Bigger gaps make PV cells more efficient

by the CADDET Japanese National Team

A new photovoltaic (PV) module design, developed in Japan, has increased PV cell output by 4% and reduced the number of cells in each module by 11%. The new reflector­concentrator PV module has larger gaps between its PV cells. Solar radiation passing through these gaps is diffused by a reflection film and is picked up by the surrounding cells.

Background

Improving the conversion efficiency of PV cells and reducing the costs of PV power generation systems are essential steps if this renewable technology is to be more widely used. The Sharp Corporation has developed a new PV module, based on an idea which came out of their research on exploiting insolation on PV modules more effectively. This PV module won the Agency of Natural Resources and Energy Director General Prize in 'the New Energy Vanguard 21' of 1997 - the award programme for most advanced new energy equipment, systems and applications.

The new PV module module (NT51FL6).

Until now, PV modules have been designed so that the gaps between PV cells connected in series are as narrow as possible, in order to fit the maximum number of cells onto the module and achieve the highest possible electrical output. In contrast, Sharp's new design has wider gaps which allow solar radiation to pass through to a reflecting film where it is diffused and received by the surrounding cells. By effectively exploiting the incident solar radiation in this simple way, the number of PV cells on the new module can be reduced without loss of power output, compared to a conventional module.

Motivation

The module was developed with a view, in part, to coping with the short supply and rising price of silicon (the raw material for PV cells) caused by the rapid growth of the semi-conductor industry in recent years.

Reducing the number of cells on a module also improves the lifecycle energy efficiency of the PV system, as the energy it generates 'pays back' the energy used in its manufacture more quickly.

The principle

The new reflecto-concentrator PV module (type NT51FL6) uses single crystal silicon PV cells with a high conversion efficiency of 17%. These are arranged at intervals 10 times wider than those in a conventional module. The wider gaps make it easier for the PV cells to collect the insolation received by the module, thereby enhancing the short-circuit current (Isc) each PV cell generates by about 4% (from 5.70 A on a conventional module to 5.93 A). The number of cells on the module can thus be reduced by 11% (from 54 to 48). The size and arrangement of cells in the new module are shown in figure 1.

Figure 1: The new reflector-concentrator PV module NT51FL6

Solar radiation passing through the gaps between cells is reflected and diffused by a backing film with fine reflection grooves laid under the PV cells. It is reflected again by the module's glass front cover and is received by the cells surrounding the gaps, contributing to the module's power generation. The principle of how this is achieved is illustrated in figure 2.

Figure 2: Reflector­concentrator principle of concentration.

How it compares

The effect of the width of the interval between cells (L) on the maximum power (Pmax) of a cell was measured by using an experimental module to investigate the concentration efficiency. The results are shown in figure 3: in essence, the effect on power output of increasing the interval starts to decline beyond 30 mm. The new module was designed to take account of these results and to use the same frame components and single crystal silicon cells as the company's conventional module (type NT51EM6).

The specifications for the new PV module are compared with those of the conventional one in the table.

Figure 3: Variation of the interval between the cells and the relative value of the
 maximum power (Pmax).

 

 

Reflector-concentrator
 module
 NT51FL6

Conventional
 module
 NT51EM6

Number of cells connected in series

 

48

54

Dimensions (mm)

 

1200 x 802 x 46

1200 x 802 x 46

Weight (kg)

 

12.5

12.5

Power output characteristics

Open circuit voltage (V)

29.9

33.6

 

Short circuit current (A)

5.93

5.70

 

Maximum power voltage (V)

23.9

26.9

 

Maximum power current (A)

5.27

5.05

 

Maximum power (W)

126

136

Applications

The Sharp Corporation has 4 models of PV power generation systems on the home-use market: a 2 kW system with 16 PV modules; a 3 kW system with 24 modules; and a 4 kW system with 32 modules. In the most commonly used 3 kW system, the new module design reduces the number of PV cells used in the system by 144.

For more information contact the CADDET Japanese National Team in Tokyo.

A 3 kW residential system with 24 modules.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.