Chapter 4. Technical Characteristics of Renewable Energy by Source
4.1 Status of RE Technology by Source
4.1.1 Solar PV
4.1.1.1 Overview of Solar PV Power Generation
The solar PV system, a power generation technology that converts sunlight directly into electricity, consists of the solar cell module (an array of multiple modules) equivalent to a generator, the capacitor for storing electric energy, the power conditioning system (PCS) that converts the direct current from a photovoltaic array to an alternating current, the system control and monitoring, and the load.
[Figure 4-1] Solar PV power generation system and components
Source: KEA (2016), Renewable Energy White Paper
The solar cell is the critical component of the solar PV system. A photovoltaic cell is a semiconductor device that converts light into electricity. A single solar cell, which is the smallest unit of a solar panel, yields an extremely low voltage of approximately 0.5~0.6 V. Therefore, several cells are connected in series
4. Technical Characteristics of Renewable
Energy by Source
in the form of a panel (the photovoltaic module) to create additional voltages of a few volts to several tens of volts or higher. These modules are inter-connected in series-parallel with the desired loading capacity. Such a combination is called the solar array and can be considered equivalent to the generator of the rotary power-generation system. The PCS/inverter, which is a power converter, acts as a transmitter by converting the direct current generated from a solar array into an alternating current of power frequency and voltage, and subsequently connecting the resulting current to power grids. The PCS/inverter provides electrical monitoring and protection of the direct and alternating currents of the system.
[Figure 4-2] Basic structure and working mechanism of solar cell
Source: KEA (2016a), Renewable Energy White Paper
[Fig. 4-2] illustrates the basic structure of a solar cell and the production process of electricity. Solar cells can be fabricated by combining p-type and n-type semiconductors (p-n junction) and applying metal electrodes to the front and back surfaces. When light is absorbed from a semiconductor, electron-hole pair is formed, and the electrons and holes flow in opposite directions due to the electric field present around the p-n junction. As a result, electricity is generated in the external circuit connected with lead wires.
4.1.1.2 Trends in Domestic and International Technology Development
[Figure 4-3] Worldwide trend of solar cell efficiency by type
Source: NREL (2016)
❙ Table 4-1 ❙ Efficiency and characteristics of solar cells by type
Type Characteristics Conversion
efficiency Stage Major Korean and overseas corporations
Silicon based Crystalline
Mono crystalline
Using thin monocrystalline Si wafer around 180 µm thickness
Advantage: performance, reliability Challenge: potential for lower
cost
~20%
(module) Commercialization
(South Korea) Hyundai Heavy Industries, LG Electronics,
Shinsung Solar Energy (Overseas) Sunpower (US), Sharp, Panasonic, Mitsubishi
Electric (Japan)
Polycrystalline
Using polycrystalline wafer consisting of small crystals
Advantage: cheaper than monocrystalline wafer Challenge: less efficient than
monocrystalline wafer
Commercialization Commercialization
(South Korea) Hanwha Q Cells, S-Energy (Overseas) Trina, JA Solar, Ginko (China), Sharp, Kyocera,
Mitsubishi Electric (Japan)
Thin-film
Amorphous or microcrystalline Si thin- film type deposited on a
wafer Advantage: large-scale, mass
production possibilities Challenge: low efficiency
~9%
(module) Commercialization
(South Korea) None (Overseas) Sharp, Kaneka, Fuji
Batteries (Japan), GS Solar (China), Next Power (Taiwan)
Compound based
CIGS
Thin-film type made of Cu, In, and Se Advantage: resource conservation, potential for mass
production, and potential for high-performance Challenge: unavailability
~16%
(module) Commercialization
(South Korea) CIGSone (Overseas) Solar Frontier (Japan), Harnergy (China),
Miasole (US)
CdTe
Thin-film type made of Cd and Te
Advantage: resource conservation, potential for mass
production and lower cost Challenge: Cd toxicity
~15%
(module) Commercialization (South Korea) None (Overseas) First Solar (US)
Concentrated photovoltaics
Application of light harvesting and III-V compound multi-
junction Advantage: ultra-high
performance Challenge: lower cost
~38%
(cell) Research phase
(South Korea) BJ Power, Any Casting, Peru (Overseas) Sharp (Japan), Amonix (US), Soitec (Germany)
Organic- based
Dye-sensitized
New type solar cell that generates power from the dye bonding to TiO2 to absorb light Advantage: potential for lower
cost Challenge: high efficiency,
durability
~12%
(cell) Research phase
(South Korea) DongjinSemichem, Sangbo,
Eagon (Overseas) Dyesol (Australia),
Fujikura (Japan)
Organic thin- film
Thin-film type using organic semiconductor Advantage: potential for lower
cost Challenge: high-efficiency,
durability
~12%
(cell) Research phase
(South Korea) Kolon, LG Chem (Overseas) Heliatek (Germany), Mitsubishi, Sumitomo, JX energy
(Japan)
Organic/
inorganic- Perovskite
New type solar cell that generates power using light-
absorbing properties of perovskite, an organic/inorganic
compound ~22%
(cell) Research phase
(South Korea) Korea Research Institute of Chemical Technology,
Ulsan National Institute of Science and Technology, Sungkyunkwan University
4.1.1.2.1 Crystalline Si Solar Cell
The Panasonic Company (post-merger with Sanyo) of Japan holds the world record for conversion efficiency of monocrystalline Si solar cells, 25.6% (143.7 cm2), set in 2014 by adopting a back-contact HIT solar-cell structure. As regards polycrystalline Si solar cells, Trina Solar in China set a new world record for conversion efficiency of 21.25% (156 mm × 156 mm) in 2015.
