The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules

doi:10.4236/sgre.2012.31004

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Smart Grid and Renewable Energy, 2012, 3, 27-33

http://dx.doi.org/10.4236/sgre.2012.31004 Published Online February 2012 (http://www.SciRP.org/journal/sgre)

The Benefits of Integrated Methods in PV Making to

Promote Their Efficiency and Achieve Low-Cost Modules

Salah A. Vaisi1,2

1Department of Architecture and Urban Planning, Faculty of Engineering, University of Kurdistan, Sanandaj, Iran; 2Bokan Center,

University of Applied Science and Technology, Bokan, Iran.

Email: [email protected], [email protected]

Received May 6th, 2011; revised September 24th, 2011; accepted October 3rd, 2011

ABSTRACT

Active systems, such as solar thermal and photovoltaic offer a great potential in reducing of fuel energy consumption.

To improve the sustainability of buildings, one of the challenges is to address the role of renewable energies. Today, the

photovoltaic installations play an important role in creating solar renewable energy. They create 2000 MW electrical

energy per year and its annual global sales grown to approximately 5.6 GWp. This paper presents a general overview on

a serious effort to produce PV panels that could provide cheaper solar power. It also focuses on short background of PV.

Furthermore, thin film technology benefits, the method of the most absorbing of solar spectrum and the method of solar

concentration and the advantages of these systems are presented. Ultimately, a new high concentration PV power sys-

tem will be assessed.

Keywords: Photovoltaic; Renewable Energy; Energy Efficiency; Zero-Energy Building; Building Smart Materials

1. Introduction

Solar energy is free and inexhaustible. This vast, clean

energy resource represents a viable alternative to the fos-

sil fuels that currently pollute our air and water, threaten

our public health, and contribute to global warming.

Failing to take advantage of such a widely available and

low-impact resource would be a grave injustice to our

children and all future generations. In the broadest sense,

solar energy supports all life on Earth and is the basis for

almost every form of energy. The amount of energy from

the sun that falls on Earth’s surface is enormous. All the

energy stored in Earth’s reserves of coal, oil, and natural

gas is matched by the energy from just 20 days of sun-

shine. Outside Earth’s atmosphere, the sun’s energy con-

tains about 1300 watts per square meter. About one third

of this light is reflected back into space, and some is ab-

sorbed by the atmosphere. By the time it reaches Earth’s

surface, the energy in sunlight has fallen to about 1000

watts per square meter at noon on a cloudless day. Aver-

aged over the entire surface of the planet, 24 hours per

day for a year, each square meter collects the approxi-

mate energy equivalent of almost a barrel of oil each year,

or 4.2 kilowatt hour of energy every day. These figures

represent the maximum available solar energy that can be

captured and used, but solar collectors capture only a

portion of this, depending on their efficiency. For exam-

ple, a one square meter solar electric panel with an effi-

ciency of 15 percent would produce about one kilowatt-

hour of electricity per day in Arizona. In other words, if

the efficiency of plants which capture and convert solar

energy into the other forms of energy improves the low-

cost and enough energy for all of the people will be ac-

cessible. By using some integrated methods the effi-

ciency of solar photovoltaic cells could be promoted.

2. Solar Thermal Concentrating Systems

By using mirrors and lenses to concentrate the rays of the

sun, solar thermal systems can produce very high tem-

peratures as high as 3000 degree Celsius. This intense

heat can be used in industrial applications or to produce

electricity. Solar concentrators come in three main de-

signs: parabolic troughs, parabolic dishes, and central

receivers. The most common design is parabolic troughs,

curved mirrors (Figures 1 and 2) that concentrate sun-

light on a liquid inside a tube that runs parallel to the

mirror. The liquid, at about 300 degree Celsius, runs to a

central collector, where it produces steam that drives an

electric turbine or convert to other forms of energy.

Parabolic dish concentrators are similar to trough con-

centrators, but focus the sunlight on a single point. Dish-

es can produce much higher temperatures, and so, in

principle, should produce electricity more efficiently. But

because they are more complicated, they have not suc-

ceeded outside of demonstration projects. A more prom-

The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules

Figure 1. Parabolic trough concentrator.

ising variation of this method is Stirling Engine. Unlike a

car’s internal combustion engine, in which gasoline ex-

ploding inside the engine produces heat that causes the

air inside the engine to expand and push out on the pis-

tons, a Stirling Engine produces heat by way of mirrors

that reflect sunlight on the outside of the engine. These

dish-stirling generators produce about 30 kilowatts of

power, and can be used to replace diesel generators in

remote locations.

