Does Solar Rely On Light Or Heat?

Solar power is a renewable energy source that utilizes radiation from the sun to generate electricity or heat. There are two main technologies used to harness solar energy:

Photovoltaics (PV) convert sunlight directly into electricity using solar cells.

Solar thermal systems use mirrors or lenses to concentrate solar radiation to heat some type of fluid, which is then used to create steam to drive a generator.

The amount of solar energy that reaches Earth’s surface is enormous, totalling around 1000 watts per square meter. Solar power provides a clean and sustainable way to meet some of our energy needs, without emitting greenhouse gases or other pollutants. This content will examine how solar energy works through photovoltaics and solar thermal systems, comparing their efficiency and real-world applications.

Table of Contents

Solar Cells and Photovoltaics

Solar cells, also called photovoltaic (PV) cells, convert sunlight directly into electricity. PV gets its name from the process of converting light (photons) to electricity (voltage), which is called the photovoltaic effect. The photovoltaic effect was first discovered in 1839 by French physicist Edmond Becquerel. It wasn’t until 1954 that researchers at Bell Labs in the United States created the first practical silicon solar cell with 4% efficiency.

Solar cells are made of semiconductor materials, such as silicon. When sunlight hits the solar cell, photons from the sunlight are absorbed by the semiconductor material. This frees electrons from their atoms, allowing them to flow through the material to produce electricity. The photovoltaic effect causes the electrons to flow in one direction across the “P-N junction” in the solar cell, creating a DC current. This electricity generated can then be used to power electrical loads.

Solar cells are combined together into solar panels or PV modules that can be installed on rooftops, vehicles, or ground mounts. Utility-scale solar farms have thousands of solar panels that generate electricity to feed into the grid. Solar cells and panels have become more efficient and cost-competitive over the decades, making PV electricity a fast-growing renewable energy source.

Sources:

https://www.explainthatstuff.com/solarcells.html

https://unboundsolar.com/blog/how-do-solar-cells-work

Solar Thermal Energy

Solar thermal systems directly convert sunlight into heat using solar collectors ^[1]. There are two main types of solar thermal collectors: flat plate collectors and concentrating/evacuated tube collectors.^[2] The flat plate collectors consist of a dark flat surface that absorbs sunlight and converts it to thermal energy. The heat is then transferred through the plate to water or other fluid that flows through tubes embedded in the panel. This heated fluid is stored in a tank and later used for hot water and space heating. The concentrating or evacuated tube collectors have plastic or glass tubes with a reflective coating that focuses sunlight onto the absorber tube running through the center. They operate at higher temperatures which makes them more efficient but also more expensive. In solar thermal systems, it is the heat from the sun’s radiation that gets absorbed and then utilized, not the sunlight itself like in photovoltaics. This thermal energy can be used for a variety of applications including heating water for residential use, desalination, and even for electricity generation via a steam turbine.^[3]

^[1] https://www.solarsource.com/solar-basics-understanding-types-solar-systems
^[2] https://newheat.com/en/how-does-solar-thermal-energy-work/
^[3] https://www.essentracomponents.com/en-us/news/industries/renewable-energy/solar-thermal-vs-solar-pv-panels

Photovoltaic vs Thermal

There are two main types of solar technology: photovoltaics (PV) and solar thermal. The key difference between the two is how they convert the sun’s energy into usable energy.

Photovoltaic panels, also known as solar panels, convert sunlight directly into electricity through the photovoltaic effect. They are made up of solar cells, usually made from silicon, that absorb photons from sunlight and release electrons to generate an electric current. This direct current electricity can then be used to power homes, businesses or fed into the grid (InvestorVillage.com, URL).

Solar thermal technologies, on the other hand, use the sun’s energy to heat a fluid which can then be used to produce hot water, heat spaces or generate electricity via a steam turbine. Common solar thermal collectors include flat plates, evacuated tubes, parabolic troughs, Fresnel lenses and solar towers (SolarFacts.net, URL).

So in summary, PV converts sunlight into electricity whereas solar thermal converts sunlight into heat. PV panels are generally more efficient but solar thermal can offer cheaper energy storage by using the heat to warm tanks of water or other fluids. Both technologies have an important role to play in harnessing the power of the sun.

Efficiency of Light vs Heat

When comparing the efficiency of photovoltaic (PV) solar panels versus solar thermal panels, PV panels tend to be more efficient at converting sunlight into usable energy. PV panels convert sunlight directly into electricity through the photovoltaic effect, whereas thermal panels use sunlight to heat up a liquid which can then heat water or spaces.

Typical PV solar panels have an efficiency rating of 15-20%, meaning they can convert 15-20% of the sunlight that hits them into electrical energy. On the other hand, solar thermal panels tend to have efficiencies around 60-80%. However, the thermal efficiency rating refers to how much of the absorbed heat is transferred, not how much total solar energy is converted. Factoring in absorption and other losses, the overall solar-to-thermal efficiency of solar thermal systems ends up being around 20-40% ^[1].

Therefore, when considering total efficiency from sunlight to usable energy, PV and thermal solar end up having comparable overall efficiency in the 15-40% range. However, PV solar panels tend to sit at the higher end of this range, giving them an edge when maximizing energy output from available sunlight. The type of solar chosen depends on the specific application and needs.

