and most importantly the maximum power point - the point
along the
current-voltage response where the product of the two values is maximized.
50
1
2.5
1.5.
R
MPP
- ------.------------------- ----e
0.5
0
0.2
0.4
Voltage (V)
0.6
0.8
Figure 23: Typical current -voltage relationships for different devices at different illuminations that
are obtained by the precision source-meter.
4.3 EXPERIMENTAL RESULTS
A few different experiments were performed in order understand the conversion process in great
detail. The first test was a TPV experiment, consisting of a simple temperature measurement and
output power measurement. Regarding STPV experiments, as discussed in the previous section, the
properties of the absorber surface greatly determine which operating regime it will operate most
efficiently within. To explore this further, we tested both broadband absorbers as well as photonic
crystal (spectrally selective) absorbers. We show that these different devices do in fact operate in
completely different optical concentrations at very similar demonstrated efficiencies.
4.3.1 THERMOPHOTOVOLTAIC CONVERSION
A TPV experiment consists of recording the output power from the PV cell at different emitter
temperatures. From our model, we may predict this performance given the optical properties of the
emitter as well as the spectral response of the PV cell. In the experiment, we bond a fine gage
special limits thermocouple (CHAL-005,
Omega Engineering Inc.) to the absorber-side of the
51
sample. We use a zirconia-based ceramic epoxy (516 Ultra-temp, Aremco Products Inc.) to bond
the thermocouple to the sample. The manufacturer's thermal annealing instructions were slightly
modified in order to anneal in an inert environment (N 2) to avoid oxidation of the carbon nanotubes.
a)
0.4
b)
I
I
0.3
0'
0.2
0
a
0.1
-
600
-
Al
800
1000
1200
1400
Emitter Temperature (K)
Figure 24: a) Optical image of the TPV setup. A thermocouple is bonded on the absorber to monitor
the temperature of the device during testing. b) TPV experimental results. The solid points are
experimental data while the dashed line is the prediction from the model. The good agreement with
the model not only serves as validation but also provides an indirect temperature measurement of
the device during full STPV testing.
The experimental results from the TPV experiment provide a few important points. Firstly, the good
agreement with the model provides a decoupling of the absorber performance from the emitter
performance. In other words it allows us to understand whether our emitter/PV radiation and
electrical model (as described in the previous section) is accurate.
Secondly, the good agreement with the model provides us an indirect temperature measurement.
When full STPV experiments are performed, it is best not to interfere with the conversion process
52
by placing a thermocouple directly in the path of the incoming light. Instead, for a given output
power, we are provided with a good sense of the equilibrium temperature of the device.
4.3.2 HIGH CONCENTRATION REGIME
As described in detail in the previous section, high optical concentrations can benefit photo-thermal
efficiency so long as black absorbers are implemented. Therefore, we created samples with carbon
nanotube absorbers (optical properties discussed in chapter 2 of this thesis). We experimentally
investigated the effect of decoupling the emitter to absorber area ratios (AR). While the effect of
increasing area ratio was discussed in greater detail in the previous section, we see experimentally
that the operating points shift to increased irradiance relative to the thermal re-emission loss. From
T abs ~
1
a-T4
~
(1)
.
before, the absorber collection efficiency for a black surface can be described as:
CGs
Therefore decreasing the ratio of the emissive power to the incident irradiance we move the
operating points to the lower right of the graph of figure 28. The estimated absorber efficiency for
the ARlO device is approximately 75%.
From this data there are two interesting cross-sections. The first is vertical lines of constant optical
concentration. We see in fact that for a given optical concentration, an optimum area ratio must
exist to maximize the conversion efficiency. This is due to the competing trends of thermal
efficiency and TPV efficiency. Namely, as the area ratio increases the thermal efficiency is
drastically enhanced at first since the thermal emission is suppressed. However, the operating
temperature of the device decreases since the absorbed power scales with absorber area for a given
heat flux. In general, the optimum area ratio increases with increasing optical concentration.
53
0.5
AR
3
1
Tae (K)
0.45
5
-1300
0.4
7
E 0.35
0.3
1200
0.25
0.2
CL
-3
10'
-1100
0.15
0.1
1000
0.05
20
40
Optical Concentration
60
80
(W/cm 2
)
00
Figure 25: Output power and temperature as a function of incident heat flux that impinges the
absorber surface for different geometry devices.