The third type of concentrator systems is a central re-

ceiver. One such plant in California features a “power

tower” design in which a 17-acre field of mirrors con-

centrates sunlight on the top of an 80-meter tower. The

intense heat boils water, producing steam that drives a

10-megawatt generator at the base of the tower. The first

version of this facility, Solar One, operated from 1982 to

1988 but had a number of problems. Reconfigured as

Solar Two during the early to mid-1990s, the facility is

successfully demonstrating the ability to collect and store

solar energy efficiently [1]. Solar Two’s success has

opened the door for further development of this technol-

ogy.

To date, the parabolic troughs have had the greatest

commercial success of the three solar concentrator de-

signs, in large part due to the nine Solar Electric Gener-

ating Stations (SEGS) built in California’s Mojave Desert.

Ranging from 14 to 80 megawatts and with a total capac-

ity of 354 megawatts, each of these stations is still oper-

ating effectively [2]. Modified versions of the SEGS

plants are being constructed in Arizona and Nevada. In

addition, Stirling Energy Systems received approval from

the California Public Utility Commission in October

2005 to build a 500-megawatt facility (with the option to

add 350 megawatts) in the Mojave Desert using the pa-

rabolic dish design. Beginning in January 2009, the plant

will supply power to Southern California Edison under a

20-year contract that will help the utility meet its re-

quirements under the state’s renewable electricity stan-

dard [3].

3. An Overview on Photovoltaic

In 1839, French scientist Edmund Becquerel discovered

that certain materials would give off a spark of electricity

when struck with sunlight. This photoelectric effect was

used in primitive solar cells made of selenium in the late

1800s. In the 1950s, scientists at Bell Labs revised the

technology by using silicon produced solar cells that

could convert four percent of the sunlight energy directly

into electricity. Within a few years, these photovoltaic

(PV) cells were powering spaceships and satellites.

The most important components of a PV cell are two

layers of semiconductor material generally composed of

silicon crystals. On its own, crystallized silicon is not a

very good conductor of electricity, but when impurities

are intentionally added—a process called doping—the

stage is set for creating an electric current. The bottom

layer of the PV cell is usually doped with boron, which

bonds with the silicon to facilitate a positive charge (P).

The top layer is doped with phosphorus, which bonds

with the silicon to facilitate a negative charge (N). The

Figure 2. Three commonly used reflecting schemes for concentrating solar energy to attain high temperatures.

The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules 29

Figure 3. Close up of Photovoltaic cell.

surface between the resulting “p-type” and “n-type” semi-

conductors is called the P-N junction (Figure 3). Elec-

tron movement at this surface produces an electrical field

that only allows electrons to flow from the p-type layer

to the n-type layer.

When sunlight enters the cell, its energy knocks elec-

trons loose in both layers. Because of the opposite charges

of the layers, the electrons want to flow from the n-type

layer to the p-type layer, but the electrical field at the P-

N junction prevents this from happening. The presence of

an external circuit, however, provides the necessary path

for electrons in the n-type layer to travel to the p-type

layer. Extremely thin wires running along the top of the

n-type layer provide this external circuit, and the elec-

trons flowing through this circuit provide the cell’s own-

er with a supply of electricity.

Most PV systems consist of individual square cells

averaging about four inches on a side. Alone, each cell

generates very little power (less than two watts), so they

are often grouped together as modules. Modules can then

be grouped into larger panels encased in glass or plastic

to provide protection from the weather, and these panels,

in turn, are either used as separate units or grouped into

even larger arrays. The three basic types of solar cells

made from silicon are single-crystal, polycrystalline, and

amorphous which are described in the following:

· Single-crystal cells are made in long cylinders and

sliced into round or hexagonal wafers. While this

process is energy-intensive and wasteful of materials,

it produces the highest-efficiency cells—as high as 25

percent in some laboratory tests. Because these high-

efficiency cells are more expensive, they are some-

times used in combination with concentrators such as

mirrors or lenses. Concentrating systems can boost ef-

The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules

ficiency to almost 40 percent. Single-crystal accounts

for 29 percent of the global market for PV [4].

· Polycrystalline cells are made of molten silicon cast

into ingots or drawn into sheets, then sliced into

squares. While production costs are lower, the effi-

ciency of the cells is lower too—around 15 percent.

