Solar Energy - chapter 5

Electricity from the sun

CONTENTS

5.1 Introduction

5.2 The case for PV

5.3 Solar cells

5.4 Balance of system

5.5 PV-system characteristics

5.6 Implementation

5.7 Applications

5.8 References

5. 1 INTRODUCTION

In a solar cell light is converted into (electricity by means of the so called photovoltaic (PV) effect. PV is still enjoying large research and development efforts in order to produce more efficient and cheaper solar cells.
But solar electricity is already economically feasible compared to other energy sources for a number of applications.
In the past, inadequate system design and sizing of system components has led to unfavourable experiences. However in recent years PV has proved to be reliable if sufficient attention is paid to the design.

In this chapter a closer look will be taken at those situations in which PV comes into consideration. Next the technology of the solar cells and other components of a PV-system will be discussed. Subsequently some characteristics of a PV- system are discussed and some attention is paid to those aspects, which are important in designing a system. Finally some interesting applications will be examined.

Figure 13. Some solar panels (Source: ATOL, 1980)

5.2 THE CASE FOR PV

In rural areas and at remote locations solar electricity is an energy source, which can be a good alternative to other energy sources. At the moment PV is used mostly for small appliances such as refrigerators, telecommunication and navigation equipment. Nevertheless in California (USA) a large PV plant has been built which feeds the grid. In general however the use of PV in large projects and grid connected applications is too expensive to be of interest. When a grid is available, a connection with the grid is generally preferred to the installation of a PV-system.
When the reliability of the energy supply is crucial PV can offer a solution. This is for instance the case in vaccine refrigeration. When the vaccine temperature becomes too high, the vaccine can no longer be used for immunization. In many developing countries the performance of the grid is very poor, so PV might even be of interest in areas where a grid is available. The alternatives include back-up generators, accumulator batteries and other renewable energy sources.
The production of solar cells requires high technology. Only a few developing countries, i.e. India and Brazil, have production facilities for solar cells. Most countries can produce only the non-PV-system parts.
Another obstacle for the use of PV in developing countries is the high initial investment cost.
Nevertheless PV has some specific advantages which make it attractive for the developing world:

• PV can be used for very small appliances, where other sources, for instance diesel, are operating far below their nominal capacity and are therefore relatively expensive
• thanks to the modular character it is easy to enlarge a system to provide more energy or add more applications to it
• PV has no moving parts, therefore repair is almost never needed and the maintenance requirements are moderate
• when well designed a system operates very reliably
• no fuel is needed

The design of a PV-system, modules together with modules storage and electronic equipment, has to be carried out very carefully. Many of the unfortunate experiences with PV were related to a bad system design. PV can be very reliable as has been demonstrated in telecommunication projects where PV was chosen specifically because of its reliability, the same applies to buoys along the rivers and coasts. Even in The Netherlands with its small amount of sunshine and its large electricity grid, newly installed buoys are all PV powered.

5.3 SOLAR CELLS

Solar radiation can be converted directly into electricity using semiconductor devices, which are known as photovoltaic (PV) cells. The most commonly used material is silicon. By diffusing phosphorus or boron into the silicon it is possible to create p- and n-type silicon, each with its own electrical characteristics. A thin silicon wafer is divided into two layers. Both layers are provided with metallic contacts.
When sunlight falls upon the solar cell a part of the light is absorbed. The energy of the light releases electrons inside the silicon. When both sides of the cell are connected an electric current will start flowing.
The size of the current depends upon the intensity of the incoming radiation. Not all the energy of the light is converted into electrical energy.

Figure 14. Schematic cross view of a silicon solar cell

There is a number of semiconductor materials from which solar cells can be made. Until recently the most commonly used was mono-crystalline silicon. At the moment poly-crystalline and amorphous silicon are becoming more important. Table 2 gives the theoretical and achieved conversion efficiencies for a few types of solar cell materials.

Table 2. Efficiency of solar cells under standard irradiation (at sea level) for several semiconductor materials. The theoretical efficiency and the achieved efficiencies under laboratory conditions and in industrial production are given.

Instead of falling directly onto the flat plate modules, the sunlight can be concentrated first by the use of lenses or mirrors. The concentrated sunlight can be focussed on a solar cell, which increases the efficiency of the cell. In this way a record efficiency of 31% was recently achieved for a silicon-gallium arsenide tandemcell. This method enables a reduction of the costs of the array but on the other hand extra costs are incurred by the lenses; the system as a whole also becomes more complex. The technology for concentrating sunlight is still under research and is not commercially available.