The next interesting cross-section is to consider horizontal lines of constant PV output power (or
equivalently constant absorber / emitter temperature). Since for a given temperature, the PV output
power is fixed, then any increase in efficiency when the area ratio is altered must be completely as a
result of this change in geometry. By keeping the output power constant and monotonically
increasing the absorber efficiency, we see a corresponding boost in overall device efficiency.
As the area ratio is increased, we observe that the absorber performance asymptotically approaches
about 80% (80% of the incident sunlight is delivered as heat to the emitter). This is understood by
the non-zero emission from the inactive absorber-side surface. If tungsten is to be used as the
inactive surface, diminishing returns in terms of efficiency are observed at area ratios above about
10 and absorber efficiencies around 75% are achieved.
design with decoupled absorber and emitter areas.
54
The 25% loss is inherent to our planar
3
)
z
2.5
C2
2
UJ
1.5
model (SQl1DD)
--
0
>0
0
1
1
2
I
experiment
3
4
5
6
Emitter-to-Absorber Area Ratio
b)
3
T
= 1285 K
T
=1055 K.
4)2.5
u 2
1.5
(Dtso212
.
0 1
2
4
6
8
10
Emitter-to-Absorber Area Ratio
Figure 26: a) Demonstration of an optimum area ratio for a specific incident heat flux. For this
operating point the incident flux was 373 suns. b) Monotically increasing STPV efficiency for a two
different emitter surface temperatures. Over 100 % improvement in absorber efficiency is
demonstrated by reducing the area of the absorber relative to the area of the emitter.
55
100
--
80
-
CNT
Inactive
Thermal Transfer
0
0.
S60-
o
0
40
*
200
10
1
10
10
Area Ratio (AR)
2
10
3
Figure 27: A breakdown of where the input power goes for different area ratio devices. 'CNT'
indicates emissive losses from the black absorber, 'Inactive' indicates radiative power lost at the
non-absorbing front-side surface, 'Thermal Transfer' is the useful energy delivered to the emitter.
This loss can be avoided only if we were to re-engineer our device into a 3-dimensional structure. A
macro-cavity (as used in many previous studies) could be taken advantage of, again pushing the
device towards high solar concentration operating conditions and very high thermal transfer
efficiency. We used our developed model to help predict the performance with all of the same
materials only a different geometric configuration. At the same solar concentration and the same
area ratio our overall efficiency is estimated to be above 6%.
Additionally, we could improve the performance of the device by implementing sub-bandgap
photon reflectors such as those used in previous TPV demonstrations [18]. We see the dominant
effect of incorporating these filters is to shift the device to higher temperatures at lower input
powers. This agrees with our intuition since recycled photons will contribute to heating the emitter
surface.
56
AR=10
10
8
0
4
0
----
Planar, I1D-PhC
---- Cauity, 1 D-PhC
'Caxity, Ideal Rugate
01
500
0
1000
1500
Solar Concentration (Suns)
Figure 28: Projected efficiency improvements through implementing cavity geometry and subbandgap photon reflector above PV cell.
4.3.3 Low CONCENTRATION REGIME
As described in chapter 3 of this thesis, lower optical concentrations could be feasible if spectral
selectivity is incorporated at the absorber surface in order to help suppress thermal re-emission. The
photonic crystal absorber surfaces (both 1-dimensional and 2-dimensional) that were introduced in
chapter 2 which were fabricated in the research group of Professor Soljacic at MIT were integrated
into our experimental setup in order to validate the lessons learned from our modeling work. We
sought to shift away from the high concentration regime where a blackbody absorber was sufficient.
We measured the output power as a function of input power and calculated the corresponding
efficiency of these devices. We notice the drastic effect of the suppression of thermal re-emission
from the absorber surface. This suppression forces the device to lower concentrations for a given
absorber / emitter temperature. Since the output power is fixed for a given temperature, this implies
that any photon that is not emitted reduces the required input power (and therefore concentration)
required to remain at the same temperature. We were able to record efficiencies as high as 3.75%
which is currently the highest recorded STPV conversion efficiency [27].
57
a
0.5
90.4
[
S0.3
100.2
0
0
2dPhC
.11.
U
5
3(
15
10
25
20
4
.
Ta(Coa Med)
MW-CN T
Model( 2dPhC)
-Model(
MW-CNT)
e
5
Input Power (W)
4
3.5
ii
b)
3
2.5
0--
21.51
-- Mdel (2dPhC
MWdel (MW-C NT),
S2dPhC
SMW-CNT
0.5
Al-
0
*
0.1
0.2
Ta 1f1at
'.
