The solar potential
There is more than enough solar radiation available around the world to satisfy the demand for solar power systems. The proportion of the sun’s rays that reaches the earth’s surface can satisfy global energy consumption 10,000 times over. On average, each square metre of land is exposed to enough sunlight to receive 1,700 kWh of energy every year.
The greater the available solar resource at a given location, the larger the quantity of electricity generated. Subtropical regions offer a better resource than more temperate latitudes. The average energy received in Europe is about 1,000 kWh per square metre per year, for example. This compares with 1,800 kWh in the Middle East. It has been calculated that if 0.71% of the European land mass was covered with PV modules, this would meet Europe’s entire electricity consumption. Furthermore, International Energy Agency (IEA) calculations show that if only 4% of the world’s very dry desert areas were used for PV installations, this would meet the whole world’s total primary energy demand. Considering the vast areas of unused space (roofs, building surfaces, fallow land, deserts etc) the potential is almost inexhaustible.
As a Technology
Solar technologies can supply the energy for all of a building’s needs—heating, cooling, hot water, light and electricity—without the harmful effects of greenhouse gas emissions created by fossil fuels. Solar applications can be used almost anywhere in the world and are appropriate for all building types such as single-family homes, multi-family residences, office and industrial buildings, schools, hospitals, and other public buildings.
In the agriculture sector, solar technology is being used to dry crops ranging from coffee and tea to wool and chicken manure. Companies in Europe, North America, and numerous developing countries see this technology as a cost-effective and environmentally sensitive process.
The majority of the energy used in commercial and industrial processes is below 250°C, a temperature range well suited for solar technologies. Solar technologies are being used for specific industrial processes, such as food processing, textile cleaning and drying, pharmaceutical and biochemical processes, desalination, and heating and cooling of factories.
SOLAR WATER HEATING – “water heated by the use of solar energy”
Solar energy can be captured as heat using thermal collectors. The two main types of collectors used in building applications are flat-plates and vacuum (or evacuated) tubes. The most common application of both types is for domestic hot water heating. Other applications include hot water services in schools, hotels and hospitals, etc, and for heating swimming pools. A solar water heater (SWH) is mainly made up of a solar panel and a tank. The solar panel contains a flat-plate collector enclosed in an insulated case and covered with glass. Water can pass through pipes bonded to the collector. An insulated storage tank is connected to the solar panel by a system of pipes, so that water can be circulated, heated and stored.
‘In many climates, a solar heating system can provide up to 85% of domestic hot water energy.’
• How do solar water heating systems work?
Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: ACTIVE, which have circulating pumps and controls, and PASSIVE, which don’t.
Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.
Three types of SOLAR COLLECTORS are used for residential applications:
• Flat-plate collector
Glazed flat-plate collectors are insulated, weatherproof boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors—typically used for solar pool heating—have a dark absorber plate, made of metal or polymer, without a cover or enclosure.
• Integral collector-storage systems
Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.
• Evacuated-tube solar collectors
They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin’s coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for U.S. commercial applications.
There are two types of ACTIVE solar water heating systems:
• Direct circulation systems
Pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes.
• Indirect circulation systems
Pumps circulate a non-freezing, heat-transfer fluid through the collectors and a heat exchanger. This heats the water that then flows into the home. They are popular in climates prone to freezing temperatures.
There are two basic types of PASSIVE systems:
Passive solar water heating systems are typically less expensive than active systems, can be more reliable and may last longer.
• Integral collector-storage passive systems
These work best in areas where temperatures rarely fall below freezing point. They also work well in households with significant daytime and evening hot-water needs.
• Thermosyphon systems
Water flows through the system when warm water rises, as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank. These systems are highly reliable.
Quick Facts and Figures:
A solar water heater works best on clear days regardless of the ambient temperature. On cloudy days, it still works but the water temperature will be relatively low.
A back-up electric heater placed in the solar storage tank reduces the optimum benefit that could be gained from the sun, since it heats all the water in the early morning hours.
The inclination, at which the solar panel is placed, depends on the purpose and season for maximum hot water usage. For winter, an angle of 60o to the horizontal is adequate. For summer, this angle should decrease to 30o. For Malta an average value of 40o could be used for the whole year.
Best orientation of solar water heating panel is towards the South, but differences of ±10o East or West, have little effect on the overall performance.
Shading by trees and buildings, affects the performance of solar water heaters. In such cases, one can change the orientation to face the sunniest area.
In most cases, installing a SWH requires minimum alterations in the water piping system of the building. Use of anti-reflective glass instead of plain glass sheets on flat plate solar panels, increase the system efficiency by about 7%.
