Solar photovoltaic (PV)
Photovoltaic is a convenient, quiet and clean way of providing long-term electrical power, both for remote locations and as a form of distributed power for houses connected to the electricity grid.
This technology converts light energy directly into electricity by transferring sunlight “photon” energy into electrical energy.
This conversion takes place within cells of specially fabricated semiconductor crystals. The main construction types of PV cells are:
Monocrystalline – this silicon crystal structure is formed by melting semiconductor grade silicon, and causing a single crystal growth around a small “seed” crystal
Polycrystalline – this produces a multiple-grained structure by controlling the cooling rate of silicon material that has been melted in a crucible
Amorphous or thin-film
New enhancements to crystalline cells include buried grid cells, triple-junction cells and “Slivercr” cells, which provide higher efficiency or lower production costs.
A number of cells (generally 36) make up a “PV module”, also known as a solar panel. PV modules can be interconnected to form arrays of different voltage and current capacities. Photovoltaic systems produce DC electricity, so power conditioning equipment such as inverters are required to produce AC for normal household appliances. Power output is very dependent on the solar radiation hitting the PV cells, so modules must be installed to face north and minimise shading to ensure maximum possible energy output.
Tracking systems can sometimes be used to allow the modules to point toward the sun and provide extra energy output. Photovoltaic systems are an ideal match to many summer electrical loads, producing maximum power when demand is also highest, so they are being increasingly used in grid-connected systems as distributed generators. In off-grid systems, some form of energy storage will be required for night or periods of low sunlight. This is usually in the form of battery storage.
While panels are the most commonly installed format for PV cells, there is large potential for “Building-integrated PV” (BiPV), where photovoltaic modules are incorporated into building fabrics or roof tiles. Concentrating systems allow small high-efficiency modules to be used, though extra cooling and specialised mounting construction are required.
Solar water heaters
A solar water heater uses the radiant heat from the sun, so requires less energy from fossil fuels. Heat is directly transferred from the collectors to potable water, or transferred from the collectors to a heat transfer liquid, then via a heat exchanger to the potable water. Other systems (‘heat pumps”) use the reverse of a refrigeration process, transferring heat from the air to the hot water tank.
In “close-coupled” systems, hot water flows naturally via the “thermosyphon” effect. The cold (and denser) heat transfer liquid or potable water flows down to the collectors, where it is heated. The heated liquid flows upward from the collectors into the roof-mounted hot water tank.
Pumped systems require some electrical energy for pumping water, but allow tanks to be located on the ground. In the storage tank, hot and cold water are kept separated through temperature stratification.
The main solar collector types are:
Flat plate – these comprise a flat metal box with a heat-absorbing surface, with tubes or in-built channels for water flow. The use of “selective” surfaces increases the panel’s heat absorption, and allows the collectors to operate at temperatures of up to 90C. Heat is transferred from the absorbing surface to the flowing water. Glazing is installed at the top of the panel to allow incoming solar radiation to pass through to the heat absorbing surface, while reducing outgoing heat loss from re-radiation or convection.
Evacuated tube – these comprise an outer tube, with internal concentric cylinders or a copper tube and fin arrangement. There is a vacuum layer between the inner and outer tubes, or between the copper tube and fin arrangement and the outer tube respectively. A transfer fluid or small quantity of water is heated in these systems, and flows as fluid or steam to a heat exchanger. In the heat exchanger, heat energy is transferred either directly to potable water, or to a heat transfer fluid. Tubes can be arranged in a bank to provide collector size modularity.
Heat Pumps – these systems transfer heat from the air to the hot water tank by the reverse of a normal refrigeration process. A refrigerant liquid absorbs heat from the air and expands. The refrigerant is then compressed, transferring heat to the hot water tank. Heat pump systems require some use of electricity, but provide typically three to four times that amount of energy as heat.
Boosting is available when solar input is lower than usual due to cloud cover, with systems designed for boosting by natural gas, liquefied petroleum gas (LPG), solid fuel heaters or an electric element. Even in southern parts of Australia such as Melbourne, solar energy is still able to supply up to 80% of annual hot water energy needs. Though solar input is maximized when collectors are installed facing north, there is some flexibility in orientation, so almost all houses should have a suitable roof area for collectors.
Solar pool heating systems are designed to continually pump water through unglazed lower cost collectors – generally multiple parallel tubes – to provide continuous low temperature rises when collector outlet temperatures are higher than pool temperatures.