What Are The Three Types Of Active Solar Energy?
Active solar energy refers to technologies that convert energy from the sun into usable heat and electricity for buildings and other applications. There are three main types of active solar energy systems: solar thermal systems, photovoltaics (PV), and concentrated solar power (CSP).
Solar thermal systems, sometimes called solar thermal collectors, use sunlight to directly heat water or indoor air. Solar thermal collectors are the most direct way to harness energy from the sun, and are used widely for residential and commercial heating applications.
Photovoltaic systems, also known as PV or solar electric, use solar cells to convert sunlight directly into electricity. PV systems generate clean, renewable electricity for homes, businesses, and utilities.
Concentrated solar power (CSP), also known as concentrated solar thermal, uses reflective surfaces like mirrors to concentrate sunlight onto a receiver, which then generates heat that is used to spin a turbine to produce electricity. CSP is mostly used for utility-scale power plants.
Solar Thermal Energy
Solar thermal energy utilizes the sun’s thermal radiation to heat water or air. Solar thermal collectors, such as flat plates, evacuated tubes, or concentrating collectors, absorb the thermal radiation and transfer the heat to a fluid passing through tubes in the collector. This heated fluid is then used for various applications like heating swimming pools, domestic hot water, or space heating.
Some key advantages of solar thermal technology are its simplicity and reliability. Solar thermal systems require minimal maintenance and have a long lifespan. The technology is also versatile and suitable for residential, commercial, and industrial applications. Solar thermal does not generate any greenhouse gas emissions or pollution during operation. It can reduce electricity or gas consumption significantly when displacing conventional water and space heating.
However, solar thermal systems have high upfront costs and can be less efficient in colder climates. They require adequate solar radiation and sunny weather to function optimally. Solar thermal only collects heat energy from the sun and cannot generate electricity like solar photovoltaics. Overall, solar thermal technology is a mature, reliable, and environmentally-friendly technology for domestic and commercial heating applications.
Solar Thermal Collectors
There are several different types of collectors used in active solar thermal systems to absorb and transfer solar radiation:
Flat plate collectors
Flat plate collectors are the most common type of solar thermal collector. They consist of a black absorber plate with tubes running through it or bonded to it, enclosed in an insulated casing with a glass or polymer cover. The glass allows shortwave radiation from the sun to pass through and hit the absorber plate while limiting convection and infrared heat losses. Flat plate collectors are relatively inexpensive and efficient, with typical efficiencies around 60-80%.
Evacuated tube collectors
Evacuated tube collectors are made up of rows of parallel transparent glass tubes, each containing a glass or metal absorber tube connected to a fin. The air is evacuated from the space between the two tubes to essentially eliminate convection and conduction heat losses. This allows evacuated tube collectors to operate at higher temperatures and achieve higher efficiencies than flat plate collectors, typically around 60-90%.
Concentrating collectors
Concentrating or focusing collectors use reflective surfaces like mirrors or lenses to concentrate solar radiation onto a small absorber area. The concentration increases temperatures and efficiencies, allowing the collectors to reach 225-400°F. Parabolic troughs, parabolic dishes, and linear Fresnel reflectors are common types of concentrating collectors. They require direct beam radiation and sun tracking systems to focus sunlight properly.
Solar Thermal Applications
Solar thermal energy has a wide variety of applications for heating water and spaces. Here are some of the most common uses:
Water Heating – Solar thermal collectors are often used to heat water for residential and commercial use. Solar water heating systems can be active systems that circulate water or antifreeze through collectors and into a storage tank, or passive systems that allow water to be heated as it flows directly through the collectors.
Space Heating – Solar space heating systems use solar thermal collectors to absorb heat and then transfer that heat directly to interior air or to a thermal mass, like tiled floors. These systems help reduce conventional heating needs.
Industrial Processes – Solar thermal technology can provide heat for industrial processes up to 300°F. Common applications include generating hot water and steam, drying crops and lumber, pasteurizing dairy products, and providing heat for industrial processes.
Desalination – Solar thermal energy can provide heat for desalination processes that remove salts and impurities from water. This makes saline water safe for drinking and makes seawater usable for irrigation.
Solar Air Conditioning – Thermally-driven air conditioning and refrigeration systems are available that use solar thermal collectors. These systems concentrate solar radiation to generate high temperatures to power an absorption cooling cycle.
Photovoltaics (PV)
Photovoltaic (PV) cells, also known as solar cells, are semiconducting devices that convert sunlight directly into electricity using the photovoltaic effect. When sunlight strikes the semiconductor material in a solar cell, electrons are energized and flow, creating an electric current that can power electrical loads. The photovoltaic effect refers to photons of sunlight energy exciting electrons in the semiconductor material into a higher state of energy, allowing them to move freely. This generates electricity in the cell.
The most common material for solar cells is crystalline silicon (c-Si), but other types include thin-film solar cells, multi-junction solar cells, organic and dye-sensitized solar cells. Crystalline silicon cells are the traditional technology with higher efficiencies but relatively higher costs.
Thin film solar cells use extremely thin layers of photosensitive materials on a substrate like glass or plastic. They are lighter and flexible but less efficient. Common thin-film materials include cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous and micromorph silicon. Organic PV and dye-sensitized solar cells are emerging technologies with potential for low costs but lower efficiency levels currently.
Photovoltaic (PV) System Components
The key components of photovoltaic systems that convert sunlight into usable electricity include:
Inverters
Inverters convert the DC output of solar panels into AC and also connect to and interact with the utility grid. Inverters are a critical component that account for approximately 25% of a PV system’s cost. Most inverters today have smart capabilities to maximize energy harvest.