0.3
... ~,
0.4
0.5
Output Power Density (W cm-,)
Figure 29: a) Output power versus input power for three different devices intended to operate in
very different regimes. The two dimensional photonic crystal absorber device reaches the same
temperature as the carbon nanotube absorber device at approximately half the input power due to its
ability to suppress thermal emission. b) Corresponding efficiency as a function of output power
density for the same experiments.
58
These experiments represent the first successful demonstration of the STPV conversion process
using a monolithic absorber/emitter device that imparts 2-dimensional photonic crystals both for
selective absorption and emission processes.
As the discussion in the modeling chapter of this thesis suggested, we were able to achieve high
thermal emitter temperatures at substantially lower optical concentrations using an area ratio of 1
(emitter and absorber areas equal). At optical concentrations of about 100x, we were able to reach
our elevated operating temperatures.
4.4 CONCLUSIONS
This important thesis chapter presents results from successful TPV and STPV conversion processes.
We have integrated nanophotonic components to a planar STPV device to achieve the highest
efficiencies for STPV conversion to date. We have discussed our experimental setup as well as our
procedure for testing these devices. Additionally, through an exploration of various materials and
geometries, we have demonstrated two different optical regimes based on the absorber surface
properties, extending the discussion from the modeling chapter. While our results are modest, we
believe that these demonstrations are a crucial step in the right direction for eventual commercially
viable STPV converters.
59
60
5. EFFICIENCY ENHANCEMENT THROUGH SPECTRAL CONVERSION
Next we sought to demonstrate the ultimate motivation for STPV energy conversion - a method to
enhance the efficiency of a single junction PV cell through an intermediate spectral conversion
process. To date, there have been no experimental demonstrations of this enhancement. To our
knowledge, all of the previously mentioned STPV experiments both in this thesis as well as past
literature were unable to provide a higher solar-to-electrical efficiency using the STPV device than
the conversion rate of the underlying PV cell.
The Shockley-Queisser limit [10] that was introduced in the first chapter of this thesis is primarily
due to the spectral losses associated with converting a broad solar spectrum using a single bandgap.
These spectral losses are ultimately responsible for heat generation in the PV converter through
processes known as either sub-bandgap photon absorption or thermalization (heat generated from
the relaxation of an electron down to the conduction band edge).
In an STPV device, the single
junction PV cell has the potential to host the same heat generation mechanisms.
In order to
improve the efficiency of this process, spectral control is introduced between the emitter and the PV
cell that should limit sub-bandgap photon absorption. This suppression has two effects: 1) it reduces
heat generation associated with these sub-bandgap photons and 2) it improves the overall
conversion efficiency by keeping the emitter hot (since less heat is lost). The reduced thermal load
on the PV cell and system efficiency goes hand in hand.
61
By integrating the optical filter that was introduced in chapter 2 of this thesis, we sought to
introduce a finer level of spectral control to the TPV conversion process in order to enhance the
efficiency of the device. Interestingly, we found that by incorporating this sub-bandgap reflector,
we could substantially reduce the heat generation within the PV converter.
We sought to directly compare the conversion process of our PV cell illuminated by the solar
spectrum versus that of the same exact cell that was converting a more manicured radiation
spectrum. In other words, if the PV cell could convert solar power at a certain efficiency, could the
addition of an intermediate absorber/emitter device between the sun and the cell improve upon that
overall efficiency?
5.1 CONCENTRATED PHOTOVOLTAIC CHARACTERIZATION
The first experiment that was performed was used to characterize the PV cell (InGaAsSb, Eg= 0.55
eV). This PV cell was exposed to a simulated AM1.5D solar spectrum and the input optical
concentration (and therefore solar power) was varied over a wide range. For each data point we
took current-voltage (I-V) sweeps in order to quantify the maximum power we could extract from
the single junction cell.
1.6
E
1.4.
1.2
1S
0.8
0.6
0
0
10
5
Input Solar Power (W)
15
Figure 30: Measured photocurrent density as a function of input simulated AM1.5 solar power. The
generated photocurrent should scale linearly with increasing illumination.
62
The photocurrent density scales linearly with increasing solar illumination in this range, since we
are far away from the limit where excited charge carriers have a non-negligible effect on photon
absorption. With a relatively constant fill factor, we observed that the solar-to-electrical efficiency
was relatively constant as a function of this input solar power.
Ne
0
Co0
5
7
9
11
13
15
Input Solar Power (W)
Figure 31: Ratio of maximum power point to input solar power. In this range, the PV cell exhibits a
relatively constant efficiency.
