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The solar energy flux reaching the Earth's surface represents a few thousand times the current use of primary energy by humans. The potential of this resource is enormous and makes solar energy a crucial component of a renewable energy portfolio aimed at reducing the global emissions of greenhouse gasses into the atmosphere. Nevertheless, the current use of this energy resource represents less than 1% of the total electricity production from renewable sources. Even though the deployment of photovoltaic systems has been increasing steadily for the last 20 years, solar technologies still suffer from some drawbacks that make them poorly competitive on an energy market dominated by fossil fuels: high capital cost, modest conversion efficiency, and intermittency. From a scientific and technical viewpoint, the development of new technologies with higher conversion efficiencies and low production costs is a key requirement for enabling the deployment of solar energy at a large scale. This report summarizes the state of the research in some mature and emerging solar technologies with high potential for large- scale energy production, and identifies fundamental research topics that are crucial for improving their performance, reliability, and competitiveness.
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STANFORD UNIVERSITY
Global Climate & Energy Project
Technical Assessment Report
An Assessment of Solar Energy Conversion
Technologies and Research Opportunities
GCEP Energy Assessment Analysis
Summer 2006
Abstract
The solar energy flux reaching the Earth’s surface represents a few thousand times the
current use of primary energy by humans. The potential of this resource is enormous and
makes solar energy a crucial component of a renewable energy portfolio aimed at
reducing the global emissions of greenhouse gasses into the atmosphere. Nevertheless,
the current use of this energy resource represents less than 1% of the total electricity
production from renewable sources. Even though the deployment of photovoltaic systems
has been increasing steadily for the last 20 years, solar technologies still suffer from some
drawbacks that make them poorly competitive on an energy market dominated by fossil
fuels: high capital cost, modest conversion efficiency, and intermittency. From a
scientific and technical viewpoint, the development of new technologies with higher
conversion efficiencies and low production costs is a key requirement for enabling the
deployment of solar energy at a large scale. This report summarizes the state of the
research in some mature and emerging solar technologies with high potential for large-
scale energy production, and identifies fundamental research topics that are crucial for
improving their performance, reliability, and competitiveness.
Issued by the Global Climate and Energy Project
http://gcep.stanford.edu




Table of Contents


Foreword....................................................................................................................... 3
Introduction .................................................................................................................. 4
Solar Radiation .................................................................................................................................... 5
Potential of Solar Energy..................................................................................................................... 6
Deployment ..................................................................................................................................... 6
Cost of electricity ............................................................................................................................ 8
Efficiency ........................................................................................................................................ 9
Environmental Aspects of Solar Energy ............................................................................................ 10
Energy payback............................................................................................................................. 11
Carbon payback............................................................................................................................. 12
Safety and environmental issues ................................................................................................... 12
Solar technologies overview ...................................................................................... 13
Photon-to-Electric Energy Conversion.............................................................................................. 14
Photon absorption and carrier generation...................................................................................... 16
Charge transfer and separation ...................................................................................................... 20
Charge transport ............................................................................................................................ 22
Overall efficiency, stability and other technological challenges ................................................... 24
Photon-to-Thermal-to-Electric Energy Conversion........................................................................... 26
Parabolic troughs........................................................................................................................... 27
Power towers................................................................................................................................. 28
Heat transfer fluids (HTF) and thermal storage............................................................................. 29
Dish-engine systems...................................................................................................................... 30
Solar chimneys .............................................................................................................................. 32
Photon-to-Chemical Energy Conversion ........................................................................................... 32
Photo(electro)chemical water splitting.......................................................................................... 33
Thermal and thermochemical processes........................................................................................ 34
Biological systems ........................................................................................................................ 35
Conclusion................................................................................................................... 36
Appendix ..................................................................................................................... 38
Index ............................................................................................................................ 39
References ................................................................................................................... 40



GCEP Solar Energy Technology Assessment - Summer 2006

2




Foreword
This report is one of a series of assessments on various areas of the energy landscape
prepared by GCEP staff. The assessments are intended to provide an introduction to the
energy area as well as context for future fundamental research activity towards reducing
greenhouse gas emissions. By examining the goals and potential of the energy
transformations in question as well as the current progress and research towards these
ends, the assessments take a step toward elucidating the most promising areas for future
research. This report, produced by GCEP Energy Analysis staff, was written by Paolo
Bosshard with contributions from Wes Hermann, Emilie Hung, Rebecca Hunt, and AJ
Simon. GCEP is also grateful to Professor Martin Green, UNSW, Australia, for his
suggestions and comments about the sections on photovoltaics. Please address all
correspondence to gcep@stanford.edu.