Because the cells are square, they can be packed more

closely together. Polycrystalline cells make up 62

percent of the global PV market.

· Amorphous silicon (a-Si) is a radically different ap-

proach. Silicon is essentially sprayed onto a glass or

metal surface in thin films, making the whole module

in one step. This approach is by far the least expen-

sive, but it results in very low efficiencies—only

about five percent. Of course, to day the efficiency of

this method is improved.

A number of exotic materials rather than silicon are

under development, such as gallium arsenide (Ga-As),

copper-indium-diselenide (CuInSe2), and cadmium-tel-

luride (CdTe). These materials offer higher efficiencies

and other interesting properties, including the ability to

manufacture amorphous cells that are sensitive to differ-

ent parts of the light spectrum. By stacking cells into

multiple layers, they can capture more of the available

light. Although a-Si accounts for only five percent of the

global market, it appears to be the most promising for

future cost reductions and growth potential. In the 1970s,

a serious effort began to produce PV panels that could

provide cheaper solar power. Experimenting with new

materials and production techniques, solar manufacturers

cut costs for solar cells rapidly, as the following graph

(Figure 4) shows. On the other hand, the number of di-

rect and indirect jobs has been increased dramatically in

recent years (Figure 5).

One approach to lowering the cost of solar electric

power is to increase the efficiency of cells, producing

more power per dollar. The opposite approach is to de-

crease production costs, using fewer dollars to produce

the same amount of power. A third approach is lowering

the costs of the rest of the system. For example, building-

Figure 4. PV manufacturing cut cost.

Figure 5. PV job creating.

integrated PV (BIPV) integrates solar panels into a build-

ing’s structure and earns the developer a credit for re-

duced construction costs. Innovative processes and de-

signs are continually reaching the market and helping

drive down costs, including string ribbon cell production,

photovoltaic roof tiles, and windows with a translucent

film of a-Si. Economies of scale from a booming global

PV market are also helping to reduce costs. More re-

cently, thanks to lower costs, strong incentives, and net

metering policies, the PV industry has placed more focus

on home, business, and utility-scale systems that are at-

tached to the power grid. In some locations, it is less ex-

pensive for utilities to install solar panels than to upgrade

the transmission and distribution system to meet new

electricity demand. In 2005, for the first time ever, the

installation of PV systems connected to the electric grid

outpaced off-grid PV systems in the United States [5]. As

the PV market continues to expand, the trend toward

grid-connected applications will be continuing. This dis-

tributed-generation approach provides a new model for

the utilities of the future. Small generators, spread through-

out a city and controlled by computers, could replace the

large oil and nuclear plants that dominate the landscape

now. Solar energy technologies are poised for significant

growth in the 21st century. More and more architects and

contractors are recognizing the value of passive and ac-

tive solar and learning how to effectively incorporate it

into building designs. Solar hot water systems can com-

pete economically with conventional systems in some

areas. And as the cost of solar PV continues to decline,

these systems will penetrate increasingly larger markets.

In fact, the solar PV industry aims to provide half of all

new U.S. electricity generation by 2025 [6].

4. Three Methods Mostly Used to Pr o moted

4.1. Advanced Thin Film Technologies for Cost

Effective Photovoltaics

The overall challenge is to provide the scientific and

The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules

technological basis for industrial mass production of

cost-effective and highly efficient, environmentally sound

and economically compliant large-area thin film solar

cells and modules. By drawing on a broad basis of ex-

perience, the entire range of module fabrication and sup-

porting will be covered: substrates, semiconductor and

contact deposition, monolithic series interconnection,

encapsulation, performance evaluation and applications

by different factories. Photovoltaics have become an in-

creasingly important industrial sector over the past ten

years [7]. Therefore the main challenges are:

· Significantly reducing the cost/efficiency ratio to-

wards €0.5/WP in the long run.

· Providing the know-how and the scientific basis for

large-area PV modules by identifying and testing new

materials and technologies with maximum cost re-

duction.

· Developing the process know-how and the production

technology, as well as the design and fabrication of

specialized equipment, resulting in low costs and high

yield in the production of large area thin film mod-

ules.

To meet these challenges, existing concepts for mate-

rials and technology will be improved and brought to

maturity in close cooperation with industry, and new

options will be investigated for materials and new types

of solar cells to provide the scientific and technological

basis for the next generation of PV devices. Accordingly,

the research activities range from basic research to in-

dustrial implementation [7].

