The Sun is the ultimate energy provider. The development of the renewable energy market is, to a large extent, based on that fact. Most of the green technologies currently operating harvest solar energy, directly or indirectly. Among indirect solar energy sources, wind, low grade geothermal (used in ground source heat pump systems), tidal and wave are either being largely employed and (wind) well established (low grade geothermal) or under development (tidal and wave).
Direct solar energy source consist of solar rays reaching the atmosphere. They can be collected by two different technologies: solar photovoltaics (also known as solar PV) and solar thermal systems. If you're interested in solar systems, we offer you a free of charge, easy to use and personalized service that will allow you to receive quotes from different solar energy suppliers. You just have to fill in the form on the right. Let’s see how they work and what are the differences between them.
The core difference between solar PV and solar thermal can be found in their working principles. Solar PV is based on the photovoltaic effect, by which a photon (the basic unit of light) impacting a surface made of a special material generates the release of an electron. Solar thermal, on the other hand, uses sunlight to heat a fluid (depending on the particular application, it can be water or other fluid).
The photovoltaic effect only takes place in a reduced number of materials, called semiconductors (such as silicon -monocrystalline, polycrystalline and amorphous- and cadmium telluride), that, after specific chemical procedures , allow the generation of an electric current when exposed to light. Those semiconductors are shaped into thin layers that conform the core element of solar cells, the basic element of a solar PV system, that produce a direct current.
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Solar cells are integrated into larger structures known as solar panels, where the desired values of current and voltage can be achieved. For grid- connected systems (that account for a large portion of the market), the use of an inverter is necessary (to convert DC generated by cells into AC, used in the grid).
The working principle of solar thermal systems is far less sophisticated, but as useful for the generation of consumable energy as the photovoltaic effect. It consists on the direct heating of water (or other fluids) by sunlight. That energy conversion takes place at different devices depending on the range of temperatures at which the working fluid is being heated.
Low and medium temperature collectors can be flat plate panels or evacuated tubes. High temperature collectors consist of concentrated solar systems, such as parabolic trough, Fresnel reflectors, dish Stirling and solar towers.
Regarding the use of those technologies, the first step is to separate small, household solar systems from power plants, in both PV and thermal.The domestic utilization of them is discussed below.
As to power plants, PV and thermal systems share the production of electricity as their aim. While PV systems generate that electricity directly from solar energy, thermal systems heat a fluid(water, oil, sats, air, etc.) that will run a steam engine, gas turbine or similar. That electricity is transferred to the grid, as AC and with the required value of voltage.
In the case of photovoltaic power stations, the capacity of the largest ones is above 500 MW, whereas that of the largest thermal power stations is lower than 400 MW. Both types are experiencing a rapid growth in many parts of the world, with many are under construction and more planned.
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Residential PV installations produce DC electricity directly from solar energy. There are two types of residential PV systems: grid-connected and “off grid” or “stand-alone”. The first ones offer the possibility of feed the grid (which is being encouraged by economic incentives) through an inverter. The latter ones are completely dedicated to self-consumption (lighting, heating, water heating, etc.) and require the use of battery banks to separate consumption from generation.
Residential solar thermal systems’ main use is water heating. They usually work combined with central heating systems (gas or fuel) oil, that start working when temperature in the water tank fall below a certain value. Thus, this systems can provide hot water all year round even in cool climates. Alternatively, thermodynamic systems can heat water using the built-in compressor, should you use up all the water; this way, you can establish a fossil-independent system. Solar thermal systems can also be used for space heating purposes (this is relatively common in countries such as Sweden).
Advantages and Disadvantages
Some of the most important advantages of solar PV:
- Regarding power plants, PV design is much simpler than that of thermal. A PV power station is formed of many solar panels connected in parallel and in serie. On the other hand, electricity generation using solar thermal technology implies the transmission of energy from a hot fluid to a generator. That has led to different and complex designs, some of them mentioned above.
- At a smaller scale, domestic PV systems are more versatile than thermal systems, since they can power many appliances (solar thermal is limited to space and water heating). They also have a larger lifespan.
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Some of the most outstanding advantages of solar thermal are:
- Its possibility to store the energy generated, since thermal power plants don’t convert solar energy directly into electricity and domestic thermal systems always include a water tank where heat can be stored. PV systems, on the contrary, suffer from an inherent intermittency that stems from sunlight patterns. The storage of electricity is normally not considered an option, due to energy losses that make it really inefficient. That results in a more uniform and reliable supply of electricity by thermal plants.
- Solar thermal technology for domestic use is much less complicated than solar PV. It’s important to point out that the energy generation for residential uses vary from thermal to PV systems. Thermal generate heat, while PV electricity. Thermal panels are also more space efficient.