Applications

Photovoltaic (PV) cells and solar thermal technologies are used in a variety of applications, each with their own advantages and limitations. Solar PV applications directly convert sunlight into electricity using semiconducting materials. They are commonly used in rooftop solar systems, solar farms, satellites, calculators and watches (Timilsina, Kurdgelashvili and Narbel 2012). PV systems can generate electricity even in low, diffuse light conditions, making them suitable for cloudy climates. However, they are less efficient at converting diffuse light compared to direct beam sunlight. Solar PV works best in applications that require electricity directly, rather than heat.

Solar thermal technologies like concentrated solar power (CSP) use mirrors or lenses to concentrate sunlight to heat a transfer fluid like oil or molten salt. The heated fluid is then used to generate electricity via a steam turbine or heat engine. Solar thermal is often used for utility-scale electricity generation as well as district heating systems, water heating, space heating, industrial process heat, desalination and enhanced oil recovery (Mekhilef, Safari, Mustaffa et al. 2012). Compared to PV, solar thermal is generally better suited for applications that require heat energy directly. The concentration and absorption of diffuse sunlight is also more effective. However, solar thermal cannot produce electricity in cloudy conditions without energy storage.

In general, PV is advantageous for small-scale distributed electricity generation while solar thermal plays a greater role in centralized, bulk power supply. Hybrid PV-thermal systems are also gaining interest, providing both electricity and heat from the same installation (Tyagi, Buddhi and Kothari 2013). Understanding the end-use application is key to selecting the optimal solar technology.

References:

Timilsina, Govinda R., Lado Kurdgelashvili, and Patrick A. Narbel. “Solar energy: Markets, economics and policies.” Renewable and sustainable energy reviews 16.1 (2012): 449-465.

Mekhilef, Saad, Riyanto Safari, W. E. S. Mustaffa, Rizalman Mamat, and Ahmad N. Abdulateef. “Solar energy in Malaysia: Current state and prospects.” Renewable and Sustainable Energy Reviews 16.1 (2012): 386-396.

Tyagi, V. V., D. Buddhi, and R. P. Kothari. “Work status on different solar cooker designs and characterization.” Renewable and Sustainable Energy Reviews 16.3 (2012): 1379-1388.

Costs

The cost of solar photovoltaic and solar thermal systems is an important factor when considering which technology to use. According to the 2018 GenCost report, solar PV has become one of the cheapest sources of new electricity generation. The levelized cost of energy for utility-scale solar PV in 2018 was around $40-$50 per MWh, making it very competitive with fossil fuels.

For residential systems, the cost per watt of installed solar PV has dropped dramatically in the past decade, from over $7 per watt in 2010 to around $2.70 per watt in 2020. This is largely due to improvements in panel efficiency and manufacturing economies of scale. The average solar PV system for a home costs between $15,000-$25,000 for a 5-8 kW system as of 2022.

Solar thermal systems that provide hot water and heating have a higher upfront cost than solar PV. A typical residential system runs $6,000-$12,000 for equipment and installation. Larger commercial solar thermal installations are in the range of $20,000-$1 million. Operation and maintenance costs are also higher for solar thermal due to the mechanical components and heat transfer fluids required.

When comparing total 20-30 year lifetime costs, both technologies can provide savings versus conventional energy. However, solar PV currently has a clear advantage in upfront system costs.

Environmental Impact

Solar energy has a positive environmental impact by reducing greenhouse gas emissions compared to fossil fuel sources. According to Sunnova, solar power emits no greenhouse gases during operation. The Solar Energy Industries Association estimates solar generation avoided over 50 million metric tons of carbon dioxide in 2016 alone. This is equivalent to taking nearly 10 million cars off the road.

Solar projects do require land, which can impact habitats. However proper siting, planning, and mitigation can reduce these effects. The amount of land needed depends on the solar technology. Photovoltaic panels mounted on rooftops have minimal land impact. Larger utility-scale projects may require a few hundred acres. According to Dataforma, this is still small compared to other energy sources when factoring in mining and drilling impacts.

Overall, solar power delivers clean renewable energy with manageable land requirements. The emissions avoided provide significant environmental benefits over fossil fuel energy sources.

Future Outlook

The future looks bright for solar power. According to the About Solar Energy / Why Solar Energy report, solar power capacity has grown at an annual rate of over 40% in recent years and projections point to continued strong growth. Many experts predict that solar will become one of the main sources of electricity globally within the next 10-20 years.

One key driver of growth in solar power is rapidly declining costs. According to research cited in the Greening the Internet Solar Power Role in Digital Connectivity article, costs of solar photovoltaics have dropped 82% in the last decade. As solar technology improves and economies of scale further reduce costs, solar power is expected to eventually achieve grid parity with fossil fuel sources.

Government incentives and policy support also play a major role. Many countries have set ambitious renewable energy and carbon reduction targets, with solar power being a centerpiece of these efforts. Continued policy support through tax credits, feed-in tariffs, renewable portfolio standards and other mechanisms will help drive growth well into the future.

In summary, with its falling prices, environmental benefits and policy momentum, solar power is poised for tremendous global growth over the coming decades.

Conclusion

In summary, solar energy relies on both light and heat to generate electricity and heat. Photovoltaic solar cells convert sunlight directly into electricity using the photoelectric effect. Meanwhile, solar thermal collectors absorb heat from the sun to warm up water or other fluids that can be used for heating applications. While photovoltaics are more efficient at converting light to electricity, solar thermal systems can provide heat at a lower cost than photovoltaics in some situations. Both technologies have seen dramatic growth and cost reductions in recent years, making solar energy increasingly competitive. With continuing advances, solar energy is expected to play a major role in the global transition to renewable energy.