Monocrystalline silicon

Monocrystalline silicon solar cell technology is based on the semiconductor technology used in the transistor and integrated circuit industry.
Using monocrystalline silicon wafers solar cells can be manufactured with a conversion efficiency of 13 - 15%. The conventional processes employed to obtain single crystal wafers are slow and very energy and material consuming.

Polycrystalline silicon

Monocrystalline silicon is gradually being replaced by polycrystalline silicon (sometimes also called semicrystalline silicon). Polycrystalline silicon can be produced at lower costs.
The efficiency of polycrystalline cells is 1 to 2% lower than the efficiency of monocrystalline. However combined with the use of cheaper silicon feedstock material, large cost reductions compared to conventional production methods are expected.

Amorphous silicon

Another option to reduce the costs of the cells is the use of amorphous silicon solar cells. These cells are very thin, and thus use very little material.
Amorphous silicon has made considerable progress. The first cells were produced in 1974. In 1985 the market share already had reached 30%. Commercial applications have been found in pocket calculators, watches and battery chargers.
One of the problems of amorphous silicon at the moment is the degradation of the cells. The cell efficiency decreases when light is falling upon the cell, especially during the first months of operation.

Other materials

Besides silicon other materials are under research for use as solar cells. CuInSe2, CdTe and GaAs look very promising in the long term, but in the coming years large-scale application of these types of cells is not expected.
The same applies to stacked solar cells. In these structures two or more cells with different characteristics are combined in order to utilize as much of the solar energy as possible.

5.4 BALANCE-OF-SYSTEM

All components of the system together, besides the modules, are called the balance-of-system (BOS). The composition of the balance-of-system depends on the kind of application and on the location of the PV-system.

The balance-of-system may comprise:

• array support structure
• connections/wiring
• power conditioning
• energy storage.

We will have a closer look at several elements of the balance-of-system.

Array support structure

The solar-cell modules rest on a array support structure. The array support structure is generally made out of aluminium or steel struts, resting on a concrete foundation. Research is being done to develop low cost constructions of wood and bamboo.
Another way of reducing costs is to mount the modules on the roofs of buildings. At the moment only limited experience with this kind of construction has been gained.
At present most systems have fixed arrays. In case of a tracking system it must keep the modules in an optimal orientation towards the sun. There are several options.

• Seasonally-adjusted tilt. A few times a year the arrays can be adjusted to the elevation of the sun.
• Single-axis or two-axis tracking. A drive mechanism keeps the modules in the direction of the sun during the whole day. The array structure can rotate in one or two directions.

Figure 15. Schematic of PV-system for household electrification (Source: Solar Energie Technik)

Power conditioning

The power conditioning can be composed of the following elements:

• controllers
• maximum power point tracking
• DC-AC converters
• interface between the PV-system and the grid
• electronic protection of the system.

The maximum power point tracking ensures that at any given moment, with any given amount of sunlight and any given cell temperature the maximum power is extracted from the modules.
In general electricity is supplied as AC (alternating current). Therefore a lot of equipment has been developed for AC- application. The PV modules, however, supply DC (direct current)-power. The consequence is that a choice has to be made between the use of DC-apparatus, not available for all appliances, and the installation of an inverter to convert DC into AC. To connect a PV- system with the grid, a special interface is needed including a DC-AC inverter.
To obtain the highest possible system efficiency it is important to lose only small amounts of energy in the power conditioning. At the moment an efficiency of 95% is possible. When the system is not working on full power the efficiency of the power conditioning does fall; sometimes only about 70% efficiency is left. The cost of the power conditioning depends on the need for AC or DC-voltages.

Energy storage

If electrical power is required when the sun is not shining or if there is a short peak demand, for instance to start an electric motor, some form of energy storage is needed or a back-up supply from a diesel or gasoline generator must be provided.
When a PV-system is used to pump up water, in many cases the choice will be to store water instead of electricity.

Several types of storage batteries are available; the lead acid battery is the most common, but Nickel-Cadmium (NiCd) batteries are also suitable.

The operation of the batteries requires much attention during the design of a PV-system. In a battery a certain amount of energy can be stored: this is the capacity of the battery. Lead acid batteries can only be discharged to 30% of the total capacity. From a technical point of view deeper discharge is possible, but the lifetime of the batteries then decreases dramatically. Moreover the total capacity of the battery will decline.
Batteries can also be overcharged. This also has a bad influence on the performance of the battery.
To keep the state of charge of the battery within the allowed range a battery controller can be used. This controller is part of the power conditioning.
A NiCd battery has a better performance. Its design makes it impossible to overcharge or discharge the NiCd-batteries to deeply. Also 100% of the capacity can be used. However NiCd batteries are at the moment (1989) two to three times as expensive as lead acid ones.
Many different batteries are available. A distinction can be made between open and closed batteries. The hermetically closed batteries need no refilling, because the water can not evaporate. Therefore closed batteries in general require less maintenance than open ones.
For uses in developing countries it is often better to transport the battery and the acid apart, so the battery will not age during the often lengthy transport time. When air mail is used, it is not even permitted to transport ready-to-use batteries. Because of safety precautions battery and acid have to be transported separately.
The number of charge/discharge cycles specifies the lifetime of the battery. Another factor of importance to the lifetime is the temperature in which the battery has to operate. The higher the temperature the shorter the lifetime. Here too NiCd has better characteristics than lead acid.

