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Solar Definitions

Renewable portfolio standards (RPS) programs

Renewable portfolio standards (RPS) programs obligate utility companies to derive a significant portion or target percentage of their total electricity from clean fuels by a given date.

For example, here in Colorado, Amendment 37 mandates that the utilities generate 10% of their electric energy from renewables by the year 2015.

In the United States, 28 states have implemented RPS programs to date.

One of the pitfalls (from the solar energy prespective only) is that much of the percentage of energy generated through RPS programs comes from centrally generated power, such as wind turbines. However, Colorado’s Amendment 37 made an explicit “carve-out” for solar contribution to the RPS target.

Types of Solar Energy

Different types of solar energy systems provide power for specific applications. Among these types of solar power are passive and active solar, as well as concentrating and nonconcentrating solar. To clear up some of the confusion regarding the types of solar energy systems and their respective applications, check out the breakdown below.

Passive Solar Energy – passive solar energy is used to convert sunlight into usable heat, cause air-movement for ventilation or cooling, or store heat for future use. Passive solar works best and proves to be most cost effective with building designs that are intended to effectively capture the sun’s heat and light. A simple example of a passive solar energy system is a greenhouse.

Key features of passive solar energy systems include:

  • No conversion of solar energy into electricity – the energy is simply collected and used or stored
  • Site selections and building placements maximize synchronized heating & lighting
  • Windows are placed in south-facing walls
  • Vents and ducts are moved to capture heat through the building
  • Trombe walls – dark, south-facing walls that absorb light and heat
  • Wide eaves
  • Heat-storing slabs
  • Superinsulation

Active Solar Energy (Photovoltaic or Thermal) – active solar energy captures the sun’s energy in order to store or convert it to thermal or electric power. In active solar energy systems, there is an active and intentional collection and redirection of energy.

There are two main types of active solar energy:

  • Thermal – used to generate heat for hot water, cooking, heating, melting, steam engines, etc.
  • Photovoltaic – used to generate electricity for both grid-tied and off-grid systems

Photovoltaic solar energy systems work by allowing light to hit the specific molecular structure, which consistes of a semiconductive substrate (usually silicon) that has been “doped” with chemical additives that create opposing positive and negative layers (known as the P and N types). When the photons of light strike the surface of the substrate, this causes electrons to move from P layer to N layer. This movement of electrons creates the electric current that powers many solar homes and buildings, as well as orbiting satellites.

Concentrating & Nonconcentrating Solar

While nonconcentrating solar does not involve the use of mirrors or other means to directly focus the sun’s light, concentrating solar uses mirrors to either focus sunlight on a PV array or to heat water or other fluids to create steam that drives turbine generators.

Concentrating solar is more complicated to build & manage, which translates into higher cost. Furthermore, concentrating solar involves moving parts, resulting in more maintenance. Accordingly, concentrating solar is more often used in larger-scale, centralized systems at commercial energy plants that tend to serve upwards of tens of thousands of homes and businesses.

Energy Return on Investment (EROI)

Energy Return on Investment (EROI) is the ratio between the amount of energy expended to obtain a particular energy resource and the amount of usable energy acquired from that resource. When the EROEI of a resource is equal to or lower than 1, that energy source becomes an “energy sink”, and can no longer be used as a primary source of energy.

A related concept is the energy pay-back time, i.e. the time required to produce an amount of energy as great as what was consumed during production.

In photovoltaics, crystalline silicon PV systems give energy payback times of 1.5-3.5 years, depending on the installation location. Expected efficiency improvements in silicon solar production techniques will reduce the required energy inputs, thereby bringing an energy payback of 1 year or less for silicon-based PV.

Peak watt

Peak watt is the maximum rated output of a photovoltaic device, such as a solar cell or array, under standardized test conditions, usually 1000 watts per square meter (0.645 watts per square inch) of sunlight with other conditions, such as temperature specified. Typical rating conditions are 68°F (20°C), ambient air temperature, and 1 m/s (6.2 x 10-3 miles/sec.) wind speed.

The Encylopedia of Alternative Energy and Sustainable Living


Microgeneration is the generation of zero or low-carbon heat and power by individuals, small businesses and communities to meet their own needs. Microgeneration technologies include small scale wind turbines, water turbines, ground source heat pumps, solar thermal collectors, solar electricity and MicroCHP installations.

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