GCEP Solar Energy Technology Assessment - Summer 2006

3




Introduction
Solar radiation represents the largest energy flow entering the terrestrial ecosystem.
After reflection and absorption in the atmosphere, some 100,000TW hit the surface of
Earth and undergo conversion to all forms of energy used by humans, with the exception
of nuclear, geothermal, and tidal energy. This resource is enormous and corresponds to
almost 6,000 fold the current global consumption of primary energy (13.7TW [1]). Thus,
solar energy has the potential of becoming a major component of a sustainable energy
portfolio with constrained greenhouse gas emissions.

Solar radiation is a renewable energy resource that has been used by humanity in all
ages. Passive solar technologies were already used by ancient civilizations for warming
and/or cooling habitations and for water heating; in the Renaissance, concentration of
solar radiation was extensively studied and in the 19th century the first solar-based
mechanical engines were built [2]. The discovery of photovoltaic effect by Becquerel in
1839 and the creation of the first photovoltaic cell in the early 1950s opened entirely new
perspectives on the use of solar energy for the production of electricity. Since then, the
evolution of solar technologies continues at an unprecedented rate. Nowadays, there exist
an extremely large variety of solar technologies, and photovoltaics have been gaining an
increasing market share for the last 20 years. Nevertheless, global generation of solar
electricity is still small compared to the potential of this resource [3]. The current cost of
solar technologies and their intermittent nature make them hardly competitive on an
energy market still dominated by cheap fossil fuels. From a scientific and technological
viewpoint, the great challenge is finding new solutions for solar energy systems to
become less capital intensive and more efficient. Many research efforts are addressing
these problems. Low-cost and/or high-efficiency photovoltaic device concepts are being
developed. Solar thermal technologies are reaching a mature stage of development and
have the potential of becoming competitive for large energy supply. Intermittency is
being addressed with extended research efforts in energy storage devices, such as
batteries and other electric storage systems, thermal storage, and the direct production of
solar fuels (typically hydrogen). All these are valuable routes for enhancing the
competitiveness and performance of solar technologies.

The aim of this report is to evaluate the potential of solar energy for low-carbon
intensive and large-scale energy production and to provide a picture of the state of
research in the most significant solar technologies. More than a comprehensive review,
this document is intended to be an attempt at identifying interdisciplinary and
fundamental research topics with high breakthrough potential for the improvement of the
performance, the reliability, and the competitiveness of solar technologies. For this
reason this analysis is a bottom-up approach; solar technologies are organized by energy
conversion paths and the discussion focuses, when possible, on the fundamental
processes and the related technical challenges. Also, the cited references are meant to
indicate the state-of-the-art and not to be a comprehensive snapshot of the ongoing
research. Where possible, reviews were referenced that will provide the reader with more
detail.

GCEP Solar Energy Technology Assessment - Summer 2006

4




Solar Radiation
Solar radiation is an electromagnetic wave emitted by the Sun’s surface that originates
in the bulk of the Sun where fusion reactions convert hydrogen atoms into helium. Every
second 3.89.1026J of nuclear energy is released by the Sun’s core [4]. This nuclear energy
flux is rapidly converted into thermal energy and transported toward the surface of the
star where it is released in the form of electromagnetic radiation. The power density
emitted by the Sun is of the order of 64MW/m2 of which ~1370W/m2 reach the top of the
Earth’s atmosphere with no significant absorption in the space. The latter quantity is
called the solar constant.

The spectral range of the solar
radiation is very large and
encompasses nanometric wavelengths
of gamma- and x-rays through metric
wavelengths of radio waves. The
energy flux is divided unevenly
among the three large spectral
categories. Ultraviolet (UV) radiation
(λ<400nm) accounts for less than 9%
of the total; visible light (VIS)
(400nm<λ<700nm) for 39%; and
infrared (IR) for about 52%.