The state-of-the-art for advanced thin film PV tech-

nology and the enhancement within the proposed project

is summarized in Table 1.

Prototypes of a small junction box especially suited for

thin-film modules were developed. Limiting the by-pass

diodes to only one per box allows a reduction in both size

and cost. It is also possible to use the box for parallel

inter-connection of the modules.

4.2. Towards the Production of Cost-Competitive

Photovoltaic Solar Energy by Absorbing the

Most of the Solar Spectrum

Solar radiation is a significant energy source: only appr-

oximately 1000 Joules of energy per second per square

meter are accessible. It is clear to us that strategies to

reach the ultimate goal of a module cost of €1/Wp will

necessarily have to go through the development of con-

cepts capable of extracting the most of every single pho-

ton available. In this respect, each of the three activities

envisaged in this paper to achieve the general goal has to

confront its own challenges. The multi-junction activity

pursues the development of solar cells that approach 40%

efficiency. To achieve this, it faces the challenge of find-

ing materials with a good compromise between lattice

matching and band-gap energy. The thermo photovoltaic

activity bases part of its success on finding suitable emit-

ters that can operate at high temperatures and/or adapt

their emission spectra to the cell’s gap. The other part

relies on the successful recycling of photons so that those

that cannot be used effectively by the solar cells can re-

turn to the emitter to assist in keeping it hot. The inter-

mediate-band solar cell approach addresses the challenge

of proving a principle of operation which would see a

Table 1. Expected enhancement of the state-of-the-art.

Technology State-of-the-art Substrate, process (efficiencies) Planned enhancement in IP (for Europe)

Lab cells

a-Si/μc-Si 12% (Kaneka)

11% (UniNE, FZJ

On glass, PE-CVD 14%

Poly Si 9% (Sanyo) On metal substrate, SPC 15% on foreign substrates

CIGS low gap 19.2% (NREL)

16% - 17% (NREL)

On glass, co-evaporation

On metal foil, co evaporation

18% on metal foil

9% on polyimide foil

CIGS wide gap 12% - 13% (HMI) On glass, sputtering, PVD 13% - 14%, advanced equipment. 10% @ 60% IR transparency for

tandem applications

CIGS tandem 7% (HMI) On glass, co-evaporation 15%

Prototypes, pilot production

a-Si/μc-Si 10% (Kaneka, FZJ) On glass 30 × 30 cm2 (FZJ)

On glass 3738 cm2 (Kaneka)

Equipment for cost-effective production of 10% modules (1 m2 @

costs towards €0.5/Wp)

CIGS wide gap 10% (Sulfurcell) On glass 5 × 5 cm2, sputtering, PVD10% on 125 × 65 cm2

Commercial product

a-Si 6% - 7% (Unisolar,

SCHOTT, Kaneka, ...)

On glass, PE-CVD

CIGS low gap 10% (Shell, Würth) On glass, co-evaporation 11% - 12%, cost-effectiveness, environmentally sound

The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules

significant improvement in the performance of the cells.

In a Project which coordinated by Prof. Antonio Luque

et al. the activity devoted to the search for new molecules

engenders the challenge of identifying molecules capable

of undergoing two-photon processes: that is molecules

that can absorb two low-energy photons to produced a

high-energy excited state or, for example, dyes that can

absorb one high-energy photon and re-emit its energy in

the form of two photons of lower energy. Among all of

the above concepts, the multi-junction approach appears

to be the most readily available for commercialization.

For that, the activity devoted specifically to speeding up

its path to market is the development of trackers, optics

and manufacturing techniques that can integrate these

cells into commercial concentrator systems1.

The multi-junction solar cell approach pursues the

better use of the solar spectrum by using a stack of sin-

gle-gap solar cells incorporated in a concentrator system,

in order to make the approach cost-effective (Figure 6).

The project, at its outset, aimed at cells with an effi-

ciency of 35%. This result has already been achieved by

FhG-ISE in the second year of their project and their

consortium now aims to achieve efficiencies as close as

possible to 40%. In the thermo-photovoltaic approach the

sun heats up, through a concentrator system, a material

called the ‘emitter’, leading to incandescence (Figure 7).

The radiation from this emitter drives an array of solar

cells, thus producing electricity. The advantage of this

approach is that, by an appropriate system of filters and

back-reflectors, photons with energy above and below

the solar cell band-gap can be directed back to the emit-

ter, helping to keep it hot by recycling the energy of

Figure 6. Schematic illustrating the operation of a multi-

junction solar cell in a concentrator system.