5.2 SOLAR THERMOPHOTOVOLTAIc DEMONSTRATION
Next we ran a full STPV experimental characterization. We fabricated an absorber emitter device
with an area ratio of 12 (ratio of emitter area to absorber area, see chapter 3) that was 4x larger than
the samples that were characterized in our previous demonstrations. Thus the emitter footprint area
was 4 cm 2 , despite only having access to a 1 cm 2 PV cell. In order to use this PV cell to characterize
the STPV device performance, we took great care in making sure that the system energy balance
was not affected by the remaining inactive surface area. This was achieved by bonding dummy cells
to match the emitter area. Furthermore, the sub-bandgap photon reflector (cut to match the emitter
area, 4 cm 2) was bonded above these cells. This bond was achieved using an optically transparent
epoxy (PDMS). Due to the extremely high reflectivity of this material at sub-bandgap wavelengths
63
(less than 2.2 um), we believe our assumption that the inactive PV cell area does not unfairly affect
the energy balance of the emitter.
Next we needed to address the issue of thermal gradients that are developed due to the thermal
spreading resistance as absorbed heat in the carbon nanotube forest conducts throughout the emitter
area. To do this, we took advantage of the symmetry of the situation - that the thermal gradients in
any one of the 4 quadrants shown below should be equivalent and therefore the average
photocurrent density that would be generated in the active PV cell could in fact be generated over
the entire emitter area. Thus we extrapolate the electrical performance from this quadrant to
quantify what power could be extracted from a PV cell that matched the emitter area.
As in our previous experiments, the emitter was held approximately 300 microns apart from the
surface of the PV cell (with a bonded filter on top of it) and placed in an evacuated environment. As
in the PV cell test, we varied the input solar power incident on the absorber and recorded the
electrical characteristics of our active PV cell.
a)
a)
~~Series-connected
InGaAsSb PV Cell (1 uc2)
b)
b)d*
)
Bonded rupate
Interferencefilter (4 cm 2
Figure 32: a) Optical image of the PV cell used in the experiment. b) Optical image of the bonded
sub-bandgap photon reflector above the active and inactive PV cells.
The average photocurrent generated in the PV cell is plotted as a function of input solar power to
the MW-CNT absorber surface. Using the isothermal radiative exchange model that was described
in a previous chapter of this thesis, we see that the generated photocurrent falls short of the
64
predicted model. This is due to the aforementioned thermal gradients associated with the spreading
resistance in our samples.
a)
b)
0.7
*
0.6
Expenrmert
lsotm-l Model
0.5
OA
0.3
&
d
-C-
0.2
0.1
d)
S
C)
0
15
-8
x10
0.7
--
T =1300 K
T=1150K
-- 4T
=1000 K
0.6
NiL
0.5
E3
0.A
IL
5
10
Input Power (W)
0.3
72
02
S1
0.1
40
0
0
1
2
3
4
Wavelength (pm)
800
900
1000
11'00
1200
Emitter Temperature (K)
13 )0
Figure 33: a) Optical image of the experimental setup with exaggerated gap spacing for more clear
view. For scale, outlined PV cell rectangle is 1 cm in length. b) Average generated photocurrent in
the active PV converter as a function of input power. c) Simulated spectral heat flux that is incident
on cell surface, taking into account both spectrally selective emission and transmission by the
photonic crystal and filter, respectively. d) TPV experimental results showing the generated
photocurrent as a function of the emitter temperature for this emitter / filter / PV cell setup.
The simulated spectral heat flux at different emitter temperatures that is incident on the PV surface
is calculated using the emitter optical properties which were measured at 1300 K and the filter
transmission data collected by an FTIR spectrometer. Also shown is a representative concentrated
AM1.5D spectrum. This plot shows the spectral conversion which the absorber / emitter device
provides. The illumination spectrum appears to mirror the solar spectrum but is shifted for a better
match with the low bandgap of our PV cell.
65
5.3
HEAT GENERATION AND THERMAL MANAGEMENT
From a heat generation point of view, these two spectra (solar and modified) would produce a
different heat load on the PV cell as they are converted differently. The bandgap of the PV cell is
almost entirely below (i.e., at higher wavelengths) than the light being received by the solar
spectrum. Therefore, upon illumination by this spectrum, nearly all of the energy will result in a
thermalization process: all excess energy (above the bandgap) turns to heat and this heat must be
removed in order to keep the cell at a constant temperature.