As shown in Fig. 1, the pattern of
the solar spectrum resembles closely
the radiation of a perfect black body
at 5800K. In the figure, AM0
indicates the Air Mass Zero reference
spectrum measured – and partially
Fig. 1. Extraterrestrial (AM0) and ground-level
modeled – outside the terrestrial
(AM1.5) spectra of the solar radiation [5].
atmosphere [5]. Radiation reaching
The dashed line represents the emission
spectrum of a black body at 5800K.

the Earth’s surface is altered by a
number of factors, namely the
inclination of the Earth’s axis and the atmosphere that causes both absorption and
reflection (albedo) of part of the incoming radiation. The influence of all these elements
on solar radiation is visible in the ground-level spectrum, labeled AM1.51 in Fig. 1, where
the light absorption by the molecular elements of the atmosphere is particularly evident.
Accounting for absorption by the atmosphere, reflection from cloud tops, oceans, and
terrestrial surfaces, and rotation of the Earth (day/night cycles), the annual mean of the
solar radiation reaching the surface is 170W/m2 for the oceans and 180W/m2 for the
continents2 [4]. Of this, about 75% is direct light, the balance of which is scattered by air
molecules, water vapor, aerosols, and clouds.

1 AM1.5 is the reference spectrum measured at a solar zenith angle of 48.19°
2 U.S. average is 205W/m2
GCEP Solar Energy Technology Assessment - Summer 2006

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Fig. 2. Solar radiation exergy flow diagram (units in TW) [6]. Shaded surfaces represent natural
exergy destruction; arrows represent human use for energy services.

The diagram in Fig. 2 illustrates the flow of the work potential, or exergy, of the solar
energy into the atmosphere and the terrestrial ecosystem. This quantity represents the
upper limit to the work obtainable from solar radiation conversion, a limit that is imposed
by the 2nd law of thermodynamics and is independent of any conceptual device.

Of the 162PW of solar radiation reaching the Earth, 86PW hit its surface in the form
of direct (75%) and diffused light (25%). The energy quality of diffused radiation is
lower (75.2% of exergy content instead of 93.2% for direct light [7]), with consequences
on the amount of work that can be extracted from it. 38PW hit the continents and a total
exergy of 0.01TW is estimated to be destroyed during the collection and use of solar
radiation for energy services. This estimation includes the use of photovoltaics and solar
thermal plants for the production of electricity and hot water. Similar estimates are shown
for wind energy (0.06TW), ocean thermal gradient (not yet exploited for energy
production), and hydroelectric energy (0.36TW) [6].

Potential of Solar Energy
Deployment
The global solar energy potential ranges from 2.5 to 80TW (see Appendix). The
lowest estimate represents around 18% of the total current primary energy consumption
(13.7TW [1]), and exceeds 10% of the estimated primary energy demand by 2030
(21.84TW [1]). More optimistic assumptions give a potential for solar energy exceeding
5 fold the current global energy consumption.

Despite the relatively low power density of the solar flux, solar energy has the
potential of supplying a non-negligible fraction of our energy needs. In the case of the US
for example, the total electricity demand (418GW in 2002) could be satisfied by covering
a land surface of 180km square with photovoltaics. This surface represents 0.35% of the
total land area and roughly corresponds to the surface covered by roads in the country
(3.6.1010m2 [8]). All US electricity could hence be potentially produced by covering the
paved roads with photovoltaic (PV) modules. Of course this cannot be applied to all
GCEP Solar Energy Technology Assessment - Summer 2006

6




countries, where the required land fraction can be more important (e.g. 24% for Belgium
[9]), with subsequent large social and environmental impacts.

The market share of solar energy is still low. Current electricity generation from PVs
is only of the order of 2.6GW3 compared to 36.3GW for all renewable energies,
hydroelectric power excluded [1,10]. Developed countries are steadily increasing their
investments in solar power plants, and IEA projections for 2030 give an enhancement of
solar electricity generation up to 13.6GW (80% of which will be from photovoltaics, and
the rest (2.4GW) from solar thermal plants). However, this amount will not exceed 6% of
the total electricity production from non-hydro renewable energies (see Fig. 3). It is
worth noting that passive solar technologies for water heating, not included in these
statistics, represent a fairly large amount of power. IEA estimates a power production of
5.3GW in 2002 and an increase up to 46GW by 2030 [1].

2002
2030
36.28GW
214.12GW
Fig. 3. Current and projected non-hydro renewable electricity production for 2030 (in GW) [1].