Currently, the back-surface-field p-type screen-printed cell accounts for more than 90% of the total global production of crystalline Si solar cells. The average efficiencies of monocrystalline (156 mm × 156 mm) and polycrystalline cells in production are 19.5~20% and 18%~19%, respectively.
The maximum theoretical efficiency of crystalline Si solar cells has been reached almost, which leaves little room for significant improvements in performance through research and development. Therefore, the focus of current research and development is on cost reduction through material development and manufacturing technology advancement rather than efficiency improvement. In addition, studies are being conducted on protective materials to increase battery life. Other current areas of research include reducing polysilicon consumption by decreasing the wafer thickness and searching for a solution to the decline in efficiency during the module production (cell-to-module [CTM] loss).
4.1.1.2.2 Thin-Film Solar Cell
Thanks to its low production cost and versatile applications, the thin-film solar cell is considered a promising candidate for the next-generation solar cell to replace the crystalline Si solar cell. The manufacturing process of thin-film solar cells is relatively simple and they can be manufactured with low- cost wafers, such as glass, instead of silicon wafer, which, in turn, reduces the manufacturing cost. Their light-weight and flexible properties are expected to contribute to the application of thin-film solar cells in a wide range of products, such as exterior cladding materials for buildings. Thin-film solar cells are classified into amorphous silicon, compound semiconductor (CIGS and CdTe), dye-sensitized (DSSC), organic (OPV), and perovskite solar cells. The compound semiconductor solar cells (CIGS and CdTe) and the amorphous Si solar cells are currently being mass produced. The CdTe and CIGS solar cells are expected to capture a higher market share in the future. The amorphous Si solar cells has dominated the early thin-film solar cell market, but delayed efforts to improve cost competitiveness and efficiency have gradually undermined their market position. However, amorphous Si solar-cell technology is being integrated into Si solar-cell technology because of its usefulness in improving the efficiency of crystalline silicon. Although CdTe solar cells currently has the largest market share of the thin-film solar cells, market expansion is expected to be limited because of the toxicity of Cd and the difficulties to procure raw material. The CIGS- type solar cells are expected to become a leading contributor to the thin-film solar cell market thanks to their
high efficiency and potential for low-cost production. However, challenges remain, such as that competitiveness must be ensured, particularly in the mass-production system to achieve breakthrough growth. Several companies have started to release various semi-mass-produced solar cells (dye-sensitized and organic solar cells) ahead of their commercialization. Generally, it is expected that the commercialization hurdles, such as low efficiency and stability, which have offset the benefits in terms of manufacturing cost and applicability, will be overcome to some extent. At the beginning of 2016, since the discovery of the organic-inorganic perovskite compound from doing research to solidify dyes for dye- sensitized solar-cells, a research institute in South Korea recorded the world-class level efficiency of 22.1%.
This raises the expectations about future high-efficiency and high-stability thin-film solar-cell technology.
Furthermore, the research on quantum dot solar cells is at the early stages. In pursuit of high photovoltaic conversion efficiency through the arrangement of different size particles, researchers attempt to take advantage of the phenomenon that smaller semiconductor particles can absorb light with shorter wavelengths and larger particles can absorb light with longer wavelengths.