However, when the PV cell is illuminated by lower energy spectra which are better tuned to its
spectral response, the resulting heat generation is substantially reduced. The higher energy photons
from the sun have been down converted and sub-bandgap photon absorption has been almost
entirely suppressed. The resultant thermal load on the PV cell is about 65% lower than from
illumination by the solar spectrum.
To quantify this heat generation reduction, in both our PV and STPV demonstrations we monitored
the amount of heat dissipation required to keep our PV converter at a fixed, equilibrium temperature
using a cooling loop. By measuring the inlet and outlet temperature of the water that passes by the
back of the PV cell surface and setting its flow rate, we determine the heat generated.
We can see that the PV cell thermal load is quite large relative to the input solar power that
impinges the cell. With the addition of the intermediate spectral converter device, we are able to
substantially reduce the thermal load (and heat generation) in the PV converter. At high powers, the
STPV device requires a 65% lower heat dissipation than the concentrated PV case. From a practical
thermal engineering point of view, this is an effective thermal management technique. Heat is
simply dissipated in the hot absorber during the spectral conversion process. We realize, however,
that this reduction in heat load is meaningless unless we are able to maintain comparable conversion
efficiencies between the two devices.
66
10
8
PV
00
S61
o 4v
*0l
12
01
.
0
STPV
10
5
Input Solar Power (W)
15
Figure 34: Heat dissipated in both the PV and STPV experiments in order to keep the PV converter
at an equilibrium temperature. This 65% reduction in thermal load is meaningless unless the device
performances are comparable.
5.4 PERFORMANCE COMPARISON
Recall that the efficiency of the PV cell remains relatively constant over this range of input solar
powers. At low powers that efficiency remains higher than that of the STPV device since the
emitter remains at insufficiently low temperatures. However, as the emitted photon flux becomes
more energetic at higher temperatures, we transition to a regime where the intermediate
absorber/emitter process enhances the overall efficiency of the solar to electrical conversion. The
experimentally demonstrated 50% enhancement in system performance represents the first
successful demonstration of a spectral conversion scheme for solar to electrical conversion.
67
4
L
STPV
JqPV
0
0
(a
5
7
9
11
13
15
Input Solar Power (W)
Figure 35: Converter device performances for both STPV and PV systems. 50% performance
enhancements are observed once the emitter reaches a sufficiently high temperature.
This demonstration of spectral enhancement is approximately independent of the quality of the PV
cell used in the device. Therefore it is reasonable to consider the radiative limit where a defect free
semiconductor device is illuminated by different spectra. If this cell faces the solar spectrum, then
we expect the system to operate at the Shockley-Queisser limit. But if that same idealized cell were
used in our experiments with a transformed radiation spectrum, we see that at sufficiently high
input powers we would have been able to exceed the efficiency limits of the PV cell alone, just as
we experimentally demonstrated. This was simulated using the isothermal radiative model that was
introduced in Chapter 3. Additionally, if we were to consider a device consisting of the same
material set, but scaled up to the point of negligible parasitic heat losses (losses from the supports
and device edges), we observe a further improvement of this converter. This substantial
performance enhancement relative to the Shockley-Queisser limit of the single junction PV cell is
due to the successful spectral down shifting.
68
35
STPV
(Scaled-Up)
,,,00
o
00
130
TPV
Wj
(Lab-Scale)
PV
(SQ-Limit)
02
73 20--
U)
5
10
15
20
25
30
Input Power (W)
Figure 36: Conversion efficiencies of the PV converter as well as the STPV converter in the
radiative limit. Also shown is a prediction of a scaled up device that has negligible parasitic heat
loss.
69
5.5 CONCLUSION
In summary, we made a direct, experimental comparison of PV to STPV conversion. Using an
intermediate absorber/emitter device, we down-shifted the solar spectrum which drastically reduces
the amount of thermalization of hot charge carriers, and by introducing a sub-bandgap photon
reflector, we greatly suppress sub-bandgap photon heat generation processes, thus reducing the load
relative to the solar exposed PV by 65%. The recycling of these photons boosted the conversion
efficiency resulting in a 50% enhancement when compared with the PV cell alone, demonstrating a
successful spectral conversion scheme for the first time ever.
While this demonstration was performed on a low bandgap cell, we believe this enhancement can
be extended to larger bandgap materials especially with the rapid development of refractory
photonic materials with unprecedented control over emission and absorption processes. We look
forward to future successful demonstrations of STPV conversion, each taking steps in the direction
towards high efficiency solar energy conversion.
70
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