Photo Voltaic (PV) cells

Key Issues:

Photovoltaic systems have a number of merits and unique advantages over conventional power-generating technologies. PV systems can be designed for a variety of applications and operational requirements, and can be used for either centralised or distributed power generation. PV systems have no moving parts, are modular, easily expandable and even transportable in some cases. Energy independence and environmental compatibility are two attractive features of PV systems. The fuel (sunlight) is free, and no noise or pollution is created from operating PV systems. In general, PV systems that are well designed and properly installed require minimal maintenance and have long service lifetimes.

At present, the high cost of PV modules and equipment (as compared to conventional energy sources) is the primary limiting factor for the technology. Consequently, the economic value of PV systems is realised over many years. In some cases, the surface area requirements for PV arrays may be a limiting factor. Due to the diffuse nature of sunlight and the existing sunlight to electrical energy conversion efficiencies of photovoltaic devices, surface area requirements for PV array installations are on the order of 8 to 12 m² per kilowatt of installed peak array capacity.

 

How PV works This interactive animation (a 2Mb Flash file) shows how PV works and how adjusting design parameters can affect energy output. Click image to play.

 

Introduction 

The word photovoltaic is a marriage of the words 'photo', which means light, and 'voltaic', which refers to the production of electricity. Photovoltaic technology generates electricity from light.

Electricity is the existence (either static or flowing) of negatively charged particles called electrons. Certain materials, called semi-conductors, can be adapted to release electrons when they are exposed to light. One of the most common of these materials is silicon (an element found in, amongst other things, sand), which is the main material in 98% of solar PV cells made today.

All PV cells have at least two layers of such semiconductors: one that is positively charged and one that is negatively charged. When light shines on the semi-conductor, the electric field across the junction between these two layers causes electricity to flow - the greater the intensity of the light, the greater the flow of electricity.

Types of PV system

Grid Connected

The most popular type of solar PV system for homes and businesses. The solar system is connected to the local electricity network allowing any excess solar electricity produced to be sold to the utility. Electricity is taken back from the network outside daylight hours. An inverter is used to convert the DC power produced by the solar system to AC power needed to run normal electrical equipment.

Grid Support

The solar system is connected to the local electricity network and a back-up battery. Any excess solar electricity produced after the battery has been charged is then sold to the network. Ideal for use in areas of unreliable power supply.

Off-grid

Completely independent of the grid, the solar system is directly connected to a battery which stores the electricity generated and acts as the main power supply. An inverter can be used to provide AC power, enabling the use of normal appliances without mains power.

Hybrid System

A solar system can be combined with another source of power - a biomass generator, a wind turbine or diesel generator - to ensure a consistent supply of electricity. A hybrid system can be grid connect, stand alone or grid support.

Designing with PVs

suitable projects for solar installations

A project with a brief incorporating social and technical innovation is an ideal candidate for building-integrated PV (BIPV). The most important aspect to consider is the location of the site. The solar installation must receive as much light as possible. Shadows cast by tall trees and neighbouring buildings must be kept in mind during the design process. The best location for solar PV is obviously on the south-facing roof or side of a building.

estimating energy outputs from different products

The following energy outputs can be used as a rough rule of thumb for the UK (assuming a reasonable tilt, orientation and system efficiency):
• 1m2 crystalline array will provide a useful output of 90-110 kWh per year.
• 1m2 triple-junction thin film array will provide a useful output of 60-80 kWh per year.

finding optimum panel inclination for different latitudes in the UK

The maximum total annual solar radiation is usually at an orientation due south and at a tilt from the horizontal equal to the latitude of the site minus approximately 20 degrees. For example 30 degrees is an optimal tilt in Southern England, increasing to almost 40 degrees in Northern Scotland.

 how panels perform at different angles and orientations

If the optimum angle is not achievable, over 90% of the maximum annual energy can still be achieved at 10 degree and 50 degree tilts.
South-facing vertical facades generate around 70% of the maximum.