Figure 7. Emitter heated up by the sun through a concen-

trator system.

these photons that otherwise would not be converted op-

timally by the solar cells. By the conclusion, it is ex-

pected that the system, made up basically of the concen-

trator, emitter and solar cell array can be integrated and

evaluated. The “intermediate-band” approach pursues

better exploitation of the solar spectrum by using inter-

mediate-band materials. These materials are character-

ized by the existence of an electronic energy band within

what otherwise would be a conventional semiconductor

band-gap. According to the principles of operation of this

cell, the intermediate band allows the absorption of low

band-gap energy photons and the sub sequent production

of enhanced photocurrent without voltage degradation.

This method also expects to identify as many intermedi-

ate-band material candidates as possible, as well as de-

monstrate experimentally the operating principles of the

intermediate-band solar cell by using quantum dot solar

cells as workbenches.

4.3. High Concentration PV Power System

(HICONPV)

Existing and innovative solar concentrators were evalu-

ated for their properties in high-concentration photovol-

taics. Plant types were identified that fulfill the technical

requirements of homogenous irradiation distribution with

solar concentration factors of 500 to 2000 suns and cost-

effective implementation perspectives. The conclusions

were that Modified Spherical Dish (Tailored Concentra-

tor, Figure 8) configurations look more suitable for

meeting current technology requirements than classical

Parabolic Dish solutions. The results shown with this

design are promising. It has been proposed to build and

test a tailored concentrator for HICONPV technology

with this design.

1The Project is coordinated by Professor Antonio Luque Instituto de

Energía Solar assisted by Projektgesellschaft Solare Energiesysteme

GmbH (PSE).

The Benefits of Integrated Methods in PV Making to Promote Their Efficiency and Achieve Low-Cost Modules 33

Figure 8. Tailored concentrator.

Figure 9. Layout of a solar concentrator.

An innovative heliostat variant was evaluated for its

properties in high-concentration photovoltaics, demon-

strating that the proposed Torque Tube Heliostat design

concept promises significant cost advantages over exist-

ing heliostat designs. This can be achieved with a much

lower construction height of the TTH, which reduces

drastically the wind loads on the structure and the re-

quired specific drive power. The aim of this tailor con-

centrator is to prove the real possibilities of this innova-

tive conceptual design, and to see the performance of the

concept under real manufacturing constraints. The pro-

posed final configuration was not optimized for 1000x

(X equals approximately the area of the optic collector

divided by area of the semiconductor solar cell. Zenith-

Solar system is designed to a concentration factor of

1000X), but rather close, so it is necessary to take into

account the optimized structural heliostat concept, where

the shape of the concentrator is no longer round but rec-

tangular. Rectangular concentrators allow us to keep the

gravity centre lower for the same aperture area (Figure

9). This has a strong influence on the structural design

and the final cost2. In conventional CPV systems, the

excess heat generated in the solar cell needs to be re-

moved to avoid damaging the cell and to maintain high

efficiency of electricity conversion. ZenithSolar utilizes

the heat generated at the solar cell receiver to provide

usable hot water heating, improving overall solar power

conversion efficiency to 75%.

5. Expected Results

The concept of this research project focuses specially on:

· New monolithic integrated modules with efficiencies

of 20% and above.

· Module design for irradiation up to 1000 suns.

· Adaptation of already proven concentrator’s concepts

that promise high quality and high reliability.

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2006.

http://www1.eere.energy.gov/solar/pdfs/set_myp_2007-2

011_proof 1.pdf

[3] Renewable Energy Access, “World’s Largest Solar Pro-

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http://www.renewableenergyaccess.com/rea/news/story?i

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[4] Solar Energy Technologies Program, “Solar Energy Tech-

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http://www1.eere.energy.gov/solar/pdfs/set_myp_2007-2

011_proof 1.pdf

[5] The Prometheus Institute, “U.S. Market Analysis,” PV

News, Vol. 25, No. 5, 2006, pp. 4-5.

[6] Solar Energy Industries Association, “Our Solar Power

Future: The US Photovoltaics Industry Roadmap through

2030 and beyond,” 2004. http://www.seia.org

[7] Renewable Energy Technologies, “Long Term Research

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2RESEARCH TRIANGLE PARK, N. C., DuPont Microcircuit Materi-

als (DuPont) today announced the introduction of DuPont™ Solamet®

PV412 photovoltaic (PV) metallization paste, Raleigh, 2009.