Batteries
Batteries store energy for use when the sun is not shining. They allow solar PV systems to provide power at night and during cloudy weather. Batteries also help stabilize PV system output and integrate with the wider grid. Lithium-ion batteries are the predominant technology today.
Charge Controllers
Charge controllers regulate how much current flows into and out of batteries. They prevent overcharging batteries and regulate voltage. They are essential for ensuring batteries are cycled properly and prolonging their lifespan.
In addition to the components above, photovoltaic systems require racking, wiring, safety disconnects, surge protection, and monitoring. Careful integration of all components is key for optimal system performance.
PV Applications
There are two main types of PV systems: off-grid (standalone) and grid-tied (connected to the utility grid).
Off-grid PV systems operate independently of the utility grid. They are commonly used in remote locations where connecting to the electrical grid is impractical or very expensive. Off-grid systems store solar energy in batteries for use when the sun is not shining. They are sized to meet the electric loads present.
Grid-tied PV systems are connected to the utility grid via a bi-directional meter. During the daytime, the solar panels generate electricity to power the building’s loads. If more electricity is produced than needed, the excess is fed back into the grid. At night or on cloudy days, electricity is drawn from the grid to meet the building’s electric needs.
Grid-tied systems do not use battery storage. The utility grid effectively serves as the battery backup. Grid-tied systems are commonly installed on rooftops of homes, businesses, and other buildings in urban and suburban areas with access to the electric grid.
With net metering policies, grid-tied PV system owners receive credit for excess electricity fed into the grid, which offsets electricity drawn from the grid at other times. This allows PV system owners to effectively use the grid for energy storage.
Concentrated Solar Power (CSP)
Concentrated solar power (CSP) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight onto a small area. Electrical power is produced when the concentrated light is converted to heat, which drives a heat engine (usually a steam turbine) connected to an electrical power generator.
There are four main types of CSP technologies:
Parabolic trough – Long rectangular, curved mirrors that concentrate the sun’s energy onto a tube containing a fluid that runs the length of the trough. The heated fluid is used to produce steam to drive a turbine.
Power tower – Hundreds or thousands of sun-tracking mirrors (heliostats) focus sunlight onto a central receiver atop a tower, heating a fluid to create steam to drive a turbine.
Parabolic dish – A parabolic dish of mirrors concentrates solar energy onto a receiver at the focal point of the dish. The receiver collects the heat, which is used to generate electricity in an engine generator.
Linear Fresnel reflector – Long, thin segments of mirrors placed at different angles to concentrate sunlight onto elevated receivers. Fluid in the receivers is heated to produce steam for turbines.
Concentrated Solar Power (CSP) Plants
Utility-scale concentrated solar power (CSP) plants use mirrors or lenses to concentrate sunlight onto a receiver, generating high temperatures to operate a traditional power cycle. The concentrated light is converted into heat, which drives a steam turbine or engine connected to an electrical power generator.
There are four main types of CSP technologies: parabolic trough, linear Fresnel reflector, power tower, and dish/engine systems. Parabolic trough plants are the most common, using U-shaped mirrors to reflect and concentrate sunlight onto a receiver tube with a heat-transfer fluid that is used to produce steam. Linear Fresnel reflector systems are similar but use long, thin segments of flat mirrors in place of the parabolic-shaped mirrors.
Power tower systems utilize a central receiver atop a tower surrounded by a field of flat mirrors called heliostats. The heliostats track and reflect sunlight onto the receiver to heat a transfer fluid that generates steam. Dish/engine systems use dish-shaped mirrors that concentrate sunlight onto a receiver at the focal point, where the heat converts a fluid to gas that drives an engine/generator.
CSP plants allow energy storage by using molten salt as the transfer fluid, which retains heat efficiently. This thermal energy storage provides the ability to shift solar power to peak demand periods when electricity prices are highest. CSP with storage offers a solution to overcome intermittency issues and enable reliable, dispatchable renewable energy generation.
Comparisons and Conclusions
Each of the three types of active solar energy have their advantages and disadvantages and are suited to different applications. Here is a comparison of the pros and cons of each:
Solar Thermal
Pros: Simple technology, low maintenance, cost-effective for water and space heating applications.
Cons: Only useful for heating applications, not electrical generation. Large installations require significant roof space.
Best Uses: Water heating, space heating, heating pools.
Photovoltaics (PV)
Pros: Directly generates electricity from sunlight. Modular and scalable. Can be installed on homes, businesses, and large-scale solar farms.
Cons: Relatively low sunlight-to-electricity efficiency. Performance depends on weather and latitude. Higher cost per watt compared to CSP.
Best uses: Distributed electrical generation, especially in remote off-grid locations. Rooftop home and business solar. Large utility-scale PV solar farms.
Concentrated Solar Power (CSP)
Pros: Very high efficiency for utility-scale electricity generation. Thermal storage enables power on demand.
Cons: Complex technology. Only suitable for large centralized power plants. Higher land use requirements than PV.
Best uses: Large-scale solar power plants, especially with thermal storage.
In terms of growth projections, PV is expected to continue strong growth globally, especially for distributed rooftop applications. CSP will see growth in sun-rich regions with increasing demand for solar thermal storage. Solar thermal for heating will remain stable or decline in maturing markets with increasing electrification. Overall, solar power is projected to provide over 10% of global electricity by 2030, with PV making up the large majority of installed capacity.