the effects of shade

Shading is critical. Minor shading can result in significant loss of energy. This is because the cell with the lowest illumination determines the operating current of the series string in which it is connected. Many modern modules use bypass diodes to minimise shade effects; but these effects must still be considered, preferably in the first stages of building design. Watch for landscaping, trees, even handrails. If shading is unavoidable, or poor light is expected on a regular basis, the best types of PV to use are amorphous thin-film, or multicrystalline thick-film.

how to maximise the energy benefits of pv cells

The more energy efficient the building, the greater the benefit of the PV cells. Bear in mind that PV glass laminates can often be doubly beneficial - providing thermal insulation as well as electricity - since they can be made of low emissivity glass.

relevant codes of practice and articles of legislation

- British Standards
- BS EN 61215
- BS EN 61646
- The Electric Supply Regulations 1988
- The Building Regulations 1991 (and amendments)
- The Construction (Design and Management) Regulations 1994
- A “Power purchase agreement” will need to be negotiated if electricity is to be sold. This is not as tricky as it sounds as electricity   networks and supply companies are legally obliged to cooperate with solar installations.
- Planning Policy Guidance Note PPG22 Renewable Energy (80k)

Health and Safety issues

The main issues to be aware of when appraising PV for CDM are:

- Weight: Modules may be heavy with some exceeding 50kg.
- Toxicity: No PV module releases sufficient toxic materials to cause any harm during installation or maintenance. A small   proportion of PV contains cadmium, the oxides of which are toxic in very small doses. Some of these may release toxic dust if   crushed during disposal.

• None of the PV products supplied by Solar Century contain any cadmium.

For a more detailed appraisal see the DTI guide : Photovoltaics in Buildings/ Safety and the CDM regulations.(available from BSRIA)

main details to consider

The main points to remember are:
• Method of fixing/ integration into the fabric must be detailed.
• Ensure that the fixing does not cover or shade any part of the PV cells.
• PV laminates are often constructed with only a narrow border.
• The fixing must allow for thermal expansion without breaking the glass.
• Weather sealing involves standard construction practices but all materials must be suitable for the temperatures likely to be met (i.e. temperatures at the back of the modules can rise to 80‹ if they are poorly ventilated or higher if they are directly insulated).
• The mounting option must allow for safe maintenance and possible replacement of individual modules. The life of the support structure must be at least that of the PV array. The preferred materials are aluminium, stainless steel or glass-fibre. Protection from corrosion is important especially as residual currents may be present.
• Wind loading
• Any extra weight
• How and where to run electrical wiring ( this may penetrate the waterproof skin)
• Where to place junction boxes

using PV glass laminates and flexible thin film PV architecturally

PV glass laminates are attractive and well suited to facades and transparent roof tops. They can be fitted to standard curtain walling structures and are suitable for any application where glass is used, as long as there is a reasonable level of light present. Low emissivity glass can be used to give additional thermal insulation benefits, or a PVB laminate can be used for the extra strength required by a roof top.
Thin-film PV is durable and flexible and is encased in a water-proof, self-cleaning polymer. It can be used in shingle form for roofing or in more unusual designs that exploit its flexibility.

how PV cells are affected by soiling

The degree of soiling will depend on the location but usually dust accumulation and self-cleaning reach a steady state after a few weeks if the array tilt is at least 15 degrees. In extreme cases dust may cause a power reduction of about 10%. At low tilts horizontal glazing bars can trap debris which could lead to shading of part of the array. The design of the system should aim to minimise uneven soiling.

the lifetimes of PV products

Most solar products have a lifetime of around thirty years. Modules of all types usually have a twenty year warranty, as do most thin-film integrated products. Crystalline PV slates and PV glass laminates usually have a ten year warranty. These times are only a rough guide and should be checked for each specific product.

©  Solar Century 2003 - reproduced with kind permission from Solar Century