5.5 PV-SYSTEM CHARACTERISTICS

Lifetime

Since PV-systems have been used only relatively shortly for terrestrial applications, there is little experience on the lifetime.
The lifetime of the modules which are commercially available at the moment (crystalline silicon) and of the power conditioning is expected to be about 15 years. A longer lifetime is thought to be not unlikely.
Batteries have an expected lifetime of five to ten years at a temperature of 25°C. When operating at higher temperatures the lifetime is shortening. At 40°C the lifetime of a lead-acid battery reduces to about one third of the lifetime under standard conditions, while the lifetime of a NiCd battery reduces to about three quarters.
Failures which can occur in the modules are broken connections, cracked solar cells and corrosion. Moisture penetration is the biggest problem in the long term.
Apart from the possibility of sudden failure, the performance of the cells will slowly decline over the years, but this effect is relatively small. Bad connections between the modules and the other components of the system can reduce the performance of the system as a whole.
Most systems have a battery controller to prevent the batteries against over-charging and excessively deep unloading. There are many controllers on the market that are not properly designed and may cause system failures.

Modularity

PV-systems can easily be scaled to the electricity demand. A single module provides enough energy to light one house, a number of modules can provide enough energy for an entire medical centre. A new system could begin with one or two modules for the most urgent purposes. The system can be expanded when more applications are envisaged, or demand grows, or when additional funds become available. The original system in the mean time does not need to be replaced.
When the expansion does take place the composition of the whole system, modules, storage and power conditioning, must be taken into consideration, in order to maintain an optimal performance.

Maintenance and reliability

Because a PV-system does not have moving parts and therefore no mechanical wear, maintenance requirements are minimal. The necessary maintenance comprises:

• cleaning the collector surface
• electrical check on the modules; the wiring/connections and the power conditioning
• visual inspection of the modules for broken cells or surface, humidity, electrical connections, and so on
• visual inspection of the mechanical connections and the supporting structure, especially on corrosion,
• repairing or changing broken parts,
• maintenance and repair of the batteries.

Apart from the battery maintenance and (possibly) the collector cleaning a yearly service should be sufficient.

Cost

The most important cost factor is the initial investment. These costs are high compared to other energy sources. However, the costs over a life time are more favorable for PV. Because no fuel is used, the only costs after the installation are the maintenance costs. To make a real comparison, therefore, between different energy sources, all the costs during the life of a system have to be reckoned with.
The high initial investment is a large obstacle to the application of PV in developing countries.
It is sometimes suggested that PV has no costs after the investment. But in general that is not true. The batteries need to be replaced every few years. While the price of modules is declining, the batteries are a cost factor, which is becoming more and more important.

5.6 IMPLEMENTATION

Designing a PV-system requires care if disappointment in the performance of the system is to be avoided. The most important aspect is the amount of sunlight. The irradiation varies during the year and is also dependent on the latitude and the weather (climate). On a clear day in the tropics with the sun shining directly overhead the irradiation might exceed 1000 W/m; on cloudy days this will be less.
The number of solar cells needed depends on the demand for energy and the amount of sunlight in the same period. The use of energy storage might be necessary to overcome cloudy days or just to overcome the night.
The sizing of the batteries is of crucial importance, whereby there has to be reckoned with the worst season concerning the irradiation and allowance made for the worst season for sunlight.
The energy demand has to be looked at very carefully. Because of the high investment costs of PV and batteries it is important to reduce the demand as much as possible by the use of energy efficient apparatus, for instance TL-lights, and to try to fit the demand to the amount of solar radiation during the day.

5.7 APPLICATIONS

Vaccine refrigeration

Vaccines require refrigeration during transport and storage. Many vaccines are not resistant to heat. The cold chain therefore has to be reliable in order to make vaccination a success.
When the performance of the grid is poor as is the case in many developing countries, or there is no grid at all, a kerosene or bottled gas (LPG) powered refrigerator can be used. However their performance is also frequently inadequate for vaccine refrigeration.
PV refrigerators have therefore an important role in the vaccination programmes of rural areas in developing countries.
The World Health Organization (WHO) is one of the most experienced organization in this field. They have tested and evaluated PV refrigerators of over SO types for 12 suppliers in 30 countries. As a part of their Expanded Programme on Immunization (1), product information sheets are published in which test results are given for those refrigerators approved by the WHO.
At the moment there are about 1000 PV refrigerators installed throughout the world. At least 20 companies can supply PV refrigerators for vaccine storage.