The major causes of the slow deployment of solar technologies are:

• The current relative high capital cost per kW installed compared with other fossil
fuel based and renewable technologies;
• The intermittent nature of the energy input, and hence the requirement for energy
storage systems to match the energy supply with the electricity demand and to
decrease the capital cost. In a medium term, energy storage will be a key
requirement for intermittent renewable energies to become more competitive
versus fossil fuels. This report is not intended to analyze this issue in more detail,
but the assessment of energy storage technologies is the object of a separate
GCEP report [11].


3 Sandia published an exhaustive list of solar power plants (PV and/or solar thermal) with output power
larger than 100kW installed worldwide in 2004
(http://www.sandia.gov/pv/docs/PDF/Solar%20Power%20Plants%20Worldwide.pdf).
GCEP Solar Energy Technology Assessment - Summer 2006

7




If we want solar energy to significantly contribute to the world’s energy supply,
massive increases in manufacturing capacity are needed. From the research standpoint,
more effort has to be put into improving efficiencies while reducing the manufacturing
costs. This is a great technological challenge that requires investment of larger financial
and intellectual resources to find innovative solutions.

Cost of electricity
The current higher capital cost of PV technologies compared to fossil fuels is a major
barrier to large-scale deployment of solar energy. Today’s price of electricity from solar
energy4, as reported by the IEA, ranges from $0.35/kWh to $0.60/kWh for solar PV and
from $0.085/kWh to $0.135/kWh for solar thermal, compared to $0.045/kWh -
$0.055/kWh for wind and $0.040/kWh for natural gas [1]. This large range of cost of
solar energy is due to differences in the local insolation and to the estimation of the
Balance Of System (BOS) cost relative to specific applications (namely stand-alone or
grid-connected, ground-mounted or rooftop systems).

The price of the active material, the manufacturing, and the BOS components are the
main elements determining the total price of PV technologies. Since the 1970’s research
has been exploring new processes for producing low-cost wafer silicon (both single-
crystal silicon, sc-Si, and polycrystalline silicon, pc-Si), and the use of low-cost materials
for thin-film PV applications, such as amorphous silicon (α-Si), III-V compounds (e.g.
GaAs, InP), CIGS (Cu[In1-xGax][Se1-ySy]2), cadmium telluride (CdTe), and more recently
organic materials. The efficiency of thin-film laboratory cells has increased steadily in
the last 20 years (see Fig. 4) and these technologies are believed to have the potential of
bringing down the cost of solar energy to $0.03/kWh - $0.05/kWh [12].

Manufacturing scale is another key requirement for decreasing the cost of solar
technologies since large-scale production lowers the cost of active materials and
production. This is demonstrated by the price drop that silicon PV modules (sc-Si, pc-Si,
α-Si) experienced since the early 1980’s following a progress ratio5 of ~77% with
cumulative production growing from 10MW to more than 1GW in 2000 [13]. If the trend
continues, the price of $1/W (~ $0.06/kWh) will be reached when the cumulative
production reaches 100GW [14], which in return will push further the deployment of
solar energy systems.

Some studies (see for example [15]) suggest that this price can be reached entirely
through manufacturing scale, without the need for any significant new invention.
However, more research and development will increase competitiveness of solar
technologies. Design and technological innovations could also decrease the cost of BOS
components, and in particular of energy storage systems that represent a major fraction of

4 Cost of solar electricity is expressed either in $/W (dollars per Watt-peak or Watt installed) or in $/kWh.
The translation from one unit to the other is straightforward and depends on local insolation and on the
cost of money. In the case of the US, $1/W corresponds approximately to $0.06/kWh.
5 Given a progress ratio PR, PV modules’ price decreases by 1-PR for each doubling of cumulative power;
1-PR is called the learning factor
GCEP Solar Energy Technology Assessment - Summer 2006

8




the total installation cost of systems where storage is required (up to 70-80% with
batteries accounting for 30-40% [16]). Such research efforts exist in the field of thermal
storage associated with solar thermal technologies (in particular central receivers and
parabolic troughs) and on new PV technologies with built-in storage systems, such as the
dye-sensitized-cell based photocapacitors developed at Toin University in Yokohama
[17]. Additionally, cell stability and low-cost encapsulation processes have also to be
improved to maximize the lifetime of PV panels, with an incisive impact on system cost.

