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Solar Electricity Basics

SEPA focuses exclusively on solar electric technologies, of which there are two basic types – photovoltaics and solar-thermal electric. 

1. Solar Water heating

Solar hot water is water heated by the use of solar energy. 

Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage. The system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use. The systems may be used to heat water for a wide variety of uses, including home, business and industrial uses. Heating swimming pools, under floor heating or energy input for space heating or cooling are more specific examples. 

In many climates, a solar heating system can provide up to 85% of domestic hot water energy. This can include domestic non-electric concentrating solar thermal systems. 

Residential solar thermal installations can be subdivided into two kinds of systems: compact and pumped systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting such as 50 °C. Hence, hot water is always available. The combination of solar hot water heating and using the back-up heat from a wood stove chimney to heat water can enable a hot water system to work all year round in cooler climates without the supplemental heat requirement of a solar hot water system being met with fossil fuels or electricity. 

Among pumped options, there is an important distinction to be made regarding the sustainability of the design of the system. This relates to what source of energy powers the pump and its controls. The type of pumped solar thermal systems which use mains electricity to pump the fluid through the panels are called low carbon solar because the pumping negates the carbon savings of the solar by about 20%, according to data in a report called “Side by side testing of eight solar water heatings by DTI UK. However, zero-carbon pumped solar thermal systems use solar electricity which is generated onsite using photovoltaic to pump the fluid and to operate its control electronics. This represents a zero operational carbon footprint and is becoming an important design goal for innovative solar thermal systems. 

2. Photovoltaics

Photovoltaic (PV) is the technical word for solar panels that create electricity. Photovoltaic material, most commonly utilizing highly-purified silicon, converts sunlight directly into electricity. When sunlight strikes the material, electrons are dislodged, creating an electrical current which can be captured and harnessed. The photovoltaic materials can be several individual solar cells or a single thin layer, which make up a larger solar panel. 

Photovoltaic cells power many of the small calculators and wrist watches in use every day. More complex systems provide electricity to pump water, power communications equipment or highway construction signs, and a myriad of other small, off-grid uses, which almost always provide electricity for small and/or intermittent electricity needs that are cleaner and cheaper to operate than extending a power line or using liquid fuels. Rural solar electrification in the developing world, away from the electric grid, can also provide much needed electricity to replace or reduce the use of kerosene lamps, diesel generators, and wood fires for lighting, refrigeration, communication, etc. 

Photovoltaic Resource Map (Source: NREL)

Photovoltaic technologies are being developed that can be built right into building roofs, facades, canopies and windows. Thin-film technologies that don’t use any silicon at all are the focus of large amount of research, as well as concentrating photovoltaics (CPV), which uses lenses or mirrors to increase the sunlight intensity, thus increasing the amount of electricity produced. 

2. Solar Thermal Electricity

Solar thermal electricity technologies (also called concentrating solar power, or CSP) produce electric power by converting the sun`s energy into high-temperature heat using various mirror configurations, which is then channeled to an on-site power plant and used to make electricity through traditional heat-conversion technologies. The plant essentially consists of two parts: one that collects solar energy and converts it to heat, and another that converts the heat energy to electricity. 

Concentrating solar power systems can be sized for village power (10 kilowatts) or grid-connected applications (hundreds of megawatts). Some systems use thermal storage during cloudy periods or at night to produce electricity outside of the traditional solar daytime window. Others can be combined with natural gas and the resulting hybrid power plants provide high-value, dispatchable power. These attributes, along with high solar-to-electric conversion efficiencies, make concentrating solar power an attractiverenewable energy option in the southwest United States and other sunbelt regions worldwide. 

Concentrating Solar Power Resource Map (Source: NREL)