Figure 16. Schematic of photovoltaic refrigerator system (Source: McNelis 1988)

System

In figure 16 a schematic diagram of a photovoltaic refrigerator is given.

The power supply for the refrigerator comprises a photovoltaic array and a battery for storage. The PV array charges the batteries via a charge regulator, to prevent the batteries from overloading and underloading. Via a regulator a DC-motor is powered. The motor is generally directly coupled to the compressor, which is responsible for the cooling inside the refrigerator. The regulator is used to keep the motor within the power range and to prevent overdischarge of the batteries. A thermostat is used to regulate the motor to keep the temperature inside the refrigerator at a constant level.

The World Health Organization (WHO) has outlined several specifications for refrigerators used for vaccine cooling:

• Continuous operation must be guaranteed during the lowest periods of insolation in the year. If other applications, such as lights, are included they should operate from a separate battery set.
• Five days of continuous operation must be guaranteed when the PV array is disconnected and the batteries are fully charged.
• The supplier is required to give a warranty for the replacement of any failing component. The minimum period is to be ten years for the PV array, five years for the batteries and two years for the other components.

Besides these three specifications the WHO has several technical specifications for the modules, the battery set, the freezer, the voltage regulator and the instrumentation. They have also prepared a list of essential spare parts for each of ten installed systems.

The refrigerators are available with capacities of 3.6 to 200 litres. For a village with 150 births a month, space for packed vaccines is necessary, but other medicines may be stored in the refrigerator too. It is important that the refrigerators are capable of freezing ice packs. These are used in transporting vaccines from the health centre to the field for immunization.
Not all DC-refrigerators can be used for vaccine storage as some suffer from internal temperature variations. These are only suitable for domestic purposes.

Telecommunications

Applications for PV-powered telecommunications have been found in a wide range, from small systems (one module, one battery) in health care communication projects to large systems operated by governments, public companies and private companies. PV is a commercial option in areas where the reliability of the power supply from the grid is poor or where there is no grid available.

System

A PV-powered telecommunication system comprises a PV array, a power conditioning unit, a battery storage unit and the telecommunication apparatus. The power conditioning can vary from a simple voltage regulator to controllers that optimize system performance. The system can also include equipment for remote control and monitoring.

Battery chargers

In developing countries a large number of batteries are used for radios, cassettes, flashlights and other applications. Often non-rechargeable batteries are used. Sometimes batteries can be recharged or car batteries (lead acid) are used. People often have to travel long distances to a charging station to reload these batteries.
A battery charging station on PV is a very simple system. Only PV-modules are needed. Storage of energy is not necessary. To charge one battery in one day a module of about 200 Wp is needed.
In several countries PV charging stations have proven to be a profitable option for local traders. A charging station can be part of a larger system by which the owner can earn some money to offset, for instance, his own battery costs.

Lighting

Electric light is an important improvement in the quality of life in areas, especially when compared to kerosene lights. Electric lighting is therefore one of the first uses for electricity in homes and other buildings. Because lighting does not need a lot of energy, in many cases extending the grid is too expensive. The use of PV as an energy source can be of help.
PV-lighting can be used for domestic and community buildings, such as schools and health centres. PV is also used as an energy source for streetlights (security) and tunnel lighting.

Water Pumping

Solar pumps are used to pump water from boreholes or from surface waters for rural drinking water supply and for irrigation. The use of PV for pumping should be considered in sunny locations for a not too large water demand in regions where the supply of fuel is inconvenient. When the wind regime at the location is sufficient a wind pump is usually more attractive than a PV pump.
There have been many problems in applying solar pumps in developing countries, but they have been overcome. More than 5,000 pumps are operating at the moment throughout the world. The guarantee of satisfactory performance depends on reliable data on insolation, water resources, water demand and well characteristics. The proper design of the whole system is complex, not least because of the many types of pumps available.

5.8 REFERENCES

• Davidson J./Komp R., The solar electric home. A photovoltaics how-to handbook, AATEC Publications, Ann Arbor, USA, 1984.
• Derrick A./Francis C./Bokalders V., Solar photovoltaic products. A guide for development workers, Intermediate Technology/The Beijer Institute/Swedish Missionary Council, London, United Kingdom, 1989.
• McNelis B./Derrick A./Starr M., Solar-powered electricity. A survey of photovoltaic power in developing countries, Intermediate Technology/Unesco,London, United Kingdom, 1988.