Organic-based photovoltaics (OPVs) are an alternative to present-day p-n junction
photovoltaic devices for reducing the cost of solar energy. They can be deposited on
lightweight, flexible and low-cost plastic substrates and thus have the potential to drop
the manufacturing and installation cost by 10 to 20 fold.

The manufacturing cost for OPVs can be very low using large-throughput roll-to-roll
manufacturing technology enabled with printable semiconductors and low-cost materials
such as plastic substrates and polymer alternatives to Transparent Conducting Oxide
(TCO) electrodes. Concerning BOS costs, packaging will remain a major concern due to
the sensitivity of organic materials to oxygen and water vapor. Cost projections for
electricity from organic photovoltaics based on the use of printing techniques and
decrease of material cost with scaleup, are significantly below $1/W [18].
Efficiency
Fig. 4. Progress in photovoltaic cell efficiencies [19].

Over the past 30 years, solar cell efficiencies have continuously improved for all
technologies. Among the most important accomplishments to be noted ([20] – see below
GCEP Solar Energy Technology Assessment - Summer 2006

9




for more detail about the specific technologies) are the 24.7%-efficient c-Si solar cell
(University of South Wales, Australia), the 18.4%-efficient CIGS solar cell (NREL), the
16.5%-efficient CdTe solar cell (NREL), and the 39%-efficient GaInP/GaAs/Ge triple-
junction solar cell under 241-suns concentration (Spectrolab) [21]. Research on dye-
sensitized solar cells (DSSCs) and organic solar cells (OSCs) began only during the last
decade. The last reported record efficiencies are 10.4% for DSSCs (Ecole Polytechnique
Fédérale de Lausanne, Switzerland) [22], and 5.7% for OSCs (Princeton University) [23].

Despite the notable progress made in the improvement of the efficiencies of all these
technologies, achieved values are still far from the thermodynamic efficiency limits of
~31% for single junctions6, 50% for 3-cell stacks, impurity PVs, or up- and down-
converters, and 54-68% for hot carrier- or impact ionization-based devices [24].
Furthermore, the efficiencies of commercial (or even the best prototype) modules are
only about 50% to 65% of these “champion” cells [20]. Closing these gaps is the subject
of ongoing research.

The solar-to-electric efficiency of solar thermal technologies varies largely depending
upon the solar flux concentration factor, the temperature of the thermal intermediary, and
the efficiency of the thermal cycle for the production of mechanical work and electricity.
Parabolic troughs and power towers reach peak efficiencies of about 20%. Dish-Stirling
systems are the most efficient, with ~30% solar-to-electric demonstrated efficiency. The
performance of these systems is highly influenced by the plant availability. In the case of
parabolic troughs and power towers, thermal storage increases the annual
7
capacity factor
from typically 20% to 50% and 75%, respectively.

Environmental Aspects of Solar Energy
Solar energy is promoted as a sustainable energy supply technology because of the
renewable nature of solar radiation and the ability of solar energy conversion systems to
generate greenhouse gas-free electricity during their lifetime. However, the energy
requirement and the environmental impact of PV module manufacture can be further
reduced, even though recent analysis of the energy and carbon cycles for PV technologies
recognized that strong improvements were made both in terms of energy and carbon
paybacks.

6 Estimation by Shockley and Quiesser for a bandgap of 1.1eV [W. Shockley, H.J. Queisser, “Detailed
balance limit of efficiency of p-n junction solar cells”, J. Appl. Phys., 32(3), 1961, p. 510]; their approach
can be used to estimate the efficiency limit for single-junction organic photovoltaics if the bandgap
energy is replaced by the exciton energy [B.A. Gregg, “The photoconversion mechanisms of exciton solar
cells”, MRS Bulletin, 30, 2005, p. 20]; calculations by Peumans and Forrest give a ~20%-efficiency limit
([P. Peumans, S.R. Forrest, “Separation of geminate charge-pairs at donor–acceptor interfaces in
disordered solids ”, Chem. Phys. Lett., 398(1), 2004, p. 27],[S.R. Forrest, “The limits to organic
photovoltaic efficiency”, MRS Bulletin, 30, 2005, p. 28])
7 Annual energy output divided by the theoretical maximum output, if the plant were running at its rated
(maximum) power during all of the 8766 hours of the year.
GCEP Solar Energy Technology Assessment - Summer 2006

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